Offshore Structures Volume II • Strength and Safety for Structural Design

Gunther Clauss, Eike Lehmann and Carsten Ostergaard Translated by M. J. Shields, FIInfSc, MITI

Offshore Structures Volume II Strength and Safety for Structural Design

With 176 Figures

Springer-Verlag London Berlin Heidelberg New York Paris Tokyo Hong Kong Barcelona Budapest

Gunther Clauss, Prof. Dr.-Ing. Technische Umversltat Berlin, InstItut fur SchIfTs- und Meerestechmk, Salzufer 17-19, 10587 BerlIn 10, Germany

Eike Lehmann, Prof. Dr.-Ing. ,Arbeitsbereich SchIfTstechmsche KonstruktlOnen und Berechnungen, Technische Umversitat HamburgJHarburg, Lavenbruch Ost, 21071 Hamburg 90, Germany

Carsten Ostergaard, Dr.-Ing. Germamscher Lloyd AG, Vorsetzen 32, D-20459 Hamburg 11, Germany

Translator M. J. Shields, FllnfSc, MITI LIterary ana Techmcal Language ServIces, 199 The Long Shoot, Nuneaton, Warks CVll 6JQ, UK

Cover Illustrations· Ch.2, FIg. 3 (Vol. I). ComplIant piled tower. Ch.7, FIg. 8. Safety domaIns in an A-W (stress strength) safety format.

lSBN-13: 978-1-4471-2000-1 e-lSBN-13: 978-1-4471-1998-2 DOT: 10.1007/978-1-4471-1998-2 BritIsh LIbrary Cataloguing In Publication Data Clauss, Gunther

Offshore Structures. -Vol 2· Strength and Safety for Structural DesIgn I. TItle II. ShIelds, M. J 627.98

lSBN-13: 978-1-4471-2000-1

LIbrary of Congress CatalogIng-In-PublicatIon Data (RevIsed for volume 2) Clauss, Gunther

Offshore structures. Rev. translatIOn of: Meerestechmsche Konstruktionen. Includes bibliographIcal references and Indexes Contents. v. 1. Conceptual deSIgn and hydromechamcs

-v. 2. Strength and safety for structural deSIgn 1. Offshore structures - DeSIgn and constructIOn.

2. Ocean engIneerIng. I. Lehmann, E (Elke). II. Ostergaard, C (Carsten) III TItle. TCI165.C5313 1992 627'.98 91-37296

Apart from any faIr dealIng for the purposes of research or prIvate study, or CrItICIsm or review, as permitted under the CopYrIght, Design and Patents Act 1988, thIs publIcatIOn may only be reproduced, stored or transmItted, In any form or by any means, WIth the prior permission In WrItIng of the publIshers, or in the case of repro graphIc reproductIOn In accordance WIth the terms of lIcences Issued by the Copyright LIcenSIng Agency. EnqUIries concermng reproductIOn outSIde those terms should be sent to the publishers.

EnglIsh translation © SprInger-Verlag London LimIted 1994

FIrSt publIshed in German as M eerestechmsche KonstruktlOnen by SprInger-Verlag Berlin HeIdelberg New York © 1988 Softcover reprint of the hardcover 1st edition 1988

The publIsher makes no representatIOn, express or ImplIed, WIth regard to the accuracy of the InformatIOn contaIned In this book and cannot accept any legal responSIbility or hablllly for any errors or omiSSIOns that may be made.

Typeset by Thomson Press (IndIa) Ltd., New Deihl

69/3830-543210 Printed on aCId-free paper

Contents

Preface to Volumes I and II . . . . . . . . . . . . . . . . . . . . . IX

Note on Terminology . . . . . xiii

4 Marine Structural Analysis . . . . . . . . . . . . . . . . . 1 4.1 Time-Independent Elastic Problems . . . . . . . . . . . . 2

4.1.1 Frameworks ........ .......... 3 4.1.2 Membranes ... . . . . . . . . . . . . . . . . . 13 4.1.3 Plates ....................... 20 4.1.4 Cylindrical Shells . . . . . . . . . . . . . . 28

4.2 Stability and Second-Order Stress Theory ..... 35 4.3 Time-Dependent Elastic Problems . . . . . . . . . . 59

4.3.1 Natural Frequencies of Beams and Plates .. 59 4.3.2 Forced Oscillations ............... 67

4.4 Ultimate Load Analysis ................ 76 4.4.1 Plastic Capacity under Combined Loading . 78 4.4.2 Ultimate Loads of Simple Structures 82

4.5 Numerical Methods ................... 88 4.5.1 Finite Element Method . . . . . . . . . . . . . 88 4.5.2 Structural Modelling with Finite Elements 109

4.6 List of Symbols ..................... 120

5 Environmental Conditions Affecting Marine Structures . . .. 125 5.1 Evaluating Stochastic Processes ........ 126

5.1.1 Stationary Random Processes .......... 126 5.1.2 Stationary Gaussian Random Processes .... 128 5.1.3 Stationary Poisson Random Processes and

Markov Chains . . . . . . . . . . . . . . . 132 5.1.3.1 The Poisson Random Process. . . . . . .. 132 5.1.3.2 Markov Chains ................ 133

5.1.4 Linear Systems with One Degree of Freedom . .. 135 5.2 Evaluating Random Processes in the Marine

Environment . . . . . . . . . . . . . . . . . . . . . . . . . 139 5.2.1 Probabilistic Description of the Stationary

Seaway. . . . . . . . . . . . . . . . . . . . . . . . . 140

VI Contents

5.2.1.1 The Superposition Model of the Seaway. 140 5.2.1.2 The State Space Model of the Seaway . 145 5.2.1.3 Probabilistic Seaway Parameters . . . . . 146

5.2.2 Statistical Analysis of the Seaway . . . . . . . . . 149 5.2.2.1 Short-Term Statistics 149 5.2.2.2 Long-Term Statistics 156

5.2.3 Wind and Sea Currents . 164 5.2.3.1 Wind ......... 164 5.2.3.2 Sea Currents . . . . . . . . . .. 168

5.3 List of Symbols .. . . . . . . . . . . . . . . . . . . . . 170

6 Evaluation of Marine Structures . . . . . . . . . . . . . . . . 177 6.1 Classical Methods of Short-Term Evaluation 178

6.1.1 Floating Structures ................. 178 6.1.2 Flexible Mooring of Floating Structures . . . . . 182 6.1.3 Fixed Structures ................... 192

6.1.3.1 Monopod Platforms in a Stationary Seaway .................... 193

6.1.3.2 Linearly Elastic Structures with more than One Degree of Freedom ........... 196

6.2 Classical Methods of Long-Term Evaluation. . . . . .. 199 6.2.1 Design Values for Environmental Loads . . . . .. 200

6.2.1.1 Design Value of an Individual Wave Load. . . . . . . . . . . . . . . . . . . . . .. 200

6.2.1.2 Comparative Evaluation of Wave Loads.. 201 6.2.1.3 Superposition of Load Processes ...... 205

6.2.2 Fatigue Strength Models ............... 211 6.2.2.1 Fatigue Strength Modelling . . . 211 6.2.2.2 Crack Propagation Modelling ....... 214 6.2.2.3 Stochastic Evaluation of Fatigue

Strength . . . . . . . . . . . . . . . . . . . .. 215 6.2.3 Fatigue Strength under Seaway Loads . . . . . .. 218

6.2.3.1 Deterministic Method of Analysis 219 6.2.3.2 Spectral Analysis Method . . . . . . . . .. 221 6.2.3.3 Simulation .. . . . . . . . . . . . . . . . .. 222 6.2.3.4 Design for Fatigue Strength . . . . . . . .. 225

6.3 Modern Methods of Reliability Analysis ... 228 6.3.1 Reliability of Load-Bearing Structural

Elements .................. 229 6.3.2 Reliability of Load-Bearing Structural

Systems ................... 240 6.3.2.1 Structures Behaving Like Series or

Parallel Systems . . . . . . . . . . . 241 6.3.2.2 Structures Behaving Like Redundant

Systems . . . . . . . . . . . . . . . . 245 6.3.3 Reliability and Risk as Functions of Time . . . . 252

Contents vii

6.3.3.1 Fatigue Failure Probability Modelling with Stress and Strength being Functions of Time . . . . . . . . . . . . . . . . . . . . 254

6.3.3.2 Hydrodynamic and Structural Analysis .. 257 6.3.3.3 Multi-Dimensional Response Surfaces

for Spectral Moments ............ 259 6.3.3.4 Stochastic Models of Basic Parameters

and Data . . . . . . . . . . . . . . . . . . .. 260 6.3.3.5 Numerical Estimation of Fatigue Failure

Probability as a Function of Time 264 6.3.3.6 Adaptive Inspection Planning . . . 266

6.4 List of Symbols . . . . . . . . . . . . . . . 271

7 Dimensioning of Marine Steel Structures . . . . . . . 281 7.1 Fabrication and Materials ............ 282 7.2 Dimensioning in Accordance with Regulations 283

7.2.1 Dimensioning of Slender Stiffeners for Plates Subject to Pressure ............ 283

7.2.2 Dimensioning of Cylindrical Structures 285 7.2.3 Tube Joints . . . . . . . . . . . . . . . . . 290

7.3 Fatigue Strength Evaluation on the Basis of Regulations ..................... 293 7.3.1 Fatique Design on the Basis of GL

Requirements . . . . . . . . . . . . . . . . . 293 7.3.2 Fatigue Design on the Basis of API-RP2A

Recommendations . . . . . . . . . . . . . . 294 7.3.3 Fatigue Analysis Based on API-RP2A

Recommendations . . . . . . . . . . . . . . 296 7.4 Development of Modern Regulations . . . . . . 298

7.4.1 Development of Regulations Based on Reliability Technology . . . . . . . . . . . . . . . . . 299

7.4.2 Principles of Quality Assurance and Classification . . . . . . . . . . . . 304

7.5 Examples of Structural Components 306 7.6 List of Symbols . . . . . . . . . . . . . . 315

Appendix .......................... 319 Al Selected Principles of Probability Theory 319 A2 Selected Principles of Matrix Calculus . . . . . . 324

Subject Index ............................. 333

Preface to Volumes I and II

As always with new technology, research and development needs a certain time for specialist knowledge arising from individual, arbitrary cases of practical or engineering activity to develop into a clear field of knowledge that can be placed in a general context. We are convinced that this point has now been reached in the field of analysis and evalua­tion of offshore structures, and we hope to produce the proof in this edition. Our main aim is to present the basics of the technology as broadly as possible, so as to facilitate technical communication between specialists in different and diverse disciplines relevant to the develop­ment of offshore structures.

In order to maintain this broad approach, which is also applicable to many other marine structures (including ships), we have deliberately but with considerable regret omitted some special fields that could have been covered in depth on the basis of current knowledge. It seemed more important, in terms of explaining the analysis and evaluation of offshore structures, that the reader should develop practical solutions directly from as many concrete examples as possible, or be referred to further special literature. For these, we have turned especially to design and development engineers who are trained or in training situations, and who are familiar with the basics of the mathematical and physical approach to technical problems.

In this way, the book serves as an introduction for students and practising engineers, or as a source of information for experts in this field, or for specialists in adjacent fields who wish to obtain an overall picture of where their own work on individual problems of offshore structures stands in relation to the field as a whole. We know that the text will at times be difficult for the reader, but we also believe that these difficulties will be repaid at the very next encounter with the practical problems of offshore structures.

The development of a structure basically requires an iterative process which can be roughly defined in terms of three stages - concept specifi­cation, analysis, and evaluation. The term analysis signifies the mathe­matical modelling and expression of the behaviour of a structure under arbitrary boundary conditions, while the term evaluation describes the

x Preface to Volumes I and II

determination of particular boundary conditions, the fulfillment of which can be accepted for data used for analysis or for the results obtained by analysis.

In support of this process, and in order to explain comprehensively the practically relevant basics for the development of offshore structures, we have arranged these books such that after an introductory survey of the field of marine science and ocean engineering (Chapter 1), alter­native concepts in offshore structures are laid down and explained (Chapter 2). We then proceed to cover as extensively as possible the main points in the analysis of offshore structures - hydromechanical analysis (Chapter 3) and structural analysis (Chapter 4) - in the frame­work of the special requirements of this field. In design practice, such analyses are bound up in a stochastic evaluation concept, which divides into the evaluation of the various environmental conditions under which offshore structures operate (Chapter 5), and the evaluation of the struc­tures themselves (Chapter 6). From this basis we go on to discuss new developments and peculiarities of dimensioning practice according to Regulations (Chapter 7). In all chapters our objective is to keep the material as self-contained and as comprehensible as possible for the reader, who is then referred to further literature.

The themes of this book, integrating different classical and modern fields of engineering science, arise from lectures we have given on the basics of hydromechanics, and on structural analysis and mechanical reliability evaluation, as well as from our long design practice in offshore structures and their components. The work in manuscript was divided according to our individual fields of interest: overall control of Chapters 1-3 went to G. Clauss, while Chapter 4 and Sections 7.1, 7.2 and 7.5, together with Appendix A2, were the responsibility of E. Lehmann. In Chapters 5 and 6, along with Sections 7.3 and 7.4 plus Appendix AI, C. Ostergaard had overall control. From these basics, we proceeded from individual sections to an integrated whole. Because of field-specific conventions which could not simply be ignored, it was impossible to use all symbols consistently: in some cases the same symbol means different things in different disciplines, so that the nomen­clature and basic methodology had to be separate for each chapter. This is particularly useful for readers who wish to use the books as a reference work on methods of rational analysis and evaluation of offshore structures within specific problem areas.

The books arose from the initiative of Springer-Verlag, who proved to be an understanding partner and support in all questions of form and production during the development of the manuscript. Basic to the text is an extensive treasure-trove of experience which we had access to at the Technical University of Berlin, the Technical University of Hamburg-Harburg, and Germanischer Lloyd, as well as through published research and development work and participation in projects of the offshore construction and shipbuilding industries. For many

Preface to Volumes I and II Xl

reasons we must be grateful for the facilities of the above universities and of Germanischer Lloyd used in the development of these books. We would especially like to thank the following colleagues and fellow­workers who have contributed towards various parts of this work:

At the Institute of Naval Architecture and Ocean Engineering of the Technical University of Berlin it is especially necessary to thank Mr H. Hohne, who with skill and endurance prepared the illustrations and diagrams for the first three chapters, overcoming with great patience the often contradictory requirements regarding layout. Corrections and completion of die manuscripts of the first three chapters lay in the hands of Messrs T. Riekert, L. Birk, and J. Heeg, who in an outstanding example of cooperation and commitment produced the final text.

In the Department of Marine Structures and Structural Analysis of the Technical University of Hamburg-Harburg, we were especially grateful to Messrs G. Nickel and W. Prediger for typing of the manu­script and preparation of diagrams for Chapter 4 and parts of Chapter 7.

In the Hydromechanics and Reliability Department of Germanischer Lloyd, Dr. T. E. Schellin and Dr. T. Jiang discussed special hydro­dynamical questions of Chapters 3,5 and 6, and Dr. G. Schall of the Technical University, Munich made valuable contributions to the last part of Chapter 6. M~ K. Hamann of Germanischer Lloyd prepared most of the illustrations for Chapters 5 and 6, which were eventually adapted to this English edition by Mr H. Hohne of the Technical University Berlin.

This English edition in two volumes is the translation of the one volume German text of 1988, which was carefully reviewed and updated, and to which essentially new parts have been added in Chapters 2,3,6 and 7.

G. Clauss E. Lehmann

C. Ostergaard

Note on Terminology

In the process of translating these books it was necessary to consider not only the correspondence between technical terms in German and English but also terminology in English itself. This proved especially difficult in the terms 'marine technology' and 'ocean engineering', which are often used interchangeably in English, yet which can have quite different implications.

For example, the term 'ocean engineering' is used in the names of technical universities and. institutions and for many committees, con­ferences, and journals which cover engineering in the marine environ­ment, while 'marine technology' is often associated with instrumentation in marine science. Yet, by definition, 'marine technology' must cover non-engineering activities in the marine environment, while 'ocean engineering' cannot.

For this reason, we eventually decided to use the terms 'marine science' and 'marine technology' in exact correspondence with the terms 'science' and 'technology' in the broader sense. In other words, all applications of technology in the marine environment come under the umbrella of 'marine technology'. This includes 'ocean engineering', which refers specifically to marine-based engineering operations, and not to other areas of marine technology such as marine biotechnology, or navigation.

We hope this definition will find general acceptance in the field, as it seems to us to be both logical and consistent.

G. Clauss, E. Lehmann, C. Ostergaard (authors) M. J. Shields (translator)