A tanker is a ship designed to transport liquids in bulk.
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. 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 proposed several problems:
In the end, the tanker had it's beginnings in the oil industry, as oil companies sought cheaper ways to transport their refinery product to their customers. The Oil Tanker was born. Nowadays, 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.
Even the Guinness brewery company in Dublin had a tanker fleet to export the famous stout to the UK.
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.
In 1954 Shell Oil developed the afra system (average freight rate assessment) for fiscal reasons. Tankers were divided in groups 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 deadweight tonnage (dwt); Medium Range for ships between 25,000 and 45,000 dwt; and Large Range for the then-enormous ships that were larger then 45,000 dwt. The ships became larger during the 1970s, and the list was extended, where the tons are long tons:
Based on the 2007 status list of the 500 largest supertankers prepared by Visser there are almost 380 vessels in the size range 279,000-320,000 dwt, making this by far the most popular size range among the larger VLCCs. Only 7 vessels are larger than this, and about 90 between 220,000 and 279,000.