Fatigue reliability using a multiple surface approach

Abstract

Reliability analysis for offshore wind turbines fatigue is an effort demanding task. New trends in the design of these systems, such as, the usage of alternative computational fluid dynamics or finite element methods, are expected to further increase the effort needed to design these systems to fatigue. As a result, design techniques that enable practicable fatigue analysis are on demand. The present paper researches on how to use fatigue damage surfaces in order to assess stress-cycle fatigue reliability. A Gaussian process model is applied as a surrogate of fatigue damage. It allows to enclose multiple normally distributed interpolated surfaces. Probabilistic SN curves are considered, creating a double surface model, where the Gaussian process model is built on top of the curve. Analysis is performed on a 5MW turbine with a monopile foundation, and stress-cycle fatigue is assessed for the tower component. Results of the implementation show that there is a significant advantage in using surrogates of fatigue damage as only a limited number of time domain simulations is required for design. Fatigue design was assessed using a subset of 25 load cases. Moreover, the predictor surrogates accurately the design procedure within different material probabilistic characteristics, and accounting for loading uncertainty. Fatigue reliability assessment with these models may be performed with approximately 10% to 40% of the binned environmental conditions computational effort, which is of interest in the wind engineering sector. Nevertheless, the approach implemented may be applied to any component on any system, with the only requirement of defining a representative fatigue indicator.