RIASSUNTO
ABSTRACT
This paper describes results from a study of ice gouge interactions with marine pipelines, carried out for Canada Oil and Gas Lands Administration. It defines the conditions under which pipelines can withstand the forces applied by gouging processes.
INTRODUCTION
Surveys of the seabed of the shallow seas bordering the Arctic Ocean reveal large areas which have been heavily scarred by ice gouging. Ice masses are driven into shallow water by the force of other pieces of drifting ice, in turn driven by wind and current. The masses ground in shallow water, and cut deep gouges into the seabed. In the Beaufort Sea and in the Arctic Islands, gouging extends from shore out to water depths of 50 m. The greatest intensity of deep gouging is found in water depths between 20 and 35 m. A few gouges more than 5 m deep are found, but most of them are rather shallower. Still deeper gouges occur in much deeper water in areas where icebergs ground, such as Davis Strait and the Grand Banks.
Pipelines are generally agreed to be the most safe, reliable and economical method of bringing Arctic offshore hydrocarbon resources to land, but it has long been recognized that seabed ice gouging is a threat to marine pipelines. A further concern is that it might not be sufficient to set a pipeline just below the maximum expected gouge depth, because of the possibility that large seabed loadings from the ice might damage a pipeline, even though the ice did not itself reach the line.
It was at one time hoped in some quarters that the formation of deep gouges would turn out not to be at active contemporary process, so that the gouges we see today would be a relic of earlier climatic and oceanographic conditions, perhaps with a different sea level and larger ice masses than occur today. Repeated mapping has proved that hope to be unjustified, and has shown that new gouges are being formed now. It was also questioned whether the ice forces currently present are large enough to cut the gouges that are observed, or whether the ice itself is strong enough to do so. That again has proved to be incorrect: the forces are indeed large, but not large in comparison with forces measured on offshore structures, and the ice stresses present are within the limits set by ice fracture and deformation.
The depth of trenching required to set a pipeline below the largest gouges in the Beaufort Sea is much greater than the depth of trenching routinely used to protect pipelines against fishing gear in heavily fished areas such as the North Sea. It is beyond existing pipeline trenching technology, because there has been no demand for very deep pipeline trenches except over relatively short distances in shallow water for shore approaches.