GB2598807A - Underwater mooring system - Google Patents

Abstract

An underwater mooring system, for mooring a platform 7 underwater at variable depths within a body of water, comprises the platform, and a coupling 4 to couple the platform to one or more submerged anchor points 2 having a vertical position that is underwater but above a bed of the body of water. The platform extends out laterally from the one or more submerged anchor points, and the coupling permits vertical movement 6 of the platform relative to the one or more submerged anchor points to allow the platform to be positioned at different depths underwater. Coupled to the platform is at least one buoyancy chamber 5 having variable buoyancy to control a depth at which the platform is positioned. The platform may be a marine agriculture platform for growth of a marine agricultural product.

Description

UNDERWATER MOORING SYSTEM

The present technique relates to the field of underwater mooring systems. More particularly, it relates to an underwater mooring system for mooring a platform underwater at variable depth within a body of water.

It may be desirable to provide a platform underwater at a certain depth within a body of water. For some applications it may be useful to support variable depths. Typical approaches for implementing variable depth mooring are very large, complex and expensive, and so may not be appropriate for some applications.

At least some examples provide an underwater mooring system for mooring a platform underwater at variable depth within a body of water, comprising: the platform; a coupling to couple the platform to one or more submerged anchor points having a vertical position that is underwater but above a bed of the body of water, the platform extending out laterally from the one or more submerged anchor points, the coupling permitting vertical movement of the platform relative to the one or more submerged anchor points to allow the platform to be positioned at different depths underwater; and coupled to the platform, at least one buoyancy chamber having variable buoyancy to control a depth at which the platform is positioned.

At least some examples provide a method for variable depth mooring of a platform underwater within a body of water, comprising: the platform being coupled to one or more submerged anchor points having a vertical position that is underwater but above a bed of the body of water, the platform extending out laterally from the one or more submerged anchor points, the coupling permitting vertical movement of the platform relative to the one or more submerged anchor points to allow the platform to be positioned at different depths underwater; and varying buoyancy of at least one buoyancy chamber coupled to the platform, to control a depth at which the platform is positioned.

Further aspects, features and advantages of the present technique will be apparent from the following description of examples, which is to be read in conjunction with the accompanying drawings, in which: Figure 1 shows an example of a variable depth underwater mooring system; Figure 2 shows an isometric view of a modular design for the variable depth underwater mooring system; Figure 3 shows a plan view of a modular design for the variable depth underwater mooring system; Figure 4 shows a plan view of a rectangular array of platform modules of a variable depth underwater mooring system; Figure 5 shows an isometric view of the variable depth underwater mooring system shown in Figure 4; Figure 6 shows an elevation view of the variable depth underwater mooring system shown in Figure 3; Figure 7 illustrates the operation of an example of a buoyancy chamber of the variable depth underwater mooring system, Figure 8 illustrates an example of an access providing line that may be provided on the underwater mooring system; Figure 9 shows an example of a variable depth underwater mooring system with an offshore structure as one submerged anchor point; Figure 10 shows an alternative example of a variable depth underwater mooring system using a movable coupling which can travel along guide lines; Figures 11A to 11C illustrate alternative examples of implementing coupling between the platform and the anchor point; Figures 12A to 12D illustrates the launch of a flexible platform for a variable depth underwater mooring system; Figure 13 shows an alternative example of securing the anchor points for a variable depth underwater mooring system; Figure 14 shows an isometric view of the guideline mechanism using multiple guidelines from a single stopper allowing large arrays to be formed; and Figure 15 shows a plan view of an array of guideline type mechanisms, with a zoomed close up showing how the guidelines and platforms are independent of each other. An underwater mooring system, for mooring a platform underwater at variable depth within a body of water, comprises the platform itself, a coupling to couple the platform to one or more submerged anchor points having a vertical position that is underwater but above a bed of the body of water, and coupled to the platform, at least one buoyancy chamber having variable buoyancy to control a depth at which the platform is positioned. The platform extends out laterally from the one or more submerged anchor points and the coupling permits vertical movement of the platform relative to the one or more submerged anchor points to allow the platform to be positioned at different depths underwater.

This approach provides a mechanism for providing variability in depth of the platform which can be implemented at relatively low cost, which is scalable to varying sizes, and which does not rely ropes, pulleys, gears or other mechanical moving parts positioned below the waterline. By coupling the platform to one or more submerged anchor points which are at a vertical position underwater but above a bed of the body of water, the platform can extend out from the anchor point laterally and is free to rise and fall to variable depths in response to changes in buoyancy of the at least one buoyancy chamber. Hence, control of the buoyancy of the at least one buoyancy chamber is sufficient to control the motion of the platform between different depths, avoiding the need for any more complex mechanical arrangement, and hence reducing installation and maintenance costs.

The platform could be used for supporting any kind of item to be held at a position underwater. For example the platform could be used to support sensors, cameras or other devices.

However, in one example the platform may comprise a marine agriculture platform for supporting growth of a marine agriculture product (for example, seaweed or kelp, oysters or mussels or other marine organisms, including animals and/or plants). For example, offshore marine farming of seaweed or kelp may be useful not only for harvesting of the crop itself, but also for capturing carbon for environmental reasons. There is an increasing interest in offshore marine agriculture to allow larger scale operation compared to marine agriculture in inshore waters, but this provides a harsher environment for the platform and the marine agriculture product. Providing a variable depth system may be useful for protecting the marine agriculture platform and its product from storms by allowing the platform to be lowered to deeper positions when required. When the storms have passed the platform can be returned to a shallower position in the water, for improved growing conditions. Also, variable depth options can be useful to allow the platform to be brought nearer to the surface to make seeding or harvesting of the agricultural crop more straightforward. Hence, the techniques used in this application are particularly useful for systems for mooring a marine agricultural platform.

However, the underwater mooring system could also be used for other fields of application, where again variable depth may be useful for protecting equipment from adverse weather and waves or for bringing the platform towards the surface to make maintenance of equipment more straightforward, for example.

The platform may be coupled between at least two submerged anchor points at different lateral positions within the body of water. In some cases, there may be more than two submerged anchor points. Providing more than two submerged anchor points can be useful to increase the scale of the underwater array and also can allow parts of the system to be held in tension between the anchor points which can be useful for positional stability.

For example, at least when the platform is at a minimum depth or maximum depth supported by the coupling, the coupling may maintain at least one of the platform and the coupling itself in tension between at least two submerged anchor points to restrict lateral movement of the platform. It may be that while the platform is being moved between different depths, then the coupling or the platform itself may go slack and so may no longer be in tension. Nevertheless, the spacing between the anchor points, and the size of the coupling and/or the platform may be chosen to ensure that the coupling and/or the platform is maintained in tension, which improves lateral stability.

The system may be configured such that, at least when the platform is positioned at a deepest position (maximum depth) supported by the underwater mooring, the at least one buoyancy chamber is fully submerged underwater. This is useful because if the buoyancy chambers are fully submerged then this means there are no portions poking up above the surface which may susceptible to being buffeted by wind for example, which improves lateral stability of the platform. Also, having submerged buoyancy chambers typically means that the buoyancy chambers may be smaller structures which do not need to extend from the submerged position all the way up to above the surface, so that the cost of manufacturing and fitting the system is lower and the buoyancy chambers also cause less of an obstacle to surrounding items.

Nevertheless, in other examples it may be possible for some buoyancy chambers to occasionally protrude above the water surface (at least when the platform is positioned at minimum depth below the surface).

In some examples the platform may comprise an interconnected array of platform modules, each platform module coupled to the one or more submerged anchor points either directly, or indirectly via at least one neighbouring platform module of the array. The interconnected array of platform modules could be arranged in different topologies, for example a linear array with all the platform modules connected in a line, a ring of platform modules in a circular or substantially circular arrangement, or a rectangular or square array defining a grid of interconnected platform modules. By providing an interconnected array of platform modules, this makes it relatively straightforward to scale the array to different sizes depending on the area of the body of water to be covered by the array, which can be particularly useful for applications such as marine agriculture where the installer may wish to tailor the size of the array based on the space available. This can allow the system to scale to potentially large areas (e.g. tens to hundreds of hectares) for example.

The buoyancy chambers could be coupled in different ways to the platform or to particular platform modules of the platform. For example it could be useful to provide multiple buoyancy chambers at opposed corners of the platform or at opposed corners of individual platform modules to allow the platform or platform module to be aligned substantially horizontally within the water with substantially equivalent buoyancy at opposite corners for example. In some examples, each platform module could have at least one buoyancy chamber attached to it. Alternatively, depending on the rigidity of the platform modules and/or any couplings between platform modules it may not be essential for every platform module to have a buoyancy chamber and some platform modules could share buoyancy chambers depending on the design. Also, some platform modules may have multiple buoyancy chambers. The number of buoyancy chambers may also depend on the relative size of the buoyancy chamber compared to the amount of positive or negative buoyancy required to reach the desired positions within the body of water, and depending on the expected water pressures at those positions for the particular application.

In one example, at least two platform modules of the array could have respective buoyancy chambers with independently variable buoyancy to permit different platform modules to be positioned at different depths. This could be useful, for example, in sheltered waters so that the deeper depth position could be set for growth of marine agricultural products on some platform modules of the array while other modules of the same array could be raised up to shallower depths for harvesting or seeding operations for instance. For example, the control infrastructure for varying the buoyancy in the respective buoyancy chambers could include independent ducts for separately controlling pumping of a fluid in and out of the buoyancy chambers to permit independent control of depths of different modules.

Other examples may not choose to implement independently variable buoyancy, and may provide a control infrastructure that raises and lowers all of the buoyancy chambers together so that all the platform modules could remain at the same depth as each other, but the platform as a whole is positionable at different depths.

The coupling may be a flexible coupling. For example the coupling could include at least one rope, cord, tape or chain which can flex as the platform moves between positions at different depths in response to changes of buoyancy of the at least one buoyancy chamber.

The coupling can be implemented in different ways. In one example, for at least one of the one or more submerged anchor points, the coupling may couple the platform to a fixed position on that submerged anchor point. In this case, there may be no freedom for the end of the coupling that is fixed to the submerged anchor point to travel along or relative to the submerged anchor point. In this case, the range of depths supported for the platform may be limited (at least in part) based on the length of the coupling and the elasticity of the material used to form the coupling (the range of depths could also depend on other factors such as spacing between adjacent anchor points which may control the tension in the coupling).

In other examples, for at least one of the one or more submerged anchor points, a guide line may extend from that anchor point towards the surface of the body of water or towards the bed of the body of water, and the coupling may be fixed to the at least one guide line by a moveable fixing which permits the coupling to travel along the guide line when the platform rises or falls in response to changes of buoyancy of the at least one buoyancy chamber. For example, one end of the coupling could include a ring which has an aperture through which the guide line passes. This approach could help to reduce the spacing required between neighbouring platform modules which could help make more efficient use of space in a given area in the body of water. The guide line may comprise at least one stopper to prevent passage of the moveable fixing beyond the stopper, to limit a range of depth within which the platform can be positioned while moored by the system.

The submerged anchor points can be implemented in different ways. In some examples, the submerged anchor point could simply be a position on a taut guide line which extends vertically between shallower and deeper positions within the body of water. For example such a guide line could be extended between the bed of the body of water (or a suspended block raised up from the bed) and a buoy near or on the surface, and the coupling for the platform could be coupled to a position on the guide line partway along it to suspend the platform at an intermediate depth with freedom to rise up and down in response to changes of buoyancy.

In other examples, at least one of the anchor points could comprise a post embedded into the bed of the body of water, with the coupling fixed to a point on the post raised up from the bed of the body of water.

In other examples, the submerged anchor point could comprise a suspended anchor block of solid material which is suspended under water above the bed of the body of water. Here, the term "block" does not imply that the block needs to be a cuboid or a particular shape. The block could be any shape, including irregular shapes. Such a suspended anchor block can provide a more solid attachment for coupling the platform, and can be anchored to the bed of the body of water, to provide increased lateral stability for the platform. An advantage of using such a suspended anchor block is that it is relatively simple to install and to remove if it is desired to uninstall the mooring system, compared to more permanent structures such as posts driven into the bed of the body of water.

In some examples, the suspended anchor block could be a negatively buoyant block suspended from a surface buoy, vessel or fixing and anchored by an anchor line extending to the bed of the body of water. Alternatively, the suspended anchor block could be a positively buoyant block fixed to the bed of the body of water by a cable or line maintained in tension by positive buoyancy of the suspended anchor block. Here, a "negatively buoyant" block refers to a block whose average density is greater than the density of the surrounding fluid and a "positively buoyant' block refers to a block whose average density is less than the density of the surrounding fluid. Hence it will be appreciated that the suspended anchor block could be implemented either as a relatively weighty block suspended from a position near the surface of the water or as a relatively light block rising up from the bed of the body of water and held in position by a cable or line fixed to the bed of the body of water.

In some examples, at least one of the submerged anchor points comprises an oil platform or oil rig, offshore turbine or other offshore structure. Hence, while the other examples discussed above allow installation of the system in open water where there is no existing structure, in locations where there is already some kind of offshore structure installed, the existing structure could be reused and the platform can be positioned around that structure to reduce the number of additional anchor points that need to be newly installed. Hence, the anchor points themselves need not necessarily be part of the underwater mooring system itself as the coupling could be attached to existing submerged anchor points that are already available.

In some examples, the underwater mooring system may include at least one access providing line, e.g. a flexible line, such as a rope, cord, tape or chain. The access providing line may pass through a guide element fixed to a point on the platform, where the guide element allows the access line to travel through the guide element relative to the platform.

For example the guide element could be a ring, fairlead or other element which fixes the access line to the platform to prevent the access line drifting off from the platform but nevertheless allows movement of the access line through or along the guide element. The access providing line extends towards a surface of the body of water and has a surface-accessible portion for pulling up the access line to gain access to one or more items connected to the access providing line. For example the surface-accessible portion could itself be located near or at the surface, or could be connected by a further line to a location near or at the surface. In general, the access providing line may be arranged so that a person or machinery located at the surface is able to obtain hold of the surface-accessible portion or a part connected to the surface-accessible portion, so that the access providing line can be pulled up towards the surface. The access providing line also has at least one weighted end to pull the access providing line back towards the platform when the access providing line is released at the surface. In some examples only one end of the access providing line could be weighted, while in other examples both ends of the access providing line could be weighted with the surface-accessible portion being at an intermediate point of the access line (e.g. this could allow the same access providing line to be shared between two adjacent platform modules with the weighted ends causing both ends of the line to drop down towards their respective platform modules).

With this approach, it is possible to gain surface access to items connected to the access providing line without needing to dive down to the level of the platform and without needing to raise the platform itself, which can be useful seeding or harvesting operations for a marine agriculture application of the underwater mooring system or for accessing sensors, cameras or other devices attached to the access providing line, for example.

The platform may be made of different materials and can be implemented in different ways. For example, in some cases a platform may comprise at least one rigid support structure. For example the platform could comprise a board, surface, grid, grate, truss structure or framework of rigid material, which can provide structural support for supporting the weight of an object, organism or person on the platform.

However, in some examples it can be useful for the platform to comprise at least one flexible support. For example the platform may include one or more ropes, cords, lines, chains or nets. Providing flexible support as the platform can be useful because this may make it less expensive to launch and recover the platforms when installing the mooring system or uninstalling it. By providing flexible modular cells for the platform modules, this allows platform modules which can be laid down from a drum or reel, which simplifies installation. When a flexible support is provided as the platform, it can be useful to hold the platform in tension between two or more submerged anchor points (at least when the platform is in a stable position at the minimum or maximum depth supported), to provide lateral stability.

It will be appreciated that, in some examples, the platform may itself be the object or item of interest that is desired to be supported underwater (e.g. where the platform is a marine agriculture platform providing a support for growth of seaweed or other agricultural product). However, in other examples, the platform may merely be a support structure and then other objects which are actually the functional components of interest (e.g. sensors or cameras) could then be attached to the platform. Hence it does not matter whether the platform is the object of interest in its own right or merely a support for other objects. Either option is encompassed in this application.

A controller may be provided to control motion of the platform between different depths solely by varying the buoyancy of the at least one buoyancy chamber. For example, the controller may control pumping of fluid in and out of the buoyancy chamber(s). As motion of the platform is controlled solely by buoyancy this means there is no need for any contacting moving parts such as gears or pulleys below the water line which could be difficult and expensive to maintain where they are prone to corrosion and fouling (seaweed and other debris sticking in the moving parts). Use of buoyancy variation as the sole mechanism for moving the platform between different depths means that the mechanical moving parts of the control system could be located above the surface (above the waterline) which may be easier to maintain.

In one example each buoyancy chamber may comprise one or more ducts to supply a working fluid less dense than water to the buoyancy chamber or expel the working fluid from the buoyancy chamber. Although any working fluid less dense than water could be used, to reduce cost it can be useful to use air as the working fluid. In some cases, a given buoyancy chamber may have two separate ducts, one for supplying working fluid and one for expelling working fluid. At least one opening (e.g. a grate or set of holes in a wall of the buoyancy chamber) may be provided in the buoyancy chamber to allow transfer of water between the buoyancy chamber and its surroundings in response to changes in pressure of the working fluid within the buoyancy chamber, as controlled by the at least one duct. Pumping equipment may be provided for controlling the regulation of pressure of working fluid supplied to the buoyancy chamber(s). In some examples this pumping equipment may be located above the surface of the body of water which may be easier to maintain, although it would also be possible for the pumping equipment to be located at least partially below water.

More detailed examples are discussed below, for providing a mechanism which achieves the variability in depth, in which the depth of the platform is variable beneath the surface using only buoyancy forces, no ropes, pulleys, gears etc. are required for locomotion, and control of buoyancy is capable of remote or autonomous operation. The design of a variable depth underwater mooring system is capable of being deployed on a large scale (10s to 100s of hectares) for marine agriculture or other applications.

It provides improvements over current subsurface mooring systems such as those used for wind turbines and tidal turbines by providing a stable platform underwater which can have a variable depth from the sea surface (many typical subsurface mooring systems only support a single depth). For example, tidal turbines are sometimes suspended within the water column by sinking the unit and attaching it to fixed anchoring points on the sea bed. Once the anchors are attached the turbine is de-ballasted and becomes positively buoyant, raising it towards the water surface. The turbine is held by the tension in the anchor lines, creating a fixed location within the water column. It is not possible to vary the depth at which the turbine is mounted, as this is determined by the length of the anchor lines.

Floating onshore wind turbines are anchored using either catenary anchors, or tension anchors. Catenary anchors work in the same way they do for ships, with long lengths of chain on the sea bed between the anchor and the turbine. As the turbine is pulled by wind/tide/waves, more anchor line is lifted off the sea bed from the direction under tension, creating more resistance to the lateral forces. This allows a high degree of positional stability, preventing the turbine from drifting off station. Floating oil platforms also use catenary anchor systems for positional stability. However, again this does not support at multiple selectable depths underwater.

Semi-submersible oil platforms and heavy lift ships consist of a single, large, buoyant structure, which varies its ballast to achieve the correct depth. These rely on penetrating the water surface and are very large, complex and expensive. In contrast, the technique discussed below is much smaller in scale and less costly to manufacture, install and uninstall. Unlike such semi-submersible system, the variable depth system described below does not rely on penetrating the surface, provides a massive reduction in structure required between buoyancy chambers, and has no concerns over lateral stability (typical ship considerations for seakeeping). Separate structures gives global flexibility so operation of large arrays in waves is possible. The rigid part of the design is submerged so forces due to waves are reduced. There are no moving parts (gears, pulleys, valves etc.) below the waterline, and the design can be easily scaled up by designing arrays of various dimensions.

In the examples below, variable depth on subsurface structures is achieved by creating a tensioned set of anchor points from which the movable structures are mounted. This allows the movable units to move within fixed limits using ballast/buoyancy chambers only. Variations in the design cover a number of different applications from shallow water to deep water as well as independent mooring sites or using existing infrastructure such as offshore rigs or wind turbines. Design of details such as harvesting and seeding operations is included.

This is particularly useful for the field of offshore marine agriculture -e.g. growth of oysters, mussels, seaweed or other products in a marine environment. These products are typically farmed in inshore waters which are protected, but scalability can be limited due to restricted areas. Costs of farming in inshore waters are higher as the waters fall under the Local Authority and rents are chargeable. Offshore farming allows more freedoms and potential for larger scales but is a harder environment to operate within. A variable depth mooring system is useful to be able to protect the array and crop from storms and waves on the surface which tend to be more severe than in-shore waters where marine agriculture typically occurs.

Figure 1 shows an example of the system in use when installed in a body of water. A catenary style chain (anchor line 1) is secured at its end with an anchor, which provides limited transverse movement while allowing the platform 7 to rise and fall with the fide/waves. A suspended anchor point 2 is held vertically in tension between a surface buoy or barge 3 and the anchor chain 1. Lateral stability is provided by providing at anchors at opposing corners and sides of the array (Figure 1 only shows the anchor on one side of the array for conciseness), which produce lateral tension which is carried through flexible connections 30 (such as ropes, cables or chains) between the suspended anchor points 2 (the flexible connections 30 are shown in Figure 2). The surface buoy/barge 3 may be fitted with location beacon, radar reflectors, communications equipment etc. Also, the surface buoy/barge 3 may house the pump room for ballasting/de-ballasting the buoyancy chambers 5 of the platform 7. A flexible connection (coupling) 4 (e.g. rope or chain) allows the platform 7 to rise/sink freely. This connection will go slack during ballasting operations and be taut again when the platform is fully raised/sunk. Ballast tanks (buoyancy chambers 5) are used for raising/lowering the platform 7. These are kept separate to maintain the level of the platform. The variable depth of the platform is indicated by arrows 6, showing transition between a raised position and lowered position.

Figure 2 shows the modular capability of the design, allowing arrays to be combined in any number of units from a single platform to a linear array of platform module 70 or rectangular array of platform modules 70. The isometric view helps to illustrate how the whole array will be kept in tension by the opposing sides.

Figure 3 shows the plan view of an array of platforms with the buoys 3 (and hence the anchor points 2 below the buoys 3) as circles and the platform modules 70 as squares, connected by the flexible coupling lines 4. The suspended anchor points 2 are connected by flexible connections 30, shown by dotted lines. This shows that the individual platforms are separated, and if separate ducting is provided to enable independent control of pumping of fluid in and out of the buoyancy chambers 5 for different platform modules 70, the platform modules can therefore be raised/lowered independently if required.

Figure 4 shows a plan view of a rectangular array of platform modules suspended between anchor points 2 by flexible couplings 4. Figures 5 and 6 show isometric and elevation views of the rectangular array of platforms shown in Figure 4. Figures 4-6 show how tension is created throughout the array by anchoring opposing sides and corners appropriately. This ensures reduced movement in tidal flows as the excess anchor chain 1 on the seafloor is lifted creating extra resistance. As shown in Figure 5, it is possible to raise a first subset 71 of the platform modules 70 to shallower depth while a second subset 72 of the platform modules remain at a deeper position, by independent control of the air pressure supplied to different buoyancy chambers 5.

Figure 7 shows an example of the ballasting system using open bottomed ballast tanks (buoyancy chambers 5). Each chamber 5 has tank walls 14, ducts 8 including an inlet duct 12 and an outlet duct 13, and an opening (e.g. grate) 10 in a base wall of the chamber allowing the water 11 to be pushed out when air (working fluid 9) is pumped in from above (in other examples the opening 10 could be in a side wall). The water is also free to flood back in when the air is released from the top of the tank. Due to the open nature of this arrangement, a variable buoyancy system is possible using an appropriate control system, working in a similar way to a buoyancy control device used in Scuba diving, as the air will be compressed as it descends to deeper depths (the increased pressure being needed to compensate for increased water pressure at deeper depths), and expand as it rises to lower depths. This also illustrates that no net pressure will be exerted on the tank walls 14 as the working fluid and surrounding water are at the same pressure, determined by the hydrostatic head, so the structure required will be minimal as there is no risk of crushing/bursting by holding high/low pressure fluid. Air inlet and outlet pipes connected to the ducts 12, 13 run up to the top side so all moving parts in the pumping equipment can be out of the water.

Hence, water is driven out through permanently open grate 10 in the bottom of the ballast tanks 5 when air (working fluid 9) is pumped in. \Mien air outlet 13 is released on the topside (therefore at atmospheric pressure), hydrostatic pressure drives the air out and the ballast tank 5 fills with water 11 through the grate (opening) 10.

Figure 8 shows an example of a harvesting system for use with seaweed or kelp growing lines. The kelp grows up from the seabed in its natural environment and this is recreated by attaching the kelp 29 to growing lines 15 which rest on the platform 7. When it is time to harvest the lines can be lifted using a guide line 16 attached to a small surface buoy 17. The growing lines feed through a fairlead (an example of guide element) 18 at the edge of the platform 7 which is weighted at the end (weighted end 19), and has a stopper 20 placed near the section of the line to be seeded. Once the crop is seeded or harvested and the line is released, the weight 19 pulls the line back through the fairlead 18 and reaches the stopper 20, positioning the line on the platform. If periodic maintenance is desired to remove growth from a portion of the line near the weighted end 19 which remains at the platform when the surface-accessible part of the growing line 15 is brought up to the surface, then it may be possible to provide a second guide line extending from the weight 19 to the buoy to allow the weighted end to also be accessed for maintenance. In use, as shown on the right hand side of Figure 8, a harvesting vessel picks the small buoy 17 up off the water surface and winches up the guide line 16 to retrieve the kelp line 15 for harvest. Upon release, the weight 19 at the end of the line pulls the kelp line back into position.

In Figure 8, the kelp growing line 15 is an example of an access providing line which has a surface-accessible portion and at least one weighted end 19. It will be appreciated that such an access providing line, which is accessible from the surface by pulling up a surface buoy 17 attached to the accessing providing line, can also be used to provide access to other items attached to the access providing line 15, such as sensors or cameras or other equipment.

In the examples, the anchor points use suspended blocks which are independently anchored within the water, without needing any rigid support structure to be permanently or semi-permanently mounted in the water.

However, as shown in Figure 9, potential array arrangements could also use a repurposed offshore oil platform 21 as its central anchor point 2. The outside of the array can be anchored to maintain tension, but the primary support will come from the rig itself Other examples could use other offshore structures such as turbines, oil rigs, pylons, etc. Figure 10 shows another way of implementing the coupling between the platform 7 and the anchor points 2. In this example, rather than using a flexible coupling which couples to a fixed point on an anchor block, a taut vertical guide line 22 is run between the submerged anchor point 2 and a surface buoy 25 (which could be the same buoy that houses pumping equipment etc. as discussed above or could be a separate buoy), and the growing lines 15 (acting as the platform 7 in its own right in this example, without any further rigid structure) are suspended between the guide lines. The growing lines/platform 15, 7 have large fairleads (a moveable fixing 23 / coupling 4) at the end which are free to rise and fall along the guidelines between stoppers 24. Multiple guide lines can be used for a single anchor point to allow the individual growing lines to be raised/lowered independently. The buoyancy chambers are not shown in Figure 10 but may be attached to the growing lines 15.

Figures 11A-11C shows the comparison between the flexible coupling and guideline arrangement. Figure 11A shows the flexible coupling 4 with the array tension pulling the platform 7 out laterally so that it rests with the flexible coupling 4 at an angle. Figure 11B shows an example where spacing between adjacent anchor points of the array is compressed, so that the coupling 4 rises and falls along a vertical path. Figure 110 shows the progression to a guideline 22 arrangement, where the coupling now couples the platform to the guideline 22 using a fairlead or other movable fixing 23 which is attached to the platform 7 and is free to move between two fixed points 24.

Figures 12A to 12D illustrate a development to allow reduced costs in build and launching the design. The platform 7 can be made to be flexible, being constructed of ropes, cables or chains so that it can be launched like a large net, before being tensioned between the anchor points to provide the stable platform. Figure 12A shows launch from reel 27 on a launch vessel 26. Figure 12B shows spreading out the platform to ensure the flexible platform is free of tangles. Figure 120 shows stretching of the flexible platform to the anchor points. Figure 120 shows securing the platform 7 against the anchor points 2 to create a stable platform. Hence, as rigid cells may be more expensive to launch and recover, the approach shown in Figure 11 allows flexible cells to be laid from a drum (similar to the laying of cable, fish nets, chain etc.).

Figure 13 shows an alternative anchoring arrangement. Rather than using catenary anchors which rise and fall with the tide, anchors can be screwed in the seabed (similar systems are used on wind and tidal turbines). From the secure anchor point (screw anchor 31) the suspended anchors points 2 are made positively buoyant, providing tension between the seabed anchor 26 and the suspended anchor point 2. From the suspended anchor point a buoy 25 (at or just below the surface) is attached which creates the taut guideline 22. A location buoy 28 can be floated to the surface if buoy 25 is underwater (keeping the buoy 25 underwater can help with stability as it prevents exposure to wind or wave action). The platform 7 can be attached to the guideline 22 via fairleads (movable fixing 23), or the suspended anchor point 2 could be used with the flexible coupling 4 as shown in Figure 1.

Figure 14 shows the implementation of the guideline 22 system using multiple guidelines 22 from a single fixed point 24 so that a rectangular array of platform modules 70 can be constructed where each platform module 70 is free to move independently of each other. The platform modules 70 are guided by the fairlead or other moveable fixing 23 along the guidelines 22.

Figure 15 shows a plan view of an array of guideline 22 arrangements, created using four guidelines 22 per single fixed point 24, with platform modules 70 guided by fairleads or other moveable fixings 23. The left hand side of Figure 15 shows a zoomed in view to show that the platform modules 70 are independent and can therefore be independently actuated if so required.

In all the examples discussed above, a control barge may be provided containing the pumping equipment, control systems, and communications equipment.

In summary, a mooring system is designed with a stable, underwater platform 7 for marine agriculture or other applications. This allows multiple depths to be used. Some examples may support only two different depths. Other examples may support three or more depth settings by varying the buoyancy of the ballast tanks 5 appropriately.

Contacted moving parts (e.g. pulleys and ropes, gears, valves etc. which are susceptible to fouling and corrosion) are removed from below the waterline.

Features include: * Variable depth mechanism -e.g. see Figure 1.

* Suspended platforms within modular cells -see Figures 2 and 3.

* In some examples, rigid, square platforms 7 are supported by suspended anchor points 2 and the whole platform rises and falls with ballast system 5.

* Alternatively, flexible platforms could be provided to reduce cost and complexity of installation.

* Guide lines 22 can be used to couple the platforms to the anchor points 2, as shown in Figures 10 and 11A-11C. The guide lines 22 may be kept under tension using buoyancy. The guide rings 23 which run up and down the guideline may be large enough that they are not compromised by fouling or corrosion (allowing passage of weeds or other debris through the guide ring 23).

* In the guide line examples shown in Figures 10 and 11A-11C, all lines may be connected so all need to be submerged or raised simultaneously. Alternatively, it is possible to use multiple guide lines from a single anchor point to allow individual cells of array to raise and fall.

* A harvesting line attachment (access providing line 15) may be provided as shown in Figure 8. In some examples, a rail may be fixed around the edge of the platform modules for such access providing lines 15 to be attached to. If the lines are fixed in place and not surface-accessible, this is not easy for harvesting. Hence, fairleads (guide elements 18) may be provided to allow raising growing lines 15 for harvesting, as shown in Figure 8. The growing lines 15 have weighted ends 19 and a guy line 16 runs to a buoy 17 on the surface. The surface buoy 17 is hooked, and can be reeled in to raise the growing line and plant/harvest the crop. Wien released, the weighted end pulls the end of the growing line through the fairlead until it reaches a stopper 20.

* The array can be held in tension between anchor points, as shown in Figures 4-6. By anchoring opposing sides and corners of the array with appropriate spacing between the anchor points, tension is created throughout the array. This anchoring style ensures reduced movement in tidal flows as the excess anchor chain 1 on the seafloor is lifted increasing the force opposing the tidal flow force.

* The array of platform modules can be implemented in different topologies or arrangements, e.g.: o Linear arrays as shown in Figure 10, with individual arrays can be laid in lines. This works well for shallower waters where the chances of the array drifting are decreased due to short anchor lines.

a Rectangular arrays (square or non-square) as shown in Figures 2 and 3. This can provide stable positioning even in deep water (km deep). Anchor chain/cable/rope can be selected appropriately to not create too much weight pulling the array down in deep water.

* Ballast system including active buoyancy control, as shown in Figure 7. Fully ballasting or deballasfing the tanks allows the platform to move from one position to the other. Active buoyancy control can be achieved similar to BCD use in SCUBA diving. Control systems for buoyancy control may use depth sensors and pump control over individual ballast tanks.

* Control barge equipment may be stored above the water line for reliability and ease of maintenance, including: o Pump system including valves and control systems.

o Communication equipment.

o Radar reflectors.

* Variants: o Anchoring: * Catenary anchors.

* Tension anchors.

o Depth change mechanism: * Flexible coupling from a single suspended anchor point.

* Ring translates along guide wire.

* Applications: o Independent: moored entirely independent of other structures.

o Offshore oil platform repurposing (Figure 9): use offshore oil platform 21 as a central anchoring point 2 and build the array around it. Tensioning anchors provided at the perimeter to maintain the correct tension across the array for positioning and depth control. Repurposed rig can be used as a processing and maintenance base, reducing the frequency required of supply, crew, and cargo transfers.

o Combined with wind farm: stretch the platforms between wind turbines in an offshore wind array.

Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims.

Claims (23)

1. CLAIMS1. An underwater mooring system for mooring a platform underwater at variable depth within a body of water, comprising: the platform; a coupling to couple the platform to one or more submerged anchor points having a vertical position that is underwater but above a bed of the body of water, the platform extending out laterally from the one or more submerged anchor points, the coupling permitting vertical movement of the platform relative to the one or more submerged anchor points to allow the platform to be positioned at different depths underwater; and coupled to the platform, at least one buoyancy chamber having variable buoyancy to control a depth at which the platform is positioned.
2. 2. The underwater mooring system according to claim 1, in which the platform comprises a marine agriculture platform for supporting growth of a marine agriculture product.
3. 3. The underwater mooring system according to any of claims 1 and 2, in which the platform is coupled between at least two submerged anchor points at different lateral positions.
4. 4. The underwater mooring system according to claim 3, in which at least when the platform is at a minimum depth or maximum depth supported by the coupling, the coupling is configured to maintain at least one of the platform and the coupling in tension between the at least two submerged anchor points to restrict lateral movement of the platform.
5. 5. The underwater mooring system according to any preceding claim, in which, at least when the platform is positioned at a deepest position supported by the underwater mooring system, the at least one buoyancy chamber is fully submerged underwater.
6. 6. The underwater mooring system according to any preceding claim, in which the platform comprises an interconnected array of platform modules, each platform module coupled to the one or more submerged anchor points directly or indirectly via at least one neighbouring platform module of the array.
7. 7. The underwater mooring system according to claim 6, in which at least two platform modules of the array have respective buoyancy chambers with independently variable buoyancy to permit different platform modules to be positioned at different depths.
8. 8. The underwater mooring system according to any preceding claim, in which the coupling is flexible.
9. 9. The underwater mooring system according to any preceding claim, in which, for at least one of the one or more submerged anchor points, the coupling couples the platform to a fixed position on that submerged anchor point.
10. 10. The underwater mooring system according to any preceding claim, in which, for at least one of the one or more submerged anchor points, a guide line extends from that submerged anchor point towards a surface of the body of water or towards the bed of the body of water, and the coupling is fixed to the at least one guide line by a movable fixing permitting the coupling to travel along the guide line when the platform rises or falls in response to changes in buoyancy of the at least one buoyancy chamber.
11. 11. The apparatus according to claim 10, in which the guide line comprises at least one stopper to prevent passage of the movable fixing beyond the stopper to limit a range of depth within which the platform is positionable.
12. 12. The apparatus according to any preceding claim, in which at least one of the one or more submerged anchor points comprises a suspended anchor block suspended underwater above the bed of the body of water.
13. 13. The apparatus according to claim 12, in which the suspended anchor block comprises a negatively buoyant block suspended from a surface buoy, vessel or fixing and anchored by an anchor line extending to the bed of the body of water.
14. 14. The apparatus according to claim 12, in which the suspended anchor block comprises a positively buoyant block fixed to the bed of the body of water by a cable or line maintained in tension by positive buoyancy of the suspended anchor block.
15. 15. The apparatus according to any of claims 1 to 11, in which at least one of the one or more submerged anchor points comprises an oil platform or rig, offshore turbine or other offshore structure
16. 16. The underwater mooring system according to any preceding claim, comprising an access providing line passing through a guide element fixed to the platform and extending towards a surface of the body of water, the guide element allowing the access providing line to travel through the guide element relative to the platform; the access providing line having: a surface-accessible portion for pulling up the access line to gain access to one or more items connected to the access providing line; and at least one weighted end to pull the access providing line back towards the platform when the access providing line is released at the surface.
17. 17. The apparatus according to any preceding claim, in which the platform comprises at least one flexible support.
18. 18. The apparatus according to any of claims 1 to 16, in which the platform comprises at least one rigid support structure.
19. 19. The apparatus according to any preceding claim, comprising a controller to control motion of the platform between different depths solely by varying the buoyancy of the at least one buoyancy chamber.
20. 20. The apparatus according to any preceding claim, in which each buoyancy chamber comprises: one or more ducts to supply a working fluid less dense than water to the buoyancy chamber or expel the working fluid from the buoyancy chamber; and at least one opening to allow transfer of water between the buoyancy chamber and its surroundings in response to changes in pressure of the working fluid within the buoyancy chamber as controlled via the one or more ducts.
21. 21. The apparatus according to claim 20, comprising pumping equipment for controlling the pressure of the working fluid in the at least one buoyancy chamber.
22. 22. The apparatus according to claim 21, in which the pumping equipment is located above the surface of the body of water.
23. 23. A method for variable depth mooring of a platform underwater within a body of water, comprising: the platform being coupled to one or more submerged anchor points having a vertical position that is underwater but above a bed of the body of water, the platform extending out laterally from the one or more submerged anchor points, the coupling permitting vertical movement of the platform relative to the one or more submerged anchor points to allow the platform to be positioned at different depths underwater; and varying buoyancy of at least one buoyancy chamber coupled to the platform, to control a depth at which the platform is positioned.