Limit State Design of oil and gas well casings

Abstract

The casing is widely used as a protective conduit during all phases of drilling operations and production in the oil and gas industry. Traditionally casings were designed using working stress design, which had a number of shortcomings such as poor economics, inflexibility and uneven risk. This thesis was initiated by the increasing demand for an improved design of casings for the oil and gas industry. A new approach using the concept of limit state design is proposed to remedy the limitations of the present design code. Limit state design employs the probability of failure rather than the usage of a safety factor, from which the designer can gain an overall idea of the safety and adequacy of the design. The main objective of this thesis is to set up a set of limit state design equations for casings under different loadings. This objective is tackled by way of investigations into three fundamental casing failure modes, i. e. casing collapse, casing burst and casing axial tensile failure. Simple equations are proposed for the calculation of the load terms in the limit state design equations for the three failure modes. A comprehensive finite element methodology is developed to investigate the ultimate strength of casings with imperfections under different loadings. Extensive comparisons between finite element models and historical experimental data demonstrate that, if the variables are known, the ultimate strength of a casing can be predicted to a satisfactory degree of accuracy using the finite element method. Detailed parametric studies have been performed to investigate the effects of major factors (i. e., the ratio of outside diameter to wall thickness, ovality, eccentricity, material hardening, anisotropy and residual stress) on the casing strength. Existing design equations are assessed by means of full-scale test data, where they are found to be only accurate within a certain region. The investigations for new limit state design equations have been performed by employing a new concept of generalized material behaviour, which is constructed from experimental data and implemented in the finite element simulation. A set of limit state design equations are derived after regression analysis of the numerical results. Comparisons demonstrate that, the new design equations are capable of providing more accurate predictions of casing strength without compromising safety. The limit state design approach is provided in a structured way with a detailed design flow chart to enable a casing designer with a conventional engineering background to assess casing design using the limit state design methodology. It is anticipated that the implementation of a limit state design methodology in the design of casings will lay the foundation for an increased safety awareness whilst enhancing cost savings.