<P> By comparison, in 1970 1.44 billion metric tons of oil were shipped by tanker . This amounted to 34.1% of all seaborne trade for that year . In terms of amount carried and distance carried, oil tankers moved 6,487 billion metric - ton - miles of oil in 1970 . </P> <P> The United Nations also keeps statistics about oil tanker productivity, stated in terms of metric tons carried per metric ton of deadweight as well as metric - ton - miles of carriage per metric ton of deadweight. 2006 In 2005, for each 1 DWT of oil tankers, 6.7 metric tons of cargo was carried . Similarly, each 1 DWT of oil tankers was responsible for 32,400 metric - ton miles of carriage . </P> <P> The main loading ports in 2005 were located in Western Asia, Western Africa, North Africa, and the Caribbean, with 196.3, 196.3, 130.2 and 246.6 million metric tons of cargo loaded in these regions . The main discharge ports were located in North America, Europe, and Japan with 537.7, 438.4, and 215.0 million metric tons of cargo discharged in these regions . </P> <P> International law requires that every merchant ship be registered in a country, called its flag state . A ship's flag state exercises regulatory control over the vessel and is required to inspect it regularly, certify the ship's equipment and crew, and issue safety and pollution prevention documents . As of 2007, the United States Central Intelligence Agency statistics count 4,295 oil tankers of 1,000 long tons deadweight (DWT) or greater worldwide . Panama was the world's largest flag state for oil tankers, with 528 of the vessels in its registry . Six other flag states had more than 200 registered oil tankers: Liberia (464), Singapore (355), China (252), Russia (250), the Marshall Islands (234) and the Bahamas (209). The Panamanian, Liberian, Marshallese and Bahamian flags are open registries and considered by the International Transport Workers' Federation to be flags of convenience . By comparison, the United States and the United Kingdom only had 59 and 27 registered oil tankers, respectively . </P>

How many barrels of oil can a tanker hold