An Improved Design Equation for Tubular Collapse

Abstract

Abstract This paper presents a new equation for predicting the collapse of tubulars under external pressure. The development of the equation is based on a large number of nonlinear finite element simulations of tubulars with different geometrical tolerances and mechanical properties. The simulation results for the collapse pressure and post-collapse geometry of the tubulars were verified with full scale physical tests. A nonlinear regression analysis was then performed on the data obtained from the simulations to optimize (he parameters of tile collapse equation. The new equation accounts for variations in the tubular diameter, wall thickness, ovality, eccentricity, and material elastic-plastic behavior. This new collapse equation provides significant advantages over existing formulas, mainly because it was developed based on non-linear mechanics solutions of the collapse problem as compared to equations derived from statistical analysis. In addition, the equation gives the true collapse pressure of the tubular, whether the tubular collapses elastically or after it has yielded. The new equation could be used to design tubulars based on manufacturing tolerances with respect to wall thickness, ovality, and eccentricity as well as material mechanical properties. This allows for design optimization which could account for significant cost savings, especially when designing expensive non-API tubulars such as corrosion resistant alloy tubulars. The new collapse equation presented in this paper provides accurate predictions of the collapse pressure of tubulars and accounts for tolerances in the tubular geometry and material elastic-plastic behavior. This new equation is simple and could be used in optimizing tubular design. Introduction One of the most important considerations involved in the design of casing and tubing for deep oil and gas wells, and drilling risers in deep water, is the resistance to collapse under external pressure. at the depths at which tubulars are now commonly used, the collapse of tubulars largely determines the wall thickness and material grade that is required for a given application. In addition, there is a tremendous cost incentive to design casing strings properly with respect to collapse. Overdesign leads to costly tubular purchases while under-design can lead to failures and costly repair operations. For these reasons, an accurate understanding of the collapse properties of tubulars is extremely valuable. The collapse problem presents difficulty for two reasons. First, the collapse of tubulars is an instability type of failure and is sensitive to imperfections such as ovality, eccentricity, and localized wall reduction. The impact of these imperfections is difficult to evaluate analytically. The second source of difficulty arises because collapse can occur in two distinct modes - the elastic mode in which failure occurs under elastic deformation and the elastic-plastic mode in which failure is preceded by permanent deformation of the tubular. Which failure mode actually occurs depends on the ratio of the tube diameter to wall thickness, the tube material yield strength, and the elastic modulus. For tubulars that collapse elastically, analytical solutions have succeeded in predicting collapse pressures, but failed to account for the sensitivity of these pressures to imperfections. P. 7^