1.2 Hydraulics
1.2.1 Optimizing Hydraulics
The Bias Unit uses the mud flow to activate the steering
pistons or pads, exerting a side force on the bit. This
operational principle requires a pressure drop between the
steering unit and the annulus, which can be achieved by a
combination of bit nozzling and a flow restrictor inside the
steering unit.
The amount of steering force is proportional to the
differential pressure across the pistons or pads. Insufficient
piston or pad pressure will result in a reduced BHA steering
response, and excessive pressure will increase the risk of
causing damage to the tool. The following table should be
used when optimizing hydraulics.
Recent US Land experience has shown that excessive
pressure with Orbit may also lead to reduced steering
response by breaking down the formation rather than
pushing off of it. For this reason, it is advised to stay in the
green or lower-yellow regions unless local experience
suggests that running in the upper-yellow range is truly
necessary.
1.2.2 Pressure Calculator on the PowerDrive App
The Pressure Calculator on the PowerDrive App should be used during
the planning stages and on the rig for real-time hydraulics adjustments.
PD
2
is the old hydraulic calculation method, it requires installing a
separate program on a personal computer. The PowerDrive App has
the most accurate output and is the preferred method for calculating
PowerDrive hydraulics.
When planning an Orbit job, it is safe to use 0.08 mm as a default ball-
sleeve radial gap. But the DD’s must be sure to re-run hydraulics once
they know the gaps for their specific tool. These can be found on the
first page of the PD Outgoing Systems Test (OST) paperwork. Entering
incorrect ball-sleeve gaps can lead to inaccurate pad pressures.
Request the calculated bearing loss from the motor vendor for all mud-
lubed or sealed bearing motors. Be sure the motor vendor considers
the mud properties and higher than normal pressure drop below the
motor when providing their estimate. These should be entered into the
PowerDrive App for accurate pad pressure calculations. “Sealed”
Bearings may have more leakage than mud-lubed bearings. It is
suggested to request new radial bearings when ordering motors to
avoid excessive leakage. Some motors may leak over 25% fluid and
still meet the vendor’s re-use criteria.
The PowerDrive App is color coded as shown in the top image to turn
green when the pressure drop is within the optimal range. The
pressure drop will increase/decrease as the nozzle sizes are changed
or a flow restrictor is added. Important: The number of nozzles and
their sizes must be reflected on the downhole setup of the bit.
The Pressure Drop Chart can be utilized while drilling to update the
Real-Time Pressure Drop across the PowerDrive. This is critical as the
Mud Weight, Pump Rate and Total Leakage change during a run. It is
important to maintain a Pressure Drop in the green zone as these
parameters change.
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