Powerful forces challenge buried line

Alaska gas pipeline needs to remain safely buried for decades in miles of chilled earth, permafrost and discontinuous permafrost

Bill White

Researcher/writer for the Office of the Federal Coordinator

A tricky challenge for moving Alaska’s North Slope natural gas to market is designing a pipe that will remain safely buried for decades in hundreds of miles of chilled earth, permafrost and discontinuous permafrost.

If the gas in the pipe — and thus the pipe itself — is too warm, it could melt permafrost, weakening the ground’s support. The pipe could sag, possibly causing it to leak or burst.

If the gas is too cold, water in the soil could draw toward the frigid steel, like iron shavings to a magnet, and form an ice bulb under or along the buried pipe, pressing it toward the surface, possibly causing it to split or crack.

Call it the Goldilocks challenge. The temperature of the gas should be neither too hot nor too cold for the soil, but just right.

Besides keeping the temperature in and around the pipe just right, the project developers must prove to federal regulators that the steel pipe is strong enough — with a smart margin of safety — to handle the stresses as underground ice pushes it around. Soil movement from earthquakes and landslides also can put extreme bending stresses on the pipe.

Nobody wants the pipe to leak or break, not the pipeline owners, gas shippers, utilities that ultimately buy the gas, regulators, project financiers or anyone else with a stake in the project’s success.

Controlling the gas temperature is a partial answer. That’s been the plan for a chain of Alaska North Slope gas pipeline projects that have come and gone over the past 40 years.

In early 2012, a partnership of TransCanada and ExxonMobil said it had just that idea in mind — temperature control — for its now-dormant idea to pipe North Slope gas down the Alaska Highway route to Alberta.

“North of the Brooks Range, the natural gas in the pipeline is proposed to be cooled to below freezing to maintain the stability of thaw-sensitive soils, thereby reducing thaw-related movement of the pipeline,” the companies said in filings with regulators. “For (compressor) stations located south of the Brooks Range, seasonal variation in station discharge gas temperature is planned to range from approximately 25 degrees Fahrenheit (°F) in the winter to approximately 45°F in the summer.”

That effort since has morphed into a similarly designed pipeline project to export LNG — liquefied natural gas — to Asia. An 800-mile gas pipeline would cross Alaska north to south in permafrost — ground that remains frozen year-round — for the first 180 miles or so, then through soil that gradually warms south of the Brooks Range. The liquefaction plant and shipping terminal would be at the end of the line, at tidewater.

That’s the general picture.

But a temperature-control solution is not the only issue. If the project developers operate the pipe close to and slightly above the steel’s ultimate strength limit, as is likely, federal regulators will require that an Alaska gas pipeline get a special permit for safety. The permit would address how the engineers and metallurgists will outfox thaw settlement and frost heave and the extraordinary strain these can place on the pipe.

To the regulators, caution is the prudent path. The pipeline could endure stresses from gas inside the pipe compressed to breathtaking pressures. It could face bending and warping forces from outside the pipe from soil and frost-heave conditions unlike those found anywhere else in the country.

Why jockeys are small

Steel pipe will expand when pressurized natural gas pulses through it, pushing against the pipe’s interior wall. The pipe will stretch and bend into new shapes without breaking, given the right conditions and forces. That’s why steel pipe can be contoured to rise up hills or plunge below rivers.

“All pipes flex a little. ... That’s the beauty of metal,” said an official with the federal Pipeline and Hazardous Materials Safety Administration. “It’s not like glass.”

In 2011, the Exxon/TransCanada venture tested pipe samples from mills around the world at an Edmonton, Canada, lab. The trials aimed to appraise the steel’s strength by replicating stresses on the pipe samples — built to the companies’ specs — from frost heaves and thaw settlements.

In one test, the lab used lasers and strain gauges to measure how 33-foot lengths of pipe responded to internal hydrostatic pressure as well as to external pressure from rams pushing and bending the pipe. At what point would the pipe deform and be unable to maintain its original shape?

Other trials were designed to test how the pipe would elongate under pressure. Bolt it to a heavy-duty frame and then pull. The specs called for the 33-foot lengths to stretch to just over 34 feet and still be OK.

Stresses and strains that bend, compress and otherwise distort seemingly rigid objects are commonplace in this world. Take bones, for example. Racetrack touts know a bit about the subject. The more weight a racehorse carries, the more its leg bones will deform at each stride before regaining their normal shape and length when the hoof rises. The weight is the stress. The bone compression is the strain. More weight means more compression. More compression means shorter strides. Shorter strides can mean the difference between a win, place or also-ran race. The horse can still perform — that is, run the race. But bone stress and strain affect its performance. That’s why jockeys don’t weigh very much.

Trial and error

Some lessons were born of mistakes. Roads that slumped away. Buildings that tipped on their axes. Smooth runways that were here today but gone in the spring. Forest clear-cuts that exposed soil to more solar heat and spawned soupy quagmires.

Oil-patch engineering has its own history with permafrost and cold soils.

Russia laid its first Arctic gas mainlines in the 1970s. Frost heaving burped some of them to the surface.

A 1979 Russian professional journal noted the challenges: “Permafrost conditions cause stresses of varying magnitude in the pipeline and ... sometimes lead to breaks.”

Also in the 1970s the oil industry developed North America’s first Arctic oil field — Prudhoe Bay on Alaska’s North Slope — and built the trans-Alaska oil pipeline to an ice-free port 800 miles south of Prudhoe. (This field’s natural gas would be the main reserves behind an Alaska LNG export project; for now, without a pipeline, the produced gas mostly is reinjected into the Prudhoe reservoir.)

The Alaska oil pipeline has operated continuously since 1977. Half of it is buried in soil that isn’t frozen so won’t thaw or slump as warm oil courses through the pipe. The other half is in permafrost areas and troublesome discontinuous permafrost soils. For those miles, the pipe is above ground, propped on 78,000 frames — called vertical support members, or VSMs — that themselves are engineering marvels. To keep the VSMs anchored during summer warmth, they’re outfitted with 124,000 special pipes that draw heat from the ground to keep it frozen.

Planning and constructing the oil pipeline served up many lessons on Arctic construction. For example, an initial idea of burying the pipeline along the entire route was soon dropped as unworkable. In a few places totaling about four miles of the 800-mile-long route, the pipe actually is buried in permafrost, but only after placing it in a refrigerated trench to keep the soil from thawing.

Even after the pipeline began carrying oil in 1977, the lessons kept coming. During the pipeline’s first six years of operations, the owners dug up at least eight buried sections that had buckled or cracked due to thaw settlement or frost heaves.

A gas pipeline experience will be different, because gas, unlike some oil, moves just fine when chilled below 32 degrees. So in theory the gas pipeline can be buried along almost its entire route.

But getting the temperature just right is trickier than it sounds.

Builders of a 500-mile oil pipeline through discontinuous permafrost from Northwest Territories to Alberta in Canada planned to flow the oil at ground temperature when the line started up in 1985. But the ground temperature actually varied more than thought, depending on the surface and soils. The oil wound up changing ground temperatures, according to Peter J. Williams in his book “Permafrost & Pipelines: Science in a Cold Climate.”

Permafrost thawing occurred when the pipe crossed from unfrozen to frozen ground. “However, a larger amount of thawing occurs initially because of the disturbance of the ground surface caused by the laying of the pipe and backfilling. Generally speaking, disturbance of the natural vegetation upsets the exchange of energy at the ground surface in such a way as to raise the temperatures of the ground slightly, and this is very important if the ground is almost at thawing point anyway,” Williams wrote.

Editor’s note: This is a reprint from the Office of the Federal Coordinator, Alaska Natural Gas Transportation Projects, online at www.arcticgas.gov/buried-alaska-gas-line-could-face-powerful-bending-forces. Part 2 of this story will appear in the Aug. 11 issue of Petroleum News.