Wave mapping in UK waters

1 Executive Health and Safety Wave mapping in UK waters supporting document Prepared by PhysE Limited for the Health and Safety Executive 2008 RR621 Research Report Executive Health and Safety Wave mapping in UK waters supporting document Martin O Williams Metocean Advisor PhysE Limited The Old Customs House The Quay Yarmouth Isle of Wight PO41 0PG This work updates the contour map of 50-year extreme significant wave height that is provided in OT 2001/010 (previously Section 11 of the Guidance Notes). The updated map, now presented for the 100 year return period, presents extreme significant wave height in UK waters derived from 373 data sets from the NEXTRA hindcast, calibrated against measured wave data and verified against established criteria.

2 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 2008 First published 2008 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: Licensing Division, Her Majesty s Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to ii ACKNOWLEDGEMENTS The author and editor gratefully acknowledge the valuable contribution made to this study by the following industry representatives: Dr Colin K Grant Metocean Specialist /Deepwater Facilities BP Exploration Mr Ian M Leggett Discipline Head -Metocean Engineering Shell EP Europe iii iv TABLE OF CONTENTS EXECUTIVE SUMMARY.

3 VII 1. 1 2. DATA SOURCES .. 2 THE NESS, NEXT AND NEXTRA WAVE HINDCASTS .. 2 VERIFICATION DATA .. 5 ESTABLISHED CRITERIA .. 8 3. THE mapping DERIVATION OF EXTREME VALUES .. 10 GRIDDING AND mapping .. 10 4. ASSESSMENT OF NEXTRA PERFORMANCE .. 14 COMPARISON WITH MEASUREMENTS .. 14 CONTOURS OF NEXTRA WAVE 15 SELECTION OF OPTIMUM GRIDDING 5. CALIBRATION OF NEXTRA HS100 .. 18 CALIBRATION METHOD .. 18 CALIBRATION 18 RESULTS OF THE CALIBRATION EXERCISE .. 20 6. THE FINAL MAP OF 100 YEAR v FIGURES Figure 1: 100 year extreme significant wave height (m) ..viii Figure 2:The NEXTRA archive Figure 3:NEXTRA grid points selected for :Locations of verification data sets.

4 6 Figure 4:Locations of established criteria .. 8 Figure 5 Figure 6: Contours of Hs100 from the software gridding options .. 12 Figure 7: Hs100 (m) measured verification data vs. closest unadjusted Figure 8: Contours of unadjusted NEXTRA Hs100 (m) from Kriging Figure 9: Contours of unadjusted NEXTRA Hs100 (m) from Local Polynomial gridding 16 Figure 10: Tested calibrations for adjusting NEXTRA Hs100 .. 20 Figure 11: Comparison of the potential NEXTRA Hs100 calibrations .. 20 Figure 12: Grid values of NEXTRA Hs100 (m) adjusted with the four 22 Figure 13: Contours of adjusted NEXTRA Hs100 (m) .. 23 Figure 14: Adjusted NEXTRA Hs100 compared to established 100yr design criteria.

5 24 Figure 15: Relationship between NEXTRA Hs50 and Hs100 (m) .. 24 Figure 16: Comparison of NEXTRA Hs50 contours (m) and those from OT 2001 25 Figure 17: Comparison of OT 2001/010 and NEXTRA Hs50 grid values (m).. 26 Figure 18: Hs100 (m) the final contour map .. 28 TABLES Table 1:NEXTRA 3 Table 2:Verification data .. 7 Table 3:Established Table 4: Data gridding options .. 11 Table 5:Hs100 (m) measured data vs. closest unadjusted Table 6:Verification and unadjusted NEXTRA Hs100 (m) .. 19 Table 7: Adjustment of wave heights by application of the four calibrations .. 21 Table 8: Tabulated comparison from Figure 26 vi EXECUTIVE SUMMARY A contour map of 100 year extreme significant wave heights in UK waters has been produced by processing, interpolating and contouring 373 data sets from the NEXTRA model hindcast.

6 The work updates the contour map of 50 year extreme significant wave height that was provided in: Department of Energy: Offshore installations: Guidance on design, construction and certification (4th Edition, 1990). HMSO Consolidated Edition 1993 (plus Amendment No. 3, 1995). Following withdrawal in 1998, this document was republished as Offshore Technology Report OT2001/010, with the 50 year map reproduced therein. The values of 100 year extreme significant wave height (Hs100) obtained directly from the NEXTRA data were found to be at variance with current industry criteria and with extreme values based on measured verification data sets.

7 In broad terms, in the North Sea, NEXTRA was found to produce approximately the same value as the verification data when 100 year significant wave height was of the order of 14 metres. Below 14m NEXTRA tended to produce higher values, and above 14m NEXTRA tended to produce lower values. A precautionary approach was therefore adopted; a calibration was derived through the assessment of the NEXTRA Hs100 values against equivalent values from measured data. The resulting algorithm was applied to adjust the NEXTRA results to a level consistent with those indicated by the measured data, and it is from these adjusted values from that the contour map has been derived.

8 The validity of extrapolating the calibration curve beyond the range for which verification data are available is questionable, and therefore the region of the map in which Hs100 is shown to be over 18 metres has been blanked off. The resulting contour map is presented below as Figure 1. vii Figure 1: 100 year extreme significant wave height (m) Important note: the information displayed on this map is intended to provide guidance and should not be treated as a substitute for site specific study. viii 1. INTRODUCTION BACKGROUND The UK Department of Energy provided guidance on design, construction and certification of offshore structures between 1974 and June 1998. This guidance was first published to support the Offshore Installations (Construction and Survey) Regulations 1974 (SI 1974/289) by providing a consistent basis for the certification of offshore installations by government-appointed certifying authorities.

9 The guidance was regularly updated to keep up with evolving technical knowledge and the fourth and final edition was published in 1990. The document , in its final format, was entitled: Department of Energy: Offshore installations: Guidance on design, construction and certification (4th Edition, 1990). HMSO Consolidated Edition 1993 (plus Amendment No. 3, 1995). The document was generally referred to as Guidance or The Guidance Notes ; sometimes as The Fourth Edition . Section 11 of that document was entitled Environmental Considerations and included maps of indicative values of environmental parameters with a 50-year return period; this return period being selected in accordance with the requirements of SI 1974/289.

10 Section 11 of the Guidance remained unchanged since initial publication in 1974. HSE withdrew the Fourth Edition from publication in June 1998 at the end of the certification regime. However, withdrawal was not a reflection on the soundness of the technical information it contained. Section 11 that addressed Environmental Considerations was republished as Offshore Technology Report 2001/010. On re-publication additional text was added to OT 2001/010 in the form of a warning as follows: It should be noted that the technical content of the Guidance has not been updated as part of the re-formatting of the OTO The therefore assess the appropriateness and currency of the technical information for any specific application.