Wave Loads on Offshore Wind Power Plants

Doctoral thesis, 2006

The thesis gives a short introduction to waves and discusses the problem with non-linear waves in shallow water and how they effect an offshore wind energy converter. The focus is on the realisation of non-linear waves in the time domain from short-term statistics in the form of a variance density spectrum of the wave elevation. For this purpose the wave transformation from deep water to the near-shore site of a wind energy farm at Bockstigen has been calculated with the use of the software SWAN (Simulating WAves Near Shore). The result is a wave spectrum, which can be used as input to the realisation. The realisation of waves is done using perturbation theory to both the first and second-order. The calculated properties are the wave elevation, water particle velocity and acceleration. The wave heights from the second-order perturbation equations are higher than those from the first-order perturbation equations. This is also the case for the water particle kinematics. The increase of variance is significant between the first-order and the second-order realisation. The calculated wave elevation exhibits non-linear features as the peaks become sharper and the troughs flatter. Comparisons with measurements at the offshore wind farm of Bockstigen have been made. They show that the second-order realisation at least captures some of the non-linearities of the waves at the site. The simulation in SWAN compares quite well with measurements. The effect of wave loading was studied at Bockstigen, and it can be seen that for perpendicular wind and wave directions, the crosswise bending moment is larger than for aligned wind and waves. The fatigue life of a generic 5 MW wind turbine, in 20 metres water depth, was investigated focusing on the effect of using different wave models and formulations of the water particle kinematics to the free surface. The resulting forces are calculated using Morison’s equation. There is a significant increase in the loads when the second-order wave model is applied. It can be concluded that both the distribution of wave heights and the water-particle kinematics are important for the fatigue calculation. This is something that should be investigated further.