Extension of Current Defect Assessment Methods for Gouge and Corrosion Defects in X80 Grade Pipeline

Abstract

The increasing demand for natural gas affects the type of transportation, both from the strategic and the economic point of view. Long-distance pipelines are a safe and economic way to transport the gas from production sites to end users. Hence, pipes producers need to supply the market executing projects where high strength material is involved, to reduce the steel use. Among high strength steel grade pipes (X80 - X100 - X120), the X80 grade is already in use for a number of gas pipelines in the world since many years. There is a need to evaluate the suitability of extending the current Fitness For Purpose methods to X80 grade steel linepipe, since the existing guidance was developed and validated mainly on test data coming from steel pipes of grade lower than X80. Hence they could not be directly applied to X80 grade pipes, but should be experimentally verified, otherwise their straightforward extrapolation would be questionable. EPRG recognized the need to cover this gap and launched a specific project, aimed at verifying the applicability of the presently available criteria to X80 grade, with particular focus on corrosion and gouge types of defects, longitudinally oriented. The project includes the collection and review of available tests data and FFP criteria, and the identification of the most promising among those collected. Four hydraulic full scale burst tests on X80 representative pipes (helically and longitudinally welded) containing simulated corrosion and gouge defects have been carried out to experimentally verify the applicability of the criteria to the X80 grade pipes. The selected criteria for the corrosion (DNV RP-F101) and for the gouge (Battelle NG-18) defects revealed to be suitable for X80 grade pipes too, as demonstrated by the accuracy in predicting the failure pressure of the experimental tests and of the literature database. The criteria showed to be even more accurate for X80 grade than they were for lower grades. Finally, it is worth mentioning that the selected criteria did not need any correction factor for obtaining the best prediction. Such a result is a demonstration of the sound theory behind the criteria.