UT in Place of RT to Test Pipe Weld Joints

Radiography Testing of more than 500 pip weld joints is to be done as per contract and is taking a lot of time. Can we adopt Ultrasonic Testing in place of RT? Following are the details of the weld joints:

1) Line sizes : 6" to 28"

2) Pipe materials : CS and SS

3) Type of welded joints : Butt welded and fillet welded joints

4) Service fluids : Nitrogen, Argon, Oxygen, Air & Water

5) Working pressures : 1 barg to 52 barg

6) Working temperatures : + 40 Deg C to -196 Deg C

Kindly give your views on

a) Whether UT is acceptable in place of RT.

b) Any prerequisites for UT.

c) Any limitations of UT.