Tankers are vessels designed for carrying any liquid cargo such as petroleum and products derived from it, liquefied gases, chemicals, wine and water. There are gas tankers designed for carrying liquefied gas, either LPG or LNG, both of which need to be kept at higher higher pressure and at low temperatures to mantaing the cargo in a liquefied state, and there are crude oil tankers. The latter usually carry crude oil from a loading port near oil fields or from the end of a pipeline to a refinery. Gas tankers are often steam turbine ships. The boil-off, which is the gas evaporated from the cargo in order to keep the temperature low, can be used as fuel for the boilers.
Tankers come in all sizes, ranging from bunkering tankers of 1000DWT used for refueling larger vessels to the real giants:

• The VLCC - Very Large Crude Carrier, 200.000 - 300.000 DWT

• The ULCC - Ultra Large Crude Carrier, over 300.000 DWT

Crude oil tankers are the largest of all cargo ships. Their capacity has risen right up to 500.000 tons and, consequently, their large draught limits their sailing routes. There are only a few ports that supertankers can enter ans thus they are mostly loaded and unloaded from off-shore pumping stations. The liquefied cargo is loaded by means of pipes from shore facilities and through flexible pipelines mounted on the jetty.

A further step in the development of the oil industry is the Floating, Production, Storage and Offloading vessel (FPSO), designed for off-shore purposes. When a large vessel like a crude oil tanker is damaged by collision or grounding, vast amounts of oil my leak out straight into the sea. This explains the strict requirement for them to have a double hull.