NON-GAUSSIAN EXTREME WAVES IN THE CENTRAL NORTH-SEA

The area of the Central North Sea is notorious for the occurrence of v ery high waves in certain wave trains. The short-term distribution of these wave trains includes waves which are far stepper than predicted by the Rayleigh distribution. Such waves are often termed ''extreme wa ves'' or ''freak waves.'' An analysis of the extreme statistical prope rties of these waves from the Moersk Olie og Gas AS operated Gorm Fiel d which is located in the Danish sector of the Central North Sea. From the wave recordings more than 400 freak wave candidates were found. T he ratio between the extreme crest height and the significant wave hei ght (20-min value) has been found to be about 1.8, and the ratio betwe en extreme crest height and extreme wave height has been found to be 0 .69. The latter ratio is clearly outside the range of Gaussian waves, and it is higher than the maximum value for steep nonlinear long-creat ed waves, thus indicating that freak waves are not of a permanent form , and probably of short-created nature. The extreme statistical distri bution is represented by a Weibull distribution with an upper bound, w here the upper bound is the value for a depth-limited breaking wave. B ased on the measured data, a procedure for determining, the freak wave crest height with a given return period is proposed. A sensitivity an alysis of the extreme value of the crest height is also made.