Analysis of Causes for Mill-end Dropout Failure of Casing-used Long Round Thread Connection
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摘要: 分析长圆螺纹套管接头工厂端发生脱扣事故的原因。观察失效试样的宏观形貌，检测套管材质的理化性能，对比分析脱扣接箍不同位置的外径、失效试样螺纹锥度，并对同批次套管试样进行实物对比试验，对螺纹接头进行拉伸条件下的有限元模拟分析。结果显示：失效套管材料屈服强度小于API Spec 5CT—2011标准要求值；螺纹接头工厂端上扣位置满足相关标准要求；脱扣接箍不同位置外径差异较小；脱扣接箍工厂端螺纹锥度波动范围较大；有限元力学模拟分析反推接头工厂端不会在上扣痕迹位置发生脱扣失效。长圆螺纹套管接头工厂端发生滑脱失效的主要原因是套管材料屈服强度偏低和现场浮动上扣导致螺纹接头发生松动。Abstract: Analyzed in the paper are the causes for mill-end tropout failure of the casing-used long round thread connection.The analysis involves such activities as observing the macro-morphology of the failure sample,testing the physical and chemical properties of the casing material,conducting a contrastive analisis of the dropout coupling's OD at different positions,checking the thread tamper of the failure sample,performing contrative physical obeject tests to samples from casings of the same lot,and making FE simulation analysis of the thread connection under tensile conditions,etc.The result shows that the yield strength of the failure casing's material is less than the value as specified under API Spec 5CT-2011;the make-up position of the thread connection mill-end is up to the requirement under the applicable specification;the dropout couple's outside diameter at different axial location is consistent and shows little difference;and variation scope of dropout couple's thread taper at mill-end is rather high.Moreover,it is inversed via the FE simulation analysis that it is imposible for the thread connection mill-end to drop out at the make-up trace location.It is therefore concluded that the said dropout failure is caused mainly due to the rather low yield strength of the casing material,and looseness of the thread connection itself as caused by the on-the-site float make-up operation.
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Keywords:
- casing /
- round thread /
- connection /
- dropout /
- analysis of failure /
- yield strength /
- float makeup
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