VIGILENT Back - the Strategic Submarine VIGILENT sailed from Plymouth in late March 2012, after nearly three and a half years undergoing work to replace her nuclear fuel and to upgrade key areas.   Taken in hand by Babcock in 2008 this was the third Vanguard Class submarine (of the four) to undergo a long overhaul period and refuel at Devonport.    The submarine moved out of dry dock in November 2011 to conduct final reactor commissioning and crew training prior to departure for sea trials in April 2012.   As one of the MoD's most complex projects, costing over £ 300m and consuming 2.5 million man hours the “boat” has been refuelled using a Rolls-Royce designed upgrade to her nuclear reactor which will power the submarine to the end of her life.   Additionally, around 80 major design improvements have been incorporated, including significant enhancements to strategic and combat weapon systems and sensors that will ensure the submarine remains highly capable to meet the exacting demands that will be placed on her during her next commission and fulfil her vital role in national defence.    The comprehensive sea trials programme, is scheduled to be completed by June this year, though this will be followed by intensive sea training by the staff of Flag Officer Sea Training to forge the crew and submarine into a single operational team.