An International Maritime Organization committee study will endorse the still-untested mid-deck tanker design as being "equivalent" in environmental safety to the double hulls mandated by U.S. law, committee members said.

But an environmentalist who served on the committee dissented from the conclusion, and the U.S. representative to the committee says continuing model testing may alter the findings.The choice between mid-decks and double hulls is a trade-off between having fewer oil spills with double hulls or less total oil spilled with mid-decks, said Gerard Peet, an engineer who served on the committee and a consultant to environmental group Friends of the Earth.

"I prefer to have less spills," Mr. Peet said. Five separate spills would cause much more environmental damage and cleanup costs than one spill, even if the total amount of oil spilled was the same, he added.

The mid-deck tanker, a concept not yet put into use, would have double sides and a horizontal bulkhead, or deck, splitting the cargo space in two. Studies indicate that mid-decks would perform as well as or better than double hulls at preventing spills from damage to the side of a tanker.

But unlike double hulls, a mid-deck tanker will always spill some oil when there is bottom damage, although it might spill less than a double-hulled ship would in a particular accident.

Current tests of mathematical models may yet change the committee's conclusion, said Joseph Angelo, chief of the vessel inspection and documentation division at the U.S. Coast Guard. The testing took place in Carderock, Md., last week and may run into this week. The final round of testing is looking at how much oil would be spilled at the moment a cargo tank is breached.

If the tests find that the study's assumptions about the amount of oil spilled during an initial breach were off by a few percentage points, it could alter the study's findings, said Mr. Angelo.

But other committee members did not think the model testing would make a difference.

"The odds of it altering the findings are quite low," said Bill Gray, executive vice president at Skaarup Ship Mangement Corp., Greenwich, Conn.

The study committee is due to make its report to the IMO's marine environment protection committee in March. If the environment protection committee endorses the mid-deck design as expected, the United States and the rest of the world could be left with two radically different approaches to tanker design.

Some people hope the IMO's action may lead to changes in U.S. law.

"I know what I'd like to see," said Mr. Gray, who has been one of the strongest advocates of the mid-deck design.