Page 99 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 75

Casing Design [ ]
1.4.13

76.60 x 40 = 3064 lbs. Note that by multiplying the vertical interval by
the nominal weight in air, we arrive at the same figure as resolving the
vectors for axial load.
Therefore, for weight in air, to calculate the tension due to the
weight of the pipe in an inclined bore, it is only necessary to multiply
the weight per unit length by the vertical interval.
The axial force due to the effect of fluid acts at the bottom of the
pipe. This puts the bottom section in compression by: (hydrostatic
pressure x cross-sectional area).
Tension due to overpull if stuck. For an unmixed string, calculate
the tension of the top joint due to the weight in air of the casing. For
a mixed string, calculate the tension of the top of each component of
the combined string (using weight in air). In a deviated well the ten-
sion at the top due to the weight of the casing equals the nominal wt/ft
multiplied by the TVD for each casing below. As is discussed above,
casing in a deviated well is partially supported by the wellbore and the
resultant axial force can be calculated simply by using TVD x nominal
weight. The difference is significant in high-angle wells.
The minimum yield strength divided by a 1.10 safety factor should
exceed this tension by an overpull allowance (usually at least 100,000 lbs)
to account for overpull while running. Weight in air is used rather than
buoyant weight because if the casing gets stuck the effect of buoyancy is
lost and the overpull allowance will need to be added to the weight in air.
For a mixed string of different weights/grades, the same calculation
should be made for the top joint of each section. For the top section
joints at depth, divide minimum yield by the safety factor (1.10) and
then by the temperature correction factor; ensure that this exceeds the
calculated tension by at least the overpull allowance.
Tension due to buoyant string weight plus test pressure.
Calculate the tension at the top joint of each different section of casing
string, when in mud and cement (refer to Section 1.4.10, “Methods of
Applying Buoyancy Effects”). Add to this tension the force exerted
when testing to the anticipated test pressure after bumping the plug.
This will equal the cross-sectional area inside the casing in square inch-
es multiplied by the test pressure in psi, or

2
Load =.7854 d × psi

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