Comment on wes-2021-125

doi:10.5194/wes-2021-125-rc1

Abstract

Present verification of the fatigue life margins on wind turbine structures utilizes damage equivalent load (DEL) computations over 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-minute DEL to determine its probability of exceedance and through 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.

Highlights

A procedure to determine long term fatigue damage and remaining life is presented as a combination of stochastic extrapolation of the 10-minute damage equivalent load (DEL) to determine its probability of exceedance and through computationally fast synthesis of DELs using level-crossings of a Gaussian process
The fatigue damage on wind turbine structures is strongly influenced by the stochastic inflow to the turbine, which is mainly composed of wind turbulence and spatial variations such as shear and veer. [Dimitrov et al (2017)]
A load measurement campaign will provide a wide scatter in DELs [Barber et al (2016)], so that it may be difficult to ascertain based on the limited simulations, as to what is the design DEL value that is to be considered for assuring that the magnitude of the measured DELs are within acceptable limits to ensure structural integrity

Summary

Introduction

The fatigue damage on wind turbine structures is strongly influenced by the stochastic inflow to the turbine, which is mainly composed of wind turbulence and spatial variations such as shear and veer. [Dimitrov et al (2017)]. The irregular ocean waves results in dynamic loads leading to fatigue damage. Since the Miner damage variable is not a physically measurable quantity, a substitute for damage is used, which is the Damage Equivalent Load (DEL). This is the load level at a particular number of cycles, which results in the same damage as the original summation of a multitude of different load cycles with different amplitudes.

Methods

Results

Conclusion