GB2500185A

GB2500185A - A Downhole Back Off Sub

Info

Publication number: GB2500185A
Application number: GB1204286.7A
Authority: GB
Inventors: John Hanton
Original assignee: Tercel IP Ltd
Current assignee: Tercel IP Ltd
Priority date: 2012-03-12
Filing date: 2012-03-12
Publication date: 2013-09-18
Also published as: GB201204286D0

Abstract

A downhole back off sub10 comprises a housing 2 couplable with a drive shaft 3, the housing 2 comprising a pair of housing parts 21, 22 rotatable relative to each other about a common rotation axis 23 and engaged together via a kinematic pair, which may be provided by cooperating threads 24 arranged to translate the relative rotation of the housing parts 21, 22 into longitudinal displacement. A transmission mechanism 5, which may be provided by a harmonic drive 51, isarranged to selectively transmit the rotation of the drive shaft 3 to at least one of the pair of the housing parts 21, 22 to induce the relative rotation and the resulting relative longitudinal of the housing parts 21, 22. Rapid reconnection/disconnection can be achieved by interrupting the transmission mechanism 7 and locking one housing part directly to the drive shaft 3.

Description

1

A Downhole Back Off Sub

The present invention relates to the field of oil and gas exploration, and in particular downhole activities, such as, for example, drilling, logging, 5 fishing, completions, etc. More specifically, the present invention is a back off sub for release and re-connection of components of workstring downhole.

Safety subs, also referred to in the industry as back-off safety subs 10 (BOSS), are used in downhole activities to disengage or connect components of downhole string whenever it becomes necessary. This tool is used in various workstrings such as drill, fishing, washover string. During normal downhole activities, the tool transmits torque from the upper string to the equipment below the tool. Disconnection may for example 15 be required to recover the part of the string above the equipment which has been or stuck or otherwise fixed downhole.

Some known safety subs comprise upper and lower mating parts couplable together by way a thread and a stop device for preventing the 20 thread from loosening until it is required to disengage the mating parts.

Such a stop device can comprise a spring or a friction ring or some other stop means which typically maintains bias between the mating parts and thus prevents the mating parts from unscrewing and maintains torsional integrity of the joint between the mating parts. When it is required to 25 disengage the mating parts, the stop device can be acted upon from surface, for example by application of a force overcoming the bias, so as to permit the mating parts to rotate relative to each other. The upper string can then be rotated from surface while the stuck portion remains stationary thus permitting the mating parts to disengage.

30

Disadvantages associated with such known back off subs include, for example, the risk of accidental disengagement of the mating parts of the back off sub during the normal downhole activity and the associated financial losses due to the non-production time required to re-connect the string.

Accordingly, the present invention provides a downhole back off sub comprising a housing couplable with a drive shaft, the housing comprising a pair of housing parts arranged to rotate relative to each other about a common rotation axis;

the housing parts being engaged together via a kinematic pair arranged to translate the relative rotation of the housing parts into longitudinal displacement of the housing parts relative to each other and relative to the common rotation axis; and a transmission mechanism arranged to selectively transmit the rotation of the drive shaft to at least one of the pair of the housing parts thereby inducing the relative rotation of the housing parts and the resulting relative longitudinal displacement of the housing parts.

The drive shaft is typically that of a workstring, which can be for example a drillstring, fishing string, washover string, logging string etc. The drive shaft typically is or is connected to a drive shaft of a motor which may be a motor at surface, such as e.g. a top drive. In drilling activities, the rotation of the motor at surface is typically transferred downhole by a drill pipe, i.e. the drill pipe serves as a drive shaft. It is envisaged that in other cases, e.g. in coiled tubing drilling or other downhole activities, the drive shaft may be that of a mud motor, that is, a downhole motor driven by a column of mud within a workstring.

3

In use, the back off sub of the invention is assembled with the workstring so that the housing is coupled with the drive shaft via the transmission mechanism.

5 Preferably, one part of the housing remains stationary during the relative rotation and the other part of the housing rotates about the rotation axis.

Preferably, the transmission mechanism comprises a gear box. Preferably, the transmission mechanism is a reducer.

10

Preferably, the transmission mechanism includes a locking means for selectively locking or unlocking at least one housing part, preferably the rotatable part of the housing so as to restrict or permit rotation of the housing part with respect to the drive shaft.

15

Further preferably, the transmission mechanism includes an interruption means for interrupting or engaging the transmission mechanism to respectively disable or enable the transmission from the drive shaft to the housing part through the transmission mechanism.

20

Preferably, the locking means and the interruption means are arranged in a cooperative relationship so that the back off sub selectively operates in a number of modes and more preferably, in at least a locked mode and a back off mode. Further preferably, the locking means and the interruption 25 means are arranged in a cooperative relationship so that the back off sub selectively operates in a re-connection mode.

Preferably, in a locked mode, the locking means locks the at least one housing part to the drive shaft whereas the transmission mechanism is 30 engaged, with the result that the housing parts are rotatable as a unit with

4

the drive shaft. Thus, in the locked mode, which is required during normal operation of the workstring (e.g. during drilling), torque and rotation from the drive shaft is transmitted to the relevant equipment downstring of the back off sub.

5

If for whatever reason it is required to disconnect the equipment downstring of the back off sub, e.g. if it has been fixed in place or becomes stuck downhole the back off sub is brought into a back off mode. In the back off mode,the locking means is actuated and the housing part 10 previously locked to the drive shaft becomes released therefrom, i.e. the housing part becomes unlocked for rotation relative to the other housing part, whereas the transmission mechanism remains engaged.

Accordingly, in the back off mode, torque and rotation from the drive shaft are transmitted through the transmission mechanism to cause the relative 15 rotation of the housing parts. Preferably, one part of the housing is in use non-rotatably attached to equipment downstring of the back of sub. Accordingly, this part of the housing, which in use is typically a lower part, remains stationary downhole if the equipment has been fixed in place or becomes stuck downhole, whereas the other part of the housing, which in 20 use is typically an upper part, is rotated by the transmission mechanism. The relative rotation of the housing parts becomes converted into the relative axial movement of the housing parts by the kinematic pair. Thus, rotation of the drive shaft in one direction, for example, clockwise, causes one housing part (i.e. the upper, rotatable part) to move away and 25 eventually disconnect from the other (i.e. the lower, stationary) housing part. Accordingly, in the back off mode, the drive shaft rotates but does not transmit rotation to the stationary housing part. Rotation of the drive shaft in the opposite direction, e.g. anticlockwise, results in the reversal of the relative rotation of the housing parts, so that the housing parts move 30 axially toward each other and reconnect. The rotation is transferred from

5

the drive shaft to the housing parts at the reduction ratio provided by the transmission mechanism. Thus, if the transmission mechanism is a reducer, as in a preferred embodiment of the present invention, the relative rotation of the housing parts is slower than the rotation of the drive 5 shaft, and a certain time interval elapses before the housing parts disconnect.

Such a delay is advantageous, for example, if the back off action starts accidentally, because the delay provides an operator with extra time to 10 assess the situation and take the necessary steps to prevent the undesired disengagement. The delay, for example, makes possible an attempt to be made to retrieve the stuck portion of the workstring by application of an axially directed force to the stuck portion, e.g. pushing or pulling force or a dynamic force using a jarring tool, while the housing 15 parts are still in the process of disconnecting, i.e. while the workstring is still one-piece. The delay in disconnection of the housing parts also means that the part of the workstring above the fixed or stuck portion of the workstring rotates during the disconnection process. The relative rotation of the housing parts is advantageous during an attempt to retrieve 20 the stuck portion of the workstring because during the application of such an axial force, a considerable amount of friction is created between the workstring and the walls of the wellbore. The rotation of the upper part of the workstring during the retrieval attempts not only provides additional force in the immediate proximity to the stuck portion of the workstring 25 which may help to release the stuck portion, but also reduces friction in the axial direction and thus facilitates the axial movements of the upper part of the workstring during the retrieval attempt.

In the re-connection mode, which is required, for example, when it is 30 necessary to re-connect the housing parts, the locking means locks at

6

least one housing part to the drive shaft whereas the transmission mechanism is interrupted with the result that the housing part locked to the drive shaft is rotatable as a unit with the drive shaft, that is, rotation and torque are transmitted directly from the drive shaft to that housing part. In 5 the re-connection mode, the stationary housing part remains stationary downhole whereas the rotatable part of the housing is rotated directly by the drive shaft. The relative rotation of the housing parts becomes converted into the relative axial movement of the housing parts by the kinematic pair. Thus, when the drive shaft rotates in one direction, for 10 example, clockwise, one housing part (i.e. the upper, rotatable part) to moves towards and eventually re-connects with the stationary (i.e. the lower, stationary) housing part. The rotation of the drive shaft is transferred from the drive shaft to the rotatable housing part directly, i.e. the angular speed of the rotatable housing part is the same as that of the 15 drive shaft. This is advantageous when it is required to re-connect the housing parts at a speed different than that provided by the transmission mechanism. If the transmission mechanism is a reducer, in the re-connection mode, re-connection of the housing parts occurs quicker than in the back off mode, which is desirable in the production environment. In 20 the re-connection mode, the drive shaft thus rotates but does not transmit rotation to the stationary housing part. Rotation of the drive shaft in the opposite direction, e.g. anticlockwise, results in the reversal of the relative rotation of the housing parts, so that the housing parts move axially away from each other and disconnect. This may be required when, for 25 example, the transmission mechanism is a reducer and for whatever reason it is required to disconnect the housing parts faster than the disconnection time provided for by the transmission mechanism.

Advantageously, the kinematic pair is selected such that the relative axial 30 displacement of the housing parts occurs for a predetermined time before

7

the housing parts disengage. Such an arrangement is advantageous in case, as explained above in connection with the transmission mechanism, the locking means unlocks inadvertently because the prolonged axial displacement provides sufficient time for an operator at surface to detect a 5 lowered torque exerted by the drive shaft indicating that the disengagement process has begun and to take measures to prevent further disengagement and to re-connect the housing parts by switching the direction of rotation of the drive shaft to rotate in the opposite direction or by bringing the back off sub into the re-connection mode in which the 10 drive shaft is coupled directly to the rotatable housing part and thus to cause the housing parts to re-engage. Also as explained above in connection with the transmission mechanism, the delay, combined with relative rotation of the housing parts facilitates an attempt to retrieve a stationary portion of the workstring.

15

In one preferred arrangement, the kinematic pair is provided in the form of a pair of cooperating threads between the housing parts. Preferably, the pitch of the threads is selected so that the longitudinal displacement of the housing parts per revolution of the drive shaft is relatively small and the 20 housing parts move a relatively small total axial distance during the predetermined time interval as the housing parts engage or disengage. This adds to the compactness of the design of the back off sub of the invention while ensuring that the housing parts remain engaged for the predetermined time interval. Such threads facilitate a slow pace of the 25 axial disengagement of the housing parts compared to the angular speed of rotation of the drive shaft. In one arrangement, the threads are fine-pitch threads. For the back off sub of the present invention, a pitch of 1mm, 2mm or 3mm would be regarded as a fine-pitch. Combined with appropriately selected length in the axial direction of the threaded 30 connection and transmission ratio of the transmission mechanism, the

8

required disconnection time is achieved. For example, for 1 mm pitch and 100mm long thread connection and a 100:1 reduction ratio of the transmission mechanism, rotation at input at 100rpm results in 50min thread disconnection (or connection) time involving 50 thread revolutions 5 and 5000 drive shaft revolutions.

Further preferably, the kinematic pair is selected such that it has tensile strength in the axial direction in a state when the kinematic pair has disconnected by about 75-85% approximately the same or greater than 10 the tensile strength of the part of the workstring which becomes stationary downhole. Such an arrangement helps to ensure that the kinematic pair does not break when an attempt is made to retrieve a stuck portion of the workstring while the housing parts of the back off sub are in the process of disconnecting.

15

Preferably, the housing is tubular and the drive shaft extends axially through the housing. Preferably, the pair of housing parts comprise a pair of sleeves connected end-to-end, preferably, co-axially. Preferably, the transmission mechanism is disposed within the housing and is coupled to 20 each housing part.

In an advantageous embodiment, the transmission mechanism comprises a harmonic drive comprising a wave generator (an elliptical disc, plug or hub sometimes also referred to as an inner gear, albeit without teeth), a 25 flexible gear and an outer gear (also referred to as a circular gear),

wherein one of the wave generator, flexible gear and outer gear serves as a rotary input component and another of the wave generator, flexible gear and outer gear serves as a rotary output component; and wherein the harmonic drive is couplable with the drive shaft so that rotation of the drive 30 shaft results in the relative rotation of the housing parts. In principle, in its

9

broader aspect, the present invention is not limited to the use of a harmonic drive as a transmission mechanism and indeed can comprise such various transmission mechanisms suitable for co-axial transmission as would be envisaged by a skilled person, such as, for example only, 5 planetary gearing. However, there are certain advantages associated with the use of a harmonic drive in a back off sub of the present invention.

A harmonic drive is a special type of drive and typically comprises a wave generator, also known as an inner gear, an intermediate gear, aka a 10 flexible gear, and an outer gear, commonly referred to as an outer gear. In a harmonic drive, when the outer gear is fixed, the wave generator and the flexible gear rotate in opposite directions; when the flexible gear is fixed, the outer gear and the wave generator rotate in the same direction; and when the wave generator is fixed, the outer gear and the flexible gear 15 rotate in the same direction. Due to its unique principle of construction, a harmonic drive provides very high or very low, depending on what is used as an input, transmission ratios (typical ratios include 100:1, 200:1, 300:1 or vice versa etc.) along with high torque transmission (due to a plurality of teeth meshing at the same time), torque multiplication (or reduction 20 depending on what is used as an input), very compact construction,

rotation precision, low vibration and absence of backlash. Since harmonic drives are known, it is not necessary to describe its construction and operation in detail.

25 Preferably, in one arrangement, the wave generator of the harmonic drive is coupled to the drive shaft and thus serves as the rotary input component and the flexible gear and the outer gear are each coupled to one of the housing parts and one of the flexible gear or the outer gear serves as a the rotary output component. Thus, the harmonic drive operates as a 30 reducer (and torque multiplier).

10

In one particular variation, the wave generator of the harmonic drive is coupled to the drive shaft, the outer gear is coupled the housing part which remains stationary during relative rotation and the flexible gear serves as 5 the rotary output component and is coupled to the part of the housing which rotates about the rotation axis relative to the stationary part of the housing. In this variation, the drive shaft and the flexible gear rotate in opposite directions. It is of course in principle possible to couple the outer gear of the harmonic drive to the rotating part of the housing and the 10 flexible gear to the stationary part of the housing. In this case, the rotating part of the housing will rotate in the same direction as the drive shaft. The kinematic pair between the housing parts must be adapted accordingly to suit the applicable direction of rotation of the rotating housing part.

15

The considerable reduction and torque multiplication provided by the harmonic drive result in the relative rotation of the housing parts much slower than the rotation of the drive shaft but with higher torque compared to other gear boxes. Low speed at the output is beneficial because it 20 provides for a time delay in case the disconnection has been induced accidentally which allows an operator at surface to detect the undesired disconnection process and to take measures to reconnect the housing parts. Also, the delay in disconnection provides for a possibility of an attempt to be made to retrieve the stuck portion of the workstring, for 25 example, by jarring or by straightforward pushing or pulling, while the housing parts are still connected, while the relative rotation of the housing parts provides extra force for retrieval and friction reduction as described above. Torque multiplication may be necessary to translate the rotary motion of the housing parts into the relative axial movement since the 30 connection (i.e. the kinematic pair) between the housing parts may be

11

such as to require high torque to induce the relative movement of the housing parts

It is further advantageous to include a combination of a harmonic drive as 5 the transmission mechanism and a threaded connection, preferably, a fine-pitched threaded connection, as the kinematic pair between the housing parts to achieve the necessary disconnection time sufficient for an inadvertent disconnection to be detected at surface and prevented while maintaining compactness of the back off sub of the invention. Also, while

10 such an arrangement provides for the desired relatively slow disconnection of the housing parts, the provision of the re-connection mode, provides for a quick re-connection of the housing parts when it is necessary to re-connect the housing parts rapidly compared to the disconnection time.

15

It will be appreciated that the present invention, in its broader aspect, is not limited to the combination of a threaded connection and a harmonic drive. The required disconnection time, in principle, can be achieved by suitably adapting one or each of the transmission mechanism and the

20 kinematic pair.

Also, preferably, the kinematic pair and/or the transmission mechanism are adjustable to provide for the necessary disconnection time interval.

25 In one variation, the locking means is provided in the form of a first spline coupling arranged between at least one housing part and the drive shaft to selectively lock and unlock the rotary connection between the housing part and the drive shaft. The splines arrangement may take many forms as will be readily envisaged by a person skilled in the art.

30

12

In one variation, the interruption means is provided in the form of a second spline coupling arranged in the transmission mechanism.

Accordingly, in the locked mode, the first spline coupling locks at least one 5 housing part to the drive shaft and the second spline coupling holds transmission mechanism in an engaged mode, with the result that the housing parts are rotatable as a unit with the drive shaft, as described above.

10 In the back off mode, the first spline coupling is actuated, thus disengaging the first spline coupling, and the housing part previously locked to the drive shaft becomes released therefrom, i.e. the housing part becomes unlocked for rotation relative to the other housing part, whereas the transmission mechanism remains engaged. The first spline coupling can 15 be disengaged by, for example, arranging the drive shaft to move axially within the housing, so that, for example, by pulling the drive shaft towards surface (i.e. applying tension to the workstring) or, as the case may be, pushing it downwardly (i.e. compressing the workstring), away from surface, the first spline coupling becomes disengaged. At the same time, 20 the second spline coupling is preferably configured so that it remains engaged when the first coupling set is disengaged. This can be achieved by selecting appropriate axial dimension and relative position of the second spline coupling taking into account the distance travelled by the drive shaft to disconnect the first spline coupling. Accordingly, torque and 25 rotation from the drive shaft are transmitted through the second spline coupling and through the transmission mechanism to cause the relative rotation of the housing parts at the transmission rate provided by the transmission mechanism, as described above.

13

In the re-connection mode the first spline coupling locks at least one housing part to the drive shaft, whereas the second spline coupling is interrupted, with the result that the housing part locked to the drive shaft remains rotatable as a unit with the drive shaft, as described above. The 5 second spline coupling can be disengaged by, for example, arranging the drive shaft to move axially within the housing, so that e.g. by pushing the drive shaft downwardly (i.e. compressing the back off sub), i.e. away from surface, or as the case may be, pulling the drive shaft towards the surface (i.e. applying tension to the back off sub), the second spline coupling 10 becomes disengaged while the first spline coupling moves together with the drive shaft and thus remains engaged when the second coupling set is disengaged. This is achieved by selecting appropriate axial dimensions and relative positions of the first spline coupling taking into account the distance travelled by the drive shaft to disconnect the second spline 15 coupling. Accordingly, the stationary housing part remains stationary downhole whereas the rotatable part of the housing is rotated directly by the drive shaft to permit rapid re-connection or disconnection of the housing parts as described above.

20 Where the transmission mechanism comprises a harmonic drive, the second spline coupling can be provided between the drive shaft and the wave generator. Alternatively, instead of providing a separate second spline coupling, the interruption means can be provided by arranging the wave generator and the flexible gear to be movable axially relative to each 25 other so as to engage or disengage. The wave generator and the flexible gear function in the same manner as described above in connection with the second spline coupling in the locked mode, the back off mode and the re-connection modes. In the interruption means provided by the wave generator and the flexible gear, the flexible gear preferably includes a 30 flared mouth opening section for receiving and guiding the wave generator

14

into engagement with the flexible gear. Since the flexible gear and the wave generator are always in register and can be engaged readily irrespective of their relative angular positions (simply by moving the flexible gear and wave generator together so that the wave generator is 5 received within the flexible gear), the use of the wave generator and the flexible gear as the interruption means eliminates the requirement for timing the harmonic drive in order to re-connect interrupted transmission as is the case with a splined type of coupling and, indeed, the need to provide a separate second coupling, whether or not a splined coupling, in 10 the first instance. Preferably, the flared mouth opening section is configured so that the wave generator is disengaged from the flexible gear when positioned within the flared mouth opening section.

Ideally, the locking means comprises an actuation mechanism adapted to 15 switch the locking means between locked and unlocked modes.

Preferably, the actuation mechanism includes a trip mechanism,

preferably a hydro-mechanical trip mechanism, actuatable upon a condition indicating that equipment below the back off sub has become stationary and to cause the locking mechanism to unlock. To detect such 20 condition, a sensor, which may, advantageously, be a mechanical sensor because such a sensor does not require power to operate, is preferably provided sensitive for example to the change in the fluid pressure inside the work string or to the change in the torque, or a specific flow pulse. The trip mechanism can be actuated by application of a force, for example, 25 tensile or compressive force, or upon receipt of a signal which can be provided in the form of a pressure differential, flow pulse, electric,

magnetic or electromagnetic field pulse, rate or differential, a specific flow rate or differential. In one specific arrangement, the trip mechanism is configured to induce the axial movement of the drive shaft in relation to the 30 housing upon detection of the condition indicating that the equipment

15

below the back off sub has become stationary. Preferably, the trip mechanism is adjustable to be actuated upon a specific condition.

Ideally, the trip mechanism comprises a delay mechanism configured to 5 prevent the locking means from unlocking for a predetermined time delay interval from the detection of a condition indicating that equipment below the back off sub has become stationary. This prevents the housing parts from beginning to disconnect immediately after the equipment is fixed or stuck. This is advantageous when the equipment below the back off sub 10 is stuck and an attempt to retrieve the stuck objects is necessary or desired, typically by pulling or pushing the workstring from surface with force or by applying a dynamic force, for example, through the use of a jarring tool. If after the predetermined time delay interval the attempt to retrieve the stuck objects is not successful, the trip mechanism causes the 15 locking means to unlock. Preferably, the delay mechanism is adjustable to provide a required time delay interval.

A plurality of such downhole back off subs can be incorporated into a workstring to provide for multiple back off locations along the workstring.

20

In another aspect, the present invention provides a method of using a back off sub according to the first aspect of the invention for releasing a part of a workstring which has been deposited downhole or disconnecting parts of workstring downhole, the method comprising 25 providing a back off sub according to the first aspect of the invention;

detecting when a part of the workstring below the back off sub has become stationary downhole;

16

allowing a predetermined time delay interval to elapse since the workstring below the back off sub has become stationary downhole and before the housing parts of the back off sub begin to disengage; and applying force from surface to the workstring within the 5 predetermined time delay interval in an attempt to release the part of the workstring which has become stationary.

Preferably, the method further comprises actuating the back off sub to disengage the housing parts of the safety sub upon expiry of the 10 predetermined time delay interval.

Preferably, the method further comprises allowing the housing parts to disengage within a predetermined disconnection time.

Preferably, the method further comprises applying force from surface to 15 the workstring while the housing pars are disengaging in an attempt to release the part of the workstring which has become stationary.

Preferably, the method comprises the step of:

within the predetermined disconnection time re-connecting the 20 housing parts.

Preferably, the method comprises the step of:

rotating the drive shaft in the opposite direction to cause reconnection of the housing parts via the transmission mechanism at the 25 transmission ratio determined by the transmission mechanism.

Preferably, the method comprises the step of:

interrupting the transmission mechanism of the back of sub; and

17

rotating the drive shaft to cause reconnection of the housing parts by directly transferring rotation from the drive shaft to the kinematic pair of the back off sub at the same rotational speed as that of the drive shaft.

5 In yet another aspect, the present invention provides a method of using a back off sub according to the first aspect of the invention for releasing a part of a workstring which has been deposited downhole or disconnecting or reconnecting parts of workstring downhole, the method comprising providing a back off sub according to the first aspect of the 10 invention;

actuating the back off sub to initiate disengagement of the housing parts of the safety sub;

15 allowing the housing parts to disengage within a predetermined disconnection time.

Preferably, the method comprises the step of:

within the predetermined disconnection time, applying axial force to 20 the workstring from surface while the housing parts of the back off sub are being disengaged in an attempt to release the part of the workstring which has become stationary.

Preferably, the method comprises the step of:

25 within the predetermined disconnection time re-connecting the housing parts.

Preferably, the method comprises the step of:

18

rotating the drive shaft in the opposite direction to cause reconnection of the housing parts via the transmission mechanism at the transmission ratio determined by the transmission mechanism.

5 Preferably, the method comprises the step of:

interrupting the transmission mechanism of the back of sub; and rotating the drive shaft to cause reconnection of the housing parts by directly transferring rotation from the drive shaft to the kinematic pair of the back off sub at the same rotational speed as that of the drive shaft.

10

In still a further aspect, the present invention provides a method of using a back off sub according to the first aspect of the invention for disconnecting or reconnecting parts of workstring downhole, the method comprising providing a back off sub according to the first aspect of the 15 invention;

interrupting the transmission mechanism of the back off sub; and rotating the drive shaft to cause disconnection of the housing parts 20 by directly transferring rotation from the drive shaft to the kinematic pair of the back off sub at the same rotational speed as that of the drive shaft.

Preferably, the method comprises the step of:

rotating the drive shaft in the opposite direction to cause 25 reconnection of the housing parts by directly transferring rotation from the drive shaft to the kinematic pair of the back off sub at the same rotational speed as that of the drive shaft.

The present invention provides a back off sub which is practically fail safe, 30 i.e. it cannot disengage accidentally. The invention also provides a back

19

off sub which enables an attempt to be made to release a part of the workstring which has been lodged downhole before the back off starts disconnecting by providing a delay between the moment when the workstring part becomes lodged and the initiation of the disconnection 5 process. The invention also provides a back off sub which facilitates attempts to release a part of workstring which has become stationary downhole, by reducing friction and providing extra force for the release attempt. The back off sub of the invention provides sufficient time for an attempt to release the stationary part of workstring in case a part of 10 workstring below the back off sub becomes stuck downhole or to reconnect the back off sub in case the disconnection process has initiated by accident before the disconnection is accomplished. The provision of the cooperating threads, preferably, small pitched threads as a kinematic pair and a harmonic drive as the transmission mechanism provides for a 15 compact configuration which is important downhole. The provision of the harmonic drive further provides for low speed and high torque transmission while at the same time allowing the back off sub to remain compact.

20 The present invention will now be described by way of examples only, with reference to the accompanying drawings in which:

Figure 1 is a schematic cross-sectional elevation of a first embodiment of the back off sub of the invention in a locked mode in which 25 the back off sub is rotatable with a workstring as a unit;

Figure 2 is a schematic cross-sectional elevation of the back off sub of Figure 1 in a back off mode in which housing parts of the back off sub rotate relative to each other in order to disengage;

Figure 3 is a schematic cross-sectional elevation of a second 30 embodiment which is a modification of the first embodiment of the back off

20

sub of the invention in a locked mode in which the back off sub is rotatable with a workstring as a unit;

Figure 4 is a schematic cross-sectional elevation of the back off sub of Figure 3 in a back off mode in which housing parts of the back off sub 5 rotate relative to each other in order to disengage;

Figure 5 is a schematic cross-sectional elevation of a third embodiment which is a modification of the second embodiment of the back off sub of the invention in a locked mode in which the back off sub is rotatable with a workstring as a unit;

10 Figure 6 is a schematic cross-sectional elevation of the back off sub of Figure 5 in a back off mode in which housing parts of the back off sub rotate relative to each other in order to disengage and in which tension is applied to the housing in order to bring it into the back off mode; and

Figure 7 is a schematic cross-sectional elevation of the back off sub 15 of Figure 5 in a back off mode in which housing parts of the back off sub rotate relative to each other in order to disengage and in which compression is applied to the housing in order to bring it into the back off mode.

20 With reference to Figures 1 and 2, a first embodiment of a downhole back off sub of the present invention will be described and is indicated generally by reference numeral 1. The back off sub 1 comprises a housing 2 which in use is mounted on a drive shaft 3 of a workstring. The workstring can be for example a drillstring, a fishing string, a washover string, a logging 25 string etc. The drive shaft 3 typically is or is connected to a drive shaft of a motor which may be a motor at surface, such as e.g. a top drive. In the presently described embodiment, the rotation of the motor at surface is transferred downhole by a drill pipe 4, i.e. the drill pipe 4 serves as a drive shaft. It will be appreciated that in other cases, e.g. in coiled tubing 30 drilling or other downhole activities, the drive shaft 3 may be that of a mud

21

motor, that is, a downhole motor driven by a column of mud within a workstring.

The housing 2 comprises a pair of tubular housing parts, a lower part 21 5 and an upper part 22. The tubular housing parts 21, 22 are provided in the form of co-axial sleeves, connected end-to-end with each other and arranged to rotate relative to each other about a common rotation axis 23. The drive shaft 3 extends axially through the housing 2. The housing parts 21, 22 are engaged together via a kinematic pair, in this 10 embodiment, co-operating threads 24, provided on each housing part 21, 22 and arranged to translate the relative rotation of the housing parts 21, 22 into longitudinal displacement of the tubular parts 21, 22 relative to each other and relative to the common rotation axis 23. A transmission mechanism 5, provided by in the form of a harmonic drive 51 in the 15 presently described embodiments, as will be described in more detail below, is arranged within the housing 2 to selectively transmit the rotation of the drive shaft 3 to at least one of the pair of the housing parts 21, 22, as will be described in more detail below, in order to induce the relative rotation of the housing parts 21, 22 and the resulting relative longitudinal 20 displacement of the housing parts 21, 22. The housing 2 is mounted on the drive shaft 3 via the transmission mechanism 5, which in the presently described embodiment is coupled to each housing part 21, 22 as will be described below. Seals 61, 62 isolate the interior of the housing 2 from the rest of the workstring.

25

The transmission mechanism 5 includes a locking mechanism 7, as will be described below, is provided for selectively locking or unlocking the housing part 22 so as to restrict or permit the rotation of the housing part, 22 with respect to the drive shaft 3. The transmission mechanism 5 30 includes an interruption mechanism 77 for interrupting or engaging the

22

transmission mechanism 5 to respectively disable or enable the transmission from the drive shaft 3 to the housing part 22 through the transmission mechanism 5. Preferably, the locking mechanism 7 and the interruption mechanism 77 are adapted to cooperate so that the back off 5 sub 1 selectively operates in a number of modes, more specifically, in at least a locked mode, a back off mode and, preferably, a re-connection mode.

In a locked mode, the locking mechanism 7 locks the housing part 22 to 10 the drive shaft 3 while the transmission mechanism 5 is engaged, with the result that the housing parts 21, 22 cannot rotate relative to each other and, instead, rotate as a unit together with the drive shaft 3 as indicated by arrows in Figure 1. The locked mode is maintained during normal operation of the workstring so that torque and rotation from the drive shaft 15 3 are transmitted through the back off sub 1 to the relevant equipment below the back off sub 1.

If and when it is required to disconnect the equipment below the back off sub 1, typically, if it is stuck downhole or has been installed or fixed in 20 place the back off sub is brought into a back off mode. In the back off mode, the locking mechanism is actuated and the housing part 22 becomes released from the drive shaft 3, whereas the transmission mechanism 5 remains engaged. In the back off mode, the torque and rotation (indicated by arrow A in Figure 2) is transmitted from the drive 25 shaft 3 through the transmission mechanism 5 to cause the relative rotation of the housing parts 21, 22. The part 21 of the housing 2, i.e. a lower part which is positioned adjacent the equipment downstring of the back off sub 1, is non-rotatably connected to the equipment. Thus, when the equipment is stuck or fixed, the lower part 21 of the housing 2 30 becomes stationary. When the housing parts 21, 22 are unlocked in

23

order to disconnect, the lower part 21 remains still, whereas the housing part 22, i.e. an upper part, is rotated about the rotation axis 23 by the transmission mechanism 5. The rotation of the upper part 22 (indicated by arrow B in Figure 2) becomes converted into the relative axial 5 movement of the housing parts 21, 22 by the cooperating threads 24.

During the relative axial movement of the housing parts 21, 22, the upper part 22 is moved away from the lower part 21 as indicated by arrow X in Figure 2, thereby causing the upper part 22 to disconnect from the lower fixed or stuck part 21. Meanwhile, the drive shaft 3 rotates within the 10 housing 2 but does not transmit rotation to the lower part 22.

Rotation of the drive shaft 3 in the opposite direction, e.g. anticlockwise, results in the reversal of the rotation of the housing part 22, so that the housing parts 21, 22 move axially toward each other and reconnect.

15

The rotation is transferred from the drive shaft 3 to the housing part 22 at the reduction ratio provided by the transmission mechanism 5. If the transmission mechanism is a reducer, as the harmonic drive 51 of the presently described embodiment, the relative rotation of the housing parts 20 21, 22 is slower than the rotation of the drive shaft 3, and, accordingly a certain time interval elapses before the housing parts 21, 22 disconnect. This delay is advantageous, for example, if the back off action starts accidentally, because the delay provides an operator with extra time to assess the situation and take the necessary steps to prevent the 25 undesired disengagement. The delay in disconnection of the housing parts 21, 22 also means that the part of the workstring above the fixed or stuck portion of the workstring rotates during the disconnection process. The relative rotation of the housing parts 21, 22 is advantageous during an attempt to retrieve the stuck portion of the workstring because during the 30 application of such an axial force, a considerable amount of friction is

24

created between the workstring and the walls of the wellbore. The relative rotation of the housing parts 21, 22 during the retrieval attempts not only provides additional force in the immediate proximity to the stuck portion of the workstring which may help to release the stuck portion, but 5 also reduces friction in the axial direction and thus facilitates the axial movements of the upper part of the workstring during the retrieval attempt.

Although not shown in Figures 1 and 2, the back off sub 1 can be readily modified to operate in a re-connection mode (which is required, for 10 example, when it is necessary to re-connect the housing parts 21, 22, the locking mechanism 7 locks the housing part 22 to the drive shaft 3 whereas the transmission mechanism 5 is interrupted with the result that the housing part 22 locked to the drive shaft 3 is rotatable as a unit with the drive shaft 3, that is, rotation and torque are transmitted directly from 15 the drive shaft 3 to the housing part 22. In the re-connection mode, the stationary housing part 21 remains stationary downhole whereas the rotatable part 22 of the housing is rotated directly by the drive shaft 3. The relative rotation of the housing parts 21, 22 becomes converted into the relative axial movement of the housing parts 21, 22 by the cooperating 20 threads 24. Thus, when the drive shaft 3 rotates in one direction, for example, clockwise, the housing part 22 moves towards and eventually reconnects with the stationary housing part 21. The rotation of the drive shaft 3 is transferred from the drive shaft 3 to the rotatable housing part 22 directly, i.e. the angular speed of the rotatable housing part 22 is the same 25 as that of the drive shaft 3. This is advantageous when it is required to reconnect the housing parts 21, 22 at a speed different than that provided by the transmission mechanism 5. If the transmission mechanism 5 is a reducer, as in the presently described embodiment, in the re-connection mode, re-connection of the housing parts 21, 22 occurs quicker than in the 30 back off mode, which is desirable in the production environment. In the

25

re-connection mode, the drive shaft 3 thus rotates but does not transmit rotation to the stationary housing part 21. Rotation of the drive shaft 3 in the opposite direction, e.g. anticlockwise, results in the reversal of the relative rotation of the housing parts 21, 22, so that the housing parts 21, 5 22 move axially away from each other and disconnect. This may be required when, for example, the transmission mechanism 5 is a reducer, as in the presently described embodiments, and it is required to disconnect the housing parts 21, 22 faster than the disconnection time provided for by the transmission mechanism 5.

10

The cooperating threads 24 are selected such that the relative axial displacement of the housing parts 21, 22 occurs for a predetermined time before the housing parts 21, 22 disengage. Such an arrangement, as discussed above, is advantageous in case the locking mechanism 7 15 unlocks inadvertently or accidentally, because the prolonged disengagement process provides sufficient time for an operator at surface to detect a lowered torque exerted by the drive shaft 3 indicating that the disengagement process between the housing parts 21, 22 has begun, and to take measures to prevent further disengagement, i.e. to re-connect the 20 housing parts 21, 22 by switching the direction of rotation of the drive shaft 3 to rotate in the opposite direction or by bringing the back off sub 1 into the re-connection mode in which the drive shaft 3 is coupled directly to the rotatable housing part 22 with the transmission mechanism 5 being interrupted, and thus causing the housing parts 21, 22 to re-engage. Also 25 as explained above in connection with the transmission mechanism, such a prolonged disconnection time of the housing parts 21, 22, combined with the relative rotation of the housing parts 21, 22 facilitates an attempt to retrieve a stationary portion of the workstring, e.g. by jarring or by simple pushing or pulling.

30

26

In the presently described embodiment, the pitch of the cooperating threads 24 is selected so that the longitudinal displacement of the housing parts 21, 22 per revolution of the drive shaft 3 is relatively small and the housing parts move a relatively small total axial distance during the 5 predetermined time interval as the housing parts engage or disengage. This adds to the compactness of the design of the back off sub of the invention while ensuring that the housing parts 21, 22 remain engaged for the predetermined time interval. The threads 24 can be fine-pitch threads. The threads 24 provide for a slow pace of the axial 10 disengagement of the housing parts 21, 22 compared to the angular speed of rotation of the drive shaft 3. For the back off sub of the present invention, a pitch of 1mm, 2mm or 3mm would be regarded as a fine-pitch. Combined with appropriately selected length in the axial direction of the threaded connection and transmission ratio of the transmission 15 mechanism 5, the required disconnection time is achieved. For example, for 1mm pitch and 100mm long thread connection and a 100:1 reduction ratio of the transmission mechanism, rotation at input at 100rpm results in 50min thread disconnection (or connection) time involving 50 thread revolutions and 5000 drive shaft 3 revolutions.

20

Furthermore, the threads 24 are selected such that the threaded connection has a tensile strength in the axial direction in a state when the threads have disconnected by about 75-85% approximately the same or greater than the tensile strength of the part of the workstring which 25 becomes stationary downhole. This prevents the threaded connection from breaking when an attempt is made to retrieve a stuck portion of the workstring while the housing parts 21, 22 of the back off sub are in the process of disconnecting.

27

In the presently described embodiment, the transmission mechanism 5 comprises a harmonic drive 51 comprising a wave generator 52, a flexible gear 53 and a circular (outer) gear 54. The wave generator 52 is mounted on the drive shaft 3 and serves as a rotary input component of 5 the harmonic drive 51. The flexible gear 53 is coupled with the upper housing part 22 and serves as a rotary output component of the harmonic drive 51. The outer gear 54 is coupled with the lower housing part 21. When the harmonic drive 51 is in operation, i.e. when the housing parts 21, 22 are unlocked for relative rotation, the outer gear 54 remains 10 stationary together with the lower housing part 21, which, in turn, is non-rotatably coupled to the equipment below the back off sub 1, while the flexible gear 53 rotates the upper housing part 22. Inherent to the principle of construction of harmonic drive, the wave generator 52 and the flexible gear 53 rotate in opposite directions as indicated by arrows A and 15 B respectively in Figure 2. It is of course in principle possible to couple the outer gear 54 of the harmonic drive 51 to the upper housing part 22 and the flexible gear 53 to the lower part 21 of the housing. In this case, the upper part 22 of the housing 2 will rotate in the same direction as the drive shaft 3. The threaded connection between the housing parts 21, 22 20 must be adapted accordingly to suit the applicable direction of rotation of the rotating housing part 21.

Due to the principle of its construction, the harmonic drive 51 provides considerable reduction, which can be for example, 100:1, 200:1, 300:1, 25 which means that the upper housing part 22 rotates very slowly compared to the drive shaft 3 the rotational speed of which is typically 30-150 rpm. The reduction combined with the small pitch threaded connection result in that disconnection of the housing parts 21, 22 does not occur instantly or soon after the disconnection process has begun. Instead, a certain time 30 period elapses before the housing parts 21, 22 are disconnected and this

28

time is sufficient for an operator at surface to become aware that the disconnection process has started and to react by taking measures to stop the disconnection and to re-connect the housing parts 21, 22 in case the disconnection process started accidentally. Also, the delay in 5 disconnection provides for a possibility of an attempt to be made to retrieve the stuck portion of the workstring, for example, by jarring or by straightforward pushing or pulling, while the housing parts 21, 22 are still connected, while the relative rotation of the housing parts 21, 22 provides extra force for retrieval and friction reduction as described above.

10

Due to the principle of its construction, along with considerable reduction, the harmonic drive 51 provides torque multiplication (which is inversely proportional to the reduction). The plurality of teeth meshing at the same time in the harmonic drive 51 facilitates high torque transmission. High

15 torque is required to rotate the housing parts 21, 22 relative each other via the cooperating threads 24 to translate the rotary motion of the housing parts 21, 22 into the relative axial movement of the housing parts 21, 22. The plurality of meshing teeth also provide for low vibration and absence of backlash, unlike other gear boxes.

20

Due to the co-axial arrangement of the rotating components of the harmonic drive 51, very compact configuration is achieved which is crucial in downhole tools.

25 In the embodiment of Figures 1 and 2, the locking mechanism 7 is provided in the form of a first set of cooperating splines (first spline coupling) 71, 72 arranged between the housing 2 and the drive shaft 3 to selectively lock and unlock the rotary connection between the housing parts 21, 22. A first spline 71 is provided on the drive shaft 3 and a

30 second spline 72 is provided on the upper part 22 of the housing 2. In the

29

present embodiment, the second spline 72 is provided on the flexible gear 53. The splines 71, 72 are releasably engageable. This is achieved by arranging the drive shaft 3 to be movable axially with respect to the housing 2. When the splines 71, 72 are engaged, the upper part 22 of the 5 housing 2 is locked to the drive shaft 3 and the entire housing 2 thus rotates as a unit with the drive shaft 3 during the normal operation of the downhole equipment. This is a locked mode of the back off sub 1.

When it is required to disengage the housing parts 21, 22 via the harmonic 10 drive 51 the back off sub 1 is switched to a back off mode in which the drive shaft 3 is caused to slide axially in relation to the housing 2 (in the present embodiment, upwardly as indicated by arrow D in Figure 2), thereby withdrawing the first spline 71 from engagement with the second spline 72 and unlocking the rotatable upper part 22 of the housing 2 to 15 permit the relative rotation of the housing parts 21, 22. The interruption mechanism 77 is provided in the form of another set of cooperating splines 73, 74 (second spline coupling) provided between the drive shaft 3 and the wave generator 52. The cooperating splines 73, 74 remain engaged when the drive shaft 3 is moved axially in relation to the housing 2 to 20 disengage the splines 71, 72 of the first set. This is achieved by selecting appropriate axial dimensions and relative positions of the splines 73, 74 taking into account the distance travelled by the drive shaft 3 to disconnect the first splines 71, 72. With the splines 73, 74 engaged and the splines 71, 72 disengaged, torque and rotation from the drive shaft 3 is 25 transmitted through harmonic drive 51 (i.e. the harmonic drive works as a reducer and torque multiplier) to cause the relative rotation of the housing parts 21, 22 at the transmission rate provided by the harmonic drive 51.

To be capable of operating in the re-connection mode, the back off sub 1 30 of Figures 1 and 2 cab be readily modified, e.g. by appropriately adjusting

30

the dimensions and relative axial positioning of the splines 71, 72, 73 and 74 so that the splines 71, 72 remain engaged while the splines 73 and 74 come out of engagement to interrupt the transmission between the drive shaft 3 and the rotatable housing part 22, with the result that the housing 5 part 22 locked to the drive shaft 3 remains rotatable as a unit with the drive shaft. This is achieved by selecting appropriate axial dimensions and relative positions of the first splines 71, 72 taking into account the distance travelled by the drive shaft 3 to disconnect the second splines 73, 74. In the presently described example, the splines 73, 74 are disengaged by 10 pushing the drive shaft 3 downwardly (i.e. compressing the workstring) while the splines 71, 72 move together with the drive shaft 3 and thus remain engaged when the splines 73, 74 become disengaged.

Accordingly, the stationary housing part 21 remains stationary downhole whereas the rotatable part 22 of the housing 2 is rotated directly by the 15 drive shaft 3 to permit re-connection or disconnection of the housing parts 21, 22 at a the transmission ratio 1:1, as described above.

The locking mechanism 7 comprises an actuation mechanism 75 for actuating the locking mechanism 7 between locked and unlocked modes. 20 In the presently described embodiment, the actuation mechanism 75 is provided in the form of a trip mechanism, such as a hydro-mechanical trip mechanism, actuatable upon a condition indicating that equipment below the back off sub has become stationary and to cause the locking mechanism 7 to unlock. To detect such condition, a sensor (not shown) 25 can be provided sensitive for example to the change in the fluid pressure inside the drill string or to the change in the torque. The trip mechanism can be actuated by application of a force, for example, tensile or compressive force, or upon receipt of a signal which can be provided in the form of a pressure differential, flow pulse, electric, magnetic or 30 electromagnetic field pulse, rate or differential, a specific flow rate or

31

differential. Upon detection of the condition indicating that equipment below the back off sub 1 has become stationary the trip mechanism 75 induces the axial movement of the drive shaft 3 in relation to the housing 2 as indicated by the arrow D in Figure 2.

5

Furthermore, the trip mechanism 75 comprises a delay mechanism (not shown) configured to prevent the locking mechanism 7 from unlocking for a predetermined time delay interval (e.g. 30min or 1 hour) from the detection of the condition indicating that equipment below the back off sub 10 1 has become stationary. This prevents the housing parts 21, 22 from disconnecting immediately after the workstring has become fixed or stuck. This is advantageous when equipment below the back off sub 1 becomes stuck and an attempt to retrieve the stuck objects is necessary or desired by pulling or pushing the workstring from surface with force, or by applying 15 a dynamic force using a jarring tool. If after the predetermined time delay interval the attempt to retrieve the stuck objects is not successful, the trip mechanism 75 causes the locking mechanism 7 to unlock. In the present embodiment, the drive shaft 3 is caused to slide axially upwardly thereby causing the splines 71, 72 to disconnect thereby permitting the housing 20 parts 21, 22 to rotate relative to each other. The delay mechanism is preferably adjustable to provide a required time delay interval.

In Figures 3 and 4, a modification of the back off sub 1 is schematically illustrated and is indicated generally by reference numeral 10. The back 25 off sub 10 of Figures 3 and 4 is the same as the back off sub of Figures 1 and 2 in most aspects but for the arrangement of interruption mechanism indicated generally 80 in Figures 3 and 4. Therefore, components of the back off sub 10 are indicated in Figures 3 and 4 using the same reference numerals as those used to indicate corresponding components of the back 30 off sub 1 of Figures 1 and 2. In the interruption mechanism 80 of the back

32

off sub 10, the spline 73 is configured as a unit with the wave generator 52 and the interruption mechanism 80 is provided by arranging the wave generator 52 and the flexible gear 53 to be movable axially relative to each other so as to engage or disengage. The wave generator 52 and the 5 flexible gear 53 function in the same manner as described above in connection with the back off sub 1 of Figures 1 and 2 in the locked mode, the back off mode and the re-connection modes. The flexible gear 53 includes a flared mouth opening section 90 for receiving and guiding the wave generator 52 into engagement with the flexible gear 53. Since the 10 flexible gear 53 and the wave generator 52 are always in register and can be engaged readily irrespective of their relative angular positions (simply by moving the flexible gear 53 and wave generator 52 together so that the wave generator is received within the flexible gear), the use of the wave generator 52 and the flexible gear 53 as the interruption mechanism 80 15 eliminates the requirement for timing the harmonic drive 51 in order to reconnect interrupted transmission as is the case with a splined type of coupling of Figures 1 and 2 and, indeed, the need to provide a separate second coupling, whether or not a splined coupling, in the first instance. The flared mouth opening section 90 is configured so that the wave 20 generator 52 is disengaged from the flexible gear 53 when positioned within the flared mouth opening section 90.

In Figures 5, 6 and 7, a modification of the back off sub 10 is schematically illustrated and is indicated generally by reference numeral 100. The back 25 off sub 100 of Figures 5, 6 and 7 is similar to the back off sub of Figures 3 and 4 but for the arrangement of the interruption mechanism indicated generally 85 in Figures 5, 6 and 7. Therefore, components of the back off sub 100 are indicated in Figures 5, 6 and 7 using the same reference numerals as those used to indicate corresponding components of the back 30 off sub 10 of Figures 3 and 4. In the interruption mechanism 85 of the

33

back off sub 100, the spline 73 is configured as a unit with the wave generator 52 and the interruption mechanism 80 is provided by arranging the wave generator 52 and the flexible gear 53 to be movable axially relative to each other so as to engage or disengage, as in the back off sub 5 10 of Figures 3 and 4. Additionally, a second wave generator 52a is provided axially spaced from the first wave generator 52. Such an arrangement allows the back off mode to be achieved in two ways. The first way is illustrated in Figure 6 and is the same as described above in connection with Figures 2 and 4, i.e. by pulling the drive shaft 3 upwardly 10 (i.e. along arrow D in Figure 6) thereby disconnecting the splines 71, 72 while the wave generators 52, 52a remain engaged with the flexible gear 53. The second way is illustrated in Figure 7 and involves pushing the drive shaft 3 downwardly, i.e. compressing the back off sub 100 (arrow F in Figure 7) thereby disconnecting the splines 71, 72, while the second 15 wave generator 52a remains engaged with the flexible gear 53 and the first wave generator 52 becomes disengaged from the flexible gear 53.

Modifications and improvements are envisaged without departing from the scope of the present invention as defined in the appended claims.

20

34

Claims

CLAIMS:

1. A downhole back off sub comprising a housing couplable with a drive shaft, the housing comprising 5 a pair of housing parts arranged to rotate relative to each other about a common rotation axis;

the housing parts being engaged together via a kinematic pair arranged to translate the relative rotation of the housing parts into longitudinal displacement of the housing parts relative to each other and 10 relative to the common rotation axis; and a transmission mechanism arranged to selectively transmit the rotation of the drive shaft to at least one of the pair of the housing parts thereby inducing the relative rotation of the housing parts and the resulting relative longitudinal displacement of the housing parts.

15

2. A back off sub as claimed in Claim 1, wherein the back off sub is configured such that one part of the housing remains stationary during the relative rotation and the other part of the housing rotates about the rotation axis.

20

3. A back off sub as claimed in Claim 1 or Claim 2, wherein the transmission mechanism includes a locking means for selectively locking or unlocking at least one housing part so as to restrict or permit rotation of the housing part with respect to the drive shaft.

25

4. A back off sub as claimed in Claim 3, wherein the transmission mechanism includes an interruption means for interrupting or engaging the transmission mechanism to respectively disable or enable the transmission from the drive shaft to the housing part through the

30 transmission mechanism.

35

5. A back off sub as claimed in Claim 4, wherein the locking means and the interruption means are arranged in a cooperative relationship so that the back off sub selectively operates in at least a locked mode and a

5 back off mode.

6. A back off sub as claimed in Claim 5, wherein in a locked mode, the locking means locks the at least one housing part to the drive shaft whereas the transmission mechanism is engaged, with the result that the

10 housing parts are rotatable as a unit with the drive shaft, so that torque and rotation from the drive shaft can be transmitted to the relevant equipment downstring of the back off sub.

7. A back off sub as claimed in Claim 5 or Claim 6, wherein in the 15 back off mode, the locking means is actuated and the housing part previously locked to the drive shaft becomes released therefrom, i.e. the housing part becomes unlocked for rotation relative to the other housing part, whereas the transmission mechanism remains engaged, so that torque and rotation from the drive shaft can be transmitted through the 20 transmission mechanism to cause the relative rotation of the housing parts.

8. A back off sub as claimed in Claim 7, wherein the kinematic pair and the transmission mechanism are mutually arranged so that in the back

25 off mode, rotation of the drive shaft in one direction causes one housing part to move away and eventually disconnect from the other housing part and wherein rotation of the drive shaft in the opposite direction results in the reversal of the relative rotation of the housing parts, so that the housing parts move axially toward each other and reconnect.

30

36

9. A back off sub as claimed in any preceding claim, wherein the transmission mechanism is a reducer and the relative rotation of the housing parts is slower than the rotation of the drive shaft and wherein a predetermined time interval elapses before the housing parts disconnect.

5

10. A back off sub as claimed in any preceding claim, wherein one part of the housing is in use non-rotatably attached to equipment downstring of the back of sub whereas the other part of the housing is rotatable by the transmission mechanism whereby the relative rotation of the housing parts

10 becomes converted into the relative axial movement of the housing parts by the kinematic pair.

11. A back off sub as claimed in any one of Claims 5 to 10, wherein the locking means and the interruption means are arranged in a cooperative 15 relationship so that the back off sub selectively operates in a re-connection mode.

12. A back off sub as claimed in Claim 11, wherein in the re-connection mode, the locking means locks at least one housing part to the drive shaft

20 whereas the transmission mechanism is interrupted with the result that the housing part locked to the drive shaft is rotatable as a unit with the drive shaft and rotation and torque are transmitted directly from the drive shaft to that housing part.

25 13. A back off sub as claimed in Claim 12, wherein in the re-connection mode one housing part remains stationary downhole whereas the other part of the housing is rotatable directly by the drive shaft, wherein the relative rotation of the housing parts becomes converted into the relative axial movement of the housing parts by the kinematic pair.

30

37

14. A back off sub as claimed in Claim 13, wherein the drive shaft and the kinematic pair are mutually arranged so that when the drive shaft rotates in one direction the rotatable housing part moves towards and eventually re-connects with the stationary housing part, and rotation of the

5 drive shaft in the opposite direction results in the reversal of the relative rotation of the housing parts, so that the housing parts move axially away from each other and disconnect.

15. A back off sub as claimed in any preceding claim, wherein the

10 kinematic pair is selected such that the relative axial displacement of the housing parts occurs for a predetermined disconnection time before the housing parts disengage.

16. A back off sub as claimed in any preceding claim, wherein the 15 kinematic pair is provided in the form of a pair of cooperating threads between the housing parts.

17. A back off sub as claimed in Claim 16, wherein the threads are fine-pitch threads.

20

18. A back off sub as claimed in Claim 16 or Claim 17, wherein the kinematic pair is selected such that it has tensile strength in the axial direction in a state when the kinematic pair has disconnected by about 75-85% approximately the same or greater than the tensile strength of the

25 part of a workstring which becomes stationary downhole.

19. A back off sub as claimed in any preceding claim, wherein the transmission mechanism comprises a gear box.

38

20. A back off sub as claimed in any preceding claim, wherein the transmission mechanism comprises a reducer.

21. A back off sub as claimed in any preceding claim, wherein the

5 transmission mechanism comprises a harmonic drive comprising a wave generator, a flexible gear and an outer gear, wherein one of the wave generator, flexible gear and outer gear serves as a rotary input component and another of the wave generator, flexible gear and outer gear serves as a rotary output component; and wherein the harmonic drive is couplable 10 with the drive shaft so that rotation of the drive shaft results in the relative rotation of the housing parts.

22. A back off sub as claimed in Claim 21, wherein the wave generator of the harmonic drive is coupled to the drive shaft and thus serves as the

15 rotary input component and the flexible gear and the outer gear are each coupled to one of the housing parts and one of the flexible gear or the outer gear serves as a the rotary output component so that the harmonic drive operates as a reducer and torque multiplier.

20 23. A back off sub as claimed in Claim 2 and in Claim 21 or Claim 22, wherein the wave generator of the harmonic drive is coupled to the drive shaft, the outer gear is coupled the housing part which remains stationary during relative rotation and the flexible gear serves as the rotary output component and is coupled to the part of the housing which rotates about 25 the rotation axis relative to the stationary part of the housing, whereby the drive shaft and the flexible gear rotate in opposite directions.

24. A back off sub as claimed in any preceding claim, wherein the kinematic pair and/or the transmission mechanism are adjustable so that

39

the relative axial displacement of the housing parts occurs for a required disconnection time before the housing parts disengage.

25. A back off sub as claimed in any preceding claim in combination 5 with Claim 4, wherein the locking means is provided in the form of a first spline coupling arranged between at least one housing part and the drive shaft to selectively lock and unlock the rotary connection between the housing part and the drive shaft.

10 26. A back off sub as claimed in Claim 25, wherein in the back off mode, the first spline coupling is actuated, thus disengaging the first spline coupling whereby the housing part previously locked to the drive shaft becomes released therefrom, i.e. the housing part becomes unlocked for rotation relative to the other housing part, while the interruption means 15 holds transmission mechanism in an engaged mode.

27. A back off sub as claimed in Claim 25 or Claim 26, wherein in the locked mode, the first spline coupling locks at least one housing part to the drive shaft and the interruption means holds transmission mechanism in

20 an engaged mode, with the result that the housing parts are rotatable as a unit with the drive shaft.

28. A back off sub as claimed in any one of Claims 25 to 27, wherein in the re-connection mode, the first spline coupling locks at least one housing

25 part to the drive shaft, whereas the transmission mechanism is interrupted, with the result that the housing part locked to the drive shaft remains rotatable as a unit with the drive shaft.

40

29. A back off sub as claimed in any one of Claims 25 to 28 wherein the interruption means is provided in the form of a second spline coupling arranged in the transmission mechanism.

5 30. A back off sub as claimed in Claim 29 and any one of Claims 21 to 23, wherein the second spline coupling is provided between the drive shaft and the wave generator.

31. A back off sub as claimed in any one of Claims 25 to 28 and any 10 one of Claims 21 to 23, wherein the interruption means is provided by arranging the wave generator and the flexible gear to be movable axially relative to each other so as to engage or disengage.

32. A back off sub as claimed in Claim 31, wherein the flexible gear 15 includes a flared mouth opening section for receiving and guiding the wave generator into engagement with the flexible gear.

33. A back off sub as claimed in Claim 32, wherein the flared mouth opening section is configured so that the wave generator is disengaged

20 from the flexible gear when positioned within the flared mouth opening section.

34. A back off sub as claimed in Claim 3, wherein the locking means comprises an actuation mechanism adapted to switch the locking means

25 between locked and unlocked modes.

35. A back off sub as claimed in Claim 34, wherein the actuation mechanism is actuatable upon a condition indicating that equipment below the back off sub has become stationary and to cause the locking

30 mechanism to unlock.

41

36. A back off sub as claimed in Claim 35, wherein to detect such a condition, a sensor is provided sensitive to the change in the condition.

5 37. A back off sub as claimed in any one of Claims 34 to 35, wherein the actuation mechanism includes a trip mechanism actuatable by a signal.

38. A back off sub as claimed in Claim 37, wherein the trip mechanism 10 is configured to induce the axial movement of the drive shaft in relation to the housing upon detection of the condition indicating that the equipment below the back off sub has become stationary.

39. A back off sub as claimed in Claim 37 or Claim 38, wherein the trip 15 mechanism comprises a delay mechanism configured to prevent the locking means from unlocking for a predetermined time delay interval from the detection of a condition indicating that equipment below the back off sub has become stationary.

20 40. A back off sub as claimed in Claim 39, wherein the delay mechanism is adjustable to provide a required time delay interval.

41. A back off sub as claimed in any preceding claim wherein, the housing is tubular and the drive shaft extends axially through the housing.

25

42. A back off sub as claimed in Claim 41, wherein the pair of housing parts comprise a pair of sleeves connected end-to-end.

42

43. A back off sub as claimed in any preceding claim, wherein the transmission mechanism is disposed within the housing and is coupled to each housing part.

5 44. A back off sub as claimed in any preceding claim, wherein the drive shaft is that of a workstring and the back off sub of the invention is assembled with the workstring so that the housing is coupled with the drive shaft via the transmission mechanism.

10 45. A back off sub as claimed in any preceding claim in combination with each of Claims 2 and 3, wherein, the locking means is configured to selectively lock or unlock the rotatable part of the housing so as to restrict or permit rotation of the housing part with respect to the drive shaft.

15 46. A workstring comprising a plurality of downhole back off subs in accordance with any preceding claim.

47. A method of using a back off sub for releasing a part of a workstring which has been deposited downhole or disconnecting parts of workstring

20 downhole, the method comprising providing a back off sub having a housing couplable with a drive shaft, the housing comprising a pair of housing parts arranged to rotate relative to each other about a common rotation axis;

25 detecting when a part of the workstring below the back off sub has become stationary downhole;

allowing a predetermined time delay interval to elapse since the workstring below the back off sub has become stationary downhole and before the housing parts of the back off sub begin to disengage; and

43

applying force from surface to the workstring within the predetermined time delay interval in an attempt to release the part of the workstring which has become stationary.

5 48. A method as claimed in Claim 47, wherein the method further comprises actuating the back off sub to disengage the housing parts of the safety sub upon expiry of the predetermined time delay interval.

49. A method as claimed in Claim 48, wherein the method further

10 comprises allowing the housing parts to disengage within a predetermined disconnection time.

50. A method as claimed in Claim 49, wherein the method further comprises applying force from surface to the workstring while the housing

15 pars are disengaging in an attempt to release the part of the workstring which has become stationary.

51. A method as claimed in Claim 49 or Claim 50, wherein, the method comprises the step of:

20 within the predetermined disconnection time re-connecting the housing parts.

52. A method as claimed in Claim 51, wherein in the back off sub the housing parts are engaged together via a kinematic pair arranged to

25 translate the relative rotation of the housing parts into longitudinal displacement of the housing parts relative to each other and relative to the common rotation axis; and the back off sub comprises a transmission mechanism arranged to selectively transmit the rotation of the drive shaft to at least one of the pair of the housing parts

44

thereby inducing the relative rotation of the housing parts and the resulting relative longitudinal displacement of the housing parts; and the method comprises the step of:

rotating the drive shaft in the opposite direction to cause 5 reconnection of the housing parts via the transmission mechanism at the transmission ratio determined by the transmission mechanism.

53. A method as claimed in Claim 51 or Claim 52, wherein in the back off sub the housing parts are engaged together via a kinematic pair

10 arranged to translate the relative rotation of the housing parts into longitudinal displacement of the housing parts relative to each other and relative to the common rotation axis; and the back off sub comprises a transmission mechanism arranged to selectively transmit the rotation of the drive shaft to at least one of the pair of the housing parts 15 thereby inducing the relative rotation of the housing parts and the resulting relative longitudinal displacement of the housing parts; and wherein, the method comprises the step of:

interrupting the transmission mechanism of the back of sub; and rotating the drive shaft to cause reconnection of the housing parts 20 by directly transferring rotation from the drive shaft to the kinematic pair of the back off sub at the same rotational speed as that of the drive shaft.

54. A method of using a back off sub, wherein back off sub is according to any one of Claims 1 to 46.

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55. A method of using a back off sub for releasing a part of a workstring which has been deposited downhole or disconnecting or reconnecting parts of workstring downhole, the method comprising providing a back off sub having 30 a housing couplable with a drive shaft, the housing comprising

45

a pair of housing parts arranged to rotate relative to each other about a common rotation axis;

detecting when a part of the workstring below the back off sub has become stationary downhole;

5 actuating the back off sub to initiate disengagement of the housing parts of the safety sub;

allowing the housing parts to disengage within a predetermined disconnection time.

10 56. A method as claimed in Claim 55, wherein the method comprises the step of:

within the predetermined disconnection time, applying axial force to the workstring from surface while the housing parts of the back off sub are being disengaged in an attempt to release the part of the workstring which 15 has become stationary.

57. A method as claimed in Claim 56, wherein the method comprises the step of:

within the predetermined disconnection time re-connecting the 20 housing parts.

58. A method as claimed in Claim 57, wherein in the back off sub the housing parts are engaged together via a kinematic pair arranged to translate the relative rotation of the housing parts into longitudinal

25 displacement of the housing parts relative to each other and relative to the common rotation axis; and the back off sub comprises a transmission mechanism arranged to selectively transmit the rotation of the drive shaft to at least one of the pair of the housing parts thereby inducing the relative rotation of the housing parts and the resulting relative 30 longitudinal displacement of the housing parts; and

46

wherein, the method comprises the step of:

rotating the drive shaft in the opposite direction to cause reconnection of the housing parts via the transmission mechanism at the transmission ratio determined by the transmission mechanism.

5

59. A method as claimed in Claim 57 or Claim 58, wherein in the back off sub the housing parts are engaged together via a kinematic pair arranged to translate the relative rotation of the housing parts into longitudinal displacement of the housing parts relative to each other and

10 relative to the common rotation axis; and the back off sub comprises a transmission mechanism arranged to selectively transmit the rotation of the drive shaft to at least one of the pair of the housing parts thereby inducing the relative rotation of the housing parts and the resulting relative longitudinal displacement of the housing parts; and 15 wherein the method comprises the step of:

interrupting the transmission mechanism of the back of sub; and rotating the drive shaft to cause reconnection of the housing parts by directly transferring rotation from the drive shaft to the kinematic pair of the back off sub at the same rotational speed as that of the drive shaft.

20

60. A method of using a back off sub, wherein back off sub is according to any one of Claims 1 to 46.

61. A method of using a back off sub for disconnecting or reconnecting 25 parts of workstring downhole, the method comprising providing a back off sub having a housing couplable with a drive shaft, the housing comprising a pair of housing parts arranged to rotate relative to each other about a common rotation axis;

47

detecting when a part of the workstring below the back off sub has become stationary downhole;

interrupting the transmission mechanism of the back off sub; and rotating the drive shaft to cause disconnection of the housing parts 5 by directly transferring rotation from the drive shaft to the kinematic pair of the back off sub at the same rotational speed as that of the drive shaft.

62. A method as claimed in Claim 61, wherein in the back off sub the housing parts are engaged together via a kinematic pair arranged to

10 translate the relative rotation of the housing parts into longitudinal displacement of the housing parts relative to each other and relative to the common rotation axis; and the back off sub comprises a transmission mechanism arranged to selectively transmit the rotation of the drive shaft to at least one of the pair of the housing parts thereby 15 inducing the relative rotation of the housing parts and the resulting relative longitudinal displacement of the housing parts; and wherein the method comprises the step of:

rotating the drive shaft in the opposite direction to cause reconnection of the housing parts by directly transferring rotation from the 20 drive shaft to the kinematic pair of the back off sub at the same rotational speed as that of the drive shaft.

63. A method of using a back off sub, wherein back off sub is according to any one of Claims 1 to 46.

25

64. A back off sub substantially as herein described with reference to and/or as shown in the accompanying drawings.

65. A method of using a back off sub substantially as herein described 30 with reference to the accompanying drawings.