Geology Reference
In-Depth Information
Siren motion is driven electrically as opposed to hydraulically; azimuthal
position, torque and 'p signal strength, i.e., the differential pressure between
upstream and downstream sides of the siren, are measured and recorded
simultaneously. This data is important to the design of control and feedback
loops for actual modulation software. At the bottom of Figure 1.5a, a black
PVC tube turns to the right into the wall. This emerges outside of the test shed,
as shown in Figure 1.5b, into a long flow section more than 1,000 feet in length.
Because the waves are acoustically “long,” they reflect minimally at bends, even
ninety-degree bends. The long wind tunnel wraps itself about a central facility
several times before exhausting into open air. This boundary condition is not, of
course, correct in practice; we therefore minimize its effect by reducing signal
amplitude, so that end reflections are not likely to compromise data quality.
Also shown in Figure 1.5b.1 are “a single transducer” near the test shed
(bottom left) and a three-level “multiple transducer array” (bottom right). The
former monitors signals that leave the MWD collar, as they are affected by
constructive or destructive wave interference, while the latter provides data for
echo cancellation and noise removal algorithm evaluation. For the simplest
schemes, two transducers are required; three allow redundancies important in
the event of data loss or corruption. Additional (recorded) noise associated with
real rigsite effects is introduced in the wind tunnel using low frequency woofers.
Numerous concepts were evaluated. Several sirens shown are impractical
but were purposely so; a broad range of data was accumulated to enhance our
fundamental understanding of rotating flows as they affect signal, torque and
erosion. We re-evaluated conventional four-lobe siren designs and developed
methods that incrementally improve signal strength and reduce torque. Results
reinforced the notion that the technology has reached its performance limits.
Radically different methods for signal enhancement and minimization of
resistive torque were needed.
As noted earlier, constructive wave interference provides “free” signal
amplitude without erosion or power penalties. This is cleverly implemented in
two ways. First, FSK with alternating high-low amplitudes is used. High
amplitudes are achieved by determining optimal frequencies from three-
dimensional color plots such as those in Figures 1.2b,c,d. Design parameters
include sound speed, source position and frequency, MWD collar design, and
whenever possible, drillpipe inner diameter and mud density. This information
is used in the waveguide model of Figure 1.2a and also in a model for non-
Newtonian attenuation applicable over the length of the drillpipe.
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