STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

MWD and LWD 925
where m is the decelerated mass. For a 2-kg mass and am = 500 g (5,000 m/s*)
F = 2 x 0.66 x 5000 = 660 daN
Note: 1 daN = 2.25 Ib
Measuring devices run inside the drillstring are mainly subject to axial
impacts. These shocks come from sudden halts in the mule shoe or from an
obstruction in the string. The measuring devices used while drilling are generally
subjected to axial and angular impacts caused by the bouncing of the bit on
the bottom and by the catching of the rollers and stabilizers on the borehole
walls. There is very little information in the literature about measuring impacts
while drilling.
Table 4-12 1 gives the specifications compiled by manufacturers for several
measuring devices. It can thus be seen that such devices must be equipped with
an axial braking system capable of having a stroke of 10 and even 20 cm.
Future Developments. Orientation measurements while drilling are practically
impossible with gimbal gyroscopes. Two-axis flexible-joint gyroscopes should be
able to withstand vibrations and impacts while maintaining a sufficiently accurate
heading provided that periodic recalibration is performed by halting drilling
and switching on the north seeking mode. In the more distant future, laser or
optical-fiber gyroscopes that have been suitably miniaturized should provide a
solution.
Example 5: Vibration and Shock Analysis-Measurement
Package Design
An MWD sensor package has the following specifications:
package mass: 0.906 kg (weight: 2 Ib)
maximum vibrations allowable (all axes)
0.5411 p-p for 2 to 10 Hz
20 g for 10 to 200 Hz
shocks: 1000 g, 0.5 ms (all axes) (g = 9.81 m/s' = 32.17 ft/s2)
Tool
Table 4-1 21
Resistance of Some Directional Tools or
Components to Axial Shocks or Impacts
Deceleration Braklng Initial Velocity
(9. G) Time (ms) Distance (m) (ds)
Azintac 60 11 0.024 4.35
Drill-director 700 10 0.23 46.2
Q-Flex accelerometer 250 11 0.10 18.15
Develco accelerometer 400 1 0.0013 2.64
4-Gimbal accelerometer 50 10 0.016 3.3
2-Axis gyroscope 100 10 0.033 6.6
Military specifications 30 to 100 10