Hi submadmans this a example of a bolted flange or frame on a submarine
and an explanation how the building process is..

Three parts 1, 2 and 3

- roll part 1.) the main hull
- cut part 2.) from the main hull
- make part 3 from a plate without the bolts holes and 
  about some milimeters stronger as needed for strength. 
- weld part 2.) with part 3.) laying in horziontal position, part 
  3 will get a little out of direction during the weld process..
- put part 2/3 in a lathe and make the surface of part 3 flat  
  and if the seal is a o-ring design make the trench also, if it is    
just a flat gasket design make the surface just flat. 
- drill the holes for the bolts. 
- weld part 2/3 to the part 1
- make the same on the other side of the hull 

In the depths the waterpressure will press both parts together 
- the bolts are just required on the surface. 

The frame on the picture is made from normal steel - for a 
smaller sub it should be made from stainless if you can pay the bill..

The frame on the picture has a diameter of 2500 mm or 8,2 feet
the force of the waterpressure in longitundinal direction to the 
seal in the workdeep of 250 meter (820 feet) is just : 1260 metric
tons.. 

regards Carsten