The properties of extreme waves

In the safety assessment of both fixed and floating offshore structures it is necessary to ensure that the structure has sufficient strength to withstand the most extreme combination of environmental loads likely to be experienced during the design life. However significant uncertainties remain concerning the characteristics of real, extreme, three-dimensional waves. The research described in this report focuses upon: � Wave crest heights and the potential loss of air gap for fixed structures by examining the distribution of wave crest elevations in storms. � Directional spreading of wave energy and the effect upon particle kinematic field by examining the wave spreading factor, (also known as wave kinematics factor) for translating between 2-D and 3-D seas. In parallel research has been undertaken on wave front steepness using the same full scale data and this is being published [Stansell et al, (2003)]. 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. ii ACKNOWLEDGEMENTS The authors are grateful to Total for supporting the operation of the environmental monitoring station and data collection programme at North Alwyn platform and to Health and Safety Executive Offshore Safety Division, United Kingdom, for funding this research. The authors are grateful to Jun Zhang and his research group, Ocean Engineering Program, Texas A&M University, USA for the permission to use the Hybrid wave model program. The authors extend their thanks to Marc Prevosto, IFREMER and George Z. Forristall, Shell International exploration and production, The Netherlands, for helpful discussions on the crest height probability models. iii iv CONTENTS Page No. Executive Summary vii 1. Introduction 1 1.1 General background 1 1.2 Project objectives 2 1.3 The North Alwyn Metocean Station and data collection 2 2. Storm Data Analysis 5 2.1 Storm data selection and methodology 5 2.2 Directional spectra and spreading functions 5 2.3 Variations in wave parameters 6 2.4 Variation in other directional parameters during storms 7 2.5 Spreading index 7 2.6 Wave kinematics factor 9 3. Crest Height Distribution 11 3.1 Wave crest height probability 11 3.2 Comparison of …