With the increasing size of wind turbines, the demand for reliable mechanical performance and lifetime design are increasingly important. Due to stochastic wind loads, the random cyclic loads acting on wind turbine tower may cause unexpected fatigue damages and thus reduce the usage lifetime of the wind turbine.

The present work shows a study on cumulative fatigue damage for a bottom-fixed wind turbine based on the NREL 5 MW baseline wind turbine under wind conditions with turbulence. The nonlinear aero-hydro-servo-elastic computation tool FAST is used to obtain the loads over the wind turbine tower in the time domain. The tower structure is simulated by beam finite element model and the time-domain response is exported on nodes between tower base and tower top. Each node of the beam model represents actually a transversal cross-section of wind turbine tower. A time history of structural stress is hence evaluated, which allows to investigate the cumulative fatigue damage in different directions on the cross-section plane. The cumulative fatigue damage is calculated using Rainflow counting and Miner's rule coupled with Wöhler curve and Goodman relation. This approach results in a spatial representation of cumulative fatigue damage on cross-section. Contrary to other methods that evaluate a unique fatigue damage value to represent the whole structure, the present method provides not only the fatigue damage accumulated at any level above ground, but also the in-plane distribution of cumulative fatigue damage at a given height.

Under stochastic wind conditions with mean speed between cut-in and cut-out speed, the present approach offers a way to get a probability distribution of cumulative fatigue damage on cross-section. This can be used to investigate the reliability of wind turbine considering the cumulative fatigue induced by wind from different directions.