Load transfer mechanism of flexible drill rod with hinges based on dynamic relaxation method

Flexible drill rod with hinges is widely used in the drilling technology of ultra-short radius horizontal wells. The flexible drill rod is different from the conventional drill rod in that its deformation process is not only accompanied by the mutual transition between the mechanism and the structure, but also has random contact with the outer pipe. Considering the structural characteristics of the flexible drill rod with hinges, flexible drill rod and outer pipe are treated as spatial beam elements after processing by the finite element method, and the hinges are discretized into universal joint connection elements. The contact relationship between the flexible drill rod and the outer pipe is described using the contact gap element; a contact nonlinear finite element simulation model of flexible drill rod with hinges is established and further solved by introducing dynamic relaxation method. A plane model for the contact between the articulated beam and the outer beam is established to verify the feasibility of the finite element model and dynamic relaxation method. According to the superposition principle and deformation coordination conditions, the analytical expressions of deflection, contact force and other parameters are derived, and the numerical solution is compared with the theoretical solution, with the maximum error of only 0.14%. Simulation analysis is performed for the flexible drill rods with hinges that have six hinge rotation limits in the building up section. The results show that the contact force between the flexible drill rod and the outer pipe is intermittent and discontinuous and randomly distributed along the axis. The larger the hinge rotation limit is, the less change in torque is transferred to the bottom end of the flexible drill rod, and the axial force transferred to the bottom end of the flexible drill rod gradually decreases. When the hinge rotation limit is greater than 5°, the axial force loss rate is greater than 59.5%.