INSPECTION, REPAIR AND MAINTENANCE OF SHIP STRUCTURES by PA Caridis BSc MSc PhD MRINA CEng Associate Professor Department of Naval Architecture and Marine Engineering National Technical University of Athens Greece Witherby Seamanship International Ltd. 4 Dunlop Square, Livingston, Edinburgh, EH54 8SB, Scotland. ii First Published 2001 Second Edition 2009 ISBN 978 – 1 – 905331 – 37 – 6 © Witherby Seamanship International Ltd. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Notice of Terms of Use All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher and copyright owner. While the principles discussed and the details given in this book are the product of careful study, the author and the publisher cannot in any way guarantee the suitability of recommendations made in this book for individual problems, and they shall not be under any legal liability of any kind in respect of or arising out of the form of contents of this book or any error therein, or in the reliance of any person thereon. Printed & bound in Great Britain by Bell & Bain Ltd. Glasgow Published by Witherby Seamanship International Ltd. 4 Dunlop Square Deans Estate, Livingston Edinburgh, EH54 8SB Scotland, UK Tel No: +44(0)1506 463 227 Fax No: +44(0)1506 468 999 Email: info@emailws.com www.witherbyseamanship.com iii INTRODUCTION In recent years, anyone who has in any way been involved in ship operations, and in particular in repairs, maintenance and surveying, is aware of the precipitous increase in the volume of regulations and international requirements that ship operators are called upon to comply with. Regulations cover all aspects of maritime operations and affect the conventional methods of managing ?eets in profound ways. The maritime world is deeply traditional and has evolved steadily through time, with past experience acting as a valuable guide to prospective ventures, whether these are of a technical or of an economic nature. However, recent developments have forced changes in thinking and in the approach to activities such as ship management and the operation of shipping companies themselves. Recent advances, not least the revolution in information technology, have facilitated progress in ship design and enabled operators to use computer-based tools as an aid to economic decisions. The ability to store and rapidly manipulate large quantities of information has meant that calculations that formerly were prohibitively time-consuming have now become commonplace and are carried out on a routine basis. In the case of ship operations this has had a profound effect The form, size and complexity of modern ships means that huge quantities of data are required to describe their technical characteristics and their various operations. The study of ship-related problems has therefore proved to be a prime candidate for computer- based procedures from the very beginning; during the past two or three decades, procedures have been developed that enable engineers to analyse complex situations rapidly and accurately. From an early stage computer-based tools were orientated at solving problems related to design or at analysing the response of ships under realistic operating conditions. It is only recently that efforts have been targeted at developing tools that can be of assistance in the repair and maintenance phase of a ship’s life, which after all comprises a signi?cant proportion of the total ?nancial outlay involved. The recent implementation of the International Safety Management (ISM) Code has made ship operators more aware of the need to rationalise resources and plan the technical management of their ?eets in more ef?cient ways. Of the expenses that relate to repairs and maintenance, the most important are hull structure repairs. Enormous sums of money are spent in hull repairs with very little effort made in planning these expenditures over time in a rational manner. Techniques such as those described in the later chapters of this book can assist in this effort. It is hoped that the economies that can be achieved will assist operators in making ships and the sea a safer and cleaner place as well as ensuring their desired return on investment. This book is aimed at people involved in the repair, maintenance and classi?cation of ocean-going merchant ships. Shipyard project managers, marine superintendents and Classi?cation Society surveyors ought to ?nd useful background information on this subject. Younger engineers who are embarking on a career in ship surveying will be provided with an understanding of the phenomena that cause deterioration in the condition of ship structures through time. Finally, students of naval architecture and related disciplines with an interest in ship operations will get a glimpse of the spectrum of problems that may lie in store for them in the future. It ought to be added at this point that, as far as I am aware, there are at present no university courses offered in this subject anywhere. Naval architecture is taught with the design of new ships and marine structures in mind, as well as the fundamental scienti?c disciplines that provide the basis for design, structural analysis and hydrodynamics. The addition of this text to the literature could provide an impetus for the introduction of courses in ship structural repair and maintenance. The material included in this book is concerned with the following topics: 1. Failure modes in ship structures and the effect of the marine environment 2. Damages to the hull structure of bulk carriers and oil tankers 3. Surveys of ship structures 4. Condition evaluation and repair/maintenance planning of ship structures. The ?rst four chapters describe corrosion, fatigue, buckling and fractures. Emphasis is placed on the nature of these failure types in ship structures and on their relative frequency. A qualitative description of each phenomenon is given, making extensive use of diagrams and photographs. The aim is to enable the reader to recognise the damage and determine its cause in order to be able to propose suitable measures for its avoidance in future. In the case of fatigue and its consequences, it was considered necessary to devote two chapters, because of its importance. Chapter 2 is devoted to a description of the mechanics of fatigue while Chapter 4 contains material on cracks that result from fatigue and other causes. Chapter 3 provides a qualitative description of buckling phenomena in ship iv structures. As any naval architect knows, buckling in ship structures is a subject fraught with dif?culties of a theoretical as well as a practical nature. In this text the subject is presented without any reference to the underlying mathematical theory. The text is accompanied by photographs of buckling in real ship structures. The two chapters that follow deal with the particular nature of these failures in bulk carriers and oil tankers. These vessel types comprise a large proportion of the world merchant ?eet of today. It should be said that, in the case of bulk carriers, an intensive effort aimed at resolving the various failure problems has been made in recent years by international organisations such as IMO, IACS and several Classi?cation Societies. These efforts are now beginning to bear fruit and bulk carrier losses are reduced in comparison to those of the early 1990s. However, it is important to discuss the origin of these problems and the remedies proposed. Useful lessons can be learned from past experience. The same is true for the problem of brittle fracture. Around ?fty years ago, brittle fractures occurred in a large number of early welded ships. At that time, welding was a new fabrication process and not all questions had been resolved. As a result, it was necessary to direct research at producing answers. Brittle fracture is now a rare occurrence, although previous experience should not be sidelined. Oil tankers are considered next. The serious environmental problems that have arisen are well known, as are the measures taken to avoid pollution from accidental oil spillage. In this chapter emphasis is placed on the effects of corrosion within cargo tanks. Fatigue fractures in oil tanker structures are also considered and cases involving buckling collapse of oil tankers are discussed. Surveys and inspections of the hull structure are the theme of the next three chapters, in which are discussed a) general survey requirements for merchant ships, b) particular requirements for surveys of bulk carriers and c) surveys of oil tankers (both single and double skin). It is important to plan carefully the extent of surveys because ship size and accessibility makes this a dif?cult job. A number of sections in Chapter 7 are devoted to a description of surveys and repairs required for hull life extension programmes. This topic is of immediate concern to owners who may wish to consider upgrading their vessels to continue trading them. The cost of new ship construction and the uncertainty surrounding their ability to provide a competitive return on investment makes such considerations necessary. The last two chapters deal with maintenance and repairs. Chapter 10 is concerned with the use of protective coatings. Surface preparation, the various types of coatings and, ?nally, a case study are considered. In the last chapter, ways of carrying out condition evaluations in a rapid and ef?cient manner, and of performing strategic maintenance and repair planning, are described. The methodology is based on the use of a database that consists of modules capable of manipulating data obtained during surveys as well as data on repair costs. Four case studies are described, involving the prediction of the hull condition and of the repair/maintenance strategy to be followed. The text is accompanied by a number of appendices that include extracts of recent regulations issued by the IMO and IACS on bulk carrier and tanker surveys. One appendix is used to describe the use of the HullCon database, the following one contains conversion factors for SI and Imperial units and the ?nal one is a Glossary of Terms used in the text. v AUTHOR’S PREFACE TO FIRST EDITION It was not my original intention to write this book in the English language, it was rather the result of coincidences and a series of meetings with a number of people that led me to believe that it would be a worthwhile aim. In recent years, a large number of investigations have been carried out in the ?eld of ship structural repair and maintenance. Problems have arisen in the operation of particular ship types and coordinated efforts have been made to counter these problems. Uppermost in mind has been the safety of those who sail the ships; in recent years this aim has received indirect support through the efforts aimed at pollution avoidance and preservation of the marine environment. Several years ago, when I started out on my career as a graduate naval architect, I joined a major Greek shipping concern, was trained and subsequently assumed the responsibilities of a Marine Superintendent. At that time, I vainly searched for reading material that would provide a background and an introduction to the world of ships and shipping operations. There really was no text that could facilitate the transition from university studies to the practical considerations and accumulated wisdom that governs many of the decisions that are taken in ship repair and maintenance activities on an everyday basis. Years later I came across the texts published by Witherby & Co as well as the numerous reports and publications that meanwhile were appearing on the subject. I realised, at that point, that it would be possible to prepare a text for young graduate engineers who would be employed in the Greek merchant marine with the job of maintaining and repairing the ?eets of various owners. As a result of this I embarked upon the preparation of the text in Greek. While on a trip to South America some time ago, I commented in passing to colleagues that I had been preparing a book on ship repairs in order to help our graduates from NTUA and also to provide reference material for more experienced engineers. The reaction was very positive and I was asked to co-operate in producing a Portuguese edition. My reaction was that it would be an impossible task to do the job directly from the Greek text. An intermediate stage involving the preparation of an English language version would be required. On my return to Europe I contacted Alan Witherby and explained my predicament. Alan was very keen on the idea as soon as he saw the proposed list of contents. We agreed that the English language text would not include the material that is already available in other published books, except where expressly required. Since Alan Witherby brushed my reticence aside I got down to preparing the text that follows. Piero Caridis Athens, January 2001 AUTHOR’S PREFACE TO SECOND EDITION It is very gratifying to be asked to write a preface to the second edition of one’s book. Since the ?rst edition was published, both I and the publisher have received favourable comments and there have been some very kind reviews in the technical press. Students who attend a course on inspections and repairs of ship structures enroll in part to receive their (free) copies of the book – that in itself is particularly satisfying given the tough standards that they usually apply to anything related to their studies. In the current economic climate, questions relating to ship hull maintenance and condition will receive particular attention, especially in the dry bulk cargo sector. Owners are reluctant to invest heavily in high maintenance standards and, as a result, the number of casualties and losses may well increase. International bodies will ?nd it more dif?cult to have new, environmentally-oriented measures accepted by the shipping fraternity, whose ?rst priority is to maintain acceptable standards of pro?tability. Nevertheless, it is hoped that the present text will continue to serve its purpose by providing descriptions of the problems that occur in the operation of ships in the particularly detrimental environment of the sea. vi ACKNOWLEDGEMENTS I wish to express my gratitude to a number of people and organisations. First of all I would like to thank Alan Witherby for the encouragement and support he has given throughout. I also want to thank him for making available material contained in other texts published by Witherbys Publishing. A number of publishers have kindly given permission to reproduce photographs and diagrams. I especially wish to mention the US Ship Structures Committee from whose reports I have used a large amount of material, photographs and diagrams. I can only hope that this book acts as a vehicle for the wider dissemination of the extensive efforts made in producing all these results. Thanks are due to Lloyd’s Register of Shipping for providing several photographs concerning damages to the hull structure and Professor John Caldwell for providing photographs of the ‘Energy Concentration’. I also wish to thank the International Maritime Organization (IMO), the International Association of Classi?cation Societies (IACS) and the Tanker Structures Co-operative Forum (TSCF) for giving permission to include extracts of their recent regulations and recommendations. Material has also been included from documents produced by a number of other Classi?cation Societies (Nippon Kaiji Kyokai, American Bureau of Shipping, Bureau Veritas, Hellenic Register of Shipping). My family has by now become used to being disturbed in their daily routine by me working at unorthodox hours and on unexpected occasions. I thank them all for their patience and for the understanding they have shown of the vicissitudes of writing a book such as this. P.A.C. vii CONTENTS Page INTRODUCTION iii AUTHOR’S PREFACE v ACKNOWLEDGEMENTS vi CHAPTER 1 CORROSION OF METAL STRUCTURES 1 1.1 A Brief Description of Corrosion Mechanisms 1 1.2 Types of Corrosion 2 1.2.1 Uniform or general corrosion 2 1.2.2 Pitting corrosion 2 1.2.3 Stress Corrosion Cracking (SCC) 2 1.2.4 Cavitation erosion - impingement attack 2 1.2.5 Bacterial corrosion 3 1.3 Factors that In?uence Corrosion 3 1.3.1 Corrosion and the environment 3 1.3.2 Corrosion mechanisms in marine structures 4 1.4 Shipbuilding Materials and their Properties 5 1.4.1 Steel 5 1.4.2 Aluminium 7 1.4.3 Copper 7 1.4.4 Brass 8 1.4.5 Copper alloys without zinc 8 1.4.6 Stainless steels 9 1.5 Design Against Corrosion 9 1.6 The Effects of Corrosion on Ships at a Global Scale 10 References 15 CHAPTER 2 FATIGUE; A HIDDEN ENEMY OF SHIP STRUCTURES 17 2.1 Introduction 17 2.2 Fatigue in the Marine Environment 18 2.3 Factors that Contribute to Fatigue 19 2.3.1 Loads 19 2.3.2 Environmental factors 21 2.3.3 Factors related to materials and fabrication 24 2.4 The Mathematical Analysis of Fatigue 25 2.4.1 Classical approach using S-N curves 26 2.4.2 Fracture mechanics approach to crack initiation 27 2.4.3 Advantages and disadvantages of fatigue analysis methods 28 2.5 Fatigue in Ship Structures 28 References 30 CHAPTER 3 BUCKLING OF SHIP STRUCTURES 31 3.1 Loads Acting on Ship Structures 31 3.1.1 Classi?cation of loads 31 3.1.2 Design loads for primary bending strength 32 3.1.3 Local buckling effects 32 3.2 The Behaviour of Metals 34 3.3 The Response of Structural Components Subjected to Compressive Loading 37 3.3.1 Columns and beam-columns 38 3.3.2 Flat and stiffened panels 40 viii 3.4 Two Cases of Hull Girder Failure 43 3.4.1 Collapse and sinking of a small tanker following longitudinal failure 43 3.4.2 The loss of a bulk carrier due to overall transverse collapse of the hull girder 44 References 47 CHAPTER 4 FRACTURES IN SHIP STRUCTURES 49 4.1 Introduction 49 4.2 Mechanisms of Crack Growth in Metals 51 4.2.1 Slip, plastic deformations and dislocations 51 4.2.2 Ductile transgranular fractures by microvoid coalescence 52 4.2.3 Transgranular brittle fracture (cleavage) 52 4.2.4 Transgranular fatigue cracks 54 4.2.5 Intergranular fracture 56 4.2.6 Sustained load fractures 56 4.2.7 Fatigue cracks 56 4.3 Fractures in Ship Structures: General Aspects 59 4.3.1 Brittle fractures 59 4.3.2 Ductile fractures and other failures in general cargo vessels 60 4.3.3 Measures for the avoidance of fatigue cracks 65 4.3.4 The repair of fractures 67 References 70 CHAPTER 5 DAMAGE TO THE HULL STRUCTURE OF BULK CARRIERS 71 5.1 Introduction and Overview 71 5.1.1 Statistical information 73 5.1.2 Regions of the structure that are prone to frequent damage 73 5.2 Causes of Damage to Bulk Carriers 74 5.2.1 The in?uence of corrosion 74 5.2.2 Cracks in bulk carrier structures. Some general observations 74 5.2.3 Other causes of damage. Observations regarding corrosion and fatigue 75 5.3 Damage in Various Parts of the Structure 81 5.3.1 Strength deck 81 5.3.2 Damage to cargo holds 82 5.3.3 Local design of stiffeners at snip ends. Fatigue strength 90 References 91 CHAPTER 6 DAMAGE TO THE HULL STRUCTURE OF OIL TANKERS 93 6.1 General 93 6.2 Corrosion in Oil Tanker Structures 93 6.2.1 Water ballast tanks 93 6.2.2 Cargo/clean ballast tanks 94 6.2.3 Cargo/dirty ballast tanks 95 6.2.4 Cargo tanks 95 6.3 Integrity of the Structure Following Wear and Corrosion 95 6.3.1 General 95 6.3.2 Tank bottom structures 95 6.3.3 Side shell, longitudinal and transverse bulkheads 95 6.3.4 Strength deck 96 6.3.5 Corrosion in various parts of tanker structures 96 6.3.6 Examples 100 6.4 Fractures and Related Failures in Tanker Structures 103 6.4.1 Fatigue fractures 103 6.4.2 Brittle fractures in tankers 106 6.5 Buckling Collapse in Tanker Structures 112 6.5.1 Buckling of the strength deck of a 100,000 dwt ton tanker 112 6.5.2 Hull girder collapse of the VLCC Energy Concentration 113 References 116 INSPECTION, REPAIR AND MAINTENANCE OF SHIP STRUCTURES ix CHAPTER 7 SURVEYS AND INSPECTIONS OF THE HULL STRUCTURE 117 7.1 Introduction. Purpose and Types of Hull Structure Surveys 117 7.2 Owner’s Surveys 117 7.2.1 Surveys conducted on behalf of insurers 117 7.2.2 On-hire surveys 117 7.2.3 Off-hire surveys 117 7.2.4 Sale and purchase survey and general condition survey 118 7.2.5 Damage surveys 118 7.2.6 Hull structure life extension schemes 118 7.3 Statutory Surveys 123 7.3.1 Annual Hull and Machinery Surveys 123 7.3.2 Intermediate Surveys 123 7.3.3 Dry-Docking Surveys 124 7.3.4 Hull Special Surveys 124 7.3.5 Special Surveys of machinery equipment 125 7.3.6 Boiler Surveys 125 7.3.7 Tail-shaft Surveys 125 7.4 Assessment of Survey Data 126 7.4.1 Assessment method 126 7.4.2 Integrity of the structure 126 7.4.3 Acceptance criteria 126 7.5 The Effect of Corrosion – Inspection, Evaluation and Prediction 128 7.5.1 General 128 7.5.2 Background to corrosion surveys on board ships 128 7.5.3 Corrosion data requirements 129 7.5.4 Corrosion rate prediction and survey techniques 130 7.5.5 Thickness measurement using ultrasound techniques 131 7.6 The Practical Investigation of Fractures in Ship Structures 137 7.6.1 Preparations for a fracture inspection 137 7.6.2 The fracture inspection 137 7.6.3 Circumstances at the time of fracture 141 7.6.4 Causes of large (brittle) fractures 141 7.7 Local Buckling in Structural Members 141 7.8 Effectiveness of Hull Structure Inspections 143 7.8.1 Scope of the problem 143 7.8.2 The Probability of Detection (POD) as a measure of inspection effectiveness 144 7.8.3 Factors affecting inspector performance 144 7.8.4 A realistic scenario 147 References 147 CHAPTER 8 SURVEYS AND MAINTENANCE OF BULK CARRIER STRUCTURES 149 8.1 Introduction. The Requirements of International Organisations for Bulk Carrier Surveys 149 8.2 Survey Requirements for Bulk Carrier Structures 150 8.3 Technical Background of Surveys 150 8.3.1 General 150 8.3.2 Nomenclature 151 8.3.3 Structural damages and deterioration 151 8.4 Preparation and Execution of Surveys 153 8.4.1 The survey programme 153 8.4.2 Principles for planning document 153 8.4.3 Conditions for survey 153 8.4.4 Access arrangement and safety 153 8.4.5 Equipment and tools 154 8.4.6 Survey at sea or at anchorage 155 8.4.7 Documentation onboard 155 8.5 Prevention of Accidents by Owners and Crew 155 8.5.1 Company practice 155 CONTENTS x INSPECTION, REPAIR AND MAINTENANCE OF SHIP STRUCTURES 8.5.2 Practice onboard vessel 156 8.5.3 Maintenance of hull structure by crew 156 8.5.4 Ship operations in port (loading/discharging) 157 8.5.5 Ship operations (at sea) 157 8.5.6 Detection of damage 157 References 158 CHAPTER 9 SURVEYS OF THE HULL STRUCTURE OF OIL TANKERS 159 9.1 Introduction. Class and Statutory Requirements 159 9.2 Vessel Geometry and Nomenclature 159 9.2.1 Conventional (single skin) oil tankers 159 9.2.2 Double skin tankers 160 9.3 Technical Background for Surveys 163 9.3.1 Structural defects 163 9.3.2 Critical areas in double hull tankers 165 9.4 Safety and Access 167 9.4.1 Safety during surveys 167 9.4.2 Access to the structure 169 9.4.3 Access methods for the structure of double skin tankers 169 9.5 Forms and Procedures for Data Collection and Reporting 170 9.5.1 Examples of the IACS Uni?ed Requirements 170 9.5.2 Planning Booklet for the IACS Enhanced Special Survey 182 9.5.3 General Condition Survey 198 9.5.4 Detailed Condition Survey 198 References 198 CHAPTER 10 MAINTENANCE PLANNING. THE USE OF PROTECTIVE COATINGS AND CATHODIC PROTECTION 199 10.1 Basic Concepts of Maintenance Planning 199 10.1.1 Background 199 10.1.2 Maintenance of ship structures 199 10.1.3 Repairs 203 10.2 Protection of the Hull Structure Using Coatings and Surface Preparation 203 10.2.1 Preparation methods for metal surfaces 205 10.2.2 Surface roughness and steel surface preparation 210 10.2.3 Preparation of aluminium surfaces 212 10.3 Anticorrosive Coatings 212 10.3.1 Protection requirements for the various parts of hull structures 213 10.3.2 Primers for steel structures 214 10.4 Antifouling Paints 215 10.4.1 Action of antifouling paints 216 10.4.2 Basic types of antifouling paints 216 10.4.3 Recent legislation concerning the use of organic-metallic antifouling paints 218 10.5 Classi?cation of Coatings on the Basis of the Binder Used 219 10.5.1 Paints that have dry oils as a base 219 10.5.2 Bituminous paints 219 10.5.3 Alkyd resin paints 219 10.5.4 Chlorinated rubber (CR) paints 220 10.5.5 Vinyl paints 220 10.5.6 Epoxy paints 221 10.5.7 Coal tar epoxy paints 221 10.5.8 Polyurethane paints 221 10.5.9 Polyurethane tar paints 222 10.5.10 Unsaturated polyester resin coatings 222 10.5.11 Zinc silicate paints 222 10.5.12 Silicon resin paints 222 xi CONTENTS 10.6 Coating Application Techniques 225 10.6.1 Use of brush and roller 225 10.6.2 Spraying 225 10.6.3 Conditions of application of protective coatings 226 10.7 Film Thickness 227 10.7.1 Measurement of ?lm thickness 227 10.7.2 Distribution of membrane thickness 227 10.7.3 Wet and dry ?lm thickness 227 10.7.4 Mean ?lm thickness and paint consumption 227 10.8 Identi?cation of Critical Regions of Hull Structures Due to Corrosion 228 10.8.1 Anticorrosive coatings in water ballast tanks 229 10.8.2 Condition of existing ships using corrosion as a criterion 229 10.8.3 The condition of protective coatings onboard existing ships 229 10.9 Types of Damage to Protective Coatings 232 10.9.1 Environmental factors 233 10.9.2 Damage related to material properties and coating application 233 10.9.3 Damage due to poor workmanship 233 10.10 Prediction of the Condition of a Protective Coating Within Water Ballast Tanks 234 10.11 Coating of a Medium Sized Bulk Carrier During Dry-Docking 235 10.12 Cathodic Protection of the Hull Structure 237 10.12.1 The effect of the properties of seawater 237 10.12.2 Aeration and oxygen content 237 10.12.3 Effect of variations in temperature and oxygen content 238 10.12.4 Effect of material and protective coating properties 238 10.13 Cathodic Protection Below the Waterline 239 10.13.1 Calculation of the required protection current density 239 10.13.2 Hull protection using galvanic anodes 239 10.13.3 Protection using impressed current systems 242 10.14 Cathodic Protection of Other Regions of the Hull Structure 244 10.14.1 Interior surfaces of tanks 244 10.14.2 Bilges 245 10.14.3 Floating docks 245 References 246 CHAPTER 11 CONDITION EVALUATION AND REPAIR PLANNING USING A DATABASE APPROACH 247 11.1 Data Acquisition 247 11.1.1 Planning of corrosion surveys and hull structure inspections 247 11.1.2 Preparations for safety and access during inspections 247 11.1.3 Instrumentation 248 11.2 General Requirements of a Ship Structure Database 248 11.2.1 Data entry 248 11.2.2 Data types and codes for their classi?cation 248 11.2.3 Documentation of corrosion parameters 249 11.3 Repair Planning Using Engineering Economy Calculations 250 11.3.1 Time value of money. Methods of evaluation and project assessment 250 11.4 Examples of Use of the Hullcon Database 253 11.4.1 Condition assessment and evaluation of bulk carrier structures 253 11.4.2 Repair/maintenance strategic planning for an oil tanker 262 11.5 Conclusions and Recommendations 264 References 264 xii INSPECTION, REPAIR AND MAINTENANCE OF SHIP STRUCTURES APPENDIX A THE ISM CODE 267 APPENDIX B ANNEX A OF IMO RESOLUTION A.744 (18) (SECTIONS 1-5) 271 APPENDIX C ANNEX B OF IMO RESOLUTION A.744 (18) (SECTIONS 1-5) 277 APPENDIX D IACS RECOMMENDED THICKNESS MEASUREMENTS FOR DETAILED CONDITION SURVEYS OF OIL TANKERS 283 APPENDIX E THE HULLCON DATABASE 293 APPENDIX F UNITS AND CONVERSION FACTORS 299 APPENDIX G GLOSSARY OF TERMS 301 CHAPTER 7 – SURVEYS AND INSPECTIONS OF THE HULL STRUCTURE 143 7.8 Effectiveness of Hull Structure Inspections 7.8.1 Scope of the Problem This section considers brie?y the factors that affect the effectiveness of the periodic inspections of the hull structure, ie the ability to locate the damages that arise and that need to be documented and repaired. A planned system of inspection and maintenance has been shown in some circumstances to result in lifetime costs of about one sixth of the cost of neglecting the damage and replacing the structure when Figure 7.18 – Local buckling of transverse girder web resulting from impact loading (a) bulkhead ?atbar stiffener (b) transverse web frame Figure 7.19 (a + b) – Local buckling in structural members INSPECTION, REPAIR AND MAINTENANCE OF SHIP STRUCTURES 144 Figure 7.20 – Variation of Probability of Detection curve The POD can be estimated using theoretical or practical considerations. Different defects (buckling, corrosion, cracks) will have different values of POD; similarly, the POD will vary and will depend on environmental factors as well as factors that are related directly to the inspector. To establish such a database, a great deal of practical work is required, involving working with inspectors in real life scenarios or else carrying out theoretical analyses. To date, this is not available and we shall limit our discussion to a qualitative treatment of the factors that affect the likelihood or probability of detection. 7.8.3 Factors Affecting Inspector Performance The factors that affect inspector performance can be grouped in the following manner: ? Vessel factors (design, structural layout, size) ? condition/maintenance factors (age, cargo, defects) ? personnel factors (overall experience, experience with vessel, training, fatigue) ? environmental factors (external, procedural). We shall consider next the effect that these groups of factors have on inspector performance. Vessel factors These include design, structural layout and size. Of these, the most important is vessel size, because this determines the time available to inspect each compartment of the vessel. Size also has an indirect effect by affecting the performance of the inspector himself through personal fatigue. Figure 7.21 shows the proportion of the hull structure that is inspected based on vessel size. The design of a vessel affects inspection performance indirectly. For example, the internal surfaces of a cargo oil tank that has just been emptied cannot be as easily inspected as those of a container carrier hold. The structural layout, that also depends on vessel design, may or may not permit ease of access. The existence of ladders, catwalks and large openings can facilitate access to remote parts of the structure. The layout can also affect the degree to which cargo residues remain, following discharge. Figure 7.22 shows the lines of access in a double hull oil tanker. In closing, we ought to add that the protective coatings used have a direct impact on inspection performance and the ability to detect ?aws. Therefore, coatings may in certain cases assist the detection of cracks (signs of rust on light-coloured coating) while in other cases they may hinder their detection (tar epoxy coating in water ballast tanks). required to do so.10 The savings come from preventive maintenance and from improved planning and execution of required repairs, including the ability to plan and implement alternative repair strategies. Statutory periodic inspections are required by Classi?cation Societies (see earlier sections). Periodic inspections are particularly important in fatigue loading where the detection and repair of cracks before they reach a critical length is crucial. The inspection of a complete hull structure is an overwhelming task because of the size of some modern vessels (Table 9, Figure 7.21). This is not the only dif?culty, however. Table 9 – Extent of hull structure inspection for a 250,000 dwt ton crude oil carrier (pre-MARPOL design) Total vertical length to be covered10,700 m Surface area of cargo tanks to be inspected300 m2 Total length of welds1,200 km Total length of welds per cargo tank390 km Total length of longitudinal stiffeners58 km Bottom surface area10,700 m2 Number of pits to be measured (1% of total area)85,000 Inspection is a physically demanding task, involving climbing and entering con?ned spaces, often under dif?cult conditions without suf?cient lighting, ability to arrange and operate instruments and record measurements, draw sketches and take photographs. One other serious constraint is the time that is available. If the inspection is made at sea with the vessel proceeding to a repair yard, certain compartments will be available for inspection while others may not be (ballasted or loaded). Related questions concern the accuracy of the inspection and its relation to the durability of the vessel. For example, setting a target of locating 75% of all cracks of length 50 mm and above will require different resources to locating cracks whose length exceeds 100 mm. The fatigue life of the vessel will be affected accordingly. The measure of success of an inspection depends on its aims, and these differ, as do the objectives. Therefore, a damage survey conducted by a surveyor acting on behalf of underwriters will consider different aspects to those that are of importance to a Classi?cation Society surveyor, even though both will be surveying the same part of the hull structure. 7.8.2 The Probability of Detection (POD) as a Measure of Inspection Effectiveness Knowledge of how likely it is for a defect or damage to be detected is useful because this will give an indication of the success of current practice. It will also indicate what types of changes in inspection techniques are required to improve effectiveness. To quantify the effectiveness of an inspection, a measure of success is required. A suitable yardstick, or measure of effectiveness, is the probability of detection, POD. As detection accuracy increases, the value of the POD will also increase, though not necessarily in a linear fashion. The value of the POD will be known with a certain degree of uncertainty, and so con?dence bounds will have to be speci?ed. A qualitative representation of the variation in POD of a particular defect (crack) with defect size is shown in Figure 7.20. 10 Weber P.F. Structural Surveys of Oil Tankers. The Institute of Marine Engineers (cited in [13]). CHAPTER 7 – SURVEYS AND INSPECTIONS OF THE HULL STRUCTURE 145 The success of an inspection is measured by the number of defects that are located in relation to the total number present. In new vessels, the number of defects will be smaller than in older ones and so it may be considered an easier task to pinpoint the defects. This would be the case if the thoroughness of the survey were the same as during later stages in the vessel’s life. Usually, inspectors do not pay as much attention to the condition at an early stage and thus defects go undetected. This makes the task of maintaining the structure in an acceptable condition more dif?cult at later stages in the life of the structure. At an early stage, the locations of defects will not be known for a particular vessel and so it will be more dif?cult to locate the fewer but more critical defects. On older ships, the trouble spots will be known and the condition can be monitored more easily. In general, as the number of defects increases, the probability of detection will decrease. The ease of detection of defects depends on the type of cargo carried in the particular compartment. Fresh water tanks are much easier to inspect than ballast tanks because of their relative cleanliness. Crude oil tanks are dif?cult to inspect because of the residue that builds up in the regions that need to be inspected. The nature of the defect is of particular importance in the probability of its being detected. The types of defects that are searched for are corrosion of surfaces, cracks and buckles. These often arise in combination. In certain cases their presence is indicated in an indirect manner. An understanding of the dispersion of loads throughout the structure enables the inspector to identify critical regions and inspect these before continuing elsewhere. An important aspect in the detection of defects is size. Microcracks that are not visible to the naked eye will pass undetected. However, for a crack to be detected it needs to have a minimum size, usually considered to be 50- 100 mm. When inspecting a structure that has just undergone steel renewals, to inspect the new welds, it is necessary to chip away at the top coating of weld material to see whether any cracks have formed in the weld material itself due, for example, to excessively high welding speeds being used. The defects themselves are therefore not always evident. Another factor of importance is that of the location of the defect. For example, a crack on the strength deck is evident for all to see. However, to locate a crack in a duct keel, it will be necessary to have the vessel free of cargo and clean. This is only possible prior to a Special Survey that takes place every 4-5 years. Hidden cracks betray their presence Condition/maintenance factors These factors re?ect the changes in the vessel with age. Thus, the presence of damage and defects and the means of preservation of the structure are included here. Figure 7.21 – Proportion of hull structure inspected based on vessel size11 11 Bell H.H. et al. Report of the Tanker Safety Study Group. U.S. Department of Transportation, U.S. Coast Guard, Washington, October 1989. Figure 7.22 – Access through side ballast space and double bottom in a double hull tanker INSPECTION, REPAIR AND MAINTENANCE OF SHIP STRUCTURES 146 An understanding of the external loading and the related structural response can be of assistance in conducting what may be termed forensic structural engineering, with application to ships. Several examples are given in other parts of this text (buckling, ultimate collapse and large fractures). The effect that vessel size has on the ability to detect defects has been discussed previously. Related to this is also the question of inspector fatigue. The extent of structure to be inspected in combination with time constraints means that survey work is intensive and tiring. The inspector will have to be in good physical condition and not be subjected to excess external pressure. Environmental factors The environment has a major role to play on the effectiveness of ship inspections. This question has been studied extensively in other industries and has been shown to be so throughout. Environmental factors can be classi?ed as those related to external conditions (weather, location of vessel) and those related to procedural questions. It is easy to see the effect of weather by comparing productivity in shipyards in various parts of the globe. The hot, humid conditions of the tropics reduce productivity substantially when compared with that achieved in climates that are more temperate. When considered in conjunction with the location of the vessel, the weather may affect in ways that are more complex. For example, a damage survey conducted onboard a vessel at sea in the midst of a storm will be less effective than one that is carried out in a repair yard. The location of the vessel will have an in?uence on the effectiveness of the inspection. Therefore, inspection of the bottom plating and all internal compartments will not be possible unless the vessel is dry-docked. At the same time, the condition of the internal spaces will be much improved when the vessel is in a shipyard, being prepared for repairs. We shall now consider procedural factors that are under the control of the inspector, to a much greater extent. These include items such as lighting, cleanliness, temperature and humidity, ventilation, means of access, inspection method, inspection strategy and type, crew support, time available. Visual inspections are affected by the availability of lighting. In dark spaces, there has to be suf?cient lighting to permit the inspector to move about safely and ef?ciently (overall diffuse lighting) and additional lighting to uncover damages to the structure (directed beam from a torch). The effect of temperature and humidity has been studied in numerous industries other than the ship repair industry. In enclosed metal spaces, the temperature may rise to uncomfortable levels for a person not experienced in arduous work. The demands of structural inspections can render conditions dif?cult even for experienced inspectors. Humidity has a similar effect. Lastly, it should not be forgotten that inspections are conducted in the presence of welders operating ?ame cutting or welding equipment. The fumes that are produced bring about a deterioration of conditions and adequate ventilation is necessary. As stressed in other parts of this book, accessibility is a critical factor in ensuring that an inspection is carried out in a satisfactory manner. Access to remote sections of the hull structure can be achieved in many different ways, other than by using permanent means provided by the ship arrangement. The various types are shown in Table 10. Table 10 – Means of access to various parts of the hull structure through the leakage of cargo or ballast water and the pollution of the seawater surrounding the vessel. Checks of ullage measurements will give an indication as to whether leakage is occurring from one tank into a neighbouring one. Local failure of structural members due to buckling is not easy to detect as the de?ection of the failed member is usually small. However, as in the case of cracks, buckling may be detected indirectly, through for example, wear in the coating. In some cases, buckling of one member, may betray buckling of another, eg a web stiffener, that of the web itself. Care has to be exercised in obtaining a picture of the full extent of damage. Different types of defects arise in combination. Corrosion in a bracket may cause thickness reductions, which through the resulting overload bring about failure through the presence of a crack, buckling, or both. In such cases, it is important to be able to understand the sequence followed. The mechanism described may arise in a corrosive environment such as an uncoated water ballast tank, but it may be the case that under different circumstances the stress ?eld in the member plays a dominant role in the failure observed. The understanding of a failure sequence is not of importance per se, but to recommend measures for repairs, improved maintenance, or even modi?cations to the structural arrangement it is helpful. Inspector factors Overall experience is often cited as a critical factor in detecting defects. A novice inspector needs to spend some time becoming acquainted with the nature of hull structures, paying attention to personal safety. After a certain period, he will be in a position to pay attention to the actual job of inspecting. The hull structures of large merchant ships are designed with the express purpose of carrying goods and so minimal allowances are made for ease of access for individuals. The size of modern vessels means that safety precautions are vital; any accident may result in a serious injury and inexperienced inspectors should not attempt to work on their own. There are circumstances when environmental conditions can make the work of the inspector or superintendent considerably more dif?cult. For these reasons, overall experience and acquaintance of the particular vessel is important. Knowledge of ship operations is necessary when preparing a plan of inspections. During the inspection, it is necessary to be able to identify individual items. For example, a number of pipelines traverse a duct keel. If one of these is particularly worn and requires replacement, the inspector should be in a position, following his exit from the duct keel, to give instructions as to which line has to be cleaned and isolated in order for repair work to commence. Related to this question is that of inspector experience with the particular vessel. The examples cited previously show how important it is for the inspector to be familiar with the vessel. An inspector who is not familiar with the vessel ought to become acquainted with the compartment layout and the structural arrangement by making use of plans and all related reports (condition survey and damage survey reports). These will enable him to make some headway at an early stage. Another issue that is cited to be of importance to effectiveness in hull structure inspections is training. In the opinion of the author, training in the sense of formal quali?cations is of secondary importance to experience in the actual process of detecting defects. It is, however, of considerable assistance in interpreting the description of a damage survey for example.