Parametric Modeling and Stress Analysis of Drillpipe Tool Joints Based on 3D Finite Element Method

Summary With the improvement of finite element (FE) algorithms and the increase in computational power, 3D FE analysis has become a vital tool for studying the mechanical properties of threaded joints. However, existing 3D modeling depends on manual mesh generation with commercial software, often simplifying features like helix angle of thread teeth and thread runout, leading to low efficiency and high dependence on operator skill. To address these challenges, we propose a parametric modeling method for 3D FE mesh generation of drillpipe tool joints, using shape functions to represent the thread profiles. This method accurately generates node coordinates for complex geometries, improving modeling efficiency while maintaining high accuracy. Using the NC50 drillpipe tool joint as an example, a 3D elastic-plastic FE model was developed, and its stress characteristics under complex loading conditions were analyzed. In addition, to overcome the limitation of traditional testing methods, which struggle to capture local strains at the thread roots and stress-relief grooves, strain gauges were placed at the stress-relief groove for simultaneous measurement of strain near the thread root and groove during the makeup process. The comparison between measured and FE stresses showed a maximum error of only 4.22%, confirming the accuracy of the model. The results indicate that the stress distribution of the tool joint under complex loads is asymmetric, with high stress at the large-end thread root of the pin (808.47 MPa) and significant nonuniformity at the large-end thread root of the box, where the stress dispersion coefficient is 0.202. These areas represent potential fatigue failure risks that require attention during the design and operation of drillpipe tool joints.