On the calculation of offshore wind turbine load spectra for fatigue design

Abstract

The present work researches on the definition of the load spectra used for offshore wind turbine low SN slope materials’ fatigue design. Uncertainty in the sample sized used to scale fatigue life is analyzed for the tower component. Damage density is investigated for different environmental conditions in order to understand the importance of the different regimes of op-eration. Damage density is identified to be a heterogeneous function of the load-ing environmental conditions. In some cases, even for low SN slope materials, most of the damage occurs due to high load ranges. To study on the influence of this heterogeneity, different statistical tail fits are used to compare the influence of accurately defining the tail region on a reference design time (𝑇). Results show that OWT fatigue is highly dependent on the 𝑡 shorter that 𝑇 time used to approximate 𝑇. This is mainly related to the fact that fatigue design de-pends not only on scaling stress ranges, but also cycle counts. Effort on the de-sign phase should be applied in the definition of the uncertainty of the load spectra due to the limitation imposed by using low sample sizes to cover the ex-tensive joint distribution of environmental parameters.