Rogue waves in the Dutch North Sea

Rogue waves in the Dutch North Sea: An experimental study into the occurrence of extreme waves due to abrupt depth transitions at future offshore wind farm locations along the Dutch coast

Doeleman, Madelief (TU Delft Civil Engineering and Geosciences)

Reniers, A.J.H.M. (mentor)
van den Bremer, T.S. (graduation committee)
Hofland, B. (graduation committee)
Westra, M. (graduation committee)
Groeneweg, J. (graduation committee)

Delft University of Technology

Civil Engineering | Hydraulic Engineering

2021-04-14

Large morphological features, so-called sand ridges, with an average height and length in the order of 10 meters and 3 kilometers respectively are located at future offshore wind farm locations in the Dutch North Sea. Recent literature, considering relatively deep water conditions (kh 1, with k the wave number and h the water depth), has shown that a wave field propagating over a shallowing depth transition shows an increase in kurtosis. Kurtosis is a measure for the vertical deviation from the commonly applied linear ocean wave model, and therefore describes a higher probability of occurrence of "rogue" waves. These "rogue" waves are known to the general public from sailors' tales and thrilling videos. Scientifically, rogue waves are described as statistically abnormal waves and are generally classified as waves that exceed twice the significant wave height. Because these waves have an abnormal height, they are associated with extreme forces. Considering the sand ridges at future offshore wind farm locations, and possible increased damages to wind turbines due to rogue waves, this thesis researches if the increased probability of occurrence of rogue waves due to the local bathymetry as observed in the relatively deep water literature also applies to relatively shallow water conditions and the Dutch North Sea. To achieve this objective, physical experiments have been performed in the Hydraulic Engineering laboratory of the Delft University of Technology. Also, an existing statistical model that describes a rogue wave mechanism due to the presence of depth transitions is evaluated. The model describes the rogue wave mechanism as the interaction between the transmitted wave field and second-order components: the free sub- and super-harmonic components generated by the depth transition. As the model assigns a prominent role to these sub- and super-harmonics, it was ensured that these components are generated correctly during the physical experiments. Following, this thesis also assesses and contributes to the second-order wave generation at the TU Delft laboratory facility.

Rogue Waves
Second-order wave generation
North Sea
Wave phenomena
Physical experiments
Depth transition
Sand ridges
Extreme waves
Monster freak waves
wind farm
Laboratory
AUKE/PC

http://resolver.tudelft.nl/uuid:782f48be-3271-4171-8a43-5c08fc0dd81b

52.57,3.71

https://doi.org/10.4121/14301455

Student theses

master thesis

© 2021 Madelief Doeleman