Tankers can range in size of capacity from several hundred tons, which includes vessels for servicing small harbours and coastal settlements, to several hundred thousand tons, for long-range haulage. Besides ocean- or seagoing tankers there are also specialized inland-waterway tankers which operate on rivers and canals with an average cargo capacity up to some thousand tons. A wide range of products are carried by tankers, including:
- hydrocarbon products such as oil, liquefied petroleum gas (LPG), and liquefied natural gas (LNG)
- chemicals, such as ammonia, chlorine, and styrene monomer
- fresh water
Tankers are a relatively new concept, dating from the later years of the 19th century. Before this, technology had simply not supported the idea of carrying bulk liquids. The market was also not geared towards transporting or selling cargo in bulk, therefore most ships carried a wide range of different products in different holds and traded outside fixed routes. Liquids were usually loaded in casks—hence the term "tonnage", which refers to the volume of the holds in terms of how many tuns or casks of wine could be carried. Even potable water, vital for the survival of the crew, was stowed in casks. Carrying bulk liquids in earlier ships posed several problems:
- The holds: on timber ships the holds were not sufficiently water, oil or air-tight to prevent a liquid cargo from spoiling or leaking. The development of iron and steel hulls solved this problem.
- Loading and discharging: Bulk liquids must be pumped - the development of efficient pumps and piping systems was vital to the development of the tanker. Steam engines were developed as prime-movers for early pumping systems. Dedicated cargo handling facilities were now required ashore too - as was a market for receiving a product in that quantity. Casks could be unloaded using ordinary cranes, and the awkward nature of the casks meant that the volume of liquid was always relatively small - therefore keeping the market more stable.
- Free Surface Effect: a large body of liquid carried aboard a ship will impact on the ship's stability, particularly when the liquid is flowing around the hold or tank in response to the ship's movements. The effect was negligible in casks, but could cause capsizing if the tank extended the width of the ship; a problem solved by extensive subdivision of the tanks.
Tankers were first used by the oil industry to transfer refined fuel in bulk from refineries to customers. This would then be stored in large tanks ashore, and subdivided for delivery to individual locations. The use of tankers caught on because other liquids were also cheaper to transport in bulk, store in dedicated terminals, then subdivide. Even the Guinness brewery used tankers to transport the stout across the Irish Sea.
Different products require different handling and transport, with specialised variants such as "chemical tankers", "oil tankers", and "LNG carriers" developed to handle dangerous chemicals, oil and oil-derived products, and liquefied natural gas respectively. These broad variants may be further differentiated with respect to ability to carry only a single product or simultaneously transport mixed cargoes such as several different chemicals or refined petroleum products.Among oil tankers, supertankers are designed for transporting oil around the Horn of Africa from the Middle East. The supertanker Seawise Giant, scrapped in 2010, was 458 meters (1,503 ft) in length and 69 meters (226 ft) wide. Supertankers are one of the three preferred methods for transporting large quantities of oil, along with pipeline transport and rail.
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 molassss in a fluid state for offloading.
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 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:
- 10,000–24,999 DWT: General Purpose tanker
- 25,000–54,999 DWT: Medium Range tanker
- 55,000–79,999 DWT: Long Range 1 (LR1)
- 80,000–159,999 DWT: Long Range 2 (LR2)
- 160,000–319,999 DWT: Very Large Crude Carrier (VLCC)
- 320,000–549,999 DWT: Ultra Large Crude Carrier (ULCC)
|Class||Length||Beam||Draft||Typical Min DWT||Typical Max DWT|
|Seawaymax||226 m (741 ft)||24 m (79 ft)||7.92 m (26.0 ft)||10,000 t DWT||60,000 t DWT|
|Panamax||228.6 m (750 ft)||32.3 m (106 ft)||12.6 m (41 ft)||60,000 t DWT||80,000 t DWT|
|Aframax||253.0 m (830.1 ft)||44.2 m (145 ft)||11.6 m (38 ft)||80,000 t DWT||120,000 t DWT|
|Suezmax||16 m (52 ft)||120,000 t DWT||200,000 t DWT|
|VLCC (Malaccamax)||470 m (1,540 ft)||60 m (200 ft)||20 m (66 ft)||200,000 t DWT||315,000 t DWT|
|ULCC||320,000 t DWT||550,000 t DWT|
Very Large Crude Carrier Size Range There are more ships smaller in size.
At nearly 380 vessels in the size range 279,000 t DWT to 320,000 t DWT, these are by far the most popular size range among the larger VLCCs. Only seven vessels are larger than this, and approximately 90 between 220,000 t DWT and 279,000 t DWT.
- Flag states
As of 2005, the United States Maritime Administration's statistics count 4,024 tankers of 10,000 LT DWT 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.