Joint Distribution For Wind And Waves In the Northern North Sea

This paper presents a joint probabilistic model of mean wind speed, significant wave height and spectral peak period. Simultaneous wind and wave measurements from the Northern North Sea covering the years 1973–99 are used as a database. The joint model is used to establish a contour surface, giving combinations of the weather parameters for which the exceedance corresponds to a return period of 100 years. INTRODUCTION For design purposes, it has been common to estimate the 100year response by exposing the structure to the simultaneous action of 100-year wind, 100-year wave and 10-year current. Present design codes (e.g. NORSOK N-003) recommend a less conservative approach by stating that the targeted response extremes can be predicted by accounting for the actual correlation between the environmental processes. This requires a joint probabilistic model for the weather parameters of interest for the problem under consideration. This work was carried out in connection with modifications on the Veslefrikk installation in the Northern North Sea. Because the parameters of concern were mainly wind speed, significant wave height and spectral peak period, combinations of wind, waves and currents have not been looked at. BitnerGregersen and Haver (1991) and Johannessen et al. (2000) provide relevant information regarding joint distributions of meteorological and oceanographically parameters. DEVELOPMENT OF JOINT PROBABILITY The weather is described by the following 3 parameters: • 1-h mean wind, W • significant wave height Hm0 • spectral peak period, Tp We seek a joint density distribution of the characteristic parameters, W Hm0 and Tp. This work has been connected to response analyses where the response most probably would be dominated by the variability of the wind. Thus, W is chosen as the primary parameter. Based on this the following joint density function seems reasonable: fWHm0Tp w h t = fW w · fHm0 W h w · fTp Hm0W t h w (1) MARGINAL DISTRIBUTION FOR WIND, W We will assume that the marginal distribution of the 1-h mean wind speed at 10 m can be described by the 2-parameter Weibull Received September 6, 2001; revised manuscript received by the editors January 31, 2002. The original version (prior to the final revised manuscript) was presented at the 11th International Offshore and Polar Engineering Conference (ISOPE-2001), Stavanger, Norway, June 17– 22, 2001.