GB2401387A

GB2401387A - Adjustable offshore wind tower foundation

Info

Publication number: GB2401387A
Application number: GB0310545A
Authority: GB
Inventors: Alexander Terrell
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-05-08
Filing date: 2003-05-08
Publication date: 2004-11-10
Anticipated expiration: 2023-05-08
Also published as: GB0310545D0; GB2401387B

Abstract

A platform 3 and tower 6 for offshore wind turbines is characterised in that the orientation of the tower with respect to the platform can be adjusted and fixed after the platform has been placed on the seabed 4, thus ensuring that the longitudinal axis 10 of the tower is vertical even where the seabed is not level or flat. The tower may be mounted on a ball and socket joint 1 and positioned in a collar 2 mounted on the platform. Screws or drive rams 7, mounted on the collar, may be used to apply horizontal force on the tower to facilitate the adjustment. The angle of the tower may otherwise be adjusted by means of tensioning guy cables (Fig 4) pulling on the tower.

Description

ADJUSTABLE OFFSHORE WIND TURBINE FOUNDATION

TECHNICAL FIELD

The present invention relates to a foundation structure to support offshore wind turbines.

BACKGROUND OF THE INVENTION

Offshore Wind Turbines are usually built on rigid foundations that are installed on the seabed.

These foundations have to be positioned with great precision to ensure that the tower can be installed in a vertical position, and that the tower remains in a vertical position during its operating lifetime. This requirement significantly raises costs for installation of the foundation structure.

Where a pile driven foundation construction is used, the requirement for precision generally involves the need for selfjacking ships, which are hugely expensive. Where a gravity foundation is used, the seabed needs to be levelled and cleared of debris to ensure that the foundation settles in the appropriate orientation. Both these methods become prohibitively expensive as water depth increases beyond about I em.

SUMMARY OF THE INVENTION

This invention overcomes these obstacles, enabling a lower cost, faster installation process.

The invention consists of a simple, adjustable foundation structure that can be placed on the seabed, with no seabed preparation. Attached to the foundation is the lower section of the Wind Turbine tower. Once the foundation structure has been positioned, it can be secured in place either through the addition of ballast, or through the use of piles. Once the foundation structure has been secured, the lower tower section can be adjusted and fixed so that it is vertical. The adjustment is enabled by use of a flexible joint at the base of the tower. The fixing is enabled by the use of screws or rams that push on the tower base in a horizontal direction, some distance above the flexible joint. Alternatively, the fixing can be enabled with adjustable guy ropes, which would be attached to the tower and to separate, pile driven foundations.

Page I The adjustable mechanism can either be deployed within an open steel framework, which can be attached to the seabed, or in a closed, solid structure, which can be filled with ballast.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail with reference to the accompanying drawings.

Figure I shows the essential elements of the adjustable mechanism. Figure 2 shows the adjustable mechanism housed within a ballast foundation. Figure 3a shows a cross section view of the ring, with four screw or ram devices pushing laterally on the tower. Figure 3b shows a cross section through the ball and socket joint, showing the lay out of the buttresses.

Figure 4 shows the adjustable mechanism supported by the fixing as supported by the adjustable guy cables.

DETAILED DESCRIPTION OF THE INVENTION

At the centre of the Adjustable Foundation is a flexible joint ( 1). This joint is realised through the use of a large ball and socket joint, which is able to support the required vertical and horizontal forces.

The socket of the joint is attached to the main structure of the foundation (3). This structure extends outwards and upwards a number of metres, depending on the size of the foundation to be developed. The structure could consist of a number of buttresses ( 12) or of a solid ballast chamber. In both cases, the structure is strong enough to withstand the forces to which it may be exposed.

Page 2 In the case of a foundation structure consisting of buttresses, then each buttress extends outwards and upwards from the joint. At the end of each buttress, there is a facility for attaching the buttresses to the seabed (4) using piles (5). The upper end of all the buttresses are joined to a ring shaped collar (2), which encircles the lower section of the tower, and constrains the tower to move within the ring. In most circumstances there would be three buttresses, forming a three-legged, tripod structure. The joint is raised above the outer end of the three buttresses, so that the platform rests on the seabed surface on three points, at the end of each buttress.

In the case of a solid ballast structure, the support structure (3) takes the form as shown in figure 2. This structure can be filled with ballast to ensure that foundation is able to withstand the forces imposed upon it. The joint ( I) is positioned in the centre and near the base of the structure. The joint enables the base of the tower to move around the vertical axis, and this movement is constrained a ring shaped collar (2), which encircles the lower section of the tower. Between the collar (2) and the main structure of the platform (3), the lower section of the tower (6) is able to move within a cone shaped space. The ballast structure is initially hollow, so that it floats on the surface of the water ( I 1). Once it has been transported to the chosen location, water can be admitted to sink the structure onto the seabed. Ballast can then be added from a ship, into the ballast structure to provide sufficient mass to enable the structure to withstand the forces.

In both instances, the ball of the joint is attached to the lower section of the wind turbine tower (6). This lower section extends upwards so that the top of the lower section is clear of the water line (16). The diameter of the tower increases with distance away from the joint, so that the bottom section is cone shaped. Where the tower passes through the collar (2), the form changes to tubular. At the top of the lower section of the tower is a flange (9) to which can be bolted the upper section of the tower. The flange can be wider than the tower, in order to serve as a platform for accessibility, and to enable attachment of the upper section of the tower.

Page 3 The tower is able to move within it's joint, and is constrained from moving too far by the collar (2), which is positioned a number of meters above the joint. The collar is penetrated by four or more screws or rams (7), which push against the side of the tower and thereby control its position within the collar (2). At the point of contact between the screws and the tower, or between the rams and the tower, the tower is reinforced so that it is not damaged by the pressure of the screws or rams.

The housing (8) for each ofthe screws or rams is attached to the collar. The rams or screws are operated by an external power source, which would typically be based on a ship. The motor to drive the screws, or the hydraulic systems to power the rams may be built into the housing.

After the platform has been placed in the water, the rams or screws are used to adjust the position of the tower so that it is vertical, even in the event of the seabed not being horizontal.

The vertical line ( 10) is shown in figures I and 2.

The screws or rams are thick and strong enough to support both the longitudinal forces, and the shear forces that they experience. The end of the screws or rams will be shaped to provide optimum contact with the tower. Alternatively, they may be attached with a flexible coupling to an arc shaped restraint (20), which will make contact with the tower over a larger area. The arc shaped restraint would have the same radius of curvature as the tower. Once the tower has been adjusted, the screws or rams are locked into place, so that the tower can no longer move.

If it is considered desirable that the tower orientation should be adjustable for a long time after the platform is placed at sea, then the ball and socket joint can be sealed using a flexible membrane prior to installation. Lubricant would then be inserted into the joint to immerse the ball. Once the structure is placed on the seabed, the joint will remain lubricated and operational for many years after installation.

Page 4 If it considered desirable to lock the tower position into place after installation and adjustment, then the two watertight, flexible membranes ( 19) can be placed between the collar and the tower, to produce a dry cavity. Once the orientation of the tower has been adjusted, a setting liquid, such as epoxy, can be injected into the cavity between the two membranes. Once this hardens, the structure will be permanently locked into place.

In the case of a structure supported by guy cables, then the foundation platform is reduced in size, and the collar is not used to restrain the movement ofthe tower. About halfway up the tower are attachments ( 13) for the supporting guys ( 14). Around the rim of the tower are evenly spaced attachments for 4 or more guys ( 14). These are attached to the tower at this point. The other ends of the wire rope guys are attached to the seabed (4), some distance from the tower, and are secured by piles or anchors (5). The wire rope guys provide the principal horizontal forces for keeping the tower level. The wire rope guys can be adjusted in length with the use of tensioning devices ( 15). These are standard hydraulic tensioners located on the seabed, and attached to the seabed with the aid of short piles. By operating the tensioners, the tower can be moved into a vertical position after having been installed on any quality of seabed. The tensioners are powered, either electrically or hydraulically, from a ship-based source. The wire rope guys are attached to the centre of the tower, and therefore do not interfere with the rotating rotors ( 17) of the wind turbine ( 18). The maximum bending moment in the tower is now experienced in the middle, where the guys are attached. As a result, the lower half of the tower is made significantly lighter, and with less material, than in a conventional design.

Page 5

Claims

I A platform and tower for offshore wind turbines characterized in that the orientation of the tower with respect to the platform can be adjusted and fixed after the platform has been placed on the seabed to ensure that the longitudinal axis of the tower is vertical even where the seabed is not level or flat.
A platform as claimed in claim] that uses a ball and socket joint so that the tower can be vertically aligned regardless of the orientation of the seabed.
3 A platform as claimed in claim I whereby the allowable angle of the tower (6) is constrained by a col lar (2), and whereby the angle of the tower with respect to the platform can be adjusted within the constraint imposed by the collar.
4 A platform as claimed in claim I whereby the angle of the tower can be adjusted by the use of rams or screws (7) applying a horizontal force on the side of the tower.
A platform as claimed in claim I whereby rams or screws (7) apply a lateral force to adjust the angle of the tower, and where said rams or screws are fitted with a jointed buffer (20) to distribute the load on the tower, said buffer being of an arc shape, with radius equal to the radius of the tower, to maintain a large contact area with the tower.
6 A platform as claimed in claim I whereby the angle of the tower (6) can be adjusted by the means oftensioning guy cables (14) pulling on the tower and working in conjunction with a ball and socket joint at the base of the tower.
7 A platform as claimed in Claim I that can be adjusted so as to position the tower in a vertical, upright position even in the event of the certain parts of the platform subsiding after installation of the wind turbine.

Page 6
8 A platform as claimed in Claim 1 that locked permanently into place after adjustment by the addition of a setting material around the tower and within a collar, so that once the setting material has set, the tower cannot be moved.
9 A platforrp as claimed in Claim 1 whereby the top of the tower section protrudes from the water to provide a level, stable and dry platform for the attachment of a wind turbine tower.

A platform as claimed in Claim 1 that can be built onshore to a single, standard design to support a variety of different sea depths, seabed conditions, and turbine size, and that can be adjusted after installation to accommodate variations of these.

A platform and tower for offshore wind turbines substantially as described herein with reference to the accompanying drawings labelled figures 1-4

9 A platform as claimed in Claim I that uses onboard motors to turn screws to make the adjustments to the angle of the tower.

A platform as claimed in Claim I that uses onboard hydraulics to drive rams to make the adjustments to the angle ofthe tower.

I I A platform as claimed in Claim I whereby the top of the lower tower section protrudes from the water to provide a level, stable and dry platform for the attachment of a wind turbine tower.

12 A platform as claimed in Claim I that can be built onshore to a single, standard design to support a variety of different sea depths, seabed conditions, and turbine size, and that can be adjusted after installation to accommodate variations of these.

13 A platform and tower for offshore wind turbines substantially as described herein with reference to the accompanying drawings labelled figures 1-4 Page 7 Amendments to the claims have been filed as follows; I A platform and tower for offshore wind turbines characterized in that the orientation of the tower with respect to the platform can be adjusted and fixed after the platform has been placed on the seabed to ensure that the longitudinal axis of the tower is vertical even where the seabed is not level or flat.

2 A platform as claimed in claim] in which a tower is mounted on a ball and socket joint so that the tower can be vertically aligned regardless of the orientation of the seabed.

3 A platform as claimed in claim I whereby the allowable angle of the tower (6) is constrained by a collar (2) that is fixed to the platform, and whereby the angle of the tower with respect to the platform can be adjusted within the constraint imposed by the collar.

4 A platform as claimed in claim I whereby the angle of the tower can be adjusted by the use of rams (7) applying a horizontal force on the side of the tower, and where said rams are fitted with a jointed buffer (20) to distribute the load on the tower, said buffer being of an arc shape, with radius equal to the radius of the tower, to maintain a large contact area with the tower; and where said rams may be driven by onboard hydraulics.

A platform as claimed in claim 1 whereby the angle of the tower can be adjusted by the use of screws (7) applying a horizontal force on the side of the tower, and where said screws are fitted with a jointed buffer (20) to distribute the load on the tower, said buffer being of an arc shape, with radius equal to the radius of the tower, to maintain a large contact area with the tower; and where said screws may be turned by onboard motors.

6 A platform as claimed in claim 1 whereby the angle of the tower (6) can be adjusted by the means oftensioning guy cables (14) pulling on the tower and working in conjunction with a ball and socket joint at the base of the tower.

7 A plum as Claimed in Claim i that can be adjusted pa as to position the waver in vertiC4!, upright position even in the eyent of Certain pr!tS of the platforms $bSjdig after installation of the wind turbine 8 Pitf0I as rlaime-d! in Claim 1 ulh9Fein the toW9F is lopped perrnaTgntly into peace Fur diustmept try the addition of setting material around the tower and within a collar encircling the tower and mounted on the platform, so that once the setting material has set' the tower cannot be Moved relative to the platform.