A Service-Life Model for Casing Strings
- Erich F. Klementich (Oil Technology Services Inc.) | Michael J. Jellison (Oil Technology Services Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- April 1986
- Document Type
- Journal Paper
- 141 - 152
- 1986. Society of Petroleum Engineers
- 1.6 Drilling Operations, 1.14.1 Casing Design, 1.14 Casing and Cementing, 1.10 Drilling Equipment, 4.2.3 Materials and Corrosion
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Summary. Conventional casing design is based solely on the achievement of adequate design factors in burst, collapse, and tension from the loads generated by the hanging weight of the pipe, internal and external surface pressures, and fluid densities. The effects of cementing, temperature pressures, and fluid densities. The effects of cementing, temperature changes, ballooning, changes in cross-sectional area, bending, and helical buckling are virtually never considered. This paper describes a service-life model for the design analysis of casing strings that include the significant factors that affect the performance of the string.
The service-life model and analysis method is applicable to the design of any casing string but is especially useful for deep, high-pressure wells. Conventional casing design is often inaccurate-too conservative for shallow strings, too liberal for deep strings. Moreover, some of the design factors used currently in the industry can lead to dangerously undersized strings. Because the calculations in the service-life model and in the subsequent triaxial stress analyses are complex, a computer program helps determine a feasible string design. Computer analysis also liberates the designer from the drudgery of repetitious calculations so that he or she can concentrate on achieving a more accurate estimate of service-life conditions. An accurate service-life model that considers the significant variables and a precise analysis method is essential in obtaining an optimum casing-string design.
Conventional Design and Analysis Methods
The evaluation of a conventional design depends on a comparison between the applied load and the load rating of the pipe. Because most load ratings are based on API equations, I conventional design factors can be referred to as "API load-capacity design factors." Principal and equivalent stress intensities are almost never involved in the evaluation of the design. The applied loads-i.e., the service loads-are calculated from simplistic assumptions based on the hanging weight of the pipe for the tension design, internal and external surface pressures, and fluid densities for the burst and collapse design (Figs. I and 2). In design of a casing string for tension, it is usually assumed that the pipe is suspended in a uniform fluid i.e., buoyancy is considered. Sometimes it is even assumed that the string is hanging free in air-a valid assumption for tubing strings but only in very limited cases. Usually a tension design factor (TDF) of 1.50 to 1.80 is maintained on the joint or pipe-body yield strength.
Experience has shown that a minimum TDF of 1.5 is required to avoid string problems with API threaded and coupled (T and C) connections. Thus a 1.60 TDF is often used. On the other hand, flush-joint casing-especially larger sizes (8% in. [21.9 cm] or greater)-requires a higher TDF to avoid joint problems. The effects of temperature changes, Poisson's effect (lateral expansion or contraction of the casing), and changes in the cross-sectional area of the pipe are not normally considered. Nevertheless, these effects can significantly influence the axial load on the casing string. A minimum design factor of 1.00 to 1.33 is usually maintained on the maximum differential burst pressure to which the casing may be subjected in a conventional design. Note that a burst design factor of 1.0 results in an automatic 10% underdesign. Even if the pipe were hydrostatically tested to the API maximum alternative test pressure, at a 1.0 burst design factor the pipe could be pressure, at a 1.0 burst design factor the pipe could be subjected to an in-service pressure greater than the test pressure. It is a principle of pressure-piping systems never pressure. It is a principle of pressure-piping systems never to work the pipe to a pressure higher than the test pressure. Because minimum internal-yield pressure is based pressure. Because minimum internal-yield pressure is based on 87.5 % of nominal wall thickness and the hydrostatic test pressure is equivalent to 80 % of nominal pipe, a 1.094 internal-pressure design factor must be used to avoid working the pipe to a pressure higher than the test pressure. The recent adoption by the API of modified couplings with teflon seal rings has further complicated the selection of an adequate burst design factor. For some sizes, weights, and grades where the coupling partially controls performance properties, the maximum alternative API performance properties, the maximum alternative API hydrostatic test pressure is 80% of the internal-pressure resistance rating. Consequently, as a practical minimum, a burst design factor of 1.25 is required, unless the exact pressure ratings of the casing and connection are known. pressure ratings of the casing and connection are known. Moreover, 1.30 would be preferable and is used by many operators. The effect of axial load on the internal-pressure resistance of the casing is generally not considered. However, it can be very significant.
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