Vibration mitigation of spar-buoy floating wind turbines using a nonlinear energy sink

This work investigates the possibility of mitigating the wind-wave exerted vibration of floating offshore wind turbines using a nonlinear energy sink (NES). The vibration mitigation of floating wind turbines using passive linear control strategies have been intensively investigated, however, the effectiveness of the linear absorbers only works in a narrow frequency band, resulting in limited vibration suppression performance. The NES comprising a mass, a damper and a cubic stiffness element has the ability to mitigate vibrations in a broadband frequency range, which has been adopted in multiple engineering structures with significant performance benefits identified. In this paper, taking spar-buoy floating wind turbines as prototype structure, a simplified model with a NES mounted in nacelle subjected to wind and wave loads is presented. The harmonic balance method is then employed to analyse its nonlinear responses to the influence of NES parameters, together with the pseudo-arc-length method. The optimum NES parameter values are subsequently identified. Results show that the performance improvement can be up to 74.2% and 14.0%, respectively, comparing to that with no absorber and the TMD. Furthermore, to demonstrate the vibration suppression ability of the optimised NES in more realistic spar-buoy wind turbines, Orcaflex-the world’s leading package for the dynamic analysis of offshore systems, is adopted as the simulation tool. The spar-buoy wind turbine as well as the NES are established in Orcaflex and it is demonstrated that the performance benefits with the NES preserve under various loading conditions. •Absorber design methodology of the nonlinear energy sink (NES) for the floating offshore wind turbines is proposed.•The harmonic balance method combined with the pseudo-arc-length method is used to analyse system’s nonlinear responses.•Orcaflex is adopted as the simulation tool, with the spar-buoy wind turbine and the NES established in it.•The proposed NES outperforms an optimised TMD in a wide range of frequency for the wind turbine vibration mitigation under various loading conditions.