The present invention concerns an adjustable depth fish farming water duct for a floating, closed fish farm requiring supply and disposal of water. Closed in this context is intended to cover fish farms with a controlled water flow through a residence chamber for the fish. A fish farming structure with an adjustable depth fish farming water duct and a method of installing an adjustable depth fish farming water duct onto a fish farming structure is also described.

Water flow and quality is essential for the fish welfare when operating fish farms. The water intakes and outlet ducts normally have a fixed length, but solutions where water is taken at two depths with two intakes has also been suggested.

Typical fish farms are of a raceway type with one or several longitudinal fish residence chambers, but the invention is also applicable to other types of fish farms with an inlet and/or an outlet for seawater.

An ideal solution would allow water to be taken from various depths. By enabling the depth to be selected, may an operator choose water at a depth with desired parameters such as temperature, oxygen content, predators, pathogens and parasites.

The ducts are typically used in connection with flow machines, such as large axial flow turbines with “propellers” or radial pumps.

NO333490 (same applicant) disclose a semi closed fish farm with lifting-and lowering devices adapted to give an inlet pipe and an outlet pipe variable length. The publication does however not suggest how such pipes can be designed to facilitate installation and maintenance.

Inlet pipes and outlet pipes of the above type are of dimensions that typically prevent the fish farm from being installed in one piece and it is an object of the present invention to provide an inlet pipe and an outlet pipe design that allows and simplifies the installation of the pipes after the fish farm has been launched into water while at the same time ensuring that an inner duct area is controlled and maintained within a design specification. Typical dimensions are 10m diameter and 30m length, and the present invention provides a solution and a method allowing large diameter pipes to be compressed, installed and expanded on the fish farm in situ, both for installation and maintenance purposes. Furthermore, it is an object of the invention to provide embodiments that ensure longevity that reduce wear on materials and allows simple control of the stresses on the pipe material both when the pipes are expanded and contracted.

The present invention thus concerns an adjustable depth fish farming water inlet or outlet duct adapted to be removably attached to a fish farming structure. The adjustable length water duct comprises an upper releasable duct connection ring adapted to be secured to a structure water port on the fish farming structure. A lower ring forms one of an inlet port and an outlet port at a lower end of the adjustable length water flow duct and is adapted to be located in a body of water where a fish farming structure is located.

The adjustable length water flow duct allows the distance between the upper releasable duct connection ring and the lower ring be varied between a first distance and a second distance.

The at least one duct lifting wires is adapted to pull the adjustable length water flow duct between an extended position and a retracted position. At least one lifting wire is provided and is secured on a portion of the fish farming water inlet or outlet duct. The at least one duct lifting wire is adapted to be secured to at least one wire pulling device on the fish farming structure.

The adjustable depth fish farming water inlet or outlet duct may include at least three external lifting wires, wherein each lifting wire is secured on an external portion of the fish farming water inlet or outlet duct, and wherein the at least three external duct lifting wires are adapted to be secured to at least one wire pulling device on the fish farming structure.

The at least one duct lifting wire may extend inside the fish farming water inlet or outlet duct.

The at least one duct lifting wire extending inside the fish farming water inlet or outlet duct may extend at the centre of the adjustable depth fish farming water inlet or outlet duct and may be secured to a harness between the wire and the duct.

The adjustable depth fish farming water inlet or outlet duct may further include a plurality of pliable duct sections between a plurality of stiffening rings and wherein the lower ring and the upper releasable duct connection ring are secured to a pliable duct section.

A pliable duct section is considered as the portion of the duct between the stiffening rings. The pliable material forming the duct sections may extend over several duct sections and does not necessarily need to be divided at the stiffening rings.

The at least two lifting wires externally of the duct may be secured to the lower ring.

The adjustable depth fish farming water inlet or outlet duct may further include distance elements limiting a minimum distance between each stiffening ring.

The distance elements limiting a minimum distance between each stiffening ring may include wire guide rings secured to the stiffening rings and wire distance elements secured to the length adjustment wires.

The distance elements limiting a minimum distance between each stiffening ring may include stiffening ring spacers secured to a number of the stiffening rings. The adjustable depth fish farming water inlet or outlet duct may, further include an upper, stiff duct section and a lower pliable duct section, at least partly located on an outside of a circumference of the upper, stiff duct section, the lower pliable duct section including a number of stiffening rings, including an upper ring and the lower ring, and wherein the at least three external lifting wires are secured to the upper ring.

The adjustable depth fish farming water inlet or outlet duct may further include an upper, stiff duct section and a lower stiff duct section at least partly located on an inside of a circumference of the upper, stiff duct section, and the at least three external duct lifting wires may be secured to the lower ring on the lower stiff duct section.

The adjustable depth fish farming water inlet or outlet duct may further include a flexible connecting element above the upper, stiff duct section, adapted to be removably attached to the fish farming structure, forming a duct section and adapted to allow the duct to move in relation to the fish farming structure to reduce stresses on the duct and the fish farming structure.

The lower ring may include a streamlined portion shaped as a bellmouth.

The adjustable depth fish farming water inlet or outlet duct may further include a protection element on a lower portion of the duct, forming a bumper to protect against a neighbouring duct when two ducts are located close together.

The protection element may be shaped as a horseshoe and may be secured to the lower ring.

Furthermore, the invention concerns a fish farming structure with at least one residence chamber for fish, at least one water inlet and one water outlet. An adjustable depth fish farming water duct is removably attached to at least one of a residence chamber inlet and a residence chamber outlet. At least one of the adjustable depth fish farming water inlet and the adjustable depth fish farming water outlet comprise an upper releasable duct connection ring adapted to be secured to a structure water port on the fish farming structure. A lower ring forms a port at the end of the adjustable length water flow duct and is adapted to be located in a body of water where a fish farming structure is located, and wherein the adjustable length water flow duct allows the distance between the upper releasable duct connection ring and the lower ring be varied between a first distance and a second distance. At least one duct lifting wire is provided, wherein and is secured on a portion of at least one of the fish farming water inlet duct and the fish farming water outlet duct. The at least one duct lifting wire is adapted to pull the adjustable length water flow ducts between an extended position and a retracted position. The at least one lifting wire is secured to at least one wire pulling device on the fish farming structure.

The upper releasable duct connection ring may be integrated in the duct. In an alternative embodiment the upper releasable duct connection ring is simply an upper part of a rigid duct.

The fish farming structure may further include an adjustable depth fish farming water inlet or outlet duct as described above.

Similarly, in an alternative embodiment, may the lower ring be integrated in the duct.

The at least one wire pulling device may be at least one winch.

Furthermore, the invention concerns a method of installing an adjustable depth fish farming water duct onto a fish farming structure. The method comprises strapping the adjustable depth fish farming water duct in a retracted position, launching the adjustable depth fish farming water duct into the sea, locating the adjustable depth fish farming water duct next to the fish farming structure, attaching a first central wire extending from a winch on the fish farming structure to a second central wire extending through the adjustable depth fish farming water duct to form a central wire secured to the adjustable depth fish farming water duct with a harness to locate the central wire at a centre of the duct, ballasting at least one of the adjustable depth fish farming water duct and the fish farming structure to allow the adjustable depth fish farming water duct to be located below a structure flow port with a structure connection ring, pulling the central wire to pull a releasable duct connection ring on the adjustable depth fish farming water duct towards the structure connection ring, and secure the releasable duct connection ring to the structure connection ring.

The method may further include attaching a second end of the second central wire to a bottom weight located on a support vessel, launching the bottom weight from the support vessel with a vessel winch wire and hoisting the bottom weight down into the sea until the central wire is tight. The weight is then pulled into the adjustable depth fish farming water duct until a harness secured to the lower ring and the second central wire is tight, the strapping on the adjustable depth fish farming water duct holding the duct in a retracted position is released, the central wire to extend the adjustable depth fish farming water duct is lowered and the central wire is lowered further to lower the bottom weight and to expand a bottom net across the lower ring.

The method is particularly adapted for installing an adjustable depth fish farming water inlet or outlet duct as disclosed in claims 1-16 onto a fish farming structure.

Short description of the enclosed figures:

Fig. 1 is a perspective view of an adjustable depth water intake or outlet duct for a fish farm;

Fig. 2 is a side elevation of an embodiment of the invention with pliable sections and a partly retracted duct secured to a fish farming structure;

Fig. 3 is a side elevation of an embodiment of the invention with a partly retracted duct and stop elements on the lifting wires;

Fig. 4 is a side elevation of a detail of fig. 3;

Fig. 5 is a side elevation of a detail of the invention with an expanded duct secured to a fish farming structure and with spacers on the stiff rings;

Fig. 6 is a side elevation of a detail of fig. 5, in a retracted position;

Fig. 7 is a side elevation of an embodiment of the invention with a partly retracted duct secured to a fish farming structure and with one rigid element and one pliable element; Fig. 8 is a side elevation of the embodiment of fig. 7, where the pliable duct sections are pulled on the outside of the stiff duct section;

Fig. 9 is a side elevation of an alternative embodiment with a lower stiff duct section;

Fig. 10 is a side elevation of the embodiment of fig. 9, where the lower stiff duct section is pulled on the inside of upper stiff duct section;

Figs.11 -18 are side elevations showing steps in a method of installation of a water intake duct on a fish farming structure;

Fig. 11 shows a step where the water intake duct is launched into the sea and is located next to the fish farming structure;

Fig. 12 shows the duct floated in place below the fish farming structure and the structure connection ring of the structure flow port and the releasable duct connection ring are aligned;

Fig. 13 shows the installation wire having to pulled the duct and the releasable duct connection ring towards the structure connection ring;

Fig. 14 shows the clamping ring clamped to the structure connection ring and the releasable duct connection ring together;

Fig. 15 shows the weight attached to the installation wire and a vessel winch wire holds the weight;

Fig. 16 shows the weight pulled into the duct, and the braces holding the duct in a contracted position are released;

Fig. 17, shows the duct in the extended position;

Fig. 18 corresponds to fig 17, but the weight has been lowered further and expands a bottom net;

Fig. 18b corresponds to fig. 18, with the addition of a second winch in addition to the first winch;

Fig. 19 shows a complete fish farming structure with four ducts of the invention;

Fig. 20 is a cross-section showing a detail of the joint between the releasable duct connection ring and the structure connection ring; and

Fig. 21 is a perspective view of a portion of the clamping ring.

Fig. 1 disclose an adjustable depth water intake or outlet duct 10 for a fish farm.

The depth is adjustable by adjusting the length of the duct 10. The duct 10 includes a lower ring 13 forming one of an inlet port and an outlet port at the lower end of the adjustable length water flow duct and is furthermore shown with six stiffening rings 12. Seven pliable duct sections 11 are located between the six stiffening rings 12, the lower ring 13 and a connection ring 16 adapted to connect the duct 10 to a fish farm. The lower ring 13 includes a streamlined portion 15 to reduce the flow resistance in the pipe and the tendency the water has to reduce the diameter of the duct. Analysis has shown that omitting the streamlined portion may result in locally stationary water in areas and this is found to reduce the required flow and increase energy consumption. The streamlined portion 15 of the lower ring 13 may be considered to be shaped as a bellmouth, a velocity stack, a throat flaring outwards or a portion of a trumpet. A typical distance between the rings is 1 ,5-3 meters.

Furthermore, the lower ring may include a protection element 19 shaped as a horse-shoe, forming a bumper to protect a neighbouring duct 10 when two ducts are located close together, (see fig. 19) A situation with two neighbouring ducts occurs when they form inlets or outlets for two longitudinal residence channels as shown in fig. 19. The ducts will typically deflect due to current and waves and will rub against each other. The protection elements 19 of two adjoining ducts should be facing each other and will prevent damage of the pliable duct sections 11.

Fig. 2 disclose an embodiment of the invention with a duct 10 secured to a fish farming structure 20. The duct includes six stiffening rings substantially maintaining the inner diameter of the duct 10. The distance between the stiffening rings affects how much the pliable sections are allowed to fold inwards and how much the inner diameter is reduced when the duct is contracted. The length of the duct 10 is reduced as the lower ring 13 is pulled upwards by external lifting wires 26 and winch 21. The pulling action on the lower ring 13 compresses the lower sections of the pliable duct sections forming compressed sections 22. The three upper pliable sections 11 are extended to their full length. The external lifting wires 26 extend through wire guide rings 27 secured to the stiffening rings. A bottom weight 24 holds a cone shaped bottom net 25 extended. The bottom net may prevent unwanted elements from entering the duct. 10. The winch 21 may be a permanently installed winch or may be a winch that is installed when the length needs to be adjusted.

Typically, three to six external lifting wires 26 lift the stiffening rings. A solution may additionally include a first central wire 28 for better control of the working operation including length adjustment and duct exchange.

The duct of Fig. 3 corresponds to the duct of fig. 2 and 1 . The duct however furthermore includes wire distance elements 30 to prevent the stiffening rings 12 from being pulled to close together to maintain the inner diameter of the duct and to prevent the pliable duct sections from being stressed beyond design conditions. The winch 21 may constitute one or more winch drums to pull the external duct lifting wires 26 running over external lifting wire pulleys 31. In an alternative embodiment may a plurality of lifting winches be located in a circle and pull directly upwards and the pulleys 31 may be omitted. The structure water port 14 forms the inlet or the outlet of the fish farming structure.

Fig. 4 shows the lower part of the duct of fig. 3 in more detail. The distance elements are shaped as balls 41 , 43, 45, 47 secured to the length adjustment wires. The balls are gradually smaller, with the largest ball 41 at the bottom and the smallest ball 47 at the top. The wire eyes 42, 44, 46, 48 are attached to the stiffening rings and are of different diameter adapted to size of the distance elements, i.e. the balls. The smallest ball 47 is allowed to run through all but the smallest wire eye 48 and will thus lift the uppermost stiffening ring shown in the figure.

The second smallest ball 45 is allowed to run through all but the second smallest wire eye 46, and will thus lift the second uppermost stiffening ring shown in the figure etc. The wires are secured to the lower ring in the lower ring wire attachment point 40.

The distance between the distance elements on the wire determines the distance between the stiffening rings and thus the folding of the pliable duct sections. The control of the folding of the pliable duct sections ensure longevity that reduce wear on materials and allows simple control of the stresses on the pipe material both when the pipes are expanded and contracted. The wires may take up some of the stresses when the pipes are in the expanded position if required. The stresses in the pipe material is more predictable when the pipe material is in tension and the material is prevented from rubbing against itself providing unwanted abrasion.

Fig. 5 shows an alternative embodiment to the distance elements on the wires for maintaining the distance between the stiffening rings 12 when the sections are collapsed. Stiffening ring spacers 50 extend up from the stiffening rings 12 between the pliable duct sections 11 that are to be collapsed. The stiffening ring spacers 50 are adapted to abut the neighbouring ring to provide the required space. The stiffening ring spacers 50 are shown extending upwards, but could also be extending downwards. The stiffening ring spacers 50 can be shaped as rings I short tubes or as individual elements secured to the stiffening rings.

Fig. 6 is a detail of the embodiment of fig. 5, showing the lower pliable duct sections in a collapsed state and with the stiffening ring spacers 50 holding the predetermined distance between the stiffening rings 12. The stiffening ring spacers 50 helps to maintain the distance between the spacers and thus an inside diameter 60 inside the duct, determined by the inward folding of the pliable duct sections.

Fig. 7 shows an alternative embodiment designed to maintain the inner diameter of the duct. A stiff duct section 70 is secured to the fish farming structure. Three pliable duct sections 71 and three stiffening rings 12 with a larger inner diameter than the stiff duct section 70 form the lower part of the duct. Pliable duct section lifting wires 17 extending on the outside of the rigid duct section allow the winch to pull the lower duct section on the outside of the stiff duct section 70. The duct connection ring 16 may be flexible to reduce the stresses on the duct and the fish farming structure due to water currents and wave action on the fish farming structure. As opposed to the external duct lifting wires 26 shown in fig. 2, are pliable duct section lifting wires 17 shown in fig. 7 secured to an upper portion of the pliable duct section 71 .

Fig. 8 shows the embodiment of fig. 7, where the pliable duct sections are pulled on the outside of the stiff duct section 70, thus making the duct shorter. The first central wire 28 is used for installation purposes when the duct needs to be exchanged. Longitudinal tracks or rails between the inner and outer section may be used to reduce friction and to facilitate adjustment of the length.

Fig. 9 shows an alternative embodiment designed to maintain the inner diameter of the duct. In fig. 9 has a lower stiff duct section 92 an outer diameter adapted to extend into an upper stiff duct section 91 , thus forming a telescopic arrangement. The lower stiff duct section 92 is pulled into the upper stiff duct section 91 with winch 21 pulling a wire (not shown) at the centre of the duct. A connecting element 93 may be flexible to allow a certain motion of the duct in relation to the fish farming structure. The flexible connecting element may be a formed as a sleeve of rubber or another flexible material such as an elastomer. The flexible connecting element 93 may be permanently secured to the duct. The flexible connecting element 93 may be the same material as used for hovercraft skirts made by a company called “Checkmate Flexible Engineering”. Longitudinal tracks or rails between the inner and outer section may be used to reduce friction and to facilitate adjustment of the length.

Wire fixing ears or padeyes 90 for the length adjustment wires are provided around the circumference of the lower ring 13.

Fig. 10 shows the embodiment of fig. 9, where the lower stiff duct section 92 is pulled on the inside of upper stiff duct section 91 , thus making the duct shorter. The first central wire 28 is used for installation purposes when the duct needs to be exchanged.

The fish farming structure is too bulky to be transported from a dockyard with connected/installed water intake and outlet ducts. The ducts must be installed when the fish farming structure is at sea. All operations are preferably performed by a working vessel on the site. Such a working vessel will typically not have the capacity to carry the ducts on deck and the ducts must thus be floated to the fish farming structure. At the first installation must four ducts be transported to the fish farming structure.

Figs.11-18 show steps in a method of installation of a water intake duct on a fish farming structure 20. The water intake duct may in one embodiment as shown have a diameter of 10m and is strapped to its shortest length of 6m. Temporary or permanent spacers between the stiffening rings are installed to prevent the pliable duct sections from being damaged. The packing of the water intake duct ensures that all the pliable duct sections (the canvas/fabric/awning cloth/tarpaulin cloth) are on the inside of the stiffening rings to prevent any abrasion of the sections during transport. The releasable duct connection ring 16 and the lower ring are strapped together to allow the water intake duct 10 to be lifted and handled at a manufacturing or maintenance site and onto a vessel I barge or to be launched into the sea at a quay. Bottom net and the required ropes/wires for the bottom weight, for operation and installation are installed before the water intake duct is launched.

Fig. 11 shows a step where the water intake duct is launched into the sea and is located next to the fish farming structure 20. A vessel 110 carries the bottom weight 24 that spans out the bottom net and attenuates the motions of the intake duct when the duct is installed. Floating bodies (not shown) may be attached to the duct to provide the required buoyancy to allow the duct to float at the required height. The floating bodies may include some mechanism to vary the buoyancy to adjust the floating level. In fig 11 , is the releasable duct connection ring 16 at the top of the duct located at sea level 112. The fish farming structure is ballasted to allow a structure connection ring 113 at a bottom of a structure flow port 114 to be at water level 112. The first central wire 28 from a winch 21 on the fish farming structure extends down into the middle of the structure flow port 114. A second central rope or wire 115 is secured to the middle of the duct with ropes and to the bottom net and extends further up to an accessible location at the sea-level 112. The first central wire 28 and the second central wire 115 are joined. The winch 21 can then be operated to aid installation of the duct onto the fish farming structure 20. Fig. 20 shows an upper ring with added buoyancy.

On fig. 12 is the duct 10 floated in place below the fish farming structure 20 and the structure connection ring 113 of the structure flow port 114 and the releasable duct connection ring 16 (the rings having equal diameter) are aligned. The winch has tightened the first central wire 28.

In fig. 13 has the winch 21 been driven further to tighten the installation wire and to pull the duct and the releasable duct connection ring 16 towards the structure connection ring. The installation wire is secured to the releasable duct connection ring 16 with braces or a harness 131 between the installation wire at the centre of the releasable duct connection ring and the lower ring. A connecting element (ref 93, fig. 9) in the form of a clamping ring 130 clamps the releasable duct connection ring 16 to the structure connection ring. The clamping ring may be a two-part ring, each connected to one of the structure connection ring and the releasable duct connection ring 16. The two parts may include a plurality of brackets spaced around the circumference of the rings and turnbuckles or bolts between the brackets as shown in figs. 20, 21 .

In fig. 14 is the clamping ring 130 holding the structure connection ring and the releasable duct connection ring together, and the winch 21 may release the tension in the first central wire 28.

In fig. 15 has the weight 24 been attached to the installation wire and a vessel winch wire 150 also holds the weight 24. The vessel winch 151 releases the vessel winch wire 150 to lower the weight 24 until the first central wire 28 is tight and the winch 21 can pull the weight upwards and the tension in the vessel winch wire 150 can be released.

In fig. 16 is the weight 24 pulled into the duct, and a second harness, the lower ring harness 160, securing the lower ring 13 to the installation wire is tight and the braces holding the duct in a contracted position are released. The winch 21 may then be used to control the extension of the duct as the lower ring 13 may be lowered carefully to extend the duct.

The lifting wires (not shown) are secured to the lower ring 13 at this stage.

Fig. 17, shows the duct in the extended position and the first central wire 28 and the winch 21 has lowered the weight 24, and extended the duct while the vessel winch wire 150 still is secured to the vessel.

Fig. 18 corresponds to fig 17, but the weight 24 has been lowered and expands the bottom net 25. The vessel winch wire still 150 is secured to the vessel. At this stage, the vessel winch wire 150 may be released from the vessel and may be hanged off and secured to the fish farming structure.

Fig. 18b corresponds to fig. 18, with the addition of a second winch 182 in addition to the first winch 21 . A central hight adjustment wire 181 extends from the second winch 182 and to a lifting harness 180 secured to the lower ring 13. The second winch 182 can thus lift or lower the lower ring 13 without pulling the bottom net 25 into the duct. If the bottom net is pulled into the duct, there is an increased risk that unwanted growth and fouling from the bottom net would be entrained in the water flow and enter the residence chamber. It is thus an advantage to be able to adjust the length of the duct without pulling the bottom net 25 into the duct. Using one central hight adjustment wire 181 instead of a plurality of external wires, simplifies the at least one wire pulling device on the fish farming structure. In some situations, may also the central hight adjustment wire 181 and the second winch 182 be omitted, and the lifting harness 180 secured to the lower ring 13 may then be secured to the first central wire 28.

One, or both of the winches 182, 21 may be permanently installed, or may be temporarily installed as needed. One winch may also serve several purposes and pull different wires as required. A removal or exchange of a duct is a reverse procedure of the procedure or method explained above.

In the event the duct includes one or several rigid sections will the procedure still be the same.

Fig. 19 shows a complete fish farming structure 20 with four ducts 10 as discussed above. The weights 24 spans out the bottom nets and attenuated the motion of the ducts 10. The fish farming structure 20 includes two longitudinal, tubular residence chambers 192 with an inlet end 190 and an outlet end 191. Each of the two residence chamber inlets 193 are secured to an adjustable depth fish farming water inlet duct 10. Padeyes 90 for the length adjustment wires (not shown) are provided around the lower rings.

In the present disclosure is releasable and releasable joint intended to describe repeatable connection and disconnection. When the disclosure explains that the releasable duct connection ring is adapted to be releasably secured to a structure water port, it is intended to exclude adhesive bonding, welding etc. generally forming non releasable connections.

Fig. 20 is a cross-section showing a detail of the releasable joint between the releasable duct connection ring 16 and the structure connection ring 113. The releasable duct connection ring 16 includes an annular buoyancy cavity 207 to provide buoyancy for the duct to aid installation and to provide stiffness. A conical transition between the releasable duct connection ring 16 and the structure connection ring 113 ensures sealing between the components. Additionally, may a sealant be used between the mating surfaces. A stiffening rib and a floating foam is shown inside the buoyancy cavity 207. The clamping ring 130 includes an upper conical metal ring 200 on the outside of the conical portion of the structure connection ring 113, and a lower conical metal ring 201 on the outside of the conical portion of the releasable duct connection ring 16. Upper padeyes or upper brackets 202 are spaced around the upper conical metal ring 200 and lower padeyes or lower brackets 204 are spaced around the lower conical metal ring 201 . Bolts or turnbuckles 205 extend between the upper brackets 202 and the lower brackets 204. A harness 206 may be used to hold the duct in the retracted position during installation. The pliable duct section 11 is secured to the releasable duct connection ring 16.

Fig. 21 is a perspective view of a portion of the clamping ring 130, showing the upper brackets 202 attached to the upper conical metal ring 200 and the lower padeyes or lower brackets 204 spaced around the lower conical metal ring 201. The bolts or turnbuckles 205 are attached to the upper brackets 202 and the lower brackets 204 and are tightened to pull the upper conical metal ring 200 towards the lower conical metal ring 201 .

The stiffening rings, the stiff duct sections or the lower ring are typically be made of GRP.

The adjustment of the length of the ducts may be automated and the length Sensors, computers, and software can be used to control the length of the ducts based on elements such as wave influence, water current, water temperatures, water salinity, anticipated amount and location of parasites, weather forecast etc. Sensors may include strain gauges measuring stresses on the structures, thermometers, water current sensors, salinity sensors etc. Stress/strain sensors typically measures the conditions in the transitions of the duct.