Geology Reference
In-Depth Information
10.4 Turbines, Batteries and Closing Remarks
In this closing section, we offer comments on hands-on tool design, again,
for siren-type MWD pulser systems capable of 10-20 “real” bits/sec (as opposed
to “compressed” bits). Our advice is practical, but not meant to be
comprehensive; nor do we claim to have performed exhaustive trade-off studies
- our observations are simply offered “as is.”
10.4.1 Siren drive.
Assumed in our discussion is a turbine alternator/generator for supplying
the required power to operate the siren. Very likely, the siren should be
powered by a brushless-DC stepper-type motor. This type of motor has the
desirable mechanical characteristics of very high starting torque and extremely
low inertia - ideally suited for rapid angular accelerations. This same type of
motor is used in computer data drives, robotics, etc. Included with this motor is
a high resolution resolver for precise angular positioning and control. Although
we have not performed calculations on power requirements, our belief is that
this motor will require 300-500 W of instantaneous power to provide the desired
data rate results. These motors require a rigid supply of electrical power, say 50
VDC or perhaps as high as 200 VDC. Sophisticated switching electronics,
probably microprocessor controlled, will be required to run the stepper motor.
10.4.2 Turbine-alternator system.
The turbine power supply is somewhat more direct. As is well known, the
mud flow provides an abundance of available power. The mechanical
difficulties include providing a blade design suitable for a wide range of flow
rates. Some experimentation using the wind tunnel methods of Chapter 8 will
be required or, perhaps, novel airfoil concepts involving twisted blades. In
addition, problems with rotary seals are well known. The major difficulty is
providing consistent sealing in the presence of pressure fluctuations contributed,
not only by the drilling environment, but more severely by the pulser itself
which will create variations in hundreds of psi downhole.
The alternator/generator is more straightforward. There are sophisticated
alternator designs available, e.g., homo-polar, but a basic rotating field design is
preferred. A major difficulty in the alternator design is the conditioning
electronics. The peak-to-peak AC voltage out of the alternator is proportional to
the rotational speed, and thus, clever electrical engineering will be required to
convert this varying input into a rigid DC output for powering the siren pulser.
If multiple turbine stages cannot be used because of size constraints and
high power is required to turn “sirens in series,” self-spinning sirens which draw
on the energy of the flowing mud are imperative. In this case, modulation can
performed by using mechanical or magneto-rheological braking as discussed
previously - an electric motor is still required to regulate rotation rates precisely
or to provide “assists” when additional torques are required momentarily.
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