If I read your question correctly
API 1104 would be what you are asking for.
There is also ASME. sect IX.
I do belive the choice of codes is yours. Might not hurt to see if there are any pre-existing contracts between your company and the contractor defining which standard to use.
I suggest you consult an independent testing Lab. They most likely have the answers you need for your application or will point you in the right direction.
Let me clear up a couple of things.
ASME/ANSI B.31 (formerly ANSI B.31, ASME entered the picture long later) is the "Code for Pressure Piping", and is divided into 8 setions. Each section is related to a different kind of piping. Secion 1, for example, is for Power Piping. Section 3 for Chemical Plant and Petroleum Refinery. Section 4 for Oil and Oil Products Transportation. Section 8 for Gas Transmission and Distribution.
The B.31 Code speaks of DESIGN AND CALCULATION of piping and piping systems, leaving the welders qualification procedures to the applicable Code or Standard. In the case of oil, oil products and gas pipelines, it would be API 1104, and in all other cases ASME IX.
Now, as part of the design of piping systems, the B.31 Code treats also of quality control procedures, including acceptability criteria of welds radiographs.
Giovanni S. Crisi
Sao Paulo - Brazil