DK201770918A1

DK201770918A1 - Offshore fish farm

Info

Publication number: DK201770918A1
Application number: DKPA201770918A
Authority: DK
Inventors: Johannesen Thormund; Olsen Marni
Original assignee: Knud E. Hansen A/S; P/F Bakkafrost
Priority date: 2017-12-06
Filing date: 2017-12-06
Publication date: 2019-06-20
Also published as: WO2019110065A1; DK179794B1; EP3720278A1

Abstract

An offshore fish farm (F,N) comprises a flexible netting (N) suspended to a floating frame structure (F). An upper netting part is above sea level and a lower closed netting part (32) is submersed. A floating frame structure (F) comprises at least three towers (1,2,3,4,5,6) having a top end part (1b,2b,3b,4b,5b,6b) with a free top end (1d,2d,3d,4d,5d,6d) above sea level and an opposite submersed bottom end part (1c,2c,3c,4c,5c,6c) with a bottom end (1a,2a,3a,4a,5a,6a) floating above the sea bed. The submersed bottom end parts (1c,2c,3c,4c,5c,6c) of the at least three towers (1,2,3,4,5,6) are circumferentially connected by means of crossbars (13,14,15,16,17,18) into a polygonal bottom frame (P). Upper flexible lines (26,27,28,29,30,31) extend tangentially to the top end parts (1b,2b,3b,4b,5b,6b) to circumferentially connect the towers (1,2,3,4,5,6) at a distance from the respective free top ends (1d,2d,3d,4d,5d,6d). The bottom ends of the at least three towers (1,2,3,4,5,6) are radially connected to a common centre (25) by means of lower flexible lines (19,20,21,22,23,24), and the lower flexible lines (19,20,21,22,23,24) are heavier than the upper flexible lines (26,27,28,29,30,31).

Description

The present invention relates to the field of aqua-culturing.

In particular the present invention relates to an offshore fish farm comprising a flexible netting suspended to a floating frame structure, wherein

- the suspended flexible netting has an upper netting part above sea level and a lower submersed netting part with a closed netting bottom,

- the floating frame structure comprises:

- at least three towers having a top end part with at least a free top end above sea level and an opposite submersed bottom end part with a bottom end floating above the sea bed, and

- the submersed bottom end parts of the at least three towers are circumferentially connected into a polygonal bottom frame by means of crossbars.

Overfishing is a growing global problem and various solutions to breed fish exist today.

The amount of pollution from aqua-culturing depends on how the fish are contained. Open-net pens are anchored in natural environments, typically coastal areas, and juveniles are sourced from hatcheries or wild populations, protected from predators and fed regularly. The fish are kept in the open-pens until they reach marketable size and are harvested. The disadvantage is that the direct exchange of water causes free and unregulated exchange between the fish farm and the surrounding environment of high concentrations of waste and disease at the anchored location.

An alternative to such stationary open-net pens is fish cages that can be moved to new locations at the ocean, however it is very complicated and time-consuming to move these fish cages.

DK 2017 70918 A1

However aqua-culturing also has many advantages. Both kinds of fish farms utilize natural currents, which provide the fish with oxygen and other appropriate natural conditions, so that fish farming can create large amounts of fish quickly, cheaply and efficiently compared to wild caught fish.

From Korean patent no. KR101341945 is known a submersible type fish cage in which the cage floats on the surface of the water. Reinforcing piles are vertically arranged at an interval from the outer peripheral portion of the cage, which piles are anchored at the sea bed. Guide units provided at the outer peripheral portion of the cage allow the cage to move up or down on the piles. The center of the bottom side of the cage is connected with a buoy by means of ropes so that the fish cage can be found in case the piles get free of the sea bed and the cage flows away. The rope is forcibly held in a guide to follow a U-shaped curve below the reinforcing piles that are anchored in the sea bed. This known fish cage however relies on being anchored in the sea bed, and setting the fish cage up or moving it is a time-consuming and heavy task.

There is a need for new commercial offshore fish farms.

It is a main aspect of the present invention to provide an offshore fish farm of the kind mentioned in the opening paragraph that can withstand even severe wind and wave conditions.

It is a further aspect of the present invention to provide an offshore fish farm of the kind mentioned in the opening paragraph that can operate without being anchored to the sea bed.

It is a further aspect of the present invention to provide an offshore fish farm of the kind mentioned in the opening paragraph that can operate with a minimum of personnel.

DK 2017 70918 A1

It is a further aspect of the present invention to provide an offshore fish farm of the kind mentioned in the opening paragraph that is safe, environmentally friendly, and offers high animal welfare.

It is a further aspect of the present invention to provide an offshore fish farm of the kind mentioned in the opening paragraph that provides for optimum breeding conditions.

It is a further aspect of the present invention to provide an offshore fish farm of the kind mentioned in the opening paragraph that has high production capacity and provides the farmer with high financial returns.

It is a further aspect of the present invention to provide an offshore fish farm of the kind mentioned in the opening paragraph that has a stable cage netting irrespective of weather conditions.

It is a further aspect of the present invention to provide an offshore fish farm of the kind mentioned in the opening paragraph from which fish is easily harvested.

It is a further aspect of the present invention to provide an offshore fish farm of the kind mentioned in the opening paragraph that is towed to a breeding location.

The novel an unique whereby these and other aspects are achieved according to the present invention consist in that

- upper flexible lines extend tangentially to the top end parts to circumferentially connect the towers at a distance from the respective free top ends,

- the bottom ends of the at least three towers are radially connected to a common centre by means of lower flexible lines, and

DK 2017 70918 A1

- the lower flexible lines are heavier than the upper flexible lines .

When the fish farm is set up at the offshore location the flexible netting is suspended from and distended by the frame structure. The frame structure has a polygonal circumference due to the rigidity of the upright towers and the rigidity of the crossbars used to connect the towers in a plane offset the bottom ends below sea level. Preferably all crossbars are connected to the towers at same distance from the bottom ends of said towers whereby the crossbars define a polygonal frame substantially parallel to a plane taken through the bottom ends. When the fish farm is floating on calm water the towers are substantial vertical and the polygonal frame substantial horizontal.

All lower flexible lines has one end secured to a respective bottom end of a tower and the corresponding opposite ends of the lower flexible lines are joined in a common center. The lower flexible lines protrude as flexible heavy slacked spokes from the common center, and together the arrangement of lower flexible lines constitutes a floating sinker body for the offshore fish farm. None of the towers or lines is in contact with or being anchored to the sea bed.

The floating sinker body stabilizes the polygonal outline of the offshore fish farm due to components of forces acting radially towards the common center. However when the bottom ends of the towers are forced against each other due to the load of the sinker body the opposite free top ends of the towers are forced away from each other.

The upper flexible lines that extend tangentially to the top end parts to circumferentially connect the towers at a distance from the respective free top ends, serve to counteract that the free top ends of the towers are forced away from each other.

DK 2017 70918 A1

So the causative and interdependent continued interaction of the arrangement of the upper flexible lines and lower flexible lines constitutes a balancing system that keeps the fish farm floating, and with the top end parts of the towers spaced apart even under very rough sea conditions. Even in situations where the top end parts tend to move towards the central axis of the fish farm, the flexible lower lines counteract such movement. The top end parts of the towers can without it affects the functionally of the offshore fish farm swing to and from the central axis as much as the flexible lower chains permit.

To optimize the balancing function and capacity of the respective heavy lower flexible lines versus the less heavy upper flexible lines the lower flexible lines can be at least double as heavy as the upper flexible lines. Preferably the lower flexible lines can be at least three times as heavy as the upper flexible lines, or the lower flexible lines can be at least four times as heavy as the upper flexible lines, or the lower flexible lines can be at least five times as heavy as the upper flexible lines, or the lower flexible lines can be at least six times as heavy as the upper flexible lines, or the lower flexible lines can be at least seven times as heavy as the upper flexible lines, or the lower flexible lines can be at least eight times as heavy as the upper flexible lines, or the lower flexible lines can be at least nine times as heavy as the upper flexible lines, or the lower flexible lines can be at least ten times as heavy as the upper flexible lines, or even heavier.

The choice of weight ratio of lower flexible lines in relation to upper flexible lines can be adjusted in accordance with the environment and offshore location of use, the dimensions of the fish farm, and its intended use. Test, trial and experience are useful when determining to design the proper offshore fish farm for a given task. Optionally weight and length of the respective lines for a given fish farm can even be altered on

DK 2017 70918 A1 site. A given acceptable certain deviation from vertical of the circumferentially spaced apart towers, that prevent collapse of the yielding offshore fish farm, is defined by the length and weight of the lines, which length and weight normally are adjusted or replaced when on an onshore location, but at least the upper flexible lines may also be adjusted or replaced when the fish farm is already off shore, should the need arise.

In a simple but preferred embodiment the lower flexible lines and/or the upper flexible lines can be heavy or less heavy chains, respectively. Chains offer the required degree of flexibility, there are many commercially available kinds, chains are not expensive, and they are easy to cut to length due to the links.

It should be noted that within the scope of the present invention any kind of lines able of providing the required flexibility and weight relation are within the scope of the present invention. Should e.g. the lower flexible lines need more weight, such weight is easy to add on, e.g. by hanging loads to these flexible lower chains. The eyes of the chain links can be used for this purpose. Strong wires provided with spaced apart detachable loads can also be used as alternative to chains.

The lower flexible chains should be kept free of the bottom of the netting and of the sea bed, but if this is observed extra loads can be hung on lines, such as secured to the links of the chains if one or both of the upper flexible lines or lower flexible lines turn out to be too light in a given environment.

Expediently a tower can be joined to the polygonal frame of crossbars by means of a connection member, such as a pipe piece or a bushing. The tower can then be inserted into a pipe piece or a bushing adapted for connection to two opposite crossbars of the polygonal frame. Optionally, the pipe piece or bushing

DK 2017 70918 A1 is slidable arranged on the respective tower to provide freedom to choose a position of the polygonal frame on the towers, e.g. in accordance with the depth of the netting, the shape of the sinker body and the position of the common centre of the joined lower flexible lines. Once the desired position of a pipe piece or a bushing is chosen the pipe pieces or bushings may be locked to the towers, but until final locking a pipe piece or a bushing can slide along a length of a tower. The slidability also allows for easy dissembling of the offshore fish farm and for refitting the offshore fish farm for use at another offshore breeding location.

The towers provide the required buoyancy to the offshore fish farm of the present invention. The weight distribution of the upper flexible lines and the lower flexible lines can be selected to adjust the buoyancy of the towers so that the offshore fish farm floats free of the sea bottom, e.g. so that about 20 % of the tower length is above sea level.

In a preferred embodiment of the offshore fish farm of the present invention the number of towers is six or higher. Six towers provide a hexagonal outline and a hexagonal polygonal frame. Even and odd numbers of towers are possible. A tower need not have a diametrically arranged opposite tower in order to set-up a proper balanced offshore fish farm.

A suitable buoyancy of the towers can be obtained when the towers are hollow pipes. The hollowness, optionally also encapsulated air, confers the buoyant property. The interacting system of flexible lines keeps the offshore fish farm erected and the netting distended without the need for further securing and distending means. Thus the offshore fish farm of the present invention is self-supported and self-adaptable to various conditions and environments.

DK 2017 70918 A1

A resilient member can be interposed between a crossbar and a tower to flexibly and resiliently connect the crossbars to the towers. The resilient members allow the towers to move out of the planes normal to the bottom plane through the bottom ends of the towers and allow the crossbars to deviate away from said bottom plane through the bottom ends of the towers. The movement of the towers and the crossbars take place in response to movements of the waves and the current, and the offshore fish farm of the present invention is configured to auto-adapt to changing weather conditions, and to other changes in the environment of use by balancing external forces impacting on the offshore fish farm. The resilient member may have shockabsorbing and/or damping properties.

At least some of the towers may have a top end part equipped with mooring and/or docking floating vessel. The mooring expedient for maintenance of harvesting the fish population, for feeding the fish, for inspection of facilities for at least one and/or docking facilities are the offshore fish farm, when for securing a towing vessel, fish, for surveillance of fish, in particular surveillance of dead fish and their removal, and for transporting personnel, etc.

The flexible netting may be secured to the frame structure in various ways and at various positions.

For example can an upper rim of the flexible netting be secured to at least some of the upper flexible lines, along the entire length of the upper flexible lines, or at just one position to a flexible line, or on more positions to a flexible line, and/or a lower circumference of the flexible netting can be kept distended by being secured to any of the pipe piece or the bushings, the polygonal frame, and/or the towers.

The flexible netting of the offshore fish farm has a submersed closed bottom and an opening above sea level. The polygonal

DK 2017 70918 A1 frame is arranged above the sea bed but below the surface of the water and the flexible netting and the frame structure float primarily due to the buoyancy of the towers. A carefully adjusted system of heavy lower chains and more light-weight flexible upper chains is responsible for balancing the offshore fish farm so that it does not collapse and so that it does not sink below the water surface.

Optionally the towers can accommodate ballast, e.g. seawater, to further control the degree of submersion of a tower, thus the ratio between the length of a tower above the surface of the sea and the length of a tower below the surface of the sea.

Furthermore the tower may serve as storage for fish feed.

A vessel, such as e.g. a tugboat, may be used to dynamically keep the floating offshore fish farm against drifting, by pulling against the current of the sea. The vessel serves as a constantly operating bollard for the off-shore fish farm. As an alternative means to keep the offshore fish farm of the present invention in substantially same position at sea without drifting the towers can be equipped with propellers in a manner similar to the manner used for offshore oil rigs.

The invention will now be described in further details by way of an exemplary embodiment consisting of six towers, six crossbars, six connection members, six heavy lower flexible lines in form of chains, and six upper flexible lines less heavy than the lower flexible lines. Other numbers than six are within the scope of the present invention, and other types of lines than chains can be used. To provide the towers with adequate buoyancy the towers are assumed to be hollow pipes.

Fig. 1 is a perspective view of a frame structure without flexible netting,

DK 2017 70918 A1 fig. 2 is a perspective view of an offshore fish farm in a first momentary position, which offshore fish farm includes the frame structure seen in fig. 1 with a suspended flexible netting, fig. 3 shows the same in a second momentary position, fig. 4 shows the floating offshore fish-farm, fig. 5 shows schematically how floating vessels dock at the offshore fish farm, and fig. 6 schematically illustrates various ships and boats involved in the aqua-culturing by means of the offshore fish farm.

The frame structure F shown in perspective in fig. 1 consists of six upright towers 1,2,3,4,5,6 provided with a respective connection member 7,8,9,10,11,12, that can slide at least a distance up and down along a tower 1,2,3,4,5,6 between a top end part 1b,2b,3b,4b,5b,6b and an opposite bottom end 1a,2a,3a,4a,5a,6a of bottom end part 1c,2c,3c,4c,5c,6c of the towers 1,2,3,4,5,6 until locked into a suitable use position spaced from said bottom ends 1a,2a,3a,4a,5a,6a.

members

7',7'';8',8'';9',9'';10',10'';11',11'

Each connection member 7,8,9,10,11,12 has oppositely protruding resilient the connection members 7,8,9,10,11,12,

12',12'' adapted to secure the crossbars 13,14,15,16,17,18 and thus establish to stable connection between the towers

1,2,3,4,5,6 and the crossbars 13,14,15,16,17,18 that allows the polygonal bottom frame P to yield at least forces so that the offshore slightly in fish farm can float as steady as response to external possible on the sea while maintaining a 3-dimensional shape.

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A lower flexible line in form of a heavy lower flexible chain 19,20,21,22,23,24 of chain links each having one end secured to a bottom end 1a,2a,3a,4a,5a,6a of a circumferentially arranged six upright towers 1,2,3,4,5,6. The opposite ends of the heavy lower flexible chains 19,20,21,22,23,24 meet in a common center 25, to apply weight and a drawing power to the bottom ends la, 2a,3a,4a,5a,6a of the six upright towers 1,2,3,4,5,6. The centrally joined, and firmly linked together, heavy lower flexible chains 19,20,21,22,23,24 serve as a sinker body, dimensioned to be free of the sea bed, and contributing in keeping the offshore fish farm freely floating and partly submersed without drifting and in substantially upright position despite the long towers 1,2,3,4,5,6 and the drag of the flexible netting seen in e.g. figs. 2 and 3.

Upper flexible chains 26,27,28,29,30,31 extend between the towers 1,2,3,4,5,6 a distance below the top end parts lb, 2b,3b,4b,5b,6b of said towers 1,2,3,4,5,6. Optionally the upper flexible chains 26,27,28,29,30,31 are provided by one continuous chain that passes through the towers. Alternatively the upper flexible chains 26,27,28,29,30,31 are separate chains secured to the outside of the towers 1,2,3,4,5,6.

The upper flexible chains 26,27,28,29,30,31 are lighter than the lower flexible chains 19,20,21,22,23,24. The upper flexible chains 26,27,28,29,30,31 keep the frame structure open and prevent the towers from getting together. If one tower moves against the other, other towers will be affected and counteract this movement. The upper flexible chains 26,27,28,29,30,31 and the lower flexible chains 19,20,21,22,23,24 co-operate and interact to keep the frame structure stable, partly submersed, the flexible netting open, and the offshore fish farm floating.

As an example a 100 000 m³ offshore fish farm of the present invention may breed 2 500 ton fish. Thus the fish density is 25 kg/m³. The towers of such a fish farm may be 2.5 m in diameter

DK 2017 70918 A1 and 50 m long. The lower flexible chains may e.g. have a weight per meter of 350 kg, and the upper flexible chains may have a weight per meter of about 35 - 50 kg. Thus the lower flexible chains are in this example about 10 to 14 times heavier than the upper flexible chains. How heavy chains and how long chains that is needed depend on the specific use and size of the offshore fish farm. The flexible netting may be synthetic polymeric netting with a mesh size that keeps the fish inside the flexible netting.

The weight of the respective chains is selectable by parameters such as wire size, pitch and width, where the wire size is the size of the wire used to create a link, the pitch is the measurement of a link length measured internally, and the width is the width of a link and is measured externally.

Figs. 2 and 3 illustrate the flexible netting N suspended by the frame structure F and the offshore fish farm in two different momentary float positions that the offshore fish farm can achieve on sea (not shown). The flexible netting is secured to the frame structure F by means of wires and/or hawsers to transmit loads to the frame structure.

In the float position seen in fig. 2 at least some of the top end parts 1b,2b,3b,4b,5b,6b of the towers 1,2,3,4,5,6 have moved towards the central axis of the offshore fish farm thereby tilting the corresponding bottom ends 1a,2a,3a,4a,5a,6a of the towers 1,2,3,4,5,6 radially away from the central axis of the offshore fish farm resulting in tensioning of the lower flexible chains 19,20,21,22,23,24. Maximum tensioning is when a lower flexible chain 19,20,21,22,23,24 is straight and parallel to the plane of the bottom ends 1a,2a,3a,4a,5a,6a.

In the float position of fig. 3 the lower flexible chains

19,20,21,22,23,24 have been slackened again which allows the top end parts 1b,2b,3b,4b,5b,6b of the towers 1,2,3,4,5,6 to

DK 2017 70918 A1 move away from the central axis of the offshore fish farm again.

In the configuration of an offshore fish farm F,N seen in fig. 2 the bottom 32 of the flexible netting N is not shown being secured to the frame structure F. It should be noted that within the scope of the present invention such securing is intended to transfer load from the flexible netting to the frame structure. As illustrative examples of ways to secure the bottom of the flexible netting to the polygonal bottom frame P, fig. 3 shows, to the left, a bottom hawser 33 that extends between the connection member 7 on the tower 1 and the bottom of the flexible netting N at the polygonal bottom frame P. Another example of how so secure the bottom 32 of the flexible netting N to the submersed part 1c,2c,3c,4c,5c,6c of the frame structure F is shown in relation to tower 2 where crossbars 17,18 on opposite sides of the tower 2 has a bottom hawser or chain 33a,33b connected to the bottom 32 of the netting N thereby securing said bottom 32 of the netting N to the polygonal bottom frame P of the frame structure F. The bottom 32 of the netting N is conveniently tapering away from the opposite opening 35 and is free and out of contact with the heavy lower flexible chains 19,20,21,22,23,24. At the upper rim of the flexible netting N, said flexible netting is suspended to the frame structure F by means of the light upper flexible chains 26,27,28,29,30,31 and to the towers by means of upper hawsers 34a,34b, as illustrated at tower 5.

Figs. 4 and 5 show the floating offshore fish-farm F,N at sea S .

As seen best in fig. 4 the bottom end parts 1c,2c,3c,4c,5c,6c are the submersed parts of the towers 1,2,3,4,,5,6 and the top end parts 1b,2b,3b,4b,5b,6b are the parts of the towers 1,2,3,4,,5,6 above the surface of the sea S. The free top ends 1d,2d,3d,4d,5d,6d of the top end parts 1b,2b,3b,4b,5b,6b are

DK 2017 70918 A1 provided with platforms 1e,2e,3e,4e,5e,6e and docking and/or boarding facilities 1f,2f,3f,4f,5f,6f for floating vessels 35,36 as seen in fig. 5.

Fig. 6 schematically illustrates various ships and boats involved in the aqua-culturing by means of the offshore fish farm F,N, and in movement and maintenance at sea of said offshore fish farm F,N.

The large ship 36 to the left in fig. 6 is a Live Fish Carrier (LFC) that transports fish to and from the offshore fish farm. The term Live Fish Carrier” is known to the person skilled in the art of aqua-culturing. The Live Fish Carrier allows for fast loading, circulation and unloading of seawater, and for loading, storing and unloading live fish, such as salmon, in a gentle manner through hoses and pipes.

A Man Overboard Boat/Fast Rescue Craft (MOB/FRC) 35 is used to transport personnel between the towers and the Live Fish Carrier in a safe manner.

The offshore fish farm F,N of the present invention is able to respond to external forces, such as wind, waves and current at the breeding position at the sea, and instantaneously alter and modify its shape in response to external forces. Thus the offshore fish farm of the present invention has a dynamic shape partly dictated by the external forces of the surrounding environment, partly by the dimensions, configuration and arrangement of the structure itself.

A towing vessel 37 has towing lines secured to at least some of the towers 1,2,3,4,5,6 to dynamically keep the offshore position of the fish farm F,N. The towing vessel 37 also serves as a feed unit so that proper fish food is ready available to the offshore fish farm at all times. A general service boat 38 may be responsible for maintenance of the offshore fish farm,

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e.g. removing dead fish and monitoring and servicing the health of the fish.

Because the feed unit is always at the location of the offshore fish farm, and because the offshore fish farm is a freely floating offshore fish farm, tugging can be done if the need arises to move the offshore fish farm fast at sea.

The ideal orientation of the towers are vertical, and during floating at the sea the offshore fish farm of the present invention aims for this orientation due to the arrangement of the upper flexible lines and the lower flexible lines, whereby the towers are kept in balance.

Below is summarized how this can be achieved by an embodiment of the present invention.

The bottom end parts of the towers are connected tangentially by means of crossbars to each other in a closed circumferential arrangement that defines a polygonal submersed frame. Connection members in form of collars around the towers at a lower securing point about a quarter of the length of a tower measured from the bottom ends of the respective towers serve to connect the crossbars to obtain the polygonal frame. The lower flexible lines are secured at lower securing point to the bottom ends of the respective towers and extend radially into a common lower center substantially vertically below the upper center of the polygonal frame.

The top end parts of the towers are interconnected with upper flexible lines that go in tangential direction from a tower to the neighboring towers.

The lower flexible lines are secured to the bottom ends of the respective towers whereby a force is created that has a horizontal component directed towards the center axis of the

DK 2017 70918 A1 fish farm. This force and the arm defined by the distance from the point of the securing of a lower flexible line to a bottom end of a tower establish a lower moment of force.

Similarly, because the upper flexible lines are secured to the top end parts of the respective tower a force is created that also has a horizontal component directed towards the center axis of the fish farm. This force and the arm defined by the distance from the point of the securing to the tower of an upper flexible line to the point of the securing of the lower flexible line establishes an upper moment of force that counteracts the lower moment of force.

The weight and tightening of the flexible lines determines the respective upper moment of force and lower moment of force to keep the offshore fish farm in balance towards having vertical towers .

The weight and tightening of the flexible lines, thus determines the flexibility and elasticity of the assembled offshore fish farm, its ability to return to equilibrium when it is forced out of equilibrium due to external forces. In the perfect equilibrium position, the upper moment of force should correspond to the lower moment of force.

The offshore fish farm of the present invention will try to bring the towers back to equilibrium when influenced by external forces.

The offshore fish farm of the present invention provides for aqua-culturing with high fish welfare while preserving the surrounding environment.

Claims

Claims

An offshore fish farm (F,N) comprising a flexible netting (N) suspended to a floating frame structure (F), the suspended flexible netting (N) has an upper netting part above sea level and a lower submersed netting part with a closed netting bottom (32), the floating frame structure (F) comprises at least three towers (1,2,3,4,5,6) having a top end part (1b,2b,3b,4b,5b,6b) (1d,2d,3d,4d,5d,6d) above with a free top end and an opposite (1c,2c,3c,4c,5c,6c) with a sea level submersed bottom end part bottom end (1a,2a,3a,4a,5a,6a) floating above the sea bed, and the the submersed bottom end parts (1c,2c,3c,4c,5c,6c) of at least three towers (1,2,3,4,5,6) are circumferentially connected by means of crossbars (13,14,15,16,17,18) into a polygonal bottom frame (P) , characterized in that lines extend upper flexible tangentially to to circumferentially connect the towers (1,2,3,4,5,6) at a distance from the respective the top end parts (1b,2b,3b,4b,5b,6b) free top ends (1d,2d,3d,4d,5d,6d), the bottom ends of the at least three towers (25) a common by (19,20,21,22,23,24), and are radially connected to flexible centre means of lower lines the lower flexible lines (19,20,21,22,23,24) are
2.

heavier than the

An offshore fish farm characterized in that upper (F,N) the flexible lines according to claim 1, lower flexible lines

DK 2017 70918 A1 (19,20,21,22,23,24) and/or the upper flexible lines (26,27,28,29,30,31) are chains.
3. An offshore fish farm (F,N) according to any of claims 1 or 2, characterized in that a tower (1,2,3,4,5,6) is connected to a crossbar (13,14,15,16,17,18) by means of a connection member (7,8,9,10,11,12), such as a pipe piece or a bushing, optionally the pipe piece or bushing is slidable arranged on a tower (1,2,3,4,5,6).
4. An offshore fish farm (F,N) according to any of the preceding claims 1, 2 or 3, characterized in that the weight ration of the lower flexible lines (19,20,21,22,23,24) in relation to the upper flexible lines (26,27,28,29,30,31) is selected to adjust the buoyancy of the offshore fish farm (F,N), preferably the offshore fish farm (F,N) floats free of the sea bottom, more preferred about 20 % of the tower (1,2,3,4,5,6) length is above sea level.
5. An offshore fish farm (F,N) according to any of the preceding claims 1 - 4, characterized in that the number of towers (1,2,3,4,5,6) is six or higher.
6. An offshore fish farm (F,N) according to any of the preceding claims 1 - 5, characterized in that the towers (1,2,3,4,5,6) are hollow pipes, optionally the towers (1,2,3,4,5,6) are adapted to accommodate a ballast such as seawater.
7. An offshore fish farm (F,N) according to any of the preceding claims 1 - 6, characterized in that a tower (1,2,3,4,5,6) is flexibly and resiliently connected to a crossbar (13,14,15,16,17,18) by means of a resilient member (7',7'';8',8'';9',9'';10',10'';11',11'';12',12'') interposed to connect a crossbar (13,14,15,16,17,18) to a

DK 2017 70918 A1 tower (1,2,3,4,5,6), optionally the resilient member connects to the connection member (7,8,9,10,11,12).
8. An offshore fish farm (F,N) according to any of the preceding claims 1 - 7, characterized in that at least some of the towers (1,2,3,4,5,6) have a top end part (1b,2b,3b,4b,5b,6b) equipped with docking and/or mooring facilities (1f,2f,3f,4f,5f,6) for at least one floating vessel (35,36,37,38).
9. An offshore fish farm (F,N) according to any of the preceding claims 1 - 8, characterized in that an upper rim of the flexible netting (N) is secured to at least some of the upper flexible lines (26,27,28,29,30,31), and/or a lower circumference of the flexible netting (N) is kept distended by being secured to any of the connection member (7,8,9,10,11,12), e.g. the pipe pieces or the bushings, the polygonal frame, the towers (1,2,3,4,5,6) and/or the crossbars (13,14,15,16,17,18).
10. An offshore fish farm (F,N) according to any of the preceding claims 1 - 9, characterized in that the lower flexible lines (19,20,21,22,23,24) are at least double as heavy as the upper flexible lines (26,27,28,29,30,31), preferably the lower flexible lines (19,20,21,22,23,24) are at least three times as heavy as the upper flexible lines (26,27,28,29,30,31), (19,20,21,22,23,24) are at the upper flexible lines lower flexible lines (19, five times as heavy as (26,27,28,29,30,31), or (19,20,21,22,23,24) are at the upper flexible lines lower flexible lines (19, or the lower flexible lines least four times as heavy as (26,27,28,29,30,31), or the 0,21,22,23,24) are at least the upper flexible lines the lower flexible lines least six times as heavy as (26,27,28,29,30,31), or the 0,21,22,23,24) are at least

DK 2017 70918 A1 seven times as heavy as the upper flexible lines (26,27,28,29,30,31), or the lower flexible lines (19,20,21,22,23,24) are at least eight times as heavy as the upper flexible lines (26,27,28,29,30,31), or the

5 lower flexible lines (19,20,21,22,23,24) are at least nine times as heavy as the upper flexible lines (26,27,28,29,30,31), or the lower flexible lines (19,20,21,22,23,24) are at least ten times as heavy as the upper flexible lines (26,27,28,29,30,31).