ballast tanks won’t be on the top of the hull in either single or double hull submarines. if it were it would be difficult to flood quickly to dive the boat quickly, and it would reduce the boat’s stability when underwater.
on Every submarine, the ballast tanks are situated so that they will be almost completely underwater even when the submarine is fully surfaced. this allows the ballast tanks to be flooded quickly by simply opening doors on the bottom of it if the submarine has to dive quickly. It also allow the ballast tanks to provide buoyancy to the submarine even on the surface when it is fully blown
On modern single hull nuclear submarines the ballast tanks are between the hydrodynamic bow snd stern casings and the pressure hull. The fact that forward ballast tank is At the front end of the boat, and below water even when the sun rains at the surface, is how so many modern submarines can get away with mounting their forward dive planes above water when the submarine is on the surface
Ballast tanks should wrap around the sub on front, center and tail sections.
The top should free flood.
The forward ballast tank would be between the bow and the front of the pressure hull which is exactly indicated by the thick limber hole line in this photograph.
The hump would then be a free flood area, with all the holes allowing the water to get in and drain out.
The back of the submarine you have this other limber line here which are the holes for the rear ballast tank. There are going to be holes on the bottom where the water comes in, while the holes on the top are where the air gets out.
There is second limber line on the center of the submarine underneath the drainage holes for the hump, and that might be for the center ballast tank. In that sense, the 094 already looks like a hybrid. The 093A also has the forward, middle and aft limber lines that indicates where the ballast tanks are.