When assembling tubing strings in the oilfields, threaded connections are used to connect the pipes with each other. The possibility to reuse these connections is often required and a certain degree of leak tightness is required, even without the use of a sealing surface or shoulder. For this reason, the total plasticity within the connection should be limited and relative movement between pin and box ought to be restricted.
Within this publication, a finite elements analysis is conducted using a 4.5 inch buttress threaded connection as defined in API 5B together with a connection using the enhanced SR23 buttress thread. In addition, an experimental validation of the make-up stage is conducted by comparing the strains generated during make-up using Digital Image Correlation and infrared monitoring.
In order to determine the optimal make-up, values found in literature are compared with a developed method using the magnitude and size of the plastically deformed zones during make-up. In addition, the effect of external axial and pressure loading is examined to identify the effects and critical areas.
As a result, it is observed that pressure loading and make-up tend to have similar effects and in order to determine the optimal make-up torque, the pressure ratings of the assembly should be taken into account to prevent overtorqueing the connection. For the case of axial loading, a critical zone is visible near the last engaged thread and excessive loading of this thread can cause premature failure within this zone. Overall, the SR23 connection shows limited, yet visible, advantages over the standard BTC connection as described in literature.