Hi Rob, I think Steve's got you covered there - if you zoom in to that top area really closely like he shows there is a small area of weirdness right in that "pole" area where the surface kind of folds back over itself.

That kind of a thing can tend to happen when you've got a lot of shapes converging together and trying to collapse down to a single "pole" point.

Once you've got a micro fold in a surface, offsetting or shelling will become problematic because they try to make a surface that follows the surface normal, and the surface normal flips around very wildly and kind of violently in that little area.

When you do run into a problem with Shell, it can help to try Offset like Steve did there, because it will tend to generate the surface even though it is messed up and you can see what spot is messed up.

It tends to be more Offset/Shell friendly to try and construct things without poles when it is possible - often times you can instead build a larger more rectangular kind of sheet and then trim it to make the final outer boundary, rather than trying to construct a surface that hugs it more directly (which tends to cause bunching/compression in the surface in cases like this).

So for example here you could take a curve like this:



Sweep it to make a rectangular base surface:



Then Trim that surface with your other bounding curves:





Something created in that way where the outer boundary is a trim curve rather than the underlying surface itself trying to be stretched and compressed can be handled a lot better by a variety of further operations, especially Offset/Shell.

- Michael