the royal navy dockyard consists of fourteen dry docks (docks numbered 1 to 15, but there is no 13 dock), four miles (6 km) of waterfront, twenty-five tidal berths, five basins and an area of 650 acres (2.6 km²). the dockyard employs 2,500 service personnel and civilians, supports circa 400 local firms and contributes approximately 10% to the income of plymouth. it is the base for the nuclear-powered hunter-killer submarines and the main refitting base for all royal navy nuclear submarines. work was completed by carillion in 2002 to build a refitting dock to support the trident missile nuclear ballistic missile submarines. devonport serves as headquarters for the flag officer sea training, which is responsible for the training of all the ships of the navy and royal fleet auxiliary, along with many from foreign naval services. the nuclear submarine refit base was put into special measures in 2013 by the office for nuclear regulation (onr) and it could be 2020 before enhanced monitoring ceases. safety concerns on ageing facilities, stretched resources and increasing demand are blamed for the measures.