Reliability-based design optimization of tuned mass-damper-inerter for vibration mitigation of spar-type floating offshore wind turbines

Tuned mass-damper-inerter (TMDI) becomes a promising control device in vibration mitigation of floating offshore wind turbines (FOWTs) due to its merits of well cost-benefit. However, existing studies lack reasonable consideration of randomness associated with external excitations such as wind and wave loads; in particular, the design optimization of TMDI to improve the control efficiency itself and strengthen the structural safety of FOWTs still remains open. To this end, the reliability-based design optimization (RBDO) of TMDI for vibration mitigation of spar-type FOWTs under wind and waves is carried out in this study. A fully coupled modeling of the FOWT-TMDI system is first performed and validated. The large-scale stochastic wind and wave fields are then simulated by integrating multi-variate copula modeling of elementary random variables and dimension-reduced spectral representation methods. To accelerate the design optimization, an optimization scheme based on twostage enriched Kriging is applied. For illustrative purposes, the design optimization of TMDI under a harsh environment in the South China Sea is considered, and relevant design parameters include the inertance-to-mass ratio, frequency ratio, damping ratio, and the normalized displacement ratio denoting the inerter's connection configuration. Numerical results reveal the advantages of the TMDI optimized through RBDO in the sense of probability, such as performance improvement, mass reduction, and stroke limitation; in addition, the inerter's connection within the nacelle, rather than to the base of the tower as suggested in previous studies, can gain a desirable control efficiency.