Physical Scale Modelling of Electrothermic Thawing of Permafrost For Alleviation of Frost Heave Problems In Chilled Gas Pipelines
Authors: Vermeulen F.E. , Chute F.S. , Cervenan M.R. 
Published: March 2010

The artificial thawing of permafrost beneath chilled gas pipelines by electrical current flawing through the frozen medium Is studied by means of scaled physical models. Techniques are described whereby, by use of a minimal number of electrodes embedded in the frozen medium, extended zones can be thawed beneath the pipeline so that the pipe can settle and frost heave stresses can be relieved.

INTRODUCTION

The design and construction of pipelines in cold regions entail special problems. It is well known, for instance, that pipelines passing through zones of continuous and discontinuous permafrost may cause ground slumping or frost heaving if adequate design measures are not taken. As an example, over half the length of the 1.22-m-diameter Alyeska oil pipeline has had to be elevated on special piles to prevent the warm oil from thawing the underlying ground ice, which in turn would have caused unacceptable subsidence of the pipe. Plans for the ill-fated 1.22-m-diameter Mackenzie Valley gas pipeline called for chilling the gas so that the cold pipe could be buried and not cause thawing of permafrost and ground ice. It was evident, however, that in zones of discontinuous permafrost, freezing of the soil around the chilled pipe would, over a period of years, cause frost heave and attendant unacceptable differential stresses in the pipeline. The route of the recently approved Alcan gas pipeline presents geotechnical problems -that are similar to those of the Mackenzie Valley route. It is again deemed advisable to bury the pipeline and to chill the gas in order to avoid ground thawing and subsidence. As before, this leads to problems in zones of discontinuous permafrost due to soil freezing and frost heave.

Various proposals have been advanced for overcoming the frost heave problem. Initially, it was thought that an earth berm placed above the pipeline would weight down the pipe to counter eventual frost heave pressures. It now seems, however, that frost heave pressures are too great to be controlled in this fashion. Other approaches that have been suggested include fishing.

Construction of the packer element is very similar to a high-pressure hydraulic hose with steel braided carcass layers. Unlike a hydraulic hose, the packer expands as required when internal pressure is applied. Expansion is allowed by special braid angles in the packer carcass.

Like a hydraulic hose, the packer element also has metal collars on both ends. The collars anchor both ends of the steel braid cables, and are threaded to fit the packer assembly.

Limitations of the Equipment

Inflatable packer equipment is limited mainly by the differential pressure holding ability of the packer elements. However, this is affected to a large extent by the diameter that the packer element has been inflated to. With the hole in good condition and close to gauge, the packer elements have had no trouble holding differential pressures in excess of 30,000 kPa.

However, because one of the main uses of inflatable packer equipment is in washed-out holes.