with high vapor pressure at ambient temperatures may require pressurized tanks or vapor recovery systems. Tank heaters may be required to maintain heavy crude oil, residual fuel, asphalt, wax, or molasses in a fluid state for offloading. Tanker capacity Tankers used for liquid fuels are classified according to their capacity. In 1954, Shell Oil developed the average freight rate assessment (AFRA) system, which classifies tankers of different sizes. To make it an independent instrument, Shell consulted the London Tanker Brokers’ Panel (LTBP). At first, they divided the groups as General Purpose for tankers under ; Medium Range for ships between 25,000 and and Large Range (later Long Range) for the then-enormous ships that were larger than . The ships became larger during the 1970s, and the list was extended, where the tons are metric tonnes: Under : Extra small tanker 10,000–: Small tanker 25,000–34,999 DWT: Intermediate tanker 35,000–44,999 DWT: Medium Range 1 (MR1) 45,000–: Medium Range 2 (MR2) 55,000–: Long Range 1 (LR1) 80,000–: Long Range 2 (LR2) 160,000–: Very Large Crude Carrier (VLCC) 320,000–: Ultra Large Crude Carrier (ULCC) 550,000-: Hyper Large Crude Carrier (HLCC) 900,000-: Mega Crude Carrier (MCC) Over : Giga Crude Carrier (GCC) Very Large Crude Carrier size range At nearly 380 vessels in the size range to , these are by far the most popular size range among the larger VLCCs. Only seven vessels are larger than this, and approximately 90 between and . Fleets of the world Flag states As of 2005, the United States Maritime Administration's statistics count 4,024 tankers of or greater worldwide. 2,582 of these are double-hulled. Panama is the leading flag state of tankers, with 592 registered ships. Five other flag states have more than two hundred registered tankers: Liberia (520), The Marshall Islands (323), Greece (233), Singapore (274) and The Bahamas (215). These flag states are also the top six in terms of fleet size in terms of deadweight tonnage. Largest fleets Greece, Japan, and the United States are the top three owners of tankers (including those owned but registered to other nations), with 733, 394, and 311 vessels respectively. These three nations account for 1,438 vessels or over 36% of the world's fleet. Builders Asian companies dominate the construction of tankers. Of the world's 4,024 tankers, 2,822 (over 70%) were built in South Korea, Japan and China. Further reading Petroleum Tables, a book by William Davies, an early