Vibration mitigation of monopile offshore wind turbine tower using inerter-assisted double-tuned mass damper under wave-wind loading

Offshore wind turbines (OWTs) are continuously exposed to dynamic loading from waves and wind. The vibrations induced by those loadings can degrade the stability of power generation, reduce fatigue life, and even lead to structural failure in OWTs. Thus, vibration suppression under those loading conditions is an important aspect of OWT design. The installation of tuned mass dampers (TMDs) is an established approach to structural vibration suppression and has proven effective for OWTs as well. In this paper, an inerter-assisted double-tuned mass damper (dTMDI), an improvised configuration of a TMD, is considered for suppressing vibrations of a monopile offshore wind turbine tower (MOWTT) under wave-wind loadings. In a dTMDI, the damper mass is split into two parts. The first part is attached to the structure through a spring and a dashpot, while the second part is attached to the first part through a spring, a dashpot and an inerter. A mathematical model of the MOWTT-dTMDI is generated by discretizing the tower into 2-node Euler-Bernoulli beam elements and assuming the springs and inerter of the damper to be linear. For illustrative purposes, a 5-MW monopile-supported OWT is considered, and wave and wind loadings are generated using OpenFAST. The characteristic parameters of the dTMDI are identified and optimized using genetic algorithm. The responses of the uncontrolled and dTMDI-controlled OWTT are obtained using a MATLAB code developed based on the Newmark-β method. The effectiveness of the dTMDI is compared with that of a conventional TMD and a double-TMD without an inerter, and the dTMDI is found to be more effective than both. For instance, with a mass ratio of 0.2% and its optimum design parameters, the dTMDI achieves a 33.8% reduction in the mean peak tower-top displacement, whereas a conventional TMD and a double-TMD with the same mass ratio achieve reductions of 26.7% and 15.6%, respectively.