The present invention relates to fish farms and to methods for farming fish.
Aquaculture has progressed with considerable success from ponds and rivers to estuaries and inshore cages. However, the largest cages in the open sea are expensive, vulnerable to storm damage and require a high density of fish in order to be economic. The resulting problems of disease and concentrated detritus is being combated with chemicals and antibiotics etc. Although such aquaculture is developing relatively fast, fish reared in captivity cannot yet adequately replace the quality, diversity and survivability of wild fish.
Ideally, the stocks of each species of fish in the wild should be maintained at a naturally sustainable level, and the fishing fleets of the world should be able to catch enough fish to satisfy demand. However, many species are now endangered and quotas have had to be imposed. Some stocks are so low that fishermen are struggling ' to catch even their quota. There is no solution as yet proposed that will quickly and surely restore the stocks of wild fish to a naturally sustainable level and secure the future livelihood of the fishing community.
We have now devised a method and equipment for enabling the rearing of fish to a size and maturity and in sufficient numbers so they can be released unfettered into the sea.
According to the invention there is provided a structure for rearing fish which comprises an enclosure attached to or otherwise associated with an oil or gas platform in the open sea, which platform is anchored or tethered or otherwise fixed or connected to the sea floor. The platform can be any kind of platform including piled, jack-up, gravity and world war two naval platforms and is preferably a platform designed for drilling, production, storage or transmission of hydrocarbons. By
'associated with' is meant that the enclosure need not be permanently attached to the platform, but is in the vicinity of the platform so that maintenance and supervision of the enclosure can take place from the platform and supplies to and control of the enclosures can take place from the platform.
The method of the invention enables the use of very large enclosures for rearing fish in the open sea, in relatively deep waters that are fully exposed to the full effects of storms, supported by offshore platforms.
Because of the size of the enclosures that can be used, the system can be used to replenish the stocks of wild fish, as well as to supplement fish supply, provide research data and prepare for marine farming. The method and equipment of the invention is referred to, hereinafter, as a "Platform Fish Ranch" or "PFR". A typical schedule of occupancy of the PFR, for Cod, Haddock and Halibut is illustrated in Table (A). The brood stock will be taken from the fishery area into that in which the juveniles are to be released and a wide sampling of brood stock should ensure that the genetic range remains diverse.
Species Hatch 1st Enclosure ά Enclosure release harvest maturity S0X50x20m 100x100x25m 50,000m3 250,000m3
Weaning Post weening % "mesh PA "mesh
Cod & @ 10°C @ 5 to 20 gms @ 20 to 100 gms @ 100 gms Haddock 8-
-2 months -2 months -2 months *4 months
-2 years ft
@ -4 Kgs 5- i-3 O
-354 years CD
5 °C @ 10 to 40 gms ! 40 to 200 gms @ 200 gms
•~ months ~3 months ~3 months 6 months o
@ -5 Kgs o 1/1
-3 years n
@ -10 Kgs
Low maximum enclosure fish densities of: 0.5 to 2 Kgs/m3 0.4 to 2 Kgs/m3 Tonnage per batch equates to: D" p n
Cod & Haddock: 25 to 100 tons 100 to 500 tons 500 tons 10,000 tons 20,000 tons
Halibut: 25 to 100 tons 100 to 500 tons 500 tons 12,500 tons 25,000 tons
Hence, to increase the biornass of Cod, on the North West Europe Continental Shelf, by 100,000 tons of mature 4Kg Cod in a particular year, requires that a Platform Fish Ranch (PFR) has released some 5,000 tons of Juvenile 100gm Cod, 3 years before (allowing for a 50% mortality). Such a PFR would release 10 batches of juveniles during the year, from 5 large enclosures, fed by 5 smaii enclosures.
To rear enough juveniles to have a significant effect on re-stocking large areas, such as the N European Continental Shelf, the size of each enclosure has to be of the order of 1000 times larger than the largest conventional cages. Secondly, to eliminate the need for medication and 'unnatural treatments' and oxygenation of the water, it will be necessary to keep the density of fish low. Thirdly, it will be essential that the equipment used is not vulnerable to storm damage and, for commercial success, the cost per ton of fish released must be far less than is currently being experienced in aquaculture with cages. Fourthly, it will be convenient to be able to tow the net enclosure slowly for hundreds or even thousands of miles in order to release the fish into a specific natural fishery location, by using two or more tugs, keeping the net enclosure in tension.
An essential element of the Platform Fish Ranch is the platform. The fish and the enclosure moored in the open sea require continuous monitoring, easy access, supplying with food regularly and protecting from storm damage. Such enclosures will contain a considerable asset value of fish and will need local policing and protecting. The provision of a vessel moored permanently on location, in all weathers would be very expensive if not impractical. The existence, therefore, of many platforms in the North Sea and on other continental shelves of the world, provides a valuable opportunity and an economic solution.
The new mesh net enclosure is held in shape by tension, unlike conventional fish cages, which rely on rigid compression members or struts to keep their shape. The size of the new mesh net enclosure can be less than 2,500 cubic metres but is preferably very much larger, with useful sizes ranging from 25,000 cubic metres to 1 million cubic metres, or more.
The shape of the mesh net enclosure is not critical and, although the open top, roof, wall, floor or partition is preferably rectangular, the shape can be regular, irregular, oval, or polygonal, etc.
If the enclosure is approximately square it can have a plan area of preferably at least 25 meters by 25 meters and it can have an average depth from the top of the enclosure to the bottom when in position at sea of preferably at least 10 metres, e.g. 100 metres by 100 metres by 20 metres, which is about 100 times larger than the largest Russian submersible cage, (SADCO), or 200 metres by 200 metres by 25 metres amounting to 1 million cubic metres, or larger.
The sheer size required necessitates a structure in tension, utilising weight, buoyancy, mooring and fluid dynamic forces to maintain its shape.
Preferably the enclosure consists of nets with preferably a finer mesh net mounted on a coarser net mesh or lattice. This enables the containment of the young fry or fingerlings, and the coarser mesh net or lattice can transmit and spread the load of the moorings. A net enclosure of finer mesh to contain young juveniles may be located within, or adjacent to, a net enclosure of coarser mesh so that they may be easily reared and transferred from one enclosure to another.
The enclosure preferably has a roof structure to prevent the fish from escaping over the top of the enclosure when the enclosure becomes submerged.
In order to avoid damage by storms, which can occur in open sea with very high waves, preferably there are submerging means which enable the enclosure to be lowered beneath the sea surface and raised up again. The roof structure will prevent the escape of the fish fry or fingerlings etc. and prevent the ingress of predators.
The height or depth at which the roof of the net enclosure is located can be altered remotely in order to allow the fish to access the sea surface in reasonable weather but, in rough seas, avoid damage to the enclosure by sinking the roof adequately below the surface. The designs of the equipment enable the net enclosure to be submerged to
escape storm damage and they can have a fixed mooring system, which remains permanently on the surface or can submerge entirely and have a roof design which avoids impact with the sea surface when in the surface mode. The mooring design caters for the high drag forces of the large fine mesh 'sail areas', with algal growth, drifting seaweed and significant tidal currents.
The enclosure can be moored or tethered to the sea floor using conventional technology for the moorings and anchors so that the enclosures keep their shape.
Single point mooring of the enclosure is possible, provided there is a means of keeping the enclosure in tension, which means can be the pressurisation of members of the roof or enclosure with air or water to act as struts, or the over-pressure of air trapped beneath an impervious roof, or over-pressure of water within the net enclosure provided by continuous pumping of water into the enclosure. The advantages of the single point mooring is that it can weathervane, roam widely to spread detritus, and be directional in design to reduce drag forces. Submergence in bad weather would be achieved through variable buoyancy controlled from or via the platform.
The choice of platform will depend on the fish and the conditions and not all platforms are suitably located for fish ranching. As well as oil or gas production platforms any offshore platform can be used, but disused oil and gas platforms are most suitable. The initial preferences will be where the tidal flow is not excessive, where the preferred fish species naturally choose to hatch and grow, where the sea is shallow enough to economise on moorings but deep enough to submerge to escape from storms and where the platform is already shut in and possibly redundant.
The space required on the platform for the fish ranch can be used for fish food, winches, and a control centre for the ranch supervisors and storage for a small seagoing boat and ROVs and/or AUNs with a launching system. If the platform is
disused and stripped of most oil and gas production equipment, there may also be space for a fish hatchery and weaning tanks.
The net enclosure can be fabricated, transported and deployed using state-of-the-art fishing net deployment and recovery methods. The introduction of the young fish and release of the older fish will be based on experience of the Fishing Industry. However these enclosures will be larger than the largest fishing nets and, unlike most fishing nets, must stay 'open' without the assistance of moving water. The design of the moorings is based on offshore experience within the Oil and Gas industry.
In use, the enclosures are set up and fish fry released into the enclosure. The fish fry can be produced in fish hatcheries on shore as in conventional fish farming or on the platform if there is space. When they have grown to juvenile size, e.g. after about three to twelve months depending on the species, the juveniles may be released into the open sea; a small proportion of juveniles may be retained in an enclosure of coarser mesh for a further 1 to 2 years for harvesting.
It is a feature of the invention that the design allows individual enclosures to be replaced easily and, without affecting or dismantling the mooring array, the design ensures that the enclosure or enclosures substantially do not have to take any of the mooring forces and the forces on the net enclosures are limited to sufficient tensions to keep their shape in moderate sea currents.
The enclosure can be a sound enclosure which generates walls of low frequency sound and is formed by installing walls of low frequency sound that cause fish to feel pain if they approach too close. The sound appears to be focussed in the plane of the wall, with the power, amplitude, or 'loudness', attenuating fast with increasing distance away from the wall, on either side. The initial Sound Enclosure is designed to enclose a space of up to 200m x 200m x 100m (about 2000 times larger than the largest SADCO), or more; and it is totally unaffected by ocean storms. Sound walls
will enable various fish species and sizes and predators to be segregated and the ocean to be farmed in future in a more constructive and sustainable way. There is no limit to the size of Sound Enclosures and they could be envisaged in sizes of several square kilometres in plan area. The depth of water is limited by the ability of the emitter design to focus the sound to avoid it spreading excessively across the wall width before reaching the sea surface.
Two fundamentally basic aspects are important in the Sound Wall design; firstly that the frequency or family of frequencies causes pain to the particular species and size of fish that are to be contained or repelled, without causing permanent harm, and, secondly, that the frequencies are so chosen and generated that they interfere and cancel out quickly in the direction perpendicularly away from the wall and so produce a steep gradient of noise and pain that enable the fish to easily appreciate which is 'the wrong direction'.
The various suitable frequencies will be those that target the natural frequencies of those organs that the fish can feel, such as the swim bladder. These vary with species and size of fish but a suitable range and mix of frequencies can be established, as has been found in the prior art of 'seal scarers', etc.
Transponders can be hung in a buoyant wall of coarse netting so that there is an array of transponders at regular intervals, both horizontally and vertically, with a power output such that any two adjacent transponders can fail without creating a gap through which fish can escape. The advantages of such a wall is that the power attenuates relatively fast with distance from the wall and the course net serves to exclude predators. However it is an expensive solution and vulnerable to storm damage near the surface.
The preferred design is to firmly 'plant' a line of transponders in the sea floor and focus the sound output into the plane of the wall with minimum loss away from the
wall as the sound rises to the sea surface. The transponders would be powered by different power cable 'ringmains' to minimise the probability of any two adjacent transponders failing. At the corners of the enclosure the transponders would continue for a short distance in each wall so as to maintain the 'noise' volume right up to the sea surface.
The intended frequencies, being those that cause sympathetic vibrations that 'hurt' but don't harm young juveniles, are a fairly low frequency, compared with sound that is audible to humans. These frequencies are less 'directional' than higher frequencies, but should be adequately directional to form a wall when installed in a row, particularly for a large sound enclosure in fairly shallow water.
The width of the sound wall can be significantly reduced by placing two sound emitters at each emitting location, half a wavelength apart, so that the two sounds are in phase when 'heard' within the plane of the wall but out of phase with each other in the direction perpendicular to the wall. The design of emitters that effectively generates sound with such precision is to be researched but the preferred solution will be an electric discharge to generate enough energy in a small enough space with millimetre precision.
Fig. 27 shows the corner between two sound walls, at which the sound goes beyond the corner and one or more emitters have to be installed beyond the corner, outside the sound enclosure, to maintain the strength of the sound up to the surface of the sea.
The emitters require redundancy of electrical power supply and emitting capability so that at least two adjacent emitters, on different supplies, could fail without opening up a hole in the wall. Two or more separate walls could be created close to each other to provide further redundancy and security.
The outer wall, or walls, can be operated at lower frequencies to deter predators and preferably the frequencies or mix of frequencies can be fairly easily adjusted or at least switched from one frequency to another. Additionally the power output could be adjustable so that the fish can be herded away from a side, while adjustments or openings or maintenance are carried out to individual emitters.
Additionally, in order to condition or educate new recruits more efficiently, it will be desirable to emit light or light flashes in the plane of the wall so that the fish can associate pain with the location of the light flashes and so learn faster to see and avoid the wall before feeling pain.
The acoustic system can also be used to signal to the fish. Sound and light can be used to associate with the arrival of fish food, so that the food is consumed relatively quickly before the sea currents or tidal flow have carried it beyond the walls of the sound enclosure.
By locating sound enclosures next to each other and by opening and closing walls and varying the sound level or amplitude, the fish can be corralled and herded from one enclosure to another and several adjacent enclosures can contain juveniles of different ages.
Because of the size potential of sound enclosures, very large stocks of fish can be envisaged. Indeed the larger the enclosure the easier it is to use sound walls since it is the attenuation of the sound at right angles to the wall, which will take up significant space. The solution to effective corralling is the gradient of the attenuation with increasing distance from the wall; such that, even a young juvenile can tell in which direction it needs to swim to lessen the pain.
A further advantage of the large sound enclosure is that it can be used to section off part of a natural fishery area so that the requirement for artificial feeding is greatly
reduced and as such even the platform can be omitted, provided a reliable power source or supply is installed at the seabed and an automatic feeder supplements the natural food supply.
There are already 30 'non-operating' (abandoned or shut-in) platforms on the N.W. Europe Continental Shelf (NWECS) in water depths of 50 to 150m (25 piled, 3 gravity, 1 TLP & 1 Jack-up).
Additionally there are now 164 'operating' platforms on the NWECS, in water depths of 50 to >350m, from which fish ranching could also be managed.
When Platform Fish Ranching becomes proven, there are over 8,000 offshore platforms on the continental shelves of the world many, or most of which could be used to manage net enclosure and/or sound enclosure fish ranching.
The invention is illustrated in the accompanying drawings in which are described four basic designs of net enclosure all of which submerge the net enclosure to escape storm damage. One has a fixed mooring system, which remains permanently on the surface and the other three submerge entirely but have different roof designs to avoid impact with the sea surface when in the surface mode.
In the drawings
Fig. 1 shows different types of net enclosures.
Figs. 2 and 3, shows a design of a buoy mooring system. Figs. 4 and 5, show another design of a buoy mooring.
Figs. 6 to 10 show the sequence of installing the moorings. Figs. 11 to 16 show net enclosures that are fixed to the seabed. Figs. 17 to 19 show floating net enclosures. Figs. 20 to 22 show the major buoy mooring concepts. Fig. 23 shows a design of fish feeder frame.
Fig. 24 shows a method of accessing the interior of a net enclosure. Fig. 25 shows the operation of a robot cleaner. Fig. 26 shows the location of the fish feeders. Fig. 27 explains the mooring design of figs. 6 to 10. Fig. 28 illustrates the Sound Enclosure concept.
Referring to Fig. 1 there are three different designs of net enclosure (1) with different roof structures, moored close to a platform (2) and in which mooring cables (3) and flexible hoses (4) connect the net enclosures (1) to the platform (2). At least two of the mooring cables (3) for each net enclosure (1) pass to the platform (2) so that their lengths can be adjusted from the platform (2) and hence facilitate the installation of buoys (5) and net enclosures (1) and also enable the net enclosure (1) to be pulled well below sea level to survive a storm by means of winches etc. acting on the mooring cables attached to the enclosures.
The net enclosure (1) can be easily removed by detaching it from the buoys (5) and the buoys will remain in place due to their mooring cables (6). Each buoy (5) or net enclosure (1) has a warning beacon, being at least a navigation light and/or radar reflector and/or sound warning system (7).
Referring to Figs. 2 and 3, these show a design of a buoy mooring system (11) that remains permanently on the surface, consisting of 3 or more buoys (5), (5 or 6 buoys may be optimum). The net enclosure (1) is installed by attaching the lower corners to the halyards (12) above water and then lowering them below water. The upper corners are then attached to the halyards (12), which are then pulled in and tensioned, so that the main mooring tension is transferred from the base cables (13) to the stretched roof (14) of the enclosure (1) to keep it above water in relatively calm weather.
When the weather deteriorates and before the waves (15) become high enough to overlap with the roof (14), the halyards (12) lower the enclosure by some 15m so that the roof (14) is about 10m below the surface. The halyards (12) can lower the enclosure (1) by a further 20m, if necessary, to avoid up to 100ft waves (16) or more. The actuation of the halyards (12) can be by electric, hydraulic, pneumatic or mechanical means and can be remotely powered and or controlled from the platform (2).
The benefits of this design are that there need not be any running moorings to the platform and the mooring tension is not increased when submerging the net enclosure. The high tension in the net enclosure roof (14), necessary in the surface mode, is transferred to the top cable (17) in the submerged mode and this allows the net enclosure roof (14) to be more compliant and flexible when riding out storms. Lastly, the warning beacons (7) remain effective throughout a storm.
Referring to Figs. 4 and 5, these show the design of a buoy mooring (11), consisting of 3 or more buoys, (4 may be optimum), that submerges in bad weather along with the net enclosure (1).
When the weather deteriorates and before the waves (15) reach the roof (14) of the enclosure (1), the whole net enclosure (1), buoys (5) and mooring system (11) are lowered below sea level by pulling in on at least all but two of the moorings (18) using a winch on the platform (2). The net enclosure (1) can be lowered to 30 m below sea level to avoid the worst storm and a 30m wave (16).
Most of the tension in the roof (14), necessary in the surface mode, is transferred to the base cable (13) in the submerged mode, thus allowing the roof (14) to be more compliant beneath a storm.
The warning beacons (7) remain effective until the enclosure is 15 m below sea level, thus protecting the enclosure from any vessels straying into the area in bad weather. If the weather is more severe, the whole assembly is lowered by a further 15 m to provide the same clearance from shipping for the navigation lights and radar reflectors, themselves.
The benefits of this design are that the buoys (5) can be very much shorter and lighter and there are no mechanical or moving parts on the net enclosure (1) and buoy mooring assembly (11). Also there are no parts left exposed to bad weather.
Referring to Figs. 6 to 10, these show the sequence of installing the moorings, buoys and net enclosure. After the seabed anchoring points have been established (by suction or, preferably, by drilling or piling), the running moorings (18) from the platform, can be let fully out and the floating buoys (5) can be attached by base cables (13), above water as shown in Fig. 6. In Fig. 7 the runmng mooring lines (18) are shortened to stretch the base cables (13) to ensure their correct installation. In Fig. 8, by further pulling in on the running moorings (18), the base cables (13) can be pulled under water, sufficiently to allow the installation vessel to pass over them. In Fig. 9 the vessel can launch the net enclosure (1), attach the lower comers to the buoys (5) above water, using the marker buoys (19) and the upper comers to the top of the buoys (5).
In Fig. 10, the cables attaching the net enclosure (1) to the buoys (5) are pulled in, working from the installation vessel, to pull the buoys (5) into the vertical. The running moorings (18) are then pulled in, from the platform, to hold the buoys (5) down and increase the tension in the roof cables (14) to hold the roof of the net enclosure (1) above water.
Further shortening of the running moorings (18) will pull the whole assembly of buoys (5) and net enclosure (1) under water.
By reversing the operation, the net enclosure can be easily disconnected, weather permitting, above water, removed and replaced, leaving the buoy mooring system in place. This will be necessary for maintaining the net enclosure about once or twice a year.
Referring to Figs. 11 to 16, these show net enclosures that are fixed to the seabed:
Fig. 11 shows the basic concept of a net enclosure surrounding the platform. The advantage is that the net enclosure is supported from the platform deck (20) and access from the platform to the inside of the net enclosure is easy. The net enclosure is however vulnerable to storm damage.
Fig.12 shows the net enclosure terminating with a roof (21) well below the storm zone. This assists in surviving storms but the fish are permanently cut off from the surface and access to the structure for inspection and maintenance would be difficult.
Fig.13 shows the net enclosure supported by the platform deck (20) but able to be lowered in bad weather. On many platforms the deck cantilevers out sufficiently to ensure that the lowered and loose net enclosure would not come into contact with, and be damaged by, the platform jacket (22) in a storm.
Fig. 14 shows a 'dome' type of net enclosure, of which the entire roof (23) is fabricated from a membrane as used in inflatable indoor tennis courts or in balloons and blimps, with a large valve (24) to allow rapid deflation. The roof buoyancy can be partly counterbalanced by chains (25) so that the depth of submergence is inversely proportional to the roof buoyancy. The maximum submergence can be designed to be when all four chains are entirely on the seabed. Other aspects of the 'dome design' are discussed under Fig. 19.
Fig. 15 shows a 'tent' type of net enclosure, in which the edge of the roof (26) is fabricated from an inflatable tube, of the type used in inflatable military boats and zodiacs, which maintains the shape but is fairly compliant in this large scale. The centre of the roof is supported by one or more floats (27), which can be deflated, and/or pulled down by a cable (28) from a platform winch.
Fig. 16 shows a 'waffle' type of net enclosure, in which the roof is made up of several relatively thin (lm or less in diameter) tubes that facilitate a roof design that could be repeated and/or extended to cover a very large area, with sections of mesh (29) between the tubes that can pass air (or water if submerged).
Wherever significant sea currents and/or tidal flows exist, the large 'sail area' of the fine mesh walls will produce a large drag force and, with the addition of algal growth and trapped seaweed, these forces will be very high. In such circumstances, the net enclosures shown in Figs. 14 and 15 will require moorings like those shown in Fig. 16 (30). Chains or weights may be added to allow the net enclosure to rise and fall with the tide.
Additionally, the attaching of the net enclosure to the seabed, however, will not be simple and the detritus will accumulate in a concentrated area.
Referring to Figs. 17 to 19, these show moorings which allow the floating net enclosures to roam laterally within a reasonable envelope which can be extended with wider spread moorings. By floating well above the seabed the detritus is carried laterally by sea currents and tidal flow and spread over a wider area. Also, these designs of net enclosure are simpler and easier to install or replace.
Fig. 17 shows the 'tensioned' roof net enclosure, in which the roof is kept above sea level by the tension in the roof cables (14), which necessitate large buoys (5) and high mooring loads. These loads are increased further on submerging or, alternatively, the
buoys can be ballasted (displacing air with water), and refilled with compressed air for re-surfacing. Fixed buoyancy, such as rigid foam, at the top of the buoy and weighting, such as pig iron, at the base can assist in increasing stability. The addition of large discs (32) at the base of the buoys (5) can assist in dampening the effect of swells, or heave, to reduce relative motion between each buoy and the comer of the net enclosure that it is moored to.
Fig. 18 shows a combination of lower tension force and 'tent' type structure. As with all of the designs it is important to ensure that the roof cannot 'slap' the surface of the water and harm the fish, which is likely when swells affect the buoys out of phase. Also, only the roof is tethered, leaving the base free to tilt away from sea currents and reduce the drag. The moorings are simpler and the mooring force reduced. The moorings, themselves, may be tethered (33) in order to restrain them while enclosures are disconnected and/or replaced.
Fig. 19 shows a preferred design of net enclosure. The 'Dome' design of roof, is simple and uses membrane materials that are well proven in other applications. The air supply from the platform can be heated or cooled. The air can be fast vented from one or more vents (24) in the roof of the dome. Only the roof is moored so that the walls (34) can tilt away from a strong tidal flow and avoid the drag becoming excessive (however much the wall is covered in algal growth or seaweed). The least mooring forces of the examples shown need only consist "of marker buoys (35) and retaining cables (33) to allow the net enclosure to be easily detached and replaced. The preferred design shown in Fig. 19 requires the least mooring forces of the examples shown and does not increase with submergence as this is achieved by venting air; the roof (23) cannot collapse too far as the peak of the roof contains one or more permanent buoyancy aids (25); adjusting the moorings from the platform can move the net enclosure over a significant area to spread detritus; constant tension winches or compensators can ensure that the net enclosure rises and falls with the tide. Any tendency to tilt due to a strong tidal flow can be compensated by shortening
the downstream moorings and by incorporating fixed or variable buoyancy around the edge of the roof (23).
Referring to Fig. 20 this shows the two elevations of the mooring required to connect with both the roof and the base of the net enclosure, whereby the mooring force (M) intersects with the addition (F) of the two net enclosure forces (FI and F2) and the mooring cable forces between the buoys (F3), very close to the centre of buoyancy (b) of the buoy. On submergence, stability relies almost entirely on the buoy being a double buoy, kept upright by the cables between the buoys.
Referring to Fig. 21 this shows the two elevations of the mooring required if the net enclosure is not tethered at the base. A mooring failure in the cases shown in Figs. 20 or 21 would upset the geometry considerably and is the main reason, apart from the high tensions, for only using a tensioned roof in relatively small net enclosures.
Referring to Fig. 22 this shows the simplicity achieved by having the net enclosure roof being self supported, for example by air pressure. Only a small marker buoy is required to support each mooring cable while the net enclosure is being installed or replaced. The mooring system is simple and stable, even when submerged. The mooring forces, although less than in the other designs, are still considerable because of the large size of the enclosure, so the transition between the mooring cable and the net enclosure will pass through a reinforced section (36) to spread the load. At this location, within the splash zone when in the surface mode, the membrane (37) will be a thicker version of the dome membrane (38) and of the material typically chosen for inflatable boats or hovercraft skirts. This membrane (37) can preferably incorporate an amount of fixed buoyancy to assist in supporting the roof weight so that there is no tendency for the roof to become unstable, with slack moorings, strong tidal flow or when submerged.
Fig. 23 shows an appropriate design of fish feeder frame (40), which can retain buoyant food inside a net enclosure, where very much more food has to be spread over a larger area than is common in fish cage aquaculture, without being swept away by the current. This design ensures that the feeder can still contain a free air / water interface (41), even after the net enclosure has submerged, provided the air pressure is increased as the feeder sinks down. Since, in this design, the excess air can escape under the floats (42), it will be fairly easy to maintain the air / water level (41) in the feeding frame.
Fig. 24 shows a method of accessing the interior of the 'dome' net enclosure, through a simple air lock (50) using zippable or "Nelcro" (RTM) sealed doors (51), with sufficient buoyancy in the structure to keep it erected while a small boat (52) enters the airlock with the outer door open. The materials indicated are balloon or blimp material (53), reinforced inflatable boat or hovercraft skirt material (54) for the splash zone and Fishing Net (55) for the walls and floor of the net enclosure.
Fig. 25 shows the operation of a robot cleaner of the type conventionally used now offshore and very much easier to use on such a net enclosure, in the absence of any structural struts both inside and outside.
Fig. 26 shows the location of the fish feeders previously shown in Fig. 23 and moored between the main net enclosure mooring points. Provided the roof is slightly buoyant, the feeders will remain upright and working when submerged.
Fig. 27 explains the mooring design of Figs. 6 to 10, more clearly, whereby the mooring array (70) in Fig. 27 of buoys (71) and mooring cables (72) can remain in place while an enclosure or enclosures (73) (Fig. 27a) can be individually removed or replaced by a vessel or vessels, such as a fishing boat (74) (Fig. 27b). A benefit of the design is that the mooring forces are not transmitted to the net enclosures and the net enclosures only require sufficient tension to keep their shape. Narious sized
enclosures are shown, the smaller enclosures (75) generally being of smaller mesh as suited to smaller and younger fish and the larger enclosures (76) generally being of coarser mesh for larger and older fish. The mooring cables (72) pull the mooring array (70) and enclosures (73) below sea level whenever required; for example, to avoid damage to the installation or the fish during rough seas.
Fig. 28 illustrates, the Sound Enclosure Concept, in which the platform is in, or near, an enclosure (60) formed of walls of sound produced by lines of sound emitters (61) fixed to the seabed. The bottom of the sound walls is indicated by the dotted lines (62) along the seabed and the top of the sound walls is indicated by the dotted lines (63) at the surface of the sea. The sound walls extend for a short distance beyond the comer of the Sound Enclosure but are omitted in the illustration to try and improve clarity.