Nonlinear Low-Frequency Response of a Floating Offshore Wind Turbine Integrated With a Steel Fish Farming Cage

In this article, the effects of Morison drag force and the second-order hydrodynamics on a state-of-the-art floating system integrating a floating offshore wind turbine with a steel fish farming cage (FOWT-SFFC) are studied. To numerically solve the second-order hydrodynamic problem with ease, a simplified structure is adopted. Convergence study is carried out on the full difference-frequency quadratic transfer function (QTF). The middle-field method is recommended to he used in the calculation of QTF. For the aero-hydro-servo-elastic simulations of FOWT-SFFC, the time-domain solver OrcaFlex is used. The effects of the Morison drag force and second-order hydrodynamics on the dynamic responses under a variety of conditions are discussed. The comparison between the results with and without Morison drag force shows that Morison drag force has twofold effects on the low-frequency responses, making the excitation and damping level both larger. The results obtained by including first-order wave loads only and involving both first-order and second-order wave loads are also compared. The comparison reveals that the low-frequency contents of the dynamic responses, particularly heave and pitch, can be boosted when the second-order hydrodynamic forces are included. In contrast to the full QTF method, the known Newman's approximation leads to underprediction of the low-frequency responses of FOWT-SFFC.