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Geographic Overview of Oil Tankers

Learn about Oil Tankers, the World's Largest Ships

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Oil Tanker

The very large Liberian-flagged oil tanker MV Sirius Star is at anchor off the coast of Somalia November 19, 2008 in the Indian Ocean.

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Updated July 25, 2010

An oil tanker is a large ship designed to move oil around the world. Their size depends on whether they travel along inland or coastal routes and the amount of oil they carry. Oil tankers carry around 2,000,000,000 metric tons of oil each year, making them a very important subject in transportation geography.

When compared to other methods of moving oil, oil tankers are considered the most efficient method aside from pipelines because of the amount of oil they can move at any given time. Due to their size, it costs an average of only two to four cents per gallon to move oil with an oil tanker.

History of Oil Tankers

The first oil tankers were developed in the 1860s. The earliest oil tankers were developed in the 1860s and were driven by wind with sails. In 1873, the Vaderland, a ship built by Palmers Shipbuilding and Iron Company, was the first oil tanker powered by steam engines.

The world's first modern oil tanker was named the Zoroaster, built in 1878. The Zoroaster was designed by Ludvig and Robert Noble of Sweden. The ship was capable of carrying 242 long tons of oil in two iron tanks.

In 1883, Colonel Henry F. Swan designed an oil tanker with several holds for oil. The holds were also subdivided to prevent oil from sloshing around and causing the ship to capsize while moving at sea. Today's oil tankers feature similar holds for their oil.

In 1903, the first oil tankers to run on internal combustion engines as opposed to steam engines were developed by the Nobel brothers. The Vandal was the first diesel-electric ship and it was able to carry 750 long tons of oil.

By World War I, oil tankers were diesel powered and large. During the war itself, the technique of "underway replenishment" was developed with the USS Maumee in 1915 as a way to carry oil to battleships and re-fuel them while still at sea. This enabled the ships to be ready for battle for longer periods. Like in WWI, oil tankers also played a significant role in World War II for their ability to refuel battleships while still at sea.

Since the end of WWII, oil tankers continued to grow in size. During the war, ships had an average capacity of 16,500 DWT. Ultra large crude carriers today can carry 500,000 DWT or more. Today, the world's largest oil tankers are known as supertankers .

How Oil Tankers Work

Oil is moved onto an oil tanker in one of several ways after the owner of the tanker enters into a contract or charter with the oil producer. Once the charter is established and a variety of safety checks are completed, oil is pumped into the tanks on the oil tanker. As the oil enters these tanks, it emits vapors that are either released into the atmosphere or captured and discharged back into the pump via vapor recovery lines.

The loading of oil onto an oil tanker usually begins at low pressure to ensure there are no leaks or other equipment problems. Once the tanks are almost full, the pressure is increased until loading and "topping off" occurs. During the topping off phase, crew members monitor how much space is left in the tanks and begin to close all valves and complete the flow of oil onto the tanker.

After full, oil tankers travel to either the market or to refineries, depending on the tankers type. Unloading oil from an oil tanker is similar to the loading procedures - safety checks are completed and oil is released via pumps first at low pressure that gradually increases as the tanks on the ship become empty. Tank levels are carefully monitored by the ship's crew and when empty, all valves are closed. From time to time after oil transfer, tanks on an oil tanker must undergo cleaning, especially if a tanker carries more than one product type.

Types of Oil Tankers

In addition to the two main types of oil tankers, crude and product tankers, there are some subcategories of oil tankers in use today. Replenishment tankers for example are used for the previously mentioned underway replenishment of oil for other ships at sea. In addition, there are ore-bulk-oil carriers that are designed to carry both oil and iron ore. The iron ore is usually carried on return trips.

Other types of oil tankers include Large Range 1 and 2 (LR1 and LR2). These are tankers with a DWT of between 49,604 and 176,369 tons. Very Large Crude Carriers (VLCC) are large oil tankers that are known as supertankers and are capable of carrying between 176,370 and 352,739 tons. The largest oil tankers are called Ultra Large Crude Carriers (ULCC) and they can carry 352,740 tons of oil and above.

Pollution and Oil Spills

Despite their efficiency in moving oil to destinations around the world, oil tankers are often criticized because of the possibility pollution, accidents and oil spills. For example in 1989, an Exxon oil tanker named Valdez ran aground in Alaska's Prince William Sound and spilled nearly 11 million gallons (41.6 million liters) of oil. The spill killed much of the area's wildlife and coated shorelines in a slick of oil.

In addition to oil spills, fires and explosions as a result of accidents on board oil tankers are another threat to crew members and wildlife. Because of past oil spills and concerns over accidents, the United States and international bodies like the European Union and the United Nations have entered into various safety agreements to prevent such catastrophes. Some of these agreements include forcing tankers to be double-hulled to prevent spills and have inert gas systems to prevent explosions and fires.

To learn more about oil tankers, visit the Discovery Channel's How Stuff Works website on oil tankers.

References

Parker, Akweli. (n.d.). "How Oil Tankers Work." How Stuff Works. Retrieved from: http://science.howstuffworks.com/oil-tanker.htm

Wikipedia. (14 July 2010). Oil Tanker - Wikipedia, the Free Encyclopedia. Retrieved from: http://en.wikipedia.org/wiki/Oil_tanker

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