There are a few problems. First of all you only placed points on the surface of the cylinder. This results in a lot of cells meeting somewhere on the axis of the cylinder. The cells have short edges which cause a lot of problematic intersections.
Second problem: duplicate edges. Every pipe was created twice.
What you could do: Add some points inside the cylinder. It leads to more uniform cell size:
I also used a slightly larger bounding box for the voronoi since it also improves the intersection. It creates cleaner intersections since the two sets of geometry overlap and never coincide.
You can remove duplicate curves, vertical curves and short segments and then finally create all pipeswith round ends. Instead of trying to solid union the NURBS pipes, create a single mesh and try to print this. A lot of 3D printing softwares can handle overlapping situations and there is no need to boolean everything into a single solid, unless you have other plans with it…
