Complex Weld Defect Analysis And Safety Assessment Of Oil&Gas Pipeline

Weld defect is a important influencing factor for the oil&gas pipeline failure. Especially for the pipeline still in operation with long history, there are many weld defects remained in the pipeline, such as the crack, misalignment, incomplete fusion, angular distortion and slag because of the state of art about the metallurgy, manufacturing, welding and inspection at the time of pipeline construction. With the operation time extended, the weld burst and leak of pipeline occurred, and these incidents lead to casualty, pollution and economic loss. In order to ensure the older pipeline can operate safely, the periodically inspection and safety assessment for the pipeline is needed. At present, the assessment technique for the single weld defect is complete. However, the research for the assessment technique for the complex defect is still need to develop.In this paper, the mechanism of the ordinary weld defects was analyzed, and the influence of misalignment and angular distortion for the pipeline stress distribution were studied. On the theory of fracture mechanics, the fracture theory models of the’crack+misalignment’and ’crack+angular distortion’were discussed. On the basis of the API579<fitness-for-service>, and with the introduction of weight function and finite element analysis, the stress intensity factor of the’crack+misalignment’and’crack+angular distortion’were established, and provided the important fundamental model for the safety assessment of complex weld defect.This paper also carried out the finite analysis for the fracture parameters of complex weld defect, and illustrated the calculation examples of stress intensity factor of crack+misalignment of girth weld and crack+angular distortion.