A nuclear-armed Vanguard submarine went into an unplanned dive as it was preparing to go on patrol, when a faulty depth gauge incorrectly led the control room to believe the submarine at a safe depth. The submarine was reportedly approaching maximum safe depth when engineers who had access to a separate working depth gauge contacted the control room.
The incident was initially reported by the Sun Newspaper, which claimed that the submarine was approaching “crush depth” at the time the error was discovered. Submarine hulls are designed to withstand the pressure of sea water up to a certain depth. They typically undergo test dives which will demonstrate their ability to operate safely at depths some way short of their theoretical limit, but are expected to usually remain at shallower depths than their ‘crush depth’. The report does not explicitly say how deep the submarine was in relation to these limits, but the submarine was said to be “entering the ‘danger zone'”, and that “the sub was still at a depth where we know it can operate, but if it ever has to go that deep the whole crew is piped to action-stations”.
Although there might be expected to be multiple instruments displaying depth within a Vanguard control room, the incident suggests that the point of failure was a single common sensor, and that the depth gauge visible to the engineers relied on a separate sensor that did not fail. It is common for multiple redundant systems to be designed into submarines to minimise the potential risk from failures. However, the incident illustrates one potential risk that arises from running the Vanguard fleet beyond their initially designed service life.
It is not clear whether the incident, which apparently happened more than a year before the report, led to the submarine returning to Faslane instead of going on patrol as planned. There have been numerous examples of extended Vanguard patrols over recent years.