Bizzare OffsetSrf

I am trying to represent a riveted angle on the side of ship hull. I have used offsetsrf to set the limits of the angle and did sweep2 to get the inside face. That is what I have here:

problem offset.3dm.zip (152.3 KB)

If I try to do an offsetsrf 3/8” I get caca galore. I don’t see what would be especially challenging here.

zooming in at (x)
shows a piece of abstract line-art:

clean up those small surface stripes.
delete them and use matchSrf to fit the surfaces.

and as you have 4 none-tangential surfaces connected in one point - offset with corner = round is the only option:

check the following file - be carful it s not 100% the geometry you posted !
problem offset_tp.3dm (632.1 KB)

kind regards - tom