An investigation of wave conditions and wave induced loads for design of wind turbine foundations at 15 – 40m depth

Abstract

The motivation for this thesis is to investigate how storm sea states in deep water transforms as the waves propagate towards shallow water. This is connection with the design of bottom fixed wind turbines in finite water depths. In order to investigate how the sea state is transformed, there have been performed a model test where the generated waves are measured as they propagate over a sloping beach.

Theory behind different shallow water effects and how these will transform the sea state, is presented. The results obtained from the present model test have also been compared to similar model tests, and the comparison generally shows the same behavior.

The results show that the surface process of the waves transforms into a nonlinear process, and the deviations from a Gaussian process shows this clearly in terms of values for skewness and kurtosis. It is seen that wave breaking will be an dissipation important in the wave spectra, significant wave height and the distributions of wave and crest heights in the sea state. Where wave breaking is seen to reduce the energy content in the wave spectrum, and contributes to make the proposed conventional distribution functions for both wave and crest height distributions conservative. The significant wave height is also seen to be transformed by effects from shoaling.

For the largest individual waves it is seen that the waves in the measured time series are asymmetrical with respect to the front and back of the wave. This effect along with the calculated Ursell number for these waves indicates that there is a need for sophisticated wave model in order to model the surface elevation of the waves with corresponding wave kinematics.