Page 254 - Drilling Technology in Nontechnical Language
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Chapter 10 – EVALUATION 245
Fig. 10–9. Kinley caliper
The driller then applies torque at the surface, and the small amount
of twisting in free pipe can be detected. By moving the tool to different
places in the drillpipe, it is possible to detect where the pipe is free in
tension and in torsion.
Once the pipe depth is known above which the pipe is free, it is then
possible to apply left-hand torque in the string (that is, torque tending to
unscrew the pipe). A small explosive charge is set off on wireline as low as
possible within the free pipe, and the shock and vibration of this explosion
will allow one (or more!) of the drillpipe connections to unscrew. This
process is called an explosive backoff.
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