Coated Pipes

From your description the pipes are now no longer capable of supporting their design expectations. If the flaking and chalking has progressed from 3.2mm (original) to 2.7mm, then the true degradation will be substantially deeper.

The pipes might be "disguised" to work by wrapping or some other technique, but this will not restore the pipes to their capability.

The thing with pipes is that they are expected to perform for very long periods of time with no service or activity on them.

If UV exposure has happened, then this would only be one side of the pipe. Hoop stress will thus be concentrated into that zone, fatigue failure will then be localised to that area and progress at a faster rate. The pipes might last 10 years or 10 weeks or 10 days, while their design life would be more like 100 years.

In my opinion, the pipes are now scrap and should be disposed to someone with an alternate use where the strength is not important and replacement new pipe obtained.

You should have said that to begin with! So what is the pipe itself made of, what is its diameter and wall thickness, what is the protective coating intended to protect from, where is it to be installed, what fluid is it to carry, under what pressure, etc.?

Thank you for the clarification. In a way you have answered your own question with the additional information.

If the parent pipe is coated with three layers, then each layer has an expected function.

Epoxy is probably to protect the pipe "forever" once it is burried.

Adhesive is probably meant to ensure the PP is gas tight against the Epoxy.

PP outer layer is probably a mechanical protection so that in laying the epoxy is not damaged in any way.

NOTE: These are my GUESS, not necessarily fact.

Consult with the pipe supplier as to the purpose of each layer and with that understanding you can then make an appropriate decision.

For instance if the PP is a mechanical protection only, then additional wrapping may be acceptable, or even special attention while laying the pipe and specification of different compacted backfill with a smaller particle size that cannot penetrate the PP.

The pipe supplier will be conservative in their response, but remember, once these things are in the ground, they are meant to last. Apply good Engineering practice and you will have no problems. Take the short cut at you peril.

Yes, the stacked pipeline can be used provided the operating pressure of the pipeline is less as compared to the usually designed parameters. Also, galvanic cathodic protection can be provided to prevent further corrosion of steel pipeline and frequent monitoring is required and DCVG survey should be done as and when required (i.e. when pipe to soil potential decreases suddenly at any location) then coating rehabilitation should be done accordingly.