Ice Rubble Field Stability

ABSTRACT Artificial islands, built as exploratory drilling platforms in the shallow waters of the southern Beaufort Sea, cause the broken ice pile-ups to form around them. These "ice rubble fields" generally form in the early winter when the ice is thin and the movements are extensive. Previously, the sliding resistance of the ice rubble was calculated by surveying pile-up profiles and drilling through the pile-ups to obtain a keel profile. This is a tedious and expensive job. We have collected sufficient data over the past eight years to verify that the excess weight on seabed can be determined solely from aerial analysis. The sliding resistance is then calculated by using excess weight and friction factors obtained from laboratory tests. CHARACTISTICS OF RUBBLE FIELDS During the past decade the search for hydrocarbons under the world's oceans has been extended to the ice infested waters of the Beaufort Sea. Esso Resources Canada Ltd. has been exploring in the Canadian Beaufort Sea since l973. Esso has drilled 18 wells from seventeen artificial islands (Figure 1). The islands are usually constructed in the summer and drilled in the winter l, 2, 3). Artificial islands have been found to induce the formation of ice rubble fields around their periphery in early winter, while the surrounding ice is thin and subject to extensive movements7. These ice rubble fields often extend to the point where the toe of the island's slope meets the sea bottom. Grounding of individual rubble piles is common within this zone. Recent observations at abandoned island locations where the fill has eroded below sea level show that even over these submerged shoals ice rubble features continue to form and remain in place until spring break-up. The stability shown by these features, particularly in late winter when the surrounding ice sheet thickness reaches its maximum thickness, has led to the conclusion that the resistance offered by these features should be considered in the design of an artificial island. New ice formation in the southern Beaufort Sea begins in late September or early October. It initially forms near the shoreline and extends slowly seawards. The rate and direction of growth is dependent on meteorological conditions. As the winter progresses the ice sheet between the shoreline out to about the 20 meters depth contour becomes land fast. The land fast ice is so termed because its motion is reduced by its boundary with land. This ice can reach a thickness of 2.2 meters by the end of the winter. The area known as the shear zone lies between the land fast ice and the polar pack. This zone is characterized by extensive ice movements and deformations. Figures 2 and 3 are photographs taken during the winter of 1980/81 at the Netserk South location, an abandoned island which has eroded to below mean sea level. These photographs show a rapid growth in the size of the rubble field within a period of five days. This rubble pile and other similar ones are formed during periods of ice movement, the greater the movement, the faster the rubble formation.