ISPECTION METHOD

feels like you want some sort of magic (and tell your customer everything is OK) that is what he wants to know.

What is the fluid being carried and what is the length of pipe ?

Since the pipe is with insulation, I assume it has to be steam or any hot liquid. With carbon steel ?

Pipeline inside can be checked with a boroscope - for any corrosion and cracks if any but it will be very time consuming.

There is something of Accoustic Emission done on the pipelines- but with pipeline underground, I don't know- doesnot look like it will give meaningful signal.

With pipeline in live condition even boroscopy is not possible (even if it is short length)

I don't find any of the NDT techniques suitable -

Visual, LPI,MPI,Eddy - ruled out

RT,UT - Ruled out (no approach, underground)

Leak testing - Ruled Out

Accoustic Emission - depending upon, doubtful

Only way is the guided wave Ultrasonic Test - check

http://www.conaminsp.com/downloads/Advanced%20NDT%20Apps/GUL%20164-07.pdf

etc or google "Guided Wave UT"

but these are specialist service and not available everywhere.

if it is over 4" diameter, use a smart pig

Guest is correct, the only way is a smart pig, a clever pig an intelligent pig or as a last resort a very very small inspector.

It goes without saying that the joints and bends in the line must be suitable for a pig. You get into trouble with the wild life crowd if your pig gets stuck.