Friction Considerations in Rotary Shouldered Threaded Connections

The new connection designs introduced by the drill pipe manufacturers (double shoulder connections, intelligent drill pipes or any new design for increased torque resistance) make the use of the Farr formula for calculating proper assembly torque more problematic. Additionally, severe drilling conditions like HPHT, directional drilling and extreme environments are affecting critical thread compound performance properties, which can make it impossible to attain the optimum makeup torque for the connection. The new drilling conditions are exposing the drill string components to higher loads and consequently reduce the safety margins. Since the makeup process is the only way to control and achieve the maximum tool joint loading capacity, it becomes important to understand the effect of friction on optimum makeup torque calculation. This paper presents a modified equation for optimum makeup torque calculation by using the latest research on thread compound frictional properties. Also, the thread compound lubrication mechanism will be explained. For the first time the thread turn load will be related to the optimum makeup torque. The paper will also explain why there is a need for a new API standard for determining thread compound frictional properties. The formula developed herein, based on experimental results, demonstrates that the newly introduced overall factor may differentiate between used and new connections.