Nonlinear Dynamical Absorber for Floating Offshore Wind Turbines
Résumé
Floating Offshore Wind Turbines (FOWT?s) are subjected to different types of dynamic loads such as environmental loads (wave and wind), aero-structure interaction loads and mechanical loads (inertial and controller effects). These loads induce vibrations in the WT mast that are transmitted in the floating foundation and mooring lines, increasing the overall ultimate loads and fatigue cycling. In this communication, the potentiality of a passive nonlinear mechanical system for the reduction of WT mast vibrations is studied. This kind of absorber called Nonlinear Energy Sink (NES) is based on the energy pumping phenomenon. The feasibility and efficiency of the absorber is demonstrated by means of a simplified full-scale numerical model composed of two degrees of freedom, one for the WT mast (first bending mode) and one for NES. The sizing of an optimized absorber for the full-scale system is presented and a sensitivity analyses considering variations of the mast stiffness characteristics is performed to investigate the NES efficiency limits, confirming the performances of the NES: reduction of vibratory level around 15 to 20 dB over a large variation range for the first resonance frequency of the WT and a large range of functioning loads.