Hi Martin, or actually in your case here it's not too difficult to repair the messed up offset because the messed up parts can be just deleted and copied from one of the good parts. Also because your pieces are all meeting up smoothly it should be possible to offset the surfaces individually and have their offsets match up with each other.

That's how I build the attached shelled version, I did an offset of just these separated individual surfaces (that is a more simple operation than offsets of joined surfaces which then have a lot of boundary processing to be done) with Distance = 0.5mm and Flip on:



Then I used Transform > Array > Circular to make the other faces from those, joined them all together (needed to scale down by 1/10 in size first before all would join), then trimmed off the bottom piece with a line to make it coplanar to the original part, built a planar surface between the 2 rings and then joined the outside part with the inner part to make the attached shelled result.

So anyway these types of steps can get you a final shelled result when the regular shell operation fails which is not particularly unusual I'm afraid.

Hope that helps!

- Michael