Assessment of the energy balance gap for ship-FOWT collision simulations with LS-DYNA/MCOL

Non-linear finite element (NLFE) codes are a consolidated tool in the crashworthiness analysis of ships and offshore structures, employed in the design of fixed offshore wind turbines (OWT) and, more recently, for floating offshore wind turbines (FOWT). In this latter case, additional complexities associated to large rigid body rotations and strong hydrodynamic effects require the use of algorithms capable of coupling external dynamics and internal mechanics. One of the available methodologies uses the NLFE package LS-Dyna combined with the rigid body dynamics program MCOL. This approach has been previously validated for ship-ship collisions. However, some inconsistencies arise for ship-FOWT impacts, particularly in the global energy balance. A parcel of the total energy input is seemingly lost, contradicting the expected energy conservancy. Therefore, the objective of this study is to understand the issue by performing LS-Dyna/MCOL simulations of ship-FOWT collisions, where main influencing factors can be observed independently. Simplified numerical models are developed for this purpose. The study provides a detailed understanding of the issue and its source.