Evergreen proposed to convert up to four of its Boeing 747-200 Freighters into Supertankers. "Supertankers" are the largest tankers, and the largest man-made mobile structures. The majority of the oil is shipped via supertankers to refineries around the world. The Batillus class supertankers are the biggest ships ever constructed by gross tonnage. Modernisation of the shipyards took place in the 1960s allowing construction of supertankers. The first vessel of homonymous Batillus class supertankers. In July 1984, the Navy began conversion of two supertankers to hospital ships. This was designed to accommodate the largest supertankers sailing directly from the Middle East. Shortly after, he inherited his father's 30-ship fleet, which included the world's five largest supertankers. Starting in 1952, he had the world's biggest supertankers built for his fleet. Yet because of the draft of today's supertankers, it needs to be dredged constantly. Some supertankers are too large to traverse the canal. Because of their size, supertankers often cannot enter port fully loaded. This plan never materialized, being dropped in favor of converting two "San Clemente" class supertankers. Most FSOs are converted "single hull" supertankers.