A reserve fleet or (less formally) mothball fleet is a collection of naval vessels (both warships and support vessels) that are fully equipped for service but are not currently needed and thus partially or fully decommissioned. In earlier times and especially in British usage, these ships were said to be laid up in ordinary, whilst a similar phrase in unofficial modern U.S. naval parlance is ghost fleet.
Such ships are generally held in reserve against a time when it may be necessary to call them back into service, and are usually tied up in backwater areas near naval bases or shipyards to speed the reactivation process. They may be modified, for instance, by having rust prone areas sealed off or wrapped in plastic (or, in the case of sailing warships, removal of the masts). While held in the reserve fleet, the ships will typically have a minimal crew (or, less formally, a skeleton crew) that makes sure the ship stays in usable condition, if nothing else, the bilge pump needs to run continuously to prevent the ship from sinking.
When a ship is placed in reserve status, the various parts and weapon systems that the vessel uses are also placed in a storage facility so that if the ship is ever reactivated, the proper spare parts and ammunition can be loaded aboard the vessel, though like the ships themselves, these stored parts and equipment are prone to fall into disrepair and obsolescence. For example, during the United States’ arms buildup under President Ronald Reagan (specifically, the 600-ship Navy plan), the US reactivated its Iowa-class battleships to serve with the fleet, but since the ships had not been used since the 1960s, the US Navy had trouble finding the various specialty items that were needed to make the ships operational and had to salvage parts from earlier battleships that had been relegated to museum ships.
In practice, the fate of most reserve ships is to be scrapped, since their technology quickly becomes so outdated they may not even be worth updating. Scrapped reserve fleet ships are sometimes used for experiments or target practice, or are sold to other nations (and occasionally to private companies for civilian conversion), or become museum ships or even artificial reefs. In recent decades, the US Navy has begun to scrap dozens of these vessels, many of which date to World War II. Exporting the vessels for shipbreaking, or dismantling, has caused international protests due to the toxic nature of the dismantled materials.1 More recently, the Navy has established a program to allow ships like Oriskany to be sunk in selected locations to create artificial reefs.
Daniel Madsen. Forgotten Fleet. The Mothball Navy. U.S. Naval Institute Press. 1999.
To Sail No More. Seven volumes. Maritime Books. United Kingdom.