Alaskan gas line routes
The article "New incentives move Alaskan gas line toward negotiation" quotes David T. Lupia, a corporate financial advisor, saying that the northern route of the gas pipeline "Uwould be much more economical," apparently than the southern route along the existing pipeline right-of-way (OGJ, Nov. 1, 2004, p. 26). He went on to say that the US and the state of Alaska are attempting "Uto improve a less attractive investment alternative."
Mr. Lupia's view is consistent with statements in other articles about the proposed natural gas pipeline. It has been widely accepted that a pipeline routed to the east of Prudhoe Bay to the Mackenzie Delta in Canada and southward along the Mackenzie River is more attractive economically than a southerly route along the existing Alaska pipeline to a new right-of-way southeastward that parallels the Alaska Highway.
Given the controversy over constructing and operating oil infrastructure in the Arctic National Wildlife Refuge (ANWR), the northerly pipeline route would have to skirt the refuge offshore for a distance of some 250 miles. Furthermore, the pipeline would have to be laid at a depth, either in gravel or on the seabed, safely below the depth limit of ice scour. That would keep the pipeline 15-20 miles offshore between the Canning River and Mackenzie Bay.
At the west and east ends of the offshore reach, the pipeline would have to pass through critical transition zones from the shoreline to a depth below the ice-scour zone offshore. On the North Slope, the industry operates pipelines in shallow waters near shore where ice remains fast with relatively minor pressure ridging. The pipelines are built upon massive gravel causeways to protect against ice forces. Offshore pipelines have yet to enter deeper arctic waters where "screwing pack ice" (a descriptive term used by arctic mariners) scours the seabed and generates great pressure ridges and ice forces.
Two construction options exist for a northern route of the Alaska gas pipeline. First, remain in water close to shore and shallow enough so that a gravel causeway could be built to protect the pipeline from ice forces. The causeway would be 250 miles in length from near Point Thomson (west of the Canning River) to a point east of Canada's Northern Yukon National Park. If Canada were to grant permission to pass through the park onshore, the causeway could be shortened by some 80 miles. Such a massive and lengthy barrier between the shore and open water would have untold environmental impacts on ANWR and native subsistence, impacts that would likely prove to be prohibitive.
The second option would be to lay the pipeline on the seabed in deeper waters below ice scour. The transition sections at the east and west ends of the offshore reach would consist of causeways and caissons that would carry the pipeline from above ground to the seabed, while always insulating the pipeline from ice forces. An offshore compressor station might also be necessary to boost pipeline pressure.
At this early stage, little has been published to demonstrate that the northern route is more economical than the southern route. I suspect that insufficient consideration has been given to the unprecedented conditions that pipeline designers and contractors must overcome to construct a pipeline through year-round, ice-infested waters.
It may turn out that the southern route along the Alaska Highway is the more attractive investment alternative.
Reference: Ivanhoe, L.F., "Sea ice will increase operating costs in Alaska's Bering Sea waters," OGJ, Oct. 26, 1981, p. 267.
Tom Standing
San Francisco Public Utilities Commission