Page 105 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 81

1.4.14
Casing Design [ ]

1.4.14. Calculating for Buckling (N )
b
A long, unconstrained string of thin walled pipe (casing, tubing) will
buckle if it is put in compression. The inherent capacity of casing itself
to resist buckling is effectively zero due to the lengths involved.
Conductors may be put in compression and not buckle until a significant
load is imposed, but in the case of conductors, the diameter and wall
thickness are significant when compared to the unsupported length.
The better the pipe is supported (e.g., centralized in gauge hole;
lying against the hole in deviated wells) the higher the load that can be
applied before buckling occurs. Where pipe is well supported, high
loads can be carried without buckling.
For practical purposes, buckling in casings need only be consid-
ered in vertical wells where the temperature and/or internal pressure
will significantly increase after cementing. In a deviated well the casing
lies against the wall and is supported by it.

Buckling in freestanding conductors. Conductors are mostly
either very short (e.g., land well, subsea wellhead) or are well support-
ed (run in platform guides). Occasionally a conductor is designed to be
freestanding for a significant height above the seabed while carrying
the wellhead loads at the top. In these circumstances a full engineering
analysis of all the service and environmental loads is required. The cir-
cumstances where a long freestanding conductor is required are not
common and the calculations are complex. Therefore, these calcula-
tions are outside the scope of this manual.
Buckling in casings. There are two sets of forces to consider when
calculating whether an unsupported casing will buckle. These forces
can be calculated for any tubular; tubing or drillpipe can also be con-
sidered in this way.
1. The force tending to make a casing buckle—the axial (compres-
sive) force at the depth of interest. In these calculations, compres-
sive force is given a positive value, tensile force is negative. This is
consistent with the signing convention in the literature and is
termed the buckling force (F ).
b
2. The force that resists buckling forces—called the stabilizing force
(F ). This is the average of the radial and tangential stresses in the
s
casing at the depth of interest.

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