WO2011147481A2

WO2011147481A2 - Offshore foundation structure, offshore foundation and method of establishing the same

Info

Publication number: WO2011147481A2
Application number: PCT/EP2010/066900
Authority: WO
Inventors: Henrik Stiesdal
Original assignee: Siemens Aktiengesellschaft
Priority date: 2010-05-28
Filing date: 2010-11-05
Publication date: 2011-12-01
Also published as: WO2011147481A3; EP2558648A2; EP2558648B1

Abstract

An offshore foundation structure, in particular a wind turbine foundation structure, with at least one leg (11) to be positioned on the seabed (6) is provided. The at least one leg (11) comprises: - a main tube (3) extending along an axial direction, - a seabed plate (1) which is located at the seabed (6) side end of the main tube (3) and prepared to hold the structure at least temporarily on the seabed (6), - a seabed plate tube (2) which extends from said seabed plate (1) into or around said main tube (3) and is prepared to be moved in relation to said main tube (3) in the axial direction of the main tube (3), and - an adjusting means (5) which is able to, and prepared for, adjusting the relative axial position of the main tube (3) and the seabed plate tube (2) to each other.

Description

Offshore foundation structure, offshore foundation and method of establishing the same

The present invention relates to an offshore foundation structure and, in particular, to a wind turbine offshore foundation structure. In addition, the invention relates to an offshore foundation, in particular to a wind turbine off- shore foundation, and to a method of establishing such an offshore foundation.

It is known to establish jacket constructions or tripod con^¬ structions on the seabed as foundations in particular for offshore wind turbine installations. A jacket construction is, for example, shown in US 5,988,949 or in EP 2 067 914 A2, a tripod construction, for example, in DE 10 2004 042 066 Al .

A typical method for establishing a foundation based on a jacked construction or a tripod construction is to prepare the seabed prior to establishing the foundation which includes levelling out variations of the height of the seabed in order to achieve a construction in level. Furthermore, the method comprises establishing piles very accurately posi- tioned in the seabed such as by means of a pre-fabricated and pre-positioned pile positioning template structure on the seabed .

The piles are established by using e.g. hydraulic driven means which literally hammer the pile down into the seabed. When the pile is established, a jacket or a tripod construc^¬ tion is set over a part of the pile (or piles in case of a tripod construction) and the space between the leg and the pile is grouted to establish a secure connection.

With respect to the mentioned state of the art it is an ob^¬ jective of the present invention to provide an advantageous offshore foundation structure and an advantageous offshore foundation, in particular for wind turbines. It is a further objective of the present invention to provide an advantageous method of establishing an offshore foundation. The first objective is solved by an offshore foundation structure as claimed in claim 1 and by an offshore foundation as claimed in claim 8. The second objective is solved by a method of establishing an offshore foundation as claimed in claim 13. The depending claims contain further developments of the present invention.

An inventive offshore foundation structure which may, in par^¬ ticular, be a wind turbine foundation structure, i.e. a foun^¬ dation structure for carrying a wind turbine tower comprises at least one leg to be positioned on the seabed. Typically, such a foundation structure is formed as jacket structure or a tripod structure with three legs resting on the seabed. The at least one leg of the inventive offshore foundation struc^¬ ture comprises an adjusting means which is able to, and pre- pared for, adjusting the axial length of the leg.

In the state of the art, prior to establishing a foundation, variations of the height of the seabed are levelled out in order to achieve a construction in level. Compared to this state of the art the invention is advantageous in that the preparation of the seabed prior to establishing the foundation can be minimized as the adjusting means is able to level out even large variation of the height of the seabed in order to achieve a construction in level. The need for levelling the seabed prior to establishing a foundation can thus be reduced or even completely eliminated.

In a concrete implementation of the invention, the at least one leg comprises: a main tube extending along an axial di- rection and a seabed plate which is located at the seabed side end of the main tube and prepared to hold the structure at least temporarily on the seabed. A seabed plate tube ex^¬ tends from said seabed plate into or around said main tube and is prepared to be moved in relation to said main tube in axial direction of said main tube. An adjusting means is pre^¬ sent which is able to, and prepared for, adjusting the rela^¬ tive axial position of the main tube and the seabed plate tube to each other.

The adjusting means can be a jack means which acts between the seabed plate tube and the main tube. In particular, the seabed plate tube and/or the main tube can comprise a jack base means which is prepared for holding the jack means and/or for providing support for the jack means. Other adjusting means like, for example, gear wheels acting between the seabed plate tube and the main tube, are in general con^¬ ceivable .

To allow for secure anchoring of the foundation structure in the seabed the at least one leg may comprise a pile which is adapted to be driven into the seabed. Such a pile may, for example, extend at least partly through the seabed plate tube and/or the main tube. This allows the pile to be located in^¬ side the leg or the legs of the offshore foundation structure when the foundation structure is positioned on the seabed and to drive the piles into the seabed after positioning the foundation structure whereby the piles and the driving means for driving the piles can be guided by the main tube and/or the seabed plate tube.

As an alternative to the pile extending at least partly through the seabed plate tube and/or the main tube the off- shore foundation structure may further comprise a separate pile tube extending from the seabed plate. This separate pile tube can then be used for holding and/or guiding a pile into the seabed after the offshore foundation structure has been positioned on the seabed. As a further alternative, piles can be driven into the seabed using a pile-positioning template, and the offshore foundation structure is then placed over the piles already placed in the seabed. The adjusting means for adjusting the relative axial position of the main tube and the seabed plate tube to each other can be designed such as to be removable from the leg so that it can be reused in another leg. However, the adjusting means can also be an integral part of the leg so that it remains in the leg after the foundation has been established.

Although jacket structures and tripod structures have been mentioned as typical offshore foundation structures other structures having two, four, or more legs are also conceiv^¬ able .

According to a second aspect of the present invention, an offshore foundation established on a seabed is provided. The offshore foundation may, in particular, be a wind turbine offshore foundation. The offshore foundation comprises an in^¬ ventive offshore foundation structure resting on the seabed. The axial length of the at least one leg is so adjusted as to level out variations of the seabed on which the foundation structure rests.

In a concrete implementation, the length of the leg is ad^¬ justed by adjusting a relative axial position of a main tube and a seabed plate tube to each other so as to a level out variations of the height of the seabed.

The inventive offshore foundation reduces, or even elimi^¬ nates, the need for levelling out the seabed area on which the foundation is to be established.

A space between the main tube and the seabed plate tube of the offshore foundation structure leg may be grouted in order to increase stability of the foundation. The at least one leg of the offshore foundation may comprise a pile which is driven into the seabed so as to function as an anchor for the foundation. Such a pile may partly extend through the seabed plate tube and/or the main tube. Alterna- tively, a separate pile tube may extend from the seabed plate. In this case the pile extends at least partly through the pile tube. According to a third aspect of the present invention a method of establishing an offshore foundation on the seabed, in particular, a wind turbine offshore foundation as described above is provided. The method comprises the steps of posi^¬ tioning an inventive offshore foundation structure and ad- justing the offshore foundation structure by use of the ad^¬ justing means.

The inventive method reduces the need for precisely levelling the seabed since the adjusting means can be designed such as to be able to level out even large variations of height of the seabed in order to achieve a construction in level. Lev^¬ elling out the seabed before establishing the foundation can therefore be reduced, or even omitted if the seabed varia^¬ tions of the site in which the foundation is to be estab- lished are not too large.

The inventive method may further comprise the step of anchor^¬ ing the offshore foundation structure by driving piles into the seabed. This can either be done by driving the piles into the seabed and then subsequently placing the offshore founda^¬ tion structure over the piles. As an alternative, the founda^¬ tion structure may first be placed on the seabed and then the piles are driven into the seabed through the seabed blade tube and the main tube or through a separate pile tube. The offshore foundation structure, for example a space between the pile and the seabed blade tube and/or the main tube or between the pile and the pile tube, may be grouted in order to increase stability of the structure after the pile has been driven into the seabed or the foundation structure has been placed on the pile.

Further features, properties and advantages of the present invention will become clear from the following description of exemplary embodiments in conjunction with the accompanying drawings .

Figure 1 shows a jacket structure of an offshore wind turbine foundation in a highly schematic view.

Figure 2 shows a tripod structure of an offshore wind turbine foundation in a highly schematic view.

Figure 3 shows a section of a first embodiment of the inventive offshore foundation structure.

Figure 4 shows a section of a second embodiment of the inventive offshore foundation structure.

Figure 5 shows a section of a third embodiment of the inventive offshore foundation structure.

The present invention will now be described by way of exem^¬ plary embodiments. These embodiments are mainly depicted in Figures 3 to 5 and relate in general to an advantageous wind turbine tower offshore foundation structure, an advantageous wind turbine tower offshore foundation established by using the wind turbine tower offshore foundation structure and an advantageous method of establishing an offshore wind turbine tower foundation such as a tripod or jacket structure on the seabed .

A possible offshore foundation structure by which the inven^¬ tion can be realised is a jacket structure as it is shown in Figure 1 in highly schematic view. The jacket structure shown in this figure comprises three legs 11 each forming a tubular hollow foundation member. Another possible offshore foundation structure by which the invention can be realised is the so called tripod structure that is schematically shown in Figure 2. In a tripod structure, three legs 11 forming tubu^¬ lar hollow foundation members support a tube 9 carrying an installation like a wind turbine. More specifically the first embodiment of the invention the invention is explained by reference to Figure 3 which sche^¬ matically illustrates this embodiment. Figure 3 shows a leg 11 of an inventive foundation structure, which could be ei^¬ ther a tripod - or jacket type structure. The leg 11 is equipped with an open longitudinal end 12 which is placed on the seabed 6 such that the open longitudinal end 12 shows to^¬ wards the seabed 6. The legs 11 are slightly tilted with re- spect to the vertical direction. Piles 7 may project through the open longitudinal ends 12 of the legs 11 into the seabed 6 to form anchors for the foundation structure. The interior of the longitudinal hollow foundation members 11 may be grouted, in particular the space between the piles 7 and the inside wall of the legs 11, in order to increase stability of the structure. Apart from using piles as anchors securing the foundation structure in place can also be achieved by anchor^¬ ing means other than piles or by means of gravitation founda^¬ tions .

In particular, Figure 3 shows the seabed end of a leg 11 of a wind turbine foundation structure (e.g. tripod or jacket) . At its end nearest the seabed 6, one or more of the legs 11 of the wind turbine foundation structure according to the first embodiment comprise (s) for a minimum configuration:

- A main tube 3 of the foundation structure leg 11,

- a seabed plate 1 prepared to hold the structure at least temporarily on the seabed 6,

- a seabed plate tube 2 prepared to be moved in relation to the said main tube 3 in the axial direction of the main tube 3, and

- a jack means 5 which is able to, and prepared for, adjust^¬ ing the relative axial position of the main tube 3 and the seabed plate tube 2 to each other. Although the seabed plate tube 2 of the present embodiment is located inside the main tube 3 it may as well be located out^¬ side the main tube 3 so as to surround the main tube 3. For a further embodiment of the invention the main tube 3 and/or the seabed plate tube 2 further comprise (s) jack base means 4 which is prepared for holding the jack means 5 and/or providing support for the jack means 5 and which can withstand the loads applied to the base means 4 by e.g. the weight of the foundation structure through the jack means 5.

A second embodiment of the invention is illustrated in Figure 4 below. Like Figure 3, Figure 4 shows the seabed end of a leg 11 of a wind turbine offshore foundation structure (e.g. tripod or jacket) .

For the second embodiment of the invention, the foundation structure is further supported in its position by one or more piles 7 penetrating the seabed 6. The piles 7 can be made of e.g. steel, concrete or reinforced concrete or a combination hereof .

For one embodiment of the invention, the piles 7 may be es^¬ tablished in the seabed 6 before the foundation structure is placed over the piles.

For another embodiment of the invention, the piles 7 may be established in the seabed 6 after the foundation structure is placed on the seabed such as through the main tube 3 and the seabed plate tube 2 as indicated by the arrow in the figure.

For various embodiments of the invention the main tube 3 and/or the seabed plate tube 2 may be secured to the pile 7 by grouting the space 10b between the pile 7 and the seabed plate tube 2 and/or the space 10a between the pile 7 and the main tube 3. A third embodiment of the invention is schematically illus^¬ trated in Figure 5. Like Figures 3 and 4, Figure 5 shows the seabed end of a leg 11 of a wind turbine foundation structure (e.g. tripod or jacket).

For the third embodiment of the invention, the seabed plate 1 further comprises a separate pile tube 8 which extends verti^¬ cally or near vertically from the seabed plate 1 and which is prepared for receiving a pile 7. The piles 7 can be made of e.g. steel, concrete or reinforced concrete or a combination hereof .

For one embodiment of the invention, the piles 7 may be es^¬ tablished in the seabed 6 before the foundation structure and the pile tubes 8 of the seabed plate 1 are placed over the piles .

For another embodiment of the invention, the piles 7 may be established in the seabed 6 after the foundation structure is placed on the seabed such as through the pile tube 8.

For various embodiments of the invention the pile tube 8 may be secured to the pile 7 by grouting the space 10c between the pile 7 and the pile tube 8.

For various embodiments of the invention, the jack means 5 may be e.g. hydraulic operated, pneumatic operated or manu^¬ ally operated. For even further embodiments of the invention, the jack means 5 may be removable so that it can be removed after position^¬ ing and possible grouting, or the jack means 5 may be non^¬ removable so that it stays installed after positioning and possible grouting.

Examples for advantages which arise from the technical fea^¬ tures according to the invention: The invention is advantageous in that the preparation of the seabed prior to establishing the foundation, can be minimized as the jack means 5 is able to level out even large varia^¬ tions of the height of the seabed 6 in order to achieve a construction in level.

The invention can furthermore be advantageous in that (for some of the embodiments) the piles are established after the foundation structure has been positioned on the seabed.

Thereby the piles do not need to be very accurately posi^¬ tioned on/in the seabed prior to positioning the foundation structure such as by a pre-fabricated and prepositioned pile- positioning template structure.

Claims

1. An offshore foundation structure, in particular a wind turbine foundation structure, with at least one leg (11) to be positioned on the seabed (6), the at least one leg (11) comprising :

- an adjusting means (5) which is able to, and prepared for, adjusting the axial length of the leg (11) .

2. The offshore foundation structure as claimed in claim 1, wherein the at least one leg (11) comprises:

- a main tube (3) extending along an axial direction,

- a seabed plate (1) which is located at the seabed (6) side end of the main tube (3) and prepared to hold the structure at least temporarily on the seabed (6),

- a seabed plate tube (2) which extends from said seabed plate (1) into or around said main tube (3) and is prepared to be moved in relation to said main tube (3) in the axial direction of the main tube (3) , and

- an adjusting means (5) which is able to, and prepared for, adjusting the relative axial position of the main tube (3) and the seabed plate tube (2) to each other.

3. The offshore foundation structure as claimed in claim 2, wherein the adjusting means is a jack means (5) acting between said seabed plate tube (2) and said main tube (3) .

4. The offshore foundation structure as claimed in claim 3, wherein said seabed plate tube and/or said main tube com- prise (s) a jack base means (4) which is prepared for holding the jack means (5) and/or for providing support for the jack means (5) .

5. The offshore foundation structure as claimed in any of the claims 1 to 4, wherein the at least one leg (11) comprises a pile (7) which is adapted to be driven into the seabed (6) .

6. The offshore foundation structure as claimed in claim 5, wherein the pile (7) extends at least partly through the sea^¬ bed plate tube (2) and/or the main tube (3) .

7. The offshore foundation structure as claimed in claim 5, further comprising a separate pile tube (8) for receiving a pile (7), said pile tube (8) extending from said seabed plate (1) .

8. The offshore foundation structure as claimed in any of the claims 1 to 5, wherein the adjusting means (5) is removable.

9. An offshore foundation, in particular a wind turbine foundation, established on a seabed comprising an offshore foun- dation structure as claimed in any of the claims 1 to 5 which rests on the seabed (3), wherein the lengths of the leg (11) is so adjusted as to level out variations of the height of the seabed ( 6) .

10. The offshore foundation as claimed in claim 9, wherein the lengths of the leg (11) is adjusted by an adjusted rela^¬ tive axial position of the main tube (3) and the seabed plate tube (2) to each other.

11. The offshore foundation as claimed in claim 10, wherein there is a space between the main tube (3) and the seabed plate tube (2) and said space is grouted.

12. The offshore foundation as claimed in any of the claims 9 to 11, wherein the at least one leg comprises a pile (7) which is driven into the seabed (6) .

13. A method of establishing an offshore foundation, in particular a wind turbine offshore foundation, as claimed in any of the claims 9 to 12 on the seabed, the method comprising the steps of positioning an offshore foundation structure as claimed in any of the claims 1 to 8, adjusting the offshore foundation structure by use of the adjusting means (5) .

14. The method as claimed in claim 13, further comprising the step of anchoring the offshore foundation structure by driv^¬ ing piles (7) into the seabed (6) .

15. The method as claimed in claim 14, wherein the offshore foundation structure is grouted.