Experimental and simulation research on connection strength of coiled tubing

Coiled tubing is frequently connected to tool strings via a dimple connector. The insufficient connection strength of coiled tubing would cause falling accidents of tool string. This study comprehensively considers the coupling relationship between screw shear and axial tension to investigate the connection strength of coiled tubing and its influencing factors. Accordingly, this research establishes a tensile simulation model of the coiled tubing with dimple connector. The simulation results are compared with the tensile experiment results of the connection strength of coiled tubing, and the relative error is only 3.05%. The simulation model is then used to examine the effects of pit depth, pit radius, steel grade, wall thickness, and internal pressure of coiled tubing on the connection strength. The findings reveal that the highest equivalent stress occurs at the pit closest to the loaded end and increases monotonically with applied displacement. Stress concentration and necking phenomena indicate that failure will occur at this location. The increase of pit depth and pit radius will increase the deformation of the pits, resulting in material accumulation around the pits and a reduction in wall thickness at the pit locations. An increase in steel grade and wall thickness enhances the connection strength of coiled tubing. Similarly, higher internal pressure causes the coiled tubing to fit more closely with the dimple connector body, which also enhances the connection strength. These conclusions are essential for improving the safety and stability of coiled tubing operations.