Rotary shouldered thread connections: Working limits under combined static loading

Rotary shouldered connections (RSC), used in the oil and gas industries, are probably the most stressed components of the drill string because they are subjected both to make-up torque and to axial and bending loads. Since loads can vary and can result in fatigue crack initiation and propagation, there is often severe damage to the first threads engaged. Such damage leads to shoulder load reduction and discontinuity in the drill string. Once we know the geometric dimensions and the materials of the pin and box elements composing the RSCs, API standards make it possible to evaluate the working limits of RSCs when they are subjected to make-up torque, torsion and tension. It is not, however, possible to establish the stress state of the connection for extreme working limits. The aim of this paper is to propose a numerical procedure, confirmed by full-scale experimental tests, which enables the evaluation both of the working limits, combined make-up and axial tensile loads, and of the stress state of RSCs for any load condition and, in particular, when RSCs are subjected to extreme combinations of make-up, torsion and axial tensile loads.