Pipeline Threatened By Melting Permafrost North Of Fairbanks

The Pipe where it passes through Glennallen. (File photo, Country Journal)  When the 800-mile long Trans Alaska Pipeline was built in the mi...

The Pipe where it passes through Glennallen. (File photo, Country Journal) 


When the 800-mile long Trans Alaska Pipeline was built in the mid-1970s, engineers were well aware that the huge project faced major environmental issues.

For one, the Pipeline passes through a corridor that is underlain with permafrost – including the Copper River Valley. 

Almost all Copper River people know that we live only feet and yards above huge permafrost ice cubes that exist in the ground under our homes and roads. 

When those permafrost ice lenses warm up and melt, we notice it. We've all seen sections of our highways suddenly slumping down – almost overnight – as the hot sun of summer hits the blacktop and heats the permafrost below.


Permafrost impact on building in Dawson City, Canada. (File photo, Country Journal) 

Copper Valley people really notice the melting permafrost under our feet when one side of a home or cabin begins to list to the side, as the foundations slowly drop into the sagging ground.

THERMOSYPHONS
To combat the predictable and damaging hazards of permafrost, when the Pipeline was built the struts that hold it up were specially designed. 

Thousands of "thermosyphons" hold up the pipe. The familiar-looking finned pipes reach down into the permafrost, and keep the ground chilled, directly under the strut, through the use of a refrigerant. 

Over the years, the pipe has weathered a number of problems: earthquakes, a bullet hole into the pipe's casing, corrosion. But it hasn't until recently, run into that long-feared melting of permafrost.

Now, though, the pilings of an 810-foot section of the Pipeline have started to move due to thawing permafrost on a nearby slope. This "slope creep" is shifting the sturdy Pipeline supports for the first time. 

The problem isn’t caused by the pipe struts. The hill near the pipe is thawing and starting to move, creating pressure on the pipeline pilings. 

To solve the movement caused by the melting under the nearby hill, Alyeska has received approval from DNR to put in 100 thermosyphons near that section of pipe to keep the ground frozen on the nearby slope, in an attempt to use technology to solve the problem. 

Permafrost temperatures have been warming in Alaska, which is underlain – over almost 85% of the state, not just here – with chunks of permafrost ice. 

The section of melting slope that needs attention is near the Dalton Highway, north of Fairbanks. 


Explanation Of Heat Pipe From Alyeska Pipeline "Safety First" Website 


There are 78,000 vertical support members (VSM) along TAPS – most of them south of Atigun Pass have heat pipes to help maintain the permafrost around the VSMs. The heat pipes act to reduce soil temperature when the temperature of the air drops below that of the ground.

The heat pipe is a natural convection, two-phase heat transfer loop, which transfers heat by vaporization and condensation within the closed system. It consists of a sealed tube, charged with a working fluid, which functions as a two-phase (gas-liquid) system at operating temperatures.

Heat from the soil enters the lower end of the tube, causing the fluid to boil. The vapor travels to the upper, radiator end of the tube in the air, where it condenses on the cooler surface, releasing energy. The condensate then returns to the lower end of the tube as a film along the tube wall.

Alyeska's Civil Integrity Team monitors air and ground temperatures at 52 locations between Pipeline Milepost 85 and 725. As reported by the Alaska National Weather Service, the past three winters have been relatively mild.

"These extreme temperatures present significant challenges to maintaining our homes and vehicles," said Phil Hoffman, Senior Engineer Advisor. "But on TAPS we are ecstatic when these cold temperatures set in, giving the heat pipes a chance to chill the soil."


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