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torsional and lateral vibration, strongly dependent on the details of the
bottomhole assembly used, is far from simple. The uncertainties are especially
the case in the highly deviated holes drilled offshore. However, the practical
consequences of vibration are significant. Many twist-offs occur downhole,
particularly near the “neutral point,” and expensive, time-consuming “fishing
jobs” are needed at times to keep the drilling program on schedule.
4.3.3.1 Physical features observed at failure.
The above unknowns would suggest that each drilling situation means a
uniquely different problem requiring a customized description. Despite the
apparent differences, however, drillers have long recognized two general
features common to rotary drilling:
(i) Far downhole, lateral bending vibrations are the most severe; they are
responsible for the greatest frequency of twistoffs and operational failures. And
many of these occur at or near the neutral point, which we define by the
vanishing of static axial loading.
(ii) Uphole, at the surface, only axial and torsional oscillations can be
detected, despite the predominance of severe bending vibrations downhole. This
almost contradictory, but universally accepted observation, is true even in
vertical wells, where damping due to pipe and borehole wall contact is minimal.
Thus, the most damaging downhole vibrations cannot be anticipated from
surface measurements and corrected.
4.3.3.2 Field evidence widely available.
The first observation is supported by post-mortem examinations of many
fished assemblies, and by direct measurement in field experiments, where
recording instruments placed close to the drillbit sampled all vibration modes
simultaneously. Further evidence is found in the computational experiments of
Mitchell and Allen (1985), using a six degree-of-freedom dynamical model.
These authors noted that axial and torsional vibration analysis of three
bottomhole assemblies was unable to identify the cause of repeated washouts
and twist-offs. After detailed study, finite element analysis established lateral
vibrations as the most likely culprit.
The second observation - vanishing lateral disturbances at the surface -
almost contradictory but universally accepted, is true even in vertical wells,
where damping due to pipe and borehole wall contact is minimal. Wolf,
Zacksenhouse and Arian (1985), for example, noted from their experiments that
downhole bending moments cannot be observed at the surface. Also, they
indicated that the nature of downhole lateral vibrations is not well understood;
for example, bending moments estimates based on static models were an order
of magnitude smaller than actual values. Their closing remarks were emphatic,
stressing that high bending moments can be observed downhole, with no
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