Structural vibration control of spar-buoy floating offshore wind turbines

Floating offshore wind turbines (FOWTs) have complex dynamics problems and strong coupling effects within their entire systems, and they are sensitive to winds and sea waves that may cause structural fatigue damages and power production fluctuations. Structural vibration control is a promising way to reduce FOWT responses, but most previous studies focused on linear control strategies with the limitations of requiring large strokes and having narrow effective frequency bandwidth. Nonlinear energy sinks (NESs) have emerged as an alternative to overcome the above limitations of linear control methods. In the present study, an OC3-Hywind spar-buoy FOWT is selected as a prototype structure for developing an in-house model to investigate its associated aerodynamics, hydrodynamics, and structural dynamics. The wind turbine blades and tower are modelled by three-dimensional Euler-Bernoulli beam elements and the floater is assumed as a rigid body. The blades' pre-twist, pitch, and rotating angles are also considered to develop the system's time-varying mass, stiffness, and damping matrices. The nonlinear quasi-static model is adopted to calculate the forces generated by the mooring cables. The straightforward and efficient in-house model is validated against FAST. Subsequently, a bistable track NES with a profile combining both negative second-order and positive fourth-order terms is proposed, designed, and incorporated into the model to mitigate FOWT responses under two investigated environmental conditions. Results show that the in-house model is of high accuracy in estimating dynamic characteristics and responses of FOWTs, paving the way for preliminary design and analysis of FOWTs with different platform types. Moreover, the bistable track NES can effectively control the fore-aft displacements at the tower top with practical strokes.