Analysis of Limit Setting Force of Casing Inside Slips Based on Cylindrical Shell Theory

ABSTRACT

Casing will be damaged in a well because of the setting force, and may even be broken when the force is too large. To clearly demonstrate the interaction of the mechanism of packer and casing, a mechanical model of casing as thin-wall cylinder was established with the consideration of the axial force and additional bending moment induced by the bending of casing near the slips of packer. And, the equation of limit setting force on the casing exerted by slips was deduced by bending moment theory of cylindrical shell. The relationships of the limit setting force of N80, P110, TP140 casing and thickness were analyzed. It was found that the bending stress and the hoop stress were the biggest near the bottom of the slips, and that position was the dangerous section. The limit setting force of casing exerted by slips increased with the thickness of casing, when the packer and the outer diameter of casing remained the same. And the limit setting force was higher in high grade casing. The limit setting force calculated in this paper was decreased by 5 % to 14 % than by thick-wall theory, and it is deduced to be safer. The method proposed in the paper can be used to evaluate the limit setting force and provide the basis for the choice of hanging load.

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