Hull Damage and Repair
Content
Lloyd’s Register
Marine Training Services
Hull
Inspection,
Damage and
Repair Course
Delegate Handout – Damage and Repair
Document Version 2.0 created on 13-Sep-04
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© Lloyd’s Register 2004. All rights reserved. Except as permitted under
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transmitted, recorded or reproduced in any form or means, without the
prior permission of the copyright owner.
Disclaimer
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referred to in this clause as the ‘Lloyd's Register Group’. The Lloyd’s
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Table of Contents
1
Overview ................................................................................................... 4
2
Types of Damage ..................................................................................... 5
3
Factors Contributing to Damage and Corrosion................................. 10
4
Areas of Stress Concentration ............................................................. 15
4.1
Hard Spots .......................................................................................................... 15
4.1.1
Definition ..............................................................................................................15
Solution..............................................................................................................................15
Solution..............................................................................................................................16
4.2
4.3
Bracket Toes and Stiffeners................................................................................ 17
4.2.1
Welding Connection between Bracket or Stiffener and Plating is Fractured .......17
4.2.2
The Bracket or Stiffener Breaks the Support Element .........................................19
4.2.3
The Bracket or Stiffener Breaks the Support Plate ..............................................20
4.2.4
Bracket Breaks at Ends........................................................................................21
Change of Section............................................................................................... 22
4.3.1
4.4
4.5
Change of Thickness .......................................................................................... 23
4.4.1
Problem ................................................................................................................23
4.4.2
Solution ................................................................................................................23
Openings............................................................................................................. 24
4.5.1
4.6
4.7
Problem ................................................................................................................22
Problem ................................................................................................................24
Misalignment ....................................................................................................... 26
4.6.1
Problem ................................................................................................................26
4.6.2
Solution ................................................................................................................26
Three Planes Meeting ......................................................................................... 27
4.7.1
Problem ................................................................................................................27
4.7.2
Solution ................................................................................................................27
5
Repairing Damage ................................................................................. 28
5.1
Recommended Approaches................................................................................ 28
5.2
Recommended Repair Methods ......................................................................... 28
6
Summary................................................................................................. 30
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1
Overview
Many ships do not meet classification requirements due to the cumulative
effects of corrosion and structural damage. In many cases, this damage is
preventable with timely maintenance and an understanding of the causes of
corrosion and damage.
This document looks at:
• The type of damage sustained on ships
• The main reasons this happens
• Methods of prevention and repair
This section covers the following topics:
• Types of damage sustained on ships
• The main factors contributing to damage and corrosion
• An examination of the main areas of stress concentration, including the
fractures that can occur and how to repair them
• Recommended methods of prevention and repair
• A summary of the measures you can take to lengthen the life of an
element
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2
Types of Damage
The table below lists the main causes of damage and for each one explains
how damage is caused and the best approach to repair it.
Cause of Damage
How Damage is Caused
Prevention and Repair
Structural overload
Overload often puts greater stress on the
ship than it was designed to cope with.
Damage from overloading may be a result of
the following:
•
Grounding
You should repair damages due to
overload in accordance with the
original approved plans, except if the
damage is due to heavy or bad
weather. In this case, you should
make an assessment of the damage
to decide if the structure needs
reinforcing.
•
Collision
•
Contact (for example with the quay or
with tugs)
•
Operational overload (for example; poor
loading sequence, too high a rate of
loading, variable ballast levels during
loading).
Too little ballast can cause problems just as
easily as too much ballast.
Heavy weather also contributes to overload
damage, particularly at the forward end of
the ship.
Design faults
Poor workmanship
Vibration fatigue
Poor design can cause damage for the
following reasons:
•
The actual loads are not known
•
The design tolerances have been
exceeded
•
Standards have not been complied with
•
There are inadequacies in the initial
design
Regardless of the quality of the design of a
ship, the workmanship may cause many
problems, as there is the strong possibility of
the following:
•
Use of sub-standard materials
•
Poor alignment
•
Poor welding
•
Poor finishing and attention to detail
•
Internal deformations
There are two main sources of vibration
fatigue:
•
Hydrodynamic
•
Mechanical
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If the design does not comply with
existing IACS standards, you should
repair the damage following a review
of the original structural plans to
determine the cause of the problem.
If there are inadequacies in the
design, these should be rectified.
You can repair the damage resulting
from poor workmanship only by
correcting the causes of the original
defects.
As well as reducing the vibration, you
can eliminate damage caused by
vibration fatigue by reinforcing the
structure of the ship. It is often the
case that the ship was inadequately
designed to withstand vibration.
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Cause of Damage
How Damage is Caused
Prevention and Repair
Wear and tear
The amount of corrosion on a ship generally
indicates the amount of wear and tear.
There are different types of corrosion:
•
General wastage
You can treat localized damage
caused by wear and tear by coating
the structure or renewing and
reinforcing the corroded area.
General wastage requires the
renewal of the structure as
necessary.
•
Localised corrosion, which is
aggravated by stress on the structure
and lack of access to areas which need
protecting against corrosion.
•
Localised pitting
Ballast tanks in certain vessels need extra
attention; for example those adjacent to
heated tanks. Tanks that are protected by
sacrificial anodes but are not used
permanently for ballast may be particularly
subject to corrosion if they are not
adequately coated.
Check the following areas:
•
General cargo ships: ballast peak and
deep tank areas; side and ‘tween deck
tanks
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•
Ro-Ro and container ships: as
general cargo, plus sides of the double
hull, if used as a ballast tank, anti-roll
and/or trim tank
•
Bulk carriers and Oil/Bulk/Ore ships:
ballast, peak, deep tanks, double hull,
transverse bulkheads, and especially
the top side tanks
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Cause of Damage
How Damage is Caused
Prevention and Repair
Pitting
Pitting is generally caused
by corrosion. It can be
localised and light or
generalised and deep.
When localised pitting is confined to the bottom of a
tank, and the depth of the pitting is less than 50% of
the original plate thickness, you can fill in the pitting
with a suitable epoxy compound in accordance with
the manufacturer’s recommendations.
You can repair isolated pitting with a depth less than
50% of the original plate thickness by welding,
provided the residual thickness of the remaining plate
exceeds 6 mm. You should observe the following
‘golden rules’:
•
The pitting must be adequately prepared for
welding (usually by grinding).
•
The electrodes used must be the appropriate low
hydrogen grade for the steel of the bottom
plating.
•
No less than four runs must be deposited in each
pit.
You must always renew the affected plate when:
•
the intensity of the pitting is excessive (above
30%)
•
the pitting is deeper than 50% of the original
thickness of the plate
•
the residual thickness is less than 6 mm
After the repairs have been carried out, the
mean thickness of bottom shell plating across the
breadth of the bottom of the tank should not be less
than 85% of the original Rule thickness.
The LR surveyor would ensure that a suitable note is
made in the Hull Memoranda in the ships records.
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The diagram below shows a range of intensities of pitting.
1% pitting
3% pitting
5% pitting
10% pitting
15% pitting
20% pitting
25% pitting
30% pitting
40% pitting
50% pitting
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The diagram below shows a range of extent of breakdown of coating.
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3
Factors Contributing to Damage and Corrosion
This section looks at:
• the factors that contribute to damage and corrosion
• ways to reduce the damage they cause
Factor that Contributes
to Damage
Issues to Consider
Fatigue
The structure of a ship may fail well below the calculated safe limit of
nominal stress. This can be the result of any of the types of damage
looked at earlier; design, stress concentration, overloading, poor
workmanship and so on. In order to combat fatigue we need to
understand:
•
the different aspects of fatigue
•
the definition of fatigue
•
the fatigue curve
•
the stages of fatigue
•
the factors that affect fatigue
Aspects of Fatigue
Fatigue should be considered as a probable cause of damage when:
•
loads are alternating or cyclic
•
loads are less than the breaking load
•
fractures appear non-ductile (brittle) with no elongation
Definition Of Fatigue
Fatigue can be defined as failure under alternating or cyclic loads or the
propagation of cracks through a component due to cyclic loads.
Stages of Fatigue
There are three stages of fatigue:
•
Initiation of the crack
•
Propagation of the crack
•
Fracture
In all types of fatigue fracture, cracks start because of a build-up of
localised stress in the components of the structure.
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Factor that Contributes
to Damage
Issues to Consider
Fatigue (continued)
Fatigue Curve
The fatigue curve is used to demonstrate the relationship between stress
(σ) and the cycle of loads.
In normal operation, a ship is subject to alternating loads. These result
from cargo operations and the cyclic effect of wave loading.
LEGEND
σ
σ
= Stress
= Fatigue limit
Number of Cycles
(equivalent to Age)
The fatigue strength is the maximum stress under which the material
will fail after a specified number of cycles.
The fatigue limit is the fatigue strength for an infinite number of cycles.
For the purpose of design the fatigue strength is set at the maximum
stress under which the material will only fail after 107 cycles.
Solutions to Fatigue Damage
You need to repair damage resulting from fatigue but more importantly,
you need to prevent the damage from re-occurring by reducing or
eliminating stress on the affected area.
You can reduce stress on a structure by:
•
Increasing the thickness of the area
•
Closing any openings
•
Modifying the shape of an area or realigning the area
•
Reinforcing the structure
•
Improving the design of the structure
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Factor that Contributes
to Damage
Issues to Consider
Stress Corrosion
Corrosion at the point of stress is greater than in other areas of a ship.
This can be seen particularly where the corrosive agent (sea water) is
ever present, such as inside ballast tanks that have not been sufficiently
coated.
Wastage from corrosion leads, in turn, to more stress on the area and
therefore more corrosion, it is cyclical and the area affected becomes
worse and worse.
The Stress Cycle
STRESS
CORROSION
CORROSION
STRESS
Often the stress cycle produces fracture lines at the point of the highest
initial stress.
In other cases the area around the initial stress concentration becomes
corroded.
Areas of excessive
corrosion and probable
subsequent buckling
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Factor that Contributes
to Damage
Issues to Consider
Progressive Corrosion
Once corrosion is established it will become progressively worse unless
prevented from doing so. Corrosion will continue to develop when:
•
the coating is not in good condition
•
the load that the structure is carrying is too high
•
there is humidity and/or heat
The stress/concentration cycle applies here too: more corrosion leads to
more stress and more stress to more corrosion.
Solutions to Stress and Progressive Corrosion
The solutions to stress and progressive corrosion include the following:
•
Adequate recoating
Humidity and Heat
•
Increasing thickness
•
Increasing the radius of openings
•
Increasing the face bars of the reinforcement on the edge of the
opening
•
Closing openings
Humidity and heat can both increase corrosion. If they are present
together the effect is greater and the rate of corrosion is vastly
accelerated.
Areas on the ship which are prone to increased humidity and heat will
suffer most, for example, tanks. In general, tanks above the waterline
are more affected than those below.
The following types of tank may suffer from the effects of heat and
humidity:
•
Fore and aft peak tanks
•
Deep tanks
•
Side tanks
•
Tween deck tanks
•
Top side tanks
•
Tanks adjacent to heated fuel oil tanks
•
Ballast tanks adjacent to heated cargo tanks
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Factor that Contributes
to Damage
Issues to Consider
Stress Concentration
Stress concentrations occur where there are points of very high stress.
In any given area the stress concentration factor (SCF) is measured by
the ratio between the maximum stress and the nominal stress in the
surrounding structure.
SCF = maximum
nominal
In general, stress concentration occurs where there is a geometrical
discontinuity such as:
•
A change in thickness
•
A change in section
•
The toe of a bracket
•
An opening
Areas of stress concentration can also occur where there is surface
roughness caused by:
•
Pores
•
Cuts
•
Overheating
•
Grinding
The age of the ship is an important factor when establishing the cause of damage.
In the case of defects due to fatigue then the age of the ship gives an indication of the
actual levels of stress.
Damage occurring on a young ship may indicate that there is a structural fault, or a poor
design feature.
If damage occurs on an older ship, the damage is more likely to be age-related.
Estimating how much longer a ship is expected to stay in service may influence the type
of repair.
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4
Areas of Stress Concentration
Stress concentration is at its most intense at the point of discontinuity, often
resulting from the abruptness of the change. For this reason the following
areas may cause problems and need to be carefully checked:
• Hard spots/points
• Bracket and stiffener toes
• Points at which there is a change of section
• Areas where there is a change of thickness
• Openings
• Misalignments
• Areas where three planes meet
4.1
Hard Spots
4.1.1
Definition
A hard spot or hard area can be defined as one of two things:
• Any point or area which is rigid in a flexible or less rigid structure
• A point or area where the deflection curve of a plate is abruptly interrupted
by the effect of a very rigid member supporting the plate
In general, a hard spot can be defined as a point or area where there is an
abrupt change of rigidity.
Generally a hard spot will occur when there is a distance of more than 80 mm between
the end of a bracket and the nearest supporting member.
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Solution
The simple solution is to eliminate the hard spot, or avoid it in the first
instance. In the illustration above, there is a hard spot where the bracket toe
meets the floor. There are a number of solutions to this as shown below:
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If the hard point is not caused by a bracket toe, you can eliminate the hard
spot by doing the following:
• Spreading the load over a wider area
• Increasing the size of brackets
• Making the change in rigidity more gradual
The chosen solution will depend on the costs and the best practice given the
age of the ship.
At the design or repair stage, brackets and trunk corners should be arranged
to avoid ending on unsupported plating.
4.2
Bracket Toes and Stiffeners
Not only can bracket toes and stiffeners result in a hard spot, they are also
prone to fractures. It is therefore necessary to give them special attention.
There are four types of frequent failure. Each type of failure, with suggested
solutions, is shown below.
4.2.1
Welding Connection between Bracket or Stiffener and Plating is Fractured
In general the solution is to modify the shape of the bracket, or to add a new
one. Sometimes it is necessary to increase the size of the bracket.
In some cases it is necessary to extend the bracket to the adjacent stiffener,
or to incorporate a new stiffener at the end of the bracket.
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Examples of some typical defects and proposed repairs are shown below.
Bracket
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Stiffener
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4.2.2
The Bracket or Stiffener Breaks the Support Element
Examples of some typical defects and proposed repairs are shown below.
Bracket
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Stiffener
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4.2.3
The Bracket or Stiffener Breaks the Support Plate
Examples of some typical defects and proposed repairs are shown below.
Bracket
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Stiffener
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4.2.4
Bracket Breaks at Ends
Examples of some typical defects and proposed repairs are shown below.
Fracture
Solution
.
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4.3
Change of Section
4.3.1
Problem
A change of section is always a problem because it creates a point of stress.
If the change is abrupt, this can cause a significant increase in stress. You
should always try to ensure that any changes in section are as ‘soft’ as
possible to keep increases in stress to a minimum.
The change of section in this example is very abrupt and the resulting stress
has caused a crack.
Solution
The solution is to make the change of section more gradual. In the above
example this can be achieved by fitting a bracket.
The size of the bracket used varies according to the:
• size of the crack
• age of the vessel
• speed of crack propagation
The bracket is fitted at the point of stress at the change of section. The
younger the vessel, the longer the bracket must be.
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4.4
Change of Thickness
4.4.1
Problem
A change of thickness can lead to stress. The stress increases in proportion
to the difference in the thickness. The more abrupt the transition, the greater
the stress that is caused.
In the example below, the dramatic change in thickness in the side of the
ship has resulted in fractures and buckling.
4.4.2
Solution
The solution to an abrupt change in thickness is to lessen the change and
make the transition as gradual as possible. The chamfer ratio should be
around 3:1. If required, an insert of intermediate thickness should also be
fitted.
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4.5
Openings
4.5.1
Problem
An opening in a structure can increase stress in two ways:
• The structure in the area of the opening has to support a higher load
compared to the surrounding structure away from the opening.
• There is stress concentration at the corners of the opening.
In the example below, the hatch cover opening creates stress and results in
a fracture.
Solutions
4.5.1.1
Choosing a Solution
The area that is lost because of the opening needs to be compensated for in
some way. There are various methods available to achieve this:
• Increasing:
•
the thickness of the area around the opening stress point using an
insert
•
the grade of material
• Adding reinforcement or a doubler plate
• Welding a collar plate over the opening
• Adding a framework of stiffeners around the opening
• Changing the shape of the opening and/or increasing the distance
between openings
• Closing small openings, such as scallops, with a collar
The repair you select should depend on:
• the size & position of the opening
• what the opening is used for
• the age of the ship
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Some examples are given below.
4.5.1.2
Openings with a Small Diameter
Openings with a diameter less than d can be compensated for by fitting
closing plates in the openings of the adjacent stiffeners.
4.5.1.3
Restoring the Original Section Modulus
The other openings can be compensated for by restoring the original section
modulus by adding one or more of the following:
• Face Bar (spigot ring)
• Reinforcement (doubler) at the web
• Reinforcement (doubler) at the flange
You can also restore the original section modulus by using of the following:
• A thicker insert in the web
• A thicker and wider face bar, that is no more than 150% of the original
thickness
•
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4.6
Misalignment
4.6.1
Problem
The misalignment of structural elements causes an increase in stress, which
can be excessive in combination with the shear effect.
The examples below show how stress concentration increases in proportion
to misalignment.
To recap, the formula for calculating the stress concentration factor (SCF) in
any given area is measured by the ratio between the maximum stress and
the nominal stress in the surrounding structure:
SCF = σ maximum
σ nominal
½ T increases SCF by a
factor of 1.6
Alignment
1 T increases SCF by a
factor of 2.1
P
P
P
T
T
T
½T
T
SCF
3.2
SCF
2.0
SCF
4.3
P = Load
T = Thickness
SCF = Stress Concentration Factor
4.6.2
Solution
The solution is to correct the misalignment. In new builds misalignment
should be prevented at both the design and construction stages.
If the misalignment is not too severe, full penetration welding may repair the
damage.
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4.7
Three Planes Meeting
4.7.1
Problem
When three planes meet, such as at the intersection of a longitudinal and
transverse bulkhead and a platform deck, there is always the possibility of
the stress concentration arising. This can lead to fractures. The type of
damage depends on the size and type of the ship.
The example below shows how three planes meeting can cause a crack. The
fractures always occur in the weld and do not normally propagate more than
120 mm.
4.7.2
Solution
The solution is to fit brackets without scallops to the area on the fractured
side, ensuring that the brackets are in line with the plane on the other side to
prevent further hard spots. Extend the brackets to the nearest stiffeners.
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5
Repairing Damage
When repairing damage, your main focus should be to eliminate the cause of
the damage. Once the cause is eliminated the damage should not re-occur,
or at least not for a very long time.
If you make a repair without eliminating the cause of the damage, the damage will
certainly return in less time than it took the original damage to develop.
This section lists the recommended:
• approaches
• repair methods
5.1
Recommended Approaches
In general, when making repairs there are two approaches which when
combined will remove most possibilities of damage:
• Reduce stress by reducing the overall load on the structure
• Improve the ability of the structure to withstand the stress by putting in
steel of greater thickness or more supporting structure
5.2
Recommended Repair Methods
The main methods of repair are listed in the table below:
Type of Repair
Recommended Repair Methods
Coating Repairs
Soft coated areas that are peeling or have become unprotected
should be re-coated.
Hard coated areas that are peeling or have become unprotected
should be grit-blasted and re-coated.
Soft coating is typically a flexible painted coating such as
Floatcoat or lanoline based coating.
Pitting/Grooving Repairs
Anode Repairs
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Hard coating is typically epoxy resin, which bonds to the structure to
which it is applied so that it becomes brittle and fractures when
stretched.
•
Weld the affected area
•
Weld and coat the area
•
Fill pitting or grooving with epoxy resin
•
Renew worn anodes
•
Add new anodes to areas requiring protection
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Type of Repair
Recommended Repair Methods
Steel Repairs
If the damage is due to collision or grounding, renew the steel to the
same original thickness as given in the approved plans.
Reinforcement
If the damage is due to fatigue or stress corrosion, fit more brackets
and renew the steel to a greater thickness than the original.
•
Fit reinforcements (doublers) to weak areas
•
Add stiffeners between existing supports to reinforce the original
strength.
Reinforcements (doublers) should not be used as permanent
repairs where:
•
Areas are corroded at the threshold of or beyond the allowable
limit
•
Shell plating or tanks are holed
•
A fracture appears in the shell envelope, deck or bulkhead
Welding Repairs
Replace fillet welding with partial penetration welding or replace partial
penetration welding with full penetration welding, as appropriate to
increase the Joint Factor.
Repairs to Design Faults
The re-design of an area which has been damaged will prevent that
damage re-occurring.
The following changes should be considered where applicable:
•
Add brackets.
•
Add stiffeners.
•
Add lugs, or close openings.
•
Change the shape of the area to increase the radius, by
increasing the area, the bracket or the stiffener.
•
Improve the scantlings by increasing the size, thickness and the
grade of the material.
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6
Summary
The measures you can take to lengthen the life of an element are listed
below:
• Fit brackets where there are no brackets.
• Extend existing brackets to decrease stress.
• Fix the ends of stiffeners by adding brackets where possible.
• Increase the thickness at the ends of brackets to 40% more than the
original thickness.
• Close all unnecessary openings.
• Use full or partial penetration welding as appropriate.
• Provide chamfers or transitions to avoid an abrupt change in thickness.
• Improve the sniping of stiffeners where required.
• Soft-coat unprotected areas or areas where the soft coating is peeling.
• Touch up or re-apply hard coatings where these have started to break
down.
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© Lloyd’s Register 2004. All rights reserved. Except as permitted under
current legislation no part of this work may be photocopied, stored in a
retrieval system, published, performed in public, adapted, broadcast,
transmitted, recorded or reproduced in any form or means, without the
prior permission of the copyright owner.
Disclaimer
Lloyd's Register, its affiliates and subsidiaries and their respective
officers, employees or agents are, individually and collectively,
referred to in this clause as the ‘Lloyd's Register Group’. The Lloyd’s
Register Group assumes no responsibility and shall not be liable to any
person for any loss, damage or expense caused by reliance on the
information or advice in this document or howsoever provided, unless
that person has signed a contract with the relevant Lloyd’s Register
Group entity for the provision of this information or advice and in that
case any responsibility or liability is exclusively on the terms and
conditions set out in that contract.
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Table of Contents
1
Overview ................................................................................................... 4
2
Types of Damage ..................................................................................... 5
3
Factors Contributing to Damage and Corrosion................................. 10
4
Areas of Stress Concentration ............................................................. 15
4.1
Hard Spots .......................................................................................................... 15
4.1.1
Definition ..............................................................................................................15
Solution..............................................................................................................................15
Solution..............................................................................................................................16
4.2
4.3
Bracket Toes and Stiffeners................................................................................ 17
4.2.1
Welding Connection between Bracket or Stiffener and Plating is Fractured .......17
4.2.2
The Bracket or Stiffener Breaks the Support Element .........................................19
4.2.3
The Bracket or Stiffener Breaks the Support Plate ..............................................20
4.2.4
Bracket Breaks at Ends........................................................................................21
Change of Section............................................................................................... 22
4.3.1
4.4
4.5
Change of Thickness .......................................................................................... 23
4.4.1
Problem ................................................................................................................23
4.4.2
Solution ................................................................................................................23
Openings............................................................................................................. 24
4.5.1
4.6
4.7
Problem ................................................................................................................22
Problem ................................................................................................................24
Misalignment ....................................................................................................... 26
4.6.1
Problem ................................................................................................................26
4.6.2
Solution ................................................................................................................26
Three Planes Meeting ......................................................................................... 27
4.7.1
Problem ................................................................................................................27
4.7.2
Solution ................................................................................................................27
5
Repairing Damage ................................................................................. 28
5.1
Recommended Approaches................................................................................ 28
5.2
Recommended Repair Methods ......................................................................... 28
6
Summary................................................................................................. 30
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1
Overview
Many ships do not meet classification requirements due to the cumulative
effects of corrosion and structural damage. In many cases, this damage is
preventable with timely maintenance and an understanding of the causes of
corrosion and damage.
This document looks at:
• The type of damage sustained on ships
• The main reasons this happens
• Methods of prevention and repair
This section covers the following topics:
• Types of damage sustained on ships
• The main factors contributing to damage and corrosion
• An examination of the main areas of stress concentration, including the
fractures that can occur and how to repair them
• Recommended methods of prevention and repair
• A summary of the measures you can take to lengthen the life of an
element
Page 4 of 30
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2
Types of Damage
The table below lists the main causes of damage and for each one explains
how damage is caused and the best approach to repair it.
Cause of Damage
How Damage is Caused
Prevention and Repair
Structural overload
Overload often puts greater stress on the
ship than it was designed to cope with.
Damage from overloading may be a result of
the following:
•
Grounding
You should repair damages due to
overload in accordance with the
original approved plans, except if the
damage is due to heavy or bad
weather. In this case, you should
make an assessment of the damage
to decide if the structure needs
reinforcing.
•
Collision
•
Contact (for example with the quay or
with tugs)
•
Operational overload (for example; poor
loading sequence, too high a rate of
loading, variable ballast levels during
loading).
Too little ballast can cause problems just as
easily as too much ballast.
Heavy weather also contributes to overload
damage, particularly at the forward end of
the ship.
Design faults
Poor workmanship
Vibration fatigue
Poor design can cause damage for the
following reasons:
•
The actual loads are not known
•
The design tolerances have been
exceeded
•
Standards have not been complied with
•
There are inadequacies in the initial
design
Regardless of the quality of the design of a
ship, the workmanship may cause many
problems, as there is the strong possibility of
the following:
•
Use of sub-standard materials
•
Poor alignment
•
Poor welding
•
Poor finishing and attention to detail
•
Internal deformations
There are two main sources of vibration
fatigue:
•
Hydrodynamic
•
Mechanical
Document Version 2.0 created on 13-Sep-04
If the design does not comply with
existing IACS standards, you should
repair the damage following a review
of the original structural plans to
determine the cause of the problem.
If there are inadequacies in the
design, these should be rectified.
You can repair the damage resulting
from poor workmanship only by
correcting the causes of the original
defects.
As well as reducing the vibration, you
can eliminate damage caused by
vibration fatigue by reinforcing the
structure of the ship. It is often the
case that the ship was inadequately
designed to withstand vibration.
Page 5 of 30
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Cause of Damage
How Damage is Caused
Prevention and Repair
Wear and tear
The amount of corrosion on a ship generally
indicates the amount of wear and tear.
There are different types of corrosion:
•
General wastage
You can treat localized damage
caused by wear and tear by coating
the structure or renewing and
reinforcing the corroded area.
General wastage requires the
renewal of the structure as
necessary.
•
Localised corrosion, which is
aggravated by stress on the structure
and lack of access to areas which need
protecting against corrosion.
•
Localised pitting
Ballast tanks in certain vessels need extra
attention; for example those adjacent to
heated tanks. Tanks that are protected by
sacrificial anodes but are not used
permanently for ballast may be particularly
subject to corrosion if they are not
adequately coated.
Check the following areas:
•
General cargo ships: ballast peak and
deep tank areas; side and ‘tween deck
tanks
Page 6 of 30
•
Ro-Ro and container ships: as
general cargo, plus sides of the double
hull, if used as a ballast tank, anti-roll
and/or trim tank
•
Bulk carriers and Oil/Bulk/Ore ships:
ballast, peak, deep tanks, double hull,
transverse bulkheads, and especially
the top side tanks
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Cause of Damage
How Damage is Caused
Prevention and Repair
Pitting
Pitting is generally caused
by corrosion. It can be
localised and light or
generalised and deep.
When localised pitting is confined to the bottom of a
tank, and the depth of the pitting is less than 50% of
the original plate thickness, you can fill in the pitting
with a suitable epoxy compound in accordance with
the manufacturer’s recommendations.
You can repair isolated pitting with a depth less than
50% of the original plate thickness by welding,
provided the residual thickness of the remaining plate
exceeds 6 mm. You should observe the following
‘golden rules’:
•
The pitting must be adequately prepared for
welding (usually by grinding).
•
The electrodes used must be the appropriate low
hydrogen grade for the steel of the bottom
plating.
•
No less than four runs must be deposited in each
pit.
You must always renew the affected plate when:
•
the intensity of the pitting is excessive (above
30%)
•
the pitting is deeper than 50% of the original
thickness of the plate
•
the residual thickness is less than 6 mm
After the repairs have been carried out, the
mean thickness of bottom shell plating across the
breadth of the bottom of the tank should not be less
than 85% of the original Rule thickness.
The LR surveyor would ensure that a suitable note is
made in the Hull Memoranda in the ships records.
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The diagram below shows a range of intensities of pitting.
1% pitting
3% pitting
5% pitting
10% pitting
15% pitting
20% pitting
25% pitting
30% pitting
40% pitting
50% pitting
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The diagram below shows a range of extent of breakdown of coating.
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3
Factors Contributing to Damage and Corrosion
This section looks at:
• the factors that contribute to damage and corrosion
• ways to reduce the damage they cause
Factor that Contributes
to Damage
Issues to Consider
Fatigue
The structure of a ship may fail well below the calculated safe limit of
nominal stress. This can be the result of any of the types of damage
looked at earlier; design, stress concentration, overloading, poor
workmanship and so on. In order to combat fatigue we need to
understand:
•
the different aspects of fatigue
•
the definition of fatigue
•
the fatigue curve
•
the stages of fatigue
•
the factors that affect fatigue
Aspects of Fatigue
Fatigue should be considered as a probable cause of damage when:
•
loads are alternating or cyclic
•
loads are less than the breaking load
•
fractures appear non-ductile (brittle) with no elongation
Definition Of Fatigue
Fatigue can be defined as failure under alternating or cyclic loads or the
propagation of cracks through a component due to cyclic loads.
Stages of Fatigue
There are three stages of fatigue:
•
Initiation of the crack
•
Propagation of the crack
•
Fracture
In all types of fatigue fracture, cracks start because of a build-up of
localised stress in the components of the structure.
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Factor that Contributes
to Damage
Issues to Consider
Fatigue (continued)
Fatigue Curve
The fatigue curve is used to demonstrate the relationship between stress
(σ) and the cycle of loads.
In normal operation, a ship is subject to alternating loads. These result
from cargo operations and the cyclic effect of wave loading.
LEGEND
σ
σ
= Stress
= Fatigue limit
Number of Cycles
(equivalent to Age)
The fatigue strength is the maximum stress under which the material
will fail after a specified number of cycles.
The fatigue limit is the fatigue strength for an infinite number of cycles.
For the purpose of design the fatigue strength is set at the maximum
stress under which the material will only fail after 107 cycles.
Solutions to Fatigue Damage
You need to repair damage resulting from fatigue but more importantly,
you need to prevent the damage from re-occurring by reducing or
eliminating stress on the affected area.
You can reduce stress on a structure by:
•
Increasing the thickness of the area
•
Closing any openings
•
Modifying the shape of an area or realigning the area
•
Reinforcing the structure
•
Improving the design of the structure
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Factor that Contributes
to Damage
Issues to Consider
Stress Corrosion
Corrosion at the point of stress is greater than in other areas of a ship.
This can be seen particularly where the corrosive agent (sea water) is
ever present, such as inside ballast tanks that have not been sufficiently
coated.
Wastage from corrosion leads, in turn, to more stress on the area and
therefore more corrosion, it is cyclical and the area affected becomes
worse and worse.
The Stress Cycle
STRESS
CORROSION
CORROSION
STRESS
Often the stress cycle produces fracture lines at the point of the highest
initial stress.
In other cases the area around the initial stress concentration becomes
corroded.
Areas of excessive
corrosion and probable
subsequent buckling
Page 12 of 30
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Factor that Contributes
to Damage
Issues to Consider
Progressive Corrosion
Once corrosion is established it will become progressively worse unless
prevented from doing so. Corrosion will continue to develop when:
•
the coating is not in good condition
•
the load that the structure is carrying is too high
•
there is humidity and/or heat
The stress/concentration cycle applies here too: more corrosion leads to
more stress and more stress to more corrosion.
Solutions to Stress and Progressive Corrosion
The solutions to stress and progressive corrosion include the following:
•
Adequate recoating
Humidity and Heat
•
Increasing thickness
•
Increasing the radius of openings
•
Increasing the face bars of the reinforcement on the edge of the
opening
•
Closing openings
Humidity and heat can both increase corrosion. If they are present
together the effect is greater and the rate of corrosion is vastly
accelerated.
Areas on the ship which are prone to increased humidity and heat will
suffer most, for example, tanks. In general, tanks above the waterline
are more affected than those below.
The following types of tank may suffer from the effects of heat and
humidity:
•
Fore and aft peak tanks
•
Deep tanks
•
Side tanks
•
Tween deck tanks
•
Top side tanks
•
Tanks adjacent to heated fuel oil tanks
•
Ballast tanks adjacent to heated cargo tanks
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Factor that Contributes
to Damage
Issues to Consider
Stress Concentration
Stress concentrations occur where there are points of very high stress.
In any given area the stress concentration factor (SCF) is measured by
the ratio between the maximum stress and the nominal stress in the
surrounding structure.
SCF = maximum
nominal
In general, stress concentration occurs where there is a geometrical
discontinuity such as:
•
A change in thickness
•
A change in section
•
The toe of a bracket
•
An opening
Areas of stress concentration can also occur where there is surface
roughness caused by:
•
Pores
•
Cuts
•
Overheating
•
Grinding
The age of the ship is an important factor when establishing the cause of damage.
In the case of defects due to fatigue then the age of the ship gives an indication of the
actual levels of stress.
Damage occurring on a young ship may indicate that there is a structural fault, or a poor
design feature.
If damage occurs on an older ship, the damage is more likely to be age-related.
Estimating how much longer a ship is expected to stay in service may influence the type
of repair.
Page 14 of 30
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4
Areas of Stress Concentration
Stress concentration is at its most intense at the point of discontinuity, often
resulting from the abruptness of the change. For this reason the following
areas may cause problems and need to be carefully checked:
• Hard spots/points
• Bracket and stiffener toes
• Points at which there is a change of section
• Areas where there is a change of thickness
• Openings
• Misalignments
• Areas where three planes meet
4.1
Hard Spots
4.1.1
Definition
A hard spot or hard area can be defined as one of two things:
• Any point or area which is rigid in a flexible or less rigid structure
• A point or area where the deflection curve of a plate is abruptly interrupted
by the effect of a very rigid member supporting the plate
In general, a hard spot can be defined as a point or area where there is an
abrupt change of rigidity.
Generally a hard spot will occur when there is a distance of more than 80 mm between
the end of a bracket and the nearest supporting member.
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Solution
The simple solution is to eliminate the hard spot, or avoid it in the first
instance. In the illustration above, there is a hard spot where the bracket toe
meets the floor. There are a number of solutions to this as shown below:
Page 16 of 30
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If the hard point is not caused by a bracket toe, you can eliminate the hard
spot by doing the following:
• Spreading the load over a wider area
• Increasing the size of brackets
• Making the change in rigidity more gradual
The chosen solution will depend on the costs and the best practice given the
age of the ship.
At the design or repair stage, brackets and trunk corners should be arranged
to avoid ending on unsupported plating.
4.2
Bracket Toes and Stiffeners
Not only can bracket toes and stiffeners result in a hard spot, they are also
prone to fractures. It is therefore necessary to give them special attention.
There are four types of frequent failure. Each type of failure, with suggested
solutions, is shown below.
4.2.1
Welding Connection between Bracket or Stiffener and Plating is Fractured
In general the solution is to modify the shape of the bracket, or to add a new
one. Sometimes it is necessary to increase the size of the bracket.
In some cases it is necessary to extend the bracket to the adjacent stiffener,
or to incorporate a new stiffener at the end of the bracket.
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Examples of some typical defects and proposed repairs are shown below.
Bracket
Page 18 of 30
Stiffener
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4.2.2
The Bracket or Stiffener Breaks the Support Element
Examples of some typical defects and proposed repairs are shown below.
Bracket
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Stiffener
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4.2.3
The Bracket or Stiffener Breaks the Support Plate
Examples of some typical defects and proposed repairs are shown below.
Bracket
Page 20 of 30
Stiffener
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4.2.4
Bracket Breaks at Ends
Examples of some typical defects and proposed repairs are shown below.
Fracture
Solution
.
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4.3
Change of Section
4.3.1
Problem
A change of section is always a problem because it creates a point of stress.
If the change is abrupt, this can cause a significant increase in stress. You
should always try to ensure that any changes in section are as ‘soft’ as
possible to keep increases in stress to a minimum.
The change of section in this example is very abrupt and the resulting stress
has caused a crack.
Solution
The solution is to make the change of section more gradual. In the above
example this can be achieved by fitting a bracket.
The size of the bracket used varies according to the:
• size of the crack
• age of the vessel
• speed of crack propagation
The bracket is fitted at the point of stress at the change of section. The
younger the vessel, the longer the bracket must be.
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4.4
Change of Thickness
4.4.1
Problem
A change of thickness can lead to stress. The stress increases in proportion
to the difference in the thickness. The more abrupt the transition, the greater
the stress that is caused.
In the example below, the dramatic change in thickness in the side of the
ship has resulted in fractures and buckling.
4.4.2
Solution
The solution to an abrupt change in thickness is to lessen the change and
make the transition as gradual as possible. The chamfer ratio should be
around 3:1. If required, an insert of intermediate thickness should also be
fitted.
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4.5
Openings
4.5.1
Problem
An opening in a structure can increase stress in two ways:
• The structure in the area of the opening has to support a higher load
compared to the surrounding structure away from the opening.
• There is stress concentration at the corners of the opening.
In the example below, the hatch cover opening creates stress and results in
a fracture.
Solutions
4.5.1.1
Choosing a Solution
The area that is lost because of the opening needs to be compensated for in
some way. There are various methods available to achieve this:
• Increasing:
•
the thickness of the area around the opening stress point using an
insert
•
the grade of material
• Adding reinforcement or a doubler plate
• Welding a collar plate over the opening
• Adding a framework of stiffeners around the opening
• Changing the shape of the opening and/or increasing the distance
between openings
• Closing small openings, such as scallops, with a collar
The repair you select should depend on:
• the size & position of the opening
• what the opening is used for
• the age of the ship
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Some examples are given below.
4.5.1.2
Openings with a Small Diameter
Openings with a diameter less than d can be compensated for by fitting
closing plates in the openings of the adjacent stiffeners.
4.5.1.3
Restoring the Original Section Modulus
The other openings can be compensated for by restoring the original section
modulus by adding one or more of the following:
• Face Bar (spigot ring)
• Reinforcement (doubler) at the web
• Reinforcement (doubler) at the flange
You can also restore the original section modulus by using of the following:
• A thicker insert in the web
• A thicker and wider face bar, that is no more than 150% of the original
thickness
•
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4.6
Misalignment
4.6.1
Problem
The misalignment of structural elements causes an increase in stress, which
can be excessive in combination with the shear effect.
The examples below show how stress concentration increases in proportion
to misalignment.
To recap, the formula for calculating the stress concentration factor (SCF) in
any given area is measured by the ratio between the maximum stress and
the nominal stress in the surrounding structure:
SCF = σ maximum
σ nominal
½ T increases SCF by a
factor of 1.6
Alignment
1 T increases SCF by a
factor of 2.1
P
P
P
T
T
T
½T
T
SCF
3.2
SCF
2.0
SCF
4.3
P = Load
T = Thickness
SCF = Stress Concentration Factor
4.6.2
Solution
The solution is to correct the misalignment. In new builds misalignment
should be prevented at both the design and construction stages.
If the misalignment is not too severe, full penetration welding may repair the
damage.
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4.7
Three Planes Meeting
4.7.1
Problem
When three planes meet, such as at the intersection of a longitudinal and
transverse bulkhead and a platform deck, there is always the possibility of
the stress concentration arising. This can lead to fractures. The type of
damage depends on the size and type of the ship.
The example below shows how three planes meeting can cause a crack. The
fractures always occur in the weld and do not normally propagate more than
120 mm.
4.7.2
Solution
The solution is to fit brackets without scallops to the area on the fractured
side, ensuring that the brackets are in line with the plane on the other side to
prevent further hard spots. Extend the brackets to the nearest stiffeners.
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5
Repairing Damage
When repairing damage, your main focus should be to eliminate the cause of
the damage. Once the cause is eliminated the damage should not re-occur,
or at least not for a very long time.
If you make a repair without eliminating the cause of the damage, the damage will
certainly return in less time than it took the original damage to develop.
This section lists the recommended:
• approaches
• repair methods
5.1
Recommended Approaches
In general, when making repairs there are two approaches which when
combined will remove most possibilities of damage:
• Reduce stress by reducing the overall load on the structure
• Improve the ability of the structure to withstand the stress by putting in
steel of greater thickness or more supporting structure
5.2
Recommended Repair Methods
The main methods of repair are listed in the table below:
Type of Repair
Recommended Repair Methods
Coating Repairs
Soft coated areas that are peeling or have become unprotected
should be re-coated.
Hard coated areas that are peeling or have become unprotected
should be grit-blasted and re-coated.
Soft coating is typically a flexible painted coating such as
Floatcoat or lanoline based coating.
Pitting/Grooving Repairs
Anode Repairs
Page 28 of 30
Hard coating is typically epoxy resin, which bonds to the structure to
which it is applied so that it becomes brittle and fractures when
stretched.
•
Weld the affected area
•
Weld and coat the area
•
Fill pitting or grooving with epoxy resin
•
Renew worn anodes
•
Add new anodes to areas requiring protection
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Type of Repair
Recommended Repair Methods
Steel Repairs
If the damage is due to collision or grounding, renew the steel to the
same original thickness as given in the approved plans.
Reinforcement
If the damage is due to fatigue or stress corrosion, fit more brackets
and renew the steel to a greater thickness than the original.
•
Fit reinforcements (doublers) to weak areas
•
Add stiffeners between existing supports to reinforce the original
strength.
Reinforcements (doublers) should not be used as permanent
repairs where:
•
Areas are corroded at the threshold of or beyond the allowable
limit
•
Shell plating or tanks are holed
•
A fracture appears in the shell envelope, deck or bulkhead
Welding Repairs
Replace fillet welding with partial penetration welding or replace partial
penetration welding with full penetration welding, as appropriate to
increase the Joint Factor.
Repairs to Design Faults
The re-design of an area which has been damaged will prevent that
damage re-occurring.
The following changes should be considered where applicable:
•
Add brackets.
•
Add stiffeners.
•
Add lugs, or close openings.
•
Change the shape of the area to increase the radius, by
increasing the area, the bracket or the stiffener.
•
Improve the scantlings by increasing the size, thickness and the
grade of the material.
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6
Summary
The measures you can take to lengthen the life of an element are listed
below:
• Fit brackets where there are no brackets.
• Extend existing brackets to decrease stress.
• Fix the ends of stiffeners by adding brackets where possible.
• Increase the thickness at the ends of brackets to 40% more than the
original thickness.
• Close all unnecessary openings.
• Use full or partial penetration welding as appropriate.
• Provide chamfers or transitions to avoid an abrupt change in thickness.
• Improve the sniping of stiffeners where required.
• Soft-coat unprotected areas or areas where the soft coating is peeling.
• Touch up or re-apply hard coatings where these have started to break
down.
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Document Version 2.0 created on 13-Sep-04
