Extreme and Freak Waves: Results of Measurements and Simulation

Abstract

Main statistical characteristics of wave climate are considered in respect of offshore and ship design. Sophistication of ships and marine platforms and expansion of offshore activities to non-investigated regions means increasing of probability of being damaged by high waves. Hindcasting of wave fields, using the hydrodynamic models is main approach to wave climate investigation. Offshore wave measurements are used, mainly, for model verification. In compliance with existent regulatory documents and accepted practice applied statistical characteristics of wind waves are prescribed to operational and extreme. Operational statistics describe wind and wave conditions for the life span of a ship or an offshore structure. Extreme characteristics determine the so-called “structure survival regime”. There are a lot of approaches to calculations of extreme wave heights at a point (classical unconditional extremes). Their comparison shows the advantages and disadvantages of each of them. Freak (rogue) waves have some principal difference from extreme wave, mainly due to their form and asymmetry. In this sense freak wave is a multidimensional extreme. Contaminated distribution may be used for probability density approximation of joint extreme and freak wave. The example of recent freak wave event is the loss of ship “Aurelia” (Class of Russian Register of shipping) in February 2005 in the North Pacific. “Aurelia” sunk during passing of atmospheric front with veering wind, changing wind waves. Any wave has at least three dimensions: height, length, and crest length. The last parameter in mean is 3 times greater than wave length. Any information about three dimensional waves is of interest, as such measurements are unique. Some results of unique stereo wave measurements in the South Pacific where the wave as high as 24.9m was fixed (probably still almost the highest measured in the World Ocean), is presented and discussed.