A tank ship or tankship, often referred to as a tanker, is a ship designed to transport liquids in bulk. Major types of tankship include the oil tanker, the chemical tanker, and the liquefied natural gas carrier.
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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. Beside 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:
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 the amount of tuns of wine (casks) that 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:
In the end, the tanker had its beginnings in the oil industry, as oil companies sought cheaper ways to transport their refinery product to their customers. The Oil Tanker was born. Today most liquids are cheaper to transport in bulk and dedicated terminals exist for each product. Large storage tanks ashore are used to store the product until it can be subdivided into smaller volumes for delivery to smaller customers.
Different products require different handling and transport. Thus special types of tankers have been built, such as "chemical tankers" and "oil tankers". "LNG carriers", as they are typically known, are a relatively rare tanker designed to carry liquefied natural gas.
Among oil tankers, supertankers are designed for transporting oil around the Horn of Africa from the Middle East. The floating storage and offloading unit (FSO) Knock Nevis, formerly the ULCC Jahre Viking, is the largest vessel in the world. The supertanker is 458 metres (1504 feet) in length and 69 m (226 ft) wide.
Supertankers are one of the three preferred methods for transporting large quantities of oil, along with pipeline transport and rail. However such tankers can create environmental disasters from oil spills especially if an accident causes the ship to sink. See Exxon Valdez, Braer, Prestige oil spill, Torrey Canyon, and Erika for examples of coastal accidents.
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:
|Class||Length||Beam||Draft||Typical Min DWT||Typical Max DWT|
|Seawaymax||226 m||24 m||7.92 m||10,000 DWT||60,000 DWT|
|Panamax||294.1 m||32.3 m||12 m||60,000 DWT||80,000 DWT|
|Aframax||80,000 DWT||120,000 DWT|
|Suezmax||16 m||120,000 DWT||200,000 DWT|
|VLCC (Malaccamax)||470 m||60 m||20 m||200,000 DWT||315,000 DWT|
|ULCC||320,000 DWT||550,000 DWT|
At nearly 380 vessels in the size range 279,000 DWT to 320,000 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 DWT and 279,000 DWT.
As of 2005, the United States Maritime Administration's statistics count 4,024 tankers of 10,000 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.
Greece, Japan, and the United States are the top three owners of tankers, with 733, 394, and 311 vessels respectively. These three nations account for 1,438 vessels or over 36% of the world's fleet.
Asian companies dominate the construction of tankers. Of the world's 4,024 tankers, 2,822 or over 70% were built in South Korea, Japan or China.