Did the Draupner wave occur in a crossing sea?

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Adcock, T. A. A. ; Taylor, P. H. ; Yan, S. ; Ma, Q. ; Janssen, P. A. E. M. (2011)

The ‘New Year Wave’ was recorded at the Draupner platform in the North Sea and is a rare high quality measurement of a ‘freak’ or ‘rogue’ wave. The wave has been the subject of much interest and numerous studies. Despite this, the event has still not been satisfactorily explained. One piece of information which was not directly measured at the platform, but which is vital to understanding the nonlinear dynamics is the wave’s directional spreading. This paper investigates the directionality of the Draupner wave and concludes it might have resulted from two wave-groups crossing, whose mean wave directions were separated by about 90◦ or more. This result has been deduced from a set-up of the low frequency second order difference waves under the giant wave, which can be explained only if two wave systems are propagating at such an angle. To check whether second order theory is satisfactory for such a highly non-linear event, we have run numerical simulations using a fully non-linear potential flow solver, which confirm the conclusion deduced from the second order theory. This is backed up by a hindcast from ECMWF which shows swell waves propagating at ∼ 80◦ to the wind sea. Other evidence which supports our conclusion are the measured forces on the structure, the magnitude of the second order sum waves and some other instances of freak waves occurring in crossing sea states.
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