UNDERWATER REMOTELY OPERATED VEHICLE

Abstract: An underwater remotely operated vehicle (ROV) (100) has a buoyancy block (102) connected to a frame (120). The buoyancy block (102) has a density less than that of the water in which the vehicle is to operate, and the centre of buoyancy of the vehicle submerged in water is higher than the centre of gravity of the vehicle. The buoyancy block (102) can be moved relative to a main vehicle frame (103) by means of a hydraulic cylinder (122), in order to adjust the horizontal position of the centre of buoyancy relative to the centre of gravity.

Description: UNDERWATER REMOTELY OPERATED VEHICLE The present invention relates to underwater remotely operated vehicles and relates particularly, but not exclusively, to underwater earth moving machines for burying cables and/or other elongate products under the surface of the seabed. Remotely operated submarine vehicles, generally referred to as ROVs, are well known. These vehicles generally consist of a rigid block of buoyant material below which is mounted a boxlike frame to which several independently driven and controlled ducted propellers, known as thrusters, are attached. The vehicle is connected to a ship or platform by means of an umbilical cable through which power, information and control signals are passed. The most common version of such prior art machines is known as a work class ROV and is mainly used for manipulative tasks in connection with the offshore oil and gas industry. Such machines typically weigh between 1 and 3 tonne and have up to 50k of installed electric power, mainly used to drive a normal complement of eight thrusters. On powerful machines, the drive is achieved by means of hydraulic pumps and motors. A prior art swimming vehicle of this type is shown in Figures 1 and 2. The vehicle 1 has a buoyancy block 2 positioned on top of the vehicle and proportioned to provide just sufficient buoyancy to make the vehicle float. A frame 3 arranged below the buoyancy block 2 carries typically four reversible thrusters 4 arranged in a generally horizontal plane so as to thrust at an angle of 45° to the horizontal axis (i.e. the preferred direction of travel) of the machine. Two further reversible thrusters 5 at the front and two reversible thrusters 6 at the rear are arranged in a generally horizontal plane, above the horizontal plane in which the reversible thrusters 4 are arranged, and generate vertical thrust. This arrangement makes it possible to apply forces and couples to oppose any such forces or couples that may be applied to the vehicle by hydrodynamic drag due to currents or the vehicle's own velocity, or forces from any tool, such as a manipulator 7, that the vehicle may be operating and supported by the vehicle. The vehicle is connected to a ship (not shown) by means of an umbilical cable 8, which in strong currents is a major source of unwanted forces applied high up on the vehicle. The vehicle also occasionally rests on skids 9 on the seabed, which are provided to keep delicate parts of the vehicle out of mud on the seabed. The ROV is lifted out of the water by means of a lift rope attached at lift point 16 which is connected to the load carrying part of the frame 3 by through- lift members 17. There is an important requirement for such vehicles to be able to bury cables, pipes and other elongate products lying on the seabed. The vehicle achieves this by making a trench below the pipe or cable, usually using jets of water to break up and fluidise the soil at the seabed, so that the pipe or cable can sink or be pushed down into the trench. This type of ROV is generally constructed by attaching a jetting package underneath the machinery of a work class ROV, as shown in Figure 3. The jetting package comprises water pumps (not shown) and two jet legs 10 supporting rows of jets 11 which can be moved from a working position (shown in solid line in Figure 3) to a stored position 12 (shown in dotted line) when not required. The jet legs 10 straddle a cable 13 and make a trench full of fluidised soil. A depressor 14 then pushes the cable 13 down through this material to ensure proper burial. Machines of this type are relatively inexpensive to buy because they are closely based on work class ROVs, which are made in relatively large numbers . However, the prior art ROV shown in Figure 3 has serious disadvantages which become unacceptable when the jetting performed by the vehicle is required to bury the linear product deep in strong soil in a single pass at a reasonably high speed, a process that requires several hundred kilowatts of power. In such situations, there is a substantial force shown as R in Figure 3 which opposes the forward motion of the machine. Reaction force R is caused by the reaction of the jets and the drag caused by forcing the jet legs and depressor through fluidised soil in the trench. This force R has a vertical component V which must be opposed by the vertical thrusters 5, 6. Horizontal component H of force R is opposed by th'e horizontal thrusters 4. However, because these thrusters are raised vertically above the line of action of the forces in the ground by a distance a, there is a large moment Ha tipping the vehicle forward. This moment must be resisted by an upward force from the front vertical thrusters 5 and a downward force from the rear vertical thrusters 6. The vertical thrusters therefore consume a great deal of power without doing any useful work, and simply replace the effect of weight in the normal dry land vehicle. It is generally so difficult to bring this type of vehicle into equilibrium that resort is often made to jetting backwards from the bottom of the jet legs as shown at 15 on Figure 3. This is a very wasteful use of installed power, it being considerably more efficient to use an appropriately positioned thruster . Also, a horizontal component of force U, shown on Figure 3, is applied high up on the vehicle 1 by the umbilical cable 8 and is another source of large couples in both the longitudinal and transverse planes. Because the buoyancy and the weight of the vehicle are approximately equal and opposite, so that weight is not available to oppose these forces, the thrusters must be used. The prior art ROV behaves in an inconvenient way when working up and down, and across, steep slopes as shown in Figures 4 and 5. Instead of tilting with the slope, the vehicle only tilts part way until a couple Wb (approximately equal to Bb) places the force from the front of the skids (in the case of the vehicle travelling down a slope as shown in Figure 4), or from the outer skid, (in the case of the vehicle travelling across a slope as shown in Figure 5) . The skids can only be forced down onto the ground by using the vertical thrusters to overcome the couple Bb. Attempts have been made to overcome this problem by moving part of the weight of an ROV in order to alter the angle at which it floats. However, since it is undesirable to add weight to an ROV in the form of moveable ballast, this has been accomplished by moving a heavy component of the ROV, such as an electric motor or transformer. This suffers from the drawback that the distance that the component can be moved in practice is only of the order of 200 mm. On an ROV weighing 4000 kg, the heaviest component may weigh 200 kg, and the total moment achieved by such means is of the order of 40 kgm. The external couple Ha shown in Figure 3 in such a vehicle, however, is of the order of 600kgs x 1.5 m. i.e. 900 kgm. As a result, moving a heavy component of the vehicle can therefore generally only contribute about l/20 th of what is required. Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art. According to the present invention, there is provided an underwater remotely operated vehicle comprising: a vehicle body; at least one solid buoyancy member connected to the vehicle body and having a density less than that of the water in which the vehicle is to operate, wherein the centre of buoyancy of the vehicle in use is higher than the centre of gravity thereof; and moving means for moving at least one said buoyancy member relative to the vehicle body to adjust the horizontal position of said centre of buoyancy relative to said centre of gravity. By adjusting the position of the centre of buoyancy of the vehicle relative to its centre of gravity, any unbalanced couples in the pitch and roll planes produced by the displacement of the resultant force provided by the thrusters from the resultant force of all the external forces on the vehicle can be balanced out in a way that does not consume power. Furthermore, by moving one or more buoyancy members relative to the vehicle body, this provides the advantage that the position of the centre of buoyancy of the vehicle can be moved a greater distance relative to the centre of gravity than in the case of moving a heavy component of the vehicle relative to the vehicle body. For example, by moving all of the buoyancy (typically representing about half of the weight of the vehicle) , it is only necessary to move the buoyancy about 450mm in the case of an ROV weighing 4000kg, so that the total moment which can be generated is about 2000kg x 0.45m, i.e. 900kg.m. This is sufficient to totally counterbalance the moment Ha without using the front vertical thrusters 5 to lift up and the rear thrusters 6 to press down, consuming and wasting a significant quantity of power. The vehicle body may comprise a support portion passing through an aperture in at least one said buoyancy member. The moving means may comprise at least one linear actuator. At least one said linear actuator is preferably a hydraulic actuator. In a preferred embodiment, at least one said linear actuator is provided with indicator means for providing an output signal representing the position of the corresponding said buoyancy member relative to the vehicle body. The moving means may be adapted to adjust the position of the centre of buoyancy relative to the centre of gravity in two directions substantially perpendicular to each other. At least one said buoyancy member may be connected to said vehicle body by means of a plurality of rigid links, each said rigid link having a universal joint at each end thereof. Preferred embodiments of the invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:- Figure 1 is a schematic side elevation view of a prior art work class ROV; Figure 2 is a schematic plan view of the ROV of Figure 1; Figure 3 is a schematic side elevation view of the ROV of Figures 1 and 2 fitted with a jetting package; Figure 4 is a schematic side elevation view of the ROV of Figure * 3 descending a steep slope; Figure 5 is a schematic side elevation view of the ROV of Figure 3 working along a steep side slope; Figure 6 is a schematic side elevation view of an ROV of a first embodiment of the present invention; and Figure 7 is a perspective view of part of an ROV of a second embodiment of the present invention. Referring to Figure 6, an ROV 100 comprises a buoyancy block 102 around the bottom of which is a strong frame 120 capable of supporting the weight of the buoyancy block 102 when it is out of the water and the uplift forces when it is submerged. This frame 120 has four bearing blocks 121 strongly attached to it which can slide along top member 125 of main vehicle frame 103. The buoyancy block 102 is moved fore and aft relative to the ROV main frame 103 by means of a hydraulic cylinder 122, which contains a transducer that signals its length to an ROV control system (not shown) . By moving the buoyancy block 102 relative to the main frame 103 of the ROV, a couple can be applied in the plane of the drawing of Figure 6. This couple can replace the couple provided by the difference between the forces exerted by the front 5 and rear 6 thrusters of the prior art arrangement of Figure 3. It can also remove the necessity for the backward pointing jets 14 of Figure 3, and can save about one third of the total installed power of a typical jetting ROV. The power thus saved can be applied to the forward soil cutting jets and to the forward pointing thrusters. The movable buoyancy block 102 adds an additional adjustment to the jetting ROV. Without it there are four controls that the operator must adjust in an attempt to obtain optimum use of the installed power. These are speed of the jetting water pumps (and hence the jetting power expended on the jets which do the useful work of making the trench) , the forward thrust from the horizontal thrusters 4, a generally downward thrust from the vertical thrusters 5 and 6, and the required balancing couple obtaining by increasing the downward thrust of the rear 6 and decreasing that of the front 5 thrusters. In this situation, the buoyancy member 102 is moved forward until the thrust exerted by the front 5 and rear 6 thrusters becomes equal. If the vehicle moves onto a downward slope as shown in Figure 4, it is simply necessary to bring the buoyancy member 102 backwards until the fronts of the skids touch the ground. In order to overcome the problem shown in Figure 5 and to deal with the moment resulting from sideways forces through the umbilical cable 108, it is necessary to be able to move the buoyancy member 102 sideways as well as backwards and forwards as shown in Figure 6. This can be achieved by means of the embodiment shown in Figure 7, in which parts common to the embodiment of Figure 6 are denoted by like reference numerals but increased by 100. Referring to Figure 7, the strong subframe 220 of the movable buoyancy member 202 is connected to the main machinery frame 203 by three links 224, or by fo r if the frame is sufficiently flexible. The links 224 are provided with ball joints at each end. The buoyancy block 202 is shown in dotted lines in the Figure for the sake of clarity. The buoyancy frame 220 is fitted with four lugs 223, one at each corner, which connect to the tops of four suspension links 224 through ball joints and pins. These links 224 connect to similar support brackets 225 on the main frame 203 through similar ball joints and pins. This arrangement allows the buoyancy frame 220 to move forwards and backwards, and to rotate, relative to the main frame 203 in a horizontal plane that moves up and down slightly. This movement is controlled by three hydraulic cylinders 222, 226, each of which is connected at one end to the buoyancy frame 220 and at the other end to the main frame 203. Longitudinal cylinder 222 corresponds to that of Figure 6 and controls the forwards and backwards location of the buoyancy member 202. Two lateral cylinders 226 control the sideways location of the buoyancy member 202, and are normally kept the same length as each other. It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.

Claims: Cl aims 1. An underwater remotely operated vehicle comprising: a vehicle body; at least one solid buoyancy member connected to the vehicle body and having a density less than that of the water in which the vehicle is to operate, wherein the centre of buoyancy of the vehicle in use is higher than the centre of gravity thereof; and moving means for moving at least one said buoyancy member relative to the vehicle body to adjust the horizontal position of said centre of buoyancy relative to said centre of gravity. 2. A vehicle according to claim 1, wherein the vehicle body comprises a support portion passing through an aperture in at least one said buoyancy member. 3. A vehicle according to claim 1 or 2, wherein the moving means comprises at least one linear actuator. 4. A vehicle according to claim 3, wherein at least one said linear actuator is a hydraulic actuator. 5. A vehicle according to claim 3 or 4, wherein at least one said linear actuator is provided with indicator means for providing an output signal representing the position of the corresponding said buoyancy member relative to the vehicle body. 6. A vehicle according to any one of the preceding claims, wherein the moving means is adapted to adjust the position of the centre of buoyancy relative to the centre of gravity in two directions substantially perpendicular to each other. 7. A vehicle according to any one of the preceding claims, wherein at least one said buoyancy member is connected to said vehicle body by means of a plurality of rigid links, each said rigid link having a universal joint at each end thereof. 8. An underwater remotely operated vehicle substantially as hereinbefore described with reference to Figures 6 and 7 of the accompanying drawings .

Inventors: ATKINSON PAUL ROBERT, ATKINSON, PAUL, ROBERT
Applicants: ATKINSON PAUL ROBERT, SOIL MACHINE DYNAMICS LTD, ATKINSON, PAUL, ROBERT, SOIL MACHINE DYNAMICS LIMITED
Application year: 2001
Main classification: E02F5
IPC codes: B63C11, B63G8, E02F5