hull damage and repair

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Lloyds 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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Hull Inspection, Damage and Repair Course - Damage and Repair

Page 2 of 30 Document Version 2.0 created on 13-Sep-04

Lloyds Register 2004. All rights reserved. Except as permitted undercurrent legislation no part of this work may be photocopied, stored in aretrieval system, published, performed in public, adapted, broadcast,

transmitted, recorded or reproduced in any form or means, without theprior 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 LloydsRegister Group assumes no responsibility and shall not be liable to anyperson for any loss, damage or expense caused by reliance on theinformation or advice in this document or howsoever provided, unlessthat person has signed a contract with the relevant Lloyds RegisterGroup entity for the provision of this information or advice and in that

case any responsibility or liability is exclusively on the terms andconditions set out in that contract.


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Document Version 2.0 created on 13-Sep-04 Page 3 of 30

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 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

4.3 Change of Section............................................................................................... 22

4.3.1 Problem................................................................................................................22

4.4 Change of Thickness .......................................................................................... 23

4.4.1 Problem................................................................................................................23

4.4.2

Solution ................................................................................................................23

4.5 Openings............................................................................................................. 24

4.5.1 Problem................................................................................................................24

4.6 Misalignment ....................................................................................................... 26

4.6.1 Problem................................................................................................................26

4.6.2 Solution ................................................................................................................26

4.7 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 cumulativeeffects of corrosion and structural damage. In many cases, this damage is

preventable with timely maintenance and an understanding of the causes ofcorrosion 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 thefractures 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 anelement


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2 Types of Damage

The table below lists the main causes of damage and for each one explainshow 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 theship than it was designed to cope with.Damage from overloading may be a result ofthe following:

Grounding

Collision

Contact (for example with the quay orwith tugs)

Operational overload (for example; poorloading sequence, too high a rate ofloading, variable ballast levels duringloading).

Too little ballast can cause problems just aseasily as too much ballast.

Heavy weather also contributes to overloaddamage, particularly at the forward end ofthe ship.

You should repair damages due tooverload in accordance with theoriginal approved plans, except if thedamage is due to heavy or badweather. In this case, you shouldmake an assessment of the damageto decide if the structure needsreinforcing.

Design faults Poor design can cause damage for thefollowing reasons:

The actual loads are not known

The design tolerances have beenexceeded

Standards have not been complied with

There are inadequacies in the initialdesign

If the design does not comply withexisting IACS standards, you should

repair the damage following a reviewof the original structural plans todetermine the cause of the problem.

If there are inadequacies in thedesign, these should be rectified.

Poor workmanship Regardless of the quality of the design of aship, the workmanship may cause manyproblems, as there is the strong possibility ofthe following:

Use of sub-standard materials

Poor alignment

Poor welding

Poor finishing and attention to detail

Internal deformations

You can repair the damage resultingfrom poor workmanship only bycorrecting the causes of the originaldefects.

Vibration fatigue There are two main sources of vibrationfatigue:

Hydrodynamic

Mechanical

As well as reducing the vibration, youcan eliminate damage caused byvibration fatigue by reinforcing thestructure of the ship. It is often thecase that the ship was inadequatelydesigned to withstand vibration.


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Wear and tear The amount of corrosion on a ship generallyindicates the amount of wear and tear.There are different types of corrosion:

General wastage

Localised corrosion, which isaggravated by stress on the structureand lack of access to areas which needprotecting against corrosion.

Localised pitting

Ballast tanks in certain vessels need extraattention; for example those adjacent toheated tanks. Tanks that are protected bysacrificial anodes but are not usedpermanently for ballast may be particularlysubject to corrosion if they are not

adequately coated.

Check the following areas:

General cargo ships: ballast peak anddeep tank areas; side and tween decktanks

Ro-Ro and container ships: asgeneral cargo, plus sides of the doublehull, if used as a ballast tank, anti-rolland/or trim tank

Bulk carriers and Oil/Bulk/Ore ships:ballast, peak, deep tanks, double hull,transverse bulkheads, and especiallythe top side tanks

You can treat localized damagecaused by wear and tear by coatingthe structure or renewing andreinforcing the corroded area.General wastage requires therenewal of the structure asnecessary.


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Pitting Pitting is generally caused

by corrosion. It can belocalised and light orgeneralised and deep.

When localised pitting is confined to the bottom of a

tank, and the depth of the pitting is less than 50% ofthe original plate thickness, you can fill in the pittingwith a suitable epoxy compound in accordance withthe manufacturers recommendations.

You can repair isolated pitting with a depth less than50% of the original plate thickness by welding,provided the residual thickness of the remaining plateexceeds 6 mm. You should observe the followinggolden rules:

The pitting must be adequately prepared forwelding (usually by grinding).

The electrodes used must be the appropriate lowhydrogen grade for the steel of the bottomplating.

No less than four runs must be deposited in eachpit.

You must always renew the affected plate when:

the intensity of the pitting is excessive (above30%)

the pitting is deeper than 50% of the originalthickness of the plate

the residual thickness is less than 6 mm

After the repairs have been carried out, themean thickness of bottom shell plating across thebreadth of the bottom of the tank should not be lessthan 85% of the original Rule thickness.

The LR surveyor would ensure that a suitable note ismade 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 Contributesto Damage

Issues to Consider

Fatigue The structure of a ship may fail well below the calculated safe limit ofnominal stress. This can be the result of any of the types of damagelooked at earlier; design, stress concentration, overloading, poorworkmanship and so on. In order to combat fatigue we need tounderstand:

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 thepropagation 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 oflocalised stress in the components of the structure.


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Issues to Consider

Fatigue (continued) Fatigue Curve

The fatigue curve is used to demonstrate the relationship between stress() and the cycle of loads.

In normal operation, a ship is subject to alternating loads. These resultfrom cargo operations and the cyclic effect of wave loading.

The fatigue strengthis the maximum stress under which the materialwill fail after a specified number of cycles.

The fatigue limitis the fatigue strength for an infinite number of cycles.

For the purpose of design the fatigue strength is set at the maximumstress under which the material will only fail after 10

7cycles.

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 oreliminating stresson 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

LEGEND

= Stress

= Fatigue limit

Number of Cycles(equivalent to Age)


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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) isever present, such as inside ballast tanks that have not been sufficientlycoated.

Wastage from corrosion leads, in turn, to more stress on the area andtherefore more corrosion, it is cyclical and the area affected becomesworse and worse.

The Stress Cycle

Often the stress cycle produces fracture lines at the point of the highestinitial stress.

In other cases the area around the initial stress concentration becomescorroded.

STRESS

CORROSION CORROSION

STRESS

Areas of excessivecorrosion and probablesubsequent buckling


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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 tomore 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

Increasing thickness

Increasing the radius of openings

Increasing the face bars of the reinforcement on the edge of theopening

Closing openings

Humidity and Heat Humidity and heat can both increase corrosion. If they are presenttogether the effect is greater and the rate of corrosion is vastlyaccelerated.

Areas on the ship which are prone to increased humidity and heat willsuffer most, for example, tanks. In general, tanks above the waterlineare more affected than those below.

The following types of tank may suffer from the effects of heat andhumidity:

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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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 bythe ratio between the maximum stress and the nominal stress in thesurrounding structure.

SCF = maximumnominal

In general, stress concentration occurs where there is a geometricaldiscontinuity 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 surfaceroughness 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 theactual levels of stress.

Damage occurring on a young ship may indicate that there is a structural fault, or a poordesign 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 typeof repair.


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4 Areas of Stress Concentration

Stress concentration is at its most intense at the point of discontinuity, oftenresulting 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 interruptedby 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 betweenthe 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 firstinstance. In the illustration above, there is a hard spot where the bracket toemeets 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 theage of the ship.

At the design or repair stage, brackets and trunk corners should be arrangedto avoid ending on unsupported plating.

4.2 Bracket Toes and StiffenersNot only can bracket toes and stiffeners result in a hard spot, they are alsoprone to fractures. It is therefore necessary to give them special attention.

There are four types of frequent failure. Each type of failure, with suggestedsolutions, is shown below.

4.2.1 Welding Connection between Bracket o r Stiffener and Plating is Fractured

In general the solution is to modify the shape of the bracket, or to add a newone. 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 Stiffener


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4.2.2 The Bracket or Stiffener Breaks the Support Element


Bracket Stiffener


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4.2.3 The Bracket or Stiffener Breaks the Support Plate


Bracket Stiffener


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4.2.4 Bracket Breaks at Ends


Fracture

Solution

.


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4.3 Change of Section

4.3.1 ProblemA 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. Youshould always try to ensure that any changes in section are as soft aspossible to keep increases in stress to a minimum.

The change of section in this example is very abrupt and the resulting stresshas caused a crack.

Solution

The solution is to make the change of section more gradual. In the aboveexample 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. Theyounger 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 proportionto the difference in the thickness. The more abrupt the transition, the greaterthe stress that is caused.

In the example below, the dramatic change in thickness in the side of theship has resulted in fractures and buckling.

4.4.2 Solution

The solution to an abrupt change in thickness is to lessen the change andmake the transition as gradual as possible. The chamfer ratio should bearound 3:1. If required, an insert of intermediate thickness should also befitted.


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4.5 Openings

4.5.1 ProblemAn opening in a structure can increase stress in two ways:

The structure in the area of the opening has to support a higher loadcompared 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 ina fracture.

Solutions

4.5.1.1

Choosing a SolutionThe area that is lost because of the opening needs to be compensated for insome way. There are various methods available to achieve this:

Increasing:

the thickness of the area around the opening stress point using aninsert

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 distancebetween 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 dcan be compensated for by fittingclosing 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 originalthickness


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4.6 Misalignment

4.6.1 ProblemThe misalignment of structural elements causes an increase in stress, whichcan be excessive in combination with the shear effect.

The examples below show how stress concentration increases in proportionto misalignment.

To recap, the formula for calculating the stress concentration factor (SCF) inany given area is measured by the ratio between the maximum stress andthe nominal stress in the surrounding structure:

SCF = maximumnominal

T increases SCF by afactor of 1.6

1 T increases SCF by afactor of 2.1

Alignment

4.6.2 Solution

The solution is to correct the misalignment. In new builds misalignmentshould be prevented at both the design and construction stages.

If the misalignment is not too severe, full penetration welding may repair thedamage.

T

P

SCF3.2

T

T

P

SCF4.3

T

P

T

SCF2.0

P = Load

T = Thickness

SCF = Stress Concentration Factor


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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 andtransverse bulkhead and a platform deck, there is always the possibility ofthe stress concentration arising. This can lead to fractures. The type ofdamage depends on the size and type of the ship.

The example below shows how three planes meeting can cause a crack. Thefractures always occur in the weld and do not normally propagate more than120 mm.

4.7.2 SolutionThe solution is to fit brackets without scallops to the area on the fracturedside, ensuring that the brackets are in line with the plane on the other side toprevent 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 ofthe 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 willcertainly return in less time than it took the original damage to develop.

This section lists the recommended:

approaches

repair methods

5.1 Recommended ApproachesIn general, when making repairs there are two approaches which whencombined 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 insteel 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 unprotectedshould be re-coated.

Hard coated areas that are peeling or have become unprotectedshould be grit-blasted and re-coated.

Soft coating is typically a flexible painted coating such asFloatcoat or lanoline based coating.

Hard coating is typically epoxy resin, which bonds to the structure towhich it is applied so that it becomes brittle and fractures whenstretched.

Pitting/Grooving Repairs Weld the affected area

Weld and coat the area

Fill pitting or grooving with epoxy resin

Anode Repairs 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 thesame original thickness as given in the approved plans.

If the damage is due to fatigue or stress corrosion, fit more bracketsand renew the steel to a greater thickness than the original.

Reinforcement Fit reinforcements (doublers) to weak areas

Add stiffeners between existing supports to reinforce the originalstrength.

Reinforcements (doublers) should not be used as permanentrepairs where:

Areas are corroded at the threshold of or beyond the allowablelimit

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 partialpenetration welding with full penetration welding, as appropriate toincrease the Joint Factor.

Repairs to Design Faults The re-design of an area which has been damaged will prevent thatdamage 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 thegrade of the material.


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6 Summary

The measures you can take to lengthen the life of an element are listedbelow:

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 theoriginal 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 breakdown.

hull damage and repair

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