Response analysis of slender structures on a spar-type floating offshore wind turbine under three types of freak waves

The Spar-type floating offshore wind turbine (FOWT) is an offshore structure that utilizes wind energy in deep water area. It is affected by the coupling of wind, waves and other environmental factors, leading to complex dynamic response characteristics, particularly during extreme sea conditions like freak waves. In our work, the phase modulation method is employed to obtain freak waves with extreme wave height. Based on this method, we propose a new approach called moving interval focusing method to generate three types of freak waves, including amplitude-close freak wave, peak-dominated freak wave and trough-dominated freak wave. Focusing on the Spar-type FOWT as the subject of this study, the response analysis on slender structures including blade and tower is conducted under these freak waves and steady wind. We examine the frequency-domain characteristics of the Spar-type FOWT when subjected to three types of freak waves using wavelet analysis. Our findings indicate that response peaks significantly rise for three freak waves, with the trough-dominated wave showing the most pronounced effect. Additionally, energy density would noticeably increase at the onset of these freak waves. Compared to the amplitude-close freak wave, the blade deformation produces a high frequency in impact region under the peak-dominated freak wave and the trough-dominated freak wave. When the wave trough deepens, the bending moments at blade root and tower base increase, by 12.1% and 34.5%, respectively. There is a time difference between the peak position of shear force and bending moment. Compared with the impact of amplitude-close freak waves, the changes of peak and trough both induce the increasement on the tower shear force. These observations highlight the varying impacts of different wave types on the structural responses of slender components in offshore wind turbines.