Drilling vessels and production platforms conducting oil and gas operations in the Arctic will need to keep station in drifting pack ice. A rapid and safe disconnection system is needed as ice loads can exceed the capacity of the mooring system used for stationkeeping. Connecting all lines to a disconnectable submerged buoy has been proven effective for this purpose in open water. Using a similar system for vessels/platforms operating in ice requires special considerations as the disconnection of the buoy may need to take place at high offsets due to the high magnitude of ice loads.
There is no published work on the dynamics of submerged buoy for disconnection at high offsets. This paper presents new findings which were derived from numerical simulations for dynamic response of two disconnectable mooring buoy systems: a conventional medium size buoy with a compliant mooring system for a shuttle tanker in 200 m water depth (WD) and a larger buoy with a semi-taut mooring system for a drilling vessel in 500 m WD. Results for the global motion of the buoy and the dynamic contact loads between the buoy and the mating cone of the vessel soon after disconnect for various magnitudes of vessel offsets are presented in this paper. The results for the medium sized buoy with compliant mooring demonstrate strong nonlinear dependence of dynamic contact load on the disconnection offset in the range of 0 to 25% of the WD. The analysis results presented in this paper also show that a medium size buoy can have significant upward motion soon after exiting the mating cone and before it reaches its equilibrium position when the buoy is disconnected at high vessel offsets. For disconnections at very large vessel offset, the buoy during the upswing could impact the keel of an ice ridge. The large buoy with semi-taut mooring shows smaller upward motion for the 0 to 6% WD offset range considered in the simulations.
