Validation of in-line inspection data quality and impact on steel pipeline diagnostic intervals

Abstract

The aim of this research is to evaluate the inaccuracies of pipe wall flaws sizing derived from an axial excitation magnetic flux leakage technology (MFL), in respect to the defect field checking. During direct assessment, construction and metallurgical features such as rolling anomalies and ingot imperfections were discovered on the natural gas pipeline serviced for many years. MFL tool sizing accuracy of pipe wall losses was individually compared with a limited number of excavations. The inspection equipment errors were estimated based on a small-scale of the verified anomalies within tolerances and the correct pipe surface classifications. The quality of diagnostic results and their correspondence to the pipeline maximum admissible operating pressure calculations was highlighted. However, certain flaws on the considered pipeline are not metal loss-type and, for this reason, cannot be assessed by the code based level 1 engineering method. A deterministic approach to the pipeline pigging time intervals was analyzed. The impact of the defect growth rate on the inspection schedule was presented in the current paper in the simplest way. Based on the considered analysis and author's experience, expert judgment guidelines for conducting in-line inspection on underground gas transmission pipelines were formulated in this publication.