Abstract
In 2018, the PNG LNG project sustained a Mw7.5 earthquake, and ca. 300 aftershocks, epicentered directly under key facilities. Around 150 km of high-pressure gas and condensate pipelines were affected. In anticipation of such an earthquake event and due to the challenging terrain that the pipeline traverses, two design methodologies were used in specifying the pipe and welds for the onshore pipelines: strain-based design and allowable stress design with robust materials selection. The strain-based design approach was used for segments crossing faults and was the subject of IPC2014-33550 [1]. In this paper, the robust allowable stress design that was used for the remainder of the onshore pipeline route will be discussed along with the performance of the pipeline designed with this methodology when it was subjected to the earthquake. Robust allowable stress design involved the selection of line pipe and welding procedures that would reduce the risk of failure during unanticipated ground movements. Lower grade, thicker wall pipe was selected, and enhanced weld properties were specified to increase weld strength overmatch and toughness. Additionally, enhanced testing of pipe and weld properties was performed in order to enable prediction of pipeline strain capacity and assessment of fitness for service of any portion of the pipeline that experienced longitudinal plastic strains due to ground movement. These efforts enabled the pipeline to safely sustain the ground movement experienced during the earthquake and allowed safe project operations to be rapidly restored.
This paper provides details of the selection of pipe grade and wall thickness and the specification of material properties for pipe and girth welds. The property distributions achieved and the impact on strain capacity are presented along with estimates of the strain experienced by the pipeline due to the earthquake. The performance of the pipeline during the earthquake illustrate the benefits of the robust allowable stress design approach for pipelines in challenging environments.
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