The term supertanker originally applied to the class of tankers too large to transit international canals while carrying cargo, and currently defined by two ship classes: Very Large Crude Carriers (VLCCs) between ~200,000 and ~300,000 deadweight tons (dwt) and Ultra Large Crude Carriers (ULCCs) greater than ~300,000 dwt. Supertankers are a remarkable technological response to market conditions that promoted economies of scale without apparent bound in the 1960s, 70s, and 80s.
The first modern tanker (tanks integral with hull) was the ~3000 dwt 'Glukauf', built in 1886. Until the 1950s, most crude oil was refined at source and transported to market in products tankers, sized between 12,000 and 30,000 dwt. Larger vessels became economically feasible when oil companies began locating refineries near energy markets, although the Suez Canal restricted tanker size. Energy market shifts and the 1956 closure of the Suez Canal created new routes, removing geographic barriers to construction of the first VLCCs and ULCCs. Fully-loaded supertankers (especially efficient diesel-powered VLCCs built in 1990s) reduced unit shipping costs dramatically, but partial loads could not sustain economies of scale; many were scrapped in the 1980s and 90s or used for storage.
The largest supertanker ever built was the 555,843 dwt 'Seawise Giant', refitted in 2004 as a floating storage and offloading unit named the 'Knock Nevis'. Current supertanker sizes are defined by market conditions providing an economic upper bound and geophysical limitations defining the number of routes and ports (or offshore terminals) that these very large vessels can safely serve.