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The Effect of Directionality on Northern North Sea Extreme Wave Design Criteria

[+] Author Affiliations
Kevin Ewans

Shell International Exploration and Production, Rijswijk, The Netherlands

Philip Jonathan

Shell Technology Centre Thornton, Chester, UK

Paper No. OMAE2007-29657, pp. 479-488; 10 pages
  • ASME 2007 26th International Conference on Offshore Mechanics and Arctic Engineering
  • Volume 2: Structures, Safety and Reliability; Petroleum Technology Symposium
  • San Diego, California, USA, June 10–15, 2007
  • Conference Sponsors: Ocean, Offshore and Arctic Engineering Division
  • ISBN: 0-7918-4268-1 | eISBN: 0-7918-3799-8
  • Copyright © 2007 by ASME


The characteristics of hindcast data for extreme storms at a Northern North Sea location are shown to depend on storm direction, reflecting storm strength and fetch variability. Storm peak HS over threshold is modelled using a generalised Pareto distribution, the parameters of which are allowed to vary smoothly with direction using a Fourier form. A directionally-varying extreme value threshold is incorporated. The degree of smoothness of extreme value shape and scale with direction is regulated by roughness-penalised maximum likelihood; the optimal value of roughness selected by cross-validation. The characteristics of 100 -year storm peak HS , estimated using the directional model differ from those estimated when ignoring the directionality of storms. In particular, the extreme right hand tail of omnidirectional HS100 is longer using the directional model, indicating in this case that ignoring directionality causes underestimation of design criteria. Although storm peak data alone are used for extreme value modelling, the influence of a storm, in directional design sectors other than that containing its storm peak direction, is incorporated by estimating the storm’s directional dissipation directly from the data. An automated approach to selection of directional design sectors is described. Directional design criteria are developed using three different approaches, all consistent with an omni-directional storm peak HS non-exceedence probability of 0.5 . We suggest a risk-cost criterion, which minimises design cost for a given omni-directional design specification, as an objective basis for optimal selection of directional criteria.

Copyright © 2007 by ASME
Topics: Waves , Design , North Sea



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