Spatial Evolution of an Extreme Sea State With an Embedded Rogue Wave

Abstract

In the last years the existence of freak waves has been affirmed by observations, registrations and severe accidents. Many publications investigated the occurrence of extreme waves, their characteristics and their impact on offshore structures, but their formation process is still under discussion. One of the famous real world registration is the so called “New Year Wave”, recorded in the North Sea at the Draupner jacket platform on January 1st, 1995. Since there is only a single point registration available, it is not possible to draw conclusions on the spatial development in front of and behind the measurement point which would be indispensable for a complete understanding of this phenomenon. This paper presents a spatial development of the “New Year Wave” being generated in a model basin (L = 120 m, W = 8 m, d = 1 m, scale 1:70). To transfer the recorded “New Year Wave” into the wave tank, an optimization approach for the experimental generation of wave sequences with predefined characteristics is applied. The method is utilized to generate scenarios with a single high wave superimposed to irregular seas. At the end of this optimization process, a control signal for a deterministic wave sequence is obtained. The generated sea state with the embedded “New Year Wave” is measured at different locations in the tank, in a range from 2163 m (full scale) ahead of to 1470 m behind the target position — altogether 520 registrations. The focus lies on a detailed description of a possible evolution of the “New Year Wave” over a large area and time interval. It is observed that the extreme wave at the target position develops mainly from a wave group of three smaller waves. In particular the group velocity, wave propagation and the energy flux of the wave group is analyzed. In addition, the WAVE FORECAST METHOD is applied. This method is based on linear wave propagation and provides a prediction of the wave train a few minutes in advance from a single surface elevation snap shot. The capability of the prediction of approaching extreme wave heights is shown.