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 inorder to know the failure cause, and one sample of cracked tool joint is taken for failure analysis. Failure analysis comprises the macroand micro analysis, the experimental analysis of chemical composition, mechanical properties and microstructure of drill pipe joint, andthe analysis of load borne on drill pipe during drilling. It is reckoned that the early fatigue fracture of square shoulder root is caused bythe irrationality of the size of the root and the severe stress concentration at the square shoulder root of joint. Fatigue cracks easily occurat the square elevator shoulder root of joint, because the loads borne on drill string are tension, torsion, bend, and vibration that arecontinuously changed in drilling. It is suggested that taper elevator shoulder be used to prevent the crack of tool joint in essential.