Coating manufacturers furnish lists of the products and temperatures for which their coatings are suitable. If the cargo and coatings are not compatible the cargo may damage the coatings or the coatings contaminate or discolour the cargo. Not all charterers have the same criteria regarding suitability of certain coatings for specific cargoes. If a coating starts to fail the tank surface is no longer fully protected and also cleaning problems increase. The merest trace of some substances can throw a cargo off specification and a single blister in a coating may conceal potentially contaminating quantities of the previous cargo. These vessels fall into two classes, liquid natural gas (LNG) carriers and liquid petroleum gas (LPG) carriers. There are three designs of LNG carriers in general use. The most widely used have the Moss Rosenberg spherical tank system and the other two best known systems are the Technigas and Gaz Transport integral tank systems. The LNG is carried at minus 163o celsius and the boil-off gas is either used as fuel in the main propulsion or put through a cryogenic plant on board and returned to the tanks. LNG has a specific gravity in the area of 0.50 depending on the composition of the gas, so LNG carriers need to have a large cubic in relation to their deadweight. Most of the existing LNG schemes use dedicated ships of a common design. The shipping fleet for a gas project always has spare capacity to take into account drydocking and repairing time but if there should be an unexpected loss of part of such a dedicated fleet, it could be difficult and expensive to obtain replacement tonnage which would be suitable, or could be made suitable, for the project terminals which have been designed for one class and size of ship.