Later this year the British Royal Navy is retiring its only remaining forward support vessel; the 10,000 ton RFA Diligence. These ships carry parts, special tools and sailors trained to do repairs on ships at sea that do not require a trip back to port or naval base but do require special tools and specialists most warships do not have. Support ships like this became popular during World War II when American and British warships operated far from port for months at a time. Many navies that regularly send their warships to distant and remote areas still maintain forward support vessels. These are often civilian vessels, like RFA Diligence, that was originally built as an offshore oil rig support vessel. These are equipped to make repairs on these rigs. This is important because while these rigs can be towed back to port that isn’t practical because the offshore rigs are actually floating oil wells that have equipment for drilling into the sea bottom of offshore waters and pumping the oil into pipelines leading back to shore. Militarizing an oil rig support ship means adding equipment needed to repair warships (including submarines) at sea or an anchorage and travelling long distances to provide that support.
RFA Diligence was built in 1981 as an oil rig support vessel and purchased by the Royal Navy in 1983 and converted to a forward support vessel. Since 1984 RFA Diligence has spent most of its time far from home, often in the Indian Ocean or Pacific. Between 2010 and 2013 RFA Diligence was refurbished to extend its useful life to 2020. But it is being retired four years early for cost reasons. The Royal Navy cannot afford to send many of the few warships it has left on distant voyages. Thus RFA Diligence is not as essential as it once was.
Many major navies also have self-propelled floating dry dock ships capable of doing even more extensive repairs at sea. In early 2016 the Chinese Navy put its first self-propelled floating dry dock (Huachuan No. 1) into service. Huachuan enables ships of up to 20,000 tons to obtain relatively simple repairs (usually to the hull) that require the use of a dry dock. Normally this would require travelling back to port for the repairs. These floating dry docks were very popular during World War II in the Pacific and the U.S. Navy had over a dozen of them. These were mostly towed, not self-propelled. The U.S. Navy retired the last of these (which mostly handled nuclear sub repairs) in 2001.
There are many commercial floating docks in service, many of them built in China. Like the new Chinese Navy floating dock these vessels (especially Huachuan) include space for workshops and personnel needed to make the repairs on ships and ship components as well.