A New OCTG Strength Equation for Collapse Under External Load Only

The influence of manufacturing technology and imperfections on casing collapse strength is not considered in API bulletin 5C3. In the meantime, the current casing true collapse strength cannot be predicted accurately by API bulletin 5C3. For these cases, the joint API/ISO work group ISOTC67 SC5 WG2b has proposed the current API bulletin 5C3 and presented a new collapse strength model with manufacturing imperfections, such as ovality, eccentricity, residual stress, etc., which improves the casing strength calculation accuracy and benefits much more for casing strength design rather than just using API bulletin 5C3. The study on the new ISO collapse model has found that it cannot be used to predict the collapse strength of all pipe sizes, and the current study results are greatly different from actual conditions, which is not the last goal to propose API bulletin 5C3 for the joint API/ISO work group ISOTC67 SC5 WG2b. So, based on the new ISO collapse model, a new oil country tubular goods (OCGT) strength equation, “new equation” for predicting all OCTG sizes collapse strength under external load only has been presented in this paper. Numerical and experimental comparisons show that there is a safety margin appropriate to the desired target reliability level between the predictive values calculated by the new equation and test collapse data, and the new equation calculation accuracy is higher than that of API and ISO, which will make great improvements in the casing design of deep and superdeep wells on the basis of guaranteeing casing material safety.