And that's just compounded by the low availability of breathable air.
Actually, you're close. Underway (that means out to sea) subs purposefully keep their oxygen levels low - very low. So low that a cigarette will immediately extinguish when the smoker is not inhaling. It must be re-lit before each puff.
But that's not important. The important part is that whatever is attempting to catch fire would smoulder for a bit before flaming up - thereby catching the eye/ear/nose of the watch or any other passing crew member.
In port, oxygen levels are normal to the atmospheric oxygen levels of the surrounding city. (By the way, Norfolk, VA smells bad. - Norfolk sub sailors know what I'm talking about. ;P )
This is not correct. Actually the atmosphere in the boat while underway is 19-21% oxygen. Atmospheric oxygen levels are around 20% worldwide. The atmosphere on the ship is no different in any way from the atmosphere that you are currently in most likely (gas wise). What IS different is the pressure which changes with the depth the ship is at, the amount of time since the ship has "equalized pressure" by putting the exaust mast up, and the amount of people on the ship (you all breathe out more than you breathe in).
And this is where *your* boat took chances. Our boat kept the oxygen levels at about 13% to 15%. Yes, you read that correctly. Again, the smokers had to inhale *deeply* while attempting to light their cigarettes so they could create enough air-draw across the surface of their *lighters* to get the lighter to even light so they could light their cigarette. Low oxygen levels starve fires.
You say you've been in 3 fires and they were extinguished within 10 minutes? I'm very glad you did. However, wow. Amazing. How many captains did your boat(s) go through? While we had our own scares we only had 1 real fire on board while underway and it was nothing more than a smoking rag. Someone left it on top of the CO2 Candle where it began to smoke. It was amazing. I was one of the few who showed up in an EAB. Three guys showed up in their skivvies. People were on it *instantly*.
The only other time we had a near miss (and the scariest moment of my life, hands down) was when our 4500lbs Hydraulic line ruptured in the engine room. It was spraying 4500lbs PSI hydraulic fluid into the engine room. If the roving watch underway hadn't been standing *right* next to the kill switch when it ruptured I might not be here today. We surfaced and remained surfaced for 3 days drawing circles on nav charts in sea state 3 to sea state 4 seas. If the oxygen levels were any higher AND (I stress AND) the fluid would have sprayed at 4500psi for more than 30 seconds, it would have been a flame-thrower.
It's purely up to the CO on what level of O2 he wants the boat to run around at. Maybe they've enacted some regulation since I got out in late 1996; but, don't sit there and say I'm not correct. Certainly, in port the ship's O2 levels are in keeping with the surround local atmosphere - ~20%. Our boat kept O2 levels low purposefully under-weigh.
But maybe they had problems getting the people out first. Subs don't have too many doors on them, and if the fire is between 25 crew and the door and there's no other route, sealing off isn't an option.
I find it hard to come to a conclusion where this would become a problem. There are multiple exits in most areas that are 'dead ends'. There'd have to be a pretty messed up situation that prevented ~25 people from escaping a location without them trying the emergency route *before* the emergency route became blocked.
Okay so oddly enough there isnt really an "emergency route" on the ship for reasons that I wont detail here (it would take too long to explain). I do have to say that there are contingency plans in place for this kind of thing under normal in-port and at-sea times. But more importantly, the question asks why wasnt the hatch just shut? The answer is pretty simple actually. When the ship went into dry dock, they most likely removed the hatch all together. In other words, even if they used the "suffocate the fire" approach, it was probably done by putting a large piece of steel or other object over the place where the hatch would normally be. That would mean that more than likely what ever they used to suffocate it, was manufactured on location within the period of time that they state that the fire was burning.
Why remove the hatch? Because the hatch is only so big around which limits the size of the equipment and items that you can get on board in one piece. Try fitting a diesel engine through the hatch only wide enough to allow a MK-48 torpeedo to fit through. Square peg in a round hole kind of thing.
As a submarine sailor (nine years total, onboard the USS Ohio (SSBN 726), USS Pittsburgh (SSN 720) and USS James K. Polk (SSN 645))...
The goat locker (Senior enlisted quarters) on our boat (SSN-702) had a hatch that allowed personnel to escape from the chief's quarters to the torpedo room. The officers quarters bathroom had a panel that could be removed so they could escape into middle-level passage if their door was blocked. 21-man and 9-man berthing quarters were the only places that I know of where you were blocked in if the main door was blocked - oh, and the sonar array access tube.
USS Phoenix SSN-702