NL2009579C2

NL2009579C2 - Sand bag wind turbine foundation.

Info

Publication number: NL2009579C2
Application number: NL2009579A
Authority: NL
Inventors: Willem Jan Lenstra
Original assignee: Stichting Energie
Priority date: 2012-10-05
Filing date: 2012-10-05
Publication date: 2014-04-08

Description

Sand Bag Wind Turbine Foundation Field of the invention

The present invention relates to a method of providing a foundation for an 5 offshore installation, such as a wind turbine, comprising providing a foundation having a foundation end part, erecting the foundation at an offshore site, and placing the foundation end part of the foundation on a seabed. In a further aspect, the present invention relates to a foundation for an offshore installation, such as a wind turbine, wherein the foundation comprises a foundation end part.

10 Prior art

The article Offshore wind turbine foundations - The COWI experience’, by J. H. Thomson et al. Proceedings of the 26th International Conference on Offshore Mechanics and Arctic Engineering, June 10-15, 2007 (OMAE2007-29567) discloses installation methods used for offshore foundation projects in European waters. Several 15 embodiments of gravity based foundations for wind turbines are discussed, where large concrete structures are used, which are sunken in position on the seabed.

International patent publication W02010/097199 discloses an anti-scour system for a seabed-mounted foundation. An arrangement of interconnected sedimentation elements is provided around a foundation, in order to prevent scouring of seabed 20 material around the foundation. In an exemplary embodiment, a mat of interconnected discarded vehicle tires is used.

The current state of the art typically uses large quantities of concrete for building gravity based foundations. Even though such heavy concrete foundation have been proven to provide durable and reliable stabilization to offshore installations, the 25 production of large quantities of concrete can be expensive and is not particularly eco-friendly. Furthermore, heavy concrete foundations often require equally heavy and expensive transportation and lifting equipment, putting definite limits on the feasible size of a gravity based foundation. Also, due to the weight of prior art gravity based foundations, it is generally not feasible to simultaneously install or remove the 30 foundation and offshore installation, i.e. the offshore installation is installed or removed separately from the foundation.

2

Summary of the invention

The present invention seeks to provide an improved gravity based foundation specifically suited for off shore applications, such as wind turbines.

According to a first aspect of the present invention, a method is provided as 5 defined above, further comprising surrounding (and/or (partly) covering) the foundation end part with a plurality of bags filled with ballast material. The ballast material is a material heavier than seawater, e.g. seabed material. Advantageously, the seabed material is seabed material locally extracted from the seabed in the vicinity of the foundation location.

10 The present invention embodiments thus comprise providing the foundation with onshore construction activities, transportation (e.g. towing) and overall logistics, forming a dedicated production and supply chain that reduces cost and increases production speed. The foundation may be erected separately from the offshore installation, or alternatively the foundation is integrally formed or permanently 15 connected with the offshore installation and both are erected simultaneously.

According to a further aspect of the present invention, a foundation is provided as defined above, further comprising a plurality of bags filled with ballast material surrounding the foundation end part.

The foundation rests on a seabed as a known gravity based foundation. The 20 ballast material may comprise (locally extracted) seabed material, e.g. sand extracted by means of a dredging installation in close vicinity of the offshore site. The plurality of bags surrounding and/or covering the foundation end part not only provide the stabilizing load to the foundation, but also provide scour protection near and around the foundation. As a result, the plurality of bags obviate the need for additional scour 25 protection solutions, such as concrete block, layers of rock, special mats, nets and/or skirts and the like. Furthermore, the plurality of bags are readily removed in case the foundation and offshore installation require transportation to an onshore facility for maintenance and/or repair work. Moreover, the plurality of bags can even be recycled when the foundation is permanently removed or replaced.

30 The gravity based foundation according to the present invention embodiments is much lighter than conventional gravity based foundations, because the stabilizing gravitational load is added to the foundation once it has arrived at the offshore site. In particular, the gravity based foundation of the present invention defines an empty 3 weight, which is the weight of the foundation without the plurality of bags that provide the stabilizing weight. Separating the stabilizing weight from the foundation allows for integrated designs, such as a foundation which is integrally formed or permanently connected with an offshore installation (e.g. wind turbine). The integrated design 5 significantly reduces the construction, transportation and installation time and complexity, since the foundation and offshore installation are handled as one single structure.

In an embodiment, the foundation end part of the foundation comprises a support structure for supporting the plurality of bags. The support structure comprises a 10 substantially flat support member perpendicular to a longitudinal axis of the foundation in an embodiment. The flat support member (e.g. a flat round foot) is arranged to rest on the seabed, wherein a plurality of bags filled with ballast material covers (and surrounds) the support member and provide the required stabilization and scour protection.

15 The flat support member may in a further embodiments accommodate a plurality of elongated bags (e.g. cylindrical bags), wherein each one of the plurality of bags comprises an open bag end and a buoyant bag rim disposed at the water surface. Typically, each bag extends from the water surface all the way toward the support member, forming a column of ballast material that imposes a gravitational load on the 20 foundation. The open bag end which is accessible from above the water surface allows for filling of the bag with ballast material. In an embodiment, the buoyant bag rim may be substantially hollow and comprises a gas such as air or helium for maximum buoyancy. In another embodiment, the bag rim may be substantially solid having a density lower than the water surrounding the foundation.

25 In a further embodiment, the foundation end part of the foundation comprises a mono-pile, which at installation is partly driven into the seabed. A plurality of bags filled with ballast material surrounds the foundation end part and covers a region of the foundation end part which is disposed above the seabed, providing scour protection and lateral stabilization. In a related embodiment, the foundation end part of the foundation 30 comprises a mono-pile partly driven into the seabed, wherein the foundation end part further comprises a collar around the mono-pile resting on the seabed. A plurality of bags filled with ballast material provide scour protection as well as lateral and axial 4 stabilization by covering the collar and a region of the foundation end part which is disposed above the seabed.

In yet a further embodiment, the support structure further comprises a containment body connected to the support member. The containment body is e.g.

5 implemented as a side wall disposed along the circumference of the support member. The support member and side wall form a bucket for accommodating the plurality of bags filled with ballast material. This embodiment allows for a relatively high stacked arrangement of bags which provides a high gravitational load on the foundation. Scour protection is obtained by completely covering the support member and side wall with a 10 plurality of bags filled with ballast material.

In yet a further embodiment, the foundation comprises a central bore having an open lower end and an open upper end, the open lower end being disposed at the seabed and the open upper end being disposed above the water surface. The central bore may be used for enclosing a power cable (previously provided at the location from 15 the seabed, and brought to the surface e.g. using a floating element) which connects to electrical systems of the offshore installation.

In yet a further embodiment, the foundation end part comprises a beam structure (e.g. jacket structure or tripod) having a plurality of support pads resting on a seabed, wherein a plurality of bags filled with ballast material cover the support pads and 20 provide the required stabilization and scour protection.

According to yet a further aspect of the present invention, a vessel for installing a foundation according to one of the foundation embodiments (and associated offshore installation) is provided, comprising a loading part for lifting, moving, and carrying the foundation, and foundation installing equipment for placing a foundation end part of 25 the foundation on a seabed. The foundation installing equipment comprises a bag management system adapted to lift, move, and position each bag at a desired location, allowing to cover the foundation end part with a plurality of bags filled with ballast material.

In an embodiment, the vessel for installing the foundation and offshore 30 installation further comprises a dredging installation arranged for locally extracting seabed material in close proximity of the offshore site. The foundation installing equipment is in an even further embodiment further arranged for placing the foundation 5 end part of the foundation on a seabed, filling a plurality of bags with locally extracted seabed material, and covering the foundation end part with the plurality of filled bags.

Short description of drawings

The present invention will be discussed in further detail hereinafter based on a 5 number of exemplary embodiments with reference to the drawings, wherein:

Fig. lisa side view of an embodiment of a foundation and offshore installation according to the present invention.

Fig. 2 is a side view of an embodiment of a foundation according to the present invention 10 Fig. 3a is a side view of another embodiment of a foundation according to the present invention.

Fig. 3b is a cross sectional view of the embodiment of Fig. 3a.

Fig. 4 is a side view of yet another embodiment of a foundation according to the present invention.

15 Detailed description of exemplary embodiments

According to a aspect of the present invention, an improved gravity based foundation is provided. Referring to the drawings, Fig. 1 shows a side view of an embodiment of an offshore installation 1 and a foundation 2, wherein the foundation 2 rests on a seabed 10 at an offshore site. The offshore installation 1 can be embodied by 20 a wind turbine 1 for the production of e.g. electric power. The foundation 2 comprises a platform 3 disposed above the water surface 11 and provides an access point for e.g. construction and maintenance personnel. Other access points at different locations of the foundation 2 and/or offshore installation 1 are conceivable. In the embodiment shown, the offshore installation 1 (e.g. wind turbine) and foundation 2 can be separate 25 components that are connected at an onshore or offshore site, but they can also be integrally formed as a single structure. In an exemplary embodiment, the wind turbine 1 comprises an integrated foundation 2, forming a single structure to be transported and installed at an offshore site.

The foundation 2 further comprises a foundation end part 4 arranged for 30 transferring forces and moments from the offshore installation 1 toward the seabed 10. In the embodiment shown, the foundation end part 4 comprises a substantially flat support member 5 (e.g. a flat round foot) perpendicular to the longitudinal axis of the 6 foundation 2. The support member 5 rests on the seabed 10 and is covered with a plurality of bags 6 filled with ballast material. The support member 5 is substantially rigid and adapted to withstand a large gravitational load imposed by the plurality of bags 6. The geometry and materials of the support member 5 are dependent on 5 application requirements, but in most embodiments it can be a substantially flat member made of e g. steel and/or concrete, or even fiber reinforced materials and the like.

In typical embodiments, the plurality of bags 6 are tightly stacked onto each other in a layered fashion, yielding a dense and heavy structure of bags 6 covering the 10 support member 5 of the foundation end part 4. The organized and layered stacking of the plurality of bags 6 may not be needed for all applications, wherein a less dense, random stacking may suffice in certain situations.

In an embodiment, the ballast material is locally extracted from the seabed 10 in close proximity of the offshore site by means of e.g. a dredging installation. Typically, 15 the dredged seabed material will be sand, but other local seabed constituents are conceivable, such as rocks and stones. However, larger seabed constituents such a rocks may have sharp edges, so using a plurality of nets 6 instead of bags 6 is certainly conceivable in view of the present invention.

Each bag 6 is made of a strong, flexible, durable, eco-friendly material and 20 adapted to hold heavy and possibly abrasive ballast materials. Each bag 6 is capable of resisting erosive and saline environments, such as abrasive particles in the seawater surrounding the foundation 2. The actual size and shape of each bag 6 may vary depending on requirements, but the geometry of each bag 6 should be optimized for e.g. strength, handling, filling, stacking and positioning. In typical embodiments, the 25 handling, filling, stacking and positioning of each bag 6 is automated and mechanized with minimal human intervention.

According to the present invention, the plurality of bags 6 obviate the need to build heavy gravity based foundations at an onshore facility, and to transport the heavy foundations to an offshore site with a large and expensive barge or vessel. Due to the 30 enormous weight of current gravity based foundations, the carrying capacity of current transportation barges, vessels and lifting equipment becomes a decisive and limiting factor.

7

The present invention enables the use of smaller transportation vessels and lifting equipment, because the ‘empty weight’ of the foundation 2 is important for transportation and lifting requirements. Indeed, the total combined weight of the plurality of bags 6 is determined at the offshore site and does not form a structural 5 characteristic of the foundation 2. The reduced weight of the foundation 2 facilitates integrated solutions, such as an offshore installation 1 having an integrally formed or permanently connected foundation 2, thereby further reducing construction, transportation and installation complexities and costs.

Furthermore, the plurality of bags 6 according to the present invention obviate the 10 need for specialized scour protection near and around the foundation 2. It is well- known that sea or ocean currents may form scour holes around a foundation structure, thereby weakening its support and stabilization. Dumping layers of rock around the foundation structure can be an effective anti-scour measure. However, no rocks are locally available for a foundation 2 placed on a sand seabed 10, and so a large number 15 of rocks will need to be transported to the offshore site, possibly in batches as the transportation vessel has limited loading capacity. Other anti-scour solutions are obviated by the present invention as well, including specialized concrete or granite blocks, mats, nets, and/or skirts etc. The stacked structure of bags 6 not only provides the necessary stabilization to the offshore installation 1 and foundation 2, but 20 simultaneously acts as a barrier between sea or ocean currents and the foundation end part 4, thereby shielding the seabed 10 from erosion near and around the foundation 2.

Another important advantage of the present invention is that the plurality of bags 6 can be readily removed when the foundation 2 reaches the end of its lifecycle or the foundation 2 and offshore installation require maintenance at an onshore facility. Also, 25 each bag 6 can be recycled for a new gravity based foundation, so that local dredging activities are minimized and further disturbances of marine life are avoided.

To briefly summarize, the plurality of bags 6 filled with local seabed material not only reduce the weight of the gravity based foundation 2 but also provide natural scour protection. In addition, the plurality of bags 6 are easily removed or recycled when the 30 offshore installation 1 and foundation 2 require maintenance at an onshore site or can be disposed.

Fig. 2 shows a further embodiment of a foundation 2 according to the present invention. In this embodiment the foundation 2 is a mono-pile, comprising a platform 3 8 disposed above the water surface 11 and a foundation end part 4 driven into the seabed 10. In the embodiment shown, a region of the foundation end part 4 which is disposed above the seabed is covered with a plurality of stacked bags 6 filled with ballast material, preferably locally extracted seabed material in close proximity of the 5 foundation 2. The plurality of bags 6 are arranged to provide a gravitational load against the foundation 2 for lateral stabilization. In another embodiment not shown, the foundation end part 4 of the mono-pile 2 is provided with a collar 5 rigidly disposed around the foundation end part 4. The collar 5 rests on the seabed and is covered with a plurality of bags 6 for lateral and axial stabilization of the mono-pile 2.

10 As described in forgoing paragraphs, the plurality of bags 6 covering the foundation end part 4 simultaneously provide scour protection and prevent the formation of erosion and scour holes near and around the foundation 2. Specialized scour protection is obviated, such as concrete or granite blocks, layered rocks, mats, nets, and/or skirts. As a result, a gravity based foundation 2 comprising a plurality of 15 bags 6 filled with ballast material is easier and faster to install as no additional steps are required for providing scour protection.

Fig. 3a shows another embodiment of a foundation 2 according to the present invention, comprising a platform 3 disposed above the water surface 11 and a foundation end part 4. The foundation end part 4 comprises a substantially flat support 20 member 5 perpendicular to the longitudinal axis of the foundation 2, and optionally a side wall 7 disposed along the circumference of the support member 5.

In the embodiment shown, a plurality of elongated bags 6 are disposed around the foundation 2 and supported by the substantially flat support member 5, wherein the side wall 7 may prevent the bags 6 from sliding off the substantially flat support member 5. 25 Alternatively, the bags 6 are attached with their closed off bottom sides at least to the flat support member 5 (in which case, the optional side wall 7 is no longer necessary).

Each bag 6 comprises an open bag end 6b and bag rim 6a disposed at the water surface 11 in operation. In an exemplary embodiment, the bag rim 6a is substantially hollow and comprises a gas such as air (or even helium). In another embodiment, the 30 bag rim 6a is substantially solid and comprises buoyant material having a density lower than sea water. As a result of the buoyant bag rim 6a, the bags 6 can be easily filled when the foundation 2 is positioned on the sea bed. Before installation, the bags 6 can remain folded as a result of which the foundation takes less space during transportation.

9

The foundation 2 may further comprise a central bore 13 having an open lower end 16 and an open upper end 14. Inside the central bore 13, a power cable 12 may be provided using e.g. a buoy 15 to which electrical systems of the offshore installation 1 can be connected.

5 The bag rim 6a is adapted for exerting an upward buoyancy force on the bag 6 for keeping the open bag end 6b accessible from above the water surface 11. In an exemplary installation scenario, the bags 6 are initially folded and disposed at the support member 5 of the foundation end part 4. When the foundation 2 is lowered into the water, the buoyancy forces acting on the bag rims 6a are sufficient for keeping the 10 open bag ends 6b at the water surface 11 and to unfold the bags 6 toward their full length once the foundation 2 is placed on the seabed 10. It is readily seen that each bag 6 may at least span the total distance from the support member 5 all the way to the water surface 11. The gravitational load may be adapted to specific applications by changing the width or diameter of the bags 6 for identical foundation 2 sizes.

15 The central bore 13 reduces the total weight of the foundation 2 and allows for smaller transportation vessels and installation equipment. The central bore 13 also facilitates connecting the offshore installation 1 to the power cable 12 which emerges from the seabed 10. Typically, the power cable 12 is connected to a buoy 15 which floats on the water surface 11 before the foundation 2 is placed on the seabed 10. The 20 buoy 15 may act as a position marker at which the foundation 2 is to be placed. When the foundation 2 is lowered into the water, the buoy 15 moves into the central bore 13 through the open lower end 16, where the open upper end 14 provides access to the buoy 15 and the power cable 12 once the foundation is placed on the seabed 10. The power cable 12 is fully enclosed and protected by the foundation 2 against e.g. debris in 25 the water, thus improving the reliability of the electrical connection to the offshore installation 1.

Fig 3b is a top view of the embodiment shown in Fig 3a, wherein the foundation 2 further comprises bag support members 5a disposed in a longitudinal or lengthwise direction of the foundation 2 between the bags 6. The bag support members 5a are 30 arranged for providing lateral support to the bag 6 and/or for fixing the bags 6 to the foundation 2. In the embodiment shown, each bag 6 is substantially cylindrical when filled with ballast material, but other shapes are conceivable depending on requirements. In an embodiment, the bag support members 5a may be envisaged as e.g.

10 rectangular plates longitudinally connected to the foundation 2. In another embodiment, the bag support members 5a may be envisaged as e.g. truss structures longitudinally connected to the foundation 2.

According to the present invention, the arrangement of bags 6 depicted in Fig. 3a 5 and 3b is also referred to as a stacked arrangement for surrounding or covering the foundation end part 4 with a plurality of bags 6 filled with ballast material. This specific embodiment of the bags 6 with long bags extending parallel to the side of the foundation, may also be applied to the foundation construction examples as shown in the embodiments of Fig. 1, 2, and 4.

10 In a further embodiment, the support member 5 and side wall 7 define a bucket for holding a plurality of bags 6 filled with ballast material. In an advantageous embodiment, the bucket formed by the support member 5 and side wall 7 is partly or completely embedded in the seabed 10. The plurality of bags 6 are in a (densely) stacked arrangement and impose a stabilizing gravitational load on the foundation 2.

15 Scour protection is obtained by fully covering the side wall 7 and support member 5 such that the transition from the seabed 10 to the stacked arrangement of bags 6 is relatively gradual, preventing erosive turbulence near the edge of the plurality of bags 6.

It is readily seen that larger stabilizing forces and moments can be imposed by 20 having a larger support member 5 and a higher side wall 7 capable of accommodating a larger number of bags 6.

Fig. 4 shows another embodiment of a foundation 2 according to the present invention, comprising a platform 3 disposed above the water surface 11 and a foundation end part 4, wherein the foundation end part 4 comprises a foundation 25 structure 8, such as a jacket structure or tripod. The foundation structure 8 can be made of e.g. steel, concrete, fiber reinforced materials, or any other material deemed suitable for the application. The foundation structure 8 comprises a plurality of support pads 9 resting on the seabed 10., wherein a plurality of bags 6 filled with ballast material are provided covering each support pad 9. Specialized scour protection for each support 30 pad 9 is obviated by virtue of the plurality of bags 6 as discussed in foregoing paragraphs.

According to another aspect of the present invention, a method of providing a foundation 2 for an offshore installation 1 is provided, comprising providing a 11 foundation 2 having a foundation end part 4, erecting the foundation 2 at an offshore site, placing the foundation end part 4 on the seabed 10, and covering the foundation end part 4 with a plurality of bags 6 filled with ballast material, the ballast material being extracted from a local seabed in close proximity of the offshore site by means of 5 a dredging installation.

In an embodiment, the foundation 2 and offshore installation 1 are provided in a way such that the installation time is minimized, making it is possible to quickly transport, install and stabilize the foundation 2 and offshore installation 1 by means of a plurality of bags 6 filled with ballast material. Conversely, the foundation 2 and 10 offshore installation 1 are provided in a way such that removal time is minimized, making it possible to quickly remove the plurality of bags 6 filled with ballast material and to transport the foundation 2 and offshore installation 1 (e.g. wind turbine) to an onshore facility for maintenance and/or repair work.

According to the present invention, providing the foundation 2 comprises onshore 15 construction activities, transportation (e.g. towing) and overall logistics, forming a dedicated production and supply chain that reduces cost and increases production speed. The overall cost of providing the foundation 2 and offshore installation 1 is appreciably lower than conventional methods. One of the advantages of the present invention is that the foundation 2 can be integrally formed or permanently connected 20 with the offshore installation 1 at an onshore facility due to its considerably lower weight. As a result, labour costs are further reduced as complex and possibly dangerous offshore activities are avoided which often require highly skilled and experienced workers.

In an embodiment, erecting the foundation 2 is accomplished by means of a 25 transportation vessel comprising suitable equipment, such as a crane and a dredging installation. In an embodiment, the foundation 2 is erected separately from the offshore installation 1. In another embodiment, the foundation 2 is integrally formed or permanently connected with the offshore installation 1 and both are erected simultaneously. The reduced weight of the foundation 2 further minimizes the time in 30 which the foundation 2 and offshore installation 1 can be erected.

The foundation end part 4 of the foundation 2 is placed on the seabed using foundation installing equipment on a vessel, which lowers the foundation 2 with or 12 without the offshore installation 1 toward the seabed 10 in a controlled and safe manner.

In an embodiment, the foundation end part 4 is first placed in/on the seabed 10 and subsequently covered with a plurality of bags 6 filled with ballast material. The 5 foundation installing equipment is arranged to fill and position each bag 6, and to form a stacked arrangement of bags 6 covering the foundation end part 4. The plurality of bags 6 positioned by the foundation installing equipment provide the necessary gravitational load for stabilization as well as scour protection.

In another embodiment, the foundation 2 is a buoyant foundation wherein the 10 foundation end part 4 is covered with a plurality of bags 6 filled with ballast material while the foundation 2 is still floating. The buoyancy of the foundation 2 diminishes due to the increasing weight of the plurality of bags 6, enabling a controlled sinking and placement of the foundation 2 on the seabed 10. The placement of the foundation 2 is completed once additional bags 6 are positioned for scour protection.

15 According to yet another aspect of the present invention, a vessel for installing a foundation 2 and offshore installation 1 is provided, comprising a loading part for lifting, moving and carrying the foundation 2, and foundation installing equipment for placing the foundation end part 4 of the foundation 2 in or on a seabed. The foundation installing equipment comprises a bag management system adapted to lift, move, and 20 position each bag 6 at a desired location, covering the foundation end part 4 with a plurality of bags 6 filled with ballast material.

In an embodiment, the vessel for installing the foundation 2 and offshore installation 1 further comprises a dredging installation arranged for locally extracting seabed material in close proximity of the offshore site. The foundation installing 25 equipment is further adapted to fill a plurality of bags 6 with the locally extracted seabed material.

According to another present invention embodiment, an additional vessel may be provided comprising a dredging installation and bag management system for filling a plurality of bags 6 with locally extracted seabed material. The additional vessel further 30 comprises a storage area for storing a plurality of bags 6 filled with locally extracted seabed material. The additional vessel can be seen as a specialized facility providing a plurality of bags 6 filled with locally dredged seabed. In this embodiment, the vessel for installing the foundation 2 and offshore installation 1 comprises a receiver and 13 storage area for a plurality of bags 6 filled with locally extracted seabed material provided by the additional vessel. This embodiment has the advantage that installation and dredging activities can be done simultaneously.

Various embodiments are envisaged, of which features may be combined: 5 1. Method of providing a foundation (2) for an offshore installation (1), such as a wind turbine, comprising: providing a foundation (2) having a foundation end part (4), erecting the foundation (2) at an offshore site, 10 placing the foundation end part (4) of the foundation (2) on a seabed (10), and surrounding the foundation end part (4) with a plurality of bags (6) filled with ballast material.

2. Method according to embodiment 1, further comprising filling the plurality of bags (6) with the ballast material, wherein the ballast material comprises seabed 15 material.

3. Method according to embodiment 2, wherein the seabed material comprises sand.

4. Method according to embodiment 2 or 3, wherein the seabed material is locally extracted.

5. Method according to any one of embodiments 1-4, wherein the plurality of bags 20 (6) are positioned in a stacked arrangement covering the foundation end part (4).

6. Method according to any one of embodiments 1-5, wherein each one of the plurality of bags (6) comprises an open bag end (6b) and a buoyant bag rim (6a).

7. Method according to any one of embodiments 1-6, further comprising removing the plurality of bags (6) away from the foundation end part (4).

25 8. Foundation for an offshore installation, such as a wind turbine, wherein the foundation (2) comprises a foundation end part (4) and a plurality of bags (6) filled with ballast material surrounding the foundation end part (4).

9. Foundation according to embodiment 8, wherein the foundation end part (4) comprises a support structure (5; 7) for supporting the plurality of bags (6).

30 10. Foundation according to embodiment 9, wherein the support structure comprises a substantially flat support member (5) perpendicular to a longitudinal axis of the foundation (2).

11. Foundation according to embodiment 9 or 10, wherein the support structure further comprises a containment body (7) connected to the support member (5).

14 12. Foundation according to any one of embodiments 8-11, wherein the ballast material comprises seabed material.

13. Foundation according to any one of embodiments 8-12, wherein each one of the plurality of bags (6) is made of seawater resistant material.

5 14. Foundation according to any one of embodiments 8-13, wherein each one of the plurality of bags (6) comprises an open bag end (6b) and a buoyant bag rim (6a).

15. Foundation according to any one of embodiments 8-14, wherein the foundation (2) comprises a central bore (13) extending there through having an open lower end (16) and an open upper end (14).

10 The present invention embodiments have been described above with reference to a number of exemplary embodiments as shown in and described with reference to the drawings. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.

Claims

A method for providing a foundation (2) for an offshore installation (1), such as a wind turbine, comprising: providing a foundation (2) with a foundation end part (4), erecting the foundation (2) on an offshore location, placing the foundation end portion (4) of the foundation (2) on a seabed (10), and surrounded by foundation end portion (4) with a plurality of bags (6) filled with ballast material.

The method of claim 1, further comprising filling the plurality of bags (6) with the ballast material, wherein the ballast material comprises seabed material.

The method of claim 2, wherein the seabed material comprises sand.

Method according to claim 2 or 3, wherein the seabed material is extracted locally. 20

The method of any one of claims 1-4, wherein the plurality of bags (6) are positioned in a stacked arrangement and cover the foundation end portion (4).

6. Method according to any of claims 1-5, wherein each of the plurality of bags (6) has an open bag end (6b) and a floating bag edge (6a).

The method of any one of claims 1-6, further comprising removing the plurality of bags (6) from the foundation end portion (4).

A foundation for an offshore installation, such as a wind turbine, wherein the foundation (2) comprises a foundation end portion (4) and a plurality of bags (6) filled with ballast material and surrounding the foundation end portion (4).

The foundation of claim 8, wherein the foundation end portion (4) comprises a support structure (5; 7) for supporting the plurality of pockets (6).

The foundation of claim 9, wherein the support structure comprises a substantially flat support body (5) that is perpendicular to a longitudinal axis of the foundation (2).

11 Foundation according to claim 9 or 10, wherein the support structure further comprises a retaining body (7) connected to the support body (5).

12. Foundation as claimed in any of the claims 8-11, wherein the ballast material comprises seabed material.

The foundation of any one of claims 8-12, wherein each of the plurality of bags (6) is made from seawater resistant material.

The foundation of any one of claims 8-13, wherein each of the plurality of bags (6) comprises an open bag end (6b) and a floating bag edge (6a). 20

The foundation of any one of claims 8-14, wherein the foundation (2) comprises a central bore (13) extending therethrough with an open bottom end (16) and an open top end (14).