Simulations of the Effect of the Rate of Change of Ice Direction on Stationkeeping of Drillships

Abstract

AbstractThis paper examines the response of a drillship to the action of an ice cover undergoing a gradual change of direction of motion. For effective stationkeeping, the drillship must continuously change its heading to face the oncoming ice. The simulations consider a vessel that maintains position using Thruster-Assisted Mooring (TAM). A turret mooring system resists the surge and sway movements, while the thrusters act to correct the heading. The simulations of ice deformation and drift solve equations that describe conservation of mass and momentum, and a failure criterion. The drillship is treated as a three-degrees-of-freedom (surge, sway and yaw) rigid body. The results give distributions of ice drift, deformation and stresses around the vessel as well as the forces and offsets of the drillship. The resulting peak ice forces and moments on the drillship show clear dependence on the rate of change of ice drift direction. As may be expected, ice forces and moments increase for high rates. They also increase for higher ice velocities. Analysis of the results shows that the ratio of the length of the drillship to the radius of curvature of ice drift trajectories can be used to estimate ice forces and moments in an environment of changing ice drift direction. The present work additionally included a cursory examination of the effects of ice cover conditions; namely ice thickness and the existence of ridge fragments and icebreaking tracks, which are often formed as part of ice management operations.