Investigating the fate of oil under ice

Arctic oil spill JIP researches model for assessing conditions for successful mechanical recovery or dispersion of spilled oil

Alan Bailey

Petroleum News

As part of a series of reports on Arctic oil spill response technologies, the Arctic Oil Spill Technology Joint Industry Program, or JIP, has published a new report on the state of current knowledge about what may happen to spilled oil that becomes trapped under sea ice. The fate of oil under or in ice is of great concern in contingency planning for the possibility of an oil spill in Arctic seas.

The JIP, with a membership composed of nine major oil companies, including Shell, BP, ExxonMobil, Statoil and ConocoPhillips, was formed in 2012 as a collaborative venture under the International Association of Oil and Gas Producers, to expand industry knowledge of Arctic oil spill response. The JIP is conducting laboratory research projects into the use of oil dispersants, environmental effects, oil slick trajectory modeling, remote sensing of oil and in-situ burning.

Two-phase study

“Oil spill response in the presence of sea ice is potentially both more complex and more simple than in open water,” the report says.

Essentially, ice might simplify a response by limiting the spread of the spilled oil. But the presence of the ice may complicate efforts to deal with the spill through mechanical removal of the oil, through the use of dispersants or through in-situ burning.

Droplet size and water turbulence

Oil spilled in Arctic waters may also impact sea ice characteristics by, for example, affecting the roughness profile of the ice subsurface, and perhaps by slowing the summer melt of the ice, the report says.

With oil being lighter than water, oil droplets will tend to float to the surface. But the question of whether the oil will form a continuous slick as it accumulates below an ice sheet will depend on the distance between individual oil droplets and the spacing between the features under the ice that cause the ice surface to be rough, the report says. Field tests have demonstrated that when oil and gas is released at a depth of about 60 feet below an ice sheet, the oil plume spreads out, with the largest oil droplets accumulating within 150 feet of the plume centerline and slightly smaller droplets within 350 feet. Subsequently, over the course of the winter, the distribution of oil under the ice may alter, as the subsurface profile of the ice changes, forming waves and packets in which the oil could accumulate, the report says.

Many types of ice

The research team identified several technologies that might be used for making observations that could achieve a better understanding of sub-ice turbulence, and hence the fate of oil under ice, in these different ice conditions. The techniques consist of the use of fluorescent dyes, observable from the air; the use of clusters of instruments for measuring the sub-ice water turbulence; the use of autonomous underwater vehicles for measuring the turbulence; the use of an acoustic system, suspended under the ice, to detect the water turbulence profile; and the use of tracer materials in oil released under ice to estimate the rates of horizontal and vertical oil dispersion.

Model possibility

This model should work well in situations where ice covers 90 percent of the sea surface, but is likely to also work with a substantially lower ice cover than this, the report says.

The use of such a model for predicting the behavior of spilled oil would require as inputs information about wind conditions; the oil droplet size distribution; information about the type and concentration of ice; and the turbulence and velocity profiles of the water, the report says. And, given the wide variety of ice types found in the Arctic, the report recommends prioritizing research, to first focus on the types of ice likely to be encountered in areas where oil development is currently proceeding.

Editor’s note: Part 1 of this story appeared in the Feb. 23 issue.

USGS develops model for oil ball tracking

The survey says that a solid material consisting of a mixture of oil and sand, forming bodies ranging in size from particles a few millimeters in diameter to mats several meters across, has been found in the Gulf of Mexico coastal surf zone, the zone where waves tend to break. And balls of this heavier-than-water material have continued to emerge at some beach locations more than three years after the first oil from Deepwater Horizon reached the shore, the survey says.

The mathematical model which the survey has developed indicates that the mobilization and redistribution of the balls requires the wave action from tropical or winter storms, and that coastal inlets would tend to trap the balls. The model also suggests that the larger balls, centimeters in size, will tend to be found close to where they originally formed, with minimal transportation of these balls along the coast. The balls will tend to be periodically covered and uncovered by sand that is easily moved by wave and current action, the model indicates.

Essentially, sand grains are smaller and more mobile that the larger oil balls, which are dense enough to rest on the seafloor, explained P. Soupy Dalyander, the lead author of a paper in the Marine Pollution Bulletin describing the results of the survey’s modeling project. In non-storm conditions the balls will thus tend to remain stationary, with the sand moving around them, Dalyander said.

—Alan Bailey