Three-dimensional vibration control of offshore floating wind turbines using multiple tuned mass dampers

Offshore floating wind turbines (FWT) subjected to misaligned wind and wave loadings experience extreme three-dimensional vibrations which will increase the fatigue loadings on the structure and mooring cables. The present study uses a three-dimensional pendulum tuned mass damper (3d-PTMD) and dual linear pounding tuned mass dampers (2PTMDs) to mitigate the three-dimensional vibrations of a spar-type FWT under wind and wave loadings. An analytical model of a spar-type FWT coupled with the 3d-PTMD and 2PTMDs is established using Euler-Lagrangian equation. Blade Element Momentum method is utilized to determine the aerodynamic loading and Morison equation is used to compute the hydrodynamic loading. Mooring cables are modeled using an open source module MoorDyn. The optimal design of the 2PTMD is determined using a numerical search approach. Performance of the proposed controlling devices in reducing the three dimensional vibrations of the spar FWT is evaluated via using the NREL 5 MW OC3-Hywind spar buoy wind turbine. Results show that a well-designed 3d-PTMD and 2PTMDs can effectively reduce the three-dimensional vibrations of the tower and the platform. More importantly, it is shown that the proposed 2PTMDs experiences a smaller stroke which is 50% smaller than that of traditional linear TMDs. This is of significant value for practical applications.