Also, I should have added that this pipe has a wall thickness, which varies in the "bulge" section. I again have an equation describing this variation and have drawn a set of offset circles outside of the inner circles in order to represent and inner and outer wall.
Therefore once I have my inner and outer surfaces created I need to fill them in to create the solid wall thickness region.
Unfortunately I have very limited experience of drawing in 3D, as up until now I have always been able to stay in the comfort of 2 dimensions!
I'm using AutoCAD 2004 Full Version.