Natural Gas Pipeline Design And Construction In Permafrost And Discontinuous Permafrost
Authors: Mathews Amos C. 
Published: December 2007

The paper discusses some of the special techniques Arctic Gas has designed to construct and operate large diameter, high pressure natural gas pipelines in permafrost and discontinuous permafrost environments. Particular mention is made of operating the line at subfreezing temperatures, using winter construction, use of snow/ice roads and methods of mitigating frost heave and thaw settlement.

Introduction

In this paper I shall discuss some of the more interesting special techniques which Arctic Gas designed to construct and operate a large diameter, high pressure natural gas pipeline in permafrost and discontinuous permafrost environments. I shall first cover permafrost and its interactions with a pipeline and then describe how the line was designed to overcome the problems caused by permafrost. Secondly, we will consider the necessarily innovative techniques that would be used to construct the line, including-the use of snow roads. As the line proceeds south from the Arctic, it encounters a region of discontinuous permafrost from which an entirely new set of constraints arise, and the paper will conclude with a discussion of them and their resolution.

The Gas Arctic/Northwest Project is a consortium of sixteen major Canadian and American producers and gas transmission and distribution companies. It formed two companies, Canadian Arctic Gas Study Limited and Alaskan Arctic Gas Study Company to study, design and make application for a pipeline to carry Prudhoe Bay and Mackenzie Delta gas to southern Canada from where it would be delivered to U.S. and Canadian consumers by new and existing pipelines. Approximately $150 million have been spent on engineering and environmental work since the project was first conceived in 1968.

Since it was realized that movement of gas from the Arctic would require new and innovative procedures, the study process began with a comparison of all possible modes of transporting gas energy south. LNG tankers, airplanes, submarines and unit trains; high voltage direct current electrical transmission; methanol, dense phase and chilled pipelines all were studied. To no one's surprise, the most conventional of them all, the chilled pipeline, was found superior for each of the three major criteria: environmental effect, timeliness and transportation cost. Having chosen the mode, Arctic Gas then undertook a massive joint engineering/environmental study based on the same criteria to select the optimum pipeline route. The result, shown at Figure 1, is as short as possible and avoids mountains and active seismic zones. Also shown on Figure 1 are the connecting U.S. and Canadian pipelines, both new and existing, used to complete the transport of frontier gas to the major population centers of the two countries. In all some 4,512 miles of line are planned, the majority being 48"-diameter pipe.

PERMAFROST

Permafrost refers to ground with a temperature remaining below 32 degrees F for a period of more than a year. The ground may be any kind of soil or rock. It may be dry, or it may be saturated and, in addition, it may contain massive ice lenses to the extent that 90% or more of its volume is water. In general, permafrost, if it remains frozen, has very substantial structural strength. But when it melts its strength depends on whether it is thaw-stable or thaw-unstable. An example of thaw-stable permafrost would be frozen gravel with a fairly low water content. When heat is imparted to it, the pore ice will melt; nevertheless, the gravel particles will continue to bear against one another and the soil mass will exhibit substantial support for a structure resting on it. On the other hand, frozen silt is a very thaw-unstable permafrost.