Fully-Coupled Time-Domain Simulations of the Transient Response of Floating Wind Turbines
Compared with land-based wind turbines, offshore ones have two clear advantages: the access to steady and strong offshore wind resources, and the proximity to heavily populated coastal areas, so that the loss from transportation of electricity is diminished. Structurally, offshore floating wind turbines are different from their land-based counter parts and existing offshore structures. Thus, current design guidelines cannot be applied for offshore floating wind turbines, and extensive numerical studies are required to predict the dynamic behavior of these novel structures. In this research, a fully coupled time domain hydrodynamic, aerodynamic and mooring cables model is developed to study the transient response of offshore floating wind turbines.
In hydrodynamic module of the coupled model, Boundary Element Method is employed to solve the boundary-value problem, and the 4th order Runge–Kutta time marching technique is used to update the position of the free surface. An unsteady wind-blade interaction model based on boundary elements has been developed to calculate the aerodynamic forces. This method achieves fully-3D and fully-unsteady simulations of the wind-blade interactions. A fully nonlinear cable dynamics model which accounts for bending, stretching, and torsional stiffnesses of the cables is employed to simulate the dynamics of the mooring system. Compared to current quasi-static approaches used in cable modeling, the fully nonlinear cable model proves a higher fidelity as it captures all the dynamics of the mooring cables. The information from the aerodynamic, hydrodynamic, and mooring system modules are passed to the dynamic equation of motion in each time step to calculate the responses of a 5MW offshore floating wind turbine.
Various relaxation tests have been carried out to investigate the dissipation effects. For the floating turbine we studied, the relaxation tests indicate that in the pitching mode there is no sufficient damping effects to dissipate disturbances (caused by gusts etc.), even if the aerodynamic damping is counted for. However, by approximately including the viscous damping through a Morison-type approach, the decaying rate in pitch motion is significantly increased (indeed, it becomes comparable to the decaying rate in heave motion). Therefore, we conclude that viscosity is the most important source of damping in pitch motion.
The response of the 5MW system to an incoming wave train generated by a pulsating pressure distribution on the free surface nearby is studied. This wave train causes responses in surge, heave and pitch directions. However, after it leaves the vicinity of the system the only remaining motion is in the heave direction, which will quickly decay due to the damping effects according to relaxation tests.