In his guest editorial for the special Arctic pipelines issue of the Journal of Pipeline Engineeringpublished in June, Professor Andrew Palmer of the National University of Singapore points out that the industry is currently facing another collapse of confidence in ambitious Arctic projects, particularly underwater. That has been brought about by two factors: the Macondo catastrophe in April 2010, and the unexpected development of alternative gas reserves in northeastern and western USA.
It is hard to overestimate the impact of Macondo: in the Arctic context, it was pointed out that the situation would be far worse if there should be a similar mishap in the Beaufort Sea, most of all if it were to happen when the sea ice is too thick to be easily broken by ships but too thin for vehicles. Some technology for Arctic oil clean-up exists, but experience is extremely limited.
Professor Palmer continues: “Many of the technical difficulties remain. Progress has been disappointingly slow, and many of the issues identified 40 years ago have not been solved satisfactorily. On land, one issue is frozen ground and differential settlement. The amount of ice contained in frozen ground varies enormously, and there can be big variations within a few metres, both horizontally and vertically. It is difficult to follow the first principle of construction on permafrost – to change the thermal regime as little as possible and therefore to make the pipeline temperature coincide with the ground temperature. If the pipeline is warmer than the ground, the permafrost under and around the pipeline thaws, and the pipeline settles further where the ground had been ice-rich, and less far where there had been less ice. The pipeline bends in response, and the bending can overstress the pipe wall and cause it to buckle. If on the other hand the pipeline is colder than the ground, the soil beneath the pipe freezes progressively, and migration of water toward the freezing front causes the soil to heave, and again the pipeline is compelled to bend and may buckle locally. That bending can damagingly interact with other forms of buckling such as lateral and upheaval buckling. Much remains to be learned about how to carry out the enormous amount of geotechnical survey required in permafrost areas, particularly in discontinuous permafrost. Yet another complication is the effect of climate change.
“An alternative is to support the pipeline above the ground surface. The supports can be simple piles, or they can be more sophisticated thermopiles that actively keep frozen the ground around the bases. Thermopiles can be subject to internal corrosion, and thermal surveys have suggested that they do not invariably operate satisfactorily. Over several hundred kilometres of the Alaska oil pipeline, those options were selected conservatively whenever it was uncertain if a buried line would be safe. They are expensive, they are visually intrusive, there have to be crossings for wildlife (though caribou turn out to be rather intelligent), an above-ground system may be adversely affected by earthquakes, and the pipeline remains vulnerable to malicious damage and to people armed with rifles. Much remains to be learned about how to design and construct an above-ground system and to be sure that it functions correctly.
“Turning to Arctic pipeline construction in the sea, one of the hazards is ice gouging. Floating ice runs aground in shallow water and scrapes along the seabed, driven by wind, current and the pressure of other pieces of ice driven along behind it. The ice cuts into the seabed, and forms a dense network of gouges, a few of them very large indeed and in an extreme case 50 m broad, 5 m deep and hundreds of metres long. A back-of-the-envelope calculation shows that the force required to make such a gouge can reach several thousand tonnes, so that if the gouging ice mass should encounter a pipeline, the line would inevitably be damaged severely. Worse still, a pipeline below the level at which the ice might strike it is still not necessarily safe, because the ice drags along some of the soil beneath it, and would carry with it a buried pipeline and could bend it severely.
“Gouging and subgouge deformation remain difficult and controversial issues, and it is not an accident that several of the papers in this issue of the Journal of Pipeline Engineering are devoted to it. At the end of the day, the engineers responsible for a project have to make a concrete decision and select a trenching depth, rather than hypothesising about encounter probabilities. A possibility that has received less attention has been that of proactively altering the design strategy by doing something more than just burying the pipeline. Research indicates that at least the effects of subgouge deformation can be eliminated, by interposing a weak layer than cannot transmit large forces downwards, above the pipeline and below the maximum gouging depth. More needs to be done to look imaginatively at alternatives.
“That remark applies to other geohazards that might be a threat to Arctic marine pipelines. One of them is strudel scour. In the early summer, ice on the Arctic rivers melts first while the sea is still frozen. Fresh water flows out over the sea ice, finds holes and cracks in the sea ice, and flows downward, generating powerful rotating whirls (‘strudels’). Under each strudel is a downward-point jet, and that jet erodes the seabed. If the scour hole it creates intersects a pipeline, the pipeline might be damaged by vortex-induced oscillation, the line might be overloaded, and the strudel might interact with other forms of sediment transport, such as the formation of sandwaves. Some research suggests that the probability of damage from that source is relatively low, but on the other hand it has been suggested that the presence of a pipeline might encourage the formation of a strudel immediately above, because heat from the pipeline would be convected upward and thin the ice.
“These and other technical issues will rightly be scrutinised with great care, before any decisions to build Arctic pipelines are reached, whether offshore or onshore. Handwaving and appeals to industry experience and competence will not be enough. The unfortunate experience of the past year will heighten awareness. There is much to do!”
