Experimental and analytical studies on the pullout strength of round thread casing connections

Pullout failure is one of the primary failure modes of API round thread casing connections. Make-up torque and thread tolerances are the two most important factors that can affect the pullout strength. In this paper, the pull-out failure process and mechanism are studied using the finite element analysis (FEA) method. In addition, the effects of make-up torque and thread tolerances on the pullout strength of API round thread connections are analyzed. The FEA results are validated through tension tests on a total of 34 pins and 17 couplings. The results show that pull-out failure initially occurs at the first and second teeth near the coupling end. At the same time, thread tolerances especially unmatched taper have notable effect on pullout strength. The tests show that higher make-up torque levels are beneficial to the pullout strength. Make-up torque 15~20% higher than the maximum API limit is recommended when round thread casing connections are used in oilfield.