Effects of various freak waves on dynamic responses of a Spar-buoy floating offshore wind turbine

Abstract

There are various kind of waves in the actual ocean. Changes on freak wave profiles affect the motion of floating offshore wind turbines (FOWTs) and threaten the operation safety. To investigate this issue, the phase modulation method is used to simulate freak waves in our work. The original freak wave is first generated, then the wave profile is modulated to obtain five other types of freak waves, such as the reversed wave, the high-crested wave, the deep-troughed wave, the fluctuated wave, the elevated wave. A coupled time-domain model of a Spar-buoy FOWT is established to simulate the motion under steady wind and various freak waves. The dynamic responses are analyzed in the time domain, including the three-degree-of-freedom (3-DOF) motions of the floating foundation, the tension of mooring line and the nacelle acceleration. The frequency-domain properties under various freak waves are investigated using wavelet analysis. The effects of various wave profiles are compared with the original freak wave. Freak waves in low sea states are also generated to research their effects on pitch motion in different sea states. Results show that the response peaks increase in the impact region under freak waves. The reversed wave increases the maximum value of the 3-DOF motions, especially in pitch motion, and causes the high-frequency energy core of wavelet plots to move. The dynamic responses under the fluctuated wave combines the properties of increased response peaks at high-crested wave and increased amplitude of low-frequency motions at deep-troughed wave. The elevated wave greatly increases the heave motion of the FOWT. The increase in sea state level causes a nonlinear amplification in the transient oscillation.