Failure behavior analysis of the main shaft threaded joint of ultra-deep top drive drilling rig under complex loading conditions

This study aims to address the issues of premature failure and fatigue cracking of ultra-deep top drive main shaft threaded joint under complex and extreme operating conditions. A three-dimensional finite element model of the main shaft threaded joint is developed based on API standards to systematically investigate the stress distribution characteristics under complex combined loads, including make-up torque, axial tension, drilling fluid pressure, and radial disturbance. Fatigue life prediction was then carried out using Fe-safe with multiple multiaxial fatigue models, and the influence of surface roughness and residual stress was quantified. The results indicate that the root of the first thread is a high-risk region for stress concentration and fatigue failure, where the maximum stress reaches 972.6 MPa under the axial load of a 15,000 m drill string. According to the Brown–Miller model, failure occurs after 27,073 cycles. Validity of the simulation predictions is confirmed by representative field failure cases. The findings of this study provide a theoretical basis and engineering reference for fatigue life prediction and reliability design of tapered threaded joints in ultra-deep top drive drilling systems.