Stories from the Field,Fundamental Questions and Solutions - Measurement While Drilling Signal Analysis, Optimization and Design - page 31

The sirens shown in Figures 1.5.d.1 and 1.5.d.2 are standard ones, typically
several inches in diameter, intended for use in conventional tools. In some
applications, the need for ultra-slim MWD devices arises and very small sirens
are required. Example designs are shown in Figure 1.5.e.1, where tapers and
notches are milled into rotor sides to examine effects on torque and signal. Over
twenty were built. Some spontaneously spin due to wind action, drawing kinetic
energy from the flow, while others jam due to “stable closed” behavior.
Miniature sirens, mounted on fine bearings, are easily tested even without
wind tunnels, and various novel designs have been identified. The candidate
shown in Figure 1.5.e.2, interestingly, remains stable-open regardless of wind
direction, as demonstrated with the lead author breathing in-and-out as the siren
spins and creates signals without interruption (note the presence and absence of
condensation in the plastic tube associated with breathing in-and-out). A
continuous air-hammer “roar” can be heard, which is indicative of low resistive
torque and low turbine power demand - an ingredient for high-data-rate
telemetry. A short movie for this experiment is available from the lead author.
In the figures presented here, we provide photographs of actual sirens
tested and devices used. Many are self-explanatory and are followed by brief
comments. In addition, numerous videos are available for viewing upon
request. All provide a useful “engineering feel” for the types of testing
conveniently and economically performed in our challenge to find high-data-rate
solutions. Improvements to both short and long wind tunnel testing have been
and are continuously being made in light of our experiences described below.
Figure 1.5f.1. Flow straighteners for upstream and downstream use.
Comments: Flow straighteners eliminate large-scale vortical structures in
the air flow induced near pipe bends and fan blades. Small scale turbulence
remains - much as it does in real drillpipe flows, while acoustic plane waves
associated with MWD signals pass effortlessly. For turbine testing, flow
straighteners are essential to eliminate any azimuthal biases to torque that are
inherent in an uncorrected wind.