Siren Design and Evaluation in Mud Flow Loops and Wind Tunnels - Measurement While Drilling Signal Analysis, Optimization and Design - page 244

6. Calculate the worst-case starting static torque using the earlier formula T mud

= T air (U mud /U air ) (Q mud /Q air ) 2 . Are electric or hydraulic motors available
commercially to provide this torque, given the packaging constraints known
for the MWD drill collar? If not, this is an unacceptable design.

7. Calculate the moment of inertia I of the siren rotor and any attachments to

the rotor shaft expected in the actual MWD tool, e.g., alternative shaft,
motor parts, etc. For the torque T mud and the calculated inertia I, do rotor
angular accelerations appear reasonable?

8. Our discussion assumed an initially opened stable-closed rotor attempting to

close. Alternatively, one might have an initially opened stable-open rotor
attempting to remain open. The downward weight shown in Figure 9.4a
would be placed on the opposite side and one would measure the torque
attempting to close the siren. Remember, the entire range of azimuthal
angles must be considered whether the siren valve is stable open or closed.

9. It is important to remember that not all siren valves are stable-open or

stable-closed. Figures 9.4c and 9.4d show sirens that rotate by themselves
without motor drives, drawing upon the power in the oncoming flow to
aerodynamically turn the rotor without further mechanical assistance.
Signal modulation for such “turbosirens” would be accomplished by
braking the rotor using mechanical or electrical means.

The above discussions focus on static torque , that is, torques needed to
open a closed stator-rotor pair or to close an opened pair. These are important
because, after all, the siren rotor must stop, start, speed up or slow down in order
to convey information. Dynamic torque is the resistive torque encountered by
the rotor once it is in steady-state rotary motion. For example, if a phase-shift
keying telemetry scheme is used, the rotor might turn three complete revolutions
before changing phase; dynamic torque would be the torque acting on the rotor
during these three rotation cycles. In general, static torques exceed dynamic
torques; thus, if an electric or hydraulic motor can overcome static torques, then
supplying the torque needed for steady rotation is not a problem. This fact was
not apparent to early MWD designers, who went through great effort to collect
dynamic torque data. Such measurements are not trivial, since expensive
dynamometers must be used to test metal models under actual mud flow
conditions. Testing is time-consuming and labor-intensive and, in this author's
opinion, unnecessary. But interestingly, theoretical considerations could reduce
test matrices substantially if dynamic results were actually deemed important.
Flowfields associated with blunt body sirens are typically separated,
rotating and highly transient and, as a result, hardly amenable to analysis or
computational simulation. Nonetheless, their flow properties do follow well
defined rules which can be extrapolated experimentally. Thus, their complicated
physical nature does not imply that hundreds of mud loop tests with metal
models are necessary. For a mud siren test, the parameters that enter are the