On March 28th the U.S. Navy decommissioned one of its nuclear submarines; the USS Miami (SSN 755). Entering service in 1990, Miami was not supposed to be decommissioned so soon and the navy has been trying to keep the boat in service after the sub was damaged by a shipyard fire in 2012. But no solution worked. In 2013 the navy concluded that recent budget cuts were making it impossible to spend half a billion dollars to repair the fire damage.
It was not for want of trying and some innovative thinking. In late 2012 the navy thought it had a way to keep those Miami repair bills affordable. The plan was to salvage components from the recently (2011) decommissioned USS Memphis. This boat entered service in 1977, 13 years before the Miami. While the Memphis was one of the "original" 31 Los Angeles boats and the Miami is one of the third generation (Improved Los Angeles) designs, both share many common components, especially in the forward part of the boat where Miami suffered most of its fire damage.
The 2012 Miami blaze occurred while the sub was in the Portsmouth (Maine) Naval Yard for maintenance and upgrades. Initially the navy estimated that the sub suffered $400 million in damage. But a more detailed examination revealed that it would cost at least $450 million and probably north of half a billion to fix the sub. The navy thought cannibalizing the similar Memphis would help, but the savings were not sufficient and the budget cuts forced the navy to prioritize. Getting an elderly and burned-out SSN back into fighting trim no longer looked like a good investment.
The USS Miami fire (set by a deranged shipyard worker) took place in May 2012, and early on there were fears that the 22 year old Miami might be scrapped because of the high cost of repairs. It's not just fires that these old Los Angeles class boats have to worry about. In 2009 a 25mm (one inch) hairline crack was found on the pressure hull of the (then) 14 year old USS Toledo. The crack was in the metal plate, not a weld, which was replaced. Above the crack there was a 53 cm (21 inch) hairline crack in the outer (non-pressurized) hull, which was under the sail. The USS Toledo had just undergone a three year refit, costing $179 million, when these cracks were discovered. The sub was sent to a nearby (to New London, Connecticut) shipyard for repairs. At first it was thought some of these cracks were related to a recent scandal where shipyard workers failed to check for substandard welds but that was not the case with the Toledo. The navy began having doubts about the reliability of its aging fleet of Los Angeles class subs.
Such a crack in the pressure hull is a serious problem because it makes it more likely that the pressure hull would fail and flood the boat, at less than the "test depth" (about two thirds the "design depth," which is the maximum depth the sub can operate at). Going a little deeper gets you to the collapse (or "crush") depth, at which the pressure hull is crushed and implodes. The deepest diving U.S. subs, the Seawolf class, are believed to have a test depth of 490 meters (1,600 feet) and a collapse depth of 730 meters (2,400 feet). During World War II collapse depths were never more than 300 meters (a thousand feet). Since then, larger boats, built of stronger metals, have greatly increased the depth subs can operate at. But that only works if the crew knows the limits of their boats and discovering cracks in the pressure hull reduces those limits. Using Memphis components for the Miami repairs would have made possible close inspection of those items, to see if there was any more unexpected deterioration. The Miami hull has already been checked for any damage from the fire and none was found. But the older a sub gets, the more likely metal fatigue and years at sea will cause undiscovered weaknesses.
Both the Miami and the Toledo were among the latest "improved Los Angeles" boats. With the Miami being retired, a much older (in terms of technology) boat may have to delay retirement and fill in. Upgrading one of these older boats would also be expensive. If the repairs for the Miami did indeed cost less than half a billion, then it was worth keeping the Miami in service. But as much as the navy wants to maintain the size of its SSN force, there is no major threat for these SSNs in the foreseeable future. Retiring the Miami a decade early will not be a major loss.
The navy is putting most of its cash into building new Virginia class boats to replace the 40 (of 62) remaining Los Angeles subs. The most recent Los Angeles boats entered service in 1996, and will be gone by the end of the 2020s. Ten Virginias are in service and another 20 are planned. If the navy can scrounge up enough cash it can build two a year and they can have all the Virginias in service before the Los Angeles class is gone. Otherwise, the SSN fleet will shrink because additional old Los Angeles subs will be retiring compared to new Virginia's entering service.