Cause Analysis on Pricking Leakage of Double-shoulder Drill Rod Joint

In comparison with API drill rod joints, double-shoulder drill rod joints have higher tensional strength and bigger hydrophthalmia. Two double-shoulder drill rod joints were found to have pricking holes during service. In order to find out the failure cause, failure analysis was performed on one drill rod joint by macro and micro morphology analysis, chemical composition test, mechanical property test, metallographic examination and finite element stress analysis. The results show that lower make-up torque resulted in lower contact pressure of the main shoulder and the reduction of thread fatigue strength. Multisource fatigue cracks formed at the root of thread due to stress concentration. Finally, the high pressure drill mud run out from the secondary shoulder and through cracks. It is suggested that recommended make-up torque should be applied on double shoulder drill rod joint before using in well drilling.