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How to respond to an Arctic oil spill In-situ burning is the technique of choice for cleaning up oil in sea ice, but drift ice conditions present significant challenges Alan Bailey Petroleum News
Everyone hopes that it will never happen. But how would people respond to an oil spill in the offshore Arctic?
In the Arctic the presence of sea ice and snow can make the techniques for oil detection and recovery very different from those used in a marine spill in warmer climates.
At the United States and Canada Northern Oil and Gas Research Forum in Anchorage, Alaska, on Oct. 28, Arctic oil spill response expert Ian Buist of S.L. Ross Environmental Research Ltd. summarized the results of 20 to 30 years of research into dealing with an oil spill in Arctic offshore conditions. Complicated “The behavior of oil spilled on ice is extremely complicated,” Buist said. The fate of the oil depends on factors such as the ice conditions and whether the oil lies above or below the ice, he said. And oil on ice behaves differently from oil on water.
“Oil spilled on top of ice or on snow spreads much, much more slowly and stays much thicker than when it’s spilled on water,” Buist said. “Thicknesses on ice are 100 times more than they are on water.”
And any snow on the ice will rapidly and effectively absorb oil, he said.
Oil discharged below sea ice will float upward to accumulate in pools on the bottom surface of the ice. Downward growth of the ice will then encapsulate the oil as layers with the ice sheet. The oil will later emerge during the spring melt, rising through brine channels or becoming exposed as the ice melts downward from the surface.
Oil spilled on water between ice floes may be contained by the floes, depending on the concentration of ice on the water surface — in high ice concentrations floes tend to touch, thus forming natural boom structures.
Delineating the oil When it comes to delineating the extent of an oil spill, the spill responders can effectively locate oil on or under landfast ice — stable ice that is attached to the coast — using techniques such as ice coring, aerial surveys or subsurface ice inspection by divers, Buist said. Ground penetrating radar is also showing considerable promise as a technology for locating oil in or under landfast ice, he said.
And the successful recovery of oil from the surface of landfast ice can be achieved by the direct pumping of oil from oil pools, or by the manual scraping out of oil that has become absorbed in snow.
But locating oil in pack and drift ice is much more challenging, especially during winter freeze up, when limited daylight and small temperature differences between the oil and ice make visual and infrared detection techniques difficult to apply.
However, the good news is that once the location of oil in ice is known, the oil can be tracked for extended periods of time by simply tracking the ice movements.
“You just have to track the ice. The oil stays with the ice,” Buist said.
And whereas containment of the oil, to prevent the oil spreading over wide areas, is a prime consideration in open water, containment is not generally an issue when sea ice is involved.
“Spills on ice are naturally contained by the ice and snow structures,” Buist said. “Additional containment is not usually needed.”
Low ice concentrations However, containment and recovery of the oil can be difficult when ice concentrations are low. Even quite small quantities of ice floating on the sea surface can play havoc with a conventional cleanup involving the gathering of oil using boom — the boom tends to collect and concentrate the ice, which can then disrupt the booming operation or prevent skimmers from removing the oil from the water.
Current research is investigating the use of some novel techniques such as underwater bubble barriers and chemical herding agents to address the question of gathering oil in relatively low concentrations of floating ice, Buist said.
The main issue for the subsequent recovery of spilled oil from around floating ice, once the oil has been contained in some way, is the speed at which skimmers can move through the oil slick in ice-laden water.
“All of the recovery devices available for spills in drift and pack ice are extremely limited in the rate at which they can encounter oil,” Buist said.
There are skimmers that have been designed for use in pack and drift ice. But because these skimmers can only pick up oil along a 2- to 3-meter-wide path, as opposed to the 200- to 300-meter-wide path of a boom and skimmer system used in open water, oil recovery rates tend to be low.
Research continues in the development of new skimmer designs, Buist said.
In-situ burning However, in-situ burning has become the technique of choice for removing oil that has been spilled around, under or on sea ice, Buist said. Research experiments and experience from actual spills has shown that burning can remove from 60 percent to 80 percent of the oil. That compares with recovery rates of perhaps 15 to 20 percent from a conventional open water spill response using boom and skimmers, he said.
“Unless a net environmental benefit analysis indicates that burning would cause more harm than good, in situ burning is the countermeasure technology of choice for larger spills in ice conditions,” Buist said.
Even oil mixed with up to 70 percent of snow can be burned, he said. And where oil is trapped in ice, responders would have plenty of time to plan a burn operation for the spring melt, when the oil would appear at the ice surface.
However, current research is focusing on the question of how to ensure that an oil slick on water is thick enough to ignite in situations where there is too much ice to deploy fire boom but too little ice for the ice floes to corral the oil, Buist said.
The past five years have seen a program to test the use of chemical herder agents to gather the oil for burning in this type of situation, Buist said. The herder chemical is sprayed in small quantities onto the water around the oil. The herder changes the properties of water in a way that causes the oil slick to contract into small areas.
Four sets of experiments have evaluated the feasibility of the technique. In one test the herder increased the thickness of the slick by a factor of 10 and the subsequent burn removed 90 percent of the oil, Buist said.
On the other hand the use of chemical dispersants, to disperse the oil into the water column, has not yet been proven in sea ice conditions, Buist said. Current research in this area includes the use of stern-driven icebreakers to mix the water-oil-dispersant combination, he said.
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