Experimental and numerical investigation on the deformation behaviors of large diameter steel tubes under concentrated lateral impact loads

Large diameter steel tubes are widely used in bottom fixed and floating offshore wind turbines. Offshore wind turbines (OWTs) operating in the oceans are exposed to the risk of collisions when ships pass and dock at turbines. Thus, it is extremely important to investigate the impact mechanics of ship-OWT collisions and propose practical designs for methods to protect OWTs from collision loads. This paper presents a series of experimental and numerical studies on the deformation behaviors of large diameter steel tubes from a NREL 5 MW spar-type floating offshore wind turbine (FOWT) under lateral impact loads. These studies consider the effects of different impact velocities, attached masses, diameters and thicknesses of the tubes on their response to impact loading. In these experiments, a rigid indenter was mounted on a pendulum system and accelerated to strike the tubes; the scale was 1:30. The dimensions of the indenter head were much smaller than the tube diameter in order to concentrate the impact load. Global motions of the impacted tubes were modeled by springs introduced at the boundaries, and their stiffnesses were determined according to an equivalent single-degree-of-freedom (SDOF) model. Numerical simulations of the experiments were conducted using the nonlinear finite element (FE) software LS-DYNA. The experimental and numerical results were compared and discussed with respect to forcedeformation curves, deformation modes and energy dissipation. Existing theoretical solutions for the lateral indentation resistance of tubes were also compared to the experimental and numerical simulation data. The results indicated needs for new solutions when the impact loads become concentrated.