Abstract

Retardation is a load sequence effect, which causes a reduced fatigue crack growth rate after an overload is encountered. Retardation can be cancelled when the overload is followed by an underload. The net effect is beneficial to the fatigue lifetime of Offshore Wind Turbines (OWTs). To be able to take this into account, computationally demanding cycle-by-cycle approaches are required.

This paper presents a methodology which aims at reducing a very long variable amplitude stress signal, such that it can be used to estimate the cycle-by-cycle fatigue damage, without jeopardizing accuracy. Filtering and reduction techniques are combined, based on typical events seen in the loading pattern of an OWT, e.g. during a storm. The effectiveness of the method is shown by comparing the number of cycles and the fatigue damage estimation, before and after reducing.

Introduction

The fatigue lifetime of welded joints is a design driver for Offshore Wind Turbine (OWT) substructures. An important simplification in the current Linear Fatigue Estimation Method is that load sequence effects are not accounted for. One of these load sequence effects is the retardation effect, which causes a lower fatigue crack growth rate after an overload is encountered, resulting in a longer fatigue lifetime. This effect can be included into the fatigue damage estimation by using certain Fracture Mechanics approaches. These methods use a cycle-by-cycle approach, which makes the processing of long variable amplitude signals a computationally demanding task. This paper presents a methodology that aims at reducing a large variable amplitude stress signal of an OWT support structure in such a way that it can be used to estimate the fatigue damage by using these cycle-by-cycle methods, without jeopardizing accuracy.

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