RESEARCH REPORT 220 - Health and Safety …

1 HSE Health & Safety executive Ship collision and capacity of brace members of fixed steel offshore platforms Prepared by Visser Consultancy for the Health and Safety executive 2004 RESEARCH REPORT 220 HSE Health & Safety executive Ship collision and capacity of brace members of fixed steel offshore platforms Dr W (Pim) Visser VISSER CONSULTANCY Zomervlinderberm 66 3994 WR Houten The Netherlands Supply boat collision is one of the most critical accidents that can affect structural Safety and integrity of fixed installations Pim Visser, an experienced engineer has disentangled much of the RESEARCH complexity and provided a practical andaccessible resource for assessing impact resistance. The capacity of members damaged due to vessel impact has been researched extensively because of the significant Safety implications.

2 However different approaches have been taken and, whilst many of the findings are in the public domain, it has not been an easy task for designers or assessment engineers to locate the information or apply it consistently. The study was therefore commissioned by HSE to collate and assess the available RESEARCH and methodologies, focusing particularly on bow or stern impacts on splash zone braces. Practical demonstrations draw on insight from Safety cases, with the recommended approach being delivered in the form of a spreadsheet with worked examples for wider application. Having compared the implications of four different norms for brace assessment, viz: Maximum joint rotation Ductility ratio Maximum brace deflection Maximum weld strain and strain hardeningresults from sample calculations were in sufficiently close agreement for an averaged norm to be adopted in the final recommendation.

3 Denting as well as member deformations are considered. Concerns for risers within the jacket can best be checked against a maximum deflection criterion of one metre. Material and tubular joint considerations relevant to brace impact assessments are also addressed. It was concluded, for example, that: CTOD tests are performed at far too slow a rate to represent vessel impact conditions and use of Charpy test results isto be preferred. Although prior to brace failure tension may be significant, its contribution in impact energy absorption can generally bedisregarded simplifying the calculations significantly. Based on dynamics considerations, brace impact may be assessed as a purely local problem. Even if a node fails before the member, further failure at the joint in shear may not follow and subsequent bracebehaviour can be calculated.

4 Engineers designing for or assessing the capacity of vessel impact of jacket structures will find this REPORT to be invaluable. This REPORT and the work it describes were funded by the Health and Safety executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy. HSE BOOKS Crown copyright 2004 First published 2004 ISBN 0 7176 2838 8 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 written permission of the copyright owner. Applications for reproduction should be made in writing to.

5 Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to ii CONTENTS 1 INTRODUCTION 1 2 SUMMARY 3 Major impact and platform findings 3 Norms for brace assessment under impact 4 On methodologies 4 Other findings 4 3 VESSEL AND SERIOUS INCIDENTS 7 Ship impact energy absorption capacity 7 Serious incidents and ship impact zone 7 Guidance on impact energy 8 Other considerations 9 4 PLATFORM GEOMETRY 11 Platform aspects 11 Other platform features 13 Risers and riser deflection effects 15 Risers.

6 Riser caissons and riser protection structures 16 Conductors 16 5 IMPACT DETAILS 17 Denting 17 Miscellaneous aspects of denting 19 6 BRACES - BEAM BENDING ANALYSES 21 Introduction 21 Properties of thin walled tubulars 21 Principle of Virtual Work 21 An elastic beam in three point bending 23 iii Elasto-plastic three-point bending problem 24 Three-point bending with tension - elastic 25 Three-point bending with tension - tubular, plastic to API 26 Three-point bending with tension - joints.

7 Plastic to HSE guidance notes 29 Cross-bracing capacity 31 Simplified energy expression 32 7 MATERIAL AND JOINTS 35 Introduction 35 Material and fracture toughness 35 Geometry and local modelling 35 Strain rate effect 36 CTOD tests 36 Charpy test 37 Maximum strain criterion 37 Strain hardening 37 Joints 38 Impact close to a node 40 8 SAMPLE PROBLEMS 43 Description of the test cases 43 Vessel impact analysis 43 Variations in the sample problems 47 9 REFERENCES 51 APPENDIX A BEAM BENDING ANALYSIS iv I ABBREVIATIONS API American Petroleum Institute LAT low astronomical tide CTOD crack tip opening displacement LRFD load and resistance factor design DP DSV HSE EA DE Edu &KMA.

8 MEMC MMMMMe el maxp u WTTPPPPPP 1 2 d e u Q R S T p u E WI dynamic positioning MSL mean sea level diving support vessel SCF stress concentration factor Health and Safety executive LIST OF SYMBOLS tube cross sectional area tube diameter Young s modulus dent energy brace energy associated with plastic deflection (u) tube moment of inertia rate of stress intensity factor (for CTOD assessment) plastic moment at the tube end points A and E plastic moment at the centre of the brace maximum elastic moment maximum elastic moment maximum moment tube full plastic moment joint ultimate out-of-plane moment capacity lateral force due to impact lateral force before joint failure lateral force after joint failure force to cause a dent dd maximum elastic lateral force ultimate lateral force force equal to P/2 shear resistance of tubular elastic section modulus tensile force plastic tensile force of tubular ultimate joint tensile force external work (in Principle of Virtual Work) internal work (in Principle of Virtual Work)

9 ZZ plastic section modulus of tubular d plastic section modulus of dented tubular wwwuma quarter brace length (l /4) b a characteristic length l brace length p plastic moment of the tube wall t wall thickness timpact impact duration u central plastic deflection mean mean value of u centre central elastic deflection B elastic deflection at point B max maximum elastic deflection dd dent depth & strain rate j angle over cross-section gA, gE ratio between joint strength and tube strength at points A and E (gA, gE < ) l characteristic eigen value for beam qqqqqqin tension / bending 1 end rotation in point A 2 end rotation in point E ave average end rotation crit.

10 Critical vale of end rotation max max. value of end rotation max maximum value of end rotation to qMarshall min minimum value of end rotation to Marshall su ultimate stress sy yield stress v INTENTIONALLY BLANK vi 1 INTRODUCTION In the former HSE Guidance Notes1 and the Safety Case Regulations2 a number of accidental scenarios for fixed steel offshore structures have been identified. One of the potentially more critical accidental events is supply boat collision3. Common designs, when compliance with the HSE Guidance Notes is sought, address the supply boat collision through a combination on described forces, described impact energy4 together with elastic and plastic deformation of tubulars, the platform as a whole and the supply boat.