Experimental Evaluation of Fitness-for-Service Assessment Procedures for Line Pipes With Circumferential Crack-Like Features

Abstract Circumferentially oriented crack-like features in pipe body, such as circumferential stress corrosion cracking, are a growing integrity concern to pipeline operators. Engineering assessment procedures exist in common standards, such as API 579 [1] and BS 7910 [2], for assessing the fitness-for-service (FFS) of these features. Analogous FFS assessment procedures for axially oriented features are widely used and have been the subject of extensive full-scale validation testing. However, a need for full-scale validation testing for circumferentially oriented crack-like features in pipe body material was identified by the first phase of a Joint Industry Project (JIP), which was sponsored by 10 North American pipeline operators. In the recently completed second phase of this JIP, full-pipe specimens with natural and machined circumferential features were destructively tested under biaxial (combined internal pressure and axial tension) loading conditions. Details concerning the specifics of the test designs, execution, and results are presented in a companion paper [3]. Test results were then compared with predictions from two FFS assessment procedures, BS 7910 and API 579, to determine the degree of precision and conservatism with which the assessments predicted the failure load. The results show that, of the FFS assessment procedures studied, the BS 7910 procedure using the Option 1 Failure Assessment Diagram (FAD) provides the most conservative results with the smallest variation (highest average test-to-predicted ratio with the smallest coefficient of variation). In some cases, such as when measured tensile properties were used with the option “e” load ratio cut-off value, the API 579 Level 2 FAD produces non-conservative predictions (average test-to-predicted ratio less than 1). As most of the test specimens had failure regimes in the “plastic collapse” region (bottom-right corner) of the FAD, the conservatism of the FFS assessment procedure predictions was found to be significantly influenced by the choice of the load ratio cut-off value. Further full-scale tests with specimens failing in the central region of the FAD are recommended to evaluate the FFS assessment procedures for a broader predictive range. Additional tests including shallower natural C-SCC colonies with sizes within the applicable range of the FFS assessment procedures are also recommended. Additional analysis based on these test results is also recommended, such as a detailed evaluation of the impact of material property assumptions on assessment results, and inclusion of additional FFS assessment procedures (such as strain-based approaches). This paper provides further discussion of the outcomes of this evaluation work, including the relative accuracy and conservatism of the FFS assessment procedures, the key differences between the procedures that may lead to differences in conservatism and accuracy, the range of applicability of these findings, and recommendations for further full-scale testing. A hypothetical case study is also included in Appendix A describing how the evaluation findings may be practically applied.