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A large ship that moves large amounts of oil from one location to another is an oil tanker. These ships may either transport oil to a refinery, or they may transport from a refinery to an oil purchaser’s location. The former type of transporter is known as a crude tanker, whereas the latter is a product tanker. Several functional additions aid tanker capacities and abilities, including specialized hull designs and storage tanks. The creation in 1878 of the Zoroaster by Sweden’s Ludvig Nobel marked the first true oil tanker.
An oil tanker can carry anywhere from around 1100 tons (about 1000 metric tons) for general purpose product tankers to over 550,000 tons (about 500,000 metric tons) for ultra large crude carriers. This bulk is called dead weight. In order to carry so much cargo, these large ships can span over a thousand feet in length. Due to their large weights and valuable cargo, oil tankers are especially vulnerable to oil spills like the Exxon Valdez tanker spill of 1989, pirate attacks, and the resulting environmental and humanitarian consequences.
Each oil tanker contains around a dozen tanks for storing oil. The vapors emitted by the fuel inside the tanks can be explosive when mixed with air, however. Therefore, an inert gas system within the tanks helps prevent this potentially flammable interaction by lowering the oxygen content of the tank air. Spaces called cofferdams are built between the tanks to provide an extra layer of heat or collision protection.
Tanks are separated from the water by a hull, which is the portion of the ship that contacts water. A tanker can have a single or a double hull, with the latter preferred because it offers greater protection for the tanks. The double hull simply provides more space between the water and the tanks.
Unloading oil is one of the most important processes on an oil tanker. Pumps are used to get oil into or out of the tanks, and these devices are contained within a pump room. Most tankers also have large loading arms that connect to loading arms or hoses on other devices so that oil may be moved on or off the ship.
The fuel transfers may take place on piers, with other ships, or underwater. A chief officer aboard the ship oversees the transfer process, assisted by about two dozen crew members. When transfers occur at ports or docks, they are known as marine transfer operations.
Oil tankers operate on a charter system. In other words, they must be hired as a merchant vessel to haul cargo. Oil companies are one common customer of these shipping systems. Organizations may charter an oil tanker for a specified amount of time, for a certain amount of cargo delivered, or for total expenses. Due to their bulk and the demands placed on them, most oil tankers are only usable for about ten years, after which they are retired.
Different ship types are used based on the client's needs. When raw, crude oil is taken from an oil outlet, it must be relocated to an oil refinery so that it can be prepared for public use as petroleum. Crude tankers serve as the transportation vessel for crude oil. In contrast, product tankers are responsible for moving refined oil to places where it is used, such as gas companies.
Some tankers also serve specialty purposes. For example, replenishment tankers can provide oil to another sea-faring vehicle while the vehicle is still in motion. Some tankers even function as semi-permanent storage containers.