Siren Design and Evaluation in Mud Flow Loops and Wind Tunnels - Measurement While Drilling Signal Analysis, Optimization and Design - page 231

Various design concepts using the sirens shown in Figures 1.5d.1 and
1.5d.2 were evaluated in detailed test matrixes. These were designed to answer
numerous questions, each associated with significant hardware design
consequences, and perhaps, to raise even more questions. For instance, for a
fixed signal frequency, is it better to spin a siren with few lobes rapidly or is it
more advantageous to rotate a siren with many lobes slowly? Different choices
affect torque, and hence power, and finally turbine or battery options. Tests for
'p indicate that “fewer lobe sirens” produce much stronger signals - this is the
standard solution for single-siren systems. However, in high-data-rate
applications, greater lobe numbers may be required; for these problems, signal
data helps estimate the number of sirens to use in multiple “sirens-in-series”
designs, which enhance net signal by local constructive wave interference. This
motivated our re-examination of “larger lobe sirens” and, in particular, the
testing of signal strength when sirens are connected in tandem.
Another interesting question is this: is it better to use two sirens with large
gaps mounted on the same shaft or a single siren with an extremely small gap as
is done conventionally? The latter is simpler mechanically. But here, erosion
lifespan, torque and power issues are raised. And if multiple sirens are used,
should they all operate simultaneously at an identical frequency, or separately at
separate frequencies for very high data rate? Our point is this: many design
premises that have become commonplace need to be completely reassessed
without prejudice and wind tunnel analyses offer convenient, fast and accurate
solutions. Wind tunnel tests and apparatus must be developed around MWD
specific needs. The photographs in Chapter 1 show different components of the
experimental system. Their designs are not final; they, themselves, represent
improvements to earlier implementations.
As an example, the use of squirrel cage blowers as shown in Figure 9.1 is
acceptable for sirens that are spinning quickly. If not, the intermittent blockage
produced by the lobes induces a significant back-interaction on the blower
blades that is noticeable by eye. When this occurs, it is impossible to maintain a
steady flow and all experimental results are suspect. In later designs, different
compressors were used to provide flow; some required lengthy charging even
for short test times, and other more powerful models, more often than not,
produced air discharges interdispersed with small oily droplets. Often, the
choice of blower is limited by what is already available. The powerful blue
blower shown in Figure 9.2a, although noisy, provided an acceptable solution.
In addition, the layout of the wind tunnel system often depends on facilities and
cost constraints.
Figure 9.2a shows the siren test section (within the short wind tunnel).
Differential pressure transducers appear in Figure 1.5h. As noted elsewhere in
this topic, when a positive overpressure is found on one side of the siren, an
underpressure is found on the opposite side; this difference is the so-called “'p”
signal strength associated with the valve. This strength depends on flow speed,