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Casing and tubing strings are the main parts of the well construction. All wells drilled for the purpose of oil/gas production (or injecting materials into underground formations) must be cased with material with sufficient strength and functionality. Therefore, this chapter provides the basic knowledge for practical casing and tubing strength evaluation and design. Casing is the major structural component of a well. Casing is needed to maintain borehole stability, prevent contamination of water sands, isolate water from producing formations, and control well pressures during drilling, production, and workover operations. Casing provides locations for the installation of blowout preventers, wellhead equipment, production packers, and production tubing. The cost of casing is a major part of the overall well cost, so selection of casing size, grade, connectors, and setting depth is a primary engineering and economic consideration. Tubing is the conduit through which oil and gas are ...
Abstract The ANSI/API Technical Report 5C3 was updated with addendum Annex M in October 2015 (informative). This is a significant step forward, since the effect of internal pressure on collapse is now calculated on the basis of Lubinski (1975) theory instead of using an inaccurate approximation. This theory is utilized in the present study to demonstrate three-dimensional model with API design formulas expressed as a circle instead of the conventional ellipse. The presented three-dimensional method is easy to use and the main user group is drilling and well engineers working in the field or in well tubular design. Industry practice is to use the effective axial tension (buoyed force) for buckling calculations, while for yielding and collapse analysis the actual axial force is used. In this study we use the effective tension for yielding and collapse calculations, thus employing a stress reference datum that is in agreement with buckling equilibrium and buoyancy. Using the effective tension, a yield circle is produced instead of the conventional yield ellipse. The circle is easy to use and a full set of design equations including buckling stresses are developed in this study. Effective force is equivalent to oilfield definitions such as fictitious force, buckling force, stability force and buoyed force. Note that for some of these terms compression is defined positive. A comprehensive set of equations for yielding and collapse are presented in this study, where the theory includes bending stresses from helical buckling and wellbore curvature. Tension is defined positive herein while compression is negative.
Tubular metallurgy is usually considered to be the dominant design requirement associated with deep sour-gas wells and as such has received extensive treatment in the industry's literature. Other requirements vital to the assurance of a successful well design, however, have not been treated adequately in the literature. This paper addresses some of the nonmetallurgical requirements incorporated in our drilling operations in the deep, sour, geopressured areas south of Jackson, MS, and in the Tuscaloosa trend in Louisiana. The following topics are included: (1) the need and use of the maximum distortion strain energy - von Mises-technique for designing production tubulars; (2) a testing technique used to evaluate connections for use with these tubulars; (3) unusual design and operational features of these wells; and (4) the inspection and handling requirements to ensure the integrity of the wellbore tubular.
The design of the wellbore tubulars for deep sour-gas service is an application of quality assurance. First, materials that are resistive to sulfide stress-cracking are selected. Operating procedures and limits are specified for the design that might include as a design parameter the maximum anticipated bullheading pressure of the well or the need for continuous corrosion-inhibitor circulation. Then the size, weight, and connection for each wellbore tubular string are selected so that the stress in these tubulars is limited to some value below the minimum yield strength of the material, thus giving added assurance of resistance to sulfide stress-cracking. Finally, special inspection and handling procedures to ensure the tubular's physical integrity at the time of running are specified. If the design fails to address any one of these items adequately, a catastrophic failure of the tubulars and blowout of the well may result. The first of these design items-the material's metallurgical requirement-has received extensive discussion in the literature, but discussion of the other three has been much more limited. To provide additional information on these items, we summarize those less-conventional design aspects associated with our exploration activities in the deep, sour, geopressured area south of Jackson MS, and in the Tuscaloosa trend. The first design task is the selection of the wall thickness, weight, and connection for a tubular string, assuming the size has already been selected. As many engineers with experience in designing deep, sour-gas service wells already know, the reality of the situation is that the tube and connection are selected separately-i.e., the tube's size and wall thickness are selected so that the stress is limited to some specified value and then the connection is selected to satisfy the same or possibly some other stress requirement. This separation is necessary as many connections do not limit the maximum stress to below the minimum yield strength of the material. And for very-high-pressure wells, the needed connection for the selected tubular may not exist and has to be custom-designed by one of the thread manufacturers. For this reason, tube selection and connection selection will be treated separately. The next topics are some of the unique operational limits and procedures required to control the axial load on the long casing and tubing strings typical of deep sour-gas wells. And finally, the special inspection and handling procedures we use to ensure physical quality of the procedures we use to ensure physical quality of the tubulars are described.
Summary This paper proposes a tubular-design ellipse dependent on backup pressure (internal pressure for collapse and external pressure for burst), so that the latest American Petroleum Institute (API) collapse-resistance equations (API TR 5C32015) with dependence on internal pressure can be shown simultaneously on the same plot with the ellipse. Unlike the current ellipse used widely in the industry for casing-and-tubing design, which is approximate, the proposed ellipse is exact for collapse, burst, and axial loads. Without the proposed ellipse, use of the approximate ellipse will continue, requiring separate plots for the API collapse-resistance equations. Example cases demonstrate the advantages of the proposed ellipse, including increased accuracy. Results help clarify questions regarding the origin and use of the 2015 API collapse-resistance equations.
Summary This paper develops a new von Mises ellipse based on backup pressure (internal pressure for collapse and external pressure for burst), so the new API collapse equations with dependence on internal pressure can be shown simultaneously on the same plot with the ellipse. Unlike the current ellipse used for casing and tubing design which is approximate, the new ellipse is exact for collapse, burst, and axial loads. Without the new ellipse, the approximate ellipse will continue to be used, and the new API collapse equations must be plotted separately. Example cases demonstrate the advantages of the new ellipse, including increased accuracy. Results help clarify many questions about the new API collapse equations, namely where they come from and how to use them.