CA3067093A1 - Tank for fish farming - Google Patents

Abstract

A tank (1) for fish farming, wherein the tank (1) comprises: a. a polygonal or circular ring pontoon (2) forming the lower part of the tank, b. sidewalls (4) being substantially vertically arranged on the ring pontoon (2) to form a substantially vertically arranged tubular member having a circular or polygonal cross section, and c. vertical columns (3) being arranged perpendicular upwards on the ring pontoon (2) and being connected to the sidewalls (4), and extends from the ring pontoon to the top of the sidewalls, where the ring pontoon and the vertical columns comprise closed buoyance elements, and where the ring pontoon is adopted to be ballasted by means of water being pumped into the buoyance element(s) therein, is described.

Description

TANK FOR FISH FARMING
Technical Field [0001] The present invention relates to fish farming. More specifically, the invention relates to a partly closed semi-submersible tank structure for fish farming.
Background Art

[0002] Fish farming has become very important for production of fish in a growing marked. Fish farms are found both onshore in closed tanks, and in coastal areas where the fish is enclosed in net cages. In coastal areas the farmed fish is most often held in net cages. The net cages allow fresh water to flow into the cages and used water to flow out of the cages by means of the current in the area, only. This reduces the additional measures needed to be taken to farm fish, but exposes the fish for the surrounding environment, and allows sea lice naturally occurring in the waters, to pass into and out of the net cage.

[0003] In areas having high density of fish farms parasites, for farming of salmon and trout, sea lice constitutes a big and economically expensive problem.
The sea lice shifts between free-swimming stages and parasitic stages in their life cycle. In the parasitic stage the sea lice is attached to the skin of the fish and feed on the skin resulting in damage to the local area where they are localized and resulting in erosion of the protective mucus and skin layers with loss of scales. This may result in haemorrhage and exposure of the underlying muscle, and thus create starting points for secondary infections by bacteria, all of which may result in discomfort, pain and even physical deformation of affected parts of the fish. The damage to the skin increases the stress for the farmed fish, and reduces the appetite and thus growth, may damage the osmoregulation and increase mortality substantially.

[0004] Accordingly, parasites, such as fish lice, may have an important and negative impact both to fish health, mortality and on the economy in fish farming. Huge resources have therefor been used in reducing the parasite problem in fish farming, both in developing mechanical devices for the removal of lice from the skin of the fish as well as anti-parasitic chemicals, and in developing of SUBSTITUTE SHEET (RULE 26) solutions that reduces infection and thus the problems related to the parasites.

[0005] The presently most important fishes for fish farming in Norway and in other coastal areas in relatively cold waters, belongs to the Salmonidae family, and includes salmon, rainbow trout, and sea char. Sea lice is a group of species that are parasites in parts of their life cycle and that have different specificity towards species of the Salmonidae family.

[0006] Measures to combat sea lice infection include both chemical and mechanical measures, and other measures. The chemical measures includes in situ treatment with chemicals such as hydrogen peroxide, or toxins developed as insecticides that are introduced into the net cage, which preferably is temporarily surrounded by a tarpaulin, or by addition of toxins to the feed.
The chemical measure all depends on that the fish has a higher tolerance to the chemicals than the sea lice. However, large amounts of chemicals are used and over time, the lice develop resistance to the chemicals, and the result of chemicals being released into the surrounding water after the treatment, may cause substantial damage in the surroundings.

[0007] Mechanical measures includes arranging a cage shielding, i.e. a fine meshed fabric around a net cage, the fabric having a mesh size stopping sea lice from passing through, but allowing water to pass, at the upper part of the net cage. In using such cage shielding, only the upper few meters of the sea cage is covered by the fabric, as free floating stages of sea lice normally lives in the upper layers of sea water, such as to about 5 m below sea level.
However, a cage shielding reduces the flow through of oxygen rich water to ascertain sufficient oxygen supply to the fish inside the cage, and may result in a need for adding additional oxygen into the cage.

[0008] Another technical measure is the use of a "snorkel sea lice barrier ", where a net "roof" is arranged 5 ¨ 10 m below sea level to keep the fish at this depth or deeper. A "snorkel" having a substantially smaller diameter than the cage, allows the fish to get to the surface to snap air, which is important for filling the air bladder of Salmonidae species.

[0009] Closed tanks are also used. Closed tanks may be arranged onshore or floating partly submerged in water. However, closed tanks need additional equipment and control systems for introducing oxygen into the water, circulation means for circulating the water, and means for removal of faeces and excess food collected at the bottom of the tank. Additionally, a purification plant for cleaning of water discharged from the tank into the surroundings, are needed. Both the construction cost for the tanks and the additional measures needed for operation of the tanks, are prohibitive compared to the well-known net cages.

[0010] N0336739B1 describes a net cage for housing live fish a short period before being slaughtered. The net cage comprises a net arranged inside a frame, and a bottom part. The frame comprises vertically arranged lifting tubes (5), one or more horizontally arranged stabilization tubes and a vertically arranged ballast ring connected to the lowermost parts of the vertical lifting tubes. The net cage may lifted by means of the lifting tubes to lift the bottom part up to or even above sea level to allow for easy removal of the fish to be slaughtered. However, no means are provided to reduce the action of waves on the net cage.

[0011] An article in iLaks.no, "MNH-produksjon soker om atte utviklingskonsesjonerD, by Alsak Berge, published 2016.04.12, describes a fish cage comprising a cylindrical upper part of the fish cage made of water tight plates, and a lower perforated part of the net cage. The fish cage are connected to a floating ring made of steel surrounding the upper part of the fish cage. The fish cage may be lifted up from the floating ring by means lifting arms connected to both the floating ring and the fish cage. However, the floating ring do always float at the water surface making the whole construction vulnerable to the action of waves.

[0012] N0158201 B relates to a net cage for fish farming, comprising a net cage connected to a frame. The frame comprises s bottom frame connected to vertical frame members and a floating ring. The vertical frame members are connected to the floating ring and may be lifted relative to the floating ring to reduce the volume of the net cage below the sea surface. However, the floating ring do always float at the water surface making the whole construction vulnerable to the action of waves.

[0013] Other mechanical / technical means for combating sea lice infection in fish farms are mostly related to remove sea lice from infected fish. An example is a floating unit comprising cameras and lasers. Sea lice on the fish are identified and localized by image analysis, and are thereafter "shot" and killed with the laser. Other means for washing and scrubbing of fish is also known, such as transferring fish to separate treatment units for washing and scrubbing of the fish to remove sea lice, and then transferring the thus treated fish to another net cage. Moving of net cages to new locations having low salinity water, such as close to the mouths of a river, is also used as sea lice is killed or inactivated by low salinity water.

[0014] An object of the present invention is to provide a semi-closed tank fish cage for fish farming, which allows for the necessary change of water during operation. Another object is to provide fish cage arrangement which is less vulnerable for the action of waves and thus may be placed at location more exposed to the action of waves than the prior art solutions allows for. Other objects of the invention will be clear for the skilled person in reading the present description and claims.
Summary of invention

[0015] The above object have been met according to the present invention by beans of a tank for fish farming, wherein the tank comprises:
1. a polygonal or circular ring pontoon forming the lower part of the tank, 2. sidewalls being substantially vertically arranged on the ring pontoon to form a substantially vertically arranged tubular member having a circular or polygonal cross section, and 3. vertical columns being arranged perpendicular upwards on the ring pontoon and being connected to the sidewalls, and extends from the ring pontoon to the top of the sidewalls, where the ring pontoon and the vertical columns comprise closed buoyancy elements, and where the ring pontoon is adopted to be ballasted by means of water being pumped into the buoyancy element(s) therein.

[0016] The present tank for fish farming has sidewalls that are closed towards the surrounding sea and thus prevents parasites living in the upper layer of the surrounding sea, to get into the tank, even though the tank is floating in the sea as the presently most used net cages. By dividing the buoyancy volume into the ring pontoon representing the lowermost part of the tank and being submerged during normal use, and vertical columns representing the buoyancy waterline of the tank, stabilisation of the floating tank preventing or reducing the impact of wave forces onto the tank, is obtained.

[0017] According to one embodiment, the tank is adopted to be adjusted between a lower position where only the upper part of the tank and the vertical columns are above the sea surface, an upper position where the vertical columns and the sidewalls are lifted above the sea surface by adjustment of the volume of ballast water in the ring pontoon. Allowing adjustment of the vertical position of the tank and having sufficient buoyancy in the ring pontoon to lift the vertical columns and the sidewalls above the surface is important both in operations, such as medical treatment and emptying the tank for fish, where it is required / desirable to reduce the volume of water in the tank, and for maintenance of the tank.

[0018] According to one embodiment, the ring pontoon has sufficient buoyancy to lift the vertical columns, the sidewalls and parts of the ring pontoon above sea level. Having sufficient buoyancy in the ring pontoon to lift both the sidewalls, the vertical columns and a part of the ring pontoon above the sea level, improves the availability of the tank for maintenance and for emptying the tank for fish, when required.

[0019] According to one embodiment, a grid with a calibrated mesh allowing circulation of water into the tank, but hindering escape of fish from the inside of the tank, is arranged at the bottom of the tank. The grid might be made of a rope net, a metal grid or the like, and has a mesh size allowing maximal water flow into and out of the tank, at the same time as the fish in the tank is stopped from escaping from the tank.

[0020] According to one embodiment, circulation openings are arranged at the lower part of the sidewalls, and where remotely controllable doors are arranged for individual opening or closing of the doors. Circulation openings being arranged below the depth where sea lice are found, will allow for more efficient water flow and thus exchange of water, in the tank. By having remotely controllable doors at the openings, each circulation opening may be opened or closed to optimize the water flow and water exchange in the tank.
Additionally, all circulation openings may be closed in conditions where sea lice or life stages thereof, are found in deeper water than usual to stop or substantially reduce the flow of sea lice into the tank.

[0021] The tank may further comprises a roof covering the top of tubular member. A
roof at the top of the tank will form a shield for weather conditions and may also be a shield for light. The roof will make it possible to have a shielded working space for personnel, and will also be a shield stopping birds from trying to catch the fish.

[0022] According to one embodiment, the tank further comprises a central column being connected to the roof and to beams at the level of the ring pontoon.
The central column may give extra support for a roof, and for other constructions in the tank.

[0023] According to one embodiment, wave dampening means are arranged in the tank to reduce internal wave motions. Internal wave motions are not wanted as internal waves may be a challenge for the construction as such, for the fish in the tank, and for personnel. Wave dampeners are therefor preferably arranged to substantially reduce internal waves in the tank.

[0024] According to one embodiment, the wave dampening means comprise separation plates arranged across the cross section of the tank, the plates being vertically arranged, arranged so that they are breaking the water surface in the tank and are extending 0.5 to 5 meters above and below the sea level.

[0025] According to one embodiment, the wave dampening means comprise double walled section(s) stretching from about 1-5 meters above sea level to about 1-5 meters below the sea level, and where the wall of doubled wall facing the inside of the tank is perforated. The skilled person will understand that separation plates and a double walled section including a perforated inner wall may be used both as the only wave dampening means, or be used in combination to give optimal wave dampening. The choice of wave dampening means and the actual construction thereof, will be dependent on the waters where the tank is intended to be used, and the normal wave size and wave pattern in the area of use.

[0026] According to one embodiment, skimming pipes are arranged in the sidewall for removing of material collected at the water surface inside the tank.
Organic material as dead fish, remains of faeces, fish food, and other waste products tend to be collected at the sea surface, both as clots, particles and water soluble material. Some of the material tend to form foam which is unwanted. Skimming pipes withdrawing surface water and the waste collected at the surface are preferably used to reduce this problem.

[0027] According to one embodiment, the skimming pipes are arranged in the double walled wave dampener section(s).

[0028] According to one embodiment, water circulation pipes are arranged for pumping water into and out of the tank to improve water circulation and exchange of water in the tank. In some conditions, the water circulation and replacement of the water inside the tank with water from the surroundings is too low. The water circulation pipes are arranged to improve the water circulation by pumping water from below the ring pontoon into the tank, and/
or to pump water from the inside of the tank out into the surroundings.
Brief description of drawings

[0029]
Figure 1 is a perspective view of a fish farm tank according to the present invention, figure 2 is a cut¨through view of the tank shown in figure 1, and Figure 3 is a cut-through view of the tank illustrating the flow of water caused by the current.
Detailed description of the invention

[0030] The present invention is related to a semi-submersible tank 1 designed for fish farming. The structure comprises a ring pontoon 2 onto which vertical columns 3 are arranged. The ring pontoon 2 and the vertical columns 3 serve as structural support, buoyancy and stabilizers for the tank 1. The ring pontoon 2 is illustrated as a substantially circular ring, but the ring pontoon may also be polygonal, having four or more straight sections. The ring pontoon is arranged substantially horizontal, or parallel to the sea surface at the lowermost part of the tank 1. The vertical columns 3 are connected to the ring pontoon and are arranged vertically upwards from the ring pontoon.

[0031] Four vertical columns are illustrated in the drawings. However, more than four vertical columns may be used. Semisubmersible structures according to the present invention having polygonal ring pontoons, such as square, hexagonal or octagonal ring pontoons may have one vertical column arranged in each corner of the ring pontoon.

[0032] Sidewalls 4 are arranged to give a compartment that is closed towards the sides of the tank 1. The sidewalls are connected to the ring pontoon 2 and the vertical columns 3. The sidewalls are arranged to make up a substantially vertically arranged tubular member, or tank, that is open at the bottom and at the top. A grid 5 with a calibrated mesh is arranged at the bottom of the tank, to permit circulation of water at deeper draft from outside into the tank, as well as to hinder escape of fish from the inside of the tank.

[0033] Circulation openings 6 are preferably arranged in the sidewalls 4 at the lower part of the tank to improve the circulation of water between the surrounding sea and the inside of the tank in case the surrounding current cannot ensure satisfactorily circulation. Circulation openings grids 7 a with a calibrated mesh to allow circulation of water through the circulation openings as well as to hinder escape of fish from inside of the tank, are arranged in the circulation openings 6. Preferably, not illustrated doors are preferably arranged at the circulation openings 6 for opening and closing of the doors according to the need for open circulation openings 6. The doors are preferably independently and remotely controllable. The doors are preferably sliding doors.

[0034] A roof 8 is preferably arranged to cover the top of the tank 1. The cover may be made of a transparent or non-transparent material, and is arranged to give a protected space for inside equipment and personnel working at the fish farm. Additionally, by using a non-transparent or only partly transparent material for the roof 8, the light inside the tank may be adjusted according to the needs. The roof 8 and the connection thereof to the tank is not air tight, but protect the inside of the tank from the weather outside to give a working space for personnel below the roof protected from the weather.

[0035] A central column 9 is arranged centrally and vertically inside the tank. The central column 9 is supported at the top thereof by not illustrated beams in the roof and at the ring pontoon level, and optionally by additional beams arranged at a level between the roof and the ring pontoon level. Water for increasing the circulation and replacement of water in the tank, and/or air may be introduced into the tank via the central column.

[0036] Separation plates 10, for dampening of waves inside of the tank 1, may also be arranged between the sidewalls 4 or vertical columns 3, and the central column at the surface level inside the tank so that the separation plates are arranged to break the water surface when the tank is in its lower position used under normal operation for fish farming. The separation plates are arranged vertically, and are sticking 0.5 to 5 meters downwards into the tank.

[0037] Wave dampeners 11 may also be arranged at the side walls 4, here illustrated by a double walled section in the splash zone inside the tank, where the wall facing the inside of the double wall section is perforated.
According to a preferred embodiment, the double walled section of the wall of the present tank stretches from about 1 to 5 meters below the water level, and to about 1 to 5 m above the water level, when the present tank is in its lower, or operative depth. The separation plates and the wave dampeners reduce the waves and splashing inside the tank.

[0038] When the present tank 1 is out in the sea, the ring pontoon 2 is at least partly submerged. The buoyancy of the ring pontoon may be adjusted by filling the ring pontoon partly with water, or by pumping out water from the ring pontoon. Preferably, the buoyancy of the ring pontoon is sufficient to allow the ring pontoon to be partly above the water level when little or no water is present inside the ring pontoon. The tank is normally only in this "upper"
position during transport, cleaning of the tank, or for maintenance. During normal operation of the tank for fish farming, the tank is in its "lower"
position where the ring pontoon is ballasted, or partly filled with water, so that only the upper part of the tank, including the upper part of the side walls 4, the upper part of the vertical columns 3 and the roof are visible above the sea level.

[0039] Typically, the diameter of a substantially circular tank, or the length of the sidewalls in a tank having a substantially square cross section, will be from 1.5 to 4 times the height of the tank, such as 2 to 3 times the height. The diameter or length of the sidewalls will be measured as inner diameter or length, whereas the height will be measured from the bottom of the ring pontoon to the sea level during normal operation for fish farming. Presently, a substantially circular tank, having a substantially circular ring pontoon is preferred. Typically, the height of the tank will be 15m or more, and the diameter will thus be 22.5 m or more. More typically, the height will be from m or more, and the diameter will be from 40 m or more. A presently preferred tank height is about 25 m, or more, and the diameter is 50 m or more. Accordingly, the ring pontoon 1 is at a depth of 15 m or more, such as 20 m or more, or even 25 m or more, in the normal operating position, i.e.
during stable periods in fish farming.

[0040] For normal operation, the tank is lowered to a required depth in the sea by partly filling the ring pontoon and/or columns with water. A closed and watertight volume in the ring pontoon and vertical columns 3 will give buoyancy to the tank when lowered into the sea. In normal operation the tank is submerged so that only the part of the tank, such as 1-10 meters of the upper part of the sidewalls, and the roof, are above sea level. In this position, the open bottom of the tank and the circulation openings 6 are at least 10 meters, such as at least 12 or at least 15 meters, such as about 20 meters below the sea level, i.e below the water depth were sea lice are normally present, to avoid or at least reduce the possibility of access to the inside of the tank of parasites, such as sea lice, living close to the surface.

[0041] Semi-submersible structures are well known for oil and gas related vessels, such as for drilling vessels, production vessels or platforms for use in deep waters. Semi-submersible vessels / structures all have one or more pontoons, which in operation is submerged below the action of surface waves, and a plurality of vertical columns both being buoyancy and stability bodies and structural elements for a part of the vessel being over the surface.
Typically, the one or more pontoons constitutes most of the buoyancy of the construction even when submerged, and the vertically arranged columns constitutes a minor part of the buoyancy and act for stabilization of the construction. According to the present invention, the one or more pontoons is one ring pontoon as described above. The ring pontoon according to the present invention constitutes between 60 and 80 % of the buoyancy of the tank construction when the tank is in its lower, or operating position.

[0042] The action of the waves decreases exponential with depth. Accordingly, the wave forces exciting the heave motion are substantially reduced by having the main part of the buoyancy at a depth as mentioned, above, i.e. at least 15 m, such as more than 20, or more than 25 m during operation of the fish cage for fish farming.

[0043] Additionally, the dynamic wave pressure (pressure fluctuations due to the waves) has an exponential decrease with depth. This means that the dynamic pressure variations on top of the ring pontoon will be higher than the dynamic pressure variations at the bottom of the ring pontoon. The difference between the two depends on the wave length/period. The vertical columns reduce the effective area on the top of the ring pontoon. This cancels out some of the effect of the higher pressure variations between the top and bottom of the ring pontoon. At a certain wave length/period there will actually be zero vertical dynamic pressure force since the pressure on top of the pontoon multiplied by the reduced area there (the area occupied by the vertical columns) exactly equals the pressure on the bottom of the pontoon multiplied by the bottom area of the ring pontoon. The sides of the pontoon and the columns will not contribute to the vertical force since the water pressure on these elements create a horizontal force (the normal to the surfaces is horizontal).

[0044] The skilled person will know how to tune the column area to obtain the desired the cancellation period, i.e. the wave period where the dynamic vertical pressure force is zero, with regard to the overall heave response.
When doing this tuning one need to keep in mind the eigenperiod of the vessel in heave and keep it away from where the major part of the wave energy is (at what wave periods do you have the major part of the wave energy).

[0045] Due to the counter action of the mass forces acting on the submerged pontoons and the pressure forces acting under the vertical columns, and the deep draft, a semi-submersible vessel or platform, is substantially less influenced by waves at the surface than a surface vessel, at least at the conditions, i.e. wavelength, and wave height, for which they are constructed.
A skilled person is able to construct a semi-submersible for a specific location based on statistics on weather and wave at the intended location. A skirt 12 may also be arranged at the sidewalls to further dampen wave action on the present tank, and to reduce vertical down flow of surface water, thus preventing parasites mainly present in the upper water layers to enter the tank. The optional skirt 12 is in the form of a horizontally arranged rim arranged at the sidewall 4 functioning as a barrier against water vertical flow of water along the sidewall 4 facing the surrounding sea.

[0046] By placing the tank at a location with significant current, the flow of water underneath the tank will establish a flow into and out of the tank and thereby exchange the water inside. The forced water flow will circulate inside the tank from downstream to upstream side and leave the tank from the upstream side. This effect has been confirmed by computerized fluid dynamics simulation performed by the applicant, and the result of this analysis is illustrated in figure 3. The direction of the current is indicated by an arrow, and the simulation shows that a substantial part of the water being forced to flow below the ring pontoon will circulate into the tank to cause exchange of the water therein with "fresh" water from the surrounding sea.

[0047] Individual activation or opening of one or more of the circulation openings 6 in the lower part of the tank may be performed to improve the circulation of water in the tank. Opening of one or more of the upstream circulation openings 6, and closing the downstream circulation openings, will cause trapping some of the current water and thereby force a better circulation of water into the tank. An equivalent effect may be obtained by equipping the tank with a shield extending below the ring pontoon at the downstream part of the circumference thereof. The tank may also be equipped with outlets for circulation purposes through either its center column and/or the side columns. The skilled person will also understand that forced circulation /
exchange of water and/or introduction of oxygen, such as by introduction of air into the water in the tank, may be necessary in some situations. Water may be pumped into the tank through the central column and/or via channels in the vertical columns, or by any other means well known for the skilled person. Additionally, in an emergency case with reduced or very low current, sections of the sidewalls 4 may be opened for additional circulation.

[0048] To further increase water circulation, especially in situations with little or no current velocity, circulation tubes may be arranged from below the ring pontoon, upwards along the sidewall or inside the vertical columns, opening into the inside of the tank. Pumps are arranged at the circulation pipes to actively promote water circulation. Typically, the openings of the circulation pipes into the inside of the tank are arranged 3 to 10 meters below the sea surface when the tank is in its lower, or operating, position. Preliminary calculations have indicated that the best effect for water circulation is obtained by arranging water circulation pipes for pumping water out of the tank at the upstream part of the tank, and arranging circulation pipes for pumping water into the tank at the downstream side of the tank. The circulation pipes for pumping out water are preferably arranged about 45 degrees at both sides of the main incoming current direction. The circulation pipes for pumping in water are preferably arranged within an angle of 30 degrees of the current at the downstream end of the tank.

[0049] Organic material from the fish farming and pollution in the sea are often collected at the surface and may cause foaming inside the tank. This mainly organic material may be removed by skimming the water by means of skimming pipes arranged close to the water surface in the circumference of the tank. Preferably, the skimming pipes are arranged inside wave dampers 11, as described above. The openings of the skimming pipes are preferably arranged from 0 to 0.5 m below the water surface.

[0050] The tank may be elevated by reducing the volume of water introduced in the ring pontoon as ballast. Elevation is done when needed for discharging fish from the tank, for cleaning and maintenance of the tank, or for treatment of the fish if infected by parasites. Elevating the tank reduces the water volume of the entrapped fish and accommodate more efficient catching of fish for emptying purposes (emptying case), or for chemical treatment of the farmed fish if infected by parasites, as done for traditional net cages. Ultimately, one may bring the whole part of the tank from above ring-pontoon, above sea surface for cleaning purposes (cleaning case) or for maintenance purposes.

[0051] Dynamic calculations of integrated system; a moored tank excited by waves, current and wind, shows a possible interaction between the wave response of the tank with the oscillating water inside and the spring of characteristics of the mooring, which may cause sloshing problems inside, as well as strength problems of the mooring system and tank itself. The design of the system is particularly considered to suppress the effect of such dynamic interaction by the introduction of damping devices at the surface level inside the tank to reduce the effect of the oscillating surface water, and float/weight devices attached to the mooring lines, to soften the spring stiffness of the mooring.

[0052] The present tank is held in place by means of a mooring system connected to the tank via fairleads on the columns and anchor winches at the top of the columns. The mooring system is tuned with respect to line configuration, line angles, buoyancy and stiffness, in order to reduce mooring forces caused by the tank dynamics and thereby improve motions. Typical; (1) In shallow water (water depth less than 100m), one buoyant body with 10 to 100 ton buoyancy is attached to each mooring line at a distance of 30-100m from the fairlead, thereby reducing the tension variations with varying positions of the SST (i.e. reduced stiffness). Natural periods of the system in surge/sway/roll/pitch/yaw can then be kept well above the periods with significant first order wave energy. The magnitude of the buoyancy can be tuned to achieve desired stiffness. (2) In shallow water (water depth less than 100m), a second solution is to combine floats of 10 to 100 ton buoyancy, with to 200 ton weight and another set of buoyancy modules of 10 to 100 ton buoyancy attached to each mooring line, thereby reducing the tension variations with varying positions of the SST (i.e. reduced stiffness). In between the buoyancy, weight and buoyancy there are 5 to 80m long mooring lines. Natural periods of the system in surge/sway/roll/pitch/yaw can be kept well above the periods with significant first order wave energy.
Mooring lines (including buoyancy and weight) can be kept well below the sea surface. The magnitude of the buoyancy and weight as well as the length of the in between mooring lines can be tuned to achieve desired stiffness.

Claims (14)

Claims

1. A tank (1) for fish farming, characterised in that the tank (1) comprises a. a polygonal or circular ring pontoon (2) forming the lower part of the tank, b. sidewalls (4) being substantially vertically arranged on the ring pontoon (2) to form a substantially vertically arranged tubular member having a circular or polygonal cross section, and c. vertical columns (3) being arranged perpendicular upwards on the ring pontoon (2) and being connected to the sidewalls (4), and extends from the ring pontoon to the top of the sidewalls, where the ring pontoon and the vertical columns comprise closed buoyance elements, and where the ring pontoon is adopted to be ballasted by means of water being pumped into the buoyance element(s) therein.

2. The tank according to claim 1, wherein the tank is adopted to be adjusted between a lower position where only the upper part of the tank and the vertical columns are above the sea surface, an upper position where the vertical columns and the sidewalls are lifted above the sea surface by adjustment of the volume of ballast water in the ring pontoon.

3. The tank according to claim 2, wherein the ring pontoon has sufficient buoyancy when not ballasted, to lift the vertical columns and the sidewalls above the sea level.

4. The tank according to claim 3, wherein the ring pontoon has sufficient buoyancy to lift the vertical columns, the sidewalls and parts of the ring pontoon above sea level.

5. The tank according to any of the preceding claims, wherein a grid (5) with a calibrated mesh allowing circulation of water into the tank, but hindering escape of fish from the inside of the tank, is arranged at the bottom of the tank.

6. The tank according to any of the preceding claims, wherein circulation openings (6) are arranged at the lower part of the sidewalls (4), and where remotely controllable doors are arranged for individual opening or closing of the doors.

7. The tank according to any of the preceding claims, wherein tank further comprises a roof (8) covering the top of tubular member.

8. The tank according to claim 6, wherein the tank further comprises a central column being connected to the roof and to beams at the level of the ring pontoon.

9. The tank according to any of the preceding claims, wherein wave dampening means are arranged in the tank to reduce internal wave motions.

10.The tank according to claim 8, wherein the wave dampening means comprise separation plates arranged across the cross section of the tank, the plates being vertically arranged, arranged so that they are breaking the water surface in the tank and are extending 0.5 to 5 meters above and below the sea level.

11.The tank according to claim 8 or 9, wherein the wave dampening means comprise double walled section(s) stretching from about 1-5 meters above sea level to about 1-5 meters below the sea level, and where the wall of doubled wall facing the inside of the tank is perforated.

12.The tank according to claim 11, wherein skimming pipes are arranged in the sidewall for removing of material collected at the water surface inside the tank.

13. The tank according to claim 12, wherein the skimming pipes are arranged in the double walled wave dampener section(s).

14.The tank according to any of the preceding claims, wherein water circulation pipes are arranged for pumping water into and out of the tank to improve water circulation and exchange of water in the tank.