Recently significant interest has been paid to abnormal waves, often called rogue waves or freak waves. These waves represent operational risks to ship and offshore structures, and are likely to be responsible for a number of accidents. As shown by several authors, in ‘the second order world’ the freak waves are pretty rare events. The present study focuses on statistical properties of freak waves. The analyses are based on second order time domain simulations, short term distributions for crest statistics obtained from the literature, and long term field data. Time series of wave elevations are generated using the Pierson-Moskowitz, JONSWAP and two-peak Torsethaugen frequency spectrum for long-crested seas and deep water. Effects of combined seas (swell and wind sea) on wave statistics are discussed. Assuming 2nd order wave theory, the short term and long term probability of occurrence of a freak wave is estimated. The difference between a “freak wave” and a “dangerous wave” is pointed out. Finally, 100 year and 10000 year crest events obtained by analysis procedures used in the offshore industry are discussed in relation to freak waves.
The wave time series recorded at “Draupner”, January 1, 1995 at 15:20 exhibits an unsymmetrical, freak wave event. The statistics of the peak event is studied within the framework of traditional non-Gaussian process models. Assuming that the sea surface is adequately described by (one of) commonly applied non-Gaussian wave models, the paper shows that the probability of occurrence of a freak event like the one observed at Draupner January 1995 is very low.
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