Dynamic response of floating offshore wind turbines under freak waves with large crest and deep trough

Abstract

Floating offshore wind turbines (FOWTs) are at high risk being attacked by freak waves with profiles of large crest or deep trough, which could damage the structures or weaken the efficiency of power generation. This study developed a numerical model to address the dynamic response of an in-place FOWT under the action of freak waves with large crest and deep trough. Based on the modulation method of freak wave profiles, various random wave trains embedded freak waves were firstly generated in the numerical wave tank. Noted that these waves generated match Gaussian seas. Then, the effect of freak waves was investigated extensively through the FOWT motions, axial acceleration on nacelle, tether forces as well as wave loads and wave fields in time domain and time-frequency domain. The results show that the occurrence of freak waves significantly amplified the dynamic response of FOWT and the impact lasted for approximately 20 spectral peak periods. Under freak waves, the effect of quadratic phase coupling on nacelle acceleration was first observed. The large crest led to a larger horizontal excursion. While the deep trough caused a larger pitch inducing a larger axial acceleration on nacelle. In addition, the nonlinear interaction (between FOWT and freak waves) caused by the large crest was stronger than that caused by deep trough.