The term supertanker usually refers to the world's largest ships, those tanker ships above 250,000 in deadweight tonnes (dwt) and capable of transporting two or three million barrels of oil. "Supertanker" is an unofficial term, and in the shipping industry, it is common practice to refer to supertankers using size class designations such as Very-Large Crude Carriers (VLCC), Ultra-Large Crude Carriers (ULCC). Smaller classes of tanker, such as Aframax or Suezmax, are no longer regarded as 'supertankers'.
Supertankers are capable of transporting vast quantities of liquids, and in practice are used to move crude oil. The largest supertanker - indeed the world's largest ever ship - was the Jahre Viking (now the permanently moored storage tanker Knock Nevis), weighing in at 564,763 deadweight tonnes. In the 1950s, tankers with only a tenth of that capacity would have been called supertankers.
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 of 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, which prompted rescaling.
The system was developed for tax reasons as the tax authorities wanted evidence that the internal billing records were correct. Before the New York Mercantile Exchange started trading crude oil futures in 1983, it was difficult to determine the exact price of oil, that could change with every contract. Shell and BP, the first ones to use the system, abandoned the AFRA system in 1983, later followed by the US oil companies. However, the system is still used today. Besides that, there is the flexible market scale, that takes typical routes and lots of 500,000 barrels.
Merchant oil tankers carry a wide range of hydrocarbon liquids ranging from crude oil to refined petroleum products. Their size is measured in deadweight metric tons (DWT). Crude carriers are among the largest, ranging from 55,000 DWT Panamax-sized vessels to ultra-large crude carriers (ULCCs) of over 440,000 DWT.
Supertanker is an informal term used to describe the largest tankers. Today it is applied to very-large crude carriers (VLCC) and ULCCs with capacity over 250,000 DWT. These ships can transport two million barrels of oil. By way of comparison, the combined oil consumption of Spain and the United Kingdom in 2005 was about 3.4 million barrels (540,000 m3) of oil a day.
Because of their great size, supertankers can often not enter port fully loaded. These ships can take on their cargo at off-shore platforms and single-point moorings. On the other end of the journey, they often pump their cargo off to smaller tankers at designated lightering points off-coast. A supertanker's routes are generally long, requiring it to stay at sea for extended periods, up to and beyond seventy days at a time.
Smaller tankers, ranging from well under 10,000 DWT to 80,000 DWT Panamax vessels, generally carry refined petroleum products, and are known as product tankers. The smallest tankers, with capacities under 10,000 DWT generally work near-coastal and inland waterways. Although they were in the past, ships of the smaller Aframax and Suezmax classes are no longer regarded as supertankers.