Determination of operating load limits for rotary shouldered connections with three-dimensional finite element analysis

With the development of ultra-deep and extended reach drilling technology, rotary shouldered connections (RSCs) must withstand more and more complex mechanical and chemical loads. Unfortunately, RSCs often fail when the combination of torsion and tension loads beyond a certain value. It is thus necessary to know the reasonable operating load limits of RSCs to decrease the failure risk of RSCs. Although API RP 7G provides the ultimate working torque for API RSCs, the relationship between the ultimate working torque and axial tension is simplified to be linear, which is only true when the axial tension is not too large. Moreover, the ultimate working torque for the widely used double shoulder connections (DSCs) has not been described in API RP 7G. In the present study, a nonlinear elasto-plastic finite element analysis is employed to reveal the nonlinear characteristic of RSCs under complex mechanic conditions. The ultimate working torque of RSCs combined with the impact of make-up torque and axial tension is calculated. Two limit operating load profiles, which describe the permissible working torque under various axial loads for API RSC and DSC, were provided by numerical calculations. Two useful diagrams were provided, in which three operation load zones were distinguished: the red zone stands for dangerous, the yellow one means admitted and the green one is recommended area. The deviation between the present results and those given by API RP 7G increases as the axial tension increases; e.g., it may reach 17.5 percent at an axial tension of 2182 kN. Moreover, a nonlinear relation is obtained when the axial tension exceeds a critical value (2182 kN for API RSC and 3255 kN for DSC). The results have been validated by an application case in Tarim oilfield of China. With the help of operating load limits described in this paper, effective decisions can be made in more complex drilling situations. (C) 2015 Elsevier BY. All rights reserved.