The vessel, one of the UK's four Vanguard Class nuclear submarines, has been officially handed back to the Navy for operational use. Known as a Long Overhaul Period (Refuel), the work package was an enormous engineering task, with the submarine emerging as effectively a new boat inside. Around 2.2 million man-hours were involved in the overhaul under a £350m contract with Babcock Marine at Devonport Dockyard in Plymouth. The work that was carried out included the installation of improved strategic weapons equipment and the integration of the tactical weapons command systems.