ASSEMBLY

Abstract

A transport assembly comprising: an attachment member attachable to a downhole tool; normally two rotatable members, connected to the attachment member, the rotatable members being arranged to rotate around their main longitudinal axis; a plurality of engaging members attached to each rotatable member and extending radially outwards therefrom with respect to the main longitudinal axis; the engaging members being helically arranged around the rotatable members. The transport assembly can thus be driven into a well in a ‘corkscrew’ type manner. Embodiments include a jarring mechanism which may be reset downhole and can jar attached tools in either direction.

Description

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to a transport assembly for use in deploying tools into a well and/or a jarring apparatus to aid passage of tools in a well; especially in non-vertical wells.

2. Description of the Prior Art

It is often desirable to support and move equipment such as tool strings in a well by using the assistance of some form of transport assembly, such as for example, a roller or wheel assembly.

In the case of a deviated wellbore where a longitudinal axis of the wellbore is non-vertical (and sometimes horizontal), gravity tends to force the string toward a lower wall of the wellbore, leading to high friction between the wall of the wellbore and the string. In such circumstances, it can be a particular challenge to move a string through the wellbore, and roller-based transport assemblies are of particular use.

Such transport assemblies have a “roller” that contacts the wellbore wall and provides support for the string to keep sections of the string off the wall easing resistance against the string. In addition, the roller may turn on engagement with the wellbore wall.

On occasion the string may become stuck in the well during transport and a jarring tool used to attempt to free the string. The jarring tool is activated to suddenly jolt the tool and the string to attempt to free it from the obstacle preventing the string from moving.

Whilst generally satisfactory the inventor of the present invention has noted that the traction assemblies and effectiveness of the jarring mechanisms may be improved.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a transport assembly comprising:

an attachment member attachable to a downhole tool;

a rotatable member, connected to the attachment member, the rotatable member being arranged to rotate around its main longitudinal axis;

a plurality of engaging members attached to the rotatable member and extending radially outwards therefrom with respect to the main longitudinal axis;

the engaging members being helically arranged around the rotatable member.

Helically arranged means at least two, optionally at least three, engaging members longitudinally and rotationally spaced from each other.

There may be two helically arranged series of engaging members attached to the rotatable member.

The engaging members may extend radially outwards by being attached to the surface of the rotatable member. In other embodiments they may be attached to the rotatable body via an arm linkage; the arm linkage and engaging members of such latter embodiments being referred to as a ‘hub assembly’.

The rotatable member may be a first rotatable member and there may be a second rotatable member; each rotatable member normally having said engaging members.

The helical arrangement of the engaging members defines a rotational pattern, and the rotational direction of the rotational pattern of the engaging members of the first and second rotatable members may be opposite each other—that is one is clockwise and the other is anti-clockwise.

The engaging members may be roller members and thus they may be arranged to rotate. Normally the engaging members rotate around an axis perpendicular to the axis of rotation of the rotatable member.

Alternatively the engaging members may be tracks.

The first and second rotatable members may be independently operable.

Rollers on the first and second rotatable members may be independently rotatable.

Thus normally the assembly comprises at least one motor to drive the rotatable members. The other rotatable member may be driven by a separate motor provided within the assembly or by power from outwith the transport assembly, such as by wireline from the surface.

The motor for the rotatable member(s) may be powered by a battery, optionally a rechargeable battery. The battery may be trickle charged.

For alternative embodiments the engaging members may be powered. Thus the roller members may drive the rotation of the rotatable members, normally by the roller members contacting an outer structure, such as a side of a borehole, and rotation of the roller members in contact with the structure will cause rotation of the rotatable member.

For embodiments comprising a hub assembly, that is arm linkages connecting the engaging members to the rotatable member, the arm linkage is normally pivotally mounted on the rotatable member and normally pivotally connected to the respective engaging member.

There may be a plurality of hub assemblies each comprising at least one arm linkage mounted on the rotatable member. For one particular embodiment there are four hub assemblies, optionally spaced around the circumference of the rotatable member. Optionally there may be more than four.

Thus the at least one engaging member can radially extend or retract with respect to the rotatable member, by a pivoting action of the arms. Nevertheless even in the retracted position at least a portion of the engaging member is still, to a lesser extent, radially extending with respect to the rotatable member. Thus even in the retracted position, the engaging members of preferred embodiments will tend to contact an outer structure, such as a borehole, before other elements of the assembly.

Optionally, one or each hub assembly has a second arm linkage, also normally pivotally mounted to the rotatable member and normally pivotally connected to the same engaging member of that hub assembly. Thus in such embodiments the engaging member is connected to the rotatable member by two arm linkages.

The two linkages may be connected to the rotatable member at axially spaced locations (that is spaced apart along the main longitudinal axis of the rotatable member), but at the same radial position. Thus a line defined by said spaced apart locations would be parallel to the main longitudinal axis of the rotatable member. Moreover the first and second arm linkages of the or each hub assembly may pivot in the same plane as each other and normally extend on the same side of the rotatable member.

The rotatable member may be provided as two rings which are independently axially movable with respect to each other. Normally, the first arm linkage of each hub assembly is mounted to the first ring of the rotatable member and the second arm portion of each hub assembly is mounted to the second ring of the rotatable member.

Where a plurality of hub assemblies are provided, each first arm portion of each assembly is normally attached to the same first ring and each second arm portion of each assembly is normally attached to the same second ring.

A proactive and/or reactive mechanism may be provided to move the engaging member from a relatively retracted radial position to an extended radial position. The proactive mechanism may be operated through hydraulic, electric or mechanical means.

A (proactive) mechanism may be provided to move the engaging member from a relatively retracted radial position to an extended radial position and back. This may be combined with a resilient element, for example between the engaging member or the arm portion and the rotatable member. Thus the proactive mechanism can be used to hold the engaging members in the retracted position when being deployed, as the engaging members may not always need to be extended at certain points in the trip. They can then be deployed as required.

Between the rotatable member and the attachment member, a reactive mechanism can comprise a resilient element mounted, optionally around an outer side of the rotatable member. The resilient element can be a spring. Before use, the tension of the spring can be adjusted by moving of a spring adjuster, which may be in the form of a tension support ring. Adjusting the spring tension may be done with a view to the diameter of the wellbore, e.g. if variances in the diameter are known to which the hub assemblies may adapt in their degree of extension. The spring adjuster may optionally be mounted next to each attachment member.

Thus the springs may be resiliently biased to extend towards the centre of the rotatable member, normally corresponding to an extended position of the engaging members. Thus the engaging members will normally extend in use to contact the wellbore and be compressed by an outside force (such as a narrower wellbore) which causes the arms to pivot and the rings to move axially against the action of the spring. When the outside force is removed, the compressed springs push axially on the rings, causing the engaging members to extend again.

At the opposite end of the spring from the rings, the attachment members normally provide a reaction force for the spring.

Embodiments of the present invention comprise a jarring mechanism, normally provided between two rotatable members.

Embodiments of the invention benefit in that they provide an integrated transport and jarring assembly.

Moreover embodiments of the invention benefit in that the jarring mechanism may be reloaded in use, even when the assembly is in a horizontal position in a horizontal borehole.

Typically the jarring mechanism can be activated by rotation of the rotatable members with respect to each other. Normally this is by rotating the rotatable members to compress or extend the jarring mechanism.

According to a second aspect of the present invention there is provided a method of operating a plurality of transport assemblies according to the first aspect of the invention, the method comprising activating the jarring mechanisms simultaneously.

According to a third aspect of the present invention there is provided a method of operating a plurality of transport assemblies according to the first aspect of the invention, the method comprising activating the jarring mechanisms sequentially.

Successive mechanisms may be activated within less than 10 seconds of each other, optionally less than 5 seconds of each other; to provide a wave effect to help dislodge the assembly.

The engaging members are typically configured to contact with and move in relation to a surface of a wellbore.

Normally the engaging members engage with the rotatable member so as to provide for relative rotation between the engaging members and the rotatable member.

Normally at least one retaining member is disposed between the engaging members and the rotatable member, and is operable to resist separation of the engaging members from the rotatable member.

The transport assembly may be being further configured such that, when the transport assembly is in use moving along a wellbore, the engaging members bear against the at least one retaining member and the at least one retaining member bears against the rotatable member.

The retaining member may be substantially spherical and arranged to both retain the engaging members on the rotatable member and reduce friction between the engaging members and the rotatable member.

Normally the engaging members are configured to rotate around a radial axis which is perpendicular to the main longitudinal axis of the rotatable member. This can reduce friction between the downhole tool and the surface of the wellbore resulting from movement of the downhole tool through the wellbore along the longitudinal axis of the wellbore.

In use, a weight of the transport assembly and a downhole tool to which the transport assembly is attached may be borne by the engaging members with the engaging members bearing against the at least one retaining member, which in turn bears against the rotatable member.

More specifically, the transport assembly may be configured such that the at least one retaining member provides for freedom of movement of the engaging members and the rotatable member in relation to each other.

More specifically, the at least one retaining member may be movable in relation to each of the engaging members and the rotatable member.

The retaining member is typically substantially spherical. Thus, the retaining member may be a ball-bearing.

The use of ball-bearings facilitates rotation of the engaging members when forces are imparted at an acute angle to an axis of rotation of the engaging members.

Alternatively or in addition, the engaging members and the rotatable member may define a space in which the at least one retaining member is held.

More specifically, one of the engaging members and the rotatable member may define a recess for receiving a part of the other of the engaging members and the rotatable member.

More specifically, the engaging members may define a part receiving recess for receiving a part of the rotatable member.

More specifically, the part receiving recess may extend in a direction substantially in line with and/or parallel to a surface over which the transport assembly moves when in use. The surface may be an outer cylindrical surface of the support member.

More specifically and where one of the engaging members and the rotatable member defines a part receiving recess, the retaining recesses may extend substantially radially of a direction in which the part receiving recess extends. Thus, separation of the engaging members and the rotatable member may be resisted.

Thus, the retaining recess may be formed in a side wall of the part receiving recess.

The rotatable member and/or the engaging members may comprise a slot, rim, flange, circumferential groove, track, bore, hole or the like, for defining the retaining recess defined in the rotatable member and/or the engaging members. The retaining recess of the rotatable member and/or the engaging members may define a space in which the retaining member is held and/or is occupied.

Alternatively or in addition, the transport assembly comprises a plurality of retaining members, the retaining members being spaced apart around the part of the rotatable member received in the engaging members.

Alternatively or in addition, the transport assembly may comprise an aperture configured to admit the at least one retaining member.

‘Boreholes’ detailed herein, unless described otherwise, may be cased or uncased boreholes.

Embodiments of the present invention may be used in open-hole as well as cased wells.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described, by way of example only, and with reference to the accompanying figures in which:

FIG. 1a is a side view of a transport assembly in accordance with the present invention;

FIG. 1b is a perspective view of the FIG. 1a transport assembly

FIG. 1c is a second side view of the FIG. 1a transport assembly with the traction mechanism at a different rotation setting compared to FIG. 1a;

FIG. 1d is a plan view of FIG. 1a transport assembly in a well;

FIG. 2 is a partially cut away perspective view of the FIG. 1a transport assembly in a well casing;

FIG. 3 is a perspective view to the FIG. 1a transport assembly;

FIG. 4 is a perspective view of a second embodiment of a transport assembly of FIG. 1a;

FIG. 5 is a sectional view of the FIG. 1a transport assembly;

FIG. 6 is an enlarged sectional view of section AD shown in FIG. 5;

FIG. 7 is a sectional view of the FIG. 4 transport assembly;

FIG. 8 is a perspective view of a second embodiment of a transport assembly;

FIG. 9 is an end view of the FIG. 8 embodiment;

FIG. 10 is cross-sectional drawing a roller apparatus in operation in a wellbore looking along a longitudinal axis of the wellbore; and,

FIG. 11 is a partial cross-sectional representation of a roller apparatus as shown in FIG. 10, but assembled with an alternative roller wheel.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of a transport assembly 100 the present invention is shown in FIGS. 1a-1d, 2, 3, 5 and 6 in the form of an elongate tool 100 comprising a first traction mechanism 10, a second traction mechanism 20 coupled by a hammer mechanism 30 and tubes 40a, 40b. A head 50 is provided at one end of the tool 100 whilst FIGS. 1b, 1d, and 2 illustratively show a well casing 90 through which the tool 100 is deployed. The tube 40b forms an attachment member for attachment to other downhole tools (not shown).

Each traction mechanism 10, 20 comprises a set of helically arranged roller apparatus 60. The direction of each resulting helical shape or ‘thread’ of each traction mechanism 10, 20 are opposite each other. In this embodiment the set of roller apparatus 60 of the first traction mechanism 10 are arranged in an anti-clockwise direction, whilst the set of helically arranged roller apparatus 60 of the second traction mechanism 20 are in a clockwise direction.

Clockwise and anti-clockwise directions as referred to herein are said directions when viewed from the head 50 end, or lower end in use when no head is present.

The traction mechanisms 10, 20 each comprise four pairs of roller apparatus 60. Each pair are disposed on the side of the tool 100 opposite each other. The neighboring pairs are spaced around the tool by 22.5 degrees such that the fourth pair are spaced 90 degrees around the tool 100 from the first pair.

As shown in FIG. 6, the traction mechanisms 10, 20 comprise a motor 12, gearbox 14 and clutch 16 in order to allow each to be rotatable independently of the other.

As a primary function the tool 100 may be used to deploy tools in a well casing 90, especially in a non-vertical deviated well, by attachment to the tool 100 to another well tool (not shown) and deployment in a well casing 90 by known means. The tool 100 is deployed into the well and the traction mechanisms 10, 20 are counter rotated with respect to each other, the roller apparatus 60 engage with the well and rotating the tool 100 and move the tool downwards in a “corkscrew” type manner. As the helical shapes of the traction mechanisms 10, 20 are opposite one another, they provide traction for the tool 100 to move into the well when the roller apparatus 60 contact the well casing 90, especially when the well is deviated from being vertical. The tools are thus deployed in the well.

The rotation of the traction mechanisms can each be reversed to move the tool 100 in the opposite direction. (The traction mechanisms 10, 20 counter-rotate in the opposite direction.)

In certain alternative embodiments each roller apparatus 60 may be powered.

As a secondary function the tool 100 functions as a jarring tool. When deploying well tools with the tool 100 they can become stuck in the well and difficult to move. Jarring or jolting the tool with a jarring tool is, in general, a known procedure. In the present case, the traction mechanisms 10, 20 are rotated in parallel in order to draw the respective mechanisms 10, 20 together until they spring apart with a hammer force. The tool 100 thus “jars” the tool 100 and any connected tools, in order to dislodge them, to attempt to move the tool onwards into or out of the well as required.

As well as jarring in a generally axial/longitudinal direction, the jarring mechanism also twists the tube 40b and attached tool string, largely by virtue of the helical arrangement of the roller apparatus 60 engaging with the well casing 90. This occurs without powering the roller apparatus 60, but the twisting can be emphasized by their powered rotation too for certain embodiments.

Thus a particular benefit of embodiments of the present invention is that they can more effectively dislodge a stuck tool because they not only jar the tool in a generally axial direction but also twist it rotationally.

Thus the jar assembly—which can be hydraulic or electrically controlled—produces a hammer like force in either direction by storing potential energy and releasing this energy rapidly.

Moreover if the tool is rotated in the opposite direction (still parallel) the tool will jar the opposite end of the tool. An advantage of such a mechanism is that it can be used to direct the jarring movement both in in-hole and up-hole directions and so towards the source of the problem.

The clutch 16 can disengage each traction mechanism from the respective motor so that it is free to rotate should it collide with the well or well casing during movement through a vertical or near-vertical section of the well.

FIG. 8 shows a perspective view of an alternative embodiment of the present invention and like parts will share common reference numerals except be preceded by a 1. In this embodiment, the assembly 101 comprises roller apparatus 160 a portion of which are spaced away from the traction mechanisms 110, 120 by arms 180, 182 for each roller mechanism 160. The arms 180, 182 are pivotally connected to a sleeve 111, 121 of the respective traction mechanisms 110, 120 and also to each roller apparatus 160. Moreover the sleeves 111, 121 are provided as two rings 111a, 111b and 121a, 121b axially movable with respect to each other.

Thus the roller apparatus 160 can adopt a retracted position, as shown for traction mechanism 110, or relatively extended configuration, as shown for traction mechanism 120. The optimum angle of the arms to the rotatable member in the most extended position is 45 degrees although this may be varied.

One advantage of such an embodiment is that the system can operate of boreholes of varying diameter, also shown in FIG. 8.

The jarring mechanism 130 and traction mechanisms 110, 120 otherwise work as described for the earlier embodiment.

The roller apparatus are may be the roller apparatus described in GB 2 450 532 which is incorporated in its entirety herein by reference. FIG. 10 shows one embodiment of a pair of roller apparatus 1000 in an assembled configuration located in a wellbore 201 (four pairs are used with preferred embodiments of the present invention). In this position, the apparatus 200 is connected at upper and lower ends to adjacent sections of a tubing string (not shown).

The roller apparatus 200 comprises a main body in the form of a tubular main body 202, which in turn is comprised of two body halves 206a,b. In FIG. 1, the roller apparatus 200 is shown as assembled with roller wheels 204 (forming a part of engaging members) connected to the main body 202.

Further, the apparatus 200 is positioned toward a lower side of the wellbore off the central axis 205, and an outer surface 207 of the roller wheels 204 are in contact with a wall of the wellbore providing stand off of the main body of the apparatus and adjacent tubing string sections from the wellbore wall for facilitating movement of the apparatus and string through the wellbore.

With further reference to FIG. 11, a roller apparatus 300 (separated from the wellbore), is shown comprising similar components as that of FIG. 10, except the roller wheels 304 are configured differently with a different outer surface profile. In this embodiment, components corresponding to those of FIG. 10 have the same reference numerals incremented by one hundred.

In FIG. 11, the internal structure of the apparatus 300 is shown. In particular, the apparatus is shown to comprise ball-bearings 308 (constituting at least one retaining member) located between an inner surface of the roller wheels 304, and an outer surface of a protruding support member 310 of the main body 302. The roller wheel 304 bears against the ball-bearings, which in turn bear against the support member 310. The ball-bearings 308 function to prevent or resist separation of the roller wheels 304 from the main body 302. In this embodiment, the ball bearings are received in a space 312 extending circumferentially around an outer surface of the support member 310, which has generally a cylindrical form. A number of ball bearings 308 are received in the space to spread the loads. The apparatus is provided a first retaining recess 314 formed in the inner surface of the roller wheel 304, and a second retaining recess 316 formed in the outer surface of the support member 310, which are aligned with each other defining the circumferential space 312 for the bearings.

The roller wheels 304 are connected to an outer end of the support member 310 via the ball-bearings in a close fitting relationship such that there is little more than a clearance gap between the inner surface of the roller wheel 304 and the outer surface of the support member 310. More specifically, the roller wheels 304 are provided with a recess 305 (constituting a part receiving recess) into which an end 311 of the support member 310 is received. Further, the ball-bearings are of a similar diameter to the circumferential space 312. In this configuration as shown in FIG. 11, the ball bearings 322 will therefore abut the walls of the retaining recesses so that the bearings function to prevent separation and detachment of the wheels by forces applied to the wheels, for example while the apparatus 300 is being run in the wellbore.

In addition to keeping the wheels 304 in position and coupled to the main body 302, the ball bearings 308 act to provide a low resistance coupling between the wheel and the support member 310 and allows for the wheels 304 to rotate with a degree of freedom about the support member 310, upon engagement with a wall of the wellbore.

The ball-bearings are inserted into the space 312 through an internal passageway 330 formed in the support member 310 and main body 302 of the downhole apparatus 300. An access aperture 332 providing access to the passageway 330 is provided in an internal wall of the tubular main body 302. This aperture can be seen in the embodiment of FIG. 10 at reference numeral 232.

The passageway 330 extends between the aperture 332 through the main body and the support member 310 and the space 312 for receiving the ball-bearings 308. A second outlet aperture 334 at the other end of the passageway 330 is provided in the retaining recess 316. Thus, the passageway 332 is used to insert and/or remove the ball bearings 322 from the space, for example if maintenance is required or to install or change out the roller wheels 304.

Once the balls have been inserted in the space 312 as described, and the apparatus 300 is assembled and ready for use as shown in FIGS. 10 and 11, an insert 340 (constituting a closing body) is located in the passageway 330 to fill the outlet aperture such that the ball bearings are not able to escape unwontedly from the space 312 and out through the passageway 330.

Thus an advantage of certain embodiments is that the tool the bidirectional hammer can be reset not just under gravity but also hydraulically (eg controlling hydraulic piston displacement), or electrically or mechanically (eg mechanical springs). Also, the hammer like force can be varied and/or increased as required to free tool sections.

Also the tool a string with any combination of multiple pairs of rotating units which can be controlled to achieve multiple critical objectives that allow the safe operating envelopes of the downhole equipment to be expanded to achieve ever increasing safe operating envelopes in more challenging well conditions (high deviated, horizontal or in an environment where tool sticking or wireline sticking is an issue):

a. Additional in-hole force or activating in hole hammer when tool below is stuck

b. Additional up-hole force or activating uphole hammer when tool below is stuck

c. compressive motion (eg freeing tools or resetting up-hole hammer system)

d. tensile motion (eg freeing tools or resetting in-hole hammer or activating up-hole hammer system

Thus embodiments of the invention allows downhole toolstrings to reach a target depth more reliably with minimal delay during oilfield operations. Similar benefits can also be achieved when recovering tools.

Moreover embodiments significantly reduce sticking risks of the downhole systems becoming stuck by reducing both the frequency and the associated consequence of equipment hanging up or getting stuck downhole.

Multiple tools in accordance with the present invention can be used and each can be fired in individual or in combination “bursts” to release a stuck tool string.

Thus an advantage of certain embodiments is that the tool allows the rotating wheel/devices to be controlled to via a clutch mechanism that will allow a full firing stroke action.

Improvements and modifications may be made without departing from the scope of the invention. For example the arms connecting the roller apparatus may be independently operable (or different lengths) in order to position the assembly off-centre within the borehole or for other reasons. Moreover the roller apparatus may be magnetic to aid connection of the roller apparatus with a metal lined borehole such as a casing.

Claims

1. A transport assembly comprising:

an attachment member attachable to a downhole tool;

a rotatable member, connected to the attachment member, the rotatable member being arranged to rotate around its main longitudinal axis;

a plurality of engaging members attached to the rotatable member and extending radially outwards therefrom with respect to the main longitudinal axis; the engaging members being helically arranged around the rotatable member.

2. A transport assembly as claimed in claim 1, wherein the rotatable member is a first rotatable member and there is a second rotatable member; the first and second rotatable members each having a helically arranged series of engaging members.

3. A transport assembly as claimed in claim 2, wherein the helical arrangements of the engaging members define rotational patterns having rotational directions, the directions being opposite each other, one being clockwise and the other being anti-clockwise.

4. A transport assembly as claimed in claim 2, wherein the first and second rotatable members are independently operable.

5. A transport assembly as claimed in claim 1, wherein the engaging members are roller members which are arranged to rotate.

6. A transport assembly as claimed in claim 5, wherein the roller members rotate around an axis perpendicular to the axis of rotation of the rotatable member.

7. A transport assembly as claimed in claim 5, wherein each roller member is independently rotatable.

8. A transport assembly as claimed in claim 1, comprising at least one motor to drive the rotatable member.

9. A transport assembly as claimed in claim 1, comprising a jarring mechanism.

10. A transport assembly as claimed in claim 9, wherein the jarring mechanism is provided between two rotatable members and is activated by rotation of the rotatable members with respect to each other.

11. A transport assembly as claimed in claim 1, wherein the engaging members are attached to the surface of the rotatable member.

12. A transport assembly as claimed in claim 1, wherein at least one engaging member is attached to the rotatable body via an arm linkage; the engaging member and arm linkage together forming a hub assembly.

13. A transport assembly as claimed in claim 12, wherein the arm linkage is pivotally mounted on the rotatable member and pivotally connected to one of the engaging members.

14. A transport assembly as claimed in claim 12, wherein there is a plurality of hub assemblies each comprising at least one engaging member and at least one arm linkage, the arm linkage mounted on the rotatable member.

15. A transport assembly as claimed in claim 12, wherein at least one hub assembly has a second arm linkage mounted to the rotatable member and connected to the same engaging member of that hub assembly.

16. A transport assembly as claimed in claim 12, wherein the first and second arm linkages of the or each hub assembly pivot in the same plane as each other and extend on the same side of the rotatable member.

17. A transport assembly as claimed in claim 12, wherein the rotatable member is provided as two rings which are independently axially movable with respect to each other.

18. A transport assembly as claimed in claim 17, wherein the first arm linkage of the or each hub assembly is mounted to the first ring of the rotatable member and the second arm portion of the or each hub assembly is mounted to the second ring of the rotatable member.

19. A transport assembly as claimed in claim 12, comprising at least one of a proactive and a reactive mechanism to move the engaging member from a relatively retracted radial position to an extended radial position.

20. A transport assembly as claimed in claim 12, wherein a resilient element is mounted between the rotatable member and the attachment member around an outer side of the rotatable member.

21. A transport assembly as claimed in claim 1, comprising at least one retaining member disposed between the engaging members and the rotatable member operable to resist separation of the engaging members from the rotatable member.

22. A transport assembly as claimed in claim 21, wherein the retaining member is substantially spherical and arranged to both retain the engaging members on the rotatable member and reduce friction between the engaging members and the rotatable member.