Formulas for Rupture, Necking, and Wrinkling of Oil Country Tubular Goods Under Combined Loads

Abstract

SummaryAdvanced well-design methods based on risk assessment of actual failure mechanisms require accurate formulas for burst, collapse, yield, and fracture. Moreover, these strength formulas should be generalized to account for the combined internal pressure, external pressure, and axial load that well tubulars experience.This paper describes the development of a set of formulas for burst strength [i.e., the failure envelope and the design limit for oil country tubular goods (OCTG) under internal overpressure and any axial tension or axial compression]. Phenomena covered are ductile rupture, pipe necking, and local pipe-wall wrinkling, thus spanning the first and second quadrant in the effective-axial-load/pressure-differential-load space.This set of formulas is based on a large-strain, elastic/plastic, <3D> formulation of pipe equilibrium, deformation, and buckling that has been solved analytically in closed form for the situation of combined loads. The formulas describe features such as a maximum internal-pressure load (rupture), a maximum axial-tension load (necking), and local buckling of a pipe under axial compression (wrinkling).Approximate burst-strength formulas are presented that together span the entire axial-load range and closely match the more complex, exact model predictions.The joint American Petroleum Institute/International Organization for Standardization SC5 Work Group 2B (API/ISO SC5 WG2B) tasked with modernizing the API 5C3 property equations has incorporated the approximate burst-strength formulas presented in this paper into the new ISO TR 10400/API TR 5C3 document (API 2008; ISO 2007).