The impact of passive tuned mass dampers and wind-wave misalignment on offshore wind turbine loads

Offshore wind turbines experience complex external loading from a variety of sources, especially wind and waves. To be economical, offshore wind turbine must operate reliably under these loading conditions. The external loading is complicated by the directionality of the wind and the waves. Metocean data in Europe and the US shows that the wind and the waves are often misaligned by significant amounts. This misalignment causes large loads on the tower in the side-side direction, which has very little structural damping compared to the fore-aft direction. Recent papers have highlighted the importance of considering wind-wave misalignment when analyzing the loads of offshore wind turbines. A variety of approaches are feasible and have been investigated to mitigate loading on offshore wind turbines due to the wind and waves. Many approaches control the aerodynamic loading on the rotor, either via control of the blade pitch or other aerodynamic actuation. This paper analyzes an alternative approach using structural control, in which passive tuned mass dampers are used to absorb and dissipate structural vibrations. In particular, this study investigates the load mitigation potential of passive tuned-mass-dampers for a 5 MW offshore wind turbine supported by a monopile, and subjected to realistic external conditions that include wind-wave misalignment. A comprehensive set of operational simulations is used to demonstrate that optimally tuned passive tuned-mass-dampers are capable or reducing tower fore-aft and side-side fatigue loads by approximately 5% and 40%, respectively. A discussion of the practical feasibility of such an approach is also provided. (C) 2014 Elsevier Ltd. All rights reserved.