Subsea substation

Abstract

A submersible substation 103 for use in relation to an offshore energy generation plant, the submersible substation 103 comprises a substation support structure 105; one or more substation components

Description

FIELD OF THE INVENTION The present invention relates to the field of offshore electrical power generation and, more particularly, to the management of electrical power generated by an offshore power generation plant. More particularly, the present invention relates to a submersible substation for use with an offshore electrical power generation plant, to methods of assembly, installation and maintenance and to submersible support structures for installation of underwater infrastructure. BACKGROUND OF THE INVENTION With the increasing call for low carbon renewable energy and the need for larger generation capacity, along with an increasing intolerance for onshore wind generation development, there is a drive for offshore renewable energy generation solutions, including tidal, wave and wind. Offshore wind farms, arrays of wind turbines, are already installed using offshore wind turbine generators manufactured with rated powers of 5MW to 10MW and more. Such offshore wind farms or plants may have rated powers of 100 MW or more. To reduce losses and enable efficient transport of generated energy to the shore, such offshore wind farms may be provided with an offshore substation connected by cable to each of the wind turbines in the array (array cables) and connected by an export cable to transfer the power to an electric grid onshore, usually to a shore-based remote substation. An offshore substation typically includes transformers, switchgear and RCDs, for performing various operations on the generated electrical energy and to facilitate its safe and efficient transport to shore. These substantial and heavy components are typically installed on a raised platform in the vicinity of the windfarm, typically on a monopile or jacket, depending upon the water depth. As the developers of offshore wind farms explore opportunities in deeper waters (where winds are generally greater and more consistent) in which floating wind turbines may be deployed, the viability of a seabed piled monopile or jacket support structure diminishes (especially in depths of greater than 60 m). One solution to this problem is to locate substations on the seabed, such as is described in EP-A-2717401, which is intended to supply power to subsea devices and in WO-A-2023 / 110262, which collects power from power generation units for export. Such seabed-located substations represent a challenge in terms of the cost of deployment, requiring substantial ships and cranes and the difficulty and cost in recovery or servicing. In an alternative approach to address this problem, there is a move to develop floating substation structures, moored to the seabed. These may in principle be, for example, semi-submersibles, tension-leg platforms (TLPs), spar buoys or barges / ships. A challenge with establishing substations on such floating structures is the exposure of the substation equipment or components to significant movement caused by wave motion or currents (e.g. rolling and pitching, jolts etc) and the difficulties associated with connecting array cables and export cables to such an arrangement. One attempted solution to address these problems is described in WO-A-2024 / 134421. This aims to provide a floating support structure for an offshore substation that has improved stability and dynamic behaviour, thereby improving the installation of electrical cables. This is said to be achieved by providing a floating platform having a hull configured to receive a hydrostatic force from the bottom upwards when at least partially immersed in water, thereby lowering the centre of gravity and limit the shifts and accelerations of the platform arising from wave and tidal movements. Another example of a floating support structure for an offshore substation is described in WO-A-2017 / 074237, which is concerned with an array of wave energy convertors. Such floating platform solutions require significant weight for ballasting of the semi-submersible support structure and significant corresponding requirement for buoyancy and retail challenges in the management of dynamic stresses on both array and export cables. There is a need for improvements in offshore renewable generation substations, especially in deepwater renewable energy generation systems, and for support structures for substations that address the above challenges. The present inventors have identified a solution, by providing a modular and deployable submersible substation, that addresses the aforementioned shortcomings. PROBLEM TO BE SOLVED BY THE INVENTION It is an object of the invention to provide an improved offshore renewable energy generation array substation. It is an objection of the invention to provide an offshore renewable energy generation array substation having improved efficiency. It is an object of the invention to provide a substation and support therefore that has improved stability and reduces dynamic stresses on cables. It is an object of the invention to provide a method of installing and servicing a substation that is low cost and available in broad weather windows. It is an object of the invention to provide a system and method for the deployment of deepwater seabed infrastructure on a recoverable support structure. SUMMARY OF THE INVENTION In accordance with a first aspect of the invention, there is provided a submersible substation for use, preferably, in relation to an offshore energy generation plant, the submersible substation comprising: a substation support structure; one or more substation components disposed on the substation support structure and preferably one or more array and / or export cable connectors, wherein the substation support structure is configured for submersed deployment and to be raised and lowered using variable buoyancy control. In a second aspect of the invention, there is provided a submersible substation system comprising: a submersible substation as defined above; a service umbilical extending from the submersible substation to an umbilical connection member; and optionally, a seabed mounted cable collector. In a third aspect of tire invention, there is provided an offshore renewable energy generator plant comprising: a submersible substation system or a submersible substation as defined above; an array of offshore renewable energy generators connected to the submersible substation system or submersible substation by one or more array cables; and an export cable connector disposed on the submersible substation for export of power generated by the plant. In a fourth aspect of the invention, there is provided a submersible support structure for underwater infrastructure installation, the submersible support structure configured for disposal of one or more infrastructure components thereon, wherein the substation support structure is configured for submersed deployment and to be raised and lowered using variable buoyancy control. In a fifth aspect of the invention, there is provided a method of installation of a submersible substation in an offshore renewable generator plant, the method comprising: providing a submersible substation as defined above to a site of an offshore renewable generator plant; connecting a plurality of array cables and an export cable to the submersible substation, preferably via dry-mate connectors; adjusting the buoyancy of the support structure of the submersible substation to provide the submersible substation with a slight negative buoyancy, allowing it to descend to the seabed; and preferably, when at the seabed, adjusting the buoyancy of the support structure of the submersible substation to anchor it on the seabed. In a sixth aspect of the invention, there is provided a method of assembly of a support structure (e.g. for a submersible substation), the method comprising: providing one or a plurality of lower frame members defining recesses for a plurality of buoyancy elements; providing a plurality of buoyancy elements in the recesses of the lower frame member(s); providing one or a plurality of cooperating upper frame members having corresponding recesses for the plurality of buoyancy elements and fitting over the buoyancy elements into cooperation with the lower frame member(s); and securing the upper and lower frame members together to secure the buoyancy elements in place and form the structure. In a seventh aspect of the invention, there is provided a modular submersible support structure for the installation and retrieval of underwater infrastructure, the modular submersible support structure comprising an assembly of one or a plurality of framework components and a plurality of buoyancy elements, wherein the number or size of framework components and number or size of buoyancy elements may be adjusted according to the size and weight requirements of the underwater infrastructure components or equipment to be installed or retrieved, wherein the modular submersible support structure is configured for deploying infrastructure on the seabed and / or is configured to be raised and lowered using variable buoyancy control. In an eighth aspect of the invention, there is provided a method of installing an underwater infrastructure component on the seabed, the method comprising: providing a submersible support structure as defined above; loading or installing on the submersible support structure one or a plurality of underwater infrastructure components for installation at a seabed location; towing, or otherwise transporting, the submersible support structure to a site for infrastructure installation; adjusting the buoyancy of the submersible support structure to provide it with a slight negative buoyancy, allowing it to descend to the seabed; and preferably, when at the seabed, adjusting the buoyancy of the support structure to anchor it on the seabed. In a ninth aspect of the invention, there is provided a submersible substation for use, preferably, in relation to an offshore energy generation plant, the submersible substation comprising: a substation support structure; and one or more substation components disposed on the substation support structure, wherein the submersible substation further comprises one or any combination of the following features: a) the substation support structure is configured for submersed deployment and to be raised and lowered using variable buoyancy control; b) the substation components are housed in one or more water-impermeable enclosures or water-impermeable chambers, the or each enclosure or chamber being, optionally independently, cooled, e.g. by a heat exchange circuit comprising a heat exchange in thermal contact with the surrounding environment (e.g. seawater, when in situ); c) the submersible substation comprises one or more power cable connectors (e.g. array and / or export cable connectors) which are preferably dry-mate connectors; and d) the support structure is configured with an on-board propulsion system, optionally configured for autonomous transport and optionally configured with a self-deployment capability. ADVANTAGES OF THE INVENTION The submersible substation and method of installation of a submersible substation (or other underwater infrastructure) using a submersible support structure configured for submersed deployment and to be raised and lowered using variable buoyancy control, in accordance with the invention, enable the installation on the seabed of a substation (or other underwater infrastructure) in a controlled manner such that is recoverable for servicing and can be towed to site and installed in position using small ships, such as a harbour tug, and where the buoyancy-controlled support structure is configured for simple and modular assembly according to desired size and buoyancy. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional schematic illustrating the deployment of a submersible substation according to one embodiment the invention, showing deployed and transit configurations; Figure 2 is a plan view schematic illustrating the submersible substation of Figure 1 in deployed configuration on the seabed; Figure 3 is a top view of the submersible substation of an embodiment of the invention with array and export cables connected; Figure 4A to 4G are a cross-sectional top view, and cross-sectional end views at each of five sections along the length, of an arrangement of cylindrical vessels forming a support structure of an embodiment of the invention; Figure 5 is a side view of two chambers of a submersible substation support structure as used in an embodiment of the invention; Figure 6 is a side schematic, section-view of a power control unit for use in a submersible substation of an embodiment of the invention, illustrating the interior chambers; and Figures 7A to 7F illustrate the steps in assembly of a submersible substation according to an embodiment of another aspect of the invention. DETAILED DESCRIPTION OF THE INVENTION The invention concerns a submersible substation and also a submersible support structure forming part of a submersible substation or for use as support structure for installation of underwater infrastructure. The submersible support structure will be described hereinafter in relation to the submersible substation, but it should be understood that, in the alternative, it may be utilized with (and is thereby described m relation to) other underwater infrastructure, where the context allows. A submersible substation according to the invention comprises a substation support structure (or submersible support structure), and one or more substation components (or other underwater infrastructure components) disposed on the substation support structure, where the support structure is configured for submersed deployment (e.g. on the seabed) and to be raised and lowered using variable buoyancy control. The support structure, therefore, is configured to float when loaded with the underwater infrastructure (such as substation components) and so can be used to transport the underwater infrastructure to an offshore site ready for deployment. Optionally, the support structured may be transported by towing using another ship, such as a harbour tug, or, in some embodiments, the support structure may be configured with its own propulsion system, which may enable autonomous transport and use. In this latter embodiment, the support structure may take on a self-deployment capability. The support structure is furthermore configured to be adapted in situ to adjust or vary its buoyancy so as to give the loaded support structure a negative buoyancy allowing the support structure and the underwater infrastructure (e.g. substation components or other bulky subsea payload or equipment) loaded and mounted thereto to descend to the seabed for deployment, in a controlled manner. The support structure may be used for any suitable marine infrastructure deployment application. Preferably, it is used for the underwater deployment of substation components. Thus, the submersible substation may find application in any offshore application requiring a substation, such as in providing power distribution to power-demand devices in the underwater environment (e.g. in association with oil and gas production and exploration). Hie submersible substation finds particular application in relation to offshore renewable energy power plant and in a preferred embodiment, therefore, the submersible substation comprises one or more array and / or export cable connectors. An offshore renewable energy power plant, for which the submersible substation finds particular application, may comprise of an array of renewable energy generators and a submersible substation system (which comprises a submersible substation). The renewable energy generators may be of any type or a combination of types. For example, renewable energy generators may be selected from one or a combination of tidal energy devices, wave energy devices, floating solar energy devices and wind energy generators. The wind energy generators (or wind turbines) may be seabed-mounted wind turbines or floating wind turbines. For each renewable energy generator in the array, there will typically be an array cable which transfers power generated from the renewable energy generator to a substation, which may be onshore but more typically for larger distances offshore or where there is a larger array, the array cables may connect to an offshore substation or substation system. An export cable may typically then transport generated power to a location for further use, typically to an onshore application (e.g. for grid electricity) via an onshore substation. A submersible substation system, e.g. for serving an offshore renewable energy power plant, may comprise a submersible substation and a service umbilical extending from the submersible substation to an umbilical connection member. Preferably, the submersible substation system further comprises a seabed-mounted cable collector. The seabed-mounted cable collector is a means for gathering or anchoring at least the array cables and preferably also an export cable (or export cables). There may be multiple cable collectors (or cable anchors), one for each or each sub-group of array cables served by the submersible substation, or, alternatively, a single cable collector configured to collect or anchor multiple (or all) array cables served by the submersible substation. Where multiple cable collectors or cable anchors are used, they are preferably in the same vicinity and orientated toward a similar or common point. The cable collectors or cable anchors serve to define a static point for each cable. The portion of each array cable that is further from the static point away from the renewable energy generator (i.e. between the static point and a terminal end of the array cable for connection to a substation) may be referred to as the dynamic array cable portion. Similarly, the portion of the export cable from the static point to its terminal end for connection to a substation may be referred to as the dynamic export cable portion. Those dynamic array cable portions and the dynamic export cable portions may individually or together be referred to as dynamic cable portions. Each array cable may have a terminal end for connection to a substation (e.g. the submersible substation). The or each export cable may have a terminal end for connection to a substation (e.g. the submersible substation). Preferably, each array cable and the or each export cable have cooperating connectors formed on the terminal ends thereof for forming connections with array or export cable connectors provided on the submersible substation. The cable connectors and cooperating connectors may together form a connection arrangement making an electrical connection. Any suitable connection arrangement may be used. The connection arrangement may be a wetmate connection arrangement or a dry-mate connection arrangement. In a preferred embodiment, dry-mate connectors are used (to form a dry-mate connection arrangement). An advantage of dry-mate connectors is that they are readily available in 66 kV and 132 kV and they are lower cost than wet-mate connectors. It is a particular benefit of the present invention, and in particular, the method of the present invention, that the connection can be made between the array cables and the submersible substation and the export cable and the submersible substation at the sea surface, using dry-mate connectors, prior to placing the submersible substation in position at the seabed. By making the connection at the surface, it is also possible to test the system is working at the surface before placing the submersible substation in location at tine seabed. In a preferred embodiment of the invention, the support structure comprises substation components mounted onto the support structure, preferably disposed in one or more water-impermeable enclosures. In one preferred embodiment, there is provided a cable connectormating system. According to this embodiment, there may be a cable connector housing on the support structure, for housing connectors (e.g. a male or female dry-mate or wet-mate connectors for connecting with cooperating connectors on the terminal ends of the array or export cables), which connectors are linked with appropriate parts of the substation components. Optionally, the housing is, aside from an opening, recess or socket bearing the cable connectors, a water-impermeable enclosure and optionally the same water-impermeable enclosure housing die substation components, or the housing is a discrete housing and houses the on-board cabling between the cable connectors and the substation components (optionally connected to the substation components through sealed apertures in the water-impermeable enclosures housing the substation components). A cable connector housing on the support structure preferably comprises a registration facing, which accommodates the cable connectors, typically orientated in the same direction and optionally in a parallel arrangement. The cable connector-mating system preferably further comprises a cradle arrangement for receiving cooperating connectors that are on the terminal ends of array and / or export cables. The cooperating connectors of the array / export cables may be arranged in the cradle arrangement to be presented in a manner for cooperating with the cable connectors of the cable connector housing and the preferred registration facing thereof, and preferably in a parallel arrangement. Once the cooperating connectors of the array / export cables are in the cradle arrangement, the cradle arrangement may be moved toward the cable connector housing until there is registration between the cradle arrangement and the registration facing (or between the cable connectors and cooperating connectors) and the cradle arrangement and cable connector housing pulled together until the cable connections are made. Optionally, the cradle arrangement maybe removed, leaving the cable connection arrangements secured, or, preferably the cradle arrangement may be secured to the cable connector housing to hold the cable connection arrangements in place (optionally with an emergency release mechanism also provided). Preferably, there is a guiding system for guiding the cradle arrangement during its movement toward cooperative connection of the cable connectors with the cooperating connectors (e.g. during movement of the cradle arrangement toward registration with cable connector housing and its registration facing), which guiding system is configured to ensure registration. The guiding system may be any suitable arrangement, such as rails or cables provided on the support structure along which the cradle arrangement is guided, or a funnel member for funneling the cradle into registration. The cradle arrangement may be moved toward the cable connector housing by any suitable mechanism. In one embodiment, the cradle arrangement may be winched toward the cable connector housing by a winch and winch-cable arrangement. The winch may be mounted on the cradle arrangement with a winch-cable connected to the cable connector housing, but preferably the winch is mounted on the support structure (e.g. on or within the cable connector housing, e.g. behind its registration facing) and one or more winch-cables extending therefrom to the cradle arrangement. Optionally, the cradle arrangement is buoyant, whereby it can be released from the support structure (e.g. by releasing or unwinding the winchcable) so as to float near the floating support structure to allow the cooperating connectors of array and export cables to be placed into the cradle, which may then be winched / hoisted aboard the support structure and into the guiding system. Onboard the support structure, there is optionally a faring for the cradle arrangement to be drawn up along to a prominent position for registration with the cable connector housing. Preferably, to facilitate onboarding of the cables and the associated cradle arrangement, a cable ramp is provided at one end or edge of the support structure. This serves to facilitate onboarding and reduces the risk of bending of cables. In a preferred embodiment, the dynamic portions of the array cables and preferably also export cable may be grouped or bundled together and secured in a bundle to form a dynamic cable bundle to facilitate handling. Optionally, the support structure may be provided with booster motors or orientation motors for facilitating the movement, directionality or orientation of the structure during surface movement and optionally also during subsurface movement. Preferably, however, the support structure is free of motorized propulsion mechanisms. Preferably, the support structure is provided with a towing cable to enable the support structure, when afloat, to be towed to site ready for deployment. The towing cable may also be used to guide the support structure during its descent / dive to its deployment site on the seabed. In use, the support structure may be loaded with underwater infrastructure (preferably substation components) for deployment, which is / are typically securely fixed to the support structure. In the case of substation components, the support structure, when loaded, can be considered a submersible substation (and will be referred to hereafter, although it should be understood that the substation components may be substituted, where the context allows, with other underwater infrastructure for subsea deployment). The submersible substation, having been loaded with its substation components at quay, may optionally be tested for functionality, both in terms of the substation componentry and the buoyancy control features (either at quay or nearby), and may be transported (e.g. towed via its towing cable, using, for example, a harbour tug, or other suitable ship) to its site for deployment. At site, a tug and submersible substation may be orientated in a direction away from which the cables are expected to join the submersible substation, which will typically be into (i.e. against) the prevailing wave / wind direction. The array and export cables may be connected (via their cooperating connectors at their cable ends) to the cable connectors provided on the submersible substation, for example by the method described above. For example, a dynamic cable bundle may be captured on a released, floating cradle arrangement and then winched on board the support structure to bring it into registration with a cable connector housing on the support structure and then to form a connection or connection arrangement, preferably by dry-mate connectors between the cable connectors and respective cooperating connectors on the terminal ends of the array / export cables. Once connected, optionally the operation of the submersible substation may be live tested prior to deployment. The support structure is then adapted to adjust or vary its buoyancy so as to give the submersible substation a slight negative buoyancy allowing it to slowly descend or dive toward the seabed. At the same time, the tug preferably provides tension (or, in the case of a self-propelled support structure, this tension is provided by propulsion by the support structure) by gently motoring in a direction of a proposed ‘landing strip’, being an area of seabed to a non-cable connection side (and preferably the direction opposing a cable connection side) of the submersible substation’s seabed deployment site, with the support structure under tow, in order to introduce tension to the dynamic portions of the array and export cables, or the dynamic cable bundle. By introducing tension into the array and export cables or the dynamic cable bundle, by which it is meant tension relative to its catenary (or relaxed) form, buckling and twisting of the cables can be avoided during descent. Once at the seabed, where due to maintaining the support structure under two (and the dynamic cable portions or dynamic cable bundle under tension), the submersible substation comes to rest at its deployment site, with the dynamic cable portions laying flat on the seabed surface between their respective static points and the submersible substation. Furthermore, the support structure may optionally be further adapted to further reduce its buoyancy to render it anchored to its site at the seabed. The towing cable used for towing the submersible structure and maintaining the system under tension during descent is preferably at least 1.25 times the depth of the deployment site and more preferably up to 3 times the deployment site depth, more preferably up to 2.5 times deployment site depth, more preferably in the range of 1.3 to 2 times, still more preferably 1.4 to 1.75 and most preferably 1.45 to 1.65 times, or about 1.5x. Upon completion of deployment, the towing cable may be released for storage on the seabed, by disconnecting it from the tug and lowering it to the seabed, preferably through a buoy line again motoring forward to keep the towing cable under relative tension. Hie tug-end of the towing cable may then come to rest on the seabed. Preferably, the tug-end of the towing cable has a weighted member attached thereto to act as an anchor. In order to raise the submersible substation (or support structure), the support structure on the seabed may be adapted to adjust or control its buoyancy so as to increase its buoyancy until it has a slight positive buoyancy, causing the submersible substation to begin to rise to the surface. Again, the ascending submersible substation will be maintained with the dynamic cable portions or dynamic cable bundle maintained under tension relative to its catenary form, which may involve a towing vessel (e.g. harbour tug) moving back, in the direction of the submersible substation during the ascent. Once the submersible substation returns to tire water surface, it may be serviced as need be, optionally disconnected from the array and / or export cables (optionally reversing the connection process mentioned above). The buoyancy may be controlled during both descent and ascent in order to maintain a desired rate of descent or ascent through the entire distance and / or to balance any tendency to pitch or roll. In case of emergency, an optional emergency release of the cable connection arrangement may be actuated. Any on-board functions, such as controlling of buoyancy, emergency release or any substation-related functions may preferably be controlled via a remote controller which may be situated on a surface vessel (such as a towing vessel). Communication between a remote controller may be by any suitable means, such as by wireless communication, but is preferably by way of a service (or control) umbilical. In a preferred embodiment, the towing cable and the service umbilical cooperate to form a tow umbilical, a single line that provides both the towing function and the umbilical providing service (e.g. a control of function) to the submersible substation (or support structure) while at the seabed or during ascent or descent, but optionally also at the water surface. Preferably, the service umbilical or tow umbilical has an umbilical connection member (or control connection mounting) at the tug-end thereof, or a connection manifold, which may optionally be in the form of a metal plate connector for mounting to a cooperating connector on the towing vessel (or tug) for providing registration with a controller connection on board the vessel and preferably also a towing cable connection. Preferably, the control connection mounting (or umbilical connection member) serves also as an anchor for anchoring the tow umbilical at the seabed. The support structure, when in situ at the water surface ready for deployment or when in situ on the seabed ready for recovery, may be adapted to change or vary' its buoyancy accordingly by any suitable means. For example, in order to reduce its buoyancy and reach a state of slight negative buoyancy, e.g. at the water surface, the support structure may be adapted by adding ballast or ballast elements to the support structure, which elements may be modularly attached, and / or by removing buoyant elements from the support structure which may be modularly attached. In order to increase its buoyancy, e.g. while at the seabed, in order to facilitate ascent and recovery of the submersible substation or support structure, the support structure may be adapted by dumping or releasing ballast (which may be achieved by actuating a ballast release mechanism) or ballast elements from the support structure, or by installing buoyant elements on the support structure. Such buoyant elements may be formed by, for example, causing a flexible container (or balloon, for example), or a plurality thereof, to be inflated from a compressed air cylinder which may be modularly attached. Preferably, however, the support structure’s buoyancy may be varied or controlled where the support structure comprises a plurality of buoyancy-control elements, the buoyancy of which may be changed or controlled, which elements are preferably in the form of buoyancy chambers, which may act, depending on the nature of their contents, to provide ballast or buoyancy, that is to enable the support structure to have negative buoyancy or positive buoyancy, as required. Preferably, the buoyancy chambers may be varied in their relative buoyancy by changing the contents thereof so that the average density of the contents increase (to reduce the buoyancy) or decreases (to increase the buoyancy). Therefore, preferably, the buoyancy chambers are configured for containing a material and for changing the material or the average density of the material within the buoyancy chamber. Preferably, this is achieved by substituting (or displacing) at least a portion of a first material having a first density within a buoyancy chamber with a second material having a second density, which is less than that of the first density, in order to increase the buoyancy of the buoyancy chamber, or vice versa, to decrease the buoyancy of the buoyancy chamber. Preferably, the first material is seawater. The second material may be any material having a density of less than seawater (at a given pressure - being the pressure at sea level or at which the support structure is deployed on the seabed or somewhere in between). For example, the second material may be fresh water, or more preferably, a compressible gas, such as air, nitrogen or carbon dioxide. Where the second material is a gas, it may be supplied to a chamber through a compressed gas conduit that may supplied through an umbilical from a compressor or compressed gas storage unit on a surface vessel (e.g. the towing vessel or tug), or it may be supplied from a compressed gas storage tank located on the support structure. The relative buoyancy of multiple buoyancy chambers on or attached to the support structure may be controlled to control or vary the buoyancy of the support structure in order to initiate ascent or descent or control the rate of ascent or descent, as well as, preferably, to maintain the balance of the support structure during ascent or descent against any undesired pitch or roll movements. The submersible support structure used in the submersible substation (or for use in deploying other underwater infrastructure) in accordance with the invention may take any suitable form, provided that its buoyancy may be controlled to facilitate descent and ascent to and from the seabed, and preferably provided that it comprises a plurality of buoyancy chambers, the contents of which may be varied in order to control or change the buoyancy of the chambers. For example, the support structure may comprise a solid core of any suitable shape, for example an elongate core, which may be made of or contain a ballast material, and have mounted on each of two or more sides (preferably at least on opposing sides) one or more buoyancy chambers for controlling the buoyancy of the device. There may be one buoyancy chamber on each side or multiple buoyancy chambers. Optionally, instead of a ballasted core, the core may comprise buoyancy chambers. The base of the support structure may be configured so that it may be rested on die seabed on a desired level. This may be achieved by having a flat bottom surface or a plurality of projections at a similar level, arranged to provide stability, or a plurality of legs on which the support structure may stand on the seabed. In a preferred embodiment, the support structure comprises a framework and a plurality of elements, preferably vessels, supported and linked by the framework. Optionally, the plurality of elements, preferably vessels, comprise one or a plurality of ballast elements or vessels and one or a plurality of buoyancy vessels. By ballast elements or vessels, it is meant elements or vessels made of or containing a material that is of greater density than seawater such that the average density of the element or vessel is greater than that of seawater so as to cause it to act as ballast on the support structure. By buoyancy vessels, it is meant a vessel having one or more buoyancy chamber, the buoyancy of which may be independently and variably controlled, for example so as to vary the average density of material within the buoyancy chamber to be the same as or less than that of sea water. Preferably, the support structure comprises a framework having one or a plurality of lower frame members defining recesses for the receipt of the plurality of elements or vessels and defining mounting projections and one or a plurality of upper framework members defining corresponding recesses and mounting projections, whereby the lower frame members may be loaded with a plurality of elements or vessels in the recesses and the upper frame members placed over the loaded elements or vessels in a cooperating manner and the corresponding mounting projections secured together to form the support structure. In one perspective, the framework may be considered to be an exoskeleton for the arrangement of elements or vessels (providing ballast and buoyancy). As well as simplicity in construction and facilitating modular assembly, the framework may serve to protect the buoyancy vessels from damage or puncture in use. In a preferred embodiment, the support structure is formed of a modular framework and / or modular elements or vessels, whereby the size of the support structure may be varied according to the size of the load which it is intended to transport and deploy and which may be varied in buoyancy according to the weight of the load which it is intended to transport and deploy. Preferably, the framework is such as to define a generally planar arrangement of recesses for forming a generally planar arrangement of elements of vessels. Optionally, there may be provided some vertically extended buoyancy vessels to provide improved stability in the event of a vertically projecting load (e.g. of substation components or underwater infrastructure) on the support structure. The support structure and any assembled framework forming the support structure may be of any suitable shape. For example, it could be circular, pentagonal, square or rectangular. The elements or vessels making up the support structure may be of any suitable shape. For example, they may be torus shaped (which in the case of a buoyancy vessel may have a plurality of buoyancy chambers) or spherical and would preferably have corresponding frame members to support and connect said elements or vessels. In a preferred embodiment, the elements and vessels are cylindrical and arranged in parallel to define an array of cylindrical elements and vessels, preferably in a planar array. Optionally the cylindrical elements and vessels are provided with tapered and preferably conical or frustoconical ends. Any suitable number of cylindrical elements and vessels may be used, which may depend upon the size of the elements / vessels and the desired size of the support structure, the cylindrical elements / vessels optionally being in one row or level or multiple rows or levels (e.g. two or three). Preferably there are 3 to 10 cylindrical elements / vessels, more preferably 4 to 7 and in one embodiment, there are five cylindrical elements / vessels. The vessels / elements are preferably separated from their nearest neighbour by a separation distance of from 0.1 to lx the width or diameter of the vessel / element, preferably by 0.25 to 0.5x the width or diameter. Preferably, where one level of cylindrical elements / vessels are provided, they are separated from their nearest neighbour by from 0.25 to 0.5x their diameter. By providing a separation between the vessels in the submersible support structure, water may then pass through the structure during ascent / descent, which can reduce the damping effects of the support structure and thereby aid in deployment and recovery of the support structure. The framework according to this preferred embodiment in which the elements / vessels are cylindrical, may comprises a plurality of planar lower frame members which are shaped (or cut) to define, when disposed with their planes perpendicular to the planar dimension of the support structure, a series of semi-circular or arc recesses each sized to receive a cylindrical element / vessel and, separating each semi-circular or arc-shaped recess, an upward projection for securing to a corresponding downward projection on an opposing upper frame member. In one embodiment, the lower and upper frame members each define five corresponding semi-circular or arc-shaped recesses to accommodate five cylindrical elements / vessels side-by-side. Preferably, the framework comprises a plurality of planar lower and upper frame member pairs configured for disposal in alignment so that they are disposed along the length of five cylindrical elements / vessels, preferably equally spaced. Tire upper and lower frame members may be secured together (e.g. bolted together or via turnbuckle ties). Adjacent frame member pairs may be secured together by connecting arms or tensile cables or turnbuckle ties. Where more than five side-by-side elements or cylinders are desired, two or three of the five-recessed frame members may be secured together in overlapping arrangement to define seven, eight or nine-recessed frame members. Thereby different sized support structures may be assembled. Preferably, in one embodiment, the elongate planar frame members have a length of up to 15 m, more preferably from 10 m or more preferably 12 m to 14 m. Thereby, they can be readily transported by truck. This arrangement also enables the support structure to be assembled at the quayside by arranging the array of lower frame members, placing the cylindrical elements / vessels in the recesses and securing on top the upper frame members before craning into the quay. The frame members are preferably formed of steel, but may be of any suitable material capable of meeting the requirements of strength. Preferably, the elements / vessels are all vessels and are each provided with a plurality of chambers for either filling with ballast (e.g. Magnetite, or other suitably dense material) which may be done once the light vessel support structure is placed m the water (to reduce lifting requirement), or used as a buoyancy chamber. In one embodiment, the cylindrical vessels are modular and made up of a plurality of cylinder portions (or cylindrical tanks), each preferably defining a single chamber. The cylinder portions may be placed in the arrangement of lower frames (which preferably are separated by less than the length of a cylinder portion) and end-to-end adjacent cylinder portions secured together end-to-end, e.g. using a bolted flanged joint. Thereby any length of support structure can be built simply by using more frame member pairs and more cylinder portions. The cylinder portions (or cylindrical tanks) preferably have a diameter of up to 6 m, preferably up to 5 m and more preferably up to 4.5 m and typically at least 2 m (e.g. from 3 to 4.5 m). Tire cylinder portions (or cylindrical tanks) preferably have a length of at least 3 m, more preferably at least 5 m and preferably up to 15 m, more preferably up to 12 to 14 m and optionally up to 10 m. Preferably they have a length of a least 6 m and more preferably at least 8 m. With such proportions, the cylindrical portions (and thus the cylindrical vessels) can be readily transported by road transport to quaysides for assembly. Preferably, each cylinder portion comprises a chamber. The chamber may be used as a ballast chamber, as mentioned above, or as a buoyancy chamber. The ballast chambers may be filled with magnetite on the water and then may be sealed closed. The buoyancy chambers may be formed with at least one valve on the lower surface (for inlet and outlet of seawater) and one valve on the top for venting air / gas and optionally a second valve on top with pressurized gas conduit for supplying gas to the chamber. Optionally a single valve formed in the top of the buoyancy chamber may provide both functions. The cylindrical vessels (and cylinder portions) may be made of any suitable material subject to it being of suitable strength to withstand the pressures at the seabed (e.g. below 40 m depth, and optionally up to 1000 m depth, e.g. from 100 m to 500 m depth). For example, they may be formed of steel (e.g. high tensile steel), which may optionally be internally reinforced, with reinforcement structures, and / or with external strengthening rims. Preferably, especially for use at greater depths, the cylinder portions may be formed of a composite construction of a steel inner hull, a plastic or composite reinforced plastic external hull and concrete or foam core. In one preferred embodiment, there are five cylindrical vessels, each having five chambers along their length. According to this preferred embodiment, the chambers of the central cylindrical vessels are ballast chambers and the midship chamber of each laterally adjacent intermediate chamber. The remainder of the chambers are buoyancy chambers which may be utilized for different purposes. The lateral midship chambers may be filled with seawater at site ready for deployment and may serve to make the submersible substation slightly negatively buoyant. Once on the seawater, the remaining intermediate chambers may be filled with seawater to firmly anchor the support structure on the seabed. The fore and aft intermediate and lateral chambers may be referred to as trim chambers and the buoyancy may be adjusted for the purpose of fine-tuning the buoyancy and improving stability by addressing any tendency to pitch or roll / tilt. As discussed above, the buoyancy-controlled support structure maybe used to transport and deploy any underwater infrastructure on the seabed, but finds particular application as a support structure for a submersible substation. The submersible substation comprises substation components mounted onto the support structure, preferably disposed in one or more water-impermeable enclosures. The water-impermeable housings may be of any suitable shape and configuration. Typically, the substation components include transformers and switchgear and may, in particular, include rectifiers and inverters. Preferably the transformer may be housed in a discrete water-impermeable chamber, discrete from the switchgear or other components and switchgear housed in another water impermeable chamber. As many water-impermeable chambers as may be desired may be provided. Preferably, the water impermeable chamber housing the transformer is thermally isolated from other water impermeable chambers - since tire transformer is likely to produce a lot of heat. Different or discrete cooling arrangements may be provided in respect of each water-impermeable chamber. There may be provided sealed apertures to allow passage of cables into and out of the discrete water-impermeable chambers and, optionally, therebetween. The water-impermeable housings and water-impermeable chambers housing the substation components may be oil / dielectric-filled (e.g. silicon oil) or may be air or inert gas environments. In any case, the water-impermeable chambers may be configured to be cooled. In one option, where the water-impermeable chambers are air or inert gas environments, an air-cooling system may be provided. In another option, which may be used additional to the first option in an air or inert gas environment, the water-impermeable chambers are cooled by one or more heat exchange circuits (e.g. of a cooling fluid) between the water-impermeable chambers and heat exchangers to the exterior allowing seawater-cooling of the interior of the chambers. Optionally, there are multiple heat exchange circuits, one for each water-impermeable chamber, which may be sized according to the expected cooling requirement of the respective substation component. Optionally, a heat exchange circuit involves a coolant fluid circulating between the interior of the water-impermeable chamber (e.g. through a thermally conductive conduit, e.g. coil) and a heat exchanger to the exterior of the chamber. Where the water-impermeable chamber is oil or dielectric filled, the heat exchange circuit may optionally comprise direct circulation of the oil / dielectric through the heat exchange circuit. In a preferred embodiment, the water-impermeable housings are cylindrical housings, which are preferably formed of a plurality of cylinder portions of a similar dimension to the cylinder portions of the above cylindrical vessels, with each cylinder portion providing a discrete water-impermeable chamber. In one preferred embodiment, there are two water-impermeable housings, each having a transformer and switchgear in discrete water-impermeable chambers and a third w ater-impermeable chamber, one of which may contain control systems for control of buoyancy levels in the buoyancy chambers as well as heat exchange circuits and any on-board power requirements. Optionally, each water-impermeable cylindrical housing, of three water-impermeable discrete chambers, may serve a plurality of renewable energy generators (e.g. 5 to 20) and may be disposed at opposing lateral positions on the support structure. Preferably, there is a power source onboard for providing any necessary power for functions on the submersible substation, which may be provided by a small onboard tidal turbine, power drawn from the system or sacrificial power. In one embodiment, in which the substation components are housed in discrete water-impermeable chambers filled with oil, the water-impermeable housing serve to provide a negative or slightly negative buoyancy contribution to the buoyancy of the submersible substation. In another embodiment, in which the substation components are housed in discrete water-impermeable chambers filled with air or inert gas, the water-impermeable housings may serve to provide a positive buoyancy contribution to the buoyancy of the submersible substation. The invention will now be described in more detail, without limitation, with reference to the accompanying Figures. In Figure 1, there is shown in side cross-sectional view a representation of a submersible sub-station system 101 and a method of deploying or installing a submersible substation 103 in its deployed configuration 103a on the seabed 121 from a deployment-ready in-transit configuration 103b at the surface 123. Tire system 201, showing the submersible substation 203 in its deployed configuration 203a, is also shown in Figure 2. The submersible substation 103 has a buoyancy-controlled support structure 105 and mounted thereon two power control units 107 containing substation components (not shown). Connected to the power control units 107 on the submersible substation 103 are array cables 109, which extend from renewable energy generators, e.g. floating wind installations (not shown), to the submersible substation 103, and export cable 111, for exporting power from the submersible substation 103 to a shore-based substation (not shown). The array cables 109 and export cables 111 have on their terminal ends cooperating connectors (not shown) for connecting to respective cable connectors (not shown) on the submersible substation 103, which form dry-mate connections. The connection mechanism of the cooperating connectors and cable connectors is illustrated in more detail in Figure 7F. The array cables 109 and export cable 111 may be formed in a dynamic cable bundle 113 extending from their respective cooperating connector ends, or when connected, from the submersible substation 103 to a cable anchor 115 or static point, which is disposed on the seabed 121. The support structure 105, which may be assembled quayside according to the size of the load it needs to carry in order to ensure there is sufficient platform area for power control units and cable access and to ensure there is sufficient buoyancy for carrying and raising the load from the seabed. The power control unit or units 107 may be loaded onto and secured to the support structure 105 at quay and then towed, e.g. with a standard harbour tug 117, using a towing cable, which may be a tow umbilical 119, to the site of the offshore renewable energy power plant. On site, tire dynamic cable bundle 113, which may be stowed on the seabed 121 or buoyed to the surface 123 and adopting a catenary fonn, may be located and secured in a cradle (not shown) of a cable-mating system (not shown), which may be winched and guided onto the support structure 105 via cable ramp 125 and connected with onboard cable connectors (not shown) in order to connect the cables to the power control unit (s) 107. On the seabed 121, from the cable anchor 115 a static portion 127 of the unbundled array cables 109 and export cable 111 extend respectively on the seabed to the floating wind turbines (not shown) and their respective device cable anchors (not shown) and to the shore, in the case of the export cable 111. The static portion 127 will typically be arranged to leave unoccupied a landing site and cable runway on the seabed fortlie submersible substation 103 and dynamic cable bundle 113, which landing site and cable runway will typically be selected to be in a direction that may facilitate installation, which is preferably in the direction against the prevailing wind / wave direction. In UK waters, for example, the landing site and cable runway will be located generally to the southwest of the cable anchor 115. The tug 117 will then orientate the subsea substation 101 on the water surface 123 so that it is directly above the landing site and cable runway, accounting for expected tidal movements. The tug 117 will pull the subsea substation 103 in a direction typically against the prevailing wind / wave direction accordingly, so that the dynamic cable bundle 113 is in tension relative to its catenary, in readiness for descent or dive. The tow umbilical 119 will typically have a length of about 1.5x the depth of the water the submersible substation 103 is being installed in. To deploy the submersible substation 103, the buoyancy in the buoyancy-controlled support structure 105 is adjusted, for example by allowing seawater to enter through inlet valves in the underside of tanks (not shown) and venting air from valves in the upper side of the tanks (not shown) in a controlled way and in order to keep the balance of the structure, until the submersible substation 103 has an overall slightly negative buoyancy. The submersible substation 103 will slowly dive or descend through the water, moving forward, as the tug 117 continues to keep the dynamic cable bundle 113 under tension (to prevent any cable buckling), until it reaches its deployed site. At this point, the buoyancy tanks may be filled with seawater (and emptied of air) to effectively anchor the submersible substation 103 on the seabed 121. Once the submersible substation 103 is deployed and operational, the tow umbilical 119 may be disconnected from the tug 117 and attached to a deployment cable (not shown) which is then released to allow the tow umbilical 119 to be controllably dropped to the seabed, while under tension (i.e. relative to its catenarv form) to prevent it buckling. Hie tow umbilical 119 has an umbilical connection module 129 for connecting to the tug, for towing and for any power and signal connections required for operation of the submersible substation 103, which umbilical connection module 129 also serves to anchor the two umbilical 119 to the seabed 121, which may located by cable to a location buoy at the surface. To raise the submersible substation 103 from its seabed configuration 103a, the reverse process to the above is carried out. The umbilical control module 129 is retrieved from the seabed 121, connected to the tug which is facing toward the prevailing wave / wind and the buoyancy of the support structure 105 by pumping compressed air into certain buoyancy tanks until the submersible substation 103 achieves a slightly positive buoyancy and slowly rises to the surface 123 with the dynamic cable bundle 113 maintained in slight tension (relative to catenary’ form) until the submersible substation 103 reaches the surface 123. The cable connections may then be released. In Figure 2 in which the submersible substation system 201 is illustrated in plan view on the seabed 221, the submersible substation 203 in its deployed position 203a, has tethered to it the tow umbilical 219 which extends in one direction outward from the submersible substation 203 to the umbilical connection module 229 which acts as an anchor for the tow umbilical. The dynamic cable bundle 213 is resting on the seabed 221 where it extends from the submersible substation 203, to which it is connected, to the cable anchor 215 from which static array cables 209 extend to renewable energy generators (not shown) and a static export cable 211 extends to shore (or another substation). The submersible substation 303 is illustrated in more detail in Figure 3, which shows the substation support structure 205 formed of an array of parallel cylindrical vessels 231,331, of which at least some include controllable buoyancy function and at least one provides a ballast function, which vessels 231,331 are linked by a framework of transverse frame members 233,333, which are disposed spaced along the length of the parallel cylindrical vessels 231,331. Arranged at opposing lateral portions of the support structure are two power control units 207,307 which are comprise of cylindrical housings affixed in a parallel arrangement with the cylindrical vessels 231,331. Tire power control units 307 are each connected by a number of array cables 209,309 and also connected to at least one of the power control units is export cable 211,311, which array cables 209,309 and export cable 211,311 descend from the support structure 205 to the seabed via cable ramp 225,325. Figures 4A to 4G illustrate plan and cross-sectional views of the arrangement of parallel cylindrical vessels 431 of the buoyancy-controlled support structure 405. Five cylindrical vessels 431 are illustrated m this embodiment, although the size can be adapted according to the particular needs in terms of weight and size of substation (or other infrastructure to be deployed on the seabed) - a central cylindrical vessel 435, two opposing lateral cylindrical vessels 437 and therebetween two intermediate cylindrical vessels 439. Each cylindrical vessel 431 is split into five buoyancy or ballast chambers 441, which are optionally interchangeably used as buoyancy chambers or may be solely used as ballast chambers. The buoyancy or ballast chambers 441 are formed along the length of and to the interior of the cylindrical vessels 431 and are isolated from one another. The five buoyancy or ballast chambers 441 along the length of each cylindrical vessel 431 may be considered as fore and aft chambers 443, midship chamber 445 and therebetween intermediate chambers 447. In the embodiment illustrated, all five of the chambers 441 in the central cylindrical vessel 435 are ballast chambers, which may be filled with, for example, a solid ballast, such as Magnetite, along with the midship chambers 445 of the intermediate cylindrical vessels 439, so as to provide some core stability and weight to the substation support structure. These may be filled quayside after launch to keep the lightship weight to a minimum prior to launch (thereby enabling the use of smaller cranes than otherwise). The remaining midship chambers 445, the remaining intermediate chambers 447 and all ten four and aft chambers 443 are buoyancy control tanks. The intermediate chambers 447 in the lateral and intermediate cylindrical vessels 437,439 may ty pically be filled with seawater as ballast. The remaining trim chambers, being the ten fore and aft chambers 443 may be air-filled or partially filled with water and then adjusted in buoyancy according to requirements. In Figure 5, two such buoyancy-control chambers 549 are illustrated in side cross-sectional views within a single cylindrical vessel 531. During the descent or dive of the submersible substation, seawater valve 551 may be opened in an otherwise air-filled chamber and sea water may flood in until an equilibrium pressure is reached. To further reduce the buoyancy, air outlet vent 553 may be opened allowing more water to enter the seawater valve 551 as air exists the air outlet vent, until a desired reduced level of buoyancy is reached. To increase the buoyancy of the chamber 549, compressed air may be forced into the chamber via compressed air conduit 555 and compressed air valve 557, while air outlet vent 553 is closed, forcing water out of the seawater valve 551 until the desired level of buoyancy is reached. The balance of the support structure within and at the water may be adjusted by adjusting the relative buoyancy of different chambers 549. Where adjacent, the neighbouring chambers may be adjusted in relative buoyancy by performing the above buoyancy-adjustment process independently on each of them, or together with use of an inter-chamber valve 559. The pitch and roll or tilt of the support structure or submersible substation may be controlled or adjusted by fine-tuning the relative buoyancy in the fore and aft chambers 443 and the buoyancy chambers 441 in the lateral cylindrical vessel. This may assist in maintaining the level of the support structure during winching on of the dynamic cable bundle 113 for connection, for example. The compressed air for pressurizing the chamber 549 through compressed air conduit 555 may be supplied from a compressed air cylinder on the vessel or more preferably a compressor on the tug 117 at the surface 123 via a compressed air conduit 555 extending through the tow umbilical 119. A Substation Control System may be provided on the tug 117 along with an interface, for monitoring and controlling the various buoyancycontrol factors, chamber level and pressures, and platform orientations. Figure 6 illustrates in cross-sectional schematic a power control unit 607, which contains substation components. The power control unit 607 comprises a water impermeable cylindrical housing 661 comprising a first discrete water-impermeable chamber 663 housing a transformer 664, a second discrete water impermeable chamber 665 housing a ballast control plant 666 and a third discrete impermeable chamber 667 which may house switchgear. Figures 7A to 7F illustrate the submersible substation 703 assembly process. As shown in Figure 7A, eleven transverse lower frame members 769 of steel arranged in parallel with their five arc-shaped recesses 770 aligned. Two cylindrical steel tanks 771 are placed across the middle arc-shaped recesses 770 of five of the lower frame members 769 and secured together with bolted flanges (not shown). A further twenty-three cylindrical steel tanks 771 are laid onto the lower frame members 669 in the same way and secured together with bolted flanges (not shown) to form five cylindrical vessels 731, each formed of five cylindrical tanks 771. Steel frustoconical ends 772 are fitted to the end of each cylindrical vessel 731, again by bolted flanges (not shown). Eleven upper frame members 773 having corresponding arced recesses are placed in a transverse arrangement over the top of the five parallel cylindrical vessels 731 to oppose the eleven lower frame members and secured together with turnbuckle ties (not shown), to form eleven transverse frame members 733 and complete the support structure 705. Of the twenty five cylindrical steel tanks 771, which define buoyancy / ballast chambers, several will be filled with solid ballast (not shown), typically at least those of the central cylindrical vessel 735, while the buoyancy chambers formed in fore and aft and lateral cylindrical steel tanks 771 will be used as buoyancy chambers and configured for interchangeable occupancy by seawater and air. Two water-impermeable cylindrical housings 761 for housing substation components (not shown) are secured to the support structure 705 by disposing each of them in a sequence of aligned upper cradle recesses 773 formed in the upper side at each end of seven adapted upper frame members 774 and secured in place. In the space between the water-impenneable cylindrical housings 761, a cable faring 775 may be formed defining a flat upper surface 776 and fore and aft orientated slopes 777. Continuing outward from the aft orientated slope is a downwardly curved cable ramp 779 to facilitate cables extending over the edge of the support structure 705, without causing excessive cable bending. As illustrated in Figure 7F, a cable connector housing 781 containing cable registration plate 783 having apertures to allow access by cooperating connectors (not shown) on the terminal ends of array and export cables (not shown) to form connection arrangements with cable connectors. The cable connectors 785, which are dry-mate connectors link to substation cables 786 which feed into the water-impermeable cylindrical housings 761 to connect with substation components. To connect the array / export cables (not shown) to the cable connectors, a cable end cradle (not shown) may be floated in the water to the aft of the submersible substation 703 and terminal ends of the cables, with associated cooperating connectors (not shown), may be placed in the cradle, which may then be winched onto the support structure 705 via the cable ramp 779 using a winch and cable (not shown) and drawn up the aft orientated slope 777 via a guide system (not shown) until the cable end cradle (not shown) is registration with the registration plate 783 whereupon the cooperating connectors (not shown) may be connected to the cable connectors thereby connecting the array cables and export cable to the submersible substation 703. The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.

Claims

1. A submersible substation for use in relation to an offshore energy generation plant, the submersible substation comprising:a substation support structure;one or more substation components disposed on the substation support structure andpreferably one or more array and / or export cable connectors, wherein the substation support structure is configured for submersed deployment and to be raised and lowered using variable buoyancy control.

2. A submersible substation as claimed in claim 1, wherein the substation is configured for deployment on the seabed.

3. A submersible substation as claimed in claim 1 or claim 2, wherein the substation support structure comprises one or more buoyancy-control elements, the buoyancy of which can be varied or changed.

4. A submersible substation as claimed in claim 3, wherein the buoyancycontrol elements are buoyancy chambers for containing a material and which buoyancy chambers are configured for remotely changing the average density of the material within the buoyancy chamber, preferably by substituting a first material, having a first density, within the buoyancy chamber, with a second material, having a second density, which is less than the first density.

5. A submersible substation as claimed in claim 4, wherein the first material is seawater and the second material is a fluid having a density less than seawater at a given pressure (e.g. air).

6. A submersible substation as claimed in any one of the preceding claims, wherein the substation support structure comprises a framework and a plurality of vessels supported and linked by the framework, wherein the plurality of vesselsoptionally comprises one or more ballast vessels and / or one or more buoyancy control vessels.

7. A submersible substation as claimed in claim 6, wherein the plurality of vessels is a plurality of cylindrical vessels arranged in parallel and the framework comprises a plurality of transverse frame members disposed at positions along the length, from fore to aft, of the plurality of cylindrical vessels.

8. A submersible substation as claimed in any one of the preceding claims, wherein the one or more substation components are disposed in one or more water-impermeable enclosures mounted on the substation support structure.

9. A submersible substation as claimed in claim 8, wherein the substation components comprise a transformer and switchgear, wherein the transformer and switchgear are disposed in discrete water-impermeable chambers.

10. A submersible substation as claimed in claim 8 or 9 wherein the water-impermeable enclosures or water-impermeable chambers serve to provide buoyancy to the submersible substation.

11. A submersible substation as claimed in any one of claims 8 to 10, wherein the water-impermeable enclosures or water impermeable chambers containing substation components are provided with cooling via a heat-exchange circuit comprising a heat exchanger in thermal contact with seawater surrounding the water-impermeable enclosure or water-impermeable chamber.

12. A submersible substation as claimed in any one of the preceding claims, wherein the substation support structure comprises a cable ramp disposed on at least one edge thereof.

13. A submersible substation as claimed in any one of the preceding claims, wherein the one or more cable connectors are dry-mate connectors.

14. A submersible substation as claimed in any one of the preceding claims, which comprises a cable-mating system comprising:a winch disposed on the substation support structure;a cradle arrangement for receiving cooperating connectors on the terminal end of array and / or export cables, the cradle arrangement being connected or connectable to the winch by way of winch cables; anda guiding system for guiding tire cradle arrangement during winching to the cable connectors to enable cooperative connection of the cable connectors with the cooperating connectors.

15. A submersible substation as claimed in any one of the preceding claims, wherein the offshore energy generation plant comprises an array of renewable energy generators.

16. A submersible substation as claimed in claim 15, wherein the renewable energy generators are wind energy generators, preferably floating wind energy generators (i.e. wind turbines).

17. A submersible substation system comprising:a submersible substation as defined in any one of claims 1 to 16;a service umbilical extending from the submersible substation to an umbilical connection member; andoptionally, a seabed mounted cable collector.

18. An offshore renewable energy generator plant comprising:a submersible substation system as defined in claim 17 or a submersible substation as defined in any one of claims 1 to 16;an array of offshore renewable energy generators connected to the submersible substation system or submersible substation by one or more array cables; andan export cable connector disposed on the submersible substation for export of power generated by the plant.

19. A method of installation of a submersible substation in an offshorerenewable generator plant, the method comprising:providing a submersible substation as defined in any one of claims 1 to 16 to a site of an offshore renewable generator plant;connecting a plurality of array cables and an export cable to the submersible substation, preferably via dry-mate connectors;adjusting the buoyancy of the support structure of the submersible substation to provide the submersible substation with a slight negative buoyancy , allowing it to descend to the seabed; andpreferably, when at the seabed, adjusting the buoyancy of the support structure of the submersible substation to anchor it on the seabed.

20. A method of assembly of a support structure for a submersible substation,the method comprising:providing one or a plurality of lower frame members defining recesses for a plurality of buoyancy elements;providing a plurality of buoyancy elements in the recesses of the lower frame member(s);providing one or a plurality of cooperating upper frame members having corresponding recesses for the plurality of buoyancy elements and fitting over the buoyancy elements into cooperation with the lower frame member(s); andsecuring the upper and lower frame members together to secure the buoyancy elements in place and form the structure.Application No: GB2414037.8Examiner: Dr Harry ProudClaims searched: 1-20Date of search: 22 January 2025Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance X 1-20 EP 2518310 Al (FUNDACION TECNALIA RES &INNOVATION) See the whole document. X 1-20 CN 114837478 A (POWERCHINA HUADONG ENGINEERING CORP) See figures 3-1 to 7-1, and related portions of the description. X 1-20 WO 2023 / 156474 Al (HEEREMA MARINE CONTRACTORS) See figures 23-29C, and page 36 lines 24-36. A US 2015 / 0252791 Al (TAUB) See figures 5 and 6, and paragraph 0093.Categories:X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if P Document published on or after the declared priority date but combined with one or more other documents of same category. before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:International Classification:Subclass Subgroup Valid From H02G 0009 / 12 01 / 01 / 2006 B63B 0035 / 44 01 / 01 / 2006 F03D 0013 / 25 01 / 01 / 2016 H02G 0001 / 10 01 / 01 / 2006 H02G 0009 / 00 01 / 01 / 2006

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