Modeling and vortex-induced vibrations of semi-submersible floating offshore wind turbines

Abstract

A rigid-flexible coupling dynamic model of a semi-submersible floating offshore wind turbine (FOWT) is established, and the characteristics of vortex-induced vibration (VIV) of the tower are studied. Lagrange’s equation is used to establish the equations of motion of a semi-submersible FOWT, a wake oscillator model is applied to simulate the interaction between tower and wind field, and the semi-empirical parameters in the wake oscillator model are optimized by computational fluid dynamics (CFD) method. The VIV characteristics of the semi-submersible FOWT before and after hoisting are investigated, and the influence of waves on VIV is analyzed. The results show that the VIV of tower bending mode with large amplitude of the semi-submersible FOWT will occur when the wind speed is in certain ranges, and the possibility of VIV of rigid-body modes with large amplitudes of the tower is low. The amplitudes of VIV and corresponding wind speed ranges can be greatly reduced, and the frequency lock-in domains will become narrower after hoisting the rotor system. The action of waves can reduce the amplitudes of VIV and cause the deviation of frequency lock-in domains, which may widen the coverage of wind speed of lock-in domains.