Effects of aerodynamic damping on the tower load of offshore horizontal axis wind turbines

Aerodynamic damping has an important effect on the dynamic response of offshore Horizontal Axis Wind Turbines (HAWTs). In this paper, an analysis of the loads on offshore HAWTs is presented. The analysis combines the aerodynamics, hydrodynamics and structural dynamics of the structure, and includes the effects of aerodynamic damping. The aim is to better understand the role of aerodynamic damping during the interaction of wind and wave and the structure, and to quantitatively evaluate the effects of aerodynamic damping on the lifetime fatigue load on offshore HAWT towers. The aerodynamic loads are estimated using the Blade Element-Momentum (BEM) theory, including the effects of dynamic inflow and dynamic stall. The wave dynamics is estimated assuming 'random sea state' described by the JONSWAP spectrum, with wave loads calculated using Morison's equation and water kinematics modelled using linear wave theory. Two aerodynamic damping models are proposed: (1) a model based on the analysis of the rotor aerodynamics incorporating the tower-top motion of a constant-speed wind turbine, which is then modified for variable-speed wind turbines by introducing a correction factor; and (2) a model based on Salzmann and van der Tempel's method (Salzmann and van der Tempel, 2005) to calculate the aerodynamic damping as the increase in the thrust per unit increase in the wind speed. The models are incorporated into a transient load analysis. The effects of aerodynamic damping on the lifetime fatigue loads of the tower are then investigated through load analysis of a 5 MW offshore HAWT. In addition, the influence of different aerodynamic damping calculation methods on the prediction of fatigue loads is studied.