Design considerations[edit]
Many modern tankers are designed for a specific cargo and a specific route. Draft is typically limited by the depth of water in loading and unloading harbors; and may be limited by the depth of straits along the preferred shipping route. Cargoes 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.[2]
Tanker capacity[edit]
Tankers used for liquid fuels are classified according to their capacity.
The small coastal tanker Pegasus on the River Weser.
The Very Large Crude Carrier (VLCC) MV Sirius Star in 2008 after her capture by Somali pirates.
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 25,000 tons deadweight (DWT); Medium Range for ships between 25,000 and 45,000 DWT and Large Range for the then-enormous ships that were larger than 45,000 DWT. The ships became larger during the 1970s, and the list was extended, where the tons are long tons:[3]