FIELD

The present disclosure relates to a downhole drag reduction apparatus for reducing the effect of drag on downhole strings in a bore.

BACKGROUND

In some industries, such as the oil and gas industry, wellbores are drilled from surface (land or subsea) to intercept specific subterranean regions, for example for the extraction of hydrocarbons from those regions, for the injection of material (water, carbon dioxide etc.), for geothermal applications and/or the like. Numerous procedures are followed, such as drilling, installing casing, running desired tooling and infrastructure and the like. Mid-life wellbore operations may involve intervention, workover and/or replacement of existing infrastructure, and at the end of the wellbore life decommissioning may require equipment and infrastructure to be removed and measures taken to adhere to environmental and legislative requirements. Typical wellbore operations thus require the use of a variety of tools, such as drilling tools, running tools, jarring tools, fishing tools, pulling tools and the like which are deployed and operated from surface.

In many cases drag forces may hamper operations, such as drag forces which resist the rotation and/or linear advancing or retraction of a rotating string of tubing or pipe, such as a drill string, running string, pulling string etc. Such drag may require excessive drive forces to be applied, for example excessive drive torque, weight, pulling force etc. which brings about significant problems, such as exceeding capacity of drive equipment, backing off connections, reaching critical load limits, such as buckling load limits, tensile failure limits, and/or the like. Such drag forces can also represent a limiting factor on the maximum reach of some wellbores, particularly in deviated bore trajectories. Also, friction between the string and the wellbore can cause damage, such as wear in casing in cased-hole sections, key-seating/washout/wear in open-hole sections and wear to the rotating string itself. Operations in open-hole can cause accelerated failure due to the aggressive nature of the wellbore conditions and heightened level of vibration causing loose components to break and/or fall-off. Benefits can therefore be provided if steps are taken to reduce the effect of drag forces during downhole operations.

SUMMARY

An aspect of the present disclosure relates to a downhole drag reducing apparatus, comprising: a mandrel; a bearing sleeve mounted on the mandrel such that the mandrel and bearing sleeve are rotatable relative to each other, the bearing sleeve defining a bore wall engaging surface; a reciprocating piston mounted within a piston housing to define a piston chamber; and a rotary valve assembly operated by relative rotation between the mandrel and the bearing sleeve to cyclically pressurise and depressurise the piston chamber to provide reciprocating movement of the reciprocating piston and the generation of vibration within the apparatus.

During downhole use in a wellbore, engagement with a bore wall of a wellbore will cause the bearing sleeve to be rotationally held. Thus, in this situation rotation of the mandrel will produce relative rotation between the mandrel and the bearing sleeve, thereby operating the rotary valve assembly to provide reciprocating movement of the reciprocating piston, as described above. Such reciprocating movement of the reciprocating piston generates vibration within the apparatus which may assist with overcoming friction, both static and dynamic friction, experienced when the apparatus is engaged with the bore wall. Moreover, one or more downhole drag reducing apparatus as disclosed herein may be connected along a work string. The relative rotation between the mandrel and the bearing sleeve may function to minimise friction between the work string, when rotated, and the bore wall of a wellbore, thus reducing the drag torque experience by the work string. In this respect, the bearing sleeve may isolate the mandrel from drag torque interactions with the bore wall.

Furthermore, it will be appreciated that the relative rotation between the mandrel and bearing sleeve required to generate the vibration within the apparatus is provided without requiring any additional tools (e.g. downhole motors). Instead, the apparatus capitalises on the relative rotation between the mandrel and bearing sleeve obtained from the bearing sleeve being engaged with the bore wall, which itself may provide a means to reduce drag torque. Therefore, since no additional tools are required to operate the apparatus, full flow and/or passage of tools through the downhole drag reducing apparatus may be facilitated.

Furthermore, as alluded to above, the relative rotation between the mandrel and the bearing sleeve may function to reduce friction with the bore wall in two ways. Firstly, the friction between the work string, when rotated, and the bore wall of the wellbore is reduced by the fact that the mandrel and bearing sleeve are rotatable relative to each other, thereby reducing the drag torque experienced by the work string. Secondly, the apparatus is configured such that it capitalises on the relative rotation between the mandrel and bearing sleeve to generate the vibration within the apparatus to further assist in overcoming friction within the wellbore.

The reciprocating piston is mounted within the piston housing to define the piston chamber. The reciprocating piston may be moveable in reverse first and second axial directions. Vibration or agitation forces are generated by relative rotation between the mandrel and the bearing sleeve. The frequency of generated vibration forces will be a function of the relative rotational speed between the mandrel and the bearing sleeve, which may be virtually infinitely variable to thus provide virtually infinite variability of the vibration frequency, providing significant advantages. Furthermore, as vibration forces are generated as a result of fluid pressure, the downhole drag reducing apparatus may be defined as a fluid actuated vibration apparatus, for example a hydraulically actuated vibration apparatus.

The mandrel may comprise one or more end connectors to permit connection with a rotating string. Such an end connector may comprise a threaded end connector, such as a pin/box type connector, premium threaded connector and/or the like. In some examples the mandrel may comprise opposite end connectors to permit the mandrel to be connected integrally and in-line within the work string. In this example, the mandrel may form a joint within the work string.

The mandrel may be a tubular. The mandrel may comprise a plurality of components. Alternatively, the mandrel may comprise a single or unitary component. The mandrel may be configured to be connected to a rotating work string, such as a drill string (e.g., formed of drill pipe, drill collars etc.), rotary casing string (e.g., used in casing while drilling applications), prop shaft (e.g., from a motor) and/or the like. When the mandrel is connected to the work string the mandrel may be rotatable together with the work string. In some examples, when the mandrel is connected to a work string the mandrel may be considered to form part of the work string.

The mandrel may be configured to transmit axial loading, for example axial tensile and/or compressive loading, therealong. Such axial loading may be applied via the work string. The mandrel may be configured to transmit torque, for example torque applied by the work string. Axial loading and/or torque may be transmitted along the work string and the mandrel between a load source (e.g., rotary drive, hoisting system etc.) and downhole assembly, such as a bottom hole assembly (BHA), tool string, completion infrastructure, wellbore infrastructure and/or the like. The load source may be located topside in some examples. Alternatively, the load source may be located downhole, for example in the form of a motor, such as a mud motor.

The bearing sleeve may be tubular. The bearing sleeve may comprise a plurality of components. The bearing sleeve may define a unitary component. Similarly, the mandrel may define a unitary component. In some examples the provision of a component in unitary form may permit that component to be defined as a solid state component (e.g., solid state bearing sleeve and/or solid state mandrel). Providing the bearing sleeve as a unitary component may provide a number of advantages, for example in terms of improved strength.

The piston housing may be a tubular. The piston housing may circumscribe the reciprocating piston. The piston housing may be a separate component to the bearing sleeve. The piston housing may be rotabably fixed to the bearing sleeve. The piston housing may be axially fixed to the bearing sleeve.

The piston housing and bearing sleeve may define a unitary component. The bearing sleeve may comprise an extended section forming the piston housing. Therefore, hereinafter unless stated or implied otherwise, any reference to the “bearing sleeve” should be construed as including the bearing sleeve and the piston housing. As described above, the rotary valve assembly is operated by relative rotation between the mandrel and the bearing sleeve to cyclically pressurise and depressurise the piston chamber. In particular, the rotary valve assembly may be operated between a pressure configuration and an exhaust configuration.

In the pressure configuration, the piston chamber may be in pressure communication with a valve inlet, and isolated from a valve exhaust, to permit the reciprocating piston to move in the first axial direction in accordance with the piston chamber being pressurised via the valve inlet.

In the exhaust configuration, the piston chamber may be isolated from the valve inlet and in pressure communication with the valve exhaust to permit the piston chamber to be depressurised and the reciprocating piston to move in the second axial direction.

A biasing arrangement may bias the reciprocating piston in one of the first and second axial directions. In particular embodiments, the biasing arrangement may bias the reciprocating piston in the second axial direction. The reciprocating piston may be biased by a mechanical biasing arrangement, e.g. a spring arrangement. The spring coefficient or stiffness of the spring arrangement may be selected in accordance with vibration requirements of the apparatus, such as frequency and force of vibration. The spring arrangement may comprise one or more (e.g. a series of) springs circumferentially arranged with respect to the reciprocating piston.

The biasing arrangement may be at least partly received in one or more apertures formed in the reciprocating piston. The apertures may be formed in an end surface of the reciprocating piston. The biasing arrangement may be supported by an impact shoulder provided on or formed with the mandrel.

The biasing arrangement may be disposed in one or more chambers. The mandrel may define or comprise the one or more spring chambers.

The bearing sleeve may be provided with one or more lateral ports. The lateral ports may be arranged adjacent the one or more spring chambers. The lateral ports may provide pressure balancing of the spring chamber as the reciprocating piston moves back and forth in the piston chamber in the first and second axial directions, in order to prevent hydraulic lock.

In the pressure configuration, pressurisation of the piston chamber may urge the reciprocating piston to move in the first axial direction. That is, the reciprocating piston may be urged to move in the first direction by virtue of the pressure chamber being pressurised via the valve inlet. As the reciprocating piston moves in the first direction, the biasing arrangement may be activated or energised. In the exhaust configuration, the reciprocating piston may be urged to move back in the second direction by virtue of the potential energy stored in the biasing arrangement being released. The biasing arrangement may act on the reciprocating piston with a force sufficient to move the reciprocating piston in the second axial direction to depressurise the piston chamber, in accordance with operational parameters.

The reciprocating piston may comprise two or more parts permitting the reciprocating piston to be mounted on the mandrel and then assembled, e.g. bolted and/or pinned together. For example, the reciprocating piston may be of a clamshell design, i.e. where two halves of the reciprocating piston are fitted around the mandrel and are bolted and/or pinned together.

In use, the apparatus may be used in combination pressure and exhaust regions, wherein the valve inlet communicates with the pressure region and the valve exhaust communicates with the exhaust region, such that a pressure differential may be applied across the rotary valve assembly. Specifically, the pressure within the pressure region may be elevated above the pressure in the exhaust region. In particular, the pressure within the pressure region may be sufficient (for example sufficiently high) to pressurise the piston chamber to cause the reciprocating piston to overcome the biasing force of the biasing arrangement and move in the first axial direction, and the pressure within the exhaust region may be sufficient (for example sufficiently low) to permit the piston chamber to be depressurised and the reciprocating piston to move in the second axial direction by virtue of the potential energy stored in the biasing arrangement being released.

The downhole drag reducing apparatus may be configured to operate in accordance with the direction of the pressure differential applied across the rotary valve assembly. As an example, in one mode of operation, suggested above, the pressure of the pressure region may be higher than the pressure of the exhaust region such that the reciprocating piston can overcome the biasing arrangement. However, should the pressure differential be reversed then what was previously the pressure region becomes the exhaust region, and vice versa, and what was previously the valve inlet becomes the valve exhaust, and vice versa.

In this respect, it should be recognised that the valve inlet and the pressure region, and valve exhaust and exhaust region, may be defined as such in accordance with the direction of an applied pressure differential applied across the rotary valve assembly. With this in mind, although features will be defined herein as relating to the valve inlet and valve exhaust (and pressure and exhaust regions), this is done so for clarity and brevity purposes and it should be understood that the function and thus identity of the valve inlet and valve exhaust (and pressure and exhaust regions) could switch depending on the operational conditions.

As described above, the downhole drag reducing apparatus comprises a mandrel and a bearing sleeve mounted on the mandrel such that the mandrel and bearing sleeve are rotatable relative to each other, the bearing sleeve defining a bore wall engaging surface. It will be understood that reference to relative rotation between the bearing sleeve and the mandrel may include the downhole drag reducing apparatus being configured such that the mandrel rotates while the bearing sleeve is stationary with respect to the a bore wall, or such that the mandrel and the bearing sleeve both rotate but at different rates.

It will be understood that while the terms “bearing sleeve” and “mandrel” have been used herein for convenience, these components may alternatively be referred to as a first structure or first body portion and a second structure or second body portion of the downhole drag reducing apparatus, the first structure or body portion and the second structure or body portion being rotatable relative to each other.

The downhole drag reducing apparatus may comprise an axial throughbore. The mandrel may define the axial throughbore. The mandrel may be disposed and/or mounted concentrically or substantially concentrically within the bearing sleeve. The mandrel may be disposed and/or mounted eccentrically within the bearing sleeve.

The mandrel and the bearing sleeve may be rotatably fixed such that the mandrel and the housing rotate together, until released. The mandrel and the bearing sleeve may be rotatably coupled, and wherein: in a first configuration the mandrel and the bearing sleeve may be rotatably fixed such that the mandrel and the bearing sleeve are configured to rotate together; and in a second configuration the mandrel and the bearing sleeve may be released for relative rotation.

The rotary valve assembly may comprise the valve inlet. The rotary valve assembly may comprise the valve exhaust.

The rotary valve assembly may be configured to provide selective communication between at least one of the valve inlet and valve exhaust and the pressure chamber.

The rotary valve assembly may be configured to cyclically permit communication with at least one of the valve inlet and valve exhaust and the piston chamber. Relative rotation of the mandrel and bearing sleeve may cause the rotary valve assembly to cyclically permit communication with at least one of the valve inlet and valve exhaust and the piston chamber

The rotary valve assembly may comprise a rotary valve member. The rotary valve member may be operatively associated with the valve inlet. The rotary valve member may be operatively associated with the valve exhaust. The rotary valve member may be configured to provide selective communication between at least one of the valve inlet and valve exhaust and the pressure chamber. The rotary valve member may be configured to cyclically permit communication with at least one of the valve inlet and valve exhaust and the piston chamber.

The mandrel may comprise or define the valve inlet. The valve inlet may be rotatably fixed with respect to the mandrel. The valve inlet may be integrally formed with the mandrel. In other examples, the bearing sleeve may define the valve inlet, or the bearing sleeve and mandrel may together define the valve inlet. The mandrel may comprise one or more flow passages. The flow passages may be obliquely oriented. One or more of the flow passages of the mandrel may define or communicate with the valve inlet. The one or more flow passages may extend through a wall of the mandrel. The one or more flow passages may extend from an inner surface of the mandrel.

The mandrel may comprise a plurality of flow passages. The flow passages may be arranged circumferentially with respect to the mandrel. At least one of the flow passages of the mandrel may be disposed at a first, uphole location relative to the reciprocating piston.

In some examples, the one or more flow passages of the mandrel may be provided on a circumferential surface (e.g. an inner circumferential surface) of the mandrel. This may allow the flow area to be readily increased and/or decreased simply by axially extending or reducing the dimensions of the one or more flow passages. The number and/or configuration, e.g. dimensions or form, of the flow passages of the mandrel may be selected in accordance with considerations such as total required inlet or exhaust area, vibration frequency and/or the like. Moreover, the ability to increase the flow area may allow a more rapid pressurisation and depressurisation which may be more explosive in terms of the vibration forces generated.

The plurality of flow passages may comprise one or more inlet openings associated with the valve inlet. The inlet openings may be provided on a circumferential surface (e.g. an inner circumferential surface) of the mandrel. The flow passages may be defined as pressure ports.

The valve inlet of the rotary valve assembly may be configured, for example by its position, construction and/or the like, to be arranged in pressure communication with the pressure region. The pressure region may be located in the axial throughbore of the mandrel. Such a pressure region may provide suitable pressure conditions to permit the piston chamber to be pressurised. The mandrel may define or communicate with the valve exhaust. The bearing sleeve may define or communicate with the valve exhaust. The bearing sleeve and mandrel together may define or communicate with the valve exhaust.

The mandrel may comprise one or more flow channels. The one or more flow channels may be provided on an outer surface of the mandrel. The mandrel may comprise one or more recesses or indents forming the flow channel(s). The flow channel(s) may be axially oriented. One or more of the flow channels may define or communicate with the valve exhaust.

The mandrel may comprise a plurality of the axial flow channels. The axial flow channels may be arranged circumferentially. At least one of the axial flow channels may be disposed at a first, uphole, location relative to the reciprocating piston.

The valve exhaust may comprise one or more outlet ports. The bearing sleeve may comprise the one or more outlet ports (e.g. lateral outlet ports) defining or in communication with the valve exhaust. The outlet ports of the bearing sleeve may be in communication with the one or more flow channels of the mandrel. At least one of the outlet ports of the bearings sleeve may be disposed at the first, uphole, location relative to the reciprocating piston.

The outlet ports may be dimensioned to limit or prevent ingress of debris from the exhaust region, e.g. annulus into the piston chamber. For example, the outlet ports may be provided with a screen, such as a mesh, and/or a filter, so as to limit debris ingress from the exhaust region, e.g. annulus, getting into the piston chamber.

In some examples, the outlet ports may be provided on a circumferential surface of the bearing sleeve. This may allow the area to be readily increased simply by axially extending the dimensions of the outlet ports. This may allow a more rapid pressurisation and depressurisation which may be more explosive in terms of the vibration force generated.

The valve exhaust of the rotary valve assembly may be configured, for example by its position, construction and/or the like, to be arranged in pressure communication with an exhaust region. Such an exhaust region may provide suitable pressure conditions to permit the piston chamber to be depressurised.

The number and dimension of flow channels and outlet ports may be selected in accordance with considerations such as total required inlet or outlet area, vibration frequency and/or the like.

The downhole drag reducing apparatus may be configured so that the flow area of the valve inlet and the valve exhaust are the same or substantially the same.

As described above, movement of the reciprocating piston in the first and second axial directions generates a vibration force within the apparatus. In particular but not exclusively, when the apparatus is configured to cyclically move the reciprocating piston in first and second axial directions, the flow areas of the valve inlet and valve exhaust may be matched for each direction.

The opening and closure timing of the valve inlet and the valve exhaust may be configured to reduce damping or choking of fluid flow for example where it is desired to facilitate more rapid exhaust of fluid, such that the impulse of the reciprocating piston, and corresponding vibration force generated, is increased.

The opening and closure timing of the valve inlet and the valve exhaust may be configured to increase damping or choking of fluid flow for example where it is desired to dampen or control movement of the reciprocating piston, and thereby limit the impulse and corresponding vibration force, generated.

As described above, the rotary valve assembly may comprise a rotary valve member operatively associated with the valve inlet and the valve exhaust. The rotary valve member may be configured to facilitate selective fluid communication between the valve inlet and the piston chamber and/or the valve exhaust and the piston chamber.

The rotary valve member may be radially interposed between the mandrel and the bearing sleeve. The rotary valve member may be disposed in an axial flow passage formed between the outside of the mandrel and the inside of the bearing sleeve. The rotary valve member may be a tubular, e.g. a tubular ring. In this regard, the rotary valve member may be defined as a rotary ring valve member.

The rotary valve member may be integrally formed with the bearing sleeve. Alternatively, the rotary valve member may be a separate component to the bearing sleeve.

The rotary valve member may be rotabably fixed with respect to the bearing sleeve. The rotary valve member may be rotabably fixed with respect to the bearing sleeve via a spline connection, key connection, or other suitable connection.

The rotary valve member may be formed of two or more parts. For example, the rotary valve member may be of a clamshell design, i.e. where two halves of the rotary valve member are fitted around the mandrel and are bolted and/or pinned together.

The rotary valve member may comprise a body. The rotary valve member may comprise one or more flow passageways. The one or more flow passageways may comprise one or more through bores (e.g. axial through bores) in the rotary valve member. Where there is more than one flow passageway, the flow passageways may be arranged or distributed circumferentially with respect to the rotary valve member. The one or more flow passageways may be circular. The one or more flow passageways may be axial flow passageways. The one or more flow passageways may selectively permit at least one of the valve inlet and valve exhaust to communicate with the piston chamber. The body of the rotary valve member may form one or more gate members, circumferentially interposed between each of the one or more flow passageways.

The one or more flow passageways may be operatively associated with the valve inlet. The valve inlet may be rotatable with respect to the one or more flow passageways. The one or more flow passageways and the valve inlet may be configured for relative rotation with respect to each other such that rotation causes the body of the rotary valve member to selectively block or obturate the valve inlet. That is, during one phase of relative rotation, the valve inlet may define an open configuration in which pressure communication with the piston chamber is permitted and in another phase the valve inlet may define a closed configuration in which pressure communication is prevented, substantially prevented or obturated.

In other examples, the valve inlet may be provided on an inlet body and the rotary valve assembly may comprise an inlet selector sleeve, wherein the inlet body and inlet selector sleeve are arranged to be rotatable relative to each other to selectively open and close the valve inlet. The inlet body and the inlet selector sleeve may be configured such that relative rotation therebetween cyclically opens and closes the valve inlet. In this respect, the open condition may be such that pressure communication with the piston chamber is provided. The closed condition may substantially or fully prevent fluid communication with the piston chamber.

The one or more flow passageways may be operatively associated with the valve exhaust. The valve exhaust may be rotatable with respect to the one or more flow passageways. The one or more flow passageways and the valve exhaust may be configured for relative rotation with respect to each other such that rotation causes the body of the rotary valve member to selectively block or obturate the valve exhaust. That is, during one phase of relative rotation, the valve exhaust may define an open configuration in which pressure communication with the piston chamber is permitted and in another phase the valve exhaust may define a closed configuration in which pressure communication is prevented, substantially prevented or obturated.

The downhole drag reducing apparatus may comprise a retaining shoulder. The rotary valve assembly may be configured to co-operative with the retaining shoulder. The retaining shoulder may be configured to retain the rotary valve assembly in place.

The retaining shoulder may be located radially between the mandrel and the bearing sleeve. The retaining shoulder may be located axially between the rotary valve member and the piston chamber.

The retaining shoulder may be integrally formed with the mandrel. The retaining shoulder may be a separate component to the mandrel. The retaining shoulder may be rotabably fixed with respect to the mandrel. The retaining shoulder may comprise or be in the form of a tubular ring. The retaining shoulder may comprise a plurality of circumferentially spaced notches or indents, e.g. forming a castellated (outer) surface. Interposed between each of the notches, the retaining shoulder comprises one or more impact structures.

The retaining shoulder may comprise one or more flow paths. The flow paths may be circumferentially spaced flow paths. The one or more circumferentially spaced notches or indents may form or define the one or more circumferentially spaced flow paths. The one or more flow paths may selectively permit at least one of the valve inlet and valve exhaust to communicate with the piston chamber.

The one or more circumferentially spaced flow paths may be configured in alignment with at least one of the valve inlet and valve exhaust. The one or more flow paths may rotate together with the valve inlet and valve exhaust, so as to maintain alignment therewith. Thus, in the pressure configuration, the retaining shoulder may permit communication between the valve inlet and piston chamber via the rotary valve member, and, in the exhaust configuration, the retaining shoulder may permit communication between the valve exhaust and the piston chamber via the rotary valve member.

During relative rotation between the mandrel and the bearing sleeve, wherein the apparatus is between the pressure configuration and the exhaust configuration, the retaining shoulder may enclose the piston chamber. In particular, the one or more impact structures, circumferentially interposed between the one or more flow paths of the retaining shoulder, may enclose the piston chamber.

As described above, reciprocating movement of the reciprocating piston generates vibration within the downhole drag reducing apparatus. As described herein, vibration of the apparatus may be provided or achieved by repeated impacts generated within the apparatus, for example impact of a hammer, operated by movement of the reciprocating piston, against an anvil. Alternatively, vibration may be provided by reciprocating movement of a mass, driven by the reciprocating piston (which reciprocating piston may define the mass), without any impact. The downhole drag reducing apparatus may comprise first and second sets of cooperating impact surfaces, wherein engagement of the impact surfaces results in the vibration force.

A first impact surface may be provided on the reciprocating piston. In particular embodiments, the first impact surface may be provided on a hammer coupled to or forming part of the reciprocating piston.

A second impact surface may be formed on a surface of the bearing sleeve and/or the mandrel. In particular embodiments, the second impact surface may be formed on a surface or anvil of the mandrel. The second impact surface may be coupled to or formed by part of the impact shoulder provided on or formed with the mandrel, mentioned above. The first and second impact surfaces may define the first set of cooperating impact surfaces.

A third impact surface may be provided on the reciprocating piston, at an opposing end to that of the first impact surface. In particular embodiments, the third impact surface may be provided on a hammer coupled to or forming part of the reciprocating piston.

A fourth impact surface may be formed on a surface of the bearing sleeve and/or the mandrel. The fourth impact surface may be formed on the mandrel (e.g. an anvil). The fourth impact surface may be coupled to or formed by the retaining shoulder. In this regard, the retaining shoulder may function as a second impact shoulder for the reciprocating piston to impact. The third and fourth impact surfaces may define the second set of co-operating impact surfaces.

As described above, in use the rotary valve assembly may be operable by relative rotation between the mandrel and the bearing sleeve to be cyclically reconfigured between the pressure configuration and the exhaust configuration, so as to move the reciprocating piston in the first and second axial directions, said movement of the reciprocating piston causing engagement of the first set of co-operating impact surfaces and engagement of the second set of co-operating impact surfaces. In some instances, however, the downhole drag reducing apparatus may be configured so that the impact surfaces do not engage, said movement of the reciprocating piston itself being sufficient to generate vibration or agitation forces.

The downhole drag reducing apparatus may be configured to dampen and/or control movement of the reciprocating piston. The vibration arrangement may comprise means for controlling and/or dampening the movement of the reciprocating piston in one of the axial directions. The downhole drag reducing apparatus may comprise a damper arrangement. The damper arrangement may comprise or take the form of a mechanical damper arrangement and/or a fluid damper arrangement, for example hydraulic damper arrangement.

The downhole drag reducing apparatus may comprise an end stop. The end stop may comprise or take the form of a buffer, for example a rubber buffer. Alternatively, the end stop may comprise or take the form of co-operating impact surfaces similar to the first and second, or third and fourth, impact surfaces described above.

The damper arrangement may comprise a dash-pot assembly. The dash-pot assembly may be coupled to or operatively associated with the reciprocating piston. In use, the dash-pot assembly may provide hydraulic dampening at the end of the travel of the reciprocating piston in the direction of buffering.

Alternatively or additionally, and as described above, the valve inlet and the valve exhaust flow areas may be choked so as to dampen and/or control the vibration force generated within the downhole drag reducing apparatus.

Alternatively or additionally, the opening and closure timings of the valve inlet and the valve exhaust may be configured to dampen and/or control the vibration force generated within the downhole drag reducing apparatus.

The downhole drag reducing apparatus may comprise a bearing arrangement. The bearing arrangement may be radially interposed between the mandrel and the bearing sleeve.

The mandrel may define a circumferentially continuous inner bearing race. The bearing sleeve may define a circumferentially continuous outer bearing race. The bearing sleeve may be mounted on the mandrel such that the outer bearing race of the bearing sleeve circumscribes the inner bearing race of the mandrel.

The bearing arrangement may be radially interposed between the inner bearing race of the mandrel and the outer bearing race of the bearing sleeve to permit the bearing sleeve and the mandrel to be rotatable relative to each other. The bearing arrangement may be a rolling bearing arrangement.

As noted above, the inner and outer bearing races are circumferentially continuous. The term “circumferentially continuous” should be understood to mean unbroken or uninterrupted around the circumference. That is, the inner and outer bearing races do not include any seam, join, interface or the like which would otherwise be present where multiple separate components are joined together.

The provision of circumferentially continuous inner and outer bearing races may afford more stability and strength within both the mandrel and the bearing sleeve, minimising failure modes associated with clamshell type devices and their corresponding connection means. For example, the full hoop strength of the bearing sleeve and the mandrel may be retained. Furthermore, the continuous construction of the bearing races may provide continuous running surfaces for the roller bearing arrangement, resulting in more even load distribution within the rolling bearing arrangement. Also, the ability to provide a form of sealing between the mandrel and the bearing sleeve may be improved. An example of such sealing will be described below.

The rolling bearing arrangement may comprise a plurality of rolling bodies radially interposed between the inner and outer bearing races. The rolling bodies may comprise one or more of rollers, balls, needles and/or the like. The rolling bearing arrangement may comprise a circumferential array of rolling bodies arranged between the inner and outer bearing races. The rolling bearing arrangement may comprise multiple (i.e., two, three, four etc.) axially distributed circumferential arrays of rolling bodies. Such axially distributed bearing arrays may provide for increased load bearing capability and allow for more stable support of the bearing sleeve about the mandrel. At least one of the outer bearing race and inner bearing race may comprise a bearing raceway for axially captivating the rolling bearing arrangement. The bearing raceway may comprise or be defined by a circumferential groove. The circumferential groove may be configured to accommodate a circumferential array of rolling bodies. Where multiple circumferential arrays of rolling bodies are provided multiple bearing raceways (e.g., grooves) may be provided. In such an arrangement multiple bearing raceways may be axially distributed along the inner and/or outer bearing races.

In one example only one of the inner and outer bearing races comprises a bearing raceway. Such an arrangement may facilitate a degree of axial movement to be achieved between the mandrel and the bearing sleeve. This might minimise axial loading applied on the rolling bearing arrangement which may minimise rotational drag, increase operational longevity and the like.

In one example the inner bearing race defined by the mandrel may comprise one or more bearing raceways, for example in the form of one or more circumferential grooves, and the outer bearing race defined by the bearing sleeve may comprise a cylindrical bearing surface (i.e., without any bearing raceway such as a groove formed therein). This example may provide benefits in terms of allowing a thinner walled bearing sleeve to be utilised, in that additional wall thickness might not be necessary to accommodate the formation of one or more raceways. Nevertheless, in an alternative example the outer bearing race defined by the bearing sleeve may comprise one or more bearing raceways, for example in the form of one or more circumferential grooves, and the inner bearing race defined by the mandrel may comprise a cylindrical bearing surface (i.e., without any bearing raceway such as a groove formed therein).

In a further example both the inner and outer bearing races may comprise one or more bearing raceways, for example in the form of one or more circumferential grooves. A bearing raceway on the inner bearing race may be axially aligned with a bearing raceway on the outer bearing surface to thus form a radially opposing raceway pair. In such an arrangement the rolling bearing arrangement will be received within the opposing raceway pairs and will thus function to axially lock the mandrel and the bearing sleeve together. In a further example, bearing raceways formed on the inner and outer bearing races may be axially misaligned (i.e., a bearing raceway may be aligned with an opposing cylindrical bearing surface). In other examples, a bearing cage, or a series of bearing cages, may be provided to retain the rolling bearing arrangement in place. The bearing cage may be provided in two or more parts. The two or more parts may permit the bearing cage to be mounted on, for example, the mandrel and assembled together.

The rolling bearing arrangement may comprise one or more rolling elements, such as rollers, balls, needles, etc. Where the rolling elements comprise one or more ball bearings, the ball bearings may be provided without the need for inner and/or outer bearing races. The rolling elements may comprise a specific number or quantity of ball bearings.

The downhole drag reducing apparatus may further comprise a bearing cavity defined between the mandrel and the bearing sleeve, wherein the rolling bearing arrangement is provided within the bearing cavity. The bearing cavity may be configured to be at least partially (incompletely or completely) filled with a lubricant, such as oil, grease and/or the like. While the term “lubricant” is used, this may provide a function other than lubrication, such as cooling, acting as a barrier medium to isolate the components within the bearing cavity from ambient fluids and debris, and/or the like. As such, the term “lubricant” may be interchangeable with the term “medium”. The bearing cavity may be associated with a lubricant port for permitting lubricant to be delivered (e.g., injected) into the bearing cavity. The lubricant port may be formed in the bearing sleeve, for example extending through (e.g., radially through) a wall thickness of the bearing sleeve. The bearing cavity may be associated with a vent port for permitting fluid (e.g. air) to be vented from the bearing cavity. The vent port may be formed in the bearing sleeve, for example extending through (e.g., radially through) a wall thickness of the bearing sleeve. Such an arrangement may provide ease of access in that the lubricant port is readily accessible from the outer surface of the bearing sleeve. The lubricant port may be sealed or sealable with a plug or equivalent structure. The vent port may be sealed or sealable with a plug or equivalent structure.

The downhole drag reducing apparatus may comprise a pressure regulator for regulating the pressure within the bearing cavity. The pressure regulator may be configured to regulate the pressure within the bearing cavity with respect to a separate region, for example a region externally of the downhole drag reducing apparatus, which might be defined by a wellbore region (e.g., annulus region) when in use. The pressure regulator may be configured to pressure balance the bearing cavity with respect to a separate region. The pressure regulator may be configured to accommodate thermal expansion and/or contraction of lubricant within the bearing cavity. The ability to regulate pressure within the bearing cavity may provide benefits such as reducing the risk of ingress or egress of material (fluids, debris etc.), providing safety measures to avoid high pressure decompression during or following retrieval, minimising the risk of deformation or other pressure induced damage to components due to high pressure differentials and/or the like.

In use, as the downhole drag reducing apparatus is deployed deeper into a wellbore the ambient hydrostatic pressure will increase, wherein the pressure regulator may function to allow the bearing cavity to be maintained in pressure balance with the ambient hydrostatic pressure. As such, the pressure differential across any seals within the downhole drag reducing apparatus may be minimised, which may facilitate improved sealing performance. When exposed to high hydrostatic (or other) pressures the risk of wellbore fluids entering the bearing cavity under pressure is minimised, thus minimising the effect such wellbore fluids might have on components within the bearing cavity. This pressure balance effect during use may also provide a similar benefit when the downhole drag reducing apparatus is retrieved towards surface, in which case the hydrostatic pressure will reduce, such that the risk of lubricant being ejected under pressure is minimised.

The pressure regulator may comprise one or more moveable barriers forming a boundary of the bearing cavity. The pressure regulator may comprise a pressure transfer arrangement. The pressure regulator may comprise a piston, bellows, diaphragm and/or the like. In some examples, as described in more detail below, the pressure regulator may be incorporated within a sealing arrangement which functions to also seal the bearing cavity.

The downhole drag reducing apparatus may comprise a sealing arrangement for sealing the bearing cavity. Such sealing may protect the inner and outer bearing races and the rolling bearing arrangement from wellbore fluids and from any aggressive solid media or debris that may be present. The sealing arrangement may be a dynamic sealing arrangement configured to provide a sealing function during relative rotation between the mandrel and the bearing sleeve. The sealing arrangement may operate in combination with a unitary bearing sleeve and mandrel to provide improved sealing, for example relative to known clamshell designs in that sealing across a split, join or seam is not required.

The sealing arrangement may be bi-directional. That is, the sealing arrangement may be configured to restrict both ingress and egress of material relative to the bearing cavity. For example, the sealing arrangement may be capable of providing sealing when exposed to a pressure differential in reverse directions.

The sealing arrangement may be configured to regulate pressure within the bearing cavity. That is, the sealing arrangement may provide a dual function of both sealing and pressure regulating the bearing cavity, thus avoiding the requirement for providing these different functions independently.

The sealing arrangement may accommodate volumetric changes within the bearing cavity, for example volumetric changes of a lubricant contained within the bearing cavity. Such volumetric changes may be as a result of thermal expansion and/or contraction of the lubricant within the bearing cavity, such that volumetric changes are accommodated without altering, or significantly altering the bearing cavity pressure, which might otherwise cause leakage past the sealing arrangement.

The sealing arrangement may facilitate pressure balancing of the bearing chamber with a separate region, for example a region externally of the downhole drag reducing apparatus (e.g., a wellbore annulus region). The sealing arrangement may define a moveable barrier of the bearing cavity to allow pressure transference between the bearing cavity and the separate region.

The ability of the sealing arrangement to accommodate volumetric and/or pressure changes within the bearing cavity may provide for a more compact and less complex design, in that separate means to provide these functions may not be required. However, as disclosed herein, separate sealing and pressure regulating arrangements may be provided. The sealing arrangement may comprise first and second seals positioned at opposing axial ends of the bearing cavity. In this example the sealing arrangement may define axial extremes of the bearing cavity.

At least one of the first and second seals may comprise one or more sealing components. At least one of the first and second seals may comprise a sealing member. The sealing member may be a bi-directional sealing member. The sealing member may be deformable and/or moveable, for example axially moveable, to accommodate volumetric changes within the bearing cavity, for example volumetric changes within a lubricant within the bearing cavity. This deformable and/or moveable capability of the sealing member may accommodate thermal expansion and contraction of a medium, such as a lubricant, contained within the bearing cavity. The deformable and/or moveable capability of the sealing member may facilitate pressure balancing of the bearing cavity with respect to an external location.

The sealing member may be formed of any material which suits the application. In some examples the sealing member may comprise a polymeric material, such as PTFE, an elastomer and/or the like.

Both the first and second seals may comprise a deformable and/or moveable seal member. This arrangement may maximise the volumetric change capabilities.

At least one of the first and second seals may comprise a wiper seal, for example a scarf cut wiper seal. The wiper seal may function in combination with a deformable and/or moveable seal member in order to ensure a clean sealing surface on one or both of the mandrel and bearing sleeve is maintained.

The bearing sleeve may be formed of a metallic material, although any other suitable material may be utilised. The bearing sleeve may be appropriately treated to facilitate improved longevity (e.g., wear resistance), accommodate improved sealing engagement with a sealing arrangement, accommodate improved running engagement with the rolling bearing arrangement and/or the like. The bearing sleeve may comprise hardened regions, honed/ground regions and/or the, for example on one or both of the inner bearing race and outer bore wall contacting structure. The bearing sleeve may define a uniform wall thickness between opposing axial ends thereof. Alternatively, the bearing sleeve may comprise a varying wall thickness between opposing axial ends thereof. Such a varying wall thickness may be provided via variations in the outer surface of the bearing sleeve. In this example the inner surface of the bearing sleeve may comprise a constant or uniform profile. A varying wall thickness may provide benefits during use, such as providing regions of the bearing sleeve which may not come into contact with a bore wall, such as regions adjacent sealing structures. This may minimise lateral loading on such regions which may minimise radial loading imparted on the sealing structure.

The bearing sleeve may comprise a first axial region which accommodates engagement or is aligned with a sealing arrangement. That is, the first axial region may circumscribe a sealing arrangement. The bearing sleeve may comprise a second axial region which accommodates engagement or is aligned with the rolling bearing arrangement. That is, the second axial region may circumscribe the rolling bearing arrangement. In this example the second axial region may define the outer bearing race. In some examples the second axial region may define a greater outer dimension (i.e., gauge dimension) than the first region. This may minimise contact between the first region of the bearing sleeve and a bore wall, which may thus minimise radial forces imparted on the sealing arrangement.

The bearing sleeve may comprise a third axial region which accommodates engagement or is aligned with a sealing arrangement. In this example the first and third axial regions may circumscribe separate portions of a sealing arrangement. The second axial region may be interposed between the first and third axial regions. In some examples the second axial region may define a greater outer dimension (i.e., gauge dimension) than the third axial region.

The bearing sleeve may define an outer bore wall contacting structure. In some examples the outer bore wall contacting structure may be provided on the second region of the bearing sleeve, as defined above.

The outer bore wall contacting structure may comprise a cylindrical outer surface of the bearing sleeve. In some examples the outer bore wall contacting structure may comprise one or more protruding structures, such as ribs, vanes, fins, humps, posts, dimples and/or the like. The protruding structures, when engaged with a bore wall, may assist in providing relative rotation between the mandrel and the bearing sleeve.

The bearing sleeve may be mounted on the mandrel by being axially slid over an end of the mandrel. Alternatively, the bearing sleeve may be directly formed or manufactured on the mandrel, for example using additive manufacturing (e.g., 3D printing) techniques.

The bearing sleeve may be axially positioned between a first axial shoulder and a second axial shoulder provided on the mandrel. In examples where the piston housing and the bearing sleeve are not provided as a unitary component, the piston housing may be axially positioned alongside the bearing sleeve and between the first and second axial shoulders provided on the mandrel. In this respect the bearing sleeve and piston housing may be held in place on the mandrel between the first and second axial shoulders.

Such station keeping of the bearing sleeve may be such that the inner and outer bearing races are maintained in alignment. The downhole drag reducing apparatus may comprise at least one bush member interposed between an axial end of the bearing sleeve/piston housing and one or both of the first and third axial shoulders. The bush member may comprise a wear bush.

At least one of the first and second axial shoulders may be integrally formed with the mandrel. At least one of the first and second axial shoulders may be provided separately and secured to the mandrel. Such separate forming and mounting of at least one of the first and second axial shoulders may permit the bearing sleeve to be slid onto one end of the mandrel during assembly.

At least one of the first and second axial shoulders may be provided via a separate tool component which is secured to the mandrel. That is, connecting of the downhole drag reducing apparatus to a separate component may form at least one of the first and second axial shoulders.

At least one of the first and second axial shoulders may comprise or be formed on a retaining arrangement. The retaining arrangement may comprise a retaining ring, for example a unitary retaining ring. The retaining ring may be threadedly mounted on the mandrel. The downhole drag reducing apparatus may comprise a torque shoulder, against which torque shoulder the retaining ring may be torqued. The torque shoulder may be formed directly on the mandrel. Alternatively, the torque shoulder may be formed on a separate component, such as a retaining mount, as described below.

In one example the mandrel may comprise a threaded portion for threaded engagement with the retaining ring.

The retaining arrangement may comprise a retaining mount configured to be mounted on the mandrel, wherein the retaining ring is configured to be mounted and secured on the retaining mount. In examples where the mandrel comprises an end connector the provision of a separate retaining mount may assist to allow the retaining arrangement to be axially set back form the end connector, which may assist to maximise the strength of the end connector. The retaining mount may define a connecting profile to facilitate connection with the retaining ring. The connecting profile may comprise a thread, for example a male thread, for threaded engagement with the retaining ring.

The retaining mount may be configured to be axially secured to the mandrel. For example, the retaining mount may comprise a radial extension configured for engagement with a radial slot within the mandrel. The retaining mount may be axially fixed to the mandrel via a suitable connection, such as a screw connection, pinned connection, keyed connection, welded connection and/or the like.

The retaining mount may be configured to be rotatably secured to the mandrel. Such an arrangement may allow torqueing of the retaining ring onto the retaining mount via a threaded connection. The retaining mount may comprise an internal non-round profile configured to engage a non-round profile formed on an outer surface region of the mandrel. The retaining mount may be keyed, pinned or the like relative to the mandrel to provide a rotary connection.

The retaining mount may be generally ring shaped.

The retaining mount may be provided as a unitary component. Alternatively, the retaining mount may be provided in multiple components (i.e., two, three, four etc.) which are assembled together on the mandrel. The multiple components may comprise multiple circumferential segments. In one example the retaining mount may comprise a pair of segments. Providing the retaining mount in multiple segments may facilitate assembly. The retaining mount components may be secured relative to the mandrel, for example individually secured relative to the mandrel. Alternatively, or additionally, separate retaining mount components may be collectively held together on the mandrel by the retaining ring.

The retaining mount may comprise or define a torque shoulder, against which torque shoulder the retaining ring is torqued.

The rolling bearing arrangement may be axially positioned between the impact shoulder provided on the mandrel and the second axial shoulder.

As noted above, the downhole drag reducing apparatus may be coupled to or within a rotating string. In some examples multiple downhole drag reducing apparatus may be connected along a length of the string. A downhole drag reducing apparatus may be located at any desired location along the length of the string. In some examples a downhole drag reducing apparatus may be located adjacent a bottom hole assembly (BHA) supported by the string.

As set out above, the mandrel defines an inner bearing race and the bearing sleeve defines an outer bearing race. The inner and outer races may be integrally formed with the mandrel and bearing sleeve, respectively. Alternatively, one or both of the inner and outer races may be provided on a separate component.

In some examples the downhole drag reducing apparatus may be defined without reference to inner and outer races. For example, in substitution for the inner race the mandrel may be defined as including an outer bearing structure, and in substitution for the outer race the bearing sleeve may be defined as including an inner bearing structure.

The downhole drag reducing apparatus or any aspect defined herein, or any individual component or groups of components, may be manufactured in any suitable manner. In some examples the disclosed downhole drag reducing apparatus, or any individual component or groups of components may be manufactured by additive manufacturing. Such described additive manufacturing typically involves processes in which components are fabricated based on three-dimensional (3D) information, for example a three-dimensional computer model (or design file), of the component.

Accordingly, examples described herein not only include the downhole drag reducing apparatus and associated components, but also methods of manufacturing the downhole drag reducing apparatus or associated components via additive manufacturing and computer software, firmware or hardware for controlling the manufacture of the downhole drag reducing apparatus and associated components via additive manufacturing. All future reference to “product” are understood to include the described downhole drag reducing apparatus and all associated components.

The structure of the product may be represented digitally in the form of a design file. A design file, or computer aided design (CAD) file, is a configuration file that encodes one or more of the surface or volumetric configuration of the shape of the product. That is, a design file represents the geometrical arrangement or shape of the product.

Design files may take any now known or later developed file format. For example, design files may be in the Stereolithography or “Standard Tessellation Language” (.stl) format which was created for stereolithography CAD programs of 3D Systems, or the Additive Manufacturing File (.amf) format, which is an American Society of Mechanical Engineers (ASME) standard that is an extensible markup-language (XML) based format designed to allow any CAD software to describe the shape and composition of any three-dimensional object to be fabricated on any additive manufacturing printer.

Further examples of design file formats include AutoCAD (.dwg) files, Blender (.blend) files, Parasolid (,x_t) files, 3D Manufacturing Format (,3mf) files, Autodesk (3ds) files, Collada (.dae) files and Wavefront (.obj) files, although many other file formats exist.

Design files may be produced using modelling (e.g. CAD modelling) software and/or through scanning the surface of a product to measure the surface configuration of the product. Once obtained, a design file may be converted into a set of computer executable instructions that, once executed by a processer, cause the processor to control an additive manufacturing apparatus to produce a product according to the geometrical arrangement specified in the design file. The conversion may convert the design file into slices or layers that are to be formed sequentially by the additive manufacturing apparatus. The instructions (otherwise known as geometric code or “G-code”) may be calibrated to the specific additive manufacturing apparatus and may specify the precise location and amount of material that is to be formed at each stage in the manufacturing process. The formation may be through deposition, through sintering, or through any other form of additive manufacturing method.

The code or instructions may be translated between different formats, converted into a set of data signals and transmitted, received as a set of data signals and converted to code, stored, etc., as necessary. The instructions may be an input to the additive manufacturing system and may come from a part designer, an intellectual property (IP) provider, a design company, the operator or owner of the additive manufacturing system, or from other sources. An additive manufacturing system may execute the instructions to fabricate the product using any of the technologies or methods disclosed herein.

Design files or computer executable instructions may be stored in a (transitory or non- transitory) computer readable storage medium (e.g., memory, storage system, etc.) storing code, or computer readable instructions, representative of the product to be produced. As noted, the code or computer readable instructions defining the product that may be used to physically generate the object, upon execution of the code or instructions by an additive manufacturing system. For example, the instructions may include a precisely defined 3D model of the product and may be generated from any of a large variety of well-known computer aided design (CAD) software systems such as AutoCAD®, TurboCAD®, DesignCAD 3D Max, etc. Alternatively, a model or prototype of the component may be scanned to determine the three-dimensional information of the component.

Accordingly, by controlling an additive manufacturing apparatus according to the computer executable instructions, the additive manufacturing apparatus may be instructed to print out the product. In light of the above, embodiments include methods of manufacture via additive manufacturing. This includes the steps of obtaining a design file representing the product and instructing an additive manufacturing apparatus to manufacture the product in assembled or unassembled form according to the design file. The additive manufacturing apparatus may include a processor that is configured to automatically convert the design file into computer executable instructions for controlling the manufacture of the product. In these embodiments, the design file itself may automatically cause the production of the product once input into the additive manufacturing device. Accordingly, in this embodiment, the design file itself may be considered computer executable instructions that cause the additive manufacturing apparatus to manufacture the product. Alternatively, the design file may be converted into instructions by an external computing system, with the resulting computer executable instructions being provided to the additive manufacturing device.

Given the above, the design and manufacture of implementations of the subject matter and the operations described in this specification may be realised using digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. For instance, hardware may include processors, microprocessors, electronic circuitry, electronic components, integrated circuits, etc. Implementations of the subject matter described in this disclosure may be realised using one or more computer programs, i.e., one or more modules of computer program instructions, encoded on computer storage medium for execution by, or to control the operation of, data processing apparatus. Alternatively or in addition, the program instructions may be encoded on an artificially generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. A computer storage medium may be, or be included in, a computer-readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them. Moreover, while a computer storage medium is not a propagated signal, a computer storage medium may be a source or destination of computer program instructions encoded in an artificially generated propagated signal. The computer storage medium may also be, or be included in, one or more separate physical components or media (e.g., multiple CDs, disks, or other storage devices).

Although additive manufacturing technology is described herein as enabling fabrication of complex objects by building objects point-by-point, layer-by-layer, typically in a vertical direction, other methods of fabrication are possible and within the scope of the present subject matter. For example, although the discussion herein refers to the addition of material to form successive layers, one skilled in the art will appreciate that the methods and structures disclosed herein may be practiced with any additive manufacturing technique or other manufacturing technology.

An aspect of the present disclosure relates to a downhole rotary string assembly comprising a downhole drag reducing apparatus according to any other aspect.

An aspect of the present disclosure relates to a method for performing a wellbore operation, comprising engaging a bore wall with a bore wall engaging surface of a bearing sleeve, wherein the bearing sleeve is mounted on a mandrel; rotating the mandrel relative to the bearing sleeve while the bearing sleeve is engaged with the bore wall; operating a rotary valve assembly by relative rotation between the mandrel and the bearing sleeve to cyclically pressurise and depressurise a piston chamber to provide reciprocating movement of a reciprocating piston mounted within a piston housing defining the piston chamber; and generating vibration within the apparatus by the reciprocating movement of the reciprocating piston.

An aspect of the present disclosure relates to a method for reducing drag downhole in a wellbore, comprising engaging a bore wall with a bore wall engaging surface of a bearing sleeve, wherein the bearing sleeve is mounted on a mandrel; rotating the mandrel relative to the bearing sleeve while the bearing sleeve is engaged with the bore wall; operating a rotary valve assembly by relative rotation between the mandrel and the bearing sleeve to cyclically pressurise and depressurise a piston chamber to provide reciprocating movement of a reciprocating piston mounted within a piston housing defining the piston chamber; and generating vibration within the apparatus by the reciprocating movement of the reciprocating piston.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic illustration of a wellbore under construction using a drill string which includes multiple downhole drag reducing apparatus;

Figure 2 is a longitudinal cross-sectional view of a downhole drag reducing apparatus;

Figure 3 is a lateral cross-sectional view of the downhole drag reducing apparatus, taken along line A-A;

Figure 4 is a lateral cross sectional view of the torque reducer of Figure 2, taken along line B-B;

Figure 5 is a perspective view of the downhole drag reducing apparatus of the Figure 2;

Figure 6 is a perspective view of a rotary valve member used in the downhole drag reducing apparatus of Figure 2;

Figure 7 is a lateral cross-sectional view of the rotary valve member of Figure 6;

Figure 8 is a perspective view of the a reciprocating piston used in the downhole drag reducing apparatus of Figure 2;

Figure 9 is a lateral cross-sectional view of the reciprocating piston of Figure 8;

Figure 10 is an enlarged longitudinal cross-sectional view of the downhole drag reducing apparatus of Figure 2, in a pressure configuration; Figure 11 is a lateral cross-sectional view of the downhole drag reducing apparatus of Figure 10, taken along line C-C;

Figure 12 is a lateral cross-sectional view of the downhole drag reducing apparatus of Figure 10, taken along line D-D;

Figure 13 is an enlarged longitudinal cross-sectional view of the downhole drag reducing apparatus of Figure 2, in an exhaust configuration;

Figure 14 is a lateral cross-sectional view of the downhole drag reducing apparatus of Figure 13, taken along line C-C;

Figure 15 is a lateral cross-sectional view of the downhole drag reducing apparatus of Figure 13, taken along line D-D;

Figure 16 illustrates an alternative embodiment of a drag reducing apparatus; and

Figure 17 illustrates the downhole drag reducing apparatus used in a retrieving operation.

DETAILED DESCRIPTION

Figure 1 diagrammatically illustrates a wellbore 10 under construction using a drill string 12 which is rotated and advanced from a drilling rig 14 located at surface. The drilling rig 14 may be of any suitable form, including a land based rig, offshore rig such as a jack up, a semi-submersible platform, a drillship and/or the like. In the present example, the wellbore 10 is a deviated bore which includes a vertical bore section 10a, a build-up section 10b and a horizontal section 10c. However, the present disclosure extends to any form of wellbore. Further, although not illustrated the wellbore 10, in some sections, may be cased or lined, with or without cementing.

The drill string 12 may be formed of multiple jointed drill pipe and collars and includes a drilling bottom hole assembly (BHA) 16 which may incorporate conventional equipment such as a drill bit, stabilisers, measurement while drilling (MWD) equipment, directional drilling equipment and/or the like. The drill string 12 further includes multiple drag reducing apparatus 18 distributed along its length. As will be described in more detail below, the drag reducing apparatus comprises a sleeve rotatably mounted on a mandrel which is rotatably connected to the drill string 12, such that when the sleeve engages a wall of the wellbore 10, the mandrel, and the drill string 12, may more freely rotate. The relative rotation between the sleeve and mandrel also causes reciprocation of a reciprocating piston which generates a vibration effect, which may also assist in reducing the effect of drag forces, for example by more readily breaking static friction, by lowering dynamic friction, etc..

A longitudinal cross-sectional view of a drag reducing apparatus 18 used in the example of Figure 1 is illustrated in Figure 2.

The drag reducing apparatus 18 comprises a mandrel 20 and a bearing sleeve 24 mounted on the mandrel 20 such that the mandrel 20 and bearing sleeve 24 are rotatable relative to each other. The bearing sleeve 24 defines a bore wall engaging surface. The drag reducing apparatus 18 further comprises a reciprocating piston 23 mounted within a piston housing 25 to define a piston chamber 27. As discussed in more detail below, a rotary valve assembly 29 is operated by relative rotation between the mandrel 20 and the bearing sleeve 24 to cyclically pressurise and depressurise the piston chamber 27 to provide reciprocating movement of the reciprocating piston 23 and the generation of vibration within the apparatus 18.

In this example, the piston housing 25 is formed as a unitary component with the bearing sleeve 24. Therefore, hereinafter, unless stated or implied otherwise, any reference to the “bearing sleeve 24” should be construed as including the bearing sleeve 24 and the piston housing 25.

The drag reducing apparatus 18 may function to minimise friction between the drill string 12 and the wall of the wellbore 10 (which may be open-hole or cased/lined). During downhole use in a wellbore, engagement with the bore wall will cause the bearing sleeve 24 to be rotationally held. Thus, in this situation rotation of the mandrel 20 will produce relative rotation between mandrel 20 and the bearing sleeve 24, thereby operating the rotary valve assembly 29 and providing reciprocating movement of the reciprocating piston 23, as described in more detail below. Such reciprocating movement of the reciprocating piston 23 generates vibration within the apparatus 18, which may assist in overcoming the friction, both static and dynamic friction, experienced when the apparatus 18 is engaged with the bore wall. Moreover, the relative rotation between the mandrel 20 and the bearing sleeve 24 may function to minimise friction between the work string 12, when rotated, and the bore wall of the wellbore 10, thus reducing the drag torque experience by the work string 12. In this respect, the bearing sleeve 24 may isolate the mandrel 20 from drag torque interactions with the bore wall, to thus maximise energy transfer between the drilling rig 14 and the drilling BHA 16. This may in turn reduce the drive torque requirements at the surface. The reduced friction effect may be of particular benefit in the build-up and horizontal sections 10a, 10c of the wellbore, and may permit further extended reach wellbores to be formed.

The mandrel 20 defines an inner bearing race 22, and the bearing sleeve 24 defines a corresponding outer bearing race 26. The bearing sleeve 24 is mounted on the mandrel 20 such that the inner and outer bearing races 22, 26 are aligned. A rolling bearing arrangement 28 is radially interposed between the mandrel 20 and the bearing sleeve 24 to permit relative rotation therebetween.

The mandrel 20 includes opposing end connectors 30, 32 (in the form of pin and box type connectors) for facilitating connection to the drill string 12. The mandrel 20 defines an axial throughbore 31.

The mandrel 20 and the bearing sleeve 24 are each of unitary construction such that the inner and outer bearing races 22, 26 are circumferentially continuous. However, it should be recognised that in some examples the mandrel and/or bearing sleeve 24 may be formed of multiple connected components while still allowing the bearing races 22, 26 to be circumferentially continuous. For example, one or both of the mandrel 20 and the bearing sleeve 24 may comprise separate interconnected axial portions, wherein the inner and/or outer bearing races 22, 26 are defined by a unitary axial component.

The provision of circumferentially continuous inner and outer bearing races 22, 26 may afford more stability and strength within both the mandrel 20 and the bearing sleeve 24, minimising failure modes associated with clamshell type devices and their corresponding connection means. For example, the full hoop strength of the bearing sleeve 24 and the mandrel 20 may be retained. Furthermore, the continuous construction of the bearing races 22, 26 may provide continuous running surfaces for the rolling bearing arrangement 28, resulting in more even load distribution. Also, the ability to provide a form of sealing between the mandrel 20 and the bearing sleeve 24, as described below, may be improved. However, in other examples, the inner and outer bearing races may not define a continuous structure (e.g., they could be of a clam shell design).

In the present example the inner bearing race 22 formed by the mandrel 20 comprises a plurality of axially arranged bearing raceways in the form of circumferential grooves 34. The rolling bearing arrangement 28 comprises a plurality of rolling elements 36 (e.g., rollers, balls, needles etc.) located within each circumferential groove 34, such that a plurality of circumferential arrays of rolling elements 36 is provided, with one circumferential array illustrated in Figure 3, which is a lateral sectional view of Figure 2 taken along line A-A. Such axially distributed arrays of rolling elements 36 may provide for increased load bearing capability and allow for more stable support of the bearing sleeve 24 about the mandrel 20.

The outer bearing race 26 formed by the bearing sleeve 24 does not include any bearing raceway but rather a cylindrical bearing surface 38, along which the rolling elements 36 may roll. This arrangement may permit a thinner walled bearing sleeve 24 to be provided, in that additional wall thickness to accommodate the formation of one or more raceways therein is not required. Further, by providing the raceway grooves 34 only in the inner bearing race 22 a degree of axial movement may be achieved between the mandrel 20 and the bearing sleeve 24. This might minimise axial loading applied on the rolling bearing elements 36 which may minimise rotational drag, increase operational longevity and the like.

A bearing cavity 40 is formed between the mandrel 20, bearing sleeve 24 and first and second axial seals 42, such that the rolling bearing arrangement 28 is provided within the bearing cavity. The bearing cavity 40 is configured to be at least partially (incompletely or completely) filled with a lubricant, such as oil, grease and/or the like. The bearing sleeve 24 comprises one or more lubricant ports 46 extending from an outer surface thereof to facilitate delivery (e.g., injection) of lubricant into the bearing cavity 40. The lubricant port(s) 46 may be sealed or sealable with a plug or equivalent structure. In the present example the first and second seals 42 are the same, although this need not be the case. The first and second seals 42 each comprise a sealing member 44, which may have bi-directional sealing capabilities. The sealing members 44 are deformable and/or moveable, for example axially moveable, to accommodate volumetric changes of the lubricant within the bearing cavity 40 which may be caused by thermal expansion and contraction. By virtue of both the first and second seals 42 having deformable and/or moveable sealing members 44 the volumetric changes which can be accommodated may be maximised.

The deformable and/or moveable capability of the sealing members 44 also facilitates pressure balancing of the bearing cavity 40 with respect to an external location, such as an annulus region within a wellbore. Such pressure balancing may be achieved by virtue of the sealing members 44 being in pressure communication with both the bearing cavity 40 and the external region. In use, as the drag reducing apparatus 18 is deployed deeper into a wellbore the ambient hydrostatic pressure will increase, wherein the sealing members 40 function to allow the bearing cavity 40 to be maintained in pressure balance with the ambient hydrostatic pressure. As such, the pressure differential across the seals 42 may be minimised, which may facilitate improved sealing performance. When exposed to high hydrostatic (or other) pressures the risk of wellbore fluids entering the bearing cavity 40 under pressure is minimised, thus minimising the effect such wellbore fluids might have on components within the bearing cavity 40. This pressure balance effect during use may also provide a similar benefit when the drag reducing apparatus 18 is retrieved towards surface, in which case the hydrostatic pressure will reduce, such that the risk of lubricant being ejected under pressure is minimised.

The sealing members 44 may be formed of any material which suits the application. In some examples the sealing members 42 may comprise a polymeric material, such as PTFE, an elastomer and/or the like.

The first and second seals 42 each further comprise a wiper seal 46, for example a scarf cut wiper seal, adjacent an outer side (relative to the bearing cavity 40) of the associated sealing members 44. The wiper seals 46 may function in combination with the sealing members 44 in order to ensure a clean sealing surfaces on the mandrel 20 and bearing sleeve 24 are maintained.

In the present example the bearing sleeve 24 comprises a varying wall thickness between opposing axial ends thereof, wherein such a varying wall thickness is provided via variations in the outer surface of the bearing sleeve 24 such that the inner surface of the bearing sleeve 24 may comprise a constant or uniform profile. More specifically, a central axial region of the bearing sleeve 24 defines a region of increased wall thickness relative to the axial end regions such that the central region defines the maximum outer gauge diameter of the drag reducing apparatus 18. As such, this central region may define the bore wall engaging surface which engages a bore wall during use. Further, this central region also circumscribes the rolling bearing arrangement 28 such that the thicker wall section has increased load bearing capacity. In addition, the bore wall engaging surface comprises one or more protruding structures or fins 49 to engage with the wall of the bore. The protruding structures 49, when engaged with a bore wall, may assist in providing relative rotation between the mandrel 20 and the bearing sleeve 24. In other examples, the protruding structures 49 may comprise ribs, vanes, humps, posts, dimples and/or the like.

The thinner walled axial end regions of the bearing sleeve 24 are aligned with the first and second seals 42. That is, the thinner walled axial end regions circumscribe the first and second seals 42. These thinner walled end regions are less likely to engage a bore wall (in view of the thicker central region) such that radial forces imparted on the seals 42 may be minimised.

The bearing sleeve 24 may comprise an extended section extending from one of the axial ends thereof, the extended section forming the piston housing 25. The bearing sleeve 24 is held in place between first and second axial shoulders 50, 52 provided on the mandrel 20, wherein a wear bush 54 is interposed between the axial ends of the bearing sleeve 24 and a respective axial shoulder 50, 52.

In the present example, the first axial shoulder 50 is integrally formed on the mandrel via an annular stepped profile, whereas the second axial shoulder 52 is separately formed and securable on the mandrel 20. Such an arrangement may permit the bearing sleeve 24 to be slid over an end of the mandrel 20 (e.g. over connector 30) with the second axial load shoulder 52 subsequently being installed to retain the bearing sleeve 24 in place. However, in other examples the second axial load shoulder may also be integrally formed with the mandrel 20. For example, some or all of the drag reducing apparatus 18 may be formed by additive manufacturing techniques.

The second load shoulder 52 comprises or is formed on a retaining arrangement 58 which includes a retaining mount 60 axially and rotatably secured to the mandrel 20, and a retaining ring 62 threadedly secured over the retaining mount 60. Referring additionally to Figure 4, which is a lateral cross-sectional view of Figure 2 taken along line B-B, the retaining mount is formed of two half segments 60a, 60b which can be mounted around the outer surface of the mandrel 20 to form a mounting ring structure. The retaining mount segments 60a, 60b each include a radial tab portion 64 which are received within a circumferential groove 66 formed in an outer surface of the mandrel 20 in order to axially fix the segments 60a, 60b to the mandrel 20. The retaining mount segments 60a, 60b also collectively define a faceted or non-round internal profile 68 which is mounted on a corresponding faceted or non-round outer profile 70 on a portion of the mandrel 20, to thus allow the segments 60a, 60b of the retaining mount 60 to be rotatably secured to the mandrel 20.

The retaining mount segments 60a, 60b collectively define an outer threaded surface of the retaining mount 60 which is engaged by an inner threaded surface of the retaining ring 62, wherein the retaining ring 62, once threaded onto the retaining mount 60 functions to retain the retaining mount segments 60a, 60b on the mandrel 20. The retaining mount segments 60a, 60b also collectively define a torque shoulder 72 against which the retaining ring 62 may be torqued or tightened.

The rotary valve assembly 29 comprises a valve inlet 74, a valve exhaust 76 and a rotary valve member 80 operatively associated with the valve inlet 74 and valve exhaust 76. The valve inlet 74 is defined by one or more flow passages 73 formed through the mandrel 20, which define inlet openings on an inside (circumferential) surface of the mandrel 20.

The valve exhaust 76 is defined by one or more flow channels 77 (visible in Figure 5) formed on the mandrel 20 and one or more outlet ports 78 formed through the bearing sleeve 24. The rotary valve member 80 is rotatably connected to the bearing sleeve 24 and operatively associated with the valve inlet 74 and valve exhaust 76 so as to selectively prevent communication with the piston chamber 27. The mandrel 20 includes a retaining shoulder 81 formed on a surface thereof (discussed in more detail below).

The reciprocating piston 23 is operatively connected to a spring arrangement 82 biasing the reciprocating piston 23 in the second axial direction B. The spring arrangement 82 is disposed in a spring chamber 84 of the mandrel 20 and may be partially disposed in one or more apertures 87 formed in an end of the reciprocating piston 23. The spring arrangement 82 is connected to and supported by an impact shoulder 90 formed on the mandrel 20. The spring coefficient or stiffness of the spring arrangement 82 may be selected in accordance with vibration requirements of the apparatus 18, such as frequency and force of vibration.

The bearing sleeve 24 is provided with one or more lateral ports arranged adjacent the spring chamber 84 to provide pressure balancing of the spring chamber 84 as the reciprocating piston 23 moves back and forth in the piston chamber 27 in the first and second axial directions A, B. Moreover, the piston chamber 27 is sealed by the provision of a dynamic seal arrangement 91 provided on the reciprocating piston 23 and configured to seal against the mandrel 20 and bearing sleeve 24.

The rotary valve assembly 29 is operated by relative rotation between the mandrel 20 and the bearing sleeve 24 to be cyclically reconfigured between a pressure configuration and an exhaust configuration. As described above, such relative rotation is beneficially provided by the bearing sleeve 24 when it is engaged with a wall of the wellbore.

In the pressure configuration, the piston chamber 27 is in pressure communication with the valve inlet 74, and isolated from the valve exhaust 76, to permit the piston 24 to move in the first axial direction A in accordance with the piston chamber 27 being pressurised via the valve inlet 74. In Figure 2, the apparatus 18 is illustrated in the pressure configuration, with the reciprocating piston 23 being urged towards the impact shoulder 90. In the exhaust configuration, the piston chamber 27 is isolated from the valve inlet 74 and in pressure communication with the valve exhaust 76 to permit the piston chamber 27 to be depressurised. To achieve this, the reciprocating piston 23 is urged by the spring arrangement 82 to move in the second axial direction B and to displace the pressure in the piston chamber 27 out via the valve exhaust 76. Such movement of the reciprocating piston 23 in the first and second axial directions A, B generates vibration within the apparatus 18.

In use, the apparatus 18 is used in combination with a pressure region P and an exhaust region E, wherein the valve inlet 74 communicates with the pressure region P and the valve exhaust 76 communicates with the exhaust region E, such that a pressure differential is applied across the rotary valve assembly 29. Specifically, the pressure within the pressure region P may be elevated above the pressure in the exhaust region E. In particular, the pressure within the pressure region P may be sufficient (for example sufficiently high) to pressurise the piston chamber 27 to cause the reciprocating piston 23 to overcome the biasing force of the spring arrangement 82 and move in the first axial direction A. Moreover, the pressure within the exhaust region E may be sufficient (for example sufficiently low) to permit the piston chamber 27 to be depressurised and the reciprocating piston 23 to move in the second axial direction B by the force provided by the spring arrangement 82 releasing potential energy stored from the down stroke of the reciprocating piston 23.

However, the present downhole drag reducing apparatus 18 may be configured to operate irrespective of the direction of the pressure differential applied across the rotary valve assembly 29. In one mode of operation, suggested above, the pressure of the pressure region P is higher than the pressure of the exhaust region E. However, should the pressure differential be reversed then what was previously the pressure region P becomes the exhaust region E, and vice versa, and what was previously the valve inlet becomes the valve exhaust, and vice versa.

Referring to Figure 5, a perspective view of the downhole drag reducing apparatus 18 is illustrated, in which some components have been removed for clarity, such as the reciprocating piston 23, the rotary valve member 80, the bearing sleeve 24 and the roller bearing arrangement 28. Referring to Figure 6, the rotary valve member 80 comprises a series of circumferential teeth or grooves 82 for connection with the bearing sleeve 24 via a spline arrangement 83 (illustrated in Figure 7). As such, when the apparatus 18 is engaged with a bore wall, the rotary valve member 80 is rotationally held with the bearing sleeve 24, such that rotation of the mandrel 20 produces relative rotation between the mandrel 20 and the rotary valve member 80. The rotary valve member 80 includes a tubular ring having first and second parts 80a, 80b, which form a clamshell design.

The rotary valve member 80 comprises a series of flow passageways 67 formed through a body 69 of the rotary valve member 80 for selectively permitting the valve inlet 74 and valve exhaust 76 to communicate with the piston chamber 27. During one phase of relative rotation (e.g. in the pressure configuration), the valve inlet 74 defines an open configuration in which pressure communication with the piston chamber 27 is permitted via the flow passageways 67 of the rotary valve member 80, and the valve exhaust 76 defines a closed configuration in which pressure communication with the piston chamber 27 is prevented by the body 69 of the rotary valve member 80, such that the piston chamber 27 may be pressurised. In another phase (e.g. at some point during the transition between the pressure configuration and the exhaust configuration), both the valve inlet 74 and the valve exhaust 76 may define a closed configuration in which pressure communication is prevented by the body 69 of the rotary valve member 80, such that pressure is contained in the piston chamber 27. In yet another phase (e.g. the exhaust configuration), the valve exhaust 76 defines an open configuration in which pressure communication with the piston chamber 27 is permitted via the flow passageways 67 of the inlet selectors 85, and the valve inlet 74 defines a closed configuration in which pressure communication with the piston chamber 27 is prevented by the body 69 of the rotary valve member 80, such that the piston chamber 27 may be depressurised.

As mentioned above, the mandrel 20 includes a retaining shoulder 81 formed on a surface thereof. The retaining shoulder 81 is axially interposed between the reciprocating piston 23 and the rotary valve member 80, and is configured to retain the rotary valve member 80 in place. The retaining shoulder 81 is in the form of a tubular ring having a series of one or more circumferentially spaced notches forming flow paths 86 for pressure to be communicated between the piston chamber 27 and the valve inlet 74 or the valve exhaust 76 (depending on the relative rotation of the mandrel 20 and rotary valve member 80). Interposed between each of the flow paths, the retaining shoulder 81 comprises one or more impact structures 71. The flow paths 86 are arranged in alignment with the valve inlet 74 and valve exhaust 76, such that when the valve inlet 74 or valve exhaust 76 are communicating with the rotary valve member 80, the flow paths 86 are configured to permit pressure to be communicated to or from the piston chamber 27. However, when neither of the valve inlet 74 and valve exhaust 76 are communicating with the rotary valve member 80 (i.e. at a point during the transition between the pressure and exhaust configurations), the impact structures 71 of the retaining shoulder 81 and the body 69 of the rotary valve member 80 may together enclose the piston chamber 27. In both the pressure and exhaust configurations, fluid pressure is communicated to or from the piston chamber 27 via the flow paths 86 of the retaining shoulder 81. In this example, the retaining shoulder 81 is integrally formed with the mandrel 20, and thus rotates together with the mandrel 20.

Referring to Figure 8, the reciprocating piston 23 comprises first and second parts 100a, 100b, also forming a clamshell design. Such a design permits the reciprocating piston 23 to be mounted on the mandrel 20 between the retaining shoulder 81 and the impact shoulder 90. The reciprocating piston 23 comprises a first impact surface 100 (e.g. a hammer formed with or coupled to the reciprocating piston 23), which in use is arranged to impact a second impact surface or anvil formed on the impact shoulder 90. Moreover, the reciprocating piston 23 comprises a third impact surface 102, opposing the first impact surface 100, and configured to impact a fourth impact surface or anvil formed on the retaining shoulder 81 of the mandrel 20. In this regard, the retaining shoulder 81 may function as a second impact shoulder for the reciprocating piston 23 to impact.

In use, the rotary valve assembly 29 is operable by relative rotation between the mandrel 20 and the bearing sleeve 24 to be cyclically reconfigured between the pressure configuration and the exhaust configuration, so as to move the reciprocating piston 23 in one of the first and second axial directions A, B, said movement of the reciprocating piston 23 engaging the respective impact surfaces to generate vibration force within the apparatus 18. However, the apparatus 18 may be configured so that the impact surfaces do not engage, said movement of the reciprocating piston 23 itself being sufficient to generate vibration or agitation forces. For example, the stiffness of the spring arrangement 82 might be such that the third and fourth impact surfaces are prevented from impacting one another.

Figure 10 illustrates an enlarged longitudinal cross-sectional view of the reciprocating piston 23 and rotary valve assembly 29. In Figure 10, the apparatus 18 is shown in the pressure configuration, in which the relative rotation between the mandrel 20 and the bearing sleeve 24 is such that the flow passages 73 of the valve inlet 74 are aligned with the flow passageways 67 of the rotary valve member 80. In this configuration, pressure communication between the pressure region P is permitted with the piston chamber 27 via the flow passages 73. Therefore, the reciprocating piston 23 is urged in the first axial direction A by virtue of fluid pressure in the piston chamber 27. Figures 11 and 12 illustrate cross-sectional views of the apparatus 18 shown in Figure 10, taken along lines C-C and D-D, respectively.

Figure 13 illustrates the exhaust configuration, in which the mandrel 20 has been rotated relative to the bearing sleeve 24 such that the axial channels 77 are now aligned with the flow passageways 67 of the rotary valve member 80. In this configuration, pressure communication between the exhaust region E is permitted with the piston chamber 27 via the flow channels 77 and outlet ports 78. Therefore, the reciprocating piston 23 is urged in the second axial direction B by virtue of the potential energy stored in the spring arrangement 82 from the down stroke of the reciprocating piston 23 being released. Figures 14 and 15 illustrate cross-sectional views of the apparatus 18 shown in Figure 10, taken along lines C-C and D-D, respectively.

Figure 16 illustrates an alternative embodiment of a drag reducing apparatus 118, in which a first rotary valve assembly 29a and first reciprocating piston 23a (of the same description as above) are provided on one side of the rolling bearing arrangement 28 (of the same description as above) and a second rotary valve assembly 29b and second reciprocating piston 23b (of the same description as above) are provided on the other side of the rolling bearing arrangement 28.

The first rotary valve assembly 29a may be arranged in an out-of-phase relationship with the second rotary valve assembly 29b, so that each of the reciprocating pistons 23a, 23b simultaneously travel in the first axial directions A, B together. That is, when the valve inlet 74a of the first rotary valve assembly 29a is in pressure communication with its respective piston chamber 27a (i.e. when the valve inlet 74a is aligned with the flow passageways 67a of the first rotary valve member 80a), the valve inlet (not visible) of the second rotary valve assembly 29b is prevented from communicating with its respective piston chamber 27b. Likewise, when the valve exhaust 76b of the second the rotary valve assembly 29b is in pressure communication with its respective piston chamber 27b (i.e. when the valve exhaust 76b is aligned with the flow passageways 67b of the second rotary valve member 80b), the valve exhaust (not visible) of the first rotary valve assembly 29a is prevented from communicating with its respective piston chamber 27a. The provision of two reciprocating pistons 23a, 23b may provide for additional impacts in the first and second axial directions A, B, and therefore increased vibration forces within the apparatus 118.

The example above illustrates the downhole drag reducing apparatus 18 used in the process of drilling a wellbore 10. However, the downhole drag reducing apparatus 18 may be used in any number of other operations. For example, Figure 17 illustrates the apparatus 18 (defined in accordance with the description above) used in a retrieving operation, where an object 200 is to be retrieved from the wellbore 210.

Figure 17 diagrammatically illustrates a wellbore comprising a vertical bore section 210a and a build-up section 210b (however, the wellbore 210 may also comprise a horizontal section). A number of drag reducing apparatus 18 are disposed along a rotating string 212 providing an axial pulling force on the object 200 in the direction of arrow 204, in order to retrieve the object 200 from the wellbore 210. A swivel 202 may be located between the object 210 and the rotating string 212 so as not to impart rotational movement to the object 200 during the retrieving operation.

The apparatus 18 may function to minimise friction, in particular static friction, experienced by the apparatus 18 and/or rotating string 212 when engaged with a bore wall of the wellbore 210, thus improving the efficiency of the retrieving operation.

In other applications, for example, the apparatus 18 may be used with a run-in operation.