Mud Siren Torque and Erosion Analysis - Measurement While Drilling Signal Analysis, Optimization and Design - page 180

tool.” Finally, “by allowing the gap to be large during the time the tool is not
operating, the passages available for flow through the valve during this time are
increased and the problem of plugging the valve is substantially decreased.”
Again, the plugging or jamming is a fluid-dynamic or aerodynamic
characteristic of the mud siren. It is not necessarily caused by debris or lost
circulation material in the mud, as the above discussion emphasizes - jamming
of the valve is possible even in clean water! U.S. Patent 3,792,429 provides an
alternative solution. “The tool includes a means for both biasing the valve
toward an open position and holding it there when the tool is not operating and
for canceling or substantially reducing the torque loads applied to the drive train
of the tool when the tool is operating. This means comprises a magnetic unit
which develops a magnetic torque characteristic which opposes the normal
hydraulic torque characteristics of the valve.” The authors point out additional
problems associated with this jamming. “Due to the composition of standard
drilling mud, solid material is normally present therein which tends to strain out
of the mud as it is forced through restricted passages in the valve which are
present when the valve is in its closed position. This solid material may
continue to collect in the valve and does present a real problem in that it may
plug the valve to such a extent that the valve cannot be opened by the motor
when operation of the tool is commenced.”
The proposed solution is a means that “is comprised of a magnetic unit
which has a magnet attached to the tool housing and a cooperating magnetic
element attached to the drive train of the tool. The unit develops magnetic
torque characteristics which are greater than the hydraulic torque characteristic
of the valve when the tool is not operating so that the valve will be biased
toward and held in its open position when the tool is not operating.” It is noted
that “the hydraulic torque characteristic of the valve is an increasing function of
the flow rate through the valve. Since the maximum flow rate will normally
occur during operation of the tool, the torque characteristic of the unit is
designed to be roughly equal to the hydraulic torque at this operating flow rate.
By positioning the unit so that the magnetic torque is 180 o out of phase with the
hydraulic torque, the resulting torque applied to the drive train at any time
during operation will be negligible.”
Although this solution is reasonable, it implies significant operational
difficulties and added cost: magnets affect navigational measurements and
requires additional shielding offered by expensive lengths of nonmagnetic drill
collar. U.S. Patent No. 3,867,714 provides still another solution, namely, a
torque producing turbine upstream of the rotor designed to maintain an open
position using hydraulic torque drawn from the mudstream. From the Abstract,
“a mud conditioning means comprising a jet and a spinner is positioned in the
drill string above the valve of said tool wherein said means imparts angular
motion to at least a portion of the drilling fluid in such a manner that the power