A New OCTG Strength Equation for Collapse Under Combined Loads

Abstract

Abstract Wells are becoming more challenging and casing designers are faced with increasing design pressures. Deep hydrocarbon targets lead to requirements for the casing to resist collapse under external pressure while significant internal pressure and axial compression or tension may exist at the same time. This paper describes the development, and its evaluation, of a new collapse strength equation for oil country tubular goods (OCTG). It is based on a generalization of a model previously proposed by Tamano et al.1. The new model is evaluated through comparisons with both finite element analyses and test data. It is more accurate in dealing with combined internal pressure, external pressure and axial load than, for example, the model currently provided in API Bulletin 5C32. The joint API/ISO SC5 Work Group 2B tasked with modernizing the API 5C3 property equations has evaluated a number of collapse models available in the literature on their performance against several different collapse databases. As a result the model presented here is recommended for developing collapse ratings in the new ISO 10400 standard.