An Engineering Method for Structural Analysis of Semisubmersible Floating Offshore Wind Turbine Substructures

This work proposes a mid-fidelity load-mapping method for the structural analysis of semisubmersible floating offshore wind turbine substructures. Building on a hybrid linear potential flow and strip-theory dynamic analysis, the method maps hydrodynamic, current, hydrostatic, gravitational, inertial, mooring, and turbine loads onto a shell-based finite element (FE) model. The functionality of the proposed method is demonstrated through two case studies involving ultimate limit state analysis of a structurally reinforced OC4 DeepCwind semisubmersible platform. The analyses were conducted for two design load cases (DLCs) formulated to represent the metocean conditions at the Utsira Nord site, located off the coast of Norway. The accuracy of the mapped hydrostatic and potential flow loads is validated against dynamic simulation data, while a mesh convergence study is used to ensure reliable FE model performance. Results show that the highest von Mises stresses occur at unsupported heave-plate regions, internal stiffeners, and welded joints, with peak stresses safely below the steel’s yield strength. The more severe conditions of DLC 6.1 lead to a broader distribution of high-stress locations compared to DLC 1.6 but only a modest increase in peak stress.