Patent Publication Number: US-8522460-B2

Title: Underwater excavation apparatus

Description:
RELATED APPLICATIONS 
     The present application is a 35 U.S.C. §371 national phase application of PCT International Application No, PCT/GB2009/01102, having an international filing date of Apr. 30, 2009, claiming priority to Great Britain Patent Application No. 0807969.1, filed May 1, 2008. The disclosures of each application are incorporated herein by reference in their entireties. The above PCT International Application was published in the English language as International Publication No. WO 2009/133373A2. 
     FIELD OF INVENTION 
     This invention relates to an improved excavation apparatus, device or tool, and in particular, though not exclusively, to an improved underwater excavation apparatus, device or tool. The invention also relates to an improved excavation system comprising such an excavation apparatus, and to a method of underwater excavation, e.g. using such an excavation apparatus. 
     The invention also relates to an improved underwater subsea mass flow excavation apparatus, device or tool, to a related excavation system comprising means for removing spoil, and to a related method of underwater or subsea excavation. 
     BACKGROUND TO INVENTION 
     Herein by “underwater” is meant below or under a surface of a body of water, whether moving or static, natural or man-made, e.g. a sea bed, ocean floor, river bed, canal bottom, lake or loch floor, dam floor, or the like. However, the invention finds particular use in seas or oceans. 
     “Mass flow” excavators operate by directing a flow of high volume fluid under low pressure at the sea bed or at a subsea structure or surface to displace material such as sea bed material. This is in contradistinction to “jet” type apparatus which direct a flow of low volume fluid under high pressure at the sea bed. “Mass flow” and “jet” or “jetting” are therefore distinct terms, known in the art. In terms of differences between mass flow excavators and jetting excavators, in mass flow (as the name suggests) it is the mass or volume of flow which moves or removes material. In jetting it is the speed, and thus pressure of the jets which does the cutting. In jetting pressures can be of the order of 3,000 psi (2.07×10 7  Pa), whereas mass flow excavators typically operate at pressures in the order of 10 to 20 psi (6.89×10 4  to 1.37×10 5  Pa). 
     It will appreciated that power is a function of pressure and flow rate. Therefore, for a given available power in order to transfer power from the device into seawater and into the soil to be disturbed, it is possible to select high flow rate and low pressure (i.e. mass flow) or to select high pressure and low flow rate (i.e. jetting). 
     A mass flow excavator is typically tethered from a vessel by means of a crane wire, which is used to lower and retrieve the excavator, and to maintain a given distance from the sea bed or structure or object requiring excavation, such as a subsea oil or gas pipeline. In order to control the excavator, sonar detection means can be used to allow the excavator operator to view the excavation in real time. Cameras and metal detection means can also be used to assist the operator. 
     Underwater mass flow excavation apparatus are known. For example, GB 2 297 777 A and WO 98/027286, also by the present Applicant (Assignee), the content of which is incorporated herein by reference. 
     Mass flow excavation is a means of creating cavities in the sea bed or deburying objects. In the trade of mass flow excavation it is accepted that excavated material is spread in a circular manner around the cavity. The material is displaced to a distance far enough to retain depth of the created cavity. There are, however, limits to the distance to which the material can be thrown, which then limits the size and depth of the cavity to be created. Current applications of mass flow excavation are restricted to those excavations which do not require the sea bed material to be excavated, collected and deposited in a particular area, such as is required for excavation of harbour areas or canals, where it is important that the excavated material is removed to particular locations. 
     The present Inventor has identified that where the excavation requires a large cavity to be created, in order to overcome this limitation in mass flow excavators a means is required to collect and carry the excavated material through a duct means away from the excavated cavity. The distance by or over which the material requires to be carried is determined by the size of the cavity to be created. 
     US 2007166107 (JACOBSEN et al) discloses a subsea excavation and suction device which includes a suction head with an inlet opening at an outer, free end and an outlet opening connected to a suction hose arranged at a distance from the inlet opening. The suction head is mounted on a hydraulic controller arm and has at the inlet opening provided with mechanical and hydraulic means to disintegrate solid material (sediment). The hydraulic means includes a number of jet nozzles, while the mechanical means includes bars. The cross-sectional area of the inlet opening is larger than the cross-sectional area of the outlet opening. 
     U.S. Pat. No. 4,479,741 A (BERTI et al) discloses a self-propelling device for burying and digging up subsea conduits laid on beds of an incoherent material. The device has: disintegrating members using high pressure water jets to create a slurry of material; digging members having suction members which draw the suspension prepared by the disintegrating members, thus leaving a trench behind; and displacement members for moving the device on the sea bed astride the conduit. 
     EP 1 857 598 A1 (IHC HOLLAND IE) discloses a suction dredger comprising a dredging tube which at one end carries a suction head and which at the other end is connected to the suction dredger hull through a hull pivot with a pivot axis which is generally transverse with respect to said hull. 
     www.toyopumpseurope.com/toyo exca.html discloses a submersible excavator having a mechanical agitator. 
     The above apparatus are mechanically complex and provide a slow means of excavation in comparison to their relative expense. 
     It is an object of at least one embodiment of at least one aspect of the present invention to seek to obviate or at least mitigate one or more of the aforementioned problems in the prior art. 
     It is an object of at least one embodiment of at least one aspect of the present invention to seek to obviate or at least mitigate one or more problems in the prior art. 
     It is an object of at least one embodiment of at least one aspect of the present invention to provide a means to effect a desire for excavating a location or “deburying” an object and optionally for collecting and transporting excavated material in a rapid and comparatively inexpensive manner. 
     SUMMARY OF INVENTION 
     One or more objects of the present invention are sought to be addressed by providing the general solution of an underwater excavation apparatus comprising: 
     means for disturbing or excavating an underwater location, such as a sea bed, ocean floor or river bed; 
     means for extracting or sucking excavated material from the location to another location. 
     According to a first aspect of the present invention there is provided an apparatus, device or tool, such as and beneficially an excavation apparatus, device or tool, such as and more beneficially an underwater excavation apparatus or tool, the apparatus or tool comprising: 
     at least one and preferably one mass flow excavation means; and 
     at least one and preferably one suction or collection means. 
     The term “mass flow” used herein is a known term of art, distinguished from “jetting” as hereinbefore explained. 
     The mass flow means may comprise means for blowing or directing fluid, e.g. at a predetermined or selected location to be excavated. 
     The mass flow or fluid may comprise underwater fluid, e.g. from the body of water, e.g. sea water, under or within which the location is positioned. 
     The mass flow means may disturb or disrupt material(s) at and/or around the location. 
     The disrupted material(s) may be referred to as, or comprise spoil. 
     The apparatus or tool may comprise means for restricting spoil or directing spoil to the suction means. 
     The apparatus or tool may comprise a baffle or hood. The baffle or hood may comprise the means for restricting and/or directly spoil. 
     The apparatus or tool may comprise a housing, enclosure or cowling, which may comprise or define a space or cavity. 
     The housing, enclosure or cowling may comprise a closed top which may comprise the baffle or hood. 
     The housing, enclosure or cowling may be made from a sheet material, e.g. sheet metal. The housing, enclosure or cowling may comprise a skeleton or frame. 
     The housing, enclosure or cowling may comprise an access means, e.g. hatch or door, e.g. in a side wall thereof. Such access means may allow access to the space or cavity, e.g. for maintenance. 
     The housing may be rectilinear or domed. The space may be rectilinear. This arrangement is believed to be advantageous. 
     The housing may comprise a wall or walls or skirt which may depend downwardly from a top. 
     The housing may comprise a base which may be at least partly open. In this way the housing may be positioned, in use, such that the housing may rest on or above the location and spoil may be removed from the location via the base into the space or cavity by the action of the mass flow excavation means. 
     The housing may comprise a planar, e.g. substantially rectangular, top. The housing may comprise a planar, e.g. substantially rectangular, base. The top may, in use, be positioned above the base. The top may be smaller than the base. This may make the housing more stable, in use. The housing may comprise first and second opposing side walls, which may taper (e.g. outwardly) from the top to the base. The housing may comprise third and fourth opposing side walls, which may depend substantially vertically between the top and the base. The first and second side walls may be bigger than the third and fourth side walls. 
     In use, a direction of intended movement of the apparatus, device or tool may be substantially parallel to a longitudinal axis of the first and second side walls. In use, a direction of intended movement of the apparatus, device, or tool may be substantially parallel to the top and the base. 
     In use, a direction of intended movement of the apparatus, device or tool may be substantially perpendicular to the third and fourth side walls. 
     The apparatus or tool may comprise means for moving the apparatus substantially vertically and/or means for moving the apparatus substantially horizontally. 
     An inlet of the mass flow excavation means may be located external of or at least communicable with external of the housing. An outlet of the mass flow means may be located internal of or at least communicable with internal of the housing, e.g. within the space. The outlet of the mass flow means may be provided in a lower portion of the space. 
     An inlet of the suction means may be located internal of or at least communicable with internal of the housing. An outlet of the suction means may be located external of or at least communicable with external of the housing, e.g. within the space. 
     The outlet of the mass flow means may be nearer the base than the inlet of the suction means. 
     A screen or filter may be provided between the mass flow excavation means and the suction means, e.g. between an outlet from the mass flow excavation means and an inlet of or to the suction means. 
     A face or side of the screen or filter closer to the mass flow excavation means may face at least partially downward or be inclined towards the base. 
     The apparatus may comprise means for facilitating movement of the apparatus such as skis, skids or runners, which may be provided on the housing, e.g. at, on, or adjacent the base. 
     The mass flow excavation means may be substantially vertically disposed, e.g. on the top of the housing. 
     In one embodiment the suction means may be substantially horizontally disposed, e.g. on a side of the housing, e.g. on one of the third or fourth side walls. 
     In an alternative embodiment the suction means may be substantially vertically disposed, e.g. on the top of the housing. 
     The mass flow excavation means may comprise a hollow body (e.g. cylindrical body) having an inlet and an outlet, at least one impeller rotatably mounted in the hollow body and means for driving the at least one impeller. The mass flow excavation means hollow body may be mounted through the housing. 
     An inner diameter (“nozzle”) diameter of at least the outlet of the mass flow excavation means of the hollow body may be at least 450 mm, or 660 mm or greater. 
     In one implementation the mass flow excavation means may comprise a device comprising a hollow body having an inlet and an outlet, at least one pair of impellers coaxially displaced one from the other and rotatably mounted in the hollow body and means for driving the impellers of the/each pair in contrary rotating or contra-rotating directions. Such a device is disclosed in GB 2 297 777 A, the content of which is incorporated herein by reference. 
     The inlet and outlet of the hollow body may be provided at opposing ends thereof, the common axis of the impellers extending between the inlet and the outlet. 
     The means for driving the impellers may comprise a motor. 
     The motor may be selected from one of a “Moineau”, a hydraulic or an electric motor. 
     In another implementation the mass flow excavation means may comprise a hollow body having at least two inlets and at least one outlet, at least one pair of impellers rotatably mounted in the hollow body, and means for driving the impellers, wherein the at least two inlets are substantially symmetrically disposed around an axis extending from the at least one outlet. Such a device is disclosed in EP 1 007 796 B1, the content of which is incorporated herein by reference. 
     The driving means may cause the impellers to be driven in contrary rotating or contra-rotating directions. 
     One of the impellers may be provided within one of the inlets and another of the impellers may be provided within another of the inlets. There may be provided one pair of inlets. 
     The mass flow means may comprise a pair of horizontally opposed inlets communicating with a single outlet, the outlet being disposed substantially midway between, and preferably perpendicular the two inlets, in use, such that the means is substantially “T” shaped in profile. 
     Alternatively the mass flow means may comprise a pair of inlets communicating with a single outlet, the inlets being substantially symmetrically disposed around an axis extending from the outlet, the outlet being disposed vertically downwards substantially midway between the two inlets, in use, such that the means is substantially “Y” shaped in profile. 
     An/the at least one impeller may be provided within each outlet. 
     The/each suction means may comprise a hollow body (e.g. cylindrical body) having an inlet and an outlet, at least one impeller rotatably mounted in the hollow body and means for driving the at least one impeller. The suction means hollow body may be mounted through the housing. 
     An inner (“nozzle”) diameter of at least the outlet of the suction means hollow body may be at least 450 mm, or may be 600 mm, or greater. 
     The/each suction means may be of a substantially similar or same structure to the mass flow excavation means. The suction means may comprise a further mass flow means. 
     In one implementation the suction means may comprise a device comprising a hollow body having an inlet and an outlet, at least one pair of impellers coaxially displaced one from the other and rotatably mounted in the hollow body and means for driving the impellers of the/each pair in contrary rotating or contra rotating directions. 
     The inlet and outlet of the hollow body may be provided at opposing ends thereof, the common axis of the impellers extending between the inlet and the outlet. 
     The means for driving the impellers may comprise a motor. 
     The motor may be selected from one of a “Moineau” motor, a hydraulic motor, or an electric motor. 
     In another implementation the suction means may comprise a hollow body having at least two inlets and at least one outlet, at least one pair of impellers rotatably mounted in the hollow body, and means for driving the impellers, wherein the at least two inlets are substantially symmetrically disposed around an axis extending from the at least one outlet. 
     The driving means may cause the impellers to be driven in contrary or contra-rotating directions. 
     One of the impellers may be provided within one of the inlets and another of the impellers may be provided within another of the inlets. 
     There may be provided one pair of inlets. 
     The suction means may comprise a pair of horizontally opposed inlets communicating with a single outlet, the outlet being disposed substantially midway between and preferably perpendicular to the two inlets, in use, such that the means is substantially “T” shaped in profile. 
     Alternatively the suction means may comprise a pair of inlets communicating with a single outlet, the inlets being substantially symmetrically disposed around an axis extending from the outlet, the outlet being disposed substantially midway between the two inlets, in use, such that the means is substantially “Y” shaped in profile. 
     An/the at least one impeller may be provided within each outlet. 
     Preferably, in use, the suction means may act or operates at a higher (mass) flow rate than the mass flow excavation means. 
     In a beneficial implementation the suction means may operate at approximately double the flow rate of the mass flow excavation means. 
     A mass flow rate of the mass flow excavation means may be at least 2,000 litres/second, and typically in the range of 2,000 to 16,000 litres/second. 
     A mass flow rate of the suction means may be at least 2,000 litres/second, and typically in the range of 2,000 to 16,000 litres/second. 
     Preferably, in use, a pressure of the flow from the mass flow means may be less than 100 psi (6.89×10 5  Pa), preferably less than 50 psi (3.44×10 5  Pa), preferably in the range 5 to 25 psi (3.44×10 4  to 1.72×10 5  Pa), and most preferably, in the range 10 to 20 psi (6.89×10 4  Pa to 1.37×10 5  Pa). 
     Preferably, in use, a pressure of flow into the suction means may be less than 100 psi (6.89×10 5  Pa), preferably less than 50 psi (3.44×10 5 ), preferably in the range 5 to 25 psi (3.44×10 4  to 1.72×10 5  Pa), and most preferably in the range 10 to 20 psi (6.89×10 4  to 1.37×10 5  Pa). 
     Preferably, in use, the action of the mass flow excavation means acts to reduce a size of spoil or distributed material, e.g. particulate thereof. 
     In a preferred implementation wherein the apparatus comprises a/the hood or housing and a/the filter or screen, in use, the mass flow excavation means may disturb and cause recirculation and reduction in size of spoil or disturbed material within the hood or housing. This may act to seek to make spoil or disturbed material small enough to pass through the screen or filter, and preferably of a maximum predetermined size to make the spoil suitable for transportation along a transport means. 
     The housing may be rectilinear or domed. The space may be rectilinear or domed. The latter may be of benefit to recirculation. 
     In a modification, the housing may be provided with means to at least partially fit over at least a portion of a pipe, pipeline, or tubular to be or which is being excavated or deburied. 
     The means for fitting over may be provided with sealing means. The sealing means may act to seal between the housing and the pipe, pipeline, or tubular, in use. The sealing means may be elastomeric. 
     For example, suitably shaped apertures may be provided in the third or fourth side walls of the housing. The apertures may be transversely aligned with one another. The apertures may extend from the base of the housing. The apertures may be substantially U-shaped. 
     This arrangement may allow the housing to be moved along the pipe as excavation or deburying thereof progresses, in use. 
     At least a portion of a/the transportation means or pipe may be trailed rearward of a direction of movement of the housing, in use. 
     According to a second aspect of the present invention there is provided a system, such as an excavation system, such as an underwater excavation system, comprising: 
     at least one apparatus according to the first aspect or general solution of the invention; and 
     means for transporting spoil from the suction means to a remote location. 
     The transport means may comprise a pipe or hose. The hose may be a collapsible or a lay flat hose. 
     The transport means may comprise at least one further suction means positioned along the transport means, e.g. in series with the suction means. 
     In one implementation the remote location may comprise a location on the sea bed, ocean floor or river bed, e.g. below the level of the location being excavated. This is particularly beneficial in seeking to obviate or mitigate refilling of the excavated location. 
     Alternatively, the remote location may comprise an above surface location or a vessel, e.g. surface vessel, e.g. boat, ship, barge or hopper. 
     An inlet of the transport means may communicate with an outlet of the suction means. 
     An outlet of the transport means may communicate to or with the remote location. 
     According to a third aspect of the present invention there is provided a method of excavating a location, such as an underwater location, comprising: 
     providing a system according to the second aspect of the present invention; using the system to move material from the location to a remote location. 
     According to a fourth aspect of the present invention there is provided a combination of a mass flow excavator and a suction means. 
     Optionally and beneficially the combination comprises an enclosure or housing. 
     According to a fifth aspect of the present invention there is provided an apparatus, device, or tool, such as an excavation apparatus, device, or tool, such as an underwater excavation apparatus, device, or tool comprising: 
     a first mass flow means; and 
     a second mass flow means. 
     The first mass flow means may direct or cause flow, e.g. of fluid, towards a location to be excavated. 
     The second mass flow means may direct or cause flow, e.g. of spoil, away from the location and/or adjacent the location. 
     The apparatus or tool may comprise a housing. 
     The first mass flow means may be a “blowing” means. 
     The second mass flow means may be a “sucking” means. 
     According to a sixth aspect of the present invention there is provided a method of excavating an underwater location comprising: 
     surveying the location; 
     excavating the location. 
     The step of surveying the location may comprise dividing the location and the environs thereof (or surrounding area) into a plurality of sectors, e.g. grid sectors. 
     The step of surveying may also comprise establishing a height, e.g. an average height, of a surface or position, e.g. below a surface of a body of water, such as a sea bed, ocean floor, lake bed, or river bed, or the like within at least a sector in which the location lies and at least one and preferably a plurality of another sector(s). Preferably the step of surveying comprises selecting one of the another sectors distal or remote from the location sector, e.g. not adjacent thereto, which (one) another sector has a lower height (at least average height) than a height (at least average height) the location sector. In other words, the another sector may be at least on average deeper below sea level, or below a surface of a body of water than the location sector. 
     Preferably also the step of selecting the one another sector comprises selecting the another sector dependent upon said another sector being in a downstream disposition or diagonally downstream disposition of the location sector in one tidal stream direction. 
     The method may also comprise providing an excavation apparatus, and preferably excavating the location with the excavation apparatus. 
     The excavation apparatus may comprise an excavation apparatus, device, tool or system according to any preceding general solution or aspect of the present invention. 
     The step of excavating the location may comprise using the excavation apparatus to remove material or spoil from the location sector to the selected another sector. 
     The method may comprise repeating the steps of the method for a plurality of locations in a plurality of sectors. In such case, each another location may be different and/or the same. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the invention will now be described by way of example only, and with reference to the accompanying drawings, which are: 
         FIG. 1  a partial cross-sectional side view of an excavation apparatus according to a first embodiment of the present invention; 
         FIG. 2  an end view of the excavation apparatus of  FIG. 1 ; 
         FIG. 3  a partial cross-sectional top view of the excavation apparatus of  FIG. 1 ; 
         FIG. 4  a cross-sectional side view of a mass flow excavation means or suction means of the excavation apparatus of  FIG. 1 ; 
         FIG. 5  a partial cross-sectional side view of an excavation apparatus according to a second embodiment of the present invention; 
         FIG. 6  a schematic perspective view of an underwater excavation system according to the present invention, in use; 
         FIG. 7  a schematic diagram of an excavation area divided into sectors; 
         FIG. 8  a further schematic diagram of the excavation area of  FIG. 7  divided into sectors; 
         FIG. 9  a further schematic diagram of the excavation area of  FIG. 7  subdivided into sub sectors; and 
         FIG. 10  an end view of an excavation apparatus according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF DRAWINGS 
     Referring initially to  FIGS. 1 to 4 , there is shown an excavation apparatus, device, or tool, particularly an underwater excavation apparatus, device, or tool, generally designated  5 , according to a first embodiment of the present invention. 
     The excavation apparatus  5  comprises: means  10  for disturbing or excavating an underwater location, such as a sea bed, ocean floor or river bed; and means  15  for extracting or sucking excavated material (suction means) from the location to another location. The disturbing or excavating means  10  comprise mass flow excavation means or mass flow means  20 . The suction means  15  comprise suction or collection means or further mass flow means  25 . 
     The mass flow means  20  comprise means for blowing or directing fluid, e.g. at a predetermined or selected location to be excavated. The fluid comprises underwater fluid, e.g. from the body of water under or within which the location is positioned. In use, the mass flow means  20  disturbs or disrupts material(s) at and/or around the location. The disrupted material(s) is referred to as spoil. 
     The apparatus  5  comprises means  30  for restricting spoil and/or directing spoil to the suction means  25 . The restricting/directing means  30  comprises a baffle or hood  35 . The hood  35  comprises part of a housing, enclosure or cowling  40  which defines a space or cavity  45 . The housing  40  comprises a closed top  50  which comprises the baffle or hood  35 . 
     The housing  40  comprises a side wall or walls or skirt  55 , which depend downwardly from the top  50 . The housing  40  also comprises a base  60  which is at least partly open. In this way the housing  40  can be positioned, in use, such that the housing  40  rests on or above the location, and spoil removed from the location via the base  60  into the space  45  by the action of the mass flow means  20 . 
     The housing  40  is typically made from a sheet material, e.g. sheet metal. The housing  40  comprises a skeleton or frame  61  for the sheet material. The housing  40  has an access means  65 , e.g. hatch or door, e.g. in a side wall thereof. Such access means  65  allows access to the space  45 , e.g. on shore, above surface and/or below surface. 
     The housing  40  comprises a planar, e.g. substantially rectangular, top  50 . The housing  40  comprises a planar, e.g. substantially rectangular, base  60 . The top  50  is, in use, positioned above the base  60 . The top  50  is in this embodiment smaller than the base  60 . This makes the housing  40  more stable, in use. The housing  40  comprises first and second opposing side walls  70 , 75 , which taper outwardly from the top  50  to the base  60 . The housing  40  comprises third and fourth opposing side walls  80 , 85 , which depend substantially vertically between the top  50  and the base  60 . The first and second side walls  70 , 75  are longer than the third and fourth side walls  80 , 85 . In use, a direction of possible or intended movement of the apparatus  5  along or adjacent the sea bed is substantially parallel to longitudinal axes of the first and second side walls  70 , 75 . 
     The apparatus  5  comprises means  90  for moving the apparatus  5  substantially vertically comprising padeyes and/or means  95  for moving the apparatus  5  substantially horizontally comprising further padeyes. 
     An inlet  100  of the mass flow means  20  is located external of the housing  40 . An outlet  105  of the mass flow means  20  is located internal of the housing  60 ; in this embodiment in a lower portion of the space  45 . 
     An inlet  110  of the suction means  25  is located internal of the housing  40 . An outlet  115  of the suction means  25  is located external of the housing  40 . As can be seen from  FIG. 1 , the inlet  100  of the mass flow means is provided nearer the base  60  than is the inlet  110  of the suction means  25 . 
     A screen or filter  120  is provided between the mass flow means  20  and the suction means  25 , e.g. between the outlet  105  from the mass flow means  20  and the inlet  110  of the suction means  25 . A face or side  121  of the screen  120  closer to the mass flow means  20  faces at least partially downward or is inclined towards the base  60 . 
     The apparatus  5  comprises means  125  for facilitating movement of the apparatus  5  such as skis, skids or runners, which are provided on the housing  40 , e.g. at, on, or adjacent the base  60 . 
     The mass flow means  20  are, at least in use, substantially vertically disposed, and in this embodiment positioned on the top  50  of the housing  40 . Further, in this embodiment the suction means  25  are, at least in use, substantially horizontally disposed on a side of the housing  40 , i.e. on the fourth side wall  85 . 
     As can best be seen from  FIG. 4 , the mass flow means  20  comprises a hollow body  130  having the inlet  100 , the outlet  105 , at least one impeller  135  rotatably mounted in the hollow body  130  and means  140  for driving the at least one impeller  135 . 
     In one alternative implementation the mass flow means  10  comprises a device comprising a hollow body having an inlet and an outlet, at least one pair of impellers coaxially displaced one from the other and rotatably mounted in the hollow body and means for driving the impellers of the/each pair in contrary rotating directions. Such a device is disclosed in GB 2 297 777 A, the content of which is incorporated herein by reference. 
     The inlet and outlet of the hollow body can be provided at opposing ends thereof, the common axis of the impellers extending between the inlet and the outlet. 
     The means for driving the impeller(s) can comprise a motor. The motor can be selected from one of: preferably a “Moineau” motor, a hydraulic motor, or alternatively, an electric motor. 
     In another alternative implementation the mass flow means  10  comprises a hollow body having at least two inlets and at least one outlet, at least one pair of impellers rotatably mounted in the hollow body, and means for driving the impellers, wherein the at least two inlets are substantially symmetrically disposed around an axis extending from the at least one outlet. Such a device is disclosed in EP 1 007 796 B1, the content of which is incorporated herein by reference. The driving means can cause the impellers to be driven in contrary rotating directions. One of the impellers can be provided within one of the inlets and another of the impellers can be provided within another of the inlets. There can be provided one pair of inlets. 
     The mass flow means can comprise a pair of horizontally opposed inlets communicating with a single outlet, the outlet being disposed substantially midway between and perpendicular to the two inlets, in use, such that the means is substantially “T” shaped in profile. 
     Alternatively the mass flow means can comprise a pair of inlets communicating with a single outlet, the inlets being substantially symmetrically disposed around an axis extending from the outlet, the outlet being disposed substantially midway between the two inlets, in use, such that the means is substantially “Y” shaped in profile. 
     The at least one impeller can be provided within the or each inlet of the mass flow means. 
     Referring again to  FIG. 1 , the/each suction means ( 15 ) comprises a hollow body  145  having the inlet  110  and the outlet  115 , at least one impeller  150  rotatably mounted in the hollow body and means  155  for driving the at least one impeller  150 . 
     The/each suction means  15  is typically of a similar structure to the mass flow means  20 —e.g. as shown in  FIG. 4 . 
     In one alternative implementation the suction means  15  comprises a device comprising a hollow body having an inlet and an outlet, at least one pair of impellers coaxially displaced one from the other and rotatably mounted in the hollow body and means for driving the impellers of the/each pair in contrary rotating directions. 
     The inlet and outlet of the hollow body can be provided at opposing ends thereof, the common axis of the impellers extending between the inlet and the outlet. 
     The means for driving the impellers typically comprise a motor. The motor can be selected from one of: preferably a “Moineau” motor, a hydraulic motor, or alternatively, an electric motor. 
     In another alternative implementation the suction means  15  alternatively comprises a hollow body having at least two inlets and at least one outlet, at least one pair of impellers rotatably mounted in the hollow body, and means for driving the impellers, wherein the at least two inlets are substantially symmetrically disposed around an axis extending from the at least one outlet. 
     The driving means can cause the impellers to be driven in contrary rotating directions. One of the impellers can be provided within one of the inlets and another of the impellers may be provided within another of the inlets. There can be provided one pair of inlets. 
     The suction means can comprise a pair of horizontally opposed inlets communicating with a single outlet, the outlet being disposed substantially midway between and perpendicular to the two inlets, in use, such that the means is substantially “T” shaped in profile. 
     Alternatively the suction means can comprise a pair of inlets communicating with a single outlet, the inlets being substantially symmetrically disposed around an axis extending from the outlet, the outlet being disposed substantially midway between the two inlets, in use, such that the means is substantially “Y” shaped in profile. 
     The at least one impeller can be provided within the or each outlet of the suction means. 
     In use, the suction means  25  can act or operate at a higher flow rate than the mass flow means  20 . For example, in a beneficial implementation the suction means  25  can operate at approximately double the flow rate of the mass flow excavation means  20 . 
     The mass flow rate of the mass flow means may be typically at least 2,000 litres/second, and typically in the range of 2,000 to 16,000 litres/second. 
     The mass flow rate of the suction means may be typically at least 2,000 litres/second, and more typically in the range of 2,000 to 16,000 litres/second. 
     The pressure of flow from the mass flow means  20  is less than 6.89×10 5  Pa (100 psi), preferably less than 3.44×10 5  Pa (50 psi), e.g. in the range 3.44×10 4  to 1.72×10 5  Pa (5 to 25 psi), and most typically in the range 6.89×10 4  to 1.37×10 5  Pa (10 to 20 psi). 
     The pressure of flow into the suction means  25  is less than 6.89×10 5  Pa (100 psi), e.g. less than 3.44×10 5  (50 psi), e.g. in the range 3.44×10 4  to 1.72×10 5  Pa (5 to 25 psi), and typically in the range 6.89×10 4  to 1.37×10 5  Pa (10 to 20 psi). 
     In use, the action of the mass flow means  20  acts to reduce a size of spoil or distributed material, e.g. particulate thereof. The hood  35 /housing  40  and a/the filter screen  120 , in use, co-act with the mass flow means  20  and suction means  25 , such that the mass flow means  20  disturbs and causes recirculation and reduction in size of spoil or disturbed material within the hood  35  and housing  40 . This acts to seek to make spoil or disturbed material small enough to pass through the screen or filter  115 , and advantageously of a maximum predetermined size to make the spoil suitable for transportation along a transport means. 
     In this embodiment the housing  40  and space  45  are rectilinear. However, in a modification the housing  40  and/or space  45  can be domed in shape. 
     The mass flow means  20  produces a high speed water flow, with a velocity typically in the order of 5 to 10 meters per second, being directed at the sea bed, and in doing so loosening material from the sea bed and throwing it up in the form of a precipitating cloud around the mass flow means  20 . 
     The mass flow means  20  comprises a propeller or impeller pump means as hereinbefore described, or can be a (large) centrifugal pump type, or a combination thereof. The mass flow means  20  is typically driven by hydraulic motor means, or alternatively, an electric motor means. The inlet of the mass flow means  20  tool is on the outside of the hood  35  and the mass flow means  20  outlet or exhaust is under the hood  35 . 
     The invention provides a means whereby the aforementioned cloud around the mass flow means  20  is captured under housing  40  which contains the mass flow means  20 . The housing  40  is suspended on a cable (S) (not shown) via padeyes  90  controlling the height and position of the housing  40  above the location where a cavity is to be created. The housing  40  can be pulled along the sea bed with further cables (not shown) secured to pulling padeyes  151 , and for this purpose the housing  40  is provided with skis or runners  152 . 
     Also connected to the housing  40  is suction means  25 , i.e. additional pump means, which can also be in the form of a propeller or centrifugal pump means or combination thereof, with its inlet ( 110 ) connected to or communicable with the space  45  under the housing  40  to ingest the disturbed sea bed material, and an exhaust or outlet  115  connected to a hose or pipe in order to transport the disturbed material to another location away from or remote from the space or cavity  45  at a distance controlled by the length of the hose which can exhaust to a second location on the sea bed or into a hopper or barge means on the water surface for further transport. The hose can be of a lay flat type which can be moved into position by divers or may be of a rigid construction. The hose can be buoyant, in order to float on the water surface, or it can be negatively buoyant in order to sit on the sea bed. 
     It is understood that most suction means  25  still have a limitation with respect to ingested particle size, and to this end screen  120  is positioned between a suction area or space and an excavating area or space within the space  45  under the hood  35  which prevents particles greater than the mesh size of the screen  120  from being ingested by the suction means  25 . Generally particles greater than 70 mm are captured by the screen  120  and so prevented from entering the suction means  25 . As can be seen from  FIG. 1 , the screen  120  is positioned at an angle in such a manner that when the suction means  25  is temporarily stopped the particles caught by the screen  120  will fall harmlessly back into the space  45 . In the case of larger particles being clumped clay or sand/clay aggregate, it is intended that the subsequent circulation caused by the excavation apparatus or tool  5  will break up the aforementioned particles until they are at the size that will pass through the screen  120  for subsequent removal and transport by the suction means  25 . 
     It will be understood that the housing  40  or hood  35  can be of a variety of shapes, such as dome shaped or rectangular, and that the housing  40  or hood  35  can be made of steel or high strength plastics, and that the housing  40  or hood  35  can be supported by support members, i.e. skeleton or frame  61 . The hood  35  is provided with an access hatch  65  to allow personnel to access the inside of the housing  40  or hood  35 , and particularly the inlet  110  of the suction means  25  for maintenance. 
     It will also be understood that there may be one or more mass flow means  20  introducing water into the hood  35  and one or more suction means  25  extracting water from under or within the hood  35 . While in the beneficial disclosed embodiment the mass flow means  20  is the sole excavation means  10 , it is also possible to introduce additional higher velocity jets of water in order to break up harder or stiffer clays, such as clays of 70 to 100 kPa or higher. For harder soils it is also possible to use a mechanical means or agitator to disturb the sea bed for suspension in the fluid under the hood  35 . 
     It will be understood that, in order to transport the excavated material along the transportation pipe, the ratio of sea bed to water being transported should advantageously not exceed a ratio of approximately 15% to 20% solids to water. This ratio can be controlled by varying the power supplied to a mass flow pump and the power supplied to a suction pump. 
     To transport material over long distances, say 200 meters or further, it may be necessary to add another suction pump in series with suction means  25  to overcome pressure losses in the transportation pipe. The additional pumps can be directly coupled after the first suction pump  25  or can be some distance along the transportation pipe. 
     In order to minimise damage caused by abrasion and wear of impellers and guide vanes of the mass flow means  20  and suction means  25 , the impellers and guide vanes can be made of a hard material or a material with a hard coating such as nitride coating or tungsten carbide coating. 
     Referring now to  FIG. 5  there is shown an excavation apparatus, generally designated  5   a , according to a second embodiment of the present invention. 
     The excavation apparatus  5   a  is similar to the excavation apparatus  5  of the first embodiment, like parts being denoted by like numerals, but suffixed “a”. 
     In this second embodiment the suction means  25   a  is substantially vertically disposed on the top  50   a  of the housing  40   a . This can be suitable for excavation of deep cavities and vertical lifting of disturbed material or spoil. 
     Referring now to  FIG. 6  there is shown an underwater excavation system generally designated  200 , comprising: 
     at least one apparatus  5 ;  5   a  according to  FIGS. 1 to 4  or  FIG. 5 ; and 
     means  205  for transporting spoil from the suction means  25  to a remote location L R . 
     The transport means  205  comprises a pipe or hose  210 . The hose  210  is typically a collapsible or lay flat hose, e.g. handlable by divers. The transport means  205  optionally comprises at least one further suction means (not shown) positioned along the transport means  205 . 
     In one implementation the remote location L R  comprises a location on the sea bed, ocean floor, lake floor, or river bed, or the like, e.g. below the level of the location being excavated. This is particularly beneficial in seeking to obviate or mitigate refilling of the excavated location. Alternatively, the remote location can comprise a vessel, e.g. barge or hopper. 
     In use the invention also provides a method of excavating the underwater location, comprising: 
     providing the system  200 ; 
     using the system  200  to move material from the location L E  to a remote location L R . 
     Referring now to  FIGS. 7 to 9 , there is exemplified a method of excavating an underwater location L E  comprising: surveying the location L E ; excavating the location L E . 
     The step of surveying the location L E  comprises dividing the location and the environs thereof into a plurality of sectors, e.g. grid sectors, A; i 1 , ii 1  . . . ; i 2 , ii 2 , . . . . 
     The step of surveying also comprises establishing a height, e.g. an average height, of a surface or position, e.g. sea bed, ocean floor, lake floor, or river bed, or the like within at least a sector i 1  in which the location lies and at least one and preferably a plurality of another sector(s) i 2 . The step of surveying comprises selecting one of the another sectors i 2  distal or remote from the location sector i 1 , i.e. not adjacent thereto, which another sector i 2  has a lower height than the location sector i 1 . 
     Also the step of selecting the one another sector i 2  comprises selecting the another sector i 2  dependent upon said another sector i 2  being in a non direct or diagonally downstream disposition or diagonally downstream disposition of the location sector i 1  in one tidal stream direction. 
     The method also comprises providing an excavation apparatus  5 , and excavating the location L E  with the excavation apparatus  5 . 
     The step of excavating the location L E  comprises using the excavation apparatus  5  to remove material or spoil from the location sector i 1  to the selected another sector i 2 . 
     The method typically comprises repeating the steps of the method for a plurality of locations in a plurality of sectors ii 1 , . . . . In such case, each another location can be different and/or the same. 
     In use, to remove excavated sea bed material, the excavation system  200  is deployed to the sea bed  300  from a vessel V 1 . The hose  210  can be a lay flat type, and can be rolled out sub sea by divers. A discharge diffuser with a handle or ROV latch (not shown) can be fitted to the discharge end of the hose  210 . After the hose  210  has been laid out, and divers have confirmed that the discharge lines are flowing freely, the excavation apparatus  5  can be powered up and excavation commenced. A work boat V 2  can be used to move a discharge end of the hose  210 . Prefabricated saddles (not shown) can be deployed beneath the hose  210  at intervals, for example, of approximately 100 metres, to assist with hose  210  movement and handling. 
     Planning and pre-job mapping of the area surrounding the location L E  to be excavated is key to successful excavation work. The area is divided into a plurality of sectors i 1 , ii 1  . . . ; i 2 , ii 2 , . . . by a grid. The tidal direction is determined, a topography of the area is determined, and a plan of material movement from a sector i 1  to sector i 2  etc is planned. 
     As shown in  FIGS. 7 to 9 , the respective sectors are spaced from one another and diagonally displaced from one another in relation to tidal direction. Further, the sector i 2  to which the material is removed is most preferably at a lower level than the sector i 1  from which the material is removed. By this method, efficient movement of material is provided and back filling of excavated obviated or mitigated. 
     Referring to  FIG. 9 , each of the sectors i 1 , ii 1  . . . ; i 2 , ii 2 , . . . can be further sub divided into sub sectors in a modified implementation of the method of excavation, if so desired. 
     Pumps of the mass flow means  20  and suction means  25 , can operate at around at least 2,000 litres per second, and typically, up to a maximum of 8,000 litres per second. Spoil transportation rates are dependent upon a number of factors, particularly spoil characteristics. Tons of spoil pumped per minute are dependent upon volume achieved. For example, for soil by volume percentage 5, 10 and 15%, tons of soil pumped per minute for pumps of 2,000 litres per second would be in the region of 6, 12, or 18 tons of soil pumped per minute. 
     Referring now to  FIG. 10 , there is shown an end view of an excavation apparatus generally designated  5   b , according to a third embodiment of the present invention. The excavation apparatus  5   b  is similar to the excavation apparatus  5  of the first embodiment, like parts being denoted by like numerals, but suffixed “b”. 
     In this third embodiment the housing  40   b  is adapted to at least partially fit over at least a portion of a pipe  41   b  to be, or which is, being excavated or deburied. A pair of apertures  42   b  are provided in the third and fourth side walls  80   b ,  85   b  of the housing  40   b . The apertures  42   b  are transversely aligned with one another, extend from the base  60   b  of the housing  40   b  and are substantially U-shaped. 
     This arrangement allows the housing  40   b  to be moved along the pipe  41   b  as excavation thereof progresses. The pipe  41   b  typically will have an outer diameter in the range 8 inches (20.32 cms) to 42 inches (106.68 cms). 
     Each aperture  42   b  is provided with a peripheral sealing means  43   b.    
     The sealing means  43   b  act to seal between the housing  40   b  and the pipe  41   b , in use, so as to improve the efficiency of the excavation apparatus  5   b.    
     At least a portion of transportation means or pipe (not shown) extending from the suction means  15   b  can, in use, extend or trail rearward of a direction of movement of the housing  40   b.    
     It will be appreciated that the embodiments of the present invention hereinbefore described are given by way of example only, and are not meant to limit the scope of the invention in any way. Further, any features of the invention recited in the Summary of Invention may form part of the disclosed embodiments. 
     The disclosed embodiments provide an apparatus or tool, such as an excavation apparatus or tool, such as an underwater excavation apparatus or tool, comprising: 
     a first mass flow means; and 
     a second mass flow means. 
     The first mass flow means may direct or cause flow, e.g. of fluid, towards a location to be excavated. The second mass flow means may direct or cause flow, e.g. of spoil, away from the location. The apparatus or tool may also comprise a housing. The first mass flow means can be referred to as a “blowing” means. The second mass flow means can be referred to as a “sucking” or “suction” means.