Buckling Load
From the stability analysis it was observed that casing buckles if the axial stress is less than the average of radial and tangential stresses. Eq. ‘2.179. When casing is installed and the cement is still fluid, the axial load at anv point x between the surface and the bottom of the string is the summation of all loads acting below t his point. If no change in pressures and fluid densities occurs, then once the cement is set the axial load at all points in the casing string will remain the same as at the time of installation. Subsequent drilling and production operations with fluids of different densities leads to changes in pressure differentials in the casing string. Changes in axial load also occur when the average ambient temperature changes. Thus, major sources of axial force are buoyancy effects, piston effect, changes in pressure and fluid densities, and changes in temperature. The summation of all these forces can be termed as the effective axial force.
Chesney and Garcia (1969) presented the first systematic method for determining the effective axial force and presented a general relationship between the effective axial stress and the radial and tangential stresses. Later, Goins (1980) and Rabia (1987) used the same method to analyze the buckling tendency of casing. In the following section, an analysis of axial loads and their effect on casing buckling is presented.