Ansys Employee

It's not too complex, the issue tends to be cell  count: you've got a (usually) 6" bore that's about 8000 ft long. With 10 cells across the diameter and aspect ratios of 5 the cells are only 5-6" long. 

As Keyur suggests 2d is probably a good start, equally, 3d is also fine but you may want to reduce the domain size. I'd also be careful with the facet tolerance, and turn the Facet Quality (DesignModeler -> Tools -> Options -> Graphics up to maximum.