STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

MWD and LWD 959
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Wob Gages
Bending
Gages
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Torque
Gages
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Figure 4-260. Sketch of theoretical strain gage position in a sub to read
WOB, torque, and bending moment.
R, = 1.5 in.
AR = 9.36 X
R
Using two gages in opposite legs of the bridge will double the sensitivity.
The axial load (compression) gives a uniform stress and strain in the absence
of a bending moment. If a bending moment exists, then one side is extended
while the other is compressed.
During the rotation an alternative signal for the axial load is superimposed
on the DC signal. By filtering, both the axial load and the bending moment
can be measured. In practice, the strain gages are placed in holes drilled in
the measuring sub as shown in Figure 4-261.
Mud Pressure. Internal and external mud pressures are usually measured with
strain gages mounted on a steel diaphragm. Figure 4262 shows a sketch of principle.