Frank Iarossi, president of Exxon Shipping Co. and a focus of criticism over the grounding of the Exxon Valdez last year, has resigned to become chairman and chief executive officer of the worldwide American Bureau of Shipping.

Mr. Iarossi said he plans to take up his duties on April 2 with the Paramus, N.J.-based organization, a group of companies that provides worldwide technical research, design analysis and inspection of ships and barges worldwide. He will succeed Richard T. Soper, who is scheduled to retire.Mr. Iarossi said in an interview Thursday that intense political pressure and press criticism of Exxon Shipping's manning and operational policies had nothing to do with his departure. Mr. Iarossi, 52, has been president there since 1982.

Asked whether he plans changes relative to ship design and research at the organization, Mr. Iarossi said the American Bureau of Shipping "has made a remarkable recovery in the 1980s. I intend to continue that momentum. Eighty percent of the ABS work is outside the United States, and it will truly be an international job."

As for recent renewed demands that tankers be fitted with double hulls and double bottoms since the Valdez grounding, Mr. Iarossi said his view "is that we are talking about trade-off risks . . . whether we talk about single hulls, side ballast tanks, double bottoms or double hulls. None of these offers a perfect solution.

"The single hull with defensively located side ballast tanks, the way the Exxon Valdez is designed, protects for collision but not a grounding," he said. "The double bottom protects for a grounding, but not a collision. So there is a clear trade-off in what you are trying to protect for, and a double

hull will protect for both a collision and grounding."

Mr. Iarossi said the "problem with a double hull is that you use up so much carrying capacity of the vessel, you'd need a lot more tankers to carry the same amount of oil."

Douglas Bailey, Alaska attorney general, said Thursday night that he thought that criticism of Exxon Shipping Co.'s operating policies may have forced Mr. Iarossi out.

Mr. Iarossi, however, insisted that the move to the American Bureau of Shipping is a "career move; I got a wonderful opportunity."

Les Rogers, an Exxon spokesman in Houston, said Exxon did not fire Mr. Iarossi and that his departure was "absolutely not" related to the Alaskan oil spill.

The announcement of his resignation was made only two days after the shipping giant - the largest in the industry - was indicted by a federal grand jury in Alaska on five criminal counts for the massive spill of crude oil last March.

The counts include two felony charges that Exxon Shipping and its parent, Exxon Corp., "willfully and knowingly" violated U.S. law by hiring crewmembers who were "mentally incapable of performing duties," and that the company "willfully and knowingly" violated laws requiring that ships be staffed at all times while at sea by "competent persons who controlled vessel movement."

Mr. Iarossi had been negotiating with the shipping bureau for several months and was elected on Feb. 26 to the posts of chairman of the board and president, Mr. Rogers said.

After serving in the U.S. Coast Guard, Mr. Iarossi joined Exxon's tanker division in 1968. From 1972 to 1976, he managed a tanker construction program for Exxon in Japan. He later was a marine adviser to Exxon and became assistant manager of Exxon's corporate planning department in 1981.