Information
-
Patent Grant
-
6604921
-
Patent Number
6,604,921
-
Date Filed
Thursday, January 24, 200222 years ago
-
Date Issued
Tuesday, August 12, 200321 years ago
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Inventors
-
Original Assignees
-
Examiners
- Denion; Thomas
- Trieu; Theresa
Agents
- Salazar; J. L. Jennie
- Jeffery; Brigitte L.
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CPC
-
US Classifications
Field of Search
US
- 418 48
- 418 178
- 418 153
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International Classifications
-
Abstract
A stator for a positive displacement motor including an external tube. The external tube includes an outer surface and an inner surface, and the inner surface includes at least two radially inwardly projecting lobes extending helically along a length of the external tube. A liner is positioned adjacent the inner surface, and the liner conforms to the radially inwardly projecting lobes formed on the inner surface and to the helical shape of the inner surface. A thickness of the liner is at a maximum at the at least two radially inwardly projecting lobes.
Description
BACKGROUND OF INVENTION
1. Field of the Invention
The invention relates generally to stators for use with positive displacement drilling motors. More specifically, the invention relates to selecting an optimized liner thickness for a stator so as to increase the power available from a positive displacement motor while increasing longevity of the stator.
2. Background Art
Positive Displacement Motors (PDMs) are known in the art and are commonly used to drill wells in earth formations. PDMs operate according to a reverse mechanical application of the Moineau principle wherein pressurized fluid is forced though a series of channels formed on a rotor and a stator. The channels are generally helical in shape and may extend the entire length of the rotor and stator. The passage of the pressurized fluid generally causes the rotor to rotate within the stator. For example, a substantially continuous seal may be formed between the rotor and the stator, and the pressurized fluid may act against the rotor proximate the sealing surfaces so as to impart rotational motion on the rotor as the pressurized fluid passes through the helical channels.
Referring to
FIG. 1
, a typical rotor
10
includes at least one lobe
12
(wherein, for example, channels
14
are formed between lobes
12
), a major diameter
8
, and a minor diameter
6
. The rotor
10
may be formed of metal or any other suitable material. The rotor
10
may also be coated to withstand harsh drilling environments experienced downhole. Referring to
FIG. 2
, a typical stator
20
comprises at least two lobes
22
, a major diameter
7
, and a minor diameter
5
. Note that if the rotor (
10
in
FIG. 1
) includes “n” lobes, the corresponding stator
20
used in combination with the rotor
10
generally includes either “n+1” or “n−1” lobes. Referring to
FIG. 3
, the stator
20
generally includes a cylindrical external tube
24
and a liner
26
. The liner
26
may be formed from an elastomer, plastic, or other synthetic or natural material known in the art. The liner
26
is typically injected into the cylindrical external tube
24
around a mold (not shown) that has been placed therein. The liner
26
is then cured for a selected time at a selected temperature (or temperatures) before the mold (not shown) is removed. A thickness
28
of the liner
26
is generally controlled by changing the dimensions of the mold (not shown).
A lower end of the rotor may be coupled either directly or indirectly to, for example, a drill bit. In this manner, the PDM provides a drive mechanism for a drill bit independent of any rotational motion of a drillstring generated proximate the surface of the well by, for example, rotation of a rotary table on a drilling rig. Accordingly, PDMs are especially useful in drilling directional wells where a drill bit is connected to a lower end of a bottom hole assembly (BHA). The BHA may include, for example, a PDM, a transmission assembly, a bent housing assembly, a bearing section, and the drill bit. The rotor may transmit torque to the drill bit via a drive shaft or a series of drive shafts that are operatively coupled to the rotor and to the drill bit. Therefore, when directionally drilling a wellbore, the drilling action is typically referred to as “sliding” because the drill string slides through the wellbore rather than rotating through the wellbore (as would be the case if the drill string were rotated using a rotary table) because rotary motion of the drill bit is produced by the PDM. However, directional drilling may also be performed by rotating the drill string and using the PDM, thereby increasing the available torque and drill bit rpm.
A rotational frequency and, for example, an amount of torque generated by the rotation of the rotor within the stator may be selected by determining a number of lobes on the rotor and stator, a major and minor diameter of the rotor and stator, and the like. An assembled view of a rotor and a stator is shown in FIG.
3
. Rotation of the rotor
10
within the stator
20
causes the rotor
10
to nutate within the stator
20
. Typically, a single nutation may be defined as when the rotor
10
moves one lobe width within the stator
20
. The motion of the rotor
10
within the stator
20
may be defined by a circle O which defines a trajectory of a point A disposed on a rotor axis as point A moves around a stator axis B during a series of nutations. Note that an “eccentricity”e of the assembly may be defined as a distance between the rotor axis A and the stator axis B when the rotor
10
and stator
20
are assembled to form a PDM.
Typical stators known in the art are formed in a manner similar to that shown in FIG.
2
. Specifically, an inner surface
29
of the external tube
24
is generally cylindrical in shape and the stator lobes
22
are formed by molding an elastomer in the external tube
24
. Problems may be encountered with the stator
20
when, for example, rotation of the rotor
10
within the stator
20
shears off portions of the stator lobes
22
. This process, which may be referred to as “chunking,” deteriorates the seal formed between the rotor
10
and stator
20
and may cause failure of the PDM. Chunking may be increased by swelling of the liner
26
or thermal fatigue. Swelling and thermal fatigue may be caused by elevated temperatures and exposure to certain drilling fluids and formation fluids, among other factors. Moreover, flexibility of the liner
26
may lead to incomplete sealing between the rotor
10
and stator
20
such that available torque may be lost when the rotor compresses the stator lobe material, thereby reducing the power output of the PDM. Accordingly, there is a need for a stator design that provides increased power output and increased longevity in harsh downhole environments.
SUMMARY OF INVENTION
In one aspect, the invention comprises a stator for a positive displacement motor. The stator comprises an external tube comprising an outer surface and an inner surface, and the inner surface comprising at least two radially inwardly projecting lobes extending helically along a selected length of the external tube. A liner is disposed proximate the inner surface, and the liner conforms to the radially inwardly projecting lobes formed on the inner surface and to the helical shape of the inner surface. A thickness of the liner is at a maximum proximate the at least two radially inwardly projecting lobes.
In another aspect, the invention comprises a positive displacement motor. The positive displacement motor comprises a stator including an external tube comprising an outer surface and an inner surface. The inner surface comprises at least two radially inwardly projecting lobes extending helically along a selected length of the external tube. A liner is disposed proximate the inner surface, and the liner conforms to the radially inwardly projecting lobes formed on the inner surface and to the helical shape of the inner surface. A thickness of the liner is at a maximum proximate the at least two radially inwardly projecting lobes. A rotor is disposed inside the stator, and the rotor comprises at least one radially outwardly projecting lobe extending helically along a selected length of the rotor. The at least one radially outwardly projecting lobe formed on the rotor is adapted to sealingly engage the at least two radially outwardly projecting lobes formed on the liner.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1
shows a prior art rotor.
FIG. 2
shows a prior art stator.
FIG. 3
shows an assembled view of a prior art positive displacement motor.
FIG. 4
shows a cross-sectional view of an embodiment of the invention.
DETAILED DESCRIPTION
FIG. 4
shows an embodiment comprising at least one aspect of the present invention, A positive displacement motor (PDM)
30
comprises a stator
32
and a rotor
34
. The stator
32
comprises an external tube
38
that may be formed from, for example, steel or another material suitable for downhole use in a drilling environment. The stator also comprises a liner
36
that may be formed from an elastomer, a plastic, or any other suitable synthetic or natural material known in the art. In some embodiments, the liner may also be formed from a fiber reinforced material such as the materials described in co-pending U.S. patent application Ser. No. 10/097,480, and assigned to the assignee of the present application.
The external tube
38
comprises a shaped inner surface
44
that comprises at least two lobes
46
formed thereon. The lobes
46
are helically formed along a selected length of the external tube
38
so that the lobes
46
define a helical pattern along the selected length. The helical form of the inner surface
44
generally corresponds to a desired shape for stator lobes. The liner
36
typically comprises at least two lobes
40
, and a thickness
42
of the liner
36
is non-uniform throughout a cross-section thereof. The lobes
40
(and the liner
36
) are helically formed along a selected length of the external tube
38
such that the liner
36
conforms to the helically shaped inner surface
44
so that the at least two lobes
46
formed on the shaped inner surface
44
correspond to the lobes
40
formed in the liner
36
. The external tube
38
, including the inner surface
44
, may be helically shaped by any means known in the art including machining, extrusion, and the like.
In some embodiments, the shaped inner surface
44
of the external tube
38
is adapted to provide additional support for the liner material. The shaped inner surface
44
“stiffens” the liner
36
by providing support for the liner
36
(e.g., by forming a metal backing), thereby increasing power available from the PDM. For example, shaping the inner surface
44
to form a contoured backing for the liner
36
may stiffen the liner material proximate the lobes
40
by reducing an amount by which the liner
36
may be compressed when contacted by the rotor
44
so that a better seal may be formed between the rotor
44
and the stator
32
. Moreover, reduced flexibility increases an amount of torque required to stall the PDM.
The thickness
42
of the liner
36
may be increased at selected locations that are exposed to, for example, increased wear and shear (e.g., proximate the lobes
40
,
46
), so that the longevity of the stator
32
and, therefore, the longevity of the PDM
30
may be increased. In some embodiments, the thickness of the liner
36
is selected so as to maximize a shear strength of the liner
36
proximate the lobes
46
The shaped form of the inner surface
44
typically results in a thinner liner
36
than is commonly used in prior art stators (such as that shown in FIG.
3
). Fluid pressure is less likely to deform the liner
36
and, accordingly, the liner
36
is less susceptible to deformation that could reduce the efficiency of the seal formed between de rotor
34
and stator
32
(thereby producing an additional loss in power output of the PDM
30
).
As shown in
FIG. 4
, the thickness
42
of the liner
36
may be varied so that a thickness TA of the portion of the liner
36
proximate the lobes
46
is greater than a thickness of other portions of the liner
36
(e.g., a thickness TB of the portion of the liner
36
proximate channels
48
). The thickness
42
of the liner
36
may be selected to generate a desired amount of contact (or, if desired, clearance) between the liner
36
and the rotor
34
. For example, the thickness
42
of the liner
36
may be selected to form a seal between the rotor
34
and the stator
32
while maintaining a desired level of compression between the rotor
34
and stator
32
when they are in contact with each other. Moreover, the thickness
42
of the liner
36
may be selected to permit, for example, swelling or contraction of the liner
36
caused by elevated temperatures, contact with drilling fluids and other fluids, and the like.
In some embodiments, the thickness TA of the liner
36
proximate the lobes
46
is selected to be at least 1.5 times the thickness TB of the liner
36
proximate the channels
48
. In other embodiments, the thickness TA of the liner
36
proximate the lobes
46
may be selected to be less than or equal to 3 times the thickness TB of the liner
36
proximate the channels
48
. Other embodiments may comprise other thickness ratios depending on the type of material (e.g., elastomer, plastic, etc.) selected to form the liner
36
.
Note that the embodiment in
FIG. 4
is generally referred to as a “5:6” configuration including 5 lobes formed on the rotor and 6 lobes formed on the stator. Other embodiments may include any other rotor/stator combination known in the art, including 1:2, 3:4, 4:5, 7:8, and other arrangements. Moreover, as described above, stators may generally be formed using “n+1” or “n−1” lobes, where “n” refers to a number of rotor lobes. Accordingly, the embodiment shown in
FIG. 4
, and other embodiments described herein, are intended to clarify the invention and are not intended to limit the scope of the invention with respect to, for example, a number of or arrangement of lobes.
Accordingly, the present invention allows for an inner surface of an external stator tube to be shaped so as to enable optimization of a liner thickness and to provide a stiff backing for the liner material. Optimizing liner thickness leads to increased power output and increased longevity of the power section.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
Claims
- 1. A stator for a positive displacement motor comprising:an external tube comprising an outer surface and an inner surface, the inner surface comprising at least two radially inwardly projecting lobes extending helically along a selected length of the external tube; and a liner disposed proximate the inner surface, the liner conforming to the radially inwardly projecting lobes formed on the inner surface and to the helical shape of the inner surface, wherein a thickness of the liner is at a maximum proximate the at least two radially inwardly projecting lobes.
- 2. The stator of claim 1, wherein a thickness of the liner is selected to form a desired level of compression between the liner and a rotor.
- 3. The stator of claim 1, wherein a thickness of the liner is selected to maximize a shear strength of the liner proximate the at least two radially inwardly projecting lobes.
- 4. The stator of claim 1, wherein a thickness of the liner is selected so as to maximize a power output of a positive displacement motor.
- 5. The stator of claim 1, wherein the inner surface is shaped so as to reduce an amount of fluid pressure deformation of the liner.
- 6. The stator of claim 1, wherein a thickness of the liner proximate the at least two radially inwardly projecting lobes is at least 1.5 times a thickness of the liner proximate channels formed between the at least two radially inwardly projecting lobes.
- 7. The stator of claim 1, wherein a thickness of the liner proximate the at least two radially inwardly projecting lobes is less than or equal to 3 times a thickness of the liner proximate channels formed between the at least two radially inwardly projecting lobes.
- 8. A positive displacement motor comprising:a stator comprising an external tube comprising an outer surface and an inner surface, the inner surface comprising at least two radially inwardly projecting lobes extending helically along a selected length of the external tube, and a liner disposed proximate the inner surface, the liner conforming to the radially inwardly projecting lobes formed on the inner surface and to the helical shape of the inner surface, wherein a thickness of the liner is at a maximum proximate the at least two radially inwardly projecting lobes; and a rotor disposed inside the stator, the rotor comprising at least one radially outwardly projecting lobe extending helically along a selected length of the rotor, the at least one radially outwardly projecting lobe formed on the rotor adapted to sealingly engage the at least two radially outwardly projecting lobes formed on the liner.
- 9. The positive displacement motor of claim 8, wherein a thickness of the liner is selected to form a desired level of compression between the liner and a rotor.
- 10. The positive displacement motor of claim 8, wherein a thickness of the liner is selected to maximize a shear strength of the liner proximate the at least two radially inwardly projecting lobes.
- 11. The positive displacement motor of claim 8, wherein a thickness of the liner is selected so as to maximize a power output of the positive displacement motor.
- 12. The positive displacement motor of claim 8, wherein the inner surface is shaped so as to reduce an amount of fluid pressure deformation of the liner.
- 13. The positive displacement motor of claim 8, wherein the inner surface is shaped so as to maximize a power output of the positive displacement motor.
- 14. The positive displacement motor of claim 8, wherein a thickness of the liner proximate the at least two radially inwardly projecting lobes is at least 1.5 times a thickness of the liner proximate channels formed between the at least two radially inwardly projecting lobes.
- 15. The positive displacement motor of claim 8, wherein a thickness of the liner proximate the at least two radially inwardly projecting lobes is less than or equal to 3 times a thickness of the liner proximate channels formed between the at least two radially inwardly projecting lobes.
US Referenced Citations (22)
Foreign Referenced Citations (6)
Number |
Date |
Country |
2017620 |
Apr 1970 |
DE |
2713468 |
Sep 1978 |
DE |
4006339 |
Aug 1991 |
DE |
WO 9840273 |
Oct 1997 |
WO |
WO 9931389 |
Jun 1999 |
WO |
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Dec 1999 |
WO |