Growth of through-wall fatigue cracks in brace members

The total fatigue life of a brace in an offshore jacket structure is conventionally considered in four parts. N 1 is the number of cycles to initiate the first discernible surface cracking as noted by any available method. N 2 is the number of cycles to detect surface cracking by visual examination without the use of crack enhancement or optical aids. N 3 is the number of cycles until the first through wall cracking and N 4 is the total number of cycles to the end of test or final separation of the member. In UK waters, flooded member detection (FMD) by ultrasonic inspection with a remotely operated vehicle is used to check whether through cracks are present; however, in practice, fatigue cracks are likely to continue to grow around the brace circumference after breaking through-wall. A review by Sharp (Ref.1) concluded that detailed knowledge of the crack shape development after breakthrough together with a value for the ratio N 4 /N 3 are required. From the structural safety viewpoint, there is clearly a need to quantify the rate of fatigue crack growth after development of a through wall crack, but prior to the point at which final separation becomes a possibility. The present study was designed to examine these factors for circumferential welds in tubular members, and hence allow the efficacy of the FMD strategy to be assessed. 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. 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.