Experimental and stochastic approaches to assessing the strain demand of pipelines and flexibility requirements for coatings

A criterion for selecting a coating for an energy pipeline is that the coating should have a suitable flexibility to meet the high strain demand during hydrostatic testing and during field bending. This requires knowledge of the level of strain demand for the pipeline, and also the maximum strain that could be tolerated by the coating system. Whereas average strains imposed during manufacturing and construction are reasonably well predicted, there is insufficient understanding on the factors leading to localised deformation of the pipe. Significant work has been carried out in the past to develop tests for assessing the coatings’ ability to handle a certain amount of strain based on bend testing, tensile testing and burst testing. However, there is a concern as to whether these tests properly represent localised micro-strains associated with construction activities including field bending and pressure testing, particularly pressure testing of pipelines designed for operation at 80% of specified minimum yield strength (SMYS). Consequently coatings considered "suitable" for modern pipelines may fail. The first issue discussed in this paper is main factors affecting strain localisation. The non-deterministic distributions of heterogeneities over the pipe provide a ground to consider the mechanisms of localisation as a stochastic process. An approach is proposed to quantify the maximum localised strain demand through cold field bending and hydrostatic experiments. Another issue discussed in this paper is the experimental assessment of coating flexibility under the effects of localised strains. Preliminary mandrel tests have been carried out to assess the uniformity of the imposed strain. Although mandrel testing has been shown to be a useful method for relative comparison of coating flexibility, it has several weaknesses that could significantly affect the reliability and reproducibility of the results