EP2559814B1

EP2559814B1 - Gravity foundation

Info

Publication number: EP2559814B1
Application number: EP12177998.7A
Authority: EP
Inventors: Andrew Grigsby; Henrietta Ridgeon
Original assignee: Gravitas Offshore Ltd
Current assignee: Gravitas Offshore Ltd
Priority date: 2011-08-15
Filing date: 2012-07-26
Publication date: 2015-04-15
Anticipated expiration: 2032-07-26
Also published as: GB201113993D0; GB2493720A; EP2559814A1

Description

The present invention relates to a gravity foundation for an offshore structure and to a method for deploying a gravity foundation.
Gravity foundations are used for many types of offshore structure when the location and the type of the structure make it feasible or advantageous to support the structure on the sea-bed. This type of foundation is generally towed into place in a floating state or transported using a specialist barge. It is then submerged and located on the sea-bed. Typically the height of the foundation will ensure that a tower or platform remains above sea level after submersion of the lower parts of the structure. Example applications include wind turbines, weather stations, lighthouses, offshore platforms for the oil and gas industry and so on. The offshore wind industry is now developing windfarms that are intended to be located some distance from land in relatively deep water (>20m). It is beneficial to deploy large wind turbines offshore as the constraints of transporting components by land are eliminated. Planning regulations and other potential restrictions for a land based windfarm can also be avoided. Designs of horizontal and vertical axis wind turbines rated to 10 MW or more are in development and will be available for commercial deployment in such offshore locations in the near future.
Known foundation structures for wind turbines installed nearer to shore are predominantly steel monopile or concrete gravity foundations. The existing concrete gravity foundations are typically lowered to the sea-bed using high-capacity cranes or specially modified barges with lowering equipment erected on them. Monopile foundations are likely to prove unsuitable for the deeper water applications and larger turbines and the weather conditions are too exposed away from shore to contemplate lowering a concrete foundation to the sea-bed by the method adopted hitherto. Hence, there is a need for an improved foundation structure for offshore structures.
EP-A-2236676 discloses a gravity foundation, especially for an offshore wind turbine. The foundation comprises a cylindrical base topped by a frustroconical section which is joined to a shaft. The interior of the foundation can be flooded in order to sink the foundation. In the figures, the base is shown as including a central cylindrical walled section, with radial walls extending therefrom.
US-A-4117691 discloses a floating offshore drilling platform. The platform has a hollow, generally circular, horizontal floating body. The body includes a central vertical passage therethrough and the interior of the body spaced outwardly of and about the central portion includes structures defining ballast tanks which may be selectively flooded and evacuated of water.
Viewed from a first aspect, the present invention provides a gravity foundation for an offshore structure, the gravity foundation comprising: a lower portion for submersion beneath water, the lower portion including an inner compartment and a plurality of outer compartments arranged around the inner compartment; characterised by a ballast water distribution system for flooding the compartments of the lower portion with water during deployment of the foundation; and in that the ballast water distribution system comprises: a first fluid connection for introducing water into the inner compartment; and a plurality of second fluid connections each for passage of water from the inner compartment to an outer compartment; the second fluid connections each including: an outlet in an outer compartment, the outer compartment being at a first side of the inner compartment; and an inlet positioned within the inner compartment at a higher level than the outlet and at second side of the inner compartment generally opposite to the location of the outer compartment.
With the ballast water distribution system described above the deployment of the foundation by ballasting to submerge the foundation is essentially self-righting. Water may be introduced to the inner compartment until it reaches the level of the pipe inlets. It will then flow through the second fluid connections and via the outlets to fill the outer compartments. If the foundation is at zero trim then the compartments will flood evenly. If the foundation moves away from zero trim then water will preferentially flow into the pipe inlets on the lower side of the inner compartment. Since these inlets convey water to an outlet on the opposite side, then incoming water is preferentially directed to the outer compartments on the higher side of the foundation, causing the foundation to trim to restore itself level. In this way, the ballast water can be distributed to all outer compartments in a way that requires no valving or controls and maintains the foundation both statically stable and at near zero trim. The foundation can be ballasted from the sea surface to the sea-bed in this way and can then be filled up to still water level and/or ballasted with solid ballast for permanent installation, as required. In preferred embodiments the foundation has no internal moving parts for deployment thereof.
This gravity foundation can cater for greater water depths than known foundations and for the larger turbines that will be deployed further from shore. The risk of weather delays is greatly reduced since the foundation is far less at risk of capsizing. Also, since the gravity foundation is essentially self-deploying and has no internal moving parts this simplifies the marine operations associated with its installation. No specialist vessel is required to deploy the foundation, just a conventional tug for towing it to the deployment location. Also, complicated internal moving parts and valve assemblies are conventionally used for deployment of foundations for offshore structures of a similar scale. Such internal parts necessitate either manned deployment, with personnel present on the foundation as it deploys to the sea-bed, or at least the possibility of a manned intervention to address unexpected events and/or system failures during deployment. Since the new foundation requires no complicated internal parts there is no need to design the foundation to permit manned internal intervention during deployment, and this further simplifies the design.
The first fluid connection may be a connection between the inner compartment and the exterior of the foundation at a point below water level for the foundation in its floating state. In the floating state the foundation will typically be empty of ballast. With a connection to the exterior of the foundation in this way water can be conveyed into the inner compartment easily without the need for pumping. In a preferred embodiment the first fluid connection has an outlet in the inner compartment that is below the level of the inlets of the second fluid connections. The inlet for the first fluid connection may be generally level with the outlet when the foundation is trim.
Preferably, the first fluid connection includes a valve outside of the main body of the foundation. With the use of a valve exterior to the main body ballasting of the foundation can be easily controlled from the outside of the foundation. During deployment of the foundation it is straightforward to cease ballasting by closing the valve for the first fluid connection. The valve may be adapted for remote operation, for example by hydraulic or mechanical means. This permits the ballasting to be controlled from the water surface, via a support vessel or via the tug used to tow the foundation to its installation site. Advantageously, no personnel need to be in direct attendance on or near the foundation as it is ballasted and submerged. There may be multiple first fluid connections, for example there may be two first fluid connections that may be located on opposite sides of the foundation.
The second fluid connections may have inlets that are all at about the same height, preferably at the same height. Keeping the inlets at the same height ensures that all of the second fluid connections react in the same way when the foundation is out of trim.
By the phrase generally opposite it is meant that the inlet of the second fluid connection is at a opposed side to the outlet, preferably at a position between 160° to 200° (for example, about 180°) away from the outer compartment when the angular position of inlet and outlet is measured about a central axis of the structure. The inlet for the second fluid connection may be placed generally opposite the centre of the outer compartment where the outlet is located. The second fluid connection may comprise a pipe extending from the inlet to the outlet, preferably a pipe extending about an outer edge of the inner chamber.
The outlets of the second fluid connections are preferably located generally at the same height. The height of the inlets of the second fluid connections is higher than the height of the outlets of the second fluid connections in order to avoid back-flow of water through the second fluid connections. The inlets may be above the height of the outlets by at least a quarter of a maximum width or diameter of the inner compartment, preferably by at least a third of the width or diameter, for example the difference in height of the inlets and outlets of the second fluid connections may be about half the maximum width or diameter of the inner compartment. The required height difference relates to the maximum width or diameter of the inner compartment since it is necessary to provide a minimum flow angle along the length of the second fluid connections. The tendency of water to flow back from the outer compartments into the inner compartment will depend on the angle to which the foundation can go out of trim and the amount of sloshing in the outer compartments as the foundation, for example, responds to wave excitation.
In a preferred embodiment the lower portion of the gravity foundation is formed of a symmetrical arrangement of outer compartments. The symmetry should be rotational symmetry about a centre of gravity of the foundation when viewed in plan. The compartments may form a circular structure in plan view, with outer compartments formed as a ring of compartments about a circular inner compartment. Preferably, there are at least three outer compartments so that the ballast water distribution system can correct for non-zero trim in pitch and/or roll occurring at any angle. There may be more outer compartments to further improve the reaction to non-zero trim and also to ensure the foundation can be installed in a controlled manner without a loss of static stability arising from sloshing during the ballasting operation. For example, there may be six or eight compartments, or more.
Preferably the inner and/or outer compartments are open-topped. The compartments allow separation of water within the compartments when the ballasting operation begins, since at this stage the foundation is floating and is vulnerable to instability caused by sloshing. When the compartments are fully flooded then the foundation will be sufficiently submerged to be stable and hence further ballasting can be done without the need to utilise the distribution system. Instead, the compartments can overflow into one another. Preferably, the height of the walls dividing the inner and outer compartments and/or the height of the walls dividing the outer compartments from one another is set so that the foundation remains stable and upright when the ballast water fills the compartments to the tops of the walls. Thus, if the foundation has not reached the sea bed when the ballast water reaches the top of the walls then it will be in a stable and upright condition whilst partially submerged. The volume of the compartments enclosed by the walls may be set so that when they are full of water the foundation is no longer buoyant and/or so that the draft of the partially deployed foundation is larger than a predetermined minimum, for example the draft may be at least 20 m, at least 25 m or at least 30 m.
The compartments within the lower portion of the structure may be separated by structural walls. The use of structural walls between the compartments increases the rigidity of the structure and ensures satisfactory load distribution from the upper parts of the foundation structure to the sea-bed soils when the foundation is deployed.
The foundation is arranged to be installed directly onto the sea-bed preferably without any prior preparation of the sea-bed such as dredging or the installation of a levelling course of granular material. In preferred embodiments, the foundation includes a flat base slab with a skirt extending downward away from the base. The skirt may extend downward by at least 0.3 m, preferably by about 0.5 m or more. The skirt may extend around the outer edge or circumference of the base slab and may be subdivided into a number of compartments. Advantageously, the skirt will penetrate into the sea-bed soils to a depth where suitable soil strata are encountered that can ensure foundation stability when extreme wind and wave loads act on the offshore structure and the foundation. The flat base slab preferably extends out beyond the area of the compartments to form a flange about a lower edge of the lower portion.
The foundation may be arranged such that the skirt penetrates the sea-bed when the foundation is ballasted with water. Thus, the skirt may penetrate using a combination of the foundation's own self-weight and reduced water pressure within the skirt compartments. Preferably however the foundation, when in use, includes further ballast to provide additional weight and improve stability. A sand or gravel ballast may be used to fill a lower part of the foundation such that some or all of the foundation below the water line is ballasted with solid material.
Preferred embodiments of the foundation include an upper portion for supporting an offshore structure. Preferably, the upper portion extends above the lower portion to such a height that it is above still water level in the intended installation location. Thus, when the foundation is in use the upper portion may remain above still water level, preferably above high tide level in tidal waters. The upper portion of the foundation may include a tower section. The tower section is preferably arranged to permit the offshore structure to be secured to it. It may be a concrete or steel tower. The tower is preferably hollow. This provides a light rigid structure and also enables air to be expelled from the lower portion as it is flooded with water.
Typically the lower portion will have a larger width than the tower section. A middle portion may be provided to join the lower portion and the upper portion of the foundation. In a preferred embodiment the lower portion is generally cylindrical and the tower is generally cylindrical. With this arrangement, the middle portion may be a frustoconical section.
The foundation is preferably configured so that it can float unassisted when unballasted, preferably so that it can float in the moderate water depths found in typical port facilities, for example at depths of 10 m. The foundation can advantageously be transported to an offshore installation location using readily-available standard tugs.
Preferably the gravity foundation is a concrete gravity foundation. Concrete is well established as a suitable material for offshore gravity foundations of this type. The use of concrete provides the necessary density and durability for the foundation. Concrete may also easily be cast or formed into a structure of the required shape and size. In a preferred embodiment, lightweight reinforced concrete is used for upper parts of the foundation, for example lightweight reinforced concrete may be used for all parts aside from a base slab, which may be standard reinforced concrete.
The foundation has been developed with a focus on the offshore wind industry and hence in a preferred embodiment the foundation is a gravity foundation for an offshore wind turbine. The invention extends to an offshore wind turbine structure incorporating the described gravity foundation. However, the foundation may also be beneficially employed as a support structure for other offshore sectors such as oil and gas.
A preferred embodiment is intended for use in water depths of 25 m to 60 m, and may have a cylindrical lower portion with a diameter of 25 m to 45 m, with a tower section of 3 m to 8 m diameter. The inner compartment may have a diameter of 10 m to 20 m, with the outer compartments being formed as sectors of a ring around the inner compartment. An upper portion of the foundation, for example a tower section, may be designed to extend 10 m to 25 m above water level when the foundation is installed on the sea-bed.
Viewed from a second aspect, the invention provides a method of deployment of an offshore gravity foundation, the gravity foundation comprising: a lower portion for submersion beneath water, the lower portion including an inner compartment and a plurality of outer compartments arranged around the inner compartment; and a ballast water distribution system for flooding the compartments of the lower portion with water during deployment of the foundation; wherein the ballast water distribution system comprises: a first fluid connection for introducing water into the inner compartment; and a plurality of second fluid connections each for passage of water from the inner compartment to an outer compartment; the second fluid connections including: an outlet in an outer compartment, the outer compartment being at a first side of the inner compartment; and an inlet positioned within the inner compartment at a higher level than the outlet and at second side of the inner compartment generally opposite to the location of the outer compartment; the method comprising: filling the inner compartment with water to thereby permit water to flow along the second fluid connections from inlet to outlet in order to flood all the compartments with water.
The method may involve a foundation with any or all features of the gravity foundation described above in the first aspect and preferred embodiments thereof.
The method may comprise controlling flow of water through the first fluid connection from a remote location. In a preferred method, the deployment of the foundation is monitored and flow of water through the first fluid connection is slowed or halted when adverse conditions are detected. Adverse conditions may include pitch and/or roll of the foundation beyond a preset maximum angle, wind speed and/or wave motion exceeding preset limits, and/or a prediction of weather exceeding a preset level of severity. A weather station on the foundation or in the vicinity may be used to monitor weather conditions.
The foundation is preferably ballasted with water until it is no longer buoyant. As the foundation submerges, buoyancy will increase again due to submersion of hollow parts of the structure above the lower portion. The method may hence include continued ballasting of the foundation until it reaches the sea-bed. The foundation is preferably installed directly onto the sea-bed without any prior preparation of the sea-bed such as dredging or the installation of a levelling course of granular material. In preferred embodiments, the foundation includes a flat base slab with a skirt extending downward away from the base and the method includes using the foundation's self-weight to push the skirt into sea-bed soils in order to provide foundation stability.
After the foundation has reached the sea-bed, the method may include continuing to flood with water until the internal water level equalises with the external water level. In preferred embodiments the method includes a step of ballasting with solid ballast, for example sand ballast, preferably after flooding with water is complete.
Preferred embodiments of the method include steps relating to transportation of the foundation from an assembly or manufacturing site to a desired offshore installation location. The foundation may be launched from land and floated in a port or similar, before being towed to the installation location, for example by a standard tug. No heavy lifting is required, since the foundation can be launched directly from a suitable slipway, launch facility, or dry dock and after towing to the installation location it can then be deployed in a controlled and stable manner using the ballast water distribution system.
The method may include installation of an offshore structure on the foundation subsequent to the ballasting and submersion operation. The offshore structure is preferably installed on an upper portion of the foundation that extends above water level, for example on a tower section. The foundation has been developed with a focus on the offshore wind industry and hence in a preferred embodiment the method may include installation of an offshore wind turbine. The upper portion and/or tower section can be of conventional construction and hence the installation of the offshore structure may proceed in a conventional fashion, without the need for special procedures or equipment.
Whilst sea level and the sea-bed is referred to above it will be understood that the foundation may also be used in any suitable body of water, such as seas, oceans, estuaries, inland lakes and reservoirs. Hence, any reference to sea level should be understood to mean a datum water level for the desired location of the foundation and similarly any reference to the sea-bed should be understood to refer to the bottom of the body of water.
Certain preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which:

Figure 1 is an isometric view of an embodiment of a gravity foundation;
Figure 2 is a close up view of an upper tower section of the gravity foundation;
Figure 3 shows a cutaway view of a lower part of the gravity foundation, with the ballast water distribution system not shown;
Figure 4 is an isometric view showing the lower portion of the gravity foundation including the ballast water distribution system;
Figure 5 is a cross-section of the lower portion of the gravity foundation including the ballast water distribution system;
Figure 6 is a diagram showing the foundation (in cross-section) being towed;
Figures 7 to 14 are a sequence of cross-sectional views illustrating the ballasting operation; and
Figure 15 is a cross-section of the deployed foundation including an indication of the preferred structural materials and ballast.

The gravity foundation of Figure 1 includes a lower portion 2 and an upper portion 4. In this preferred embodiment the gravity foundation has a circular cross-section. Hence the upper portion 4 includes a cylindrical tower section 6 and the lower portion 2 includes a cylindrical outer ring wall 8. A frustoconical section 10 connects the outer ring wall 8 and the tower section 6.
The lower portion 2 also includes a circular base slab 12, which extends across the entire underside of the outer ring wall 8 and extends outwardly therefrom to form a flange. The base slab is fitted with two tow points 14, which are mounting points for a tow line used when the foundation is floated to its installation location.
Figure 1 shows the foundation when installed on the sea-bed. It therefore also illustrates ballast used to surround and stabilise the base slab 12. This ballast comprises a layer of natural sea dredged aggregate 16, and a layer of olivine ballast 18 on top of the aggregate 16.
Further detail of the tower section 6 is shown in Figure 2. The tower section 6 in the preferred embodiment is fitted with an external platform 20 having a crane 22 and lay down area 24. Access ladders 26 are also included, which are for access to the platform 20 and hence to the offshore structure. The lower ladder 26 has a fender 28 to protect personnel from movement of adjacent vessels. An external intermediate platform 30 separates ladder stages. Advantageously parts such as the external platform 20 and ladders 26 are installed on the tower section 6 whilst the foundation is constructed on land. This means that the foundation can be ready for use and easily accessible as soon as it has been installed.
Further detail of the lower portion 2 and frustum 10 can be seen in Figure 3, which is cut-away to show the internal arrangement. Within the outer ring wall 8 various open compartments are formed. A central, inner, compartment 32 sits within a ring of outer compartments 34. Outer compartment walls 36 divide the outer compartments 34 from one another. The inner compartment 32 is encircled by an inner compartment wall 38. In this embodiment the inner compartment 32 is cylindrical and the outer compartments 34 are sectors of a ring about the inner compartment 32. The compartments are arranged symmetrically and there are eight outer compartments 34, all with the same size and shape. Two J-tubes 40, only partially visible in Figure 3 extends down from the upper part of the foundation into the inner compartment 32 and then through one of the outer compartments 34 before terminating outside of the outer ring wall 8. The J-tubes 40 are used for routeing electrical cables or control umbilicals from equipment supported by the foundation to the sea-bed.
Figure 3 also shows additional detail of arrangement of the olivine ballast 18 and the aggregate 16, which are formed as ring shaped layers about the outer edge of the flange of the base slab 12. Beneath the base slab 12 is a skirt 42, which is a circular protrusion aligned with the outer ring wall 8. The skirt 42 penetrates into the sea-bed when the foundation is deployed. The self-weight of the foundation and ballast within the foundation is used to push the skirt 42 into the sea-bed. The use of the skirt 42 helps avoid the need for preparation of the sea-bed and enables the foundation to cope with local unevenness in the sea-bed.
The ballast water distribution system 44 is shown in Figures 4 and 5. Figure 4 is an isometric view showing the pipe layout within the compartments 32, 34 and Figure 5 is a cross-sectional elevation. The ballast water distribution system 44 includes two first fluid connections 46 that extend between the exterior of the outer ring wall 8 and the inner compartment 32. By means of the first fluid connections 46 water is allowed from the surrounding sea into the inner compartment 32, to flood the inner compartment 32 as the foundation is ballasted and deployed. The first fluid connections 46 have an outlet in the inner compartment 32 part way up the inner compartment wall 38 as seen in Figure 5. The ballast water distribution system 44 further includes multiple second fluid connections 48 that join each of the outer compartments 34 to the inner compartment 32.
The second fluid connections 48 each have an inlet 50 and an outlet 52. The inlets 50 are all at the same height and are located toward the top of the inner compartment wall 38. Pipes extend around the wall 38 of the inner compartment 32 and pass through the wall 38 to join the inlets 50 to outlets 52 in each of the outer compartments 34. The outlets 52 are at a lower level than the inlets 50 and the second fluid connections 48 are arranged so that the inlets 50 are placed generally opposite their respective outlets 52, and hence are generally opposite the outer compartments 34 that they supply with water. Herein, the reference to a lower and higher level of the outlets 52 and inlets 50 is with reference to the normal orientation of the foundation. Thus, a higher level is a level nearer the upper portion 4 and a lower level is a level nearer the lower portion 2 and/or base 12. In this preferred embodiment the inlets 50 are located at the opposite side of the circular central compartment 32 to their respective outlets 52.
Figure 6 shows the foundation in a floating configuration being towed by a standard tug 54. A tow line 56 connects to the tow points 14 via a monkey plate 58. As the foundation is shown in cross-section in Figure 6 a greater extent of one of the J-tubes 40 can be seen. The J-tube 40 has an end 60 that protrudes outside of the outer ring wall 8. The towing distance T can vary in conventional fashion, from perhaps 50 m in an estuary to as much as 150 m or more in offshore waters. The draft D of the empty foundation structure is sufficiently small to enable towing in typical port and dock water depths, for example it may be about 9.5 m or below to allow towing in water depths with lowest astronomical tide (LAT) of as small as 10 m. Figure 6 indicates LAT and also ground level GL in a typical towing scenario. The centre of gravity 62 of the empty foundation is above LAT and sits above the base slab 12 by a height G, which is about 9.8 m in this embodiment. The radius of the base slab 12 for the illustrated embodiment is 19 m and it is intended for deployment in waters with LAT of perhaps 35 m. The skirt 42 protrudes from beneath the base by 0.5 m in this embodiment. Figure 15 shows further example dimensions for other parts of the gravity foundations
Deployment of the foundation will now be described with reference to Figures 6 through 15. First, the foundation is constructed ashore and floated as shown in Figure 6. It is towed to the installation location by a standard tug 54. At the chosen installation site a ballasting operation takes place leading to controlled submersion of the foundation and deployment to a chosen location on the sea-bed.
Figure 7 to 14 illustrate the ballasting operation using a sequence of cross-sectional elevations. The sea level at LAT is used as a reference level RL and in this example the depth of the sea-bed beneath LAT is 35 m. For clarity, the secondary steel work such as the external platform 20 and ladders 26 are not shown. Figure 7 shows the fully floating stage. The foundation floats at about the same draft D as the draft during towing and the inlets to the first fluid connections 46 are just below sea level LAT. Valves 64 for the first fluid connections 46 are closed. Flexible lines connect the first fluid connections 46 to floats or support vessels (not shown) and permit control of water flow into the inner compartment 32 from a remote location. Valves 64 can be present at the inlets of the first fluid connections 46 and at a point on the lines near to the water surface.
The second stage is flooding of the inner compartment 32, as shown in Figure 8. The valves 64 of the first fluid connections 46 are opened. As water ballast 66 fills the inner compartment 32 the draft D increases. When the inner compartment 32 is flooded up to the level of the inlets 50 for the second fluid connections 48 then water begins to flow into the outer compartments 34. Figure 9 shows the point where the inner compartment 32 is filled to the height of the inlets 50 by the water 66. Figure 10 shows the next stage, where the outer compartments 34 are filling. The draft D continues to increase steadily and the foundation is kept stable since the if the foundation is not at zero trim, water will preferentially flow into the pipe inlets 50 on the lower side of the foundation such that the water is directed to the outer compartments 34 on the higher side of the foundation, causing it to trim to restore itself level. The ballast water 66 can be distributed to all outer compartments 34 in a way that requires no valving or controls and maintains the foundation both statically stable and at near zero trim. Separation of the lower portion 2 of the foundation into multiple compartments 32, 34 acts to reduce instabilities arising from sloshing of the ballast water 66.
After the ballast water 66 has filled the inner compartment 42 to the height of the inlets 50 then the water level in the outer and inner compartments 34, 32 will equalise as shown in Figures 11 to 13. The foundation continues to submerge in a controlled manner as more water enters via the valves 64. Figures 12 and 13 show continued filling and increase of draft D of the foundation. In Figures 12 and 13, as the foundation approaches near to the sea-bed, at RL -35 m in this example, the outer compartments 34 are filled to the tops of the outer compartment dividing walls 36. In Figure 13 the skirt 42 begins to contact and penetrate the sea-bed. As the foundation touches down on the sea-bed the level of the ballast water 66 continues to increase, which pushes the skirt 42 into the material of the sea-bed until the base slab 12 of the foundation contacts and settles into the sea-bed. The foundation is then ballasted with water up to sea level, before being further ballasted with sand ballast 68, and then surrounded by the natural sea dredged aggregate 16 and olivine ballast 18 described above.
Figure 15 shows the final installed and ballasted configuration of the foundation in cross-section. By way of example, the entire structure may be about 50-60 m in height, with the outer ring wall 8 being about 9 m high, the frustoconical section being about 18 m high and with sides angled at about 30-35° from the vertical, and the remainder of the height being made up of the tower and a short (perhaps about 4 m high) transition section between the frustoconical part 10 and the tower section 6. The diameter of the base in this example can be about 19 m to the edge of the base slab 12, with the outer ring wall having a diameter of about 16 m and inner compartment wall having a diameter of about 6 m. The tower section 6 may have a diameter of about 3-3.5 m. The tower section 6 protrudes from sea level to a height of about 20 m at LAT. Sea water ballast 66 fills the tower section to sea level and also fills the majority of the frustoconical section 10. Sand ballast 68 fills the lower portion 2 of the foundation and a part of the frustoconical section 10, up to a level 70. The different materials used for the foundation are also illustrated in Figure 15. Different shading shows the different materials. The preferred material is concrete, with the base slab 12 and skirt 42 being cast in normal weight reinforced concrete and the remaining parts of the foundation, including the compartment walls 36, 38, the outer ring wall 8, the frustoconical section 10 and tower section 6, being cast in lightweight reinforced concrete.
When the foundation is installed the desired offshore structure can be completed by installation of appropriate structures on the tower section. The preferred embodiment illustrated herein is suited for receiving a wind turbine tower. Other applications are also possible.

Claims

A gravity foundation for an offshore structure, the gravity foundation comprising:
a lower portion for submersion beneath water, the lower portion including an inner compartment and a plurality of outer compartments arranged around the inner compartment; and characterized by:
a ballast water distribution system for flooding the compartments of the lower portion with water during deployment of the foundation;

wherein the ballast water distribution system comprises: a first fluid connection for introducing water into the inner compartment; and a plurality of second fluid connections each for passage of water from the inner compartment to an outer compartment; the second fluid connections each including: an outlet in an outer compartment, the outer compartment being at a first side of the inner compartment; and an inlet positioned within the inner compartment at a higher level than the outlet and at a second side of the inner compartment generally opposite to the location of the outer compartment.
A gravity foundation as claimed in claim 1, wherein the second fluid connections have inlets that are all at about the same height.
A gravity foundation as claimed in claim 1 or 2, wherein the inlet for each of the second fluid connections is placed generally opposite the centre of the outer compartment where the respective outlet is located.
A gravity foundation as claimed in claim 1, 2 or 3, wherein the inlets are above the height of the outlets by at least a quarter of a maximum width or diameter of the inner compartment.
A gravity foundation as claimed in any preceding claim, wherein the first fluid connection comprises a connection between the inner compartment and the exterior of the foundation at a point below water level for the foundation in an unballasted floating state.
A gravity foundation as claimed in any preceding claim, wherein the lower portion of the gravity foundation comprises a symmetrical arrangement of at least three outer compartments.
A gravity foundation as claimed in any preceding claim, wherein the height of walls dividing the inner and outer compartments and/or the height of walls dividing the outer compartments from one another is set so that the foundation remains stable and upright when the ballast water fills the outer compartments to the tops of the walls.
A gravity foundation as claimed in any preceding claim, wherein the foundation includes a flat base slab with a skirt extending downward away from the base.
An offshore wind turbine structure incorporating a gravity foundation as claimed in any preceding claim.
A method of deployment of an offshore gravity foundation, the gravity foundation comprising:
a lower portion for submersion beneath water, the lower portion including an inner compartment and a plurality of outer compartments arranged around the inner compartment; and

a ballast water distribution system for flooding the compartments of the lower portion with water during deployment of the foundation;

wherein the ballast water distribution system comprises: a first fluid connection for introducing water into the inner compartment; and a plurality of second fluid connections each for passage of water from the inner compartment to an outer compartment; the second fluid connections including: an outlet in an outer compartment, the outer compartment being at a first side of the inner compartment; and an inlet positioned within the inner compartment at a higher level than the outlet and at second side of the inner compartment generally opposite to the location of the outer compartment;

the method comprising: filling the inner compartment with water to thereby permit water to flow along the second fluid connections from inlet to outlet in order to flood all the compartments with water.
A method as claimed in claim 10, comprising deployment of a gravity foundation as claimed in any of claims 1 to 9.
A method as claimed in claim 10 or 11, wherein the foundation is installed directly onto the sea-bed without any prior preparation such as dredging or the installation of a levelling course of granular material.
A method as claimed in claim 10, 11 or 12, wherein the foundation includes a flat base slab with a skirt extending downward away from the base and the method includes using the foundation's self-weight to push the skirt into sea-bed soils.
A method as claimed in any of claims 10 to 13, comprising: installation of an offshore structure on the foundation subsequent to the ballasting operation.
A method as claimed in claim 14 wherein the offshore structure is a wind turbine.