This study is motivated by importance of an accurate assessment of extreme waves and the assooiated (impact) loads on offshore structures. Wavelet transform is adopted for studying the wave time histories from a major storm in the Central North Sea, as well as the data simulated according to the second order wave theory. One of the recorded time series of surface elevation includes a possible freak wave event. Such an extreme wave is looked at in terms of wavelet coefficients, in comparison to other peaks in the records and in a simulated time history. The study also addresses the question on how well the simulated time series represent the field data, and possible strategies for simulating wave time series with a freak wave event.
An important load case to be dealt with in design of offshore structures is wave-deck impacts. For fixed offshore platforms the height from the mean water level to the cellar deck (lowest level of topside) is a fixed height and it is essentially the crest height of the undisturbed incoming wave that suggest whether or not a wave-deck impact is likely to take place. Usually fixed offshore platforms are designed such that the underside of the cellar deck is not hit by the 10000-year wave crest. This means that the substructure is not designed to withstand significant wave-deck impacts. Regarding floating offshore structures e.g. semi-submersibles, the effective height of, the cellar deck relative to wave surface is, in addition to the height of the incoming wave, very much affected by wave induced motions of the floater. Furthermore, the surface piercing elements of the floater are typically of a much larger diameter than those of a fixed platform, i.e. the diffracted wave field is also important regarding an accurate assessment of the wave-deck impact scenario.
