Anchorage Daily News

A big worry about the oil industry's heightened interest in finding and producing oil off Alaska's northern coast involves how to clean up a spill that happens in broken sea ice.

In the Arctic, sea ice and snow can make the techniques for detecting and recovering spilled oil very different from responding to offshore spills in warmer climates.

At the United States and Canada Northern Oil and Gas Research Forum in Anchorage last week, 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.

The fate of spilled oil depends on factors such as the ice conditions and whether the oil lies above or below the ice, Buist said.

"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 of the ice. Ice growing downward will 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 stable ice that is attached to the coast -- called landfast ice -- 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.

Containment -- keeping the oil from spreading over wide areas -- is not generally an issue when sea ice is involved.

"Spills on ice are naturally contained by the ice and snow structures," Buist said.

LIGHT BROKEN ICE

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, which gathers 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 gathered oil.

Research is investigating the use of some novel techniques such as underwater bubble barriers and chemical herding agents in low concentrations of floating ice, Buist said.

The main issue for the recovery of spilled oil from floating ice, once the oil has been contained in some way, is the speed at which skimmers can move through the ice.

"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.

Some skimmers have been designed for use in pack and drift ice. But because they can pick up oil only along a 6- to 10-feet-wide path, as opposed to the 600- to 1,000-feet-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.

However, 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 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.

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