Investigating the Effectiveness of Techniques Used in Assessing the Integrity of Non-Piggable Pipelines

A substantial part of the high pressure gastransport network of Gastransport Services as part of N.V. Nederlandse Gasunie in the Netherlands is not piggable. It is therefore foreseen that an ECDA approach including coating survey techniques, CP measurements and bell hole excavations, will be implemented to establish the integrity of these non-piggable lines. In order to get a better understanding of the performance of some techniques for our specific situation, a test program was carried out on a pipeline of 33 kilometres with a diameter of 8” and bitumenous coating, constructed in the sixties. DCVG, Pearson and a combination of CIPS and Pearson (CIPP) were tested in combination with 3 MFL intelligent pigruns, current attenuation measurements by the Stray Current Mapper (SCM) and bell hole excavations. Initial results for the three coating survey techniques showed that the detection probabilities for a coating defect ranged from 18% to 68% under the assumption that no false calls were generated. Using results from repeat measurements and taking into account the possibility of false calls, the detection probabilities increased to potential maximum values of 48% to 94%. Better estimations of the values could be obtained after verification of some indications from CIPP, leading to maximum values in the range of 70% to 84% for the best two techniques. Additional measurements at pre-selected locations by means of pipe-to-soil-potentials and pin-current measurements did not indicate active corrosion. Verification excavations at 14 locations indicated that in all situations a coating defect existed and that mild general corrosion was present in eight situations due to the very aggressive soil (soil resistances of several Ωm’s were measured). Current attenuation measurements by the SCM suggested a uniform distribution of coating quality. The results from the three intelligent pigruns were of relatively little use because of the detection threshold of 15% wall thickness and the fact that the suppliers often did not agree on the interpretation of metal loss. Therefore the results could not be considered to be an absolute true reference for relating the coating defects to. Although the results of the program gave very valuable insight into the characteristics of the different techniques it is recognized that not all results can be translated to other circumstances/pipelines. Therefore additional tests and surveys will be carried out in the coming year(s) to further work out the ECDA procedure for the specific situation of the non-piggable pipelines of Gastransport Services.