Stories from the Field,Fundamental Questions and Solutions - Measurement While Drilling Signal Analysis, Optimization and Design - page 19

1.2.5.3 Pressure, torque and erosion computer modeling.
The mud siren, conceptualized in Figure 1.4a, is installed in its own MWD
drill collar and consists of two parts, a stationary stator and a rotor that rotates
relative to the stator. The rotor periodically blocks the oncoming mud flow as
the siren valve opens and closes. Bi-directional pressure pulses are created
during rotation. At the very minimum, the cross-sectional flow area is half-
blocked by the open siren; at worst, the drill collar is almost completely blocked,
leaving a narrow gap (necessary for water hammer pressure signal creation)
between stator and rotor faces for fluid passage. This implies high erosion by
the sand-laden mud and careful aerodynamic tailoring is needed. Because there
are at least a dozen geometric design parameters, testing is expensive and time-
consuming. Thus, the computational method in Chin (2004), which solves the
three-dimensional Laplace equation for the velocity potential in detail, is used to
search for optimal designs. Computed results, displayed for various degrees of
valve closure, are shown in Figures 1.4b and 1.4c. Other results include
“resistive torque vs angle of closure” important to the design of fast-action
rotors. Results are validated and refined by “short wind tunnel” analyses
described later.
Figure 1.4a. Early 1980s “stable closed' siren (left) and improved 1990s
“stable-opened” downstream rotor design.
While apparently simple in design, unanticipated flow effects are to be
found. The upstream rotor design used in early designs produces numerous
operational hazards, the least of them being stoppage of data transmission.
When rock debris or sudden jarring occurs, the rotor is known to stop at a closed
azimuthal position that completely blocks mud flow. This results in severe tool
erosion, extremely high pressures that affect well control, not to mention surface
safety issues associated with high pressure buildup at the mudpump. Early
solutions addressed the symptoms and not the cause, e.g., mechanical springs or