Abstract:
A system and method for stockpiling material on the seafloor, the system and method using seafloor collection machines, such auxiliary or bulk cutters or collection machines, to capture seafloor material to be stockpiled. The captured seafloor material is carried in slurry form over a flexible transfer pipe to an outlet at a desired seafloor site. In a preferred form the outlet is mounted in a seafloor stockpiling hood that sits on the seafloor at the desired seafloor site and captures and contains slurry from the outlet while allowing egress of water. The captured seafloor material can then be extracted to a surface vessel.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to underwater mining, and in particular relates to a system and method for seafloor stockpiling. In particular the invention relates, but is not limited, to mining, gathering, and stockpiling resources on the seafloor using a plurality of cooperating seafloor tools. 
       BACKGROUND OF THE INVENTION 
       [0002]    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. 
         [0003]    Most dredging equipment typically operates only to depths of tens of metres, with even very large dredges having maximum dredging depths of little more than one hundred metres. Dredging is thus usually limited to relatively shallow water. 
         [0004]    Subsea boreholes such as oil wells can operate in deeper water of up to several thousand metres depth. However, subsea borehole mining technology does not enable seafloor mining. 
         [0005]    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. 
         [0006]    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 
       [0007]    According to a first aspect, the present invention provides a system for seafloor stockpiling, the system comprising:
       a flexible transfer pipe for carrying slurry from a slurry inlet to a slurry outlet; wherein   the slurry inlet receives slurry from a seafloor collection machine; and   the slurry outlet, positioned at a desired location distal from the slurry inlet, delivers the slurry to a seafloor site.       
 
         [0011]    According to a second aspect, the present invention provides a method for seafloor stockpiling, the method comprising:
       capturing seafloor material in a slurry form;   carrying captured slurry through a flexible transfer pipe to a slurry outlet; and   positioning the slurry outlet at a desired seafloor site distal from the slurry inlet.       
 
         [0015]    Preferably the outlet is mounted in a seafloor stockpiling hood. The seafloor stockpiling hood preferably has an open bottom and preferably captures and contains the slurry on a seafloor surface of the seafloor site. The seafloor stockpiling hood preferably allows egress of water from the slurry in the hood. 
         [0016]    The flexible slurry transfer pipe permits the slurry outlet to be moved relative to the slurry inlet, for example to accommodate varied seafloor topography, environmental conditions and/or seafloor device operating conditions. Embodiments of the first and second aspects of the invention may thus be applied in a broad range of seafloor mining applications in which it is desired to transfer a slurry from one seafloor site to another. 
         [0017]    In embodiments of the first and second aspects of the invention the slurry inlet may be mounted upon a seafloor gathering tool configured to gather slurry from more than one seafloor location for delivery to the slurry outlet. 
         [0018]    In embodiments of the first and second aspects of the invention, the desired location to which the slurry outlet delivers the slurry may comprise a naturally occurring seafloor site at which the slurry is released. In such embodiments the slurry outlet may simply be anchored at or proximal to the desired location to deliver slurry. The desired location may comprise a naturally occurring seafloor depression in order to promote settling of solids in the slurry into the depression. 
         [0019]    The desired location could be artificially formed and could for example be a walled area with the walls comprising solid material placed in order to form walls. The walled area could have an open wall and for example may have a wall only to a downstream side of the desired location when a prevailing current is known to occur, such that solids settling out of the slurry delivered to the desired location will tend to gather against the open wall and thus tend to remain at the desired location. Alternatively, the walled area could be substantially surrounded by the wall and function as a settling tank for slurry delivered into the desired location. In further embodiments the desired location may comprise a substantially enclosed volume into which the slurry is pumped so as to capture solids in the slurry. 
         [0020]    The slurry may contain waste material which is desired to be relocated on the seafloor. Alternatively, the slurry may comprise valuable solids which are desired to be recovered from the seafloor to a surface vessel, via a seafloor stockpiling site at the desired location. 
         [0021]    According to a third aspect, the present invention provides a system for seafloor mining, the system comprising:
       at least one seafloor tool that captures seafloor material in a slurry form;   a seafloor stockpiling hood for receiving seafloor material in slurry form that captures and contains seafloor material present in the slurry at a seafloor site while permitting egress of water present in the slurry from the hood;   at least one flexible stockpiling transfer pipe for transport of slurry from the seafloor tool to the seafloor stockpiling hood;
           a gathering tool for extracting seafloor material captured by the hood and delivering the gathered seafloor material to a riser and lifting system that lifts the seafloor material to the surface; and   
           a surface vessel for receiving the seafloor material from the riser and lifting system.       
 
         [0027]    According to a fourth aspect, the present invention provides a method for seafloor mining, the method comprising:
       at least one seafloor tool capturing seafloor material in a slurry form;   a seafloor stockpiling hood receiving the seafloor material in slurry form from the seafloor tool and capturing and containing seafloor material present in the slurry at a seafloor site while permitting egress of water present in the slurry from the hood;   extracting seafloor material from the hood and delivering the gathered seafloor material to a riser and lifting system; and   a surface vessel receiving the seafloor material from the riser and lifting system.       
 
         [0032]    Preferably the seafloor material is extracted in slurry form. Preferably the seafloor material extracted in slurry form is delivered to the riser and lifting system via a riser transfer pipe. 
         [0033]    The third and fourth aspects of the present invention recognise that slurry flow rates desired for capturing seafloor material can be significantly different to the slurry flow rates desired for lifting a slurry in a riser and lift system, and thus provides for decoupling of these flow rates by use of a seafloor stockpiling hood. The respective flow rates may thus be separately optimised. 
         [0034]    Moreover, significant operational benefits result from removing the dependence of the gathering system from the operation of the seafloor tool, such that the gathering of stockpiled material for delivery to the riser and lift system may occur even when the seafloor tool is not capturing seafloor material. This is particularly important for seafloor tools with highly variable production capacity, such as a peak capacity of around 10,000 tonnes per day but an average production of 3,000 tonnes per day, as the present invention permits a gathering system and riser and lift system to be designed to meet the average production value rather than the peak production value. 
         [0035]    Moreover, in the case of small seafloor sites, the use of stockpiling can afford particular operational benefits in permitting a single tool to work a bench for extended lengths of time, reducing the need for multiple seafloor tools to co-habit a small bench or the need for large number of tool movements to permit alternating tools to work the small site. With use of seafloor stockpiling and suitable stockpiling transfer pipes each seafloor tool can work with considerably reduced interdependence at varying sites in the proximity of the stockpile. For example, in some embodiments the, or each, stockpile pipe may be configured to permit the associated seafloor tool to work up to 200 m away from the stockpile and up to 50 m above or below the stockpile in elevation. 
         [0036]    The hood preferably has an open bottom and is configured such that, when positioned on a relatively flat portion of the seafloor, the hood and seafloor define a stockpiling cavity. The walls of the hood preferably completely enclose a stockpiling volume in a manner to minimise the loss of slow-settling fine particles (referred to herein as “fines”). In such embodiments, to accommodate large volumes of slurry inflow, the hood preferably permits the egress of water from the stockpiling volume so as to filter and capture the seafloor material from the slurry. To this, end, preferably a significant surface area of the walls of the hood are formed of filter material which contains fines while permitting egress of water from the hood. 
         [0037]    A grade of the filter material, being a dimension below which solid particles can pass through the filter material, is preferably selected in order to maximise fines containment while permitting the necessary water flow rate out of the hood to accommodate slurry inflows into the hood. For example the filter material may comprise a silt curtain of 50 micron grade. The seafloor hood preferably comprises a space frame supporting the filter material, with the walls of the hood being formed by the filter material. 
         [0038]    Capture of fines from a slurry inflow into the hood can be advantageous both environmentally in avoiding escape of plumes of the seafloor material, and operationally as such fines may represent 30% or more of the seafloor material desired to be gathered. 
         [0039]    The, or each, seafloor tool delivering captured seafloor material to the stockpiling hood may comprise an auxiliary cutter, a bulk cutter, or a collection machine. 
         [0040]    The gathering tool for delivering seafloor material from the seafloor hood to the riser and lift system may extract seafloor material directly from the hood. The gathering tool may be a portion of the seafloor hood, for example a suction inlet positioned within the hood and connected to a suitable transfer pipe and slurry pumping system. Additionally or alternatively, the gathering tool for delivering seafloor material from the seafloor hood to the riser and lift system may be a collection machine separate to the hood, the collection machine having a collection head configured to be brought within the hood via a collection port in the hood, the collection head comprising a suction inlet. Alternatively there may be no gathering tool of the hood, and the hood may simply be removed to leave the seafloor ore pile freely accessible to a gathering machine. 
         [0041]    The slurry flow rate in the stockpiling transfer pipe may for example be about 3,000 m 3 /hour, with an ore concentration of about 3%. In contrast, in such an embodiment the flow rate in the riser transfer pipe may be around 1000 m 3 /hour at an average ore concentration of about 12%. 
         [0042]    The stockpile hood may have angled walls forming a substantially frustoconical or frustopyramidal shape, the walls being at an angle to approximate the expected rill angle of an ore heap so as to avoid a stockpiled ore heap exerting significant outward pressure on the walls. 
         [0043]    In alternative embodiments the seafloor stockpiling hood may comprise a settling tank with an encircling wall, whereby delivery of a slurry into the settling tank permits gathered material to settle to the base of the settling tank and permits water of the slurry to rise out of the tank, the tank having a sufficient cross section that a flow rate of water out of the tank is slow, to permit fines to settle. Preferably, the cross sectional area of the tank is sufficient, relative to an inlet slurry flow rate, that the flow rate out of the tank is about 12 m/hour or less, so that fines settling in water at a rate greater than 12 m/hour are captured. 
         [0044]    Further, the present invention provides a system 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 multi-tool system of the present invention may also present a useful seafloor mining option in water as shallow as 100 m or other relatively shallow submerged applications. Accordingly 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. 
         [0045]    The, or each, seafloor tool may be an untethered remotely operated vehicle (ROV), or may be a tethered vehicle operated by umbilicals connecting to the surface. 
         [0046]    The seafloor gathering tool preferably comprises a mobile slurry inlet which can be controllably positioned proximal to stockpiled material to be gathered. Thereby, suction at the slurry inlet causes water and proximal solids to be drawn into the inlet in the form of a slurry. The seafloor gathering tool preferably has a remote attachment and disconnection system for connection of a riser transfer pipe for transfer of the slurry from the stockpile to the riser base. In such embodiments, the remote connection system enables deployment and recovery of the gathering machine to and from the seafloor without recovery of the slurry riser system. The suction at the slurry inlet may be generated by a pump of the gathering tool, or alternatively may be generated by a subsea transfer pump at the riser base. 
         [0047]    The riser and lift system preferably comprises a subsea slurry lift pump to pump slurry to the surface through a riser pipe. In preferred embodiments the seafloor stockpiling hood receives seafloor material in slurry form from the seafloor tool via a flexible stockpile transfer pipe. The stockpile transfer pipe preferably has remote connection/disconnection ability at both the seafloor tool and the hood. 
         [0048]    The surface vessel may be a navigable vessel, a platform, a barge, or other surface hardware. The surface vessel preferably comprises dewatering equipment to dewater the slurry received from the riser, and may further comprise ore transfer and/or processing facilities such as an ore concentrator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0049]    An example of the invention will now be described with reference to the accompanying drawings, in which: 
           [0050]      FIG. 1  is a simplified overview of a subsea system in accordance with one embodiment of the present invention; 
           [0051]      FIG. 2  illustrates another embodiment involving simultaneous operation of seafloor tools sharing a single stockpiling device; 
           [0052]      FIGS. 3   a  to  3   d  illustrate the example operational positions of the stockpiling system; 
           [0053]      FIG. 4  illustrates the seafloor mining system of  FIG. 2  from an elevated perspective; 
           [0054]      FIGS. 5   a - 5   d  illustrate the collection machine in greater detail; 
           [0055]      FIG. 6  illustrates the collection machine dredge pumping system; 
           [0056]      FIG. 7  illustrates another embodiment in which the stockpiling device is a settling tank; and 
           [0057]      FIG. 8  illustrates fluid flows and settling rates in the embodiment of  FIG. 7 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0058]    The following abbreviations and acronyms are used throughout the following detailed description: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 m 
                 Metres 
               
               
                   
                 PSV 
                 Production Support Vessel 
               
               
                   
                 RALS 
                 Riser and Lifting System 
               
               
                   
                 ROV(s) 
                 Remotely Operated Vehicle(s) 
               
               
                   
                 RTP 
                 Riser Transfer Pipe 
               
               
                   
                 SMS 
                 Seafloor Massive Sulphide 
               
               
                   
                 SMT(s) 
                 Seafloor Mining Tool(s) 
               
               
                   
                 SSLP 
                 Subsea Slurry Lift Pump 
               
               
                   
                 CM 
                 seafloor Collecting and cutting Machine 
               
               
                   
                 AM 
                 seafloor Auxiliary Mining machine 
               
               
                   
                 BC 
                 seafloor Bulk Cutting machine 
               
               
                   
                   
               
             
          
         
       
     
         [0059]      FIG. 1  is a simplified overview of a subsea system  100  in accordance with one embodiment of the present invention. A derrick  102  and dewatering plant  104  are mounted upon an oceangoing production support vessel  106 . Production support vessel (PSV)  106  has ore transfer facilities to load retrieved ore onto barge  108 . The present embodiment provides a system  100  operable to 2500 m depth, however alternative embodiments may be designed for operation to 3000 m depth or greater. During production operations, one or more seafloor mining tools (SMTs) are used to excavate ore from the seabed  110 . The SMTs comprise a seafloor bulk cutting (BC) machine  112 , a seafloor collection machine (CM)  114  and a seafloor auxiliary mining (AM) machine  116 . 
         [0060]    Ore mined by the BC  112  is gathered upon being cut and pumped, in the form of slurry, from the BC through a stockpile transfer pipe (STP)  128  to a seafloor stockpiling device  124   a , which captures ore from the slurry while releasing water from the slurry. CM  114  inserts a boom-mounted suction inlet into stockpile  124   a  to gather ore in slurry form and transfers this slurry to the base of the riser  122 . A subsea lift pump  118  then lifts the slurry via a rigid riser  122  (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. AM  116  works another area of the mine site and delivers it&#39;s cuttings to the stockpile device  124   a  or to another stockpile device  124   b  for later gathering by CM  114 . 
         [0061]    An inlet grizzly sizing screen is used on the CM  114  inlet to prevent over-size particles being introduced into the slurry system  120 ,  118 ,  122 ,  104 . The system  100  is designed so that this grizzly screen size is interchangeable. 
         [0062]    The CM  114 , the BC  112  and the AM  116  each have a pump and control system which maintains the integrity of slurry flow and accounts for anticipated variability in inlet slurry conditions. The pump/gathering system incorporates automatic slurry inlet dilution and bypass valves to prevent loss of flow integrity associated with blockages and/or instantaneous changes in slurry intake density outside of the system&#39;s specified operating limits. Alternative slurry density control systems may be employed in other embodiments. 
         [0063]    In order to minimise risk of blocking the riser transfer pipe (RTP)  120  and/or CM  114 , in this embodiment the CM  114  has a dump valve that is activated when the slurry flow integrity is compromised. In alternative embodiments of the invention a dump valve may be omitted. The CM  114  of this embodiment further incorporates a back flow system to assist in clearing any slurry system blockages within the CM  114 . This system is a configuration of pipes and valves that direct high pressure water from the slurry discharge line back to the suction head of the gathering machine  114 . Dump valves and backflow systems are similarly provided for the stockpile hoses  126 ,  128  and stockpile system  124  in this embodiment. 
         [0064]    The Riser and Lift System (RALS)  118 ,  122  lifts the seawater-based slurry containing the mineral ore particles to the Production Support Vessel (PSV)  106  at the surface via a vertical steel riser  122  suspended from the vessel. The ore particles mined by the SMT are collected using suction, and the particles thus become entrained in seawater-based slurry which is then pumped to the base of the riser via a Riser Transfer Pipe (RTP)  120  in a “lazy-S configuration”. A Subsea Slurry Lift Pump (SSLP)  118  suspended below the base of the riser  122  will drive the slurry from the base of the riser  122  to the vessel  106 , which will be over a height of up to 2500 m in this embodiment. Once at the surface, the slurry passes through a dewatering process  104 . The solids are transferred to a transport barge  108  for shipment to shore. The waste water, topped up with additional seawater as required, is passed through a header tank system onboard the PSV  106  and pumped back down to the base of the riser  122  via auxiliary seawater pipelines clamped to the main riser pipe  122 . The return seawater, on arrival at the base of the riser  122 , is then used to drive the positive-displacement chambers of the SSLP  118  prior to being discharged into the sea close to the depth at which it was originally collected. Alternative means to drive the SSLP  118  can also be provided, for example electric, hydraulic, pneumatic or electro-hydraulic systems, among others. 
         [0065]    The riser  122  is supplied in sections (joints), each joint being made up of a central pipe for the transportation of slurry mix from the base of the riser to the surface, together with two water return lines for powering the Subsea Slurry Lift Pump  118  from the surface. Plus, a Dump Valve System to enable all slurry in the Riser pipe  122  to be flushed from the system in the event of unexpected shut down, to prevent blockages. 
         [0066]    The Subsea Slurry Lift Pump (SSLP)  118  is suspended at the bottom of the riser  122  and receives slurry from the CM  114  via the riser transfer pipe  120 . The SSLP  118  subsequently pumps the slurry to the Production Support Vessel  106 . The pump assembly  118  comprises two pump modules, each module containing a suitable number of positive displacement pump chambers driven by pressurised water delivered from surface pumps via seawater lines attached to the riser  122 . The pump  118  is controlled from the surface vessel  106  by a computerised electronic system which passes control signals through umbilical cables to a receiving control unit on the pump  118 . Functions are operated hydraulically with a bank of dual redundancy electro-hydraulic power packs located on the pump  118 . The electrical power to drive the power packs is fed through the same umbilical cables which carry the control data signals from the surface to the pump  118 . The two (dual redundancy) umbilicals for control of the SSLP  118  are secured to clamps on the riser  122  with the weight of the umbilical distributed along the riser joints. 
         [0067]    The main function of the surface pumps is to provide pressurized water to drive the Subsea Slurry Lift Pump  118 . Multiple triplex or centrifugal pumps will be installed on the Production Support Vessel  106 , all taking water removed from the slurry mix (&lt;0.1 mm residues) in the dewatering process, made up with surface seawater to the required volume before being pumped down the water return lines to the SSLP  118  at depth. The surface system incorporates a return water header tank fed from the dewatering system and topped up with the required volume to drive the SSLP  118  using centrifugal pumps extracting filtered surface seawater via a sea chest in the vessel hull. The water in the header tank is delivered to a bank of charge pumps which boost the pressure for delivery to the inlet of the surface pumps. 
         [0068]    A derrick and draw-works system  102  is installed on the support vessel  106  in order to deploy and recover the riser  122  and subsea lift pump  118 . In addition handling systems within the area of the derrick  102  move the SSLP  118  into a designated maintenance area. 
         [0069]    A surge tank is incorporated between the RALS discharge and the dewatering plant  104  to moderate instantaneous slurry variability prior to feed into the dewatering plant. The dewatering system  104  will receive ore from the RALS  122  as mineral slurry. To ensure that the ore is suitable for transport, the large volume of water within the slurry must be removed. The dewatering process of this embodiment uses three stages of solid/liquid separation:
       Stage 1—Screening—using vibrating twin double deck screens   Stage 2—De-sanding—using hydro cyclones and centrifuges   Stage 3—Filtration—using disk filters       
 
         [0073]    Vibrating screen decks are used to separate the coarse particles from the slurry stream. These coarse particles are considered to be free draining and will not require any mechanical dewatering to achieve the required moisture limit. A vibrating basket centrifuge is used to provide mechanical dewatering of the medium particle size fraction to ensure the required moisture limit is reached. 
         [0074]    Hydro cyclones are then used to separate the valuable fine particles (&gt;0.006 mm) from the slurry feed which have not been removed by the screen decks. Disk filters are used to dewater the valuable fines (between 0.5 mm and 0.006 mm) prior to loading on to the transport barge  108 . This ore size fraction requires greater mechanical input (vacuum) to remove moisture. The ore/slurry waste water is then returned to the seafloor via a pump-set and piping system. A dewatering plant  104  is installed on the topsides surface facilities, in this case the PSV  106 , to reduce the moisture content of the ore to below the transportable moisture limit (TML) of the ore. Reducing the moisture content below the TML allows safe carriage of the ore by ship. It also reduces the cost of transport due to the reduced volume of material being shipped. Alternative embodiments may utilise any suitable other configuration of dewatering plant. 
         [0075]    In the case of dewatering plant  104  failure, the gathering machine  114  will disengage the seafloor  110  and continue pumping seawater. The volume of the surge tank is sufficient to accommodate the volume of slurry in the RALS  122 ,  118  in the case of any dewatering plant  104  failure. The slurry in the RALS  118 ,  122  will be discharged to the surge tank, or vibrating screens and surge tank, until seawater only is discharged to surface, at which time the dewatering plant  104  by-pass will be engaged and water circulated back to the subsea lift pump or the RALS/gathering machine shut down. 
         [0076]    The PSV  106  remains on location for the duration of mining and supports all mining, processing and offshore loading activities to enable safe and efficient mining of the seafloor deposits  110 , recovery of cut ore to the surface, treatment (dewatering, including return of treated water to seafloor) and off-loading of the dewatered ore into the transportation barges  108  for onward shipment to stockpiling and subsequent treatment facilities. Station holding capability for the vessel is via dynamic positioning. Alternative station holding may be by mooring the vessel, or by a combination of both dynamic positioning and mooring depending on site specific conditions. 
         [0077]    The system  100  of the present embodiment thus provides a means and method for achieving steady state seafloor mining and gathering production, such as seafloor massive sulphide (SMS) production. 
         [0078]      FIG. 2  illustrates simultaneous operation of BC  112 , AM  116  and CM  114 , as made possible by the use of a single shared stockpiling device  124 . Cuttings from BC  112  and AM  116  are simultaneously delivered in slurry form into stockpiling hood  124 . As shown, new stockpiles of ore are built up within hood  124 , and on top of previously formed stockpiles. CM  114  simultaneously works to collect stockpiled cuttings and deliver them in slurry form vie RTP  120  to RALS  118 ,  122 . 
         [0079]    STPs  128  and  126  may be configured to take any suitable shape while in use, whether an inverted catenary as shown in  FIG. 2 , an “M” shape, or otherwise. 
         [0080]      FIGS. 3   a  to  3   d  illustrate example operational positions of the system  100 , primarily determined by the stockpiling hose  128  of the seafloor tool  112 , which together define an operational envelope of the system. With a STP  128  having a length of approximately 320 m and a hose inner diameter of approximately 425 mm, the horizontal freedom of movement of the BC  112  relative to a stockpiling site of the hood  124  is 50 to 200 m, in any direction, and the vertical freedom of movement of the BC  112  relative to the stockpile site of the hood  124  is +/−50 m.  FIG. 3   a  illustrates the BC  112  in a position that is higher than, but relatively close to, the hood  124 .  FIG. 3   b  illustrates the BC  112  in a position that is lower than, but still relatively close to the hood  124 .  FIG. 3   c  illustrates the BC  112  in a position that is higher than, but relatively far away from, the hood  124 .  FIG. 3   d  illustrates the BC  112  in a position that is lower than, but still relatively far away from, the hood  124 . 
         [0081]    In one seafloor mining embodiment, it is desired that both the auxiliary cutter (AC)  116  and a bulk cutter (BC)  112  are able, at certain times, to simultaneously work respective sites within a mine area, each producing a slurry flow of up to 3,000 m 3 /hour. The present invention offers a significant benefit in avoiding the need for two respective RALSs each capable of transferring 3,000 m 3 /hour. Instead, the slurry flows from the AC  116  and the BC  112  may be delivered to one or more seafloor stockpiling hoods  124 , and a single RALS  118 ,  122  may extract stockpiled ore in a slurry at around 1000 m 3 /hour. In a mine site with relatively small benches, it is to be expected that the BC  112  and AC  116  will not operate continuously due to inter-site movement, so that operation of the RALS  118 ,  122  at a lower rate for a greater period of each day can be expected to roughly maintain site throughput, with the, or each, stockpile  124  operating as an operational buffer. 
         [0082]      FIG. 4  illustrates an example of the seafloor mining system of the present embodiment from an elevated perspective. 
         [0083]      FIGS. 5   a - 5   d  illustrate an example collection machine (CM)  114  in greater detail. The CM  114  has a crown cutter collector  502 , a boom/ladder  504 , a chassis  506 , a slew yoke  508 , crawler assembly  510  and lift point  512 . In this configuration the crown cutter has a suction head grid at 50 mm working as a rock guard, a collection range height of −2 m to 5 m, and a collection range width of +/−4 m (8 m total width). Such a CM  114  can be utilised in the present invention to extract seafloor material in slurry form from and/or adjacent to the stockpile device  124 . 
         [0084]      FIG. 6  illustrates an example dredge pumping system  600  of the CM  114 . The dredge pumping system  600  has three pumps  602 ,  604 , and  606  that generate a combined outlet pressure of approximately 1750 kPa above ambient pressure. The pumping system  600  has an outlet  608  which is fluidly connected to the riser transfer pipe (RTP). A dump valve  610  is provided adjacent the outlet  608  that is activated when the slurry flow integrity is compromised. A back flush system  610  is also provided which can be used to back flush the crown cutter collector  502 , particularly when the crown cutter collector  502  is clogged or has a blockage. The back flush system  610  can also be used as a dilution system to dilute the seafloor material being extracted if desired. 
         [0085]      FIGS. 7 and 8  illustrate an alternative embodiment of the invention in which the stockpiling device  124  is, or at least includes, a settling tank  700  with open top. Slurry from the BC  112  and/or the AM  116  is delivered into the top of the tank  700  by a delivery inlet  702 . The slurry is typically delivered at up to 6000 m 3 /hour, at which rate the flow rate upwards out of the tank is 12 m/hour. In this configuration, particles less than approximately 69 micrometres in size will settle too slowly and will exit the tank, but all fines larger than approximately 69 micrometres will have suitable conditions for settling in a heap  704  and will thus be captured and contained in the settling tank  700 . 
         [0086]    The stockpiling system of the present invention could be used as part of alternative offshore system designs. For example, while the described embodiment addresses seafloor material of value which is to be recovered to a surface vessel, in accordance with the first and second aspects of the invention the slurry may simply be delivered to a desired location at a site distal from the slurry inlet, for example to relocate waste to another seafloor site distal from a site of interest. The present invention also recognises that a range of costs and losses arise from the double handling of seafloor material involved in such a stockpiling method, but recognises that such costs and losses can by use of the present systems and techniques be minimised while affording a significant net operational benefit to some seafloor mining applications. 
         [0087]    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.