Two corporations that build giant offshore oil drilling rigs are the main promoters and would be beneficiaries of a $20 million Corps of Engineers project in Louisiana that is on President Carter's "hit list"

The companies, J. Ray Mcdermott & Co., and Avondale Shipyards, want a wider and deeper channel from their rig construction sites in Morgan City, La., to the Gulf of Mexico so they can expand their already booming business.

The rigs they build are eight stories high and cost $30 million each.

The water project,which is under construction, would enlarge channels in the Atchafalaya River and bayous Chene, Boef and Black to a width of 400 feet and a 20 - foot depth.

Atpresent, with the narrower channel the companies must fabricate their rigs in two or three sections in Morgan City and float them toward the gulf. They are joined and outfitted at facilities near the open water.

If the channel dredging project is completed, the massive rigs could be completely constructed at Morgan City.

The area also could provide repair refitting facilities for large mobile rigs already in use.

President Carter, last Feb. 21, listed the project among those he wanted halted. He recently described it as the widening of a waterway "at taxpayer expense for the benefit of a very few private companies."

Up to now, the only vocal opponents to the project have been environmentalists. In a 1975 filing with the Army Corps of Engineers, the Environmental Defense Fund said the project "is a good example of how private 'local interests' use the corps and the American taxpayers' money to subsidize projects which they, as shrewd businessmen, know are unwise, foolish and money - losing investments."

The project's most fervent supporters agree the benefits would accrue primarily to the two large companies. They argue, however, that the geographic area stands to benefit economically from that.

The corps, is justifying the dredging, found the prime benefits, totaling an estimated $2.7 million each year, would come from "use of facilities in the area by large mobile rigs for purposes of maintenance, repair and storm refuge."

The yards of the two big rig builders, plus facilities run by Teledyne Corp., and Raymond, International - two other large companies - are the only ones capable of performing such services.

But the two companies are not now engaged in those activities.

Ed kyle, part owner of the local radio station and a member of the Morgan City Port Commission, is a strong booster of the project.

In a telephone interview yesterday kyle said support for the channel began in 1958 when the companies and local businessmen dreamed of becoming "the offshore rig repair and construction base of the world."

An industrial group of area businessmen to push the project was formed. It is now headed by Ernest B. Gravois, administrative assistant to the executive vice president of McDermott.

Support was developed within the Louisiana congressional delegation and the project was authorized by Congress in 1958.

In recent years, according to campaign fund records, Gravois and other McDermott officials along with officers of the Avondale shipyards have been contributors to members of the Louisiana delegation.

Gravois led a delegation of Morgan City businessmen to Washington recently to meet with Louisiana members of Congress to find out what could be done "to change the President's idea of canceling our project."

Kyle, who also has visited Washington in the past few weeks, said the delegation members told him to "keep fighting and they are going to stand with us."

The Carter administration's initial screenings of the Louisiana project found that expected benefits "were considered inadequate to offset adverse environmental effects." Those unfavorable effects apparently arose out of plans to use 7,000 acres of wetlands as the disposal site for the dredging spoils.

To date, $5 million of the projected $20 million total cost has been spent on the project. Another $5 million was appropriated last year. Dredging contracts were opened, but not awarded, 10 days ago.