Directional fatigue accumulation in wind turbine steel towers

Wind turbines are subject to fatigue loads during their entire lifetime of 20-25 years. A main source of the fatigue loads is the turbulence, which varies with direction due to the surrounding terrain and wake effects inside wind farms. A common approach to assess wind turbine fatigue loads is to simulate the structural response based on a site-specific wind climate, described in the IEC 61400-1 standard. To reduce the amount of needed simulations the standard introduces an "effective turbulence" approximation that integrates directional variation of turbulence, resulting in an omnidirectional value. This method implicitly assumes that all wind turbine components face the wind directly, which is a conservative simplification for components below the yaw bearing.

Using wind measurements from almost one hundred international sites, we show how this simplification leads to over-predictions of tower fatigue loads of up to 23% compared to directional fatigue accumulation. Three simplified models are developed to approximate the directional fatigue damage using various levels of information ranging from only the wind rose to full sector wise simulations. The first two recommended models may be used as proxies to decide if sector wise simulations are feasible, and the last model accurately predicts the full directional fatigue damage. The simplified models can contribute to a reduced material consumption of wind turbine towers, thereby reducing the cost of wind energy.