Hi Lars - that's getting to be a quite awfully dense surface at 13 Mb just for that one shape.

That is probably too dense, the offset surface is usually going to be denser than the original surface so if you try to offset such a complex thing it is going to really be too much.

I'd try the original one that you had but trying to true up the seam by using MakePeriodic in Rhino.

- Michael