Cracks or crack-like defects have been associated with failures in gas transmission pipelines and gas distribution piping systems, some incidents being consequential. Cracks are identified in natural gas pipeline safety regulations and safety standards as conditions that must be dealt with, however critical sizes or other specific evaluation criteria are not generally given. A hydrostatic test pressure to specified margins above the operating pressure could establish a benchmark for critical sizes that are a potential threat to the integrity of the pipe. Cracks that are too small to fail at the hydrostatic test pressure may still remain present in the pipeline. Ideally, determination of a sufficient margin of safety to identify defects that should be monitored after taking into consideration the potential growth of crack or crack like defects that are too small to fail a hydrostatic test pressure will address defects of concern that may remain present in the pipeline.
The development of an integrated robotic internal inspection tool that can be used to accurately detect and size cracks or crack like features in natural gas pipelines is of mutual interest to pipeline regulators and operators. NDE technology carried by inline inspection tools should be at least capable of identifying cracks or crack like features that would fail in a hydrostatic pressure test or could potentially grow to failure with a high degree of probability prior to the next ILI reassessment. The tool and its integrated software algorithms must be capable of indicating the location, position, orientation, size, and characteristics of defects of interest or concern to minimize validation and/or repair costs and to preserve the overall integrity of pipelines in a gas transmission operator’s integrity management program.
This paper develops criteria for the minimum detection requirements of cracks or crack-like defects by an ILI tool to ensure equivalent integrity to that provided by a hydrostatic pressure test performed to standard requirements. The critical defect sizes will be estimated using fracture mechanics (e.g., the Modified NG-18 equation) considering different toughness categories. The paper addresses the challenge of variability in the real material properties through two approaches. The first approach is to conduct three sets of analyses for each material grade based on the lower 5% percentile, mean and upper 95% percentile properties. The second approach is to conduct the analysis based on Monte Carlo simulations to generate random combinations of material properties as well as crack dimensions.
