Fracture criterion for XFEM to predict burst pressure of dents with cracks in pipeline steels

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Cracks in pipelines constitute one of their major integrity threats as their existence can lead to loss of containment capability. When cracks exist, operators need to ensure that the associated burst pressure is high enough compared to the operating pressure of the pipeline. Currently, there are several analytical methods for predicting the burst pressure of cracked pipelines. Cracks, however, are often associated with other integrity threats such as corrosion or dents for which no agreed upon assessment method exists. In order to predict the behaviour of a pipeline in these situations operators either rely on experience making the integrity decisions inconsistent, or on numerical tools. Current numerical tools, however, are extremely time rendering their application impractical. The more recently developed eXtended Finite Element analysis Method (XFEM) provides a more robust approach in which crack propagation can be modelled efficiently. This research project aims at developing calibrated XFEM based fracture strain criteria for predicting crack propagation in pipelines. The developed criteria will take into consideration the constraints at the crack-tip specific to various loading conditions and applicable to different grades of pipeline steel. The criteria will provide significant numerical convenience with accuracy in predicting the brittle or ductile fracture related to the crack-tip constraints, independent of the specimen geometry, pre-cracking size, and loading conditions. The developed criterion will be validated and applied for the assessment of dent-crack flaws in thin walled pipelines. Different parameters of the dent-crack flaw, namely: crack length, crack depth, dent length, dent depth, crack location in the dent, and operating pressure during dent formation will be varied to understand their effect on the burst pressure. Comparison of predicted burst pressures for different scenarios will be done to identify critical flaw scenarios to develop a guide for dent-crack flaw assessment. This research will provide assessment criteria for dents associated with cracks, in addition to providing an accurate tool for evaluating the safe dig pressure for pipelines with crack like features.