Mud Siren Torque and Erosion Analysis - Measurement While Drilling Signal Analysis, Optimization and Design - page 178

Possible transmission methods are several in variety, e.g., mud pulse,
electromagnetic, and drillpipe acoustic. These methods send encoded
information obtained from near-bit sensors to the surface without the operational
difficulties associated with wireline methods. In this chapter, we introduce and
solve special problems associated with the most popular, namely, mud pulse
telemetry, which presently experiences wide commercial application. In
particular, we consider special design issues important to “mud siren” signal
sources, which unlike competing positive and negative pulsers, enable the
highest data rate and the potential for even faster transmissions.
A vintage 1980s mud siren is shown in Figure 7.1 with its key elements
housed in the tool drill collar. Water hammer dominated pressure pulses are
created by the moving upstream rotor while rotating about the stationary
downstream stator , as it interrupts the downward flow of drilling mud. In the
schematic, the resulting pressure wave is shown propagating upward, although,
of course, an equally strong signal of opposite sign propagates downward. If the
rotation continues at constant rate without change, the only wave form created is
a periodic one, which obviously contains no useful information.
Encoded '0' and '1' data is transmitted by non-periodic siren (also referred
to as “valve”) movements using any number of telemetry schemes, e.g., “phase
shift keying,” “frequency shift keying,” “pulse width modulation,” “pulse
amplitude modulation,” and so on. This chapter does not deal with telemetry
issues and modulation schemes, which have been addressed previously. Instead,
it focuses on the fundamental problem of efficient mechanical mud siren source
design. Here, the basic design problems and their solutions, and fast and
efficient computational methods needed to assess the practical viability of new
siren pulsers, are considered in detail.
Figure 7.1. Early-to-late 1980s “stable-closed” design.