So, something like:

50' pipe 1" walls 40' diameter aluminum: 88,239 lbs
400' length 3/4" Aluminum walls 40' diameter: 529,584 lbs
400' length 1/2" Aluminum walls 40' diameter: 353,056 lbs
400' length 3/8" Aluminum walls 40' diameter: 235,371 lbs
The 50' pipe with 1" walls will be a pressurized structure,
it's two ends will be about equal to 2" thick wall of Aluminum.
With 2 40' diameter disc 2" thick Aluminum: 70,591 lb.

This 50' length pipe structure will also inner pressure vessel which is 10' in diameter and made of 1/2" thick aluminum walls:
50' length pipe 1/2 walls 10' diameter: 11,030 lbs
Total mass 1,196,632

The yield strength of the aluminum will considered be 40,000 psi or 250 MPa

Aluminum alloy 2014-T6 has 400 MPa [about 64,000 psi]
http://en.wikipedia.org/wiki/Yield_(engineering)

So, the 50' length 1" wall can hold 166 psi but it's max operating psi will be around 120 psi. The inner container- 10' diameter 1/2 walls can hold 333 psig. It's operating psi will be 250 psig- 250 psi higher than outside vessel- so max operating pressure of 370 psi.

The purpose of this pressurize air is not to power the Pipelauncher, but rather to provide a means of delivering enough air in a very short time period. The pipelauncher is a heat engine- there will be burning involved.

Access for maintenance purposes could from the "flight deck" thru the larger pressurized vessel [assuming it isn't pressurize at this time].
One could use hand holds, stairs, or if you want to be fancy, an elevator.
You would have hatches that went thru both of those 2" thick discs.
You need to reinforce around it and have a sealing mechanism or "airlocks".

So, gross mass of pipelauncher without a rocket is about 1.2 million lbs.

Sea water weighs 64.08 lbs. Divide 1.2 million by 64.08 lbs you get 18727 cubic feet of sea water you need to displace in order for this to float.
The area of 40' diameter is 1256.6 sq ft, 10' high by 40' diameter is 12,566 cubic ft.
The volume of the 3 aluminum 400' sections of pipe is:
6634.5 cubic ft solid aluminum.
So if had enough air pressure inside the pipe to push the
sea water 10' below the surface level, the pipelauncher floats.
For every 33' depth there is 14.7 psi [1 atm].
So if the air pressure inside the pipe was 4.45
psi above sea level pressure, it would push the water down by 10'.
So with this the pipelauncher would be vertical and the top [or flight deck] would 50' above the water.

Now, if you increase to pressure inside the pipe from 4.45 psi to 10 psi what happens? It depends on how quickly this is done. If done instantaneously the water inside could go down somewhere around 20' and this forces the
pipelauncher to go up, it will go up until the volume is increase so it's at 4.45 psi again- so pipelauncher bounce up and the water will bounce down. The water
will end up at around the same level it was at before. And a pipelauncher will end up floating a little bit higher.
Btw regardless of how quickly the air pressure is increased
if you were to apply force to prevent the pipelauncher from going up, it require around a million lbs weight or force to stop it.

For purpose of determining acceleration, the 6 gees needed, we will ignore the displacement of the pipes: 6634.5 cubic ft.
It's mass whether floating or not and it's complicated as more pipe weight gets above the water. So, to accelerate 1.2 million lbs by 6 gees, sea level must be pushed down by 12,566 cubic ft times 6.
Which is 77396 cubic ft, which is pushing the water down 60 more feet.
So as long as the water is pushed down 70' under the sea level the pipelaucher will accelerate at 6 gees. So 70' requires 31.2 psi of air pressure.

Of course we haven't added the rocket yet, but the rocket weighted 1.2 million lbs which is pretty big rocket- for 6 gees the water need to pushed down to 140' and you need a constant 62.4 psi of air pressure to maintain 6 gees.
And since the pipelauncher is going up exponentially you will need an exponential amount of volume air at 62.4 psi for that 2 seconds.

The first 400' pipe has 3/4" walls have yield strength of 125 psi
The second 400' has 1/2" walls have yield strength of 83.3 psi
The last 400' has 3/8" walls which have yield strength of 62.5