Multiaxial Fatigue Initiation-Propagation Modelling With Microstructure Effects for Welded Monopile Offshore Support Structures

Abstract This paper presents a combined crack initiation-propagation finite element (FE) modelling methodology for multiaxial fatigue analysis in weldments of offshore wind turbine (OWT) monopile support structures, for thermo-mechanical controlled processed (TMCP) S355 G10+M steel, under realistic operational load cases. A key objective is to account for the effects of TMCP- and welding-inducing microstructural inhomogeneities within a critical-plane multiaxial fatigue methodology. The effects of combined cycles are incorporated using the Palmgren-Miner method for cumulative damage over wind turbine service life. Fatigue crack propagation is incorporated via the extended finite element method (xFEM). The results are compared to the DNV standard for welded joint fatigue in offshore structures and previously published results. The results confirm a significant level of conservatism of current design standards for these weldments, alongside the microstructural effects on the fatigue of these weldments. The ability of the proposed methodology to provide a preliminary physical basis for effect of thickness is demonstrated. This is important for safe upscaling of offshore wind.