WO2001025083A1

WO2001025083A1 - Apparatus and method for positioning subaqueous articles

Info

Publication number: WO2001025083A1
Application number: PCT/GB2000/003723
Authority: WO
Inventors: Richard Walker; Andrew Gordon Walker; Nicholas Edward Ward
Original assignee: Richard Walker; Andrew Gordon Walker; Nicholas Edward Ward
Priority date: 1999-10-01
Filing date: 2000-09-29
Publication date: 2001-04-12
Also published as: JP2003511298A; GB2354751A; EP1216187A1; AU7536400A; GB9923127D0

Abstract

Known apparatus for aligning equipment lowered from the ocean surface comprises lifting tackle and guidelines attached to the equipment or supporting framework. The lifting tackle and the guidelines are used to control the position of the equipment. In relatively shallow waters divers use the guidelines to rotate the equipment into the correct orientation. In much deeper waters remote-operational vehicles (ROV) are used. Each ROV comprises a video feedback and one or more articulated arms that are remotely controlled from the surface. The ROV is used to position the equipment using the guidelines. It is very difficult to manoeuvre the ROV and control the orientation of the equipment. A thrust assembly (1) comprises two thrust units (2, 4) mounted on respective ends of a metal pipe (6) which is suspended in water by lifting rigging (8). The thrust units (2, 4) are each connected to a manifold box (10) by hydraulic supply and return tubes (12). The manifold box (10) is centrally mounted on the pipe (6). Extending from the manifold box (10) are hydraulic supply and return tubes (14). The distal end of the hydraulic supply tube (14) comprises a male coupling element that is received by a corresponding female coupling element of a remote-operational vehicle (ROV) (16). The ROV (16) is suspended in the water distant from the pipe (6) and the thrust units (2, 4).

Description

APPARATUS AND METHOD FOR POSITIONING SUBAQUEOUS

ARTICLES

The present invention relates to apparatus and a method for positioning subaqueous articles and is concerned particularly with apparatus and a method for positioning subaqueous articles using power supplied by a remotely operated vehicle.

Known apparatus for aligning equipment lowered from the ocean surface comprises lifting tackle and guidelines attached to the equipment or supporting framework. The lifting tackle and the guidelines are used to control the position of the equipment. In relatively shallow waters divers use the guidelines to rotate the equipment into the correct orientation. In much deeper waters remote-operational vehicles (ROV) are used. Each ROV comprises a video feedback and one or more articulated arms that are remotely controlled from the surface. The ROV is used to position the equipment using the guidelines. It is very difficult to manoeuvre the ROV and control the orientation of the equipment.

Known underwater work assemblies comprise a primary lift module and a secondary work module. Patent specification US 4010619 discloses such an assembly. The primary lift module and secondary ROV work module are permanently attached to each other by a cable tether. The tether supplies the secondary ROV work module with power. The primary lift module and the secondary ROV work module are united during ascent and descent. It can be difficult using the secondary ROV work module to manoeuvre and control the orientation of some equipment being lowered on a separate cable tether. According to a first aspect of the present invention there is provided a remotely controlled thrust assembly for a subaqueous article positioning system that comprises a remote subaqueous device, characterised in that the thrust assembly comprises thrust means and means for attaching the thrust assembly to an article, the arrangement being such that in use the thrust means is operable via the remote subaqueous device and the thrust assembly is remotely detachable from the article.

Preferably, the thrust assembly comprises a power supply line connectable to the remote subaqueous device, the arrangement being such that in use the remote subaqueous device supplies power to the thrust means .

The thrust means is preferably controlled via the remote subaqueous device.

A male coupling element and a female coupling element preferably forms a connection between the remote subaqueous device and the power supply line.

Preferably the power supply line extends from the thrust means, the distal end of the power supply line being receivable by the remote subaqueous device.

The distal end of the power supply line preferably comprises a male connection element and the remote subaqueous device preferably comprises a co-operative female connection element.

The remote subaqueous device is preferably a remote-operational vehicle (ROV) . In an embodiment of the present invention the thrust assembly comprises article-lifting means attachable to an article, the thrust means being fixed to the article-lifting means and the arrangement being such that in use the article-lifting means is remotely detachable from the article.

The article-lifting means preferably comprises a section upon which the thrust means is mounted.

It will be appreciated that the article-lifting means may comprise suitable load lifting rigging.

The section upon which the thrust means is mounted is preferably an adjustable planar section comprising attachment means for the thrust assembly.

Preferably the adjustable planar section comprises the attachment means disposed at either end thereof.

Alternatively, the planar section comprises two parts, the first part having the attachment means at one end thereof and the second part being free to articulate with respect to the first part.

In an embodiment of the present invention the thrust assembly is detachable from the article by direct manipulation of the attachment means.

In an alternative embodiment the thrust assembly is detachable from the article when a remote signal is received by the attachment means. Preferably, the remote-operational vehicle comprises an acoustic transmitter and the thrust assembly comprises an acoustic receiver, the arrangement being such that, in use, the thrust assembly is detached from the article when the acoustic transmitter sends a signal to the acoustic receiver.

Preferably, the thrust means is hydraulically powered.

Alternatively, the thrust means is electrically powered.

Preferably, the thrust means comprises at least one thrust unit comprising a propeller.

In an embodiment of the present invention the thrust assembly comprises a network of thrust means connected to the remote subaqueous device via a manifold or a junction box.

The thrust assembly preferably comprises hydraulic pump means.

The hydraulic pump means is preferably powered by hydraulic pressure from the remote subaqueous device.

In an embodiment of the present invention hydraulic pressure from the remote subaqueous device or electrical power from the remote subaqueous device preferably powers the hydraulic pump means .

Preferably, the thrust assembly comprises buoyancy means, the arrangement being such that in use when the thrust assembly is detached from an article the buoyancy means operates to move the thrust assembly in a direction towards the surface of the water. According to a second aspect of the present invention there is provided a method of positioning a subaqueous article using a remote subaqueous device and a thrust assembly according to the first aspect of the present invention, the method comprising attaching the thrust assembly to an article, lowering the article through a body of water, connecting the thrust assembly to the remote subaqueous device and operating the thrust assembly via the remote subaqueous device.

The method preferably comprises remotely detaching the thrust assembly from the article once the article is in the required position.

The method preferably comprises remotely detaching the thrust assembly from the remote subaqueous device once the article is in the required position.

The method preferably comprises retrieving the thrust assembly from the water.

According to a third aspect of the present invention there is provided a subaqueous article positioning system comprising a remote subaqueous device, a thrust assembly operable via the remove subaqueous device, means for attaching the thrust assembly to an article and means for lifting and lowering the article and thrust assembly.

The invention may include any combination of the features or limitations referred to herein.

The present invention may be carried into practice in various ways, but three embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a perspective view of a pair of thrust units mounted on a pipe suspended by lifting rigging and a subaqueous remote-operating vehicle,

Figure 2 shows a plan view of the of a pair of thrust units mounted on a piece of equipment suspended on lifting rigging and a subaqueous remote-operating vehicle as shown in Figure 1,

Figure 3 shows a plan view of a second embodiment of the present invention comprising two thrust unit assemblies connected to a subaqueous remote-operating vehicle and lifting rigging disposed above a piece of equipment,

Figure 4 shows a side view of the thrust unit assembly and lifting rigging of Figure 3 with the assemblies detached from the subaqueous remote-operating vehicle and disposed above the piece of equipment,

Figure 5 shows a perspective view of one of the thrust unit assemblies shown in Figure 4,

Figure 6 shows a side view a thrust unit connected to a central manifold box, and

Figure 7 shows a perspective view of a third embodiment of the present invention comprising of two pairs of thrust units mounted on respective rigging struts and a subaqueous remote-operating vehicle. Referring to Figure 1 , a first embodiment of the present invention is shown wherein a thrust assembly 1 comprises two thrust units 2,4 mounted on respective ends of a metal pipe 6 which is suspended in water by lifting rigging 8. The thrust units 2,4 are each connected to a manifold box 10 by hydraulic supply and return tubes 12. The manifold box 10 is centrally mounted on the pipe 6. Extending from the manifold box 10 are hydraulic supply and return tubes 14. The distal end of the a hydraulic supply tube 14 comprises a male coupling element that is received by a corresponding female coupling element of a remote- operational vehicle (ROV) 16. The ROV 16 is suspended in the water distant from the pipe 6 and the thrust units 2,4.

In an alternative embodiment of the invention the thrust units are powered by electrical means. The electrical power is supplied by the ROV via a suitable electrical supply line that is 'hot stabbed' into the ROV.

The ROV 16 comprises a tether cable 18, a video camera 20 and a robotic arm 22 with an articulating claw (not shown) at the end of the arm 22. An umbilical cable extends from a vessel on the surface of the water to an intermediate subaqueous frame that contains a junction box and a reel of the tether cable 18 extending therefrom. The umbilical cable 18 comprises power lines and communication lines.

The thrust units 2,4 and the manifold box 10 are mounted on the pipe 6 before the pipe is lowered through the water by the lifting rigging 8. Means used for mounting the thrust units 2,4 and the manifold box 10 onto the pipe may be electromagnetic means or mechanical means such as a releasable clamp assembly. The pipe 6 is lowered through the water until it reaches the seabed or a landing zone. The claw on the articulating arm 22 of the ROV 16 is used to grab the male coupling element of the supply line 14 and insert the male coupling into the corresponding female coupling element within the ROV 16. There is a watertight seal between the male and female connection elements (this is known as hot stabbing).

Hydraulic power is supplied to the thrust units 2,4 from the ROV 16. Hydraulic fluid passes through the supply line 14, the manifold box 10 and the respective tubes 12. The hydraulic supply line 14 comprises a small diameter supply tube and a large diameter return tube. The operator on the surface vessel uses the video camera 20 to view the orientation of the pipe 6. The operator uses a joystick (or other suitable switching/control mechanism) to send control signals to the ROV 16 in order to operate the respective thrust units 2,4.

A first degree of orientation of the pipe 6 is in the circular rotational directions 24 (either clockwise or anti-clockwise) . The pipe 6 rotates about the vertical axis of the lifting rigging 8. The first degree of orientation is achieved when the thrust units 2,4 drive in opposing directions. A second degree of orientation of the pipe 6 is in the arcuate direction 26. The second degree of orientation is achieved when the thrust units 2,4 drive in the same direction such that the pipe 6 swings on the lifting rigging 8. The curve of the arc 26 will depend upon the length of the lifting rigging 8 because this substantially defines the radius of the arc 26.

When the pipe 6 is correctly positioned the ROV 16 removes the male coupling of the supply line 14 from the female coupling of the ROV 16. The lifting rigging 8, the thrust units 2,4 and the manifold box 10 are then removed from the pipe 6 and are lifted back to the surface. The thrust units 2,4 and the manifold box 10 may be released from the pipe 6 by mechanical means, for example by the remote arm of ROV 16, or remotely by a signal sent from the operator. The signal may cause the opening of hydraulic locking mechanisms so releasing the thrust units 2,4 and manifold box 10 from the pipe 6.

Additional buoyancy is provided for the thrust units 2,4 and the manifold box 10 such that when released they float upwardly in a direction away from the positioned article. Once the trust units 2,4 and the manifold box 10 are distant from the pipe the lifting rigging 8 takes the thrust units 2,4 and the manifold box 10 to the surface where they can be mounted on a further section of pipe. The ROV 16 remains below the surface and can be used to provide power and control means to a second set of thrust units whilst the first set of thrust units are being mounted onto the further section of pipe.

Referring to Figure 2, the thrust units 2,4 and respective equipment described herein above are being used to position a large piece of subaqueous equipment 30. The thrust units 2,4 are mounted on the piece of equipment and the equipment 30 is being positioned within a pipe run 32.

Referring to Figures 3 to 5, a second embodiment of the present invention is shown. Some features of this embodiment are substantially the same as those described with reference to Figures 1 and 2 of the accompanying drawings and therefore similar reference numerals have been used to describe similar components and further description will not be given. However, with this particular embodiment a thrust assembly 41 comprises two thrust units 2,4 each attached to a rectangular mounting board 34. The length between end sections 36 of each mounting board 34 may be adjustable in order to fit pieces of equipment of various sizes. The two thrust units 2,4 are connected to each other by a hydraulic tube 33.

Each mounting board 34 comprises a central planar section 35 and two detachable clamping end sections 36. Each detachable clamping end section 36 is formed with a circular slot through which a lifting rigging cable 40 passes. A pair of bolts 42 clamps the end sections 36 onto the cable 40. Disposed on the cable 40 directly below each end section 36 are three tubular buoyancy sections 44. At the lowermost end of the respective buoyancy sections 44 is an attachment mechanism 46. The four attachment mechanisms 46 operate to attach the lifting rigging cable 40 to four lifting lugs 50 which extend from the respective four corners of the uppermost surface of the equipment 48 being positioned. The four attachment mechanisms 46 each comprise an acoustic receiver used to operate the mechanism 46.

In use the four attachment mechanisms 46 are fixed onto the four lugs 50 whilst the equipment is above the surface of the water. The equipment and thrust units 2,4 are then lowered through the water using the lifting rigging 8. Once at the correct depth an ROV 16 is remotely controlled to take the male coupling element of the supply line 14 and insert it into the corresponding female coupling element within the ROV 16. The thrust units 2,4 being powered by the ROV 16 are used to manoeuvre the equipment into the correct position. Once this has been achieved the operator will release the male coupling of the supply line from the female coupling by use of the ROV articulated arm 22. The mechanisms 46 are then released via a signal from the ROV 16. The ROV 16 transmits an acoustic signal 52 to the mechanisms 46. On receiving the acoustic signal 52 the mechanisms 46 disconnect from the lugs 50. After the mechanisms 46 have been disconnected from the lugs 50 the buoyancy 44 lifts the thrust units 2,4 away from the equipment 48. The tubular buoyancy sections 44 possess sufficient buoyancy to lift the mounting board 34 and thrust units 2,4 upwardly clear of the positioned article. The thrust units 2,4 are brought to the surface of the water by the lifting tackle where they are retrieved and used for a further piece of equipment.

Referring to Figure 5, the manifold box 10 may comprise a plurality of hydraulic supply and return sockets 60 used to supply a plurality of thrust units.

The thrust means can be alternatively powered by electrical means. The electrical power can be conveniently supplied via the ROV or alternatively by a separate power source.

In some situations it may be more convenient for the thrust units 2,4 to be located on the underside of the mounting board 34. Also, the mounting board 34 can comprise two pieces of tubular sections; a first section being partially disposed within a second section. The second section is attached to the lifting rigging and the first section is in reciprocating sliding contact with the second section.

Examples of the type of thrusters that can be utilised for the present invention are SEAEYE SM5 thrusters and Tecnadyne Model 2020 brushless thruster.

Examples of the type of hot stab connectors that can be utilised are the HYDROBOND ATL505/M-08P female connectors and the HYDROBOND ATL605/M-08S male connectors. Referring to Figure 7, a third embodiment of the present invention is shown. Some features of this embodiment are substantially the same as those previously herein described with reference to the accompanying drawings and therefore similar reference numerals have been used to describe similar components and further description will not be given.

However, with this particular embodiment a thrust assembly 70 comprises four thrust units 72,74, 76, 78. Thrust units 72,74 are attached to a mounting bar 80 towards the respective ends of the bar 80 and the thrust units 76,78 are attached to a mounting bar 82 towards the respective ends of the bar 82. Disposed at each end of respective bars 80, 82 there is an attachment mechanism 46 as herein before described.

At the distal end of lifting rigging 8 there is an annular ring 90. Coupled onto the ring 90 are two smaller rings 91,92. Two cables 40 are attached to ring 91 and two cables 40 are attached to ring 92. The cables 40 extend from the rings 91,92 in a direction towards the respective corners of a piece of equipment 48.

Disposed towards the lower end of each cable 40 and above each attachment mechanism 46 there is a tubular buoyancy section 44. The four attachment mechanisms 46 operate to attach the lifting rigging cables 40 to four lifting lugs 50 which extend from the respective four corners of the uppermost surface of the equipment 48 being positioned. The four attachment mechanisms 46 may each comprise an acoustic receiver used to operate the mechanism 46.

Suspended from the upper regions of the cables 40 is a hydraulic pump unit 51. Connected to the hydraulic pump unit 51 are four hydraulic supply and return tubes 12. The tubes 12 supply the thrust units 72, 74,

76, 78 with hydraulic pressure to power the respective propellers of the thrust units. Each tube 12 extends from the hydraulic pump unit 51 along one of the cables 40, through one of the buoyancy sections and into one of the thrust units. Each tube 12 is tethered to one of the cables 40 at various points along said tube 12. The hydraulic pump unit 51 also comprises a power supply cable 53 that extends from one side of the unit 51. The supply cable 53 comprises electrical power lines and hydraulic power lines. Disposed at the distal end of the cable 53 is a male connector.

An ROV 16 comprises a tether cable 18, a video camera (not shown), two horizontal thrusters 94, two vertical thrusters 96 and a robotic arm 22 with an articulating claw 98 at the end of the arm 22. The thrusters 94, 96 are used to manoeuvre the ROV 16 and hold the ROV 16 stationary in a flow of water current. An umbilical cable may extend from a vessel on the surface of the water to an intermediate subaqueous frame that contains a junction box and a reel of the tether cable 18 extending therefrom. The umbilical cable 18 comprises electrical power lines, hydraulic power lines and communication lines. In use the umbilical cable 18 supplies the ROV 16 with hydraulic power and electrical power.

In use the ROV 16 is lowered into the water and lowered through the water away from the surface of the water. The four attachment mechanisms 46 are fixed onto the four lugs 50 whilst the equipment 48 is above the surface of the water. The equipment 48 and the underwater positioning assembly 70 are then lowered through the water using the lifting rigging 8. Once at the equipment 48 and the underwater positioning assembly 70 are both at the correct depth the ROV 16 is remotely manoeuvred towards the equipment 48 and assembly 70. When the ROV 16 is close enough to the assembly 70 the robotic arm 22 is manipulated to take the free end of the male coupling element of the supply line 53 in the articulating claw 98. The male coupling is inserted into a corresponding female coupling element within the ROV 16 using the robotic arm 22. The ROV 16 can supply the hydraulic pump unit 51 with electrical power and hydraulic power via the supply line 53. Electrical or hydraulic power from the ROV 16 may be used to drive the hydraulic pump unit 51 , which in turn will supply hydraulic pressure to the respective thrust units 72, 74, 76, 78. The pump unit 51 comprises means to select and control the supply of power to the respective thrust units 72, 74, 76, 78.

The ROV 16 supplies the hydraulic pump unit 51 with power and control commands from the operator on the surface of the water. The control commands activate the respective thrust units 72,74, 76, 78 in order to manoeuvre the equipment 48 into the correct position. When the ROV 16 is subjected to a strong underwater current the horizontal thrusters 94 may use the majority of the hydraulic power. In this situation the ROV 16 can supply electrical power to operate the hydraulic pump unit 51.

In an alternative embodiment the thrust units 72, 74, 76, 78 could be powered by electrical means. The electrical power can be conveniently supplied via the ROV 16 or alternatively by a separate power source.

Once the equipment 48 is in the correct position the operator of the ROV 16 will release the male coupling of the supply line 53 from the female coupling by use of the articulated arm 22. The attachment mechanisms 46 are then released via a signal from the ROV 16. The ROV 16 may transmit an acoustic release signal 52 to the mechanisms 46. On receiving the acoustic signal 52 the mechanisms 46 disconnect from the lugs 50. In an alternative embodiment the articulated arm 22 and claw 98 may be used to mechanically disconnect the mechanisms 46 from the lugs 50.

After the mechanisms 46 have been disconnected from the lugs 50 the buoyancy sections 44 lift the thrust units 72,74, 76, 78 away from the equipment 48. The tubular buoyancy sections 44 possess sufficient buoyancy to lift the underwater positioning assembly 70 upwardly clear of the positioned equipment 46. The thrust units 72,74, 76, 78 are brought to the surface of the water by the lifting tackle where they are retrieved and used for a further piece of equipment.

Claims

1. A remotely controlled thrust assembly (1 , 41, 70) for a subaqueous article positioning system that comprises a remote subaqueous device (16) , characterised in that the thrust assembly (1 , 41, 70) comprises thrust means (2, 4) and means (46) for attaching the thrust assembly to an article (6, 30, 48), the arrangement being such that, in use, the thrust means (2, 4) is operable via the remote subaqueous device (16) and the thrust assembly (1 , 41, 70) is remotely detachable from the article (6, 30 48).

2. A remotely controlled thrust assembly (1 , 41, 70) as claimed in claim 1, wherein the thrust assembly (1, 41 , 70) comprises a power supply line (14, 53) connectable to the remote subaqueous device (16) , the arrangement being such that in use the remote subaqueous device (16) supplies power to the thrust means (2, 4) .

3. A remotely controlled thrust assembly (1, 41 , 70) as claimed in claim 1 or claim 2, wherein the thrust means (2, 4) is controllable via the remote subaqueous device (16) .

4. A remotely controlled thrust assembly (1 , 41 , 70) as claimed in claim 2 or claim 3, wherein a male coupling element and a female coupling element forms a connection between the remote subaqueous device (16) and the power supply line (14, 53) .

5. A remotely controlled thrust assembly (1 , 41 , 70) as claimed in any one of claims 2 to 4, wherein the power supply line (14, 53) extends from the thrust means (2, 4) , the distal end of the power supply line (14, 53) being receivable by the remote subaqueous device (16) .

6. A remotely controlled thrust assembly (1 , 41 , 70) as claimed in any one of claims 2 to 5, wherein the distal end of the power supply line (14, 53) comprises a male connection element and the remote subaqueous device (16) comprises a co-operative female connection element.

7. A remotely controlled thrust assembly (1, 41, 70) as claimed in any one of the previous claims, wherein the remote subaqueous device (16) is a remote-operational vehicle (ROV) .

8. A remotely controlled thrust assembly (1, 41 , 70) as claimed in any one of the previous claims, wherein the thrust assembly (1 , 41, 70) comprises article-lifting means (8, 40) attachable to an article (16, 48), the thrust means (2, 4) being fixed to the article-lifting means (8, 40), the arrangement being such that in use the article-lifting means (8, 40) is remotely detachable from the article (16, 48) .

9. A remotely controlled thrust assembly (1 , 41 , 70) as claimed in claim 8, wherein the article-lifting means (8, 40) comprises a section (35, 80, 82) upon which the thrust means (2, 4) is mounted.

10. A remotely controlled thrust assembly (1 , 41 , 70) as claimed in claim 9, wherein the section (35, 80, 82) upon which the thrust means (2, 4) is mounted is an adjustable planar section comprising attachment means (46) for the thrust assembly.

11. A remotely controlled thrust assembly (1 , 41 , 70) as claimed in claim 10, wherein the adjustable planar section comprises the attachment means (46) disposed at either end thereof.

12. A remotely controlled thrust assembly (1 , 41, 70) as claimed in claim 10, wherein the planar section comprises two parts, the first part having the attachment means (46) at one end thereof and the second part being free to articulate with respect to the first part.

13. A remotely controlled thrust assembly (1 , 41 , 70) as claimed in any one of the previous claims, wherein the thrust assembly(l , 41 , 70) is detachable from the article (16, 48) by direct manipulation of the attachment means (46) .

14. A remotely controlled thrust assembly (1 , 41, 70) as claimed in any one of claims 1 to 12, wherein the thrust assembly (1, 41, 70) is detachable from the article (16, 48) when a remote signal is received by the attachment means (46) .

15. A remotely controlled thrust assembly (1 , 41, 70) as claimed in any one of claims 7 to 14, wherein the remote-operational vehicle (16) comprises an acoustic transmitter and the thrust assembly (1 , 41 , 70) comprises an acoustic receiver, the arrangement being such that, in use, the thrust assembly (1, 41, 70) is detached from the article when the acoustic transmitter sends a signal to the acoustic receiver.

16. A remotely controlled thrust assembly (1 , 41, 70) as claimed in any one of the previous claims, wherein the thrust means (2, 4) is hydraulically powered.

17. A remotely controlled thrust assembly (1 , 41 , 70) as claimed in any one of claims 1 to 15, wherein the thrust means (2, 4) is electrically powered.

18. A remotely controlled thrust assembly (1 , 41, 70) as claimed in any one of the previous claims, wherein the thrust means (2, 4) comprises at least one thrust unit comprising a propeller.

19. A remotely controlled thrust assembly (1, 41, 70) as claimed in any one of the previous claims, wherein the thrust assembly (1 , 41, 70) comprises a network of thrust means (2, 4) connected to the remote subaqueous device via a manifold (10, 51) or a junction box.

20. A remotely controlled thrust assembly (1 , 41, 70) as claimed in any one of the previous claims, wherein the thrust assembly (1 , 41, 70) comprises hydraulic pump means (51).

21. A remotely controlled thrust assembly (1 , 41, 70) as claimed in claim 20, wherein the hydraulic pump means (51) is powered by hydraulic pressure from the remote subaqueous device (16) .

22. A remotely controlled thrust assembly (1 , 41, 70) as claimed in claim 20 or claim 21 , wherein hydraulic pressure from the remote subaqueous device (16) or electrical power from the remote subaqueous device (16) powers the hydraulic pump means (51) .

23. A remotely controlled thrust assembly (1 , 41, 70) as claimed in any one of the previous claims, wherein the thrust assembly (1 , 41 , 70) comprises buoyancy means (44), the arrangement being such that in use when the thrust assembly (1 , 41 , 70) is detached from an article (6, 30, 48) the buoyancy means (44) operates to move the thrust assembly (1 , 41 , 70) in a direction towards the surface of the water.

24. A method of positioning a subaqueous article using a remote subaqueous device (16) and a thrust assembly (1, 41, 70) according to the first aspect of the present invention, the method comprising attaching the thrust assembly (1, 41, 70) to an article (6, 30, 48) , lowering the article (6, 30, 48) through a body of water, connecting the thrust assembly (1, 41, 70) to the remote subaqueous device (16) and operating the thrust assembly (1, 41, 70) via the remote subaqueous device (16) .

25. A method of positioning a subaqueous article as claimed in claim 24, wherein the method comprises remotely detaching the thrust assembly (1, 41, 70) from the article (6, 30, 48) once the article (6, 30, 48) is in the required position.

26. A method of positioning a subaqueous article as claimed in claim 24 or claim 25, wherein the method comprises remotely detaching the thrust assembly (1, 41, 70) from the remote subaqueous device (16) once the article (6, 30, 48) is in the required position.

27. A method of positioning a subaqueous article as claimed in claim 26, wherein the method comprises retrieving the thrust assembly (1 , 41, 70) from the water.

28. A subaqueous article positioning system comprising a remote subaqueous device (16) , a thrust assembly (1 , 41 , 70) operable via the remove subaqueous device (16) , means for attaching the thrust assembly (1 , 41, 70) to an article (6, 30, 48) and means (8 40) for lifting and lowering the article (6, 30, 48) and thrust assembly (1 , 41 , 70) .