The spiral welded defect of steel oil transmission pipeline is one of the main causes resulting in pipeline leakage accident. Hence the failure assessment for known-size spiral welded defects is an important step to ensure the safety of defected pipeline. Lack of suitable criterion for assessing the spiral welded manufacture defects of pipeline network in China, is a difficult technology problem to be solved desirably. This paper first summarized the basic idea of preliminary failure assessment (Grade 1A of code BS 7910:2005) with some insight of our own understanding, and then applied the preliminary failure assessment to the spiral welded defects of oil pipeline, with the use of ultrasonic inspection data of Daqing-Tieling old pipeline from LingYuan to XinMiao, Northeastern China. The calculation of both fracture and plastic collapse failure for spiral welded defects indicates some detected flaws of pipeline are not safe as the internal pressure is greater than 4.5 MPa. A leakage accident of spiral welded pipeline in Western China is also assessed through fractography analyses and failure calculations. This paper concludes that the preliminary failure assessment provides useful outcome for reference in making decision of inspection, integrity assessment and repair of spiral welded pipeline, and hence is a step of fundamental importance and practical significance before more accurate data becomes available for higher grade assessment.

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