钻杆NC50内螺纹接头裂纹原因分析

为推广应用斜坡吊卡台肩钻杆，对某井钻杆内螺纹接头直角吊卡台肩部位断裂和裂纹事故进行了详细调查研究，并对一根内螺纹接头钻杆进行了失效分析。失效分析内容主要包括裂纹宏观分析和微观分析，钻杆接头材料化学成分、力学性能和金相组织试验分析，钻杆在钻井过程中受力条件分析等。通过失效分析，认为钻杆接头为早期疲劳失效，钻杆内螺纹接头直角吊卡台肩部位产生早期疲劳断裂和裂纹的原因是该部位结构尺寸不合理，应力集中严重。在钻井过程中钻柱会承受交变的拉伸、扭转、弯曲、震动等疲劳载荷，很容易在应力集中严重的内螺纹接头直角吊卡台肩部位产生疲劳断裂和裂纹。要从根本上防止钻杆接头断裂，建议采用斜坡吊卡台肩钻杆。

Cause analysis on crack of NC50 box tool joint

This paper gives a thorough investigation on the accident of the drill pipe crack at square shoulder root of tool joint in order to know the failure cause, and one sample of cracked tool joint is taken for failure analysis.Failure analysis comprises the macro and micro analysis, the experimental analysis of chemical composition, mechanical properties and microstructure of drill pipe joint, and the analysis of load borne on drill pipe during drilling.It is reckoned that the early fatigue fracture of square shoulder root is caused by the irrationality of the size of the root and the severe stress concentration at the square shoulder root of joint.Fatigue cracks easily occur at the square elevator shoulder root of joint, because the loads borne on drill string are tension, torsion, bend, and vibration that are continuously changed in drilling.It is suggested that taper elevator shoulder be used to prevent the crack of tool joint in essential.