Wake Influence on Dynamic Load Characteristics of Offshore Floating Wind Turbines

Abstract

Because the flow conditions of an offshore floating wind turbine and onshore fixed wind turbine differ, it is debatable whether the aerodynamic load predictions of an offshore floating wind turbine using the conventional blade-element momentum theory, which does not consider the dynamic wake effects, are accurate. Although a generalized dynamic wake method has been developed to consider the dynamic wake effect, it is only stable for lightly loaded wind turbines at high wind speeds. In contrast to the blade-element momentum theory and generalized dynamic wake method, the unsteady vortex lattice method can inherently represent the nonuniform flow effects of the trailing wake from the turbine blades. This paper aims to determine the wake influence of offshore floating wind turbines at low-wind-speed conditions by comparing three wake models: the blade-element momentum theory, generalized dynamic wake method, and unsteady vortex lattice method. The Offshore Code Comparison Collaboration Hywind model is chosen for offshore floating wind-turbine simulation. Results show that the blade-element momentum theory underestimates the rotor torque and speed. Moreover, although responses of the generalized dynamic wake method and unsteady vortex lattice method agree well at moderate wind speeds, the generalized dynamic wake method predicts higher induction factor than that of the blade-element momentum theory and unsteady vortex lattice method at low wind speeds. At low wind speeds, the blade flapwise bending moment, rotor torque, and tower side-to-side bending moment calculated by the blade-element momentum theory are considerably different from those obtained by the unsteady vortex lattice method.