Fatigue and Fracture Mechanics of Offshore Structures

Fatigue and Fracture Mechanics of Offshore Structures

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ENGINEERING

RESEARCH

SERIES

Fatigue and Fracture Mechanics of Offshore Structures L S Etube

Series Editor Duncan Dowson

Professional Engineering Publishing Limited, London and Bury St Edmunds, UK

First published 2001 This publication is copyright under the Berne Convention and the International Copyright Convention. All rights reserved. Apart from any fair dealing for the purpose of private study, research, criticism, or review, as permitted under the Copyright Designs and Patents Act 1988, no part may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, electrical, chemical, mechanical, photocopying, recording or otherwise, without the prior permission of the copyright owners. Unlicensed multiple copying of this publication is illegal. Inquiries should be addressed to: The Publishing Editor, Professional Engineering Publishing Limited, Northgate Avenue, Bury St Edmunds, Suffolk, IP32 6BW, UK. Fax: +44 (1)284 705271.

© Etube

ISBN 1 86058 312 1 ISSN 1468-3938 ERS 4

A CIP catalogue record for this book is available from the British Library.

Printed and bound in Great Britain by St. Edmundsbury Press Limited, Suffolk, UK

The publishers are not responsible for any statement made in this publication. Data, discussion, and conclusions developed by the Author are for information only and are not intended for use without independent substantiating investigation on the part of the potential users. Opinions expressed are those of the Author and are not necessarily those of the Institution of Mechanical Engineers or its publishers.

About the Author

Dr Linus Etube (BEng, PhD, CEng, MIMechE) joined the Department of Mechanical Engineering at the University of London in October 1991 as an undergraduate. After completing his BEng in 1994, he started his PhD research programme as a research student. He was appointed a lecturer in December 1997 and obtained his PhD in September 1998. His research interests include: • fatigue and the applications of fracture mechanics to engineering structures under realistic loading and environmental conditions; • development of novel fracture mechanics models for engineering applications; • variable amplitude fatigue behaviour of offshore and related structures; • structural mechanics and failure analysis of offshore and related structures; • offshore safety, structural integrity, and reliability; • risk analysis. Dr Etube has worked closely with a wide range of both UK-based and global organizations in the offshore oil and gas sector, including regulatory bodies such as the UK Health and Safety Executive (HSE), in expanding the knowledge and understanding of structural steels used offshore and in related industry sectors.

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Contents Series Editor's Foreword Foreword

xiii xv

Acknowledgements

xvii

Notation

xxi

Chapter 1 Literature Review 1.1 Introduction and background 1.2 Review 1.3 Stress analysis of tubular joints 1.3.1 Definition of stresses in welded connections 1.3.2 Definition of hot spot stress 1.3.3 Methods of stress analysis 1.4 Fatigue design 1.4.1 S-N approach 1.4.2 The Fracture Mechanics (FM) approach 1.5 Summary

1 4 5 5 6 9 16 16 36 40

Chapter 2 Service Load Simulation 2.1 Introduction 2.2 Fatigue loading in Jack-up structures 2.3 Review of previous loading models 2.3.1 COLOS/C 12-20 series 2.3.2 UKOSRP II double-peaked spectrum 2.3.3 Hart/Wischung algorithm 2.3.4 WASH sequence 2.4 The JOSH model 2.5 Generation of JOSH 2.5.1 The pseudo random binary sequence technique 2.5.2 The Morkov chain technique 2.6 Jack-up dynamic response 2.6.1 The transfer function approach 2.6.2 Modelling of structural parameters 2.6.3 Modelling of soil-structure interaction

43 44 47 48 48 48 49 49 49 50 50 52 52 55 57

Contents

2.7 2.8 2.9 2.10

Modelling of wave loading Selection of sea states Discussion Summary

Chapter 3 Large-scale Fatigue Testing 3.1 Introduction 3.2 Test specimen consideration 3.2.1 Properties of SE 702 3.2.2 Consideration of test specimen geometry 3.2.3 Fabrication of SE 702 specimens 3.3 Experimental set-up 3.3.1 Details of test rig 3.3.2 Test control and data acquisition 3.3.3 Simulation of environmental conditions 3.4 Stress analysis of Y joints 3.4.1 Experimental stress analysis procedure 3.4.2 Use of parametric equations 3.5 Experimental fatigue testing 3.5.1 Test parameters and the JOSH sequence 3.6 Fatigue test results 3.6.1 Fatigue crack initiation 3.6.2 Crack growth curves 3.6.3 Crack aspect ratio evolution 3.6.4 S-N data 3.7 Discussion 3.8 Summary Chapter 4 4.1 4.2 4.3 4.4

4.5 4.6 4.7

x

Fracture Mechanics Analysis Introduction The stress intensity factor concept Experimental results Use of empirical SIF solutions 4.4.1 The average stress model 4.4.2 The two-phase model (TPM) 4.4.3 The modified average stress model Adapted plate solutions 4.5.1 Newman-Raju SIF solution for surface cracks New semi-empirical Y factor solution Variable amplitude crack growth models 4.7.1 Equivalent stress range approach 4.7.2 Equivalent crack growth concept

60 62 64 76

77 78 78 80 82 82 82 82 83 84 84 85 87 87 91 91 94 96 98 98 103

105 106 108 114 114 115 116 117 118 122 127 127 128

Contents

4.8 4.9

Consideration of sequence effects Fast assessment of offshore structures 4.9.1 New normalized PSD equation 4.10 Sea state probability model 4.10.1 Use of sea state probability distribution model 4.10.2 Formulation of the sea state equivalent stress concept 4.11 Discussion 4.12 Summary

130 132 134 136 138 139 140 147

Chapter 5 Conclusion 5.1 Summary 5.2 Conclusions and recommendations

149 149

References

153

Index

161

xi

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Series Editor's Foreword The nature of engineering research is such that many readers of papers in learned society journals wish to know more about the full story and background to the work reported. In some disciplines this is accommodated when the thesis or engineering report is published in monograph form - describing the research in much more complete form than is possible in journal papers. The Engineering Research Series offers this opportunity to engineers in universities and industry and will thus disseminate wider accounts of engineering research progress than are currently available. The volumes will supplement and not compete with the publication of peer-reviewed papers in journals. Factors to be considered in the selection of items for the Series include the intrinsic quality of the thesis, its comprehensive nature, the novelty of the subject, potential applications, and the relevance to the wider engineering community. Selection of volumes for publication will be based mainly upon one of the following; single higher degree theses; a series of theses on a particular engineering topic; submissions for higher doctorates; reports to sponsors of research; or comprehensive industrial research reports. It is usual for university engineering research groups to undertake research on problems reflecting their expertise over several years. In such cases it may be appropriate to produce a comprehensive, but selective, account of the development of understanding and knowledge on the topic in a specially prepared single volume. Authors are invited to discuss ideas for new volumes with Sheril Leich, Commissioning Editor in the Books Department, Professional Engineering Publishing Limited, or with the Series Editor. The fourth volume in the Series comes from London University and is entitled Fatigue and Fracture Mechanics of Offshore Structures by Linus Sone Etube Department of Mechanical Engineering, University College, London. The development and operation of the North Sea oil and gas fields represents a truly remarkable technological achievement. Many new engineering challenges have been encountered at various stages in the development and operation of the fields, and the present volume illustrates an engineering science approach to one of them. An important factor affecting the integrity of these huge offshore structures is the fatigue behaviour of welded tubular joints.

Fatigue and Fracture Mechanics of Offshore Structures

The author has studied many aspects of the fatigue characteristics of such joints in Jackup offshore structures. The application of millions of cycles of variable amplitude loading is related to sea states and wave motion. The development of a standard loading history is described and realistic environmental conditions are considered. An account is given of large-scale fatigue testing and the detailed analysis of fatigue crack initiation and growth; bringing together a balance of experimental and theoretical approaches to the problem. This latest contribution to the Engineering Research Series enlarges the scope of topics covered by the early volumes. Topics covered to date deal with aspects of design, surface inspection techniques, and wettability characteristics of engineering materials.

Professor Duncan Dowson Series Editor Engineering Research Series

xiv

Foreword

The tubular welded joints used in the construction of offshore structures can experience millions of variable amplitude load cycles during their service life. Such fatigue loading represents a main cause of degradation of structural integrity in these structures. As a result, fatigue is an important consideration in their design. Jack-up legs are made from a range of high-strength steels with yield strengths up to 700 MPa. These steels were thought to exhibit fatigue resistance properties which are different when compared with conventional fixed platform steels such as BS 4360 50D and BS 7191 355D. The perceived difference in their behaviour was heightened by the discovery, in the late 1980s and early 1990s, of extensive cracking around the spud-can regions of several Jack-ups operating in the North Sea. It was thought that these steels might be more susceptible to hydrogen cracking and embrittlement. This enhanced the need to study their behaviour under representative service loading conditions. This book contains results of an investigation undertaken to assess the performance of a typical high-strength weldable Jack-up steel under realistic loading and environmental conditions. Details of the methodology employed to develop a typical Jack-up Offshore Standard load History (JOSH) are presented. The factors that influence fatigue resistance of structural steels used in the construction of Jack-up structures are highlighted. The methods used to model the relevant factors for inclusion in JOSH are presented with particular emphasis on loading and structural response interaction. Results and details of experimental Variable Amplitude Corrosion Fatigue (VACF) tests conducted using JOSH are reported and discussed, with respect to crack growth mechanisms in high-strength weldable Jack-up steels. Different fracture mechanics models for VACF crack growth prediction are compared, and a novel improved generalized methodology for fast assessment of offshore structural welded joints is presented.

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Acknowledgements

I wish to express my deep appreciation for the support offered by the following individuals and organizations. First, I would like to thank Dr F. P. Brennan and Professor W. D. Dover for the invaluable supervision and guidance given during the course of my PhD programme. Their stimulating discussions are invaluable. I would like to thank Professors L. F. Boswell and J. Sharp for taking the time to review the manuscript and for their fine suggestions and recommendations. I would also like to thank everyone in the Department of Mechanical Engineering, at University College London, especially my colleagues who all played an important role in providing the conducive and intellectually stimulating environment under which work presented in this book was conducted. I acknowledge financial support received from the Cameroon Government through the Ministry of Higher Education and Scientific Research in the form of an Overseas Scholarship, which enabled me to acquire a solid grounding in the field of mechanical engineering, subsequently developing a keen interest in structural integrity of engineering structures. The financial support of a number of organizations made the successful completion of the work reported in this book possible. I acknowledge the following organizations for their contribution (in-kind or financial): the UK Health and Safety Executive (HSE); British Steel (now The Corus Group, from a merger between British Steel and Koninklijke Hoogovens); Statoil; and Creusot Loire Industrie. Dr L S Etube Department of Mechanical Engineering University College London, UK

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To my family This book is dedicated to my entire family. To my late father, Daniel Etube, who spent the greater part of his life educating and encouraging people to aspire for a deeper understanding of the things around them. To my mother - Christina Etube - my brothers, my sisters, and my wife - Delphine Ntube Etube. Their love, support, and guidance will always remain an inexhaustible source of inspiration for me.

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Notation

Roman

af A, p Ag/AgCl

Crack depth (except where otherwise defined), half surface crack length Initial crack depth Final crack depth (except where otherwise defined) Scaling parameters (except where otherwise defined) Silver/silver chloride reference electrode

C, m CD, Cm C mj J, Cp, p

Paris law material constants Drag coefficient, mass coefficient Paris law material constants for a multi-segment da/dN curve Empirical retardation parameter, shaping parameter

da/dN D Ds

Crack growth rate Tubular joint chord diameter Miner's cumulative damage ratio

erf(x) E

Error function of x Young's modulus

f, fn,

fnB, fB, £2B

Frequency, natural frequency, peak frequency Frequency corrected non-dimensional parameters

G

Shear modulus

a, c a

ao, i

fp

Hs, TD, Tz Hr, Tr, fl

Extreme sea state parameters Significant wave height, dominant period, mean zero crossing period Non dimensional wave height, Period and frequency ratios

I

Irregularity factor

Hs ext, Tz ext

KISCC

Maximum and minimum stress intensity factors Critical mode I stress intensity factor Stress intensity factor for stress corrosion cracking

L

Tubular joint chord length

Mi M x d1

ith spectral moment ACPD crack depth modifier, one-dimensional ACPD solution

N, N1, N3

Life (number of cycles), initiation life, through thickness life

Kmax,

Kmin

KIC

Notation

P(x), p(x)

Exceedance of variable x, probability of occurrence of variable x

VQB VQr

UEG diameter ratio modifying parameter

R

Spud-can radius or chord radius (defined where applicable)

S, AS, Aa SB, tB y S(f)N

Stress range Thickness effect parameters Equivalent stress, equivalent stress for sea state i Normalized response spectrum

t T

Brace thickness or time or plate thickness (defined where applicable) Tubular joint chord thickness

Y(a), Y

Stress intensity factor correction function (Y factor)

Sh, Shi

UEG short chord modifying parameter

Greek Symbols a, p, Y, T, 0 Tubular joint dimensional parameters (except where otherwise defined) a Twice the ratio of chord length to chord diameter (2L/D) Ratio of brace diameter to chord diameter (d/D) P Ratio of chord diameter to twice chord thickness (D/2T) Y e Spectral bandwidth parameter e Angle around tubular joint chord/brace intersection v Poisson's ratio Damping ratio e Root mean square (RMS) value a Crack tip opening stress, stress intensity factor corresponding to aop Oop, Kop Yield strength Oy Ratio of brace thickness to chord thickness (t/T) T Angular position or angle of inclination o Gamma function of x r(x) Stress intensity factor range AK Threshold stress intensity factor range AKth, Effective stress intensity factor range AKeff A, Vc, Vr, ACPD probe spacing, crack voltage, reference voltage Stress spectrum given by Wirsching's equation D(f) Crack shape correction (CSC) factor ¥ Acronyms ACPD AVS CP

xxii

Aalternating current potential difference Average stress Cathodic protection

Notation

CLI FEA IPB JOSH MAVS SCF TPM UKOSRP UEG UCL UKCS UTS OPB PSD SENB WASH

Creusot Loire Industrie Finite element analysis In-plane bending Jack-up offshore standard load history Modified average stress Stress concentration factor Two-phase model United Kingdom Offshore Steels Research Project Underwater engineering group University College London United Kingdom Continental Shelf Ultimate tensile strength Out-of-plane bending Power spectral density (spectrum) Single edge notch bend Wave action standard history

xxiii

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Chapter 1

Literature Review

1.1 Introduction and background In recent years there has been considerable interest in the use of high-strength steels in the construction of offshore structures. One main reason for this is to satisfy the desire for lightweight constructions. This is particularly relevant to offshore structures, because a reduction in weight can lead to the achievement of considerable saving in support substructure. There are other potential benefits to be derived from the use of high-strength steels. Fabrication costs, for example, can be minimized through the use of reduced plate thicknesses. Historically, there has been a great deal of interest in the use of high-strength steels for the fabrication of Jack-up structures when compared with fixed platforms. However, the potential benefits of using high-strength steels have been recognized by the offshore oil and gas industry, and a fairly recent review [1.1] showed that the proportion of higher strength steels used in fixed offshore structures had gone up to 40 per cent by 1995. This is a five-fold increase when compared to 8 per cent in 1988. It is important to note that most of the high-strength steels used in fixed structures are limited to topside applications and other less critical parts of the structure where fatigue damage is not a major concern. This situation is, however, different for Jack-up platforms that have traditionally been used for short-term drilling and maintenance operations. These structures are now being increasingly used as production platforms for marginal field development. In recent designs, extended periods at the same elevation in their fatigue design philosophy are included. BP Harding is a typical example of this new generation of Jack-up platforms. It is designed to operate in a water depth of 100 m with an intended service life of thirty-five years.

1

Fatigue and Fracture Mechanics of Offshore Structures

High-strength steels used in the construction of Jack-up structures are mainly limited to the fabrication of the legs. Steels with nominal yield strength in the range of 450 to 700 MPa have commonly been used. The detailed leg structure will vary from one type of Jack-up to another. A review of different designs is presented in [1.2]. In this review the structures were classified according to the Jack-up design, some of which include Le Toumeau, CFEM, MSC, Friede and Goldman, and Hitachi designs. In general, each leg (Fig. 1.1) is made of three or four longitudinal chord members that may contain a rack plate for elevating the hull and a series of interconnecting horizontal and diagonal tubular members (Fig. 1.2). In some designs, supplementary braces are frequently used between main brace midpoints to increase the buckling resistance of the structure and to provide adequate structural redundancy.

Fig. 1.1 Typical lattice leg structure of a Jack-up platform

Fig. 1.2 Typical Jack-up leg chord with rack plate

2

Literature Review

There has recently been a remarkable increase in the overall size of these structures. The main reason for this is to satisfy the requirement to operate in deeper waters in predominantly harsher sectors of the North Sea, and worldwide. Several Jack-ups are now used for production. This role requires long-term deployment and, therefore, limits the opportunity for dry dock inspection and repair. This has increased the risk of deterioration from long-term problems such as fatigue. The vast majority of research on the fatigue performance of tubular welded joints, carried out by the offshore oil and gas industry [1.3, 1.4] has focussed on conventional fixed offshore platform steels such as BS 4360 50D [1.5] and BS 7191 355D [1.6] with typical yield strengths in the region of 350 MPa. Fatigue data on higher strength tubular joints are, therefore, very limited and this has been highlighted in reviews [1.2, 1.7]. Consequently the fatigue design guidance developed to date is not applicable to high-strength steels, and this is reflected in the guidance published in 1995 [1.8]. This document has now been withdrawn, but the basic design, curve is restricted to steels with guaranteed yield strengths of up to 400 MPa for nodal joints and 500 MPa for welded plate connections. Even the current draft ISO standard [1.9] only provides guidance for steels with yield strength less than 500 MPa. The absence of sufficient guidance on the use of high-strength steels and the lack of fatigue data is a matter for concern. This is all the more important due to the increasing proportion of higher strength steel grades used in offshore applications. This concern was strengthened by the discovery of extensive cracking in the spud-can region of Jack-ups operating in the United Kingdom Continental Shelf (UKCS) in the period of 1988-89 [1.10]. As a result, it was accepted that high-strength steels are more susceptible to corrosion fatigue and hydrogen-induced stress corrosion cracking (HISCC) when compared with conventional fixed platform steels. The main conclusion drawn from the investigation was that the generation of hydrogen from the sacrificial anode systems protecting high-strength steel structures at levels that are excessively negative (i.e.