Damage equivalent load synthesis and stochastic extrapolation for fatigue life validation

Abstract

Present verification of the fatigue life margins on wind turbine structures utilizes damage equivalent load (DEL) computations over a limited time duration. In this article, a procedure to determine long-term fatigue damage and remaining life is presented as a combination of stochastic extrapolation of the 10 min DEL to determine its probability of exceedance and computationally fast synthesis of DELs using level crossings of a Gaussian process. Both the synthesis of DELs and long-term stochastic extrapolation are validated using measured loads from a wind farm. The extrapolation for the blade root flap and tower base fore–aft damage equivalent moment is presented using a three-parameter Weibull distribution, whereby the long-term damage equivalent load levels are forecast for both simulated and measured values. The damage equivalent load magnitude at a selected target probability of exceedance provides an indicator of the integrity of the structure for the next year. The extrapolated damage equivalent load over a year is validated using measured multi-year damage equivalent loads from a turbine in the Lillgrund wind farm, which is subject to wakes. The simulation of damage equivalent loads using the method of level crossings of a Gaussian process is shown to be able to reconstruct the damage equivalent load for both blade root and tower base moments. The prediction of the tower base fore–aft DEL is demonstrated to be feasible when using the Vanmarcke correction for very narrow band processes. The combined method of fast damage equivalent load computations and stochastic extrapolation to the next year allows a quick and accurate forecasting of structural integrity of operational wind turbines.