Downhole Motors - Turbines- Turbine Characteristics

Turbine Characteristics
• Torque and RPM are inversely proportional (i.e. as RPM increases,
torque decreases and vice versa).
• RPM is directly proportional to flow rate (at a constant torque).
• Torque is a function of flow rate, mud density, blade angle and the
number of stages, and varies if weight-on-bit varies.
• Optimum power output takes place when thrust bearings are balanced.
• Changing the flow rate causes the characteristic curve to shift.
• Off bottom, the turbine RPM will reach “run away speed” and torque
is zero.
• On bottom, and just at stall, the turbine achieves maximum torque and
RPM is zero.
• Optimum performance is at half the stall torque and at half the
runaway speed, the turbine then achieves maximum horsepower.
• A stabilized turbine used in tangent sections will normally cause the
hole to “walk” to the left.

Downhole Motors - Turbines- Turbine Observations

Turbine Observations
• There is minimal surface indication of a turbine stalling.
• Turbines do not readily allow the pumping of LCM.
• Sand content of the drilling fluid should be kept to a minimum.
• Due to minimal rubber components, the turbine is able to operate in
high temperature wells.
• Pressure drop through the tool is typically high and can be anything
from 500 psi to over 2000 psi.
• Turbines do not require a by-pass valve.
• Usually, the maximum allowable bearing wear is of the order of 4mm.

Downhole Motors - Turbines- Directional Turbine

Directional Turbine
This is a short tool which has a set number of stages and its bearing section
entirely within one housing. That is, it is not a sectional tool and will be
typically less than 30 feet long. It is designed for short runs to kick off or
correct a directional well, using a bent sub as the deflection device.
Steerable turbodrills do exist and will be discussed later.

Downhole Motors - Turbines- Bearing Section

Bearing Section
Usually, thrust bearings are made up of rubber discs (Figure 5-24) which
are non-rotating (being fixed to the outer housing of the tool) and rotating
steel discs attached to the central rotating shaft. Long bearing sections
known as cartridges are used for long life in tangent or straight hole drilling
sections. These are changeable at the rigsite. If the bearings wear past the
maximum point, considerable damage can be inflicted as the steel rotors
will crash into the stators below.

Downhole Motors - Turbines- Drive Section

Drive Section
This will consist of a series of bladed stators, fixed to the outer tool housing
and bladed rotors fixed to the central rotating shaft. Mud flow is deflected
at a pre-determined angle off the stator blades to hit the rotor blades and
cause the shaft to rotate. The angle of the blades will affect the torque and
speed output of the turbine

Downhole Motors - Turbines

Turbines
A turbine is made up of several sections:
• Drive stages or motor section.
• Axial thrust bearing assembly and radial bearings.
• Bit drive sub.
As stated earlier, the drive stages, or motor section, consists of a series of
stators and rotors of a bladed design. This stator and rotor combination
form a stage. Turbines are referred to as 90 stage, 250 stage, etc. The
number of stages determines the torque generated. Each stage,
theoretically, applies an equal amount of torque to the control shaft and it is
the sum of those torques which will be output to the bit.
The drive sub is simply the bit connection and bearing shaft. Radial
bearings protect the shaft from lateral loading and the thrust bearings
support the downwards hydraulic thrust from mud being pumped through
the tool and the upward thrust of weight being applied to the bit.
Theoretically, weight on bit should be applied to equalize the hydraulic
thrust, which unloads the bearings and prolongs their life.

Downhole Motors - Positive Displacement Motors : Motor Orientation/Control

Motor Orientation/Control
All directional wells require steering during initial kick offs, correction
runs, sidetracks, and re-drills. Once the desired direction in which the tool
should be faced is determined, the next step is to actually face the tool in
that direction in order to drill the predetermined course.
For the Mach 1/AD motor, a cartridge data transmission (CDT) system has
been developed that allows orientation of the motor in a particular
direction, while still allowing drilling with drillstring rotation. This CDT
system uses a special heavy duty steering tool which provides continuous
surface readout of the drift angle and azimuth, as well as toolface
orientation while drilling ahead.
A “hard wire” from the steering tool, through the drillstring to the surface,
relays the information to computerized surface equipment. Data
transmitted from the steering tool is updated and converted instantly to
information which can be used to make any necessary corrections to the
motor.