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Received Sep 22, 2017; Revised Nov 24, 2017; Accepted Dec 4, 2017
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1. Introduction
The growing demand for energy and natural resources has been pushing exploration and production activities of oil and natural gas. In particular, application of thin-walled high-strength steel has become a trend in the oil and gas transportation system over long distance, which can improve the transportation efficiency by high-pressure operation and reduce the pipe laying cost by reducing the wall thickness of pipes [1, 2]. Research projects have then been focused on the development of API grades X80 and X100 and more recently to grade X120 [1]. The high-grade pipeline steels have high yield-to-ultimate tensile strength ratio, which means they have relatively low strain-hardening ability. For the high-strength pipelines adopted in industries, the mean diameter-to-thickness ratio D/t generally ranges from 50 to 100, such as the grade API 5L X80 steel applied on TransCanada system.
The long-distance oil and gas pipes are joined by girth weld. Common failures in pipelines result primarily from the weld defects. The failure assessment of crack-like flaws is an important issue in design and maintenance of the pipeline systems. Specifically, the fracture parameter J-integral has been widely used in the structural integrity assessment of defective pipes. Full three-dimensional finite element (FE) analyses can provide accurate results for the fracture response. However, FE analyses require large computational time, expertise, and resources, which make the numerical computation quite expensive to be used routinely; hence, they are not suitable for engineering structural integrity assessment. Therefore, the simplified J-estimation scheme with much less computational cost is highly desired from the view of engineering application.
Based upon the fully plastic J-integral solution developed by Shih and Hutchinson [3], Kumar et al. [4] introduced the widely known GE/EPRI J-estimation approach for two-dimensional geometries. Afterwards, the original work was extended by various researchers [5–14] to include additional geometries and loading conditions. Another popular J-estimation method is the reference stress approach which adopts the plastic limit load as the reference stress [15]. Based upon the FE results...