Page 84 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 60

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1.4.8 Well Design

hardening, then the crack will suddenly extend a little and stop. This
process repeats until finally the remaining steel cannot take the load
anymore. Thus, you can see the rusty original crack and a set of failure
marks, resembling rings in a cut-off tree. The final failure is a tensile
failure, which will show very rough edges characteristic of this over the
area where the final failure occurred. It is vital to clean with a bristle
brush and soapy water, dry off, and oil the failure faces lightly to pre-
serve these indications. Cleaning with a wire brush and/or allowing
continuing corrosion may make identification of the failure mode
impossible later on.
Finally, thermal strain should be mentioned as it becomes relevant
to buckling in casing design. The Coefficient of Thermal Expansion (a)
gives the thermal strain in a uniform body subjected to uniform heat-
ing. The commonly accepted Coefficient of Thermal Expansion for
steel is given by

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Strain e = 6.9 x 10 /°F (1.24 x 10 /°C)
So for every °C uniform increase in temperature, steel will expand
by 0.0000124 of its original length.

1.4.8. Safety Factors

Safety factors are arbitrary figures that have evolved with experi-
ence. A safety factor is applied to casing yield strength by dividing the
yield strength by the safety factor. (Note that some authorities quote
safety factors as a percentage (e.g. 90%) or as a number less than 1 (e.g.
0.90). In this case, multiply by the safety factor so as to reduce the
available strength.)
Recommendations in various drilling literature and operator policies
on safety factors can be quite confusing and widely varying. Reasons for
various safety factors and clear recommendations are discussed in the
following section.
Many uncertainties exist about the actual forces that a casing may
be subjected to during its design life. Also, the casing strength is like-
ly to deteriorate with time due to wear, erosion, corrosion, and reaction
with produced fluids. Therefore, when designing casings, the expected
forces are calculated and compared to the casing strength as stated in
API Bulletin 5C2 or other authoritative document that is downgraded

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