Defect management strategy of geometrical dents in in-service long-distance natural gas pipeline

The classification method of geometric dent defect in gas pipeline is introduced and the evaluation methods used abroad are compared and analyzed. The disadvantages of the evaluation criteria such as the maximum depth of a geometric dent and the maximum strain on the internal and external surfaces created by a geometric dent are pointed out. An in-sight study has been made on the expressions of equivalent strain given by Lukasiewicz-Czyz and Gao Ming and their calculation results have been compared with those calculated by use of the equations recommended by standard ASME B31.8. It is shown that the equivalent strain on internal surface calculated by ASME B31.8's equations is under-estimated by one time than that by Lukasiewicz-Czyz equation and that on the external surface under-estimated by 0.5 times. Compared to the results by Gao Ming calculation, the equivalent strain on internal surface as calculated by ASME B31.8's equations is under-estimated by two times and that on the external surface under-estimated by 0.2 times. A risk based recommendation has been proposed on treatment procedure and detection of geometric dent defect, which is of importance for the security of safety operation of natural gas pipelines in China and the promotion to the advance and perfection of pipeline integrity management.