Validation of the ColFOWT collision assessment tool for ship/offshore wind turbines

Abstract

The consequences of ship collisions against offshore wind turbines can range from minor structural damage to catastrophic failure, depending on various factors such as vessel initial kinetic energy, impact location, and geometry. To address this critical issue, the ColFOWT (Collision against Floating Offshore Wind Turbines) project focuses on developing a rapid collision assessment tool that integrates closed-form analytical models. This tool predicts the complex energy transfer processes during impacts and aims to validate its accuracy through numerical simulations using the LS-DYNA non-linear FEM package. In this mater thesis, the analytical tool for spar-buoy floating platforms is presented. The developed method adopts a semi-coupled approach, using the rigid-body dynamics program MCOL to simulate the external dynamics of the floating wind turbine. Meanwhile, internal mechanics are computed using an elasto-plastic simplified method for the impact response of standalone tubular offshore wind turbine supports. An overview of the algorithm is provided, including a detailed explanation of both internal mechanics and external dynamics solvers, along with the coupling method. The presented collision tool is validated by comparing it against simulations conducted with LS-DYNA/MCOL. The case study involves various offshore supply vessels impacting, with varying initial velocities, an NREL 5 MW baseline turbine mounted on an OC3 Hywind reference spar platform. The results demonstrate that in most of the cases, the analytical tool is able to capture quite accurately the response of both the turbine and the vessel, accounting for the action of both the surrounding water and the mooring lines. Its current limitations are also highlighted and, although there is still room for improvement, this user-friendly and cost-effective complementary tool shows good accuracy for the early-design stage.