Page 104 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 80

[ ] Well Design
1.4.13

Refer to the previous section, which shows how to calculate the
effect of increasing hole angle on sidewall forces in air.
Subtract the buoyancy forces from the sidewall forces to give the
actual force pressing the pipe against the wellbore wall.

In general,

Force due to friction between surfaces = coefficient of
friction (m) x area x force pressing the surfaces together.

However, when calculating frictional forces between casing and
wall, length is used rather than area and m is generally assumed to be
0.35 in open hole and 0.20 inside another casing.
Therefore, to calculate the frictional force acting against pipe reci-
procation, calculate the actual force pressing the pipe against the well-
bore wall, multiply by m, and then multiply by the length (in feet)
under consideration.
Compression loads. Casings are mostly in tension, except for con-
ductors. However, pure compression (as opposed to buckling that is
covered next) is rarely a problem leading to failure of pipe.
Compression will occur during running and cementing pipe (due
to buoyancy forces acting at the bottom). It can also occur if the casing
is set down on bottom or on an obstruction.
Bending through a dogleg will place the inner fibers of the casing
in compression; the effect is identical in magnitude and opposite in
effect to that experienced by the outer fibers. The calculation is the
same as was done for tension due to bending, covered previously. In
most cases this compressive effect will serve to reduce tension in the
inner fibers rather than lead to compression, except in the lower part
of the casing string when running it through the dogleg section (i.e.,
little weight below the curved section of casing).
If a section of pipe is fixed in two places and free in between,
increasing temperature will cause the casing to expand and this will
then lead to compressive forces. If the pipe is in tension to start with,
the tension will reduce and could eventually go into compression,
depending on the initial tension and the change in temperature. (Refer
to Section 1.4.7, Mechanical Properties of Steel)

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