Margins of safety in FPSO hull strength

Recent studies were carried out to determine the variation in safety factors (between the applied loading and the capacity) of FPSOs currently operating in the UK Sector of the Central North Sea, and designed using either a class rule type approach or a more advanced hydrodynamic – FE type approach. Results from these studies seem to indicate that the safety factor in both cases is similar, since the advanced hydrodynamic / FE type approach tends to result in higher loadings but also higher capacities. However, it is not clear to what extent these results can be applied to other FPSOs, or to other areas. There is also an apparent variation in reserve strength against hogging and sagging bending moments. This is particularly true for double hull FPSOs that tend to have a higher capacity at the bottom of the hull in comparison to the deck. The purpose of this report is to collate available data on the range of variation of safety factors of currently used FPSOs, and to consider how more consistent margins of reserve strength for various FPSO configurations under various loading conditions might be achieved. This report and the work it describes were funded by the Health and Safety Executive. 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.