METHOD AND ASSOCIATED DEVICE

Description

[0001] The present invention relates to a method of installation, particularly, but not exclusively, installation of Gravity Based Structures and / or bases or supports therefor; and associated apparatus.BACKGROUND

[0002] In various industries, such as the renewables, oil / gas and utilities industries, structures are often required to support the weight, and often other forces, associated with installations thereabove.

[0003] Often such structures can be in marine locations, typically protruding above the sea level. Various anchoring, tethering and support structures have been employed for such offshore structures.

[0004] For example, monopoles, tripods or other piling supports can be inserted into the seabed to provide a fixed support for the installation thereabove. Other structures, such as tension leg platforms are utilised in particular circumstances for some offshore installations. Offshore installations such as wind turbines and the like are sometimes supported on structures that acquire stability from their own weight on the seabed, typically known as "Gravity Based Structures" (GBS). Gravity based structures deployed on the seabed provide stability by virtue of their own weight. Use of gravity based structures for supporting various types of offshore installations is one option for installation on particular seabed floors.

[0005] GB 2 225 365 A discloses a method for construction and installation of a gravity platform which is in entirety being constructed on at least one floating construction and ballastable barge or the like. Subsequent to that the platform is ready-built in one or several parts, it is in separate parts lifted off from the construction barge or barges by means of one or more cranes and is immersed for assembly in correct site on the sea bottom by ballasting one barge to a water level substantially level with the top edge of at least one bottom section and pumping air into the bottom sections to reduce the effective weight of the platform.

[0006] US 4 662 789 A discloses a hydrostatic coupling device for rapidly releasing a first object immersed in water from another object resting on the sea bed. The device comprises a seal mounted in a groove in the first object, which in use will be pressed against the other object. The groove is formed as a closed loop delimiting a space at the surface of the first object facing the other object. The seal is formed as a membrane mounted along its sides and dividing the groove into an outer part, open to the ambient water, and an inner part, which via a pipe can be connected to a reservoir above the water surface. This reservoir maintains a liquid column pressure in the inner part, being a certain amount higher than the ambient water pressure. A pumping device removes water from the space delimited by the seal and both objects.

[0007] WO 2010 / 019050 A1 discloses a foundation for an offshore wind turbine generator, preferably for sea depth above 45 metres, and a method of constructing and installing the foundation. The foundation in the construction, transport and installation phases is a self floating and stabile construction of reinforced concrete comprising: a foundation structure and a hollow, substantially conical tower structure arranged on the foundation structure and having a lower external diameter less than the smallest transverse dimension of the foundation structure, since the diameter of the tower structure gradually decreases upwards toward the upper portion of the tower structure.SUMMARY

[0008] In accordance with the invention as defined by the appended independent claims, there is provided a method of installing a Gravity Based Structure, and a Gravity Based Structure ("GBS") substructure for supporting a GBS thereon. Preferred embodiments of the invention follow from the dependent claims.

[0009] In at least some examples, there is provided a method of constructing a support structure. The method may comprise providing a support pre-structure for supporting a construction thereabove. The construction may comprise a weight. The construction thereabove may be associated with a Gravity Based Structure (GBS). The support structure may comprise a substructure. The support structure may effectively comprise a pre-structure for locating below the construction, such as the GBS, thereabove. The method may comprise installing the substructure at a position to be below the GBS. The substructure may provide a foundation upon which to support the GBS when installed. The method may comprise installing the substructure to define at least an annular support region for the GBS. The method may comprise installing the GBS above the substructure with the annular support region below the GBS.

[0010] According to at least some examples, there is provided a method of installing a Gravity Based Structure (GBS), the method comprising: installing a substructure at a position to be below the GBS, the substructure providing a foundation upon which to support the GBS when installed, the method comprising installing the substructure to define at least an annular support region for the GBS; and installing the GBS above the substructure with the annular support region below the GBS; wherein the method comprises pre-installing the substructure in advance of the GBS, with the GBS being installed above the substructure subsequent to the substructure's installation.

[0011] The method may comprise installing the GBS at a marine location. The substructure may comprise an offshore substructure, such as for installation at or in a seabed. The method may comprise installing the GBS at a subsea location. The method may comprise installing the GBS to be at least partially submerged once installed. The method may comprise installing the GBS to be fully submerged once installed. The method may comprise installing the substructure at a marine location. The method may comprise installing the substructure at a subsea location. The method may comprise installing the substructure to be at least partially submerged once installed. The method may comprise installing the substructure to be fully submerged once installed.

[0012] The substructure may comprise a support element. The support element may comprise a tubular support. The tubular support may comprise a circular profile. The tubular support may comprise a skirt element. The skirt element may comprise a cylindrical profile. The skirt element may comprise a vertically-oriented cylinder with a central longitudinal axis of the cylinder being vertically oriented when installed. The cylinder may comprise a hollow cylinder. The cylinder may comprise an open central portion for accommodating foundation material therein when installed. The foundation material may comprise formation material, such as in situ formation material (e.g. soil, ground, rock / s, etc.) at the location for installation of the substructure. Additionally, or alternatively, the foundation material for at least partially filling the support element's open central portion may comprise ex situ material, such as supplied materials (e.g. ballast, gravel, concrete, etc.).

[0013] The skirt element may be open at the bottom, at least prior to and / or for / during installation.

[0014] The skirt element may be open at the bottom for facilitating insertion of the skirt element into the formation. The skirt element may comprise a thin-walled element. For example, the wall thickness of the wall defining the cylinder may comprise a thickness of at least an order of magnitude less than a radius of the cylinder. The skirt element may comprise a metal element. For example, the skirt element may be formed of or from steel, such as plate or sheet steel. Additionally, or alternatively, the skirt element may be formed from concrete. The method may comprise prefabricating the skirt elements at a location remote from the location for installation.

[0015] The substructure may comprise a plurality of support elements. The plurality of support elements of the substructure may be associated with a single structure thereabove.

[0016] The method may comprise arranging the plurality of substructure support elements circumferentially to define the annular support region for the GBS. The plurality of substructure support elements may comprise a plurality of skirt elements. The plurality of skirt elements may each define a pocket or compartment within the skirt, such that a plurality of pockets or compartments are provided beneath a single GBS. The skirt may isolate the soil beneath the foundation / GBS. The skirt may encapsulate any water pockets within the soil.

[0017] In at least some examples, the substructure may enable a reduction or even elimination of at least some bespoke or dedicated scour protection. For example, the installation methods and / or presence of the particular substructures may already mitigate a negative effect / s associated with scouring.

[0018] The skirt elements may be dimensioned to be considerably smaller than the GBS to be supported thereabove. The skirt elements may be dimensioned to each only comprise a diameter considerably smaller than that of the annular support region defined by the plurality of skirt elements. The skirt elements may each comprise a diameter less than a radius of the GBS to be supported thereabove. The plurality of skirt elements may be laterally arranged to define the annular support region. The plurality of skirt elements may be non-concentrically arranged. The center of each skirt element may be non-coincident with the other skirt elements. The plurality of skirt elements may be arranged around a common centrepoint, such as corresponding to a vertical, central longitudinal axis of the GBS when installed. The plurality of skirt elements may be distributed, such as evenly distributed, around the centrepoint. The plurality of skirt elements may be arranged in an annular pattern centered on the longitudinal axis of the GBS, the annular pattern defining the annular support region.

[0019] When installed, the plurality of skirt elements may be horizontally laterally arranged. The plurality of skirt elements may be horizontally spaced from each other. The plurality of skirt elements may all be installed at a similar height and / or to a similar depth when installed. The tops of all the skirt elements may be on a same horizontal plane once installed. The plurality of skirt elements may define the horizontal plane when installed. In at least some examples, the bottoms of all the skirt elements may be on a same horizontal plane once installed.

[0020] The present applicant has identified that, conventionally, GBS may encounter issues, such as reduced stability, or lessening of stability over time. For example, effects associated with the local environment, such as cyclic action of waves, currents, etc. can weaken the load-bearing capacity of the GBS, such as due to increased pore water pressure. In some cases, 'pore pressure build-up' in the soil below the GBS can lead to a water cushion, weakening the effectiveness of the GBS. Here, the provision of the substructure as defined herein can at least mitigate such potential issues associated with GBS.

[0021] The skirt elements may be individually installed. The substructure may be installed incrementally. Alternatively, the method may comprise simultaneously installing the plurality of skirt elements.

[0022] In at least some examples, the skirt element / s may be open at the top. The skirt element / s may be open at the top at least prior to installation. At least during installation, the skirt elements may be open at their upper ends, the upper ends being below the GBS when installed.

[0023] In at least some examples, the skirt element / s may be closed at the top. The skirt element / s may comprise an inverted cup-shape, at least prior to and / or for installation. The skirt element may comprise a top cover. Accordingly, the skirt elements may be enclosed at an upper end, the upper end being below the GBS when installed. Each skirt element may be enclosed at an upper end by a respective end wall. Alternatively, the plurality of skirt elements may be enclosed at their respective upper ends by a common end wall. The common end wall may comprise a circular or ring-shaped end wall. The common end wall may be configured to correlate to the arrangement of the skirt elements when installed. For example, the common end wall may define or circumscribe a footprint or circle within which the skirt elements are arranged when installed.

[0024] The method may comprise enclosing the skirt element prior to installation of the skirt element. The method may comprise enclosing the skirt element prior to transportation of the skirt element to the location for installation. Alternatively, the method may comprise enclosing the skirt element / s after installation of the skirt element / s. The method may comprise enclosing the skirt element / s by placing the top cover over the upper end of the skirt element / s.

[0025] The method may comprise attaching the top cover / s to the skirt element / s. Alternatively, the method may comprise locating the top cover / s on the skirt element / s to be held in place by weight. The top cover may comprise a concrete member, such as a concrete slab.

[0026] The plurality of skirt elements for forming a single GBS support structure may all comprise a similar type. For example, each of the plurality of skirt elements may comprise a similar configuration / s, such as dimension / s and / or a presence of a top cover / s. In at least some examples. The substructure comprises a plurality of similarly-proportioned skirt elements, regularly arranged about a central longitudinal, vertical axis for the GBS.

[0027] The method may comprise providing the substructure below the GBS such that the substructure falls within a footprint, such as a periphery or circumference when viewed in plan view, of the GBS when installed; and wherein a central axis of the substructure is vertically aligned with a central axis of the GBS when installed.

[0028] The method may comprise providing the substructure below the GBS such that the substructure overlaps a periphery, such as a circumference, of the GBS when installed, with a portion / s of the substructure being located internally and another portion / s of the substructure being located externally of the GBS when viewed in plan view.

[0029] The method may comprise inserting or lowering the substructure at least partially into the formation below.

[0030] The method may comprise installing the skirt element / s in a powered operation. The powered operation may comprise vibration or agitation. The powered operation may comprise piling or driving the support element / s into

[0031] Where the skirt element is enclosed at the top, the skirt element may effectively function as a suction bucket during installation. Accordingly, the method may comprise creating a pressure differential, such as with a pump / s, to create an underpressure within the skirt such the skirt is pulled downwards (onto / into the formation therebelow).

[0032] The method may comprise pre-installing the substructure in advance of the GBS. The method may comprise the GBS being installed above the substructure subsequent to the substructure. The method may comprise transporting the substructure to the location for installation separately from the GBS.

[0033] Alternatively, the method may comprise pre-assembling the substructure with the GBS thereabove. The method may comprise the substructure being installed together with the GBS. The method may comprise transporting the GBS and substructure as a combined pre-assembled unit to the location for installation. Alternatively, the method may comprise pre-assembling the substructure at or adjacent the installation location prior to installation.

[0034] The method may comprise connecting the substructure to the GBS. The method may comprise connecting the substructure to the GBS prior to installing the substructure. Additionally, or alternatively, the method may comprise connecting the substructure to the GBS subsequent to installing the substructure.

[0035] The method may comprise providing a separation between the substructure and the GBS thereabove. The separation may provide a gap, such as of at least 50mm, optionally 10mm or considerably more., between a top of the substructure and a bottom of the GBS. The separation may be between all of the substructure and all of the GBS. Accordingly, the substructure and the GBS may be entirely separated by the gap. The method may comprise providing a buffer, levelling or other deformable laver / s between the top of the substructure and a bottom of the GBS. The method may comprise providing a gravel bed between the substructure and the GBS. The method may comprise applying a gravel bed on top of the substructure prior to application or installation of the GBS thereabove.

[0036] The method may comprise improving the stiffness of region below the GBS. The method may comprise improving the stiffness of the foundation / s below the GBS prior to installation of the GBS. The method may comprise providing a uniform, or at least more uniform, stiffness of substrate or region below the GBS. The method may comprise reducing or even eliminating remedial soil works to uniformize the soil stiffness. The method may comprise improving the stiffness by installing the substructure. The substructure may effectively comprise one or more stiffeners such that the method comprises providing an improved stiffness of foundation for the GBS. The improved stiffness being relative to the formation / soil prior to the installation of the substructure. The skirt element / s may provide at least a portion of the stiffness.

[0037] The method may comprise providing a consistent stiffness of support around at least the annular support region for the GBS. The consistent stiffness of support may be provided at least partially by a consistency of stiffness of the substructure.

[0038] There may be provided a Gravity Based Structure ("GBS") substructure for supporting a GBS, the substructure providing a foundation upon which to support the GBS when installed, wherein the substructure comprises at least an annular support region to define an annular support for the GBS when installed.

[0039] The substructure may comprise a plurality of substructure elements. The plurality of substructure elements may be arranged circumferentially when installed to define the annular support region for the GBS. The plurality of substructure elements may comprise a plurality of skirt elements. The skirt elements may each comprise a vertically-oriented cylinder, with a central longitudinal axis of the cylinder being vertically oriented when installed. Each cylinder may comprise an open central portion for installation for accommodating foundation material therein when installed.

[0040] The skirt element may comprise a metal cylinder. Additionally or alternatively, the cylinder may comprise a concrete cylinder. The cylinder may be configured for insertion into the formation when installed, such that the open central portion is filled with formation and an upper end of each skirt element is adjacent an upper surface of the formation.

[0041] According to an aspect there is provided a system comprising the apparatus of any other aspect, example, embodiment or claim. The apparatus may comprise the substructure or a component thereof, such as a support / skirt element, of any other aspect, claims, embodiment or example.

[0042] According to an aspect, there is provided an installation. The installation may comprise comprising the GBS and the substructure of any other aspect, example, claim or embodiment. The installation may comprise a separation between the substructure and the GBS thereabove. The separation may provide a gap, such as of at least 50mm, optionally 10mm or considerably more, between a top of the substructure and a bottom of the GBS. The separation may be between all of the substructure and all of the GBS. Accordingly, the substructure and the GBS may be entirely separated by the gap. The installation may comprise a buffer, levelling or other deformable laver / s between the top of the substructure and a bottom of the GBS. The installation may comprise a gravel bed between the substructure and the GBS. The method may comprise applying a gravel bed on top of the substructure prior to application or installation of the GBS thereabove. The gap between the substructure and the GBS thereabove may be filled with the gravel bed.

[0043] According to an aspect, there is provided an array of support elements according to any other aspect, embodiment, example or claim. For example, there may be provided an array of skirt elements for forming a substructure beneath a GBS when installed. The array may be provided distal to the installation location. The array of support elements may be provided discretely.

[0044] According to an aspect, there is provided a method of using the apparatus, such as the substructure or portion / s thereof, according to an aspect, claim, embodiment or example of this disclosure.

[0045] The steps of the method may be in any order.

[0046] According to an aspect of, there is provided an apparatus configured to perform a method according to an aspect, claim, embodiment or example of this disclosure.BRIEF DESCRIPTION

[0047] An embodiment of the present disclosure will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows three examples of support structures for offshore installations: Figure 1a shows a monopile structure 3; Figure 1b shows a tripod structure 5; and Figure 1c shows a Gravity-based structure 7 (GBS). Figure 2 shows an example of a method according to the present disclosure; Figure 3 shows an example of a substructure according to the present disclosure; Figure 4 shows an analysis model of the substructure of Figure4; Figure 5 shows another example of a substructure according to the present disclosure; Figure 6 shows schematic plan and side views of another example of a substructure according to the present disclosure; Figure 7 shows schematic plan and side views of another example of a substructure according to the present disclosure; Figure 8 shows schematic plan and side views of another example of a substructure according to the present disclosure; Figure 9 shows schematic plan and side views of another example of a substructure according to the present disclosure; Figure 10 shows a schematic side view of the substructure of Figure 8 prior to installation; Figure 11 shows a schematic side view of the substructure of Figure 8 after installation; Figure 12 shows schematic plan and elevation views of the substructure of Figure 6; Figure 13 shows an example of a transit of a support element; Figure 14 shows a schematic example of a further transit of a plurality of support elements; Figure 15 illustrates an example of an installation step of a support element; and Figure 16 shows an example of a portion of an installation positioned above a substructure. DETAILED DESCRIPTION

[0048] Referring firstly to Figure 1, there is shown three examples of support structures for offshore installations, such as a wind turbine. Figure 1a shows a monopile structure 3; Figure 1b shows a tripod structure 5; and Figure 1c shows a Gravity-based structure 7 (GBS). The monopile structure 3 of Figure 1a and the tripod structure 5 of Figure 1b have been penetrated into the formation therebelow, which is a seabed formation 9 as shown here. Each of the monopile structure 3 of Figure 1a; the tripod structure 5 of Figure 1b; and the GBS 7 of Figure 1c includes a scour protection 11 in the examples shown here.

[0049] Referring now to Figure 2, there is depicted an example diagram of a method 2 according to the present disclosure. Initially, a plurality of skirt elements are fabricated in a first step 4. Thereafter the skirt elements are positioned in a subsequent step 4. As outlined in detail below, the skirt elements are individually positioned incrementally in a series of sub-steps; or the skirt elements are positioned in unison in a single step. Likewise, where the GBS is positioned upon the skirt elements in a subsequent step 8, in at least some examples the two positioning steps 6, 8 can be effectively combined with a single GBS being positioned along with the plurality of skirt elements in a single step. Accordingly, in contrast to embodiments whereby a single skirt is positioned beneath a single structure, such as a single skirt under a single GBS; the method here provides a plurality of skirt elements beneath a single structure. Where a skirt may have previously been employed, a single skirt has been associated with a single construction element, such as providing a single skirt for a single monopole or three skirts for a tripod (a single respective skirt for each respective leg). In contrast, here, multiple skirt elements 16 are provided for a single substructure 10 for a single GBS.

[0050] Accordingly, here, the method comprises providing a support pre-structure in the form of a substructure 10 for supporting a construction thereabove. As shown in Figure 6, the construction above here comprises a Gravity Based Structure 12 (GBS 12). The substructure 10 effectively comprises a pre-structure for locating below the GBS 12 thereabove. As shown in Figure 3, the method comprises installing the substructure 10 at a position to be below the GBS 12. Accordingly, the substructure 10 provides a foundation upon which to support the GBS 12 when installed. The method comprises installing the substructure 10 to define at least an annular support region 14 for the GBS 12. The method comprises installing the GBS 12 above the substructure 10 with the annular support region 14 vertically below the GBS 12, being directly below the GBS 12 as shown here.

[0051] In the example here, there is provided a method of installing a Gravity Based Structure (GBS 12), the method comprising installing the substructure 10 at a position to be below the GBS 12, the substructure 10 providing a foundation upon which to support the GBS 12 when installed, the method comprising installing the substructure 10 to define at least an annular support region for the GBS 12; and installing the GBS 12 above the substructure 10 with the annular support region therebelow. It will be appreciated that the location installation here is a subsea marine location, with the substructure 10 being located at the seabed. Here, the method comprises installing the substructure 10 to be fully submerged once installed. Accordingly, the method in this example comprises installing the GBS 12 to be at least partially submerged once installed.

[0052] Referring now to Figure 3, there is shown an example of a substructure 12 comprising a plurality of support elements in the form of circumferentially-arranged skirt elements 16. The substructure 10 comprises a support element. The support element comprises a tubular support. The tubular support comprises a circular profile. The tubular support comprises a skirt element 16. The skirt element 16 comprises a cylindrical profile. The skirt element 16 comprises a vertically-oriented cylinder with a central longitudinal axis of the cylinder being vertically oriented when installed. The cylinder comprises a hollow cylinder. The cylinder comprises an open central portion for accommodating foundation material therein when installed. The foundation material comprises formation material, such as in situ formation material (e.g. soil, ground, rock / s, etc.) at the location for installation of the substructure 10. Additionally, or alternatively, the foundation material for at least partially filling the support element's open central portion comprises ex situ material, such as supplied materials (e.g. ballast, gravel, concrete, etc.).

[0053] The skirt element 16 is open at the bottom, prior to and during installation. The skirt element 16 is open at the bottom for facilitating insertion of the skirt element 16 into the formation. The skirt element 16 comprises a thin-walled element. For example, the wall thickness of the wall defining the cylinder comprises a thickness of at least an order of magnitude less than a radius of the cylinder. The skirt element 16 here comprises a metal element formed from plate or sheet steel. Here, the method comprises prefabricating the skirt elements art a location remote from the location for installation.

[0054] The substructure 10 comprises a plurality of support elements. The method comprises arranging the plurality of substructure 10 support elements circumferentially to define the annular support region for the GBS 12. The plurality of substructure 10 support elements comprises a plurality of skirt elements. The plurality of skirt elements may each define a pocket or compartment within the skirt, such that a plurality of pockets or compartments are provided beneath a single GBS 12. The skirt may isolate the soil beneath the foundation / GBS 12. The skirt may encapsulate any water pockets within the soil.

[0055] It will be appreciated that the arrangement of the skirt elements 16 can improve poor soil conditions. Here, the plurality of skirt elements 16 (for forming the single support substructure for the single GBS) all comprise a similar type. Each of the plurality of skirt elements 16 comprises a similar configuration, including dimensions. Accordingly, the substructure 10 comprises a plurality of similarly-proportioned skirt elements 16, regularly arranged about the central longitudinal, vertical axis 18 for the GBS 12. In the example shown here, the substructure 10 comprise a total of fourteen skirt elements 16, each with a diameter of 7.9m. The skirt elements 16 are arranged around the central longitudinal axis 18, to define the annular region with an inner diameter (global middle) of 36m; and an outer diameter of 44m. The substructure 10 here is configured to provide a skirt penetration of 5m into the formation. The plurality of skirt elements 16 can be configured to transfer global base shear load through a soft top soil layer and down to competent soil. In the example shown here, the substructure 10 is configured for a soil zone 2B with a 2.4m weak top layer. The skirt elements 16 are lift-installed prior to installation of GBS (e.g. 4 points lift). As shown here, a 0.5m penetration (weight 1100mT) is used; whereafter suction can be applied during installation. Accordingly, a maximum suction used during penetration can be around 180kPa, as illustrated in Figure 4, which shows an analysis model of the substructure of Figure 4. Here, the load conditions of the skirt elements 16 are governed by the maximum suction (of 180kPa) during penetration. In the example shown here, the weight of the skirts is 588mT.

[0056] Figure 5 shows another example of a substructure 110 according to the present disclosure The substructure 110 shown in Figure 5 is generally similar to that shown in Figure 3, with like features referenced by like numerals, incremented by 100. Accordingly, the substructure 110 comprises a plurality of skirt elements 116 arranged around a common central longitudinal axis 118. For conciseness, definitions of all like features are not duplicated in this description. Here, the four skirt elements 116 can be made from concrete, with an individual diameter of 15m of each skirt element 116. A top slab 122 (shown here with a diameter of 36m) can be positioned intermediate the plurality of skirt elements 112 and the single GBS to be positioned thereabove.

[0057] Figure 6 shows schematic plan and side views of another example of a substructure according to the present disclosure. The substructure 210 shown in Figure 6 is generally similar to that shown in Figure 5, with like features referenced by like numerals, incremented by 100. Accordingly, the substructure 210 comprises a plurality of skirt elements 216 arranged around a common central longitudinal axis 218. Here, the four skirt elements 216 are modular; and can be made from steel, defining an outer diameter of the skirt elements 218 as 15m in the example shown. The skirt elements 216 here are modular, being individually transported and installed sequentially, prior to placement of the GBS 212 thereabove, with a gravel pad 224 being applied here intermediate the installation of the skirt elements 216 and the GBS 212. The annular support region defined by the arrangement of the plurality of the skirt elements 216 is larger than any individual support element 216. For example, the diameter of the annular support region is more than twice that of an individual skirt element 216. The elevation view of Figure 6 clearly shows the arrangement of skirt elements 216 installed into the formation 209 to a depth corresponding to the height of each skirt element 216.

[0058] Figure 7 shows schematic plan and side views of another example of a substructure according to the present disclosure. The substructure 310 shown in Figure 7 is generally similar to that 210 shown in Figure 6, with like features referenced by like numerals, incremented by 100. Accordingly, the substructure 310 comprises a plurality of skirt elements 316 arranged around a common central longitudinal axis 318. Here, Figure 7 shows an example of a substructure 310 comprising a plurality of pre-installed modular skirts 316 connected to the GBS 312. As shown here, a total of eight steel skirts 316 are interconnected by a circular concrete skirt slab 322 with a ring beam 324. The circular concrete slab 322 here has a central opening (6m in diameter), providing access to the centre of the substructure 310. The example of a skirt slab 322 here is filled with a layer of crushed stone and cement slurry before installation on the seabed. Accordingly, there is ensured an evenly distributed gravity load between the GBS 312 and the skirt slab 322. A horizontal gap between the GBS periphery and the ring beam 324 can be filled with an underwater concrete cast (not shown) to ensure fixity between the GBS 324 and the skirt slab 322. However, in other examples, no such filling is provided, with no such additional fixity required.

[0059] In the example shown here, each steel skirt element 316 has a diameter of 8.9m; and a height of 6m; formed from steel plate with a 40mm thickness. The skirt slab 322 has an outer diameter more than four times that of the individual skirt elements 316. The skirt slab 322 has an outer diameter around 42m in the example shown. The skirt slab thickness of 1.1m can be achieved with concrete / steel (e.g. reinforced concrete). Accordingly, the approximate dry weight is 3650 tons; with an approximate submerged weight of 3400 tons.

[0060] Figure 8 shows schematic plan and side views of another example of a substructure 410 according to the present disclosure. The substructure 410 shown in Figure 8 is generally similar to that shown in Figure 7, with like features referenced by like numerals, incremented by 100. Accordingly, the substructure 410 comprises a plurality of skirt elements 416 arranged around a common central longitudinal axis 418. For conciseness, definitions of all like features are not duplicated in this description.

[0061] Here, Figure 8 shows an example of a substructure 410 comprising a plurality of integrated skirt elements 416 connected to the GBS 412. As shown here, a total of six steel skirts 416 are integrated. It will be appreciated that the steel skirt elements 416 can be integrated in the GBS 412 at a construction site or pre-installed offshore.

[0062] Figure 9 shows schematic plan and side views of another example of a substructure 510 according to the present disclosure. The substructure 510 shown in Figure 9 is generally similar to that 410 shown in Figure 8, with like features referenced by like numerals, incremented by 100. Accordingly, the substructure 510 comprises a plurality of skirt elements 516 arranged around a common central longitudinal axis 518. For conciseness, definitions of all like features are not duplicated in this description. The modular steel skirt elements 516 shown in Figure 9 are generally similar to those 216 shown in Figure 6. Here, the skirt elements 516 can be open-ended at their top ends, at least for installation. Rather than any slab or common enclosure, the plurality of skirt elements 516 can be covered by the gravel pad 524 intermediate the substructure 510 and the GBS 512 (following installation of the skirt elements 516 into the formation 509).

[0063] Figure 10 shows a schematic side view of the substructure 410 of Figure 8 prior to installation; whilst Figure 11 shows a schematic side view of the substructure 410 of Figure 8 after installation. As shown in Figure 10, the plurality of skirt elements 416 are connected to the GBS 412 prior to installation at the installation, such as the seabed 409 shown in Figure 11. Here, the skirt elements 416 are installed at the GBS fabrication yard and are fixed in the GBS base slab. The method here comprises pre-assembling the substructure 410 with the GBS 412 thereabove, such as illustrated in Figure 10. The method comprises the substructure 110 being installed together with the GBS 412. The method comprises transporting the GBS 412 and substructure 410 as a combined pre-assembled unit to the location for installation. Accordingly, the integrated skirt elements 416 can be transported and installed integrally with the GBS 412. In other examples, the method comprises pre-assembling the substructure 410 at or adjacent the installation location prior to installation. In each of these cases, the method comprises connecting the substructure 410 to the GBS 412 prior to installing the substructure 410. In the example shown here, six cylindrical skirt elements 416 are provided, each with a diameter of 12m made from plate steel (e.g. S355) with a thickness of around 38mm. Accordingly, the substructure 410 comprising the six skirt elements 416 contributes around 432 mT to the weight of the GBS 412. The method here comprises simultaneously installing the plurality of skirt elements 416. Here the skirt elements 416 have a height of 6m, with full penetration into the formation 409 (as shown in Figure 11) ensuring that the substructure 410 penetrates the formation 409 to a corresponding depth of 6m. As shown in Figure 11, when installed, the GBS 412 can be directly connected to the substructure 410 directly therebelow. Accordingly, any intermediate layers or structures, such as a gravel pad, may be reduced or even eliminated.

[0064] Figure 12 shows schematic plan and elevation views of the substructure 210 of Figure 6. It will be appreciated that the arrangement of pre-installed modular skirt elements 216 is without connection to the GBS 212. The skirt elements 216 here each comprise a steel cylinder with a respective concrete top slab 217. The skirt elements 216 have an individual diameter of 15 m; and are 6m deep - allowing a penetration of the substructure 210 into the formation 209 of a corresponding 6m, as shown in the elevation view of Figure 12.

[0065] Here, each of the four free-standing skirt elements 216 can be installed prior to installation of the gravel pad and GBS 212. There is no mechanical connection between the GBS 212 and the skirt elements 216. Accordingly, the fabrication and transport of the GBS 212 can be as for without such a substructure 210 (e.g. as for a conventional GBS fabrication and transportation). Furthermore, such installation can mitigate or even eliminate additional skirt offshore work after GBS 212 installation.

[0066] Figures 13 through Figure 15 show examples of a transit of a support element, such as the skirt element 316 of Figure 7. Figure 13 shows a respective front and end view of the skirt element 316 mounted on a rolling transport 330 (e.g. multiwheeler). Accordingly, the plurality of skirt elements 316 can be individually transported from the fabrication yard, such as for marine transport on a vessel 340 as shown n Figure 14. Each skirt element 316 can be individually lifted onto the vessel 340, where it will be appreciated that the plurality of skirt elements 316 can be distributed along the vessel 340 as shown (or across other vessels, depending on dimensions). In at least some examples, all of the modular skirt elements 316 for forming a single substructure 310 for a single GBS 312 can be transported on a same vessel. In some instances, the GBS 312 can also be loaded on separately to the same vessel 340.

[0067] Figure 15 shows a single skirt element 316 of the plurality for forming the single substructure 310, being lifted into position at the marine location. It will be appreciated that each individual skirt element 316 can be sequentially installed. Accordingly, each skirt element 316 can be inserted into the seabed, such as initially under weight and optionally vibro-hammering or the like. Once a sufficient penetration of the individual skirt element 316 into the formation 309 has been achieved, then a pump can be activated to use suction within the enclosed skirt element 316 (effectively acting as a suction bucket) to fully insert the individual skirt element 316 into the formation 309 to the desired penetration. This installation procedure can be repeated for each of the individual skirt elements 316 to provide the arrangement of the substructure 310 for receiving the GBS 312 thereabove. The skirt elements 316 are individually installed, with the substructure 310 being installed incrementally. It will be appreciated that an intermediate procedure, such as a gravel pad installation can be performed prior to installation of the GBS 312. Accordingly, the gravel pad can be installed after and on top of the skirt elements 316. The gravel pad can be levelled out and adjusted for any elevation differences between the top of the skirt elements 316 if present.

[0068] Once the GBS 312 has been installed, then the remaining structure can be completed. For example, as shown in Figure 16, additional components of the installation 350, such as a wind turbine or the like, can be installed on or above the GBS 312.

[0069] It will be appreciated that the provision of such substructures below the GBS as shown can improve the stiffness of formation / soil below the GBS (prior to installation of the GBS). The installation methods provide a more uniform stiffness of substrate or region below the GBS (e.g. relative to the formation / soil prior to the installation of the substructure - and relative to conventional soil remediation operations). Accordingly, the installation methods reduce or even eliminate remedial soil works to uniformize the soil stiffness.

[0070] It will be appreciated that, although schematic, the relative proportions of the skirt elements shown are shown to scale with the relative proportions of the slabs, GBS and annular regions shown.

[0071] The invention is defined by the appended claims.

Claims

1. A method of installing a Gravity Based Structure ("GBS") (12), the method comprising; installing a substructure (10) at a position to be below the GBS, the substructure providing a foundation upon which to support the GBS when installed, the method comprising installing the substructure to define at least an annular support region (14) for the GBS; and installing the GBS (12) above the substructure (10) with the annular support region below the GBS; wherein the method comprises pre-installing the substructure (10) in advance of the GBS (12), with the GBS being installed above the substructure subsequent to the substructure's installation.

2. The method of claim 1, wherein the substructure (10) comprises at least one skirt element (16), the skirt element comprising a vertically-oriented cylinder with a central longitudinal axis of the cylinder being vertically oriented when installed, each cylinder comprising an open central portion for accommodating foundation material therein when installed, optionally wherein the skirt element comprises a metal and / or concrete cylinder.

3. The method of claim 2, wherein the substructure (10) comprises a plurality of skirt elements (16) and the method comprises arranging the plurality of skirt elements circumferentially to define the annular support region (14) for the GBS, optionally wherein each of the plurality of skirt elements is individually installed, the substructure being installed incrementally, or optionally wherein the method comprises simultaneously installing the plurality of skirt elements (16).

4. The method of any of claims 2 to 3, wherein, at least during installation, the skirt element (16) is open at its upper end, the upper end being below the GBS (12) when installed.

5. The method of any of claims 2 to 3, wherein the skirt element (16) is enclosed at an upper end, the upper end being below the GBS (12) when installed, optionally wherein the substructure (10) comprises a plurality of skirt elements (16) and each skirt element is enclosed at an upper end by a respective end wall; the method comprising enclosing the skirt element prior to installation of the skirt element, optionally prior to transportation of the skirt element, or optionally wherein the substructure comprises a plurality of skirt elements and the plurality of skirt elements is enclosed at their respective upper ends by a common end wall, such as a circular or ring-shaped end wall, the common end wall being configured to correlate to the arrangement of the skirt elements when installed.

6. The method of claim 5, wherein the skirt elements (16) are enclosed at the upper ends by a concrete end wall / s.

7. The method of any preceding claim, wherein the method comprises providing the substructure (10) below the GBS (12) such that the substructure falls within a footprint, such as a periphery or circumference when viewed in plan view, of the GBS when installed; and wherein a central axis of the substructure is vertically aligned with a central axis (18) of the GBS when installed, optionally wherein installing the substructure comprises inserting or lowering the substructure at least partially into the formation below (409), optionally wherein the method comprises installing the substructure using one or more of: vibration, piling, oscillation, pumping, suction.

8. The method of any preceding claim, wherein the method comprises transporting the substructure (10) to the location for installation separately from the GBS (12).

9. The method of any preceding claim, wherein the method comprises providing a gap between the substructure and the GBS (12) thereabove, optionally wherein the method comprises providing a gravel bed between a top of the substructure and a bottom of the GBS, the gravel bed being applied after installation of the substructure and prior to the installation of the GBS.

10. The method of any preceding claim, wherein the method comprises connecting the substructure (10) to the GBS (12) subsequent to installing the substructure.

11. The method of any preceding claim, wherein the substructure (10) effectively comprises one or more stiffeners such that the method comprises providing an improved stiffness of foundation for the GBS, the improved stiffness being relative to the formation / soil prior to the installation of the substructure; and / or wherein the method comprises providing a consistent stiffness of support around at least the annular support region (14) for the GBS, the consistent stiffness of support being provided at least partially by a consistency of stiffness of the substructure.

12. A Gravity Based Structure ("GBS") substructure (10) for supporting a GBS (12) thereon, the substructure providing a foundation upon which to support the GBS when installed, wherein the substructure comprises at least an annular support region (14) to define an annular support for the GBS when installed.

13. The substructure of claim 12, wherein the substructure (10) comprises a pre-installed substructure configured to be installed in advance of the GBS, optionally wherein the substructure (10) comprises a plurality of substructure elements (16), the plurality of substructure elements arranged circumferentially when installed to define the annular support region (14) for the GBS; and wherein the plurality of substructure elements (16) comprises a plurality of skirt elements, the skirt elements each comprising a vertically-oriented cylinder, with a central longitudinal axis of the cylinder being vertically oriented when installed, each cylinder comprising an open central portion for installation for accommodating foundation material therein when installed, optionally wherein each skirt element (16) comprises a metal and / or concrete cylinder, the cylinder being configured for insertion into the formation when installed, such that the open central portion is filled with formation and an upper end of each skirt element is adjacent an upper surface of the formation (409).

14. An installation comprising the GBS (12) and the substructure (12) of any of claims 12 to 13.

15. The installation of claim 14, comprising a gap between the substructure (10) and the GBS (12) thereabove, the gap comprising a gravel bed between a top of the substructure and a bottom of the GBS.