Page 85 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 61

1.4.8
Casing Design [ ]

by a safety factor. The actual safety factors used may be stated by com-
pany or government policy and they should be substituted for the safe-
ty factors assumed below if applicable. Following is a discussion on the
most common safety factors.
Burst. Casings may be subjected to burst pressure throughout their
design life. With time, the burst performance of casings may degrade
due to wear, corrosion and other factors. API recommends 90% of the
minimum internal yield. Experience has shown that this safety factor is
sufficient. Biaxial effects increase the burst strength of casing under ten-
sion but this is not usually accounted for; casing tension can change
with time due to thermal expansion. API does not account for biaxial
effects on burst though it can of course be calculated.
Collapse. As with burst, casings may be subjected to collapse pres-
sure throughout their design life and the same factors may degrade
performance with time. A safety factor of 1.1 applied to the minimum
collapse pressure is recommended. Experience has shown that these
safety factors are sufficient. In deeper casings, biaxial effects reduce
collapse strength of casing under tension (biaxial effects are covered
below) and in certain circumstances, they should be evaluated or the
safety factor should be increased. (Note that Table 4 of API Bulletin
5C2 has corrections for biaxial effects in collapse.)
Tension. A quick review of safety factors in tension recommended
by various authors and operating companies shows more variation
than for any other safety factor. This varies from 1.3 to 1.8, some
accounting for buoyancy and others not. Neal Adams, for instance, rec-
ommends a factor of 1.6 applied using the buoyant load or 100,000 lbs
of overpull, whichever is more conservative. Preston Moore recom-
mends 1.8 using buoyant load. API recommends 90%, which interest-
ingly does not reduce tension below the elastic limit. It makes sense to
examine what the safety factor covers under different circumstances
and to define safety factors for those particular circumstances. Using
too high a safety factor may result in overdesign of the casing, which
may increase costs unnecessarily. It can be seen from the recommen-
dations below that the more effort put into the design, the lower the
safety factor needed and, perhaps, the lower the casing’s grade, weight,
and cost will be.
If a simple casing design on a deviated well is done using only uni-
axial loads related to casing weight and pressure testing, then the
uncalculated factors that will increase the overall casing load will

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