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

管道中的裂纹构成了其主要的完整性威胁之一，因为它们的存在可能导致遏制能力的丧失。当存在裂缝时，操作人员需要确保相关的爆破压力与管道的工作压力相比足够高。目前，有几种分析方法用于预测含裂纹管道的爆破压力。 然而，裂纹通常与其他完整性威胁有关，例如腐蚀或凹痕，对此没有商定的评估方法。为了预测管道在这些情况下的行为，运营商要么依靠经验，使完整性决策不一致，要么依靠数值工具。然而，目前的数值工具非常耗时，使得它们的应用不切实际。最近开发的扩展有限元分析方法（XFEM）提供了一个更强大的方法，可以有效地模拟裂纹扩展。本研究旨在开发基于XFEM的断裂应变准则，用于预测管道中的裂纹扩展。所开发的准则将考虑特定于各种载荷条件的裂纹尖端的约束，并适用于不同等级的管线钢。该标准将提供显着的数值方便与准确性预测脆性或韧性断裂相关的裂纹尖端的限制，独立的试样的几何形状，预开裂的大小，和加载条件。所提出的准则将被验证并应用于薄壁管道中凹痕裂纹缺陷的评定。将改变凹痕-裂纹缺陷的不同参数，即：裂纹长度、裂纹深度、凹痕长度、凹痕深度、凹痕中的裂纹位置以及凹痕形成期间的操作压力，以了解它们对爆破压力的影响。将对不同情况下的预测爆破压力进行比较，以确定关键缺陷情况，从而制定凹痕-裂纹缺陷评估指南。本研究将提供与裂纹相关的凹痕的评估标准，除了提供一个准确的工具来评估具有裂纹特征的管道的安全挖掘压力。

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.