Modeling of Pressured Ice Interaction with Ships

Abstract

The paper describes numerical simulations of ship transit through ice. The simulations employ a model which is based on solving the conservation of mass and linear momentum together with constitutive equations representing plastic yield. A cohesive Mohr-Coulomb criterion with a tension cut-off is used to represent the yield condition. The numerical solution approach is based on a Lagrangian-Eulerian hybrid formulation. A depth-averaged version of the model is used, whereby the stresses and velocities are averaged over ice thickness. Ice thickness build-up and lead opening are accounted for in the model. The ice cover is driven by prescribed displacements or pressures at the boundaries. Wind and water current drag are also included. The simulations address cases of ship moving at constant velocity through a uniform ice cover, of 200 m width and 1 km length. The geometry of the Canadian Coast Guard vessel, CCGS Louis S. St- Laurent, is used in the tests. The results give the evolution of the distributions of ice concentration, thickness and pressures. The ice force-time records are also produced. The predicted forces are compared to recently reported field measurements of ice forces on the CCGS Louis S. St-Laurent. The magnitude of the simulation forces are in agreement with the measurements. A parametric study examined the role of the following variables: velocity of the ship, ice concentration, ice thickness, and properties of the ice cover (angle of internal friction). The results indicate that velocity has the most pronounced effect on ice force. The concentration and thickness also had significant effects. The angle of internal friction has somewhat less significant effects. The simulations also examined ship transit under pressured ice, or convergence, conditions. Conditions at the lateral boundaries applied ice movements against the ship during transit. The simulations show the pressure build-up against the sides of the bow due to ice convergence, and the increase in the ice force on the ship. The results indicate that ice pressures on the ship are two orders of magnitude higher than the large-scale average stresses which are calculated over a 1 km length.