Transient behaviour of grouted connections of offshore wind turbines subject to ship impact

Abstract

Grouted connections are a critical structural element of monopile-supported offshore wind turbines (OWTs), connecting turbine towers with foundation piles. Due to engineering requirements, they are normally installed at the mean water level (MWL), leading to high risk of ship collisions. In this study, transient response and damage analysis were performed on the grouted connections of monopile OWT when subjected to ship impacts. Taking into account the sandwich construction of the grouted connection of the OWT and the non-linear behaviour of the structure materials, a finite element model of a vessel of 2000-ton displacement collision with a 5 MW monopile OWT was developed using explicit finite element code LS-DYNA. The ship was assumed to strike head-on and at the middle of the grouted connection. Characteristics of the collision system in terms of energy, velocity, impact force and the response of the ship and the grouted connection were analysed. Internal contact forces, stress and effective plastic strain distribution of the grouted connection were discussed. Four phases of the collision process were identified: initial collision, motion towards maximum OWT deflection, towards vessel-OWT separation, and after separation. The ship collision response and damage of the grouted connection were considerably influenced by the strain rate effect of the structure materials. Collided by a ship, even at a low velocity of 2 m/s, the grouted connection for monopile OWT could suffer heavy or major damage. The current analysis sheds light on the dynamic response of grouted connections of monopile OWTs to ship impact. It lays a solid foundation for future analysis on the design of grouted connections of OWTs.