Safety factors are in terms of the yield stress. So, if they had a safety of factor of 3, this means that they should be able to withstand 3x the expected stres

Safety factors are in terms of the yield stress. So, if they had a safety of factor of 3, this means that they should be able to withstand 3x the expected stress. Pressure is linearly related to depth, so it should have been designed for 12,000 meters with a reasonable safety factor like that. Of course, this doesn't take in to account sharp impacts, which, especially for carbon fiber, dramatically change the strength requirements. Some ballpark typical numbers for safety factor: spacecraft ~1.2, airplanes ~1.5, automobiles ~3