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Lecture 15A.1: Offshore Structures:
General Introduction
OBJECTIVE/SCOPE
To identify the basic vocabulary, to introduce the major concepts for offshore platform structures, and to explain
where the basic structural requirements for design are generated.
PREREQUISITES
None.
SUMMARY
The lecture starts with a presentation of the importance of offshore hydro-carbon exploitation, the basic steps in
the development process (from seismic exploration to platform removal) and the introduction of the major
structural concepts (jacket-based, GBS-based, TLP, floating). The major codes are identified.
For the fixed platform concepts (jacket and GBS), the different execution phases are briefly explained: design,
fabrication and installation. Special attention is given to some principles of topside design.
A basic introduction to cost aspects is presented.
Finally terms are introduced through a glossary.
1. INTRODUCTION
Offshore platforms are constructed to produce the hydrocarbons oil and gas. The contribution of offshore oil
production in the year 1988 to the world energy consumption was 9% and is estimated to be 24% in 2000.
The investment (CAPEX) required at present to produce one barrel of oil per day ($/B/D) and the production costs
(OPEX) per barrel are depicted in the table below.
World oil production in 1988 was 63 million barrel/day. These figures clearly indicate the challenge for the offshore
designer: a growing contribution is required from offshore exploitation, a very capital intensive activity.
Figure 1 shows the distribution of the oil and gas fields in the North Sea, a major contribution to the world offshore
hydrocarbons. It also indicates the onshore fields in England, the Netherlands and Germany.
Condition CAPEX $/B/D OPEX
$/B
Conventional
Average 4000 - 8000 5
Middle East 500 - 3000 1
Non-Opec 3000 - 12000 8
Offshore
North Sea 10000 - 25000 5 - 10
Deepwater 15000 - 35000 10 - 15
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2. OFFSHORE PLATFORMS
2.1 Introduction of Basic Types
The overwhelming majority of platforms are piled-jacket with deck structures, all built in steel (see Slides 1 and 2).
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Slide 1 : Jacket based platform - Southern sector North Sea

Slide 2 : Jacket based platform - Northern sector North Sea
A second major type is the gravity concrete structure (see Figure 2), which is employed in the North Sea in the
Norwegian and British sectors.
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A third type is the floating production unit.
2.2 Environment
The offshore environment can be characterized by:
 water depth at location
 soil, at seabottom and in-depth
 wind speed, air temperature
 waves, tide and storm surge, current
 ice (fixed, floes, icebergs)
 earthquakes (if necessary)
The topside structure also must be kept clear of the wave crest. The clearance (airgap) usually is taken at
approximately 1,50 m, but should be increased if reservoir depletion will create significant subsidence.
2.3 Construction
The environment as well as financial aspects require that a high degree of prefabrication must be performed
onshore. It is necessary to design to limit offshore work to a minimum. The overall cost of a man-hour offshore is
approximately five times that of an onshore man-hour. The cost of construction equipment required to handle
loads, and the cost for logistics are also a magnitude higher offshore.
These factors combined with the size and weight of the items, require that a designer must carefully consider all
construction activities between shop fabrication and offshore installation.
2.4 Codes
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Structural design has to comply with specific offshore structural codes. The worldwide leading structural code is the
API-RP2A [1]. The recently issued Lloyds rules [2] and the DnV rules [3] are also important.
Specific government requirements have to be complied with, e.g. in the rules of Department of Energy (DoE),
Norwegian Petroleum Direktorate (NPD). For the detail design of the topside structure the AISC-code [4] is
frequently used, and the AWS-code [5] is used for welding.
In the UK the Piper alpha diaster has led to a completely new approach to regulation offshore. The responsibility for
regulatory control has been moved to the Health and Safety Executive (HSE) and the operator has to produce a
formal safety assessment (TSA) himself instead of complying with detailed regulations.
2.5 Certification and Warranty Survey
Government authorities require that recognized bodies appraise the aspects of structural integrity and issue a
certificate to that purpose.
The major certification bodies are:
 Det norske Veritas (DnV)
 Lloyds Register of Shipping (LRS)
 American Bureau of Shipping (ABS)
 Bureau Veritas (BV)
 Germanischer Lloyd (GL)
Their requirements are available to the designer [2, 3, 6, 7, 8].
Insurance companies covering transport and installation require the structures to be reviewed by warranty
surveyors before acceptance. The warranty surveyors apply standards, if available, on a confidential basis.
3. OFFSHORE DEVELOPMENT OF AN OIL/GAS FIELD
3.1 Introduction
The different requirements of an offshore platform and the typical phases of an offshore development are
summarized in [9]. After several initial phases which include seismic field surveying, one or more exploration wells
are drilled. Jack-up drilling rigs are used for this purpose for water depths up to 100 - 120 m; for deeper water
floating rigs are used. The results are studied and the economics and risks of different development plans are
evaluated. Factors involved in the evaluation may include number of wells required, fixed or floated production
facilities, number of such facilities, and pipeline or tanker off-loading.
As soon as exploitation is decided and approved, there are four main technical activities, prior to production:
 engineering and design
 fabrication and installation of the production facility
 drilling of production wells, taking 2 - 3 months/well
 providing the off loading system (pipelines, tankers, etc.).
The drilling and construction interaction is described below for two typical fixed platform concepts.
3.2 Jacket Based Platform for Shallow Water
First the jacket is installed. The wells are then drilled by a jack-up drilling unit standing close by with a cantilever
rig extending over the jacket. Slide 3 shows a jack-up drilling unit with a cantilever rig. (In this instance it is
engaged in exploratory drilling and is therefore working in isolation.)
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Slide 3 : Cantilevered drilling rig: Self-elevating (jack-up) exploration drilling platform.
Design and construction of the topside are progressed parallel to the drilling, allowing production to start soon after
deck installation. For further wells, the jack-up drilling unit will be called once again and will reach over the well
area of the production deck.
As an alternative to this concept the wells are often accommodated in a separate wellhead platform, linked by a
bridge to the production platform (see Slide 1).
3.3 Jacket and Gravity Based Platform for Deep Water
The wells are drilled from a drilling rig on the permanent platform (see Slide 2). Drilling starts after the platform is
built and completely installed. Consequently production starts between one and two years after platform
installation.
In recent years pre-drilled wells have been used to allow an earlier start of the production. In this case the
platform has to be installed exactly above the pre-drilled wells.
4. JACKETS AND PILE FOUNDATION
4.1 Introduction
Jackets, the tower-like braced tubular structures, generally perform two functions:
 They provide the substructure for the production facility (topside), keeping it stable above the waves.
 They support laterally and protect the 26-30 inch well conductors and the pipeline riser.
The installation methods for the jacket and the piles have a profound impact on the design.
4.2 Pile Foundation
The jacket foundation is provided by open-ended tubular steel piles, with diameters up to 2m. The piles are driven
approximately 40 - 80 m, and in some cases 120 m deep into the seabed.
There are basically three types of pile/jacket arrangement (see Figure 3):
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Pile-through-leg concept, where the pile is installed in the corner legs of the jacket.
Skirt piles through pile sleeves at the jacket-base, where the pile is installed in guides attached to the jacket leg.
Skirt piles can be grouped in clusters around each of the jacket legs.
Vertical skirt piles are directly installed in the pile sleeve at the jacket base; all other guides are deleted. This
arrangement results in reduced structural weight and easier pile driving. In contrast inclined piles enlarge the
foundation at the bottom, thus providing a stiffer structure.
4.3 Pile Bearing Resistance
Axial load resistance is required for bearing as well as for tension. The pile accumulates both skin friction as well as
end bearing resistance.
Lateral load resistance of the pile is required for restraint of the horizontal forces. These forces lead to significant
bending of the pile near to the seabed.
Number, arrangement, diameter and penetration of the piles depend on the environmental loads and the soil
conditions at the location.
4.4 Corrosion Protection
The most usual form of corrosion protection of the bare underwater part of the jacket as well as the upper part of
the piles in soil is by cathodic protection using sacrificial anodes. A sacrificial anode (approximate 3 kN each)
consists of a zinc/aluminium bar cast about a steel tube and welded on to the structures. Typically approximately
5% of the jacket weight is applied as anodes.
The steelwork in the splash zone is usually protected by a sacrificial wall thickness of 12 mm to the members.
5. TOPSIDES
5.1 Introduction
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The major functions on the deck of an offshore platform are:
 well control
 support for well work-over equipment
 separation of gas, oil and non-transportable components in the raw product, e.g. water, parafines/waxes and
sand
 support for pumps/compressors required to transport the product ashore
 power generation
 accommodation for operating and maintenance staff.
There are basically two structural types of topside, the integrated and modularized topside which are positioned
either on a jacket or on a concrete gravity substructure.
5.2 Jacket-based Topsides
5.2.1 Concepts
There are four structural concepts in practice. They result from the lifting capacity of crane vessels and the load-
out capacity at the yards:
 the single integrated deck (up to approx 100 MN)
 the split deck in two four-leg units
 the integrated deck with living quarter module
 the modularized topside consisting of module support frame (MSF) carrying a series of modules.
Slide 4 shows an integrated deck (though excluding the living quarters and helideck) being moved from its
assembly building.

Slide 4 : Integrated topside during load out
5.2.2 Structural Design for Integrated Topsides
For the smaller decks, up to approximately 100 MN weight, the support structure consists of trusses or portal
frames with deletion of diagonals.
The moderate vertical load and shear per column allows the topside to be supported by vertical columns (deck
legs) only, down to the top of the piles (situated at approximately +4 m to +6 m L.A.T. (Low Astronomic Tide).
5.2.3 Structural Design for Modularized Jacket-based Topsides
A major modularized topside weighs 200 to 400 MN. In this case the MSF is a heavy tubular structure (Figure 4),
with lateral bracing down to the top of jacket.
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5.3 Structural Design for Modularized Gravity-based Topsides
The topsides to be supported by a gravity-based substructure (see Figure 2) are in a weight range of 200 MN up to
500 MN.
The backbone of the structure is a system of heavy box-girders with a height of approximately 10 m and a width of
approximately 12 - 15 m (see Figure 5).
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The substructure of the deck is rigidly connected to the concrete column and acts as a beam supporting the deck
modules. This connection introduces wave-induced fatigue in the deck structure. A recent development, foreseen
for the Norwegian Troll platform, is to provide a flexible connection between the deck and concrete column, thus
eliminating fatigue in the deck [10].
6. EQUIPMENT AND LIVING QUARTER MODULES
Equipment modules (20-75 MN) have the form of rectangular boxes with one or two intermediate floors.
The floors are steel plate (6, 8 or 10 mm thick) for roof and lower floor, and grating for intermediate floors.
In living quarter modules (5-25 MN) all sleeping rooms require windows and several doors must be provided in the
outer walls. This requirement can interfere seriously with truss arrangements. Floors are flat or stiffened plate.
Three types of structural concepts, all avoiding interior columns, can be distinguished:
 conventional trusses in the walls.
 stiffened plate walls (so called stressed skin or deck house type).
 heavy base frame (with wind bracings in the walls).
7. CONSTRUCTION
7.1 Introduction
The design of offshore structures has to consider various requirements of construction relating to:
1. fabrication.
2. weight.
3. load-out.
4. sea transport.
5. offshore installation.
6. module installation.
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7. hook-up.
8. commissioning.
A documented construction strategy should be available during all phases of the design and the actual design
development should be monitored against the construction strategy.
Construction is illustrated below by four examples.
7.2 Construction of Jackets and Topsides
7.2.1 Lift Installed Jackets
The jacket is built in the vertical (smaller jackets) or horizontal position (bigger jackets) on a quay of a fabrication
site.
The jacket is loaded-out and seafastened aboard a barge. At the offshore location the barge is moored alongside an
offshore crane vessel.
The jacket is lifted off the barge, upended from the horizontal, and carefully set down onto the seabed.
After setting down the jacket, the piles are installed into the sleeves and, driven into the seabed. Fixing the piles to
the jacket completes the installation.
7.2.2 Launch Installed Jackets
The jacket is built in horizontal position.
For load-out to the transport barge, the jacket is put on skids sliding on a straight track of steel beams, and pulled
onto the barge (Slide 5).

Slide 5 : Jacket being loaded onto barge by skidding
At the offshore location the jacket is slid off the barge. It immerses deeply into the water and assumes a floating
position afterwards (see Figure 6).
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Two parallel heavy vertical trusses in the jacket structure are required, capable of taking the support reactions
during launching. To reduce forces and moments in the jacket, rocker arms are attached to the stern of the barge.
The next phase is to upright the jacket by means of controlled flooding of the buoyancy tanks and then set down
onto the seabed. Self-upending jackets obtain a vertical position after the launch on their own. Piling and
pile/jacket fixing completes the installation.
7.2.3 Topsides for a Gravity-Based Structure (GBS)
The topside is assembled above the sea on a temporary support near a yard. It is then taken by a barge of such
dimensions as to fit between the columns of the temporary support and between the columns of the GBS. The GBS
is brought in a deep floating condition in a sheltered site, e.g. a Norwegian fjord. The barge is positioned between
the columns and the GBS is then deballasted to mate with and to take over the deck from the barge. The floating
GBS with deck is then towed to the offshore site and set down onto the seabed.
7.2.4 Jacket Topsides
For topsides up to approximately 120 MN, the topside may be installed in one lift. Slide 6 shows a 60 MN topside
being installed by floating cranes.
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Slide 6 : Installation of 60MN K12-BP topside by floating crane
For the modularized topside, first the MSF will be installed, immediately followed by the modules.
7.3 Offshore Lifting
Lifting of heavy loads from barges (Slide 6) is one of the very important and spectacular construction activities
requiring a focus on the problem when concepts are developed. Weather windows, i.e. periods of suitable weather
conditions, are required for these operations.
7.3.1 Crane Vessel
Lifting of heavy loads offshore requires use of specialized crane vessels. Figure 7 provides information on a typical
big, dual crane vessel. Table 1 (page 16) lists some of the major offshore crane vessels.
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7.3.2 Sling-arrangement, Slings and Shackles
For lifting, steel wire ropes in a four-sling arrangement are used which directly rest in the four-point hook of the
crane vessel, (see Figure 8). The heaviest sling available now has a diameter of approximately 350 mm, a breaking
load of approximately 48 MN, and a safe working load (SWL) of 16 MN. Shackles are available up to 10 MN SWL to
connect the padeyes installed at the module's columns. Due to the space required, connecting more than one
shackle to the same column is not very attractive. So when the sling load exceeds 10 MN, padears become an
option.
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Table 1 Major Offshore Crane Vessels
Operator Name Mode Type Lifting capacity (Tonnes)
Heerema Thor Monohull Fix 2720
Rev 1820
Odin Monohull Fix 2720
Rev 2450
Hermod Semisub Fix 4536 + 3628 = 8164
Rev 3630 + 2720 = 6350
Balder Semisub Fix 3630 + 2720 = 6350
Rev 3000 + 2000 = 5000
McDermott DB50 Monohull Fix 4000
Rev 3800
DB100 Semisub Fix 1820
Rev 1450
DB101 Semisub Fix 3360
Rev 2450
DB102 Semisub Rev 6000 + 6000 = 12000
Micoperi M7000 Semisub Rev 7000 + 7000 = 14000
ETPM DLB1601 Monohull Rev. 1600
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Notes:
1. Rated lifting capacity in metric tonnes.
2. When the crane vessels are provided with two cranes, these cranes are situated at the vessels stern or bow at
approximately 60 m distance c.t.c.
1. 3. Rev = Load capability with fully revolving crane.
Fix = Load capability with crane fixed.
7.4 Sea Transport and Sea Fastening
Transportation is performed aboard a flat-top barge or, if possible, on the deck of the crane vessel.
The module requires fixing to the barge (see Figure 9) to withstand barge motions in rough seas. The sea fastening
concept is determined by the positions of the framing in the module as well as of the "hard points" in the barge.

7.5 Load-out
7.5.1 Introduction
For load-out three basic methods are applied:
 skidding
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 platform trailers
 shearlegs.
7.5.2 Skidding
Skidding is a method feasible for items of any weight. The system consists of a series of steel beams, acting as
track, on which a group of skids with each approximately 6 MN load capacity is arranged. Each skid is provided
with a hydraulic jack to control the reaction.
7.5.3 Platform Trailers
Specialized trailer units (see Figure 10) can be combined to act as one unit for loads up to 60 - 75 MN. The wheels
are individually suspended and integrated jacks allow adjustment up to 300 mm.

The load capacity over the projected ground area varies from approximately 55 to 85 kN/sq.m.
The units can drive in all directions and negotiate curves.
7.5.4 Shearlegs
Load-out by shearlegs is attractive for small jackets built on the quay. Smaller decks (up to 10 - 12 MN) can be
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loaded out on the decklegs pre-positioned on the barge, thus allowing deck and deckleg to be installed in one lift
offshore.
7.6 Platform Removal
In recent years platform removal has become common. The mode of removal depends strongly on the regulations
of the local authorities. Provision for removal should be considered in the design phase.
8. STRUCTURAL ANALYSIS
8.1 Introduction
The majority of structural analyses are based on the linear theory of elasticity for total system behaviour. Dynamic
analysis is performed for the system behaviour under wave-attack if the natural period exceeds 3 seconds. Many
elements can exhibit local dynamic behaviour, e.g. compressor foundations, flare-stacks, crane-pedestals, slender
jacket members, conductors.
8.2 In-place Phase
Three types of analysis are performed:
 Survival state, under wave/current/wind attack with 50 or 100 years recurrence period.
 Operational state, under wave/current/wind attack with 1 or 5 years recurrence period, under full operation.
 Fatigue assessment.
 Accidental.
All these analyses are performed on the complete and intact structure. Assessments at damaged structures, e.g.
with one member deleted, and assessments of collision situations are occasionally performed.
8.3 Construction Phase
The major phases of construction when structural integrity may be endangered are:
 Load-out
 Sea transport
 Upending of jackets
 Lifting.
9. COST ASPECTS
9.1 Introduction
The economic feasibility of an offshore project depends on many aspects: capital expenditure (CAPEX), tax,
royalties, operational expenditure (OPEX).
In a typical offshore field development, one third of the CAPEX is spent on the platform, one third on the drilling of
wells and one third on the pipelines.
Cost estimates are usually prepared in a deterministic approach. Recently cost-estimating using a probabilistic
approach has been developed and adopted in major offshore projects.
The CAPEX of an installed offshore platform topside amounts to approximately 20 ECU/kg.
9.2 Capital Expenditure (CAPEX)
The major elements in the CAPEX for an offshore platform are:
 project management and design
 material and equipment procurement
 fabrication
 transport and installation
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 hook-up and commissioning.
9.3 Operational Expenditure (OPEX)
In the North Sea approximately 20 percent of OPEX are required for offshore inspection, maintenance and repair
(IMR).
The amount to be spent on IMR over the project life can add up to approximately half the original investment.
IMR is the area in which the structural engineer makes a contribution by effort in design, selection of material,
improved corrosion protection, accessibility, basic provisions for scaffolding, avoiding jacket attachments
dangerous to divers, etc.
10. DEEP WATER DEVELOPMENTS
Deep water introduces a wide range of extra difficulties for the operator, the designer and constructor of offshore
platforms.
Fixed platforms have recently been installed in water of 410 m. depth, i.e. "Bullwinkle" developed by Shell Oil for a
Gulf of Mexico location. The jacket weighed nearly 500 MN.
The maximum depth of water at platform sites in the North Sea is approximately 220 m at present. The
development of the Troll field situated in approximately 305 m deep water is planned for 1993.
In the Gulf of Mexico and offshore California several fixed platforms in water depths of 250 - 350 m are in
operation (Cerveza, Cognac). Exxon has a guyed tower platform (Lena) in operation in 300 m deep water.
An option for deeper locations is to use subsea wells with flowlines to a nearby (approximately maximum 10 km)
fixed platform at a smaller water depth. Alternatively subsea wells may be used with flexible risers to a floating
production unit. Subsea wells are now feasible for 300 - 900 m deep water. The deepest wells have been
developed off Brasil in moderate weather conditions.
The tension leg platform (TLP) seems to be the most promising deepwater production unit (Figure 11). It consists
of a semi-submersible pontoon, tied to the seabed by vertical prestressed tethers. The first TLP was Hutton in the
North Sea and recently TLP-Jolliet was installed at a 530 m deep location in the Gulf of Mexico. Norwegian Snorre
and Heidrun fields have been developed with TLPs as well.
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11. CONCLUDING SUMMARY
 The lecture starts with the presentation of the importance of offshore hydro-carbon exploitation, the basic
steps in the development process (from seismic exploration to platform removal) and the introduction of the
major structural concepts (jacket-based, GBS-based, TLP, floating).
 The major codes are identified.
 For the fixed platform concepts (jacket and GBS), the different execution phases are briefly explained: design,
fabrication and installation. Special attention is given to the principles of topside design.
 A basic introduction to cost aspects is presented.
 Finally terms are introduced within a glossary.
12. GLOSSARY OF TERMS
AIR GAP Clearance between the top of maximum wave and underside of the topside.
CAISSONS See SUMPS
CONDUCTORS The tubular protecting and guiding the drill string from the topside down to 40 to 100m under the
sea bottom. After drilling it protects the well casing.
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G.B.S. Gravity based structure, sitting flatly on the sea bottom, stable through its weight.
HOOK-UP Connecting components or systems, after installation offshore.
JACKET Tubular sub-structure under a topside, standing in the water and pile founded.
LOAD-OUT The operation of bringing the object (module, jacket, deck) from the quay onto the transportation
barge.
PADEARS (TRUNNIONS) Thick-walled tubular stubs, directly receiving slings and transversely welded to the main
structure.
PADEYES Thick-walled plate with hole, receiving the pin of the shackle, welded to the main structure.
PIPELINE RISER The piping section which rises from the sea bed to topside level.
SEA-FASTENING The structure to keep the object rigidly connected to the barge during transport.
SHACKLES Connecting element (bow + pin) between slings and padeyes.
SLINGS Cables with spliced eyed at both ends, for offshore lifting, the upper end resting in the crane hook.
SPREADER Tubular frame, used in lifting operation.
SUBSEA TEMPLATE Structure at seabottom, to guide conductors prior to jacket installation.
SUMPS Vertical pipes from topside down to 5-10 m below water level for intake or discharge.
TOPSIDE Topside, the compact offshore process plant, with all auxiliaries, positioned above the waves.
UP ENDING Bringing the jacket in vertical position, prior to set down on the sea bottom.
WEATHER WINDOW
A period of calm weather, defined on basis of operational limits for the offshore marine operation.
WELLHEAD AREA Area in topside where the wellheads are positioned including the valves mounted on its top.
13. REFERENCES
[1] API-RP2A: Recommended practice for planning, designing and constructing fixed offshore platforms.
American Petroleum Institute 18th ed. 1989.
The structural offshore code, governs the majority of platforms.
[2] LRS Code for offshore platforms.
Lloyds Register of Shipping.
London (UK) 1988.
Regulations of a major certifying authority.
[3] DnV: Rules for the classification of fixed offshore installations.
Det Norske Veritas 1989.
Important set of rules.
[4] AISC: Specification for the design, fabrication and erection of structural steel for buildings.
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American Institute of Steel Construction 1989.
Widely used structural code for topsides.
[5] AWS D1.1-90: Structural Welding Code - Steel.
American Welding Society 1990.
The structural offshore welding code.
[6] DnV/Marine Operations: Standard for insurance warranty surveys in marine operations.
Det norske Veritas June 1985.
Regulations of a major certifying authority.
[7] ABS: Rules for building and classing offshore installations, Part 1 Structures.
American Bureau of Shipping 1983.
Regulations of a major certifying authority.
[8] BV: Rules and regulations for the construction and classification of offshore platforms.
Bureau Veritas, Paris 1975.
Regulations of a major certifying authority.
[9] ANON: A primer of offshore operations.
Petex Publ. Austin U.S.A 2nd ed. 1985.
Fundamental information about offshore oil and gas operations.
[10] AGJ Berkelder et al: Flexible deck joints.
ASME/OMAE-conference The Hague 1989 Vol.II pp. 753-760.
Presents interesting new concept in GBS design.
14. ADDITIONAL READING
1. BS 6235: Code of practice for fixed offshore structures.
British Standards Institution 1982.
Important code, mainly for the British offshore sector.
2. DoE Offshore installations: Guidance on design and construction, U.K. Department of Energy 1990.
Governmental regulations for British offshore sector only.
3. UEG: Design of tubular joints (3 volumes).
UEG Offshore Research Publ. U.R.33 1985.
Important theoretical and practical book.
4. J. Wardenier: Hollow section joints.
Delft University Press 1981.
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Theoretical publication on tubular design including practical design formulae.
5. ARSEM: Design guides for offshore structures welded tubular joints.
Edition Technip, Paris (France), 1987.
Important theoretical and practical book.
6. D. Johnston: Field development options.
Oil & Gas Journal, May 5 1986, pp 132 - 142.
Good presentation on development options.
7. G. I. Claum et al: Offshore Structures: Vol 1: Conceptual Design and Hydri-mechanics; Vol 2 - Strength and
Safety for Structural design.
Springer Verlag, London 1992.
Fundamental publication on structural behaviour.
8. W.J. Graff: Introduction to offshore structures.
Gulf Publishing Company, Houston 1981.
Good general introduction to offshore structures.
9. B.C. Gerwick: Construction of offshore structures.
John Wiley & Sons, New York 1986.
Up to date presentation of offshore design and construction.
10. T.A. Doody et al: Important considerations for successful fabrication of offshore structures.
OTC paper 5348, Houston 1986, pp 531-539.
Valuable paper on fabrication aspects.
11. D.I. Karsan et al: An economic study on parameters influencing the cost of fixed platforms.
OTC paper 5301, Houston 1986, pp 79-93.
Good presentation on offshore CAPEX assessment.
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Lecture 15A.2: Loads (I) : Introduction
and Environmental Loads
OBJECTIVE/SCOPE
To introduce the types of loads for which a fixed steel offshore structure must be designed. To present
briefly the loads generated by environmental factors.
PREREQUISITES
A basic knowledge of structural analysis for static and dynamic loadings.
SUMMARY
The categories of load for which a pile supported steel offshore platform must be designed are introduced
and then the different types of environmental loads are presented. The loads include: wind, wave, current,
earthquake, ice and snow, temperature, sea bed movement, marine growth and tide generated loads.
Loads due to wind, waves and earthquake are discussed in more detail together with their idealizations for
the various types of analyses. Frequent references are made to the codes of practice recommended by the
American Petroleum Institute, Det Norske Veritas, the British Standards Institution and the British
Department of Energy, as well as to the relevant regulations of the Norwegian Petroleum Directorate.
1. INTRODUCTION
The loads for which an offshore structure must be designed can be classified into the following categories:
1. Permanent (dead) loads.
2. Operating (live) loads.
3. Environmental loads including earthquakes.
4. Construction - installation loads.
5. Accidental loads.
Whilst the design of buildings onshore is usually influenced mainly by the permanent and operating loads,
the design of offshore structures is dominated by environmental loads, especially waves, and the loads
arising in the various stages of construction and installation. This lecture deals with environmental loads,
whilst the other loadings are treated in Lecture 15A.3.
In civil engineering, earthquakes are normally regarded as accidental loads (see Eurocode 8 [1]), but in
offshore engineering they are treated as environmental loads. This practice is followed in the two lectures
dealing with loads, Lectures 15A.2 and 15A.3.
2. ENVIRONMENTAL LOADS
Environmental loads are those caused by environmental phenomena such as wind, waves, current, tides,
earthquakes, temperature, ice, sea bed movement, and marine growth. Their characteristic parameters,
defining design load values, are determined in special studies on the basis of available data. According to
US and Norwegian regulations (or codes of practice), the mean recurrence interval for the corresponding
design event must be 100 years, while according to the British rules it should be 50 years or greater.
Details of design criteria, simplifying assumptions, required data, etc., can be found in the regulations and
codes of practice listed in [1] - [8].
2.1 Wind Loads
Wind loads act on the portion of a platform above the water level, as well as on any equipment, housing,
derrick, etc. located on the deck. An important parameter pertaining to wind data is the time interval over
which wind speeds are averaged. For averaging intervals less than one minute, wind speeds are classified
as gusts. For averaging intervals of one minute or longer they are classified as sustained wind speeds.
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The wind velocity profile may be taken from API-RP2A [2]:
V
h
/V
H
= (h/H)
1/n
(1)

where:
V
h
is the wind velocity at height h,

V
H
is the wind velocity at reference height H, typically 10m above mean water level,

1/n is 1/13 to 1/7, depending on the sea state, the distance from land and the averaging time interval. It is
approximately equal to 1/13 for gusts and 1/8 for sustained winds in the open ocean.
From the design wind velocity V(m/s), the static wind force F
w
(N) acting perpendicular to an exposed area
A(m
2
) can be computed as follows:

F
w
= (1/2) µ V
2
C
s
A (2)

where:
µ is the wind density (µ ~ 1.225 Kg/m
3
)

C
s
is the shape coefficient (C
s
= 1,5 for beams and sides of buildings, C
s
= 0,5 for cylindrical sections and
C
s
= 1,0 for total projected area of platform).
Shielding and solidity effects can be accounted for, in the judgement of the designer, using appropriate
coefficients.
For combination with wave loads, the DNV [4] and DOE-OG [7] rules recommend the most unfavourable of
the following two loadings:
a. 1-minute sustained wind speeds combined with extreme waves.
b. 3-second gusts.
API-RP2A [2] distinguishes between global and local wind load effects. For the first case it gives guideline
values of mean 1-hour average wind speeds to be combined with extreme waves and current. For the
second case it gives values of extreme wind speeds to be used without regard to waves.
Wind loads are generally taken as static. When, however, the ratio of height to the least horizontal
dimension of the wind exposed object (or structure) is greater than 5, then this object (or structure) could
be wind sensitive. API-RP2A requires the dynamic effects of the wind to be taken into account in this case
and the flow induced cyclic wind loads due to vortex shedding must be investigated.
2.2 Wave Loads
The wave loading of an offshore structure is usually the most important of all environmental loadings for
which the structure must be designed. The forces on the structure are caused by the motion of the water
due to the waves which are generated by the action of the wind on the surface of the sea. Determination of
these forces requires the solution of two separate, though interrelated problems. The first is the sea state
computed using an idealisation of the wave surface profile and the wave kinematics given by an
appropriate wave theory. The second is the computation of the wave forces on individual members and on
the total structure, from the fluid motion.
Two different analysis concepts are used:
 The design wave concept, where a regular wave of given height and period is defined and the forces
due to this wave are calculated using a high-order wave theory. Usually the 100-year wave, i.e. the
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maximum wave with a return period of 100 years, is chosen. No dynamic behaviour of the structure is
considered. This static analysis is appropriate when the dominant wave periods are well above the
period of the structure. This is the case of extreme storm waves acting on shallow water structures.
 Statistical analysis on the basis of a wave scatter diagram for the location of the structure. Appropriate
wave spectra are defined to perform the analysis in the frequency domain and to generate random
waves, if dynamic analyses for extreme wave loadings are required for deepwater structures. With
statistical methods, the most probable maximum force during the lifetime of the structure is calculated
using linear wave theory. The statistical approach has to be chosen to analyze the fatigue strength
and the dynamic behaviour of the structure.
2.2.1 Wave theories
Wave theories describe the kinematics of waves of water on the basis of potential theory. In particular,
they serve to calculate the particle velocities and accelerations and the dynamic pressure as functions of
the surface elevation of the waves. The waves are assumed to be long-crested, i.e. they can be described
by a two-dimensional flow field, and are characterized by the parameters: wave height (H), period (T) and
water depth (d) as shown in Figure 1.

Different wave theories of varying complexity, developed on the basis of simplifying assumptions, are
appropriate for different ranges of the wave parameters. Among the most common theories are: the linear
Airy theory, the Stokes fifth-order theory, the solitary wave theory, the cnoidal theory, Dean's stream
function theory and the numerical theory by Chappelear. For the selection of the most appropriate theory,
the graph shown in Figure 2 may be consulted. As an example, Table 1 presents results of the linear wave
theory for finite depth and deep water conditions. Corresponding particle paths are illustrated in Figures 3
and 4. Note the strong influence of the water depth on the wave kinematics. Results from high-order wave
theories can be found in the literature, e.g. see [9].
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2.2.2 Wave Statistics
In reality waves do not occur as regular waves, but as irregular sea states. The irregular appearance results
from the linear superposition of an infinite number of regular waves with varying frequency (Figure 5). The
best means to describe a random sea state is using the wave energy density spectrum S(f), usually called
the wave spectrum for simplicity. It is formulated as a function of the wave frequency f using the
parameters: significant wave height H
s
(i.e. the mean of the highest third of all waves present in a wave
train) and mean wave period (zero-upcrossing period) T
o
. As an additional parameter the spectral width
can be taken into account.
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Wave directionality can be introduced by means of a directional spreading function D(f,o), where o is the
angle of the wave approach direction (Figure 6). A directional wave spectrum S (f,o) can then be defined
as:
S (f,o ) = S(f).D (f,o ) (3)

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The response of the structure, i.e. forces, motions, is calculated by multiplication of the wave energy
spectrum with the square of a linear transfer function. From the resulting response spectrum the significant
and the maximum expected response in a given time interval can be easily deduced.
For long-term statistics, a wave scatter diagram for the location of the structure is needed. It can be
obtained from measurements over a long period or be deduced from weather observations in the region
(the so-called hindcast method). The scatter diagram contains the joint probability of occurrence of pairs of
significant wave height and mean wave period. For every pair of parameters the wave spectrum is
calculated by a standard formula, e.g. Pierson-Moskowitz (Figure 6), yielding finally the desired response
spectrum. For fatigue analysis the total number and amplitude of load cycles during the life-time of the
structure can be derived in this way. For structures with substantial dynamic response to the wave
excitation, the maximum forces and motions have to be calculated by statistical methods or a time-domain
analysis.
2.2.3 Wave forces on structural members
Structures exposed to waves experience substantial forces much higher than wind loadings. The forces
result from the dynamic pressure and the water particle motions. Two different cases can be distinguished:
 Large volume bodies, termed hydrodynamic compact structures, influence the wave field by diffraction
and reflection. The forces on these bodies have to be determined by costly numerical calculations
based on diffraction theory.
 Slender, hydrodynamically transparent structures have no significant influence on the wave field. The
forces can be calculated in a straight-forward manner with Morison's equation. As a rule, Morison's
equation may be applied when D/L s 0.2, where D is the member diameter and L is the wave length.
The steel jackets of offshore structures can usually be regarded as hydrodynamically transparent. The wave
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forces on the submerged members can therefore be calculated by Morison's equation, which expresses the
wave force as the sum of an inertia force proportional to the particle acceleration and a non-linear drag
force proportional to the square of the particle velocity: (4)
where
F is the wave force per unit length on a circular cylinder (N)
v, |v, are water particle velocity normal to the cylinder, calculated with the selected wave theory at the
cylinder axis (m/s)
are water particle acceleration normal to the cylinder, calculated with the selected wave theory at the
cylinder axis (m/s
2
)

µ is the water density (kg/m
3
)

D is the member diameter, including marine growth (m)
C
D
, C
M
are drag and inertia coefficients, respectively.

In this form the equation is valid for fixed tubular cylinders. For the analysis of the motion response of a
structure it has to be modified to account for the motion of the cylinder [10]. The values of C
D
and C
M

depend on the wave theory used, surface roughness and the flow parameters. According to API-RP2A, C
D
~
0,6 to 1,2 and C
M
~ 1,3 to 2,0. Additional information can be found in the DNV rules [4].
The total wave force on each member is obtained by numerical integration over the length of the member.
The fluid velocities and accelerations at the integration points are found by direct application of the selected
wave theory.
According to Morison's equation the drag force is non-linear. This non-linear formulation is used in the
design wave concept. However, for the determination of a transfer function needed for frequency domain
calculations, the drag force has to be linearized in a suitable way [9]. Thus, frequency domain solutions are
appropriate for fatigue life calculations, for which the forces due to the operational level waves are
dominated by the linear inertia term. The nonlinear formulation and hence time domain solutions are
required for dynamic analyses of deepwater structures under extreme, storm waves, for which the drag
portion of the force is the dominant part [10].
In addition to the forces given by Morison's equation, the lift forces F
D
and the slamming forces F
S
,
typically neglected in global response computations, can be important for local member design. For a
member section of unit length, these forces can be estimated as follows:
F
L
= (1/2) µ C
L
Dv
2
(5)

F
S
= (1/2) µ C
s
Dv
2
(6)

where C
L
, C
S
are the lift and slamming coefficients respectively, and the rest of the symbols are as defined
in Morison's equation. Lift forces are perpendicular to the member axis and the fluid velocity v and are
related to the vortex shedding frequency. Slamming forces acting on the underside of horizontal members
near the mean water level are impulsive and nearly vertical. Lift forces can be estimated by taking C
L
~ 1,3
C
D
. For tubular members C
s
~ t.
2.3 Current Loads
There are tidal, circulation and storm generated currents. Figure 7 shows a wind and tidal current profile
typical of the Gulf of Mexico. When insufficient field measurements are available, current velocities may be
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obtained from various sources, e.g. Appendix A of DNV [4]. In platform design, the effects of current
superimposed on waves are taken into account by adding the corresponding fluid velocities vectorially.
Since the drag force varies with the square of the velocity, this addition can greatly increase the forces on a
platform. For slender members, cyclic loads induced by vortex shedding may also be important and should
be examined.

2.4 Earthquake Loads
Offshore structures in seismic regions are typically designed for two levels of earthquake intensity: the
strength level and the ductility level earthquake. For the strength level earthquake, defined as having a
"reasonable likelihood of not being exceeded during the platform's life" (mean recurrence interval ~ 200 -
500 years), the structure is designed to respond elastically. For the ductility level earthquake, defined as
close to the "maximum credible earthquake" at the site, the structure is designed for inelastic response and
to have adequate reserve strength to avoid collapse.
For strength level design, the seismic loading may be specified either by sets of accelerograms (Figure 8)
or by means of design response spectra (Figure 9). Use of design spectra has a number of advantages over
time history solutions (base acceleration input). For this reason design response spectra are the preferable
approach for strength level designs. If the design spectral intensity, characteristic of the seismic hazard at
the site, is denoted by a
max
, then API-RP2A recommends using a
max
for the two principal horizontal
directions and 0,5a
max
for the vertical direction. The DNV rules, on the other hand, recommend a
max
and
0,7 a
max
for the two horizontal directions (two different combinations) and 0,5 a
max
for the vertical. The
value of a
max
and often the spectral shapes are determined by site specific seismological studies.
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Designs for ductility level earthquakes will normally require inelastic analyses for which the seismic input
must be specified by sets of 3-component accelerograms, real or artificial, representative of the extreme
ground motions that could shake the platform site. The characteristics of such motions, however, may still
be prescribed by means of design spectra, which are usually the result of a site specific seismotectonic
study. More detail of the analysis of earthquakes is given in the Lectures 17: Seismic Design.
2.5 Ice and Snow Loads
Ice is a primary problem for marine structures in the arctic and sub-arctic zones. Ice formation and
expansion can generate large pressures that give rise to horizontal as well as vertical forces. In addition,
large blocks of ice driven by current, winds and waves with speeds that can approach 0,5 to 1,0 m/s, may
hit the structure and produce impact loads.
As a first approximation, statically applied, horizontal ice forces may be estimated as follows:
F
i
= C
i
f
c
A (7)

where:
A is the exposed area of structure,
f
c
is the compressive strength of ice,

C
i
is the coefficient accounting for shape, rate of load application and other factors, with usual values
between 0,3 and 0,7.
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Generally, detailed studies based on field measurements, laboratory tests and analytical work are required
to develop reliable design ice forces for a given geographical location.
In addition to these forces, ice formation and snow accumulations increase gravity and wind loads, the
latter by increasing areas exposed to the action of wind. More detailed information on snow loads may be
found in Eurocode 1 [8].
2.6 Loads due to Temperature Variations
Offshore structures can be subjected to temperature gradients which produce thermal stresses. To take
account of such stresses, extreme values of sea and air temperatures which are likely to occur during the
life of the structure must be estimated. Relevant data for the North Sea are given in BS6235 [6]. In
addition to the environmental sources, human factors can also generate thermal loads, e.g. through
accidental release of cryogenic material, which must be taken into account in design as accidental loads.
The temperature of the oil and gas produced must also be considered.
2.7 Marine Growth
Marine growth is accumulated on submerged members. Its main effect is to increase the wave forces on
the members by increasing not only exposed areas and volumes, but also the drag coefficient due to higher
surface roughness. In addition, it increases the unit mass of the member, resulting in higher gravity loads
and in lower member frequencies. Depending upon geographic location, the thickness of marine growth can
reach 0,3m or more. It is accounted for in design through appropriate increases in the diameters and
masses of the submerged members.
2.8 Tides
Tides affect the wave and current loads indirectly, i.e. through the variation of the level of the sea surface.
The tides are classified as: (a) astronomical tides - caused essentially from the gravitational pull of the
moon and the sun and (b) storm surges - caused by the combined action of wind and barometric pressure
differentials during a storm. The combined effect of the two types of tide is called the storm tide. Tide
dependent water levels and the associated definitions, as used in platform design, are shown in Figure 10.
The astronomical tide range depends on the geographic location and the phase of the moon. Its maximum,
the spring tide, occurs at new moon. The range varies from centimetres to several metres and may be
obtained from special maps. Storm surges depend upon the return period considered and their range is on
the order of 1,0 to 3,0m. When designing a platform, extreme storm waves are superimposed on the still
water level (see Figure 10), while for design considerations such as levels for boat landing places, barge
fenders, upper limits of marine growth, etc., the daily variations of the astronomical tide are used.

2.9 Sea Floor Movements
Movement of the sea floor can occur as a result of active geologic processes, storm wave pressures,
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earthquakes, pressure reduction in the producing reservoir, etc. The loads generated by such movements
affect, not only the design of the piles, but the jacket as well. Such forces are determined by special
geotechnical studies and investigations.
3. CONCLUDING SUMMARY
 Environmental loads form a major category of loads which control many aspects of platform design.
 The main environmental loads are due to wind, waves, current, earthquakes, ice and snow,
temperature variations, marine growth, tides and seafloor movements.
 Widely accepted rules of practice, listed as [1] - [13], provide guideline values for most environmental
loads.
 For major structures, specification of environmental design loads requires specific studies.
 Some environmental loads can be highly uncertain.
 The definition of certain environmental loads depends upon the type of analysis used in the design.
4. REFERENCES
[1] Eurocode 8: "Structures in Seismic Regions - Design", CEN (in preparation).
[2] API-RP2A, "Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms",
American Petroleum Institute, Washington, D.C., 18th ed., 1989.
[3] OCS, "Requirements for Verifying the Structural Integrity of OCS Platforms"., United States Geologic
Survey, National Centre, Reston, Virginia, 1980.
[4] DNV, "Rules for the Design, Construction and Inspection of Offshore Structures", Det Norske Veritas,
Oslo, 1977 (with corrections 1982).
[5] NPD, "Regulation for Structural Design of Load-bearing Structures Intended for Exploitation of
Petroleum Resources", Norwegian Petroleum Directorate, 1985.
[6] BS6235, "Code of Practice for Fixed Offshore Structures", British Standards Institution, London, 1982.
[7] DOE-OG, "Offshore Installation: Guidance on Design and Construction", U.K., Dept. of Energy, London
1985.
[8] Eurocode 1: "Basis of Design and Actions on Structures", CEN (in preparation).
[9] Clauss, G. T. et al: "Offshore Structures, Vol 1 - Conceptual Design and Hydromechanics", Springer,
London 1992.
[10] Anagnostopoulos, S.A., "Dynamic Response of Offshore Structures to Extreme Waves including Fluid -
Structure Interaction", Engr. Structures, Vol. 4, pp.179-185, 1982.
[11] Hsu, H.T., "Applied Offshore Structural Engineering", Gulf Publishing Co., Houston, 1981.
[12] Graff, W.J., "Introduction to Offshore Structures", Gulf Publishing Co., Houston, 1981.
[13] Gerwick, B.C. Jr., "Construction of Offshore Structures", John Wiley, New York, 1986.
Table 1 Results of Linear Airy Theory [11]
Phase u = kx - e t
Relative water depth d/L
Deep water
d/L > 0,5
Finite water depth
d/L < 0,5
Velocity potential u


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Surface elevation z
Dynamic pressure
p
dyn
=

,
a
cos u

µ g,
a
e
kz
cos u

,
a
cos u


Water particle velocities
horizontal u =
vertical w =

,
a
e e
kz
cos u


,
a
e e
kz
sin u




Water particle
accelerations
horizontal u' =
vertical w' =

,
a
e
2
e
kz
sin u


-,
a
e
2
e
kz
cos u




Wave celerity c =

Group velocity c
gr

=
Circular frequency
e =
Wave length L =

Wave number k =

c
o
=

c
gr
=

e =
L
o
=

k
o
=

c =
c
gr
=

e =
L =
kd tanh kd =
Water particle
displacements
horizontal ç
vertical ,

Particle trajectories

-,
a
e
kz
sin u

,
a
e
kz
cos u


Circular orbits



Elliptical orbits
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Where ,
a
=

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Lecture 15A.3: Loads (II) - Other Loads
OBJECTIVE/SCOPE
To present and briefly describe all loads, except environmental loads, and the load combinations for which a fixed offshore
structure must be designed.
PREREQUISITES
A basic knowledge of structural analysis for static and dynamic loadings.
SUMMARY
The various categories of loads, except environmental, for which a pile-supported steel offshore platform must be designed are
presented. These categories include permanent (dead) loads, operating (live) loads, loads generated during fabrication and
installation (due to lifts, loadout, transportation, launching and upending) and accidental loads. In addition, the different load
combinations for all types of loads, including environmental, as required (or suggested) by applicable regulations (or codes of
practice) are given.
The categories of loads described herein are the following:
1. Permanent (dead) loads
2. Operating (live) loads
3. Fabrication and installation loads
4. Accidental loads
The major categories of environmental loads are not included. They are dealt with in Lecture 15A.2.
1. PERMANENT (DEAD) LOADS
Permanent loads include the following:
a. Weight of the structure in air, including the weight of grout and ballast, if necessary.
b. Weights of equipment, attachments or associated structures which are permanently mounted on the platform.
c. Hydrostatic forces on the various members below the waterline. These forces include buoyancy and hydrostatic pressures.
Sealed tubular members must be designed for the worst condition when flooded or non-flooded.
2. OPERATING (LIVE) LOADS
Operating loads arise from the operations on the platform and include the weight of all non-permanent equipment or material, as
well as forces generated during operation of equipment. More specifically, operating loads include the following:
a. The weight of all non-permanent equipment (e.g. drilling, production), facilities (e.g. living quarters, furniture, life support
systems, heliport, etc), consumable supplies, liquids, etc.
b. Forces generated during operations, e.g. drilling, vessel mooring, helicopter landing, crane operations, etc.
The necessary data for computation of all operating loads are provided by the operator and the equipment manufacturers. The data
need to be critically evaluated by the designer. An example of detailed live load specification is given in Table 1 where the values in
the first and second columns are for design of the portions of the structure directly affected by the loads and the reduced values in
the last column are for the structure as a whole. In the absence of such data, the following values are recommended in BS6235
[1]:
a. crew quarters and passageways: 3,2 KN/m
2

b. working areas: 8,5 KN/m
2

c. storage areas: H KN/m
2

where
 is the specific weight of stored materials, not to be taken less than 6,87KN/m
3
,

H is the storage height (m).
Forces generated during operations are often dynamic or impulsive in nature and must be treated as such. For example, according
to the BS6235 rules, two types of helicopter landing should be considered, heavy and emergency landing. The impact load in the
first case is to be taken as 1,5 times the maximum take-off weight, while in the second case this factor becomes 2,5. In addition, a
horizontal load applied at the points of impact and taken equal to half the maximum take-off weight must be considered. Loads
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from rotating machinery, drilling equipment, etc. may normally be treated as harmonic forces. For vessel mooring, design forces
are computed for the largest ship likely to approach at operational speeds. According to BS6235, the minimum impact to be
considered is of a vessel of 2500 tonnes at 0,5 m/s.
3. FABRICATION AND INSTALLATION LOADS
These loads are temporary and arise during fabrication and installation of the platform or its components. During fabrication,
erection lifts of various structural components generate lifting forces, while in the installation phase forces are generated during
platform loadout, transportation to the site, launching and upending, as well as during lifts related to installation.
According to the DNV rules [2], the return period for computing design environmental conditions for installation as well as
fabrication should normally be three times the duration of the corresponding phase. API-RP2A, on the other hand [3], leaves this
design return period up to the owner, while the BS6235 rules [1] recommend a minimum recurrence interval of 10 years for the
design environmental loads associated with transportation of the structure to the offshore site.
3.1 Lifting Forces
Lifting forces are functions of the weight of the structural component being lifted, the number and location of lifting eyes used for
the lift, the angle between each sling and the vertical axis and the conditions under which the lift is performed (Figure 1). All
members and connections of a lifted component must be designed for the forces resulting from static equilibrium of the lifted
weight and the sling tensions. Moreover, API-RP2A recommends that in order to compensate for any side movements, lifting eyes
and the connections to the supporting structural members should be designed for the combined action of the static sling load and a
horizontal force equal to 5% this load, applied perpendicular to the padeye at the centre of the pin hole. All these design forces are
applied as static loads if the lifts are performed in the fabrication yard. If, however, the lifting derrick or the structure to be lifted is
on a floating vessel, then dynamic load factors should be applied to the static lifting forces. In particular, for lifts made offshore
API-RP2A recommends two minimum values of dynamic load factors: 2,0 and 1,35. The first is for designing the padeyes as well as
all members and their end connections framing the joint where the padeye is attached, while the second is for all other members
transmitting lifting forces. For loadout at sheltered locations, the corresponding minimum load factors for the two groups of
structural components become, according to API-RP2A, 1,5 and 1,15, respectively.

3.2 Loadout Forces
These are forces generated when the jacket is loaded from the fabrication yard onto the barge. If the loadout is carried out by
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direct lift, then, unless the lifting arrangement is different from that to be used for installation, lifting forces need not be computed,
because lifting in the open sea creates a more severe loading condition which requires higher dynamic load factors. If loadout is
done by skidding the structure onto the barge, a number of static loading conditions must be considered, with the jacket supported
on its side. Such loading conditions arise from the different positions of the jacket during the loadout phases, (as shown in Figure
2), from movement of the barge due to tidal fluctuations, marine traffic or change of draft, and from possible support settlements.
Since movement of the jacket is slow, all loading conditions can be taken as static. Typical values of friction coefficients for
calculation of skidding forces are the following:
 steel on steel without lubrication............................................ 0,25
 steel on steel with lubrication............................................... 0,15
 steel on teflon.................................................................. 0,10
 teflon on teflon................................................................. 0,08


3.3 Transportation Forces
These forces are generated when platform components (jacket, deck) are transported offshore on barges or self-floating. They
depend upon the weight, geometry and support conditions of the structure (by barge or by buoyancy) and also on the
environmental conditions (waves, winds and currents) that are encountered during transportation. The types of motion that a
floating structure may experience are shown schematically in Figure 3.
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In order to minimize the associated risks and secure safe transport from the fabrication yard to the platform site, it is important to
plan the operation carefully by considering, according to API-RP2A [3], the following:
1. Previous experience along the tow route
2. Exposure time and reliability of predicted "weather windows"
3. Accessibility of safe havens
4. Seasonal weather system
5. Appropriate return period for determining design wind, wave and current conditions, taking into account characteristics of the
tow such as size, structure, sensitivity and cost.
Transportation forces are generated by the motion of the tow, i.e. the structure and supporting barge. They are determined from
the design winds, waves and currents. If the structure is self-floating, the loads can be calculated directly. According to API-RP2A
[3], towing analyses must be based on the results of model basin tests or appropriate analytical methods and must consider wind
and wave directions parallel, perpendicular and at 45 to the tow axis. Inertial loads may be computed from a rigid body analysis of
the tow by combining roll and pitch with heave motions, when the size of the tow, magnitude of the sea state and experience make
such assumptions reasonable. For open sea conditions, the following may be considered as typical design values:
Single - amplitude roll: 20
Single - amplitude pitch: 10
Period of roll or pitch: 10 second
Heave acceleration: 0,2 g
When transporting a large jacket by barge, stability against capsizing is a primary design consideration because of the high centre
of gravity of the jacket. Moreover, the relative stiffness of jacket and barge may need to be taken into account together with the
wave slamming forces that could result during a heavy roll motion of the tow (Figure 4) when structural analyses are carried out for
designing the tie-down braces and the jacket members affected by the induced loads. Special computer programs are available to
compute the transportation loads in the structure-barge system and the resulting stresses for any specified environmental
condition.
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3.4 Launching and Upending Forces
These forces are generated during the launch of a jacket from the barge into the sea and during the subsequent upending into its
proper vertical position to rest on the seabed. A schematic view of these operations can be seen in Figure 5.

There are five stages in a launch-upending operation:
a. Jacket slides along the skid beams
b. Jacket rotates on the rocker arms
c. Jacket rotates and slides simultaneously
d. Jacket detaches completely and comes to its floating equilibrium position
e. Jacket is upended by a combination of controlled flooding and simultaneous lifting by a derrick barge.
The loads, static as well as dynamic, induced during each of these stages and the force required to set the jacket into motion can
be evaluated by appropriate analyses, which also consider the action of wind, waves and currents expected during the operation.
To start the launch, the barge must be ballasted to an appropriate draft and trim angle and subsequently the jacket must be pulled
towards the stern by a winch. Sliding of the jacket starts as soon as the downward force (gravity component and winch pull)
exceeds the friction force. As the jacket slides, its weight is supported on the two legs that are part of the launch trusses. The
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support length keeps decreasing and reaches a minimum, equal to the length of the rocker beams, when rotation starts. It is
generally at this instant that the most severe launching forces develop as reactions to the weight of the jacket. During stages (d)
and (e), variable hydrostatic forces arise which have to be considered at all members affected. Buoyancy calculations are required
for every stage of the operation to ensure fully controlled, stable motion. Computer programs are available to perform the stress
analyses required for launching and upending and also to portray the whole operation graphically.
4. ACCIDENTAL LOADS
According to the DNV rules [2], accidental loads are loads, ill-defined with respect to intensity and frequency, which may occur as a
result of accident or exceptional circumstances. Accidental loads are also specified as a separate category in the NPD regulations
[4], but not in API-RP2A [3], BS6235 [1] or the DOE-OG rules [5]. Examples of accidental loads are loads due to collision with
vessels, fire or explosion, dropped objects, and unintended flooding of bouyancy tanks. Special measures are normally taken to
reduce the risk from accidental loads. For example, protection of wellheads or other critical equipment from a dropped object can
be provided by specially designed, impact resistant covers. According to the NPD regulations [4], an accidental load can be
disregarded if its annual probability of occurrence is less than 10
-4
. This number is meant as an order of magnitude estimate and is
extremely difficult to compute. Earthquakes are treated as an environmental load in offshore structure design.
5. LOAD COMBINATIONS
The load combinations used for designing fixed offshore structures depend upon the design method used, i.e. whether limit state or
allowable stress design is employed. The load combinations recommended for use with allowable stress procedures are:
a. Dead loads plus operating environmental loads plus maximum live loads, appropriate to normal operations of the platform.
b. Dead loads plus operating environmental loads plus minimum live loads, appropriate to normal operations of the platform.
c. Dead loads plus extreme (design) environmental loads plus maximum live loads, appropriate for combining with extreme
conditions.
d. Dead loads plus extreme (design) environmental loads plus minimum live loads, appropriate for combining with extreme
conditions.
Moreover, environmental loads, with the exception of earthquake loads, should be combined in a manner consistent with their joint
probability of occurrence during the loading condition considered. Earthquake loads, if applicable, are to be imposed as a separate
environmental load, i.e., not to be combined with waves, wind, etc. Operating environmental conditions are defined as
representative of severe but not necessarily limiting conditions that, if exceeded, would require cessation of platform operations.
The DNV rules [2] permit allowable stress design but recommend the semi-probabilistic limit state design method, which the NPD
rules also require [4]. BS6235 permits both methods but the design equations it gives are for the allowable stress method [1]. API-
RP2A is very specific in recommending not to apply limit state methods. According to the DNV and the NPD rules for limit state
design, four limit states must be checked:
1. Ultimate limit state
For this limit state the following two loading combinations must be used:
Ordinary: 1,3 P + 1,3 L + 1,0 D + 0,7 E, and
Extreme : 1,0 P + 1,0 L + 1,0 D + 1,3 E
where P, L, D and E stand for Permanent (dead), Operating (live), Deformation (e.g., temperature, differential settlement) and
Environmental loads respectively. For well controlled dead and live loads during fabrication and installation, the load factor 1,3
may be reduced to 1,2. Furthermore, for structures that are unmanned during storm conditions and which are not used for
storage of oil and gas, the 1,3 load factor for environmental loads - except earthquakes - may be reduced to 1,15.
2. Fatigue limit state
All load factors are to be taken as 1,0.
3. Progressive Collapse limit state
All load factors are to be taken as 1,0.
4. Serviceability limit state
All load factors are to be taken as 1,0.
The so-called characteristic values of the loads used in the above combinations and limit states are summarized in Table 2, taken
from the NPD rules.
6. CONCLUDING SUMMARY
 In addition to environmental loads, an offshore structure must be designed for dead and live loads, fabrication and installation
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loads as well as accidental loads.
 Widely accepted rules of practice, listed in the references, are usually followed for specifying such loads.
 The type and magnitude of fabrication, transportation and installation loads depend upon the methods and sequences used for
the corresponding phases.
 Dynamic and impact effects are normally taken into account by means of appropriate dynamic load factors.
 Accidental loads are not well defined with respect to intensity and probability of occurrence. They will typically require special
protective measures.
 Load combinations and load factors depend upon the design method to be used. API-RP2A is based on allowable stress design
and recommends against limit state design, BSI favours allowable stress design, while DNV and NPD recommend limit state
design.
7. REFERENCES
[1] BS6235, "Code of Practice for Fixed Offshore Structures", British Standards Institution, London, 1982.
[2] "Rules for the Design, Construction and Inspection of Offshore Structures", Det Norske Veritas (DNV), Oslo, 1977 (with
corrections 1982).
[3] API-RP2A, "Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms", American Petroleum
Institute, Washington, D.C., 18th ed., 1989.
[4] "Regulation for Structural Design of Load-bearing Structures Intended for Exploitation of Petroleum Resources", Norwegian
Petroleum Directorate (NPD), 1985.
[5] DOE-OG, "Offshore Installation: Guidance on Design and Construction", U.K. Department of Energy, London 1985.
8. ADDITIONAL READING
1. OCS, "Requirements for Verifying the Structural Integrity of OCS Platforms"., United States Geologic Survey, National Centre,
Reston, Virginia, 1980.
2. Hsu, H.T., "Applied Offshore Structural Engineering", Gulf Publishing Co., Houston, 1981.
3. Graff, W.G., "Introduction to Offshore Structures", Gulf Publishing Co., Houston, 1981.
4. Gerwick, B.C. Jr., "Construction of Offshore Structures", John Wiley, New York, 1986.
Table 1 Minimum design live load specification
(1) Accumulated with a point load equal to the weight of the heaviest part likely to be removed, with a minimum value of 5 kN.
Point loads are assumed as being applied to a 0,3m  0,3m surface.
Loads to be taken into
account (kN/m
2
)

For portions of the
structure
For the
structure as a
whole
Zone considered Flooring
and joists
Other
components
(3)
Process zone (around
wells and large-scale
machines)
5 (1) 5 (1) 2.5
Drilling zone 5 (1) 5 (1) 2.5
Catwalks and walking
platforms (except
emergency exits)
3 2.5 1
Stairways (except
emergency exits)
4 3 0
Module roofing 2 1.5 1
Emergency exits 5 5 0
STORAGE
Storage floors - heavy
Storage floors - light
18
9
12
6
8 (2)
4 (2)
Delivery zone 10 10 5
Non-attributed area 6 4 3
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(2) Applied on the entirety of the flooring surface (including traffic).
(3) This column gives the loads to be taken into account for the structure's overall calculation. These values are the input for the
computer runs.
Table 2 Characteristic Loads according to NPD [4]

LOAD TYPE LIMIT STATES FOR TEMPORARY PHASES LIMIT STATES FOR NORMAL OPERATIONS

Serviceability

Fatigue

Ultimate
Progressive Collapse
Serviceability

Fatigue

Ultimate
Progressive Collapse
Abnormal
effects
Damage
condition
Abnormal
effects
Damage
condition
DEAD EXPECTED VALUE
LIVE SPECIFIED VALUE
DEFORMATION EXPECTED EXTREME VALUE
ENVIRONMENTAL Dependent
on
operational
requirements
Expected
load
history
Value dependent on measures
taken
Dependent
on
operational
requirements
Expected
load
history
Annual
exceedance
probability
10
-2

Annual
exceedance
probability
10
-4

Annual
exceedance
probability
10
-2

ACCIDENTAL NOT APPLICABLE Dependent
on
operational
requirements
NOT APPLICABLE Annual
exceedance
probability
10
-4

NOT
APPLICABLE
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Lecture 15A.4 - Analysis I
OBJECTIVE/SCOPE
To present the main analysis procedures for offshore structures.
PREREQUISITES
Lecture 15A.1: Offshore Structures: General Introduction
Lecture 15A.2: Loads I: Introduction and Environmental Loads
Lecture 15A.3: Loads II: Other Loads
RELATED LECTURES
Lecture 15A.5: Analysis II
SUMMARY
Analytical models used in offshore engineering are briefly described. Acceptance
criteria for the verification of offshore structures are presented.
Simple rules for preliminary member sizing are given and procedures for static in-
place and dynamic analysis are described.
1. ANALYTICAL MODEL
The analysis of an offshore structure is an extensive task, embracing consideration
of the different stages, i.e. execution, installation, and in-service stages, during its
life. Many disciplines, e.g. structural, geotechnical, naval architecture, metallurgy
are involved.
This lecture and Lecture 15A.5 are purposely limited to presenting an overview of
available analysis procedures and providing benchmarks for the reader to
appreciate the validity of his assumptions and results. They primarily address
jackets, which are more unusual structures compared to decks and modules, and
which more closely resemble onshore petro-chemical plants.
2. ANALYTICAL MODEL
The analytical models used in offshore engineering are in some respects similar to
those adopted for other types of steel structures. Only the salient features of
offshore models are presented here.
The same model is used throughout the analysis process with only minor
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adjustments being made to suit the specific conditions, e.g. at supports in
particular, relating to each analysis.
2.1 Stick Models
Stick models (beam elements assembled in frames) are used extensively for
tubular structures (jackets, bridges, flare booms) and lattice trusses (modules,
decks).
2.1.1 Joints
Each member is normally rigidly fixed at its ends to other elements in the model.
If more accuracy is required, particularly for the assessment of natural vibration
modes, local flexibility of the connections may be represented by a joint stiffness
matrix.
2.1.2 Members
In addition to its geometrical and material properties, each member is
characterised by hydrodynamic coefficients, e.g. relating to drag, inertia, and
marine growth, to allow wave forces to be automatically generated.
2.2 Plate Models
Integrated decks and hulls of floating platforms involving large bulkheads are
described by plate elements. The characteristics assumed for the plate elements
depend on the principal state of stress which they are subjected to. Membrane
stresses are taken when the element is subjected merely to axial load and shear.
Plate stresses are adopted when bending and lateral pressure are to be taken into
account.
3. ACCEPTANCE CRITERIA
3.1 Code Checks
The verification of an element consists of comparing its characteristic resistance(s)
to a design force or stress. It includes:
 a strength check, where the characteristic resistance is related to the yield
strength of the element,
 a stability check for elements in compression where the characteristic
resistance relates to the buckling limit of the element.
An element (member or plate) is checked at typical sections (at least both ends
and midspan) against resistance and buckling. This verification also includes the
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effect of water pressure for deepwater structures.
Tubular joints are checked against punching under various load patterns. These
checks may indicate the need for local reinforcement of the chord using
overthickness or internal ring-stiffeners.
Elements should also be verified against fatigue, corrosion, temperature or
durability wherever relevant.
3.2 Allowable Stress Method
This method is presently specified by American codes (API, AISC).
The loads remain unfactored and a unique coefficient is applied to the
characteristic resistance to obtain an allowable stress as follows:
"Normal" and "Extreme" respectively represent the most severe conditions:
 under which the plant is to operate without shut-down.
 the platform is to endure over its lifetime.
3.3 Limit State Method
This method is enforced by European and Norwegian Authorities and has now been
adopted by API as it offers a more uniform reliability.
Partial factors are applied to the loads and to the characteristic resistance of the
element, reflecting the amount of confidence placed in the design value of each
parameter and the degree of risk accepted under a limit state, i.e:
 Ultimate Limit State (ULS):
corresponds to an ultimate event considering the structural resistance
with appropriate reserve.
 Fatigue Limit State (FLS):
relates to the possibility of failure under cyclic loading.
Condition Axial Strong axis
bending
Weak axis
bending
Normal 0,60 0,66 0,75
Extreme 0,80 0,88 1,00
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 Progressive Collapse Limit State (PLS):
reflects the ability of the structure to resist collapse under accidental or
abnormal conditions.
 Service Limit State (SLS):
corresponds to criteria for normal use or durability (often specified by
the plant operator).
3.3.1 Load factors
Norwegian Authorities (2, 4) specify the following sets of load factors:
where the respective load categories are:
P are permanent loads (structural weight, dry equipments, ballast, hydrostatic
pressure).
L are live loads (storage, personnel, liquids).
D are deformations (out-of-level supports, subsidence).
E are environmental loads (wave, current, wind, earthquake).
A are accidental loads (dropped object, ship impact, blast, fire).
3.3.2 Material factors
Limit State Load Categories
P L D E A
ULS (normal) 1,3 1,3 1,0 0,7 0,0
ULS (extreme) 1,0 1,0 1,0 1,3 0,0
FLS 0,0 0,0 0,0 1,0 0,0
PLS (accidental) 1,0 1,0 1,0 1,0 1,0
PLS (post-damage) 1,0 1,0 1,0 1,0 0,0
SLS 1,0 1,0 1,0 1,0 0,0
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The material partial factors for steel is normally taken equal to 1,15 for ULS and
1,00 for PLS and SLS design.
3.3.3 Classification of Design Conditions
Guidance for classifying typical conditions into typical limit states is given in the
following table:
Condition Loadings Design
Criterion P/L E D A
Construction P

ULS,SLS
Load-Out P reduced wind support disp

ULS
Transport P transport wind and
wave

ULS
Tow-out (accidental) P

flooded
compart
PLS
Launch P

ULS
Lifting P

ULS
In-Place (normal) P + L wind, wave & snow actual

ULS,SLS
In-Place (extreme) P + L wind & 100 year wave actual

ULS
SLS
In-Place
(exceptional)
P + L wind & 10000 year
wave
actual

PLS
Earthquake P + L
10
-2
quake


ULS
Rare Earthquake P + L
10
-4
quake


PLS
Explosion P + L

blast PLS
Fire P + L

fire PLS
Dropped Object P + L

drill collar PLS
Boat Collision P + L

boat impact PLS
Damaged Structure P + reduced L reduced wave & wind

PLS
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4. PRELIMINARY MEMBER SIZING
The analysis of a structure is an iterative process which requires progressive
adjustment of the member sizes with respect to the forces they transmit, until a
safe and economical design is achieved.
It is therefore of the utmost importance to start the main analysis from a model
which is close to the final optimized one.
The simple rules given below provide an easy way of selecting realistic sizes for
the main elements of offshore structures in moderate water depth (up to 80m)
where dynamic effects are negligible.
4.1 Jacket Pile Sizes
 calculate the vertical resultant (dead weight, live loads, buoyancy), the
overall shear and the overturning moment (environmental forces) at the
mudline.
 assuming that the jacket behaves as a rigid body, derive the maximum axial
and shear force at the top of the pile.
 select a pile diameter in accordance with the expected leg diameter and the
capacity of pile driving equipment.
 derive the penetration from the shaft friction and tip bearing diagrams.
 assuming an equivalent soil subgrade modulus and full fixity at the base of
the jacket, calculate the maximum moment in the pile and derive its wall
thickness.
4.2 Deck Leg Sizes
 adapt the diameter of the leg to that of the pile.
 determine the effective length from the degree of fixity of the leg into the
deck (depending upon the height of the cellar deck).
 calculate the moment caused by wind loads on topsides and derive the
appropriate thickness.
4.3 Jacket Bracings
 select the diameter in order to obtain a span/diameter ratio between 30 and
40.
 calculate the axial force in the brace from the overall shear and the local
bending caused by the wave assuming partial or total end restraint.
 derive the thickness such that the diameter/thickness ratio lies between 20
and 70 and eliminate any hydrostatic buckle tendency by imposing
D/t<170/
3
H (H is the depth of member below the free surface).

4.4 Deck Framing
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 select a spacing between stiffeners (typically 500 to 800mm).
 derive the plate thickness from formulae accounting for local plastification
under the wheel footprint of the design forklift truck.
 determine by straight beam formulae the sizes of the main girders under
"blanket" live loads and/or the respective weight of the heaviest equipments.
5. STATIC IN-PLACE ANALYSIS
The static in-place analysis is the basic and generally the simplest of all analyses.
The structure is modelled as it stands during its operational life, and subjected to
pseudo-static loads.
This analysis is always carried at the very early stage of the project, often from a
simplified model, to size the main elements of the structure.
5.1 Structural Model
5.1.1 Main Model
The main model should account for eccentricities and local reinforcements at the
joints.
Typical models for North Sea jackets may feature over 800 nodes and 4000
members.
5.1.2 Appurtenances
The contribution of appurtenances (risers, J-tubes, caissons, conductors, boat-
fenders, etc.) to the overall stiffness of the structure is normally neglected.
They are therefore analysed separately and their reactions applied as loads at the
interfaces with the main structure.
5.1.3 Foundation Model
Since their behaviour is non-linear, foundations are often analysed separately from
the structural model.
They are represented by an equivalent load-dependent secant stiffness matrix;
coefficients are determined by an iterative process where the forces and
displacements at the common boundaries of structural and foundation models are
equated.
This matrix may need to be adjusted to the mean reaction corresponding to each
loading condition.
5.2 Loadings
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This Section is a reminder of the main types of loads, which are described in more
detail in Lectures 15A.2 and 15A.3.
5.2.1 Gravity Loads
Gravity loads consist of:
 dead weight of structure and equipments.
 live loads (equipments, fluids, personnel).
Depending on the area of structure under scrutiny, live loads must be positioned
to produce the most severe configuration (compression or tension); this may occur
for instance when positioning the drilling rig.
5.2.2 Environmental Loads
Environmental loads consist of wave, current and wind loads assumed to act
simultaneously in the same direction.
In general eight wave incidences are selected; for each the position of the crest
relative to the platform must be established such that the maximum overturning
moment and/or shear are produced at the mudline.
5.3 Loading Combinations
The static in-place analysis is performed under different conditions where the loads
are approximated by their pseudo-static equivalent.
The basic loads relevant to a given condition are multiplied by the appropriate load
factors and combined to produce the most severe effect in each individual element
of the structure.
6. DYNAMIC ANALYSIS
A dynamic analysis is normally mandatory for every offshore structure, but can be
restricted to the main modes in the case of stiff structures.
6.1 Dynamic Model
The dynamic model of the structure is derived from the main static model.
Some simplifications may however take place:
 local joint reinforcements and eccentricities may be disregarded.
 masses are lumped at the member ends.
 the foundation model may be derived from cyclic soil behaviour.
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6.2 Equations of Motion
The governing dynamic equations of multi-degrees-of-freedom systems can be
expressed in the matrix form:
MX'' + CX' + KX = P(t)
where
M is the mass matrix
C is the damping matrix
K is the stiffness matrix
X, X', X'' are the displacement, velocity and acceleration vectors (function
of time).
P(t) is the time dependent force vector; in the most general case it may depend
on the displacements of the structure also (i.e. relative motion of the structure
with respect to the wave velocity in Morison equation).
6.2.1 Mass
The mass matrix represents the distribution of masses over the structure.
Masses include that of the structure itself, the appurtenances, liquids trapped in
legs or tanks, the added mass of water (mass of water displaced by the member
and determined from potential flow theory) and the mass of marine growth.
Masses are generally lumped at discrete points of the model. The mass matrix
consequently becomes diagonal but local modes of vibration of single members
are ignored (these modes may be important for certain members subjected to an
earthquake). The selection of lumping points may significantly affect the ensuing
solution.
As a further simplification to larger models involving considerable degrees-of-
freedom, the system can be condensed to a few freedoms while still retaining its
basic energy distribution.
6.2.2 Damping
Damping is the most difficult to estimate among all parameters governing the
dynamic response of a structure.
It may consist of structural and hydrodynamic damping.
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Structural Damping
Structural damping is associated with the loss of energy by internal friction in the
material.
It increases with the order of the mode, being roughly proportional to the strain
energy involved in each.
Hydrodynamic Damping
Damping provided by the water surrounding the structure is commonly added to
the former, but may alternatively be accounted as part of the forcing function
when vibrations are close to resonance (vortex-shedding in particular).
Representation of Damping
Viscous damping represents the most common and simple form of damping. It
may have one of the following representations:
 modal damping: a specific damping ratio  expressing the percentage to
critical associated with each mode (typically  = 0,5% structural;  = 1,5%
hydrodynamic)
 proportional damping: defined as a linear combination of stiffness and mass
matrices.
All other types of non-viscous damping should preferably be expressed as an
equivalent viscous damping matrix.
6.2.3 Stiffness
The stiffness matrix is in all aspects similar to the one used in static analyses.
6.3 Free Vibration Mode Shapes and Frequencies
The first step in a dynamic analysis consists of determining the principal natural
vibration mode shapes and frequencies of the undamped, multi-degree-of-freedom
structure up to a given order (30th to 50th).
This consists in solving the eigenvalue problem:
KX =  MX
For rigid structures having a fundamental vibration period well below the range of
wave periods (typically less than 3 s), the dynamic behaviour is simply accounted
for by multiplying the time-dependent loads by a dynamic amplification factor
(DAF):
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DAF =
where  = T
N
/T is the ratio of the period of the structure to the wave period.

6.4 Modal Superposition Method
A convenient technique consists of uncoupling the equations through the normal
modes of the system.
This method is only applicable if:
 each mass, stiffness and damping matrix is time-independent.
 non-linear forces are linearized beforehand (drag).
The total response is obtained by summing the responses of the individual single-
degree-of-freedom oscillators associated to each normal mode of the structure.
This method offers the advantage that the eigen modes provide substantial insight
into the problem, and can be re-used for as many subsequent response
calculations as needed at later stages.
It may however prove time-consuming when a large number of modes is required
to represent the response accurately. Therefore:
 the simple superposition method (mode-displacement) is applied to a
truncated number of lowest modes for predicting earthquake response.
 it must be corrected by the static contribution of the higher modes (mode-
acceleration method) for wave loadings.
6.4.1 Frequency Domain Analysis
Such analysis is most appropriate for evaluating the steady-state response of a
system subjected to cyclic loadings, as the transient part of the response vanishes
rapidly under the effect of damping.
The loading function is developed in Fourier series up to an order :
p(t) =
The plot of the amplitudes p
j
versus the circular frequencies 
j
is called the
amplitude power spectra of the loading. Usually, significant values of p
j
only occur
within a narrow range of frequencies and the analysis can be restricted to it.
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The relationship between response and force vectors is expressed by the transfer
matrix H, such as:
H = [-M 
2
+ i x C  + K]

the elements of which represent:
H
j,k
=

The spectral density of response in freedom j versus force is then:

The fast Fourier transform (FFT) is the most efficient algorithm associated with this
kind of analysis.
6.4.2 Time Domain Analysis
The response of the i-th mode may alternatively be determined by resorting to
Duhamel's integral:
X
j
(t) =

The overall response is then obtained by summing at each time step the individual
responses over all significant modes.
6.5 Direct Integration Methods
Direct step-by-step integration of the equations of motion is the most general
method and is applicable to:
 non-linear problems involving special forms of damping and response-
dependent loadings.
 responses involving many vibration modes to be determined over a short
time interval.
The dynamic equilibrium at an instant  is governed by the same type of
equations, where all matrices (mass, damping, stiffness, load) are simultaneously
dependent on the time and structural response as well.
All available integration techniques are characterised by their stability (i.e. the
tendency for uncontrolled divergence of amplitude to occur with increasing time
steps). Unconditionally stable methods are always to be preferred (for instance
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Newmark-beta with  = 1/4 or Wilson-theta with  = 1,4).
7. CONCLUDING SUMMARY
 The analysis of offshore structures is an extensive task.
 The analytical models used in offshore engineering are in some respects
similar to those used for other types of steel structures. The same model is
used throughout the analysis process.
 The verification of an element consists of comparing its characteristic
resistance(s) to a design force or stress. Several methods are available.
 Simple rules are available for preliminary member sizing.
 Static in-plane analysis is always carried out at the early stage of a project to
size the main elements of the structure. A dynamic analysis is normally
mandatory for every offshore structure.
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Lecture 15A.5 - Analysis II
OBJECTIVE/SCOPE
To present the analysis procedures for offshore structures relating to fatigue,
abnormal and accident conditions, load-out and transportation, installation and
local design.
PREREQUISITES
Lecture 15A.1: Offshore Structures: General Introduction
Lecture 15A.2: Loads I: Introduction and Environmental Loads
Lecture 15A.3: Loads II: Other Loads
RELATED LECTURES
Lecture 15A.4: Analysis I
SUMMARY
Methods of fatigue analysis are described including the fatigue model (structural,
hydrodynamic loading, and joint stress models) and the methods of fatigue
damage assessment.
Abnormal and accidental conditions are considered relating to earthquake, impact
and progressive collapse.
Analyses required for load-out and transportation and for installation are outlined.
Local analysis for specific parts of the structure which are better treated by
dedicated models outside of the global analysis are identified.
1. FATIGUE ANALYSIS
A fatigue analysis is performed for those structures sensitive to the action of cyclic
loadings such as:
 wave (jackets, floating structures).
 wind (flare booms, stair towers).
 structures under rotating equipments.
1.1 Fatigue Model
1.1.1 Structural Model
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The in-place model is used for the fatigue analysis.
Quasi-static analysis is often chosen; it permits all local stresses to be
comprehensively represented. The dynamic effects are accounted for by factoring
the loads by the relevant DAF.
Modal analysis may be used instead; it offers computational efficiency, but may
also overlook important local response modes, particularly near the waterline
where direct wave action causes high out-of-plane bending (see Section 5.2). The
mode - acceleration method may overcome this problem.
1.1.2 Hydrodynamic Loading Model
A very large number of computer runs may be necessary to evaluate the stress
range at the joints. The wave is repeatedly generated for:
 different blocks of wave heights (typically from 2 to 28m in steps of 2m),
each associated with a characteristic wave and zero-upcrossing period.
 different incidences (typically eight).
 different phases to determine the stress range for a given wave at each joint.
1.1.3 Joint Stress Model
Nominal joint stresses are calculated for eight points around the circumference of
the brace. The maximum local (hot spot) stress is obtained by multiplying the
former by a stress concentration factor (SCF) given by parametric formulae which
are functions of the joint geometry and the load pattern (balanced/unbalanced).
1.1.4 Fatigue Damage Model
The fatigue failure of joints in offshore structures primarily depends on the stress
ranges and their number of occurrences, formulated by S-N curves:
log N
i
= log  + mlog 
i

The number of cycles to failure N
i
corresponds to a stress range. The effect of the
constant stresses, mainly welding residual stresses, is implicitly accounted for in
this formulation.
The cumulative damage caused by n
i
cycles of stress 
i
, over the operational life
of the platform (30 to 50 years) is obtained by the Palmgren-Miner rule:
D =
The limit of this ratio depends on the position of the joint with respect to the
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splash zone (typically +/-4m on either side of the mean sea level). The ratio
should normally not exceed:
 1,0 above,
 0,1 within,
 0,3 below the splash zone.
1.1.5 Closed Form Expression
The damage may alternatively be expressed in closed form:
D =
where
, m are coefficients of the selected S-N curve.
 is the stress range exceeded once in N cycles.
k is a long-term distribution parameter, depending on the position of the joint in
the structure.
N is the total number of cycles.
1.2 Deterministic Analysis
This analysis consists of time-domain analysis of the structure. The main
advantage of this representation is that non-linear effects (drag, high order wave
theories) are handled explicitly.
A minimum of four regular waves described in terms of height and associated
period are considered for each heading angle.
1.3 Spectral Analysis
Waves of a given height are not characterised by a unique frequency, but rather
by a range of frequencies. If this range corresponds to a peak in the structural
response, the fatigue life predicted by the deterministic method can be seriously
distorted.
This problem is overcome by using a scatter diagram, in which the joint
occurrence of wave height and period is quantified. Wave directionality may also
be accounted for. Eventually the most thorough representation of a sea state
consists of:
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 the frequency spectrum constructed from the significant wave heights and
mean zero-crossing periods.
 the directionality function derived from the mean direction and associated
spreading function.
This approach requires that the physical process be approximately linear (or
properly linearised) and stationary. Transfer functions TF are determined from
time-domain analyses involving various wave heights, each with different period
and incidence:

The response has normally a narrow-banded spectrum and can be described by a
Rayleigh distribution.
The zero-upcrossing frequency of stress cycles is then approximated by:
T
z
=

where m
n
is the nth order moment of the response.

The significant stress range is readily obtained for each sea state as:

sig
=

where S(,) is the directional wave energy spectrum.
1.4 Wind Fatigue
1.4.1 Wind Gusts
The fatigue damage caused by the fluctuating part of wind (gusts) on slender
structures like flare booms and bridges is usually predicted by spectral methods.
The main feature of such analysis is the introduction of coherence functions
accounting for the spanwise correlation of forces.
1.4.2 Vortex Shedding
Vortex induced failure occurs for tubes subjected to a uniform or oscillating flow of
fluid.
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Within a specific range of fluid velocities, eddies are shed at a frequency close to
the resonant frequency of the member.
This phenomenon involves forced displacements, which can be determined by
models such as those suggested in [1].
2. ABNORMAL AND ACCIDENTAL CONDITIONS
This type of analysis addresses conditions which may considerably affect the
integrity of the structure, but only have a limited risk of occurrence.
Typically all events with a probability level less than the 10
-4
threshold are
disregarded.
2.1 Earthquake Analysis
2.1.1 Model
Particular attention shall be paid to:
 foundations: the near field (i.e. the soil mass in the direct vicinity of the
structure) shall accurately represent load-deflection behaviour. As a general
rule the lateral foundation behaviour is essentially controlled by horizontal
ground motions of shallow soil layers.
 modal damping (in general taken as 5% and 7% of critical for ULS and PLS
analyses respectively).
2.1.2 Ductility Requirements
The seismic forces in a structure are highly dependent on its dynamic
characteristics. Design recommendations are given by API to determine an
efficient geometry. The recommendations call for:
 providing sufficient redundancy and symmetry in the structure.
 favouring X-bracings instead of K-bracings.
 avoiding abrupt changes in stiffness.
 improving the post-buckling behaviour of bracings.
2.1.3 Analysis Method
Earthquake analyses can be carried out according to the general methods
presented in Lecture 15A.4.
However their distinctive feature is that they represent essentially a base motion
problem and that the seismic loads are therefore dependent on the dynamic
characteristics of the structure.
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Modal spectral response analysis is normally used. It consists of a superposition of
maximum mode response and forms a response spectrum curve characteristic of
the input motion. This spectrum is the result of time-histories of a SDOF system
for different natural periods of vibration and damping.
Direct time integration can be used instead for specific accelerograms adapted to
the site.
2.2 Impact
The analysis of impact loads on structures is carried out locally using simple plastic
models [2].
Should a more sophisticated analysis be required, it can be accomplished using
time-domain techniques presented in Section 6 of Lecture 15A.4.
The whole energy must be absorbed within acceptable deformations.
2.2.1 Dropped Object/Boat Impact
When a wellhead protection cover is hit by a drill collar, or a tube (jacket leg,
fender) is crushed by a supply boat, two load/deformation mechanisms occur
simultaneously:
 local punch-through (cover) or denting (tube).
 global deformation along plastic hinges with possible appearance of
membrane forces.
2.2.2 Blast and Fire
Owing to the current lack of definitive guidance regarding explosions and fire, the
behaviour of structures in such events has so far been only predicted by simple
models based on:
 equivalent static overpressure and plastic deformation of plates for blast
analysis.
 the reduction of material strength and elastic modulus under temperature
increase.
In the aftermath of recent mishaps however, more accurate analyses may become
mandatory, based on a better understanding of the pressure-time histories and
the effective resistance and response of structures to explosions and fire.
2.3 Progressive Collapse
Some elements of the structure (legs, bracings, bulkheads) may partially or
completely loose their strength as a result of accidental damage.
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The purpose of such analysis is to ensure that the spare resistance of the
remaining structure is sufficient to allow the loads to redistribute.
Since such a configuration is only temporary (mobilisation period prior to repairs)
and that operations will also be restricted around the damaged area, reduced live
and environmental loads are generally accepted.
In this analysis, the damaged elements are removed from the model. Their
residual strength may be represented by forces applied at the boundary nodes
with the intact structure.
3. LOAD OUT & TRANSPORTATION
3.1 Load-Out
The load-out procedure consists in moving the jacket or module from its
construction site to the transportation barge by skidding, or by using trailers
underneath it.
The barge may be floating and is continuously deballasted as the package
progresses onto it, or grounded on the bottom of the harbour.
3.1.1 Skidding
The most severe configuration during skidding occurs when the part of the
structure is cantilevering out:
 from the quayside before it touches the barge.
 from the barge just after it has left the quay.
The analysis should also investigate the possibility of high local reactions being the
result of settlement of the skidway or errors in the ballasting procedure.
3.1.2 Load-Out by Trailers
As the reaction on each trailer can be kept constant, analysis of load-out by
trailers only requires a single step to determine the optimal distribution of trailers.
3.2 Transportation
3.2.1 Naval Architectural Model
The model consists of the rigid-body assembly of the barge and the structure.
Barges are in general characterised by a low length/beam ratio and a high
beam/draught ratio, as well as sharp corners which introduce heavy viscous
damping.
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For jacket transport, particular care shall be taken in the representation of
overhanging parts (legs, buoyancy tanks) which contribute significantly to the
righting moment.
Dry-transported decks and modules may be simply represented by their mass and
moments of inertia.
This analysis shall provide the linear and angular accelerations and displacements
of the structure to be entered in the structural model as inertia forces, and also
the partition and intensity of buoyancy and slamming forces.
3.2.2 Structural Model
The jacket model is a simplified version of the in-place model, from which
eccentricities and local reinforcements may be omitted.
The barge is modelled as a plane grid, with members having the equivalent
properties of the longitudinal and transversal bulkheads.
As the barge passes over a wave trough or a crest, a portion only of the barge is
supported by buoyancy (long barges may be spanning over a whole trough or be
half-cantilevered).
The model therefore represents the jacket and the barge as two structures
coupled together by the seafastening members.
4. INSTALLATION
4.1 Launching
4.1.1 Naval Architectural Model
A three dimensional analysis is carried out to evaluate the global forces acting on
the jacket at various time steps during the launch sequence.
At each time step, the jacket/barge rigid body system is repositioned to equilibrate
the internal and external forces produced by:
 jacket weight, inertia, buoyancy and drag forces.
 barge weight, buoyancy and ballast forces.
 vertical reactions and friction forces between jacket and barge.
The maximum reaction on the rocker arm is normally obtained when the jacket
just starts rotating about the rocker hinge.
4.1.2 Structural Model
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The structural model is in all aspects identical to the one used for the
transportation analysis, with possibly a finer representation of the launch legs.
The rocker arm is also represented as a vertical beam hinged approximately at
midspan. Interface loads obtained by the rigid body analysis are input at boundary
conditions on the launch legs. All interface members must remain in compression,
otherwise they are inactivated and the analysis restarted for that step.
Once the tilting phase has begun, the jacket is analysed at least for each main leg
node being at the vertical of the rocker arm pivot.
4.2 Upending
No dedicated structural analysis is required for this phase, which is essentially a
naval architecture problem.
A local analysis of the lugs is performed for crane-assisted upendings.
4.3 Docking
Docking of a jacket onto a pre-installed template requires guides to be analysed
for local impact. The same requirement applied for bumpers to aid the installation
of modules.
4.4 Unpiled Stability
The condition where the jacket may for a while stand unpiled on the seafloor is
analysed for the design installation wave.
The stability of the jacket as a whole (overturning tendency) is investigated,
together with the resistance of the mudmats against soil pressure.
4.5 Piling
The piles are checked during driving for the dynamic stresses caused by the
impact wave of the hammer blow. The maximum cantilevered (stick-up) length of
pile must be established for the self-weight of the pile and hammer combined,
accounting for first and second order moments arising from the pile batter.
Hydrodynamic actions are added for underwater driving.
Elements in the vicinity of the piles (guides, sleeves) shall also be checked, see
Section 5.1.
4.6 Lifting
4.6.1 Model
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The model used for the lift analysis of a structure consists of the in-place model
plus the representation of the rigging arrangement (slings, spreader frames).
For single lifts the slings converge towards the hook joint, which is the sole vertical
support in the model and shall be located exactly on the vertical through the
centre of gracity (CoG) of the model.
For heavier dual-crane lifts, the CoG shall be contained in the vertical plane
defined by the two hook joints.
The mathematical instability of the model with respect to horizontal forces is
avoided by using soft horizontal springs at the padeyes. The force and elongation
in these springs should always remain small.
4.6.2 Design Factors
Different factors are applied to the basic sling forces to account for specific effects
during lifting operations.
4.6.2.1 Skew Load Factor (SKL)
This factor represents the effect of fabrication tolerances and lack-of-fit of the
slings on the load repartition in a statically undetermined rigging arrangement (4
slings or more). Skew factors may either be directly computed by applying to a
pair of opposite slings a temperature difference such that their
elongation/shortening corresponds to the mismatch, or determined arbitrarily
(typically 1/3 - 2/3 repartition).
4.6.2.2 Dynamic Amplification Factor (DAF)
This factor accounts for global dynamic effects normally experienced during lifting
operations. DnV [24] recommends minimum values as follows:
4.6.2.3 Tilt Effect Factor (TEF)
This factor accounts for additional sling loading caused by the rotation of the lifted
object about a horizontal axis and by the longitudinal deviation of the hooks from
their theoretical position in the case of a multi-hook lift. It shall normally be based
on 5 and 3 tilt respectively depending on whether cranes are on different vessels
or not.
Lifted Weight W
(tonnes)
up to 100 t 100 t to
1000t
1000 t to
2500t
more than
2500 t
DAF offshore 1,30 1,20 1,15 1,10
DAF inshore 1,15 1,10 1,05 1,05
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4.6.2.4 Yaw Effect Factor (YEF)
This factor accounts for the rotation of the lifted object about a vertical axis (equal
to 1,05 typically).
4.6.3 Consequence Factors
Forces in elements checked under lift conditions are multiplied by a factor
reflecting the consequence a failure of that specific element would have on the
integrity of the overall structure:
 1,30 for spreader frames, lifting points (padeyes) and their attachment to the
structure.
 1,15 for all members transferring the load to the lifting points.
 1,00 for other elements.
5. LOCAL ANALYSES AND DESIGN
Local analyses address specific parts of the structure which are better treated by
dedicated models outside the global analysis.
The list of analyses below is not exhaustive and more information can be found in
[1-24] which provide a complete design procedure in each particular case.
5.1 Pile/Sleeve Connections
Underwater pile/sleeve connection is usually achieved by grouting the annulus
between the outside of the pile and the inner sleeve.
The main verifications address:
 the shear stresses in the concrete.
 the fatigue damage in the shear plates and the attachment welds to the main
jacket accumulated during pile driving and throughout the life of the platform.
5.2 Members within the Splash Zone
Horizontal members (conductor guide frames in particular) located within the
splash zone (+/-5m on either side of the mean-sea-level approximately) shall be
analysed for fatigue caused by repeated wave slamming.
A slamming coefficient C
s
=3,5 is often selected.

5.3 Straightened Nodes
Typical straightened nodes (ring-stiffened nodes, bottle legs nodes with
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diaphragms) are analysed by finite-elements models, from which parametric
envelope formulae are drawn and applied to all nodes representative of the same
class.
5.4 Appurtenances
5.4.1 Risers, Caissons & J-Tubes
Static In-Place and Fatigue
Risers, caissons and J-tubes are verified either by structural or piping programs for
the action of environmental forces, internal pressure and temperature. Particular
attention is paid to the bends not always satisfactorily represented by structural
programs and the location of the touch-down point now known a-priori.
A fatigue analysis is also performed to assess the fatigue damage to the clamps
and the attachments to the jacket.
Pull-In
J-tubes are empty ducts continuously guiding a post-installed riser pulled inside.
They are verified by empirical plastic models against the forces generated during
pull-in by the friction of the cable and the deformation of the pull head, see [22].
5.4.2 Conductors
Conductors are analysed in-place as beam columns on discrete simple supports,
these being provided by the horizontal framing of the jacket (typically 20 to 25 m
span).
The installation sequence of the different casings must be considered to assess the
distribution of stresses in the different tubes forming the overall composite
section.
Also the portion of compression force in the conductor caused by the hanging
casings is regarded as an internal force (similar to prestressing) which therefore
does not induce any buckling tendency, see [23].
5.5 Helidecks
The helideck is normally designed to resist an impact load equal to 2,5 times the
take-off weight of the heaviest helicopter factored by a DAF of 1,30.
Plastic theories are applicable for designing the plate and stiffeners, while the main
framing is analysed elastically.
5.6 Flare Booms
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Analyses of flare booms particularly consider:
 variable positions during installation (horizontal pick-up from the barge, lift
upright).
 reduced material characteristics due to high temperature in the vicinity of the
tip during operation.
 dynamic response under gusty winds.
 local excitation of diagonals by wind vortex-shedding.
6. CONCLUDING SUMMARY
 With the trend to ever deeper and more slender offshore structures in yet
harsher environments, more elaborate theories are necessary to analyse
complex situations. There is a risk for the Engineer having increasingly to rely
on the sole results of computer analyses at the expense of sound design
practice.
 To retain enough control of the process of analysis, the following
recommendations are given:
 check the interfaces between the different analyses and ensure the consistency of
the input/output.
 verify the validity of the data resulting from a complex analysis against a
simplified model, which can also be used to assess the influence of a particular
parameter.
 make full use of "good engineering judgement" to criticise the unexpected results
of an analysis.
7. REFERENCES
[1] Skop R.A. & Griffin O.M., An Heuristic Model for Determining Flow-Induced
Vibrations of Offshore Structures/OTC paper 1843, May 1973.
[2] De Oliveira J.G., The Behaviour of Steel Offshore Structures under Accidental
Collisions/OTC paper 4136, May 1981.
[3] API-RP2A, Recommended Practice for Planning, Designing and Constructing
Fixed Offshore Platforms/18th edition, September1989.
[4] DnV, Rules for the Classification of Fixed Offshore Structures, September
1989.
[5] DnV, Standard for Insurance Warranty Surveys in Marine Operations, June
1985.
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[6] NPD, Regulation for Structural Design of Loadbearing Structures Intended for
Exploitation of Petroleum Resources, October1984 and Veiledning om Utforming,
Beregning og Dimensjonering av Stalkonstruksjoner i Petroleumsvirksomheten,
December1989.
[7] DoE, Offshore Installations: Guidance on Design and Construction/London,
April 1984.
[8] McClelland B. & Reifel M.D., Planning and Design of Fixed Offshore
Platforms/Van Nostrand Reinhold, 1986.
[9] UEG, Node Flexibility and its Effect on Jacket Structures/CIRIA Report UR22,
1984.
[10] Hallam M.G., Heaf N.J. & Wootton L.R., Dynamics of Marine Structures/ CIRIA
Report UR8 (2nd edition), October 1978.
[11] Wilson J.F., Dynamics of Offshore Structures/Wiley Interscience, 1984.
[12] Clough R.W. & Penzien J., Dynamics of Structures/McGraw-Hill, New York,
1975.
[13] Newland D.E., Random Vibrations and Spectral Analysis/Longman Scientific
(2nd edition), 1984.
[14] Zienkiewicz O.C., Lewis R.W. & Stagg K.G., Numerical Methods in Offshore
Engineering/Wiley Interscience, 1978.
[15] Davenport A.G., The Response of Slender Line-Like Structures to a Gusty
Wind/ICE Vol.23, 1962.
[16] Williams A.K. & Rhinne J.E., Fatigue Analysis of Steel Offshore Structures/ICE
Vol.60, November 1976.
[17] Anagnostopoulos S.A., Wave and Earthquake Response of Offshore
Structures: Evaluation of Modal Solutions/ASCE J. of the Structural Div., vol. 108,
No ST10, October 1982.
[18] Chianis J.W. & Mangiavacchi A., A Critical Review of Transportation Analysis
Procedures/OTC paper 4617, May1983.
[19] Kaplan P. Jiang C.W. & Bentson J, Hydrodynamic Analysis of Barge-Platform
Systems in Waves/Royal Inst. of Naval Architects, London, April 1982.
[20] Hambro L., Jacket Launching Simulation by Differentiation of Constraints/
Applied Ocean Research, Vol.4 No.3, 1982.
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[21] Bunce J.W. & Wyatt T.A., Development of Unified Design Criteria for Heavy
Lift Operations Offshore/OTC paper 4192, May 1982.
[22] Walker A.C. & Davies P., A Design Basis for the J-Tube Method of Riser
Installation/J. of Energy Resources Technology, pp. 263-270, September 1983.
[23] Stahl B. & Baur M.P., Design Methodology for Offshore Platform Conductors/J.
of Petroleum Technology, November 1983.
[24] DnV - Rules for the Classification of Steel Ships, January 1989.
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Lecture 15A.6: Foundations
OBJECTIVE\SCOPE
 to classify different types of piles
 to understand main design methods
 to cover various methods of installation
PREREQUISITES
Lecture 1B.2.2: Limit State Design Philosophy and Partial Safety Factors
Lectures 10.6: Shear Connection
Lectures 12.4: Fatigue Behaviour of Hollow Section Joints
Lecture 15A.12: Connections in Offshore Deck Structures
Lecture 17.5: Requirements and Verifications of Seismic Resistant Structures
A general knowledge of design in offshore structures and an understanding of offshore installation are also
required.
SUMMARY
In this lecture piled foundations for offshore structures are presented. The lecture starts with the classification of
soil. The main steps in the design of piles are then explained. The different kinds of piles and hammers are
described. The three main execution phases are briefly discussed: fabrication, transport and installation.
1. INTRODUCTION
1.1 Classification of Soils
The stratigraphy of the sea bed results from a complex geological process during which various materials were
deposited, remoulded and pressed together.
Soil texture consists of small mineral or organic particles basically characterized by their grain size and mutual
interaction (friction, cohesion).
The properties of a specific soil depend mainly on the following factors:
 density.
 water content.
 over consolidation ratio.
For design purposes the influence of these factors on soil behaviour is expressed in terms of two fundamental
parameters:
 friction angle.
 undrained shear strength C
u
.
Since the least significant of either of these parameters is often neglected, soils can be classified within "ideal"
categories:
 granular soils.
 cohesive soils.
1.2 Granular Soils
Granular soils are non-plastic soils with negligible cohesion between particles. They include:
 sands : characterized by large to medium particle sizes (1mm to 0,05mm) offering a high permeability,
 silts : characterized by particle sizes between 0,05 and 0,02mm; they are generally over-consolidated; they
may exhibit some cohesion.
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1.3 Cohesive Soils
Clays are plastic soils with particle sizes less than 0,002mm which tend to stick together; their permeability is low.
1.4 Multi-Layered Strata
The nature and characteristics of the soil surrounding a pile generally vary with the depth. For analysis purposes,
the soil is divided into several layers, each having constant properties throughout. The number of layers depends
on the precision required of the analysis.
2. DESIGN
Steel offshore platforms are usually founded on piles, driven deep into the soil (Figure 1). The piles have to
transfer the loads acting on the jacket into the sea bed. In this section theoretical aspects of the design of piles are
presented. Checking of the pile itself is described in detail in the Worked Example.

2.1 Design Loads
These loads are those transferred from the jacket to the foundation. They are calculated at the mudline.
2.1.1 Gravity loads
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Gravity loads (platform dead load and live loads) are distributed as axial compression forces on the piles
depending upon their respective eccentricity.
2.1.2 Environmental loads
Environmental loads due to waves, current, wind, earthquake, etc. are basically horizontal. Their resultant at
mudline consists of:
 shear distributed as horizontal forces on the piles.
 overturning moment on the jacket, equilibrated by axial tension/ compression in symmetrically disposed piles
(upstream/downstream).
2.1.3 Load combinations
The basic gravity and environmental loads multiplied by relevant load factors are combined in order to produce the
most severe effect(s) at mudline, resulting in:
 vertical compression or pullout force, and
 lateral shear force plus bending.
2.2 Static Axial Pile Resistance
The overall resistance of the pile against axial force is the sum of shaft friction and end bearing.
2.2.1 Lateral friction along the shaft (shaft friction)
Skin friction is mobilized along the shaft of the tubular pile (and possibly also along the inner wall when the soil
plug is not removed).
The unit shaft friction:
 for sands: is proportional to the overburden pressure,
 for clays: is calculated by the "alpha" or "lambda" method and is a constant equal to the shear strength C
u
at
great depth.
Lateral friction is integrated along the whole penetration of the pile.
2.2.2 End bearing
End bearing is the resultant of bearing pressure over the gross end area of the pile, i.e. with or without the area of
plug if relevant.
The bearing pressure:
 for clays: is equal to 9  C
u
.

 for sands: is proportional to the overburden pressure as explained in Section 6.4.2 of API-RP2A [1].
2.2.3 Pile penetration
The pile penetration shall be sufficient to generate enough friction and bearing resistance against the maximum
design compression multiplied by the appropriate factor of safety. No bearing resistance can be mobilized against
pull-out: the friction available must be equated to the pull out force multiplied by the appropriate factor of safety.
2.3 Lateral Pile Resistance
The shear at the mudline caused by environmental loads is resisted by lateral bearing of the pile on the soil. This
action may generate large deformations and high bending moments in the part of the pile directly below the
mudline, particularly in soft soils.
2.3.1 P-y curves
P-y curves represent the lateral soil resistance versus deflection. The shape of these curves varies with the depth
and the type of soil at the considered elevation. The general shape of the curves for increasing displacement
features:
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 elastic (linear) behaviour for small deflections,
 elastic/plastic behaviour for medium deflections,
 constant resistance for large deflections or loss of resistance when the soil skeleton deteriorates (clay under
cyclic load in particular).
2.3.2 Lateral pile analysis
For analysis purposes, the soil is modelled as lumped non-linear springs distributed along the pile. The fourth order
differential equation which expresses the pile deformation is integrated by successive iterations, the secant
stiffness of the soil springs being updated at each step.
For large deformations, the second order contribution of the axial compression to the bending moment (P-Delta
effect) shall be taken into account.
2.4 Pile Driving
Piles installed by driving are forced into the soil by a ram hitting the top. The impact is transmitted along the pile
in the form of a wave, which reflects on the pile tip. The energy is progressively lost by plastic friction on the sides
and bearing at the tip of the pile.
2.4.1 Empirical formulae
A considerable number of empirical formulae exist to predict pile driveability. Each formula is generally limited to a
particular type of soil and hammer.
2.4.2 Wave equation
This method of analysing the driving process consists of representing the ensemble of pile/soil/hammer as a one-
dimensional assembly of masses, springs and dashpots:
 the pile is modelled as a discrete assembly of masses and elastic springs.
 the soil is idealized as a massless medium characterized by elastic-perfectly-plastic springs and linear
dashpots.
 the hammer is modelled as a mass falling with an initial velocity.
 the cushion is represented by a weightless spring (see Figure 3).
 the pile cap is represented by a mass of infinite rigidity.
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The energy of the ram hitting the top of the pile generates a stress wave in the pile, which dissipates progressively
by friction between the pile and the soil and by reflection at the extremities of the pile.
The plastic displacement of the tip relative to the soil is the set achieved by the blow. Curves can be drawn to
represent the number of blows per unit length required to drive the pile at different penetrations.
The wave equation, though representing the most rigorous assessment to date of the driving process, still suffers a
lack of accuracy, mostly caused by the inaccuracies in the soil model.
3. DIFFERENT KINDS OF PILES
Driven piles are the most popular and cost-efficient type of foundation for offshore structures.
As shown in Figure 2, the following alternatives may be chosen when driving proves impractical:
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 insert piles.
 drilled and grouted piles.
 belled piles.


3.1 Driven Piles
Piles are usually made up in segments. After placing and driving the first long segment, extension segments called
add-ons are set on piece by piece as driving proceeds until the overall design length is achieved.
In recent years one-piece piles have been widely used in the North Sea since the offshore work is considerably
reduced.
Wall thickness may vary. A thicker wall is sometimes required:
 in sections from mudline down to a specified depth within which bending stresses are especially high,
 at the pile tip (driving shoe) to resist local bearing stresses while driving.
Uniform wall thickness is however preferable thus avoiding construction and installation problems.
3.2 Insert Piles
Insert piles are smaller diameter piles driven through the main pile from which the soil plug has been previously
drilled out. They are therefore not subjected to skin friction over the length of the main pile and can reach
substantial additional penetration.
The insert pile is welded to the main pile at the top of the jacket and the annular space between the tubes is
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grouted.
This type of pile is used:
 in a preplanned situation: performance is good although material and installation costs are higher than for
normal driven piles.
 as an emergency procedure: when scheduled piles cannot be driven to the required penetration, resulting
therefore in one of the following drawbacks.
 a thicker wall section of the main pile will be within the jacket height instead of below the mudline.
 reduced friction area and end bearing pressure,
 difficulties often noted for the setting-in of all the required volume of grouting, i.e. the concern is the leakage of
grout or the impossibility to fill with the calculated volume of grout.
3.3 Drilled and Grouted Piles
This procedure is the only means of installing piles with tension resistance in hard soils or soft rocks; it resembles
that for drilling a conductor well.
An oversized hole is initially drilled to the proposed pile penetration depth. The pile is then lowered down,
sometimes centred in the hole by spacers and the annular space between the pile shaft and the surrounding soil is
grouted.
Design uncertainty results because:
 hard soil formation softens when exposed to the water or mud used during drilling and exhibits lower skin
friction resistance.
 in case of calcareous sand, external grouting just crushes the sand, slightly extending the effective pile
diameter but not increasing the friction significantly.
3.4 Belled Piles
While belled piles, on land, are used to decrease the bearing stress under a pile, offshore belled piles provide a
large bearing area to increase tip uplift resistance.
The main pile, normally driven, serves here as a casing through which a rig drills a slightly oversized hole ahead. A
belling tool (underreamer) then enlarges the socket to a conical bell with a base diameter a few times that of the
main pile. A heavy reinforcement cage is lowered inside the bell which is subsequently filled with concrete made
using fine aggregate (maximum size 10mm).
4. FABRICATION AND INSTALLATION
4.1 Fabrication
The piles are usually made up of "cans" - cylinders of rolled plate with a longitudinal seam. Single cans are
typically 1,5m long or more. Longitudinal seams of two adjacent segments are rotated 90 apart at least.
Bevelling is mandatory should the wall thickness difference exceed 3mm between adjacent cans. Maximum
deviation from straightness is specified (0.1% in length).
Commonly used steel grade is X52 or X60.
The outside surface of grouted piles should be free of mill scale and varnished.
In certain instances, steel piles are protected underwater by sacrificial anodes or by impressed current. In the
splash zone additional thickness to allow for corrosion (3mm for example) and epoxy or rubberized coating, monel
or copper-nickel sheeting are provided.
4.2 Transportation
4.2.1 Barge transportation
Pile segments are choked and fastened to the barge to prevent them from falling overboard under severe
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seastates. Pile plate should be thick enough to prevent any deformation caused by stacking.
4.2.2 Self floating mode
This method is attractive where long segments of pile are to be lifted and set in guides far below the sea surface
(skirt piles for example).
The ends of the piles are sealed by steel closure plates or rubber diaphragms which should be able to resist wave
slamming during the tow.
4.2.3 Transport within the jacket
The piles are pre-set inside the main legs or in the guides/sleeves, generating additional weight and possibly
buoyancy (if closed). They are held in place by shims which prevent them from escaping from their guides during
launch and uprighting of the jacket.
Several piles are driven immediately after the jacket has touched down, providing initial stability against the action
of waves and current.
4.3 Hammers
Piles are positioned:
 either inside the jacket legs, extending the full height of the jacket,
 or encased in sleeves protruding at the bottom of the jacket, running vertical or parallel to the legs (typical
batter 1/12 to 1/6).
Piles can then be driven using any type of hammer (or a combination of types). Hammers are illustrated in Figure
3.
4.3.1 Steam hammers
Steam hammers are widely used for offshore installation of jackets. They are generally single acting with rates of
up to 40 blows/minute. Energies of current hammers range from 60 000 to 1 250 000 ft lb/blow. (82KNm to
1725KNm per blow).
During driving, the hammer with attached driving head rides the pile rather than being supported by leads. The
hammer line from the crane boom is slackened so as to prevent transmission of impact and vibration into the
boom.
4.3.2 Diesel hammers
Diesel hammers are much used at offshore terminals. They are lighter to handle and less energy consuming than
steam hammers, but their effective energy is limited.
4.3.3 Hydraulic hammers
Hydraulic hammers are dedicated to underwater driving (skirt piles terminating far below the sea surface).
Menck hydraulic hammers are widely used. They utilize a solid steel ram and a flexible steel pile cap to limit impact
forces. They are double acting. Hydraulic fluid under high pressure is used to force a piston or set of pistons, and
in turn, the ram up and down.
Properties of some hammers used offshore are shown in Table 1. A selection of large offshore pile driving hammers
driving on heavy piles is also shown in Table 2.
4.3.4 Selection of hammer size
Selection of hammer size is based on:
 experience of similar situations (see Quality Control: Section 4.6),
 numerical modelling of driving for each particular site (see Pile Driving: Section 2.4)
Typical values of pile sizes, wall thicknesses, and hammer energies for steam hammers are shown in Table 3.
4.4 Installation
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4.4.1 Pile handling and positioning
Figure 4 shows the different ways of providing lifting points for positioning pile sections. Padeyes are generally
used (welded in the fabrication yard; their design should take into account the changes in load direction during
lifting). Padeyes are then carefully cut before lowering the next pile section.

Sketch E shows the different steps for the positioning of pile sections:
 pile or add-on lifted from the barge deck.
 rotation of the crane to position add-on.
 installing and lowering of the pile add-on.
4.4.2 Pile connections
Different solutions for connecting pile segments back-to-back are used:
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 either by welding, Shielded Metal Arc Welding (SMAW) or flux-cored, segments held temporarily by internal or
external stabbing guides as shown in Figure 4. Welding time depends upon:
- pile wall thickness: 3 hours for 1in. thick (25,4mm); 16 hours for 3in. thick, (76,2mm) (typical).
- number and qualification of the welders.
- environmental conditions.
 or by mechanical connectors (as shown in Figure 4):
- breech block (twisting method).
- lug type (hydraulic method).
4.4.3 Hammer placement
Figure 5 shows the different steps of this routine operation:

 lifting from the barge deck.
 positioning over pile by booming out or in (the bell of the hammer acts as a stabling guide... very helpful in
rough weather).
 alignment of the pile cap.
 lowering leads after hammer positionment.
Each add-on should be designed to prevent bending or buckling failure during installation and in-place conditions.
4.4.4 Driving
Some penetration under the self weight of the pile is normal. For soft soil conditions, particular measures are taken
to avoid an uncontrolled run.
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Piles are then driven or drilled until pile refusal.
Pile refusal is defined as the minimum rate of penetration beyond which further advancement of the pile is no
longer achievable because of the time required and the possible damage to the pile or to the hammer. A widely
accepted rate for defining refusal is 300 blows/foot (980 blows/metre).
4.5 Pile-to-Jacket Connections
4.5.1 Welded shims
The shims are inserted at the top of the pile within the annulus between the pile and jacket leg (see Figure 6) and
welded afterwards.

4.5.2 Mechanical locking system
This metal-to-metal connection is achieved by a hydraulic swaging tool lowered inside the pile and expanding it
into machined grooves provided in the sleeves at two or three elevations as shown on Figure 7.
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This type of connection is most popular for subsea templates. It offers immediate strength and the possibility to
re-enter the connection should swaging prove incomplete.
4.5.3 Grouting
This hybrid connection is the most commonly used for connecting piles to the main structure (in the mudline area).
Forces are transmitted by shear through the grout.
Figure 8 shows the two types of packers commonly used. The expansive, non-shrinking grout must fill completely
the annulus between the pile and leg (or sleeve).
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Bonding should be excellent; it is improved by shear connectors (shear keys, strips or weld beads disposed on the
surface of the sleeve and pile in contact with the grout).
The width of the annulus between pile and sleeve should be maintained constant by use of centralizers and be
limited to:
 1,5in. minimum, (38,1mm)
 about 4in. (101,6mm) maximum (to avoid destruction of the tensile strength of the grout by internal
microcracking).
Packers are used to confine the grout and prevent it from escaping at the base of the sleeve. Packers are often
damaged during piling and are therefore:
 installed in a double set.
 attached to the base of the sleeve to protect them during pile entry and driving.
Thorough filling should be checked by suitable devices, e.g. electrical resistance gauges, radioactive tracers, well-
logging devices or overflow pipes checked by divers.
4.6 Quality Control
Quality control shall:
 confirm the adequacy of the foundation with respect to the design.
 provide a record of pile installation for reference to subsequent driving of nearby piles and future
modifications to the platform.
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The installation report shall mention:
 pile identification (diameter and thickness).
 measured lengths of add-ons and cut-offs.
 self penetration of pile (under its own weight and under static weight of the hammer).
 blowcount throughout driving with identification of hammer used and energy, as shown in Figure 9.
 record of incidents and abnormalities:
- unexpected behaviour of the pile and/or hammer.
- interruptions of driving (with set-up time and blowcount subsequently required to break the pile loose).
- pile damage if any.
 elevations of soil plug and internal water surface after driving.
 information about the pile/structure connection:
- equipment and procedure employed.
- overall volume of grout and quality.
- record of interruptions and delays.


4.7 Contingency Plan
Contingency documents should provide back-up solutions in case "unforeseen" events occur such as:
 impossibility to get the required pile penetration.
 mechanical breakdown of the hammer.
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 grout pipe blockage.
5. CONCLUDING SUMMARY
This lecture has described:
 the difficult aspects of foundations in a variety of soils.
 the multiplicity of solutions and the different kind of piles and hammers.
 the complexity of the process from design to installation.
6. REFERENCES
[1] API-RP2A, "Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms",
American Petroleum Institute, Washington, D.C., 18th ed., 1989.
7. ADDITIONAL READING
1. McClelland, B. and Reifel, M. D., Planning and design of fixed offshore platforms, Von Mostrand Reinhold
Company (1982).
2. Bowles, J. E., Foundation analysis and design, MacGraw Hill Book Company (4th edition 1988).
3. Bowles, J. E., Analytical and computer methods in Foundation Engineering, MacGraw Hill Book Company
(1983).
4. Poulos, H. G. and Davis, E. H., Pile foundation analysis and design, John Wiley and Sons (1980).
5. Graff, W. J., Introduction to offshore structures, Gulf Publishing Company (1981).
6. Le Tirant, P., Reconnaissance des sols en mer pour l'implantation des ouvrages Pétroliens, Technip (1976)
7. Pieux dans les formatines carbonates - Technip ARGEMA (1988).
8. Capacité patante des pieux - Technip ARGEMA (1988).
9. Dawson, T. H., Offshore Structural Engineering, Prentice Hall Inc (1983).
10. Gerwick, Ben C., Construction of Offshore Structures, John Wiley and Sons (1986).
A. Air/Steam Hammers
Make Model Rated
Energy
(ft-lbs)
Ram
Weight
(kips)
Max.
Stroke
(m)
Std. Pilecap
Weight
(kips)
Typical
Hammer
Weight
(w/leads)
(kips)
Rated Operating
Pressure
(psi)
Steam
Consumption
(lbs ht)
Air
Consumption
(lbs ht)
Hose
ST/F
.....
Rated
BPM
Conmaco 6850
5650
5300
300
200
510.000
325.000
150.000
90.000
60.000
85
65
30
30
20
72
60
60
36
36
57,5
59,0
12,7
12,7
12,7
312
262
92
86
74
180
160
160
150
120
31.500

8.064
6.944
5.563
7.500

1.711
1.471
1.195
2 @ 4
3 @ 4
4
3
3
40
45
46
54
59
Menck
(MRBS)
12500
8800
8000
7000
5000
4600
3000
1800
850
1.582.220
954.750
867.960
632.885
542.470
499.070
325.480
189.850
93.340
275,58
194,01
176,37
154
110,23
101,41
66,14
38,58
18,96
69
59
59
49
59
59
59
59
50
154,32
103,62
85,98
92,4
66,14
52,91
33,07
22,05
11,5
853
600
564
583
335
313
205
125
64
171
150
142
156
150
142
142
142
142
53.910
32.400
30.860
30.800
20.940
19.840
12.130
7.060
3.530
26.500
16.700
15.900
14.830
10.400
9.900
6.000
3.700
1.950
2 @ 6
8
8
4 @ 4
6
6
5
4
3
36
36
38
35
40
42
42
44
45
MKT OS-60
OS-40
18.000
120.000
60
40
36
36










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TABLE 1 Properties of some hammers used offshore
OS-20 60.000 20 36 38,65 150 3 60
C. Hydraulic Hammers
Make Model Rated Energy

(ft-lb)
Ram
Weight

(kips)
Standard
Pilecap
Weight
(kips)
Hammer Weight

(kips)
Typical Operating
Pressure
(psi)
Rated
Oil Flow
(gal. min)
Rated
BPM
HMB 4000
3000A
3000
1500
900
500
1.200.000
800.000
725.000
290.000
170.000
72.000
205
152
139
55
30,8
9,5


33
17,6

1,1
490
414

172
88
27,5

40-70
Menck MRBU
MHU 1700
MHU 900
MH 195
MH 165
MH 145
MH 120
MH 96
MH 80
760.000
1.230.000
650.000
141.000
119.000
105.000
87.000
69.000
58.000
132
207
110
22,0
19,0
16,5
13,9
11,0
9,3
84
77

6,0
6,0
6,0
6,0
1,9
1,9
415
617
386
59
51
46
40
27
24
3400
3400
3100
3550
3190
2755
2320
2830
2465
845
845
580
98
103
102
103
75
75
50-80
32-65
48-65
38
42
42
44
48
48
Hammer
Type

Blows per
Minute

Weight including
Offshore Cage, if
any (metric tons)
Rated Striking Energy Expected Net Energy
(ft-lb x 1000)
(ft-lb x
1000)
KNm On Anvil On Pile
Vulcan 3250 Single-acting steam 60 300 750 1040 673 600
HBM 3000 Hydraulic underwater 50-60 175 1034 1430 542 542
HBM 3000 A Hydraulic underwater 40-70 190 1100 1520 796 796
HBM 3000 P Slender hydraulic underwater 40-70 170 1120 1550 800 800
Menck MHU 900 Slender hydraulic underwater 48-65 135 - - 651 618
Menck MRBS 8000 Single-acting steam 38 280 868 1200 715 629
Vulcan 4250 Single-acting steam 53 337 1000 1380 901 800
HBM 4000 Hydraulic underwater 40-70 222 1700 2350 1157 1157
Vulcan 6300 Single-acting steam 37 380 1800 2490 1697 1440
Menck MRBS 12500 Single-acting steam 38 385 1582 2190 1384 1147
Menck MHU 1700 Slender hydraulic underwater 32-65 235 - - 1230 1169
IHC S-300 Slender hydraulic underwater 40 30 220 300 - -
IHC S-800 Slender hydraulic underwater 40 80 580 800 - -
IHC S-1600 Slender hydraulic underwater 30 160 1160 1600 - -
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TABLE 2 Large pile driving hammers
Note 1: With the heavier hammers in the range given, the wall thicknesses must be near the upper range of those
listed in order to prevent overstress (yielding) in the pile under hard driving.
Note 2: With diesel hammers, the effective hammer energy is from one-half to two-thirds the values generally
listed by the manufacturers and the above table must be adjusted accordingly. Diesel hammers would normally
only be used on 36-in. or less diameter piles.
Note 3: Hydraulic hammers have a more sustained blow, and hence the above table can be modified to fit the
stress wave pattern.
TABLE 3 Typical values of pile sizes, wall thickness and hammer energies
IHC S-2000 Slender hydraulic underwater - 260 1449 2000 - -
IHC S-2300 Slender hydraulic underwater - - 1566 2300 - -
Pile Outer
Diameter
Wall Thickness Hammer Energy
(in.) (mm) (in.) (mm) (ft-lb) (kN-m)
24
30
36
42
48
60
72
84
96
108
120
600
750
900
1.050
1.200
1.500
1.800
2.100
2.400
2.700
3.000
5/8 - 7/8
¾
7/8 - 1
1 - 1¼
17- 1¾
17 - 1¾
1¼ - 2
1¼ - 2
1¼ - 2
1½ - 2½
1½ - 2½
15-21
19
21-25
25-32
28-44
28-44
32-50
32-50
32-50
37-62
37-62
50.000 - 120.000
50.000 - 120.000
50.000 - 180.000
60.000 - 300.000
90.000 - 500.000
90.000 - 500.000
120.000 - 700.000
180.000 - 1.000.000
180.000 - 1.000.000
300.000 - 1.000.000
300.000 - 1.000.000
70 - 168
70 - 168
70 - 252
84 - 120
126 - 700
126 - 700
168 - 980
252 - 1.400
252 - 1.400
420 - 1.400
420 - 1.400
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Lecture 15A.7: Tubular Joints in Offshore
Structures
OBJECTIVE/SCOPE
To present methods for the design of large tubular joints typically found on
offshore structures.
PREREQUISITES
Lecture 15A.1: Offshore Structures: General Introduction
RELATED LECTURES
Lecture 15A.8 : Fabrication
Lecture 15A.12: Connections in Offshore Deck Structures
SUMMARY
The lecture defines the principle terms and ratios used in tubular joint design. It
presents the classifications for T, Y, X, N, K and KT joints and discusses the
significance of gaps, overlaps, multiplanar joints and the details of joint
arrangements. It describes design methods for static and fatigue strength,
presenting some detailed information on stress concentration factors.
1. INTRODUCTION
The main structure of a topside consists of either an integrated deck or a module
support frame and modules. Commonly tubular lattice frames are present,
however a significant amount of rolled and built up sections are also used.
This lecture refers to the design of tubular joints. These are used extensively
offshore, particularly for jacket structures. Connection of I-shape sections or
boxed beams whether rolled or built up, are basically similar to those used for
onshore structures. Refer to the corresponding lectures for appropriate design
guidance.
Two main calculations need to be performed in order to adequately design a
tubular joint. These are:
1. Static strength considerations
2. Fatigue behaviour considerations
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The question of fatigue behaviour always has to be addressed, even where simple
assessment of fatigue behaviour shows this will not be a problem. The joint
designer must therefore always be "fatigue minded".
2. DEFINITIONS
The following definitions are universally acknowledged [1]: (refer to Figure 1 for
clarification):
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The CHORD is the main member, receiving the other components. It is necessarily
a through member. The other tubulars are welded to it, without piercing through
the chord at the intersection.
Other tubulars belonging to the joint assembly may be as large as the chord, but
they can never be larger.
The CAN is the section of the chord reinforced with an increased wall thickness, or
stiffeners.
The BRACES are the structural members which are welded to the chord. They
physically terminate on the chord skin.
The STUB is the extremity of the brace, locally reinforced with an increased wall
thickness.
Different positions have to be identified along the brace - chord intersection line:
 CROWN position is located where the brace to chord intersection crosses the
plane containing the brace and chord.
 SADDLE position is located where the brace to chord intersection crosses the
plane perpendicular to the plane containing the brace and chord, which also
contains the brace axis.
2.1 Geometrical definitions
Refer to Figure 1
L is the length of the chord can
D is the chord outside diameter
T is the chord wall thickness
d is the brace outside diameter
t is the brace wall thickness (where there are several braces, a subscript identifies
the brace)
g is the theoretical gap between weld toes
e is the eccentricity · Positive when opposite to the brace side, Negative when on
the brace side
u is the angle between brace and chord axis.
2.2 Geometrical ratios
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o = Can slenderness ratio
| = Brace to chord diameter ratio (always s 1)
¸ = Chord slenderness ratio
t = Brace to chord thickness ratio
, = Relative gap
These are non-dimensional variables for use in parametrical equations.
3. CLASSIFICATION
Load paths within a joint are very different, according to the joint geometry. The
following classification is used, see Figure 2.
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3.1 T and Y Joints
These are joints made up of a single brace, perpendicular to the chord (T joint) or
inclined to it (Y joints).
In a T joint, the axial force acting in the brace is reacted by bending in the chord.
In a Y joint, the axial force is reacted by bending and axial force in the chord.
3.2 X Joints
X joints include two coaxial braces on either side of the chord.
Axial forces are balanced in the braces, which in an ideal X joint have the same
diameter and thickness. In fact, other considerations such as brace length, which
can be very different on each side of the chord, may lead to two slightly different
braces. Angles may be slightly different as well.
The important point to note is the balance of forces in the braces. If the axial force
in one brace is far higher than the one in the other brace, the joint may be
classified as a Y (or a T) joint rather than an X joint.
3.3 N and K Joints
These joints include two braces. One of them may be perpendicular to the chord
(N joint) or both inclined (K joint).
The ideal load pattern of these joints is reached when axial forces are balanced in
the braces, i.e. net force into chord member is low.
3.4 KT Joints
These joints include three braces.
The load pattern for these joints is more complex. Ideally axial forces should be
balanced within the braces, i.e. net force into chord member is low.
3.5 Limitations
For a joint to be able to be fabricated and to be effective, the geometrical ratios
given in Section 2.2 have limitations. Table 3.1 shows these limits and their typical
ranges.
Parameter Typical
range
Limitations
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(1) Physical limitation
(2) Brace shall be less or equal to chord thickness (see punching shear)
(3) Angle limitation to get a correct workmanship of welds.
Table 3.1 Geometrical Limits and Typical Ranges
3.6 How to classify a joint
This classification deals only with braces located in one plane.
It must always be remembered that this classification is based on load pattern as
well as the geometry. Engineering judgement must therefore be used to classify a
joint. For example a geometrical K joint may be classified as.
 a K joint when forces are balanced within braces.
 a Y joint when the force in one brace is reacted predominantly by the chord,
rather than by the second brace.
4. GAP AND OVERLAP
4.1 Definitions
The GAP is the distance along the chord between the weld toes of the braces
(Figure 3).
min max

0,4 - 0,8 0,2 1

12 - 20 10 30

0,3 - 0,7 0,2
1
(2)

u 40° - 90°
30°
(3)

90°
(1)

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The theoretical gap is the shortest distance between the outer surfaces of two
braces, measured on the line where they cross the chord outer surface. The real
gap is the one measured at the corresponding location, between actual weld toes.
A brace OVERLAPS another brace when one brace is welded to the other brace.
The overlapping brace is always the thinner brace.
The overlapped brace is always completely welded to the chord.
4.2 Limitations
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The minimum gap allowed is 50mm. This limitation is set to avoid two welds
clashing. This is important because the gap is a highly stressed zone.
4.3 Multiplanar Joints
The same definitions and limitations apply to multiplanar joints.
5. JOINT ARRANGEMENT
As a rule, welds in a joint have to be kept away from zones of high stress
concentration.
The following practice, see Figure 4, should be followed:
1. The chord circumferential welds are to be located at either 300mm or a
quarter of the chord diameter, whichever is the greater, from the nearest
point of a brace-chord connection.
2. The brace circumferential welds are to be located at either 600mm or a brace
diameter, whichever is the greatest, from the nearest point of the brace-
chord connection.
3. The actual gap shall not be less than 50mm. To achieve this, most designers
use a 70 or 75mm theoretical gap.
4. Eccentricity and offset are to be kept within a quarter of the chord diameter.
When higher values can not be avoided, secondary moments have to be
introduced in the structural analysis by introducing extra nodes.
5. Thickness transitions are smoothed to a 1 in 4 slope, by tapering the thicker
wall.
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6. STATIC STRENGTH
6.1 Loads taken into account
The loads considered in a joint static strength design are the axial force, the in-
plane bending moment and the out-of-plane bending moment for each brace.
The other components (transverse shear and brace torsion moment) are usually
neglected since unlike the preceding loads, these loads do not induce bending in
the chord wall. Nevertheless, their presence must never be forgotten and in some
specific cases, their effects must be assessed. The axial load, in-plane and out-of-
plane bending moments are normally the dimensioning criterion for tubular joints.
6.2 Punching shear
6.2.1 Acting punching shear
The acting punching shear is the shear stress developed in the chord by the brace
load.
The acting punching stress v
p
is written as:

v
p
= t f sin u

where f is the nominal axial, in-plane bending or out-of-plane bending stress in the
brace (punching shear for each kept separate), see Figure 5.
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6.2.2 Allowable punching shear
Allowable punching shear values in the chord wall are determined from test results
carried out on full scale or on reduced scale models.
Tests are performed on experimental rigs such as the one shown in Figure 6. They
are performed for a single load-case (axial force, in-plane bending, or out-of-plane
bending).
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The ultimate static strength obtained through these tests can then be expressed in
terms of punching shear, as defined above.
Statistical treatment of results allow formulae to be defined for the allowable
punching shear stress.
6.2.3 The API method
Several offshore design regulations are based on the punching shear concept
[1,2]. The following method is presented in API RP2A [2]:
A. Principle
 This method applies to a single brace without overlap, for a non-stiffened
joint. When the joint includes several braces, each brace connection is
checked independently.
 Punching shear for each load component (axial force, in-plane bending, and
out of plane bending) is calculated and compared to the allowable punching
shear stress for the appropriate load and geometry.
 Interaction formulae are given for combined loading, combining the three
punching shear ratio calculated for each component.
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B. Allowable punching shear stress
The allowable punching shear stress for each load component is:
V
pa
= Q
q
Q
f


where: F
yc
is the yield strength of the chord member

Q
q
is to account for the effects of type of loading and geometry, see Table 6.1.

Q
f
is a factor to account for the nominal longitudinal stress in the chord

Q
f
= 1 - ì ¸

f
AX
, f
IPB
, f
OPB
are the nominal axial, in-plane bending and out of plane bending
stresses in the chord
Value for ì and Q
q
are given in Table 6.1

Load component Axial load In-plane bending Out of plane bending
Stress in brace
f
ax

f
by

f
bz

Acting punching shear
V
px
= t f
ax
sin u

V
p
= t f
by
sin u

V
p
= t f
bz
sin u

Q
q

K joints



T & Y Joints


w/o diaphragm
X


w diaphragm
Tension Compression




ì 0,030 0,045 0,021
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Table 6.1 Values of Q
q
for allowable punching shear stress from APIRP2A
Q
g
= 1,8 - 0,1 for ¸ s 20

Q
g
= 1,4 - 4 g/D for ¸ > 20

but Q
g
must be > 1,0

Q| = for | > 0,6
Q
B
= 1,0 for | s 0,6

C. Loading Combination
For combined loadings involving more than one load component, the following
equations shall be satisfied:

where: IPB refers to in-plane bending component
OPB refers to out-of-plane bending component
AX refers to axial force component
and
ax


where: arc sin term is in radians.
6.3 Overlapping joints
The parametric formulae discussed in Section 6.2 were specifically established for
non-overlapping joints with no internal reinforcement. These formulae cannot be
used for overlapping joints.
In an overlapping joint, part of the load is transferred directly from one brace to
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the other through the overlapping section, without that part of the load
transferring through the chord. The static strength of an overlapping joint is higher
than a similar joint without an overlap.
API RP2A, [2] allows the static shear strength of the overlapping weld section to
be added to the punching shear capacity of the brace-chord connection, see Figure
7.

The allowable axial load component perpendicular to the chord, P± (in Newtons)
should be taken to be:
P
±
= (v
pa
T l
1
) + (2v
wa
t
w
l
2
)

where:
v
pa
is the allowable punching shear stress (MPa) for axial stress.

l
1
is the circumference for that portion of the brace which contacts the chord
(mm), see Figure 7.
v
wa
is the allowable shear stress for weld between braces (MPa).

t
w
is the lesser of the weld throat thickness or the thickness t of the inner brace
(mm).
l
2
is the projected chord length (one side) of the overlapping weld, measured
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perpendicular to the chord (mm), see Figure 7.
6.4 Reinforced joints
6.4.1 Definition
Large chord wall thickness may be reduced by stiffening the chord. The most usual
reinforcement consists of ring stiffening inside the chord.
Some joints may require more complex stiffening. This is the case for large
diameter chords which would otherwise require an un-economic chord wall
thickness.
There are very many different stiffening solutions for a large diameter chord.
Therefore there are no parametric formulae available for these designs. Specific
analyses must therefore be carried out for an accurate solution. This may involve
finite element analysis.
6.4.2 Ring Stiffening
Ring stiffening consists of ring plates welded in the chord can prior to welding the
braces to it.
The punching shear capacity of the chord still may be taken into account when
calculating the forces acting on the stiffeners.
Ring stiffeners can be justified through parametric formulae available in various
publications, the best known being published by Roark [3].
7. STRESS CONCENTRATION
As in any mechanical body presenting discontinuities, stresses are not uniform
along the connecting surface of a brace and chord. Figure 8 shows an example of
the stress distribution in a joint with local discontinuities at and in the vicinity of
the brace chord intersection.
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7.1 Stress concentration factor
The stress concentration factor (SCF) is defined as the ratio of the highest stress
in the connection (or hot spot stress f
HS
) to the nominal brace stress f
NOM
:
SCF = f
HS
/f
NOM

7.2 Kellog equation
This approximate formula can be used for rapidly assessing SCF, for preliminary
analyses.
f
HS
/v
p =
1,8 √¸

v
p
being the punching shear.

7.3 Parametric formulae
SCF parametric formulae have been determined based on a large number of finite
element analyses and cross-checked with either full scale or model tests. They are
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based on many man years of work by numerous research teams.
A large number of parametric formulae have been published [4]. Sections 7.3.1 to
7.3.3 give, as an example, the most commonly used and acknowledged formulae.
In using any set of formulae, care should be taken in classifying the situation and
ascertaining any limitations that apply.
The only alternatives to these formulae are to perform model tests (full size or at
reduced scale) or finite element analyses.
No parametric formulae are presently available for stiffened joints. The only ones
published to date concern non-stiffened, non overlapping joints.
7.3.1 Kuang equations for T/Y joints [4]
Axial load
SCF
CHORD
= 1,981 ¸
0,808
t
1,333

exp
(-1,2|
3
o
0,057
sin
1,694
u

SCF
BRACE
= 3,751 ¸
0,55
t
exp
(-1,35|
3
) o
0,12
sin
1,94
u

Out-of-plane bending
SCF
CHORD
= 1,024 ¸
1,014
t
0,889
|
0,787
sin
1,557
u 0,3 s | s 0,55

SCF
CHORD
= 0,462 ¸
1,014
t
0,889
|
(-0,619)
sin
1,557
u 0,55 s | s 0,75

SCF
BRACE
= 1,522 ¸
0,852
t
0,543
|
0,801
sin
2,033
u 0,3 s | s 0,55

SCF
BRACE
= 0,796 ¸
0,852
t
0,543
|
(-0,281)
sin
2,033
u 0,55 s | s 0,75

In-plane bending
SCF
CHORD
= 0,702 ¸
0,60
t
0,86
|(
-0,04)
sin
0,57
u

SCF
BRACE
= 1,301 ¸
0,23
t
0,38
|
(-0,38)
sin
0,21
u

7.3.2 Kuang equations for K joints [4]
Balanced axial load
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SCF
CHORD
= 1,506 ¸
0,666
t
1,104
|
(-0,059)
(g/D)
0,067
sin
1,521
u

SCF
BRACE
= 0,92 ¸
0,157
t
0,56
|(
-0,441)
(g/D)
0,058
Exp(1,448 sin u )

In-plane bending (bending moment applied to one brace only)
SCF
CHORD
= 1,822 ¸
0,38
t
0,94
|
0,06
sin
0,9
u

SCF
BRACE
= 2,827 t
0,35
|
-0,35
sin
0,5
u

7.3.3 Kuang equations for KT joints [4]
Balanced axial load Outer braces only loaded
SCF
CHORD
= 1,83 ¸
0,54
t
1,068
|
0,12
sin u 0° < u s 90°

SCF
BRACE
= 6,06 ¸
0,1
t
0,68
|
-0,36
{(g1+g2)/D}
0,126
sin
0,5
u 0° < u > 45°

SCF
BRACE
= 13,8 ¸
0,1
t
0,68
|
-0,36
{(g1+g2)/D}
0,126
sin
2,88
u 45° s u > 90°

SCF
BRACE
= 4,89 ¸
0,123
t
0,672
|
-0,396
{(g1+g2)/D}
0,159
sin
2,267
u

In-plane bending - as for K joint
Validity range
The above equation for T/Y, K and KT joints are generally valid for joint
parameters within the following limits:
8,333 s ¸ s 33,3
0,20 s t s 0,8
0,3 s | s 0,8 unless stated otherwise
6,667 s o s 40 unless stated otherwise
0° s o s 90° unless stated otherwise.
8. FATIGUE ANALYSIS
A fatigue analysis of a joint consists of the following steps:
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1. Calculation of nominal stress ranges in the brace and the chords
2. Calculation of hot-spot stress range
3. Calculation of joint fatigue lives using S-N curves for tubular members at
joints.
8.1 Nominal stress range
Nominal stress ranges in braces and chords are calculated by a global stress
analyses.
8.1.1 Wave histogram
A wave histogram has to be obtained for each direction around the platform. A
simple form of a wave histogram is as follows:
8.2.2 Nominal stress ranges
Nominal stress ranges can be calculated by following the steps below:
1. Wave heights are grouped in "blocks", for which just one stress range will be
calculated. Different wave directions need to be considered with a minimum
of three "blocks" per wave direction.
2. For each block one representative wave is chosen, whose action is supposed
to represent the action of the whole block. The highest wave of the block is
normally chosen.
3. Nominal stresses for each joint component are then calculated for different
phase angles of the chosen wave, for one complete cycle (360°). The nominal
stress range for the joint component is defined as the difference between the
highest and the lowest stress obtained for a full wave cycle. Four to twelve
phase angles per wave are usually considered.
Wave height
(metres)
Average
number per
year
0-1,5
1,5-3
3-4,5
4,5-6
6-8
8-10
3 100 000
410 000
730 000
5 000
800
20
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8.2 Hot spot stress ranges
Hot spot stress ranges are then evaluated for each chosen joint location by
applying parametric formulae [4] (or by applying the SCF calculated from a
detailed analysis).
When using parametric formulae, stress components (axial, in plane bending and
out of plane bending) have to be distinct throughout the calculations, as the SCF
formulae apply individually for each load component.
Where a chord and brace intersect, four to eight locations are usually chosen
around the intersection line. For each of these locations the stress response for
each sea state should be computed, giving adequate consideration to both global
and local stress effects.
8.3 S-N Curves
S-N curves to be used for offshore structures are given by statutory regulations
[1,2]. APIRP2A uses the curves shown in Figure 9.
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The X and X
1
curves should be used with hot spot stress ranges based on suitable
stress concentration factors. The permissible number of cycles is obtained from
the S-N curve by taking the hot spot stress range, and entering the graph.
It should be noted that Curve X presumes welds which merge smoothly with the
adjoining base metal. For weld without such profile control, the X' curve is
applicable.
8.4 Cumulative Fatigue Damage Ratio
The stress responses should be combined into the long term stress distribution,
which should then be used to calculate the cumulative fatigue damage ratio, D,
given by:
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D =
where
n is the number of cycles applied at a given stress range
N is the number of cycles to cause failure for the given stress range (obtained
from appropriate S-N curve).
In general the design fatigue life of each joint and member should be at least
twice the intended service life of the structure, i.e. a safety factor of 2,0.
For critical elements whose sole failure would be catastrophic, use of a larger
safety factor should be considered.
9. CONCLUDING SUMMARY
 Terminology, geometric ratios and joint classifications are now standardised
for tubular joints.
 The presence of gaps and overlaps significantly influence joint behaviour.
 Determination of static strength is generally based on the concept of
punching shear, with the allowance of overlapping joints.
 Special analysis are required for reinforced joints.
 Stress concentration factors (SCF) are defined for most commonly occurring
joints.
 Determination of fatigue strength is based on nominal stress range multiplied
by appropriate SCF.
10. REFERENCES
[1] Offshore Installations: Guidance on Design, Construction and Certification.
Fourth Edition, HMSO, 1990.
[2] Recommended Practice for Planning, Designing and Constructing Fixed
Offshore Platforms, API RP2A Nineteenth Edition.
[3] Young, Warren C, Roark's Formulae for Stress and Strain. Sixth Edition,
McGraw-Hill.
[4] Stress Concentration Factors for Simple Tubular Joints, 1989, Volumes 1 to 5,
Lloyds Register of Shipping-Offshore Division.
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STRUCTURAL SYSTEMS: OFFSHORE
Lecture 15A.8: Fabrication
OBJECTIVE
To describe the general methods of jacket fabrication. To discuss the various stages of operation
from material selection, through erection, including construction practices and equipment. To
indicate the calculations normally involved.
PREREQUISITES
Lecture 15A.1: Offshore Structures: General Introduction
Lectures 3.1: General Fabrication of Steel Structures
Lectures 3.2: Erection
Lectures 3.3: Principles of Welding
Lectures 3.4: Welding Processes
RELATED LECTURES
Lectures 15: Structural Systems: Offshore
SUMMARY
The construction philosophy and definition of the construction phases of the fabrication of offshore
structures are described. The overall execution plan and the contractor's organisation for its
implementation are introduced and constructability i.e. the more general aspects of design - the size
and transportability of components, welding access considerations, construction tolerance, is
discussed.
The fabrication of nodes and reinforced tubulars, including the fabrication procedure for a typical
node is described together with jacket assembly and erection and the procedures for "big lift".
1. INTRODUCTION
1.1 Construction Phases
Jacket construction involves the following work phases:
Procurement
The technical and commercial activities required to supply material and specialised products to
enable the execution of construction activities.
Fabrication
The processes normally carried out in a fabrication shop to produce relatively small units. Thus
fabrication includes processes such as cutting, rolling, pressing, fitting, welding, stress relieving on
such items as welded tubulars, beams, nodes, girders, cones, supports, clamps, etc.
Assembly
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The processes normally performed outside the fabrication shop but at ground level in order to
assemble groups of shop fabricated items into an (assembled) unit for subsequent erection in
accordance with a construction sequence.
Erection
The processes required to install assembled and shop fabricated items together in their final
configuration. These processes include fitting and welding. However the emphasis is on the
transportation and lifting of heavy assemblies.
1.2 Construction Philosophy
The design of a jacket, i.e. a lifted, launched or self-floating jacket, is determined primarily by the
offshore installation equipment available and the intended water depth. In general the preference is
to lift the jacket in place. The size of such jackets has being increasing as offshore lifting capacity
has grown. With modern lifting capacity now up to 14,000 tonnes, jackets approaching this order of
magnitude are now candidates for lifting into position.
For jackets destined for shallow water, where the height is of the same order as the plan
dimensions, erection is usually carried out vertically, i.e. in the same attitude as the final installation.
Such jackets may be lifted or skidded onto the barge.
Jackets destined for deeper water are usually erected on their side. Such jackets are loaded by
skidding out onto a barge. Historically most large jackets have been barge launched. This method of
construction usually involves additional flotation tanks and extensive pipework and valving to enable
the legs to be flooded for ballasting the jacket into the vertical position on site. This method of
construction is currently applicable for jackets up to 25,000 tonnes. Very large jackets, in excess of
this, have been constructed as self-floaters in a graving dock and towed offshore subsequent to
flooding the dock.
In considering the construction philosophy and contract strategy, the objectives of achieving quality
requirements and efficiency are of fundamental importance. An offshore jacket goes through a series
of very distinct stages as it moves from fabrication to load-out. These stages range from operations
which are almost totally automatic under very controlled conditions, e.g. steel production, automatic
welding, to operations which are almost totally manual in very variable conditions, e.g. yard
erection, offshore activities. Thus decreasing efficiency occurs as progress through these operations
advances. In addition, the stable conditions in repetitive processes of the early operations are more
conducive to the maintenance of high quality. A third basic consideration is that risk increases with
each progressive stage. These general trends during construction are shown in Table 1.
It is clear therefore that, as a general principle, as much work as possible should be undertaken in
the earlier more productive, higher quality, less risky phases of the project.
Some of the principles which reduce the time and cost of construction are:
 Subdivision into as large components and modules as it is possible to fabricate and assemble.
 Concurrent fabrication of major components in the most favourable location and under the most
favourable conditions applicable to each component.
 Planning the flow of components to their assembly site. Providing adequate facilities and
equipment for assembly, including such items as synchrolifts, and heavy-lift cranes.
 Simplification of configurations and standardisation of details, grades and sizes. Avoidance of
excessively tight tolerances.
 Selection of structural systems that utilise skills and trades on a relatively continuous and
uniform basis. Avoidance of procedures that are overly sensitive to weather conditions;
ensuring that processes which are weather sensitive are completed during shop fabrication, e.g.
protective coating.
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Quality management is a vital and integral component of all aspects of offshore fabrication.
Essentially it involves ensuring that what is produced is what is needed. The requirements for
documentation, hold point, audits, reviews and corrective actions are part of the quality assurance
process. They are crucial tools for controlling the project execution and providing verifiable evidence
of the fabricator's competence.
Quality control, inspection and testing should be performed during all phases of construction to
ensure that specified requirements are being met. The most effective quality scheme is one which
prevents the introduction of defective materials and workmanship into a structure, rather than
finding problems after they occur.
A general note on Quality Assurance for Offshore Construction is included in Appendix 1. It is
applicable to this lecture and also to Lecture 15A.9: Installation.
2. ENGINEERING OF EXECUTION
Engineering of execution, 'construction engineering', entails the work required during each phase of
execution to ensure that the design requirements are fulfilled. A general method of execution is
envisaged at the jacket design stage. Since the shape of the jacket, its form and properties require
quite specific methods of load-out, offshore transportation and installation (which are construction
activities executed under contractor responsibility), there is considerable interfacing of engineering
requirements in these phases. In the earlier phases, i.e. procurement through assembly and
erection, the contractor, while being limited by design specification requirements, has freedom of
choice with regard to the exact method of execution adopted. However, in all phases the contractor
is required to demonstrate that the methods which he adopts are compatible with the specification
requirements and do not affect the integrity of the structure.
Each phase of execution has its own specific engineering requirements which are determined by the
processes executed during that phase. These processes range from those which are largely repetitive
early in execution to one-off activities in the latter phases. Accordingly the engineering which
supports procurement and shop fabrication is voluminous but repetitive, e.g. material take-offs, shop
drawings, cutting plans, etc. The assembly and erection phases are supported by a mix of repetitive
engineering, e.g. scaffolding, and specific studies for limited series of activities.
The volume of contractor construction engineering on a large jacket is typically 130,000/150,000
hours. The typical organisation of a contractor's technical documents is shown in Table 2.
When designing larger components consideration must be given to their subdivision into elements
which will not distort when fabricated and which can be relatively easily assembled without
welding/dimensional problems. For instance, nodes are categorised as either complex or simple from
the execution viewpoint based on the number of separate fitting-welding-NDT (non-destructive
testing) cycles required during fabrication and the possibility of automatic welding between the node
can and the tubular during sub-assembly. The number of fitting-welding-NDT cycles depends on the
existence of ring stiffeners and the number and disposition of stubs. For reasons relating to weld
distortion and to allow automatic welding, it is almost essential that ring stiffeners be installed prior
to fitting/welding of stubs. This adds an extra cycle to the fabrication of the node. Thus ring
stiffeners are best avoided. Where this is not possible, care should be taken to define them at an
early stage on critical nodes.
Node stubs can be classified as simple or overlapping. Overlapping stubs add at least one complete
cycle to node fabrication and should therefore be avoided where possible. The minimum separation
between the weld toes of adjacent simple stubs is typically specified as 50mm, see API RP2A, Fig.
4.3.1-2 [1]. However this distance is too small to allow simultaneous welding of adjacent stubs -
150mm is a more practical distance.
3. FABRICATION
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3.1 Fabrication Processes
The specifications for fabrication of offshore jackets are determined by the designer. They are
usually based on one or more of the well known codes, with additional requirements dictated by the
specific design, client standards, statutory rules, etc. Two recognised codes which are used
extensively for establishing general requirements are the API RP2A Recommended Practice for
Planning, Designing and Constructing Fixed Offshore Platforms, [1] and AISC Specification for the
Design, Fabrication and Erection of Structural Steel for Buildings [2].
For larger jackets, the nodes tend to be fabricated separately under highly controlled shop
conditions. Alternatively cast steel nodes may be used in order to eliminate critical welding details.
Recent experience, both in the laboratory and as a result of in-service inspection, has prompted
increasingly greater attention to the welding aspects of fabrication. In particular greater attention
has been focused on the importance of complete joint penetration groove welds, elimination of
"notch effects" at the root and especially the cap of node welds, and achieving the required weld
profile. Welds which are critical for fatigue endurance may be required to be ground to a smooth
curve. This process reduces the probability of brittle failure. However it also implies increasingly
sophisticated and stringent fabrication and quality assurance/quality control (QA/QC) requirements.
Typical welding details from API RP2A [1], showing tubular members framing into or overlapping
another member with access from one side only, are shown in Figure 1. However, a lot of emphasis
is placed on designing stubs which can be welded from both sides. For instance, in the weld details
for the Bouri jacket, Figure 2, most stubs are accessible from both sides.
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Welding procedures are required, detailing steel grades, joint design, welding consumables, etc.
Welds are typically subject to 100% visual, magnetic particle inspection (MPI) and ultrasonic test
(UT) inspection. The weld acceptance criteria, e.g. maximum weld undercut length (t/2 or 10mm),
and maximum depth (t/20 or 0,25mm), imply an exceptionally high quality of welding. In addition all
welders should be qualified for the type of work assigned to them and certified accordingly.
The location and orientation of circumferential and longitudinal welds during construction is based on
minimising interferences and ensuring the minimum distance between circumferential welds. Special
attention is required on items such as pile sleeve shear plates, launch runners, mudmats, etc. where
planned avoidance of weld interference is critical.
All temporary plates and fittings should be subjected to the same requirements for weld testing as
the member to which they are being affixed. There is also an overriding necessity to ensure that
such attachments are located at a safe distance from main structural welds in order to minimise the
risk of defect propagation. This is not unduly conservative - the "Alexander Kielland" capsized due to
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a fatigue crack initiated at the attachment of a sonar device to a principal structural member.
Temporary cut-outs should be of sufficient size to allow sound replacement. Corners should be
rounded to minimise stress concentrations.
Where welds are found to be defective, they should be rectified by grinding, machining or welding as
required. Welds of insufficient strength, ductility or notch toughness should be completely removed
prior to repair.
In general, sub-assemblies are executed so that at least one of the two edges which will mate during
subsequent assembly/erection has a cut-off allowance. This procedure provides flexibility in that the
sub-assemblies can be sent to the field with the cut-off allowance in place and cut to fit on location.
Alternatively they can be cut to exact dimensions during sub-assembly where the as-built dimension
has already been determined.
3.2 Node Fabrication
The primary structure nodes are frequently geometrically complex. Accordingly their fabrication
presents particular problems, especially from the points of view of welding and dimensional control.
On a complex jacket the designer may specify the node cans, or the whole node including stubs and
ring stiffeners, in material with specified through-thickness properties. This requirement is
introduced because of tearing or punching effects likely to be sustained by these elements during
their design life and indeed during fabrication. The designer may also "thicken" or reinforce the cans
to withstand local stresses. Finally, in an effort to ensure that node welds contain minimal levels of
residual stress due to fabrication, thermal stress relieving or post-weld heat treatment (PWHT) of the
heavier more restrained welds may be prescribed. This is frequently a requirement for thicker walled
North Sea jackets.
API RP2A [1] provides specific tolerances for final fabrication. The contractor must work within
tolerances which preserve dimensional compatibility and observe weight control requirements at
each phase of construction. Bearing these requirements in mind, node fabrication tolerances are
tight, e.g. typical working points within 6mm of theoretical, stub angle within 1 minute, all braces
within 12mm of the design dimension.
The typical fabrication process for a conventional node, assuming that the can (with or without ring
stiffeners) has already been fabricated, commences with profiling of stubs and terminates with UT
inspection of the finished node after PWHT.
The intermediate stages can be performed in several different ways, some of which depend on the
specific geometry of the node and many of which depend on fabricator preference. Some fabricators
prefer to orient the can upright, maintaining that it enables more stubs to be fitted simultaneously.
However the majority of fabricators tend to fit the stubs to a can placed on horizontal rollers. The
sequential steps in the fabrication of a typical node are as follows:
 Trace generators, working points, etc. onto the can. Cut and profile the stubs. Touch up bevels
and trace generators onto the stubs. Trace node locations onto the can surface and grind or
blast areas. UT the cleaned areas to ensure that the steel is free from laminations. Particular
care is required where shrinkage strains in the through-thickness direction may lead to lamellar
tearing in highly restrained joints.
 Assemble one or two adjacent stubs in the same plane on the can. Tack-weld in position. Verify
dimensional control and weld preparations around stub.
 Weld according to predetermined sequence to limit deformation. The welding processes used
are usually shielded metal arc welding (SMAW) or flux cored arc welding (FCAW), see Lecture
3.4 Welding Processes. If the weld is double-sided, after 3 or 4 passes, back-grind and clean
weld roots from opposite side. Perform MPI test on ground roots. Complete weld body. Deposit
weld bead for cap profiling. Toe-grind profiles if required. Grind weld beads at base metal to
remove undercut. Allow welds to cool. Visually inspect finished welds. MPI and UT finished
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welds.
 Repeat previous steps for successive stubs.
 When all stubs have been fitted and welded out perform post weld heat treatment (PWHT) as
required, blast or grind welds and perform NDT re-test on all welds.
 Cut any required off-cuts on cans or stubs. Perform final dimensional control of node.
3.3 Jacket Sub-assemblies
Sub-assembly can be considered as an intermediate stage between standard shop fabrication, i.e.
nodes, tubulars, beams etc. and assembly or erection. The emphasis is on performing the maximum
number of welds in the shop. This ensures the highest weld quality since many node and tubular
welds can be double-sided and/or automatic when performed in the fabrication shop.
When defining sub-assemblies, the principal factors to be borne in mind are the following:
 Size/Weight/Dimensions: these are largely governed by considerations of transportability.
 Welding Sequence: sub-assemblies should not imply a difficult welding sequence causing
distortion or induced stresses during sub-assembly welding or the subsequent assembly or
erection.
 Constructability: certain sub-assemblies may have specific construction difficulties associated
with them, e.g. short, large diameter infillings are difficult to erect vertically and are best
included in sub-assemblies, if possible.
3.4 Dimensional Control
Of all the areas of quality control (QC) which require attention, that of dimensional control, as
emphasised in the code and specifications, tends to be exaggerated. However, it is clear that
attention must be paid to the dimensions which have structural significance, e.g. the straightness of
elements, ovality of tubulars, eccentricities at node joints, etc. It is also clear that on a jacket the
global alignment/verticality of items such as pile sleeves, conductor guides, launch runners, etc. are
also important. Finally dimensional control of items which are intended for "mating" or "removal"
offshore, for example piles/pile sleeves, jacket top/MSF base, buoyancy tank/supports, etc. is vital to
the efficiency of offshore installation. There are therefore, many aspects where the attention to
dimensional control is justified even if the overall design might occasionally benefit if the designer
did not always require that everything fitted so tightly.
The principal reason for requiring such accurate dimensional control of nodes and tubulars during
fabrication is not because of the structural consequences of out-of-tolerance but rather because the
parts may not fit together in the yard. It is one of the most vexing incongruities of the tubular steel
jacket concept that the theoretical tolerances on node stub eccentricity are generous from the
structural viewpoint while the actual tolerances are very tight because of considerations regarding
the fitting together of components during subsequent phases of construction.
The dimensional control of node fabrication, in particular, involves potentially intricate calculations in
the shop. However, the most successful systems simply involve the inclusion on the shop drawings
of several additional "checking" measurements and the correct marking of the node can and stub
generators and offsets.
4. JACKET ASSEMBLY AND ERECTION
4.1 Jacket Assembly
Shop fabricated sub-assemblies and loose items are incorporated into assemblies which constitute
the major lifts of the erection sequence. Thus for a large jacket, the assemblies are typically of four
types:
 Jacket levels incorporating conductor guide frames
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 Top frames
 Jacket rows i.e. bents or partial bents
 Pile sleeve clusters.
The assembly and erection phases are based on the following objectives:
 Maximise on-the-ground assembly (as opposed to erection) and maximise access around the
jacket during execution.
 Minimise erection joints in principal structural elements, such as jacket legs, launch runners,
rows, levels. Align critical areas such as conductor guides, pile sleeves, launch runners.
 Sub-assemble principal structural elements of jacket such as jacket legs, rows, levels. Sub-
assemble, and where possible pre-test, systems such as grouting, ballasting. Include maximum
quantity of secondary items such as anodes, risers, J-tubes, caissons. Coat or paint required
areas (top of jacket, risers) prior to erection.
 Minimise the use of temporary items which require subsequent removal, such as scaffolding,
walkways, lifting aids, etc. and pre-install such aids where they are necessary.
The assembly of a jacket frame, often having a spread at the base of 50m or more, places severe
demands on field layout and survey and on temporary support and adjustment bracing. Such large
dimensions mean that the thermal changes can be significant. Temperature differences may be as
great as 30C between dawn and afternoon and as much as 15C between various parts of the
structure, resulting in several centimetres distortion. However, the practice of 'using the sun' to fit
elements which are not dimensionally in-tolerance is common in the field. This procedure in itself
tends to induce residual stresses in the structure. Because of the difficulty associated with thermal
distortion, it is normal to "correct" all measurements to a standard temperature, e.g. 20 C.
Elastic deflections are also a source of difficulty in maintaining tolerances in the location of nodes.
Foundation displacements under the skid beams and temporary erection skids must be carefully
calculated and monitored.
The overall assembly sequence and programme requires that each assembly be completed prior to
lifting. It is normal to determine the exact location, orientation and attitude, i.e. face-up or face-
down, of each assembly in the field in anticipation of its lifting procedure.
Assembly layout drawings are usually prepared showing central co-ordinates for each assembly. The
central co-ordinates are then used as local bench marks with the object of defining the assembly,
the sub-assemblies, loose items, appurtenances and temporary attachments which comprise, field
welds, overall dimensions, weight, reference drawings, etc.
Dimensional control of the assembly both prior to and after welding, can be by means of a series of
self-checking measurements on the structure itself. Provided cross checks are adequate, the time
consuming exercise of referring measurements to an external bench mark can be avoided.
Normally the assembly is tacked in position to theoretical dimensions using allowable positive
tolerances to compensate for weld shrinkage. Perhaps the most fundamental rule in fitting is the
avoidance of "force-fitting" of members prior to welding or to force stresses into unwelded members
through the welding sequence since such conditions cannot have been foreseen by the designer.
An outline sequence of events which apply to all types of assembly is as follows:
 Preparation of assembly support and staging
 Rough setting of assembly main structure and position tacking. Dimensional control of assembly
main structure.
 Infilling of secondary structure and position tacking. Dimensional control of assembly and
secondary structure.
 Preweld inspection. Weldout of structure subject to continuous inspection and according to
approved sequence.
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 Installation of appurtenances (e.g. anodes, supports, walkways, risers, J-tubes, caissons,
grouting and ballasting) and scaffolding, lifting, aids, erection guides, temporary attachments.
 Test (e.g. hydrotest) if required. Overall NDT, dimensional control.
 Blasting and painting or touch up. Removal of temporary assembly supports and staging.
 Preparation for transport/lift/erection.
4.4 Jacket Erection
In this phase assembled, sub-assembled and fabricated structures, together with loose items, are
incorporated into the final structure according to the sequence outlined in Figure 3a - 3e.

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Jacket frames are typically laid out flat and then rolled using multiple crawler cranes. Co-ordinating
such a rigging and lifting operation requires thoroughly developed three-dimensional layouts, firm
and level foundations for the cranes and experienced, well rehearsed operators.
Twenty four cranes were involved in the two major side frame lifts during the erection of platform
Cerveza, which was 300m long.
For the Magnus platform and Bouri DP3, a different procedure known as "toast rack" was used. Here
the jacket horizontal levels were fabricated, erected in place and tied in to complete the jacket.
For the Bullwinkle jacket, one of the world's largest, sections of the jacket were fabricated in Japan,
transported by barge to Texas and assembled using jacking towers which rolled up the sections to
heights as great as 140m.
For jackets destined for shallow water erection is usually carried out vertically, i.e. in the same
attitude as the final installation. Such jackets may be lifted onto the barge or skidded out. In this
latter case, adequate temporary pads and braces must be provided under the columns to distribute
the loads for skidding.
The structural analysis associated with the erection procedure for a given assembly usually involves
a computer model with all relevant structural characteristics. The assembly is analysed for a number
of load cases which correspond (approximately) to the support conditions of the assembly at its
presumed critical attitudes, i.e. the locations of the cranes, bogies, saddles, etc. when the panel is
being transported and when it is in horizontal and vertical attitudes. The structural analysis for
lift/transport identifies the worst cases from the point of view of structural response. These cases are
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then analysed to determine the maximum stresses and displacements. The calculations should show
that global and local stresses are within allowable limits according to API/AISC codes.
Frequently, a structural analysis computer programme is used for this purpose. The analysis will
indicate where bending stresses are high and/or crane, bogie or support loads inadmissible. Thus
modifications can be made to redistribute structural stresses and loads at "supports" to optimise
both and ensure that neither the cranes nor the structure can be overloaded during erection.
An outline sequence for the erection of all major components would be:
 Technical appraisal of lift methods. Calculations for crane configuration, rigging accessories, etc.
 Preparation of cranes for lift. Preparation for rigging. Transport assembly to lift location. Roll-up
into position with scaffolding and staging in position, if possible.
 Preparation of fixing system and wind bracing (usually done by means of guy wires and
turnbuckles). Weldout at least sufficient to allow crane release.
 Crane release. Removal of rigging and temporary attachments.
Jacket structural completion is followed by a short phase during which all the jacket systems, both
permanent and those required during installation, are completed and rendered functional. The load-
out operations are covered in Lecture 15A.9: Installation.
5. CONCLUDING SUMMARY
 The design of a jacket is determined primarily by the offshore installation equipment available
and the intended water depth.
 In general, the preference is to lift the jacket in place. Jackets destined for deeper water are
usually erected on their side.
 As a general principle, as much of the execution as possible should be undertaken in the early
phases of fabrication.
 Each phase of execution has its own engineering requirements which are determined by the
processes executed during that phase.
 The specifications for fabrication of offshore jackets are determined by the designer and are
usually based on one or more of the well known codes.
 Shop fabricated sub-assemblies and hose items are incorporated into assemblies which
constitute the major lifts of the erection sequence.
 Assembled, sub-assembled and fabricated structures, together with loose items, are
incorporated into the final structure in a sequence which takes account of structural analyses of
bending stresses, and crane, bogie and support loads.
6. REFERENCES
[1] API RP2A, Recommended Practice for Planning, Designing and Construction of Fixed Offshore
Installations, latest edition.
Engineering design principles and practices that have evolved during the development of offshore oil
resources.
[2] AISC Specification for the Design, Fabrication and Erection of Structural Steel for Buildings, latest
edition.
API code refers to this specification for calculations of basic allowable stresses of all jacket members.
7. ADDITIONAL READING
[1] Det Norkse Veritas Marine Operations Recommended Practice RP5 - Lifting (June 1985).
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Principles and good practice for offshore heavy lifts.
[2] AWS Structural Welding Code AWS D1.1-88.
All jacket welding and weld procedure qualifications are required by the API code to be undertaken in
accordance with this code.
[3] Det Norske Veritas, Rules for the Design, Construction and Inspection of Offshore Structures,
1977.
Rules for construction and installation of steel jackets as required by DNV.
[4] Lloyd's Register of Shipping, Rules and Regulations for the Classification of Fixed Offshore
Installations, 1989.
Based on Lloyd's experience from certification of over 500 platforms world-wide.
APPENDIX 1
Quality Assurance and Quality Control
It is becoming increasingly common for operators to specify that the quality of construction for
offshore structures be controlled by a recognised quality system management standard.
ISO 9000/EN 29000, Standard for Quality Systems Management, is recognised as the accepted
standard in such situations. These standards set down the requirements that a soundly based quality
management system must fulfil if it is to assist in properly defining and controlling product quality.
Because the standards deal with the quality system, and are not product standards, they are
applicable to many sectors of industry including offshore construction. They apply in any situation
where management wish to adopt a clearly defined policy and an orderly approach to providing a
quality product.
All aspects of a company's activities are covered in the standards including:-
QA Management Complexity
The overall programme for a jacket construction, shows that there are a very considerable number
of offshore activities in many different locations within a very short period of time. The evaluation of
the performance of such a range of activities and at a number of centres is a major QA/QC
undertaking.
It is difficult to fully appreciate the scope of documentation on a jacket construction project.
Consider the documentation which is expected to flow from one location to another in respect to a
single node. From the time the plate is manufactured until it is located in the final structure, a
dossier must be compiled. This documentation could commence with copies of certificates from the

Design Product Traceability
Contract Review Process Control
Documentation Control Inspection/Testing
Management Responsibility Calibration of Equipment
Purchasing Control of Non-Conformances
Corrective Action Handling/Storage/Delivery
Quality Records Training
Management Review/Audits Etc.
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steel plate manufacturer and progress through several welding, NDT, dimensional control phases at
a number of successive locations, culminating in the issue of a Release Note at the node fabrication
shop.
Clearly this is necessary on some items, e.g. steel, welds, NDT Certificates for the jacket primary
structure, risers, etc. These documents may be useful during maintenance of the platform enabling
many in-service problems to be traced to abnormalities which occurred during construction.
Construction of a large jacket typically involves thousands of steel plates. Each plate inevitably
becomes an individual as it is allocated a unique number corresponding to a Material Utilisation
Schedule or Cutting Plan. The individual number of pieces of plate could be in excess of 20,000
items. The primary object of material control is to ensure that, at any stage of construction, the
origin of each and every item can be traced back to a material certificate which in turn corresponds
to a set of test/chemical composition etc. as contained in the Data Dossier. However, voluminous as
this documentation may be, it constitutes less than half of the total documentation produced for a
complete jacket. Consider for instance the number of welds in a complex buoyancy tank, the
walkways on top of the jacket, the anodes, launch runners, grout lines, etc. Each of these must be
welded, several must be individually inspected. However the requirement to produce sophisticated
documentation in respect of each is questionable. For this reason it is important that agreement is
reached at an early stage as to the individual items which require identification, that these be kept to
a minimum and that the identification system be simple. In actual practice it has proven to be very
difficult to make all materials really traceable. Much more could be done to structure such
documentation in such a way that it would really be of help throughout the platform life.
Procedures and Specifications
Within the Contractor's organisation QA/QC procedures must be developed for the project, many of
which will be specifically for jacket construction. These are divided into Management Procedures
(e.g. Management of Non Conformities, Management of Jacket Completion Onshore, etc.) and
Control Procedures (e.g. Procedure for Ultrasonic Testing of welds at Jacket Yard, Dimensional
Control Procedure for Node Fabrication at factory etc.). Construction Procedures/Specifications are
also required (e.g. Jacket Assembly and Erection Procedure, Pile Installation Procedure, etc.) in
addition to a vast number of weld procedure specifications and qualifications, welder qualifications
and inspection plans.
Even if the number of specific procedures required from each subcontractor is minimised, a
fabrication subcontractor will still be required to develop procedure and specifications for the
following typical functions/activities: subcontract organisation, material control, fabrication
method/sequence, procedures for cutting, forming, pre-heating, post-weld heat treatment along with
the more obvious welding and NDT procedures and Inspection Plans. Typically hundreds of
procedures/specifications must be developed by jacket subcontractors.
Certification
On most offshore projects, the underwriters normally agree to insure the plant during its operating
life provided it is designed, constructed and maintained to predetermined standards and certified as
such. This certification is also almost invariably required by the state authorities in whose waters the
plant is installed. It is normally performed by one of the traditional ship classification societies known
as the Certifying Authority (CA). In the widest sense, certification requires that the CA carry out
independent surveillance to ensure that the standards chosen for the project are satisfactory and
that the project is performed in accordance with them. Formerly this meant that the CA inspected
every activity likely to influence the adequacy of the final product - an enormous task. More recently
with the advent of QA, the certification function can mean audits of the construction so that, rather
than inspect everything, the CA satisfies himself that the manner in which the construction is being
managed and performed (based on incomplete but comprehensive inspection) is likely to lead to a
satisfactory product.
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Table 1 Jacket Construction Phases and Characteristics
Phase Work Centre Efficiency Quality
Variability
Risk
Engineering Office Decreasing Increasing Increasing
Procurement Factory Decreasing Increasing Increasing
Fabrication Fabrication Shop Decreasing Increasing Increasing
Assembly and
Erection
Yard Site Decreasing Increasing Increasing
Loadout and
Seafastening
Transition Decreasing Increasing Increasing
Transport and
Installation
Offshore Site Decreasing Increasing Increasing
No. Document Series Group or Individual Subject Title
1 Shop Drawings, Cutting Plan Welding standards, nodes, tubulars, piles, pile sleeves, clusters, conductor guide frames,
launch runners, buoyancy tanks, cathodic protection system, protective coating systems,
risers, j-tubes, caissons, boat landings, boat bumpers, walkways, grouting systems,
ballasting system, installation aids, as-built drawings.
2 Method and Temporary Works Drawings Subassemblies, assemblies, supports, access, scaffolding, lifting and transportation
onshore, test and commission, identification.
Onshore construction accessories.
Offshore installation (preparation, transportation, lifting, launching, anchor patterns etc.).
Offshore installation accessories (tools, guides, access, handling etc.).
3 Quality Assurance Procedures Documentation identification, distribution and approval, witness and hold points, technical
modifications and non-conformance management, material control, material identification
and traceability, procurement and subcontracting, weld parameter control, management
of specific problem areas.
4 Quality Control Procedures NDT methods (visual, UT, x-ray, dye penetrant, MPI), dimensional control, destructive
testing methods, NDT operator training and qualification, calibration of inspection
equipment, pressure testing, miscellaneous testing.
5 Manuals Testing, commissioning and preparation of jacket for tow. Load-out manuals - jacket
piles, topsides.
Installation manuals - jacket, piles, topsides.
6 Weld Procedures For each location - weld procedures
- repair procedures.
7 Design Reports, Reviews and Specifications Quay design, skidway design, mooring system design, soil improvement spec., skidding
system spec., dredging spec., transportation of jacket and piles, buoyancy tanks, jacket
launching and emplacement, on-bottom stability, pile driveability, jacket levelling study.
8 Engineering Meetings Normally held at critical phases of construction at the various construction locations.
9 Fabrication, Assembly and Erection Fabrication/welding sequence (for principal items), forming, bending, stress relieving,
coating, assembly and erection, temporary and secondary attachments, lifting and
transporting, jackdown, weight control, settlement control, jacket weighing.
10 Inspection plan Steel supply (at each supplier).
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Table 2 Jacket Construction Engineering:
Typical Organisation of Contractor's Technical Documents
Fabrication of typical jacket and pile components (at relevant centres).
Assembly and erection.
11 Technical Proposals and Non-Conformance
Resolutions
Technical Clarification Requests )
Technical Relaxation Requests ) Possible at every
Major Non-Conformance Reports ) Phase of location
Minor Non-Conformance Reports ) of the Project.
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Lecture 15A.9: Installation
OBJECTIVE
To describe the general methods of jacket installation. To discuss the various stages of operation from loadout
through offshore positioning and installation, including construction practices and equipment. To indicate the
calculations normally involved.
PREREQUISITES
Lectures 15A.1: Offshore Structures : General Introduction
RELATED LECTURES
Lectures 3.1: General Fabrication of Steel Structures
Lectures 3.2: Erection
Lecture 3.3: Principles of Welding
Lecture 3.4: Welding Processes
Lectures 15A: Structural Systems: Offshore
SUMMARY
The phases of installation of a steel jacket - loadout, seafastening, offshore transportation and installation - are
described and the associated analyses are indicated.
1. INTRODUCTION
1.1 Project Phases
A steel jacket installation usually consists of the following project phases:
Loadout - Comprises the movement of the completed structure onto the barge which will transport it offshore.
Seafastening - Comprises fitting and welding sufficient structure between the structure and the barge to prevent
the jacket shifting during transit to the offshore site.
Offshore Transportation - Comprises the tow to the location offshore and arrival of the barge at the offshore site
with the seafastened structure.
Installation - Comprises the series of activities required to place the structure in the final offshore location. These
activities include jacket lift and upending, positioning, pile installation, jacket levelling and grouting, together with
support services for these activities.
1.2 Construction Philosophy
In deciding how best to fabricate (i.e. vertical or on its side) and install (i.e. lifted, launched or self-floating) a given
jacket, the options are principally determined by the installation equipment available and the jacket's intended water
depth. In general, the preference is to lift the jacket into location. The motivation for this installation method, rather
than the more traditional barge-launching, is a reluctance to spend money on jacket steelwork which will only be
used during the temporary installation phase. The size of such lifted jackets has been increasing as offshore lifting
capacity has grown. With modern lifting capacity now up to 14000 tonnes (see Table 1), jackets approaching this
order of magnitude are now candidates for lifting into position.
Figure 1 shows how the 6000 tonne jacket for the Kittiwake field in the North Sea was lifted from the barge into the
water and up-ended in a continuous operation, ending with the jacket on the seabed ready for piling. The advantage
of this approach is that the jacket, being lowered into the water, does not require the launch frames necessary for
launching from a barge. Also, since the weight of the jacket is taken by the cranes throughout, there is no need for
special buoyancy tanks and deballasting systems.
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Jackets destined for deeper water are heavier and are usually erected on their side and launched from a barge
(Figure 2). This method of construction is currently applicable for jackets up to 25000 tonnes. A launched jacket
usually requires additional buoyancy tanks with extensive pipework and valving to enable the legs and tanks to be
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flooded in order to ballast the jacket into the vertical position on site. For instance, in the case of the Brae 'B' jacket
(a large 19000 tonne jacket installed in 100m water depth in the North Sea) it was necessary to provide 11000
tonnes of additional buoyancy. This buoyancy was primarily to limit the jacket trajectory through launch (i.e. to stop
it hitting the sea bed) but was also essential for maintaining bottom clearance during up-ending. The additional
buoyancy took the form of two 'saddle' tanks, two pairs of twin 'piggy-bank tanks' and twelve 'cigar' tubes installed
down the pile guides (Figure 3). Altogether the auxiliary buoyancy added about 3,300 tonnes additional weight to
the jacket.

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Very large jackets, in excess of launch capacity, have been constructed as self-floaters in a graving dock, towed
offshore subsequent to flooding the dock, and installed on location by means of controlled flooding of the legs (see
Figure 4).
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1.3 Installation Planning
The installation of a jacket consists of loading out, seafastening and transporting the structure to the installation
site, positioning the jacket on the site and achieving a stable structure in accordance with the design drawings and
specifications, in anticipation of installation of the platform topsides.
An important aspect is the avoidance of unacceptable risk during offshore activities from loadout through to platform
completion. It is recognised that the potential cost to the project associated with failure to successfully execute
marine activities is particularly high. Normally therefore the contractor is obliged to produce procedures for these
activities which demonstrate that the risk of failure has been reduced to acceptable levels. He is also required to
demonstrate that, prior to the commencement of an activity, all the necessary preparations have been completed.
An installation plan must be prepared for each installation. The plan will include the method and procedures
developed for the loadout, seafastening and transportation and for the complete installation of the jacket, piles,
superstructure and equipment. Depending on the complexity of the installation, detailed procedures and instructions
may be needed for special items such as grouting, diving, welding inspections, etc. Limitations on the various
operations due to factors such as environmental conditions, barge stability, lifting capacity, etc. must be defined.
The installation plan is normally subdivided into phases, e.g., loadout, seafastening, transportation and installation.
Installation drawings, specifications and procedures must be prepared showing all the pertinent information
necessary for construction of the total facility on location at sea. These drawings typically include details of all
inspection aids such as lifting eyes, launch runners or trusses, jacking brackets, stabbing points, etc. For jackets
installed by flotation or launching, drawings showing launching, up-ending and flotation procedures must be
prepared. In addition, details are also provided for piping, valving and controls of the flotation system, etc. as well
as barge arrangements and tie-down details.
The engineering input into an offshore installation project also involves the design of all temporary bracing,
seafastenings, rigging, slings, shackles and installation aids, etc. These must be designed in accordance with an
approved offshore design code, e.g. API RP2A [1].
Quality management is a vital and integral component of all offshore installation projects. A general note on Quality
Assurance for Offshore Construction is appended to Lecture 15A.8 : Fabrication. It is equally applicable to an
offshore installation project.
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2. LOADOUT AND SEAFASTENING
Loadout entails the movement of the completed structure onto the barge which will transport it offshore.
Seafastening entails fitting and welding sufficient ties between the jacket and the barge to prevent the jacket
shifting while in transit to the offshore site.
Jackets which have been fabricated on their side are usually loaded by skidding the entire structure onto a cargo or
launch barge. During loadout, the jacket is supported on the skid ways, usually on two inner legs of the jacket, see
Fig. 9 of Lecture 15A.1. The legs function as the bottom chord of a large truss, which can span between points of
support, especially when part of the jacket is on the barge and part still on the skid ways.
Where jackets are fabricated in the vertical, i.e. in the same attitude as the final installation, they may be lifted onto
the barge or skidded out. In this latter case, adequate temporary pads and braces must be provided under the
columns to distribute the loads during skidding.
Initial friction of the jacket on the skid ways may be as high as 15 per cent, especially if the jacket has been erected
with its weight bearing continuously on the skid way. In some cases the jacket is initially fabricated slightly above
the skid ways using hydraulic or sand jacks. Then, at the time of loadout, the jacket is lowered onto the skid ways.
To reduce the sliding friction, grease on hardwood, or heavy lubricating oil on steel, or even fibre-filled Teflon faced
pads, are used. Values of sliding friction as low as 3 per cent are usually attained.
The barge should be of adequate size and structural strength to ensure that the stability and static and dynamic
stresses in the barge and seafastenings due to the loading operation and during transportation remain within
acceptable limits. The barge must also have the capability to launch the jacket, if this is required, without the use of
a derrick barge. For a barge which floats during the loadout, the ballast system must be capable of compensating
the changes in tide and loading. It is usual in this case to load out on a rising tide so that the tide assists the ballast
system. In the case of a barge which will be grounded during loadout, the barge must have sufficient structural
strength to distribute the concentrated deck loads to the supporting foundation material.
The jacket must be loaded in such a manner that the barge is in a balanced and stable condition. Barge stability can
be determined in accordance with regulations such as those published by Noble Denton, The American Bureau of
Shipping, or the US Coast Guard. Allowable static and dynamic stresses in the barge hull and framing due to
loadout, transportation and launching must not be exceeded.
A simplified check list for the operations relating to jacket loadout might be:
1. Is the jacket complete? Has the structure been analysed for loadout stresses on the basis of the actual
structure as fabricated at the time of loadout?
2. Is the launch barge securely moored to the loadout dock, so that it won't move out during the loading? Is the
barge properly moored against sideways movement?
3. If compression struts are used between the barge skid ways and those on shore, are they accurately aligned
and supported so they won't kick out during loadout? Have the pull lines, shackles, and pad eyes been
inspected to ensure they are properly installed and can't foul during loadout?
4. Can the barge be properly ballasted? If the tide will vary during loadout, are ballasting arrangements adequate?
Will ballast be adjusted as the weight of the jacket goes onto the barge? Are there proper controls? Is there an
adequate standby ballast system? Are there back-up systems to pull the jacket back to shore if anything goes
wrong during loadout? If the ballast correction is to be made iteratively, step-by-step as the jacket is loaded,
are there clear paint marks so that each step can be clearly identified?
5. Have clear lines of supervision and control been established? Are the voice radio channels checked? Have the
marine surveyors been notified so that they can be present? Owner's representatives? Certifying Authority?
Have their approvals been received?
Once the jacket is on the barge, the barge must be ballasted for transportation. During loadout, many tanks will be
partially full, in order to control deck elevation and trim. However, with the jacket fully supported on the barge,
these considerations are no longer relevant and the tanks can be ballasted to suit the demands of the sea voyage.
Ballast tanks should normally either be full or completely empty, to eliminate free surface and sloshing effects. The
draft and freeboard will have been carefully selected to maximise stability, and especially to minimise submersion of
projecting members of the jacket during the tow and the consequent slamming, buoyancy and collapse forces.
Large jacket launch and cargo barges are relatively flexible structures in that the jacket structure is normally (much)
stiffer. Therefore, ballasting the barge to obtain the required draft and trim should preferably be done at the dock
side before seafastenings are attached. If one scheme of ballasting is to be used for a sheltered channel tow and
another for the open sea, the seafastenings should be freed during the reballasting to avoid imposing undue stresses
on the jacket legs or, alternatively, calculations should be performed to demonstrate that freeing is not required by
the reballasting procedure.
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Seafastenings are installed after loadout and must be completed prior to sailaway. They are major structural
systems, subjected to both static and dynamic loads. When the barge is on the high seas it must be assumed that it
can encounter conditions which are "as bad as could have been statistically foreseen". Accordingly, the gravity and
inertial forces involved must be calculated for all anticipated barge accelerations and angles of roll and pitch during
the design sea conditions adopted for the tow (usually the 10-year return storm for that season and location). In
determining this criteria, the reliability of the short term weather forecast should be considered. Since the loads are
dynamic, impact must be minimised. Seafastenings should be attached to the jacket only at locations approved by
the designer. They should be attached to the barge at locations which are capable of distributing the load to the
barge internal framing. They should also be designed to facilitate easy removal on location. Seafastenings are
normally subject to the same code requirements for fabrication as the jacket.
3. OFFSHORE TRANSPORTATION
The transportation of heavy components from a fabrication yard to the offshore site is a critical activity. It is
especially so in the case of the jacket since the behaviour of the unit usually influences the verification of barge
strength, the design of seafastenings, and indeed the design of the jacket itself. Also there are the practical aspects
of tug selection, tow route, etc. to be considered.
The size and power requirements of the towing vessels and the design of the towing arrangement must be
calculated or determined from past experience. Tug selection involves such considerations as length of tow route,
proximity of safe harbours and the anticipated weather conditions and sea states. As a minimum the tugs should be
capable of maintaining station in a 15 metre/second wind with accompanying waves. However, this criterion
depends on the location. For instance, the requirement in the Mediterranean is typically that the main tug should
maintain station against a 20 metre/second wind, 5,0m significant sea-state and 0,5 metre/second current, acting
simultaneously. Weather forecasting is provided throughout the tow so that, if exceptional weather threatens, a pre-
arranged port of refuge may be sought.
Experience has shown that the first phase of transportation is the most treacherous. There are several reasons for
this. In the harbour area a big tug can normally exercise very little control even with a shortened towline. With a
short towline between two considerable masses, the large tug and the much larger barge/jacket, the risk of
snapping is high. Thus it is standard practice to lengthen the towline once out of the port. Also, because of the
nature of many ports, close control is essential in order to avoid the possibility of running aground. Normally,
therefore, the harbour tugs take the barge out under the guidance of a pilot who knows the port. When the barge is
out of the port the problems are not totally solved since it must be assumed that the worst can happen, i.e. the
towline may break.
The tug must have sufficient time to pick up the emergency towline and control the barge before it drifts into
shallow water. Thus the departure is normally subject to strict weather forecast conditions for a period which
assumes that the speed of the tow is between 1 and 2 knots for the first 100 nautical miles from the coast.
Consequently, as a minimum, a favourable 48-hour weather forecast is required, e.g. Force 5 and decreasing.
Once the tow is under way, trim will be adjusted to optimise tow speed and give directional stability during tow.
Usually the barge will be trimmed down by the stern.
The behaviour of the jacket seafastened to the barge must be satisfactory both from the point of view of static and
dynamic stability. Both are verified by means of numerical analyses. However, particularly for larger structures, the
sensitivity of the dynamic analysis will usually warrant verification by model testing.
The intact static stability criteria usually adopted is that the righting arm be positive throughout a range of 36 about
any axis. The so-called dynamic stability of wind overturning criteria simply ensure that for a given wind, the energy
which tends to overturn the barge is at least 40% less than that which is available due to the inherent righting
stability of the barge.
In considering the motions of the jacket and barge it is intuitively plausible that roll will be the most problematical
motion (from the point of view of body accelerations) and that the largest roll will be caused by a beam sea. It may
be less obvious, but nevertheless true, that if the barge width and, to a lesser extent the length, are reduced, the
roll will diminish and if the barge is set at a (much) deeper draft, the roll will also diminish. All of these
considerations reflect static properties of the jacket and barge. Improvements can occasionally be made by choosing
a narrower barge (although obviously stability will suffer) or increasing the draft (although in this case stability may
again suffer and parts of the structure which were previously 'dry' may now be subjected to 'slamming'). Incorrect
"balancing" of these aspects can have very serious cost/risk implications in overall project terms. Thus, for a large
jacket, the barge selection process is normally performed at a very early stage of the design process.
4. OFFSHORE INSTALLATION
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This section is concerned with the stages of jacket installation commencing with removal of the jacket from the
barge to its placing on the sea bed and temporary on-bottom stability. Lecture 15A.6: Foundations covers the
subject of pile installation.
4.1 Removal of Jacket from Barge
Unless a jacket is a self floater, it must first be removed from the transportation barge. There are two basic methods
used:
 launch
 lift
4.1.1 Launch
The launch site is normally at or near the installation location. With heavy jackets in shallow water it may be
necessary to launch the jacket in deep water at some distance from the installation location and tow the jacket to
site.
Immediately prior to launch, the seafastening securing the jacket to the barge is cut. The jacket is then pulled along
the barge skid ways (which were used for loadout) by winches. As the jacket moves towards the stern of the barge,
the barge start to tilt and a point is reached when the jacket is self sliding. An initial tilt to the barge may have been
provided by ballasting immediately prior to launch. A stern trim of approximately 5 is usually aimed for.
The skid ways terminate in rocker arms at the stern of the barge. As the jacket moves along the skid ways its centre
of gravity reaches a point where it is vertically above the rocker arm pivot. Further movement causes the rocker
arm and jacket to rotate. The jacket will then slide under its own self weight into the water. Various stages in the
launch of a jacket are shown in Figure 1a to 1d.
Once in the water the self floating jacket is brought under control with lines from tugs and/or the installation vessel.
The jacket must be designed and fabricated to withstand the stresses caused by the launch. This can be achieved
either by strengthening those members which might be over-stressed by the launching operation, or designing into
the jacket a special truss, commonly referred to as a launch truss. Spacing between jacket members or launch
trusses will be dictated by the spacing between launch skid ways. Thus a jacket will generally be designed from the
outset for installation by a specific barge.
Once launched the jacket must float with a reserve of buoyancy in order to stop the downward momentum of the
jacket. This requires the jacket to be water tight. It is common practice to gain additional buoyancy by sealing
jacket legs and pile sleeves with removable rubber diaphragms. However, there is frequently a need for even more
buoyancy. This is achieved by adding buoyancy tanks. These need to be removable and are located where they give
most benefit. Buoyancy tanks from previous launches are often used.
The launch of a jacket is clearly a critical phase in the life of the jacket. Considerable design effort is required in
order to ensure that the launch sequence is feasible. A jacket launch naval analysis is required in order to:
 ensure that an adequate sliding velocity is maintained during the rocker arm rotation;
 verify that the trajectory followed has a safe seabed clearance;
 determine the jacket behaviour during launch;
 define operational requirements during launch, including ballast configuration;
 check the stability of the jacket during launch and when free floating.
The plots shown in Figures 1a to 1d are extracted from such an analysis. The jacket weight was 14,000 tonnes and
was being installed in 105 metres of water. The analysis showed that it should take approximately 2 minutes
between start of self sliding (Figure 1a) and the jacket reaching its final floating position (Figure 1d).
4.1.2 Lift
An increasing number of jackets are being installed by direct lift. This trend has been encouraged by the availability
of large crane vessels such as the Micoperi 7000. Curves showing load capacity against lifting radius are shown in
Figure 2. Another factor tending to increase direct lift jackets are savings in weight that are being achieved in jacket
design.
In a direct lift the jacket is lifted off the barge completely in air. A second form of lift is the buoyancy assisted lift. In
this case the barge is flooded and hence submerged. This results in part of the jacket being buoyant, reducing hook
loads. Buoyancy tanks may be added to the jacket if required.
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Shallow water jackets may be lifted in the vertical position. In this case no up-ending is required and installation is
straight forward. Deep water jackets will in general be lifted on their side. Two cranes will normally be used, noting
that large derrick barges such as the Micoperi 7000 are fitted with two cranes as standard. When considering a
tandem lift it should be noted that it is unlikely that both hooks will carry the same load, and that the maximum
permissible jacket weight will be less than the sum of the two crane capacities. It should also be noted that cranes
are frequently guyed back to give maximum lift capacity and carry less load if they are revolving. This can further
reduce the apparent lift capacity. Finally, the weight of lifting slings need to be considered, these contributing as
much as 7% of the lift weight.
When the jacket is to be removed from the transportation barge by lifting, it is normal for the installation vessel to
be correctly moored and positioned so that up-ending and set-down may proceed as one integral lift operation.
The selection of a suitable installation vessel is clearly essential. In addition to lift capacity, it is also necessary to
consider stability and motion response characteristics. In the harsh North Sea environments installation vessels are
usually semi-submersibles such as the Micoperi 7000. In more moderate waters they are often flat bottomed barges.
In intermediate environments, e.g., the Gulf of Mexico, ship-shaped vessels may be used.
The large semi-submersible crane vessels used in the North Sea have full dynamic positioning systems for locating
themselves on site. They also have sophisticated computer controlled ballast systems to keep the vessel level during
lifting operations. During a lift the ballast system is also used to counteract heel and increase hoisting and lowering
speeds during the crucial lift-off and set-down operations.
The natural period of large installation vessels in roll, pitch and heave tend to be close to the typical peak periods of
the sea spectra encountered offshore. These motions therefore predominate. Normally this means that beam seas
should be avoided since this excites roll which is the most disruptive motion. However, the "best attitude" is not
always possible since it depends on the work that the vessel is required to perform. Accordingly vessel operators
perform extensive studies to determine permissible sea states for specific operations and vessel captains invariably
"experiment" with different headings in a particular sea in order to minimise motions and maximise workability.
The first stages in lifting a jacket from the transportation barge involve positioning the barge and connecting the
slings to the hook. The barge will normally be controlled by tugs. Once everything is ready for lift to proceed the
seafastenings will be cut. The next stage is to transfer the weight of the jacket from the barge to the crane. The
general requirement here is to lift as rapidly as possible. However, careful control and phasing with barge and crane
vessel motions is required in order to ensure that once the jacket is lifted clear of the barge it does not hit the barge
as a subsequent wave passes through. The same lift procedure is adopted in both a direct and buoyancy assisted
lift.
Once the jacket is lifted clear of the barge, the barge is removed by tugs. Up-ending of the jacket will then normally
proceed directly.
4.2 Jacket Up-ending and Set-down
Unless a jacket is transported and lifted in its upright position, it will be necessary to up-end the jacket at the
installation location. Up-ending may be achieved by controlled flooding of buoyancy tanks, by using a crane vessel
or by a combination of both.
4.2.1 Up-ending by Ballast control and Flooding
A large crane vessel will not normally be required for either a launched or self-floating jacket. Upending is therefore
achieved by controlled flooding. A small installation vessel will usually be required for the installation of piles once
the jacket has been set-down, so this is used as the platform from which to control the various flooding operations.
This installation vessel will also be used to help position the jacket.
Figure 3 shows a sketch of the Brae 'B' jacket showing the auxiliary buoyancy tanks. In this case the flooding
system involved 42 primary and 22 contingency subsea valves under direct hydraulic control. The nitrogen power
source and associated control panels were contained in watertight capsules.
Figure 4 shows a sequence of sketches indicating how a self floating jacket is upended. In step 1 the waterline
compartments at one end of the jacket are flooded. More water line tanks are flooded in step 2 until by step 3 the
upper frame of the jacket reaches waterline and may also be flooded. The jacket is then allowed to rotate until all
legs are equally flooded as in step 4. The jackets natural position will then be floating upright as in step 5. Further
flooding of the jacket as in step 6 will enable the jacket to be lowered onto the sea bed in a controlled manner.
The up-ending of a launched jacket will be similar to that shown in Figure 4. The main difference is that there may
be less excess buoyancy with which to control the operation. In this case a combination of flooding and lift, as
shown in Figure 5, may be used to up-end and set-down the jacket.
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The crane and ballasting operations need to be clearly defined before the operation begins. This involves careful
naval analysis of the free floating position of the jacket at various stages during the up-ending procedure. A feature
of these analyses is the need to consider what happens in the event of buoyancy tanks being accidentally flooded, or
of flooding valves failing to operate. Contingency procedures and equipment must be provided.
4.2.2 Up-ending using the crane vessel
Figure 5 shows the most simple use of a crane to up-end and set-down a jacket. This is acceptable for jackets that
are launched. For horizontally oriented jackets that are lifted directly the procedure is more involved.
A horizontally lifted jacket may be upended in one of two ways. Perhaps the most straight forward is to lower the
jacket into the water so that it floats. Slings can then be removed and new slings attached at the top of the jacket.
The jacket may then be up-ended as shown in Figure 5. This may require closures to legs and some additional
buoyancy.
A second method is to up-end directly, as shown in Figure 6. This requires special padears so that the necessary
rotation between slings and jacket can occur. Careful naval analysis is also required in order to carefully determine
hook loads and to ensure that the jacket remains stable.
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Once up-ended the jacket can be set-down on the sea-bed. Since the lifting points are submerged divers may be
required to disconnect the slings from the jacket.
Although two crane hooks are shown in Figure 6, it should be noted that for light weight jackets it is possible to up-
end using a single crane. In this case the main and auxiliary hooks are used together, for example the main hook
taking the weight of the jacket with the auxiliary hook providing the upending force.
An increasing trend is to install a jacket over an existing well or wells. A pre-drilling template will have been used to
position the wells, the same template being used to position the jacket. It is necessary to ensure that the well heads
are protected from damage due to accidental contact with the jacket.
Once set-down the jacket should be positioned at or near grade and levelled within the tolerances specified in the
installation plan. Once level, care should be exercised to maintain grade and levelness of the jacket during
subsequent operations. Levelling the jacket after all piles have been installed should be avoided if at all possible as it
is costly and frequently ineffective. If necessary, levelling should take place after a minimum number of piles have
been driven by jacking or lifting. In this instance procedures should be used to minimise bending stresses in the
piles.
4.3 On-bottom Stability
Once set-down on the sea bed, it is normal for piling to proceed as rapidly as possible. However, this far into the
installation procedure the weather and hence sea conditions may be detioriating. This is a result of long term
weather forecasting being less reliable than short term forecasting. It should also be noted that any problems
encountered during the installation procedure will result in delay and that it may be some time before the jacket is
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adequately fixed to the sea bed by piling.
It is necessary for the jacket to be stable and level during piling. A separate on-bottom stability analysis is therefore
carried out. Three conditions need to be met:
(1) vertical resistance to jacket weight and piling loads;
(2) stability against sliding under wave/current loading;
(3) stability against overturning under wave/current loading.
In carrying out the above analyses it is necessary to use an appropriate sea-state to generate hydro-dynamic
loading. This should be the maximum statistical wave which may occur prior to piling being completed. Assuming
installation to occur in the summer months, a typical criteria may be a 1 year summer storm wave.
The provisions that need to be made to ensure on-bottom stability vary greatly depending on jacket location, height
and on sea-bed soil conditions. For example, with good soil conditions the jacket may be able to be supported
directly on existing jacket steel with no extra provision made. However, with poor soil conditions large 'mudmats'
may be required in order to spread the load. These can influence launch and installation dynamics.
For many jackets it is not possible to achieve stability against sliding and overturning using flat mudmats. In these
circumstances mudmats with skirts may be used. Skirts considerably improve the resistance to sliding, and in silty
or clay soils can allow nominal tension loading to resist overturning. Another option frequently used is to stab a
number of piles as soon as the jacket is set-down. These will penetrate some distance under self weight providing
additional sliding resistance. Since most piles are inclined, the piles also provide a degree of resistance to over
turning.
5. CONCLUDING SUMMARY
 There are broadly four phases to the installation of a steel jacket - loadout, seafastening, offshore
transportation and installation offshore.
 In deciding how best to fabricate and install a given jacket, the options are principally determined by the
installation equipment available and the jacket's intended water depth.
 An installation plan must be prepared for each installation. Loadout entails the movement of the completed
structure onto the barge which will transport it offshore.
 Seafastening entails fitting and welding sufficient ties between the jacket and the barge to prevent shifting
while in transit to the offshore site.
 The transportation of heavy components from a fabrication yard to the offshore site is a critical activity
requiring careful calculation and planning.
 Removal of the jacket from the barge is accomplished either by direct lifting with a derrick barge and lowering
into position, or by launching. A number of engineering studies are required for jacket launch and set-down.
6. REFERENCES
[1] API RP2A, Recommended Practice for Planning, Designing and Construction of Fixed Offshore Installations, latest
edition. Engineering design principles and practices that have evolved during the development of offshore oil
resources.
7. ADDITIONAL READING
1. Det Norske Veritas Marine Operations Recommended Practice RP5 - Lifting (June 1985). Principles and good
practice for offshore heavy lifts.
2. AISC Specification for the Design, Fabrication and Erection of Structural Steel for Buildings, latest edition. API
code refers to this specification for calculation of basic allowable stresses of all jacket members.
3. AWS Structural Welding Code AWS D1.1-88. All jacket welding and weld procedure qualifications are required
by the API code to be undertaken in accordance with this code.
4. Det Norske Veritas, Rules for the Design, Construction and Inspection of Offshore Structures, 1977. Rules for
construction and installation of steel jackets as required by DNV.
5. Lloyds Register of Shipping, Rules and Regulations for the Classification of Fixed Offshore Installations, 1989.
Based on Lloyd's experience from certification of over 500 platforms world-wide.
Operator Name Type Mode Lifting Capacity
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Notes:
1. Rated lifting capacity in metric tonnes
2. When the crane vessels are provided with two cranes, these are situated at the vessels stern at approximately
60m distance ctc.
Table 1 Major Offshore Crane Vessels
Heerema Thor Monohull Fix
Rev
2720
1820
Odin Monohull Fix
Rev
2720
2450
Hermod Semisub Fix
Rev
4536 + 3628 = 8164
3630 + 2720 = 6350
Balder Semisub Fix
Rev
3630 + 2720 = 6350
3000 + 2000 = 5000
McDermott DB50 Monohull Fix
Rev
4000
3800
DB100 Semisub Fix
Rev
1820
1450
DB101 Semisub Fix
Rev
3360
2450
DB102 Semisub Rev 6000 + 6000 = 12000
Micoperi M7000 Semisub Rev 7000 + 7000 = 14000
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Lecture 15A.10: Superstructures I
OBJECTIVE/SCOPE
To introduce the functional requirements; to identify major interfaces with the process,
equipment, logistics, and safety; to introduce the structural concepts for jacket and gravity
based structure (GBS) topsides; to elaborate on structural design for deck floors.
PREREQUISITES
Lectures 1A & 1B: Steel Construction
Lecture 2.4: Steel Grades and Qualities
Lecture 2.5: Selection of Steel Quality
Lectures 3.1: General Fabrication of Steel Structures
Lecture 6.3: Elastic Instability Modes
Lecture 7.6: Built-up Columns
Lectures 8.4: Plate Girder Behaviour & Design
Lectures 11.2: Welded Connections
Lecture 12.2: Advanced Introduction to Fatigue
Lectures 15A: Structural Systems - Offshore
SUMMARY
The topside lay-out is discussed, referring to API-RP2G [1], and to general aspects of
interface control and weight control.
The different types of topside structures (relevant to the type of substructure, jacket or
GBS) are introduced and described. These types are:
1. integrated deck.
2. module support frame.
3. modules.
Floor concepts are presented and several aspects of the plate floor design are addressed.
1. INTRODUCTION
This lecture deals with the overall aspects of the design of offshore topsides.
The topside of an offshore structure accommodates the equipment and supports modules
and accessories such as living quarters, helideck, flare stack or flare boom, microwave
tower, and crane pedestals.
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The structural concept for the deck is influenced greatly by the type of substructure (jacket
or GBS) and the method of construction, see Figures 1 and 2.

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Heavy decks, over 10,000 tons, are provided with a module support frame onto which a
number of modules are placed. Smaller decks, such as those located in the southern North
Sea, are nowadays installed complete with all equipment in one lift to minimize offshore
hook-up. Most of this lecture refers to this type of integrated deck such as is shown in
Figures 3 and 4.
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The selection of the concept for the structural deck is made in close cooperation with the
other disciplines.
2. BASIC ASPECTS OF DESIGN
2.1 Space and Elevations
The first step in developing a new design concept is to consider all the requirements for
the deck structure. The design requirements and their impact on the structural system are
discussed below.
The lay-out of the deck is influenced by the type of hydrocarbon processing to be
undertaken.
The area required for the equipment, piping and cable routings, the vertical clearance as
well as the access/egress requirements determine the deck area and deck elevations.
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The elevation on the lowest decks depends on the environmental conditions. The elevation
of the cellar deck, i.e. the lowest deck, is based on the maximum elevation of the design
wave crest, including tide and storm sway, plus an air gap of 1,5m minimum.
The vertical distance between the decks of the topside is generally in the range 6 - 9m in
the North Sea.
Consideration of the prevailing wind direction is very important in determining the position
of various components on the platform, such as the vent of the flare, cranes, helideck; and
the logistic and safety provisions.
2.2 Lay-out Requirements
The requirements for the various topside components are briefly described below, based
on API-RP2G [1].
Wells: the position of the wells depends heavily on whether the wells will be drilled and
worked with a separate cantilevered jack-up rig or with a platform-based rig. In the first
case the wells must be close to the platform edge and require significant deck area above
free of obstacles. In the second case a pair of heavy beams to support the drill rig must be
provided.
Equipment, piping and cable-supports: all devices to treat the oil or gas shall comply
with the requirements of API-RP2G [1].
Living quarters and helideck: the helideck should be in the vicinity of the living quarters
to enable fast evacuation. Usually the helideck is located in the obstacle free area on top of
the living quarters.
Gas compressor module: the pressure in gas reservoirs declines due to production.
Future compression may be needed in order to achieve acceptable gas flow through the
export pipeline.
Water or gas injection module: oil production declines after some years of operation.
The reservoir then requires stimulation by, for example, injection of water.
Deck crane: the location of the crane should be selected so as to obtain maximum deck
coverage and to enable the crane operator to keep eye contact with the lifted object and
the supply vessel. The location of the deck crane should be outside the obstacle free area
of the helideck and should not interfere with future facilities.
Vent/flare boom or stack: a vent discharges gaseous products in the air without
burning them; a flare discharges and burns these products. Both vents and flares should
be located outside hazardous areas and away from the helideck. The tip shall exceed the
elevation of the helideck by at least 100 feet. Heat radiation shall be checked.
Microwave tower: A high mounting is required to provide obstacle free support for
microwave antennae. A stiff support is required in order to comply with the stringent
deflection criteria.
Survival capsules and man-overboard crane: the supporting structures for these items
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usually cantilever from the main structure. Shock loading and dynamic amplification
increases the support reactions during operation.
Walkways, ladders and stairs: these items should be kept obstacle free, be non-
slippery and have sufficient width to allow evacuation of personnel on stretchers.
Cladding, walls, doors and louvres: the type of cladding depends on the operational
requirements and the preference of the oil company. For safety reasons, walls and doors
may have to satisfy specified explosion and fire resistance requirements. Louvres may be
used to allow natural ventilation, whilst preventing entry of rain, snow and birds.
Lay down areas for equipment, spares and consumables: these areas are provided
by cantilevering from the main structure in order to allow access to the lower deck levels
by the deck crane, without providing hatches through the decks.
Hatches: access to the lower decks within reach of the crane is required to enable
maintenance, repair and platform modification. The hatches should be identified early in
the design.
Risers, caissons, sumps: the riser section of the pipeline rises from the seabed to the
deck. It introduces vertical and horizontal loads (environmental and operational) in the
deck structure. Caissons for pumps and sumps for discharge are hung from the cellar deck
and introduce significant vertical and horizontal loads in the deck.
Drainage provisions: provision is required for spillage in drip pans under the equipment
and for collecting oil-polluted rainwater to prevent spilling into the sea.
Deck penetrations: pipes connecting process-items on different decks and, vessels, cable
routings, etc. can require significant areas to be clear of structural members. The major
penetrations should be identified early in the design and coordinated with major structural
members.
Other provisions: items such as monorails and inspection gangways may also be
required.
2.3 Loads
In Lecture 15A.3 the different types of loads have been identified and partly quantified.
Dead weight, tankful live load and wind load are discussed here. Dead weight includes the
weight of structure, equipment, piping, cables, machinery, and architectural outfitting.
Tankful live load covers weight of potable water, diesel fuel, helifuel, glycol, methanol,
well-kill mud, lubrication oil, waste, etc.
Live load also covers all sorts of miscellaneous loads such as bagged or palletized
consumables, spare parts, maintenance equipment, etc.
The application of live load is typical for topsides. For design considerable engineering
judgement is required concerning:
 the magnitude of the load to be applied to the various structural items:
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- direct loaded deck stringer
- deck beam
- deck truss
- deck leg
- jacket
- pile
- pile bearing resistance
 the area to which live load is to be applied. This area is described in the code as the
non-occupied area.
For local strength, the walkways, escape routes, etc. are considered as non-
occupied by equipment and are thus loaded by live load.
For overall strength, the walkways, escape routes, etc. are considered as
occupied (kept clear for evacuation) and consequently no live load is applied.
 the arrangement of loads that generates maximum stress. A policy on this item
should be prepared for each project, stating both variation of loads over one deck,
and variation over various decks.
Wind loads should be properly assessed. For overall structural integrity, the complex shape
of the platform creates problems in assessing the effective area for wind load. Special
elements such as communication towers and flare booms require consideration as wind
sensitive structures.
To control the design process, weight engineering as explained in Section 2.5 below, shall
be performed by the project management staff. Any structural analysis must be linked to
the latest available information in the weight report. This requires that the load file for the
structural analysis and the weight report are compatible with respect to total weight,
weight distribution and centres of gravity.
2.4 Interface Control
The many functions of the topside result in the involvement of many disciplines in the
design.
Due to the high cost of providing platform space, the facility must be designed to be very
compact. This requirement leads to several major areas of interdisciplinary control.
 Space allocation: the structure should not use space allocated for equipment or
access routes. Overhead clearance between piping, cable routes, equipment and the
floor overhead should be respected.
 Direct interface control; pumps, vessels and piping require support by the steel
structure.
 Interface between drilling and workover operations.
 Interface between platform crane and helideck, deckhouse, drilling rig, and flare
stack.
 Interface with the export riser.
 Interface between the deck and modules.
 Interface between the topside and bridge from adjacent platform.
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 Interface with the substructure.
2.5 Weight Engineering
The weight of the overall facility as well as its major components is critical. Lack of weight
control can lead to costly design changes as well as to major provisions in order to keep
within the limits of the construction strategy.
Weight engineering consists of:
 weight prognosis
 weight reporting
 weight control
 weighing
Weight prognosis is the methodology which applies an uncertainty surcharge as high as
+30% in the conceptual design phase, to +5% in the final fabrication phase, see Figure 5.

3. STRUCTURAL SYSTEMS
3.1 Selection of Topside for a Main Jacket-Based Structure
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The selection of the concept for the topside structure is the second step in the
development of a structural system. The two possible basic alternatives: a truss type
(Figure 4) or a portal-frame type without braces (Figure 3), are compared in Table 1.
Table 1 Comparison of concepts for main jacket-based structures
Note: ++ denotes greater benefit
-- denotes greater disadvantage
The selection of the topside main structure concept, truss or portal frame, is linked with
the decision of the position of the longitudinal structure in the cross-section. In a 20-25m
wide deck, trusses will generally be arranged in longitudinal rows: centre line and both
outer walls (Figure 6).
No. Item Truss type Frame type
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Discipline non-interference
Flexibility during construction
Flexibility during operation
Automated fabrication
Construction depth
Inspection
Maintenance
Weight of structure
Strength reserve
Potential for high strength steel
Structural CAPEX
Platform CAPEX
-
-
-
-
++
-
-
+
+
+
+
+
++
++
++
++
0
++
+
0
++
++
+
++
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In such decks, however, portal frames will be arranged in 2 longitudinal rows,
approximately 14-16m apart, allowing floor cantilevers of approximately 5m (Figure 3).
3.2 Selection of Topsides for Gravity Based Structures
Topsides of gravity based concrete structures (GBS) are quite different from the jacket
based topsides, see Lecture 15A.1. The topside structure is an important element in the
overall portal-type system. Gravity based substructures have been built with one to four
shafts. A rectangular or a T-arrangement of four shafts has been adopted. The basic form
is a modularized topside with a grid of heavy box girders.
A few elements only of the GBS-topside structural design are indicated below:
 due to portal frame action, the deck is subject to fatigue; a design case difficult to
control in topside design.
 equipment lay out optimization, piping and cable routes, logistic and emergency
routes require many big openings and perforations of plate walls, thus creating stress
concentrations.
 attachment of secondary structures and of equipment/pipe/cable supports to the
main structure must be strictly controlled, to avoid fatigue problems.
 the connection area with the concrete shaft must provide the transition from circular
(shaft) to square (deck). It accommodates high strength anchor bars, temporary
crushing devices for deckmating, and requires tolerances on deck and substructure
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dimensions.
 inspection and repair options must be planned carefully, especially as fatigue may
occur.
The material used at present is high strength steel typically of 355 MPa yield stress. There
is a trend to use higher strength steel (420-460 MPa).
3.3 Floor Systems
The concept for the floor-system in offshore structures is conventional: hot rolled beams,
typically at 1000-1200mm centres, are covered by a chequered or flat steel plate 6-10mm
thick.
The options are:
 conventional steel floor
 steel grating (bar-type or plate type)
 aluminium floor system
 orthotropic deck in steel
 corrugated steel plate
The conventional steel floor contributes approximately half of the weight of the steel
structure of an offshore deck.
Steel gratings, especially with the plate type, could gain increased application as their
weight per sq.m. is attractive.
Aluminium has attracted much interest recently; current development in Norway will show
its real potential.
Orthotropic decks in steel have found application in helidecks. Further study is required to
assess their actual feasibility for floors of offshore modules.
Corrugated steel plate (approximately 1-3mm thick) as sub-flooring has been used in
living quarters.
In summary, the floor concept used for a typical floor of an offshore deck of a module is a
conventional steel floor or steel grating.
3.4 Floor Panel Concept for Conventional Steel Floor
The floor panel, defined as the assembly of the floor plate and the stringer, can be
connected to the overall structure in two ways:
 stacked: stringer over the top of deckbeams.
 flush: stringer welded in between deck beam, with top flange in one plane. It is
practically impossible to change from the flush to stacked arrangement in a later
phase of the design.
All elevations and overhead clearances are involved in the choice of arrangement.
Clearances are very important for equipment height, pipe routing, pipe stress, cable
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routing, etc. The single most important structural aspect is the amount of prefabrication
that can be carried out away from the main fabrication yard. The cost is also a very
important factor.
3.5 Floor Stabilization Concept
The deck structure requires lateral stabilization of each floor with respect to:
 lateral instability of beams
 horizontal forces, e.g. wind, pipe reactions, sea transport
 horizontal components of permanent braces
 horizontal components of temporary braces, e.g. seafastening
 horizontal components of sling forces
 module skewing during installation.
There are essentially two options for floor stabilization:
 provision of separate underfloor horizontal bracings
 allocate the stabilization function to the floor plate.
There is a clear preference for the stabilization by the floor plate. Where underfloor bracing
is adopted, there are two configuration options (see Figure 11). The rhomboid solution
should be chosen for the upper deck, due to congestion at the column by the padeyes for
lifting. The underfloor bracing under a plate floor does create a very unclear structural
situation. The bracing is assumed completely to perform the stabilizing function, but in
practice the floor plate is much too stiff to allow that. It is common practice in the
structural analysis for underfloor bracing to neglect completely the floor plate.
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4. DECK FLOORING DESIGN
4.1 Introduction
The selection of the main deck dimensions have been considered above in relation to lay-
out requirements.
The interactive process of conceptual design of the jacket and deck yields the spacing of
the columns. In the Dutch sector of the North Sea, transverse column spacings are
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typically 9m for a wellhead platform to 15m for a production platform. Longitudinally
spacings are typically 15m.
Next decisions are made on:
 floor system: plate versus grating
 main structure: truss versus portal frame
 floor panel concept: stacked versus flush
 floor stabilization underfloor bracing versus plate.
The structural concept is then complete.
A principle for economic design of steel structures is that the load-paths should be short.
For a floor design of a production deck typical dimensions are:
Structural item Typical span
1. floor plate 1m
2. stringer (longitudinal) 5m
3. deck beam (transversal) 15m
4. main structure (longitudinal) 15m
5. column
These components are identified in Figure 6.
4.2 Floor Plate
Design
Options are to choose between flat plate, chequered plate or tear plate. Another option for
providing slip resistance is to coat with a sand finish. The floor plate thickness is usually 8-
10mm and 6mm for lighter loaded floors, although welding distortion may rule out the 6
mm thickness.
In practice the floor plate acts as horizontal bracing between the columns.
Special attention is required to ensure that all welds between the floor plate and the
underlying structure do not form brittle points. Failure of such welds could lead to crack
initiation in the rest of the structure.
The same attention applies to the buckling of the floor plate by stresses which are picked
up unintentionally.
Strength of Floor Plate
The strength of the floor plate is very high both for uniform as well as concentrated loads.
Elastic, small-deflection theory provides uneconomical conservative results.
API-RP2A (2) does not specify live loads. They are specified by the operator.
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For main decks generally accepted figures are:
p = 20kN/sqm, or
F = 10-25 kN on a 0,3 x 0,3m load area
Det Norske Veritas [3] presents an expression for the required plate thickness t, which
incorporates membrane effects and is of special interest for design for local loads.
Equipment and containers are regularly offloaded by the crane in some deck areas, such
as lay-down areas and food container platforms. An increased plate thickness may be
required in these areas due to larger concentrated loads (1).
4.3 Stringers
The typical stringer for a production platform is an IPE 240-270 or HE 240-280A profile
positioned at approximately 1m centres and spanning 5m.
It is important to choose, especially for stacked floor panels, a profile which allows
selection of heavier sections with practically identical depth to accommodate local heavy
equipment.
Designers should avoid choosing deeper sections or reinforcing them to accommodate late
extra load requirements by welding another section underneath. Interference with small
diameter, hard piping or with cable trays then is quite likely.
Joining floor plate and stringers requires welding. Intermittent welding is generally not
accepted. A continuous thin weld (a = 4 mm) is usually specified. The shear in this weld is
generally quite low.
The joint between the stringer and the deck beam differs with the floor panel concept
chosen.
 stacked floors have a continuous fillet weld around the flange contact area and
generally do not have web stiffening of the stringers.
If the top of the deck beam becomes inaccessible for maintenance, some operators will
require seal plates to be welded between the deck beam and the floor plate. This is quite
expensive. A typical joint is depicted in Figure 7.
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The decision on the type of stringer joint should preferably be made prior to material
ordering.
 flush floors. Welding the floor between deck beams requires removal of the top-flange
of the stringer near its end and perfect fit between the deck beams and floor. Deck
beam prefabrication is also required.
4.4 Deck Beams
Deck beams supporting the floor panels or providing direct support to major equipment
are generally provided as HE 800-1000 beams, though HL 1000 (400mm wide) or HX 1000
(450mm wide) are also used for heavier loads or greater spans.
The major joint in the deck beam is that with the main structure.
The joint configuration is strongly determined by the prefabrication concept and elevation
of the flanges. It is different for the stacked and for the flush concept.
Stacked Floor Concept
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Figures 8 and 9 illustrate the problems.

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For the full stacked concept (Figure 9), where both transverse and longitudinal main beam
are positioned lower, welding of the top flanges is straightforward.
The lower flange, typically 40mm thick, can only be welded to the web, typically 20-25mm
thick, if alignment of both flanges is ensured.
The lower flange of the main structure should be at least 250mm underneath, to enable
back welding of the root.
For the less suitable partially stacked concept (Figure 8), where only the transverse main
beam is positioned lower, connection for the top flange of the transverse deck beam is
more difficult. Direct welding of the top flange of the deck beam to the web should be
rejected. Options are shown in Figure 9 with detail (a) haunching and detail (b) slotting
the top flange through the web.
Again it is apparent that a decision on joint configuration is required prior to material
ordering.
Flush Floor Concept
Detailing is dependent of the prefabrication policy.
If the deck panel is prefabricated as an assembly of plate, stringer and deck beam, the
detail shown in Figure 10a is the more appropriate.
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To allow top flange welding a strip of the floor plate is fitted and welded last.
If the deck panel is fabricated as an assembly of plate and stringer only, the detail Figure
10b will be the most feasible.
4.5 Horizontal Bracing
In Section 3.5 the preference for the floor plate to act as horizontal bracing was indicated.
If however separate bracing members are required, the elevation must be chosen
carefully. The bracing members have to pass with sufficient clearance under the stringers,
penetrate the web of the deck beams at sufficient distance from the lower flange. They
also require good access for welding of the joint.
These requirements generate the elevation and the maximum feasible diameter of the
brace (Figure 11).
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Horizontal bracing can easily clash with vertical piping and major hatches.
Assembly of the braces is generally quite cumbersome.
5. CONCLUDING SUMMARY
 The topside lay-out was discussed, referring to API-RP2G, together with general
aspects of interface control and weight control.
 Based on the type of substructure, jacket and GBS, the different types of topside
structure were introduced and described. These types are:
integrated deck.
module support frame.
modules.
 Floor concepts were described.
 Several aspects of the plate floor design were addressed.
6. REFERENCES
[1] API-RP2G: Production facilities on offshore structures.
American Petroleum Institute 1 ed. 1974.
Presents the basic requirements.
[2] API-RP2A: Recommended practice for planning, designing and constructing fixed
platforms.
American Petroleum Institute, 18th ed., 1989.
The structural offshore code governs the majority of platforms.
[3] DNV: Rules for the classification of steel ships.
Part 5, Chapter 2.4.C, Permanent decks for wheel loading.
Det Norske Veritas.
Practical approach for economic floor plate design under static load.
7. ADDITIONAL READING
1. M. Langseth & c.s.: Dropped objects, plugging capacity of steel plates.
BOSS Conference 1988 Trondheim, pp 1001-1014.
Floor and roof plate behaviour under accidental loading.
2. D. v.d. Zee & A.G.J. Berkelder: Placid K12BP biggest Dutch production platform.
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IRO Journal, nr. 38, 1987, pp 3-9.
Presents a recent example for a portal-framed topside.
3. P. Gjerde et al: Design of steel deckstructures for deepwater multishaft gravity
concrete platform.
9th. OMAE conference Houston 1990, paper 90-335.
Most recent presentation on GBS topside structure.
4. P. Dubas & c.s.: Behaviour and design of steel plated structures, IABSE Surveys S
31/1985, August 1985, pp 17-44.
Good background to theory of plated structures.
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Lecture 15A.11 - Superstructures II
OBJECTIVE/SCOPE
To elaborate on structural steel concepts for integrated decks, module support frames, and modules. To show
principles and methods of construction (from yard to offshore site).
PRE-REQUISITES
Lectures 1A & 1B: Steel Construction
Lecture 2.4: Steel Grades and Qualities
Lecture 2.5: Selection of Steel Quality
Lectures 3.1: General Fabrication of Steel Structures
Lecture 6.3: Elastic Instability Modes
Lecture 7.6: Built-up Columns
Lectures 8.4: Plate Girder Behaviour and Design
Lecture 11.2: Welded Connections
Lecture 12.2: Advanced Introduction to Fatigue
Lecture 15A: Offshore Structures
SUMMARY
Structural systems for each type of topside structure are introduced, i.e. truss, portal frame, box girder, and
stressed skin.
Some special topics of design are addressed and the different construction phases are presented in more detail,
i.e.:
1. fabrication
2. weighing
3. load out
4. sea transport
5. offshore installation especially deckmating
6. module installation
7. hook-up
8. commissioning.
A brief discussion on inspection and repair and on platform removal concludes this lecture.
1. INTRODUCTION
This lecture deals with the structural design of jacket-based offshore deck structures, following the introduction
in Lecture 15A.10.
Heavy decks, over 10.000 tons, are provided with a module support frame onto which a number of modules are
placed, see Lecture 15A.1, Figs. 4 and 5. Smaller decks, such as those located in the southern North Sea, are
nowadays installed complete with all equipment in one lift to minimize offshore hook-up. Most of this lecture
refers to this type of integrated deck as described in Lecture 15A.10.
The selection of the concept for the structural deck is made in close cooperation with the other disciplines.
For the design of the deck structure, the in-place condition has to be considered, together with the various
previous stages such as fabrication, load-out, transport and installation.
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A structural system for a deck structure comprises several of the following elements:
2. MAIN STRUCTURE DESIGN
2.1 Introduction
Some major topics in topside structural design are reviewed below.
2.2 Main Structure-Portal Frame Design
A portal frame design has been used in recent major projects in the Dutch sector such as Amoco P15, Placid
K12 [5] and Penzoil L8.
The main girder/column joint, as shown in Figure 1, is very important in determining the height. It is most
practical to position the longitudinal and transverse main girder flanges at the same elevation.
Floors (steel plate or grating) }
Deck stringer (H beams, bulbs or troughs) } Discussed in
Horizontal bracing } Lecture 15A.10
Deck beams

}

Primary girders }
Vertical trusses or bracing } Discussed in
Deck legs } this lecture
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Haunching of the transverse main girder , which is more lightly loaded-in-plane, however is not an option as
these girders become highly loaded during transport.
The severe restraint of welding a tubular in a diaphragm requires the selection of TTP steel for the column
section.
Due to the high importance of the diaphragm plates in the overall integrity of the structure and the welding
constraints on the web plates in between, TTP-steel is chosen also for the diaphragm.
Another option is to weld the girders directly onto the unstiffened can section of the column. The assessment of
ultimate resistance as well as fatigue strength has been the subject of recent research (see Lecture 15A.12).
Further improvement of the theoretical and experimental background is required. For lighter loaded truss
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structures, this non-stiffened type of joint has been used successfully.
A third solution is to weld the girders directly to the can section of the column, which is internally stiffened by
rings. Its most severe disadvantage is the difficulty of inspecting the column interior.
The disadvantage of both direct girder-column joints is that the girder sizing is governed by the very high
moments at the column/beam transition point.
Cast steel nodes form an alternative to the welded designs.
Member selection for portal frame structures with increasing section moduli usually includes:
 300 mm wide rolled beam.
 400 mm wide rolled beam.
 450 mm /460 mm wide rolled beam.
 castellated beams fabricated from rolled beams, giving a height 1,5 times the original beam height.
 built-up girders fabricated from rolled beam T-sections with a web plate welded-in-between.
 plate girder.
The plate girder of course provides the greatest flexibility for design, material selection and procurement,
though its cost per tonne is approximately twice that of a rolled beam.
2.3 Main Structure-Truss Design
Most offshore structures of moderate size have been provided with a truss-type structure. Typically such
trusses consist of rolled beams as chords and tubulars as diagonals.
Truss design requires several choices which affect the structural efficiency and have impact on other disciplines:
 number and configuration of braces
 falling or rising braces
 intermediate load carrying of chords
 presence of external moments on joints
 braces: tubulars or H-rolled sections
 chords: rolled section or plate girders
 truss joints: locally reinforced chord or prefabricated node section.
Figure 2 shows different arrangements of braces (basically N or W-type) obtained by variation of the number of
nodes. It should be kept in mind that all diagonals and verticals form obstructions for piping and cable routings
of all kinds.
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For the transverse trusses, transparency is even more important, especially near the well area. The number of
members required should therefore be reduced to a minimum.
Providing a W-truss with light verticals should be evaluated against choosing a heavier chord section.
If a joint, e.g. at the top deck, is subject to severe moments due to lifting, ventstack, or crane pedestal for
example, much of the bracing stress would result from unintended bending. Generally the deck leg restraint
creates a similar problem in the lower deck. An evaluation should yield a preferred location therefore for the
node of the end brace.
The truss deflects under its vertical load which leads to restraint of the chord in the column and to bending of
the chord. Both effects can quite severely effect the efficiency. The chord section should be kept compact
therefore and not given too much height.
Tubulars (circular, square or rectangular) or rolled sections can be chosen for the braces.
The choice depends primarily on the loads and the chord width. A chord width of 300mm can accommodate a
10 in. brace only. Thus a wider chord flange is preferred.
2.4 Main Structure-Stressed Skin Design
A third major structural option is the stressed skin concept, where full height plate walls take the function of the
truss or the frame.
Modules for living quarters are frequently built to this concept. Other types of modules have not been built with
stressed skin since the obstruction they cause during construction is severe.
For smaller stressed skin modules, trapezoid corrugated plate can be used to provide a wall in a frame of
square hollow sections.
For bigger modules, flat plate stiffened with through-stiffeners is used for the walls.
The detailed design can only be made with a clear plan for assembling the module which shows the panels that
must be prefabricated.
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2.5 Non-Load Bearing Walls
Blast or fire walls are provided in offshore platforms. Due to their function full welding to the main structure is
often unavoidable, see Figure 3a.

Special attention is required concerning:
 the capability of the walls to comply with the deformation of the main structure during load-out, sea
transport, lifting and in-service.
 the strength of welds to the main structure being stronger than the plate to avoid rupture and potential
crack initiation of the main structure.
One solution is to provide a flexible detail, see Figure 3b and 3c, with stiffeners falling short.
2.6 Crane Pedestals
Crane pedestal, are discussed briefly below.
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It is structurally economical to put the crane pedestal on top of a main column. For a truss type the main
structure will be close to the platform periphery so a moderate length of crane boom is sufficient.
For a portal frame type with columns closer to the outer periphery, the pedestal requires a special column in
order to avoid using a crane with large boom length. Figure 4 depicts such a solution.

The functions of the main structure with respect to the crane pedestal are:
 to provide torsional support preferable at top deck level
 to provide lateral restraint at top deck level
 to provide lateral restraint at the lower end of the pedestal
 to provide vertical support, preferably at the lower end of the pedestal.
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Bending restraint by deck beams and/or main structure girders is not required and should be reduced where
possible. Torsion caused by slewing of the crane should preferably be resisted by the floor plate, the stiffest
element.
It has become practice to include the tapered top section of the pedestal in the supply package of the crane.
The top section contains the large flange for the slewing bearing.
Fatigue due to crane operations is a design criterion and requires careful detailing of the pedestal and the
adjoining structure.
3. ANALYSIS OF DECK STRUCTURES
3.1 Introduction
Although the analysis of deck structures is a standard task, several aspects require special attention:
 Plate girder design
 Strength of joints
 Strength of the floor plate
 Lifting points
 Modelling of floor plates
 Support of modules.
3.2 Plate Girder Design
Design of plate girders requires selection of many dimensional variables and of approaches for assessing load-
carrying resistance. Lectures 8.4 deal in more detail with plate girder design.
Web buckling due to bending, normal force and shear restricts the slenderness of the web which is expressed as
the height of the web (h) divided by the web thickness (t).
API-RP2A [2] refers to the AISC manual [3] which gives the figures below for material with yield-stress of 355
MPa:
Allowable bending stress 0,66 Fy 0,60 Fy
Ratio web height h to thickness t 90 138
Ratio flange width b to thickness t 18 27
Instead of the above approach, more recent research, [3] and [6], allows use of the post-buckling strength.
The depth/thickness limits given above do not then apply.
3.3 Strength of Joints
The most important joints in a topside steel structure are:
 the ring stiffened joint between rolled beams or plate girders with a circular column.
 the non-stiffened joint between rolled beams or plate girders with a circular column.
 the tubular brace joint to single web beams.
 the non-overlapped tubular joint.
These joints are discussed in Lecture 15A.12.
3.4 Lifting Points
The effect of lifting points on deck design is considerable. For example the local forces that act on the lifting
points (Figure 5) have to be transmitted safely through to the deck structure.
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There are two types of lifting points, trunnions and padeye, Figure 6.
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Trunnions, though favourable from other points of view, see Section 4, can generate considerable offset of the
sling force with respect to the topdeck system points. Significant bending is generated which is transferred to
the topdeck girders to the extent that they contribute to joint stiffness. It is most efficient to leave these
bending moments in the column, by providing stiff columns.
Padeyes generally provide a good opportunity to minimize or eliminate offset, as far as they can be situated on
top of the column. The requirement of recessed padeyes (recessed padeyes are those which are positioned
between the top and bottom flange elevation) or the presence of other structures on the top deck can lead to
very eccentric positioning and resulting heavy moments. For this reason the lifting concept must be developed
in the concept phase of the structural development.
API-RP2A [1] requires larger load factors to be used for members direct-loaded by padeyes or trunnions.
3.5 Modelling of Floor Plates
There are two points of major interest:
 representation of the floorplate in the structural model
 true elevation
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There are several ways to model the plate. The most direct is to choose a computer-program which allows
selection of plate elements. A second option is to define representative members which model the plate
stiffness by diagonals.
The deck plate is often positioned in the model at the elevation of the centre line, i.e. the mid height of the
main structure girders, in order to save nodes in the model. It should however be recognised that this "error" of
elevation, amounting to 0,5 - 1m, can affect the results. A separate evaluation should then be performed on the
effect to this deliberate "error" at least at some critical points.
3.6 Support of Modules
Modules and deck structures interact structurally. API-RP2A [1] requires that modules are modelled as elastic
structures for the analysis of the supporting deck. In the 1970's major difficulties arose in the decks for
concrete gravity structures, because modules were represented as a set of loads for the different load cases,
acting at the support points, and neglecting structural interaction. The basic phenomenon of this interaction is
that the distribution of the support reactions of the module is quite unequal and varies with the load case.
Dimensional control of the module as well as the support, with corrective measures, further provide control over
the module - deck interaction. Some modules, such as living quarter modules, gas compressor and injection
modules, are often placed on anti-vibration pads in order to isolate them from vibrations.
4. CONSTRUCTION
4.1 Introduction
In Lecture 15A.1 the principal aspects of construction of offshore structures and their major equipment was
introduced.
For topsides more specific aspects are discussed below.
4.2 Fabrication
4.2.1 Operations
The design should allow efficient prefabrication of major sections. Prefabrication will avoid congestion in one
working area and it speeds up the whole construction process.
Prefabrication and assembly shall properly incorporate the aspects of installation of major and smaller
mechanical equipment, as well as outfitting with piping, electrical and instrument cables and lines. It should be
recognized that major mechanical and electrical equipment is often not available at the start of assembly and
must be brought in during fabrication.
4.2.2 Design aspects
Since the overhead space is well covered by extensive piping routes as well as cable trays during construction,
"late" structural work should preferably not be positioned overhead in that underfloor area.
Fabrication of offshore steel structures is principally assembly by welding.
The prefabrication concept and joint detailing should maximize welding productivity with many horizontal welds
preferably made using SMAW technology.
Support to the topside during construction should be well controlled to avoid settlement and to keep within
construction tolerances.
Special consideration should be given to the selection of materials suitable for the fabrication. Where thick-
walled elements are involved requiring Post Weld Heat Treatment (PWHT), the design should position such
welding and the PWHT in the prefabrication phase.
4.3 Weight Engineering
The topside must be kept under strict weight control, as explained in Lecture 15A.10. To that end the topside is
usually weighed prior to load out. The basic design of a weighing system usually consists of a set of hydraulic
jacks with electrical load cells on top, installed between the topside and the shop floor. The accuracy of such
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systems is typically 0,5-1%.
Accuracy is necessary in order to check the actual position of the centre of gravity. Knowledge of the position is
vital for the installation.
The system for support of the topside should be similar to the anticipated method of load out.
4.4 Load Out
4.4.1 Operations
The load out usually combines two operations:
 moving the topside from the fabrication hall to the nearby quay.
 moving the topside from the quay onto the barge.
The short journey on land can be complicated when the track is not flat or curves have to be taken.
The most preferred option for load out is therefore to use a platform trailer with individual suspended wheels,
see Figure 7 and Slide 1.

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Slide 1 : General arrangement of a load out through skidding
The trailer drives from the quay over a rocker flap resting on the quay and the barge and then slowly onto the
barge. The barge is kept in right trim by ballast pumping.
When it reached the right position, the topside is set down on the beam grid of the sea fastening.
4.4.2 Design aspects load out
When using platform trailers the lower deck should be designed to meet three basic load-out requirements:
 the bottom flange plates of the transverse beams should all be in one plane.
 the distance of transverse beams should not exceed approximately 7 m.
 the lower deck should be able to take an upward reaction typically in the range of 50-60 kN/m
2
of ground
area.
A uniform distribution of loads is assumed for platform trailers. Skid systems which are not provided with a
proper load sharing system will lead to a non-uniform load distribution.
Design for load-out requires coordination with sea fastening design.
4.5 Sea Transport and Sea Fastening
4.5.1 Operations
Sea transport is a very critical operation, especially for topsides (see Slide 2).

Slide 2 : Seafastening of 105MN Brent C topside
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After completion of the load out and full fastening to the barge, the barge is ballasted to its target draft and
cleared for the transport.
The barge is towed by one or two tugs to the offshore location. There the barge is positioned close alongside
the crane vessel.
Prior to lifting, the sea fastening is cut free.
Planning the sea transport contains several steps:
 identification of critical clearances, e.g. (harbour depth, width of bridges or locks, etc inshore)
 barge selection (a.o. stability, dynamic behaviour, location of bulkends).
 evaluation of sea route (weather, length of tow).
 assessment of barge motions in sea state.
 development of a sea fastening concept.
 assessment of deck/module integrity.
 assessment of barge integrity.
There is also the option with some crane vessels to transport the top side on board. Usually an extra take over
is required as the draft of the crane vessel exceeds the depth at the fabricator's quay. The advantage however
is that sea fastening requires less effort. Furthermore, the offshore operation is simpler and quicker, as the
most critical and weather sensitive operation - lift off the barge- is avoided.
4.5.2 Design aspects of sea transport and sea fastening
Several elements of the structure are dominated by the load condition during transport, see Lecture 15A.1.
All equipment in or on the topside is also subject to heavy loads, e.g. control panels, generator skids, platform
crane, during transport.
Internal bracing of a topside for transport is not favoured since it creates obstacles and risk of damage or fire to
cables, instruments, piping and equipment during subsequent removal. External bracing is also not without
problems. The width of the topside requires an extra wide barge. It is difficult to find "strong" points on the
topside exterior. The basic concept is therefore to fix the topside to the barge by its columns only.
The designer should be aware that the bending stiffness of the topside often exceeds that of the barge.
Considerable "composite" action can result when the barge deflects in heavy head-on seas.
It is very important for any sea fastening concept to consider aspects of de-seafastening, i.e. cutting free, prior
to lift off, and the need to remain safe in a moderate sea state.
De-seafastening should not require any handling by cranes. Braces cut loose at one end should therefore
remain stable and safe while fixed at one end only.
Design of the sea fastening should not require any welding in the column joint, since the topside would not then
be ready for immediate set down onto the jacket.
When the tow is more than one or two days long, fatigue may have to be considered on critical nodes.
4.6 Installation
4.6.1 Operations
Installation on the substructure can be:
 deck mating with a deep submerged floating GBS (Slide 3)
 lifting onto an already installed jacket (Slide 4).
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Slide 3 : Deckmating of the 500MN Gullfaks-C topside

Slide 4 : Installation of 60MN K12-BP topside by floating crane
Deck mating is a floating operation in a sheltered location, e.g. a Norwegian fjord or Scottish loch. Deck mating
requires that the deck is temporarily supported with the final supports free. This requirement creates a very
awkward load situation for the deck structure.
Lifting is the usual installation method for jacket-based topsides. During development of a platform concept, the
lift strategy should be defined as part of the overall construction strategy. The lifting capacity of crane vessels is
defined by hook-load and reach.
The required reach is determined mainly by the width of the topside and/or the transport barge.
The major steps are:
 review of the weight report.
 assessment of "critical" elevations.
 assessment of feasible crane vessels.
 development of a lift concept.
 preliminary sizing of slings, shackles, trunnions, etc.
 concept design of guides and bumpers.
 analysis of deck or module structure for lift condition.
4.6.2 Design aspects of installation by lifting
The lift concept consists of several elements:
 the single or dual crane lift
 the sling configuration
 choice of topside pick-up points
 the necessity (or not) for spreader bars or even spreader frames
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 the single, double or paired slings
 the choice of padeyes, or trunnions.
Crane vessels were listed in Lecture 15A.1. Slings are available up to over 400mm nominal diameter with safe
working loads of 20-25 MN.
A basic element in all elevations is the inevitable tolerance in sling length which leads to an unequal distribution
of sling forces (typically 25%-75%) in a four sling lift. The unequal sling forces lead to significant stresses in the
module (see Figure 8).

The use of spreader bars leads to a fully balanced lift without distorting the module. However the spreader bar
is quite expensive and usually leads to a requirement for a higher hook elevation.
The use of a spreader frame should only be considered in exceptional cases and does not prevent module
distortion. The padeye/shackle option is limited by the safe working load (maximum 10MN) of the biggest
shackle. The trunnion can accommodate higher loads.
4.7 Hook up
Hook up is the completion of all joints and connections after installation.
For economic reasons, the overall construction strategy should keep hook up work to a minimum. Critical hook
up work is the work required immediately to secure the object in order to survive the next storm.
4.8 Commissioning
Commissioning is not relevant to the structural design.
4.9 Inspection Maintenance and Repair (IMR)
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These activities are a major source of operational expenditure, OPEX, as introduced in Lecture 15A.1.
Some requirements are:
 inspection of the primary structure is a statutory, fully planned activity.
 inspection is only possible when proper access to the area or joint is provided.
 gaining access is costly and requires space to be left behind equipment.
 minimum provisions, e.g. small clamps under the deck, greatly speeds up scaffolding.
 crack growth through fatigue is slow. A crack is usually detectable before one quarter of its life is passed.
 dirt accumulation promotes corrosion damage.
 maximum use should be made of the results of inspection. Evaluation should lead to modification of the
inspection programme where appropriate.
4.10 Removal
Removal requirements are different from country to country. In some depths of water full removal is required in
some countries from the mudline upward. Elsewhere only the structure 75 m or more above the mudline must
be removed.
Extensive engineering of removal is required to achieve a safe and effective operation. In the Gulf of Mexico
removed structures are dumped in the form of reefs. It is very difficult and inefficient at present to include
conceptual removal engineering in the design phase. When re-use of the facility is planned, then removal
engineering should be developed early in the design.
5. CONCLUDING SUMMARY
 Structural systems for each type of topside structure were introduced, i.e. truss, portal, box girder, and
stressed skin systems.
 In the section on design some topics were addressed in more detail.
 In the section on construction the different phases were presented in more detail, i.e.
i. fabrication
ii. weighing
iii. load out
iv. sea transport
v. offshore installation especially deckmating
vi. module installation
vii. hook-up
viii. commissioning
 A brief discussion on inspection and repair and on platform removal concluded the lecture.
6. REFERENCES
[1] API-RP2A: Recommended practice for planning, designing and constructing fixed platforms.
American Petroleum Institute, 18th ed., 1989.
The structural offshore code, governs the majority of platforms.
[2] AISC: Allowable stress design manual (ASD).
9th ed., American Institute of Steel Construction, 1989.
Widely used structural code for topsides.
[3] API-Bulletin 2V: Bulletin on design of flat plate structures.
American Petroleum Institute, 1st ed., 1987.
Valuable specialist addendum to API-RP2A.
[4] API-Bulletin 2U: Bulletin on stability design of cylindrical shells.
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American Petroleum Institute, 1st ed., 1987.
Valuable specialist addendum to API-RP2A.
[5] D.v.d. Zee & A.G.J. Berkelder: Placid K12BP biggest Dutch production platform.
IRO Journal, nr. 38, 1987, pp 3-9.
Presents a recent example for a portal framed topside.
[6] R. Narayanan: Plated structures/Stability and Strength.
Applied Science Publishers, London, 1983.
Good designers guide to plated structures design.
[7] ANON: Gullfaks C platform deckmating.
Ocean Industry, April 1989, pp 24.
Good description of the actual mating of deck to GBS.
[8] A.G.J. Berkelder: Seafastening 105 MN Brent C deck.
Bouwen met Staal, nr.24 1979.
Presentation of seafastening design for GBS topside.
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Lecture 15A.12: Connections in Offshore
Deck Structures
OBJECTIVE/SCOPE
To outline and explain the best methods for forming structural connections in offshore deck
structures; to discuss the importance of a proper choice of connection type to achieve both the
required strength and stiffness, and ease of fabrication.
PREREQUISITES
Lectures 11.2: Welded Connections
Lectures 11.4: Analysis of Connections
Lectures 13: Tubular Structures
Lectures 15A: Structural Systems: Offshore
RELATED LECTURES (covering specific items in greater detail)
Lecture 2.4: Steel Grades and Qualities
Lecture 2.5: Selection of Steel Quality
Lectures 3.6: Inspection/Quality Assurance
Lecture 4A.5: Corrosion Protection in Offshore Structures and Sheet Piling
Lecture 11.5: Simple Connections for Buildings
Lecture 12.2: Advanced Introduction to Fatigue
Lectures 12.4: Fatigue Behaviour of Hollow Section Joints
SUMMARY
Various forms of structural connections in steel offshore deck structures are discussed; these
cover the connections between deck stringers and main beams, between main beams themselves,
between main beams and deck legs, truss connections and connections between columns and
beams. The importance of designing and dimensioning to minimise fabrication and maintenance is
emphasised.
1. INTRODUCTION
Large offshore deck structures have traditionally been built up using modular components, see
Lectures 15A.10 and 15A.11; a module support frame is built on top of the deck legs of the jacket
structure on which the various modules are installed. With the high lifting capacities currently
available, the topsides of light to medium offshore structures can now be installed in one lift. This
development has had a considerable influence on the fabrication and design of deck structures,
and has resulted in heavier modules, constructed of larger and heavier members, with
consequences for the connections.
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Another aspect influencing fabrication, and thus the design, was the development of cleaner
steels, with modified chemical compositions and good through-thickness properties. This so-called
TTP steel (i.e. steel with through-thickness properties, see Lecture 2.4) has a low sulphur content
to avoid lamellar tearing. Furthermore, if the carbon and carbon equivalent (CEV) is low, the
preheat temperature of the steel can be lowered, resulting in easier welding (without preheating)
which again influences the connection design.
The increase in lifting capacity, and the exploration for gas and oil in deeper water, have both
resulted in larger structures, and have stimulated the use of higher strength steels, with yield
strengths above 355 N/mm
2
.

The joints have to be designed to withstand the various loading conditions (see Lectures 15A.2
and 15A.3) experienced during fabrication, load-out, transport, installation and the in-place
condition (operation and storm). In order to allow redistribution of stresses it is important that the
joints are stronger than the connected members; if this is not the case the joints themselves must
have sufficient deformation/rotation capacity.
The connection design should take account of all the aforementioned aspects and should be
considered as an interactive procedure involving the choice of the structural layout, the fabrication
sequence and the steel grades and qualities to be used. Other aspects such as inspection and
corrosion protection requirements must also be considered.
Since the fabrication costs are mainly governed by the costs of welding, the connections should be
simple, and where possible, avoid the use of stiffeners.
2. CONNECTIONS IN OFFSHORE DECK MODULES
The type of connections used in offshore deck modules depends directly on the type of structure
involved:
 truss types
 frame types
 stressed skin
As discussed in more detail in Lectures 15A.10 and 15A.11, the structural system for a deck
includes several of the following elements:
 floor (steel plate or grating)
 deck stringers (I-beams, bulb flats or troughs)
 deck beams
 main beams or girders (beams on main grid lines)
 vertical trusses or braces
 deck legs and columns
Depending on their function, loading, and availability of sections, these elements can be made of
rolled I or H-sections, rolled circular or rectangular hollow sections, or welded sections; for the
larger sizes, welded I or box plate girders, or welded tubular members are used.
These elements have to be connected together; since the modules are generally fabricated under
controlled conditions at the fabrication yard, welded connections are common practice. The main
connection types are discussed more in detail below. Although it is common practice in offshore
design to use the API-RP2A [1] or the AISC rules [2], the basic joint behaviour is discussed in this
lecture without reference to the safety factors to be used.
3. CONNECTIONS BETWEEN DECK STRINGERS AND BEAMS
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The deck floor structure can be designed as a floor plate with stringers, or as an orthotropic plate.
The floor plate with stringers is the most common type as it gives design flexibility for later
changes (local loads, deck penetrations, etc). Orthotropic plate structures, are generally used in
helidecks, see Lectures 15A.10 and 15A.11.
The use of stacked stringers, as shown in Figure 1, facilitates fabrication and is, therefore to be
preferred to the use of continuous connections, as shown in Figure 2.

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For ease of fabrication, stiffeners should be avoided if possible. This means that the vertical loads
have to be transmitted by the webs, as shown in Figure 1, over a length l
s
for the stringer, and l
b

for the deck beam; web crippling failure is also possible and should be checked. These are a
common details which are dealt with in Eurocode 3 [3] and other codes.
For the continuous connections, shown in Figure 2, the moment is assumed to be transferred by
the flange connection and the shear by the web connection.
The type of full penetration weld at the top flange for continuous connections depends on the
fabrication sequence and should be decided by the fabricator. The bottom flange and web can
generally be connected by fillet welds. A full penetration weld of the flange, without a 'mouse
hole', is preferred because of corrosion protection although this results in a small weld defect at
the neck between flange and web. However, even under fatigue loading such a defect can be
accepted [4] the same is also valid for static loading. Only in cases where very high strength steel
(f
y
> 500N/mm
2
) is used and a high yield to ultimate stress ratio, e.g. f
y
/f
u
> 0,9 occurs should
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this detail be evaluated rigorously. Since all loading cases are not always checked, the welds have
to be designed to have at least the same strength as the connected parts, i.e. as the flange or
web.
It should be recognized that the shear stress distribution (Figure 2) for a detail with a 'mouse hole'
is more severe than that without a 'mouse hole'. Special attention should be given to the
unsupported upper side of the web in Figure 2b, as local buckling may be a problem, see Lecture
6.2 and [5].
4. CONNECTIONS BETWEEN INTERMEDIATE AND MAIN DECK
BEAMS
The connection between the deck beams is most convenient if these beams have the same height,
as shown in Figure 3b. Here the flanges are connected with full penetration welds, and the web by
fillet welds or a full penetration weld depending on the thickness. Tolerance control is necessary to
avoid differences at the deck floor level, between stringers. The shear loads are generally too high
to allow a single or double sided notch as shown in Figure 3b since this results in a higher shear
stress, see Figure 2. In case of equal heights, no TTP requirements are necessary for the beams.
For the connection of beams with unequal heights, however, as shown in Figure 3a, the web of the
main beam should have a TTP quality due to the loads being transferred through the web
thickness. Furthermore, to satisfy the requirements for avoidance of cold cracking, etc., either the
flange thickness of the intermediate beam should be less than 1,5 times the web thickness of the
main beam, or the material should have a low carbon content (see Lecture 2.5).
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As an expensive alternative solution, a plate connecting the flanges can be slotted through the
web, as shown in Figure 4. Haunched alternatives are given in Lectures 15A.10 and 15A.11.
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All welds should be designed to have the strength of the connected parts.
As a consequence the connection is as strong as the member; only in case of large 'mouse holes'
the shear stress and possible local buckling of the unsupported web part [5] have to be checked.
5. BEAM TO DECK LEG CONNECTIONS
The main beams, either rolled H sections or plate girders, must be connected to the deck legs,
which are normally fabricated tubular members. For a frame type structure, this connection should
be rigid and capable of transmitting the yield moment resistance of the connected beams. These
connections, or nodes, are generally prefabricated, consisting of a tubular "can" with surrounding
"diamond" (diaphragm) plates for the connection with the beams, as shown in Figure 5. This type
of connection requires special material specifications and special welding procedures.
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Stiffened Connections
The shear loads are transferred by the connection of the web plates to the tube walls. The
moment is transmitted by the diamond plate in combination with an effective ring width of the
tubular "can". The design resistance, for factored loading, is normally checked with the
experimental Kamba formula, which is simplified by Kurobane [6] as follows:
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N
Rd
=

where:
N
Rd
is the design resistance for the flange for factored loading

f
y
is the yield stress of deck leg "can"

b
1
is the flange width of deck beam

d
o
is the outer diameter of tube

t
o
is the wall thickness of the deck leg "can"

t
s
is the thickness of ring plate

h
s
is the smallest width of the ring plate

b
f
=

Validity ranges:

The axial force in the flange N, is derived from N = M
cw
/(h
1
- t
1
) (see Figure 5). This formula is
based on the test results for a ring-stiffened joint with two opposite loads; more detailed research
is currently being carried out [7]. In the case of multi-planar loading, for four loads acting in the
same sense, the joint strength will be greater. However, if the two loads in one direction are
tensile and the two in the direction perpendicular to that are compressive, the joint strength may
be decreased. Reference [7] reports that this decrease was found to be a maximum of 30%.
Furthermore, if the deck leg is loaded by axial compressive stress amounting to 60% of the yield
value, the strength of the connection has to be reduced by 20%.
Non-Stiffened Connections
For truss type frames, the beam to deck leg connection has to transfer mainly axial loading and an
unstiffened connection, as shown in Figure 6, could be used; this is, however, not yet common
practice. If sufficient deformation capacity exists, the secondary bending moments can be
neglected for static loading. If fatigue loading has to be checked, however, care should be taken
with these secondary bending moments, because the stress concentration factors at the flange to
tubular connection are rather high. For practical cases these stress concentration factors can be in
the order of 10 for , see [8].
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The static design resistance for factored load of the unstiffened connection is determined by the
strength of the flange to tube connection, which can be based on Togo's ring model, see Lecture
13.2. The design resistance for flange loads in one direction (X-joint loading) is given by Eurocode
3 [3] and [9].
N
Rd
=

where:
N
Rd
is the design strength for the flange for factored loading

f
yo
is the yield stress of joint "can"

t
o
is the wall thickness of joint "can"

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 is the flange width b
1
to "can" diameter d
o
ratio
k
p
is the influence function for additional stress in the chord.

Validity ranges:
0,4    1,0
For bending moments in-plane, the axial force N is derived from N = M
cw
/(h
1
-t
1
) as shown in
Figure 5.
For an axial loading the flange connections can interact such that the connection strength (I to
tubular) is not twice the strength of one flange connection but:
N
Rd
.

Consequently the beam to deck leg connection has to be checked for:
N
Sd
 N
Rd


M
ipsd
 N
R.d
(h
1
- t
1
)

Currently, for multi-planar loading with loads and moments acting in the opposite sense, the same
30% reduction in joint strength as before is recommended, although initial investigations indicate
that this may be conservative [10]. No reduction has to be applied if the loads are acting in the
same sense.
6. CONNECTIONS BETWEEN BEAMS AND COLUMNS
Columns between decks are necessary where external surfaces of the modules are clad, or where
cantilevers or laydown areas are provided. The connection with the deck beams can be flexible in
the longitudinal direction if these columns have only to withstand lateral loading. If, however, they
are used to transfer loadings from cantilevers to both decks, the connections should have the
same strength as the column or they should have sufficient deformation capacity.
Figure 7 shows a possible full strength detail for columns connected to a plate girder, with possible
connecting side beams and an extended cantilever. Here the web of the plate girder is ended
before the flange to allow a tubular section to be welded between the flanges. I-beam sections,
even with different depths, can be easily welded to this tubular section, and the columns can be
welded to the flanges.
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The "joint can" should have about the same diameter and thickness as the column. In Figure 7
longitudinal beams and a cantilever beam are also connected to this can. The bending moment
resistance is here determined by the connection of the bottom flange to the tubular can, similarly
as discussed in Section 5.
7. TRUSS CONNECTIONS
Since the chords of the trusses are part of the deck floors, they are almost always made from an I
or H-section; in exceptional cases, welded box sections are used. The diagonals are tubular,
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rectangular hollow sections, or H sections; all have their advantages and disadvantages with
regard to material costs, maintenance and fabrication. Where these diagonals are connected to an
I section chord, the chord should be stiffened to obtain a full strength connection; it should be
kept in mind that intermediate beams may have to be connected to the chord at this location. The
connection should be designed in such a way that fabrication and inspection will be easily possible.
Figure 8 shows some connection details for lightly loaded trusses.

These connections generally do not develop a strength equal to or larger than the yield strength of
the diagonals. Consequently the connection should have sufficient deformation capacity. However
experimental evidence is only available for the connection according to Figure 8a.
From a fabrication point of view, the connections with a gap between the braces are preferred.
However the connections with overlapped braces as shown in Figures 8c and 8d are stronger.
The connection strength may be governed by various criteria, depending on the geometry, i.e:
a. chord web strength
b. chord web crippling under a compression brace
c. chord web shear between the diagonals of a gap joint
d. chord web buckling
e. brace (diagonal) effective width
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f. brace shear failure at the flange connection
g. weld failure (to be avoided by full strength welds)
h. lamellar tearing (to be avoided by TTP material for the flange).
For connections according to Figure 8a, Eurocode 3 [3] provides design strength formulae which
can be used in a modified way for the connections of Figure 8b to 8d.
Within the scope of this lecture it is not possible to deal with all connections in detail, however one
example is given for a connection between tubular braces and an I-section chord as shown in
Figure 9.
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The strength of the connection for axial loads at the chord intersection (cross-section A) is
governed by the effective width area:
A
eff.c
= 2 (b
m1
t
p
+ b
m2
t
w
)

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For the brace intersection the effective width is given by:
A
eff.b
= 2 (b
e1
+ b
e2
) t
p

The strength of the connection is thus given by:
N
2
sin 
2
= A
eff.c
f
yo

and
N
2
sin 
2
= A
eff.b
f
yo

The effective widths b
m1
, b
m2
, b
e1
and b
e2
are given in Eurocode 3 (6.6.8 and Appendix K, Table
K.8.2).
As an additional check the chord cross-section between the braces has to be checked for shear
and shear in combination with axial loading and bending moments, see Table K.8.2 of Eurocode 3.
The chord and braces have furthermore to satisfy the limits for d/t and h/t to avoid local buckling.
Weld failure and lamellar tearing should always be avoided by choosing full strength welds and
proper selection of the steel grade and quality.
In these cases where the joint strength is lower than the brace member strength, sufficient
rotation capacity should be available if the bending moments are neglected. Since it is difficult to
show that sufficient deformation capacity exists due to a lack of research evidence, either the
bending moments have to be incorporated in the strength assessment or the joint is stiffened to
such an extent that the joint strength is larger than the brace member strength, e.g. as shown in
Figure 10.
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8. SPECIAL CONNECTIONS
The previous sections dealt with the most common types of connection; however, depending on
the platform layout, other types of connections may be necessary. Figure 11, for example, shows
the connection between two panels of stiffened plates. Here both panels are made by (semi)
automatic welding processes. Allowance is made for welding tolerances by welding the ends of the
stringers after the fitting together of the panels. This procedure can be used for modules which are
designed using the stressed skin method.
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Special provisions are necessary for lifting the modules; padeyes or trunnions, for example, can be
provided for this purpose, as shown in Figure 12; nowadays these devices are sometimes made of
cast steel. It is important that these lifting devices are designed in such a way that they can be
connected to the deck structure at a later stage when the precise location of the centre of gravity
of the module, and the lifting method, are known.
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Strength of padeyes is often assessed by means of "Lloyds" formulae, which are presented in the
SWL (safe working load) format.
The SWL is the least of the following values of N
i
:

N
1
= 0,60 (a t
L
+ 2 b t
E
) f
y

N
2
= 1,08 (c t
L
+ (D - d) t
E
) f
y

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N
3
= 0,87 d (t
L
+ 2 t
E
) f
y

where the following limitations apply:
 1,0   8,0
 and if  1,0
then put t
L
+ 2 t
E
= d in the above formulae.

 t
E
not to exceed t
L
/2

 d
HOLE
/d
PIN
 1,05
Tubular connections are not dealt with in this lecture since these are discussed in more detail in
Lectures 13.2 and 13.3.
For offshore deck structures, built up from stiffened plate panels, reference should be made to
Lectures 8.3 and 8.4.
For living quarters and helicopter decks, use can be made of the information in the previous
sections.
9. CONCLUDING SUMMARY
 The optimal design of offshore deck structures depends, to a large extent, on the
coordination between the specialists for the various disciplines; for the layout, coordination
between structural, mechanical, electrical, fabrication, load out and installation engineers is
important.
 The structural designer has to consider the fabrication sequence; the conditions for welding
and inspection (e.g. can it be welded properly?); the consequences of the choice of material
grade and quality on the fabrication; and the various load conditions.
 In general, most connections can be designed with the basic formulae used for tubular
connections and beam-to-column connections. Background information is given in [1, 2, 9,
11 - 15].
 Recently a study has been carried out to investigate the use of RHS in deck structures [16].
This shows that the use of RHS, instead of beams, for deck trusses can be economical.
However, due to restrictions in available sizes, economical solutions are mainly found for
smaller platform sizes and for secondary steelwork such as staircase towers, access platforms
and equipment supports.
10. REFERENCES
[1] API-RP2A "Recommended Practice for Planning, Designing and Constructing Fixed Offshore
Platforms". American Petroleum Institute, 18th Edition, 1989
[2] AISC "Specification for the Design, Fabrication and Erection of Structural Steel for Buildings".
American Institute of Steel Construction, Chicago, 1980
[3] Eurocode 3: "Design of Steel Structures": ENV 1993-1-1: Part 1.1, General Rules and Rules for
Buildings, CEN, 1992.
[4] Dijkstra, O.D., Wardenier, J. "The Fatigue Behaviour of Welded Splices with and without
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Mouseholes in IPE 400 and HEM 320 beams". Paper 14 Int. Conference Weld Failures, November
1988, London
[5] Lindner, F. and Gietzeit, R. "Zur Tragfähigkeit ausgeklinkter Träger" Stahlbauwz. 1985.
[6] Kurobane, Y. "New Developments and Practices in Tubular Joint Design". IIW doc. XV-488-
81/XIII-1004-81, International Institute of Welding, 1981
[7] Rink, H.D., Wardenier, J. and Winkel, G.D. de "Numerical Investigation into the Static Strength
of Stiffened I-Beam to Column Connections". Proceedings International Symposium on Tubular
Structures, Delft, June 1991. Delft University Press.
[8] Hertogs, A.A., Puthli, R.S. and Wardenier, J. "Stress Concentration Factors in Plate to Tube
Connections". Proceedings ASME/OMAE Conference, March 1989, Vol. II, pp. 719-727
[9] Wardenier, J. "Hollow Section Joints". Delft University Press, Delft, 1982
[10] Broek, T.J. van der, Puthli, R.S. and Wardenier, J. "The Influence of Multiplanar Loading on
the Strength and Stiffness of Plate to Tubular Column Connections". Proceedings International
Conference "Welded Structures 90", London, UK, November 1990
[11] DNV "Rules for the Design, Construction and Inspection of Fixed offshore Structures" 1977
(with corrections 1982)
[12] Lloyd's Register "Rules and Regulations for the Classification of Fixed Offshore Installation".
London, July 1988
[13] IIW-XV-E "Design Recommendations for Hollow Section Joints - Predominantly Statically
Loaded - 2nd edition". 1989, IIW doc XV-701-89
[14] UEG "Design of Tubular Joints for Offshore Structures". UEG, London, 1985 (3 volumes)
[15] Voss, R.P. "Lasteinleitung in geschweisste Vollwandträger aus Stahl im Hinblick auf die
Bemessung von Lagersteifen". Ph.D-Thesis, TU Berlin D83, 1983
[16] Guy, H.D. "Structural Hollow Sections for Topside Constructions". Steel Construction Today,
1990, 4
11. ADDITIONAL READING
1. Marshall, P.W. "Design of Welded Tubular Connections: Basis and Use of AWS Provisions".
Elsvier, 1991
2. Schaap, D., Pal, A.H.M. v.d., Vries, A. de., Dague. D. and Wardenier,J. "The Design of
Amoco's 'Rijn' Production Platform". Proceedings of the International Conference on Steel and
Aluminium Structures, Cardiff, UK, 8-10 July 1987, Vol. Steel Structures
3. Paul, J.C., Valk, C.A.C. v.d., and Wardenier, J. "The Static Strength of Circular Multiplanar X-
joints". Proceedings of the third IIW International Symposium on Tubular Structures,
Lappeenranta, September 1989
Page 21 of 21 ESDEP LECTURE NOTE [WG15A]
5/23/2011 http://www.haiyangshiyou.com/esdep/master/wg15a/l1200.htm

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