You look like to use turning rolls for welding.
I am using V groove for 13mm thick with FCAW with mixed gas for piping shop.
For your situation, I would like to recommend as follows:
1) use V groove with 50-60 degree to get better penetration
=== you need more time for welding but less time for edge preparation and possibly no preheating
2) you need 4-6 passes
3) for cap passes, use relatively high voltage to get small toe angle
4) consider weaving unit for better penetration and bead shape