Abstract
Methods for coupled aero-hydro-servo-elastic time-domain simulations of Floating Offshore Wind Turbines (FOWTs) have been successfully developed. One of the present challenges is a realistic approximation of the viscous drag of the wetted members of the floating platform. This paper presents a method for an iterative response calculation with a reduced-order frequency-domain model. It has heave plate drag coefficients, which are parameterized functions of literature data. The reduced-order model does not represent more than the most relevant effects on the FOWT system dynamics. It includes first-order and second-order wave forces, coupled with the wind turbine structural dynamics, aerodynamics and control system dynamics. So far, the viscous drag coefficients are usually defined as constants, independent of the load cases. With the computationally efficient frequency-domain model, it is possible to iterate the drag, such that it fits to the obtained amplitudes of oscillation of the different members. The results show that the drag coefficients vary significantly across operational load conditions. The viscous drag coefficients converge quickly and the method is applicable for concept-level design studies of FOWTs with load case-dependent drag.
Highlights
Research on floating instead of fixed-bottom offshore wind energy started more than ten years ago
The present study addresses the question of the load-case dependence of the drag coefficients for Floating Offshore Wind Turbines (FOWTs)
In [28], the heave plate drag CD,hp was identified for coupled model tests of the TripleSpar, including the wind turbine and controller
Summary
Research on floating instead of fixed-bottom offshore wind energy started more than ten years ago. There are different coupling effects between the submodels of aerodynamics, hydrodynamics, servodynamics and structural dynamics This results from the soft substructure restoring forces, especially of semi-submersible concepts, leading to a slow fore-aft motion of the rotor and an influence on the aerodynamics. One important example for the dynamic coupling effects is the influence of the wind turbine blade pitch controller on the global fore-aft mode of the FOWT. The controller pitches the blades, due to a change of the relative wind speed seen by the rotor, originated from a fore-aft oscillation of the platform. This pitching reduces the thrust and can eventually yield a fore-aft system instability as shown by [2,3,4], among others. The first was the FAST model by National Renewable Energy Laboratory, Boulder, CO, USA (NREL) (see [6])
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