SELECTION OF CASING WEIGHT, GRADE AND COUPLINGS
After establishing the number of casing strings required to complete a hole, their respective setting depths and the outside diameters, one must select the nominal weight, steel grade, and couplings of each of these strings. In practice, each casing string is designed to withstand the maximal load that is anticipated during casing landing, drilling, and production operations (Prentice, 1970).
Often, it is not possible to predict the tensile, collapse, and burst loads during the life of the casing. For example, drilling fluid left in the annulus between the casing and the drilled hole deteriorates with time. Consequently, the pressure gradient may be reduced to that of salt water which can lead to a significant increase in burst pressure. The casing design, therefore, proceeds on the basis of the worst anticipated loading conditions throughout the life of the well.
Performance properties of the casing deteriorate with time due to wear and corrosion. A safety factor is used, therefore, to allow for such uncertainties and to ensure that the rated performance of the casing is always greater than the expected loading. Safety factors vary according to the operator and have been developed over many years of drilling and production experience. According to Rabia (1987), common safety factors for the three principal loads are: 0.85—1.125 for collapse, 1 —1.1 for burst and 1.6—1.8 for tension.
Maximal load concept tends to make the casing design very expensive. Minimal cost can be achieved by using a combination casing string—a casing string with different nominal weights, grades and couplings. By choosing the string with the lowest possible weight per foot of steel and the lowest coupling grades that meet the design load conditions, minimal cost is achieved.
Design load conditions vary from one casing string to another because each casing string is designed to serve a specific purpose. In the following sections general methods for designing each of these casing strings (conductor pipe, surface casing, intermediate casing, production casing and liner) are presented.
Casing-head housing is generally installed on the conductor pipe. Thus, conductor pipe is subjected to a compressional load resulting from the weight of subsequent casing strings. Hence, the design of the conductor pipe is made once the total weight of the successive casing strings is known.
It is customary to use a graphical technique to select the steel grade that will satisfy the different design loads. This method was first introduced by Goins et al. (1965, 1966) and later modified by Prentice (1970) and Rabia (1987). In this approach, a graph of loads (collapse or burst) versus depth is first constructed, then the strength values of available steel grades are plotted as vertical lines. Steel grades which satisfy the maximal existing load requirements of collapse and burst pressures are selected.
Design load for collapse and burst should be considered first. Once the weight, grade, and sectional lengths which satisfy burst and collapse loads have been determined, the tension load can be evaluated and the pipe section can be upgraded if it is necessary. The final step is to check the biaxial effect on collapse and burst loads, respectively. If the strength in any part of the section is lower than the potential load, the section should be upgraded and the calculation repeated.
In the following sections, a systematic procedure for selecting steel grade, weight, coupling, and sectional length is presented. Table 3.3 presents the available steel grades and couplings and related performance properties for expected pressures as listed in Table 3.2.
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Table 3.3: Available steel grades, weights and coupling types and their minimum performance properties available for the expected pressures.
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LTC = long thread coupling, STC = short thread coupling, BTC = buttress thread coupling, and PTC = proprietary coupling.