Abstract
The ability of a drillship to maintain its heading to face oncoming pack ice is crucial under situations involving changes in pack ice drift direction. The performance of a vessel employing a Thruster-Assisted Mooring (TAM) system under such conditions is examined in this paper. Numerical simulations were used to determine the stresses and deformations within the moving pack ice cover, as well as the response of a drillship with characteristics similar to published information on Stena's DrillMAX. A turret mooring system is assumed to resist surge and sway direction offsets, but provides no restoring moment to vessel's yaw. In such system, thrusters would be used to provide damping and to apply the corrective moment that controls the heading of the vessel. The pack ice cover is assumed to consist of managed floes of sizes ranging from 30 m to 50 m, and with a uniform thickness of 1 m. The ice cover moves against the vessel at a steady velocity of 0.5 m/s. Simulations start with the vessel at a heading inclined to the oncoming ice direction. The simulations predict the evolution of the distributions of ice thickness and pressures, ice forces and moments, as well as the response of the vessel. Test cases examined a range of values of the initial heading of the vessel and limits on available yaw moment that can be generated by the thrusters. The results illustrate the manner in which the vessel can correct its heading, and give the corresponding offsets, ice forces and moments.
