The Southeastern New England Shipbuilding Corporation (SENESCO), Quonset, R.I., will soon enter a new phase of shipbuilding when it begins construction of its first double-hull barge for transporting liquids with a potential environment impact. The vessel- to be built under the 11th contract awarded to SENESCO - will carry 98 percent pure sulfuric acid in Baltimore Harbor between two manufacturing plants for Millenium Inorganic Chemical Company. Based in Ashtabula, Ohio, Millennium manufactures the acid in one plant and then transports it to a factory seven miles away. The 70-ft.barge will carry 7,000 gallons. "Obviously, the integrity of the welding and general manufacturing process is paramount when you consider the cargo," said Robert G. Homan, SENESCO president and CEO.