Triaxial evaluation of loads is used extensively for casing and tubing string design and analysis. The paper presents a method of determining the triaxially based collapse strength of casing and tubing subjected to simultaneous axial and internal pressure loads. The triaxially based collapse strength method was recently adopted by the American Petroleum Institute (API) and an addendum issued to API Technical Report 5C3 (TR 5C3). Casing and tubing subjected to combined loads have higher collapse strength than previous formulas predict.

The triaxially based formula presented incorporates internal pressure and axial load simultaneously into the calculation of casing and tubing collapse strength. The formula is compared to the previous First Edition TR 5C3 uniaxial method of accounting for internal pressure. The paper compares both the uniaxial method and the new triaxially based method with API combined load collapse test data, validating the triaxially based formula. Casing design examples contrast the uniaxial method with the triaxially based method, showing the higher collapse design factors calculated when using the more accurate triaxially based formula.

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