Structural performance evaluation of wind turbine towers under Nordic climate conditions

Abstract

This study evaluated the structural performance of a standardised wind turbine tower procured from international manufacturers under Finnish wind conditions. Eight months of wind data and tower geometry were obtained from a wind farm operating in Finland. From the wind data, the average and extreme wind speeds at the wind farm site were calculated to be 6.9 m/s and 40 m/s, respectively. A numerical model of the wind turbine tower was built using ABAQUS and validated based on its natural frequency obtained from the experimental results. Based on the wind speeds in normal and extreme conditions, the average and extreme wind loads were calculated and applied to the numerical model to investigate its stress and deflection. During normal operational conditions, the stress was 70 MPa, while deflection was 0.6% of the tower’s height. However, during the extreme conditions, the same parameters increased to 102 MPa and 0.65%. A fault with the pitch mechanism was also studied in extreme wind conditions. In the case of this simultaneous occurrence of extreme wind speeds and fault conditions, the stress and deflection were evaluated to be 215 MPa and 2% of the tower’s height, exceeding the maximum allowable criteria of 1% maximum deflection used in this study. The wind turbine tower connections were modelled using cohesive surface behaviour. A parametric analysis of the preloaded bolt connection revealed that the tower's performance degraded with the loss of connection stiffness, i.e., the natural frequency decreased while stress and deflection increased. Based on the wind and numerical analysis, it was concluded that wind turbine tower can resist both average and extreme conditions. However, any fault with the pitch mechanism or excessive loss of connection stiffness will adversely affect its structural performance.