Dear all,

Apart from radiograhpy ...how do we decide Ultrasonic test , Magnetic particle test, Dye pentrante tests or any other if applicable, for welds (Fillet or Grooved or both) in category D, Skirts, base rings, skirt to head, lifting lugs, saddles, etc
Is there any mandatory requirements in ASME to follow. if not what are the suggested practices...?

You need to read the AWS D1.1 Structural Welding Code, since the assembly/welding of the skirt is not regulated by the ASME VIII, except for the welding to the pressure components, like head and skirt, lug to the head/shell. For these welds, you could try to read the UW, particularly the inspection and testing requirements.
cheers,
gr2vessels