Assessment of Design Collapse Equations for OCTG Pipes Under Combined Loads

Abstract

The objective of this paper is to evaluate the design collapse equations presented in Chap. 8 and Annex F of the current standard ISO TR 10400 for casings under external pressure and axial tension. A nonlinear numerical model has been developed to analyze the performance of these equations to predict casing collapse under combined loads. Experimental tests have been performed with different diameters, d/h ratio, and steel grade to calibrate the numerical model. The design collapse equations shown in the ISO TR 10400 are replicated from the API 5C3. Due to the various limitations identified since the first publication of the American Petroleum Institute (API) equations, the API Work Group has assessed different models to be used as design collapse equations and Klever–Tamano (KT) model has shown to be reliable and more accurate. However, the API Work Group included the KT model in the API 5C3 as informative. For this reason, KT model is presented in the Annex F of ISO TR 10400. The work done in this paper has confirmed the better performance of KT model for most of the cases analyzed. For combined loading, the API collapse equation results in a simple strength derating method, while the KT model has achieved similar behavior for low values of axial tension when comparing the experimental results. The axial tension for the casings into the well is likely to be lower than 40% of yield strength. Therefore, the KT model has shown to be more convenient to well design than API equations.