Geology Reference
In-Depth Information
8
Downhole Turbine Design and
Short Wind Tunnel Testing
As an experienced professional embarking on a career in MWD in 1981,
having worked as Research Aerodynamicist at Boeing and Turbomachinery
Manager at Pratt & Whitney Aircraft, the jet engine manufacturer, and having
earned graduate engineering degrees from Caltech and M.I.T., I had envisioned
MWD turbine design as a low-tech “slam dunk” affair. Nothing could have
been farther from the truth. In fact, nothing taught in classical aerodynamics
applied that could have been reasonably used for downhole tool design. To
understand why turbine design is frustrating, one needs only to compare the
contrasting operating environments seen by MWD versus jet engine turbines.
We will do this in the following section; fortunately, it turns out that good
turbine design can be approached systematically using basic physical principles.
8.1 Turbine Design Issues
A standard MWD turbine is shown in Figure 8.1 where the mud flows
from left to right. A single “stage” consists of an upstream “stator,” which does
not rotate, and a downstream “rotor” which does. The shaft and alternator
combination to which the rotor is connected is not shown. The turbine
transforms the kinetic energy of the mud into electrical energy used to power
both logging sensors and siren pulser. The demands on the turbine are
nontrivial. Up to several horsepower, e.g., 2-3 HP, may be required in a modern
high-data-rate tool, which is significant in view of mechanical packaging
constraints and severe drill collar space limitations. As power demands
increase, erosion accelerates and life spans decrease rapidly. But very often,
generating the required power is not the problem. For an oncoming axial speed
U, it is known that torque varies with U
2
while power varies like U
3
. Since
positive displacement mud pumps rarely pump with constant speed, a
r
10%
speed fluctuation easily translates into a
r
30% variance in power that must be
regulated electrically. The designer must, of course, err on the side of excess
power, since insufficient power renders an MWD tool useless.
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