Note: Descriptions are shown in the official language in which they were submitted.
CA 02270017 1999-04-26
1 "ROTARY PUMP TOP DRIVE "
2
3 FIELD OF THE INVENTION
4 The invention relates to a form of drivehead for rotating a rod string
which in turn drives a progressive cavity pump. No gear-reduction is used and
6 should the drivehead fail, the concentric drive is immune to the high-speed
7 rotational backspin which can occur.
8
9 BACKGROUND OF THE INVENTION
A progressive cavity ("PC") pump is located in a well and is used to
11 pump subterranean fluids to the surface. Over 12,000 oil wells in Canada
use
12 PC pumps. Conventionally the PC pump is driven from a speed-reducing
13 drivehead located at the surface. A long assembly or string of rods
transmit the
14 slow rotation from the drivehead down the well to the PC pump. The top end
of
the rod string is fitted with a polish or polishing rod which is driven by the
16 drivehead.
17 The conventional rotational speed of 100 - 700 rpm is adjusted
18 according to the downhole pumping conditions including the oil
characteristics.
19 Rotational speeds of less than 100 rpm are not currently practised due to
the
large gear reduction required and lack of flexibility to attain greater speeds
as
21 pumping conditions improve. Under certain conditions, such as in the case
of
22 very viscous oils or low formation influx rates, it desirable to have even
lower
23 rotational speeds. These cases are not adequately served with the prior art
24 driveheads.
CA 02270017 1999-04-26
1 When driven, the rod string winds up in torsion like an elastic. In a
2 3,000 foot long rod string, the rods could be wound over 100 revolutions.
When
3 the drivehead ceases to drive the rod string and PC pump (such as during a
4 power failure), the rods try to unwind, spinning the drive and motor
backwards,
called backspin. If unrestrained, backspin speed of the rods can reach upwards
6 of 5,000 rpm and in combination with gear reduction to the drivehead, the
7 resulting speed at the drivehead can dangerously destructive - with most
injuries
8 being caused by sheave self-destruction.
9 Apparatus is known to control backspin. Some driveheads have
braking systems which are activated upon backspin, either when the drive shuts
11 down or the rod string physically begins to spins backwards. Substantially
all
12 modern drivehead are fitted with some sort of device which deals with
backspin.
13 Older driveheads utilise centripetally-actuated mechanical drum-
14 breaking systems. Alternately a hydraulic motor can cause brake pads to
engage a disc-braking rotor on the polish rod. Braking shoes, drums, shoes,
16 pads and discs are subject to wear and the possibility of localised hot
spots
17 which can be an ignition hazard.
18 Some driveheads are hydraulic pump driven. Upon backspin the
19 drive naturally reverts to a pump. Hydraulic fluid is directed through a
restriction
orifice or check valve for releasing pressure. Similarly, in motor-driven gear
21 boxes, a clutch activates on backspin to engage a separate hydraulic motor.
At
22 the high torque of backspin conditions, the hydraulic fluid can become very
hot,
23 or if the fluid is lost, backspin control is also lost.
24 In another instance, it is known to provide an AC drive motor with
an inline shaft. A Variable Frequency Drive or VFD manipulates line AC/DC and
2
CA 02270017 1999-04-26
1 then DC back to a variable frequency AC to control the shaft speed. No gear
2 reduction is provided thus backspin is not dangerously speed multiplied.
Further
3 the VFD can provide inductive braking. Slip associated with AC induction
motor
4 unfortunately limits the motor's ability to reach the low rotational speeds
desired
in some circumstances, such as with high viscosity oils. Low rotational speeds
6 can result in motor overheating.
7 Further, to start an AC induction motor under load can result in in-
8 rush amperage as much as 5 or 6 times the normal full load operating
current.
9 For instance, for a typical 50 Hp NEMA B motor having a full load current of
about 200 A, the in-rush could be 1000 - 1200 amperes. While this is only for
a
11 short period of time and is an accepted disadvantage of AC motors, there is
a
12 further economic impact. Often, the peak current use of an end user is
identified
13 by the utility provider as the basis for setting the per unit charge for
all power
14 consumption. Accordingly, even though steady-state use may be only 200
amperes, the price paid for the electricity could be based on a higher, albeit
16 momentary, electrical current use.
17 Thus, there is a need for a more versatile drivehead which is capable
18 of a wide range of operating speeds, particularly low rotational speeds and
is does
19 not suffer the enumerated backspin disadvantages of the gear-reduced
apparatus.
3
CA 02270017 1999-04-26
1 SUMMARY OF THE INVENTION
2 Simply, a drivehead for a downhole rotary pump is provided. The
3 present invention provides high torque with low rpm output yet avoids the
use of
4 gear-reduction. This is achieved by combining a switched reluctance motor
and
a concentric hollow drive shaft through which the rod string's polish rod
extends.
6 Should there be an interruption in the motor's power, resulting backspin
will not
7 produce a dangerous surface reaction.
8 Broadly then, a drivehead is provided for driving a downhole rotary
9 pump and a rotary rod string, the rod string extending upwardly through a
wellhead to the drivehead and downwardly through a wellbore to the rotary
11 pump, the drivehead comprising:
12 ~ a switched reluctance motor mounted in a frame and secured to
13 the wellhead;
14 ~ a hollow shaft extending through the motor's rotor and sized for
accepting the polish rod of the rotary rod string;
16 ~ a bushing for drivably and supportably connecting the hollow
17 shaft and the rod string; and
18 ~ bearings for rotatably supporting the rotor in the motor's stator
19 including a thrust bearing for supporting the rod string.
21 Figure 1 is cross-sectional view of a well, a rotary pump and a
22 wellhead with a rod string extending therethrough and having a switched
23 reluctance drivehead mounted above the wellhead.
24
4
CA 02270017 1999-04-26
1 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
2 Having reference to Fig. 1, a drivehead 1 is used to rotate the rod
3 string 2 of a rotary pump 3 located downhole in a well 4. The well 4 is
sealed
4 with a wellhead 5 through which the rod string 2 extends. A polish rod 6 at
the
upper end of the rod string 2 extends through the wellhead 5 and through a
6 packing 7. The drivehead 1 is located above the wellhead 5. The drivehead 1
is
7 secured to the wellhead 5 to absorb reaction torque and thus prevent
rotation.
8 The drive head is secured 8 to the wellhead to arrest reactive rotation,
preferably
9 using a flanged connection or chain (shown). The polish rod 6 extends
through
the drivehead 1. A rod clamp 9, secured to the polish rod 6, bears against the
11 top of the drivehead 1 to suspend itself and the rod string 2 therefrom.
12 Optionally a taper-lock bushing locks the hollow shaft to polish rod.
13 The drivehead 1 is usually spaced above the wellhead packing 7
14 using a housing or frame 10 so as to provide ease of access to the polish
rod 6
and packing 7. Accordingly, a length of polish rod 6 is exposed between the
16 wellhead 5 and the drivehead 1. The polish rod 6 is shown extending above
the
17 top of the drivehead 1.
18 The drivehead comprises a conventional switched reluctance
19 motor 11 having a stator 12 and a rotor 13 mounted in the frame 10.
Switched
Reluctance (SR) motors are known, the details of which are known to those of
21 ordinary skill in the art. The components of the SR motor are only
described
22 generally, there being many possible variations thereof.
23
5
CA 02270017 1999-04-26
1 A hollow shaft 14 extends through the rotor 13 for guiding the
2 polish rod 6 concentrically therethrough.
3 The stator 12 has a plurality of slots coil wound for forming phase
4 windings - typically three are provided. Pairs of diametrically opposing
stator
poles conventionally wired in series for forming each independent phase of the
6 multiphased switched reluctance motor. The coils are electronically switched
7 (electronically commutated) in a predetermined sequence so as to form a
8 moving magnetic field.
9 The rotor 13 also forms one or more poles and has no phase
windings but has a plurality of teeth which are closely radially spaced to the
11 stator poles.
12 The stator 12 produces a moving magnetic field induces torque
13 through the teeth of the rotor 13. The rotor rotates to move the teeth
inline with
14 the energized stator poles for mimimizing the flux path (minimum
reluctance).
Using predetermined switching of the appropriate stator poles, the desired
rotor
16 speed is achieved, as is forward or reverse rotation.
17 The SR motor 11, and hence the drivehead 1, produces high
18 starting torque, a relatively flat torque curve and thus is capable of a
wide range
19 of operational speeds. Accordingly, the drivehead is fully functional and
operable at low rotational speeds approaching zero rpm without the need for
21 extraordinary cooling means.
22 The stator 12 is supported in the frame 10 which is secured to the
23 wellhead 5 to prevent rotation. Such a frame is similar to that provided
for
24 vertical AC motors such as those offered by U.S. Electrical Motors,
Division of
Emerson Electric Co., using a NEMA WPI enclosure.
6
CA 02270017 1999-04-26
1 Systems to control the SR motor 11 include electronics to control
2 the switching, means for high frequency switching of the stator's poles and
3 means for determining the position of the rotor 12. These controls are known
in
4 the industry including those provided by Emotron AB, in Sweden.
The polish rod 6 extends upwardly from the packing 7 in the
6 wellhead 5 and then through and out of the drivehead's hollow shaft 14. The
7 polish rod clamp 9 secures to the rod 6 above the drivehead 1 for preventing
the
8 polish rod from passing downwardly through the hollow shaft 14. Accordingly,
9 the hollow shaft 14, and the rotor 13 support the weight of the rod string
2.
Bearings 15 in the frame 10 rotationally guide the rotor 13
11 concentrically within the stator 12. The bearings 15 include a least one
set of
12 thrust bearings for supporting the axial load of the rod string 2.
13 As a result of the novel combination of the present invention,
14 certain advantages are obtained:
~ very low rotational speed capability - including less than 100
16 rpm
17 ~ high starting torque and a substantially flat torque curve;
18 ~ minimal components;
19 ~ use of a concentric shaft and rod string and elimination of
speed-reduction for providing passive and safe response to
21 backspin situations; and
22 ~ elimination of the need for additional mechanical or hydraulic
23 backspin prevention devices.
7
