The nonlinear coupling effect between degree-of-freedom (DOFs) and the influence of vortex-induced loads on the motion of SPAR-type floating offshore wind turbine (FOWT) are studied based on an aero-hydro-vortex-mooring coupled model. Both the first- and second-order wave loads are calculated based on the three-dimensional (3D) potential theory. The aerodynamic loads on the rotor are acquired with the blade element momentum (BEM) theory. The vortex-induced loads are simulated with computational fluid dynamics (CFD) approach. The mooring forces are solved by the catenary theory and the nonlinear stiffness provided by the SPAR buoy is also considered. The coupled model is set up and a numerical code is developed for calculating the dynamic response of a Hywind SPAR-type FOWT under the combined sea states of wind, wave, and current. It shows that the amplitudes of sway and roll are dominated by lift loads induced by vortex shedding, and the oscillations in roll reach the same level of pitch in some scenarios. The mean value of surge is changed under the drag loads, but the mean position in pitch, as well as the oscillations in surge and pitch, is little affected by the current. Due to the coupling effects, the heave motion is also influenced by vortex-induced forces. When vortex-shedding frequency is close to the natural frequency in roll, the motions are increased. Due to nonlinear stiffness, super-harmonic response occurs in heave, which may lead to internal resonance.

Simulations are conducted in time domain to investigate the dynamic response of a spar-type floating offshore wind turbine (FOWT) under the freak wave scenarios. Toward this end, a coupled aero-hydro-mooring in-house numerical code is adopted to perform the simulations. The methodology includes a blade-element-momentum (BEM) model for simulating the aerodynamic loads, a nonlinear model for simulating the hydrodynamic loads, a nonlinear restoring model of Spar buoy, and a nonlinear algorithm for simulating the mooring cables. The OC3 Hywind spar-type FOWT is adopted as an example to study the dynamic response under the freak wave conditions, meanwhile the time series of freak waves are generated using the random frequency components selection phase modulation method. The motion of platform, the tension applied on the mooring lines, and the power generation performance are documented in several cases. According to the simulations, it is indicated that when a freak wave acts on the FOWT, the transient motion of the FOWT is induced in all degrees-of-freedom, as well as the produced power decreases rapidly. Furthermore, the impact of freak wave parameters on the motion of FOWT is discussed.

Considering the aero-hydro-mooring-control coupled performance of a floating Vertical Axis Wind Turbine (VAWT), the numerical model of the floating helical VAWT system is established, and the fully coupled simulation program of the floating helical VAWT is developed. The aerodynamic load of the wind turbine system is calculated using the unsteady BEM model, and the hydrodynamic load is calculated using the 3D potential theory. The floating foundation is considered as a rigid body, and the blades and tower are considered as flexible bodies. Based on the Kane method of a multi-body system, the dynamic responses of the VAWT could be solved in the time domain. A variable speed control model considering efficiency and load is established to match the rotating speed with the wind speed, and it could maintain the target output power under the influence of turbulent wind and large-scale movement of the floating foundation. The control strategy of limiting the target speed change rate and low-pass filtering is adopted to ensure the rapid regulation of the wind turbine under low wind speed conditions and stable regulation under high wind speed conditions.

Focusing on the transitional depth offshore area from 50 m to 75 m, types of articulated foundations are proposed for supporting the NREL 5 MW offshore wind turbine. To investigate the dynamic behaviors under various water depths, three articulated foundations were adopted and numerical simulations were conducted in the time domain. An in-house code was chosen to simulate the dynamic response of the articulated offshore wind turbine. The aerodynamic load on rotating blades and the wind pressure load on tower are calculated based on the blade element momentum theory and the empirical formula, respectively. The hydrodynamic load is simulated by 3D potential flow theory. The motions of foundation, the aerodynamic performance of the wind turbine, and the loads on the articulated joint are documented and compared in different cases. According to the simulation, all three articulated offshore wind turbines show great dynamic performance and totally meet the requirement of power generation under the rated operational condition. Moreover, the comparison is based on time histories and spectra among these responses. The result shows that dynamic responses of the shallower one oscillate more severely compared to the other designs.