A semi-analytical methodology to assess the dynamic elastic response of a monopile offshore wind turbine subjected to low-energy ship impacts

Low-energy impacts between ships and offshore wind turbines (OWT) can occur during maintenance operations, when service vessels need to approach the turbine at low speeds. While this type of accident might not result in significant permanent damage of the tower or the foundation, the whipping elastic response of the overall structure can induce high accelerations at the rotor-nacelle assembly (RNA), where sensitive subcomponents such as the generator, bearings and gearbox are contained. This study presents a two-step methodology to quickly assess the whipping elastic response of a monopile OWT subjected to a low-energy ship impact. The first step consists of modelling the monopile as a reduced set of Euler-Bernoulli beams with different cross-sections and a tip lumped mass representing the RNA. A semi-analytical framework is used to derive closed-form expressions to calculate natural frequencies and mode shapes of the beam assembly. Once these properties are obtained, the modal superposition method is applied to calculate the monopile's response to a pre-defined collision force. Validation of this semi-analytical methodology is performed using the nonlinear FEM code LS-DYNA. The proposed method is intended for pre-design stages, when risk assessments are performed and a large number of collision simulations is required. In this case, fast calculation tools are generally more suitable than FEM, allowing for a good compromise between computational cost and accuracy.