The selection of a suitable pipeline repair/replacement coating is from a technical point of view the major decision to be made for a rehabilitation project. Rehabilitation seeks to re-establish coating efficacy to its original state. This does not mean that the same coating must be used for rehabilitation; often more state-of-the-art coatings are employed.
Current thinking on the mechanism of failure of thin film liquid applied coating (epoxy) requires different coating parameters to those used for example in evaluating tapes. With thin film coatings, the penetration of water and
subsequent osmotic pressures developed will result in many small canals creating further water uptake. Eventually the thin film coating becomes transparent to moisture and becomes a semi-permeable membrane which, under the influence of cathodic protection draws water through the coating to collect at the steel coating interface as a blister which eventually bursts and allows further undermining of the coating.
Coating failure is accelerated by increasing temperature. Which is a very relevant remark in the context of rising temperatures due to changing climate conditions. This failure mechanism is not applicable to tape based coatings.
A coating used for the rehabilitation of a pipeline must fulfill the following requirements:
- The most important and fundamental requirement of a coating is that it must separate the pipe from the moisture
in the soil. To do this the coating must have a high resistance towards permeability and water uptake.
- The coating must have a good bonding to the steel pipe to prevent moisture ingress and to resist soil stresses. This is very dependent upon surface cleanliness and the surface profile prior to coating application.
- The coating must be capable of being applied in the ditch under the prevailing environmental conditions.
- The coating must be capable of being repaired and to withstand holidays over time.
- The coating or supporting techniques must be available to cover the repair of the overlap areas onto the parent coating.
- The coating must be tolerant to the high akalic conditions generated by cathodic protection
An important design goal for the coating, is that it must have a long useful lifetime. A wide variety of tests are used to evaluate and select a suitable coating for the desired application. It is very important to realize that there is not a single solution that fits all applications.