Effects of Over-Torque on Stress Relief in Conical Threaded Connections

The analysis of the stress state induced by the make-up torque of tapered threaded connections is very important for the improvement of both the static and the fatigue resistance of such connections. Make-up torque induces a nonuniform stress state; in the same way, the axial load induced by the make-up torque is distributed in a nonuniform way along the sections of the coupled elements of the connections. The most important consequence is the overloading at the first threads engaged, which has to be summed to the stress concentration in the core section due to the notch effect. The aim of this paper is to analyze the stress and strain fields induced by make-up torque in tapered threaded connections; in particular, the purpose is to investigate the effects of the application of over-torque before putting the connections into service (stress relief due to over-torque before nominal make-up torque application). The evaluation of the local stress state and of stress relief, with a good level of precision, was performed by using finite element models in order to assess the percentage of load carried by each thread after the application of the make-up torque and further axial tensile or compressive loads. Numerical results confirmed the favorable thread load distribution induced by overloading. Experimental tests carried out on full-scale connections, used in the oil and gas industry for the joining of pipes, confirmed the numerical results.