Stability of Simply Shaped Icebergs

Abstract

Abstract With the advent of petroleum drilling in the Arctic and Eastern Canadian offshore regions, iceberg towing is now being explored as a method of avoiding collisions with offshore platforms. Because of the danger of the berg rolling, more information on stability is required before the method can be completely depended upon. This study analyses the stability of sixteen simply shaped icebergs chosen to approximately correspond to above-water classifications given by the Ice Patrol. The results indicate that the stability depends largely on the above-water characteristics of the berg and is relatively independent of the below-water shape. For the shapes chosen, the minimum stable ratio of waterline width to above-water height is approximately 6:1 for blocky or tabular shapes, 4:1 for drydock shapes, 3.8:1 for dome shapes and L8:1 for pinnacled shapes. Based solely on the above-water characteristics, which are easily measured, these figures may provide a quick estimate of stability which could be essential in the field. First, the type of berg is determined from the Ice Patrol classifications. Second, the ratio of waterline width (smallest waterline dimension) to approximate height is calculated. Finally, if this ratio is near the minimum stable ratio for this particular class, the iceberg is probably close to instability. With sufficient field experience, a towing crew may be able to determine when special precautions are necessary. Introduction PETROLEUM DRILLING in the Eastern Canadian offshore has created a great deal of interest in the towing of icebergs to prevent collisions with stationary offshore platforms or vessels. In studies of this method, a number of medium- and small-size bergs have been successfully towed (Bruneau and Dempster, 1972; Duval, et al, 1970; Hydrospace, 1970), indicating that the method is feasible. The stability of icebergs under tow is an important consideration. Observations of North Atlantic icebergs (Murray, 1969; Smith, 1931) show that they roll quite frequently. If one should roll over, the towing equipment would likely be lost or thrown off. Then there is the danger of a collision with the offshore structure if too much time is lost in getting it under tow again, A large rolling berg might even endanger the towing ship. Before the towing method can be completely depended upon more information on stability is required. TABLE 1- ABOVE-WATER ICEBERG SHAPE Classifications* BLOCKY – Steep precipitous sides with horizontal or flat top. Very solid berg. Length: height ratio 2.5:1 DR YDOCK – Eroded so that a large U-shaped slot is formed, with twin columns or pinnacles. Slot extends into the water line or close to it. DOME – Large smooth rounded top. Solid type of berg. PINNACLED – Large central spire or pyramid of One or more spires dominating shape. Less massive than dome-shaped berg of similar dimensions. Tabular – Horizontal or flat-tooped berg with length: height ratio of 5:1. GROWLER- A mass of glacial ice that has calved from a berg or is the remains of a berg.