On Some Factors Affecting Casing Collapse Resistance under External Pressure

Abstract

Abstract Casing collapse under external pressure is a complex phenomenon and is one of the governing factors in the tubular design of oil and gas wells. Collapse strength of a casing depends on its outer diameter to wall thickness ratio (i.e. D/t), material Young's modulus, yield stress, shape of stress-strain curve, temperature, ovality, wall eccentricity, circumferential residual stress, as well as combined loadings. This paper studies the quantitative effects of two geometric impefections, namely (i) wear on the inside diameter (ID) of casing and (ii) wall eccentricity angle; as well as two combined loading scenarios: (iii) dogleg bending and (iv) axial compression. Although these scenarios are not covered in the industry Standards, such as API TR 5C3 (June 2018 edition) [1], they are important factors that must be considered in the tubular design of oil and gas wells. In order to quantify the effects of the four scenarios listed above, nonlinear parametric collapse Finite Element Analysis (FEA) studies were performed. Modified Riks method was utilized to predict the casing on-site collapse pressure as well as the unstable post-collapse response. Both material and geometric nonlinearities were taken into account. Elastic-plastic material property with strain hardening was incorporated in the collapse FEA models. Based on the results of the parametric FEA simulations, some important observations and conclusions can be made: (i) for the scenario of casing ID wear, reduction in worn casing collapse resistance is nearly proportional to the reduction in minimum worn thickness regardless of the casing dimensiosns and material grade; (ii) for the case of wall eccentricity irregularity, as the circumferential location of maximum wall thickness moves closer to the minimum wall thickness location, the casing collapse resistance decreases; (iii) casing collapse resistance decreases as the dogleg severity (DLS) increases. Moreover, the effect of dogleg bending on collapse resistance has a strong dependence on casing dimensions and material grade; (iv) in contrast to the effect of axial tension, dependence of collapse resistance on axial compression is quite nonlinear and non-monotonic. In other words, as axial compression increases, collapse resistance increases until the axial compression reaches approximately 50% to 60% (primarily depending on D/t ratio) of yield strength, and then decreases as axial compression increases further.