Conceptual design of floating drilling production and storage caisson for arctic waters

Abstract The use of a permanently moored, vertically floating caisson appears to be a practicable concept for a drilling, production, and storage structure for use in the moderate ice conditions of the low Arctic. An hourglass configuration at the waterline plus ballast control of draft permits use of either the upward or the downward-breaking mode. Induced heave of the massive caisson is an effective means for breaking the ice and facilitates its clearing past the structure. Model tests were conducted to investigate the icebreaking capabilities of a moored caisson vessel. Results indicate that the structure could be held on location in the presence of a moving pack ice, up to 2.5 m thick, containing unconsolidated ridges up to 14 m thick. Actively induced heaving with a heave amplitude only slightly greater than that induced by the movement of the ice sheet was found to be effective in reducing the vessel's response in pitch and surge, and the resulting mooring line forces. The downward-breaking mode was found to be more effective than the upward-breaking mode. or deepwater areas in which icebergs may be encountered, provisions may be made for lateral movement to avoid an iceberg. Calculations calibrated to prior wave-basin model tests on similar under-water configurations show that the structure can continue operations during all normal open-sea conditions and survive extreme storm waves without abandoning station.