Fatigue analysis of floating offshore wind substructures

As offshore wind energy advances, floating offshore wind turbines (FOWTs) provide a crucial solution for harnessing wind power in deep waters where fixed-bottom wind turbines are not viable. Traditional FOWT analyses model structural behaviour as a rigid substructure connected to a flexible or rigid tower, allowing easy extraction of tower base loads. Consequently, most fatigue research for FOWT substructures focuses on the tower base, neglecting welded joints, which are known to be fatigue-critical locations. Hence, accurate assessment of the loads, and particularly the local stresses at these joints, is vital. This poster introduces a multidimensional time-domain strategy to estimate the fatigue life of a FOWT substructure, highlighting a novel load mapping method that translates global dynamic simulation loads into a detailed finite element model (FEM). The initial step applies hydrostatic, inertial, mooring, tower base, and Morison drag loads to the FEM. Hydrodynamic forces like diffraction, radiation, and Froude-Krylov loads, however, are commonly oversimplified as concentrated forces at the substructure's centre of gravity. For better precision, these forces should be represented as distributed pressures across the wetted surface of the substructure. To achieve this, a boundary element method (BEM) solver calculates these distributed pressures in the frequency domain, which are then converted to time-domain data using dynamic simulation inputs, i.e. wave elevation for diffraction and Froude-Krylov loads, and platform motion for radiation loads. This high-fidelity load modelling on the FEM enables the determination of accurate stresses, and, in extension, fatigue life predictions for the FOWT substructure. These predictions are crucial for optimising the structural integrity and operational lifespan of offshore wind energy systems.