Brian,
I would recommend having the flange pieces of the "T" stiffeners
rolled if they are 1/4" thick to get a uniform radius. Yes, my hull
is 30 inch and the ID of my hatch is 21.5" . As to the inner
stiffener being changed from 10 inches to 12 inches without impacting the
ultimate depth rating, this has to do with which failure mode is giving the minimum
allowable working pressure. The ABS calculation for stiffened cylindrical
sections has five different minimum allowable working pressure valves, each of
these corresponding to a different failure mode. For your design case,
this means your inner stiffener spacing is changing the MAWP for one or more of
the failure modes, but not the failure mode that is giving the minimum MAWP for
your design case. BTW, make sure you are changing the number of circumferential
lobes for failure (line 27). When you change this value from 2 to 3
to 4 ..., you will see that one of the MAWP will start out with a large value
then get smaller than larger and larger. Pick the value that gives the
lowest MAWP for this section of the spreadsheet.
Cliff