A novel concept of floating absorber for motion mitigation in floating offshore wind turbines

This paper proposes the innovative concept of floating absorber for motion mitigation in floating wind turbines on spar supports. The floating absorber consists in a spar floater coupled with the floating wind turbine by an elastic spring and a viscous damper in parallel. It is conceived to act as a resonant mass tuned to the pitch frequency of the floating wind turbine and, for this purpose, its parameters are calibrated based on the wellestablished fixed point tuning procedure typically applied for tuned mass dampers. For validation, extensive time-domain numerical simulations are performed on a multi-degree-of-freedom computational model of the coupled system, considering thirty-six different configurations of the floating absorber and fifteen different combinations of wind-wave conditions. The results demonstrate a considerable effectiveness of the floating absorber in reducing the pitch motion of the floating wind turbine. A main advantage over conventional tuned mass devices is that the absorber is not housed in the nacelle of the wind turbine but floats close to the floating wind turbine. This not only ensures reduced costs of installation and maintenance of the floating absorber but allows, also, to increase its mass without causing extra weight on the floating wind turbine, exploiting hydrodynamic added mass as well, with consequent benefits in terms of motion mitigation in the floating wind turbine. Another relevant result is that, for a given mass of the floating absorber and a given attachment point of the parallel spring-dashpot, the performances of the floating absorber do not vary significantly with its geometry. This is important to facilitate design and manufacturing of the floating absorber, confirming the robustness of the proposed concept and its great potential for motion mitigation in floating wind turbines.