Drillstring Vibrations: Classic Ideas and Modern Approaches - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 119

We indicated that bit dynamics affects bearing life and related operational
issues (Fear, Thorogood, Whelehan and Williamson (1992) discuss drilling
economics, cost and risk analysis models). Developments in real-time
measurement can reduce these risks; Falconer, Burgess and Wolfenberger
(1986) show that tooth wear and bearing failure can be diagnosed by MWD
downhole-weight-on-bit and torque sensors. Understanding downhole
vibrations helps us design better sensors and interpretation models. For
example, what proportions of a measured strain are due to axial versus lateral
flexure? Where should different sensor types be ideally placed and mounted? It
goes without saying that improved MWD designs and improved understanding
of the environment go hand-in-hand.
Formation imaging. Inferring medium properties from incident and
scattered waves is standard in ultrasonic, medical and X-ray imaging.
Compton' s classical experiments in scattering, for example, contributed greatly
to our knowledge of atomic structure. Our one-dimensional drillstring is
geometrically simple; it does, though, support several classes of waves, but their
properties are different enough that undesired waves may be filtered. Imaging is
one area where technology transfer can reap significant gains.
Psychological discomfort. Many will acknowledge the irritation that
loose change or belt buckles in clothes dryers causes. Imagine what visions that
rattling drillpipes must evoke in terms of potential blowouts, lost time and
money, not to mention psychological discomfort! This factor appears on few
lists of worrisome factors, but history has shown that as technology advances,
environmental demands increase; this was so in jet engine “noise abatement”
design, intended to lessen “noise pollution” near airport communities!
4.1.5 Elastic line model of the drillstring.
Attempts at developing simulators modeling both drillstring dynamics and
formation response have been made. For example, Eronini, Somerton and
Auslander (1982) describe an early dynamic model for rotary drilling, using
simple lumped parameter ordinary differential equation analysis, as opposed to
distributed parameter partial differential equation analysis.
4.1.5.1 Early efforts.
The GEODYN program in Baird, Caskey, Tinianow and Stone (1984)
represents another effort. This finite element model simulated the three-
dimensional transient response of a PDC bit interacting with a nonuniform
formation. Modeling parameters included hole size, bottom surface shapes and
rock nonuniformities. Ultimately, the detailed response of the BHA and the
entire drillstring would be considered; areas for development included modeling
rock penetration, bit torque, side forces and directional tendencies. Baird,
Caskey, Wormley and Stone (1985) subsequently considered BHA behavior due