It was recently revealed that Britain almost lost one of its nuclear attack submarines (SSN) in 2011 when the air conditioning system of the HMS Turbulent failed while the boat was surfaced in the Indian Ocean. Temperatures in the sub went as high as 60 degrees (Celsius, 140 degrees Fahrenheit) before the crew found a solution. Nearly 40 percent of the crew were disabled by the heat and some systems were overheating and shutting down or acting erratically. The boat was at sea and it would take hours before any help could arrive. The cause of the problem was unknown and the temperature outside the boat was over 40 degrees (104 degrees Fahrenheit) and rising.
Not willing to risk losing the sub to the heat related problems the captain ordered the boat to submerge, as at a depth of over 100 meters the cold water would cool the boat. That worked, and gave the engineers time to figure out that the problem was a buildup of crustaceans that blocked the water inlet pipe for the air conditioning. Within 24 hours that problem was fixed and the sub was able to continue without further incident.
Then, in 2009 the Royal Navy revealed that during the previous 21 years it's nuclear submarine suffered about one fire a month. Moreover, the subs suffered two collisions every three years. None of the fires or collisions caused serious damage. Less than ten percent of the fires required more than the sailors in the vicinity to deal with. Three of the fires occurred while the ships were in port. Most of the fires, and all the collisions, occurred while the subs were underway and submerged. That is when the boats are most vulnerable to something going wrong as that is when the most equipment is in use and often under stress. Subs are built to deal with these equipment failures, and in the case of the Royal Navy boats, that worked. The 237 fires and 14 collisions took place on 21 subs (13 SSNs and 8 SSBN) in service during that period.
Then in 2012 one of Britain’s four SSBNs (ballistic missile submarines) suffered a rudder failure after test firing a SLBM (sea launched ballistic missile) off Florida (North America). The sub (the HMS Vanguard) has just undergone a midlife refurbishment that cost over half a billion dollars. After the rudder problem was discovered, the Vanguard entered an American shipyard in nearby Georgia for repairs. The Royal Navy was embarrassed that a sub fresh out of a three year refurbishment could suffer a rudder failure four months later. This was not the first such embarrassment for the Vanguard. The rudder problem three years after the sub collided with a French SSBN while submerged in the mid-Atlantic. The damage to both boats was superficial but it was embarrassing how two SSBNs could have bumped into each other in the middle of an ocean. (Strategypage)