Evaluation of Existing Fracture Mechanics Models for Burst Pressure Predictions, Theoretical and Experimental Aspects

The predictions of burst pressure and leak/rupture have significant impact on the pipeline integrity assessment results, and subsequently on the extent of the required mitigation and re-inspection interval. There are models available for burst pressure and leak/rupture prediction in the literature, namely API-579, CorLas®, and NG-18 (Modified Ln-Sec). In this paper, evaluation of existing fracture mechanics models for burst pressure and leak/rupture prediction for external crack and crack like features including stress corrosion cracks (SCC) are performed. Both theoretical and practical aspects of each model are discussed in detail. An experiment is set up to conduct fifteen full scale burst tests of 20-inch, 34-inch and 36-inch diameter joints removed from the pipeline field. Among them, seven pipe joints are with base material SCC, four joints with seam weld cracks, three joints with linearly aligned crack-like features surrounded with shallow base material SCC, and one joint with crack-like feature in the Weld area. A high speed camera is utilized to capture rupture events. The rupture events can be stable tearing, crack coalescence and unstable propagation of a critical crack or multiple closely aligned cracks in a crack colony which are responsible for the rupture. Detailed profile measurements of the critical crack/s for burst pressure predictions are done from the fracture surfaces. Relevant material properties are measured for each joint in base material or seam weld location depending on the crack location. Burst pressure predictions are performed with models available in API-579, CorLas®, and Modified NG-18. An elastic-plastic finite element model is generated to quantify the effective stress intensity factor with and without the end cap effects during the burst testing. The accuracy and conservatism of the models evaluated are analyzed. Implications of the findings are discussed.