Into the tanker category can also be placed the LPG (Liquid Petroleum Gas – such as butane) and the LNG (Liquid Natural Gas – such as methane) carriers. All tankers are equipped with pumps by which they discharge their cargoes. They are also equipped with sophisticated cargo tank cleaning apparatus. Unlike dry cargo ships, where all that is needed to clean the cargo holds is a pressure water hose and brooms, liquid cargo tanks need elaborate cleaning. In the case of oil tankers, to avoid the risk of explosion it is important to reduce the oxygen and the oil vapour content of the tanks before cleaning takes place. This is done by pumping inert gas (usually the exhaust gasses from the main engine) into the holds prior to the commencement of cleaning. Most modern crude oil tankers employ a “crude oil wash” system which also eliminates the need to dispose of large quantities of dirty ballast. In the case of chemical tankers, cleaning can be even more complicated -in some cases steam being necessary. Other equipment which can be found are cargo heating coils, needed when the cargoes become too viscous to pump or even solidify at ambient temperatures. At the other end of the scale is refrigeration for LPG. The final piece of equipment found on tankers is a crane or derrick for handling the loading/discharging hoses, and is found by the cargo manifold amidships. The scope for this heading is enormous, covering as it does everything else that floats except warships and private yachts. The main participants in this field are: Ranging from the large ocean going salvage tugs which are also used to tow oil rigs around the world, to the small harbour tugs. The power of these tugs can either be expressed by the brake horse power (bhp) of their engines or (more usually) by the bollard pull they can exert. Seldom self propelled, used in off-shore oil fields. They are usually divided into two categories, drilling and production platforms.