During the practice of oil and gas drilling and exploration, the casing wear became more and more serious and well known. A crescent-shaped wear pattern is the major wear type and leads to decrease of casing collapse strength. It is obvious that the casing design and safety assessment must take casing wear into account. However, existing empirical equations generally neglect the effect of manufacturing defects on collapse strength, which leads to a large error between calculated results and experimental data. To address the need, the wear mechanism is studied in this paper, and the effects of both wear and manufacturing defects on the collapse strength are considered. Based on this study, a new equation for calculating the collapse strength of a casing with crescent-shaped wear is presented. Comparison with experiment and alternate assessment methods shows that the new equation is much better than other methods and its calculated results are much closer to the experimental data. The new equation was used for the casing design of ultra-deep wells, especially in the north–west part of China.

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