WO1999007949A1

WO1999007949A1 - Underwater mining apparatus and method

Info

Publication number: WO1999007949A1
Application number: PCT/GB1998/002403
Authority: WO
Inventors: Charles Francis Heyes; Johannes Duijzers; Ivor John Jones; Peter Looijen
Original assignee: Namibian Minerals Corporation Limited
Priority date: 1997-08-08
Filing date: 1998-08-10
Publication date: 1999-02-18
Also published as: AU8738498A; EP1000202A1

Abstract

An underwater mining vehicle (28) is provided with a suction boom (22) carrying a tooth cutter (9) at its forward tip and having an array of nozzles (20) disposed about its periphery. In use, the boom is oriented vertically and applies a downward force of 5 to 20 tones (49 to 196 kN) on a layer of sandstone drape covering a diamondiferous gravel layer. This breaks the drape and enables the mouth of the suction boom to access the diamondiferous gravel. The two lower/rearward nozzles (20) generate high velocity jets which form a fluidised region in the gravel and suspend particles up to 200 mm in diameter which are then sucked up in suspension via the suction boom and lifted to the surface where diamonds are extracted.

Description

UNDERWATER MINING APPARATUS AND METHOD

The present invention relates to a method of extracting mineral bearing (in particular diamondiferous) material at a submerged location, to underwater mining apparatus for extracting particulate material (particularly but not exclusively diamondiferous material), and to a method of operating underwater mining apparatus. Reference is made herein to mineral bearing material. This may be diamondiferous material, as an example, but could also for example be material bearing any precious metal (such as gold and in particular alluvial gold) or precious stones. One aim of the present invention is to provide a method and apparatus for extracting mineral bearing material from beneath a covering layer of non-mineral- bearing material at a submerged location.

In one aspect, the invention provides a method of extracting mineral bearing material from beneath a covering layer of non-mineral-bearing material at a submerged location, the method comprising the steps of breaking through the covering layer by means of a downwardly-acting breaking tool carried by a submerged vessel, forming a suspension of particulate mineral bearing material and lifting the suspension to the surface.

Preferably the mineral bearing material is diamondiferous material and the non-mineral-bearing material is non-diamondiferous material.

Preferably, the suspension is formed as a fluidised region by at least one water jet. Preferably, the fluidised region is formed by two water jets.

Preferably, the submerged vessel is a vehicle supported on the covering layer. Preferably, the vehicle is a tracked vehicle. Preferably, the vessel moves the breaking tool substantially horizontally whilst the breaking tool projects into the covering layer, and thereby enlarges an opening in the covering layer. Preferably, the breaking tool comprises a boom-mounted cutting member with a serrated cutting surface and the serrated cutting surface is moved across the upper surface of the covering layer to enlarge the opening. Preferably, the cutting member has an elongate cutting surface with transverse serrations and has a generally convex profile in side view. Preferably, the cutting member extends across the mouth of a suction boom.

Preferably, the downwardly-acting breaking tool applies a (substantially vertical) downward force to penetrate the covering layer. Preferably, the downward component of force applied by the breaking tool is at least 3 tonnes (27.4 kN), more preferably at least 5 tonnes (49 kN) and most preferably in the range 5 to 20 tonnes

(49 to 196 kN).

A force of 5 tonnes is typically sufficient to break through a covering layer of sandstone of several hundred mm thickness, which commonly covers diamondiferous gravel, for example off the western coast of Southern Africa.

Preferably, said drive means is arranged to exert a force having a downward component of at least 3 tonnes (29.4 kN). Preferably, said drive means is arranged to exert a force having a downward component of at least of 5 tonnes (49 kN).

In another aspect, the invention provides underwater mining apparatus for extracting particulate material from beneath the seabed, the apparatus comprising a submersible vessel including a cantilevered boom carried by the vessel and drive means arranged to force the free end of the boom downwardly against the seabed, the free end of the boom having a breaking tool and the drive means being arranged to apply a force having a downward component of at least 3 tonnes (29.4 kN). Preferably, said downward component has a value of at least 5 tonnes (49 kN), more preferably a value of from 5 to 20 tonnes (49 to 196 kN). The breaking tool may be movable from side to side and backwards and forwards (as well as downwardly), to break the covering layer.

Preferably, the breaking tool has a serrated cutting surface and extends transversely across the tip of the boom. Preferably, the serrated cutting surface has a generally convex profile in side view.

Preferably, the boom is a tubular suction boom, which is suitably of a strengthened design.

Preferably, means are provided for fluidising particulate material adjacent the mouth of the suction boom.

Preferably, the vessel is a tracked vehicle. Another aim of the present invention is to suspend relatively coarse material and to lift such material to the surface.

In another aspect, the invention provides underwater mining apparatus for extracting particulate material, the apparatus comprising a vehicle capable of travelling on the seabed, a suction boom carried on the vehicle, at least one jetting nozzle arranged to form a fluidised region of the particulate material adjacent the mouth of the suction boom, and means for pumping water through the or each nozzle, the pumping means and the or each nozzle being arranged to generate a flow velocity (at the or each nozzle outlet) sufficient to suspend particles of density 1800 kg/m³ and diameter 100 mm in the fluidised region.

Preferably, the pumping means and the or each nozzle is arranged to generate a flow velocity (at the or each nozzle outlet) sufficient to suspend particles of 200 mm diameter and density 1800 kg/m³ in the fluidised region.

Preferably, said pumping means is arranged to generate a total volume flow rate of water from the nozzle or all of nozzles of at least 100 m³/hr. More preferably, said pumping means is arranged to generate a total volume flow rate of water from the nozzle or all of the nozzles of from 200 to 2000 m³/hr. Most preferably, said pumping means is arranged to generate a total volume flow rate of water from the nozzle or all of the nozzles of from 500 to 1600 m³/hr. In one embodiment, a volume flow rate of about 1500 m³/hr and a jet velocity of from 26 to 60, preferably 28 to 43, m/s has been successfully employed to suspend particles of 200 mm diameter and density 1800 kg/m³ in seawater. However these are preferred values and other less preferred ranges may also be used.

Preferably, the or each nozzle is disposed substantially parallel to and about the periphery of the suction boom and is arranged to form a coherent jet. Preferably, the mouth of each nozzle is set back from the mouth of the suction boom by from 1 to 9, preferably 3 to 8, nozzle diameters.

Preferably, means are provided at the mouth of the suction boom for preventing suction of particles having a particle size greater than a predetermined value. Preferably, the suction-preventing means is arranged to prevent suction of particles of maximum cross-sectional dimension greater than 100 mm, preferably 150 or 200 mm. Preferably, the suction-preventing means comprises at least one member extending transversely across the mouth of the suction boom.

Preferably, a cutting member is mounted on or adjacent the mouth of the suction boom. Preferably, the cutting member has a serrated cutting surface and extends across the mouth of the suction boom. Preferably, the cutting member is an elongate member with transverse serrations and its cutting surface has a generally convex profile in side view.

Preferably, the apparatus further comprises a drive means arranged to force the suction boom axially through a sandstone layer of thickness 200 mm or greater. Preferably, said drive means is arranged to exert a force having a downward component of at least 3 tonnes (29.4 kN).

More preferably, said drive means is arranged to exert a force having a downward component of at least of 5 tonnes (49 kN).

A further aim of the invention in another aspect is to prevent blocking of a riser carrying suspended material to the surface.

Accordingly, the invention also provides underwater mining apparatus comprising suction means arranged to suck up suspended particulate material, pumping means arranged to pump the suspension through a riser towards the surface, means for monitoring the flow rate of the suspension and valve means arranged to introduce extra water into the suspension in response to a drop in flow rate below a predetermined value.

The flow rate being monitored may for example be a linear rate (velocity) or a volume flow rate.

In one embodiment the predetermined value is from 1 to 5, preferably 3 to 4.5 m/s. Preferably, the valve means is arranged to remain open for a period sufficient to enable all the suspended material in the riser to be pumped to the surface in response to the detection of such a drop in flow rate. Preferably, a dump valve is provided situated adjacent the lower end of the riser, and arranged to open in response to failure of the pumping means, allowing solids within the riser to exit via the dump valve.

Preferably, the flow rate monitoring means comprises at least one of: a) means for sensing pressure in a flow path between a suction head and an inlet of the pumping means; b) means for sensing pressure in a flow path upstream of the pumping means; c) means for sensing flow velocity; and d) means for sensing density.

A further aim of the invention in other aspects is to alleviate problems caused by blocking the flow of the invention to the riser.

Accordingly in another aspect the invention provides a method of operating an underwater mining apparatus wherein a flow path between an inlet of a suction pump and a suction head which is normally arranged to convey a suspension of particulate material from the suction head to the inlet is substantially cut off to release suction at the suction head whilst water is introduced to the inlet of the suction pump to allow the suction pump to continue to operate. The particulate material may in particular be mineral bearing material such as diamondiferous material.

Preferably, the flow path is cut off by a valve in the flow path. Preferably, the valve is operated in response to a detected pressure condition in any flow path conveying said suspension. Preferably, the pressure condition includes at least one of: a) the pressure in the first-mentioned flow path, and b) the pressure upstream of the suction pump.

Preferably, water is injected into said first-mentioned flow path upstream of the suction head to clear the particulate material from the flow path and/or suction head.

The invention also extends to underwater mining apparatus comprising a suction arrangement including a suction pump normally communicating with a suction head and arranged to draw up a suspension of particulate material from the suction head, means for substantially cutting off suction to the suction head and means for introducing water to an inlet of the suction pump to allow the pump to operate whilst suction to the suction head is cut off. Preferably, the means for substantially cutting off suction comprises a valve responsive to a pressure condition in a flow path conveying said suspension.

Preferably, the apparatus comprises means for injecting water into said first- mentioned flow path upstream of the suction head. Preferably, the apparatus comprises means for sensing at least one of: a) the pressure in the first-mentioned flow path, and b) the pressure upstream of the suction pump, the cutting off means being responsive to an output of said sensing means.

The invention extends to diamonds or other minerals obtainable by any of the methods as aforesaid.

Preferred features of the present invention are now described below by way of example only with reference to Figures 1 to 14 of the accompanying drawings, wherein:

Figure 1 is a somewhat schematic side elevation of an underwater mining vehicle;

Figure 2 is a sketch perspective view of the vehicle of Figure 1;

Figure 3 is a plan view (that is from a direction orthogonal to the axis of the suction boom) of a nozzle arrangement and breaking tool of the vehicle;

Figure 3A is a plan view of a variant of the nozzle arrangement and breaking tool shown in Figure 3;

Figure 4 is a side elevation, partly in section, taken on Figure 3;

Figure 5 is an axial longitudinal cross-section of a nozzle used in the vehicle;

Figure 6 is a cross-section showing the typical geological strata in which the vehicle is intended to operate; Figure 7 is a rear elevation showing the vehicle in a channel cut out by its breaking tool, water jets and suction boom;

Figure 8 is a plan view showing the path of the vehicle and the operation of the suction boom and breaking tool;

Figure 9 is a diagrammatic illustration of the riser arrangement of the vehicle; Figure 10 is a front elevation of the suction boom mouth of the vehicle;

Figure 10A is a front elevation of a variant of the suction boom mouth shown in Figure 10; Figure 11 is a somewhat diagrammatic side elevation showing the initial cutting operation though a sandstone layer;

Figure 12 is a diagrammatic side elevation showing a subsequent cutting operation; Figure 13 is a diagrammatic side elevation showing the subsequent fluidisation and suction of diamondiferous material; and

Figure 14 is a somewhat schematic elevation of an alternative embodiment of underwater mining vehicle.

Referring to Figure 1, the vehicle 28 runs on tracks 13 which are driven by a standard electro-hydraulic drive (not shown). Two 500 kW, 3,300 V, 50 Hz motors 1 are mechanically coupled to the input of a gearbox 2 which combines the two drives and has a 3:1 reduction ratio. This drive is transmitted via a bearing 4 to a gravel pump 3 which is a standard pump manufactured by Warman International Limited but, like the other equipment on the vehicle, is marinised to withstand the effects of a deep sea environment (for example, a depth of 100 to 200 metres). The intake to the gravel pump 3 is connected to a flexible hose 5 which is coupled to a suction boom 22.

The suction boom 22 is carried on an arm 14 which is pivotally supported at 15 on a further arm 16 which is mounted on a pivotal support 17 on the chassis of the vehicle. The entire upper chassis of the vehicle (that is, excluding the tracks 13) is rotatable about a vertical axis. The angle of inclination of arm 14 is adjustable by a hydraulic cylinder 18 and, as shown in Figure 1, the arm can be oriented substantially vertically downwards.

A breaking tool in the form of a serrated cutting tooth 9 is carried on the tip of the suction boom 22. The tip is manoeuvrable in the vertical plane to anywhere within the region 19 shown shaded in Figure 1. The tooth is convex in side elevation. The serrations are canted such that the greater abrasive action is achieved when the tooth is being pulled towards the body of the vehicle. In an alternative embodiment the cutting tooth could be replaced by one or more elongate punching picks parallel to the suction boom, which may be either fixed or movable with respect to the boom. The machinery on the vehicle is protected by a space frame 12 which is shown more clearly in Figure 2. The outlet of the gravel pump 3 is connected to a riser 27 (Figure 2) which carries suspended material to a mother vessel (not shown) at the water surface.

The head 22a of the suction boom 22 is shown more clearly in Figures 2 and 10 and it will be seen that four jetting nozzles 20 protrude through apertures 33 in a substantially rectangular flange 21 and are disposed about the periphery of the suction boom. As shown in Figures 3 and 4, the flange and hence the entire head 22a is bolted to the remainder of the suction boom 22, thus allowing for replacement or interchanging of the head. The mouths of the nozzles are set back by approximately 400 mm from the mouth of the boom, partly to provide protection for the nozzles and partly to provide a baffling effect preventing substantial flow inwardly into the mouth of the suction boom. The nozzles are each protected by generally U-shaped shrouds 8 (not shown in Figure 2) which are attached to the exterior of the suction boom.

The serrated cutting tooth 9 extends diametrally across the mouth of the suction boom and intersects with a further member 34 which extends orthogonally across the mouth. The quadrants in the mouth of the suction boom defined by the cutting tooth 9 and member 34 allow only particles of diameter 200 mm or less to pass through the suction boom, as indicated by particle 134 shown in phantom in Figure 10. The vehicle is arranged to suspend only particles of diameter 200 mm or less, since larger particles could not easily be carried to the surface in the riser. As shown in Figures 3A and 10A, in a variant the wall of the mouth of the suction boom 22 tapers outwardly at each side (in the preferred embodiment by roughly 15%) to increase the area of each elongated quadrant as defined by member 34 and the cutting tooth 9; this is to avoid the cutting tooth covering too much of the surface area of the mouth and hence to increase the capacity of the suction boom 22. As shown in Figures 2, 3 and 4, the two jets 20 on the upper and forward region of the suction boom 22 are connected to a common manifold 7 which communicates with the outlet of a high power pump 6 (Figure 2) having a capacity of 1,500 m³/hr at 8.2 bar. Again, the two jets 20 on the lower and rearward region of the suction boom are connected to a further common manifold 7a which also communicates with the outlet of the pump. The pump 6 is powered directly by a further 500 kW motor (not shown) similar to motors 1 and a further similar motor drives the tracks 13. The exit diameter D of each nozzle bore (Figure 5) is 80 mm and it will be seen that the nozzle bore comprises an initial constant diameter portion 20A, a transition region 20B which is frusto-conical with an angle of taper of 7° and a final, reduced diameter outlet section having a diameter D of 80 mm. The pump 6 is designed to produce a jet of water from each nozzle having a velocity between 28 and 43 m/s, typically about 35 m/s, and a flow rate of between, say, 500 and 1000 m³/hr. Below a velocity of 28 m/s or so agitation can become insufficient to move the gravel; and above 43 m/s or so the agitation may be too great and gravel and diamonds may be blown away. The flow from the pump 6 can be directed to either the two jets 20 at the upper/forward end of the suction boom, or to the two jets 20 at the lower/rear side of the suction boom or to all four jets, according to the particular operation being performed.

Referring now to Figure 6, the seabed on which the vehicle is designed to operate will typically have an upper layer of gravel 2 to 3 m in thickness, an intermediate layer 25 of sandstone drape typically 300 to 400 mm in thickness and a lower layer 24 of gravel, typically 1 to 1.5 m in thickness, resting on a layer of bedrock 23. The layers of gravel 26 and 24 both contain diamonds. It is believed that only the upper layer 26 has hitherto been accessible to conventional undersea mining equipment. However, the vehicle of this embodiment is arranged to fluidise the layer of gravel 26 and suck it up, and then to punch through the layer of sandstone 25 by applying a downward pressure of 5 to 20 tonnes (49 to 196 kN) by means of the cutting tooth 9 and the substantially vertically oriented suction boom 22 to enable the diamondiferous gravel layer 24 to be suspended and extracted. In this manner, a trench extending down to the bedrock 23 can be formed, as shown in Figure 7.

The cutting and extraction operation is shown in more detail in Figures 11 to 13. Firstly, when the vehicle 28 is lowered onto the seabed and rests on the upper gravel layer 26, high pressure water is supplied to one or both pairs of jetting nozzles 20 (preferably the lower and rearward pair) to form a fluidised region of gravel which is then sucked up by the suction boom 22 and transferred to the surface by riser 27. The jets are sufficiently powerful to suspend particles up to 200 mm in diameter. At the end of this stage a trench is formed and the vehicle rests on the sandstone drape 25 covering the lower layer 24 of gravel, as shown in Figure 11. The arm 14 and associated suction boom are then oriented substantially vertically by the hydraulic cylinder 18 (Figure 1) to swing the suction boom about pivot 15 as indicated by arrow A to a nearly vertical orientation.

Downward pressure is then applied hydraulically via arm 16 and the cutting tooth 9 punches a hole in the sandstone drape 25 as shown. The downward pressure may have a (vertically) downward component of, say, 3, 5 or up to 20 tonnes at maximum reach of the arm. A practical upper limit is given by the requirement not to raise any part of the vehicle off the surface on which it is resting.

The hole in the sandstone drape is enlarged by moving the substantially vertical suction boom forwardly, rearwardly or from side to side to break up the sandstone drape, rather in the manner of an icebreaker. As shown in Figure 8, the chassis and associated boom 14 can swing by 270° and therefore all the drape within arc 29 (Figure 8) can be broken up to allow access to the gravel layer 24 beneath. Additionally, as shown in Figure 12 the cutting tooth 9 can be operated with a sawing action as indicated by arrow B to abrade the sandstone drape 25.

As shown in Figure 13, once the drape has been cleared, the suction boom can access the lower level of gravel 24 and the two lower/rearward jetting nozzles 20 generate jets of water 34 which form a vortex below the mouth of the suction boom, scouring the bedrock, and generate a fluidised region of gravel (keeping the gravel in suspension), which gravel is then sucked up by the suction boom as illustrated by arrow 35. In order to achieve the fluidisation effect, the mouth of the suction boom 22 is maintained at such a distance from the gravel that the nozzles 20 are typically no closer to the gravel than three times their diameter and typically no further away than ten times their diameter. This ensures their optimum effect and in particular can ensure that a wall of water can be generated through which neither the gravel nor diamonds can escape.

The typical path of the vehicle 28 is illustrated at 30 in Figure 8 and will be seen to comprise parallel tracks with curved transition regions during which the vehicle is steered by an appropriate differential drive to its two tracks 13. The tracks 13 are 1.6 m wide and approximately 7 metres between the forward and rear axles giving a footprint of about 22 square metres which supports a submerged weight of 85 tonnes. It will be noted that the vehicle does not need to turn around when moving from track to track.

In order to ensure that the extracted material is safely delivered to the surface via the riser 27, a control arrangement as shown in Figure 9 is employed. The pump 3 sucks up the suspension from the suction boom 22 via a conduit 128 which has an inlet upstream of the pump governed by a waste gate valve 29. Valve 29 is controlled by an output signal from a control arrangement 33 which receives an input signal from a density/velocity meter 31 which measures the flow velocity of the pumped mixture as well as its density.

The density/velocity meter 31 is suitably a radioactive density/inductive velocity transmitter which also generates a signal indicating mass flow rate of material for use by the operator of the vehicle.

If the velocity of the pumped mixture falls below say 3.8 m/s, the control arrangement 33 opens the waste gate valve 29 which allows water to be drawn into the pump suction side, thereby diluting the suspension, and therefore retains the velocity through the pump and the riser 27 to the surface. This prevents loss of flow; too low a velocity in the discharge pipe would result in the dredged material falling out of transported suspension, thus falling to the bottom of the discharge riser pipe 27 and causing blockage.

The waste gate valve 29 remains open for a period of 20 seconds before closing automatically, which allows all the dredged mixture sufficient time to rise to the surface and out of the riser 27. Then the riser contains only water and so the waste gate valve 29 can close without further risk of a blockage occurring in the riser. Normal working velocity of the pumped mixture is in the region of 6.0 to 6.5 m/s.

As a further precaution, an additional dump valve 32 is situated at the lower end of the riser 27. Should the dredge pump 3 unexpectedly stop then a monitoring signal from this pump is detected by control arrangement 33 which causes dump valve

32 to open fully and allow any solids inside the riser 27 to fall out of the riser. Once power has been restored then the dump valve 32 can be closed manually by a tool operator. A status indicator for the valve is incorporated in the operator's control display (not shown). As yet a further precaution, a pressure sensor 31a is provided which could sense a change in pressure caused, for example, by a stone blocking the conduit 128. The output from the sensor may be used to open the waste gate valve 29 or stop the pump 6. In order to enable the suction boom 22 to be unblocked if necessary, but without switching off and re-starting the dredge pump 3 which would raise power consumption and could cause overheating problems, a knife gate valve 138 is provided in conduit 128 and is operated in conjunction with the waste gate valve 29 under the control of controller 33 to block the flow in the conduit. If it becomes necessary to unblock the suction boom 22, knife gate valve 138 is closed, with the result that the flow velocity detected by meter 31 falls and triggers the opening of waste gate valve 29, which allows water into the inlet of pump 3 and maintains the flow of suspended material downstream of the pump. Instead of the waste gate valve being opened as a result of a fall in flow velocity detected by the meter 31, that valve may be opened simultaneously with closure of the knife gate valve.

Closure of the knife gate valve in addition to opening of the waste gate valve reduces the suction in the suction boom to zero. If the knife gate were not closed, in practice there might be sufficient suction in the suction boom to prevent satisfactory unblocking of the boom.

In order to improve unblocking of the boom, a conduit 130, connected to high pressure water from pump 6, is provided with an injection valve 134 which in turn is controlled by an output signal from controller 33 to inject water into the flow path (at a pressure of 15 Bar) between suction boom 22 and valve 138. This arrangement reverses flow through suction boom 22 and flushes out material causing the blockage. A pressure transducer 132 communicating with conduit 130 detects clearance of the blockage by the consequential drop in pressure and terminates the flushing operation by a signal to controller 33. Controller 33 generates an output signal in response which closes valve 132. The above process enables the flow in riser 27 to be maintained continuously and vastly shortens the time needed to unblock the suction from 22. Overall, in practice the above process has been found to save up to 30 minutes' down time per hour.

A further advantage of the unblocking arrangement described above is that it can afford a clean start and stop for the gravel pump 3, for example if the pump needs to be turned off for maintenance. By closing the knife gate valve 138 and opening the waste gate valve 29 fresh water can be introduced into the entire riser 27. This can reduce the risk that blockage will occur once the gravel pump is turned off, by reducing or eliminating the amount of gravel in the riser.

In a variant of the above described unblocking arrangement, the apparatus is provided with additional transducers to enable an adequate flow rate to be maintained, namely a differential vacuum transducer 136 which detects negative differential pressure (that is pressure below the local underwater pressure) upstream of valve 138 and accordingly provides an early indication of blockage, and a pressure transducer

140 upstream of pump 3 which provides a pressure value which correlates with the density value provided by the density/velocity meter 31 and is used to provide a check on the density value. Further, density/velocity meter 31 may if necessary comprise two separate sections, a density meter and a separate velocity meter.

Whenever the transducer 136 detects a differential pressure more negative than

- 0.8 Bar in conjunction with the detection by meter 31 of a velocity in riser section 27 of less than 3.8 metres/second, then controller 33 opens waste gate valve 29 in order to allow pump 3 to suck in water and boost the downstream velocity of the suspension.

When the flow rate of the suspension returns to its normal value of 6.5 metres/second and the transducer 140 indicates (by a normal pressure indication) that most of the suspension in riser 27 has gone to the surface then the waste gate valve

29 is closed. If however the above procedure fails to restore differential pressure in conduit 128 to the normal value then the procedure is repeated but with the knife gate valve 138 closed after the valve 29 has opened.

The injection valve 134 is opened during this procedure to flush the suction boom 22 and associated section of conduit 128 until the differential transducer 136 shows that pressure in conduit 128 has returned to a normal value. When all the transducers are showing their normal values then the arrangement is returned to its normal status and dredging is continued.

An alternative preferred embodiment of underwater mining vehicle is shown in Figure 14, in which like parts to those described with reference to the previous embodiment are accorded like reference numerals. The alternative embodiment is to all intents and purposes the same as the previous embodiment, except that the riser 27 is located in the alternative embodiment near the rear of the vehicle.

The vehicle is suitably a remotely operated vehicle (ROV) and can be provided with suitable cameras and other sensors enabling it to be controlled from the surface. It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination. Any reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

Claims

C L A I M S

1. A method of extracting mineral bearing material from beneath a covering layer of non-mineral bearing material at a submerged location, the method comprising the steps of breaking through the covering layer by means of a downwardly-acting breaking tool carried by a submerged vessel, forming a suspension of particulate mineral bearing material and lifting the suspension to the surface.

2. A method as claimed in Claim 1 wherein the suspension is formed as a fluidised region by at least one water jet.

3. A method as claimed in Claim 2 wherein the fluidised region is formed by two water jets.

4. A method as claimed in any preceding claim wherein the submerged vessel is a vehicle supported on the covering layer.

5. A method as claimed in Claim 4 wherein the vehicle is a tracked vehicle.

6. A method as claimed in any preceding claim wherein the vessel moves the breaking tool substantially horizontally whilst the breaking tool projects into the covering layer, and thereby enlarges an opening in the covering layer.

7. A method as claimed in Claim 6 wherein the breaking tool comprises a boom- mounted cutting member with a serrated cutting surface and the serrated cutting surface is moved across the upper surface of the covering layer to enlarge the opening.

8. A method as claimed in Claim 7 wherein the cutting member has an elongate cutting surface with transverse serrations and has a generally convex profile in side view.

9. A method as claimed in Claim 6, 7 or 8, wherein the cutting member extends across the mouth of a suction boom.

10. A method as claimed in any preceding claim wherein the downwardly-acting breaking tool applies a downward force to penetrate the covering layer.

11. A method as claimed in any preceding claim wherein the downward component of force applied by the breaking tool is at least 3 tonnes (27.4 kN).

12. A method as claimed in Claim 11 wherein the downward component of said force is at least 5 tonnes (49 kN).

13. A method as claimed in Claim 11 or Claim 12 wherein the downward component of said force is in the range 5 to 20 tonnes (49 to 196 kN).

14. Underwater mining apparatus for extracting particulate material from beneath the seabed, the apparatus comprising a submersible vessel including a cantilevered boom carried by the vessel, and drive means arranged to force the free end of the boom downwardly against the seabed, the free end of the boom having a breaking tool and the drive means being arranged to apply a force having a downward component of at least 3 tonnes (29.4 kN).

15. Underwater mining apparatus according to Claim 14 wherein said downward component has a value of at least 5 tonnes (49 kN).

16. Underwater mining apparatus according to Claim 14 or 15, wherein said downward component has a value of from 5 to 20 tonnes (49 to 196 kN).

17. Underwater mining apparatus according to any of Claims 14 to 16, wherein the breaking tool has a serrated cutting surface and extends transversely across the tip of the boom.

18. Underwater mining apparatus according to Claim 17 wherein the serrated cutting surface has a generally convex profile in side view.

19. Underwater mining apparatus according to any of Claims 14 to 18 wherein the boom is a tubular suction boom.

20. Underwater mining apparatus according to Claim 17 wherein means are provided for fluidising particulate material adjacent the mouth of the suction boom.

21. Underwater mining apparatus according to any of Claims 14 to 20 wherein the vessel is a tracked vehicle.

22. Underwater mining apparatus for extracting particulate material, the apparatus comprising a vehicle capable of travelling on the seabed, a suction boom carried on the vehicle, at least one jetting nozzle arranged to form a fluidised region of the particulate material adjacent the mouth of the suction boom, and means for pumping water through the or each nozzle, the pumping means and the or each nozzle being arranged to generate a flow velocity sufficient to suspend particles of density 1800 kg/m³ and diameter 100 mm in the fluidised region.

23. Apparatus according to Claim 22 wherein the pumping means and the or each nozzle is arranged to generate a flow velocity sufficient to suspend particles of 200 mm diameter and density 1800 kg/m³ in the fluidised region.

24. Underwater mining apparatus according to Claim 22 or Claim 23 wherein said pumping means is arranged to generate a total volume flow rate of water from the nozzle or all of the nozzles of at least 100 m³/hr.

25. Underwater mining apparatus according to Claim 22 or Claim 23 wherein said pumping means is arranged to generate a total volume flow rate of water from the nozzle or all of the nozzles of from 200 to 2000 m³/hr.

26. Underwater mining apparatus according to Claim 22 or Claim 23 wherein said pumping means is arranged to generate a total volume flow rate of water from the nozzle or all of the nozzles of from 500 to 1600 m³/hr.

27. Underwater mining apparatus according to Claim 25 or Claim 26 wherein the pumping means and the or each nozzle are arranged to generate a jet velocity of from

20 to 60 m/s.

28. Apparatus according to any of Claims 22 to 27 wherein the or each nozzle is disposed substantially parallel to and about the periphery of the suction boom and is arranged to form a coherent jet.

29. Apparatus according to any one of Claims 22 to 28 wherein the mouth of each nozzle is set back from the mouth of the suction boom by from 1 to 9 nozzle diameters.

30. Apparatus according to any of Claims 22 to 29 further comprising means for preventing suction of particles having a particle size greater than a predetermined value provided at the mouth of the suction boom.

31. Apparatus according to Claim 30 wherein the suction-preventing means is arranged to prevent suction of particles of maximum cross-sectional dimension greater than 100 mm.

32. Apparatus according to Claim 30 or 31 wherein the suction-preventing means comprises at least one member extending transversely across the mouth of the suction boom.

33. Apparatus according to any of Claims 22 to 32 further comprising a cutting member mounted on or adjacent the mouth of the suction boom.

34. Apparatus according to Claim 33 wherein the cutting member has a serrated cutting surface and extends across the mouth of the suction boom.

35. Apparatus according to Claim 33 or Claim 34 wherein the cutting member is an elongate member with transverse serrations and its cutting surface has a generally convex profile in side view.

36. Apparatus according to any of Claims 22 to 35 further comprising drive means arranged to force the suction boom axially through a sandstone layer of thickness 200 mm or greater.

37. Apparatus according to Claim 36 wherein said drive means is arranged to exert a force having a downward component of at least 3 tonnes (29.4 kN).

38. Apparatus according to Claim 37 wherein said drive means is arranged to exert a force having a downward component at least of 5 tonnes (49 kN).

39. Underwater mining apparatus comprising suction means arranged to suck up suspended particulate material, pumping means arranged to pump the suspension through a riser towards the surface, means for monitoring the flow rate of the suspension and valve means arranged to introduce extra water into the suspension in response to a drop in flow rate below a predetermined value.

40. Underwater mining apparatus according to Claim 39 wherein the predetermined value is from 1 to 5 m/s.

41. Underwater mining apparatus according to Claim 39 or 40 wherein the valve means is arranged to remain open for a period sufficient to enable all the suspended material in the riser to be pumped to the surface in response to the detection of such a drop in flow rate.

42. Underwater mining apparatus according to any of Claims 39 to 41 further comprising a dump valve situated adjacent the lower end of the riser, and arranged to open in response to failure of the pumping means, allowing solids within the riser to exit via the dump valve.

43. Underwater mining apparatus according to any of claims 39 to 42 wherein the flow rate monitoring means comprises at least one of: a) means for sensing pressure in a flow path between a suction head and an inlet of the pumping means; b) means for sensing pressure in a flow path upstream of the pumping means; c) means for sensing flow velocity; and d) means for sensing density.

44. A method of operating an underwater mining apparatus wherein a flow path between an inlet of a suction pump and a suction head which is normally arranged to convey a suspension of particulate material from the suction head to the inlet is substantially cut off to release suction at the suction head whilst water is introduced to the inlet of the suction pump to allow the suction pump to continue to operate.

45. A method as claimed in claim 44 wherein the flow path is cut off by a valve in the flow path.

46. A method as claimed in claim 45 wherein the valve is operated in response to a detected pressure condition in any flow path conveying said suspension.

47. A method as claimed in claim 46 wherein the pressure condition includes at least one of: a) the pressure in the first-mentioned flow path, and b) the pressure upstream of the suction pump.

48. A method as claimed in any of claims 44 to 47 wherein water is injected into said first-mentioned flow path upstream of the suction head to clear the particulate material from the flow path and/or suction head.

49. Underwater mining apparatus comprising a suction arrangement including a suction pump normally communicating with a suction head and arranged to draw up a suspension of particulate material from the suction head, means for substantially cutting off suction to the suction head and means for introducing water to an inlet of the suction pump to allow the pump to operate whilst suction to the suction head is cut off.

50. Apparatus as claimed in claim 49 wherein the means for substantially cutting off suction comprises a valve responsive to a pressure condition in a flow path conveying said suspension.

51. Apparatus as claimed in Claim 49 or 50 comprising means for injecting water into said first-mentioned flow path upstream of the suction head.

52. Apparatus as claimed in any of claims 49 to 51 comprising means for sensing at least one of: a) the pressure in the first-mentioned flow path, and b) the pressure upstream of the suction pump, the cutting off means being responsive to an output of said sensing means.

53. A method of extracting diamondiferous or other mineral bearing material substantially as described hereinabove with reference to the accompanying drawings.

54. Underwater mining apparatus substantially as described hereinabove with reference to Figures 1 to 5, 7, 10 and 14 (including Figures 3A and 10A) of the accompanying drawings.

55. Diamonds or other minerals obtainable by a method as claimed in any one of Claims 1 to 13 or 53.