SUPPORT STRUCTURE FOR USE IN THE OFFSHORE WIND FARM INDUSTRY
A support structure for use in the offshore wind farm industry, and a method of manufacturing and installing same, including a foundation for installation on a seabed below a body of water and a tower connected to and extending upwards from the foundation and being capable of supporting at least an equipment unit. The foundation includes a bottom slab and a wall extending upwards from the bottom slab, thereby defining a first cavity for holding ballast and for providing buoyancy during tow-out and installation. The foundation further includes a circumferential skirt extending downwards from the bottom slab, thereby defining at least one compartment underneath the foundation.
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The present invention relates to structures for supporting offshore wind turbines and similar equipment. More specifically, the invention relates to a support structure for use in the offshore wind farm industry, comprising a foundation for installation on a seabed below a body of water and a tower connected to and extending upwards from the foundation and being capable of supporting at least an equipment unit; as well as a method of manufacturing the support structure and a method of installing the support structure.
The increasing demand of exploitation of renewable energy sources enhances the demand of offshore wind power generation where the wind conditions are more favorable than onshore and the environmental impact is much less. There is an increasing need for structures that can support heavy wind turbines in a significant height over the sea level. The support structure consists of shaft/tower fixed to the seabed either directly by means of a foundation or the structure is made floating and connected to the seabed by means of a mooring. The present invention relates to the former type, namely the fixed support structures.
Typical fixed support structures for wind turbines applied in practice, planned for application, patented and described in publicly accessible sources are, in general terms characterized by the following:
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- 1. Demanding installation where the tower is deployed in-situ on a preinstalled foundation
- 2. The foundation is fixed to the seabed by driven or drilled piles
Existing solutions using the gravity force to fix the structure to the seabed instead of piles are known for their considerable limits of application related to their weight, water depth at the installation site as well as water depth at the load-out locations and along the transport route.
EP 1 429 024 discloses a support structure for an offshore wind turbine, comprising a caisson supported by several columns embedded in the seabed and subjected to tension and pressure loads. Selected columns are piled at an inclined angle with respect to the vertical. The caisson is supported below the water surface but above the seabed.
WO 03/080939 discloses a foundation structure for a wind turbine tower or similar, for installation on the seabed. The foundation structure can be manoeuvred to its offshore position using a vessel and separate (and removable) buoyancy means. These buoyancy elements must be rather large in order to maintain stability. When in position, the structure is lowered to the seabed and a pumping mechanism is used to sink a lower portion of the structure (e.g. skirts) into the seabed. When the foundation structure has been anchored (or piled) in position on the seabed, it is capable of supporting the wind turbine tower.
By their nature, the above solutions tend to yield high overall capital investment costs, i.e. the total costs for fabrication, load-out, transport and installation.
It is therefore provided a support structure for use in the offshore wind farm industry, comprising a foundation for installation on a seabed below a body of water and a tower connected to and extending upwards from the foundation and being capable of supporting at least an equipment unit, characterized in that the foundation comprises a bottom slab element and a wall extending upwards from the bottom slab element, thereby defining a first cavity for holding ballast and for providing buoyancy during tow-out and installation.
The tower is preferably connected to the foundation via a lower part of the tower being attached to the bottom slab element and connected to the foundation via fixing elements connected to at least an upper wall portion.
Preferably, the foundation comprises a circumferential skirt extending downwards from the bottom slab, thereby defining at least one compartment underneath the foundation. Preferably, the at least one compartment is subdivided into compartments by means of skirts extending downwards from the bottom slab and preferably extending radially from a center portion of the bottom slab to respective areas of the circumferential skirt.
In an embodiment, the foundation comprises a roof structure, extending between the upper wall and the tower, thereby enclosing the first cavity. In one embodiment, the roof structure comprises an outer shell and an inner shell defining at least one second cavity there between, said inner shell facing the first cavity. The second cavity is preferably filled with a material such as concrete. In another embodiment, the roof structure is formed by concrete cast in conventional formwork, or by single shell metal plates.
In one embodiment, the bottom slab element and the wall comprise an outer shell and an inner shell defining at least one second cavity there between, said inner shell facing the first cavity. The second cavity is preferably filled with a material such as concrete. In another embodiment, the upper wall is formed by slipform casting or by single shell metal plates.
In one embodiment, the support structure comprises a buoyant stabilizing device releasably and slidably connected to the foundation, whereby the stability of the structure is maintained during tow, and during installation when the roof structure is moved from a position above the water to a partly or fully submerged state. Preferably, the buoyant stabilizing device comprises a recessed portion having upper and lower end stops for cooperation with a flange on the foundation, whereby the buoyant stabilizing device slidable movement is restricted by said upper and lower end stops. Also, the buoyant stabilizing device preferably comprises at least one inner cavity for selective addition and extraction of a ballasting fluid, such as water.
It is also provided a method of manufacturing the invented support structure, comprising the providing of a bottom slab having downwardly extending skirts to an onshore fabrication site, characterized by the steps of:
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- a) extending a circumferential lower wall from the bottom slab to form a foundation lower part, said lower wall having a vertical extension dimensioned according to the buoyancy requirements for the completed support structure;
- b) placing the lower part in a floating position on the body of water;
- c) extending the upper wall; and
- d) connecting the tower to the foundation by attaching a lower part of the tower to the bottom slab element and by connecting a part of the tower via fixing elements to at least an upper wall portion of the foundation.
In one embodiment of the invented method, a roof structure is extended between the upper wall and the tower, thereby enclosing the first cavity.
It is also provided a method of installing the invented support structure, characterized by the steps of: towing the structure in a floating state to the installation location, and transferring the structure from a floating state to an installed state, by filling ballast into the first cavity until the structure is installed on the seabed.
If deemed necessary, the method installing comprises moving the foundation into a substantially level state by injecting a grouting material into selected ones of the compartments confined by the skirts below the bottom slab element.
The present invention introduces a number of parameters and structural compatibility by using different material types that can be applied for optimizing the supply of ready-for-operation structural supports for offshore wind farms. The following advantageous aspects are achieved:
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- 1. Large degree of completion and commissioning work can be done at the fabrication site instead at the offshore installation site, allowing integration of the tower to the foundation, cabling work and similar
- 2. Wider material selection and range of structural dimensions
- 3. Transport to the site on deck of barges and vessels is eliminated or significantly reduced
- 4. Separate buoyancy elements during tow-out are not required
- 5. Deployment into the position (transfer from the transport position to the operation position) by adding ballast, not by lifting
- 6. No piling or other forms of “fixing” to the seabed is needed
- 7. Design and outfitting for removal can be easily implemented
- 8. Need for large offshore cranes is avoided
In addition to lower overall costs the present invention resolves shortcomings associated with the known solutions by:
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- 1. Enabling delivery of the supports from fabrication sites allowing operation of shallow draft vessels thus widening the selection of fabrication sites
- 2. Reducing needs for specialized vessels
- 3. Allowing superstructure (tower, wind generator, etc) to be fitted to foundation structure at shore, prior to tow to installation location
- 4. Allowing foundation structure to be leveled following installation on seabed, to prevent unpredictable inclination of the installed support
- 5. Reducing or eliminating hydrodynamic loads acting directly on the tower
- 6. Resistance to heavy ice loads
These and other characteristics of the invention will be clear from the following description of a preferential form of embodiment, given as a non-restrictive example, with reference to the attached drawings wherein:
Although not mandatory, it is advantageous to give the foundation 4 a circular shape that can efficiently resist environmental loads in various phases during fabrication, transport and operation; typically hydrostatic water pressure, wave loads and—in some cases—, ice loads. The tower 7 is fixed to the foundation by means of a multiple-legged structure 8.
The inventive fabrication, transport and installation procedure is illustrated in
Upon completion of the lower part 22 of the foundation 4; 4′, it is transferred (e.g. lifted; cf.
Upon lowering into water the lower part 22 of foundation 4; 4′ floats with an appropriate freeboard that allows safe work with continuation of the fabrication, which may continue either with extending the vertical walls in order to increase the freeboard of the floating body or filling the cavity between the shells by concrete to increase strength of the lower part of the foundation 4; 4′. Completion of the foundation 4; 4′ involves construction of walls up to the their final height and for the foundation 4′ shown in
In
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- 1. Postpone installation of equipment units 5, 6a-c to the phase where the support structure 4; 4′ and tower 7 have been installed at the offshore site. This reduces significantly the height of centre of gravity above the center of buoyancy, reduces the wind loads on the structure during this phase, and reduces the draft during tow.
- 2. Use a telescopic tower (such as disclosed in Norwegian patent application no. 20073363) where the upper part is inserted into the lower part during tow and installation and thereafter retracted (pushed out by hydrostatic pressure). The effects of this are of the same character as in item 1 described above.
- 3. Design and fabricate the tower 7 or its upper part from lighter materials than steel, e.g. from high strength reinforced plastics.
In
The invention is particularly suitable for suitable for shallow waters in particular in the interval between 8 m and 30 m. The system can preferably be designed in the soft-stiff dynamic response regime.
Claims
1-19. (canceled)
20. A support structure for use in the offshore wind farm industry, comprising:
- a foundation for installation on a seabed below a body of water; and
- a tower connected to and extending upwards from the foundation and being capable of supporting at least an equipment unit,
- wherein the foundation further comprises: a bottom slab element and a wall extending upwards from the bottom slab element, thereby defining a first cavity wherein the foundation comprises a circumferential skirt extending downwards from the bottom slab, thereby defining at least one compartment underneath the foundation.
21. The support structure of claim 20, wherein the tower is connected to the foundation via a lower part of the tower being attached to the bottom slab element.
22. The support structure of claim 20, wherein the tower is connected to the foundation via fixing elements connected to at least an upper wall portion.
23. The support structure of claim 20, wherein the at least one compartment is subdivided into compartments by means of skirts extending downwards from the bottom slab and preferably extending radially from a center portion of the bottom slab to respective areas of the circumferential skirt.
24. The support structure of claim wherein the foundation comprises a roof structure, extending between an upper wall portion and the tower, thereby enclosing the first cavity.
25. The support structure of claim 24, wherein the roof structure comprises an outer shell and an inner shell defining at least one second cavity there between, said inner shell facing the first cavity.
26. The support structure of claim 25, wherein the second cavity is filled with a material such as concrete.
27. The support structure of claim 24, wherein the roof structure is formed by concrete cast in conventional formwork, or by single shell metal plates.
28. The support structure of claim 20, wherein the bottom slab and the wall comprise an outer shell and an inner shell defining at least one second cavity there between, said inner shell facing the first cavity.
29. The support structure of claim 28, wherein the upper wall is formed by slipform casting or by single shell metal plates.
30. The support structure of claim 24, further comprising a buoyant stabilizing device releasably and slidably connected to the foundation, whereby the stability of the structure is maintained during installation when the roof structure is moved from a position above the water to a fully submerged state.
31. The support structure of claim 30, wherein the buoyant stabilizing device comprises a recessed portion having upper and lower end stops for cooperation with a flange on the foundation, whereby the buoyant stabilizing device slidable movement is restricted by said upper and lower end stops.
32. The support structure of claim 30, wherein the buoyant stabilizing device comprises at least one inner cavity for selective addition and extraction of a ballasting fluid, such as water.
33. A method of manufacturing the support structure of claim 20, comprising the providing of a bottom slab having downwardly extending skirts to an onshore fabrication site, comprising the steps of:
- a) extending a circumferential lower wall from the bottom slab to form a foundation lower part, said lower wall having a vertical extension dimensioned according to the buoyancy requirements for the completed support structure;
- b) placing the foundation lower part in a floating position on the body of water;
- c) extending an upper wall portion; and
- d) connecting the tower to the foundation by attaching a lower part of the tower to the bottom slab and by connecting a part of the tower via fixing elements to at least the upper wall portion of the foundation.
34. The method of claim 33, wherein a roof structure is extended between the upper wall and the tower, thereby enclosing the first cavity.
35. A method of installing a support structure for use in the offshore wind farm industry, comprising a foundation for installation on a seabed below a body of water and a tower connected to and extending upwards from the foundation and being capable of supporting at least an equipment unit, comprising the steps of:
- a) towing the structure in a floating state to the installation location while controlling the buoyancy and center of gravity of the support by means of a controlled addition of a ballast material into a cavity defined by structural elements of the support structure, whereby the need for separate buoyancy elements and/or specialized vessels during transport is eliminated or significantly reduced, and
- b) transferring the structure from a floating state to an installed state, by filling a ballast material into the cavity until the structure is installed on the seabed, whereby the need for separate buoyancy elements and/or specialized vessels and cranes during installation is eliminated or significantly reduced.
36. The method of claim 35, wherein the support structure is moved into a substantially level state on the seabed by injecting a grouting material into selected compartments confined by skirts below a bottom slab element of the support structure.
37. A method of installing a support structure for use in the offshore wind farm industry, comprising a foundation for installation on a seabed below a body of water and a tower connected to and extending upwards from the foundation and being capable of supporting at least an equipment unit, comprising the steps of:
- a) towing the structure in a floating state to the installation location while controlling the buoyancy and center of gravity of the support by means of a cavity defined by structural elements of the support structure, whereby the need for separate buoyancy elements and/or specialized vessels during transport is eliminated or significantly reduced,
- b) transferring the structure from a floating state to an installed state, by filling a ballast material into the cavity until the structure is installed on the seabed, whereby the need for separate buoyancy elements and/or specialized vessels and cranes during installation is eliminated or significantly reduced; and
- c) moving the support structure into a substantially level state on the seabed by injecting a grouting material into selected compartments confined by skirts below a bottom slab element of the support structure.
38. The method of claim 35, wherein the filling in step b comprises the filling, at least partly, of water or a solid material or a combination of both.
39. The method of claim 37, wherein the filling in step b comprises the filling, at least partly, of water or a solid material or a combination of both.
Type: Application
Filed: Jun 17, 2009
Publication Date: Dec 15, 2011
Applicant: SEATOWER AS (Oslo)
Inventors: Karel Karal (Oslo), Sigurd Ramslie (Quinns Rocks)
Application Number: 13/000,286
International Classification: E02D 29/09 (20060101); E02D 5/20 (20060101);