Page 86 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 62

[ ] Well Design
1.4.8

include bending through doglegs, hole drags (overpull) when picking
up, and shock loads. In this case apply a safety factor of 1.75 to the
lesser of API minimum yield or connection tensile strength before
comparing to the tensile force due to weight + force due to pressure
testing (cross-sectional area x test pressure).
If calculations are also made to allow for bending forces due to
directional profile or if the well is vertical, apply a safety factor of 1.5
to the lesser of API minimum yield or connection tensile strength
before comparing to the maximum calculated tension.
If a full triaxial analysis is done (e.g., by using a casing design pro-
gram incorporating triaxial calculations) and a temperature correction
factor is also applied (see “Temperature correction factors for steel”
within this section), then apply a safety factor of 1.25 to the lesser of
API minimum yield or connection tensile strength before comparing
with the maximum calculated stress.
For all the cases mentioned, specifying an overpull allowance based
on the casing weight in air and compared to the lesser of API minimum
yield or connection tensile strength with a 1.1 safety factor should also
be calculated. This assumes stuck pipe (no buoyancy) and new casing.
Often 100,000 lbs is used as a reasonable minimum overpull to be able
to apply on casing and still be 10% below the minimum yield. If this
overpull allowance is more severe than the relevant safety factors
applied above, the casing weight and/or grade and/or pipe mix (in a
mixed string) should be changed accordingly to allow this overpull
with the casing on bottom. This will also ensure that shock loads will
be covered, unless the driller handles the casing very roughly.
The driller should always know how much to safely pull on the
pipe and it is a good idea to mark this on the casing running tally—
especially with a mixed string.
Compression. API figures for minimum yield in tension calculate
the load required to produce a stretch of 0.5% in normal strength steel
(grade E). In compression, Young’s Modulus of Elasticity is still the same
and so the same load when in compression would produce a contrac-
tion of 0.5%. For pure compressive failure, therefore, it is reasonable to
assume the same minimum yield strength as in tension. In most cases
the only time that compression has to be calculated is for a conductor
that supports subsequent casing, completion, BOP, and wellhead loads.
Use the figure for minimum tensile yield without modification (e.g.,
safety factor is 1).

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