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. 3a-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.