Self-propelled apparatus

Abstract

A self-propelled apparatus for traversing a bore has a fluid inlet and outlet, and a fluid driver to move a fluid from the inlet to the outlet. The self-propelled apparatus has a leading end and a trailing end, and a propulsion assembly 526 comprising an axial inlet 534 provided at one of the leading and trailing ends, an outlet 538 provided on the apparatus between the leading and trailing ends and laterally offset from the longitudinal axis, the outlet configured to discharge fluid from the apparatus in a discharge direction 540 to generate a thrust vector for propelling the apparatus in the intended direction of travel; and a fluid driver 542 configured to move a fluid through the propulsion assembly from the axial inlet to the outlet. The apparatus may have a propulsion assembly at each of the leading and trailing ends, and the driver may be an impeller. The apparatus may have a conical filter 594 and fluid driver rear standoff distance l. The apparatus may further have chamber 560 with pressure compensation mechanism, and a payload housing for deploying e.g. fibre-optic cable.

Description

FIELD The present disclosure relates to a self-propelled apparatus, in particular to a self-propelled apparatus for traversing a throughbore. BACKGROUND Many industries require equipment and / or tools to be conveyed through a throughbore. In the oil and gas industry, for example, hydrocarbon bearing formations are accessed by drilling a wellbore from surface, the wellbore typically being vertical or substantially vertical. In vertical wellbores, gravity-fed deployment methods may be utilised to advance equipment and / or tools downhole. However, in deviated or non-vertical wellbores, gravity may not be utilised. SUMMARY An aspect of the present disclosure relates to a self-propelled apparatus for traversing a throughbore, comprising: a leading end and a trailing end with respect to an intended direction of travel of the apparatus, the apparatus defining a longitudinal axis extending between the leading and trailing ends; and a propulsion assembly comprising: an axial inlet provided at one of the leading and trailing ends, the axial inlet configured to draw fluid into the propulsion assembly in an intake direction; an outlet provided on the apparatus between the leading and trailing ends and laterally offset from the longitudinal axis, the outlet configured to discharge fluid from the apparatus in a discharge direction to generate a thrust vector for propelling the apparatus in the intended direction of travel; and a fluid driver configured to move a fluid through the propulsion assembly from the axial inlet to the outlet. The apparatus may be configured for traversing any type of throughbore. For example, the throughbore may be defined by pipework or pipeline architecture, such as located at surface (e.g. in a production facility), subsea and / or subterranean. Alternatively, the throughbore may be defined by or within a wellbore, such as used in the exploration and extraction of mineral resources (e.g., oil and gas), in geothermal applications, etc. The throughbore may be defined by a drilled wellbore, a cased or lined wellbore, a tubing or pipe string extending through a wellbore, and / or the like. The apparatus may have particular advantages in deviated or non-vertical throughbores, such as horizontal wellbores, due to the provision of the propulsion assembly. The propulsion assembly comprising the axial inlet (e.g., as opposed to a radial or circumferential inlet) may provide for optimal thrust generation as well as providing other advantages. For example, an axial inlet may allow for the intake fluid area to be maximised, or more readily optimised. Also, where there is solid material (e.g., debris, sediment, sand, proppant and / or other particles) in the throughbore, the propulsion assembly may be configured to draw the solid material into the axial inlet and discharge the solid material through the outlet. As such, the propulsion assembly may be configured to shift solid material from ahead of the apparatus to the trailing end of the apparatus, thereby clearing a path through the throughbore. The apparatus may comprise or carry a payload. The apparatus may comprise a payload housing, e.g., at the trailing end of the apparatus. In some examples, the payload housing may be configured to deploy elongate media (e.g., an optical fibre) in or through the throughbore. In some examples, the elongate media may be connected to a receiver at surface and deployed through the throughbore via the trailing end of the apparatus as the apparatus traverses the throughbore. In this respect, the payload housing may be referred to as a deployment apparatus. In other examples, the payload housing may comprise other tools and / or equipment for conveying and / or deploying through the throughbore. The payload housing may comprise a spool of the elongate media. The spool may comprise a bobbin upon which the elongate media is wound. The bobbin may define an axis. The bobbin axis may be aligned with the spool axis. The bobbin axis may define the spool axis. The spool may be mounted in cantilever form within the payload housing. In this respect the spool may define a fixed or proximal end, and a free or distal end. In one example, an initial wrap segment (i.e., first wrap segment to be formed on the spool) may be provided adjacent the proximal end. Alternatively, the initial wrap segment may be provided towards the distal end. The spool may be mounted within a cavity of the payload housing. The payload housing may comprise or define a cylindrical cavity. The payload housing may define a payload outlet. Where the payload comprises elongate media, the payload housing may comprise a guide, such as a funnel, to guide the elongate media towards the payload outlet. In some examples the payload outlet may comprise a store or bath of a coating, such that the elongate media may become coated during exiting the payload housing. The payload outlet of the payload housing may comprise a resistance arrangement to provide a degree of resistance to exit of the elongate media. For example, the outlet of the payload housing may establish interference with the elongate media. A length of the elongate media may be wound around a spool axis to form a plurality of wrap segments arranged axially along the spool axis, as for example described in WO 2019 / 016538, which is incorporated herein by reference. Alternatively, the payload housing may comprise first and spools of elongate media. The elongate media may be de-spoolable simultaneously from respective first and second spools during movement of the apparatus through the throughbore, as for example described in WO 2019 / 138236, which is incorporated herein by reference. The apparatus may comprise a downhole optical transmitter configured to emit an optical signal for transmission through the optical fibre between an uphole location and the apparatus, as for example described in WO 2020 / 079410, which is incorporated herein by reference. In some examples, the cavity of the payload housing may comprise an isolating fluid for isolating the elongate media from the fluid entering the cavity from the throughbore, as for example described in WO 2021 / 043479 A1, which is incorporated herein by reference. In some examples, the apparatus may additionally or alternatively be configured for connection to a payload. The apparatus may be configured to tow the payload. The apparatus may comprise an attachment member, e.g., a towing eye, hook, etc., for connection to the payload. The attachment member may be provided at the trailing end. The payload may comprise a tool or equipment to be conveyed and / or deployed through the throughbore. The payload may comprise a deployment apparatus. The deployment apparatus may comprise a spool of elongate media, e.g., optical fibre, to be deployed throughout the throughbore. The apparatus may be used in multiple other applications. For example, the apparatus may be used in a plugging application. In this respect the apparatus may comprise a plug for plugging the throughbore. The plug may comprise a cone portion for landing on a seat or obstruction in the throughbore. The plug may comprise a seal. The plug may be provided at the leading end of the apparatus. In other applications, the apparatus may be used in a data logging application. The apparatus may comprise data logging equipment, e.g. one or more sensors. The apparatus may be configured to communicate with an external receiver, e.g., at surface. The apparatus may be configured to transmit logged data to the external receiver. In further applications, the apparatus may be used as an actuator for actuating a tool. For example, the apparatus may be configured to land on a seat associated with the tool. The seat may be connected to a sleeve. The apparatus may be configured to shift a position of the sleeve by creating a build-up of pressure within the throughbore behind the apparatus. Where the apparatus is deployed in a wellbore, the leading end may be defined as a downhole end and the trailing end may be defined as an uphole end. The self-propelled apparatus may be defined as or comprise a downhole tractor. The fluid driver may comprise a rotodynamic fluid driver. For example, the fluid driver may comprise a rotor which is driven to displace fluid in a direction from the axial inlet to the outlet. The fluid driver may comprise one or more impellers, which may be referred to herein as propellers or thrusters. The fluid driver may comprise a single stage (e.g., a single impeller) or multiple stages (e.g., multiple impellers arranged in series). The fluid driver may be configured to move fluid in an axial direction and / or in a lateral or radial direction. The fluid driver may be configured to generate a mixed flow regime. In some examples, the fluid driver may comprise a radial impeller. The fluid driver may comprise a central suction eye. The impeller may comprise two or more blades mounted to an impeller hub. The blades may comprise a curvature. The blades may be spaced apart from one another by a distance sufficient to permit solid material to pass from the axial inlet to the outlet between the blades as the impeller rotates. The propulsion assembly may comprise a propulsion housing. The fluid driver may be mounted within the propulsion housing. The propulsion housing may comprise a fluid driver receiving area for the fluid driver to be mounted within the propulsion housing. The propulsion housing may be configured to contain a fluid flow around the fluid driver, which may increase a force output of the fluid driver. The fluid driver may be defined as an internal or closed fluid driver (e.g., an internal or closed impeller). The fluid driver may be mounted within or adjacent the axial inlet. Alternatively, the fluid driver may be mounted at another location between the axial inlet and the outlet. The propulsion housing may comprise a shroud member for enclosing the fluid driver, for example to protect the fluid driver from contact with a wall of the throughbore. The propulsion housing may comprise a support member. The support member may comprise a cylindrical portion and a conical portion. The conical portion may taper towards the leading end of the apparatus. The conical portion may taper towards the shroud member. The shroud member may be connected to the support member (e.g., the conical portion) via one or more fin connectors. The one or more fin connectors may be circumferentially arranged around an exterior of the conical portion. The one or more fin connectors may be blade-shaped for reducing drag and providing stability to the apparatus. The one or more fin connectors may be located towards a trailing end of the fluid driver. A length of the fin connectors may generally extend parallel to the longitudinal axis of the apparatus. The fin connectors may extend across some or all of the conical portion in an axial direction. The fluid driver may be mounted to a leading end of the support member (e.g., the conical portion). The shroud member may enclose a portion of the support member (e.g., the conical portion). The shroud member may comprise a tubular structure with open ends. The propulsion assembly may comprise a stand-off zone between a trailing end of the fluid driver and an internal surface of the propulsion housing. In some examples, the internal surface of the propulsion housing may be defined by leading end(s) of the one or more fin connectors. The stand-off zone may comprise a region of free space. The stand-off zone may comprise a minimum distance between the trailing end of the fluid driver and the internal surface for minimising shear forces resulting from the interaction of the fluid driver and the fluid within the stand-off zone, e.g., produced by rotation of the fluid driver. The minimum distance may be defined as an axial clearance. The minimum distance may be determined in accordance with fluid mechanic equations and operational requirements of the apparatus. The propulsion assembly may comprise a flow path extending between the axial inlet and the outlet. The flow path may comprise one or more stages. A first stage of the flow path may comprise an axial flow passage in communication with the axial inlet. The first stage may be enclosed by the shroud member. A second stage of the flow path may diverge from the longitudinal axis, e.g., for directing fluid from the central flow passage away from the apparatus. The second stage may be axially offset from the shroud member on the trailing end of the shroud member. The flow path at the second stage may be radially exposed on its outer side to the throughbore. In other examples, the second stage may comprise one or more flow channels defined by the propulsion housing and between the fluid driver and the outlet. The one or more flow channels may be defined between the shroud member and the conical portion of the housing. The fluid driver may be located in the first stage (e.g., in the axial flow passage) of the flow path. Alternatively, the fluid driver may be located in the second stage of the flow path. The propulsion housing may define the axial inlet. The housing may define the outlet. The housing may define the flow path. The outlet may comprise an area sufficient to permit solid material drawn into the inlet to be discharged from the outlet. The discharge direction may be defined by an orientation of the outlet. The discharge direction may be towards the trailing end of the apparatus, to create the required thrust to propel the apparatus in the intended direction of travel. The discharge direction may be substantially parallel to the longitudinal axis. Alternatively, the discharge direction may be at an oblique angle to the longitudinal axis. The oblique angle may comprise a longitudinal component opposing the intended direction of travel. The intake direction may be substantially parallel to the longitudinal axis. The intake direction may be at an oblique angle to the longitudinal axis. The outlet may comprise one or more outlet ports. The one or more outlet ports may be circumferentially arranged around an exterior of the apparatus. The one or more outlet ports may comprise one or more jets or nozzles. The one or more outlet ports may be provided on the apparatus between the leading and trailing ends. The one or more outlet ports may be disposed adjacent the leading end, the trailing end and / or disposed centrally on the apparatus. The outlet ports may be defined circumferentially between the one or more fin connectors. The outlet ports may be or comprise an annular outlet. The annular outlet may be defined radially between the support member (e.g., the conical portion) and the shroud member. The inlet may comprise one or more inlet ports. The one or more inlet ports may be arranged on an axial end face of the apparatus. The axial end face may be at the leading end or the trailing end of the apparatus, depending on the location of the axial inlet as defined in the aspect above. The inlet may define an inlet flow area. The outlet may define an outlet flow area. The inlet flow area may be greater than the outlet flow area. Alternatively, the outlet flow area may be greater than the inlet flow area. The propulsion assembly may comprise a motor configured to drive rotation of the fluid driver. The fluid driver may be connected to the motor via a drive shaft. The drive shaft may extend in an axial direction, e.g., coaxial with the longitudinal axis of the apparatus. The drive shaft may be connected to the motor via a coupling. The coupling may be configured to permit small deviations in the axis of rotation of the drive shaft. This may reduce lateral loads acting on the drive shaft and motor. The coupling may comprise a coupling sleeve. The coupling may comprise a deformable, elastic material. The propulsion assembly may comprise one or more thrust bearings. The one or more thrust bearings may be interposed between an annular shoulder of the drive shaft and an internal surface of the housing. The one or more thrust bearings bearing may be configured to permit rotation of the drive shaft within the housing. The one or more thrust bearings may be configured to accommodate axial loads resulting from the thrust generated to be transmitted from the drive shaft to the housing. The drive shaft may be supported by a journal support member for stabilising rotation of the drive shaft, e.g., preventing shaft whirling. The propulsion assembly may comprise a seal for sealing between the fluid driver and an internal of the housing. The seal may be a dynamic seal. The apparatus may be remote-controlled, e.g. controllable by an operator at surface. Alternatively, the apparatus may be configured for autonomous operation. The propulsion assembly may comprise an electronics module. The electronics module may comprise a controller for controlling the apparatus, e.g., controlling the motor, such as a rotational speed of the motor. The electronics module may comprise a receiver for receiving a signal (e.g., from an operator). The controller may be configured to control the apparatus in accordance with the received signal. The motor may be configured to output a variable rotational speed. The motor may be bidirectional to permit the fluid driver to be rotated in forward and reverse directions, such that the apparatus can be propelled in forward and reverse directions within the throughbore. The ability to reverse the apparatus may assist in traversing obstructions in the throughbore and / or clearing the obstructions. The propulsion assembly may comprise a power supply, such as a battery. Alternatively, the apparatus may be connected to a power supply at surface, e.g., via an electrical line. The power supply may be configured to supply power to the motor and / or electronics module. The electronics module may comprise a memory for storing instructions for operation of the apparatus. In some examples, the apparatus may comprise a sensor configured to sense one or more properties of the throughbore, e.g. pressure, temperature, etc., of a fluid within the throughbore. The controller may be configured to control operation of the apparatus in response to sensing one or more parameters. The apparatus may comprise a transmitter configured to transmit sensed data to an external receiver, e.g. at surface. In some examples, the apparatus may be configured to generate pressure pulse signals through fluid within the throughbore. The pressure pulse signals may be used to transmit the sensed data for receipt at the external receiver. The propulsion assembly may be configured or adapted to generate the pressure pulse signals in the fluid within the throughbore. Alternatively, the apparatus may comprise a dedicated pressure pulse signal generator. For example, the pressure pulse signal generator may comprise a control valve for choking a flow through the throughbore. The apparatus (e.g., the housing) may comprise a chamber. The motor and / or electronics module may be disposed in the chamber. The chamber may contain a fluid. The fluid may comprise a coolant (e.g., an oil). The coolant may be configured to cool the propulsion assembly (e.g., the motor and / or electronics module). The motor and / or electronics module may be submerged in the coolant. In other examples, the chamber may comprise another fluid. In certain applications, the apparatus may be exposed to different environments as the apparatus traverses the throughbore, which may affect the state of the fluid (e.g., pressure, temperature, etc.) in the chamber. The apparatus (e.g., the housing) may comprise a pressure compensation mechanism configured to vary a volume of the chamber, e.g., to accommodate for changes of state of the fluid as the apparatus traverses the throughbore. The housing may comprise a base and an extendable part. In some examples, the extendable part may comprise the support member and the shroud member. The chamber may be defined between the base and the extendable part (e.g., the support member). The extendable part may be movable relative to the base. Movement of the extendable part relative to the base may vary the volume of the chamber. The extendable part may be exposed on a first side to a pressure within the chamber. The extendable part may be exposed on an opposing second side to a pressure within the throughbore. The position of the extendable part may be provided in accordance with a pressure differential between the chamber and the throughbore. As such, the chamber may be pressure balanced with the throughbore. The base may comprise a tubular structure. The extendable part (e.g., the support member) may comprise a tubular structure. The extendable part (e.g., the support member) may be slidably disposed within the base, or vice versa. The base and the extendable part may define a telescopic arrangement. The extendable part may be rotationally locked relative to the base. The extendable part may be provided at the leading end of the housing. The extendable part may comprise the fluid driver receiving area for the fluid driver to be mounted within the extendable part. The extendable part may define the axial inlet. The extendable part may define the outlet. The apparatus may comprise one or more wall engaging members (e.g., skis, rollers, etc.) configured to engage a wall of the throughbore. The one or more wall engaging members may be configured to stabilise the apparatus within the throughbore. The one or more wall engaging members may comprise a tapered edge for engaging the wall of the throughbore, which may assist where a layer of material, such as debris or sand, has been deposited on the wall of the throughbore. Alternatively, the one or more wall engaging members may instead comprise a flat or rounded wall-engaging portion. The one or more wall engaging members may be circumferentially arranged around an exterior of the apparatus. The one or more wall engaging members may be provided at one or more locations along the longitudinal axis of the apparatus. The apparatus may comprise one or more brakes for engaging a wall of the throughbore. The one or more brakes may comprise extendable (e.g., laterally extendable) members. The one or more brakes may function as an anchor configured to secure the apparatus within the throughbore, e.g., at a desired location for an operation to be performed, such as logging data. The apparatus may comprise a filter assembly for filtering fluid from the throughbore entering the axial inlet. The filter assembly may be provided upstream of the axial inlet. The filter assembly may be attached to a leading end of the propulsion housing, e.g., via a mechanical connection, such as a threaded connection, bolted connection, etc., or may be formed integrally with the housing. The filter assembly may be connected to a leading end of the shroud member, e.g., via a mechanical connection or may be integrally formed with the shroud member. The filter assembly may comprise a conical shape. Alternatively, the filter assembly may comprise other shapes, such as a cylindrical or axial shape. The filter assembly may taper towards the leading end of the apparatus. The filter assembly may comprise a nose tip. The filter assembly may comprise a filter portion, comprising a grille or mesh structure. The grille or mesh structure may comprise openings sufficient in size to permit solid material drawn towards the axial inlet to pass through the openings of the grille or mesh structure. The payload housing may comprise a pressure compensation mechanism. The pressure compensation mechanism may be configured to balance pressure within the payload housing as the payload is deployed from the payload housing. The payload housing may be in fluid communication with the throughbore. The payload housing may comprise a payload housing filter configured to filter fluid entering the payload housing from the throughbore. The payload housing filter may comprise a cylindrical structure having a grille or mesh structure, e.g., including circular openings. The leading end of the apparatus may comprise an enlarged diameter portion for restricting fluid flow around the apparatus within the throughbore. The enlarged diameter portion may be sized to substantially correspond to or be slightly less than a diameter of the throughbore. The enlarged diameter portion may restrict fluid flow around the apparatus such that a pressure differential is created across the apparatus between upstream and downstream locations of the apparatus. The pressure differential may assist in advancing the apparatus through the throughbore by pumping the apparatus in the intended direction of travel. The propulsion assembly referred to above may be a leading end propulsion assembly (i.e., having the axial inlet provided at the leading end). The axial inlet, outlet and fluid driver referred to above may define a first axial inlet, a first outlet and a first fluid driver. Instead of or in addition to the payload housing described above, the apparatus may comprise a trailing end propulsion assembly (i.e., having a second axial inlet provided at the trailing end). In such examples, the apparatus may be configured to tow the payload. The trailing end propulsion assembly may comprise a flow path configured to reverse a direction of fluid moving through the propulsion assembly. The flow path may comprise a turn of more than 90 degrees. The reversal of direction may be configured to generate a reverse thrust vector. That is, fluid may enter the second axial inlet in one direction and be discharged from the outlet in an opposite direction. The flow path may be tapered from the inlet to the outlet, which may assist in increasing a velocity of fluid being discharged from the propulsion assembly. The second axial inlet may be configured to draw fluid into the trailing end propulsion assembly in a second intake direction. The second intake direction may be substantially parallel to the longitudinal axis. The trailing end propulsion assembly may comprise a second outlet provided on the apparatus between the leading and trailing ends. The second outlet may be laterally offset from the longitudinal axis. The second outlet may be configured to discharge fluid from the apparatus in a second discharge direction to generate a second thrust vector for propelling the apparatus in the intended direction of travel. The trailing end propulsion may comprise a second fluid driver configured to move a fluid through the trailing end propulsion assembly from the second axial inlet to the second outlet. The housing referred to above may define a leading propulsion housing. The apparatus may further comprise a trailing propulsion housing. The leading propulsion housing may define the first axial inlet. The leading propulsion housing may define the first outlet. The leading propulsion housing may comprise the first fluid driver. The apparatus may comprise a trailing propulsion housing. The trailing propulsion housing may define the second axial inlet. The trailing propulsion housing may define the second outlet. The trailing propulsion housing may comprise the second fluid driver. The leading and trailing propulsion housings may each comprise one or more of the features described above in relation to the housing of the first aspect. The leading and trailing propulsion housings may be connected to one another or may be integral with one another. Alternatively, an intermediate structure may be interposed between the leading and trailing propulsion assemblies. The intermediate structure may connect the leading and trailing propulsion assemblies together. The intermediate structure may comprise a power supply, e.g., a battery pack, for powering the propulsion assemblies. Alternatively, the intermediate structure may comprise a payload, e.g. comprising one or more tools and / or equipment. A single motor may be provided to drive rotation of the first and second fluid drivers. Alternatively, the apparatus may comprise a respective motor for driving rotation of the first and second fluid drivers. Where respective motors are provided, the controllers may control the respective motors independently, such that the first and second fluid drivers may be rotated independently of one another. As such, the trailing and leading end propulsion assemblies may be selectively operated to provide for thrust vectoring control. In some examples, the leading and trailing end propulsion assemblies may be utilised to perform different tasks. For instance, the trailing end propulsion assembly may be operated when the apparatus is turning a corner of the throughbore. The leading end propulsion assembly may be operated when the apparatus is traversing an obstruction in the throughbore, e.g., to shift material from the leading end of the apparatus to the trailing end. Another aspect of the present disclosure relates to a self-propelled apparatus for traversing a throughbore, comprising: a leading end and a trailing end with respect to an intended direction of travel of the apparatus, the apparatus defining a longitudinal axis extending between the leading and trailing ends; a leading end propulsion assembly comprising: an axial inlet provided at the leading end, the axial inlet configured to draw fluid into the leading end propulsion assembly in an intake direction; an outlet provided on the apparatus between the leading and trailing ends and laterally offset from the longitudinal axis, the outlet configured to discharge fluid from the apparatus in a discharge direction to generate a thrust vector for propelling the apparatus in the intended direction of travel; and a fluid driver configured to move a fluid through the leading end propulsion assembly from the axial inlet to the outlet; and a payload housing. The payload housing may comprise or carry a payload. The payload housing may be configured to deploy the payload. The payload housing may be referred to herein as a deployment apparatus. In some examples, the payload housing may be configured to deploy elongate media (e.g., an optical fibre) in or throughout the throughbore. In some examples, the elongate media may be connected to a receiver at surface and deployed throughout throughbore via the trailing end of the apparatus as the apparatus traverses the throughbore. In other examples, the payload housing may comprise other tools and / or equipment for conveying and / or deploying through the throughbore. The payload housing may be at the trailing end of the apparatus. The payload housing may comprise a spool of the elongate media. The spool may comprise a bobbin upon which the elongate media is wound. The bobbin may define an axis. The bobbin axis may be aligned with the spool axis. The bobbin axis may define the spool axis. The spool may be mounted in cantilever form within the payload housing. In this respect the spool may define a fixed or proximal end, and a free or distal end. In one example, an initial wrap segment (i.e., first wrap segment to be formed on the spool) may be provided adjacent the proximal end. Alternatively, the initial wrap segment may be provided towards the distal end. The spool may be mounted within a cavity of the payload housing. The payload housing may comprise or define a cylindrical cavity. The payload housing may define a payload outlet. Where the payload comprises elongate media, the payload housing may comprise a guide, such as a funnel, to guide the elongate media towards the payload outlet. In some examples the payload outlet may comprise a store or bath of a coating, such that the elongate media may become coated during exiting the payload housing. The payload outlet of the payload housing may comprise a resistance arrangement to provide a degree of resistance to exit of the elongate media. For example, the outlet of the payload housing may establish interference with the elongate media. Another aspect of the present disclosure relates to a self-propelled apparatus for traversing a throughbore, comprising: a leading end and a trailing end with respect to an intended direction of travel of the apparatus, the apparatus defining a longitudinal axis extending between the leading and trailing ends; a leading end propulsion assembly comprising: a first axial inlet provided at the leading end, the first axial inlet configured to draw fluid into the leading end propulsion assembly in a first intake direction; a first outlet provided on the apparatus between the leading and trailing ends and laterally offset from the longitudinal axis, the first outlet configured to discharge fluid from the apparatus in a first discharge direction to generate a first thrust vector for propelling the apparatus in the intended direction of travel; and a first fluid driver configured to move a fluid through the leading end propulsion assembly from the first axial inlet to the first outlet; and a trailing end propulsion assembly comprising: a second axial inlet provided at the trailing end, the second axial inlet configured to draw fluid into the trailing end propulsion assembly in a second intake direction; a second outlet provided on the apparatus between the leading and trailing ends and laterally offset from the longitudinal axis, the second outlet configured to discharge fluid from the apparatus in a second discharge direction to generate a second thrust vector for propelling the apparatus in the intended direction of travel; and a second fluid driver configured to move a fluid through the trailing end propulsion assembly from the second axial inlet to the second outlet. The first and second discharge directions may be parallel to one another. Alternatively, the first and second discharge directions may be at different angles to one another. The first and second intake directions may be opposite to one another. The apparatus disclosed herein, or at least some components of the apparatus, may be manufactured in any suitable manner, such as using conventional manufacturing processes. Accordingly, examples described herein not only include the apparatus and associated components, but also methods of manufacturing the apparatus or associated components via conventional manufacturing processes. In some examples, the 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 apparatus and associated components, but also methods of manufacturing the apparatus or associated components via additive manufacturing and computer software, firmware or hardware for controlling the manufacture of the apparatus and associated components via additive manufacturing. The structure of the apparatus and associated components 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 apparatus and associated components. That is, a design file represents the geometrical arrangement or shape of the apparatus and associated components. In light of the above, the present disclosure includes methods of manufacture, such as via additive manufacturing. This includes the steps of obtaining a design file representing the apparatus and associated components and instructing an additive manufacturing apparatus to manufacture the apparatus and associated components 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 apparatus and associated components. In these embodiments, the design file itself may automatically cause the production of the apparatus and associated components 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 apparatus and associated components. 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. Another aspect of the present disclosure relates to a method of traversing a self-propelled apparatus through a throughbore, the method comprising: positioning the apparatus in the throughbore, the apparatus comprising a leading end and a trailing end with respect to an intended direction of travel of the apparatus, the apparatus defining a longitudinal axis extending between the leading and trailing ends; rotating a fluid driver of a propulsion assembly of the apparatus to draw fluid into an axial inlet of the propulsion assembly in an intake direction, the axial inlet provided at one of the leading and trailing ends; moving fluid through the propulsion assembly from the axial inlet to an outlet of the propulsion assembly, the outlet provided on the apparatus between the leading and trailing ends and laterally offset from the longitudinal axis; and discharging fluid from the outlet in a discharge direction to generate a thrust vector for propelling the apparatus in the intended direction of travel. An aspect of the present disclosure relates to a downhole tractor for traversing a wellbore, comprising: a leading end and a trailing end with respect to an intended direction of travel of the tractor, the tractor defining a longitudinal axis extending between the leading and trailing ends; and a propulsion assembly comprising: an axial inlet provided at one of the leading and trailing ends, the axial inlet configured to draw fluid into the propulsion assembly in an intake direction; an outlet provided on the tractor between the leading and trailing ends and laterally offset from the longitudinal axis, the outlet configured to discharge fluid from the tractor in a discharge direction to generate a thrust vector for propelling the tractor in the intended direction of travel; and a fluid driver configured to move a fluid through the propulsion assembly from the axial inlet to the outlet. Another aspect of the present disclosure relates to an optical fibre deployment system, the system comprising: a self-propelled apparatus for traversing a throughbore, comprising: a leading end and a trailing end with respect to an intended direction of travel of the apparatus, the apparatus defining a longitudinal axis extending between the leading and trailing ends; and a propulsion assembly comprising: an axial inlet provided at one of the leading and trailing ends, the axial inlet configured to draw fluid into the propulsion assembly in an intake direction; an outlet provided on the apparatus between the leading and trailing ends and laterally offset from the longitudinal axis, the outlet configured to discharge fluid from the apparatus in a discharge direction to generate a thrust vector for propelling the apparatus in the intended direction of travel; and a fluid driver configured to move a fluid through the propulsion assembly from the axial inlet to the outlet; and a deployment apparatus configured to deploy optical fibre in the throughbore as the self-propelled apparatus traverses the throughbore. The self-propelled apparatus may be attached or integrally formed with the deployment apparatus. Alternatively, the self-propelled apparatus may be configured to tow the deployment apparatus. The deployment apparatus may comprise a spool optical fibre to be deployed in or throughout the throughbore. Another aspect of the present disclosure relates to a self-propelled apparatus for traversing a throughbore, comprising: a leading end and a trailing end with respect to an intended direction of travel of the apparatus, the apparatus defining a longitudinal axis extending between the leading and trailing ends; a leading end propulsion assembly; and a trailing end propulsion assembly. It will be appreciated that features described in relation to one aspect may be equally combined with any other aspect described herein. 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 self-propelled apparatus traversing a wellbore in a formation; Figure 2 is a side view of the apparatus; Figure 3 is a cross-sectional, side view of the apparatus; Figure 4 is an isometric view of the apparatus; Figure 5 is a cross-sectional, side view of a leading propulsion assembly of the apparatus; Figure 6 is a cross-sectional, side view of a trailing propulsion assembly of the apparatus; Figure 7 is an isometric, cross-sectional view of the trailing propulsion assembly; Figure 8 is an isometric, cross-sectional view of the leading propulsion assembly; Figure 9 is an isometric view of the trailing propulsion assembly; Figure 10 is a side view of the apparatus comprising a filter; Figure 11 is a side view of the apparatus comprising an enlarged diameter portion at a leading end of the apparatus; Figure 12 is a side view of an alternative self-propelled apparatus; Figure 13 is a side view of a further alternative self-propelled apparatus; Figure 14 is a side view of a further alternative self-propelled apparatus; Figure 15 is a side view of a further alternative self-propelled apparatus; Figure 16 is a side view of a further alternative self-propelled apparatus; Figure 17 is a cross-sectional, side view of the self-propelled apparatus of Figure 16; Figure 18 is a cross-sectional, side view of a propulsion assembly of the self-propelled apparatus of Figure 16; Figure 19 is a side view of a fluid driver of the propulsion assembly of Figure 18; Figures 20 and 21 are isometric views of the propulsion assembly of Figure 18; Figure 22 is a cross-sectional, side view of a payload housing of the self-propelled apparatus of Figure 16; Figure 23 is a cross-sectional, isometric view of the payload housing of Figure 22, taken through line A-A in Figure 22; and Figure 24 is an isometric view of the payload housing of Figure 22. DETAILED DESCRIPTION OF THE DRAWINGS Aspects of the present disclosure relate to self-propelled apparatus for traversing a throughbore. Multiple applications may be possible and may facilitate transportation of equipment and / or tools through any type of throughbore. For example, the throughbore may be defined by pipework or pipeline architecture, such as located at surface (e.g. in a production facility), subsea and / or subterranean. For the purposes of providing an exemplary application, the following description relates to a self-propelled apparatus traversing a throughbore defined by a wellbore in a formation, such as used in the exploration and extraction of mineral resources (e.g., oil and gas), in geothermal applications, etc. Figure 1 is a diagrammatic illustration of a self-propelled apparatus 10 traversing a deviated, horizontal wellbore 12 in a formation 14. The apparatus 10 is attached to a payload via an attachment structure at a trailing end 22 of the apparatus 10, for conveying the payload through the wellbore 12. In this example, the payload is a deployment apparatus 16 configured to deploy optical fibre 18 through the wellbore 12, with the optical fibre 18 connected to a receiver at surface. However, in other examples, the payload may be or comprise other tools and / or equipment for conveying through the wellbore 12. Referring to Figures 2 and 3, the apparatus 10 comprises a leading end 20 and a trailing end 22 with respect to an intended direction of travel of the apparatus 10. The apparatus 10 defines a longitudinal axis 24 extending between the leading and trailing ends 20, 22. The apparatus 10 comprises a leading end propulsion assembly 26 at the leading end 20 of the apparatus 10 and a trailing end propulsion assembly 28 at a trailing end 22 of the apparatus 10, which will be described in more detail below. The apparatus 10 comprises rollers 30 configured to engage a wall of the throughbore and stabilise the apparatus 10 as the apparatus 10 traverses through the wellbore 12. The rollers 30 are circumferentially arranged around an exterior of the apparatus 10. In this example, two sets of rollers 30 are provided at different locations along the longitudinal axis 24 of the apparatus 10. However, the apparatus 10 may comprise any number of rollers 30 provided at one or more locations along the longitudinal axis 24. In some examples, the rollers 30 comprise a tapered edge (visible in Figure 6) for engaging a wall of the throughbore and piercing through any layers of material, such as debris or sand, deposited on the wall of the wellbore 12. Alternatively, the rollers 30 may comprise a flat or rounded wall-engaging portion. An intermediate structure 32 is provided between the leading and trailing end propulsion assemblies 20, 22. In this example, the intermediate structure 32 comprises a battery pack for powering the propulsion assemblies 20, 22. However, the intermediate structure 32 may additionally or alternatively comprise other tools or equipment, for instance the intermediate structure 32 may comprise a payload to be conveyed through the throughbore. Figure 4 illustrates the apparatus 10 comprising an attachment member 27 for attaching to a payload. The attachment member 27 comprises two arms 29 extending from a trailing end 22 of the apparatus 10. At a first end, the arms 29 are pivotably mounted to the apparatus 10 via a pinned connection 35 to permit rotation of the arms 29 about an axis perpendicular to the longitudinal axis 24. At a second, opposing end, the arms 29 comprise portions 31 converging at a head 33 of the attachment member 27. In other examples, alternative attachment members may be used. Referring to Figures 5 and 8, the leading end propulsion assembly 26 comprises an axial inlet 34 provided at the leading end 20 of the apparatus 10. The axial inlet 34 is provided on a leading end face of the apparatus 10 and is configured to draw fluid into the leading end propulsion assembly 26 in an intake direction 36 substantially parallel to the longitudinal axis 24. Outlet ports 38 are provided on the apparatus 10 between the leading and trailing ends 20, 22 and are circumferentially arranged around an exterior of the apparatus 10 so as to be laterally offset from the longitudinal axis 24. In other examples, the apparatus 10 may comprise a single outlet port provided on the exterior of the apparatus 10. The outlet ports 38 are configured to discharge fluid from the apparatus 10 in a discharge direction 40 towards the trailing end 22 of the apparatus 10 to create a thrust vector for propelling the apparatus 10 through the wellbore 12. In this example, the discharge direction 40 is at an oblique angle to the longitudinal axis 24. However, in other examples, the discharge direction 40 may be substantially parallel to the longitudinal axis 24. The apparatus 10 comprises a leading propulsion housing 41 defining the first axial inlet 34 and outlet ports 38. The leading propulsion housing 41 comprises a fluid driver receiving area for mounting a fluid driver, which in this example is an impeller 42, within the leading propulsion housing 41. In this example, the impeller 42 is mounted adjacent the axial inlet 34 at the leading end 20 of the apparatus 10. The impeller 42 may be defined as an internal or closed impeller. The leading propulsion housing 41 protects the impeller 42 from damage, for example due to contact with a wall of the wellbore 12, and contains a fluid flow around the impeller 42 such that flow is directed through the channels between the blades of the impeller 42. The leading propulsion housing 41 defines a flow path extending between the axial inlet 34 and the outlet ports 38 for fluid to be directed through the propulsion assembly 26. In this example, the flow path comprises a first stage and a second stage. The first stage comprises a central flow passage 21 in communication with the axial inlet and in which the impeller 42 is mounted. The second stage comprises multiple fluid channels 46 diverging from the longitudinal axis 24 for directing fluid from the central flow passage 21 to the respective outlet ports 38. A flow area of the axial inlet 34 is greater than a flow area of the outlet ports 38, with the fluid channels 46 tapering towards the outlet ports 38. This may assist in increasing a velocity of fluid being discharged from the propulsion assembly 26 thereby increasing a magnitude of the thrust vector generated. A motor 48 is provided to drive rotation of the impeller 42 via a drive shaft 50, the impeller 42 being connected to the drive shaft 50 via a nut 44 and a rotational lock member 45. The drive shaft 50 is connected to the motor 48 via a motor coupling sleeve 52. The motor coupling sleeve 52 is configured to permit small deviations in the axis of rotation of the drive shaft 50, which may reduce lateral loads acting on the drive shaft 50 and motor 48. A thrust bearing 54 is interposed between an annular shoulder 59 of the drive shaft 50 and an internal surface of the leading propulsion housing 41. The thrust bearing 54 is configured to permit rotation of the drive shaft 50 within the leading propulsion housing 41 and to accommodate axial loads resulting from the thrust generated to be transmitted from the drive shaft 50 to the leading propulsion housing 41. The thrust bearing 54 incorporates a seal 56 for sealing between the drive shaft 50 and the leading propulsion housing 41. The drive shaft 50 is supported by a journal support member 58 for stabilising rotation of the drive shaft 50, e.g., preventing shaft whirling. The leading end propulsion assembly 26 comprises a chamber 60 for receiving an electronics module. The chamber 60 comprises one or more electrical connectors 62 (e.g., power or data connectors) for connection to the electronics module. The electronics module may include a controller for controlling operation of the motor 48 and a receiver for receiving a signal (e.g., from an operator at surface) for the controller to control the motor 48. The motor 48 may be configured to output a variable rotational speed. Further, the motor 48 may be bidirectional so that the impeller 42 can be rotated in forward and reverse directions to permit the apparatus 10 to be propelled in forward and reverse directions within the throughbore. This may assist when traversing or clearing obstructions in the throughbore. In some examples, the apparatus 10 may comprise a sensor configured to sense one or more properties of the throughbore, e.g. pressure, temperature, etc., of fluid within the throughbore. The apparatus 10 may further comprise a transmitter configured to communicate with an external receiver, e.g. at surface. The apparatus 10 may be configured to generate pressure pulse signals through a fluid within the throughbore, e.g., using the propulsion assemblies 20, 22, to transmit the data to the external receiver. Alternatively, the apparatus 10 may comprise a dedicated pressure pulse signal generator. The chamber 60 may comprise a coolant (e.g., an oil) for cooling the motor 48 and / or electronics module. The motor 48 and electronics module may be disposed in the chamber 60 so as to be submerged in the coolant. The apparatus 10 may be exposed to different environments as the apparatus 10 traverses the wellbore 12, which may affect the state of the coolant (e.g., pressure, temperature, etc.). To accommodate for thermal expansion, pressure changes, etc., of the coolant as the apparatus 10 traverses the wellbore 12, the leading propulsion housing 41 comprises a pressure compensation mechanism configured to vary a volume of the chamber 60. In particular, the leading propulsion housing 41 comprises a base 41a and an extendable part 41b, and the chamber 60 is defined between the base 41a and the extendable part 41b. The extendable part 41b is movable relative to the base 41a to vary the volume of the chamber. A dynamic seal 70 is provided to seal between the base 41a and the extendable part 41b and permit relative longitudinal movement of the base 41a and the extendable part 41b. The extendable part 41b is exposed on a first side to pressure within the chamber 60 and on an opposing second side to pressure within the wellbore 12. As such, the position of the extendable part 41b may be provided in accordance with a pressure differential between the chamber 60 and the wellbore 12. In this way, the chamber 60 is pressure balanced with the wellbore 12, thus allowing the chamber 60 to expand to accommodate for thermal expansion, pressure changes, etc., of the coolant. The base 41a and extendable part 41b comprise tubular structures, and the extendable part 41b is slidably disposed within the base 41a in a telescopic arrangement. The extendable part 41b comprises a longitudinal slot 72 formed in its outer surface for receiving a key 74 of the base 41a, thereby rotationally locking the extendable part 41b relative to the base 41a. In this example, the extendable part 41b comprises the impeller 42, the axial inlet 34 and the outlet ports 38. However, other arrangements may be possible. The provision of an axial inlet 34 (e.g., as opposed to a radial or circumferential inlet) may provide for optimal thrust generation as well as providing other advantages. For example, an axial inlet may allow for the intake fluid area to be maximised, or more readily optimised. Also, where there is solid material (e.g., debris, sediment, sand or other pine particles) mixed with fluid within the throughbore, the leading end propulsion assembly 26 may be configured to draw the solid material into the axial inlet 34 and discharge the solid material through the outlet ports 38 towards the trailing end 22 of the apparatus 10. In this way, the leading end propulsion assembly 26 may shift solid material from the leading end 20 of the apparatus 10 to the trailing end 22, thereby clearing a path through the wellbore 12. This may be particularly beneficial in applications where the apparatus 10 is deployed in the wellbore 12 subsequent to a fracturing operation where proppant material may be deposited in the wellbore 12. Referring to Figures 6, 7 and 9, the trailing end propulsion assembly 28 comprises an axial inlet 76 provided at the trailing end 22 of the apparatus 10. The axial inlet 76 is provided on a trailing end face of the apparatus 10 and is configured to draw fluid into the propulsion assembly in an intake direction 78 substantially parallel to the longitudinal axis 24. Outlet ports 80 are provided on the apparatus 10 towards the trailing end 22 and are circumferentially arranged around an exterior surface of the apparatus 10 so as to be laterally offset from the longitudinal axis 24. The trailing propulsion housing 81 comprises a fluid driver receiving area for mounting a fluid driver, which in this example is an impeller 84, within the trailing propulsion housing 81. As with the leading propulsion assembly 41, the trailing end propulsion assembly 28 comprises a motor 48, drive shaft 50, motor coupling 52, thrust bearing 54, seal 56, journal support member 58 and chamber 60, which are the same as the respective components of the leading end propulsion assembly 26 described above, and have been assigned the same reference numerals. Furthermore, the trailing propulsion housing 81 comprises a base 81a and an extendable part 81b configured to vary the volume of a chamber 60 in accordance with a pressure differential between the chamber 60 and a pressure within the throughbore, as described above. Unlike the leading propulsion housing 41, the trailing propulsion housing 81 defines a flow path having a central flow passage 23 and a plurality of fluid channels 92 configured to reverse a direction of fluid moving through the trailing end propulsion assembly 28 to create a reverse thrust vector. That is, fluid enters the axial inlet 76 in one direction and is discharged from the outlet ports 80 in an opposite direction. In this example, the trailing propulsion housing 81 comprises an outlet structure 37 defining an external, bowl-shaped portion configured such that the outlet ports 80 discharge fluid from the apparatus 10 in a discharge direction 82 towards the trailing end 22 of the apparatus 10. The discharge direction 82 is illustrated at an oblique angle to the longitudinal axis 24. However, the discharge direction 82 may be substantially parallel to the longitudinal axis 24. As illustrated in Figure 9, the outlet structure 37 comprises support posts 83 circumferentially arranged around an exterior of the trailing propulsion housing 81. The apparatus 10 comprises a respective motor 48 and a respective controller for controlling the impellers 42, 84 independently of one another, which may provide a number of advantages. For instance, the trailing and leading end propulsion assemblies 20, 22 may be selectively operated to provide thrust vectoring control of the apparatus 10. Moreover, the leading and trailing end propulsion assemblies 20, 22 may be utilised to perform different tasks. For instance, the trailing end propulsion assembly 28 may be operated when the apparatus 10 is turning a corner of the throughbore, while the leading end propulsion assembly 26 may be operated when the apparatus 10 is traversing an obstruction in the throughbore, e.g., to shift solid material in front of the apparatus 10 to the trailing end 22, clearing a path forward through the wellbore 12. In some examples, the apparatus 10 may comprise a filter for filtering fluid before the fluid enters the axial inlet 34. Figure 10 illustrates the apparatus 10 comprising a filter assembly 94 provided upstream of the axial inlet 34. The filter assembly 94 is fitted to the leading end 20 of the apparatus 10 via a mechanical connection 96, such as a bolted connection. However, in other examples, the filter assembly 94 may be integrally formed with the leading propulsion housing 41. The filter assembly 94 comprises a circumferential filter 98 having a nose cone portion 100. However, the filter assembly 94 may instead comprise an axial filter. In some examples, an additional filter assembly may be provided downstream of the axial inlet 76 at the trailing end of the apparatus. The leading end 20 of the apparatus 10 may comprise an enlarged diameter portion 102 for restricting fluid flow around the apparatus 10 within the throughbore, as illustrated in Figure 11. The enlarged diameter portion 102 is sized to substantially correspond to an inner diameter of the wellbore 12. The enlarged diameter portion 102 may restrict fluid flow around the apparatus 10 such that a pressure differential is created between upstream and downstream locations of the apparatus 10. The pressure differential may assist in advancing the apparatus 10 through the wellbore 12 by pumping the apparatus 10 in the intended direction of travel. Multiple configurations of the apparatus may be possible. For instance, the apparatus may comprise only one of the leading and trailing end propulsion assembles. Figure 12 illustrates an alternative self-propelled apparatus 110 where, unlike the apparatus 10, the alternative apparatus 110 includes only a leading end propulsion assembly 126, i.e., the trailing end propulsion assembly 28 and the intermediate structure 32 are omitted. The alternative apparatus 110 comprises outlet ports 138 circumferentially arranged around an exterior of the apparatus 110 in a central region of the apparatus 110, longitudinally between two sets of rollers 130. Figure 13 is a plan view of a further alternative self-propelled apparatus 210. The apparatus 210 comprises a leading end propulsion assembly 226, two sets of rollers 230 and a payload housing 215 integrally formed with the apparatus 210. The payload housing 215 contains a payload 216 for conveying through the wellbore 12. In this example, the payload comprises a spool of optical fibre 218 connected to a receiver at surface and being deployed throughout wellbore 12 via a trailing end of the apparatus 210. In other examples, the payload 216 may comprise other tools and / or equipment for conveying through the wellbore 12. The self-propelled apparatus of the present disclosure may be used in several applications. For example, Figure 14 illustrates an alternative self-propelled apparatus 310 used in a plugging application. In this example, the apparatus 310 comprises a trailing end propulsion assembly 328 and a plug member 313 at a leading end of the apparatus 310. The plug member 313 comprises a cone portion 315 for landing on a seat or obstruction 317 within the throughbore. The plug may comprise a seal for sealing against the seat or obstruction 317. However, other forms of plug members may be used. Figure 15 illustrates a further alternative self-propelled apparatus 410 being used in a data logging application. In this example, the apparatus 410 comprises a trailing end propulsion assembly 428 and a housing 419 for containing data logging equipment. The apparatus 410 comprises communication means configured to communicate with an external receiver through a wireless communication channel. The apparatus 410 comprises one or more radially extendable brakes 421 for engaging a wall of the throughbore and securing the apparatus 410 relative to the throughbore, e.g., at a desired location for logging data. Figures 16 and 17 illustrate a further alternative self-propelled apparatus 510. As in the previous examples, the apparatus 510 comprises a leading end 520 and a trailing end 522 with respect to an intended direction of travel of the apparatus 510, and a longitudinal axis 524 extending between the leading and trailing ends 520, 522. The apparatus 510 comprises an intermediate structure 532 between the leading and trailing ends 520, 522 of the apparatus 510. The intermediate structure 532 comprises a battery pack 523 for powering the propulsion assembly 526. However, the intermediate structure 532 may additionally or alternatively comprise other tools or equipment. The apparatus 510 comprises a propulsion assembly 526 at the leading end 520 of the apparatus 510 and a payload housing 515 at the trailing end 522 of the apparatus 510. In this example, the payload housing 515 is configured to deploy an optical fibre 518 (or other elongate media) through the wellbore 12. The optical fibre 518 may be connected to a receiver at surface and deployed throughout wellbore 12 via the trailing end 522 of the apparatus 510 as the apparatus 510 traverses the wellbore 12. In other examples, the payload housing 515 may comprise other tools and / or equipment for conveying and / or deploying through the wellbore 12. The apparatus 510 comprises skis 530 for engaging a wall of the wellbore 12 and stabilising the apparatus 510 as the apparatus 510 traverses through the wellbore 1. However, in other examples, the apparatus 510 may additionally or alternatively comprise rollers. The skis 530 are circumferentially arranged around an exterior of the apparatus 510 and provided at three locations along the longitudinal axis 524 of the apparatus 510. However, the apparatus 510 may comprise any number of skis 530, provided at one or more locations along the longitudinal axis 524. The skis 530 comprise a radially outward, wall engaging portion for sliding along and / or piercing through any layers of material, such as debris or sand, deposited on the wall of the wellbore 12. Referring to Figures 18 to 21, the propulsion assembly 526 comprises an axial inlet 534 provided at the leading end 520 of the apparatus 510. The axial inlet 534 is provided on a leading end face of the apparatus 510 and is configured to draw fluid into the leading end propulsion assembly 526 in an intake direction 536 substantially parallel to the longitudinal axis 524. The apparatus 510 comprises a filter assembly 594 provided upstream of the axial inlet 534. The propulsion assembly 526 comprises a propulsion housing 541 including a base 541a and an extendable part 541b, which will be discussed below. A shroud member 543 is provided at a leading end 520 of the extendable part 541b for enclosing a fluid driver, which in this example is an impeller 542. The shroud member 543 protects the impeller 542 from damage, for example due to contact with a wall of the wellbore 512, and contains a fluid flow around the impeller 542 such that flow is directed through channels between blades 542a, 542b of the impeller 542. The filter assembly 594 is connected to a leading end of the shroud member 543, e.g., via a mechanical connection or may be integrally formed therewith. The filter assembly 594 comprises a conical shape, which tapers towards the leading end 520 of the apparatus 510. The filter assembly 594 comprises a conical grille or mesh structure 591, and a nose tip 600. However, in other examples, the filter assembly 594 may comprise other shapes. The grille or mesh structure 591 may comprise openings sufficient in size to permit solid material drawn towards the axial inlet 534 to pass between through the grille or mesh structure 591. An example impeller 542 is shown in Figure 19. The impeller 542 comprises two blades 542a,b mounted to an impeller hub 539. The blades 542a,b comprise a curvature and are spaced apart from one another by a distance sufficient to permit solid material drawn into the axial inlet 534 to pass between the blades 542a,b as the impeller 542 rotates. In this way, the propulsion assembly 526 may shift solid material from the leading end 520 of the apparatus 510 to the trailing end 522, thereby clearing a path through the wellbore 12. This may be particularly beneficial in applications where the apparatus 510 is deployed in the wellbore 12 subsequent to a fracturing operation where proppant material may be deposited in the wellbore 12. The extendable part 541b of the propulsion housing 541 comprises a support member on a trailing end of the shroud member 543. The support member comprises a cylindrical portion 547 and a conical portion 546, which tapers towards the leading end 520 of the apparatus 510 and the shroud member 543. The shroud member 543 is connected to the conical portion 546 via fin connectors 545 (visible in Figures 20 and 21). The fin connectors 545 are circumferentially arranged on an exterior of the conical portion 546 and are blade-shaped for reducing drag and providing stability to the apparatus 510. In this example, three fin connectors 545 are circumferentially disposed on the conical portion 546. The fin connectors 545 generally extend in an axial direction from the cylindrical portion 547, substantially parallel to the longitudinal axis 524 of the apparatus 510, across the conical portion 546 to connect to the shroud member 543. The shroud member 543 comprises a tubular structure with open axial ends. The impeller 542 is mounted within the shroud member 543 to define a stand-off zone between the trailing end of the impeller 542 and the leading end of the fin connectors 545. The stand-off zone comprises a minimum length / between the leading end of the connectors 545 and the trailing end of the impeller 542 for minimising shear forces resulting from the interaction of the impeller 542 and the fluid within the stand-off zone, e.g., produced by rotation of the impeller 542. The minimum length / may be determined in accordance with fluid mechanic equations and operational requirements of the apparatus. The propulsion assembly 526 comprises outlet ports 538 defined radially between the shroud member 543 and the conical portion 546, and circumferentially between the fins connectors 545. The outlet ports 538 define an annular outlet. As in the previous examples, the outlet ports 538 are laterally offset from the longitudinal axis 524. The outlet ports 538 are arranged to discharge fluid from the apparatus 510 in a discharge direction 540 towards the trailing end 522 of the apparatus 510 to create a thrust vector for propelling the apparatus 510 through the wellbore 512. In this example, the discharge direction 540 is at an oblique angle to the longitudinal axis 524. A flow path extends between the axial inlet 534 and the outlet ports 538 for fluid to be directed through the propulsion assembly 526. In this example, the flow path comprises a first stage and a second stage. The first stage comprises a central flow passage 521 in communication with the axial inlet 534 and in which the impeller 542 is mounted. The first stage 521 is enclosed by the shroud member 543. The second stage is axially offset from the shroud member 543 on the trailing end of the shroud member 543, such that the flow path at the second stage is radially exposed on its outer side to wellbore 12. Due to the shape of the conical portion 546, the second stage diverges from the longitudinal axis 524 for directing fluid from the central flow passage 521 away from the apparatus 510. As in the previous examples, a motor 548 is provided to drive rotation of the impeller 542 via a drive shaft 550, the impeller 542 being connected to the drive shaft 550 via a nut 544 and a rotational lock member (e.g., rotational lock member 45 in Figure 5). The drive shaft 550 is connected to the motor 548 via a motor coupling sleeve 552. The motor coupling sleeve 552 is configured to permit small deviations in the axis of rotation of the drive shaft 550, which may reduce lateral loads acting on the drive shaft 550 and motor 548. Thrust bearings 554a,b are provided to permit rotation of the drive shaft 50 within the propulsion housing 541 and to accommodate axial loads resulting from thrust generation to be transmitted from the drive shaft 550 to the propulsion housing 541. The drive shaft 550 is supported by a journal support member 558 for stabilising rotation of the drive shaft 550 and preventing shaft whirling. A seal 556 is interposed between the thrust bearing 554b and an internal surface of the propulsion housing 541, for sealing between the impeller 542 and an internal of the propulsion housing 541. As in the previous examples, the propulsion assembly 526 comprises a chamber 560 for receiving an electronics module. The chamber 560 comprises one or more electrical connectors 562 (e.g., power or data connectors) for connection to the electronics module. The electronics module may include a controller for controlling operation of the motor 548 and a receiver for receiving a signal (e.g., from an operator at surface) for the controller to control the motor 548. The motor 548 may be configured to output a variable rotational speed. Further, the motor 548 may be bidirectional so that the impeller 542 can be rotated in forward and reverse directions to permit the apparatus 510 to be propelled in forward and reverse directions within the throughbore. The chamber 560 may comprise a coolant (e.g., an oil) for cooling the motor 548 and / or electronics module. The motor 548 and electronics module may be disposed in the chamber 560 and submerged in the coolant. The apparatus 510 may be exposed to different environments as the apparatus 510 traverses the wellbore 512, which may affect the state of the coolant (e.g., pressure, temperature, etc.). To accommodate for thermal expansion, pressure changes, etc., of the coolant as the apparatus 510 traverses the wellbore 512, the propulsion housing 541 comprises a pressure compensation mechanism configured to vary a volume of the chamber 560. In particular, the propulsion housing 541 comprises a base 541a and an extendable part 541b, and the chamber 560 is defined between the base 541a and the extendable part 541b. The extendable part 541b is movable relative to the base 541a to vary the volume of the chamber. A dynamic seal 570 is provided to seal between the base 541a and the extendable part 541b and permit relative longitudinal movement of the base 541a and the extendable part 541b. The extendable part 541b is exposed on a first side to pressure within the chamber 560 and on an opposing second side to pressure within the wellbore 12. As such, the position of the extendable part 541b may be provided in accordance with a pressure differential between the chamber 560 and the wellbore 12. In this way, the chamber 560 is pressure balanced with the wellbore 12, thus allowing the chamber 560 to expand to accommodate for thermal expansion, pressure changes, etc., of the coolant. The base 541a and extendable part 541b comprise tubular structures, and the extendable part 541b is slidably disposed within the base 541a in a telescopic arrangement. The extendable part 541b comprises longitudinal slots 572 formed in an outer surface of the extendable part 541b for receiving keys 574 of the base 541a, thereby rotationally locking the extendable part 541b relative to the base 541a. Referring to Figures 22 and 23, the payload housing 515 comprises a cavity region 622 for accommodating a spool of the optical fibre 518, which is wound on a bobbin 624. The bobbin 624 is mounted within the cavity 622 in cantilever form such that the bobbin 624 defines a fixed proximal end 626 and a free distal end 628, and is arranged to be coaxial with the longitudinal axis 524 of the apparatus 510. A trailing end of the payload housing 515 includes an internal funnel 632 which functions to guide fibre 518 despooled from the bobbin 624 towards an exit 618. The exit 618 includes a throughbore 634 which is dimensioned to a similar diameter as the fibre 518, and in some examples the bore 634 may provide a degree of resistance to fibre 518 passing therethrough. This may assist to control the rate of fibre deployment. In some examples a volume of grease or similar material may be provided within the cavity 622, for example within the internal funnel 632. Such grease may become coated on a fibre 518 during deployment from the apparatus 510. The grease may function to provide a degree of resistance to the deployment of the fibre 518, to permit the fibre 518 to stick to a wall of the wellbore 12, to protect the fibre 518, to provide lubrication to the fibre 518 and the like. The bobbin 624 includes a winding surface 636 which has a conical portion 638 and an adjacent cylindrical portion 640 for receiving fibre thereon, wherein the conical portion 638 is located towards the distal end 628 of the bobbin 624 and the cylindrical portion 640 is located towards the proximal end 626. A length of the fibre 518 may be wound around a spool axis to form a plurality of wrap segments arranged axially along the spool axis, as for example described in the Applicant’s WO 2019 / 016538, which is incorporated herein by reference. The bobbin 624 further comprises an annular lip 644 at the distal end 628 to assist during despooling of fibre 518 from the bobbin 624. The payload housing 515 further comprises a payload housing filter 630 configured to filter fluid entering the payload housing 515 from the wellbore 12. The payload housing filter 630 is disposed upstream of the bobbin 624 relative to a direction of travel of the apparatus 510. The payload housing filter 630 comprises a cylindrical structure having a grille or mesh structure, including circular openings 631. As the apparatus 510 traverses the wellbore 12, the fibre 518 is deployed from the exit 618 as it unspools from the bobbin 624. Consequently, the volume of fibre 518 occupying the cavity 622 will be reduced as the fibre 518 is deployed. As such, fluid from the wellbore 12 may enter the cavity 622 via the payload housing filter 630 to ensure that the payload housing 515 is pressure compensated as the apparatus 510 traverses the wellbore 12 and deploys the fibre 518. Accordingly, the payload housing filter 630 may function as a pressure compensation mechanism. The payload housing filter 630 ensures only clean well fluid is allowed to fill the volume around the fibre 518. The payload housing filter 630 may be configured, for example, through selection of an appropriate material and / or pore size, to filter debris from bore fluid. In some examples, the cavity 622 of the payload housing 515 may comprise an isolating fluid for isolating the fibre 518 from the fluid entering the cavity 622 from the wellbore 12, such as in WO 2021 / 043479 A1, which is incorporated herein by reference. Figure 23 is a cross-sectional, isometric view of the payload housing 515 of Figure 22, taken through line A-A in Figure 2, showing a central flow passage 641 of the bobbin 624 in fluid communication with the wellbore 12, which communicates with radial ports 642 and axial channel 643 to allow fluid from the wellbore 12 to fill the region around the spool of fibre 518. The flow passage 641 is also in communication with the region around the spool of the fibre 518 via a space at a distal end 628 of the bobbin 624. The self-propelled apparatus disclosed herein, or at least some components of the apparatus, may be manufactured in any suitable manner, such as using conventional manufacturing processes. Accordingly, examples described herein not only include the apparatus and associated components, but also methods of manufacturing the apparatus or associated components via conventional manufacturing processes. In some examples, the apparatus, or any individual component or groups of components may be manufactured by additive manufacturing.

Claims

1. A self-propelled apparatus for traversing a throughbore, comprising:a leading end and a trailing end with respect to an intended direction of travel of the apparatus, the apparatus defining a longitudinal axis extending between the leading and trailing ends; anda propulsion assembly comprising:an axial inlet provided at one of the leading and trailing ends, the axial inlet configured to draw fluid into the propulsion assembly in an intake direction;an outlet provided on the apparatus between the leading and trailing ends and laterally offset from the longitudinal axis, the outlet configured to discharge fluid from the apparatus in a discharge direction to generate a thrust vector for propelling the apparatus in the intended direction of travel; anda fluid driver configured to move a fluid through the propulsion assembly from the axial inlet to the outlet.

2. The apparatus of claim 1, wherein the propulsion assembly comprises a housing and the fluid driver is mounted within the housing.

3. The apparatus of claim 1 or 2, wherein the propulsion assembly comprises a shroud member for enclosing the fluid driver.

4. The apparatus of any preceding claim, wherein the propulsion housing comprises a support member, the support member comprising cylindrical portion and a conical portion, wherein the conical portion tapers towards the shroud member.

5. The apparatus of any preceding claim, wherein the shroud member is connected to the conical portion via one or more fin connectors.

6. The apparatus of any preceding claim, wherein the shroud member comprises a tubular structure having open ends.

7. The apparatus of any preceding claim, wherein the propulsion assembly defines a flow path extending between the axial inlet and the outlet.

8. The apparatus of claim 7, wherein the flow path comprises a first stage and a second stage, the first stage comprising an axial flow passage in communication with the axial inlet, and the second stage diverging from the longitudinal axis.

9. The apparatus of claim 8, wherein the fluid driver is located in the first stage.

10. The apparatus of any preceding claim, wherein the discharge direction is substantially parallel to the longitudinal axis or at an oblique angle to the longitudinal axis.

11. The apparatus of any preceding claim, wherein the outlet comprises one or more outlet ports circumferentially arranged around an exterior of the apparatus.

12. The apparatus of any preceding claim, wherein the propulsion assembly comprises a stand-off zone between a trailing end of the fluid driver and an internal surface of the propulsion housing.

13. The apparatus of any preceding claim, wherein the apparatus comprises a chamber and a pressure compensation mechanism configured to vary a volume of the chamber.

14. The apparatus of claim 13, wherein the chamber comprises a fluid, such as a coolant.

15. The apparatus of any preceding claim, wherein the propulsion assembly is configured to draw solid material from the throughbore into the axial inlet and discharge the solid material through the outlet.

16. The apparatus of any preceding claim, wherein the fluid driver comprises an impeller.

17. The apparatus of claim 16, wherein the one or more impellers comprise two or more blades comprising a curvature and spaced apart from one another by a distance sufficient to permit solid material drawn into the axial inlet to pass between the blades as the impeller rotates.

18. The apparatus of any preceding claim, comprising one or more wall engaging members for engaging a wall of the throughbore.

19. The apparatus of any preceding claim, comprising a filter assembly for filtering fluid entering the axial inlet.

20. The apparatus of claim 19, wherein the filter assembly comprises a conical potion tapering towards a leading end of the apparatus.

21. The apparatus of any preceding claim, wherein the apparatus comprises a payload housing at a trailing end of the apparatus.

22. The apparatus of claim 21, wherein the payload housing is configured to deploy an elongate media in or throughout the throughbore as the apparatus traverses the throughbore.

23. The apparatus of any one of claims 1 to 20, wherein the propulsion assembly is a leading end propulsion assembly, and the apparatus further comprises a trailing end propulsion assembly.

24. The apparatus of any preceding claim, wherein the apparatus is configured for connection to a payload.

25. A method of traversing a self-propelled apparatus through a throughbore, the method comprising:positioning the apparatus in the throughbore, the apparatus comprising a leading end and a trailing end with respect to an intended direction of travel of the apparatus, the apparatus defining a longitudinal axis extending between the leading and trailing ends;rotating a fluid driver of a propulsion assembly of the apparatus to draw fluid into an axial inlet of the propulsion assembly in an intake direction, the axial inlet provided at one of the leading and trailing ends;moving fluid through the propulsion assembly from the axial inlet to an outlet of the propulsion assembly, the outlet provided on the apparatus between the leading and trailing ends and laterally offset from the longitudinal axis; anddischarging fluid from the outlet in a discharge direction to generate a thrust 5 vector for propelling the apparatus in the intended direction of travel.

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