Remotely operated vehicles (ROVs) are well known and are commonly of a type which are deployed by means of a deployment cage suspended from a surface vessel, the vehicle being tethered to the cage and the vehicle being of substantially neutral buoyancy such that slight altering of the buoyancy of the ROV determines its vertical position.

However, with such ROVs, the lifting capacity of the ROV is determined by the degree of positive buoyancy which the ROV can attain and, therefore, the lifting capacity is usually relatively low.

In US 4721055, there is disclosed an alternative remotely operated underwater vehicle which includes a body portion having a positive buoyancy and a clump weight connected to the body portion by a cable. In use, the body portion of the ROV is held in position relative to the seabed by the clump weight. When the ROV is deployed, the clump weight is located adjacent an underside surface of the body portion. Once the clump weight has reached the seabed, the cable is wound out so as to increase the distance between the clump weight and the positive buoyancy means in the ROV and thereby increase the moment of inertia of the body portion about the clump weight.

However, although the moment of inertia of the body portion about the clump weight is increased by this arrangement, when the body portion is lifting a heavy load there is also a tendency of the body portion itself to rotate about a substantially horizontal axis. In order to reduce this tendency and thereby compensate for unbalanced loads, the ROV includes hydraulic means for adjusting the position of the cable relative to the body portion of the ROV in fore and aft transverse directions. Such means are cumbersome and expensive, and the amount of load compensation which can be obtained by these means is limited.

The present invention seeks, among other things, to provide an ROV which overcomes the above mentioned disadvantage.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a stabilizing means for a submersible vehicle, the submersible vehicle including a clump weight and a body portion having positive buoyancy means, the clump weight being sufficiently heavy to overcome the buoyancy of the positive buoyancy means and thereby cause the submersible vehicle to sink when submerged, the clump weight being connected to the body portion by a connection means, characterised in that the connection means includes a substantially rigid portion extending, in use, beneath the body portion such that, a substantially horizontal axis about which the body portion may rotate is located adjacent an end of the substantially rigid portion spaced from the body portion.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a diagrammatic view of a prior art ROV in use; Figure 2 is a diagrammatic view of the prior art ROV shown in Figure 1 with the ROV lifting a heavy load in use; Figure 3 is a diagrammatic perspective view of an ROV in accordance with the present invention in use with the ROV in a contracted configuration; Figure 4 is a diagrammatic perspective view of the ROV shown in Figure 4 with the ROV in an expanded configuration; Figures 5a and 5b are diagrammatic side and front views respectively of the ROV shown in Figure 3 ; Figures 6a and 6b are diagrammatic side and front views respectively of the ROV shown in Figure 4 ; and Figure 7 is a diagrammatic side view of a portion of the ROV shown in Figures 3 to 6.

DESCRIPTION OF THE INVENTION Referring to Figures I and 2, there is shown a prior art ROV 10 in use adjacent a stack 12 of a deep sea drilling rig, the deep sea drilling rig being positioned on a sea bed 15. The ROV 10 includes a body portion 11 having positive buoyancy means 13 and is held in position relative to the sea bed 15 by a clump weight 14 to which the ROV 10 is attached by a cable 16. The cable 16 is connected to the body portion 11 at a winching means (not shown), and extends from the winching means through an entry point 34 at the base of the body portion 11. With this arrangement, it is possible to produce an ROV which has a lifting capacity of approximately 2 tonnes.

In use, the ROV 10 is lowered into position on an end of an umbilical cable 18 from a derrick 20 of a support vessel 22. Once the clump weight 14 reaches the sea bed 15, the umbilical cable 18 is wound out a little further to ensure that the ROV 10 is substantially decoupled from the heaving motion of the support vessel 22. As the weight of the umbilical cable 18 is not a significant factor, the overall structure of the umbilical cable 18, and the signal wires carried therein, can be made sufficiently strong to withstand the heaving motion of the surface vessel 22 without affecting the performance of the ROV 10. Once the ROV 10 is anchored to the sea bed 15 by the clump weight 14, the height of the ROV 10 relative to the sea bed 15 is adjusted by using the winching means to wind out the cable 16 until the desired height is achieved. Once the ROV 10 is at the desired height, a grabber arm 28 and manipulators 30 are controlled so as to perform the desired operation on the stack 12.

The prior art ROV 10 is shown during a lifting operation in Figure 2 with the grabber arm 28 engaged with and lifting an object 32. As shown, the weight of the object 32 engaged with the grabber arm 28 causes the body portion 11 to tend to rotate about a substantially horizontal axis passing through the entry point 34 of the cable 16 into the body portion 11. It will be appreciated, therefore, that with the prior art ROV shown in Figures 1 and 2, since the distance between the horizontal axis of rotation of the body portion 11 and the centroid of the buoyancy means 13 is relatively small the lifting capacity of the ROV 10 is relatively small and the range of applications to which the ROV may be applied is limited.

To reduce this tendency of the body portion 11 to rotate about a horizontal axis and thereby compensate for unbalanced loads, the body portion 11 includes hydraulic means (not shown) for adjusting the position of the cable 16 relative to the body portion 11 in fore and aft transverse directions. It will be appreciated, however, that such means are cumbersome and expensive and are limited in the amount of load compensation which can be obtained.

Referring to Figures 3 to 7, there is shown an ROV 40 in accordance with the present invention.

Operation of the ROV 40 is similar to operation of the prior art ROV 10 in that the ROV 40 is deployed in the same way by lowering the ROV 40 into position from a derrick of a support vessel or from a drill rig until its clump weight reaches the sea bed, and winding out the cable between the clump weight and the body portion of the ROV 40 until the ROV 40 is at the desired height relative to the sea bed.

Figures 3,5a and 5b show the ROV 40 in a contracted configuration and Figures 4,6a and 6b show the ROV 40 in an expanded configuration.

The ROV 40 is shown in Figures 3 and 4 in use adjacent a stack 42 of a deep sea drilling rig and in Figures Sa, 5b, 6a and 6b in more detail. The ROV 40 includes a clump weight 44 attached to a body portion 52 of the ROV 40 by a connection means including a cable 46. The ROV 40 is lowered into position on an end of an umbilical cable (not shown) extending from a support vessel or drill rig (not shown) to a coupling 48 on the body portion 52. The ROV 40 also includes positive buoyancy means in the form of flotation cells 54 which provide the ROV 40 with a positive buoyancy, rear 56 and side 57 thrusters for manoeuvering the ROV 40 in use, and manipulators 58 and a grabber arm (not shown) for carrying out operations on the stack 42. The buoyancy means may be formed of a high density, lightweight, partially cross linked, structural cellular material expanded according to a CFC free process.

A winch assembly and tube means of the ROV 40 are shown in more detail in Figure 7.

The winch assembly is provided with a winch 60 operable so as to wind the cable 46 in or out and thereby raise or lower the body portion 52 of the ROV 40 relative to the clump weight 44 and thereby the sea bed 50. The cable 46 passes from the winch 60 to the clump weight 44 through a pulley 62 which is rotatably held relative to the body portion 52 by any suitable means. The cable 46 also passes through a substantially rigid portion in the form of tube means 64 including a first tubular member 66 and a second tubular member 68 telescopically received in the first tubular member 66. The tube means 64 is slidably received in a mounting 70 attached to the body portion 52 of the ROV 40.

Preferably, the overlap between the first and second tubular members 66,68 and between the tube means 64 and the mounting 70 is at least 2.5 times the diameter of the second tubular member 68.

The tube means 64 also includes nylon rings which serve to facilitate smooth movement of the tube means 64 and restrict wear.

The arrangement is such that the tube means 64 is slidable relative to the mounting 70 such that, when the winch 60 operates so as to let out the cable 46, the body portion 52 moves away from the clump weight 44 and the tube means 64 moves from the contracted configuration as shown in Figures 5a and 5b wherein a substantial portion of the first 66 and second 68 tubular members are located inside the body portion 52 to the expanded configuration as shown in Figures 6a, 6b and 7 wherein a substantial proportion of the first 66 and second 68 tubular members are located outside of the body portion 52.

Likewise, when the winch 60 operates so as to wind in the cable 46, the body portion 52 moves towards the clump weight 44 and the tube means 64 moves from the expanded configuration as shown in Figures 6a, 6b and 7 to the contracted configuration as shown in Figures 5a and 5b.

The tube means 64 requires no hydraulic or electrical components to deploy or recover the tube means 64, the arrangement being such that the tube means 64 moves to the expanded configuration under its own weight when the cable 46 is wound out, and the tube means 64 is urged to move to the contracted configuration by the clump weight 44 as the cable 46 is wound in.

The tube means 64 may be formed of mild steel, stainless steel or aluminium.

It will be appreciated that when the tube means 64 is in the expanded configuration and the grabber arm and/or the manipulators 58 is/are engaged with a heavy object (not shown), the body portion 52 will tend to rotate about a substantially horizontal axis passing through the point of entry of the cable 46 into the second tubular member 68.

Therefore, in use, the distance between the horizontal axis about which the body portion 52 tends to rotate and the centroid of the buoyancy means 13 will be greater when the tube means 64 is in the expanded configuration than when the tube means 64 is in the contracted configuration and, accordingly, when the tube means 64 is in the expanded configuration the moment of inertia will be greater than when the tube means 64 is in the contracted configuration. The ROV 40 is also equipped with navigational equipment, fixed lights, movable lights and fixed and movable cameras (not shown). The ROV 40 may utilise an electrical/hydraulic system which is powered from the surface via the umbilical cable 48 to control operation of the ROV 40 in use. Additional hydraulic/electrical outlets may be provided on the ROV for facilitating attachment of additional tools as necessary.

Modifications and variations such as would be apparent to the skilled addressee are deemed to be within the scope of the present invention.