Fatigue damage mitigation of monopile offshore wind turbines utilizing a rotational inertia double tuned mass damper (RIDTMD) under realistic wind-wave conditions

Offshore wind turbines present undesirable vibrations under cyclic environmental loadings, which induces longterm fatigue damage and even failure issue. Monopile offshore wind turbine (MOWT) is widest utilization in engineering fields. This study presents fatigue mitigation assessment of MOWT by utilizing a rotational inertia double tuned mass damper (RIDTMD), aiming to prolong the structural fatigue life and suppress vibration. An analytical model of MOWT with RIDTMD is established using Euler-Lagrangian equation and dynamic responses are solved by the finite element method. To predict metocean condition, a statistical analysis is conducted for deriving realistic wind-wave loadings. The RIDTMD parameters are optimized by simultaneously considering the maximum of fatigue life and the minimum of damper's stroke. The fatigue damage is estimated based on rainflow cycle counting method and Miner's rule. The results show that an RIDTMD can prolong the MOWT fatigue life compared with uncontrolled system; the maximum and RMS displacement of tower top are also greatly reduced by comparison with a traditional TMD. It is demonstrated that RIDTMD can decrease MOWT's fatigue damage by using a lighter damper, which is significant for the space limitation in practical.