Graphical Approach to Casing Design: Quick Design Charts
The Quick Design Charts allow for fast design of an entire combination casing string. An example is shown in Fig. o.2. To obtain the string for a 9^-in. hole drilled with 12-ppg mud, the casing length is entered on the abscissa and the individual casing string depths are displayed on the ordinate. For each depth section, the chart also provides the casing weight, grade and thread type.
A number of factors can limit the use of these charts. However, depending upon the way in which the charts were originally developed, the following limitations may apply:
12 ppg mud CASING STRINGS DEPTH — 1000 Feet
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9 5/8’ COMBINATION |
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Combination Casing String Design Charts have been plotted ‘
from computer runs.
Theses are based upon a selection of weights & grades to provide the most economical string using the following design factors:
TENSION 1.8(a)
BURST——————- 1.0(b)
COLLAPSE———— 1.125(c)
(a) Buoyancy effect is not included
(b) An outside pressure gradient of 1/2 PSI per ft is included on all surface and intermediate strings (8.5/8" and larger.)
(c) Collapse is based upon lowered resistance due to axial loading.
Section lengths are a minimum of 1000 feet.
Minimum drift diameters for any string are indicated by arr°ws. Qj Special oversize drifts are shown by asterisk. [4]
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Pipe O. D. is 9.875". |
Pipe O. D. is 9.750".
Fig. 5.2: Qu ick design chart. (Courtesy of Lone Star Steel Co.)
• Axial loads are not used to correct for collapse resistance.
• Buoyancy is not considered.
• The cost design criteria are not mentioned.
• The charts are restricted to a very particular load scenario.
EXAMPLE 5-1: The Use of Quick Design Charts.
Using Fig. 5.2 and Table B. l (see Appendix B). design an intermediate combination string for a well that will be drilled in a well-known field. Examine all the possibilities and in particular, aim for the most economical design.
The following data for Example 5-1 was carefully chosen to illustrate the strength of the Quick Design Chart:
9|-in. intermediate casing set at 10,000 ft
Smallest casing section allowed: 1.000 ft
Design factor for burst: 1.0
Design factor for collapse: 1.125
Design factor for pipe body yield: 1.8
Production casing depth (next casing): 15,000 ft
Mud specific weight while running casing: 12 lb/gal
Equivalent circulating specific weight to fracture the casing shoe: 15 lb/gal Heaviest mud specific weight, to drill to the production depth: 15 lb/gal blowout preventer working pressure: 5,000 psi
Although this data works well for Example 5-1. real data cannot always be slotted so readily into a Quick Design Chart as will be demonstrated in Exercises 6. 7. 8 and 9.
The combination casing string obtained directly from Fig. 5.2 is shown in Table 5.1. The prices for the casings come from Table B. l, which is a printout of the file
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Table 5.1: Quick Design Chart Solution to Example 5-1.
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PRICE958.CPR. From Table 5.1. the total cost. I’SS 291,266 and total buoyant weight, 345,570 lbf can be deduced easily.
A five-section string design is more complicated than it needs to be. Further analysis can be performed to check the cost of reducing this number. This particular design chart considers the decrease in collapse resistance due to axial loading. However. the chart is not based on API Bui. 5C3 (1989). which is much more restrictive for non-API casing grades (e. g. S-95). The chart uses a higher table ratings for collapse than those that would be obtained using the API’s formulas. For API casing grades, a casing of equal weight can always be substituted for one of higher grade because the replacement will have a higher collapse resistance; this is not necessarily true for non-API casing grades. In this example, substituting N-80 with S-95 in the interval 3.850 to 1.000 ft results in the combination casing string shown in Table 5.2.
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Table 5.2: Modified Quick Design Chart Solution to Example 5-1.
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As in the earlier case, the total cost. $297,471 and total buoyant weight. 345,570 lbf are easily calculated from Table 5.2.
With this design, the engineer is challenged by the decision either to spend an extra $6,205 (an increase of 2.1391 in cost) and limit the number of sections to three, or to retain the original chart-derived five-section string. A simplified string may mean cost savings elsewhere when field operations are considered together with minimum quantities to be purchased, logistics, etc.