OK I'll bite.
I found nothing in API 650 either, but obviously you would need a bw annular plate. The breakdown would be impractical past 3/8" and the three plate laps would be a challenge for an earthling. I also checked Euronorm and found nothing. AWWA D100-05, however, Section5.9.4.5 states "Bottom plates under the sidewall that are thicker than 3/8" shall be butt welded.
Great question.