Re: [PSUBS-MAILIST] Lengthwise internal supports

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

Carsten - if I understand you correctly, you rolled I-beam sections into 
circular form, and then welded these in place by means of fillet welds at the 
edges of the flange in contact with the pressure hull?

An interesting idea, but I am concerned about the fatigue sensitivity of that 
detail, since by creating a partial penetration fillet weld you essentially 
introduce an artificial crack in the joint.

Every project I have worked on (mostly ROVs and heavy mining machinery) has 
entailed the application of full penetration welds where a prepared joint (V-
chamfer or similar) is welded from one side, then the weld is backgouged from 
the opposite side into the root pass from the first weld, then the weld is 
completed from that side, ensuring full penetration with no voids.  Further 
refinement can include profile grinding to a smooth fillet, which eliminates 
surface stress concentrations and inclusions at the bead surface, and finally 
whole-assembly stress relief.

Granted, the mining machinery is subjected to higher loads and more cyclic 
loading than a sub will ever see, but I wonder if it would be prudent to 
design for full penetration welding of any joint directly connected to the 
main pressure hull?  Does ABS have any guidelines regarding this?  PVHO?

In your specific case, I think that a crack in that weld would probably just 
lead to separation of the stiffening ring from the pressure hull surface, 
which is probably not a big deal since the ring is loaded in compression.  If 
there is, however, any chance of such a crack propagating into the main 
pressure hull, then this issue needs to be considered.

Is fatigue analysis a necessary part of PSUB design, or are PSUBS generally 
subject to so few loading cycles in their lifetime that it is sufficient to 
consider strength only?

I don't know, I am just thinking out loud (in print?).

-Sean


Quoting Carsten Standfuss <Merlinsub@t-online.de>:

> Hi Cliff, 
> 
> as some test has shown the lengthwise internal supports will not work
> properly. 
> 
> But you can use rolled H-Profile instead of T -Frames. H-Profiles are
> much easier to roll than T-Frames and there is no need to weld them
> together. U-Frames will also work this way. 
> 
> http://www.prismnet.com/~moki/20030115.120916/113-1333a.JPG
> 
> Regards Carsten





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