Numerical case studies of the ice loads to an Azimuthing thruster during the ridge interaction

An interaction between an azimuthing thruster and an ice ridge was simulated by the finite element method (FEM). An explicit solution algorithm was utilized, which enabled an efficient way to simulate ice failure in a dynamic analysis. The model described the whole ridge: the sail and keel consisting of ice rubble and the consolidated layer close to the waterline. The geometry of ridge was discretized by the finite elements. The azimuthing thruster including a part of ship body was modelled as a 3-dimensional rigid structure. The failure of rubble was modelled with a shear-cap material model. Spatially distributed material parameters together with the Concrete Damaged Plasticity -model (CDP) were applied to simulate the failure of the consolidated layer In the selected case, the ship moves backwards with an initial velocity against the ridge. It was assumed that the propeller does not rotate. The numerical study introduced basic understanding about how the ridge fails during the interaction with the ship hull and thruster body, how the ice loads are distributed and how large the overall ice loads for the thruster are. Three different ridge geometries were studied representing mild, typical and extreme conditions in Baltic Sea. Also, the parametric study included a variation of the initial velocity of the ship and the steering angle of the thruster. The simulations show a good correlation with the full-scale ice load measurements. The simulation results will be further utilized as background information for the upcoming proposal of the Finnish Swedish Ice Class Rules for Propulsion Machinery regarding Azimuthing thrusters.