Including load sequence effects in the fatigue damage estimation of an offshore wind turbine substructure

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.