An auxiliary ship is a naval ship designed to support combatant ships and other naval operations. Auxiliary ships are not primary combatant vessels, though they may have some limited combat capacity, usually for purposes of self-defense. Auxiliary ships are extremely important for navies of all sizes because if they were not present the primary fleet vessels would be unsupported. Thus, virtually every navy maintains an extensive fleet of auxiliary ships. However, the composition and size of these auxiliary fleets vary depending on the nature of each navy and its primary mission. Smaller coastal navies tend to have smaller auxiliary vessels focusing primarily on littoral and training support roles. Larger blue-water navies tend to have larger auxiliary fleets comprising longer-range fleet support vessels designed to provide support far beyond territorial waters. Roles Replenishment One of the most direct ways that auxiliary ships support the fleet is by providing underway replenishment (also known as "replenishment at sea") to major fleet units. This allow the fleet to remain in the same location, with the replenishment vessels bringing up fuel, ammunition, food, and supplies from shore to the fleet wherever it is operating. Oilers ("replenishment tanker") are vessels specifically designed to bring fuel oil to the fleet, while the earlier colliers supplied coal burning steamships. Specific role replenishment vessels include Combat stores ship, depot ship, general stores issue ship and ammunition ship. Tenders are specifically designed to support a type of smaller naval unit, like submarines, destroyer, and seaplanes, providing a mobile base of operations for these units: specifically destroyer tenders, submarine tenders, seaplane tenders, torpedo boat tenders. Transport Supporting front-line operating bases requires immense transportation capacity. Transport ships are often converted merchant ships simply commissioned (APA, APD, APH, APV) into naval service. Tankers are transports specifically designed to ship fuel to forward locations. Transport ships are often employed not only carrying cargo for naval support but in support of all forces of a nation's military (AK, AKA, AKN, AKR, AKS). In particular, troopships and attack transports are used to carry a large number of soldiers to operational theatres. Some transport ships are highly specialized, like the ammunition ships employed by the US Navy. Large ocean tugs (AT, ATO, ATF, ATA, ATR) are used to tow large auxiliary ships, like barges, floating repair docks, and floating cranes in the open sea, also disabled ships. Repair Repairing ships at sea or in conflict areas is