Re: [PSUBS-MAILIST] Lengthwise internal supports

["Sean T. Stevenson" <cast55@telus.net>]

Carsten - if I understand correctly from your images, you used an I-beam which 
you rolled into circular form, and then welded via fillet welds at the edges 
of the beam flange in contact with the pressure hull?

I was wondering if it was necessary to have full penetration welds on all 
pressure hull features, which this design would not permit?  Fillet welding 
a "T" joint without full penetration creates an artificial crack within the 
joint which is a fatigue sensitive detail.  Now - if such a weld failed, the 
crack would probably just separate the stiffening ring from the pressure hull, 
which is probably not a huge problem since it is loaded in compression, but I 
wonder if such a crack could propagate into the main pressure hull?

Every project I have been involved with at work (typically mining equipment) 
necessitates full penetration welds - i.e. weld from one side, then backgouge 
from other side into root pass from first weld and complete weld joint from 
there, thereby ensuring full penetration with no voids.  Further refinement 
can include profile grinding the weld into a smooth fillet to eliminate 
geometry artifacts (surface stress concentrations) or inclusions at the bead 
surface, and subsequently stress relieving the welded assembly.

Granted, the mining machinery is subjected to many more load cycles than a sub 
would ever see, but I wonder if it would be prudent to design for full 
penetration welds on all parts that are directly connected to the pressure 
hull?  Does ABS or the PVHO standard require this?

Is a fatigue analysis a necessary part of PSUB design, or is the lifetime of a 
PSUB short enough that we can design for strength only and ignore fatigue 
analysis since the lifetime number of load cycles on a PSUB is limited?

I don't know - I'm just thinking out loud (in print?).

-Sean





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