A complemental analysis of wave irregularity effect on the hydrodynamic responses of offshore wind turbines with the semi-submersible platform

Abstract

Changes in the spectral shape and the directional spreading are typical properties of irregular waves in nature. The effect of wave irregularity on the hydrodynamic responses of floating offshore wind turbines (FOWT) has been investigated in several studies. However, a complemental analysis of the effects of frequency spectrum shape and wave multi-directionality on the low-frequency (LF) and the wave-frequency (WF) responses due to the second order and the first order hydrodynamic loads, respectively and cable tensions of FOWT under a complete range of mean wave directions is missing. In this study, two hydrodynamic models are developed firstly using different calibration methods based on the free decay tests and wave loading tests. They are compared with the experimental data for validation. No wind loads were considered in this analysis. The validation results show that the model calibrated using wave loading has better agreement with the experimental data, especially in the LF region, and therefore used for further analysis. Then the hydrodynamic responses are investigated under irregular waves with different spectral shapes. As the spectral shape becomes narrower with pronounced wave grouping and the larger waves in the time series, the responses and tensions increase in the WF region. Furthermore, the narrower the spectrum, the more snap loads in the mooring cables occur. Hydrodynamic responses are also compared under a uni-directional and multi-directional wave excitation from all angles of attack in terms of LF and WF amplitudes. The condition that the responses under multi-directional waves are higher than the ones under uni-directional waves has appeared in multiple cases, especially in the WF region, although it does not lead to excessive responses like the uni-directional wave. Therefore, it is concluded that the wave irregularity in terms of the spectral shape and the directional spreading should be considered during the design stage for better comprehension of the actual motion of floating wind turbines.