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Fall/Winter 2001 Vol. 1 No. 2



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Tanker Mega Borg accident (photo courtesy National Oceanic and Atmospheric Administration) Preventing Disasters by Design

A New Tool for Evaluating Oil Tanker Performance

Say the words Exxon Valdez to almost anyone over the age of 18, and you're sure to evoke the same memories of oil-soaked loons, cormorants, otters, and seals struggling along the shore and in the frigid water.

People everywhere were moved by the tragedy, including Congress. Not long after the tanker ran aground, spilling more than 11 million gallons of crude oil into the Prince William Sound, the Oil Pollution Act of 1990 was passed, requiring, among other things, that all oil tankers traveling in U.S. waters be equipped with double hulls. And, because most countries conduct trade -- or would like to conduct trade -- with the United States, the double hull quickly became the de facto standard worldwide for tanker designs.

Transporting oiled wildlife to rehab center after Exxon Valdez spill (photo courtesy NOAA) The legislation seems to have accomplished what Congress intended. According to a 1996 National Research Council study, not only were there no accidents even one-tenth the magnitude of the Valdez in the five years that followed, but the amount of oil spilled from vessels -- and the number of spills of more than 100 gallons -- also declined.

Despite these results, some groups would like to see the standards revisited. They argue that new and innovative designs could meet or surpass the performance of the double hull. Currently, the International Maritime Organization (IMO), a United Nations agency responsible for improving safety and minimizing pollution in international shipping, has approved two alternative designs deemed to be as effective as the double hull in preventing oil spills. But tankers with these designs may never be built as long as the United States will only permit double-hull tankers to call on its ports.

The 1990 law made provisions for addressing these concerns. It directed the U.S. Coast Guard to look for alternative designs that could perform as well as or better than the double hull in preventing oil spills. However, there has been no systematic way to evaluate how one ship design would perform over another in a grounding or collision.

A National Research Council report offers a new method by which oil tanker designs can be compared for the amount of environmental damage they would cause if involved in an accident. By predicting how a tanker would perform -- before the tanker has even been built -- the method could give rise to innovative designs that are less costly and give equal or better environmental protection than current fleets provide.

The new methodology assesses a ship's chances of causing an oil spill by analyzing three features: the amount of structural damage to the ship if a specific collision or grounding occurred, and the resulting spillage; the environmental consequences of the spill, including physical measures such as the thickness and area of oil in the water and the extent of shoreline damage; and how ships of similar size but different design would compare in the same imaginary incident.

NOAA scientist assesses oil penetration after Exxon Valdez spill (photo courtesy NOAA) One of the most important points of the methodology -- and counterintuitive to what most people would guess -- is that the environmental consequence is not directly related to the amount of oil that is spilled. Small spills were found to cause a disproportionately higher amount of environmental damage, gallon for gallon, than one would expect. Once the environment has been heavily damaged, the relative harm done by extra oil diminishes. For this reason, a tanker design that may result in many small spills would be less desirable than one that could cause a few large ones, according to the report.

In addition to continued testing and refining of the methodology, the Coast Guard should establish a procedure by which innovative designs can be submitted for their consideration, the report says. They should also encourage the IMO to adopt the methodology so that tankers worldwide can be uniformly evaluated.   -- Jennifer Wenger


Environmental Performance of Tanker Designs in Collision and Grounding: Method for Comparison (TRB Special Report 259). Committee on Evaluating Double-Hull Tanker Design Alternatives, Marine Board, Transportation Research Board (2001, approx. 160 pp.; available from the Transportation Research Board, tel. 202-334-3214; $24.00 for single copies).

Kirsi Tikka, formerly a professor at the Webb Institute of Naval Architecture, Glen Cove, N.Y., chaired the committee. The study was funded by the U.S. Coast Guard.



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Copyright 2001 by the National Academy of Sciences