FAQ on Failures, Part Two: USS Thresher Failure
John Tauxe, Timothy R. Reeves, and Richard H. Rustad
Reference: Ballard, R. and M. McConnell Explorations: A Life of Underwater Adventure. Hyperion Books, 1998.
A ballast blow valve failure was involved, as was a reactor scram due to wetting of certain electrical systems (i.e. there were multiple failures), and I think more than 24 people died (all aboard died, and you don't take a nuclear submarine to sea with only 24 people aboard). This incident led to certification of critical components (SUBSAFE certification), and in the changing of certain emergency procedures. [Reeves]
[...] Bob Ballard's book "Explorations" [...] begins with an account of the loss of the Thresher. According to him, they were at a depth of about 1000' (if I recall correctly) and were moving very slowly (a couple of knots) when the fitting blew. This caused a reactor scram and subsequent loss of power. At such a depth, blowing ballast tanks does not work, since the pressure causes the blowing ports to ice over. The only way for the sub to reach the surface is to "power" its way up. Ordinarily you'd just trim the bow planes up and apply power, but in this case, they had to hope that they could trim up the bow with just their momentum, since it would be some fifteen minutes before they could get the reactor back on line. The bow planes were trimmed up, but it caused the sub to stall, just like an airplane. The bow went up a little, but forward momentum was lost. With no power, the sub slowly slipped backwards and down until it passed its crush depth of 1500', then [...] she imploded and quickly drifted to the bottom in 8000' (I think) depth. [Tauxe]
The emergency blow system will work whether on the surface or at crush depth. The reason is that the emergency blow air banks are at a significantly higher pressure than sea pressure. However, posters are correct when they say that the emergency blow is less efficient at great depths. Not only does it take longer to blow out sea water due to the greater back pressure, but depending on the size of the air banks, not all of the ballast tank volume will be displaced by air. Having some forward momentum is therefore a bonus because the ship can complement the added buoyancy by driving to the surface. Will a sub at all stop reach the surface from great depth with just an emergency blow? Yes, but it will take a while. However, if there is flooding, forward momentum becomes crucial to ensuring survivability of the ship. As the ship gets closer to the surface, more sea water is displaced from the ballast tanks simply due to reducing the back pressure.
The pressure drop from the emergency air banks to the ballast tanks is enough to cause ice formation and plug emergency blow lines -- such was the case with the Thresher. Two changes were made to ensure this never happened again. First, the size of the emergency blow lines was increased dramatically. Second, the air in the emergency air banks is kept as dry as possible by eliminating as much moisture as possible when charging the banks, and by periodically blowing down the tanks. [Rustad]
