Well casings are subjected to severe corrosion because of their exposure to corrosion environments – seawater, saline formation water, and the corrosive fluids that are transported. Corrosion develops pits and cavities at both the inner and outer walls of the casing. The burst and collapse loads are the most common loads for casing under operation conditions. The loads act on the pitted casing will cause stress concentration and degraded casing strength. The strength deterioration can significantly shorten the casing life; even cause failure of the well. Thus, it is highly desirable to know the stress concentration factor (SCF) for both casing designing and evaluation processes.

The degree of stress concentration depends on the cavity shape. This paper derives simple formulae for SCF around cavities of various geometries including shallow-spherical (depth is less than the open radius), exact hemispherical (depth is equal to the open radius), and deep-spherical (depth is greater than the open radius) cavities. SCF graphs are generated and results of sensitivity analyses are presented. A comparison analysis of the shallow-spherical, exact hemispherical and deep-spherical cavities SCF formulae is also presented. The analysis shows that the shallow-spherical and deep-spherical formulae give almost the same SCF results. Thus, any shallow-spherical or deep-spherical formulae can be used in designing and evaluating casing with spherical cavities. Finally, application procedure for predicting the degraded casing strength is illustrated in this paper.

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