Hi JTB, looks like a bug in the surface offset mechanism in the geometry library.

I'll take a look and see what if I can find out what is involved in the bug, but for the moment a quick workaround to get it finished now is to split it into 2 halves and use "ShrinkTrimmedSrf" (this has to be set up on a keyboard shortcut, there is not a button for it), then it can be shelled. I've attached a result.

- Michael