Oil tankers were once the only means of transporting petroleum. Even petroleum that needed to be transported across land was often loaded onto an oil tanker and shipped via water channel to a port close to its destination. However, during World War II, a great number of oil tankers were sunk by German U-boats in an effort to disrupt supply to Ally forces. These events led to cooperation between governments and private industry to install pipelines as alternative means of transportation.
Oil tankers are generally divided into two basic types. Crude tankers transport crude oil while product tankers transport refined product. In general, crude tankers are substantially larger than product tankers.
In 1954, something called the average freight rate assessment (AFRA) system was developed by Shell Oil as a means to classify tankers of different sizes. Under the AFRA, tankers are rated according to their dead weight (DWT). DWT, which is expressed in metric tons, is defined as the maximum load by weight that a tanker can safely carry. The AFRA scale divides oil tankers into six classes as follows:
- General purpose tanker (10,000 -24,999 DWT)
- Medium range tanker (25,000 -44,999 DWT)
- Large range 1 (45,000 -79,999)
- Large range 2 (80,000 -159,999)
- Very large crude carrier (160,000 -319,999)
- Ultra large crude carrier (320,000 -549,999) – so-called “supertankers”
Ultra large crude carriers are capable of transporting over two million barrels of oil, which was roughly enough to supply the United Kingdom for a single day in 2005. Supertankers are so large that they cannot enter most ports fully loaded. Instead, they load and unload cargo at offshore platforms. Many supertankers stay at sea for greater than 70 days at a time.
The largest oil tankers ever built were 379 m in length, 68 m wide, 34 m tall, and had a capacity of over three million barrels of oil. The largest of these tankers, the TI Asia and TI Africa, were both converted into floating petroleum containment vessels when their operation and maintenance became too expensive.
Oil tankers are usually divided into 8 to 12 tanks, with each tank split into two or three individual compartments. Tanks are separated from the hull of the ship and are often separated from one another by a cofferdam. A cofferdam is nothing more than a space left open between two bulkheads to provide protection from heat, fire, and collision.
Tanker hulls can be single or double. Most modern tankers are double-hulled and all single-hulled tankers are to be phased out by 2026. Double-hulled ships are easier to ballast, easier to clean, and provide better protection in low impact collisions and grounding. Nevertheless, there are drawbacks. Double-hulled ships are more difficult to maintain and there is the potential risk of explosion if a vapor detection system is not fitted to detect vapor that leaks into the space between hulls.
A great number of safety mechanisms are incorporated into oil tankers to prevent explosion of flammable hydrocarbon vapors. The most common system of preventing explosion is to introduce an inert gas, such as helium or argon, into the oil tanks. This lowers the flammable limit of vapors, which is the minimum concentration at which vapors can be ignited. It can be thought of a s a method of diluting hydrocarbon vapors so that they are less likely to explode.
The most important safety measures on at any oil tanker are the established body of rules known as best practices. These rules include standard international maritime law. Rules limit the way oil can be loaded and unloaded, as well as how it can be stored. It is important to note that whether oil is being transported, tanks are being cleaned, or product is being loaded, the atmosphere within each tank is carefully monitored to ensure that it is never capable of igniting hydrocarbon gas.
A general purpose tanker costs around 43 million USD while a supertanker can cost upwards of 120 million USD. In recent years, the economics of shipping oil in tankers has come into question. With crude becoming heavier, the volume that can be shipped at any given time is lower. In addition, heavy crude provides less profit overall, but shipping costs the same. Thus, it is becoming less economical to move heavy crude over long distances.