Advanced System Summary and Modern MWD Developments - Measurement While Drilling Signal Analysis, Optimization and Design - page 280

Following Chapter 7, each siren would have downstream rotors, which are
known for restoring torques that diminish significantly from those of upstream
rotors for the same flowrate. Special tapers can be added to rotor sides which
additionally reduce torque, as described in the lead author's work in U. S. Patent
Nos. 4,785,300 and 5,787,052. In fact, torque requirements might be further
reduced if one rotor, or all rotors, were self-spinning in the sense of the
turbosirens discussed previously, although the systems would now require non-
conventional mechanical design. Rather than complete dependence on a
brushless DC motor, which may lack sufficient power to turn all sirens, the self-
spinning system could be modulated by a mechanical braking system.
Magneto-rheological fluids-based braking provides one possibility. A mud
pulser is controlled by an electric field which may be applied to an electro-active
fluid. The electro-active fluid is employed to act as a rapid-response brake to
slow or interrupt the rotation of a mud motor or mud siren, thus creating
pressure pulses in a circulating fluid. In short, the applied electric field alters the
molecular orientation of constituent fluid molecules and very rapidly changes its
viscosity or resistance. In certain embodiments, the electro-active fluid is used
as a direct brake acting on a shaft rotating in a volume of electro-active fluid
where the shaft is coupled to the mud motor or siren. The application of a field
to the electro-active fluid impedes the rotation of the shaft, thus slowing the mud
motor and creating a pressure pulse in the circulating fluid. In another
embodiment, a Moineau pump circulating an electro-active fluid is coupled to
the mud motor. The application of a field to the electro-active fluid slows the
rotation of the pump, thus slowing the mud motor and creating a pressure pulse
in the circulating fluid. Further details are offered in the lead author's U.S.
Patent No. 7,082,078 entitled “Magnetorheological Fluid Controlled Mud
Pulser.”
Again, the principles underlying “sirens in series” designs are developed
by the lead author in U.S. Patent No. 5,583,827, “Measurement-While-Drilling
System and Method.” Essentially, the work done by the rotor on the flowing
mud should result in increased signal. Since the distance between sirens is
small, say one foot, the phase difference between the created signals can be
neglected when compared to that associated with reflections from the drillbit.
Two sirens would create twice the signal of a single siren. Together with, say,
the “1.7” gain arising from the constructive interference due to drillbit
reflections, a pulser system with 3.4 times the signal of a single unoptimized
siren is possible, resulting in significant increases in transmission distance.
10.1.4 Attenuation misinterpretation.
Serious misconceptions in MWD design are found in conventional
perceptions underlying attenuation. The paper of Desbrandes, Bourgoyne and
Carter (1987) describes tests of a fluidic pulser in the flow loop of Figure 9.18
and concludes that signals beyond 25 Hz suffer from great attenuation. In the
early 1990s, this author used the same flow loop, however, with siren and