Abstract
Abstract In the Arctic ice is abundant and cheap, and therefore has long been considered as structural material for offshore drilling platforms. In February 1979 Exxon constructed an ice island 400 m in diameter and 10 m thick, in water 3.5 m deep, about 6 km north o£ Prudhoe Bay, Alaska. The purpose of the experiment was to learn whether such an island could survive one summer and then serve as a platform for an extended drilling period the following winter. Our studies of the decay of this island are based on:surface observation and photography,diving observations and bottom photos,fathometer, sub bottom profiler and sides can sonar surveys, and spot measurements of water salinity, temperature, and turbidity around the island. Thermal erosion, promoted by turbulent heat transfer from wave action and a relatively warm surface water layer surrounding the island, resulted in a 3- to S-m-deep notch at sea level, frequent calving, and a cantilevered, vertical scarp above sea level around the island. The winds, waves, and surface currents that transport warm river water past the island are predominantly from the east. Therefore, thermal erosion and calving were most rapid on the east side; they had truncated the island to the original center point by late August. By early September, the last remnants of the island floated away. The submerged part of the island, below the sea-level notch, consisted of an extensive pedestal reaching as much as 15 m beyond the island and feathering out to 10cm thick at the lip. The outer 5 m of this pedestal was elevated above the flat sea floor, forming a gap up to 25 cm wide that exposed the smooth base of the island to divers. Average island shore erosion, effected largely by calving, ranged from 2.5- to 5-m/day. The pedestal retreated across the sea floor at the same rapid rate, but melting, rather than calving, was probably the major cause of erosion. Hydraulic processes did not have time to develop cut-and-fill structures in the sea floor. In short, the test showed that unprotected ice islands in shallow, warm coastal waters have no chance of survival through the summer. Introduction Ice interacts with the marine environment in many different ways, and through these interactions becomes an important geologic agent. The U.S. Geological Survey has conducted research on ice related processes for many years. Recent petroleum developments in northern Canada and Alaska have focused attention on ice in the marine environment and on the effects of the ice on shores and the shelf surface. During the winter of 1978-1979, a circular ice island with a diameter of 400 m was built in the shallow waters of Stefansson Sound, about 6 km north of Prudhoe Bay, Alaska (Fig. 1). This large, firmly grounded mass of ice of known age, size, and shape provided a unique opportunity to study the effects of summer conditions until the island was destroyed by natural processes before winter.
Published Version
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