Pipe welders are skilled enough to weld most structural, but many D1.1 welders do not have the skills necessary to pass a B31.1 test. Sooo......... to answer your question, you need to be able to pass a 6G GTAW root and hot with SMAW fill and cap. If you're able to make acceptable roots with GTAW 99.999% of the time ( on carbon and stainless ), capping with GTAW will be a walk in the park.  When you can master 7018 caps on 6G 2" sch 80, then you'll be runnin with the BIG DOGS!
Good Luck.