The effect of bending moment direction on tool joints: Working load limits under complex loads

• It's found bending moment direction has significant effect on tool joint performance.
• The tool joint under complex loads was analyzed with three-dimensional FEM.
• A method is developed to calculate the ultimate working torque for tool joint.

In drilling application, tool joints are often subjected to complex loads, such as axial tension, bending moment and working torque. With the development of the ultra-deep and extended reach drilling technology, bending moment plays a more and more important role. However, few studies claim to study the effect of bending moment on the tool joints' connection performance. In addition, the influence of the direction of bending moment on tool joints is not published. This paper established a three-dimensional finite element model of the tool joint, and analyzed the mechanical behaviors of the tool joint under make-up torque, axial tension and bending moment with the explicit dynamic finite element method. Compared with a two-dimensional axisymmetric model, the three-dimensional finite element model can effectively simulate the make-up performance of the tool joint by taking the helix angle of the thread into consideration. The results show that the make-up torque can make the force acting on the tool joint unbalanced. Moreover, the stress distribution of the tool joint presents a distinct asymmetric characteristic under the bending moment. Besides, the bending moment direction, which not merely influences the value of moment borne by the contact surfaces between the pin and box, also has a certain effect on the ultimate working torque of the tool joint. Hence, the objective of the work presented in this paper was to explain that the bending moment has an obviously asymmetric characteristic and a strongly influence on the tool joint. In this paper, a method for calculating the ultimate working torque of the tool joint under complex loads is developed. The ultimate working torque diagram of the tool joint is then obtained to define the limits of operation loads.

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