This paper presents an analysis of the makeup torque influence on Rotary Shouldered Connection (RSC) reliability under static and dynamic loads. Both theoretical and experimental results of torque-turn diagrams and full scale fatigue tests are shown. The paper gives recommendations for the optimum makeup torque under cyclic loads for different material characteristics and suggests an operative patented method to estimate optimum makeup torques for connections directly at the rig site. Results are presented and discussed for different tool joint and drill collar connections. All conclusions are illustrated with a set of plots describing the results of the investigations.
RSC friction capacity. A good make up quality is achieved by creating optimum preload conditions (axial loads) into connection by means of a makeup torque. This make up will guarantee maximum RSC reliability under applied static and/or dynamic loads and mud pressure. The main RSC geometrical parameters and friction coefficient on contact surfaces define the RSC friction capacity and preload condition into connection under the application of a given makeup torque. In the early days of deep drilling very little was known about RSC friction coefficient during making up and the friction coefficient was usually taken to be equal to 0.2. In 1950-60 a set of RSC makeup tests was developed with different compounds. Special connection design threads were used. Connections were made up at a given makeup torques and makeup axial loads were estimated by pin elongation measurements. These investigations showed that RCS friction coefficient without dope was equal to 0.133, while with the common dope used at the rig site (60% lead powders 40% grease base) it was equal to 0.08. This friction coefficient was accepted by API as a standard value for drilling calculations though it is not the same for different kinds of compounds, RSCs and drilling conditions.
The results of both theoretical and experimental investigations show that RSC optimum makeup torques are different under static and dynamic (fatigue) loading conditions and can be estimated. However, it is difficult to use existing recommendations at the rig site due to the influence of different technical and technological factors (well conditions, geometrical and mechanical connection parameters, frictional properties of thread compounds and so on) on the optimum makeup values. RSC friction capacity depends on different factors and may vary within a large range (more than 50%) during connection life and from one assembly to the other. While drilling, an improper break-in procedure, cleaning and lubrication of RSC may lead to local thread (or shoulder face) galling and dramatic changing of its friction capacity.