ALERT - TRIMIS · alert tca5-ct-2006-031459 alert assessment of life-cycle effect of repairs on tankers coordination action thematic priority: sustainable development, global change

ALERT

TCA5-CT-2006-031459

ALERT

ASSESSMENT OF LIFE-CYCLE EFFECT OF REPAIRS ON TANKERS

COORDINATION ACTION THEMATIC PRIORITY: SUSTAINABLE DEVELOPMENT, GLOBAL CHANGE & ECOSYSTEMS

DELIVERABLE 5.3 – CONSOLIDATED REPORT ON THE R&D REQUIREMENTS AND PRIORITIES DOCUMENT ID CODE: 5-3-RD-2008-01-01-2

Due date of deliverable: 31-10-2008 Actual submission date: 17-10-2008 Start date of project: 1-11-2006 Duration: 2 Years UNEW Revision [FINAL]

Project co-funded by the European Commission within the Sixth Framework Programme (2002-2006)

Dissemination Level PU Public x PP Restricted to other programme participants (including the Commission Services) RE Restricted to a group specified by the consortium (including the Commission

Services)

CO Confidential, only for members of the consortium (including the Commission Services)

ALERT – D5.3 Consolidated Report on the R&D Requirements and Priorities

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AUTHORS: Name Company Jon Downes UNEW REVIEWING/APPROVAL OF REPORT: Name Company Approved Date Bjarne Thygesen INTERTANKO 30-10-2008 Correia Rodrigues LISNAVE 30-10-2008 Nabile Hifi NAME-SSRC 31-10-2008 Nigel Barltrop NAME-SSRC 31-10-2008 DOCUMENT HISTORY: Revision Date Company Initials Revised pages Short description of

changes 0 22-10-2008 UNEW JD all Initial draft for

comment 1 30-10-2008 UNEW JD all Incorporation of review

comments received 2 06-11-2008 UNEW JD all Final Version DISCLAIMER Use of any knowledge, information or data contained in this document shall be at the user's sole risk. Neither the ALERT Consortium nor any of its members, their officers, employees or agents accept shall be liable or responsible, in negligence or otherwise, for any loss, damage or expense whatever sustained by any person as a result of the use, in any manner or form, of any knowledge, information or data contained in this document, or due to any inaccuracy, omission or error therein contained. The European Community shall not in any way be liable or responsible for the use of any such knowledge, information or data, or of the consequences thereof.



CONTENTS

EXECUTIVE SUMMARY .................................................................................................................... 5

1. SCOPE ........................................................................................................................................... 6

1.1. ASSESSMENT OF LIFE-CYCLE EFFECT OF REPAIRS ON TANKERS: ALERT ............................. 6 1.2. SCOPE OF THIS REPORT .......................................................................................................... 7

2. RESEARCH DOMAIN DESCRIPTION .................................................................................... 8

3. STATE OF THE ART ................................................................................................................ 10

3.1. DETECTION OF WELDING DEFECTS BY MEANS OF NON-DESTRUCTIVE TESTING ............... 10 3.1.1. NDT Examination Requirements .................................................................................... 10 3.1.2. Material Surface Preparation For NDT ......................................................................... 11 3.1.3. NDT Operators Competence .......................................................................................... 11 3.1.4. Probability Of Detection ................................................................................................. 11

3.2. CORROSION .......................................................................................................................... 12 3.2.1. Ballast Tanks .................................................................................................................. 12 3.2.2. Ballast Tank Coating – From Voluntary To Compulsory ............................................... 12 3.2.3. Coating And Anodes ....................................................................................................... 13 3.2.4. Coating Working Principle ............................................................................................. 13 3.2.5. Root Cause Of Failure .................................................................................................... 13 3.2.6. Management Tools – Software ....................................................................................... 13 3.2.7. Guidelines & Paint Inspector Certification .................................................................... 13 3.2.8. Ballast Tank Survey Requirement & Surveyor Competence ........................................... 14 3.2.9. Past Decisions ................................................................................................................ 14 3.2.10. Corrosion Coupons And Inhibitors ............................................................................ 15 3.2.11. Port State Control ...................................................................................................... 15 3.2.12. Ballast Water Exchange ............................................................................................. 15 3.2.13. Corrosion Prevention – Inert gas .............................................................................. 16 3.2.14. Ballast Tank Coating – Added Value ......................................................................... 16 3.2.15. Cargo tank Coating – Single Hull .............................................................................. 16 3.2.16. Cargo tank Corrosion – Double Hull ........................................................................ 16 3.2.17. Oil Cargo Tank Coating ............................................................................................ 17 3.2.18. Access And Common Structural Rules ....................................................................... 17 3.2.19. Exxon Valdez – OPA 90 and Imo Double Hull Requirement ..................................... 17 3.2.20. Erika And Prestige – Regulatory Consequences ........................................................ 17 3.2.21. Performance Standard For Protective Coatings – SOLAS Amendments ................... 18 3.2.22. Cargo tank Coating.................................................................................................... 18 3.2.23. Progressive Owners ................................................................................................... 19 3.2.24. Hull Flexing ............................................................................................................... 19 3.2.25. OCIMF - Scrutiny of Oil Tankers and Tanker Management ..................................... 19 3.2.26. OCIMF Sire Inspection .............................................................................................. 20 3.2.27. Tanker Management Self Assessment (TMSA) – KPI ................................................ 20 3.2.28. Feedback To Ship Builder .......................................................................................... 21 3.2.29. Environmental Impact Of Replacing Corroded Steel................................................. 21

3.3. STRUCTURAL ASSESSMENT METHODS ................................................................................. 21 3.3.1. Global Strength ............................................................................................................... 21 3.3.2. Local Strength ................................................................................................................. 21 3.3.3. Fatigue Strength ............................................................................................................. 22 3.3.4. Residual Stresses ............................................................................................................ 22

3.4. THROUGH LIFE MANAGEMENT ............................................................................................ 22

4. IDENTIFICATION OF GAPS IN THE REQUIRED KNOWLEDGE ................................. 28

4.1. TECHNOLOGIES FOR IN-SERVICE CRACK DETECTION .......................................................... 28 4.1.1. Preferred Techniques And Compatibility With Present Regulations And Practices ...... 29 4.1.2. Techniques Requiring Further Development .................................................................. 29

4.2. STRUCTURAL ASSESSMENT METHODS ................................................................................. 34



4.3. THROUGH LIFE MANAGEMENT ............................................................................................ 36 4.4. CLASSIFICATION SOCIETY NEEDS ......................................................................................... 39

4.4.1. TANKER CONVERSIONS .............................................................................................. 39 4.4.2. SENSITIVITY ANALYSIS AND RULE REVIEW/MAKING ............................................ 39

4.5. REPAIR YARD NEEDS ............................................................................................................ 40 4.5.1. Overview ......................................................................................................................... 40 4.5.2. PLANNING ..................................................................................................................... 40 4.5.3. KNOWING THE REPAIR ............................................................................................... 41 4.5.4. REPAIR DEFINITION .................................................................................................... 41

4.6. OWNERS NEEDS .................................................................................................................... 42 4.6.1. BASIC PRINCIPLES ...................................................................................................... 42 4.6.2. REPAIRING CONSTRUCTION DEFECTS ................................................................... 43 4.6.3. DATA SHARING ............................................................................................................. 43 4.6.4. STANDARDISATION ..................................................................................................... 44

5. FUTURE RESEARCH AND DEVELOPMENT NEEDS ....................................................... 45

5.1. OVERVIEW ........................................................................................................................... 45 5.2. INFORMATION GATHERING AND HANDLING FOR DAMAGES AND REPAIRS ............................. 45 5.3. CONDITION MONITORING OF SHIPS ...................................................................................... 46 5.4. STRUCTURAL ASSESSMENT METHODS ................................................................................. 48 5.5. THROUGH LIFE MANAGEMENT ............................................................................................. 49

6. CONCLUSIONS ......................................................................................................................... 50

7. REFERENCES ............................................................................................................................ 51



EXECUTIVE SUMMARY The Coordination Action project ALERT has been undertaken in response to the needs identified following various casualty investigations for better understanding in areas of the detecting of defects and weaknesses during and after survey and after repairs, the reduction of any adverse effects of repairs, and current strength requirements for deck opening securing arrangements. The primary objectives of the ALERT project have been:

• To undertake a thorough examination of current practices in the field of ship repair and to propose improvements to the underlying processes in consultation with industry.

• Review existing and emerging technologies appropriate for ship repair practices and to propose areas for the development of technologies for future application

• Improve the efficiency of tankers by considering inspection, maintenance and re-pair scheduling.

• Consider a framework that will be capable of determining the extent of repair work that an existing ship could safely undergo with minimum additional risk of structural failure for the rest of her service life, in a rational way

• Encourage best practice in the tanker shipping and ship repair community. • To effectively disseminate the results and facilitate the acceptance by

European society and by industry. It can be seen that the ALERT project (Assessment of Life-cycle Effect of Repairs on Tankers) has undertaken a thorough examination of current practices in the field of ship repair. The project has critically reviewed the current and emerging technologies in the areas of Ship Repair Practices (WP1), Condition Monitoring of Ships (WP2), Structural Assessment Methodologies (WP3) and Through Life Management (WP4). Furthermore, the project has disseminated the results through publications, seminars and prepared R&D project proposals (WP5). The project has then used this information to initially identify gaps in current knowledge and then to propose and prioritise the future R&D needs and developments in each of the areas covered by the work packages. This report brings together the information developed by each of the work packages in the ALERT project into a single consolidated report. It is intended that this consolidated report will form the basis for future research into the effect of repairs on ships. ALERT is the acronym for Assessment of Life-cycle Effect of Repairs on Tankers, supported by the European Commission under the Sustainable development, Global change and Ecosystems thematic area, Sustainable Surface Transport Programme of the 6TH Framework Programme. The support is given under the scheme of CA, Contract No. TCA5-CT-2006-031459.



1. SCOPE 1.1. ASSESSMENT OF LIFE-CYCLE EFFECT OF REPAIRS ON

TANKERS: ALERT In past tanker accidents in European waters with serious consequences such as the Erika and Prestige, structural deficiencies may have largely contributed to the accident. Casualty investigation after the Prestige accident identified the need for better understanding in areas of the detecting of defects and weaknesses during and after survey and after repairs, the reduction of any adverse effects of repairs, current strength requirements for deck opening securing arrangements, etc. This Coordination Action project ALERT was undertaken in response to this need. The ALERT project (Assessment of Life-cycle Effect of Repairs on Tankers) has undertaken a thorough examination of current practices in the field of ship repair. The project has critically reviewed the current and emerging technologies, identified and prioritised future R&D needs and developments in the areas of Ship Repair Practices (WP1), Condition Monitoring of Ships (WP2), Structural Assessment Methodologies (WP3) and Through Life Management (WP4). Finally, the project has disseminated the results through publications, seminars and workshops and prepared R&D project proposals (WP5). The objectives of the ALERT project have been:

• To undertake a thorough examination of current practices in the field of ship repair and to propose improvements to the underlying processes in consultation with industry.

• Review existing and emerging technologies appropriate for ship repair practices and to propose areas for the development of technologies for future application

• Sense the development of loss of structural integrity by reviewing existing and emerging technologies and proposing solutions for the future.

• Improve the efficiency of tankers by considering inspection, maintenance and re-pair scheduling.

• Consider a framework that will be capable of determining the extent of repair work that an existing ship could safely undergo with minimum additional risk of structural failure for the rest of her service life, in a rational way

• Promote a safe transportation system for Europe, • Reduce human losses, injuries and environmental damage risk associated with

transportation of hazardous goods by tankers • Encourage best practice in the tanker shipping and ship repair community. • To effectively disseminate the results and facilitate the acceptance by

European society and by industry. • To coordinate these efforts and to demonstrate the positive effect of this

coordination through the participation in integrated projects (IPs), Networks of Excellence (NoE), Strategic Targeted Research Projects (STREPs) and other Coordinated Actions (CAs) for effective distribution of best practice.



ALERT Partners:

Participant Role

Participant no.

Participant name Participant short name

Country

CO (Coordinator)

1 University of Newcastle UNEW UK

TCO (Technical Coordinator)

2 University of Strathclyde NAME-SSRC UK

ECO (Exploitation Coordinator)

3 Bahamas Maritime Authority

BMA Bahamas

CR (Contractor)

4 Bureau Veritas BV France

CR 5 Materiaal Metingen Europe

MME Netherlands

CR 7 International Association of Independent Tanker Owners

INTERTANKO Norway

CR 8 Alpha Marine Services Ltd

AMS Greece

CR 9 Technical University of Hamburg-Harburg

TUHH Germany

CR 10 Lisnave Estalerios Navalis SA

LISNAVE Portugal

Furthermore, the IMO Secretariat is participating as an observer/advisor to the project. The project website is available at: http://alert.ncl.ac.uk This contains both public sections and partner only sections in order to both inform the public about the project but also to facilitate communication between project partners.

1.2. SCOPE OF THIS REPORT This report brings together the information developed by each of the work packages in the ALERT project into a single consolidated report. It is intended that this consolidated report will form the basis for future research into the effect of repairs on ships.

http://alert.ncl.ac.uk/�



2. RESEARCH DOMAIN DESCRIPTION The ALERT project consisted of 5 Work packages covering the following areas of relevance to the projects needs: WP1 investigated existing ship repair practices. Specifically, critical reviewing the current knowledge and understanding, and identifying future research and development needs in the following areas

• The standard repair practices and of classification society procedural requirements. Particular areas of interest will include extent of cropping of wasted steel; adequacy of current requirements for ensuring structural continuity in way of repairs; adequacy of current requirements for ensuring the avoidance of stress concentrations in way of the joins between new and old steel; acceptable tolerances in way of repairs. Evaluation of alternative methods of repair such as straps, renewal of alternate wasted stiffening members, steel renewal procedure afloat (immersed parts) or under extreme ambient conditions (low temperature, excessive humidity) that are currently exist but not adopted as standard by ship repairers.

• Consideration and implications of common repair, inspection and maintenance procedures, requirements and acceptance by the classification societies.

WP2 studied the condition monitoring of ships. Areas such as non-destructive testing of welds, means of detecting fatigue cracks and recording presence of fatigue cracks prior to repairs or renewals, monitoring the environment in void and ballast spaces, and, specific to double hull tankers, corrosion detection and protection in double hull ballast and cargo tank spaces will be studied. WP3 investigated the structural strength assessment of tankers. Specifically, critical reviewing the current knowledge and understanding, and then undertaking the identification of future research and development needs in the following areas:

• The effect of contact damage and the effect of minor deformations imposed during towage or other operations of large tankers on the strength of a ship’s side structure and guidance on identifying and reporting such damage.

• The effects of ageing (corrosion, fatigue) and repair on the strength of the hull girder and larger components such as deck, side and bottom grillages.

• Methods for strength assessment for different modes of failure (excessive yielding, buckling, fracture).

• Means of minimising the influence of residual stresses in areas where large repairs are carried out.

• The effects of heating when welding and fairing and also jacking to fit prefabricated sections.

WP4 studied several areas of through-life management of tanker structures. The effects of operational profiles of tanker vessels such as route planning and weather profiles, partial cargo loading, and heating of cargoes on the wastage (corrosion) rates and ultimately on the structural health of the vessel have been considered.



WP4 also identified the repair, inspection and maintenance scheduling procedures used by owners and consider alternative or improved procedures to maintain a high standard of vessel’s structural integrity. WP5 aimed to integrate and prioritise the research and development needs identified in WP’s 1-4 and disseminate and exploit the project results and prepare research proposals in prioritised R&D areas. This work package will ensure that key sector groups are informed about the progress of the project. It should be noted that while the above mentioned areas are applicable to most steel ship structures; the focus of attention within the ALERT Project was on tanker structures since the consequences of tanker vessels losing their structural integrity are very high in relation to environmental pollution.


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3. STATE OF THE ART

3.1. DETECTION OF WELDING DEFECTS BY MEANS OF NON-DESTRUCTIVE TESTING

The detection of welding defects by means of non-destructive testing (NDT) relies on the methods applied, the procedures used and the capabilities of the NDT-operators and the extent of the examination. Although the application of NDT in new building is different from repair situations (due to different surface conditions and material properties), there appears to be not much difference in the written requirements and specifications applied in these various parts of the marine industry. The differences in requirements in respect to methods to be used, allowable defects and extent of examination, between the IACS Recommendations, Class requirements and the practices applied in the ship repair industry are presented in Deliverable 2.1 part A (Moerland et al, 2007). Some techniques specified for particular applications are not considered as the best available nor would they provide the best POD (Probability of Detection) results (as an example: Radiography is often specified for crack detection while this method is a specific volumetric technique).

3.1.1. NDT EXAMINATION REQUIREMENTS Although different rules and guides specify the required extent of examination differently, a commonly used rule is that the extent of examination is to be judged or modified by the surveyor (at the Surveyors discretion/satisfaction). Also the selection of areas to be examined, when less than 100% examination is required, is normally considered to be up to the individual surveyor. There is some variety in respect to extent of examination, acceptance criteria and the choice of a location in case of spot checks between the various classification societies and or IACS but the largest variation is probably introduced by the aforementioned system which leaves a lot of decision making and rule interpretation to the local surveyors. The competency of NDT operators should include knowledge of ships structures, as is normally required for operators performing thickness measurements onboard ships. Deliverable 2.1-part A also investigated the risks inherent in the non detection of cracks and considered possible ways to improve current procedures for the means of detecting and recording the presence of fatigue cracks. Although a number of well proven non destructive examination techniques exist for detecting surface and sub-surface defects, due to time and economical reasons, visual inspections are the most preferred method in inspecting large tanker structures. The factors affecting the quality of visual inspections are examined and possible improvements are recommended.


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Although some differences between rules and practices were observed, it has to be realised that in most cases the standard or code used was at least equivalent and in many cases stricter in respect to the acceptance criteria than required by the rules. Some rules or guides do not address all types of defects to be found by surface techniques and probably expect the visual inspection to be conducted by the surveyor and leave the judgment in respect to acceptability to him or her. The common practice within classification societies where decisions and judgments in respect to extent of examination and methods to be used is left to “the surveyor’s satisfaction” most probably provides the largest variation in intensity of testing and number of indications found during NDT.

3.1.2. MATERIAL SURFACE PREPARATION FOR NDT The surface conditions of the materials tested in ship repair are different from new building conditions and special repair based criteria and requirements should be developed. The work package partners have not seen that the “fitness for purpose” principle is being applied in the ship repair industry and in some occasions repeatedly repairs have de-creased the structural integrity more than the initial indication/defect might have done.

3.1.3. NDT OPERATORS COMPETENCE The level of competence of NDT operators performing the testing of welds, should not only be based on the general certification system based on one of the recognized codes or standards, but also include specific requirements in respect to the knowledge of ships structures. Crosschecking procedures should also address this specific knowledge. Means of detecting fatigue cracks and recording presence of fatigue cracks prior to re-pairs or renewals were investigated. Locating fatigue cracks by visual inspections is very difficult, especially in large ship tanks. Although a number of well proven non destructive examination techniques exist for detecting surface and sub-surface defects, due to time and economical reasons, visual inspections are the most preferred method in inspecting large tanker structures. The factors affecting the quality of visual inspections are examined and possible improvements are recommended.

3.1.4. PROBABILITY OF DETECTION Probability of detection (POD) curves are useful if reliability based fatigue assessment of a structure is required. The existing POD curves for close visual inspections, advanced visual inspection techniques such as Magnetic Particle Inspection (MPI) and Liquid Penetrant (DP) techniques and Ultrasonic and Radiographic NDE techniques were presented and discussed by Moreland et al (2007). These existing POD curves are generated from test conducted by nuclear, aerospace and offshore industries. The report highlighted the need for improved large test data for the development of improved POD curves for close visual inspections specific to ship structures.


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High quality fabrication as well as inspection and Fitness for Service evaluations are essential for maintaining safe service. One important area for future research is a better understanding of the influence of residual stresses. It is still not easy to determine how they evolve during cyclic loading with time and also how to measure and reduce their adverse effects.

3.2. CORROSION

3.2.1. BALLAST TANKS Corrosion of ship’s steel structures is nothing new in the marine environment. The corrosion mechanisms of steel structures in ships ballast tanks, cargo tanks, cofferdams and other spaces exposed to condensation and the salty atmosphere are complex, well known ranging from galvanic corrosion, aggressive cargo components to microbial induced corrosion. Ballast tank corrosion is a common challenge for all types of steel vessels that use seawater as ballast. There is no lack of experience with corrosion in the ballast tanks and cargo tanks of oil tankers (Rauta, 2004).

3.2.2. BALLAST TANK COATING – FROM VOLUNTARY TO COMPULSORY

Coating of ballast tanks used to be voluntary. It was industry practice to coat the ballast tank spaces to avoid extensive, time consuming and costly steel replacements later in the ships commercial life. Not all ships ballast tanks were coated. During the 1980s many bulk carriers were lost and some tankers experienced structural failures like M/T KIRKI. As a consequence coating of the ballast tanks became mandatory through IMO and SOLAS Chapter II-1 Regulation 3-2 and applied to all tankers and bulk carriers constructed on or after 1 July 1998. Accidents involving oil tankers like M/T ERIKA in December 1999 and M/T PRESTIGE in November 2002 drew the industry’s attention to steel structures, corrosion protection, structural maintenance, quality of class surveys, inspection intervals etc. The international regulatory consequences of the two tanker incidents are significant and among them accelerated phasing out of single hull tankers, access requirement for inspection in cargo and ballast tanks, coating of ballast tanks and other dry spaces exposed to salty, humid and corrosive atmospheres. Late in the process it was acknowledged that the coating requirement was not sufficient unless it was supplemented by a regulatory coating performance standard. The recent adopted Performance Standard for Protective Coatings (PSPC) and the requirement for all ships to have a Coating Technical File (CTF) onboard available for inspection describing the initial coating application and all repairs to the coating are expected to fill that gap.


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3.2.3. COATING AND ANODES Coating application and sacrificial anodes are tried and tested corrosion prevention methods of steel structures ashore as well as on ships. There are examples of ships where the ballast tanks coating have shown clear signs of premature deterioration long before the ship reached its first special survey at the 5th year renewal survey and there are ships where the ballast tanks have lasted for up to 20 years with minor touch-up maintenance.

3.2.4. COATING WORKING PRINCIPLE In principle the coating film separates the steel from the seawater. This film, given time will let water (H2O) through at some point in time. The paint films will not let through the salt ions. The service life of the coating system on any ship depends therefore heavily on the choice of coating, the steel surface preparation and conditions during the application of the coating. The decisions involve technical know-how, practical, commercial and financial choices and life cycle cost assumptions. The combined result of those decisions will not become apparent until some point in the future.

3.2.5. ROOT CAUSE OF FAILURE The root cause of corrosion system failures is often organizational rather than technical. It happens that mistakes are done for short term practical reasons without the decision makers fully realizing the consequences for the coating’s long term reliability. The result will often be premature coating failure followed by early coating maintenance need.

3.2.6. MANAGEMENT TOOLS – SOFTWARE Rust development on coated steel surfaces in seawater ballast tanks is visible and easy to find. The steel plate thickness of corroded areas can accurately be measured to quantify any loss of material. There are software packages available from all major class societies, and others, where the result of tank structural inspections as well as the result of thickness measurements can be entered. The software can provide three-dimensional colour coded graphics of the tank structure for survey planning and maintenance scheduling. The software can calculate surface area for coating repairs as well as steel weight if steel replacement is necessary.

3.2.7. GUIDELINES & PAINT INSPECTOR CERTIFICATION The industry has for years produced detailed coating application guidelines. The guides are easily available from for example publisher like Witherby’s Publishing & Seamanship International. Paint manufacturers have produced detailed data sheets for their products. The challenge for a ship owner is often to convince the ship builders that it is essential to follow these guidelines to achieve a lasting corrosion protection system. The larger paint manufacturers offer paint courses that are compliant with for example the requirement of National Association of Corrosion Engineers (NACE)


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and The Norwegian professional Council for Education and Certification of Surface Treatment (FROSIO) aimed at paint inspectors.

3.2.8. BALLAST TANK SURVEY REQUIREMENT & SURVEYOR COMPETENCE

There is no lack of voluntary industry best practice guidelines, mandatory and/or class survey requirements to make certain that the ships seawater ballast tanks are maintained in an acceptable condition during a tanker’s commercial life. There are numerous statutory international rules and regulations regulating the way tankers are built and operated through IMO, class societies, flag states, USCG, etc. There are even rules on rules to en-sure the first sets of rules are lived up to – like the Goal Based Standard as one example. The survey requirements are very clear. The details are, for example spelled out in IMO resolution A.744(18) (ESP Guidelines) (IMO, 2000). The option is available to class to reduce the inspection interval of ballast and cargo tanks to once every 12 months if deemed necessary. The class may also issue Condition of Class which will force an owner to take remedial action, normally within a short time limit. There is, therefore, every chance to discover early signs of coating breakdown and the start of corrosion giving ample time to deal with it based on current compulsory inspection regime. Ballast tank inspections of double hull tankers are relatively easy to conduct and can often be done with the ship in service. Ballast tanks can generally be made available for internal inspection during every loaded voyage and can often be made available also during ballasted voyages. Ballast tanks can even be inspected in port. For oil tankers, however, ballast tank inspection could be restricted by oil terminals. It is generally preferred that when a ship arrives the repair yard for scheduled dry docking that any coating and structural repairs are identified and prepared before arrival. The weakness with industry best practice guidelines is that such guidelines issue by for example Tanker Structure Cooperative Forum (TSCF), IACS and others are just guide-lines. Guidelines are free to be ignored by those who do not wish to follow industry best practice. Surveyor competence and experience, access to information and independence is important for those surveying the ships and their structures and that includes the owners’ or managers’ superintendents.

3.2.9. PAST DECISIONS A tanker’s structural condition on the day of inspection is the accumulated result of many past decisions made by people who will often in the meantime have moved to other companies as their career developed. Decisions that are often made against a spreadsheet investment calculation based on expected return at the new-building stage and too low life cycle cost estimates, often ignoring relevant past experiences. The root cause of corrosion taking hold on an oil tanker to the extent it weakens a tanker’s structure to the extent that it is in danger of collapsing may be traced to the specification and contract stage before the tanker was built, application during


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construction and the level of maintenance during its service life. Tankers with structural problems, corrosion and other deficiencies have also been brand new and looking good when they sailed on their maiden voyage.

3.2.10. CORROSION COUPONS AND INHIBITORS Ballast tank corrosion can be monitored and corrosion rate estimated based on corrosion coupons. The addition of approved corrosion inhibitors to the ballast water will slow down the corrosion rate, particularly on ships which may carry ballast for prolonged periods of time like in lay up or as permanent ballast. Ballast tank coating maintenance, once signs of rust materialise, should start by rust removal and application of a compatible surface tolerant coating when the remedial action is relatively manageable at reasonable re-investment (or cost). Failing coating due to unsatisfactory workmanship, preparations or coating product is a hidden, latent defect that will reveal itself given a little time.

3.2.11. PORT STATE CONTROL Port state control as for example the Paris MoU has been an important standard raiser for the last 25 years or so. In the 2007 annual report it is stated that “there are still some ship owners which manage to operate unsafe ships, thereby endangering the crew and the environment. Unfortunately they are assisted by poorly performing flag States and fly-by-night recognized organizations. Some banks are still willing to provide mortgage and insurance companies to provide coverage”. The Port State control operate a system whereby these ships are targeted and can be inspected as often as every 6th months or so. Reputable owners may therefore be subjected to PSC inspections every 24 to 36 months. Well established PSC inspectors have no hesitation in detaining a ship in poor condition until the condition has been corrected. These poorly performing ships and those poorly performing flag States are well known and information is available on for example the Paris MoU website. Such ships tend to gravitate towards trades in the parts of the world where the port states inspection regime is not so well implemented and controls none existing or lax.

3.2.12. BALLAST WATER EXCHANGE The introduction of the ballast water exchange to prevent the unwanted species migration has increased wear and tear effect on the coating. The ballast water exchange required ballast to be replaced either by the dilution method or the replacement method. An advantage of the ballast water exchange is reduction of the silt and sand build up onboard where ships take ballast in river estuaries or ports where there are huge tide variations and shifting of mud. The International Convention for the Control of Ships Ballast Water and Sediments (BWM Convention) requires ballast water treatment. The possible introduction of chemical treatment of the ballast water to prevent the spread of unwanted species through ballast water may cause hitherto unknown effects on the coatings’ life span. Some of the other ballast water treatment systems under development may also have effects on the coating life still to be discovered.


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3.2.13. CORROSION PREVENTION – INERT GAS Inert gas produced by burning heavy fuel oil in the ship’s boilers and passed through one (or two) seawater scrubber system(s) is proposed to be injected into the ballast tanks as corrosion protection. The thought is that by replacing the oxygen rich atmosphere normally present in the ballast tanks, whether the tanker is in loaded or ballasted condition, may reduce the corrosion rate. Injection of inert gas to the ballast tanks will represent additional challenges to the coating corrosion resistance and durability. In case of hydrocarbon gas detection in the ballast tanks inert gas may be required for safety reasons.

3.2.14. BALLAST TANK COATING – ADDED VALUE A well thought out and properly applied ballast coating system is an assurance against costly and time consuming steel repairs later in the tanker’s commercial life. Passing of SIRE, future charterers’ inspections, port state control, class surveys and CAP surveys as the tanker approaches the 15 years mark and beyond should be simple with minimal, if any, coating and structural repairs.

3.2.15. CARGO TANK COATING – SINGLE HULL There is also experience with cargo tank coating on crude oil tankers from way back in the 1970’s when it was still allowed to have combined cargo and ballast tanks for single hull tankers. It was common practice back then to apply coal tar epoxy type coating underneath the main deck and a couple of meters down on the bulkheads in combined cargo and ballast tanks. The result was often very good and minimum steel replacement was needed.

3.2.16. CARGO TANK CORROSION – DOUBLE HULL Today’s double hull tankers suffer corrosion in the cargo tanks in the same manner as the single hull tankers. Particularly in the under-deck area, or ullage space, caused by the corrosive environment present in that area. Cargo tank bottoms and other horizontal surfaces are exposed to MIC (Microbial Induced Corrosion) because that is where water collects and of the thermos bottle effect keeping the oil cargo warmer and the microbe’s active much longer compared to single hull tankers. Should there be any discovery of new types of corrosion it is very likely that the shore based oil industry has already made the discovery and have a head start on its reasons and how to deal with it. Cargo tanks are also part of the tanker’s structure. Corrosion of oil cargo tanks is an important factor because corrosion is also taking place in the crude oil cargo tanks. The crude oil cargo itself are corrosive to a varying degree, some more than others. The environment in the area of the cargo tank heads, or ullage space, is corrosive and acid due to VOC, H2S and inert gas components and humidity.


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Some crude oil cargo absorbs inert gas which means frequent topping up during the loaded passage to maintain the necessary cargo tank over pressure. Other crude oils are more volatile and generate VOC during the voyage that may necessitate pressure relief through the press/vac valves or manually through the vent riser. A correctly applied high quality coating on the cargo tank bottom will prevent the onset of corrosion, being general corrosion or (Microbial Induced Corrosion) MIC attack. Access to the bottom areas of the cargo tanks for inspection and possible coating repairs is easy.

3.2.17. OIL CARGO TANK COATING A proposal for a new SOLAS regulation II-1/3-9 introducing mandatory coating of cargo oil tanks of new oil tankers is presently under consideration in IMO (2008). The proposal is expected to follow the same principle as for the ballast tank coating requirement.

3.2.18. ACCESS AND COMMON STRUCTURAL RULES Inspection methods, access for close up inspection and corrosion prevention measures for the oil cargo tanks have been discussed since the mid 1960s when the 100.000 tdw tankers “Super tankers” first entered the scene and continued without interruption when the VLCCs in the range of 220.000 tdw in the 1970’s and up to 500.000 tdw tankers by the early 1980’s were built. The VLCC have settled around 300.000 tdw size and the access and corrosion discussions have continued without any real action until the introduction of the idea of Common Structural Rules (CSR) among the class societies around the early 2000’s. The common structural rules are expected to eliminate the competition on steel weight and which entered into force from 1 July 2008. These rules also included requirements for suitable access within the structure for inspection purposes.

3.2.19. EXXON VALDEZ – OPA 90 AND IMO DOUBLE HULL REQUIREMENT

The EXXON VALDEZ grounding incident in Alaska in 1989 resulted in the American Oil Pollution Act of 1990, known as OPA 90 with the requirement for all oil tankers above a certain size to be double hull. This was followed up internationally through IMO and resulted in a phasing out time- table for single hull tankers. The effect of the ERIKA and PRESTIGE breaking up and sinking was, among other actions accelerated phasing out time-table for single hull tankers.

3.2.20. ERIKA AND PRESTIGE – REGULATORY CONSEQUENCES Important changes are now underway as a result of several spectacular incidents involving tankers in the 20 years plus age bracket. The incidents involving the sinking of the old single hull tankers; NIKHODA in the Japanese Sea in 1997, the sinking of ERIKA in 1999 followed by the sinking of PRESTIGE in 2002 and resulting oil


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pollution have drawn public attention to tanker age, potential inadequacy of structural repairs due to material wastage, potential survey inadequacy and inadequate repair work quality control. These incidents resulted in several actions through IMO.

3.2.21. PERFORMANCE STANDARD FOR PROTECTIVE COATINGS – SOLAS AMENDMENTS

The SOLAS II-1 Regulation 3-2 “Corrosion prevention of seawater ballast tanks in oil tankers and bulk carriers”, has been amended and states that “all dedicated seawater ballast tanks in all types of ships of not less than 500 gross tonnage and double-side skin spaces in bulk carriers of 150m in length and upwards shall be coated during new construction in accordance with the PSPC”. The amendment entered into force on 1 July 2008 and the coating performance standard will apply to ships of 500 gross tonnage and above for which the building contract is placed on or after 1 July 2008; or, in the absence of a building contract, the keels of which are laid or which are at a similar stage of construction on or after 1 January 2009, or the delivery of which is on or after 1 July 2012. The target useful ballast tank coating life of 15 years is considered to be the time period, from initial application, over which the coating system is intended to remain in “GOOD” condition. “GOOD” condition is as defined in Resolution A.744(18) (IMO, 2000). The actual useful life will vary depending on a long list of factors of which the most important by far is the overall standard achieved during the initial coating work at the new building site. The future transparency of a vessel’s ballast tank condition was enhanced by the introduction of IMO Resolution MSC.215(82), Performance Standard for Protective Coatings (PSPC) in 2006. One of the requirements is that the ship builder is responsible for compiling the Coating Technical File (CTF). The CTF is required to report in detail any repairs done to the coating and will be available for inspection onboard as and when required.

3.2.22. CARGO TANK COATING Coating of the cargo tank head area and a few meters down on the bulkheads if properly applied at new building will prevent corrosion. The coating should include under- deck web frames, including flange and the toe connection. A high quality coating application in the crude oil cargo tanks applied to the deck heads, or ullage space, has the potential to prevent material loss due to corrosion for many years, always provided the coating remains in “GOOD” condition and does not break down prematurely. Coating colour can be experimented with, but it is possible that a lighter colour coating applied to the crude oil cargo tank heads will easily reveal any coating deterioration and structural cracks also in the crude oil cargo tanks. Structural cracks as well as coating failure will be easily spotted as red lines or red areas if the cargo tanks are water washed with seawater before arriving at the ship repair yard. Coating combined with permanent access will become a strong factor in discovering structural deterioration.


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3.2.23. PROGRESSIVE OWNERS There are a number of crude oil tankers that were delivered from the ship builder in 2002 where all the cargo tanks were coated underneath the main deck (or tank heads) and a couple of meter down on the bulkheads as well as the flat bottom. These tankers were also fitted with permanent access platforms for inspection by cleverly arranging a combination of inclined ladders, stringers and deep longitudinals fitted with railing for safety providing very easy access to close-up observation of the coating condition without any need for rafting or mountaineering (climbing) techniques. One of these tankers (an Aframax) passed its first special survey in 2007 (5 years old) and the cargo tank coating was in excellent condition after about 85 crude oil cargoes. The owners of these tankers are industry leaders and in the forefront of progress.

3.2.24. HULL FLEXING The tanker will often flex in heavy seas and this may cause coating cracking as the coating hardens with age. Repairs of coating failure in the cargo tank deck head area may not be a realistic option due to access difficulties, everything considered, with today’s coating repair technology. Cost of providing access and suitable environmental condition in the cargo tank suitable for re-coating are major challenges on an existing crude oil tanker. There is just one chance to get it right; during on-site application at the new building site. The positive side of coating break down is that the location is extremely easy to identify and the steel thickness can be easily measured from the main deck, at least as far as the deck plate is concerned. If the coating is intact the steel structure must be at its original thickness. The importance of choosing coating and the coating application itself to “get it right first time” cannot be underestimated.

3.2.25. OCIMF - SCRUTINY OF OIL TANKERS AND TANKER MANAGEMENT

The real drivers for change are the market forces followed by the regulatory bodies. The market forces are made up of many variables of which a free press, a vocal general public and the elected politicians are important participants as drivers for change. The other important factors are the financial markets, insurers, the stock market, a company’s share price, investor confidence and a company’s general reputation. The industry, flag states and port states will respond to public pressure. The global effect of an unnecessary incident causing pollution can have far reaching consequences, as has been shown the last 10 – 15 years in particular. It is not corrosion alone that makes a tanker slide down the quality scale as time passes, but it is the major one because it affects structural strength – the hull. Rust is nothing new and structural deterioration does not happen suddenly – it is a relatively slow process with many warning signs. The first indications that a ship has become substandard might be refusal of class by IACS member societies, repeated PSC detentions, change in trade pattern and ownership, change of flag to poorly performing flag states, financiers etc. It is people who let the standard slide for whatever reason.


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Tanker oil spill could have undesirable commercial consequences for oil majors ranging from the public’s boycott at the fuel pumps to years of litigation through the courts in many jurisdictions. Charterers’ monitoring of the tankers’ technical and operational conditions and selective chartering today has made it easier for the owner/operators’ technical and operational departments to be allocated sufficient resources beyond the need to comply with bare minimum compulsory requirements from the flag and the class society as it was in the past.

3.2.26. OCIMF SIRE INSPECTION There is today scrutiny of all operational and technical matters concerning all aspects of tanker operations as never seen before by charterers like oil majors. Major charterers, who are members of OCIMF (Oil Companies International Marine Forum), introduced the SIRE (Ship Inspection Report Programme) back in 1993 due to falling tanker standards as their risk management tool. However, it must also be said that the oil tanker market freight rate level had been depressed for many years through the late 1970’s and most of the 1980’s due to significant surplus tanker tonnage. In 1993, the OCIMF members agreed to develop a common inspection standard of the tankers they chartered and their requirements for quality have increased ever since. The SIRE Programme has developed and been revised several times. OCIMF produced the Vessel Inspection Questionnaires for Oil Tankers, Combination Carriers, Shuttle Tankers, Chemical Tankers and Gas Carriers the basis for the SIRE Programme. The 2008 Edition, Revision 1 which is the latest revision was published in July 2008. The programme is freely available from OCIMF’s website. The programme requires that participating submitting companies follow a uniform Vessel Inspection Procedure. The procedure has an Inspection Element and a Report Element. The programme has detailed guidance and requirements to the inspector including Conduct of Inspections. Passing of Ship Inspection Report (SIRE) Programme, oil majors vetting inspections, port state control (PSC), class surveys and Condition Assessment Programme (CAP) surveys as the tanker approaches the 15 years mark and beyond should be simple with minimal, if any, coating and structural repairs if the coating is kept at a condition which corresponds to a CAP 2 grading or better.

3.2.27. TANKER MANAGEMENT SELF ASSESSMENT (TMSA) – KPI OCIMF introduced in 2004 as a voluntary measure the four stage TANKER MANAGEMENT AND SELF ASSESSMENT (TMSA) scheme which is a best-practice guide for ship operators based on 12 principles of management practices. The TMSA uses the concept of Key Performance Indicators (KPI) to track a company’s effectiveness in meeting its aims and objectives. The KPIs reflect the company’s operational health. By the year 2007 several major charterers will not charter tankers that do not participate in the scheme and the list of charterers with this requirement is increasing. The second version of the TMSA was published early 2008. INTERTANKO has set up its own TMSA benchmarking database where the members can check their achievements against the participating members – their peers.


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There are other similar industry proposals under development expected to show comparable key performance indicators – KPIs between ship operators.

3.2.28. FEEDBACK TO SHIP BUILDER There is normally no feedback to the ship builder about the ship’s structural condition or the coating condition and service experience after the guarantee period has expired, normally 12 months after delivery. The shipbuilders’ motives for guaranteeing the steel structure and the coating systems for just one year for a tanker that has a commercial life span of 20 – 25 years has obvious liability reasons.

3.2.29. ENVIRONMENTAL IMPACT OF REPLACING CORRODED STEEL Steel replacement caused by unnecessary corrosion leading to steel repairs has environmental impact. The local and global environmental impact of steel replacement, cleaning, grit blasting, dust and recoating create greenhouse gas emissions counted from the steel manufacturing, transportation, preparations, coating and solvent manufacture and use of energy during replacement in a repair dry dock.

3.3. STRUCTURAL ASSESSMENT METHODS

3.3.1. GLOBAL STRENGTH Longitudinal strength can be considered as one of the fundamental components of ship structural strength. This is the ability of the hull girder to withstand longitudinal bending under operational conditions and extreme loads without suffering failure. The assessment of longitudinal strength involves the evaluation of the capacity of the hull girder subjected to longitudinal bending and the estimation of the maximum bending moment that might be experienced by it. When considering the global longitudinal strength of repaired tankers, it is seen as essential that a check of the hull girder strength is undertaken on the repaired ship. It is shown that simple section modulus approaches are inadequate to quantify the effects of the re-pair. More rigorous 2D progressive collapse, 3D finite element, or Idealised Structural Unit Method (ISUM) methodologies need to be employed but even these need further work to be fully applicable. These more rigorous methods cope with the effects of mis-alignment, imperfections, and residual stresses, with varying degrees of success. In the case of local strength, the strength of welded connections is crucial to the strength of the repaired structure. The requirements and methods for assessment of global strength are discussed in more detail in Deliverable D3.1

3.3.2. LOCAL STRENGTH The local strength of tanker structures depends highly on the buckling strength of the individual structural members, because after buckling, the structural member loses its ability to carry additional compressive loads. Therefore, the local strength is determined by the ultimate load-carrying capacity, which includes buckling in the


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compressive load domain. The requirements for local strength are discussed in Deliverable D3.1. The strength of a structure depends to a large extent on the strength of the connections between the different components, i.e. the stiffened plates in tanker structures. During structural design, much attention is paid to the connections and welded joints with respect to their static as well as fatigue strength. In this section, the static strength is dealt with. In the Deliverable D3.1, the design considerations for some typical connections and welded joints are outlined, which are considered to be relevant for tanker structures.

3.3.3. FATIGUE STRENGTH Fatigue strength assessment methods which can take account of thickness mismatches, miss-alignments, and residual stresses induced at repaired joints must be employed to investigate the effects of stress concentrations on fatigue crack initiation and growth. The joint between the repaired and the original structure is of critical importance to the strength of any repair. This is particularly true when the original structure has corrosion present which can increase the probability of weld defects and their effect on crack growth rates. A state of the art review of fatigue crack and fracture assessment methods has been carried out. This review has also noted the effect of post-weld treatments on the fatigue life of the structure.

3.3.4. RESIDUAL STRESSES The effects of residual stresses on a repaired structure are considered as detailed in Deliverable D3.1, in particular, different means of minimising the influence of residual stresses in areas where large repairs are carried out. The effect of heating when welding and fairing and also jacking to fit prefabricated sections is noted. An assessment of the effects of repaired structure on the weld-induced residual stresses has been carried out. This simplified assessment concludes that the effect of repairs will be to increase residual stresses over the shaken out residual stresses in an aged ship structure. Both the local and global strength characteristics will be strongly influenced by stiffness differences between the original (existing) structure and the repaired (renewed) structure. This can increase the local stresses and may cause local buckling and fatigue cracking to occur with catastrophic consequences. Further work is needed in this area to investigate the influence of stiffness mis-match on the local stress concentrations and hence provide input into the extent of any repair carried out.

3.4. THROUGH LIFE MANAGEMENT Structural failure refers to loss of the load-carrying capacity of either a component within a structure or the structure itself. Structural failure is initiated when the material is stressed to its strength limit, thus causing fracture or excessive deformation. Structural failures of ships are, like onshore structures, very common and these contribute to the personal risk levels of mariners, and high pollution and economic costs.


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Structural repair practices depend on the cause of the damage under consideration. The repair method and repair amount can vary depending on whether the failure is due to corrosion, an incident, a design or a construction issue. Assessment of the impact of repair on life cycle of tanker structures is recognised as a major area of research to enhance safety of life and environment protection in the future. Sharing knowledge and experience on this subject seems to be of the utmost importance to be able to investigate the existing methods aiming at constantly increasing quality of repair standards and practices. There is very little published work on the effect of repairs on the reliability of ship structures (see Paik J K., 2006 for a recent review). The effects of repairs on the structural strength of the hull girder are not well understood, and the investigation on the modes of failure typically experienced in the region of a repair seems necessary. The effects of repairs on the failure probability have been the objective of the Deliverable D4.1 (Batistatos, et. al. 2007). Ships operate in different sea condition, and in a severely corroding and (metal) fatiguing environment that reduces the strength of the ship structure (structural reliability) which can only be kept safe by regular inspection and repair of paint coatings, excessively corroded plate and fatigue cracks. Sea conditions influence on ship-structure includes the effects of wind, and climatic conditions such as temperature range, and waves, this has been studied in Deliverable D4.1. As a result, it was interesting to see that sensitivity of the reliability to the fatigue environment is very high but the sensitivity to the extreme environment is relatively low and may be at least partially compensated by shake out of residual stress. Air and sea temperatures and solar radiation are also important factors because if they affect the structural steel temperature it is likely to affect the corrosion rate and the fracture toughness (resistance of the steel to unstable fracture in the presence of cracks and high stresses). Rough estimates of the effect of temperature on corrosion rate are that it may nearly double between 0 and 20 degrees Celsius; however there are many other chemical factors that affect corrosion of the different parts of ships. Guedes-Soares et al (2005) calculated immersed outer hull corrosion rates of 0.25mm/year to 0.4mm/year for ships trading in different parts of the Pacific Ocean; the lowest rate was a Southern Pacific Australia to Chile route. The same researchers (Garbotov et al 2005) also reviewed deck plate corrosion measurements and found typical values of about 0.1mm/year but with very great variability – with some plates corroding at a rate more than 1mm/year. The corrosion mechanisms in ships seawater ballast tanks have been researched for many years and are well understood. Ballast tank corrosion is a common challenge for all types of steel vessels that use seawater as ballast and has remained so till this day. There is significant experience with corrosion in the ballast tanks (or cargo tanks) of new and old ships. The challenges are related to a list of issues concerning initial coating application, resources needed for coating maintenance during the ship’s life etc. Also, there is sufficient mandatory and/or class survey requirements to make certain that the ships’ seawater ballast tanks are maintained in an acceptable condition


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during a tanker’s commercial life. However, today’s double hull tankers suffer corrosion in the cargo tanks more than the single hull tankers (Rauta, 2004). Particularly in the under-deck area, or ullage space, caused by the corrosive environment present in that area. Cargo tank bottoms are exposed to Microbial Induced Corrosion (MIC) because of the thermos flask effect keeping the oil cargo warmer and the microbe actives much stronger compared to single hull tankers. In principle, corrosion should be limited by the IMO resolution A.744 (18) (2000) to 10% loss of section modulus. Previous experience (when converting tankers to FPSOs) was that the corrosion could be much greater than suggested by the class survey. Fatigue strength assessment methods which can take into account thickness mismatches, miss-alignments, and residual stresses induced at repaired joints could be employed to investigate the effects of stress concentrations on fatigue crack initiation and growth. The joint between the repaired and the original structure is of critical importance to the strength of any repair. This is particularly true when the original structure has corrosion present which can increase the probability of weld defects and their effect on crack growth rates. The studies carried out in Deliverable D4.1 shows that, the failure probability is relatively insensitive to the corrosion rate and corresponds closely to the change in failure probability associated with a 10% increase in fatigue and extreme stress. Both the local and global strength characteristics will be strongly influenced by stiffness differences between the original (existing) structure and the repaired (renewed) structure. This can increase the local stresses and may cause local buckling and fatigue cracking to occur with catastrophic consequences. Current knowledge is limited in this area and further research could investigate the influence of stiffness mismatch on the local stress concentrations in relation to fatigue life of the structure and hence provide input into the ex-tent of any repair carried out. Ship designs change, and specific differences associated with build year are differences in steel type and differences in construction details. The investigation reported in the Deliverable D4.1 shows that the newer steels tend to be tougher with a decrease in the ductile-brittle transition temperature of about 30oC between the years 1950 and 2000. Also different ships may have significantly different side shell connection details. These have caused a lot of problems and are now often subjected to finite element analysis in order to refine the design. However because the results of finite element analysis of ship details are difficult to interpret very different fatigue lives may result from two details that according to finite element analysis have the same fatiguing stresses. The studies carried out in the Deliverable D4.1 shows that for a ship with right angled corner details and a high SCF (Stress Concentration Factor) but with a 78 years mean fatigue life had an insignificant probability of failure whereas the ship with low SCF details but with the same fatigue life had a 1.6×10-5 annual probability of failure.


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Residual stresses result from welding and deforming the structure to correct shape errors. The effects of residual stresses that may be introduced at new building and during repairs are discussed in the Deliverable D3.1 (Downes et al 2007), and the effect of repair-related residual stresses on structural reliability is discussed in Deliverable D4.1, where two principal effects have been identified:

1. The increased tensile stresses in conjunction with the cracks that have been growing through fatigue before the repairs were made may increase the probability of a crack becoming unstable and suddenly extending.

2. The increased tensile stresses may prevent a running crack from arresting. The deliverable also discussed the sensitivity of the ship structural failure probability to:

• A change in the residual stress level. • Repairs carried out in a single highly stressed part of the structure, poor fit-up,

or welding • Introducing a single area of low toughness during the repair • Weld with both a significant defect and low toughness

To assess the sensitivity analysis carried out in the Deliverable D4.1, reliability modelling has been used and a large number of locations on a ship that might be susceptible to degradation under the effects of fatigue and corrosion have been considered. The probabilistic mathematical model included the inspection of the ship and calculated a year by year failure probability. Failure is defined using a fracture mechanics failure assessment diagram. The failure as defined may not imply the complete loss of the ship but does suggest that a crack will suddenly extend. The calculated failure probabilities were typically very low when the ship was young but increased dramatically as the ship aged and generally was less strong through corrosion loss and the presence of undetected fatigue cracks. Reliability based structural assessment and inspections have been utilized by the offshore industry for quite some time. However, it is only recently that probabilistic based rules development and assessment is considered in shipbuilding industry. It is recognised by the project partners that reliability based inspections and repair strategies will not only improve the cost effectiveness of the maintenance of ship structures but also enable the risk associated with inspections and repairs to be determined quantitatively. Contemporary tanker vessels are expected to have a hull structure maintained in good condition by ship owners and managers. The environmental and commercial implications of an oil pollution incident are so great, that ship owners and managers must organize tanker vessels’ inspection, maintenance and repair so that the probabilities of hull failure are minimized if not eliminated. This is reflected not only in recent TMSA1 - OCIMF2

1 TMSA: Tanker Management Self Assessment 2 OCIMF: Oil Companies International Marine Forum

requirements but also to the Companies’ SMS (Safety Management System) policies the majority of which have been adapted so as to include a “zero incident – zero spill” goal.


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In order to achieve these requirements, Companies’ SMS departments in co-operation with the Technical departments prepare written procedures, which prescribe in detail the actions to be followed for hull inspections, maintenance and repairs. Such procedures should at least provide concise information on the following topics: A. Hull structural inspection procedures:

1. Frequency of inspections for different spaces. 2. What kind of defects to look for (coating deterioration, corrosion, cracks, im-

pact damage, buckling etc.). 3. Where to look for these defects. 4. Reference to information / publications describing the possible root causes of

the identified defects (e.g. stress corrosion cracking). B. Structural repairs procedures:

1. Identification and recording of damage extent. 2. Identification of repairs extent and “geometry”. 3. Identification of materials required (plating & stiffeners thickness, size, shape,

material strength and grade). 4. Acceptance criteria for alternative material that may be used if material with

original characteristics is not available (strength, grade, thickness, with existing material and welding procedures and consumables required).

5. Identification of welding / cutting methods and consumables. 6. Class approved welding procedures 7. Welder’s Qualifications. 8. Steel and welding consumables acceptance – certificates – tests. 9. Repair method (e.g. welding sequence). 10. In situ inspection of fitting and welding work – NDT program during job

progress (e.g. Visual Examination (VE) after gouging and before final welding of the root, Radiation Examination (RE) & Ultrasonic Examination (UE) of butt joints at weld cross connections etc.).

11. Final inspection and tests extent (e.g. tank hydrostatic test, hose test etc.). 12. Repairs documentation and record keeping (onboard and in the office).

C. Company office and shipboard personnel required knowledge about inspection and repair procedures described in A and B and training programs to acquire the required knowledge and skills. Currently Safety Management System (SMS) and Technical Department personnel, in order to prepare efficient and thorough SMS procedures may rely on the following sources:

1. IACS and individual Classification Society Rules & Regulations. 2. IACS Guidelines and Recommendations. 3. OCIMF Guidelines and Requirements. 4. TSCF Guidelines. 5. INTERTANKO Guidelines.


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6. Charterer’s Requirements / Guidelines. 7. Company Experience.

Company experience is one of the most valuable assets and the primary source for the in-house procedures for inspection, maintenance and repairs used. All too often Company experience is acquired the hard way, i.e. through observation and rectification of defects already occurred. The observation could be a result of Company personnel inspections or repairs prescribed by external, i.e. class and charterer’s surveyors. However Companies’ experience varies widely and therefore there exists a need for a set of high standard, straightforward and universal guidelines on inspection, repair and maintenance procedures, in order to avoid serious tanker accidents. Deliverable D4.1 carried out an assessment of the extent to which the Rules and Guide-lines available address the inspection and repair topics mentioned in A and B above.


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4. IDENTIFICATION OF GAPS IN THE REQUIRED KNOWLEDGE

Following the review of the current knowledge and understanding in each of the areas covered by the work packages, the project then went on to identify the gaps in the current knowledge. These are discussed below:

4.1. TECHNOLOGIES FOR IN-SERVICE CRACK DETECTION The performance of in-service crack detection in ships is different from the crack detection during new building and/or repair. The in-service crack detection is normally performed prior to repairs and should also be used as a control method after repairs, when the vessel has been brought back into service in order to verify that repairs have not negatively affected the area adjacent to the repairs. The surface conditions and environmental parameters are completely different in the in-service situation where accessibility, lighting conditions, atmospheric circumstances and cleanliness are not comparable. The effect of these parameters on the reliability of detection of defects should be researched and investigated thoroughly. The parameters for this type of inspection and a summary of the available techniques and their respective advantages and restrictions, both technically and commercially are summarised. It is to be recognised that the performance of ultrasonic thickness measurements is a form of NDT which has been used for many years in the in-service situation, so experience gained in this field will prove to be beneficial for the evaluation of other NDT-methods. When we want to appraise the available techniques for their fitness for purpose in the in-service application, we have to define the materials to be tested, the surface conditions of these materials the types of defects to be detected and the method of reporting. As there is more and more a tendency to the digital storage of results in data management systems, NDT techniques having the ability to have results and observations recorded in a digital format have preference over methods less reproducible. This is especially applicable when instrument settings, such as sensitivity levels, range etc. can be recorded and reproduced for future comparison. It is obvious that POD’s for the various techniques under consideration will also be important but as there is no relevant information available on POD’s for the in-service application of the various techniques, one of the recommendations from the ALERT Project will have to be to have a research project wherein these will be developed. A detailed comparison of the NDT techniques currently available (not necessarily developed to a stage that can be readily applicable to marine structure inspections) in the market is given in Table 1. This comparison includes applicability of the method


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to coated and uncoated surfaces, requirement for surface preparation, level of reliability of defect detection, required level of expertise and experience for the operator, equipment cost, etc. Also, comments on each of the methods are provided in Table 2.

4.1.1. PREFERRED TECHNIQUES AND COMPATIBILITY WITH PRESENT REGULATIONS AND PRACTICES

From the detailed list of methods discussed in Tables 1 and 2, the following techniques are widely available and are currently employed by the maritime industry in relation to in-service inspections.

• Visual Inspections • Eddy Current Testing • Dye Penetrant Testing • Magnetic Particle Inspection • Radiographic Testing

4.1.2. TECHNIQUES REQUIRING FURTHER DEVELOPMENT From the list of techniques discussed in Tables 1 and 2, Eddy Current Testing has been identified as the technique which offers a big potential to be employed for in-service crack detections in ships but it requires further development. In addition to Eddy current testing, Ultrasonic testing /TOFD is a promising technique which could offer substantial benefits, especially when sizing of cracks is required.


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Table 1: Comparison of NDT methods

NDT Methods

Visual Inspections Pressure Testing Leak Testing Dye Penetrant Testing

Magnetic Particle Inspection

Eddy Current Testing

Coated and uncoated surfaces

Limited (only when coating is cracked)

Yes Limited Only uncoated Only uncoated or thin coating

Yes

Rusty and oily/wet surfaces

Large cracks only Yes Yes No No Yes

Crack detection and sizing

Sizing not No Limited No depth sizing No depth sizing Yes

Reproducible and recordable

Limited No No Limited Limited Yes

Fast and flexible

Yes No No Not fast reasonably flexible

Reasonably flexible and fast

Yes

Surface breaking defects

Limited Only penetrating No, only penetrating defects

Yes Yes Yes

Robotics and underwater

Only with optical systems

Only with optical systems

No No No Yes

Operator qualifications

Experience / NDT L I or II to EN 473

Experience Experience / NDT L I or II to EN 473




Investment in equipment

None < €5,000 < €5,000 < €5,000 < €5,000 < €10,000


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Table 1: Comparison of NDT methods NDT Methods Ultrasonic

Testing/conventional angle beam

Ultrasonic Testing/Guided

waves

Ultrasonic Testing/TOFD (Time of Flight

Difraction)

Ultrasonic Testing/Phased

Arrays

Acoustic Emission Testing

Radiographic Testing

Coated and uncoated surfaces

Uncoated only Yes Uncoated only Uncoated only Yes Yes

Rusty and oily/wet surfaces

Oily/wet: yes, Rusty :no

Yes



Yes Yes

Crack detection and sizing

Moderate detection, sizing difficult

Moderate detection, sizing difficult

Moderate detection, sizing Yes

Moderate detection, sizing Yes

Detection: growth only, sizing No

Poor

Reproducible and recordable

Yes Yes

Yes Yes Recordable: Yes, Reproducible : No

Recordable : Yes, Reproducible : Yes

Fast and flexible

No No No No No No

Surface breaking defects

Yes Yes Yes Yes Yes Yes (with minimum detection level)

Robotics and underwater

No No No No No No

Operator qualifications

Experience / NDT L II to EN 473




Experience / NDT L II to EN 473 +

Extensive experience


Investment in equipment

< €10,000 < €20,000 > €20,000 > €20,000 >> €20,000 > €20,000


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Table 2: NDT methods NDT Method

COMMENTS

Visual Inspections Visual inspection is often considered as a reliable method for the fast inspection of tanks and other compartments, provided the surface are clean and coated as cracks will show rusty stains on the coating. The use of more flexible coatings has limited the reliability of this method, while in double hull vessels the more extensive flexing of the structures will lead to cracking of the coating, not necessarily coinciding with cracking of the structure, thus leading to extra testing to determine the presence of cracks in the structures.

Pressure Testing Pressure testing is apparently only fit for the detection of full penetrating cracks between various compartments, and can therefore only be used for a very limited number of applications.

Leak Testing For leak testing the same restrictions apply; only between compartments and only crack fully penetrating a weld or plate can be detected.

Dye Penetrant Testing For Dye Penetrant testing the following restrictions also apply: -Flaw should be “open” to the surface -Condition of surface is critical ( clean and oil free ) -Inside condition of crack is highly critical

Magnetic Particle Inspection Magnetic Particle Testing using permanent magnets is less reliable than using electrical AC-magnets. This makes the technique less flexible for using inside tanks and compartments while safety is also an issue to be considered.

Eddy Current Testing Eddy Current requires more skills than the other methods but is far more flexible in respect to use and reporting. The application of eddy current testing can be carried out in the same way as ultrasonic thickness measurements are performed. There is still a choice to use a handheld instrument and direct reading of the results by the operator or using a remote system where a probe operator enters a compartment and a remote instrument operator records the results and adjusts the instrument where and as required, in direct communication with the probe operator who could even have a small slave-monitor attached to his wrist.

Ultrasonic Testing/conventional angle beam

Ultrasonic testing is more fit for finding embedded defects. The detection of in-service defects such as cracks is not always easy and sizing is extremely difficult with manual pulse-echo


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techniques. Ultrasonic Testing/Guided waves

Guided waves testing are a technique which is still under development and could in the future prove to be useful for the detection of in-service defects. At this moment the costs and complexity of operation are prohibitive for the application in in-service inspection in ships. Also the availability in area’s away from advanced industrialised centre’s will be a problem.

Ultrasonic Testing/TOFD TOFD is a promising technique readily available, but acceptance is still a problem. The current practice that TOFD applications require validation prior to use does restrict the flexibility for the application. With the introduction of 3rd party based examination and certification schemes under EN 473, and the adoption in various industrial standards this technique has the potential to become more and more important especially where defect sizing is important. As a technique for surface crack detection TOFD will be outperformed by Eddy Current, but for the defect sizing and/or evaluation of the original imperfection causing the defect, TOFD has definitely a good potential.

Ultrasonic Testing/Phased Arrays

Phased array is another new ultrasonic technique with a large potential for defect sizing. This technique is presently further developed and could prove to be valuable especially with new developments as “sampling phased array” etc. For the in-service application this technique could in the future be useful as it depends less on the accurate movement of probes over the surface under examination than TOFD.

Acoustic Emission Testing Acoustic emission is a technique widely used in the aviation and petrochemical industry. It should be considered more as a condition monitoring tool than a method for finding in service defects. This technique detects the occurrence of a crack more than finding the extent of a crack. Recent EU sponsored research has shown some promising results but further development is required before any full-scale installation on board of ships can be considered. The use of acoustic emission techniques can be compared with the use of stress monitoring systems where a number of sensors are installed and connected to a centrally located instrument. It is a technique worthwhile considering for future research efforts.

Radiographic Testing Radiographic techniques are very useful for the detection of volumetric embedded defects. The disadvantage of radiography is the fact that radiation poses a health risk and requires double sided access. For the detection of in-service defects radiation is not the best method.


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4.2. STRUCTURAL ASSESSMENT METHODS Deliverable 3.1 presented a “state of the art” review of structural strength assessment methods for assessing the effect of repair on the local and global strength of tankers. The main conclusions drawn from the report are as follows:

• For global longitudinal strength of repaired tankers, it is seen as essential that a check is undertaken on the repaired ship.

• Simple section modulus approaches are inadequate to quantify the effects of the repair. More rigorous 2D progressive collapse, 3D finite element, or ISUM methodologies need to be employed but even these need further work to be fully applicable.

• These more rigorous methods cope with the effects of mis-alignment, imperfections, and residual stresses, with varying degrees of success. All need further development before they can be applied with confidence to the strength of a repaired structure.

• In the case of local strength, the strength of welded connections is crucial to the strength of the repaired structure. These welded joints produce geometric discontinuities which can act as crack initiation points for which, depending on the detail, the initiation phase may be relatively short.

The report also presented a “state of the art” review of fatigue strength assessment methods which can be used to estimate the likelihood of fatigue cracking in the repaired structure.

• Fatigue strength assessment methods which can take account of thickness mis-matches, mis-alignments, and residual stresses induced at repaired joints must be employed to investigate the effects of stress concentrations on fatigue crack initiation and growth.

• The effect of corrosion levels is obviously a major factor when assessing both local and global structural strength. The wastage due to corrosion has to be taken into account when assessing the extent of the repair to be carried out. More work needs to be undertaken in establishing improved methods of quantifying the effects of the corrosion on both the local, and global strength, and hence the extent of any repairs to be carried out.

• Further work needs to be done on quantifying the effect of various types of corrosion on the fatigue life of ship’s structure and hence on the fatigue life of the interface between repaired structure and the original structure.

• The joint between the repaired and the original structure is of critical importance to the strength of any repair. This is particularly true when the original structure has corrosion present which can increase the probability of weld defects and their effect on crack growth rates. A state of the art review of fatigue crack and fracture assessment methods has been carried out. This review has also considered the effect of post-weld treatments on the fatigue life of the structure.

An assessment of the effects of repaired structure on the weld-induced residual stresses has been carried out in the Deliverable D3.1.


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• This simplified assessment concludes that the effect of repairs will be to increase residual stresses over the shaken out residual stresses in an aged ship structure. And as reported in the Deliverable D4.1, this increases the probability of crack initiation and reduces the likelihood of the crack arresting, thus increasing the probability of catastrophic failure.

Both the local and global strength characteristics will be strongly influenced by stiffness differences between the original (existing) structure and the repaired (renewed) structure. This can increase the local stresses and may cause local buckling and fatigue cracking to occur with catastrophic consequences. Further work is needed in this area to investigate the influence of stiffness mis-match on the local stress concentrations and hence provide input into the extent of any repair carried out. The formation of weld-induced residual stresses during the replacement of plates was further studied in Task 3.2. Today, modern numerical tools such as the finite element method and electronic data processing allow the simulation of the complex temperature distribution and the mechanical response of the material during the welding process to be performed. This leads to highly nonlinear analyses consuming long computing times. Therefore, such analyses have been applied up to now only to relatively small cases and short weld lengths and – to the knowledge of the participants – never to repair welds in ship structures. As there are gaps in the knowledge of the residual stresses induced by the repair of structures and the methods mentioned offer a high potential in this respect, first attempts were made to apply the methods to a replaced plate. The computations, which were presented during the Ship Repair Symposium (Fricke and Zacke, 2008) in Newcastle in Sept. 2008, showed quite interesting results:

• They confirm the assumption that the welding residual stresses are decreasing in the replaced plate with increasing size

• The welding sequence influences the amount of residual stresses particularly close to the welds

• In the single-layer welds which were analysed, the residual stresses are influenced by the arrangement of tack welds

The rather simplified analyses should be supplemented by investigation of the following aspects in order to complete the picture:

• effects of multi-layer welding with different weld sequences • effects of different plate thickness • effects of stiffeners

From the results, appropriate guidelines could be derived for repair procedures aiming at the reduction of welding residual stresses. Furthermore, known residual stress fields allow their effects on the strength and their re-distribution and shake-down during service loading to be investigated and areas of non-destructive testing to be defined rationally. The influence of residual stresses due to a repaired section on the global strength of the hull girder of a tanker and the effect of a renewed section with different scantlings to the surrounding hull girder on the global strength characteristics were investigated as part of Task 3.2 in a limited study as reported in report 3-32-TR-2008-01-01-0 and


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which were presented during the Ship Repair Symposium (Downes et al, 2008) in Newcastle in Sept. 2008. It was found that there are good grounds for future research into the effects of the residual stress field in the region of a repair on the strength of the hull girder. It appears that the level of influence that the residual stress has is dependent upon both the size and the location of the repair within the overall cross section and the actual level of the residual stress field itself. Areas that should be considered for future research are:

• The size of repair. • Location of the repair within the cross section. • Longitudinal location of the repair within the hull girder. • Level of residual stress within the region of the repair. • Residual stress field pattern – i.e. should this be uniform across the whole

repair section? It should also be recognised that this study only considered one of the possible variable associated with the repair of a structure. Miss-alignments are generally considered to play a significant part in the strength of a structure and should be further investigated. From this analysis, it may also be possible to conclude that global strength is not of primary importance for a repaired structure – local strength in the region of the repair may be more significant.

4.3. THROUGH LIFE MANAGEMENT The first deliverable of the work package (D4.1) presented a “state of the art” review of inspection and repair scheduling and practice with reference to rules, regulations, guidance and some example company practices. Numerous rule and regulation documents were reviewed from IMO, ABS, BV, DNV, LRS and IACS. The Guidance documents were reviewed from Classification Societies, Tanker Structure Co-operative Forum (TSCF), and Oil Companies International Marine Forum (OCIMF). Some company procedures have also reviewed through the distribution of a questionnaire. This has lead to some interesting conclusions. The key conclusions from the review of inspection and repair scheduling and practice are:

1. Good guidance is available from IMO, IACS and Class societies. 2. However the considerable additional discretion given to surveyors in agreeing

the extent and acceptability of repairs (in comparison to new build standards) may not lead to satisfactory work. There is concern that they can be subject to undue pressures from owners/managers.

3. The Tanker Structures Cooperative Forum guideline was considered particularly good, but it needs updating to include experience from double hull tankers.

4. Company procedures focus: a. more on machinery than hull structure and


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b. more on personnel safety than technical issues. 5. Better technical training (on ship structures and the effects of corrosion and

cracking) for office and shipboard personnel is recommended. 6. Repair yard procedures are difficult to monitor when there is a (now common)

long chain of subcontractors and this can lead to good procedures not being properly applied.

The deliverable also presents a “state of the art” through-life structural reliability analysis, applied to a simplified model representing a well designed and built ship, to investigate the significance of:

• operational profiles, • well executed repairs • poorly executed repairs

on the structural reliability of the ship’s hull. The reliability modelling considered a large number of locations on a ship that might be susceptible to degradation under the effects of fatigue and corrosion. The probabilistic mathematical model included the inspection of the ship and calculated a year by year failure probability. Failure is defined using a fracture mechanics failure assessment diagram. It may not imply the complete loss of the ship but does suggest that a crack will suddenly extend. The calculated failure probabilities were typically very low when the ship was young but increased dramatically as the ship aged and generally was less strong through corrosion loss and the presence of undetected fatigue cracks. The results demonstrated a high sensitivity to many of the parameters that were varied in the study. It was also noted that in many instances the inspection and repair cycle is adjusted to compensate, at least in part, for these sensitivities. So for example the ships working the TAPS trade in the late 1980s, where the wave climate was very severe, had an inspection and maintenance regime, tailored to the individual ships (where the fatigue damage was also made much worse by the high yield steel hulls) and the harsh environment, that often resulted in several times the amount of inspection that would usually be required but, although large numbers of repairs were required, did keep the ships reasonably safe. The study represented a ship whose average failure probability, assuming any repairs were perfect, is about 10-5. The key conclusions relating to the effect of repairs, which although only valid for that particular case considered, were:

1. A poor quality repair (a large defect or a large stress concentration in a normally highly stressed area) increased the failure probability by about 50 times. The effect was largest later in the life of the ship.

2. A localized area of low fracture toughness increased the failure probability by 10 times. The proportional reduction effect was largest soon after the repair.

3. Poor repair inserting low fracture toughness and defects at the same position and without owner/class being aware of the problem made failure probability increase about 104 times and so become very likely.


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4. Reintroduction of shaken down residual stresses increased failure probability by about 10 times.

Both parts of this review have provided interesting results. It appears from this work that:

1. Better guidance for surveyors and (and NDT operatives and companies) involved with inspection and repair may be needed. Existing guidance puts a lot of responsibility on the surveyor which is good in that the surveyor is enabled to make appropriate decisions when dealing with the possibly unusual circumstances of a particular repair. Writing detailed guidelines to better define the methodology for deciding what is acceptable will not be straightforward, however it appears to be needed and it is proposed that the way forward to producing a generally accepted guidance considered as a future development.

2. Better training of ship and ship management office personnel in the structural behaviour and ‘technical’ risks that a ship is subjected, is required so that their procedures can be improved and their decision making can be better informed. A specification for the training that should be provided could be an output of a future study.

3. Both good and poor repairs may have adverse effects on the long term reliability of a ship’s structure (in most cases, the repaired ship will be more reliable than if degradation or damage had been left with no repair undertaken). However, more realistic case studies are required to be performed as a future R&D study to provide greater realism and a better comparison with reality. These case studies will feed into the specifications for the required guidance and training discussed above and will provide a better understanding of the risks associated with different inspection and repair procedures and qualities achieved.


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4.4. CLASSIFICATION SOCIETY NEEDS

4.4.1. TANKER CONVERSIONS With the trade in bulk commodities being so robust at the moment, the corresponding charter rates for bulk carriers have risen dramatically. As a result, a large number of orders for large bulk carriers have been placed. However, some of these orders (BV’s information is that there are currently 39 on order throughout the world) are for conversions from oil tanker to bulk / ore carrier. These conversions require the removal of thousands of tons of steel from the ship’s structure and the fitting of new steel in different locations, to modify the ship’s arrangement from that of an oil tanker to that of a bulk / ore carrier. Of course, hatch openings are also cut into the deck to allow for loading / unloading of the cargo holds. What is more, most of these tankers are 10-15 years old with an unknown operational life before the conversion (loading, waves encountered etc.). Therefore these types of conversions usually involve significant uncertainties. Class societies are confident that the methods they employ now and the requirements being placed on the converted ships will be satisfactory in the short to medium term. However, it is felt that research should be carried out on how the risk levels associated with these conversions are expected to change over time, especially taking into account that these vessels’ operational life may be significantly extended.

4.4.2. SENSITIVITY ANALYSIS AND RULE REVIEW/MAKING The general research requirements of class are for a much clearer understanding of the importance of different maintenance and repair strategies. Class societies would like an investigation into the structural performance of ships having structure that is degraded and then repaired, either as a result of poor workmanship during construction or ineffective maintenance or accidental damage. The first degradation is a recognition that the workmanship of shipyards may not be up to the standard of the designers’ drawings or owner’s requirements. Of course, this is taken into account in class rules by allowable tolerances and implicit safety factors. However, given the current trend of designs using first principle approaches to demonstrate equivalence, the criticality of workmanship is most probably increasing. An investigation should therefore be undertaken into how ‘virtual’ sister ships, with different construction quality, perform in normal service and in accident conditions. The output of this research would be used to revise tolerances for construction standards as well as to produce a model that would allow for the analysis of construction defects. The second level of degradation is a ship that has differing levels of age-related defects. This will look to investigate the criticality of corrosion, cracks, buckles and any other structural damages. This part of the project will also complete fundamental research on the effect of replacing old steel with new. The output from this part of the project will be models that provide a better understanding of the effect that an aged


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structure will have on operation and accidental structural performance and revised guidelines for when to repair a degraded structure. The third level of degradation is a ship that has been damaged in an accident. The output of this research should be a methodology that integrates extent of damage calculations and also structural collapse calculations.

Build Quality/Degradation

Maintenance Quality/Degradation

Repair Quality/Degradation

A Design

Figure 1 Class would like to see an extensive numerical study backed up by experimental campaigns into how a ship degrades through its life and the best ways to repair it.

4.5. REPAIR YARD NEEDS

4.5.1. OVERVIEW Although new technologies are an important concern for the repair yards, they face some additional difficulties, when compared to new building yards, due to the variety of jobs and the difficulty of access on board. The main focus for repair yards is organisational issues where some quality and productivity improvements can be obtained, representing better results for repair yards and ship owners.

4.5.2. PLANNING Facing the present situation of the shipping market, ship owners are forced to confirm dry-dock space a few months beforehand. This means that work specifications are made with the known jobs at that time. This is done mainly for quotation purposes. By the time the vessel arrives at the repair yard, a new job list arises. As long as there is manpower available in the shipyard and spare parts are available on board, repair schedules can be maintained for most of the mechanical jobs. However, new (additional) steel jobs can significantly affect repair periods, especially when specific materials are needed (steel with a special grade, thickness or shape) and preparation work is required going through the construction drawings of the damaged


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/ corroded areas and or going on board to take measurements of the pieces to be replaced. This is a time consuming job that will always affect the start of work. Since time is the first priority for ship owners, everything is done to avoid or minimise delays on the dry-docking period which sometimes means reduced quality of the final surface treatment due to a reduction of the number of coats and sometimes curing time. Therefore, if it were possible to have a full list of the jobs required prior to ship’s arrival, then the work quality would improve and the risks of delay would be minimized.

4.5.3. KNOWING THE REPAIR Repair yards are deeply concerned with their reputation regarding not only quality but also timeliness. To avoid delays and overruns, it is of great importance for the shipyard to know the repair details in advance. In a large number of steel renewal repairs, time could be reduced if there were an agreement between repair yard and owner for the purchasing of materials and for the pre-fabrication of steel components. It is estimated that more than 50% of the steel jobs are not known by the repair yard on the ship’s arrival date and from the remaining steel jobs (the ones included in the work specification), less than 10% are correct or reliably described. The agreement for purchasing materials and pre-fabrication is achieved in less than 5% of the steel jobs. If it were possible to provide a complete job list as described in 4.5.2, then the next aim would be to increase the accuracy of the job description. It is well known that it is complicated to perform tank inspections and thickness measurements during operation, but there may be new technologies available that could be useful in this matter. It is important that ship maintenance managers know most of what is expected when the vessel dry-docks and share that information with the repair yard so that solutions can be obtained to avoid undesired rescheduling.

4.5.4. REPAIR DEFINITION In some of the steel renewal jobs, welding problems are faced due to the differences between the new and the old plates. When the steel job is identified in place, under the presence of an owner’s representative, a yard representative and a class surveyor, it is not realized sometimes that the plates marked for renewal are connected to corroded or deformed steel. Although unnecessary replacements should be avoided, good attention should be paid to the old steel condition to avoid bad quality welding seams. When repaired upon detection, time is wasted, whereas left undetected they could pose a structural problem.


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Deformations should also be avoided i.e. when making the connections to the new plates. When the old plates are deformed, the time and cost for the reshaping are higher than what would be necessary to renew a slightly larger area. This is also an important issue that can be improved at the time of the identification and definition, by involving all parties. If the repair yards were therefore provided with a complete list of jobs and a better description of the damaged areas, they would be able to define the most suitable repair first time around and avoid any rework or cost implications due to plan changes mid way through a repair. This subject can be misinterpreted with an intention to earn more money by increasing the renewed steel weight but quality and time have greater importance than a slightly increased amount of steel to be added / renewed.

4.6. OWNERS NEEDS

4.6.1. BASIC PRINCIPLES This paragraph addresses the two basic principles that could contribute to a proper, successful and safe upgrading of ship’s structure throughout her entire life. The principles apply for damage repairs caused by collisions/grounding, various conversions of existing ships as well as due to general corrosion:

1. Assessment of the condition of the ship’s structure Correct actions require good quality of inspection/survey and a good system for a proper assessment of findings. In other words, no matter who inspects the ship, the information package as a result of the survey/inspection on same ship performed by different companies or individuals should be similar. In addition, no matter who uses that package of data, the assessment of the condition of the structure of the ship and the extent of the repairs should be also within a certain level of similarity. Unfortunately, there is no evidence of proper monitoring on most of these aspects.

2. Extent and quality of repairs Several tanker accidents may be the result of structural deficiencies. As a result of the lessons learned from such events, IACS has issued over the last few years an increased number of Recommendations/Guidelines aimed at harmonizing survey procedures, assessment of the survey results and quality standards for ship repairs. However, these guides and recommendations are industry “best practice”, therefore their application is on voluntary basis and there is no system to check and monitor their application. Each class society has, over the years, developed its own philosophy on how to assess the structural condition of a ship and thus the decision on the extent of the repairs might still be a variable result of the different approaches by those who perform the assessment. The IACS Recommendation 96 on Guidelines for Surveys, assessment and repair of Hull Structures is relatively new (since April 2007) to assess its impact on whether it


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could iron out the current differences. The big test of the efficiency of this Recommendation and of its application will however be seen in the near future as more and more double hull tankers built early – mid 1990s will go through their 3rd special survey. Equally important but apparently even less subject to proper monitoring is the issue of quality of repairs. IACS has issued guides which, if applied properly, should eliminate defects and weaknesses after repairs. The result of a repair should bring no adverse effects on the strength of the ship’s structure. However, with current techniques and knowledge at the industries ‘coal face’ this is almost impossible to check. The first priority should be to ensure a proper monitoring of the application of existing engineering knowledge and practical experience. The second major improvement will be achieved when information on every serious incident, assessment of damage and repairs affected will be shared to those that have a vested interest.

4.6.2. REPAIRING CONSTRUCTION DEFECTS New yards entering the shipbuilding and repair markets have no more experience with what they are building or repairing than that represented by their employees. Mistakes and a steep learning curve can be expected. As a result, deficiencies concerning the hull structures are often found in relation to welds, sectional alignments, coating preparation and application and so on. The practical corrective measures, for example for alignment gaps to fill in the gaps between hull blocks with inserts or very large welds, are often not satisfactory. Too much corrective work does delay the production line and potentially the vessel’s delivery date with associated consequences. Anecdotal evidence suggests that night work is not unheard of to speed up and avoid time consuming corrections. This is problematic because there may not be the required supervision form owners representatives and class surveyors at night. The risk is that there are many reasonably priced new-buildings that potentially run the risk of becoming tomorrow’s trouble ships. This is a situation that some shipbuilders are well aware of. Insurers are also aware that many ships built in particular yards do represent a future risk, mainly concerning hull steel work. Particular attention should therefore be given to developing ‘best practice’ recommendations and rules for the repair of new ships during the new-building stage.

4.6.3. DATA SHARING The class societies have clout and represent vast resources of competence, knowledge and experience that nobody can dispute and which flag states find difficult to match. However, ships that get into difficulty quite often have all the certificates valid as issued by a class society. It should be noted that class societies do not have unrestricted access to ships. If the crew or owner would like to hide something from the class society, it is fairly easy for them to do so.


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IMO has no enforcement role on this matter, acting purely as a secretariat. The enforcement of the rules is the responsibility of the Flag Administrations while Port State Control acts as a marine police to see whether ships are compliant with the rules. Both flag administrations and port state control have a much greater right to access a ship at will. A serious accident or near miss on a vessel should be reported to IMO in accordance with the Casualty Investigation Code. In cases of pollution with subsequent broad media coverage there is often reporting to the relevant body or committee within IMO. The idea is to take advantage of the casualty investigation report’s findings to prevent similar occurrences. In most other cases, without media coverage or pollution, there is often more limited reporting, if at all, to IMO. The secretariat can for example not enforce a request for an accident report. Most administrations represented in IMO do not like to be scrutinized. In the end, reporting is totally dependent on the individual governments. Therefore, owners feel that it is necessary for owners, class societies and flag states to work together to share data on accidents so we can all learn from them and avoid the same mistakes being made over and over again.

4.6.4. STANDARDISATION Too many choices could be a concern. Ships, including tankers, should become standardized. There are as “many variations as there are ship owners, shipbuilders, class surveyors, etc”. Is it possible to standardise hull lines, hull structure and cargo block layout (for same ship type and same size)? Should only better hull shapes and structures than a standardised one be allowed?


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5. FUTURE RESEARCH AND DEVELOPMENT NEEDS

5.1. OVERVIEW From the evaluation of the current state of knowledge relating to ship repairs and the identification of gaps in this knowledge, future areas for research have been proposed. These proposed areas for future research are discussed below and have been prioritised based on the knowledge of the consortium and discussion within the industry.

5.2. INFORMATION GATHERING AND HANDLING FOR DAMAGES AND REPAIRS

The historical approach has been to use degradation information from ships in order to draw up experience-based design, inspection and maintenance rules. However this experience is not always encapsulated in researchable databases, but rather in class society survey reports. It has certainly not, as the owners highlighted in their requirements, been typical for large scale information sharing in the industry. It is proposed by Aksu and McGregor (2008) to explore the processes of collecting and analysing the data from a lot of ships and incidents. Then the confidence in that statistical data will be higher and it becomes a useful input to plans to keep all those ships safe. Contents of the Ship Database would preferably include (precise details could be determined during a future project):

• Hull structure definition • If electronic drawings are available: Initial as-built (electronic) drawings

stored in the data base so that navigation of the ship structure and finding the precise form of construction including stiffener connection details and bracket toes etc is straightforward. or

• If electronic drawings are not available then basic topological information to define locations in the ship structure with a simple cataloguing system for all structural details will be used instead.

• Survey information o Cracks by precise location: i.e.

Position (e.g. frame number and stiffener number). Crack type (from a list of typical cracks for the type of detail or

a sketch if not previously classified). Crack length and penetration through the thickness.

o Corrosion Position (e.g. plate between frame numbers and stiffener

numbers). Description of corrosion (average loss on plate, size of pits, size

of grooves).


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• Repair history o Extent of renewals o Quality of renewals

• Ship voyage and lay-up history. • Stress monitoring data – if available

The work will cover the data collection process and the analysis of the information on the structure condition of ships gathered before and after repairs, and to develop data management systems for ship structure condition monitoring, and procedures for data logging and visualisation with common protocols accessible by the owner, class society, repair yard and insurers. This research will also develop procedures for information collection and analysis in repair yards to improve the efficiency of repairs. It should also lead to the ability to predict where defects are likely to be on a ship and therefore give a repair yard a clearer idea of what may be required when the ship arrives for dry dock. However, it is not possible for research to force flag and port states and class societies to work together by sharing data but it can provide a means for them to share data if they so choose.

5.3. CONDITION MONITORING OF SHIPS The difficulty of inspecting very large enclosed structures, such as the inside of tanks on tankers, means that, historically it has been impossible to ensure that all defects are found and repaired. Improving the means of inspection will reduce the possibility of significant deficiencies escaping detection as discussed by Aksu et al (2008). As was discussed by the repair yards in Aksu et al (2008), this means that before repairs are commenced if there is both an improved visual inspection regime and the ability to ensure that thickness measurements are truly representative of the condition of the ship there will be a significant improvement to the repair process. It may also be possible to use NDT methods to detect faults before repairs are carried out. After repair, there needs to be a means to detect any defects within or around the repair, this could be done by using the best available NDT technology. Deliverable D2.1 noted the issues related to current inspection and detection methodologies and investigated the large scale applicability and benefits of emerging and new NDT techniques which are not currently employed by the shipbuilding industry. Also, an investigation into the reliability of close visual inspections is recommended. This could then lead to a decision support system that suggests, dependent on the defect being sort, techniques to improve the reliability of close visual inspections by means of special equipment. This is envisaged to provide significant help to surveyors in situations where the decisions are left to the discretion of surveyors.


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R&D Needs Priority (Low, medium, high)

1

Assessment of the reliability (probability of detection) of NDT techniques, especially in relation to inspections of deteriorated and repaired structures. Development of POD curves for different NDT techniques POD curves for close visual inspections.

Medium

2 Data logging and visualisations Medium

3 Training of NDT operator on ship specific knowledge, perhaps require qualifications High

4 Better information handling (prior to inspection knowledge of stress concentrations) Medium

5 Identification of emerging NDT techniques that are available for other engineering applications but not employed by shipbuilding industry. High

6 Development of indigenous NDT techniques specific for crack detection of large steel structures Medium

7 Investigation of large scale applicability, appropriateness of existing NDT techniques to replace or to supplement close visual inspections High

8 Cost benefit analysis of implementation of large scale application of existing and emerging NDT techniques for inspection and repair High

9 Develop techniques to improve reliability (probability of detection) of close visual inspections – use of mechanical, visual, electrical/electronic equipment to aid visual inspections by surveyors

High

10 Strain gauging ships Medium

11 Monitoring of environment in void spaces High

12 Assessment on coating repair techniques High

13 New coatings specifically for repair of ships Medium

14 Techniques for improving the condition of coating application High

15 Coating condition techniques (EIS) Medium

16 Use of coatings for alternative applications such as survey tools Medium

17 Contractual issues (guarantee period for ship structure and coating) Medium

18 Development of coatings that would last throughout the life of vessels Medium

19 Environmental footprint with regard to carrying steel around or replacing it (mine to scrapping)

High

20 Differing welding techniques in repair to newbuild (including post weld treatment techniques) Medium

21 Alternative repair practices (SPS) Medium


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5.4. STRUCTURAL ASSESSMENT METHODS As was highlighted in the general requirements and that of the class societies, the effects of repairs on the structural strength of the hull girder are not well understood. This work should investigate the modes of failure typically experienced in the region of a repair as discussed by Downes et al (2008b). The work should then consider the effect on the local and global strength of hull girder subjected to repair and degradation. The effect of repairs on the fatigue and fracture strength of a ship should be investigated in relation to:

• Weld quality in the repaired section; • Influence of residual stresses in developing small defects, which might be

present in the vicinity of the cropped and replaced area, and; • In the presence of stresses due to misalignments.

The numerical models proposed should be validated/calibrated from the findings of the experimental investigations proposed below. Onboard measurements and laboratory experiments should be carried out in order to determine effects of repairs on local and global strength of a ship experimentally and to verify the numerical analyses results. It is well known that the residual stresses due to welds can be quite high (as high as yield strength of the material) and the effects of these are not well understood in the repaired ship sections. To quantify the magnitude of residual stresses, onboard strain measurements will be conducted on a vessel before and after repair. In the laboratory environment, static and fatigue tests will be conducted on test specimens which are specifically produced from large deteriorated sections either cropped during an actual repair or cut-out section from a scrapped ship with a new welded section in the middle representing the combination of renewed and old material in the vicinity of repair. The laboratory tests will investigate the effects of parameters such as misalignment, residual stresses, weld quality, etc

R&D Needs Priority

(Low, medium, high)

1 Tests on old structure with new inserted material High 2 Systematic tests on small coupons formed from new/old materials High 3 Determination of failure modes of repaired and degraded structures High

4 Evaluate the failure potential of existing double hulled tankers subjected to repairs High

5 Quality of repair welds Medium 6 Differing welding techniques in repair to newbuild (including post weld

treatment techniques) Medium

7 Effect of repairs on the local strength Medium

8 Magnitude of residual stresses as result of repairs and their effect on local strength Medium

9 Effect of imperfections (due to contact damage) on ultimate strength Medium


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5.5. THROUGH LIFE MANAGEMENT Based on the work undertaken in Hifi et al (2008), recommendations for future research and development needs have been made in the area of through life management of ships:

R&D Needs Priority

(Low, medium, high)

1 Inspection technologies, Develop techniques to improve reliability (probability of detection) of close visual inspections – use of mechanical, visual, electrical/electronic equipment to aid visual inspections by surveyors high

2 Procedures/ guidelines to be followed by ship owners, managers, class high 3 Repair yard practices (technical and managerial) high 4 Reliability based repair methodology high 5 Material properties high

6 Local and global damage effects (understanding effects of local deterioration e.g. due to general corrosion, pitting, cracking, collision damage) on structural strength.

high

7 Effects of good and poor repairs (with misalignment and/or weld defects on ultimate local and global strength) high 8 Effect of repairs on the fatigue and fracture strength high 9 Sensitivity analysis of differing build, maintenance and repair strategies high 10 Data management for ship structure condition monitoring medium 11 Residual stresses and welding techniques medium 12 Quality of repair welds medium 13 Efficiency of the ship structure inspection medium 14 Guidelines on improved repair practice (welding, minimising residual stresses, misalignments, etc) medium 15 Numerical modelling of as-built structure and its performance to validate experiments and provide a base case medium 16 Defect sizes leading to eventual failure in the case of new and repaired ships medium 17 Efficiency of repair (information collection and analysis in repair yards) medium 18 Corrosion rates/modelling medium 19 Effect of operational profiles medium


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6. CONCLUSIONS The Coordination Action project ALERT has been undertaken in response to the needs identified following various casualty investigations for better understanding in areas of the detecting of defects and weaknesses during and after survey and after repairs, the reduction of any adverse effects of repairs, and current strength requirements for deck opening securing arrangements. It can be seen that the ALERT project (Assessment of Life-cycle Effect of Repairs on Tankers) has undertaken a thorough examination of current practices in the field of ship repair. The project has critically reviewed the current and emerging technologies in the areas of Ship Repair Practices (WP1), Condition Monitoring of Ships (WP2), Structural Assessment Methodologies (WP3) and Through Life Management (WP4). Furthermore, the project has disseminated the results through publications, seminars and prepared R&D project proposals (WP5). The project has then used this information to initially identify gaps in current knowledge and then to propose and prioritise the future R&D needs and developments in each of the areas covered by the work packages. This report brings together the information developed by each of the work packages in the ALERT project into a single consolidated report. It is intended that this consolidated report will form the basis for future research into the effect of repairs on ships.


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7. REFERENCES ALERT - “Assessment of Life-Cycle Effect of Repairs on Tankers”, Annex 1 – ‘Description of Work’. 2006. Project No TCA5-CT-2006-031459 Aksu, S., Moerland, P., Thygesen, B., Barltrop, N, and Hifi, N. (2008) “D2.2 Condition Monitoring of Ships – Future Research and Development Needs”, ALERT Project Deliverable D2.2. Project No TCA5-CT-2006-031459. Aksu, S., and McGregor, J. (2008) “D1.2 – Report on the Future R&D Needs and Requirements for Tasks 1.1, 1.2, and 1.3”. ALERT Project Deliverable D1.2. Project No TCA5-CT-2006-031459. Batistatos, N., Barltrop, N., Aksu, S., Xu, L. and Hifi, N., (2007) “State of the Art Report - Repair and Maintenance Scheduling”, ALERT Project Deliverable D4.1. 2007, Project No TCA5-CT-2006-031459. Downes, J., Dow, R., Fricke, W., Barltrop, N., and Li Xu, (2007) “State of the Art Reports – Structural Assessment Methodologies”, ALERT Deliverable D3.1, Project No TCA5-CT-2006-031459 Downes, J., and Dow, R. (2008) “Report 3.2.1 – Effects of Residual Stress Due to Repairs on the Ultimate Strength of an Example Tanker”, ALERT Report 3-32-TR-2008-01-01-0, Project No TCA5-CT-2006-031459 Downes, J., Dow, R., Aksu, S., Barltrop, N., Hifi, N., Incecik, A., Fricke, W., Zacke, S., Varsouras, C., and Tsichlis, P. (2008) “D3.2 Structural Assessment Methods – Future Research and Development Needs”. ALERT Deliverable D3.2, Project No TCA5-CT-2006-031459 Downes, J., Dow, R., Fricke, W., Barltrop, N., and Xu, L. (2008) “An Assessment of the Effect of Repairs on the Strength of Tankers”, Proc. of Int. Symp. on Ship Repair Technology (Ed. R.S. Dow and J. Downes), Newcastle University, 1st & 2nd September 2008, Newcastle upon Tyne. ISBN:978-0-7017-0220-5 Fricke, W. and Zacke, S., (2008) “Influence of Welding Sequence and Structural Stiffness on Residual Stresses of a Replaced Plate During Ship Repair”, Proc. of Int. Symp. on Ship Repair Technology (Ed. R.S. Dow and J. Downes), Newcastle University, 1st & 2nd September 2008, Newcastle upon Tyne. ISBN:978-0-7017-0220-5 Garbatov Y, Guedes Soares C and Wang G., (2005), “Non-linear time dependent corrosion wastage of deck plates of ballast and cargo tanks of tankers”, OMAE 2005-67579. Guedes Soares C, Garbatov Y, Zayed A, Wang G, (2005),” Non-linear corrosion model for immersed steel plates accounting for environmental factors”, ABS Technical papers.


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Hifi, N., Barltrop, N., Aksu, S., Incecik, A., Vasouras, C., and Batistatos, N. (2008) “D4.2 Through Life Management – Future Research and Development Needs”. ALERT Project Deliverable D4.2. Project No TCA5-CT-2006-031459. IMO resolution A.744 (18) (2000) “Guidelines on the enhanced programme of inspections during surveys of bulk carriers and oil tankers” Moerland, P., Aksu, S. and Barltrop, N. (2007) “Non Destructive Testing Of Welds And Means Of Detecting Fatigue Cracks”, ALERT Project Deliverable D2.1A Project No TCA5-CT-2006-031459. Paik J K. (Chairman), 2006, “Condition assessment of aged ships, Committee v.6”, 16th International Ship And Offshore Structures Congress, 20-25 August 2006, Southampton, UK, Proceedings VOLUME 2. Rauta, D. and Thygesen, B. (2007) “Corrosion detection and Protection, Monitoring the Environ-ment in Void and Ballast Spaces”, ALERT Project Deliverable D2.1B Project No TCA5-CT-2006-031459. Rauta, D, (2004), “Double hulls & corrosion”, The Royal Institute of Naval Architects, Conference, Design & operation of double, hull tankers, London, 25-26 February 2004.

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ALERT - TRIMIS · alert tca5-ct-2006-031459 alert assessment of life-cycle effect of repairs on tankers coordination action thematic priority: sustainable development, global change