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This application is a U.S. National Phase Application pursuant to 35 U.S.C. §371 of International Application No. PCT/AU2011/000732 filed on Jun. 17, 2011, which claims priority to Australian Patent Application 2010902668, filed Jun. 18, 2010, the disclosures of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates generally to underwater mining, and in particular relates to a system and method for seafloor mining and gathering using a bulk cutting seafloor tool. 
     BACKGROUND OF THE INVENTION 
     Seabed excavation is often performed by dredging, for example to retrieve valuable alluvial placer deposits or to keep waterways navigable. Suction dredging involves positioning a gathering end of a pipe or tube close to the seabed material to be excavated, and using a surface pump to generate a negative differential pressure to suck water and nearby mobile seafloor sediment up the pipe. Cutter suction dredging further provides a cutter head at or near the suction inlet to release compacted soils, gravels or even hard rock, to be sucked up the tube. Large cutter suction dredges can apply tens of thousands of kilowatts of cutting power. Other seabed dredging techniques include auger suction, jet lift, air lift and bucket dredging. 
     Most dredging equipment typically operates only to depths of tens of meters, with even very large dredges having maximum dredging depths of little more than one hundred meters. Dredging is thus usually limited to relatively shallow water. 
     Subsea boreholes such as oil wells can operate in deeper water of up to several thousand meters depth. However, subsea borehole mining technology does not enable seafloor mining. 
     Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. 
     Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 
     SUMMARY OF THE INVENTION 
     According to a first aspect the present invention provides a seafloor bulk mining tool for production cutting of a seafloor bench, the bulk mining tool comprising:
         a tracked locomotion system to enable locomotion of the bulk mining tool across a seafloor bench;   power and control interfaces to receive power and control signals from a surface source; and   a drum cutter for cutting a bench; and   a sizing grill adjacent the drum cutter, for sizing cuttings as they are produced by the drum cutter.       

     According to a second aspect the present invention provides a method for production cutting of a seafloor bench, the method comprising:
         a seafloor bulk mining tool receiving power and control signals from a surface source;   the seafloor bulk mining tool locomoting across the seafloor bench; and   a drum cutter of the seafloor bulk mining tool cutting the bench, and a sizing grill adjacent the drum cutter sizing cuttings as they are produced by the drum cutter.       

     The seafloor bulk mining tool of the present invention thus advantageously provides for bulk cutting of benches occurring or formed on the seafloor. 
     In preferred embodiments of the invention, the seafloor bulk mining tool comprises a slurry pump system and a slurry inlet proximal to the cutting drum, configured to capture cuttings from the proximity of the sizing grill in the form of a slurry. The slurry may be pumped a short distance from the seafloor bulk mining tool, for example simply to one side of the path taken or to be taken by the tool, or behind the tool to avoid the tool having to travel over cuttings on the seafloor. Alternatively, the slurry may be pumped to a seafloor stockpile location some distance away from the seafloor mining tool via a suitable transfer pipe. 
     In preferred embodiments, a collection shroud partially surrounds the cutting drum to optimise containment and collection of cuttings by the slurry pump system. 
     The sizing preferably sizes cuttings by crushing particles larger than a grill-to-drum distance against the cutter drum. 
     The seafloor bulk mining tool may be an untethered remotely operated vehicle (ROV) or may be a tethered vehicle operated by umbilicals connecting to the surface. 
     Further, the present invention provides a tool adaptable in some embodiments to deployment at significant water depths. For example some embodiments may be operable at depths greater than about 400 m, more preferably greater than 1000 m and more preferably greater than 1500 m depth. Nevertheless it is to be appreciated that the tool of the present invention may also present a useful seafloor cutting option in water as shallow as 100 m or other relatively shallow submerged application. Accordingly it is to be appreciated that references to the seafloor or seabed are not intended to exclude application of the present invention to mining or excavation of lake floors, estuary floors, fjord floors, sound floors, bay floors, harbour floors or the like, whether in salt, brackish, or fresh water, and such applications are included within the scope of the present specification. 
     The bulk mining cutter of the bulk mining tool in some embodiments may comprise an electrically or hydraulically driven cutting drum which trails or leads the tool during locomotion. The cutting drum may be mounted on a boom assembly allowing variable cutting depth, whereby the cutting depth may for example be chosen responsive to a hardness of material in the bench being cut. 
     The drum cutter of the bulk mining tool is preferably configured to generate cuttings of a desired size. For example, the cuttings may be of a size suitable for gathering in the form of a slurry of water and cuttings. Preferably the drum cut width is greater than the machine track width. 
     Where the material to be retrieved is of a thickness greater than a bench height, the bench height being defined by the cutting depth of the seafloor bulk mining tool, multiple layers of benches of the material may be removed by multiple bulk mining steps performed by the bulk mining tool of the present invention. Cuttings produced with each pass of the seafloor bulk mining tool may be gathered by a suction inlet of the bulk mining tool during each pass or by other seafloor tools after each pass. 
     The bulk mining tool&#39;s weight is preferably selected such that the tool has sufficient weight when submerged in order that the bulk mining tool may apply sufficient downwards force to enable production cutting of a bench. 
     The seafloor bulk mining tool is preferably designed to work on a relatively flat and relatively horizontal bench surface and to cut down into the surface to a cutting depth while traversing across the bench surface. Cuttings may be left in place for subsequent gathering by a seafloor gathering tool, or may be gathered by a suction inlet near the cutter drum during cutting and delivered away from the tool. The seafloor bulk mining tool preferably cuts substantially an entire bench by traversing the surface of the bench in one or more paths. The cutting paths of the bulk mining tool are preferably optimised to maximise ore recovery from the bench based on the unique bench size and bench shape existing at the seafloor site concerned. 
     Preferably, the gathering or stockpiling area may be distal from the ore bench, with the bulk mining tool in such embodiments having a slurry pump system or a side cast system or the like for deposition of cut ore in a gathering or stockpiling area. Alternatively, the gathering area into which the cuttings are deposited by the bulk mining tool is the same location as the ore bench, whereby the bulk mining tool cuts the ore without substantially relocating the ore. Such embodiments permit the bulk mining tool design, function and operation to focus on the cutting requirements for such bulk mining, without being complicated by considerations of relocating cuttings. 
     The bench may comprise an ore bench of valuable ore to be retrieved, or may comprise a bench of hard rock or other seafloor material to be removed for other purposes. The ore may comprise seafloor massive sulphides. 
     The present invention recognises that seafloor sites of interest can be of complex topography, and the present invention thus provides for multiple seafloor mining tools operating in concert to effect retrieval of the seafloor material. 
     In this specification, the term “drum cutter” is not intended to encompass cutters of the disc type. Disc cutters being those, for example, which provide a cut which is relatively narrow when compared with the disc cutter diameter. 
     According to a third aspect the present invention provides a seafloor bulk mining toot for production cutting of a seafloor bench, the bulk mining tool comprising:
         a tracked locomotion system to enable locomotion of the bulk mining tool across a seafloor bench;   power and control interfaces to receive power and control signals from a surface source; and   a drum cutter positioned aft of the tool during locomotion and configured for cutting a bench during locomotion across the bench and for leaving cuttings on the seafloor for subsequent gathering.       

     According to a fourth aspect the present invention provides a method for production cutting of a seafloor bench, the method comprising:
         a seafloor bulk mining tool receiving power and control signals from a surface source;   the seafloor bulk mining tool locomoting across the seafloor bench; and   a drum cutter of the seafloor bulk mining tool cutting the bench, the drum cutter being positioned aft of the tool during locomotion and leaving cuttings on the seafloor for subsequent gathering.       

     The third and fourth aspects of the invention may permit improved cutting efficiency and therefore a faster mining rate, as compared to a bulk cutter which gathers its own cuttings. 
     Some embodiments of the third and fourth aspects of the invention may comprise a sizing grill proximal to the cutting drum to size cuttings produced by the cutting drum, however in other embodiments a sizing grill may be omitted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An example of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a simplified overview of a subsea system in accordance with one embodiment of the present invention; 
         FIGS. 2   a  and  2   b  illustrate operation of a bulk miner in accordance with one embodiment of the present invention; 
         FIG. 3  is a perspective view representation of a bulk miner having a single cutter drum in accordance with another embodiment of the present invention; 
         FIG. 4  is an elevation view of a bulk miner in accordance with a similar embodiment of the invention; 
         FIG. 5  is a perspective view representation of a bulk miner having two cutter drums in accordance with a further embodiment of the present invention; 
         FIG. 6  is a schematic outlining the bulk mining machine deployment and operational system; 
         FIGS. 7   a  and  7   b  illustrate a bulk mining tool in accordance with another embodiment of the invention; and 
         FIGS. 8   a  and  8   b  illustrate overcutting and plunge cutting, respectively. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a simplified overview of a subsea system  100 , incorporating a bulk mining machine  112  in accordance with one embodiment of the present invention. A derrick  102  and dewatering plant  104  are mounted upon an oceangoing production support vessel (PSV)  106 . The PSV  106  has ore transfer facilities to load retrieved and dewatered ore onto barge  108 . The present embodiment provides a tool  112  operable to 2500 m depth, however alternative embodiments may be designed for operation from 100 m to 3000 m depth or greater. During production operations, seafloor mining tools (SMTs) will be used to excavate ore from the seabed  110 . The SMTs comprise a seafloor bulk mining machine  112 , a seafloor gathering machine  114 , a seafloor auxiliary mining machine  116  and a seafloor stockpiling device  126 . 
     Ore mined by the bulk mining machine (BM)  112  and auxiliary mining machine (AUX)  116  is pumped into stockpile  124  via stockpile transfer pipe  126 . Ore in stockpile  124  is gathered by gathering machine  114  and pumped, in the form of slurry, through a riser transfer pipe (RTP)  120  to the base of the riser  122 . A subsea lift pump  118  then lifts the slurry via a rigid riser  122 , which is shown interrupted in  FIG. 1  and may be up to 2500 m long in this embodiment. The slurry travels to the surface support vessel  106  where it is dewatered by plant  104 . The waste water is returned under pressure back to the seafloor to provide charge pressure for the subsea lift pump  118 . The dewatered ore is offloaded onto transport barge  108  to be transported to a stockpile facility before being transported to a processing site. 
     The BM  112  cuts a bench while progressing across the bench, and makes one or more traversals back and forth across the bench in order to cut substantially the entire area of the bench. The BM  112  may further make additional passes across or perpendicular to the original traversals in order to more closely trim the edges of the bench.  FIG. 2   a  illustrates the seafloor mining environment during a first bench cutting stage. 
     Given the bulk mining role of the BM  112  it is expected that some portion of the bench, particularly at lateral extremities and footwalls where the BM  112  must maintain a safety margin as well as have room to turn around to begin a new traversal of the bench, will not be fully cut by the BM  112 . This is shown in  FIG. 2   b , in which the bench edges are about 4 m high after cutting of multiple benches. 
     The BM  112  is designed to manoeuvre around the mine site and to cut mineral deposits through remote operator control on the topside Production Support Vessel  106 . In this embodiment the BM  112  requires a minimum bench area of about 750 square meters for efficient operation. In alternative embodiments the dimensions of the BM may be of a smaller scale to permit the BM to commence operations upon a bench of area less than 750 square meters, or in other embodiments the BM may be of a larger scale and require a minimum bench size of greater than 750 square meters to commence operation. Benches are then progressively removed from the high point in the manner shown in  FIGS. 2   a  and  2   b  so as to recover the mound of ore deposit. 
     Excavated particle size is controlled by the BM cutter configuration and speed of advancement. This is determined by cutter diameter, pick spacing, angle, speed of cutter rotation and rate of machine advancement. Cutting system parameters (cutter rotation speed, cut depth, advancement speed) can be manually or automatically controlled. In some embodiments, interlocking may be provided as a safety measure to prevent stalling of cutting operations and potential damage to the machines. In alternative embodiments, particle size may be controlled by a crusher or sizing device integrated within the BM. 
     Additional digging lines for the BM  112  and vehicle manoeuvring turns can be undertaken manually or by means of automated routines. Automation of the cutting is preferably maximised, and to this end a control system of the PSV  106  has the capability to incorporate automatic feedback control integrated into a mine model such that operating parameters such as cutting rate, recovered ore grade, rock hardness and particle size learned from overlying benches can be automatically used to control mining of subsequent underlying benches. 
     Overall, the aim of the cutting sequence is to maximise production rate and deliver stockpiles of cut ore on the seafloor. Once cut and left on the seafloor, the ore is then gathered by any suitable means, preferably by a separate gathering machine (GM)  114 . 
     The seafloor vehicle  112  for bulk mining, cutting and excavating material is described in more detail below with reference to  FIGS. 3 and 4 . The seafloor mining tool  112  of the embodiment provides an ore cutting/sizing function. Control systems on board the PSV ensure efficient optimisation of SMT operations whilst maximising a safe working area between machines, umbilicals and lift wires to ensure continuous operation.  FIG. 3  is a perspective view representation of a BM in accordance with an embodiment of the present invention.  FIG. 4  is an elevation view of a bulk miner in accordance with a similar embodiment of the invention. 
     As outlined in  FIG. 3  and  FIG. 4 , the BM is a high production cutting machine which is intended for the excavation of the target ore in preparation for pumping as a slurry to the PSV. The system incorporates an electrically driven cutting drum assembly  302  positioned at the rear of the vehicle  112 . The cutter drum assembly  302  is mounted on a boom assembly  304  capable of lifting and lowering the cutting drum assembly  302 . The cutter drum  302  is designed to cut a bench of up to 4 m in depth in multiple passes, leaving fragmented material in place in a uniform distribution. The fragmented material suitably has a particle size distribution suiting slurry transfer parameters and the topside recovery process. The cutting drum may be required to operate in either overcut or undercut modes. In alternative embodiments, the cutting drum assembly may be hydraulically driven. 
     A tracked locomotion system  306  is capable of propelling the vehicle  112  in a forward direction whilst the cutting drum  302  is engaged in cutting rock or ore. After cutting, the cut ore is simply left and remains on the seafloor, where it is left to be recovered and delivered to the RALS pumping system  118 —suitably by the seafloor gathering machine (GM)  114 . The primary function of the BM  112  is thus to cut and size a bench of 4 m depth in multiple or single passes, and to serve as a high production horizontal cutting machine. The BM is thus a heavy tracked machine with low centre of gravity to maximise power delivery to the rock or ore bearing body. The machine of the embodiment delivers about 900 kW to the rock face, and requires a total machine power of between 2 MW and 3 MW. 
     In an alternative embodiment shown in  FIG. 5 , the bulk miner incorporates two boom mounted cutting drums, one at each end of the vehicle. In this embodiment, the vehicle does not need to be turned around at the end of each pass across the bench, as it is possible to instead simply engage whichever cutting drum is trailing the vehicle. In the embodiments of  FIGS. 3 to 5 , the cut width is greater than the machine track width. 
     The bulk mining machine deployment and operational system is outlined in  FIG. 6 . Here, the Production Support Vessel (PSV)  106  hosts a control room from which the BM  112  is operated, along with the winches for both umbilical and lift wire, along with an A Frame for deployment and recovery of the BM  112 . The BM  112  is connected to the vessel  106  by means of an umbilical cable, and a main hoist wire. The umbilical cable provides electrical power to drive the track drive motors, hydraulic system drive motors(s), and cutter system drive motor(s). The umbilical also provides multiplexed fibre optic communication links between the BM  112  and the operational control room. 
     The BM  112  is lowered from the PSV  106  to the seafloor, via a main hoist wire. When the BM  112  is landed on the seafloor, the hoist wire can be disconnected and recovered either back to the PSV  106 , or to a safe height whereby it will not get tangled with the umbilical during bench cutting operations. When the BM  112  is ready to be recovered to the PSV  106 , the hoist wire can be reconnected. 
     During cutting operations, the cutter drum  302  is lowered, and a force applied to the rock face depending on its hardness and desired fragmentation rate during cutting. The vehicle tracks forward and the cutting drum  302  cuts at a controlled rate and force. Automatic routines are in place to maintain a constant cutting force with the boom  304  force and track tramming speed being automatically adjusted with variations in cutting force requirements. Ore is cut and ground in one pass to a bench depth up to 4 m in single or multiple passes. The BM  112  follows a plan developing strips of cut ore until the site or bench is fully cut to a single pass of cutter depth, then the gathering of ore by a separate machine occurs. 
     At the end of a cut line, the BM configured with a dual cutter drum arrangement as shown in  FIG. 5  will raise the rear cutter drum, manoeuvre onto the next cut line (in parallel with the line that has just been completed), lower the forward cutter drum, and continue operations (this time effectively in reverse so that the cutter boom is always at the rear of the direction of travel). 
     The BM configured with a single cutter drum as shown in  FIGS. 3 and 4 , the vehicle will raise the drum  302 , and turn substantially  180  degrees to begin a new cut line. 
     A water jet system may optionally be installed in BM  112  to provide cleaning of the cutter drum picks in the event they are clogged, and flushing of the vehicle tracks in the event they get covered in material. 
       FIGS. 7   a  and  7   b  illustrate a bulk cutter  700  in accordance with another embodiment of the invention. Bulk cutter  700  comprises an electrically driven cutting drum assembly  702  positioned at the front of the vehicle  700 . The cutter drum assembly  702  is mounted on a boom assembly  704  which is capable of lifting and lowering the cutting drum assembly  702 . The cutter drum assembly  702  is designed to cut a bench of up to 4 m in depth in multiple passes. A sizing grill  708  is provided adjacent the cutting drum  702  and is mounted on the boom assembly  704 , although in alternative embodiments the grill  708  may be mounted on the vehicle chassis similarly as for spade  710 . The sizing grill sizes cuttings as they are produced by the drum  702 , to a size suitable for transport in slurry form. A spade  710  separates cuttings from the seabed as the tool  700  moves forward, and an auger  712  urges cuttings within spade  710  towards a suction inlet, not visible in  FIG. 7  but shown generally at  714 . 
     Bulk cutter  700  thus cuts, sizes and sucks up cuttings in a single process. Cuttings captured by the suction inlet  714  in this embodiment are pumped via transfer pipe to a selected seafloor stockpiling location. 
     The embodiment of  FIG. 7  recognises the particular benefit of using a suction inlet  714  to capture cuttings which comprise a significant proportion of fine and small particles. In water such particles are inefficiently captured by mechanical means whereas a suitably configured and operated slurry inlet presents an efficient method for gathering cuttings of all sizes produced by the cutting drum  702 . Containment and capture of cuttings is aided by collection shroud  716 . 
     While the embodiment of  FIG. 7  comprises a suction inlet alternative embodiments such as those of  FIGS. 3 and 5  may omit such a suction inlet. 
     The bulk cutler of some embodiments of the invention may undertake overcutting, in which the cutting drum is forward of, and at a fixed height relative to, the tool  700 , and the tool travels across the bench, as shown in  FIG. 8   a . In some embodiments such as shown in  FIG. 8   b , the bulk cutter may be used in a plunging mode, in which the machine is stationary during cutting and the cutting drum is lowered down a wall while cutting the wall, up to about 4 m high and to a cutting depth up to about half the diameter of the cutting drum. After each such plunge cut the machine then travels forward by the depth of the cut and performs another plunge cut. 
     It is to be appreciated that particular terms used herein may be synonymous with other terms which equally describe the present invention and the scope of the present application is thus not to be limited to any one such synonym. For example, seafloor mining tools may also be referred to as subsea machines, a production support vessel may be referred to as a surface vessel and/or surface facilities, ore may be equally or alternatively referred to as rock, consolidated sediment, unconsolidated sediment, soil, seafloor material, and mining may comprise cutting, dredging or otherwise removing material. 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Summary:
A seafloor bulk mining tool for production cutting of a seafloor bench. The tool uses a tracked locomotion system to travel across a seafloor bench. Power and control interfaces receive power and control signals from a surface source. The tool has a drum cutter for cutting the bench, and a sizing grill adjacent the drum cutter for sizing cuttings as they are produced by the drum cutter. A drum shroud may also be provided to help contain cuttings. A suction inlet can be used to capture cuttings as they are produced, for example in conjunction with a spade and augers.