Waterborne transportation: Oil tankers

Crude oil must be moved from the production site to refineries and from refineries to consumers. These movements are made using a number of different modes of transportation. Crude oil and refined products are transported across the water in barges and tankers. On land crude oil and products are moved using pipelines, trucks and trains.

Let’s focus on the oil tankers.


An oil tanker is a large ship designed to move oil around the world. Their size depends on wether 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.

There are two basic types of oil tankers: the crude tanker and the product tanker. Crude tankers move large quantities of unrefined crude oil from its point of extraction to refineries. Product tankers, generally much smaller, are designed to move petrochemicals from refineries to points near consuming markets.

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.

  • How oil tankers work

Oil is moved onto an oil tanker in 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 ant « 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 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.

The United States and international bodies like the European Union and the United Nations have entered into various safety agreements to prevent oil spills, fires and explosions. Some of these agreements include forcing tankers to be double-hulled and have inert gas systems.

* DWT: deadweight tonnage. It’s a measure of how much weight a ship is carrying or can safely carry. It’s the sum of the weights of cargo, fuel, fresh water, ballast water, provisions, passengers and crew


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