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There are many factors that affect collapsing strength thus, the measured collapsing strength data of production casing and its test piece have a higher discreteness. Thus it cannot represent the actual casing collapsing strength. The test sample used for API 5C3 casing collapse test at the early stage was too short, and the collapse pressure obtained was on the high side due to the end face effect at both ends. In light of the collapsing test data of the pipes produced by the industry, the measured collapsing strength is higher than the API 5C3 calculation result by more than 30%. For a pipe with a D/t value of 10 to 26, the length of the test sample is more than 8 times the pipe diameter. The collapsing strength calculation method of API 5C3 cannot reflect the obvious technical advance in casing manufacture technique in recent years. In long-term practice and studies, the following problems have been found in the collapsing strength calculation method of API 5C3: 1. (D/t) TE: D/t boundary value between elastoplastic collapse and elastic collapse (D/t) PT: D/t boundary value between plastic collapse and elastoplastic collapse

(D/t) YP: D/t boundary value between yield strength collapse and plastic collapse P F: minimum elastic collapse pressure, MPa P T: minimum elastoplastic collapse pressure, MPa P P: minimum plastic collapse pressure, MPa P YP : minimum yield strength collapse pressure, MPa The meanings of the signs in the formulae are as follows. The formula of elastoplastic collapse pressure is the transitional region between elastic and plastic collapse pressures.

The formula of collapse pressure of yield and the formula of elastic collapse pressure are theoretical derivation formulae, while the formula of plastic collapse pressure is an empirical formula that was obtained by regression analysis of 2888 tests of K55, N80, and P110 test pieces produced before 1968. In the API 5C3 standard, the casing collapse pressure calculation includes four regions, that is, four formulae that include collapse pressure of minimum yield, plastic collapse pressure, elastoplastic collapse pressure, and elastic collapse pressure. Wan Renpu, in Advanced Well Completion Engineering (Third Edition), 2011 Casing Strength Calculation in API 5C3 Collapsing Strength Calculation