A permafrost expert questioned design changes to the proposed Mackenzie Valley natural gas pipeline that would cut the number of initial compressor stations from three to one, fearing that having fewer stations could affect permafrost along the valley.

Calgary-based Imperial Oil, which is leading a consortium of companies behind the proposed pipeline, appeared Tuesday in Inuvik before the Joint Review Panel, which is looking at the environmental and social impacts of the project.

The company, which had planned to build three compressor stations along the 1,200-kilometre pipeline along the Mackenzie Valley, now wants to build only one station for the first few years of operation, then add the other two in later years.

But Fons Schellekens, a permafrost expert with the federal Natural Resources Department, told the hearing that Imperial has not addressed the environmental effects having fewer stations could have.

"We will get a much larger frost bulb around the pipeline than in the three-compressor-station scenario," Schellekens told the review panel Tuesday. "Potentially drainage patterns are going to be changed. I was wondering why no additional mitigation measures are required."

Natural gas enters a pipeline hot, then continually cools until it reaches a compressor station, which would regulate the temperature of the gas. Most of the proposed pipeline would be buried underground through the Mackenzie Valley, passing through ice, winding down slopes and under creeks and rivers.

Schellekens said having only one compressor station along the pipeline would leave the cooling gas colder, freezing water in the soil around the pipeline and thus making more permafrost. Adding more stations later would then make the gas hotter, melting that ice. Both scenarios would put pressure on the pipe, he said.

"That is going to generate a lot of water around [the] pipe," he said. "The pipe may drift to the surface."

Rick Luckasavitch, Imperial's technical manager for the Mackenzie Gas Project, told the hearing that the proposed pipeline is designed for the wide range of soil temperatures that would be encountered along the length of the valley.

"That factor is accounted for in our designs, and those boundary conditions are at the same point, be it over land or under water, what's the warmest and what's the coldest that we would see," Luckasavitch said.

The Joint Review Panel hearings in Inuvik continue Wednesday.