J. Skourup et al., NON-GAUSSIAN EXTREME WAVES IN THE CENTRAL NORTH-SEA, Journal of offshore mechanics and Arctic engineering, 119(3), 1997, pp. 146-150
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.