Offshore substation

Abstract

An offshore substation 1 comprises: a foundation, such as a gravity-based structure 2, arranged on a seabed and extending above a sea level 3, a deck 8 provided above and supported by the foundation,

Description

TECHNICAL FIELD The present disclosure relates to offshore substations, and more specifically to offshore substations comprising a gravity-based structure and deck arrangements for offshore substations. BACKGROUND Offshore renewable energy farms, such as wind farms, are increasing in popularity around the world, and there is an ongoing quest for efficient, reliable and environmentally sustainable solutions in offshore renewable energy infrastructure. It has, however, proven a complicated process not only to produce electricity offshore but also to transport the generated electricity to land. A central part of an offshore renewable energy farm is the offshore electrical substation. The offshore substation receives electricity generated by, for example, wind turbines and increases the voltage before delivering the electricity to export cables. Much of the development in offshore wind farms is primarily focused on fixed-to-bottom foundations, such as monopiles and jackets, to support wind turbines, offshore substations and other infrastructure. Monopiles and jackets are relatively simple and cost-effective, but may have challenges in deeper waters and harsh environmental conditions. Gravity-based structures are an alternative to monopiles and jackets, and utilize the weight of concrete or steel structures to support themselves at the seabed, providing stability for supporting electrical infrastructure provided on a deck above sea level. Electrical equipment units which form part of the electrical infrastructure may include, for example, gas insulated switchgear, transformers, and so on. Gravity-based structures may incorporate designs that distribute weight efficiently while withstanding dynamic forces from wind, waves, and currents. Gravity-based structures may provide a stable foundation for supporting generators and / or housing electrical equipment necessary for power generation and transmission in an offshore energy infrastructure. However, most offshore substations are based on traditional designs, and electrical equipment units are arranged according to established principles. Such designs and principles may not be very cost- or space-efficient, and there is therefore a need for improved offshore substations to reduce shortcomings of traditional designs and eliminate disadvantages of known techniques. Further, to handle delay and variations in work flows in the preparation of different parts or components, there is a need for an offshore substation with a flexible design that allows electrical equipment units to be installed at various stages during manufacturing, and allows easy installation and connection of the deck and electrical equipment units to the offshore substation structure. SUMMARY It is an object to mitigate, alleviate or eliminate one or more of the aboveidentified deficiencies and disadvantages in the prior art and solve at least the above mentioned problem, or at least to provide useful alternatives to the state of the art. According to a first example, there is provided an offshore substation, comprising: a gravity-based structure arranged on a seabed and extending above a sea level, the gravity-based structure comprising an outer wall defining a cavity within the gravity-based structure; a deck provided above and supported by the gravity-based structure, the deck having electrical equipment units arranged thereon; and a cable hang-off system provided in the cavity and configured to support import and / or export cables, wherein the import and / or export cables are connected to power cables extending from at least one of the electrical equipment units. According to another example, there is provided a method of installing an offshore substation, the method comprising the steps of: installing a gravity-based structure comprising cable guide tubes to a seabed; connecting a deck to the gravitybased structure, the deck having at least two transformer modules and a gas insulated switchgear module with pre-installed power cables arranged thereon; lowering the pre-installed power cables into a cavity of the gravity-based structure; and connecting the power cables to import and / or export cables supported in a cable hang-off system provided in the cavity of the gravity-based structure. According to another example, there is provided an offshore substation, comprising: a foundation arranged on a seabed and extending above a sea level; and a deck provided above and supported by the foundation, the deck having at least three electrical equipment units arranged thereon, the at least three electrical equipment units comprising a gas insulated switchgear module and two transformer modules, wherein the gas insulated switchgear module is provided between the two transformer modules. The detailed description and appended claims outline further inventive examples and embodiments. BRIEF DESCRIPTION OF THE FIGURES Examples will now be outlined in the following detailed description and the accompanying figures, in which: Fig. 1 shows a sectioned perspective view of an upper part of an embodiment of an offshore substation. Fig. 2 shows a sectioned side view of a gravity-based structure comprising J-tubes, installed on a seabed. Fig. 3 shows a top view of a deck comprising transformer modules arranged rotationally symmetrical about a gas insulated switchgear module. DETAILED DESCRIPTION Aspects of the present disclosure will now be described with reference to the accompanying figures, in which some preferred examples are shown. The invention may, however, be embodied in other forms and should not be construed as limited to the herein disclosed examples. The disclosed examples are provided to fully convey the scope of the disclosure to the skilled person. It is also to be understood that the terminology used herein is for purpose of describing particular examples only, and is not intended to be limiting. It should be noted that, as used in the specification and the appended claim, the articles "a", "an" and "the" are intended to mean that there are one or more of the elements or steps unless the context explicitly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps. Figure 1 shows an offshore substation 1. The offshore substation 1 may be provided in the vicinity of and connected by cables to a nearby offshore wind farm or similar installation, such as wave power plants, tidal power plants, or other types. The present examples will be described in the context of offshore wind power plants. The offshore substation 1 facilitates efficient transmission of electricity from the wind turbines to an onshore grid. The offshore substation 1 may aggregate the electrical output from multiple wind turbines, and the power from the wind farm is transformed in the offshore substation 1 and is further distributed to e.g. a land-based installation by export cables. The offshore substation 1 comprises a gravity-based structure 2, and may be referred to as a gravity-based substation. The gravity-based structure 2 is configured for fixation to a seabed 19, described in more detail with reference to figure 2. The gravity-based structure 2 extends above the sea level 3. The gravity-based structure 2 is generally hollow, and comprises an outer wall 4. The outer wall 4 may be rotationally symmetrical about a central longitudinal axis of the gravity-based structure 2. The gravity-based structure 2 may preferably be made from concrete. In particular, the outer wall 4 may preferably be made from concrete. The outer wall 4 defines a cavity 5, and the cavity 5 may extend generally along the length of the gravity-based structure 2. Equipment and material accommodated inside the cavity 5 is thus protected from the sea and the surrounding environment by the outer wall 4. A lower portion 6 of the gravity-based structure 2 may comprise a flared section. The flared section may be a frustoconical portion, as in the illustrated example. The diameter of the flared section increases in a direction downwards towards the seabed 19. In figure 1, most of the lower part of the gravity-based structure 2 is not shown, but a skilled person appreciates that the lower portion 6 continues down to the seabed 19, as illustrated in figure 2. The flared section of the lower portion 6 ensures a stable and rigid support for the gravity-based structure 2, and may house solid ballast at the bottom (described further with reference to figure 2 below) to support and anchor the offshore substation 1 to the seabed 19. An upper portion 7 of the gravity-based structure 2 may comprise a flared section. The diameter of the flared section of the upper portion 7 increases in a direction upwards towards the top of the gravity-based structure 2. The flared upper portion 7 facilitates a cavity 5 with a volume inside the upper portion 7, and provides a solid foundation for a deck 8 provided above. An intermediate portion 9 provided between the lower and upper portions 6,7 connects the lower and upper portions 6,7. The intermediate portion 9 may be a cylindrical section, and may have a diameter corresponding to the smallest diameter of the lower and upper portions 6,7. Preferably, the levels of the lowest astronomical tide and the highest astronomical tide of the sea level 3 are within the vertical extension of the intermediate portion 9. Because the intermediate portion 9 is positioned at the sea level 3, it may be strengthened or otherwise constructed to withstand drifting ice, impacts from adrift vessels, etc. The deck 8 is positioned above the gravity-based structure 2. The deck 8 is configured for supporting electrical equipment units 10. The electrical equipment units 10 include electrical equipment for handling electric power which is to be supplied to or from the substation, for example electrical switchgear, transformers, shunt reactors, auxiliary equipment, etc. The electrical equipment units 10 may be arranged with a housing partially or fully enclosing electrical equipment arranged therein. The electrical equipment units 10 may also be formed as an integrated topside unit. The arrangement on the deck 8 may thus comprise or form an enclosed structure. In the illustrated example, three electrical equipment units 10 are provided on the deck 8. The electrical equipment units 10 may comprise a gas insulated switchgear (GIS) module 11. A gas insulated switchgear module 11 may be, or comprise, a gas insulated electrical switchgear assembly configured for handling electric power at the offshore substation. The gas insulated switchgear module 11 may be a module comprising compact metal encapsulated switchgear with low-voltage or high-voltage components that houses different electrical devices such as circuit breakers and disconnectors, bus bars, transformers, earth switches, surge arrestors, etc. The main function of the gas insulated switchgear module 11 is to switch, separate, transform, measure and distribute electrical energy. The gas insulated switchgear module 11 may further comprise auxiliary equipment. The gas insulated switchgear module 11 is preferably provided centered above the gravity-based structure 2. A central longitudinal axis of the gravity-based structure 2 thus intersects the gas insulated switchgear module 11. The offshore substation 1 may preferably comprise a single deck 8, and all the electrical equipment units 10 are thus provided on the same deck 8, instead of being distributed over two or more floors. As the offshore substation 1 comprises a single deck 8, the electrical equipment units 10 to be positioned on the deck 8 may be provided at different times, and in different and varying order. The construction of the offshore substation 1 may thus be planned such as not to depend on one particular electrical equipment unit being finished or installed before the next one is initiated or installed. This allows for efficient design and construction and completion of the deck 8 and offshore substation 1. The electrical equipment units 10 may further comprise transformer modules 12. A transformer module 12 may be, or comprise, an electrical transformer assembly configured for handling electric power at the offshore substation. The transformer modules 12 may comprise shunt reactors. The offshore substation 1 may comprise at least two transformer modules 12, as in the illustrated example. The transformer modules 12 are provided symmetrically about the gas insulated switchgear module 11, i.e. on two opposing sides of the gas insulated switchgear module 11. The transformer modules 12 are significantly heavier than the gas insulated switchgear module 11, but as the transformer modules 12 are arranged on opposing sides of the gas insulated switchgear module 11, the center of gravity of the deck 8 and electrical equipment units 10 is maintained substantially in the center of the deck 8 and above the gravity-based structure 2. The gas insulated switchgear module 11 is provided in the center of the deck 8, and the gas insulated switchgear module 11 may thus beneficially be provided directly above the gravity-based structure 2. More than two transformer modules 12 may also be provided on the deck 8. If more than two transformer modules 12 are provided on the deck 8, they may beneficially be arranged rotationally symmetrical about the gas insulated switchgear module 11 in a direction about the longitudinal direction of the gravity-based structure 2. As such, even though each transformer module 12 is substantially heavier than the gas insulated switchgear module 11, the center of gravity for the electrical equipment units 10 combined on the deck 8 is maintained in the gas insulated switchgear module 11 in the center of the deck 8. The deck 8 may comprise several deck segments rigidly connected together side by side. During construction of the deck 8, the deck segment for each of the electrical equipment units 10 may be finished, and the whole deck 8 assembled, even though the electrical equipment units 10 to be positioned onto the deck 8 are not present or not yet ready to be installed. The electrical equipment units 10 may further comprise other electrical equipment, such as standard electrical equipment, arranged around the gas insulated switchgear module 11. The other electrical equipment may also be arranged rotationally symmetrical about the gas insulated switchgear module 11 to ensure a balanced weight distribution. A cable hang-off system 14 is provided in the cavity 5 of the gravity-based structure 2. Because the gas insulated switchgear module 11 is provided directly above the gravity-based structure 2, power cables 13 extending from the gas insulated switchgear module 11 to the cable hang-off system 14 may extend vertically or substantially vertically, and straight into the cavity 5 of the gravity-based structure 2. The power cables 13 may comprise high-voltage cables. The upper portion 7 of the gravity-based structure 2 is at least partially open at an upper end, such that the power cables 13 extend downwards from the gas insulated switchgear module 11 into the cavity 5. The cable hang-off system 14 is provided in the cavity 5, connecting the power cables 13 to import and export cables 25 provided in cable guide tubes 15. The cable guide tubes 15 may be J-tubes. The cable guide tubes 15 may further be supported by the cable hang-off system 14. The cable guide tubes 15 support and protect import and / or export cables 25 between the seabed and the upper portion 7 of the gravity-based structure 2. The cable guide tubes 15 can provide sealing and corrosion protection between flexible umbilicals, cables and pipes. The cable guide tubes 15 facilitate connection of equipment on the gravity-based structure 2 to surrounding infrastructure. This is described in more detail with reference to figure 2. The hang-off system 14 is provided at a vertical elevation which is higher than the highest astronomical tide of the sea level 3. Most of the power cables 13 above the cable hang-off system 14 may be connected to the gas insulated switchgear module 11, and because the power cables 13 extend essentially straight down from the gas insulated switchgear module 11 to the cable hang-off system 14, a very short running of cables is ensured. The length of the power cables 13 is thus reduced or minimized, and the cable routing is very compact. The necessary area topside may also be reduced because cable overlength may be provided inside the gravity-based structure 2, instead of being provided on the deck 8 or elsewhere topside. Because the cable overlength is provided between the gas insulated switchgear module 11 and the cable hang-off system 14, further cable usage is reduced. Because the cable hang-off system 14 is provided in the cavity 5, cable overlay makes it possible to connect multiple times if the connection initially goes wrong, e.g. during installation. The pull-in space above the cable hang-off system 14 further provides straight pulling which is an advantage if the power cables 13 are large and stiff. The cable hang-off system 14 may comprise a floor 24 in the cavity 5. The floor 24 may form part of the gravity-based structure 2, and may preferably be made from concrete. The floor 24 may be provided both as a support for the cable hang-off system 14, and also act as a work floor where operators can work. The cable hang-off system 14 is preferably provided inside a flared upper portion 7 of the gravitybased structure 2. This provides a spacious and easy working condition with room for overlength and easy connection of cables. A cable hang-off system 14 provided inside the gravity-based structure 2 further ensures a low center of gravity of the offshore substation 1, and minimizes topside weight. A support frame 16 may be provided between the gravity-based structure 2 and the deck 8, to allow for easy and efficient cable routing of the cables extending from the gas insulated switchgear module 11 to the cable hang-off system 14. The support frame 16 supports and elevates the deck 8 a distance from the gravity-based structure 2, such that the two elements are spaced apart. The support frame 16 is preferably a metal truss and / or beam structure. The support frame 16 further allows cable routing down from the gas insulated switchgear module 11, through the support frame 16 and up to the nearby transformer modules 12. This minimizes occupied space on the upper side of the deck 8, where the electrical equipment units 10 are provided. The gas insulated switchgear module 11 may comprise a floor 17. The gas insulated switchgear and the various components housed in the gas insulated switchgear module 11 may further be provided on top of the floor 17. The floor 17 is preferably elevated, such that the floor 17 is provided a distance above the deck 8. When the gas insulated switchgear module 11 is installed on the deck 8, a space is thus formed between the floor 17 and the deck 8, allowing efficient routing of the power cables 13 extending out from and into the gas insulated switchgear module 11. Further, the power cables 13 can be coiled-up in the space below the floor 17, to allow efficient installation of the deck 8 onto the gravity-based structure 2. The overlength in the coiled-up power cables 13 also ensures easy installation and connection of the power cables 13 to the cable hang-off system 14 and cables in the cable guide tubes 15. If the step of connecting the power cables 13 is not successful in a first attempt, overlength of the coiled-up power cables 13 can be utilized for rejointing the power cables 13. The deck 8 may further comprise a cable opening 18. The cable opening 18 may be provided below the gas insulated switchgear module 11. The cable opening 18 may in one example be one opening, preferably centered on the deck 8. In the illustrated example, the cable opening 18 is a circular opening coincident with the central longitudinal axis of the gravity-based structure 2. The majority of the power cables 13 may preferably be guided from the deck 8, through the cable opening 18, and into the cavity 5. In another example, the cable opening 18 may comprise a plurality of openings, such as perforations, to allow the power cables 13 to extend through the deck 8. Prior to installation of the deck 8 onto the gravity-based structure 2, the power cables 13 may preferably be coiled-up below the gas insulated switchgear module 11. When the deck 8 has been installed, the power cables 13 may be lowered, such as dropped, down and connected at the cable hang-off system 14. If needed, rejoining of cables is easily achieved, due to re-drop down of cables from the gas insulated switchgear module 11. Figure 2 shows the gravity-based structure 2 installed on a seabed 19. The support frame 16 is illustrated on top of the gravity-based structure 2, but the deck 8 and electrical equipment units 10 are not illustrated in figure 2. The gravity-based structure 2 of figure 2 comprises an upper portion 7 which in this example is shaped cylindrical, and not flared. The gravity-based structure 2 comprises solid ballast 20, to secure the gravity-based structure 2 and offshore substation to the seabed 19. The solid ballast 20 is preferably provided in the lower part of the gravity-based structure 2, as is known in the art. The gravity-based structure 2 may further comprise flow holes 21. The flow holes 21 are provided on the intermediate portion 9 or the lower portion 6 of the gravity-based structure 2. The flow holes 21 are provided below the lowest astronomical tide of the sea level 3. The flow holes 21 allow seawater to enter a ballast cavity 22 inside the gravity-based structure 2, to further stabilize and anchor the gravity-based structure 2 to the seabed 19. The ballast cavity 22 may be provided above the solid ballast 20. Scour protection 23 may be provided around the bottom of the gravity-based structure 2, at the seabed 19. The gravity-based structure 2 comprises cable guide tubes 15. Preferably, the cable guide tubes 15 are pre-installed on the gravity-based structure 2, prior to installing the gravity-based structure 2 on the seabed 19. If the gravity-based structure 2 comes ready equipped with the cable guide tubes 15, installation of the gravity-based structure 2 and the offshore substation can be simplified. Further, the power cables 13 can easily be connected to import and / or export cables 25 in the cable guide tubes 15 when the deck (not illustrated in figure 2) is installed onto the gravity-based structure 2. The cable guide tubes 15 may extend through the solid ballast 20. At a lower end, the cable guide tubes 15 protrude from the gravity-based structure 2 at the bottom, near the seabed 19. At an upper end, the cable guide tubes 15 are supported in or at (e.g., adjacent) the cable hang-off system 14. The cable guide tubes 15 extend inside the gravity-based structure 2, through the solid ballast 20 and ballast cavity 22, and are thus protected and supported by the outer wall 4 of the gravity-based structure 2. An offshore substation may be exposed to impact with sea ice, ships and vessels, marine debris, etc. and cable guide tubes 15 are commonly vulnerable for such impacts. A gravity-based structure 2 comprising cable guide tubes 15 housed within the outer wall 4 thus yields a great advantage, and the inside of the gravity-based structure 2, such as the solid ballast 20 and the inside of the outer wall 4, may further support the cable guide tubes 15. Figure 3 shows a top view of a deck 8 of an offshore substation 1. The deck 8 comprises a gas insulated switchgear module 11 and two transformer modules 12. The two transformer modules 12 are provided on opposite sides of the gas insulated switchgear module 11. In the layout shown in figure 3, the transformer modules 12 are provided rotationally symmetrical about the gas insulated switchgear module 11. Such a layout is scalable, as more transformer modules 12 could be added, while maintaining the center of gravity in the centered gas insulated switchgear module 11. In some examples, an offshore substation 1 may, independent of the foundation, comprise a gas insulated switchgear module 11 and at least two transformer modules 12 arranged mirrored or rotationally symmetrical about the gas insulated switchgear module 11. A rotationally symmetrical arrangement is illustrated in figure 3. The foundation may, for example, be a gravity-based foundation 2 as described above, a jacket foundation, a monopile, or other types. The gas insulated switchgear module 11 and at least two transformer modules 12 are arranged on a single deck 8. The centre of gravity can thus be maintained in the centre of the deck 8, i.e. in the gas insulated switchgear module 11. The two transformer modules 12 may be identical modules. This can also ensure easy manufacturing and installation, while keeping the connection lengths down. Two transformer modules may be arranged on opposite sides of the gas insulated switchgear module 11. The gas insulated switchgear module 11 is preferably provided vertically above the foundation. In an alternative example, the two transformer modules 12 may be provided on the deck 8 mirrored relative to the gas insulated switchgear module 11. In any of the examples or embodiments described or claimed herein, the cable guide tubes, such as J-tubes 15, extend out of the gravity-based structure 2 at a lower portion 6 thereof, such as adjacent a seafloor 19. The cable guide tubes may extend out of the gravity-based structure 2 through the outer wall 4. In any of the examples or embodiments described or claimed herein, a combined centre of gravity of the deck 8, the gas insulated switchgear module 11 and the transformer modules 12 is located vertically above the foundation, such as above the gravity-based structure 2. In any of the examples or embodiments described or claimed herein, the gravity-based structure 2 may comprise an intermediate portion 9, such as a cylindrical intermediate portion 9. The cylindrical intermediate portion 9 may be located between a flared lower portion 6 and an upper portion 7, for example a flared upper portion 7, of the gravity-based structure 2. The intermediate portion 9 may have a diameter which is not larger than a smallest diameter of the lower and upper portions 6,7. The offshore substation 1 may be arranged such that the vertical extension of the intermediate portion 9 spans the levels of the lowest astronomical tide and the highest astronomical tide of the sea level 3. In any of the examples or embodiments described or claimed herein, the gas insulated switchgear module 11 may comprise a floor 17 which is spaced from the deck 8. In any of the examples or embodiments described or claimed herein, the cavity 5 may define a pull-in space above the hang-off system 14. In any of the examples or embodiments described or claimed herein, the preinstalled power cables 13 may be arranged below the floor 17, such as between the floor 17 and the deck 8 or below the floor 17 and vertically higher than the deck 8, when the deck 8 is connected to the gravity-based structure 2. In any of the examples or embodiments described or claimed herein, the step of lowering / dropping down the pre-installed power cables 13 into the cavity 5 of the gravity-based structure 2 may comprise lowering / dropping the pre-installed power cables 13 through a cable opening 18 in the deck 8. The cable opening 18 may be provided below the gas insulated switchgear module 11. The invention is not limited by the embodiments described above; reference should be had to the appended claims.

Claims

1. An offshore substation (1), comprising:a gravity-based structure (2) arranged on a seabed (19) and extending above a sea level (3), the gravity-based structure (2) comprising an outer wall (4) defining a cavity (5) within the gravity-based structure (2);a deck (8) provided above and supported by the gravity-based structure (2), the deck (8) having electrical equipment units (10) arranged thereon; anda cable hang-off system (14) provided in the cavity (5) and configured to support import and / or export cables (25),wherein the import and / or export cables (25) are connected to power cables (13) extending from at least one of the electrical equipment units (10).

2. The offshore substation (1) according to claim 1, wherein the cable hang-off system (14) comprises a floor (24) provided in an upper portion (7) of the gravitybased structure (2).

3. The offshore substation (1) according to any one of the preceding claims, wherein the gravity-based structure (2) comprises cable guide tubes (15) extending from the cable hang-off system (14) to a lower portion (6) of the gravity-based structure (2).

4. The offshore substation (1) according to claim 3, wherein the cable guide tubes (15) are provided inside the outer wall (4).

5. The offshore substation (1) according to any one of the preceding claims, wherein the gravity-based structure (2) comprises a flared section at an upper portion (7), and the cable hang-off system (14) is provided inside the flared section.

6. The offshore substation (1) according to any one of the preceding claims, wherein the deck (8) is spaced apart from the gravity-based structure (2) by a support frame (16) providing an additional distance between the cable hang-off system (14) and the deck (8).

7. The offshore substation (1) according to any one of the preceding claims, wherein the electrical equipment units (10) comprise a gas insulated switchgear module (11).

8. The offshore substation (1) according to claim 7, wherein the gas insulated switchgear module (11) is positioned vertically, for example centred, above the gravity-based structure (2) and the gas insulated switchgear module (11) intersects a central longitudinal axis of the gravity-based structure (2).

9. The offshore substation (1) according to claim 7 or 8, wherein at least three electrical equipment units (10) are provided on the deck (8), the at least three electrical equipment units (10) comprising the gas insulated switchgear module (11) and two transformer modules (12), and the gas insulated switchgear module (11) is provided between the two transformer modules (12).

10. The offshore substation (1) according to any one of claims 7-9, wherein the electrical equipment units (10) comprise two transformer modules (12) arranged rotationally symmetrical about the gas insulated switchgear module (11).

11. The offshore substation (1) according to any one of claims 7-10, wherein the deck (8) comprises a cable opening (18) provided below the gas insulated switchgear module (11).

12. A method of installing an offshore substation (1), the method comprising the steps of:installing a gravity-based structure (2) comprising cable guide tubes (15) to a seabed (19);connecting a deck (8) to the gravity-based structure (2), the deck (8) having at least two transformer modules (12) and a gas insulated switchgear module (11) with pre-installed power cables (13) arranged thereon;lowering the pre-installed power cables (13) into a cavity (5) of the gravitybased structure (2); andconnecting the power cables (13) to import and / or export cables (25) supported in a cable hang-off system (14) provided in the cavity (5) of the gravitybased structure (2).

13. The method according to claim 12, wherein the pre-installed power cables (13) are arranged below the gas insulated switchgear module (11) when the deck (8) is connected to the gravity-based structure (2).

14. The method according to claim 12 or 13, wherein the at least two transformer modules (12) are provided rotationally symmetrical about the gas insulated switchgear module (11).

15. The method according to claim 12, 13 or 14, wherein the gas insulated switchgear module (11) is provided between the at least two transformer modules (12).

16. An offshore substation (1), comprising:a foundation (2) arranged on a seabed (19) and extending above a sea level (3); anda deck (8) provided above and supported by the foundation (2), the deck (8) having at least three electrical equipment units (10) arranged thereon, the at least three electrical equipment units (10) comprising a gas insulated switchgear module (11) and two transformer modules (12),wherein the gas insulated switchgear module (11) is provided between the two transformer modules (12).

17. The offshore substation (1) according to claim 16, wherein the gas insulated switchgear module (11) is positioned vertically above the foundation (2).

18. The offshore substation (1) according to any one of claims 16 or 17, wherein the two transformer modules (12) are arranged rotationally symmetrical about the gas insulated switchgear module (11).

19. The offshore substation (1) according to any one of claims 16 or 17, wherein the two transformer modules (12) are arranged mirrored about the gas insulated switchgear module (11).

20. The offshore substation (1) according to any one of claims 16-19, wherein a combined centre of gravity of the deck (8), the gas insulated switchgear module (11) and the two transformer modules (12) is located vertically above the foundation (2).5 21. The offshore substation (1) according to any one of claims 16-20, wherein the two transformer modules (12) are identical modules.

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