Quantifying the Impact of Assumptions on Predicted Burst Pressure Assessments

Abstract

The ability for deterministic fracture mechanics assessments to correctly estimate the predicted burst pressure (PBP) is dependent upon the accuracy of the model as well as the assumptions made regarding mechanical properties and defect geometry. Within the assessment of defects identified through field investigation or through ILI programs, the use of extreme bounding values for all input variables can lead to unacceptably over-conservative predictions and impede the ability to achieve a target safety margin. This paper examines the effect multiple assumptions have on the bias between predicted and actual burst pressures for various kinds of defects that have caused in-service and hydrostatic pressure test failures in ERW and flash weld line pipe materials. The predicted failure pressures of defects documented within a recent U.S. Department of Transportation compendium of ERW and flash weld seam failures were analyzed using a variety of scenarios based on knowledge of key nominal parameters such as grade, outside diameter, wall thickness, and the maximum defect length and depth. The ratio of the predicted to actual failure pressure was statistically examined across the scenarios for the different defect types. Observations are made regarding the use of the PBP model in order to statistically quantify the accuracy of the model, which can be used as input for an operator to develop a process that achieves a target safety margin.