CN110913688B

CN110913688B - Tank for fish culture

Info

Publication number: CN110913688B
Application number: CN201880039215.0A
Authority: CN
Inventors: 佩尔·赫伯特·克里斯滕森; 佩尔·斯韦勒·霍尔特·克里斯滕森; 拉尔斯·E·斯塔什克
Original assignee: Wade Group Co ltd
Current assignee: Wade Group Co ltd
Priority date: 2017-06-13
Filing date: 2018-06-13
Publication date: 2022-07-26
Anticipated expiration: 2038-06-13
Also published as: GB202000220D0; NO20170955A1; GB2577462A8; CA3067093A1; GB2577462B; NO343577B1; WO2018229123A1; GB2577462A; CN110913688A; CL2019003637A1; AU2018285812A1

Abstract

A tank (1) for fish farming is described, wherein the tank (1) comprises: a. a polygonal or circular ring buoyancy tank (2) forming a lower part of the tank; b. a side wall (4) arranged substantially vertically on the annular buoyancy tank (2) to form a substantially vertically arranged tubular member having a circular or polygonal cross-section; a vertical column (3) arranged vertically upwards on the annular buoyancy tank (2) and connected with the side wall (4) and extending from the annular buoyancy tank to the top of the side wall, wherein the annular buoyancy tank and the vertical column comprise closed buoyancy elements, and wherein the annular buoyancy tank is used for ballasting by means of buoyancy elements into which water is pumped.

Description

Tank for fish culture

Technical Field

The invention relates to fish farming. More particularly, the present invention relates to a partially enclosed semi-submerged tank structure for fish farming.

Background

Fish farming has become very important for the growth of fish production. Fish farms can be found both onshore in closed tanks and in coastal areas where fish are enclosed in netpens. In coastal areas, farmed fish are usually kept in netpens. The netpen allows fresh water to flow into the netpen and used water to flow out of the netpen only by the flow in this region. This reduces the additional measures that need to be taken to breed the fish, but exposes the fish to the surrounding environment and allows sea lice, which naturally occur in the water area, to enter and exit the netpen.

Sea lice pose a big and economically expensive problem for farming salmon and trout in areas with high density of fish farm parasites. Sea lice alternate between free-swimming and parasitic phases of their life cycle. During the parasitic phase, sea lice adhere to and feed on the skin of fish, which leads to damage to the local sites where they are located and to erosion of the protective mucus and skin layers in case of loss of scales. This can lead to bleeding and exposure of underlying muscles, creating a starting point for bacterial secondary infections, all of which can cause discomfort, pain and even physical deformation in the affected parts of the fish. Damage to the skin increases stress on the farmed fish and reduces appetite, which in turn slows growth, may impair osmoregulation and greatly increase mortality.

Thus, parasites such as fish lice can have a significant and negative impact on fish health, mortality, and economics of fish farming. Accordingly, a great deal of resources have been used to reduce the problem of parasites in fish farming, both to develop mechanical means for removing lice from the skin of fish and anti-parasite chemicals, and to develop solutions to reduce infection and thus problems associated with parasites.

Currently in norway and other coastal areas of relatively cold waters, the most important fish for fish farming belong to the salmonidae family and include salmon, rainbow trout and crow's salmon. Sea lice are a class of species that are parasites during part of their life cycle and have different specificities for species of salmonidae.

Measures against sea lice infestation include chemical and mechanical measures, among others. Chemical measures include in situ treatment with chemicals such as hydrogen peroxide or toxins developed as pesticides introduced into the netpen, preferably temporarily surrounded by a tarpaulin, or by adding toxins to the feed. The chemical measures are all dependent on the fish having a higher tolerance to chemicals than sea lice. However, a large amount of chemicals are used, and lice become resistant to the chemicals over time, and the result of the chemicals being released into the surrounding waters after treatment may result in severe damage to the surrounding environment.

The mechanical measures consist in arranging a cage, i.e. a fine-meshed fabric, having a mesh size preventing sea lice from passing through, but allowing water to pass through, around the netpen, on top of the netpen. In the case of such cages only the upper few meters of the sea cage are covered by the fabric, since sea lice in the free-floating stage usually live in the upper layers of sea water, such as to about 5m below sea level. However, the cage reduces the flow of oxygen-enriched water that ensures that sufficient oxygen is supplied to the fish within the cage, and may result in the need to add additional oxygen to the cage.

Another technical measure is the use of a "snorkel sea lice barrier" in which a mesh "roof" is placed 5 to 10m below sea level to keep the fish at that depth or deeper. The "snorkel" which is substantially smaller in diameter than the cage allows the fish to reach the surface to breathe air, which is important for filling air bladders of the salmonidae variety.

Closed cans may also be used. The closed tank may be arranged onshore or floating in the water in a partially submerged manner. However, closed tanks require additional equipment and control systems for introducing oxygen into the water, circulation means for circulating the water, and means for removing manure and excess food that has accumulated at the bottom of the tank. In addition, there is a need for a purification apparatus for cleaning the water discharged from the tank into the surrounding environment. The construction costs of the tank and the additional measures required for operating the tank are prohibitively expensive compared to the well-known netpen.

NO336739B1 describes a netpen for the placement of live fish in the short term before slaughter. The netpen comprises a net and a bottom part arranged within a frame. The frame comprises a vertically arranged riser (5), one or more horizontally arranged ballast tubes and a vertically arranged ballast ring connected to the lowermost portion of the vertical riser. The netpen can be lifted by a riser to lift the bottom part even above sea level to allow easy extraction of the fish to be slaughtered. However, no means are provided to reduce the effect of waves on the netpen.

No in ilaks No. by Alsak Berge in 2016The article MNH-produksjon published 12.04.

om

A fish cage is described comprising a cylindrical upper part of the fish cage made of watertight panels and a lower perforated part of the mesh cage. The fish cage is connected with a floating ring made of steel surrounding the upper part of the fish cage. The fish cage may be lifted from the buoyant ring by lifting arms connected to the buoyant ring and the fish cage. However, the floating rings do always float on the water surface, making the entire construction vulnerable to wave action.

NO158201B relates to a netpen for fish farming, comprising a netpen connected to a frame. The frame includes a bottom frame and a floating ring connected to vertical frame members. The vertical frame members are connected to the buoyant ring and can be lifted relative to the buoyant ring to reduce the volume of the netpen below the sea surface. However, the floating ring does always float on the water surface, making the entire construction vulnerable to waves.

Other mechanical/technical means for combating sea lice infestation in fish farms are mostly associated with the removal of sea lice from infected fish. An example is a floating cell comprising a camera and a laser. Sea lice on fish are identified and located by image analysis and then "shot" and killed with a laser. Other means for washing and scrubbing fish are also known, such as transferring the fish to a separate processing unit to wash and scrub the fish to remove sea lice, and then transferring the fish so treated to another netpen. Moving the mesh cage to a new location with low salinity water, such as near the estuary, is also used because sea lice are killed or inactivated by the low salinity water.

It is an object of the present invention to provide a semi-closed tank fish cage for fish farming which allows the necessary water change during operation. Another object is to provide a fish cage arrangement that is less susceptible to wave action and can therefore be placed in a position that is more exposed to wave action than the prior art solutions allow. Other objects of the invention will be apparent to the skilled person upon reading the present specification and claims.

Disclosure of Invention

According to the invention, the above object is achieved by a tank for fish farming, wherein the tank comprises:

1. a polygonal or circular ring buoyancy tank forming a lower portion of the tank,

2. a side wall arranged substantially vertically on the annular buoyancy tank to form a substantially vertically arranged tubular member having a circular or polygonal cross-section, an

3. A vertical column disposed vertically upward on the annular pontoon and connected to the sidewall and extending from the annular pontoon to a top of the sidewall,

wherein the annular buoyancy tank and the vertical column comprise closed buoyancy elements, and wherein the annular buoyancy tank is used for ballasting by pumping water into the buoyancy elements in the annular buoyancy tank.

The tank for fish farming of the invention has a side wall that is closed towards the surrounding sea water and thus prevents parasites living in the upper layers of the surrounding sea water from entering the tank, even if the tank floats in the sea like the netpen most commonly used today. By dividing the buoyancy volume into an annular buoyancy tank representing the lowermost part of the tank and submerged during normal use, and a vertical column representing the buoyancy waterline of the tank, stabilization of the floating tank is obtained, thereby preventing or reducing the effect of wave forces on the tank.

According to one embodiment, the tanks are adapted to be adjusted by adjustment of the volume of ballast water in the annular buoyancy tank between a lower position, in which only the upper parts of the tanks and the vertical columns are above the sea surface, and an upper position, in which the vertical columns and the side walls are lifted above the sea surface. Allowing the vertical position of the tank to be adjusted and having sufficient buoyancy in the annular buoyancy tank to lift the vertical columns and sidewalls above the water surface is important in operating such as medical treatment and emptying of the tank to obtain fish where it is necessary/desirable to reduce the volume of water in the tank, as well as in maintaining the tank.

According to one embodiment, the annular buoyancy tank has sufficient buoyancy to lift the vertical columns, the sidewalls, and a portion of the annular buoyancy tank above sea level. Having sufficient buoyancy in the annular buoyancy tank to lift the sidewalls, vertical columns, and a portion of the annular buoyancy tank above sea level improves the availability of the tank for maintenance and emptying of the tank for fish when needed.

According to one embodiment, at the bottom of the tank there is arranged a grid with calibrated meshes that allows water to circulate into the tank, but prevents fish from escaping from the interior of the tank. The grating may be made of a net of rope, a metal grating, or the like, and has a mesh size that allows maximum water flow into and out of the tank, while preventing fish in the tank from escaping the tank.

According to one embodiment, a circulation opening is arranged at a lower portion of the side wall, and wherein remotely controllable doors are arranged for individually opening or closing said doors. A circulation opening arranged below the depth at which sea lice are found will allow more efficient water flow in the tank and thus more efficient water exchange. By having a remotely controllable door at the opening, each circulation opening can be opened or closed to optimize water flow and water exchange in the tank. In addition, in case sea lice or their life stages are found in deeper water than usual, all circulation openings can be closed to prevent or substantially reduce the inflow of sea lice into the tank.

The canister may further include a top cap covering the top of the tubular member. The top cover at the top of the tank will form a shield for weather conditions and may also be a shield for light. The roof will provide a shielded working space for personnel and will also be a protective shield to deter birds from attempting to catch fish.

According to one embodiment, the tank further comprises a central column connected with the top cover and with the beams at the level of the annular pontoon. The center post may provide additional support for the roof and other configurations in the tank.

According to one embodiment, wave dampening means are arranged in the tank to reduce internal wave motion. Internal wave motion is undesirable because internal waves can pose challenges to such constructions, fish in the tank, and personnel. Thus, the wave suppressor is preferably arranged to substantially reduce internal waves in the tank.

According to one embodiment, the wave suppression means comprise partition plates arranged on the cross section of the tank, which plates are arranged vertically, arranged such that they intercept the water surface in the tank and extend 0.5 to 5 meters above and below sea level.

According to one embodiment, the wave suppression means comprises a double wall section extending from about 1 to 5 meters above sea level to about 1 to 5 meters below sea level, and wherein the wall of the double wall facing the interior of the tank is perforated. One skilled in the art will appreciate that the divider plate and the double wall section including the perforated inner wall may both function as a wave dampening device alone, or in combination to provide optimal wave dampening. The choice of wave suppression means and its actual configuration will depend on the normal wave size and wave pattern of the water area where the tank is intended to be used and the area of use.

According to one embodiment a skimming pipe is arranged in the side wall for removing material accumulated at the water surface inside the tank. Organic materials such as dead fish, fecal debris, fish food, and other waste tend to accumulate at the sea surface as clumps, particles, and water-soluble materials. Some materials tend to form undesirable foam. To reduce this problem, it is preferred to use a skimming pipe that recovers surface water and waste that accumulates on the surface.

According to one embodiment, the skimming piping is arranged in the double-walled wave suppressor section.

According to one embodiment, a water circulation conduit is arranged for pumping water into and out of the tank to improve water circulation and water exchange in the tank. In some cases the water circulation and the replacement of the water in the tank with water from the surroundings is too low. The water circulation conduit is arranged to improve water circulation by pumping water into the tank from below the annular buoyancy tank and/or pumping water out of the interior of the tank to the surrounding environment.

Drawings

Figure 1 is a perspective view of a fish farming tank according to the invention,

FIG. 2 is a cross-sectional view of the canister shown in FIG. 1, an

Fig. 3 is a cross-sectional view of the tank illustrating the flow of water caused by the flow of water.

Detailed Description

The present invention relates to a semi-submerged tank 1 designed for fish farming. The structure comprises an annular buoyancy tank 2 on which vertical columns 3 are arranged. The annular buoyancy tank 2 and the vertical columns 3 serve as structural support, buoyancy and stabilizers for the tank 1. The annular buoyancy tank 2 is illustrated as a substantially circular ring, but the annular buoyancy tank may also be polygonal, having four or more straight sections. The annular buoyancy tank 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 annular buoyancy tank and are arranged vertically upwards from the annular buoyancy tank.

Four vertical columns are illustrated in the drawings. However, more than four vertical posts may be used. A semi-submerged structure according to the present invention having a polygonal annular pontoon, such as a square, hexagonal or octagonal annular pontoon, may have one vertical column arranged in each corner of the annular pontoon.

The side wall 4 is arranged to provide a compartment closed towards the side of the can 1. The side walls are connected to the buoyancy tank 2 and the vertical columns 3. The side walls are arranged to constitute a tubular member or tank arranged substantially vertically, open at the bottom and at the top. At the bottom of the tank a grid 5 with calibrated meshes is arranged to allow the water to circulate from the outside into the tank at deeper draught and to prevent the fish from escaping from the inside of the tank.

Preferably, circulation openings 6 are arranged in the side wall 4 at the lower part of the tank to improve the circulation of water between the surrounding sea and the interior of the tank in case the surrounding water flow cannot ensure a satisfactory circulation. In the circulation opening 6 a circulation opening grid 7 with calibrated meshes is arranged to allow the circulation of water through the circulation opening and to prevent the escape of fish from the interior of the tank. Preferably, a door, not illustrated, is preferably arranged at the circulation opening 6 for opening and closing the door as required for opening the circulation opening 6. Preferably, the doors are individually and remotely controllable. The door is preferably a sliding door.

Preferably, a top cover 8 is arranged to cover the top of the tank 1. The cover may be made of transparent or opaque material and is arranged to provide a protected space for the internal equipment and personnel working in the fish farm. In addition, by using an opaque or only partially transparent material for the top cover 8, the light inside the tank can be adjusted as desired. The roof 8 and its connection to the tank are not airtight but protect the interior of the tank from the outside weather, providing a weather-proof working space for the personnel below the roof.

A central column 9 is arranged centrally and vertically within the tank. The central column 9 is supported at its top by not illustrated beams in the top cover and at the level of the annular pontoon, optionally by additional beams arranged at a level between the top cover and the level of the annular pontoon. Water and/or air for increasing circulation and displacement of water in the tank may be introduced into the tank through the central column.

A partition plate 10 for dampening waves inside the tank 1 may also be arranged between the side wall 4 or the vertical column 3 and the central column at the surface level of the tank interior, so that the partition plate is arranged to intercept the water surface when the tank is in a lower position used by the tank in normal operation of fish farming. The partition plate is vertically arranged and inserted downward into the tank by 0.5 to 5 meters.

It is also possible to arrange a wave suppressor 11 at the side wall 4, which wave suppressor is here exemplified by a double-walled section in the splash zone inside the tank, wherein the inwardly facing wall of the double-walled section is perforated. According to a preferred embodiment, the double wall section of the wall of the present tank extends from about 1 to 5 meters below the horizontal plane and extends to about 1 to 5m above the horizontal plane when the present tank is at its lower or operating depth. The divider plate and wave suppressor reduce waves and splashes within the tank.

When the tank 1 is present in the sea, the annular buoyancy tank 2 is at least partially submerged. The buoyancy of the annular buoyancy tank may be adjusted by partially filling the annular buoyancy tank with water or by pumping water out of the annular buoyancy tank. Preferably, the buoyancy of the annular buoyancy chamber is sufficient to allow the annular buoyancy chamber to be partially above water level when little or no water is present within the annular buoyancy chamber. During transport, cleaning of the tank or maintenance, the tank is usually only in this "upper" position. During normal operation of the tank for fish farming, the tank is in its "lower" position, in which the annular buoyancy tank is ballasted or partially filled with water, so that only the upper part of the tank, including the upper part of the side wall 4, the upper part of the vertical columns 3 and the roof, is visible on sea level.

Typically, the diameter of a substantially circular tank or the length of a side wall in a tank having a substantially square cross-section will be 1.5 to 4 times the height of the tank, such as 2 to 3 times the height. During normal operation of the fish farming, the diameter or length of the sidewall will be measured as the inner diameter or length, while the height will be measured from the bottom of the annular pontoon to the sea level. Currently, a substantially circular tank with a substantially circular annular buoyancy tank is preferred. Typically, the height of the tank will be 15m or more and thus the diameter will be 22.5m or more. More typically, the height will be 20m or more and the diameter will be 40m or more. Currently preferred can heights are about 25m or more and diameters are 50m or more. Thus, in a normal operating position, i.e. during a stable period of fish farming, the depth of the annular buoyancy tank 1 is 15m or more, such as 20m or more, or even 25m or more.

For normal operation, the tank is lowered to a desired depth in the sea by partially filling the annular buoyancy tank and/or column with water. The enclosed and watertight volume in the annular buoyancy tanks and the vertical columns 3 gives buoyancy to the tank when it is lowered into the sea. In normal operation, the tank is submerged such that only a portion of the tank, such as 1 to 10 meters of the upper portion of the side wall and the roof, is above sea level. In this position the open bottom of the tank and the circulation opening 6 are at least 10 meters, such as at least 12 meters or at least 15 meters, such as about 20 meters, below sea level, i.e. below the water depth where sea lice are normally present, to avoid or at least reduce the possibility of parasites living near the surface, such as sea lice, coming into contact with the interior of the tank.

Semi-submersible structures are well known for oil and gas related vessels such as drill ships, production ships or platforms for use in deep water. The semi-submersible vessels/structures each have: one or more buoyancy tanks which, in operation, are submerged under the action of surface waves; and a plurality of vertical columns that are both buoyant and stable; and a structural element for a portion of the container above the surface. Typically, the buoyancy tank or tanks constitute the majority of the buoyancy of the construction, even when submerged, and the vertically arranged columns constitute a small part of the buoyancy and serve to stabilise the construction. According to the invention, the one or more buoyancy tanks are one annular buoyancy tank as described above. The annular buoyancy tank according to the invention constitutes between 60% and 80% of the buoyancy of the tank construction when the tank is in its lower or operating position.

The effect of the waves decreases exponentially with depth. Thus, by having most of the buoyancy at the depth as described above, i.e. at least 15m such as more than 20m or more than 25m, during operation of the fish cage for fish farming, the wave forces that excite the heaving motion are substantially reduced.

In addition, dynamic wave pressure (due to wave-induced pressure fluctuations) decreases exponentially with depth. This means that the dynamic pressure variations on the top of the annular buoyancy tank will be higher than the dynamic pressure variations at the bottom of the annular buoyancy tank. The difference between the two depends on the wavelength/period. The vertical columns reduce the effective area on top of the annular pontoon. This counteracts some of the effects of higher pressure variations between the top and bottom of the annular buoyancy tank. At a certain wavelength/period, the vertical dynamic pressure will be practically zero, since the pressure on the top of the buoyancy tank multiplied by the area reduced there (the area occupied by the vertical columns) is exactly equal to the pressure on the bottom of the buoyancy tank multiplied by the area of the bottom of the annular buoyancy tank. The sides and columns of the buoyancy tank will not contribute to the vertical forces because the water pressure on these elements creates horizontal forces (the surface normal is horizontal).

The skilled person will know how to adjust the column area to obtain the desired period of cancellation with respect to the overall heave response, i.e. the wave period when the dynamic vertical pressure is zero. This adjustment is made taking into account the eigenperiod of the heave for the vessel and keeping it away from where the main part of the wave energy is located (the wave period with the main part of the wave energy).

Due to the reaction of mass forces acting on the submerged pontoons and the pressure acting under the vertical columns and the deep draft, semi-submerged vessels or platforms are substantially less affected by waves at the surface than surface vessels, at least under certain conditions, i.e. the wavelength and wave height for which they are constructed. The technician can construct a semi-submerged for a particular location based on the statistics of weather and waves at the expected location. It is also possible to arrange skirts 2 at the side walls to further dampen wave action on the present tank and to reduce the vertical downward flow of surface water, thereby preventing parasites mainly present in the upper water layer from entering the tank. The optional skirt 12 is in the form of a horizontally arranged edge arranged at the side wall 4, which acts as a barrier preventing water from flowing vertically along the side wall 4 facing the surrounding sea water.

By placing the tank in a position where the water flow is large, the flow of water under the tank will establish a flow into and out of the tank, thereby exchanging the water inside. The forced water flow will circulate from downstream to upstream side in the tank and leave the tank from upstream side. This effect has been confirmed by computerized fluid dynamics simulations performed by the applicant, and the results of this analysis are illustrated in fig. 3. The direction of water flow is indicated by the arrows and the simulation shows that the vast majority of the water forced to flow under the annular buoyancy tank will be circulated into the tank to exchange the water therein with "fresh" water from the surrounding seawater.

Separate 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 one or more of the upstream circulation openings 6 and closing the downstream circulation openings will cause some water flow to be trapped, thereby forcing better circulation of water into the tank. An equivalent effect can be obtained by equipping the tank with a shield extending below the annular buoyancy tank at a downstream portion of its periphery. The tank may also be provided with an outlet for circulation through its central column and/or side columns. The skilled person will also appreciate that in some cases it may be desirable to subject the water to forced circulation/exchange and/or to introduce oxygen, such as by introducing air into the water in the tank. The water may be pumped into the tank through the central column and/or through channels in the vertical columns or by any other means known to the skilled person. In addition, in emergency situations of reduced or very low water flow, sections of the side wall 4 may be opened for additional cycles.

To further increase the water circulation, especially in cases of little or no water velocity, a circulation pipe leading to the interior of the tank may be arranged from below the annular buoyancy tank, up the side wall or inside the vertical column. A pump is arranged at the circulation pipe to actively promote circulation of the water. Typically, the opening of the circulation conduit into the interior of the tank is arranged 3 to 10 meters below the sea surface when the tank is in its lower or working position. Preliminary calculations show that an optimum water circulation effect can be obtained by arranging a water circulation pipe for pumping water out of the tank at an upstream portion of the tank and a circulation pipe for pumping water into the tank at a downstream side of the tank. The circulation pipes for the pumped-out water are preferably arranged at about 45 degrees on both sides in the main incoming water flow direction. The circulation conduit for the pumped-in water is preferably arranged within an angle of 30 degrees of the water flow at the downstream end of the tank.

Organic material from fish farming and marine pollution often accumulates on the surface and can cause blistering inside the tank. This predominantly organic material may be removed by placing a skimming pipe in the periphery of the tank near the water surface to skim water. Preferably, as mentioned above, the skimming duct is arranged inside the wave suppressor 11. The opening of the skimmer pipe is preferably arranged 0 to 0.5m below the water surface.

The tank can be raised by reducing the volume of water introduced as ballast into the annular buoyancy tank. The raising is performed when it is necessary to discharge the fish from the tank for cleaning and maintenance of the tank or for treatment of the fish if the fish is infected with parasites. As with conventional netpens, raising the tank reduces the water volume of the captured fish and enables more efficient capture of the fish for emptying purposes (empty tanks), or if the farmed fish are infected with parasites to chemically treat the farmed fish. Finally, a part of the entire tank can be brought above the sea surface from above the annular buoyancy tank for cleaning purposes (clean tank) or for maintenance purposes.

Dynamic calculation of the integrated system; mooring vessels excited by waves, currents and wind show a possible interaction between the wave response of the vessel with internal oscillating water and the spring characteristics at the mooring, which may cause internal sloshing problems as well as strength problems in the mooring system and the vessel itself. The design of the system is especially considered to dampen the influence of such dynamic interactions by introducing dampening means at the surface level inside the tank to reduce the influence of the oscillating surface water and the buoy/weight means attached to the mooring line to mitigate the spring rate of the mooring.

The vessel is held in place by a mooring system connected to the vessel by a fairlead on the column and an anchor winch at the top of the column. The mooring system is adjusted with respect to line configuration, line angle, buoyancy and stiffness in order to reduce mooring forces caused by tank dynamics, thereby improving motion. Typically; (1) in shallow waters (water depths less than 100m), a buoyant body with a buoyancy of 10 to 100 tons is attached to each mooring line 30 to 100m from the fairlead, thereby reducing the tension variations affected by the change in position of the SST (i.e. reduced stiffness). The natural period of the system in surge/roll/pitch/yaw can then be kept well above the period with significant first order wave energy. The amount of buoyancy may be adjusted to achieve the desired stiffness. (2) In shallow waters (water depths less than 100m), the second solution is to combine a buoy with a buoyancy of 10 to 100 tons with a weight of 20 to 200 tons with another set of buoyancy modules of 10 to 100 tons attached to each mooring line, thereby reducing the tension variation (i.e. stiffness reduction) affected by the position variation of the SST (i.e. reduced stiffness). Between buoyancy, weight and buoyancy there are mooring lines 5 to 80m long. The natural period of the system in surge/roll/pitch/yaw can be kept well above the period with significant first order wave energy. The mooring line (including buoyancy and weight) can be kept well below the surface of the sea. The amount of buoyancy and weight, as well as the length between mooring lines, may be adjusted to achieve the desired stiffness.

Claims

1. A semi-submersible tank structure (1) for fish farming, characterized in that the semi-submersible tank structure (1) comprises:

a. a polygonal or circular ring buoyancy tank (2) forming a lower part of the semi-submerged tank structure (1),

b. a side wall (4) arranged substantially vertically on the annular buoyancy tank (2) to form a substantially vertically arranged tubular member having a circular or polygonal cross-section, the side wall (4) being adapted to be closed towards surrounding sea water,

c. a vertical column (3) arranged vertically upwards on the annular buoyancy tank (2) and connected with the side wall (4) and extending from the annular buoyancy tank (2) to the top of the side wall,

wherein the annular buoyancy tank (2) and the vertical column (3) comprise respectively a closed buoyancy element and a commensurate stabilizing effect, such that the annular buoyancy tank (2) constitutes the majority of the buoyancy of the construction and the combination of the annular buoyancy tank (2) and the vertical column (3) acts to dampen or reduce the movement of the semi-submerged tank structure, such that the semi-submerged tank structure is less affected by waves,

and wherein the buoyancy elements of the buoyancy ring (2) are used for ballasting by pumping water into and out of the buoyancy ring,

wherein at the bottom of the semi-submerged tank structure (1) a grid (5) with calibrated meshes is arranged, which allows the circulation of water into the semi-submerged tank structure (1), but prevents the escape of fish from the interior of the semi-submerged tank structure (1),

the semi-submerged tank structure comprising wave suppression means arranged in the semi-submerged tank structure (1) to reduce internal wave motion, wave suppression means comprising a partition plate arranged on a cross section of the semi-submerged tank structure (1), the partition plate being arranged vertically, arranged such that the partition plate intercepts the water surface in the semi-submerged tank structure (1) and extends 0.5 to 5 meters above and below sea level, and

the wave suppression device comprises a double-walled section extending from 1 to 5 meters above sea level to 1 to 5 meters below said sea level, and wherein a wall of the double-walled section facing the interior of the semi-submerged tank structure (1) is perforated.

2. A semi-submerged tank structure (1) according to claim 1, wherein the semi-submerged tank structure (1) is used for adjustment by adjustment of the volume of ballast water in the annular buoyancy tank (2) between a lower position, in which only the upper part of the semi-submerged tank structure (1) and the vertical columns (3) are above sea level, and an upper position, in which the vertical columns (3) and the side walls are lifted above sea level.

3. A semi-submerged tank structure (1) according to claim 2, wherein the annular buoyancy tank (2), when unballasted, has sufficient buoyancy to lift the vertical column (3) and the side wall (4) above sea level.

4. A semi-submerged tank structure (1) according to claim 3, wherein the annular buoyancy tank (2) has sufficient buoyancy to lift the vertical column (3), the side wall and a portion of the annular buoyancy tank (2) above sea level.

5. A semi-submerged tank structure (1) according to any one of claims 1 to 4, wherein a circulation opening (6) is arranged at a lower portion of the side wall (4), and wherein remotely controllable doors are arranged for individually opening or closing said doors.

6. A semi-submersible tank structure (1) according to any one of claims 1 to 4, wherein the semi-submersible tank structure (1) further comprises a roof (8) covering the top of the tubular member.

7. A semi-submerged tank structure (1) according to claim 6, wherein the semi-submerged tank structure (1) further comprises a central column (9) connected with the roof (8) and with beams at the level of the annular buoyancy tank (2).

8. A semi-submersible tank structure (1) according to claim 1, wherein a skimming pipe is arranged in the side wall for removing material accumulated at the water surface of the interior of the semi-submersible tank structure (1).

9. Semi-submersible tank structure (1) according to claim 8, wherein the skimming piping is arranged in a double-walled section of a wave suppression device.

10. A semi-submerged tank structure (1) according to any of claims 1 to 4, wherein water circulation pipes are arranged for pumping water into and out of the semi-submerged tank structure (1) to improve water circulation and water exchange in the semi-submerged tank structure (1).