Abstract
For a horizontal well or an extended reach well, which is promising for well construction in . future, a well casing is subjected to local wear of wall thickness on its inside surface due to rotational contact with tool joint or drill pipe during drilling. Collapse strength of worn casing is analyzed as an unstable problem by the elastic-plastic finite-element method (FEM) and the decrease ratio of collapse strength due to casing wear is formulated by nonlinear programming using parameters of the wear shape. The elastic-plastic FEM is fully applicable as values calculated by the FEM program agree well with those of experiments. It was found that the decrease ratio of collapse strength of a worn casing is linearly proportional to the decrease ratio of wall thickness in both modes of plastic collapse and elastic collapse, and that the apical angle of the wear shape has a slight influence on collapse strength.
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