Ice Islands as Hazards to Arctic Offshore Production Structures

ABSTRACT Consideration is given to the hazard presented by moving ice islands. A review of ice island occurrences, as reported in the literature, is supplemented by details of our observations of several recently-discovered ice islands north of Ellesmere Island. Systematic traverses of the edge of the ice shelves for the past two decades have revealed ice island generation to be the rule. Thickness and ice type can be inferred from ice shelf studies, which include crystallography, salinity and temperature profiles. Ice island trajectories, after calving from the ice shelves, are related to winds and internal pack ice stress conditions. Suggestions for possible defensive measures are given. INTRODUCTION With the likely exploration and development of offshore oil and gas reservoirs in the Beaufort and Chukchi Seas during the next decade, it is of interest to determine the probability of encounter between an offshore production structure and a drifting ice island. Ice islands are large tabular icebergs with thicknesses ranging up to 60 meters and lateral dimensions ranging upward to approximately 40 × 40 km. Because of their relatively great thickness, they have often been observed grounded at water depths greater than 20 meters, and may represent the strongest variety of ice in the Arctic Ocean. Ice islands are generated by calving from the ice shelves along the northern coast of Ellesmere and Axel Heiberg Islands, and have been noted since the earliest explorers' journeys. Early explorers noticed the ice shelves on the coast of Ellesmere Island as having a upper surface topography appearing like hills and dales, with long linear lakes and watercourses. These same long linear features were noted on drifting islands of ice in 1883 by Greeley, in 1918 by Storkerson, and by other early explorers as well. A thorough search of aerial photos by Greenaway in 1952 yielded 59 possible ice islands as well as many more smaller fragments. "In October 1943, 3 km to the north-northwest of Mist Chelyuskin, a flat-topped iceberg was discovered, 1500 m long, 400 wide, and rising 10 high out of the sea" (Zubov 1948). A review of ice island sightings since 1946 by Speeding (1977) resulted in an estimate of 3184 km of ice islands which had been produced and/or discovered during that 30 year interval. Many of those discovered in the interval 1946 to 1966 were named, and some have subsequently been reported as drifting into the North Atlantic where they have broken up. In 1946, the ice island T-1, measuring approximately 15 × 18 nautical miles, was discovered 300 miles north of Point Barrow. Ice island T-2 was discovered in 1950 and was estimated to be 17 × 18 nautical miles in size. On July 29, 1950 the most famous ice island, T-3, was discovered at 75°24'N/173°00'W, measuring 4.5 × 9 nautical miles in size.