The Effect of Directional Spreading on Extreme Sea State Response for Floating Wind Turbines

Empirical investigations have consistently demonstrated the multi-directional nature of actual ocean waves, a phenomenon that has been paid relatively limited attention in the research field of floating wind turbines. The present study seeks to reduce this knowledge gap by experimentally investigating the response of a novel 15 MW floating wind turbine design when subjected to multi-directional wave conditions.

The experimental work was carried out in the deep water basin at DHI A/S (DHI), at a scale of 1:40. In this investigation, the wind turbine rotor was simplistically modeled as a point mass, allowing for a focused examination of the hydrodynamic performance of the floater and mooring system.

The investigation of the concept’s behavior involved wave-only tests for irregular multi-directional waves with two spreading angles and across several sea states. The effect of swell waves was also examined through standalone tests as well as the combined motion response when they were superposed with directionally spread wind wave spectra.

While the effects of directional spreading and swell are well understood for small amplitude waves, the nonlinear interactions and load effects are less explored in the literature. The study therefore also covers severe wave conditions, both in terms of long sea state realizations and three-dimensional focusing waves. For all tests, the low-frequency response that is outside of the linear wave spectrum has special interest, since the natural frequencies of the floating structure are placed here. The outcomes of the experiments are presented through comparison of time series, power spectra, exceedance probability plots, and harmonic separation.