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 chapter 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 KT model has been assessed previously against different models by API Work Group and it has shown to be reliable to be used as design equations. However, the API Work Group included the KT model in the appendix F of the code as informative. 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 de-rating method, whilst 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.