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Commercial crude oil supertanker AbQaiq.

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.

Until the 1960s, international maritime trade relied heavily on the Suez Canal, the width and depth of which effectively placed a limit on the size of merchant vessels; it was rare for oil tankers to exceed 45,000 tonnes in displacement. This was changed drastically by the Six-Day War, with the Suez Canal being closed from 1967 to 1975. Being forced to make the longer and costlier detour through the Cape of Good Hope, shipping companies opted for the next-best option, that was to build very large tankers, free from the constraint of the Suezmax, that offer better economies of scale than that of older, smaller vessels.

When first introduced, their size and draft prevented them from docking at many existing docks, requiring them to discharge their cargo into smaller tankers offshore. Some ports have developed special deep-water off-loading facilities connected to the land by pipelines (for an example, see Louisiana Offshore Oil Port). Supertankers are also very efficient ships, mostly relying on a single propeller for propulsion, and therefore supertanker transport costs typically account for only US $0.02 per gallon of gas at the pump.^[1]

Due to their size and mass, supertankers have very poor maneuverability; the stopping distance of a supertanker is legendary, typically measured in miles. When operating close to the shoreline they are vulnerable to running aground, largely because at slow speed it is impossible to control their movements. When this happens, oil spills are a risk, though accidents are far less common than they were in the 1970s and early 1980s.

In "single-hulled" tankers, the hull is also the wall of the oil tanks, and any breach will result in an oil spill. Newer tankers are "double-hulled", with a space between the hull and the storage tanks to reduce the risk of a spill if the outer hull is breached. This space is used to carry water ballast when the ship is not carrying an oil cargo. In practice the addition of an extra hull should prevent such a ship from suffering a catastophic breach of the hull. A double-hull tanker is generally safer than a single-hull in a grounding incident, especially when the shore is not very rocky[1]. However, double hulls are not a complete solution, and they are at greater risk of explosion if petroleum vapour collects in the space between the hulls.

Following the Exxon Valdez incident, the United States passed the Oil Pollution Act of 1990 (OPA-90), which included a stipulation that all tankers entering its waters be double-hulled by 2015. Following the sinkings of the Erika (1999) and Prestige (2002), the European Union passed its own stringent anti-pollution packages (known as Erika I, II and III), which which also require all tankers entering its waters to be double-hulled by 2010. The Erika packages are controversial because they introduced the completely new legal concept of "serious negligence".

The largest known double-hulled tanker in the world is the TI Asia. Completed in 2002, it boasts a deadweight tonnage of 441,893 tonnes.