Apparatus For The Eccentric Connection of Tubular Members in a Downhole Pumping Apparatus

Abstract

A downhole GCP pumping apparatus which accommodates eccentricities in the position of input and output shafts of the various elements of the assembly by connecting the drive rod string directly to noncentered shafts while maintaining the overall outside dimension of production tubing within the walls of the well hole.

Description

The present invention relates to improvements in equipment used in production wells to retrieve and deliver to the surface, production fluids from subterranean deposit, and, more particularly, to apparatus for the connection of an eccentric shaft of downhole equipment to thereby eliminate problems caused by eccentricities.

BACKGROUND OF INVENTION

Field of the Invention

A secure connection between tubular members of downhole equipment is a requirement for successful oilfield production operations. Most connections are either threaded or flange type, but may also include welded connections.

In the case of downhole pumping equipment (sometimes referred to as downhole assembly), such as an electric submersible pump (ESP), both threaded and flange connections are utilized, with threaded connections used in the production tubing, and flange connections used between the several downhole components that make up the ESP assembly.

With sucker rod pumps and progressive cavity pumps (PCP), all connections are of the threaded type, as the downhole assembly consists of only one component. The connections between components in all three of these downhole pumping systems are concentric. That is, the centerline of each connected component is collinear. This collinearity is possible because the axis of motion of the components of the downhole system is concentric with the centerline axes of the components.

In the case of the ESP, the axis of motion is the common rotational axis of the downhole centrifugal pump, electric motor and other rotating members, all of which are concentric with the centerline of the tubing. Likewise, for a PCP, the axis of motion is the rotational axis of the pump rotor, and drive rod string, which turns concentrically inside the tubing and extends to the surface prime mover. For a sucker rod pump, the axis of motion is the centerline of the reciprocating rod string within the tubing, which extends from the reciprocating downhole pump to the pumping unit at the surface.

In the case of the geared centrifugal pumping system (GCP), however, the construction and design of the downhole transmission component of the system does not easily allow concentric attachment of the downhole assembly to the production tubing string because the drive input shaft of the transmission is radially offset from the centerline of the assembly housings.

If the downhole assembly of a GCP system were concentrically connected to the production tubing, as is the case in the other types of downhole pumping systems, the drive rod string axis of rotation would not be collinear with the radially offset drive input shaft of the transmission. The consequent required match-up of these respective rotational axes would require a connecting drive shaft with flexible joints of some nature (e.g. universal joints), between the drive rod string and the transmission input shaft. The very high input torque of a typical GCP would make such a connecting drive shaft difficult to design with sufficient durability, strength and reliability and still be small enough, diametrically, to fit within the downhole assembly housing.

The present invention addresses this problem by eccentrically attaching the production tubing to the downhole assembly, allowing the drive rod string and the transmission to share a common rotational axis.

SUMMARY OF INVENTION

It is well known that when connecting the input shaft of a piece of rotating equipment to its prime mover, great care is required to assure that the respective axes of rotation are precisely aligned. If there is misalignment, there will be lateral loading of bearings and bending of shafts, both of which result in excessive wear, or fatigue, and early failure. The object is to avoid lateral loading.

The current theory of connecting the transmission of a GCP to the drive rod string requires the same precise alignment. If one were to design the GCP downhole assembly to be concentrically connected to the production tubing, as is done for all other downhole pumping systems, the rotational axis of the drive rod string, turning within the tubing, would not match up with the offset axis of the input drive shaft of the downhole transmission component of the GCP system. In accordance with current technology, this mismatch would be met by including a compensating offset shaft arrangement between the drive rods and the transmission that would bring the axis of the rods in line with that of the transmission input.

Such a connecting system might utilize a short drive shaft and two universal joints, or a drive shaft and crown spline assemblies. Neither of these alternatives is practical due to the high torque of the input drive, and the consequently large size of the universal joints or crown spline assemblies required to handle the loads.

A simpler system that provides for accurate alignment of the drive rod string and the transmission input, as well as the ability to handle maximum input torque, is to utilize an eccentric connections between the rod string and the GCP downhole assembly. That is, the tubing is radially offset from the centerline of the GCP assembly sufficiently to allow the drive rod string within the tubing, and transmission input shaft, to share a common axis of rotation, while keeping the outer edge of the tubing within the outside diameter of the downhole assembly components.

Such an eccentric connection allows the downhole assembly and tubing to be run in a well as easily as it would be were the tubing concentrically connected with the assembly components, as the maximum outside diameter of the system is no greater than the outer diameter of the largest component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a depiction of the general configuration of a geared centrifugal pumping system installed in an oil well, as originally envisioned in Morrow (U.S. Pat. No. 5,960,886) and separated in order to show both the above ground power drive system and the subsurface system for capturing the production fluid to send topside;

FIG. 2 is a partial cross-section through the upper members of the downhole assembly of the pumping system in Morrow '886, showing the detail of the intended connection between tubing and the downhole assembly, and between the drive string and the transmission;

FIG. 3 shows a concentric connection between the production tubing and the downhole assembly;

FIG. 4 is a partial cross-section through FIG. 3, showing the internal details of a geared centrifugal pump downhole assembly with a concentric tubing-downhole assembly connection, showing the mismatch of the rod string centerline and the transmission input shaft rotational axis;

FIG. 5 shows the structure that embodies the present invention, wherein there is an eccentric connection between the tubing and the downhole assembly, with the tubing radially offset relative to the centerline of the downhole assembly;

FIG. 6 shows a partial cross-section through FIG. 5. showing the alignment of the drive rod string centerline with the transmission input shaft rotational axis;

FIG. 7 shows the FIG. 5 structure, with the connections disconnected to reveal their threaded nature.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The following detailed description discloses, to those skilled in the art, a novel system for connecting a downhole assembly of a geared centrifugal pumping (GCP) system to a drive rod string in which one or more eccentricities of component rotational axes in the drive line, in current systems, create problems in delivery.

With the foregoing in mind, reference is made to the general configuration of a prior art geared centrifugal pumping system as shown in FIGS. 1 and 2, are found in Morrow (U.S. Pat. No. 5,960,886), consisting of a drive head 12 at the surface, a drive rod string 14 runs inside the production tubing string 16, which serves several vital functions in the novel system, connected to the downhole GCP assembly comprising a transmission 20 and a pump 22. The transmission 20 is essential in such systems to increases the rotational speed of the drive rod string 14 from approximately 500 RPM to approximately 3500 RPM utilized by multi-stage centrifugal pumps 22 in common use in the oil industry. Note that the GCP assembly is shown attached to the production tubing string 16, via a concentric connection 18 in the FIG. 1 configuration, where the center lines of the tubing and of the downhole assembly are collinear.

FIG. 2 is a partial cross-section through transmission 20, showing the intended configuration of a concentric connection 18 between the production tubing string 16, with the drive rod string 14 turning within, to the downhole GCP assembly. Note that the gear train 24 within the transmission 20, consists of a parallel shaft, multi-stage gear set, which, by the very nature of such a configuration, has an eccentric input shaft 26. Consequently, the axis of rotation of the input shaft 26 of the gear train 24 is significantly offset compared to the centerline of the production tubing string 16 and drive rod string 14 turning within. In accordance with the current prevailing technology (made obsolete by the present invention), the drive rod string 14 connects to the input shaft 26 of the transmission 20 by means of a compensating offset shaft arrangement, which includes a short shaft 28 joined by two universal, or constant velocity, joints 30. A similar shaft 28a with two universal joints 30a is shown at the output end of the transmission 20 to align the eccentric output of the transmission 20 to the concentric axis of the pump 12.

One significant problem with the concentric tubing-GCP assembly configuration shown in FIGS. 1 and 2 is that the required torque capacity of the universal joints 30 can be quite high—in excess of 1500 lb. ft. Consequently, universal joints that are currently available with the required torque capacity and durability, are too large to fit within the assembly housing required by the narrow confines of a typical oil well.

Other methods of torsionally joining offset shafts, such as a shaft equipped with crowned spline assemblies, suffer the same problem as the universal joints. That is, if the spline assemblies are to have the required torque capacity and service life, they are inevitably too large in diameter for the GCP housing.

The concentric tubing-downhole assembly connection and the consequent axial misalignments, which are inevitable in the FIGS. 1 and 2 embodiments, are shown in FIGS. 3 and 4. In FIG. 3, the production tubing string 16 connects to the downhole assembly 32 concentrically, where their respective centerlines are collinear. FIG. 4 shows a partial longitudinal cross-section through the structure of FIG. 3, showing the drive rod string 14 within the production tubing string 16, a short splined, or square sectioned, shaft 34 attached to the drive rod string 14 that, when inserted into the receiver 36, allows a torsional connection between the drive rod string 14 and the input shaft 26 of transmission 20. In the configuration shown in FIG. 4, there is a considerable lateral offset between the axes of rotation of the drive rod string 14 and the transmission input shaft 26, shown as 40.

Such a large offset cannot practically be accommodated by bending the respective shafts to allow connection, as the lateral loads due to the deflection would cause rapid and damaging wear of supporting shaft bearings, and the bending of the shaft would result in fatigue failure.

A highly efficient solution to the problem articulated is taught by the present invention. Accordingly, and with reference to FIGS. 5, 6 and 7, the connection between the production tubing string 16 and the downhole assembly 32 is eccentric, rather than concentric. That is, the production tubing string 16 is eccentrically connected to the downhole assembly 32 and offset to one side, such that the tubing centerline is aligned with the transmission input shaft 26, as seen in cross-section in FIG. 6. By virtue of this simple and efficacious modification of a conventional concentric connection, the upper portion of the drive rod string 14 can be directly, and collinearly, connected to the transmission input shaft 26 without need of a connecting shaft with universal joints or crown splines that a concentric attachment between the production tubing string 16 and the downhole assembly 32 would require.

Further, in keeping with the invention, care is taken to make sure that the outer diameter of the tubing couplings 42 do not extend diametrically beyond the outside diameter of the downhole assembly 32, so that the entire tubing-downhole equipment assembly has a maximum outside diameter no greater than the maximum outside diameter of the downhole assembly. This way, this eccentric assembly can be run in the same wells that a system with a concentric tubing-assembly connection can be run. FIG. 7 shows the typical threaded connections 44 between the multiple sections of production tubing string 16 and the similar threaded connection between the production tubing string 16 and the downhole assembly 32.

Having now provided a detailed description of the preferred embodiment of the present invention, it will be appreciated that those skilled in the art will perceive the existence of minor alternative elements available to achieve the same goals of the invention. It is understood that such variations fall within the scope of the claims of the present invention as described.

Claims

1. In a well having a well casing extending from the surface to a subsurface interval;

a supply of rotational power located at the surface of the well;

a string of tubing extending within the casing from the surface to a downhole assembly and fixedly attached thereto;

a drive rod string, said drive rod string being housed within the production tubing and extending between the supply of rotational power at the surface and the downhole assembly, the upper end of said drive rod string being rotationally connected to the supply of rotational power at the surface, and the lower end of said drive rod string being rotationally connected to a component of the downhole assembly for the purpose of providing rotational power to said component;

a rotationally driven component of the downhole assembly, said component being equipped with an input shaft, said input shaft being radially offset from the centerline of said downhole assembly, said input shaft being directly connected to said drive rod string for the purpose of rotational power input to said component;

wherein the tubing is fixedly attached to said downhole assembly such that the centerline of said tubing is radially offset from the centerline of said downhole assembly;

wherein said radial offset of the connection of said tubing and said downhole assembly matches in extent and direction the radial offset of the input shaft of sid rotationally driven component such that said drive rod string can connect with said input shaft of said rotationally driven component, without any part of said rod string making contact with the inner wall of said tubing.

2. The assembly of claim 1, wherein, the radial offset of the attachment of said tubing to said downhole assembly being such that the outer maximum radial extent of the diametrically largest part of said production tubing is within the maximum radial extent of said downhole assembly.

3. The assembly of claim 1, wherein said downhole assembly being a geared centrifugal pump.