Design of Casing Strings

Abstract Considerable economy can be effected by designing each casing string individually for the particular set of conditions involved. The paper discusses methods and procedures for incorporating all of the important variables in the design of individual strings, and step-by-step examples are given. If the casing is never empty, the problem is quite simple and can be solved very easily. While oil strings may or may not at sometime be empty, surface and intermediate strings are seldom, if ever, empty and, therefore, are not subject to collapse loads. Design charts and the usual design methods provide for collapse and tension loads occurring simultaneously. It is extremely improbable that these loads exist at the same time. A practical method of designing casing strings to provide both tensile strength and collapse resistance at the time each is needed is presented. Strings designed by this method will be much cheaper than those selected from charts or designed by the usual computation methods. This economy is realized along with adequate safety. Introduction The design of casing strings has long been a problem to the engineer. Many elaborate charts have been prepared to ease the task of. the designer. These charts are a form of standardization and while standardization is usually a goal much sought after, it has doubtful value in the design of casing strings. There are so many factors involved that it is impossible to include them in any set of charts. Consequently the use of available charts has resulted in thousands upon thousands of tons of steel being buried in the ground needlessly which, of course, represents dollars wasted. This can be corrected by designing strings on an individual basis. The purpose of this paper is to outline all of the required computation methods and procedures and show how the many variable factors can be incorporated into the design of individual strings. A safe, economical method for designing strings is also presented which provides for various load conditions when they occur. Factors To Be Considered In the past most casing strings, whether selected from charts or designed by computation methods, have been based on the assumption that the casing is empty (and, therefore subjected to a full head of mud) and at the same time is hanging in air. Obviously this is an inconsistency. The effect of tension on collapse resistance is considered, but the effect of buoyancy is neglected and yet if collapse acts on an empty string, a buoyant force in excess of the total weight of mud displaced by the casing wall must exist. Buoyancy puts at least some of the casing at the bottom of the string in compression. Compression stresses increase the collapse resistance just as tension stresses decrease it.