Here's the long version
There are many circumstances around this.
What we know:
1. She was cut in two, when they replaced Diesel engine/generator and batteries. After the engine refit, she was welded together again.
- A weld on a submarine hull have to be 100% perfect. Are there a minor defect in an weld, you can get problems.
2. Electric short circuit on some of her batteries.
- She have two main banks of batteries, so if one goes, should be a big problem.
- could the electric problem she experienced also caused problems with some off here pumps, eventually freeze over. (something that was put up as an alternative theory of USS Thresher, when it sunk and imploded)
- Gas from batteries that have experienced problems, and not been vented out probably (because a bad sensor for example). build up of that gas in a room full of batteries with problems...
- Can the short circuit made other type of damage?
3. Reports of mechanical problems.
- are an mechanical problems a pump, how would an short on the batteries affect pumps that are already worn out.
4. Crew members talk about that there is problem with the submarine, long before the accident.
- what are those problems? bad valves, old torpedoes, maintenance problems
5. The Submarine is 32+ years old.
- past it's hull life, without proper maintenance you can get problems.
"when Norway did a reift on there Ula class between 2012-2017, they used over 50 million dollar. in two batches. among the refit for the 4 last submarine that have to be operational until new 212CD class are build, was total removal of the outer hull, basicly a total stripdown of the submarine, just to get access to the pressure hull. then the entire pressure hull was scanned tested and re-welded in many location, including replacement of several valves. they was also striped back to original state internally, and rebuild internally. All this to get 10+ year out of them, until new submarine are operational"
6. The Argentinian military is quit underfunded and struggle with keeping equipment maintained.
- not so long ago, a Argentinian Navy vessel sunk at dock, and the military used 2 years to get it up.
- Refit of the submarine was slowed down due to not enough funding for the refit.
There are so many circumstances, that when they find here and start the investigation. They will most likely find many things that contributed to the event ARA San Juan have been trough.
My short list would be:
- defect batteries, temporary fixed before they dived again.
- hydrogen build up in battery hold
- spark on hydrogen batteries
- sever damage to either aft or front pressure hull
- if front, torpedoes with low maintenance... unstable
- no electricity to operate valves
- valves perhaps frozen or stuck, lock by an electric locking mechanism (mechanical problems)
- Can't emergency blow
- no power or not enough power for propulsion.
- sink to a new crush depth, due to bad maintenance. (the crush depth would not be around 800-900 meter but less, due to not good enough maintenance (crush dept: German built submarine have often there crush depth of the double of there test dive depth)) (not a submarine that are deeper than a certain point, and have no power. a emergency blow of the ballast, would not be enough to help the submarine up, due to the pressure on the hull from before)