Patent Application
20160319609
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.
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.
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.
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
One significant problem with the concentric tubing-GCP assembly configuration shown in
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
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
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.
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.
