The top drive for casing running system can effectively avoid borehole neck down and sticking, reducing construction period by about 50%. However, the casing will bear higher torque load, and the casing joint threads can meet the bearing performance, which have a critical impact on casing running and the life of oil and gas wells. Variable pitch can reduce thread stress concentration and improve load-bearing strength. Lacking research on mechanical behavior has greatly restricted the development of variable pitch thread. This article generated a variable pitch casing joint thread helix forming method and control equation; designed an ultrahigh-torque variable pitch casing joint thread based on the theory of screw transformation matrix; established a three-dimensional finite element model of the variable pitch casing joint; optimized the main structure parameters with the judgment of torque-bearing capacity; and conducted parameter-sensitivity evaluation of guide surface angle, bearing surface angle, pitch of box thread, and amount of varying pitch under tension load, compression load, bending load, and torque load. The designed variable pitch casing joint had been successfully used in several top driving casing running wells, downed to the maximum depth of 4375 m, the largest hole deviation angle of 55.37° and horizontal section length of about 260 m. Under a torque of 4000 N m, the top drive rotated down into the gas well successfully without failure. The research work of this article has significance on variable pitch thread and enhances the bearing capacity of the thread.
Design and mechanical behavior study of ultrahigh-torque variable pitch casing joint
