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exactly solved in Chapter 2, with true radiation conditions in both pipe and
annulus, therefore, provides a more rigorous and accurate means for evaluating
advanced telemetry concepts. Of course, final hardware testing and evaluation
must be performed under high pressure mud conditions, but given the problems
associated with a number of long flow loops, and the fact that they offer no
telemetry advantages, the author recommends the use of simpler test vessels.
Figure 9.19. Halliburton 11,000 feet flow loop.
The details associated with one industry flow loop, an 11,000 feet facility
shown in Figure 9.19 and described in “High Data Rate MWD Mud Pulse
Telemetry,” W.R. Gardner, United States Department of Energy Natural Gas
Conference, Houston, Tx, March 25, 1997, a public domain document, are
equally vague. As far as the lead author is aware, little attention had been paid
to wave propagation issues, and unless details related to impedance mismatches,
and inlet and outlet boundary conditions, and so on, are understood, care must
be undertaken in designing and interpreting experiments. Long flow loops
designed without attention to wave acoustics can lead to incorrect field
predictions. Obviously, it is simpler to design long flow loops correctly at the
outset, rather than having to correct measurements later using software.
In assessing the test-worthiness of long flow loops and long wind tunnels,
great care should be taken in eliminating spurious reflections and reverberations.
For example, even small area mismatches between the flexible and solid plastic
tubing in Figure 9.12a can lead to reverberations within the test section that lead
to errors in evaluating signal strength. On the other hand, area mismatches
between drillpipe and MWD collar cross-sectional areas which lead to acoustical
reverberations, modeled in Chapters 2 and 5, should and can be validated
experimentally. The presence of impedance discontinuities is easily detected. If
the pump piston quickly excites the mud column (or a balloon “pops” at the inlet
or outlet of a long wind tunnel), the existence of early reflections indicates
problems. For instance, if the sound speed is 5,000 ft/sec in a 10,000 ft flow
loop, the round-trip transit time should be 4 seconds - earlier arrivals time
would indicate spurious reflections that degrade data quality. Attenuation
measurements should be made “following the wave.” If a pulser is operated by
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