Arrangement for Stabilization of a Floating Foundation

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of European Patent Office Application No. 08012310.2 EP filed Jul. 8, 2008, which is incorporated by reference herein in its entirety.

FIELD OF INVENTION

The invention relates to an arrangement for the stabilization of a floating foundation, which is used offshore to carry a wind-turbine.

BACKGROUND OF INVENTION

Floating foundations for wind-turbines are known from WO 2006 132 539 A1 and from WO 2006 121 337 A1 showing so called “spar-buoy”. These spar-buoy-types are fixed at a predetermined position by the use of one set of mooring cable.

FIG. 3 shows a typical spar-buoy foundation according to the state of the art.

A wind-turbine 1 is mounted on a tower 2, which is supported by a spar-buoy foundation 3. There is a set of mooring cables 5. First endings of the mooring cables 5 are attached to the spar-buoy foundation 3 at a certain position 4, while second endings of the mooring cables are attached to anchors 6.

The level of the position 4 is below the waterline WL. There is a balancing weight 7 located on the lower ending of the spar-buoy foundation 3, which is relevant for the stability of the whole construction.

This kind of mooring-arrangement is simple and cheap. But the stability of the foundation relies only on gravity, so an equilibrium is established between overturning moments on the one hand and a restoring moment on the other hand.

The overturning moments are performed by waves and wind while the restoring moment is performed by the weight of the foundation, which acts on the lower end of the foundation.

FIG. 4 shows with reference to FIG. 3 typical operating loads of a normal spar-buoy foundation.

The whole structure is generally exposed to an arising overturning moment.

The overturning moment is defined as multiplication from a wind-load 8, acting on the wind-turbine 1, and a lever-arm 9, which length is defined by the distance between a force centre and the mooring cable attachment 4.

The overturning moment is additionally defined as multiplication from the wind load 10, acting on the tower 2, and a lever-arm 11, which length is defined by the distance between the wind-load-force-centre and the mooring cable attachment 4.

Last the overturning moment is additionally defined as multiplication from a wave and current load 12 multiplied with an arm-lever 13, which is defined by the distance between this force centre and the mooring cable attachment 4.

The restoring moment is the gravity load 14 of the balancing weight 7 multiplied with a lever-arm 15, which is defined by the distance from the gravity force centre to the mooring cable attachment 4.

When the overturning moment and the restoring moment are in equilibrium the structure shows an inclination angle 16.

If the restoring moment is “0” the foundation itself is not inclined. Any overturning moments lead to inclination of the foundation. So the operation of a wind-turbine is negatively affected by the inclination.

SUMMARY OF INVENTION

An object of the present invention is to provide an improved arrangement for the stabilization of a floating foundation to be used offshore.

This aim is solved by an arrangement as claimed in the independent claim. Advantageous embodiments of the invention are subject of the dependent claims.

According to the invention the arrangement for stabilization of a foundation is arranged to carry a wind-turbine mounted on a tower. The foundation is fixed with a set of mooring cables. First endings of the mooring cables are attached to the foundation at a certain position while second endings of the mooring cables are attached to the floor. There is a second set of mooring-cables, while first endings of the second set of mooring-cables are attached to the foundation near or at its bottom end. Second endings of the second set of mooring-cables are attached to the floor, too.

The inventive arrangement allows a higher stabilisation of the floating foundation, which is arranged to carry a wind-turbine.

The inventive arrangement is cheap and is easy to implement even at older locations, which are already at work.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described now in more detail by the help of some figures, where:

FIG. 1 shows a spar-buoy foundation according to the invention,

FIG. 2 shows with reference to FIG. 1 operating loads of the inventive spar-buoy foundation,

FIG. 3 shows a typical spar-buoy foundation according to the state of the art.

FIG. 4 shows with reference to FIG. 3 typical operating loads of a normal spar-buoy foundation.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a spar-buoy foundation according to the invention.

A wind-turbine 1 is mounted on a tower 2, which is supported by a spar-buoy foundation 3. There is a set of mooring cables 5. First endings of the mooring cables 5 are attached to the spar-buoy foundation 3 at a certain position 4, while second endings of the mooring cables are attached to anchors 6.

The level of the position 4 is below the waterline WL. There is a balancing weight 7 located on the lower ending of the spar-buoy foundation 3, which is relevant for the stability of the whole construction.

According to the invention there is at least another set of mooring cables 17. First endings of the mooring cables 17 are attached to the spar-buoy foundation 3 near or at the bottom end of the foundation 3. Second endings of the mooring cables 17 are attached to the anchors 6, preferably. It is also possible to use another set of anchors for this mooring-cables 17.

In this case the two sets of mooring cables 5 and 17 serve to stabilise the foundation. Therefore the restoring moment will be a combination of a first moment, arising from the weight of the lower end of the foundation, and a second moment, arising from differential forces in the mooring cables 5 and 17.

FIG. 2 shows with reference to FIG. 1 typical operating loads of the inventive spar-buoy foundation 3.

The whole structure is generally exposed to an arising overturning moment.

The overturning moment is defined as multiplication from a wind-load 8, acting on the wind-turbine 1, and a lever-arm 9, which length is defined by the distance between a wind-load-force-centre and the mooring cable attachment 4.

The overturning moment is additionally defined as multiplication from the wind load 10, acting on the tower 2, and a lever-arm 11, which length is defined by the distance between the wind-load-force-centre and the mooring cable attachment 4.

Last the overturning moment is additionally defined as multiplication from a wave and current load 12 multiplied with a arm-lever 13, which is defined by the distance between this force centre and the mooring cable attachment 4.

The restoring moment is the horizontal component 18 of a second mooring cable force 19, multiplied with an arm 20 from the attachment point AT of the second set mooring cables 17 to the attachment point 4 of the first set of mooring cable 5.

When the overturning moment and the restoring moment are in equilibrium the structure has a very small inclination angle.

In the normal mooring arrangement of the spar-buoy foundation as described in FIG. 3 and FIG. 4 a typical high loading situation would lead to about 13 m displacement of the tower top and about 5 degrees maximum inclination.

According to the invention the mooring arrangement of the spar-buoy foundation the same loading situation will lead to about 3 m displacement of the tower top and about 1.5 degrees maximum inclination. So the inventive arrangement leads to a higher stiffness.

In a preferred embodiment the first set of mooring-cable 5 and/or the second set of mooring cable 17 are fitted with tensioning devices, to allow an adjustment of the relative tautness of the particular set of mooring cable.