Page 62 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:55 PM Page 38

[ ] Well Design
1.4.1

1.4.1. General Points and Definitions

There are two different jobs that casing must be designed for. The
first is to allow you to safely drill the well and resist any forces or con-
ditions that are imposed on it during drilling, without sustaining signif-
icant damage. The second is to act throughout the life of the well to
meet the well objectives without requiring a workover. The design cri-
teria for each string of casing are different during drilling and during the
remainder of the life of the well. It is your job to design the most eco-
nomical casing that is fit for purpose for the full design life of the well.
Computer programs make detailed casing designs routinely possi-
ble, including triaxial analyses. This can give lower casing costs. While
it is recommended to use a recognized design program, it is important
to check the results with some hand calculations to confirm that the
results are in the right ballpark. Computer programs often have bugs
and a slip in data entry can also give invalid results.
It is useful first to briefly define the terms used in this manual and
to summarize the drilling and production purposes of each string of
casing. Some basic mechanical properties and formulae are discussed,
then each element of the design is considered and the calculations cov-
ered. Tables summarizing the suggested criteria for each casing for
drilling and production are given.
Stove pipe. The stove pipe is used in onshore drilling to protect the
surface soil from erosion and to allow fluid returns while drilling for
the conductor. It is usually set very shallow during location prepara-
tion, a couple of meters below the cellar floor, and no diverter or
blowout preventer (BOP) is nippled up on it. It does not support sub-
sequent casing or wellhead loads and generally serves no purpose once
the conductor is in place. The only design criterion is that it is big
enough to allow the conductor to be run inside it.
Conductor pipe. On a land rig or bottom-supported offshore rig,
the conductor may be drilled and cemented in place (hard seabed) or
driven to refusal with a hammer. The setting depth has to be sufficient
to withstand the extra hydrostatic pressure imposed at the bottom by
bringing returns up to the flowline. The conductor then has a diverter
nippled up to it for drilling the next hole section for surface casing.
On a floating rig, the conductor may be drilled and cemented in
place or jetted into a soft seabed. Returns to the rig are not usually
established and the next hole section (for surface casing) is therefore
drilled with returns to the seabed. If the seabed is soft enough to jet

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