METHOD OF RECOVERING A DEPOSIT FROM THE SEA BED

Abstract

A method of recovering a deposit from the sea bed using a surface vessel (20) untethered from the sea bed and first and second suction vehicles (27; 30) which traverse the sea bed to suck up the deposit and are each connected to the surface vessel by a respective flexible riser (26; 29) along which a slurry of the deposit is transferred from the suction vehicles to the surface vessel. The method comprises moving the first and second suction vehicles to and fro across the sea bed in a plurality of lanes such that they travel substantially further than the surface vessel, turning or reversing each vehicle at the end of each lane in a manner such that the lanes are adjacent to one another to mine the deposit substantially without gaps between adjacent lanes. Only the suction vessels rest on the sea bed.

Description

The present invention relates to a method of recovering deposit from the sea bed.

A method for doing this is disclosed in WO 2010/000289. This involves a surface vessel and a suction vehicle which traverses the sea bed. The suction vehicle is attached to the surface vessel by a flexible riser along which a slurry of the deposit is transferred from the suction vehicle to the surface vessel.

In order to be economically viable, the production process has to be as efficient as possible. This means that the best possible production rate should be realised against the least possible cost. A number of factors affect the production rate such as the pumping capacity, and suction vehicle speed, while operational costs are largely determined by the energy cost.

The reach of the suction vehicles with respect to the surface vessel, (i.e. the horizontal distance they can travel away from the surface vessel) is determined by the length of the flexible riser. While a longer flexible riser provides a bigger reach, this also has drawbacks such as increasing the weight of the suction vehicle, and also providing increased resistance and inertia forces on the suction vehicle when travelling and manoeuvring.

The most straightforward way of recovering the deposit would be for the suction vehicle to simply follow the surface vessel by each travelling along a single lane. However, under these circumstances, the suction vehicle must travel at the same speed as the surface vessel. This is undesirable given the safety implications caused by dragging the riser system through the water at relatively high speeds. This could easily damage a suction vehicle should the suction vehicle fail.

A report for Pisces Environmental Services (Pty) Ltd for Benguela Current Large Marine Ecosystem Programme, entitled “Data Gathering and Gap Analysis for Assessment of Cumulative Effects of Marine Diamond Mining Activities on the BCLME Region” (Project BEHP/CEA/03/02), Chapter 4: Mining Methods, pages 165-168; Published. March 2008; available from www.bclme.org discloses the use of a seabed crawler for seabed mining. However, the vehicles are moved by an operator using a joystick to manoeuvre the vehicle. As such, it will not follow a well defined pattern.

According to the present invention, there is provided a method of recovering a deposit from the sea bed using a surface vessel and first and second suction vehicles which traverse the sea bed to suck up the deposit and are each connected to the surface vessel by a respective flexible riser along which a slurry of the deposit is transferred from the suction vehicles to the surface vessel, the method comprising moving the first and second suction vehicles to and fro across the sea bed in a plurality of lanes such that they travel substantially further than the surface vessel, turning or reversing each vehicle at the end of each lane in a manner such that the lanes are adjacent to one another to mine the deposit substantially without gaps between adjacent lanes.

In the present invention, there is no longer the need for the suction vehicles to match the speed of the surface vessel. Further, by mining the sea bed in adjacent lanes, an area of sea bed can be covered without leaving gaps, thereby improving the efficiency of the operation.

The use of more than one suction vehicle is beneficial in that it provides continuity of operation. A spare suction vehicle may be kept on the deck of the surface vessel and, if one suction vehicle fails, it can be replaced with a spare vehicle while the other vehicle or vehicles continue to operate. The recovered vehicle can then be repaired while normal operation is underway. Although described with two such vehicles, the present invention is also applicable to two or more vehicles.

FIG. 1A illustrates a suction vehicle 1 which has a mouth at one end. This travels along a first lane 2 before turning through 180° at turning circle 3 and returning along a second lane 4. The suction vehicle 1 makes a further pass, but this time travelling along a third lane 5 to a second turning circle 6. As can be seen in FIG. 1A, there is a significant overlap 7 between the two turning circles 3, 6 at which time operation of the suction vehicle is not optimal. FIG. 1B shows a second vehicle 8 which has a suction mouth at either end. Instead of rotating through 180°, the vehicle 8, once it reaches the end of a first lane 9, 10 is then simply reversed into an adjacent lane 11, 12 while still facing in the same direction. As will be apparent from FIG. 1B, regions 13, 14 are then missed by the suction vehicles again, resulting in an inefficient suction operation. As a result of this, to maximise the efficiency of the recovery, the number of turns in the mining pattern should be minimised.

Therefore, preferably the method further comprises moving the first suction vehicle across a first grid, with the grid being covered by the vehicle moving in a first lane in a longitudinal direction from a first end of the grid to a second end, making a small lateral displacement before returning longitudinally to the first end along a lane adjacent to the first lane, and repeatedly moving in this way between the two ends, each time along a lane laterally adjacent to the previous lane until the first grid is completed; simultaneously moving the second suction vehicle in the same manner in a second grid laterally adjacent to the first grid, such that the lateral spacing between the first and second suction vehicles remains substantially constant throughout; and once the first and second grids are completed, moving the first and second suction vehicles to third and fourth grids longitudinally adjacent to the first and second grids respectively and covering these in the same manner as the first and second grids.

This provides an efficient mining pattern in that the first and second suction vehicles are able to be kept apart by substantially constant distance thereby eliminating the possibility of collision and entanglement.

Secondly, because the suction vehicles move longitudinally at first and second grids which are laterally adjacent to one another, there can be optimal use of the length of the riser resulting in longer lanes thereby improving the efficiency as compared to a mining pattern in which suction vehicles move in a lateral direction.

The surface vessel may simply move slowly in the longitudinal direction thereby keeping pace with the suction vehicles as they move the third and fourth grids and move progressively onto further grids, However, preferably, the method further comprises moving the surface vessel in a lateral direction while the surface vehicles are moving in first and second grids, and moving the surface vessel in a longitudinal direction as the surface vehicles move to the third and fourth grids. By moving the surface vessel laterally as the suction vehicles traverse the grids, the width of the grid can be increased for any given length of riser. This effectively allows the use of a shorter riser. As the surface vehicle moves in the longitudinal direction as the suction vehicles move to the third and fourth grids, it will generally only need to move at approximately half speed of the suction vehicles such that the vehicles have to travel from one end of one grid to the opposite end of the second grid, effectively travelling the length of two grids, while the surface vessel travels generally from the mid-point of one grid to the mid-point of the adjacent grid which is approximately the length of one grid.

When reaching the end of the first and second grids, the suction vehicles may be moved laterally across the full width of the grid such that they then begin covering the next grid in the same lateral direction in which they covered the previous grid. However, preferably, the small lateral displacement at the first and second grids takes place in the opposite lateral direction from the respective third and fourth grids. Essentially, this means that each longitudinally adjacent grid is covered in the opposite lateral direction to the previous grid. This provides a more efficient mining pattern and requires less lateral movement of the surface vessel.

For a second preferred pattern, the method preferably comprises moving the surface vessel in a longitudinal direction and moving the first and second suction vehicles in a lateral direction. With such a pattern, the surface vessel can travel along a much simpler path as the suction vehicles traverse in a lateral direction. As such, such a pattern can be employed if more complex control of the surface vessel is considered undesirable.

With such a pattern, the suction vessel can further comprise moving each of the suction vehicles one on each side of the surface vessel. Alternatively, the method may further comprise moving each of the suction vehicles on both sides of the surface vessel. The former has the advantage that the two suctions vehicles each move in their own space, while the latter provides longer lanes and hence reduces the number of turns.

For a third preferred pattern, the method preferably further comprises the steps of moving the first and second suction vehicles in an arcuate path substantially centred on the point where the flexible riser attaches to the surface vessel. This has the benefit that the “reach” of each riser remains substantially constant and also allows for longer lanes than the lateral arrangement above.

Again, the method may further comprise moving each of the suction vehicles on each side of the surface vessel, or moving each of the suction vessels on both sides of the surface vessel.

Each suction vehicle may have a mouth at each end, in which case vehicles are moved between the ends of the grid while facing substantially the same direction at all times. However, preferably, the method further comprises turning each suction vehicle through substantially 180° each time it reaches the end of a lane prior to returning to the opposite end. This method can be used with a vehicle with a single suction mouth which can have a simpler, cheaper and lighter design.

Examples of methods in accordance with the present invention will now be described with reference to the accompanying drawings, in which:

FIGS. 1A and 1B are schematic plan views showing the problems encountered at the lane ends;

FIG. 2 is a schematic cross-section of the recovering system;

FIGS. 3 a-3d are schematic plan views showing the stages of a first mining pattern;

FIG. 4 is a schematic plan view of a second mining pattern;

FIG. 5 is a schematic plan view of a variation of the second mining pattern; and

FIG. 6 is a schematic plan view of a third mining. pattern.

The overall system is shown in FIG. 2 which comprises a surface vessel 20 equipped with a pumping system 21 and a slurry processing system 22. The riser system comprises a rigid riser bundle 23 supported in part by a flotation tank 24 and flexible ducts 25 leading to the surface vessel. A first flexible riser 26 leads from the rigid riser bundle 23 to a first suction vehicle 27 which travels across the sea bed 28 while a second flexible riser 29 leads to a second suction vehicle 30. The surface vessel has a dynamic positioning system that enables it to stay in position and follow a predetermined track.

In use, the suction vehicles traverse the sea bed sucking up surface deposits which are then pumped along the flexible risers 26, 29, through the rigid riser bundle 23 and the flexible ducts 25 to the surface vessel where they are processed by a treatment plant 22 before being transported ashore for further processing. Waste water is then pumped by the pump system 21 down the riser system for disposal adjacent to the sea bed. This application is concerned with the nature of the mining pattern and no further details will be provided here. For details of the riser bundle 23, reference is made to pending application (agent's ref. 113711GB00) and the suction vehicles 27, 30 are described in the application (agent's ref 113709GB00). The overall system is also described in general terms in WO 2010/000289.

In FIG. 2, the lateral direction is depicted by arrow 31, while the longitudinal direction is the direction into and out of the plane of the page. This is depicted in FIG. 3, the arrow 32.

The mining pattern will now be described with reference to FIG. 3. Referring first to FIG. 3d, the mining pattern is divided into a first grid 33, second grid 34, a third grid 35, fourth grid 36. First and second grids 33, 34 are laterally adjacent to one another and are covered simultaneously by the first 27 and second 30 suction vehicles respectively. The third and fourth grids 35, 36 are laterally adjacent to one another and longitudinally adjacent to the first 33 and second 34 grids respectively. The third and fourth grids 35, 36 are covered simultaneously subsequently to the coverage of first 33 and second 34 grids as described below in greater detail.

Although the description below relates primarily to the formation of four grids, it will be readily understood that the process is intended to be continuously repeated over and over again until the desired area is completed. Typically, each lane will be 10 to 15 metres in width with clearance of approximately 2 metres between adjacent lanes. Each grid will be hundreds of metres in length and width. Before the recovery operation commences, the sea bed is the subject of a survey allowing an optimal course to be plotted which provides as many straight runs as possible. This will provide optimum coverage while avoiding the large scale sea bed features. The two vehicles will be equipped with sensors as described in the above mentioned co-pending application (agent's ref. 113709GB00) so that small-scale obstructions can be avoided by the suction vehicles. In these circumstances, the vehicles will follow a course around the obstructions and then return to the lanes described below as soon as possible.

When the surface vehicle 20 reaches the end of a long run, it will turn in accordance with the large-scale mining pattern determined from the survey and begin again on a new course using the mining pattern described below. This course may well be a course adjacent to the one that it has just covered, or may be in an entirely different direction.

Thus, the description below covers the basic building blocks of a mining pattern which is repeated over and over again to cover potentially thousands of square kilometres of sea bed.

First 27 and second 30 suction vehicles initially begin to recover the deposit from the first 33 and second 34 grids by moving along a first longitudinal path 37. When they reach the end of the grid they either turn through a 180° as shown in FIG. 1A, or simply reverse their direction moving to an adjacent lane as shown in FIG. 1B if a suction vehicle has a suction mouth at either end. This process of turning or reversing at each end of the grid to operate in an adjacent lane is repeated until the first and second grids are covered (just after the position shown in FIG. 3b).

There is no need for the two vehicles to be in exactly the same position as one another along their lane at any one time. One vehicle could, for example, be travelling in one direction, while the other travels in the opposite direction. The important thing is that, as the lanes are essentially in the same direction, the vehicles generally move in parallel lanes, so that they do not generally approach one another as they move.

Initially, the first vehicle 27 is at its maximum reach (i.e. it is horizontally as far from the surface vehicle as the riser 26 will allow), while the second vehicle 30 is at its minimum reach. As the recovery process proceeds, the suction vehicle 27 will get closer to the surface vessel, while the second suction vessel will get further away until they reach the position shown in FIG. 2 where the second suction vehicle 30 moves towards its maximum reach. At that time, the surface vessel 20 will make a lateral step (as depicted by line 38 in FIG. 3d) to allow the suction vehicles to complete the first and second grids 33, 34. The suction vehicles reach the first end of the last lane of the first and second grids (shown in FIG. 3b). The surface vessel 20 turns to move longitudinally, its path being represented by path 39 in FIG. 3d. The turn will not be as abrupt as shown in FIG. 3d, and will, in practice, be a smooth curve. Any deviation from this idealised path shown in FIG. 3d is compensated for in the flexibility of the risers. As the surface vessel 20 travels along this path, the two suction vehicles 27, 30 complete the final lane of the first and second grids 33, 34 and then continue longitudinally along paths 40 to form the first lane of the third 35 and fourth 36 grids. In this way, the surface vessel 20 can travel at approximately half the speed of the suction vehicles 27, 30.

The suction vehicles 27, 30 then proceed to cover the third 35 and fourth 36 grids in the same way described above in relation to the first 33 and second 34 grids, with the exception that the direction of lateral movement is opposite the direction of lateral movement of the first 33 and second 34 grids, namely right to left as shown in FIG. 3. At this time, the surface vehicle 20 will also move laterally in the same manner as described above with the lateral movement being depicted by the line 41 in FIG. 3d.

As shown in FIG. 3d, at the end of the third 35 and fourth 36 grids, the suction vehicles 27, 30 move longitudinally forward and begin to cover subsequent grids exactly the same way that they covered the first and second grids as described with reference to FIGS. 3a and 3b.

A second example of a mining pattern is shown in FIG. 4. In this case, the surface vessel 20 moves in a longitudinal direction. The first suction vehicle 27 moves to and fro in a transverse direction on one side of the surface vessel 20, while the second suction vehicle 30 does the same on the opposite side of the surface vessel 30. The suction vehicles 27, 30 turn at the end of the lanes as shown in FIG. 1A. The surface vessel 20 moves essentially in a straight line, while the two surface suction vessels move in the maximum extent allowed by their risers 26, 29.

FIG. 4 shows the suction vehicles operating such that they approach one another at a position close to the surface vessel. Alternatively, they could operate with a substantially constant separation, such that as one approaches the surface vessel, the other is at its furthest position from the surface vessel. This avoids any problems with entanglement close to the surface vessel and means that the patterns covered by the two suction vessels can be closer together.

A modification of this pattern is shown in FIG. 5. Again, the surface vessel 20 moves in a longitudinal direction, while the suction vehicles 27, 30 move laterally. In this case, both suction vehicles travel in much longer lanes which extend on both lateral sides of the surface vessel. As will be appreciated from FIG. 5, the increased length of the lanes reduces the number of turns and the associated inefficiency. On the other hand, more careful control of the position of the suction vehicles is required to avoid collision and entanglement of their flexible risers 26, 29 and umbilicals 50, 51.

A third pattern is shown in FIG. 6. This is similar to the pattern of FIG. 4, except that the suction vessels 27, 30 follow an arcuate path generally centred about the point 52 about which the risers 26, 29 are attached to the surface vessel 20 in plan view. In practice, this will actually be the point at which the risers 26, 29 are attached to the bottom of the rigid riser bundle 23 as shown in FIG. 2 as the rigid riser bundle 23 will be directly beneath the point 52 or slightly behind it owing to the drag on the bundle 23.

When the suction vehicles 27, 30 reach the far end of the lane at which point they are moving in the same longitudinal direction as the surface vessel 20, the surface vessel 20 will make a small longitudinal step (of approximately 10 metres). The suction vehicles 27, 30 will then make the return journey. At the point where they are at the opposite end of their lanes, such that they are travelling perpendicular to the direction of movement of the surface vessel 20, the surface vessel 20 again makes a small longitudinal step while the suction vehicles turn as shown in FIG. 1A to move onto the next path. As depicted in FIG. 6, the vehicles are shown reversing at one end of the lane when they are furthest apart and turning at the end of the path when they are closest to one another. It is equally possible to carry out this pattern with the vessels turning at both ends or even reversing at both ends. The arcuate lanes shown in FIG. 6 could also be achieved in a similar manner to FIG. 5, such that each suction vessel 27, 30 effectively follows a semi-circular path on both sides of the surface vessel 20. Again, this is a trade-off between the reduced number of turns and the enhanced control required to avoid collision.

Claims

1. A method of recovering a deposit from the sea bed using a surface vessel untethered from the sea bed and first and second suction vehicles which traverse the sea bed to suck up the deposit and are each connected to the surface vessel by a respective flexible riser along which a slurry of the deposit is transferred from the suction vehicles to the surface vessel, the method comprising: moving the first and second suction vehicles to and fro across the sea bed in a plurality of lanes such that they travel substantially further than the surface vessel, turning or reversing each vehicle at the end of each lane in a manner such that the lanes are adjacent to one another to mine the deposit substantially without gaps between adjacent lanes,wherein only the suction vessels rest on the sea bed.

2. The method of claim 1, further comprising moving the first suction vehicle across a first grid, with the first grid being covered by the first suction vehicle moving in a first lane in a longitudinal direction from a first end of the first grid to a second end, making a small lateral displacement before returning longitudinally to the first end along a lane adjacent to the first lane, and repeatedly moving in this way between the two ends, each time along a lane laterally adjacent to the previous lane until the first grid is completed; simultaneously moving the second suction vehicle in the same manner in a second grid laterally adjacent to the first grid, such that the lateral spacing between the first and second suction vehicles remains substantially constant throughout; andonce the first and second grids are completed, moving the first and second suction vehicles to third and fourth grids longitudinally adjacent to the first and second grids respectively and covering these in the same manner as the first and second grids.

3. The method according to of claim 2, further comprising moving the surface vessel in a lateral direction while the first and second suction vehicles are moving in the first and second grids, and moving the surface vessel in a longitudinal direction as the first and second suction vehicles move to the third and fourth grids.

4. The method of claim 1, wherein the small lateral displacement in the first and second grids takes place in the opposite lateral direction from the respective third and fourth grids.

5. The method of claim 1, further comprising moving the surface vessel in a longitudinal direction and moving the first and second suction vehicles in a lateral direction.

6. The method of claim 5, further comprising moving each of the first and second suction vehicles one on each side of the surface vessel.

7. The method of claim 5, further comprising moving each of the first and second suction vehicles on both sides of the surface vessel.

8. The method of claim 1, further comprising the steps of moving the first and second suction vehicles in an arcuate path substantially centred on the point where the respective flexible riser attaches to the surface vessel.

9. The method of claim 1, further comprising moving each of the first and second suction vehicles one on each side of the surface vessel.

10. The method of claim 8, further comprising moving each of the first and second suction vehicles on both sides of the surface vessel.

11. The method of claim 1, further comprising turning each one of the first and second suction vehicles through substantially 180° when it reaches the end of lane.

12. The method of claim 1, wherein each of the first and second suction vehicles has a mouth at each end and the method further comprises reversing each one of the first and second vessels when it reaches the end of the lane.