To keep the water around an off-shore installation (1) free from a harmful impact of ice, a vessel (5) is used to deploy an anchor (6) in a position at a distance from the off-shore installation (1) and in a direction which, seen from the off-shore installation (1), is substantially in parallel with the direction of movement (P) of the ice. By means of the machinery of the vessel, which preferably comprises azimuth propellers, the direction of the anchor line is adjusted and so is the orientation of the vessel relative to the anchor line to the effect that then propellers can be used to crush and dispose of the ice without using energy to hold the vessel up against the pressure of the ice.