Shippers are not likely to see their insurance rates leap higher despite the fire aboard the Norwegian oil tanker Mega Borg in the Gulf of Mexico, marine insurance underwriters said Monday.

''My guess is that this will have little or no effect on cargo insurance rates," said Frederick McDermott, supervising cargo underwriter for the Marine Office of America Corp., a unit of Continental Insurance Corp.The Mega Borg, which suffered major explosions and fires over the weekend, carries about $20 million in hull insurance, according to the ship's managers and Walter Michael of the American Hull Insurance Syndicate.

If firefighters lose the battle and the tanker splits, spilling its 38 million gallons of oil, it would become by far the worst spill in U.S. history, exceeding by more than three times the 11 million-gallon spill of the Exxon Valdez off Alaska in 1989.

The Exxon Valdez has cost more than $2 billion to clean up.

"While the aggregation of major incidents will have an effect on marine insurance rates in due course, there is currently an overcapacity in the market, which will hold rates down," Mr. Michael said.

"I hope (the accident) will contribute to a firming of hull (insurance) rates, but no firming has happened over the past year despite some major accidents," said Mr. Michael.

The more conservative marine insurance companies would like to see an increase in premiums to create the resources necessary to handle major catastrophes, said Mr. McDermott.

Mr. McDermott said for recent renewals of marine insurance policies have been lower than usual.

Mr. McDermott said London underwriters, who handle much of the world's marine insurance, are still competing strongly for this insurance business. ''They need to keep the foreign cash coming in," he said.

French state-controlled oil company Societe Nationale Elf Aquitaine, whose U.S. subsidiary owns the oil leaking from the tanker, cannot be held liable for spillage, a company source asserted Tuesday.

"As a simple charterer, we cannot see how we could be liable," said a company official who asked not to be named. "As far as the accident and all the damages are concerned, the shipper must a priori be held responsible."

One aspect of the marine insurance market that may see higher rates in the near future is pollution liability insurance, he said. "The liability field is where the disasters will have the most impact, and this is where we may see the market act a little bit more intelligently," Mr. McDermott said.

Problems in fighting the fire have drawn criticism of Texas as ill- prepared to handle such disasters.

Texas should prepare for oil spills the way California prepares for earthquakes, said Andy Mangan, deputy land commissioner, as emergency crews braced for a possibly catastrophic spill from the Mega Borg.

"We've got some plans. But they're not capable of handling a gigantic spill," Mr. Mangan told The Associated Press.

At the time of the explosion, emergency crews were a continent away. Equipment to spray flame-choking foam was in Louisiana.

Other oil containment and salvage equipment was brought from as far away as the Netherlands. The private companies responding to the Mega Borg fire had their equipment overseas because there are more offshore fires in the North Sea and Middle East than the Gulf of Mexico, Coast Guard Admiral Williams

Merlin said.

"This is our worst nightmare," said Garry Mauro, Texas land commissioner and a member of the governor's oil spill advisory committee.

Mr. Mangan and other state officials are urging lawmakers to unify the state's scattershot approach to dealing with spills.

"The focus has not been there. There hasn't been any concerted effort to

put such a contingency plan together," Mr. Mangan said.

Mr. Mangan said state officials have worked closely with Alaska and other states since the Exxon Valdez spill.

He said Texas probably is better prepared than some coastal states but not as well prepared as it could be.