Abstract:
A system for efficiently circulating water in a large volume of rearing space for aquatic organisms, comprising an impervious enclosure for containing the water and aquatic organisms and a pump for pumping water from an intake duct through intake ducting to a flow diverter, which then directs a flow of water radially outward within the enclosure to directly induce a circulation of water within the enclosure. The ‘center drive’ circulation pattern is sufficiently uniform to provide optimum rearing conditions for cultured finfish, while also ensuring that solid wastes are swept toward the central drain, even in a very large tank.

Description:
FIELD OF INVENTION 
       [0001]    This invention relates to the rearing of aquatic organisms in a controlled environment and more particularly to impervious closed-container rearing systems enclosing and efficiently circulating water in a large volume of rearing space. 
       BACKGROUND OF THE INVENTION 
       [0002]    Methods and equipment for induction of circulating flow in aquaculture enclosures are known in the art. Circular tanks are most commonly used, due to their inherent structural strength, and because they can maintain a characteristic rotating flow, against which finfish are induced to swim. Swimming exercise is believed to promote weight gain and feed conversion efficiency in some species of finfish. 
         [0003]    In one common design, water is introduced into a circular rearing tank at the perimeter, in a tangential direction, so as to impart angular momentum to the fluid flow, and is withdrawn from the central axis of the tank through a standpipe or floor drain. The primary flow in this design follows a spiral path from the perimeter toward the center. It is also known that such azimuthal flow in circular tanks induces a secondary, toroidal flow by a mechanism known as the ‘teacup effect’; centrifugal pressure exerted on fluid at the rotating free surface boundary is not balanced by the slower boundary layer-influenced flow adjacent to the floor of the tank. The pressure imbalance induces flow radially outward along the free surface, down the vertical tank wall, and radially inward across the floor, back to the central axis, where fluid is displaced vertically upward creating a hydraulic circuit. The teacup effect is responsible for the self-cleaning property of circular tanks, whereby settle-able solid debris, including fecal matter, uneaten feed pellets, and moribund fish, are swept in a spiral path toward the center of the floor and out through a drain. 
         [0004]    In a variant of this design, the majority of the flow exiting the tank is drawn from an overflow weir at the upper side wall, while the solids exit through the center drain with the remainder of the flow. This configuration concentrates the solid waste in a relatively small proportion of the flow stream and facilitates de-watering and treatment steps of recirculating aquaculture systems. 
         [0005]    U.S. Pat. Nos. 3,653,358 and 3,698,359 to Fremont describe a watertight liner suspended from a floatation collar of flexibly linked, foam-filled floats and provided with inlet and outlet pipes, and oxygen spargers to continuously oxygenate the enclosed water. Flow pattern is from one end of an elongate enclosure to the other, as is the case with land-based ‘raceway’ enclosures, or is not specified. 
         [0006]    U.S. Pat. No. 4,211,183 to Hoult describes a land-based recirculating aquaculture system with centrally located upwelling pump and central drain with integral bio-filter. In one implementation the bio-filter support follows a spiral path, but no mention is made of the circulation pattern within the rearing volume of the tank, or particularly of the effect of feeding circulation from the central top surface of the water volume. 
         [0007]    U.S. Pat. No. 4,798,168 to Vadseth describes a floating closed-containment aquaculture enclosure with an externally mounted vertical pump duct drawing water from depth, discharging horizontally tangentially into the perimeter of the floating enclosure. Water follows a spiral path with induced poloidal component, and exits through a center standpipe drain. 
         [0008]    U.S. Pat. No. 6,443,100 to Brenton further describes the flow pattern within floating closed-containment enclosures, and claims a design of standpipe drain for such rearing enclosures that extracts clear effluent and solids through separate pipes. 
         [0009]    None of the previously described methods specifically address the changes in intrinsic fluid behavior as aquaculture enclosures are scaled from volumes in the order of 100 cubic meters typical of land-based culture systems to volumes in the order of 10,000 cubic meters required for large scale grow-out operations typical in the modern culture of salmonids and tunas. Such tanks may have diameters of up to 40 meters, and depths to 15 meters. At this scale, two practical difficulties arise with the azimuthal flow pattern and with the teacup effect. Firstly, tangential velocity at the perimeter of the tank produced by the flow volume necessary to exchange the large volume of enclosed water volume in the time required (on the order of one hour) is higher than the preferred swimming speed of the cultured fish, particularly in the early life stages. Secondly, the teacup effect becomes less significant as the Reynolds number of the flow increases. At large scale, turbulence and momentum predominate, while viscosity, which is responsible for the boundary layer which drives the toroidal flow component, is less influential in determining the overall behavior of the flow. In practice, solids are seen to build up on the floor of the tank, the central vortex drifts from the axis or bifurcates, and in extreme cases multiple concentric toroidal vortices develop, with upwelling zones re-suspending solids. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    It is an object of the current invention to address the shortcomings of previous methods of inducing circulation when employed in larger floating seaborne tank enclosures, particularly designs in which influent water is introduced in a tangential direction at the perimeter of the tank. It is a further object of the current invention to provide a robust design of pump and ducting system and a buoyantly supported tank which floats within an enclosing water body such as ocean, lake, or reservoir, and which can withstand large environmental loads from waves, wind, tide, and ice. It is a further object of the current invention to provide a platform from which service access to said pump and ducting system is facilitated. 
         [0011]    In a basic form the invention would have a vertically oriented intake duct and submersible pump inducing vertical upward flow within the duct, located at the central axis of a radially symmetric tank, drawing influent water from some distance below the tank. A flow diverter fitted to the top end of the intake duct directs flow radially outward over the liquid surface enclosed by the tank so as to directly induce a poloidal flow pattern within the tank. The overall flow within the tank resembles the laminar boundary layer-induced ‘teacup effect’ flow observed in smaller tanks, but with a greater poloidal component, and at a much larger scale. 
         [0012]    Because the intake duct, pump, outlet duct, filtration methods, associated floatation, oxygenation and control systems may all be located at a common, central axial platform, advantages are found in construction cost, maintenance access and structural strength. The ‘center drive’ circulation pattern is sufficiently uniform to provide optimum rearing conditions for cultured finfish, while also ensuring that solid wastes are swept toward the central drain, even in a very large tank. 
         [0013]    The invention is essentially a system for efficiently circulating water in a large volume of rearing space for aquatic organisms, comprising an impervious enclosure for containing the water and aquatic organisms and a pump for pumping water from an intake duct through intake ducting to a flow diverter, which then directs a flow of water radially outward within the enclosure to directly induce a circulation of water within the enclosure. By “radially outward” is meant from a center region of the tank toward peripheral regions of the tank, whether substantially along radius lines from a center axis of the tank directly to peripheral points at the tank outer walls, or more indirectly on flow lines that are at acute or obtuse angles to tangents on the periphery of the tank. The directing of the water thus outward results in qualitative advantages that are not provided by the circulation mechanisms of the prior technology. 
         [0014]    In a preferred embodiment, the flow diverter directs a flow of water radially outward and induces a poloidal flow to circulate water within a buoyantly supported tank for use in an open body of water. The intake duct is located outside the tank, and the pump draws influent water from outside the tank via the intake duct for the flow diverter, floatation collars for buoyantly supporting the tank being secured around a periphery of the tank by brackets. The flow diverter is centrally located within the impervious enclosure, the intake duct is vertically oriented, the pump is a submersible pump inducing vertical upward flow within the intake duct, comprising a motor which is connected to and rotates an impeller blade by means of an elongated shaft inserted down the intake duct, and the pump and the intake duct are located along the central axis of the impervious enclosure, which is radially symmetric. The intake duct, the pump, and an outlet duct for the flow diverter, are all accessible from a central axial service platform within the impervious enclosure. The impervious enclosure can be an open-topped tank supported by floatation collars and containing a central circulation platform for the intake duct, the flow diverter, the pump, and having a central mast assembly that is suspended by flotation billets to enable floatation of the central circulation platform within the tank and that is anchored to maintain a central location for the circulation platform within the tank. The mast assembly comprises poles embedded in the platform, linked by cross-members, and secured together at the top by a ring which is secured by stays attached to brackets around the periphery of the wall. 
         [0015]    The tank can have a filter skirt for trapping debris, a standpipe outlet providing an exit from the impervious enclosure filtered excess water, and an annular gutter for trapping heavier solid debris not caught by the filter skirt, the filter skirt comprising a coaxial upper slot drain and an annular lower slot drain. A vertical standpipe outlet duct is located coaxially to the intake duct, and below the flow diverter. 
         [0016]    In its overall structure, the tank comprises a cylindrical wall, a circular bottom, and a central portion holding the intake duct. The floating central circulation platform holding the intake duct and the flow diverter is a structural truss of supporting floatation billets made of foam-filled rotational-molded polyethylene. The central mast assembly is connected by rope stays to brackets supporting the floatation collar, to provide structural support against wave and tidal forces. 
         [0017]    In an optimal embodiment, the flow diverter comprises a concentric series of curved vanes that divert a flow of influent liquid pumped up the intake duct and radially spreads the flow of influent liquid along a surface of water within the enclosure from the floating circulation platform to a wall of the impervious enclosure. The flow diverter induces a poloidal flow and a secondary toroidal flow of water within the impervious enclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  shows a top isometric overview of an Aquaculture Rearing Enclosure (Tank) moored to Support Buoys (Platform without Flow Diverter) 
           [0019]      FIG. 2  shows a side isometric cutaway view of an Aquaculture Rearing Enclosure (Tank) 
           [0020]      FIG. 3  shows a side isometric close-up cutaway view of the Floating Circulation Platform (Platform), Filter Skirt and associated elements. 
           [0021]      FIG. 4  shows a top isometric view of a Floating Circulation Platform with its Flow Diverter. 
           [0022]      FIG. 5  shows a top isometric prior art Hatchery Tank with its tangential outlet and downward spiral flow pattern. 
           [0023]      FIG. 6  shows a side isometric cutaway view of the Tank, with induced poloidal flow and secondary toroidal flow directions indicated. 
           [0024]      FIG. 7  shows a top isometric cutaway view of the Tank exposing elements of a flow diverter. 
           [0025]      FIG. 8   a  shows a side cutaway reference view of the Tank, while  FIG. 8   b  shows a close-up of the flow patterns around the Filter Skirt. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    All elements will now be introduced by reference to drawing figures, then how each element functions and interacts with each other element will be described where necessary. 
         [0027]      FIG. 1  shows an overview of an Aquaculture Rearing Enclosure (Tank)  10  secured by mooring lines  22  to support buoys  26  by their underwater spars  24 . The tank  10  is comprised of a cylindrical wall  12  and circular bottom  14  with an intake duct  30  at its center. Floatation collars  16  are secured around the tank  10  periphery by brackets  18 . Safety hand rails  20  are anchored to the top of the wall  12  between brackets  18 ; the latter serve as anchors for one end of each rope stay  50  which then attaches to the ring  52  at the top of the mast assembly  44 . Note that the Floating Circulation Platform  28  is without its Flow Diverter  32  in order to show the how the mast assembly  44  is anchored within. 
         [0028]      FIG. 2  shows an internal cutaway view of the Aquaculture Rearing Enclosure (Tank)  10 . The Floating Circulation Platform (Platform)  28  is comprised of numerous flotation billets  36  which provide buoyancy to support the mast assembly  44 , pump assembly  54 , filter skirt  38 , and intake duct  30 . The mast assembly  44  is comprised of poles  46  embedded in the platform  28 , linked by cross-members  48 , and secured together at the top by a ring  52  which is secured by stays  50  attached to brackets  18  around the periphery of the wall  12 . The pump assembly  54  is comprised of a motor  56  which is connected to and rotates an impeller blade  60  by means of an elongated shaft  58  inserted down the intake duct  30 . Also shown is the standpipe outlet  42  which is the main exit for filtered excess water and an annular gutter  34  which traps heavier solid debris  66  not caught by the filter skirt  38 . 
         [0029]      FIG. 3  shows a close-up cutaway view of the Floating Circulation Platform  28 , with focus on the location of the coaxial upper slot drain  40  and the annular lower slot drain  41 , both elements of the filter skirt  38 . (see  FIGS. 8   a  &amp;  8   b  for drainage details) A vertical ‘standpipe’ outlet  42  duct is located coaxially to the intake duct  30 , and below the flow diverter  32  as shown. 
         [0030]      FIG. 4  shows a close-up view of a Floating Circulation Platform  28  with its Flow Diverter  32  in place, which is comprised of a concentric series of curved vanes  82  which divert the flow of influent liquid  62  pumped up the intake duct  30  and radially spread it along the water surface  64  from the platform  28  to the wall  12 . Also visible are the numerous flotation billets  36  and the base of some poles  46  of the mast assembly  44 . 
         [0031]      FIG. 5  shows a prior art Hatchery Tank  74  with its tangential flow outlet  76  creating a spiral flow  80  pattern down towards its drain  78 . 
         [0032]      FIG. 6  shows a cutaway view of the Tank  10 , illustrating how the platform  28  induces a poloidal flow  70  and a secondary toroidal flow  72  in the directions indicated. 
         [0033]      FIG. 7  shows a cutaway view of the Tank  10  and focusing on the platform  28  with the vanes  82  of its flow diverter  32  creating the output flow patterns seen in  FIG. 6 . 
         [0034]      FIG. 8   a  shows a reference view of the Tank  10 , with  FIG. 8   b  a close-up of the circled area in  FIG. 8   a  showing the drainage flow patterns around the Filter Skirt  38 . The majority of poloidal flow  70  becomes outlet flow  84  by following the surface of the filter skirt  38 , entering the upper drain  40 , then exiting through the standpipe outlet  42 . Some of the heavier solid debris  66  of the poloidal flow  70  slides under the bottom of the filter skirt, i.e. the lower drain  41 , and then is sucked upwards to exit through the standpipe outlet  42 . The heaviest solid debris  66  follows the gutter flow  88  path shown and settles into the annular gutter  34  for later removal. 
         [0035]    The floating circulation platform  28 , as shown in  FIGS. 2 &amp; 4 , is a structural truss of metal or fiberglass construction supporting floatation billets  36 , typically constructed of foam-filled rotational-molded polyethylene. The central mast assembly  44  is connected by rope stays  50  to brackets  18  supporting the floatation collar  16 , providing structural support against wave and tidal forces which act to deform wall  12  of the tank  10 . 
         [0036]    The filter skirt  38 , as shown in  FIG. 3 , is a tensile fabric structure made of filter medium such as is commonly used for filter presses, centrifuge baskets, and the like. It is supported between the bottom  14  of the tank  10  and the platform  28 . Some portion of the effluent flow volume, preferably less than 10%, passes through the annular lower drain slot  41  at the base of the filter skirt  38 , carrying heavier settled solid debris  66  via the gutter flow  88  path to the annular gutter  34 , from which it is periodically pumped to dewatering and composting equipment located conveniently on shore or barge. Effluent flow with lighter than water debris follows the supernatant flow  86  upward to combine with the main outlet flow  84  from the coaxial upper drain slot, and then leaves the enclosure  10  through the co-axial standpipe outlet  42 . 
         [0037]      FIG. 5  shows the circulation pattern of the prior art, namely a typical land-based circular rearing tank, where water enters the hatchery tank  74  by means of a tangential flow outlet  76  which creates a spiral circulation path  80  towards the central drain  78  at the bottom of the tank  74 . Toroidal flow induces a secondary poloidal flow by the teacup effect. Solids settle vertically through the water column to the floor of the tank, and are swept in spiral path  80  toward drain  78 . 
         [0038]      FIGS. 6 through 8   a /b relate the ingoing and outgoing flow and drainage paths necessary to understanding the unique features of the present invention.  FIG. 6  shows the circulation flows generated in a large tank  10 ;  FIG. 7  shows how the flow diverter creates the flows necessary for optimal aquaculture rearing, and  FIGS. 8   a /b show how effluent is safely filtered or trapped. 
         [0039]    A tank  10  supported by floatation collars  16  encloses culture water  68 , and includes central circulating platform  28  consisting of an intake duct  30 , flow diverter  32 , pump assembly  54 , and mast assembly  44 , suspended by flotation billets  36 . Water is drawn vertically up the intake duct  30  by the impeller  60 , and then diverted radially by flow diverter  32 , then outward along the water surface  64 , thereby inducing a poloidal flow  70 , which eventually mixes with the culture water  68  and primarily exits through the coaxial upper slot drain  40 . Poloidal flow  70  also induces a secondary toroidal flow  72  which reduces acceleration of effluent arriving at the primary upper drain  40 . 
         [0040]    Floating closed-containment aquaculture systems possess proven advantages over net-pen enclosures. A steady, pumped flow of influent water may be drawn from a selected depth within the water column, thereby avoiding extreme temperatures, silt contamination, abnormal salinity, toxic plankton, and motile parasites. Influent water may be oxygenated, and maintained at a pre-determined dissolved oxygen set point by automated means. Fixed enclosure geometry allows improved accuracy of sonar biomass estimation devices. Predators are more effectively separated from the cultured fish, and are unable to see them through the opaque walls of the enclosure. Solid waste, including uneaten feed and fecal matter, may be separated from the effluent stream before it leaves the enclosure. 
         [0041]    Previous closed-containment enclosure designs (impervious to water) have not been big enough or sufficiently robust to enable production on a scale comparable with existing net-pen (water permeable) farms. Typical net-pens may enclose 10,000 to 30,000 cubic meters of water, and are stocked with 300 tonnes of live fish. The current invention enables pumped circulation of water within an enclosure of up to 10,000 cubic meters volume, while including a central structural spar, attached by means of rope stays to the perimeter floatation collar, and which supports the tank against environmental loads. 
         [0042]    In a surprising aspect of the preferred embodiment, it is found that influent liquid  62  does not travel directly to the standpipe outlet  42  via the upper drain  40 , even though a relatively short distance separates the flow diverter  32  and outlet  42 . Instead, influent  62  follows the free-surface  64  boundary radially to the perimeter of the tank  10 , where it is diverted down the wall  12 , radially back to the center axis, and then rises to the upper drain  40 . (See  FIG. 8   b ) In a further surprising aspect of the invention, the poloidal induced flow  70  gives rise to a secondary, toroidal flow  72  (i.e. azimuthal flow, about the vertical axis) of greater velocity and momentum than the driven poloidal component. By this means, the overall flow within the tank  10  resembles the laminar boundary layer-induced ‘teacup effect’ flow observed in smaller tanks, but with greater poloidal component, and at a much larger scale. 
         [0043]    The foregoing description of the preferred implementations should be considered as illustrative only, and not limiting. Other embodiments are not ruled out or similar methods leading to the same result. Other techniques and other materials may be employed towards similar ends. Various changes and modifications will occur to those skilled in the art, without departing from the true scope of the invention as defined in the above disclosure, and the following claims.