Abstract
Rogue waves are exceptionally high waves, relative to the surrounding sea state. They occur rarely, which makes them difficult to predict. Due to their particular height and limited predictability, they pose a threat to offshore structures and operations. A first step towards the prediction of rogue waves in a region is the investigation of their occurrence frequency. Knowing whether rogue waves occur as often as expected at random, or more frequently, gives a hint on the mechanisms that may have contributed to their formation. It is likely that different processes can be responsible for rogue wave formation. In deep and intermediate water, rogue wave occurrence frequencies beyond the expectations of common wave height distributions have been explained mathematically and experimentally, for example, by nonlinear focusing. In shallow water, in which wave dynamics experience the influence of the sea floor, the presence of solitons has been suspected as a cause of increased rogue wave occurrence. An improved understanding of the mechanisms that form rogue waves is crucial for their prediction. The prediction of rogue waves can prevent accidents at sea. This thesis concerns rogue waves in the southern North Sea, in both intermediate-water and shallow-water regimes. In an extensive set of surface elevation data from six buoy and five radar measurement stations, rogue waves with exceptional crest and crest-to-trough heights were identified. Their occurrence frequencies were compared to the expectations of common wave height distributions of a Weibull type. The statistical analysis in intermediate water depths revealed that wave buoys did not identify more rogue waves than expected. However, rogue wave frequencies recorded at radar stations exceeded the expectations of the common wave height distributions. The discrepancy between the results might be due to the different measurement techniques. Following the statistical analysis at intermediate-water sites, it was investigated whether nonlinear focusing, that has led to deep-water rogue wave formation in numerical and physical experiments, provides a conceivable explanation for rogue wave generation at the considered sites. An investigation of wave energy spectra at the intermediate-water stations yielded broad-banded conditions in both frequency and angular direction. These are unlikely conditions for nonlinear focusing to occur. In shallow water, the statistics revealed an exceptionally high number of rogue waves at one buoy and one radar device. In the shallow-water time series at these sites, the recorded rogue waves could be associated with the presence of solitons.
The study leads to the conclusion that intermediate-water rogue waves in the southern North Sea are probably not the result of nonlinear focusing. At some sites in shallow water above a sloping sea floor, rogue wave occurrence frequencies exceeding the expectations of common wave height distributions, could be explained by the presence of solitons.
