How Oil Tankers Work

More Types of Oil Tankers

The VLCC-class tanker MV Sirius Star at anchor off the coast of Somalia, shortly after being overrun by pirates.
The VLCC-class tanker MV Sirius Star at anchor off the coast of Somalia, shortly after being overrun by pirates.
U.S. Navy/Getty Images

In addition to classifying these mighty maritime vessels as crude tankers and product tankers, we can sub-categorize them even further. Some tankers are built to haul oil from one point to another, while replenishment oilers are designed to ferry fuel out to ships at sea and refuel them. Sometimes, if sea-bound tankers become too old or uneconomical to operate, they're used as floating storage units.

If you work in the oil transport business or have an interest in it, you'll probably hear the following terms:

  • Double Hull - A mandatory design feature on newly built oil tankers; double-hull construction means the ship has two hulls, one inside the other. This offers an extra layer of protection against damage that might otherwise result in catastrophic oil spills.
  • DWT - Deadweight tonnage refers to the maximum load of cargo, fuel, provisions and ballast a ship can carry. DWT is usually measured in metric tons.
  • OBO - The idea behind these ore-bulk-oil carriers is to give them something to carry on the return leg of their trips, so that they can make money both ways. As the name suggests, the return cargo is usually iron ore.
  • LR1/LR2 - Large Range 1 and Large Range 2 tankers have a DWT between 45,000 and 159,999 metric tons (49,604 and 176,369 tons).
  • VLCC -Very Large Crude Carriers weigh between 160,000 and 319,999 DWT (176,370 and 352,739 tons). Oil carriers of this size and above are known as supertankers.
  • ULCC - Ultra Large Crude Carriers are the largest oceangoing vessels -- with DWTs of 320,000 metric tons (352,740 tons) and above -- and are comparable in length to the height of some of the world's tallest buildings.

Two different classification scales are used to categorize ships. The ship descriptions above use the Average Freight Rate Assessment (AFRA) system. However, another method, called the flexible market scale, differs slightly in its weight limits for some size categories of ships.

In the next section, you'll learn some of the rules and regulations that govern how oil tankers are built, operated and maintained.