NO20191155A1

NO20191155A1 - Submersible cage

Info

Publication number: NO20191155A1
Application number: NO20191155A
Authority: NO
Inventors: Morten Aga
Original assignee: Searas As
Priority date: 2017-03-01
Filing date: 2019-09-26
Publication date: 2019-09-26
Also published as: NO20170298A1; WO2018169407A9; WO2018169407A1; NO344106B1; NO346650B1

Description

Submersible cage

Area of the invention

The present invention relates to a device for a submersible net cage and a method to regulate the buoyancy of said submersible net cage.

Background of the invention

Net cages for farming of marine organisms, such as fish and shellfish, are located in freshwater or seawater. Today, net cages are typically found in fjords or bays where the external conditions such as wind and waves are relatively calm. However, there is also a need to install fish farming net cages offshore, in order to adopt new areas for fish farming, and also to make the facilities larger. Furthermore, it can be desirable to move some installations offshore to prevent infection by pathogenic organisms.

The challenges of moving net cages offshore include weather conditions worsening. For example, the Norwegian coast can, in many areas, experience waves with a height of up to 30 meters. For this reason, net cages that can be submerged in the sea, so-called submersible net cages, have been designed. Below the surface, they will be subjected to lesser external forces from waves and currents.

The applicant's own patent application WO2016/063040 describes such a submersible net cage for farming of marine organisms. The net cage has walls and ceilings and is arranged floating in the sea. The net cage is equipped with a number of floats to keep the net cage afloat, and to be able to regulate the buoyancy so that the net cage can be lowered into the sea and raised again. These floats are extended horizontally and are preferably arranged in the upper section of the net cage, i.e. level with, or just below the water surface when the net cage is in a raised configuration. Furthermore, the net cage is fitted with a number of flexible elements (such as chains) which extend from the net cage and to another external support point, and the flexible elements preferably hang in a loop from the net cage to the external support point.

WO0152638 describes a floating net cage with sinking elements attached to the bottom and floating elements fastened to the top of the net cage, where the floating elements are adjustable.

WO9806254 describes a submersible net cage with a float collar at the top and a sinking element at the bottom where buoyancy is regulated by adjusting the amount of air in the float collar.

W008081229 describes a net cage where the floating ability is regulated in that floats are filled or emptied with water or air.

Aim of the present invention.

An aim of the present invention is to provide a submersible net cage, i.e. a net cage which can be lowered from the surface and down into the water, with improved options to regulate the net cage buoyancy.

It is a further aim of the present invention to improve the regulation of the horizontal position of the net cage, in particular when the net cage is in a submerged configuration.

It is also an aim of the present invention to provide a method for the precise regulation of buoyancy of a submersible net cage. One also seeks to establish a solution where the buoyancy can individually be regulated in many of the various arranged buoyancy points of net cage.

Summary of the Invention

The present invention relates in one aspect to a submersible net cage for farming of marine organisms, comprising of

- a net cage bag with walls for receiving said marine organisms

- buoyancy elements set up to establish varying buoyancy,

- sinking elements set up to establish downward forces on the net cage, characterised in that at least one of said buoyancy elements is a first buoyancy element, where said buoyancy element has an elongated shape, is attached to the net cage and is set up to be positioned vertically in the water in which the net cage floats or is submersed in.

In a preferred embodiment, said first buoyancy element is open at the lower section and closed at the upper section.

In a preferred embodiment, said sinking element comprises one or more flexible elements fastened to the net cage.

In a preferred embodiment, said flexible element is a chain.

In a preferred embodiment, an upper section of said first buoyancy element is comprised of a buoyancy means with a fixed buoyancy, where said buoyancy means with a fixed buoyancy is either a solid buoyancy element, such as polystyrene, an air-filled and watertight pipe section which is fastened to or part of the upper section of the buoyancy element, or that a buoyancy means, such as air, is fed into an upper section of the buoyancy element, in a position at a predetermined distance from the top of the buoyancy element.

In a preferred embodiment, said buoyancy element is set up to receive;

- a first medium A of equal or higher density than the surrounding water, and

- a second medium B of lower density than the surrounding water.

In a preferred embodiment, said float is comprised of means for the feeding and discharging of said first medium.

In a preferred embodiment, the buoyancy of each of said vertically positioned buoyancy elements is regulated by the quantity of the first medium A in said buoyancy element and that the buoyancy in each element can be regulated independently of each other.

In a preferred embodiment, said buoyancy element is comprised of a weight in a lower section of the buoyancy element.

In a preferred embodiment, said buoyancy element has an opening at a given depth to limit the buoyancy.

In a preferred embodiment, said flexible element is fastened to the net cage and to an external support point.

In a preferred embodiment, the net cage is also comprised of a second, horizontally arranged buoyancy element.

In a preferred embodiment, said second horizontally positioned buoyancy element is a ring-shaped float (float collar).

In a preferred embodiment, said second buoyancy element is comprised of the means for supply and discharge of a medium of lower density than the surrounding water.

In a preferred embodiment, said medium, which has a lower density than the surrounding water, is a gas, preferably air.

In a preferred embodiment, said second medium is comprised of water, i.e. the same medium as the surrounding water.

In a preferred embodiment, said first medium is air.

In a preferred embodiment, the net cage is also comprised of one or more horizontal buoyancy elements.

In a preferred embodiment, each of the buoyancy elements is positioned along the entire net cage circumference and is either fastened to the vertical buoyancy elements or to the wall of the net cage.

In a preferred embodiment, said buoyancy elements are arranged in an upper and/or lower section of the net cage or buoyancy elements.

In a second aspect, the present invention relates to a method of regulating the raising and lowering of a submersible net cage, characterised in that said submersible net cage is comprised of a net cage bag with walls for receiving said marine organisms, vertical buoyancy elements set up to establish variable buoyancy and sink elements set up to establish downward forces on the net cage, characterised in that said buoyancy elements are comprised of means set up to establish a fixed buoyancy and means to establish a variable buoyancy and the buoyancy of the buoyancy elements is regulated by adding or removing a means of lower density than the surrounding water to/from the buoyancy elements.

In a preferred embodiment said horizontal buoyancy means are filled with air when the net cage is brought to the surface with the vertical buoyancy elements, and when the net cage shall be lowered the horizontal buoyancy elements are emptied and the net cage depends on the buoyancy of the vertical buoyancy elements, and when the air is let out of the vertical buoyancy elements the net cage will sink downward.

In a preferred embodiment, the net cage is equipped with a number of buoyancy means, and the buoyancy in each of these buoyancy means is individually regulated.

Description of figures

Preferred embodiments of the invention shall be described in more detail in the following with reference to the accompanying figures, where:

Figure 1 shows schematically a fish farming net cage with first and second buoyancy elements, where the net cage is in a floating configuration.

Figure 2 shows schematically a fish farming net cage with first and second buoyancy elements, where the net cage is in a submerged configuration.

Figure 3 shows schematically a vertical, elongated, buoyancy element with a closed top and an open bottom, a rigid buoyancy element and an opening to limit buoyancy.

Figure 4 shows schematically how a medium (air) of less density than the surrounding water can be added to a first float for regulating the buoyancy of the float and the net cage it is attached to.

Figure 5 shows schematically how a first float (buoyancy element) is attached via a flexible element to an external support point, here a floating unit, where the net cage that the float is attached to is in a submerged configuration.

Figure 6 shows schematically how a first float (buoyancy element) is attached via a flexible element to an external support point, here a floating unit, where the net cage that the float is attached to is in a floating configuration.

Figure 7 shows a net cage according to the invention where horizontal buoyancy elements are also arranged.

Description of preferred embodiments of the invention.

In figure 1 a submersible net cage 10 for the farming of marine organisms is shown schematically. A volume of water, such as fresh water or seawater, is enclosed by the net cage 10, and the walls 12 form both the bottom wall, wall sections and lid/roof of the net cage. The invention relates to an arrangement for the lowering and raising of the net cage 10 in a water reservoir and buoyancy elements16, 30 and sinking elements 18 which bring about such raising and lowering. The net cage 10 is a so-called submersible net cage, i.e. in one configuration it can be floating at the water line, and it can then be lowered from this position to a sunken position at a given, often predetermined depth.

The fish farming net cage 10 may be of any type, i.e. waterproof net cages 10 can be used, but also water permeable net cages 10, i.e., the wall section 12 of net cage 10 can be waterproof or water permeable. Whether the net cage 10 is waterproof or water permeable will depend on the conditions under which it shall be used.

From a technical standpoint, submersible net cages which have a float, or a set of floats attached to the net cage top section, are known. Often, such floats are in the form of a float collar, and this is normally filled with air to ensure buoyancy. The float collar or the float on such known installations is arranged horizontally, i.e. that the air which is filled into the float has a large horizontal extent but a limited vertical extent. The typical cross-sectional diameter of a float collar is 500 mm and the length is usually 160 m (circumference upper section of the net cage). The volume for uptake of air is then 27m<3,>and the vertical extent only 0.6 m. Normally for three typical float collars this results in over 50m<3>buoyancy at the vertical extent of only 0.6 m.

Furthermore, known solutions use sinking elements attached to the net cage to exert a downwardly directed force, i.e. to apply forces acting opposite to the buoyancy effect of the floats. For example, weights can be attached to the net cage. If the net cage shall be in a floating or sunken configuration, then such known solutions are regulated by the amount of air in the horizontal floats. When the floats are filled with air, the net cage will have sufficient buoyancy to float in the water surface. It will sink when the weight of the sinking elements exceeds the buoyancy, i.e. when air is discharged from the buoyancy elements.

From WO2016 / 06304 a system is also known in which flexible sinking elements are used. These sinking elements can, for example, be chains, and they are attached at one end of the net cage, and the other end to a fixed support point (e.g., a buoy at the water surface). The net weight of the chain allows the chain to act as a sink element with a downwardly directed force. It is preferred that the chain forms an arc between the net cage and the support point.

Even with such a net cage installation with horizontal floats and flexible sinking elements it has proven challenging to accurately adjust the net cage buoyancy. The present invention provides a solution to this problem by establishing vertical buoyancy elements 16 where the amount of buoyancy means in the buoyancy element 16 can be adjusted accurately.

A net cage 10 is preferably provided with a number of such buoyancy elements 16. Preferably, this number of buoyancy elements 16 are arranged in the outer periphery of the net cage, i.e. outside the net cage bag 12 or the net cage wall 12. In the description and the claims, these buoyancy elements are referred to as the first buoyancy elements 16. They are also referred to as vertical buoyancy elements 16. The term "vertical" in connection with these first buoyancy elements 16 should not be understood narrowly. It is intended that they have a mainly vertical positioning but can also, in some cases, have a certain inclined position.

The vertical buoyancy elements 16 have an elongated form, preferably in a pipe-like form. They are hollow inside and waterproof in the upper section 16b, while they are open in the bottom section 16a. They are equipped with valves for intake of, or discharge of, one or more media, preferably two or more media. In the solution which is shown in Figure 1 , it is indicated that a buoyancy element 16 is filled with a liquid having the same or higher density as the surrounding water reservoir. This medium is in the figure referred to as "A". Moreover, the buoyancy element 16 is filled with a medium, referred to as "B", which has less density than the surrounding water reservoir. This medium will provide buoyancy. Medium A frequently uses the same liquid as the water reservoir, and medium B is often a gas, most preferably air. Thus, two separated phases will form in the buoyancy element 16 and the amount of air in the buoyancy element will determine the buoyancy.

Air is supplied to the buoyancy element 16 from a pressurised container (not shown in detail in the figures). As mentioned, a net cage 10 can be provided with a number of buoyancy elements 16. For round net cages or for square net cages, it is appropriate with 4, 8, 12 or 16 buoyancy elements 16, but any number can in principle be used. Each of the buoyancy elements can be regulated individually, i.e. air can be supplied and removed from each buoyancy element independent of each other.

It is also preferred that the buoyancy elements 16, or some of the buoyancy elements 16, have a buoyancy element of a solid material in an upper section to set up a fixed non-adjustable buoyancy effect. This can, for example, be polystyrene or a similar light material, which will ensure a certain buoyancy even if the buoyancy means B should leak out of the buoyancy element 16.

A net cage 10 can be held in a floating position, and also be lowered to a submerged position, only when using such first vertical buoyancy elements 16 (in combination with sinking elements 18), with appropriate sizing of the various elements. However, it is preferred that one also equips the net cage 10 with conventional horizontal floats 30, such as a float collar 30. Such additional buoyancy elements 30 are termed as “horizontal or other buoyancy elements” 30. One will then be able to use these other buoyancy elements 30 for lowering and raising the net cage 10. When the buoyancy element 30 is filled with air then the net cage 10 has a total buoyancy sufficient for the net cage 10 to be raised and positioned in a floating configuration (as shown in Figure 1 ). When one sucks air out of the buoyancy element 30, the net cage 10 will sink, and it will suitably sink to a predetermined depth as determined by the amount of air in the first (vertical) buoyancy elements 16. The vertical buoyancy elements 16 are therefore used to position the net cage 10 at a suitable depth, and also to individually regulate the buoyancy of each buoyancy element 16 so that the net cage 10 is evenly arranged horizontally in a sunken position (as shown in figure 2).

In a representative example of the present invention, as shown schematically in figure 1, a standard plastic ring 30 or float collar 30 is used. Heavy chains 18 between the support point 40 (mooring plate under the buoy), and float collar 30 or other point of net cage 10 are used as a flexible element 18. The chain 18 is used as a flexible element 18 and is a compensating weight for the net cage 10. Such a chain link 18 has a large stabilising weight and causes the passive stabilisation of the net cage 10, vertically. In this example, 12 vertical pipes 16 with an inner diameter of 27cm are used. The outer diameter can be 30 cm in a standard PE pipe.

On top of the vertical pipe 16, there will preferably be a buoyancy element of a certain fixed buoyancy. This will be in addition to, and independent of, the buoyancy which is established by the buoyancy medium B which gives a varying buoyancy. This solid buoyancy element ensures a certain buoyancy if the supply pipe for air to the buoyancy element 16 should break, or if the buoyancy element 16 is damaged and the air flows out of the element. The fixed buoyancy which is provided by the buoyancy medium B in pipe 16 will ensure that the system between the chain 18 and the buoyancy pipe 16 does not obtain a negative buoyancy. The fixed buoyancy can be ensured with a solid-shaped buoyancy element made of a lightweight material with a density lower than the surrounding water, such as, for example, polystyrene. Alternatively, one may ensure that the supply of air takes place at a distance from the top, so that even if this supply should break, and air was to escape from the element 16, then there will be a buoyancy generating air pocket in the upper portion of the buoyancy element 16.

The technology is based on a compensation system which works with the help of buoyancy which is established in that compressed air is pumped into vertical pipes 16 and chain 18 with a specific weight (weight/metre) which corresponds to the buoyancy. The net cage 10 can be raised or sunk to great accuracy in the water column because of air being pumped into, or discharged from, the vertical pipes 16.

Vertically positioned pipes 16 are used because these give a unique opportunity to control the amount of air (and therefore buoyancy) in each of the pipes 16.

The net cage 10 will, by the correct dimensioning of the buoyancy elements and the sinking elements, be in a stable equilibrium. When the net cage 10 is adjusted to lie at, for example 10 m water depth, it will adjust back to this depth even if the net cage is raised to the surface and immersed again (by filling and emptying of other buoyancy elements 30). The same is true if the net cage 10 was lowered to 20 m water depth, it will rise up to 10 m again and stabilise there. The airflow in the buoyancy elements 16 will be able to adjust the net cage 10 to a predetermined depth.

The challenges with a raising technology and a lowering technology for submersible fish farming net cages 10 are many. Here, we will mention the essential challenges and we will also specify that these challenges are solved with the technology that the present invention provides;

When lowering a net cage 10 into a water reservoir, the volume of air will decrease as a result of the equation of state V1 x P1 = V2x P2. This means that if one lowers a buoyancy element which is open at the bottom and which is filled with some air from the surface, down to 10m, the buoyancy is halved.

V2= (V1x P1)/ P2

V2=1/2

The present invention provides a fixed (predetermined) buoyancy at the top of the pipe 16 and a variable buoyancy down in the pipe 16, and this makes this problem easier to handle.

In the example above where the net cage 10 is on the surface, the pressure in the buoyancy pipe 16 will be equal to the water level in the pipe which can lie at 40 m.

When the cage is then lowered to 10 metres the volume of air in the vertical pipe will be reduced by:

V2= (V1x P1)/ P2

V2= (1x 4) / 5

V2= 4/5

That is, the pressure will change considerably less with the vertical buoyancy elements. This means that when one lowers the net cage one will let out air to get the net cage to sink at the same time as the pressure in the buoyancy elements increases.

With this method the stability of the net cage 10 will substantially increase while it is submersed.

The chain principle (flexible sinking element 18) will prevent the transmission of wave forces from the mooring buoy 40 to the net cage 10. This will be a substantial benefit in offshore farming where large waves occur.

Adjustment and supervision of buoyancy in deep vertical pipes 16 will be significantly easier than controlling the buoyancy by pumping air into the floating collar 30 itself. In a vertical pipe 16 one will be able to pump in, for example, air which gives 57 kg buoyancy and read a pressure increase of 0.1 bar. If as much air was to be pumped into a horizontal buoyancy element 30, it would be very difficult to read any pressure difference, and the regulation of buoyancy would therefore be difficult.

Figure 4 shows the principle of how to control the amount of air in the vertical pipes 16 so that one has full control of the net cage 10 lying horizontally. Pressure sensors read the pressure in the air hose 60a that goes along with the feeding pipe from the float and down to the vertical buoyancy pipe 16.

Typically, the air hose 60a will be 1⁄4 hose which is certified for seawater and air.

At the top of the buoyancy pipe 16 there is, as mentioned, a fixed buoyancy element (not shown in the figures) which shall ensure that if all the air leaves the pipes 16, for example, from the air hoses rupturing, the net cage will sink to a depth which is determined by how much buoyancy the fixed buoyancy elements have.

To lower the net cage, all air is released from the buoyancy element 30, or alternatively from the buoyancy elements 16 if the buoyancy element 30 is not used.

The principle for the raising mechanism and lowering mechanism is as follows for the representative example; the chain 18 between the mooring plate 30 and the net cage 10 is 70m long. In water, the chain 18 has a density of 20kg/m. The weight of the chain 18 that shall be held up by the flotation pipe 16 is 20 kg / m x (70/2) = 700 kg. This is 700 kg / 57kg / m = 12.3 m of air in the vertical pipe 16.

The buoyancy can be calculated accurately with the appropriate calculation of weight and buoyancy elements 16, 30.

A buoyancy pipe has an ID of 27 cm. This provides a buoyancy of about 57 kg/m. The pipe will then be filled with air at the top down to: 700 kg / 57 kg / m = 12.3 m. The system will now be in equilibrium. When waves act on the buoy, the mooring plate will move up and down in line with the waves. The large mass in the net cage will ensure the movements are minimal.

In the surface position (as shown schematically in figure 4), the entire chain 18 will be lifted by the air in the buoyancy pipe. 70 m x 20 kg / m gives ca. 1400 kg.

This results in 1400kg / 57kg / m = 24 m of air in the buoyancy pipe.

There can be a separate airline 60a from a control point near the net cage 10. This can be a feed float, or another station.

According to the invention, many conventional components can be used for assembling a net cage 10. The mooring frame (not shown) is installed in a normal way. It is preferred for the anchoring ring 40b under buoyancy buoy 40 to be somewhat lower in the water. It is preferable that it is placed at the desired submerged depth of the net cage 10. The net cage 10 will then maximize the ability to compensate for large waves.

The installation of net cage 10 then starts with a standard plastic ring 30 as buoyancy element 30. The spreading out of the mooring frame 40b will be somewhat larger compared to a traditional anchoring of a plastic ring 30. This is because chain 18 needs extra length between the mooring ring 40b below the buoy 40 and the net cage 12. The extra length is necessary because it shall compensate for high waves and in addition it shall be possible to raise the net cage 10 up to the surface.

After the plastic ring 30 is moored, it will be appropriate to install the net cage 12. However, it is still possible to install the net cage after the vertical pipes 16 are installed in the same manner by a net cage change.

After the plastic ring 30 is moored and net 12 is installed, the vertical buoyancy pipes 16 are installed. In the example given above, the vertical pipes 16 are comprised of PE pipes of length 40m. The upper 6m of the pipe have an OD of 400 mm. Here fixed buoyancy is installed (not shown) inside the pipe 16. At the bottom 16a of the pipe there is a length of 3m with an OD of 400 mm. A weight (not shown) is placed in this section. Between the two sections there is 31 m of PE pipes with an OD of 300mm for the uptake of media referred to as A and B. The pipes 16 are drawn and positioned into the correct side of the chain 18 and lowered.

The appropriate buoyancy of the pipes 16 is adjusted with compressed air which is pumped in during the installation. This makes it easy to connect the pipes 16 to the net cage 10. Whether the vertical pipes 16 shall be connected to the chain 18 or directly to the net cage 10 will be considered, both are possible. The buoyancy of the whole pipe 16 in a surfaced position and in a submerged position will be appropriately dimensioned, and it seems sensible to start with the pipe 16 having a buoyancy in submerged position which corresponds to a weight of half the length of the chain 18.

Once the vertical pipes 16 are in place, the bottom ring 70 shall be installed. The bottom ring 70 can be in 12 parts which are fastened on the outside of the vertical pipes 16. The purpose of the bottom ring 70 is that it will hold the shape of the vertical pipes 16.

To stabilise the net cage 10 further, particularly not to get a varying buoyancy when lowering or raising the net cage 10, or when the cage moves up or down in the water for other reasons (waves or currents), the net cage 10 also has a number of horizontal buoyancy elements 50 arranged, as shown in figure 7. There can be one or many such elements 50, and they preferably have a pipe shape which extends along the whole circumference of the net cage. Preferably, the entire buoyancy element 50 is either filled with water or filled with air, so that the contribution of the element 50 to the buoyancy of the net cage 10 is constant.

Claims

1. Submersible net cage (10) for the farming of marine organisms, comprising - a net cage bag with walls (12) for receiving said marine organisms

- buoyancy elements set up to establish varying buoyancy,

- sinking elements adapted to establish downward forces on the net cage (10), characterised in that at least one of said buoyancy elements is a first buoyancy element (16), where said buoyancy element (16) has an elongated shape, is attached to the net cage (10) and set up to be positioned vertically in the water which the net cage (10) is floating or submerged in.

2. Submersible net cage (10) according to claim 1 , characterised in that said first buoyancy element (16) is open at a lower section (16a) and closed at an upper section (16b).

3. Submersible net cage (10) according to claim 2, characterised in that said sinking element is comprised of one or more flexible elements (18) fastened to the net cage (10).

4. Submersible net cage (10) according to claim 3, characterised in that said flexible element (18) is a chain.

5. Submersible net cage (10) according to any of the preceding claims, characterised in that said first buoyancy element (16) is comprised of a buoyancy means (16d) in an upper section with fixed buoyancy, where said buoyancy means (16d) with fixed buoyancy is either a solid buoyancy element, such as polystyrene, an air-filled and watertight pipe section which is attached to, or part of, the upper section of the buoyancy element (16), or that a buoyancy means, such as air, is fed into an upper section of the buoyancy element (16), at a position at a predetermined distance from the top of the buoyancy element (16).

6. Submersible net cage (10) according to any of the preceding claims, characterised in that said buoyancy element (16) is set up to receive;

- a first medium A with equal or higher density than the surrounding water, and

- a second medium B with lower density than the surrounding water.

7. Submersible net cage (10) according to any of the preceding claims, characterised in that said float (16) is comprised of means (16a) for the supply and discharge of said first medium.

8. Submersible net cage (10) according to any of the preceding claims, characterised in that the buoyancy of each of said vertically positioned buoyancy elements (16) is regulated by the amount of the first medium A in said buoyancy element (16) and that the buoyancy in each element (16) can be regulated independently of each other.

9. Submersible net cage (10) according to any of the preceding claims, characterised in that said buoyancy element (16) has a weight (16e) in a lower section of the buoyancy element (16).

10. Submersible net cage (10) according to any of the preceding claims, characterised in that said buoyancy element (16) has an opening (16c) at a given depth to limit buoyancy.

11. Submersible net cage (10) according to claim 3 or 4, characterised in that said flexible element (18) is fastened to the net cage (10) and to an external point of support (40).

12. Submersible net cage (10) according to any of the preceding claims, characterised in that the net cage (10) is also comprised of a second, horizontally arranged buoyancy element (30).

13. Submersible net cage (10) according to claim 12, characterised in that said second horizontally positioned buoyancy element (30) is a ring-shaped float (float collar) (30).

14. Submersible net cage (10) according to claim 12 or 13, characterised in that said second buoyancy element (30) is comprised of a means (30a) for the supply and discharge of a medium of lower density than the surrounding water.

15. Submersible net cage (10) according to any of the preceding claims, characterised in that said medium of lower density than the surrounding water is a gas, preferably air.

16. Submersible net cage (10) according to any of the preceding claims, characterised in that said second medium is water, i.e. the same medium as the surrounding water.

17. Submersible net cage (10) according to any of the preceding claims, characterised in that said first medium is air.

18. Submersible net cage (10) according to any of the preceding claims, characterised in that the net cage (10) also includes one or more horizontal buoyancy elements (50).

19. Submersible net cage (10) according to claim 18, characterised in that each of the horizontal buoyancy elements (50) is arranged along the whole of the net cage circumference, and is either attached to the vertical buoyancy elements (16) or to the net cage wall (12).

20. Submersible net cage (10) in accordance with claim 18, characterised in that said buoyancy elements (50) are arranged in an upper and/or lower section of the net cage (10) or buoyancy elements (16).

21. Method for regulating the raising and lowering of a submersible net cage (10), characterised in that said submersible net cage (10) is comprised of a net cage bag (10) with walls (12) for receiving said marine organisms, vertical buoyancy elements (16) set up to establish varying buoyancy and sinking elements set up to establish downward forces on the net cage, characterised in that said buoyancy elements are comprised of means set up to establish a fixed buoyancy and means to establish a varying buoyancy and the buoyancy of the buoyancy elements is controlled by supplying or removing a means of lower density than the surrounding water to/from the buoyancy elements.

22. Method according to claim 21 , characterised in that said horizontal buoyancy means (30) are filled with air when the net cage (10) is brought to the surface with the vertical buoyancy elements (16) and when the net cage (10) shall be lowered, the horizontal buoyancy elements (30 ) are emptied and the net cage (10) then depends on the buoyancy of the vertical buoyancy elements (16), and when the air is released from the vertical buoyancy elements (16) the net cage (10) will sink.

23. Method according to claim 21 , characterised in that the net cage is equipped with a number of buoyancy means (16) and that the buoyancy in each of these buoyancy means (16) is controlled individually.