Probabilistic Fatigue Load Assessment for Floating Wind Turbines

Compared to bottom-fixed offshore wind turbines, floating offshore wind turbines are subject to increased sensitivity with respect to the ocean environment. This is in large part due to the additional degrees of freedom of the floating support structure, which result in significant amplitudes of translation and rotation due to environmental loading.

The present work elaborates, implements, and evaluates two fundamentally different approaches to calculate probabilistic fatigue damage. The first one is a Monte Carlo simulation, solving the integration problem by sequential addition of new damage events according to their occurrence probability. This is also used as the baseline case, as it can be shown that numerical Monte Carlo integration will converge to the correct solution. The second approach aims at a separation of the environmental probability space and the load regime induced by environmental conditions, by the introduction of surrogate models. This is of major benefit for fast evaluations required in system optimization and also supports a classification based design approach. A primary result of this work is the definition of reliable and automatable approaches by harmonizing alternative solutions for the abovementioned sub-problems such that they are applicable to arbitrary support structures and components. To achieve this for probabilistic fatigue assessment, a thorough understanding of the behaviour of floating wind turbines and all relevant impacts is required, as well as the methodologies to determine these in a reliable way. In this sense, an additional result of the present work is an improvement of understanding of floating wind systems in a highly uncertain load regime with high dimensionality.