The Observer, July, 2008

Spill response with dispersants largely impractical

The citizens’ council has long doubted that chemical dispersants would work on spilled oil in Prince William Sound. Now, a new study suggests dispersant effectiveness doesn’t matter much, because environmental conditions in the Sound make the chemicals impossible to apply most of the time anyway.

Dispersants are chemicals designed to dilute spilled oil so it fades from the surface slick and mixes into the water column below, where it’s biodegraded by natural processes.

The new study suggests the application of dispersants is impossible 75 percent of the time in central Prince William Sound and 60 percent of the time at Hinchinbrook Entrance, where oil tankers leave the Sound and pass into the Gulf of Alaska.

The study also found that burning spilled oil on the water is generally impossible 86 percent of the time during winter at Hinchinbrook.

The council-sponsored study, titled “Nonmechanical Response Gap Estimate for Two Operating Areas of Prince William Sound” was conducted by Nuka Research and Planning Group.

Non-mechanical response refers to the use of dispersants and the burning of oil on the water as cleanup tactics. Mechanical response refers to the use of equipment like containment booms and surface skimmers. The latest study is a follow-up to a 2007 Nuka report on the feasibility of mechanical response in Prince William Sound conditions.

“This has been a long-standing concern of the board,” said Executive Director John Devens. “The study was prompted over concerns that the oil industry can move tankers through Prince William Sound in environmental conditions that would prevent them from responding to an oil spill, if one were to happen.”

The purpose of the project was to estimate how often conditions in the Sound would prevent a successful spill response. Two key areas, Hinchinbrook Entrance and Central Prince William Sound, were the focus of the study, because of a pre-existing body of extensive weather data for the two locations.

The study looks at dispersant distribution by aircraft, the method of application outlined in the oil industry’s contingency plan for cleaning up tanker spills. The study analyzes four environmental factors to determine the limitations of applying dispersants: wind, the state of the sea, temperature and visibility.

Wind and sea state are critical to dispersant application. If the sea is too calm, dispersants will not churn into the oil and dispersion will not occur. But, if the sea is too rough, dispersants are likely to be displaced before they are able to mix with oil and dispersion also fails to occur. Winds between 12 and 25 mph create an ideal mixing environment for dispersants.

Sea state means the vertical height of waves. Waves of two feet to 10 feet are considered ideal mixing conditions for dispersants.

Visibility is a major factor for airborne dispersant application, eliminating night-time response altogether and impairing or preventing it if daytime visibility drops below about two miles. Temperature was not a limiting condition for dispersant application.

The same four environmental factors were applied to burning oil on water, which is known as in-situ burning. In-situ response often uses containment booms to corral oil into a thick slick in order to burn, which can subject in-situ response to some of the same constraints as mechanical response.

Winds over about 17 mph make it difficult to ignite and sustain a burn. At 23 mph and over, the study deemed in-situ burning impossible. Wind also hinders the effectiveness of containment booms and can damage equipment.

Calm seas provide ideal burning conditions, according to the study, while swells three to six feet high impair burning and swells over six feet make it virtually impossible.

Visibility conditions limit burning to daylight hours for practical reasons regarding vision and being able to monitor the smoke plume.

Nuka’s Tim Robertson said the latest study was limited by questions about the accuracy of available data on visibility and sea conditions in the Sound. The study took a conservative approach and considered only the darkness of night as a visibility factor and did not consider reductions in visibility caused by fog, snow, or other precipitation.

Marine forecasters in the Anchorage Region Headquarters of the National Weather Service have been noting for years that automated measurements from moored weather buoys tend to under-report sea conditions such as wind speed and wave height.

The next step in the project is to determine if the two studies require further investigation, such as a more comprehensive collection of visibility data or a closer look at data to determine which environmental factors have the greatest effect on the study’s results.

The council is in the early stages of communicating the results of the response gap studies with industry officials, in an effort to develop a collaborative approach

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