Wikipedia
SUBSAFE is a quality assurance program of the United States Navy designed to maintain the safety of the nuclear submarine fleet; specifically, to provide maximum reasonable assurance that subs' hulls will stay watertight, and that they can recover from unanticipated flooding.
SUBSAFE covers all systems exposed to sea pressure or critical to flooding recovery. All work done and all materials used on those systems are tightly controlled to ensure the material used in their assembly as well as the methods of assembly, maintenance, and testing are correct. They require certification with traceable quality evidence which track the item from the point of manufacture (including all records of the creation of the product, i.e. source materials as well as smelting and hardening process for metals) to the point of installation within a SUBSAFE boundary. These measures increase the cost of submarine construction and maintenance.
SUBSAFE addresses only flooding; mission assurance is not a concern, simply a side benefit. Other safety programs and organizations regulate such things as fire safety, weapons systems safety, and nuclear reactor systems safety.
From 1915 to 1963, the United States Navy lost 16 submarines to non-combat related causes. Since SUBSAFE began in 1963, only one submarine, the non-SUBSAFE-certified , has been lost.