This application claims priority to and the benefit of U.S. Prov. Pat. App. No.

61/848,858, filed January 14, 2013, and is incorporated herein by reference in its entirety. TECHNICAL FIELD

The present invention relates in general to downhole drilling motors and, in particular, to a system, method and apparatus for a flexible joint for a downhole drilling motor.

BACKGROUND ART

When drilling an oil or gas well it is common practice to use a positive displacement mud motor (PDM) to rotate a drill bit with respect to the rest of the drill string. There are two primary conditions under which a PDM is used. The first condition is when the operator desires to rotate the bit faster than conventional means provide. The second condition is in a directional well where the wellbore is steered by preventing the drill string from rotating while just the bit rotates in order to increase the depth of the borehole.

A mud motor comprises four primary components: a power section, a bent housing (or offset housing), a bearing section, and a driveshaft. The power section converts energy from the flow of mud into rotational energy. A central rotor in the power section spins as mud flows between it and a stator. The rotation of the central rotor is not constrained to a central axis of the motor and is thus slightly eccentric. The bent housing is located directly below the power section. The bent housing facilitates the bend in the outer body of the motor. The bend is useful at directing the drill bit to steer in a selected direction. The bearing section is below the bent housing. The bearing section secures the drill bit and stabilizes the rotation of the bit within the housing. The drive shaft is located inside the bent housing. The drive shaft connects the bearing section and the power section, and transmits power and torque from the power section to the bit. However, since the axis of the power section is displaced at an angle from the axis of the bearing section, some means of transmitting torque to this bend is required. In addition, the eccentric motion of the rotor means that an extra degree of freedom should be accounted for.

Traditionally, compensation for the extra degree of freedom is provided by one or more universal or constant velocity (CV) joints. There are several different types of CV joints for transmitting torque to the bottom of the mud motor, although each of them has negative attributes. For example, a traditional Cardan type or double Cardan type universal joint lacks the strength required to handle the heavy torsional loads required by drilling applications.

Accordingly, Cardan or double Cardan type CV joints are prone to catastrophic failure since their lugs can shear off under high impact loading. Moreover, Cardan type joints do not transmit rotational velocity with constant input speed and constant output speed. In addition, as the bend angle of the joint increases, the oscillation and velocity of the output shaft increases. Even though the angles that are used for directional drilling are small, any rotational irregularities can cause drilling problems.

A more traditional type of CV joint is a Rezzpa joint, which is typically used for the front wheels of most front wheel drive cars. These joints employ balls that slide along tracks in both mating parts of the joint. Rezzpa joints are typically very smooth and exhibit a constant output velocity over a wide range of joint angles. However, they do not allow a high amount of torque to be transmitted per unit of diameter. Because of this limitation, Rezzpa joints are not good candidates for mud motor drive shaft applications.

A common industry practice has been to use a modified version of the Rezzpa joint for mud motor drive shafts. Because the range of angles that mud motor joints are subjected to is relatively low (e.g., less than about 10 degrees), the modified Rezzpa joint has pits in the ball section of its joint rather than tracks. Thus, the ball bearings seat in the pits in fixed locations in the ball section of the joint. This design reduces flexibility by one degree of freedom that is necessary for the ball bearings to contact all of the housing groove surfaces simultaneously. However, it does add the advantage of eliminating the need for a bearing cage and increases the amount of torque that can be transmitted in a joint of a given cross-sectional size.

One problem with modified Rezzpa joints is that they are subject to wear and breakage since their contact surface area used to transmit torque is very low. Also, the use of spherical ball bearings to transmit the torque generates a high amount of hoop stress in the housing section, which necessitates a thicker section to carry torque.

An alternative design to the modified Rezzpa joint uses spline features on the ball section of the joint, rather than pits. This design somewhat improves the torque load by distributing it over a larger surface area, theoretically decreasing the amount of galling wear. However, the static splines in the ball section have at least two disadvantages. First, in order to allow for misalignment of the shafts, the splines must be cut relatively loosely with respect to one another. This requirement permits significant backlash that increases impact loading in variable drilling conditions.

The second and most important disadvantage is that the splines are subject to a high degree of wear on the splines that are disposed laterally with respect to the bend angle. That is, the high-side and low-side splines fit into their respective grooves with a good mating fit. However, as the splines move toward the position perpendicular to the plane described by the angle of the bend, a spline parallel to the shaft of the ball section will no longer be parallel to the mating spline on the socket section that is parallel to its connected shaft. This misalignment necessarily causes wear and prematurely fatigues the component.

USP 6203435 and USP 8033917 disclose swivel spline flex joint designs that overcome these low surface area and misalignment wear problems. Each of those patents describe mud motor CV, universal joint, or drive shaft devices that employ rotating keys in a ball section that mate with grooves cut in a socket section. The keys are always arranged in pairs so that the torque load is always distributed across the faces of the keys, so that there is never any side force exerted on the ball/socket assembly.

While these designs do theoretically reduce the shearing force on the face of the load bearing keys, there is a trade off in that if only one pair of keys is used, the output rotational velocity varies slightly with respect to the input rotational velocity. This type of oscillatory motion is one of the reasons that universal joints are not preferred.

Although this problem can be mitigated through the use of additional key pairs, the key size must remain at a certain dimension to withstand the torque load. Moreover, since each key of a given size requires space on the circumference of the ball where they are located, using key pairs reduces the number of keys that can be used for smoothing the output velocity. Although each of these designs is workable, improvements in flexible joints for downhole mud motors continue to be of interest.

SUMMARY OF INVENTION

Embodiments of a system, method and apparatus for a joint are disclosed. For example, a joint for a downhole mud motor may include a ball having a ball shaft extending therefrom along a ball axis. In one version, the ball has a recess formed therein. A key may be configured to be mounted to the ball. In addition, embodiments may include a socket having a socket shaft extending therefrom along a socket axis. The socket axis may be anti-aligned with the ball axis. In some versions, a slot may be formed inside the socket and configured to receive the key. The recess, key and slot may include a plurality of each of them.

The foregoing and other objects and advantages of these embodiments will be apparent to those of ordinary skill in the art in view of the following detailed description, taken in conjunction with the appended claims and the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

So that the manner in which the features and advantages of the embodiments are attained and can be understood in more detail, a more particular description may be had by reference to the embodiments thereof that are illustrated in the appended drawings.

However, the drawings illustrate only some embodiments and therefore are not to be considered limiting in scope as there may be other equally effective embodiments.

FIG. 1 is a sectional side view of an embodiment of a joint.

FIG. 2 is an exploded isometric view of an embodiment of a ball for a joint.

FIG. 3 is an isometric view of an embodiment of a socket for a joint.

FIG. 4 is a sectional side view of an embodiment of a socket for a joint.

FIG. 5 is a sectional side isometric view of another embodiment of a joint.

FIG. 6 is an isometric view of an embodiment of a disassembled joint.

The use of the same reference symbols in different drawings indicates similar or identical items.

DESCRIPTION OF EMBODIMENTS

Embodiments of a system, method and apparatus for a flexible joint for downhole mud motors are disclosed. For example, FIGS. 1 and 2 depict an embodiment of a joint 11 that may include a ball 13 attached to a shaft 15. A housing or mating socket 17 may be attached to another shaft 19. The ball 13 may sit inside the socket 17, such that torque may be transmitted through the joint 11 with one or more spline protrusions or keys 21 that can swivel. The keys 21 on the ball 13 can mate with one or more grooves 23 (FIGS. 3 and 4) formed inside the socket 17. Embodiments of the keys 21 may have a generally cylindrical outer surface 30 (FIG. 2). Each key 21 may include an underside 31 (FIGS. 1 and 5). The underside 31 may comprise different shapes. For example, the underside 31 can be flat, or a generally concave spherical profile that is complementary in shape to the outer profile of the ball 13. Each key 21 also may include flat sides 33 that mate with the grooves 23 formed in the socket 17. Embodiments of the ball 13 may include one or more holes 35 (FIG. 2) formed around its major outer diameter (i.e., equator). Holes 35 can be equally spaced apart and odd numbered and allow the keys 21 to be located therein.

In some versions, the grooves 23 in the socket 17 may be angled at a slight inclination (e.g., about 1 to 5 degrees), such that (under load) the driving torque will force the ball 13 and socket 17 towards each other. This slight angle may be equivalent to about half of the angle that the joint 11 would be expected to work under. In one embodiment, the keys 21 may be held in place in the ball 13 with spring pins or dowel pins 37. Pins 37 may be secured in holes 35 adjacent to each main recess or locating hole 39 for respective ones of the keys 21. Each key 21 may be provided with complementary circumferential grooves 41 that can correspond to the location of a respective pin 37 so that the key 21 may rotate but will not be allowed to move axially out of the hole 39.

In an embodiment, the ball 13 may retain its spherical profile from the distal tip 43 (FIGS. 1 and 5) of the ball 13 to a line or area 45 that is past the equator of the ball 13. The angle a of the spherical profile beyond the equator may correspond to a "latitude" in degrees relative to the equator of the highest amount of axial eccentricity β that the joint 11 would be expected to accommodate. For example, that angle a may be up to about 10 degrees. The spherical profile on the ball 13 up to and past the equator may be load bearing as the joint 11 experiences torque. Thus, the surface of the ball 13 may be treated to be wear resistant.

In another embodiment, an apparatus and method for sealing the joint 11 from mud invasion are disclosed. For example, one embodiment uses a metal boot shield 51. The exterior 53 (FIG. 4) of the socket 17 may be provided with a spherical profile having a center point that corresponds to the center point of the ball section. The metal boot shield 51 may include a seal 55 (FIG. 1) that slides slightly around the spherical profile exterior 53 of the socket 17. A lubricating fluid may be used to seal inside the joint and reduce or eliminate the ingress of contaminating materials. Alternatively, the apparatus and method of sealing the joint may comprise a conventional elastomeric boot. The elastomeric boot may comprise a bellows-style configuration, as is known in the art. Other aspects of embodiments may include one or more of the following items. For example, a joint 11 for a downhole mud motor may include a ball 13 having a ball shaft 15 extending therefrom along a ball axis 14. The ball 13 may include a plurality of recesses or blind holes 39 formed therein. A plurality of keys 21 may be configured to be mounted to the ball 13.

Embodiments of a socket 17 may include a socket shaft 19 extending therefrom along a socket axis 20. The socket axis 20 may be anti-aligned with the ball axis 14. The term "anti-aligned" as used herein means that the ball shaft axis and the socket shaft axis can be axially aligned or misaligned (e.g., skewed), but are not required to be aligned or coaxial. The axes 14, 20 can be anti-aligned by not greater than about β = 10 degrees. A plurality of slots 23 may be formed inside the socket 17 and may be configured to receive respective ones of the keys 21. Each key 21 may be provided with a top surface 22 (FIG. 5), a front surface 30 and a rear surface 32. One or more of these surfaces may be are arcuate. For example, the top, front and rear surfaces 22, 30, 32 of the keys 21 may be cylindrical or spherical. Each key 21 also has side surfaces 33. In an

embodiment, the side surfaces 33 are flat.

Embodiments of each key 21 may be provided with a base 26 (FIG. 2) and a head 28 protruding from the base 26. The head 28 may be larger than the base 26. For example, the head 28 can be longer but narrower than the base 26. The underside 31 (FIGS. 1 and 5) of the head 28 may circumscribe the base 26. In two examples, the underside 31 of the head 28 may be flat or spherically concave.

In an embodiment, the key 21 may further comprise chamfered surfaces 25 (FIG. 6) between the top surface 22 and the side surfaces 33. The chamfered surfaces 25 can be non-contact surfaces relative to the socket 17.

The recesses 39 and keys 21 may be generally cylindrical and complementary to each other in shape. In some embodiments, the recesses 39 may be stepped 61 (FIG. 5), such as coaxial cylinders having different diameters. The keys 21 may include stepped profiles that are complementary in shape to the steps 61 in the recesses 39. As shown in FIGS. 3, 4 and 6, each slot 23 may be provided with a helical twist relative to the socket axis 20 (FIG. 1), rather than simple rectangular grooves. In essence, the slots may be slightly rotated about the socket axis 20. For example, the helical twist can be less than or equal to about 2 degrees relative to the socket axis 20.

In another embodiment, each key 21 may be provided with a dimension 27 (FIG. 6) between the flat side surfaces 33. The dimension 27 of key 21 can be between about 0.005 inches and about 0.020 inches less than a complementary width 29 between the flat side surfaces of one of the slots 23 in the socket 17.

Some embodiments of the joint 11 have a quantity of the keys 21 that consists of an odd number of the keys 21 (e.g., 1, 3, 5, 7, etc.). Each key 21 may include a hole 28 (FIG. 5) extending therethrough. The hole 28 may be is configured to facilitate a venting of lubricant when the key 21 is installed in the ball 13.

In some embodiments, the keys 21 are not balls and the keys 21 are elongated. Each key 21 may include a key axis (e.g., along hole 28), about which the keys 21 are configured to swivel in the ball 13 when installed in the ball 13.

As shown in FIGS. 1-4, an exterior of the ball 13 and an interior of the socket 17 may be substantially spherical and complementary in shape. The ball 13 may be provided with a spherical profile from a distal end 43 (FIG. 1) thereof past an equator 44 thereof. The spherical profile may extend to an angle a beyond the equator 44 that corresponds to a latitude relative to the equator 44 of up to about 10 degrees. The recesses 39 can be approximately centered on the equator 44 of the ball 13.

Embodiments of the socket 17 may include a distal end 18 (FIG. 4) and an interior surface extending axially inward from the distal end 18. In some versions, the interior surface may be spherical up to about an equator of the socket 17.

As shown in FIGS. 5 and 6, the joint 11 may further include a compensating piston 81 configured to be located in an axial recess 83 in the socket 17. In another version, the joint 11 may further include an axial bearing 71 configured to be located between the compensating piston 81 and a distal end 43 of the ball 13. The axes 14, 20 of the shafts 15, 19 are configured to intersect in approximately a same location throughout a rotation of the shafts 15, 19. In still other embodiments, embodiments of a device for use in transmitting torque between two shafts may comprise one ball body shaped roughly spherically and connected to the end of one of the shafts. A socket body profile with a roughly spherical inner surface may accept the spherically shaped end. The socket body can contain slots located in the socket face oriented substantially in line with the shaft connected to the socket. The ball section may contain a plurality of holes drilled around the diameter of the ball on a plane that is perpendicular to the orientation of the shaft connected to the ball section.

The device may further comprise a plurality of torque transmission elements. The torque transmission elements may be rotatably fastened to the ball section by a plurality of pin elements. The number of torque transmission elements may consist of an odd number of elements, rather than an even number of elements as is known in the art.

The ball section can have a spherical profile past the equator location.

The grooves in the socket section may be angled such that the transmission of torque in the drive direction forces the ball and socket section towards or into one another.

The torque transmission elements may include a roughly cylindrical shape with one face spherically profiled to rest closely to the ball section. The torque transmission elements may include a flat face to mate with grooves cut in the socket section.

The torque elements can be rotatably fastened to the ball section with pins shaped roughly cylindrically.

The flat sides of the torque transmission elements can be sized slightly less than the thickness of the angled mating grooves in the socket section.

In another version, the device for use in transmitting torque between two parts of a downhole mud motor may include a joint where the axis of the shaft that is supplying the torque intersects the axis of the shaft that is receiving the torque in roughly the same location throughout the rotation of the shaft. The joint can include a socket section that is roughly spherical in outer profile over a length that envelopes the intersection of the axis of both shafts. The joint can further include a shield component shaped roughly conically and attached in a sealed manner to the shaft of the joint bearing the ball section of the joint and internally profiled to accept elastomer seals to exclude debris from entering the joint area. Unlike prior art designs, some embodiments provide an odd number of keys, and eliminate the need for the keys to be provided as matched pairs. Eliminating one or more keys from matched pairs of keys allows for the use of relatively larger keys for improved torque transfer. Selecting a reasonable number of keys (e.g., five radially symmetric arranged keys) may ensure smooth rotational velocity. In another example, the key may include a plurality of keys, the recess may include a plurality of recesses, and the slot may include a plurality of slots. A quantity of the keys, recesses and slots can be an odd number of keys, an odd number of recesses and an odd number of slots.

This written description uses examples to disclose the embodiments, including the best mode, and also to enable those of ordinary skill in the art to make and use the invention. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Note that not all of the activities described above in the general description or the examples are required, that a portion of a specific activity may not be required, and that one or more further activities may be performed in addition to those described. Still further, the order in which activities are listed are not necessarily the order in which they are performed.

In the foregoing specification, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of invention.

As used herein, the terms "comprises," "comprising," "includes," "including," "has," "having" or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of features is not necessarily limited only to those features but may include other features not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, "or" refers to an inclusive-or and not to an exclusive-or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

Also, the use of "a" or "an" are employed to describe elements and components described herein. This is done merely for convenience and to give a general sense of the scope of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.

Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims.

After reading the specification, skilled artisans will appreciate that certain features are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. Further, references to values stated in ranges include each and every value within that range.