NRC: DOUBLE HULLS NOT A SURE CURE FOR TANKER SPILLS

Requiring double hulls is not a cure-all for preventing oil spills from tankers, a National Research Council study says.

NRC said its study of 17 tanker design concepts found that no single design is superior for all accident scenarios. However, on the basis of cost effectiveness, the double hull is among the best values.

The 1990 Oil Pollution Act, covering tankers operating in U.S. waters, requires new vessels to have double hulls and spells out a timetable for a phaseout of older, single hull vessels.

NRC FINDINGS

NRC said double hulls will not prevent all accidental spills. And because such vessels will not be completely phased in for 25 years, other steps should be taken to reduce the risk of spills from existing vessels.

The study said tanker vessel design is only one aspect of pollution control. Proper construction, operation, maintenance, inspection, and crew training are other critical elements.

Henry Marcus of the Massachusetts Institute of Technology chaired the NRC study committee.

He noted a double hull would not have prevented the Exxon Valdez spill off Alaska.

"The impact was too great," he said. "Both hulls would have been punctured."

The report cited problems with double hulls, but they can be managed by proper design and inspection.

One problem is that hydrocarbon vapors can collect in the space between the hulls, increasing the risk of fires and explosions. There is a perception that following an accident, double hull tankers may become less stable than single hull ships. And the added internal structure of double hulls makes inspection and maintenance more difficult.

The report predicted tanker traffic will increase in U.S. waters because oil imports are expected to increase, and new tankers should be more robust.

It said modern vessels are not considered as durable as older ships, which were built with a larger margin for error, "much stronger than they needed to be, to compensate for unknown vulnerabilities." But sophisticated computer designs and advances in materials and construction have reduced that margin for error.

The report said about 65% of today's world fleet was built before safety requirements were tightened in the early 1980s by the International Convention for Prevention of Pollution from Ships (Marpol), and those tankers have not met Marpol requirements.

WHAT SHOULD BE DONE

NRC said governments should consider requiring all ships to comply with current Marpol standards, such as requiring them to have independent ballast tanks positioned to shield the cargo in case of an accident.

The committee also recommended that consideration be given to requiring existing ships to implement hydrostatic loading.

Under that procedure, oil in the cargo tank is kept at a level low enough to reduce the hydrostatic pressure exerted by the oil at the bottom of the ship to less than that of the seawater. In the event of a bottom rupture, the difference in pressure of the cargo and the seawater results in water flowing in, rather than oil escaping.

NRC also said governments should consider requiring tankers to have standardized towing fittings and emergency cargo transfer systems that would lessen the extent of spills.

The committee found no design superior to double hulls but suggested that the intermediate oil-tight deck with double sides could perform better than the double hull in certain circumstances.

Such a vessel would employ hydrostatic controls but also would have the oil separated into upper and lower chambers at midship height. However, the design is unproven and has a number of drawbacks such as a potential for spills in slow speed accidents.

Finally, the committee recommended a major research program to improve tanker designs.