Drillstring Vibrations: Classic Ideas and Modern Approaches - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 109

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Drillstring Vibrations:
Classic Ideas and Modern Approaches
The subject of drillstring vibrations has attracted increasing interest among
oilfield practitioners in recent years. For example, its roles in pipe fatigue,
borehole instability, increased rate of penetration (using controlled resonances),
and as a potential “see ahead of the drillbit” downhole imaging device (e.g.,
drillstring vibrations have been correlated with formation type), among others,
have been noted in the literature. For these reasons, isolated analytical models
have been formulated to simulate empirically observed results, in order to
improve our understanding of different aspects of the physical phenomenon.
Ultimately, integrated drilling simulators would predict penetration rate, fatigue
hazards, violent lateral vibrations, stick-slip phenomena, given the kinematics of
the drillbit, appropriate measures of formation hardness or rock-bit interaction,
drillstring configuration (i.e., “bottom hole assembly” or “BHA”) and so on.
Optimal well planning would then become a reality.
Recent years have seen increasingly powerful computers accompanied by
very sophisticated finite element software for elastic and vibrations modeling. It
is not uncommon to find studies where every single insert on a drillbit is
modeled for stress and heat transfer properties, with general axial, torsional and
lateral vibrations elsewhere along the drillstring simultaneously simulated.
While these capabilities are important in engineering design, they are not
particularly useful in understanding the physics of drillstring vibrations. For
instance, why do drillstrings often fail catastrophically and suddenly at the
neutral point without any indication at the surface? This question reminds the
author of a meeting with a well known operating company researcher years ago.
This scientist, who at the time was elated at “removing an instability” from his
numerical simulator that had just suggested this failure mechanism - a physical
phenomena he was not aware of ! One can only guess how many drillstring
failures his company subsequently endured because he “doctored” his coding
incorrectly. In this topic, we will focus on understanding fundamental principles
and will not review commercial operating software packages.
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