Cause Analysis on Thread Leakage of NC50 Drill Pipe Joint in a Well

Some cracks and punctures were found out when a patch of used drill pipe joints were detected with nondestructive testing method. By means of macroscopic observation, magnetic particle inspection, chemical composition analysis, mechanical property test, metallographic examination and fracture analysis, the causes of crack and leakage of the joints were analyzed. The results show that during drilling, due to the insufficient upper buckle torque of the drill pipe joints, the contact pressure of the main shoulder surface was small, the internal and external threads were loose to produce dynamic load, the stress distribution of the threads bottom changed, the local stress concentration intensified, the fatigue strength decreased, the fatigue crack source formed at the thread bottom, and the crack further expanded and penetrated under the tensile load. The inner wall of the joint thread caused the internal high-pressure mud liquid to flow into the thread channel from the auxiliary shoulder and through cracks, and the main shoulder surface which lost the sealing property rushed out and washed out and corrodes, finally leading to the joint leakage failure.