Tanker is a ship designed to carry liquid cargo. Most tankers transport petroleum products, but they may also carry such cargoes as molasses, wine, chemicals, and even coal, grain, and iron ore.
A tanker has as many as 25 tanks, the walls of which are formed in part by the hull. A crew of 25 to 40 people lives in the deckhouse, which usually stands above the engines at the stern. On most tankers, the deckhouse has five or six stories, with the bridge on top. Cargo pumps and piping line the main deck.
Kinds of tankers.
There are three chief kinds of tankers: (1) oil tankers, (2) combination carriers, and (3) liquefied natural gas carriers.
Oil tankers
carry crude oil and refined petroleum products. A hull forms the outside of the tanks. Bulkheads (walls) run the length and width of the ship and divide the ship into compartments. This construction strengthens the hull. It also enables an oil tanker to carry several petroleum products at once. The larger a tanker, the lower the carrying cost per pound or kilogram of oil.
Extra large oil tankers, called supertankers, were developed during the 1960’s and 1970’s. These ships are also known as very large crude carriers (VLCC’s) or ultra large crude carriers (ULCC’s), depending on their size. The largest supertanker ever built was the Seawise Giant. Now called the Knock Nevis, it measures 1,504 feet (458 meters) long and 225 feet (69 meters) wide. It can hold about 1.2 billion pounds (550 million kilograms) of oil. When filled, its hull extends 80 feet (24 meters) under the water. Supertankers cruise at speeds of about 15 knots (nautical miles per hour) and are extremely difficult to maneuver. A VLCC making an emergency stop may travel over 3 miles (5 kilometers) before stopping completely.
Most supertankers transport oil from the Middle East to Europe and Japan. In those areas, supertankers load and unload cargo through underwater pipelines provided by offshore ports. If a port is not deep enough for a supertanker, the oil is transferred to smaller tankers and brought ashore. The supertanker may deliver some of the oil itself after being made lighter by such a transfer.
Combination carriers
carry such cargoes as grain and ore, as well as oil. These ships are divided into two groups, ore/oil carriers and ore/bulk/oil carriers (OBO’s). They have tanks and pumps for storing and unloading liquids. They also have large hatches on the main deck for loading and unloading dry cargo.
Liquefied natural gas (LNG) carriers
were developed in the 1960’s. When natural gas is chilled to –260 °F (–162 °C), it shrinks to about 1/600 of its volume and becomes liquid. Plants on shore liquefy natural gas and pump it into specially insulated tanks on the tankers.
Oil spills.
Tankers spill about 21/2 billion pounds (1.1 billion kilograms) of oil into the world’s oceans annually due to accidents and normal ship operations. Many nations are cooperating to reduce this pollution. Most tankers built since the early 1990’s have double hulls to minimize oil loss in accidents. Shipping companies have developed methods to help prevent spilled oil from spreading over the water’s surface and to help remove spills. Crews also try to prevent discharge of the oily seawater carried in deep holds to stabilize the ships.
