We as RTD have experience with al these techniques we booked good results with the eddy current system to detect corrosion under isolation and we have also have digital RT available to detect CUI there is also an other solution to detect CUI that is with long range ultrasonic.
for extra information about these techniques check our website.
www.rtd-group.com

I have been through extensive CUI campaigns on our offshore platforms over the past 10 years and the reliability of NDT techniques to detect CUI is questionable.  

The radiographic techniques are very good, but time consuming, expensive and have their own inherent safety issues.

Long range ultrasonics is useful for long straight lengths of pipework, not good for short sections with numerous welded branches, pipe supports or numerous elbows/changes of direction.  The sound energy is also attenuated by wet or damp insulation.

Pulsed Eddy Current has proven useful in pipe diameters above 8", but only for areas of general corrosion, not good for localised corrosion.

Has the insulation ever been removed/replace in the past during any planned routine Fabric Maintenance Programme?

You still have to get there for any NDE and if you scaffold and do advanced inspection you can trim the work scope down. This is the most economical way to be sure. I always say seeing is believing, and while I am looking at the worst suspect areas I am also fixing them, this should be in every maintenance system. We spend 4.5 Mil a year on this. Find an experienced Inspection team to be your eyes.