Pls read again 7.3.2.1 of API650 .( Complete penetration and complete fusion are required for welds joining shell plates to shell plates. Inspection for the quality of the welds shall be made using either the radiographic method specified in 8.1 or alternatively, by agreement between the Purchaser and the Manufacturer, using the ultrasonic method specified in 8.3.1 (see Appendix U)...)
Look to the app .U again...for the butt welds of shell to shell vertical , Shell to shell horizontal ,“T” joints between shell vertical/horizontal, butt welds of Nozzle/manway.....RT is mandatory req. as per the standard.