Transient Constant Area Surface and Downhole Wave Models - Measurement While Drilling Signal Analysis, Optimization and Design - page 125

4.4.5 Note on multiple-transducer methods.
The excellent abilities afforded by multiple transducer methods in
canceling pump noise and MWD reflections from the pistons and desurger have
been demonstrated in numerous simulations. Minimal information is required -
only an accurate determination of surface mud sound speed is required. The
previous discussions focused on two-transducer methods for simplicity, but their
extension to three or more is straightforward. For example, from Equation 4.4f
or f ' = ½ (p t - cp x ), improvements to p x and p t imply better filtering. The spatial
derivative was calculated from p x = (p 2 - p 1 )/' + O(') where ' is transducer
separation and O(') is the error. Use of a high-order three-point formula for
three-transducer application, for instance, leads to better spatial accuracy.
Similar considerations apply to p t , that is, more levels of data storage can
produce higher time accuracy.
A problem with multiple transducers is their effect on the standpipe.
Drillers are reluctant to tap additional holes since this decreases its structural
integrity. One possible solution is to drill only two holes but to connect these
two by a longer hydraulic hose or metal pipe - and then tap, say, three or more
holes into it. The pressures on this “bypass loop” are used for signal processing.
This idea is described in U.S. Patent No. 5,515,336 awarded to the lead author
in 1996. From the Abstract, “An acoustic detector in a mud pulse telemetry
system includes a bypass loop in parallel with a section of the main mud line
that supplies drilling mud to a drill string. The detector includes a pair of
pressure sensing ports in the bypass line, and one or more pressure transducers
for detecting the pressure at different locations in the bypass loop so that the
differential pressure can be measured. The bypass loop has a small internal
passageway relative to the main mud supply line and may include a constriction
so as to create two regions in the passageway that differ in cross sectional areas.
Forming the pressure sensing ports in the regions of differing cross sectional
areas allows the pressure transducers to more precisely detect the mud pulse
signals. Because of its relatively small cross sectional area, only a small fraction
of the drilling mud flows through the bypass loop. The bypass loop may thus be
constructed of hydraulic hose and a relatively small rigid body having a central
through bore.” Some diagrams are reproduced in Figure 4.4d.
Figure 4.4d. Bypass loop for multiple transducer implementation.