Overcoming the challenges of Arctic pipelines

A buckled pipeline in Siberia that thawed the ground in which it was buried.

A buckled pipeline in Siberia that thawed the ground in which it was buried.

Pipeline construction in the Arctic had been little considered until the 1,300 km Trans-Alaska Pipeline was proposed 40 years ago. Professor Andrew Palmer of the National University of Singapore worked on a technical issue during the pipeline’s planning and has maintained a strong interest in the project since. Here, he explains some of the challenges the project had to overcome.

Before the construction of the Trans-Alaska Pipeline, only pipelines of short lengths had been constructed in places such as Siberia and close to Norman Wells on the Mackenzie River, Canada.

Following BP’s discovery of 25 Bbbl of oil in place at the Prudhoe Bay oil field near the Arctic Ocean coast in 1968, the Trans-Alaska Pipeline was proposed.

A demonstration project took the tanker Manhattan through the North-West Passage in 1969, and showed that it was feasible to make the voyage in the summer, but that a year-round transportation route would be difficult if not impracticable. The alternative solution was a 48 inch diameter pipeline, 1,300 km from Pump Station 1 near Prudhoe Bay to the ice-free port of Valdez, southeast of Anchorage, Alaska.

In its time, the project was high-profile and hugely controversial and when I began research on one aspect of the engineering I was bullied by colleagues (and their wives), on the grounds that the pipeline would “ruin Alaska”. That idea turned out to be completely false. There were years of hearings and lawsuits, but in the end the permits were issued, construction began in 1974 and was completed at a cost of $US8 billion. The pipeline began operation in 1977.

A land pipeline in the Arctic runs into potential technical difficulties. Most of them are to do with the flows of water and heat, like the majority of geotechnical problems are. Frozen soil is very strong, and can bear large loads. On the other hand, if a pipeline is hot and heat is conducted from the pipeline into the soil, the frozen soil thaws, and if it has a large ice content it turns into a soft mud that can carry hardly any load and lets a pipeline sink. The amount of ice in frozen soil varies enormously, even over quite short horizontal distances, and therefore the amount of sinking varies. A pipeline might become severely bent, and in extreme cases might buckle.

This problem can be dealt with by insulating the pipeline, and by raising it above the ground surface on piles. At the time of the conceptual engineering of the Trans-Alaska Pipeline, pessimists thought that it might be necessary to do this for 100 km or so, but in the end a much greater length was raised. That solution creates other difficulties, some of them unexpected. Alaskans are fond of guns and hunting, and so people shoot at the pipeline. Reindeer or Caribou migrate, and it was objected that they would see a raised pipeline as a barrier, but in reality they are intelligent enough to walk underneath.

The Trans-Alaska Pipeline has now been in operation for 30 years, without any major incident to the main line, though a 6,000 bbl oil spill from a flowline happened in 2006. Over the pipeline’s lifetime it has transported 15 Bbbl of oil. The throughput has decreased, from more than 2 MMbbl/d at the peak to around 1MMbbl/d now, and some of the original eight pump stations have been decommissioned.

Current Arctic proposals

Currently, a pipeline is being proposed to bring approximately 30 trillion cubic feet of stranded gas on the North Slope of Alaska to markets. Many different competing schemes have been investigated, but the most likely to succeed is a pipeline south to the Fairbanks area, and then southeast along the Alaska Highway into Canada. Those routes too are controversial, but the pipeline is extremely popular in Alaska, and the recently resigned Governor Sarah Palin had made it one of her target projects. New Governor Sean Parnell is expected to continue advocacy of the gas pipeline. An unlikely alternative is an offshore pipeline in the Arctic Ocean, eastward parallel to the coast, to link into a Canadian gas pipeline from the Mackenzie Delta region. A Fairbanks newspaper condemned that option as the very worst for Alaska and one that must be fought relentlessly, because it would deprive the state of jobs.

The Mackenzie Valley Gas Pipeline has been argued about for almost as long as the Trans-Alaska Pipeline has existed, but nothing has been built. Much engineering was done in the 1970s, and there were hearings in front of the Berger Commission, but in the end the Commission ruled against the project, essentially on socio-economic rather than technical grounds.

Alternative projects have invested large sums in site investigation and engineering. The current lead project is the Mackenzie Gas Project, led by ExxonMobil and its partners, and there were public hearings in Yellowknife in 2006. Some First Nations aboriginal groups are partners in the project, but others are opposed. The project appears currently to be ‘on the back burner’, in part because the estimated cost suddenly leapt from $US9 billion to $US16 billion.

Thaw settlement

The thaw settlement problem described earlier is still present, but in a gas pipeline it can be countered by chilling the gas. That creates another problem, frost heave, in which water migrates toward cold fronts in unfrozen ground, freezes to form ice lenses, and as more water arrives the lenses grow and lift and bend the pipeline. Ideally, the temperature of the gas and the pipeline wall ought to match the ground temperature exactly, so that both frost heave and thaw settlement are avoided, but that is not easy to arrange and control.

The Russians have vast gas reserves in the Yamal peninsula that juts into the Arctic Ocean, and have built an extensive network of pipelines to bring gas to the industrial cities of Siberia, European Russia, and further into Europe. Figure 1 shows a Siberian pipeline that thawed the very wet ground it was buried in, lost lateral support, and buckled dramatically.

Oil and gas development beneath the Arctic seas creates a need for underwater pipelines. The thaw settlement and frost heave problems essentially disappear, because the upper boundary of the permafrost rapidly drops away from the shore, and because the seabed temperature regime is far more stable and little affected by the seasons. The principal problems with Arctic underwater pipelines are ice gouging and construction.

Ice gouging

Ice gouging of the seabed occurs when large ice masses, mostly ridges formed by collisions between different ice sheets (and not technically ‘icebergs’), drift into shallow water, run aground, and are then pushed along by more ice behind them. They cut deep gouges into the seabed, often 5 m deep and 50 m or more wide. A rapid calculation shows that the force required to cut such a gouge is thousands of tonnes. If a pipeline were in the way, the force would be transferred to the pipeline, and it would be damaged. We already know that it is difficult and expensive to protect a pipeline against damage if a ship’s anchor is accidentally dragged into it, and the force that gouging ice might apply is ten times larger. The solution is to trench the pipe so that the ice passes harmlessly over it, but it has to be safely below the deepest gouge, and somewhat further because the seabed soil beneath the ice is heavily deformed, though it may be practicable to protect a pipeline from that by placing a weak layer directly above it.

Subsea Arctic pipelines

Finding a reliable and economical construction method for underwater pipelines is a harder problem. It depends on location: the Arctic is a huge area, with widely varying climates, and it makes no more sense to think of a unique ‘Arctic’ environment than it would to think of a unique ‘tropical’ environment. The engineer has some freedom to choose the season of the year. In some places there is a long open-water season, and there a pipeline might be constructed by conventional laybarge, reeling, or tow methods. A difficulty is to be confident that the construction vessel can be brought to site, can lay the pipeline, and can get out before the ice consolidates, so that the vessel is not expensively trapped over the following winter.

In other areas the winter ice is stable and fixed in position, and there a pipeline can be made-up on shore and dragged by an
ice-based winch along the seabed under the ice, if necessary into a pre-excavated trench. This technique was applied to install a pipeline bundle in the Drake Gas Field off the coast of Melville Island in the Canadian Arctic, in a demonstration project as long ago as 1978. That project used a plough to excavate the trench, and when many years later the well was plugged and the site was cleaned up, the plough was disposed of by bulldozing a hole, pushing the plough in, and backfilling over it – which will be an intriguing puzzle to an archaeologist in a thousand years’ time.

An alternative is to weld together the pipeline on the sea ice, cut a trench through the ice, and lower the pipeline to the seabed with sidebooms, as in conventional land construction. That technique was applied to a pipeline running to the NorthStar artificial island off the north coast of Alaska.

In yet other locations, there is too much sea ice in summer to use a laybarge, and the winter ice is not stable enough for ice-based construction, usually because the site lies close to constantly moving ice pack. There bottom pull methods may be preferred, particularly if the line is running to an artificial drilling and production island, or to a bottom-founded caisson platform.

Arctic challenges and possibilities

The Arctic is beautiful and environmentally vulnerable, and at the same time has huge resources that society will want to take advantage of. Oil companies are nowadays highly aware of the possibility of environmental damage, extremely sensitive about it, and firmly supervised. There is no lack of challenging and unexpected problems for the pipeline engineer.

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