Anchoring arrangement for floating wind turbine installations
The anchoring device for a floating wind turbine installation, such wind turbine installation comprising a floating cell (7), a tower (8) arranged over the floating cell, a generator (9) mounted on the tower which is rotatable in relation to wind direction and fitted with a wind rotor (10), and an anchor line arrangement (6) connected to anchors or anchoring points on the sea bed. The individual anchor lines (11) are each, at a certain distance from the floating cell (7) at a fixing point (5) on the individual anchor line, connected with double lines (2, 3) slanting outwards and connected to the floating cell (7) in a delta-shaped arrangement.
The present invention relates to an anchoring arrangement for a floating wind turbine installation, such wind turbine installation comprising a floating cell, a tower arranged over the floating cell, a generator mounted on the tower which is rotatable in relation to wind direction and fitted with a wind rotor, and an anchor line arrangement connected to anchors on the sea bed.
The advantage of using floating wind turbines is that this allows almost unlimited access to installation areas, since relatively deep sea areas can be used.
Normally, wind turbines are arranged such that the rotor faces the wind with the tower positioned downstream of the wind direction. This is to avoid the wind flow being disturbed before it passes through the rotor, which could otherwise lead to loss of energy and disruptive vibrations and impulses in the turbine.
To control this, active direction correction of the turbine around the tower's vertical axis is required as the wind changes direction. This is normally achieved by arranging a rotating bearing rim with a ring and pinion solution between the top of the tower and the nacelle.
Yawing is performed by a system which registers the wind direction and automatically drives the pinion by a motor to make the rotor turn into the direction of the wind.
This works well when the tower is on a fixed foundation, as in land installations and offshore installations in shallow waters.
When the tower is mounted on a floating support, it is necessary to ensure sufficient resistance to rotation round the vertical axis, so that active rotation of the nacelle is performed rather than the whole plant rotating too far when the wind turbine is subjected to oblique wind loads.
Resistance to rotation is provided by anchor lines which are pre-tensioned by a specific force. When anchor lines are fixed directly to a slim cylindrical construction, then, as described below, this gives low resistance to rotation as the anchor line is connected near the rotation axis. Resistance to rotation occurs when the tower is rotated from its equilibrium position and a righting arm occurs as a function of angle () and radius (r) from the rotation axis to the line fixing point. The righting arm (a) is in this case:
a=sin()×r
The righting force corresponds to the normal component Fn on the rotation axis of the pre-tension on the line, and the righting moment will then become:
Mr=Fn×a=Fn×sin()×r
The resistance moment against rotation is thus, as shown above, a sine function with a maximum at 90° (see below). At small angles of rotation the rotation resistance will act like a linear rotation spring.
From U.S. Pat. No. 3,082,608 an anchored platform of triangular design has previously been disclosed. From each corner, preferably at a 20° angle, two chains or cables are extended which are joined to heavy weights arranged on the sea bed, while from each weight further anchoring lines extend to heavy anchors sited further away from the platform. The purpose of this solution is primarily an anchoring solution designed to eliminate platform motion caused by waves. The solution will provide resistance to rotation but will be unsuitable for anchoring a slim cylindrical floating wind turbine, because it presupposes that there is a great distance between the three fixing points for the anchors lines. The known solution is based on a taut anchoring system, generating very large dynamic forces in the anchor lines. In addition the anchoring solution is heavy and complex, which in turn necessitates high costs of manufacture and installation.
The present invention provides a solution for anchoring a floating wind turbine installation by which it is possible to increase significantly the initial resistance to rotation round the vertical axis. It further provides a solution which is extremely simple and can be used for anchoring wind turbine installations in very deep water.
The invention is characterised in that the individual anchor lines, at a certain distance from the floating cell and at a fixing point on the individual anchor line, are each connected to at least two separate lines which slant outwards and are fixed to the floating cell in a delta-shaped arrangement, as indicated in the attached independent claim 1.
The dependent claims 2-6 define advantageous features of the invention.
The invention will be further described by means of an example and with reference to the attached figures in which:
As mentioned,
As is further evident from the anchor line arrangement 6 shown in
The length of lines 11 is relatively long, depending on the depth of the sea bed where the wind turbine is located, and the pre-tensioning in the individual anchor lines may be of the order of 1000 kN. The lines' angle to the horizontal plane is approx. 30-70° and the length of the lines 2, 3, depending on the dimensions of the wind turbine installation and of the floating cell as a whole, may be of the order of 50 m.
With these suggested values, calculation shows that the arrangement according to the invention is of the order of 9 times more resistant to rotation than it would have been with a conventional solution in which the anchor lines are fixed directly to the floating cell, without the lines being arranged in a delta shape.
The characteristics of rotational stiffness for a conventional solution and for the delta line solution according to the invention are shown in
As mentioned,
Mr=Fn×sin()×R
When the angle of rotation reaches a critical value () so that there is slack on one line, the moment increase will be small until the maximum moment is reached. The critical angle for slack in one of the delta lines in the example calculated here, as shown in
With the increased initial resistance to rotation achieved with this arrangement, active direction control of the turbine can be effected with an acceptable response angle in the tower.
For a conventional anchor system, it can further be seen from the figure that the resistance to rotation increases the slack until a maximum rotation resistance is reached close to an angle of 90°.
The invention as defined in the claims is not limited to the embodiment shown in the figures and described in the foregoing, so that instead of three anchor lines, four or more anchor lines 11 may be used, each with corresponding lines 2, 3 arranged in a delta-shaped pattern. Use of three anchor lines at intervals of 120° is however seen as representing the simplest and cheapest solution.
Claims
1-6. (canceled)
7. Anchoring arrangement for a floating wind turbine installation, such wind turbine installation comprising a floating cell (7), a tower (8) arranged over the floating cell, a generator (9) mounted on the tower which is rotatable in relation to wind direction and fitted with a wind rotor (10), and an anchor line arrangement (6) connected to anchors or anchor points on the sea bed wherein the individual anchor lines (11) are each, at a certain distance from the floating cell (7) at a fixing point (5) on the individual anchor line, connected with at least two lines (2, 3), each slanting outwards and being connected to the floating cell (7) in a delta-shaped arrangement.
8. Anchoring arrangement according to claim 7, wherein the anchor line arrangement (6) comprises three anchor lines (11) arranged symmetrically at intervals of 120°.
9. Anchoring arrangement according to claim 7, wherein lines (2, 3) in the delta-shaped arrangement are arranged at a mutual angle of between 20 and 60°.
10. Anchoring arrangement according to claim 7, wherein lines (2, 3) in the delta-shaped arrangement are arranged at an angle to the horizontal plane of between 30 and 70°.
11. Anchoring arrangement according to claim 7, wherein lines (2, 3) in the delta-shaped arrangement are fixed to the floating cell (7) at the maximum possible distance for the fixing points on the floating cell.
12. Anchoring arrangement according to claim 7, wherein lines (2, 3) in the delta-shaped arrangement are fixed in pairs to joint brackets (4) on the floating cell (7).
Type: Application
Filed: May 4, 2006
Publication Date: Apr 9, 2009
Inventor: Dagfinn Sveen (Oslo)
Application Number: 11/919,887
International Classification: B63B 21/50 (20060101);