AU2021202442B2

AU2021202442B2 - Gravity-Based Structure For Off-Shore Structures

Info

Publication number: AU2021202442B2
Application number: AU2021202442A
Authority: AU
Inventors: Nicholas O'Keeffe; Brett Wilson
Original assignee: Linch Pin Entpr Pty Ltd
Current assignee: Linch Pin Enterprises Pty Ltd
Priority date: 2021-03-16
Filing date: 2021-04-21
Publication date: 2023-10-19
Anticipated expiration: 2041-04-21
Also published as: AU2021202442A1

Abstract

A gravity-based structure to support off-shore structures, such as wind turbines and oil and gas platforms, is described. The gravity-based structure includes a submersible frame having a primary base body, an auxiliary base body spaced apart from the base body in parallel alignment therewith and a plurality of connecting members to interconnect the auxiliary body to the base body. One or more support towers is mounted on the base body, the one or more support towers being configured, in use, to support the off-shore structure. The gravity-based structure may be pre-assembled with an off-shore structure, and said assembly may be subsequently transported proximal to an off-shore installation site by a semi-submersible vessel, for example. The submersible frame of the assembly may then be progressively ballasted with water to cause the submersible frame to settle on a mudline of the off-shore installation site. 1/9 40 38 - 10 36 32 24 22 16c 17 14c 16d 20 16a 16c 30 16c 16 24 17 14d 16b 20 26 171 12 14c 178 14a 14 14b 14c 30 Figure 1

Description

1/9

40 38

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Figure 1

"Gravity-based structure for off-shore structures"

Technical Field

[0001] The disclosure relates to a gravity-based structure for off-shore structures, in particular to a fully assembled and self-installing gravity-based structure for off-shore structures such as wind turbines and oil and gas offshore platforms. The disclosure also provides a method of installing an off-shore structure and an off-shore structure assembly.

Background

[0002] The discussion of the background to the disclosure is intended to facilitate an understanding of the disclosure. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.

[0003] Large derrick barges or other vessel types with crane hook capacities sufficient for lifting and installing platforms as well as supporting equipment for foundation piling are often required to install off-shore structures. The crew rates for offshore work are high and a large number of personnel are normally required to operate the vessel, install the platform and to provide supporting services. Specialised equipment is normally also required to install piles, grout connections, or provide diving services. This equipment is expensive and may also require trained personnel to operate the equipment. The vessel charter rate combined with the high crew rate and specialised equipment rates results in an expensive day rate when using such installation techniques and related vessels.

[0004] The mobilisation and de-mobilisation cost in combination with the high day rate makes such vessels extremely expensive to use.

[0005] Foundations in Australia's North West Shelf and also in Bass Strait have often experienced problems with the installation of piles. Piles have sometimes underperformed against expectations and required substantial remedial work involving additional piling or even bracing structures to be installed. In the North West Shelf the Calcarenite Limestone formations have required expensive and time consuming drilled and grouted piles to be installed. This necessitates specialised and expensive equipment for installation.

[0006] Given the high costs of utilising vessels to lift structures or to install piles, a self-installing gravity-based structure, not requiring a derrick barge or other vessel with a crane, would be beneficial. A gravity-based structure provides a lower cost solution with less risk in the installation phase where piling problems or weather downtime are often encountered whilst having to support expensive day rate equipment and crew.

[0007] Patent number W02018/150063 describes a gravity-based structure for off shore structures, based on a triangular, flat, concrete base with three vertical concrete columns arranged on the corners. It has the ability to allow a fully assembled wind turbine to be floated at sea and transported to the required installation field. It also can be ballasted using only water for ballast with the vertical columns providing floatation stability during submergence of the flat concrete base below the water surface. No additional auxiliary support means is required. Hence the gravity base can be re floated for removal or relocation.

[0008] The gravity-based structure described in patent W02018/150063 requires the structure to be constructed from reinforced concrete. Reinforced concrete has a large mass, but a relatively low density such that when placed below water the submerged weight is significantly reduced. Hence a very large, initial in-air weight is required to be constructed necessitating either a dry dock, casting basin or very large capacity wharf for construction and subsequent floating or skidding across the wharf to a dry tow vessel.

[0009] The wind turbine tower is centrally disposed on the flat concrete base. In addition, the structure is limited to off-shore installations in a water depth of between m to 50m.

[0010] Other gravity-based structures have been used for off-shore structures in a variety of configurations and some have been constructed from steel as well as reinforced concrete. Reinforced concrete has been used to provide sufficient submerged mass for on-bottom stability. When steel gravity bases have been used, additional solid ballast has been required to be introduced subsequent to the installation onto the sea bed. In addition, auxiliary buoyancy tanks or other equipment has been used in some cases to stabilise the structure during installation.

[0011] A number of oil and gas offshore platforms have notably been constructed such that the substructure may be floated into a deep fiord prior to separately floating the superstructure and mating the two. None of these design concepts have been fully assembled with the superstructure and substructure combined in a fabrication yard.

[0012] Other structures which are self-installing often use suction bucket type foundations and utilise a telescopic type arrangement whereby the floating structure is not fully assembled in the final position, but has to be lowered and/or raised into position offshore.

[0013] The gravity-based structure as described herein seeks to overcome at least some of the problems described above.

Summary

[0014] The disclosure provides a gravity-based structure to support off-shore structures, such as wind turbines and oil and gas platforms. The disclosure also provides a method of installing an off-shore structure and an off-shore structure assembly.

[0015] One aspect of the disclosure provides a gravity-based structure to support off shore structures, said gravity-based structure comprising: a submersible frame having a primary base body, an auxiliary base body spaced apart from the primary base body in parallel alignment therewith, wherein the auxiliary base body is shorter in length than the primary base body, and a plurality of connecting members to interconnect the auxiliary body to the base body; and, a single support tower mounted on the primary base body, the single support tower being configured, in use, to support the off-shore structure.

[0016] In one embodiment, the gravity-based structure maybe provided with a ballasting system arranged, in use, to controllably flood the submersible frame with water to cause the submersible frame to settle on a mudline of an off-shore installation site.

[0017] In one embodiment, an effective height of the one or more support towers of the gravity-based structure is sufficient to dispose the off-shore structure above a waterline when the submersible frame of the gravity-based structure is disposed on the mudline.

[0018] In one embodiment, the primary base body and the auxiliary base body each comprise a respective hollow tubular portion and a hollow rectilinear portion depending from an underside of the hollow tubular portion. The hollow tubular portions and the hollow rectilinear portions may be configured to be flooded with water independently of one another.

[0019] In one embodiment, the auxiliary base body maybe shorter than or equal in length to the primary base body.

[0020] In one embodiment, the hollow rectilinear portions may contain solid ballast therein.

[0021] In a further aspect there is provided an off-shore structure assembly comprising an off-shore structure mounted on a gravity-based structure as defined above.

[0022] In one embodiment, the off-shore structure comprises a wind turbine or an oil and gas platform/topsides.

[0023] In one embodiment, said assembly may be provided with a ballasting system arranged, in use, to progressively ballast the submersible frame of the gravity-based structure with water to cause the submersible frame to settle on a mudline of an off shore installation site.

[0024] In one embodiment, said assembly is capable of being configured in a wet tow draft by partially flooding the submersible frame with water.

[0025] In one embodiment, said assembly is capable of being installed on an off shore installation site by fully flooding the submersible frame, and optionally the one or more support towers, with water.

[0026] In a further aspect, there is provided a method of installing an off-shore structure comprising the steps of: (a) pre-assembling the off-shore structure with a gravity-based structure as defined above; (b) positioning the pre-assembled off-shore structure and gravity-based structure ("said assembly") proximal to an off-shore installation site; and (c) progressively ballasting the submersible frame of the gravity-based structure of said assembly with water to cause the submersible frame to settle on a mudline of the off-shore installation site.

[0027] In one embodiment, the step of positioning said assembly proximal to an off shore installation site comprises: loading said assembly onto a semi-submersible vessel; dry towing said assembly with the semi-submersible vessel to a pre-determined location; submerging the semi-submersible vessel and partially flooding the primary base body and auxiliary base body of the submersible frame to configure said assembly in a wet tow draft; and wet towing said assembly to the off-shore installation site.

[0028] In one embodiment, the primary base body and the auxiliary base body of the submersible frame each comprise a respective hollow tubular portion and a hollow rectilinear portion depending from an underside of the hollow tubular portion, whereby the hollow rectilinear portions of the primary and auxiliary base bodies are flooded to configure said assembly in the wet tow draft.

[0029] In one embodiment, progressively ballasting the submersible frame of the gravity-based structure of said assembly comprises flooding the hollow tubular portion of the primary base body or the auxiliary base body with water to cause said base body to descend, and then flooding the hollow tubular portion of the other of the primary base body or the auxiliary base body, and optionally the single support tower, with water so that the submersible frame resides substantially horizontally on the mudline.

Brief Description of Drawings

[0030] Notwithstanding any other forms which may fall within the scope of the process as set forth in the Summary, specific embodiments will now be described with reference to the accompanying figures below:

[0031] Figure 1 shows a perspective view of one embodiment of a gravity-based structure to support an off-shore structure as described herein;

[0032] Figure 2 shows a perspective view of an alternative embodiment of a gravity based structure to support an off-shore structure as described herein;

[0033] Figure 3 shows a perspective view of another embodiment of a gravity-based structure to support an off-shore structure as described herein;

[0034] Figure 4 shows a perspective view of an off-shore wind turbine assembly installed on a seabed and supported by the gravity-based structure shown in Figure 1;

[0035] Figure 5 shows a perspective view of an off-shore oil and gas platform installed on a seabed and supported by another embodiment of the gravity-based structure disclosed herein;

[0036] Figure 6 shows a representation of an oil and gas platform being mounted on the gravity-based structure shown in Figure 5 at a wharf with a harbour based crane;

[0037] Figure 7 shows a representation of the pre-assembled oil and gas platform mounted on the gravity-based structure shown in Figure 6 being skidded onto a semi submersible vessel for dry tow;

[0038] Figure 8 shows a representation of the pre-assembled oil and gas platform mounted on the gravity-based structure shown in Figure 7 being partially submerged for wet tow via a plurality of tugs; and,

[0039] Figure 9 shows a representation of the pre-assembled oil and gas platform mounted on the gravity-based structure installed at the off-shore site by fully submerging the submersible frame of said structure.

Description of Embodiments

[0040] The disclosure relates to a gravity-based structure to support an off-shore structure, in particular a fully assembled and self-installing gravity-based structure to support off-shore structures such as wind turbines and oil and gas platforms. The disclosure also provides a method of installing an off-shore structure and an off-shore structure assembly.

GENERALTERMS

[0041] Throughout this specification, unless specifically stated otherwise or the context requires otherwise, reference to a single step, composition of matter, group of steps or group of compositions of matter shall be taken to encompass one and a plurality (i.e. one or more) of those steps, compositions of matter, groups of steps or groups of compositions of matter. Thus, as used herein, the singular forms "a", "an" and "the" include plural aspects unless the context clearly dictates otherwise. For example, reference to "a" includes a single as well as two or more; reference to "an" includes a single as well as two or more; reference to "the" includes a single as well as two or more and so forth.

[0042] Each example of the present disclosure described herein is to be applied mutatis mutandis to each and every other example unless specifically stated otherwise. The present disclosure is not to be limited in scope by the specific examples described herein, which are intended for the purpose of exemplification only. Functionally-equivalent products, compositions and methods are clearly within the scope of the disclosure as described herein.

[0043] The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

[0044] When an element or layer is referred to as being "on", "engaged to", "connected to" or "coupled to" another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being "directly on," "directly engaged to", "directly connected to" or "directly coupled to" another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., "between" versus "directly between," "adjacent" versus "directly adjacent," etc.).

[0045] Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as "first," "second," and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

[0046] Reference to positional descriptions, such as lower and upper, are to be taken in context of the embodiments depicted in the figures, and are not to be taken as limiting the invention to the literal interpretation of the term but rather as would be understood by the skilled addressee.

[0047] Spatially relative terms, such as"inner," "outer," "beneath", "below", "lower", "above", "upper" and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the example term "below" can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[0048] The term "and/or", e.g., "X and/or Y" shall be understood to mean either "X and Y" or "X or Y" and shall be taken to provide explicit support for both meanings or for either meaning.

[0049] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[0050] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

[0051] The term "about" as used herein means within 5%, and more preferably within 1%, of a given value or range. For example, "about 3.7%" means from 3.5 to 3.9%, preferably from 3.66 to 3.74%. When the term "about" is associated with a range of values, e.g., "about X% to Y%", the term "about" is intended to modify both the lower (X) and upper (Y) values of the recited range. For example, "about 20% to 40%" is equivalent to "about 20% to about 40%".

GRAVITY-BASED STRUCTURE TO SUPPORT AN OFF-SHORE STRUCTURE

[0052] Referring to the figures, where like numerals refer to like parts throughout, there are shown various embodiments of a gravity-based structure 10, 10'to support off-shore structures 100 such as wind turbines, oil and gas topsides and platforms, and soforth.

[0053] The gravity-based structure 10, 10' comprises a submersible frame 12 including a primary base body 14 and an auxiliary base body 16 spaced apart from the base body 14 in parallel alignment therewith and interconnected by a plurality of connecting members 18.

[0054] It will be appreciated that the submersible frame 12 is generally horizontally disposed relative to a ground surface (when on land), a mudline (when submerged), or a waterline (when buoyant or submerged).

[0055] The auxiliary base body 16 may be shorter than or equal in length to the primary base body 14 but otherwise their cross-sectional dimensions may be the same. The primary base body 14 and the auxiliary base body 16 each comprise a respective tubular portion 14a, 16a and a generally rectilinear portion 14b, 16b downwardly depending from the tubular portion 14a, 16a. The tubular portions 14a, 16a and generally rectilinear portions 14b, 16b are configured, in use, to be controllably flooded with water to facilitate installation of the gravity-based structure 10, ' on the mudline.

[0056] The tubular portions 14a, 16a are hollow and may be divided into compartments 14c, 16c with one or more internal diaphragms 17 (as required) to control installation flooding.

[0057] Alternatively, the tubular portions 14a, 16a may be filled with air to facilitate dry- or wet-towing of the gravity-based structure 10, 10' or re-flotation of the gravity based structure 10, 10' from the mudline. Advantageously, the tubular portions 14a, 16a are more likely to withstand the existing hydrostatic pressure prior to flooding with water or such water pressure head required for re-floating the gravity-based structure , 10'.

[0058] The generally rectilinear portions 14b, 16b include a hollow compartment 14d, 16d and a skirt depending from a periphery of an underside of the hollow compartment. In use, the hollow compartments 14d, 16d may be flooded after initial flotation, as will be described later in more detail, thereby limiting the hydrostatic water pressure acting on them during the submergence process. The skirts engage the mudline and helps the submersible frame 12 to resist lateral sliding on the mudline.

[0059] The hollow compartments 14d, 16d may have substantially vertical walls, as shown in Figures 1 to 5. In alternative embodiments, the hollow compartments 14d, 16d may have outwardly sloping walls as shown in Figure 6 so that the skirts depending therefrom has a comparatively larger footprint. These alternative embodiments are particularly suitable for gravity-based structures for supporting larger off-shore structures such as oil and gas topsides.

[0060] The generally rectilinear portions 14b, 16b may also include a plurality of stiffeners and stringers in the hollow compartment 14d, 16d for rigidification. The stiffeners may be perforated to allow free-flooding of the hollow compartments 14d, 16d.

[0061] The gravity-based structure 10, 10' may include a suitable ballasting system including a manifold, one or more pumps and valves associated with said hollow compartments 14c/d, 16c/d to facilitate ingress of water (or air) into, and egress of water from, the tubular portions 14a, 16a and the rectilinear portions 14b, 16b. It will be appreciated that the hollow compartments 14c/d, 16c/d of the tubular portions 14a. 16a and the rectilinear portions 14b, 16b may be configured to be flooded with water independently of one another.

[0062] The hollow compartments 14d, 16d of the rectilinear portions 14b, 16b may also optionally contain solid ballast material, such as scrap steel or pig iron, which may be introduced therein during fabrication. Other than water, no additional ballasting offshore is required.

[0063] The connecting members 18 may include one or more brace members 20 and/or one or more trusses 22. It will be appreciated that the one or more brace members 20 and/or one or more trusses 22 may be configured in any one of a plurality of suitable arrangements, depending on the size and weight of the gravity-based structure 10, 10', the height and weight of the off-shore structure 100, and the depth of water in which the gravity-based structure 10, 10' may be installed. In particular, the connecting members 18 may be configured so that the submersible frame 12 comprises a moment-resisting frame. The term 'moment-resisting frame', as used herein, refers to a frame that uses rigid connection between each of its constituent members and consequently is able to resist lateral and overturning forces because of the bending moment and shear strength that is inherent in its members and associated connecting joints.

[0064] For example, in the embodiments shown in Figures 1 and 2, the brace member 20 extends generally at right angles between the primary base body 14 and the auxiliary base body 16. In these particular embodiments, a pair of brace members extends from respective opposing end portions 24 of the auxiliary base body 16 to the primary base body 14. Alternatively, or additionally, the connecting members 18 may include a pair of trusses 22 extending at a diverging angle from a generally central portion 26 of the primary base body 14 to respective opposing end portions 20 of the auxiliary base body 16. In alternative embodiments, the trusses 22 may extend generally at right angles between the primary base body 14 and the auxiliary base body 16.

[0065] In the embodiment shown in Figure 3, the connecting member 18 comprises a caisson 28 extending generally at right angles between the primary base body 14 and the auxiliary base body 16. The caisson 28 comprises a respective tubular portion 28a and a rectilinear portion 28b depending from an underside of the tubular portion 28a. In this particular embodiment, a pair of caissons 28 extend from respective opposing end portions 24 of the auxiliary base body 16 to respective end portions 30 of the primary base body 14. Generally, caissons 28 may be employed in addition to or as an alternative to trusses 22 to increase the foundation bearing capacity of the submersible frame 12 for large off-shore structures, in particular oil and gas platforms. It will be appreciated that the caissons 28 may have similar cross-sectional dimensions as the primary and auxiliary base bodies 14, 16.

[0066] One or more support towers 32 are mounted on the primary base body 14 and, optionally, the auxiliary base body 16. The one or more support towers 32 are configured, in use, to support an off-shore structure, such as a wind turbine as shown in Figure 4, oil and gas topside as shown in Figure 5 or other offshore superstructure.

[0067] In the embodiment shown in Figures 1, 2, 4 and 5, the support tower 32 is equidistantly spaced between respective opposing end portions 30 of the primary base body 14 and is mounted on an uppermost side of the tubular portion 14a of the primary base body 14. Eccentric placement of the support tower 32 with respect to the submersible frame 12 may be particularly advantageous in embodiments where the gravity-based structure 10, 10' supports an oil and gas wellhead platform which frequently requires close positioning of wells for access via a jack-up drilling rig.

[0068] The support tower 32 includes a cylindrical lower portion 34 and a tapered cylindrical upper portion 36. In other embodiments, the support tower 32 may have a constant cross-sectional area along its entire length. A free end 38 of the support tower 32 may be provided with a mounting plate 40 on which the off-shore structure 100, 100' is mounted.

[0069] In the embodiment shown in Figures 2 and 5, a pair of further support towers 32' are integral with respective end portions 24 of the auxiliary base body 16. The support tower 32' may be a cylindrical column. The further support towers 32' may be shorter than or equal in height to the support tower 32 but otherwise their cross sectional dimensions may be the same.

[0070] In the embodiment shown in Figure 3, a pair of support towers 32 are mounted on respective end portions 30 of the primary base body 14 and a pair of further support towers 32' are mounted on respective end portions 24 of the auxiliary base body 16. Depending on the height of the support towers 32', it will be appreciated that additional bracing may be provided between the support towers 32'.

[0071] The support tower 32 maybe associated with the ballasting system. For example, the support tower 32 may be provided with a submersible pump to pump water out of the support tower 32 and the other flooded hollow compartments 14c/d, 16c/d.

[0072] It will be appreciated from the foregoing description that the invention may be configured for a variety of water depths and off-shore structures. The submersible frame 12 may be provided with further support towers to either support the off-shore structure 100, 100' and/or to provide additional stability during installation, as will be described later. It will be appreciated that the effective height of the one or more support towers of the gravity-based structure will be selected to be sufficient to dispose the off-shore structure above a waterline when the submersible frame of the gravity-based structure is disposed on the mudline. Unlike the gravity-based structure described in W02018/150063, the gravity-based structure 10, 10' may be located in a wide range of water depths, including and exceeding 20m-50m.

[0073] The gravity-based structure 10, 10' may be fabricated from steel. Consequently, it may be conveniently constructed at existing steel fabrication yards with access to a body of water via a wharf. In contrast to the gravity-based structure described in W02018/150063, neither an expensive dry dock nor a casting basin is required. Steel has a high density (Specific Gravity SG 7.85) compared to reinforced concrete (SG 2.4-3.0). Hence, when submerged, the resulting effective density is not reduced as much as reinforced concrete [i.e. 2.4-1.025 (sea water SG) = 1.375 for concrete compared with 7.85-1.025 = 6.825 for steel]. This allows a much lighter in-air weight for the steel gravity-based structure 10, 10' than would otherwise be possible using reinforced concrete. Additionally, depending on the how the gravity-based structure 10, 10' is configured to suit an installation water depth, superstructure weight and elevation, the gravity-based structure 10, 10' is more capable of being skidded out across moderate capacity wharves due to the relatively light weight of steel compared to reinforced concrete.

[0074] Figures 6 to 9 illustrate a sequence of stages in the assembly and installation of the off-shore structure and gravity-based structure at an off-shore site. The off shore structure, such as a wind turbine 100 or an oil and gas platform 100' may be assembled with the gravity-based structure 10, 10' at the fabrication yard prior to towing and installation. For example, the off-shore structure 100, 100' may be mounted on the mounting plate 40 of the gravity-based structure 10, 10 via land based cranes or by harbour cranes, as shown in Figure 6. Consequently, large derrick barges or other vessel types with crane hook capacities sufficient for lifting and installing off-shore structures at the off-shore installation site are not required.

[0075] Furthermore, when the gravity-based structure 10, 10' is used to support an oil or gas producing wellhead platform, the well conductors or casings thereof may be passed down through the supporting tower 32 where they may be protected from environmental loading and accidental boat impact.

[0076] The pre-assembled off-shore structure 100, 100' and gravity-based structure , 10' ("the assembly") may then be skidded or otherwise loaded out onto a semi submersible barge across a wharf as shown in Figure 7. The assembly may then be sea-fastened to the barge ready for a dry tow transportation to the intended installation site. With such an arrangement, the assembly may be transported great distances to locations remote from the fabrication yard or it may be wet towed to a nearby site without requiring a dry tow phase.

[0077] Once at or near the intended installation site, the assembly may be offloaded from the dry tow barge by submerging the deck of the barge and floating the assembly off assisted by tugs. The submersible frame 12 has a low draft in this condition by virtue of the vertical walls and skirts of the rectilinear portions 14b, 16b where only low hydrostatic pressure heads are experienced.

[0078] On completion of the float-off operation, the assembly may be ballasted with sea water to a wet tow draft wherein the hollow compartments of the rectilinear portions 14b, 16b are completely flooded. Floatation buoyancy is then provided by the tubular portions 14a, 16b of the primary base 14 and auxiliary base 16 of the submersible frame 12. The tubular portions 14a, 16b have a greater capacity to resist hydrostatic forces which allows the assembly to be installed in deep water.

[0079] The assembly may then be wet towed via tugs to the intended installation site, where the tubular portion 14a of the primary base body 14 is first flooded with water. The tubular portion 14a sinks so that the submersible frame 12 is initially at an angle to the mudline, the auxiliary base body 16 and/or any further support towers 32' being used to stabilise the submersible frame 12 as it descends to the mudline. The tubular portion 16a of the auxiliary base 16 is then subsequently flooded with water whereupon the primary base body 14 and the auxiliary base body 16 of the submersible frame 12 rest generally horizontally on the mudline as shown in Figure 9. It will be appreciated that in other embodiments, the submersible frame 12 may be progressively ballasted by flooding the tubular portion 16a of the auxiliary base body 16 followed by flooding the tubular portion 14a of the primary base body 14.

[0080] It is envisaged that no additional ballasting may be required for on-bottom stability of the assembly during its operational life.

[0081] The gravity-based structure 10, 10' may be re-floated and re-submerged at a different location, or removed from service. Re-floating the gravity-based structure 10, ' may be achieved by either pumping water from the hollow compartments 14c/d, 16c/d of the tubular and rectilinear portions 14, 16 of the submersible frame 12 or displacing water therefrom with air. Fluidisation of the mudline foundation may be required to overcome suction forces and this may be configured in the ballast system with water being pumped from the offshore structure down through ballast piping.

[0082] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

[0083] In the claims which follow and in the preceding description except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

CLAIMS:

1. A gravity-based structure to support off-shore structures, said gravity-based structure comprising: a submersible frame having a primary base body, an auxiliary base body spaced apart from the primary base body in parallel alignment therewith, wherein the auxiliary base body is shorter in length than the primary base body, and a plurality of connecting members to interconnect the auxiliary base body to the primary base body; and, a single support tower mounted on the primary base body, the single support tower being configured, in use, to support the off-shore structure.

2. The gravity-based structure according to claim 1, wherein the single support tower is equidistantly spaced between respective opposing end portions of the primary base body.

3. The gravity-based structure according to claim 1, wherein the single support tower is eccentrically mounted on the primary base body.

4. The gravity-based structure according to any one of claims 1 to 3, wherein the auxiliary base body has a pair of further support towers mounted on respective end portions thereof, wherein the pair of further support towers are shorter than the single support tower mounted on the primary base body.

5. The gravity-based structure according to any one of claims 1 to 4, further comprising a ballasting system arranged, in use, to controllably flood the submersible frame with water to cause the submersible frame to settle on a mudline of an off-shore installation site.

6. The gravity-based structure according to claim 5, wherein the ballasting system is configured to progressively ballast the primary base body and the auxiliary base body independently of one another.

7. The gravity-based structure according to any one of claims 1 to 6, wherein an effective height of the single support tower of the gravity-based structure is sufficient to dispose the off-shore structure above a waterline when the submersible frame of the gravity-based structure is disposed on the mudline.

8. The gravity-based structure according to any one of claims 1 to 7, wherein the primary base body and the auxiliary base body each comprise a respective hollow tubular portion and a hollow rectilinear portion depending from an underside of the hollow tubular portion.

9. The gravity-based structure according to claim 8, wherein the hollow tubular portions and the hollow rectilinear portions are configured to be flooded with water independently of one another.

10. The gravity-based structure according to claim 8 or claim 9, wherein the hollow rectilinear portions contain solid ballast therein.

11. An off-shore structure assembly comprising an off-shore structure mounted on a gravity-based structure as defined in any one of the preceding claims.

12. The off-shore structure assembly according to claim 11, wherein the off-shore structure comprises a wind turbine or an oil and gas platform/topsides.

13. The off-shore structure assembly according to claim 11 or claim 12, wherein said assembly is provided with a ballasting system arranged, in use, to progressively ballast the submersible frame of the gravity-based structure with water to cause the submersible frame to settle on a mudline of an off-shore installation site.

14. The off-shore structure assembly according to any one of claims 11 to 13, wherein said assembly is capable of being configured in a wet tow draft by partially flooding the submersible frame with water.

15. The off-shore structure assembly according to any one of claims 11 to 14, wherein said assembly is capable of being installed on an off-shore installation site by fully flooding the submersible frame, and optionally the single support tower, with water.

16. A method of installing an off-shore structure comprising the steps of: (a) pre-assembling the off-shore structure with a gravity-based structure as defined in any one of claims 1 to 10; (b) positioning the pre-assembled off-shore structure and gravity-based structure ("said assembly") proximal to an off-shore installation site; and (c) progressively ballasting the submersible frame of the gravity-based structure of said assembly with water to cause the submersible frame to settle on a mudline of the off-shore installation site.

17. The method according to claim 16, wherein positioning said assembly proximal to an off-shore installation site comprises: loading said assembly onto a semi-submersible vessel; dry towing said assembly with the semi-submersible vessel to a pre-determined location; submerging the semi-submersible vessel and partially flooding the primary base body and auxiliary base body of the submersible frame to configure said assembly in a wet tow draft; and wet towing said assembly to the off-shore installation site.

18. The method according to claim 16 or claim 17, wherein the primary base body and the auxiliary base body of the submersible frame each comprise a respective hollow tubular portion and a hollow rectilinear portion depending from an underside of the hollow tubular portion, whereby the hollow rectilinear portions of the primary and auxiliary base bodies are flooded to configure said assembly in the wet tow draft.

19. The method according to claim 18, wherein progressively ballasting the submersible frame of the gravity-based structure of said assembly comprises flooding the hollow tubular portion of the primary base body or the auxiliary base body with water to cause said base body to descend, and then flooding the hollow tubular portion of the other of the primary base body or the auxiliary base body, and optionally the single support tower, with water so that the submersible frame settles substantially horizontally on the mudline.