Review and Proposed Improvement of a Failure Model for SCC of Pipelines

Abstract

Oil and gas pipelines are subject to stress corrosion cracking (SCC) in groundwater environments. Recent SCC failures have emphasized the need for accurate failure prediction models that can be used to assess the integrity and safety of existing pipelines. SCC is characterized by colonies of many longitudinal surface cracks in the body of the pipeline that link up to form long shallow flaws, with length-to-depth (L/d) ratios that are typically in the range of 50 to 200. Such flaws are particularly challenging for standard failure prediction models. Because inelastic material behavior is usually associated with SCC failures of pipelines, the authors previously developed a failure prediction model that utilizes both flow strength and J fracture toughness to evaluate the failure of pipelines with crack-like flaws. Failure is predicted to occur either when the applied J value is equal to the J fracture toughness or the net-section stress is equal to the flow strength. To improve the failure model, recent work has concentrated on long, deep surface cracks. Multiple surface cracks and their possible interaction also have been considered. Interactions are modeled for both flow strength and J fracture toughness failure criteria. Methods for evaluating crack interaction are proposed, and examples of crack-interaction predictions are presented. Also, existing methods for evaluating ductile tearing instability, as well as crack initiation toughness, are described. It is proposed that these methods will improve the failure model.