Mud Siren Torque and Erosion Analysis - Measurement While Drilling Signal Analysis, Optimization and Design - page 182

Figure 7.3. A “stable-closed” four lobe siren used in flow loop tests.
These design considerations are validated from numerous (literally
hundreds of) CNPC wind tunnel tests and verified in water and mud flow loops.
To what extent each of (1) - (3) is necessary, and in which combinations, of
course, depends on geometric details, but these three items appear to be the main
relevant design parameters for the basic siren configuration in Figure 7.2. We
emphasize that, despite our appreciation for the roles of these variables, the
financial and time costs for a realistic test matrix are still substantial.
Having identified these parameters as pertinent from the aerodynamic
standpoint - and demonstrated that their effects are repeatable from a testing
perspective - we next ask if fast and efficient computational methods can be
developed to quickly identify not only new design principles, but to provide
engineering trends and details on closing and opening torques, optimum
numbers and sizes of lobes for a given flow rate and drill collar size, pressure
magnitudes, surface velocity predictions for erosion estimates, and so on.
7.1.4 Torque and its importance.
For high-data-rate telemetry, low rotor torque in addition to high signal
amplitude and frequency are required. Originally the work of Chin and Trevino
(1988) focused on achieving stable-opened designs, but with recent
requirements for high-data-rate telemetry on the horizon, research is focusing on
developing designs that not only do not jam, but which are extremely low in
torque as well. Why is low torque of paramount importance? There are three
principal reasons. Low torque (1) reduces jamming tendencies of debris
temporarily lodged in the rotor-stator gap, (2) allows mud sirens to modulate
signals faster and achieve higher data rates, and (3) implies lower power
requirements and thus decreases erosion incidence in downhole turbines.
Additionally, (4) low power consumption allows additional sensors to be
operated, while (5) low torque provides additional flexibility in selecting
optimal telemetry schemes - that is, data rate increases can also be realized
from signal processing advantages and not mechanical considerations alone.