A theoretical and experimental approach to the problem of collapse of deep-well casing

Abstract

The theory of plastic collapse of tubes under external pressure is developed. The theory of plastic collapse shows that the resistance to collapse depends on the shape of the stress-strain curve, among other factors. The results of open- and closed-end collapse tests on a number of steel tubes carefully machined to assure roundness and concentricity are reported. The results of a few tests on brass tubes also are reported. It is considered that the test results provide substantial confirmation of the validity of the plastic-collapse theory. Although the plastic case of collapse is of predominant interest in casing design, a theoretical analysis of the case of elastic collapse also has been made. The analysis shows that in the case of elastic collapse a small initial out-of-roundness does not lower the collapse pressure of the tube. It is shown also that longitudinal tension or compression stress has no effect on collapse pressure in the elastic case. The laws of plastic flow of metals have been applied to the case of plastic collapse of tubes when an axial load is acting. An axial compression stress raises the collapse pressure, and an axial tension stress lowers the collapse pressure.