Now, got fiqure strenght of the 3/8" thick spacers
If using steel with psi strength yeild of 40,000.
40,000 divide by diameter 320.85" equals 124. 67 [times .75 [3/8" times 2] equals 93.5 psi.
And the .312" wall thickness 3' pipes:
40,000 divide by diameter 36" equals 1111.1 times .624 equals 693.3 psi. But as part of this larger pipe:
40,000 divide by diameter 320.85" equals 124. 67 times 1.248 equals 155.59
So all this means that 75 psi would barely scrap by as max pressure for fabricated pipe and say around 200 psi for each of the 3' pipes. It would probably good to shoot for about 60 psi max pressure allowing for some safety margin. Now 60 psi will force the air inside the pipe about 130 below the water level, And means that the bottom 100' ft pipe will have +1 atm of seawater pressure compressing against the 60 psi max pressure- so this 100' length of pipe only needs to able to withstand 45 psi. I call this the skirt section, it will not made of the pipe and spacer, but will simply be a single large pipe with same diameter as pipe and spacer at the weld [320.85" in diameter]. 40,000 divide by diameter 320.85" equals 124. 67 [times .50 [1/4" times 2] equals 62.3 psi. Next above the 1/4" thick wall, there will be another 100' section of pipe, this section of pipe can come out of the water so it will have 3/8" thick walls. Since circumference is 84 and each sq' weighs about 15 lb that's 1260 lb per foot of length or 126,000 lbs per 100' And the 1/4" walled pipe is 50% less weight: 84,000 lbs. So for the 200' of length the total weight is 210,000 lbs {105 tons} and this section is replacing the 200' of pipe & spacer which would have weighed 215 tons, which shaves 110 tons off the 1075 tons total given in previous post, so at this point the total is 965 tons.
Now with this reduction in weight I'm getting closer, but once again I've run out of time:)