Page 97 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 73

1.4.13
Casing Design [ ]

versely a pipe in compression will have reduced burst and increased
collapse resistance.
Biaxial effect could be used to reduce the required weight and/or
grade of the casing string as net burst forces are usually highest at sur-
face (assuming there are higher supporting densities outside the casing
than are inside). As tension is also highest at the surface, an increase in
burst strength is seen where it is needed. However, care should be
taken in using this to improve a marginal design because production-
casing tension at surface could reduce due to thermal expansion under
production, reducing the tension and therefore the benefit gained.
The revised collapse resistance of casings accounting for biaxial
effects is shown in Table 4 of API Bulletin 5C2. There is normally no
need to use the formulae to hand calculate biaxial effect. A good cas-
ing design program would account for biaxial using the formulae.

1.4.13 Calculating Axial Loads

Once the burst and collapse requirements are known, the weight
and grade of the casing needs to be estimated before tension and com-
pression can be calculated. Use casing design tables (e.g., API Bulletin
5C2, Halliburton book, casing manufacturer’s data, etc.) to decide
which of the available casings will handle the worst case burst and col-
lapse pressures. Use the lowest available weight/grade of casing that is
strong enough, apply the desired safety factor, and (for a mixed casing
string) apply the temperature correction factor to the minimum burst
pressures before comparing it to the calculated maximum pressures. In
a mature area, you will probably have a stock of your commonly used
casings, otherwise you need to know what casings are available in time
for the earliest possible spud date.
Tension due to weight in a deviated wellbore. In a vertical well,
clearly the tensile force at the top of the casing equals the entire buoy-
ant weight of the casing. If the entire string was placed in a horizontal
bore, the tensile force at the “top” end would be zero. In a deviated well
greater than 0˚ and less than 90˚ inclination, calculate the actual tensile
force, accounting for the wellbore support and buoyancy.
Tensile force due to the weight of the casing in a deviated well, in
air and ignoring friction, resolves to (TVD x weight/foot). Forces due

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