A simplified method to assess the elasto-plastic response of standalone tubular Offshore Wind Turbine supports subjected to ship impact

This paper presents a simplified analytical approach to rapidly simulate the elasto-plastic response of standalone tubular Offshore Wind Turbine (OWT) supports (e.g. monopiles and spar floating platforms) that are subjected to a ship collision. The wind turbine, idealized as a constant cross-section cantilever tube with a tip mass, is assumed to be struck by a rigid impactor. In a first step, a series of preliminary finite element simulations enable the identification of three successive phases in the deformation process: (i) local elastic denting, (ii) local plastic denting combined with global elastic bending, and (iii) plastic buckling at the base of the structure. In a second step, from these observations, existing analytical formulations extracted from the literature are integrated into a time-stepping algorithm. Finally, resulting force-penetration and energy balance curves are compared to finite element results. Simulations are performed considering mid and quarter-length impacts of a 6000-tonnes Offshore Supply Vessel approaching at different velocities. Seven different OWTs are tested. A good correlation between analytical predictions and numerical simulations is found in the vast majority of the investigated collision scenarios. Limitations are encountered for stockier supports, where the contribution of the global elastic bending mode is underestimated by the proposed analytical model. Overall, the method presents a promising alternative for risk assessment and structural optimization applications.