EP2121424A1

EP2121424A1 - A mooring system for stabilizing a vessel, a vessel, method for stabilizing a vessel and use of a mooring system

Info

Publication number: EP2121424A1
Application number: EP07846436A
Authority: EP
Inventors: Anders SØE-JENSEN
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2006-12-22
Filing date: 2007-12-21
Publication date: 2009-11-25
Also published as: WO2008077405A1; US20090255454A1

Abstract

The invention relates to a mooring system for stabilizing a vessel for handling offshore structures where said system comprise at least one mooring unit, connection means for connecting said mooring unit to said vessel, and tractive means for stabilizing said vessel by establishing permanent tension in said connection means. Furthermore said tractive means comprises means for lowering said connected vessel's position in relation to the water level. The invention also relates to a method of stabilizing a vessel, a vessel for handling offshore structures and use of a mooring system.

Description

A MOORING SYSTEM FOR STABILIZING A VESSEL, A VESSEL, METHOD FOR STABILIZING A VESSEL AND USE OF A MOORING SYSTEM

Background of the invention

The invention relates to a mooring system for stabilizing a vessel for handling offshore structures, a vessel, a method of stabilizing a vessel for handling offshore structures and use of a mooring system.

Description of the related art

Vessels for transportation and mounting of offshore structures to be installed on offshore locations are known in the art.

As it is often desired to maintain the vessel in a substantially fixed position during loading/unloading and/or during mounting of said offshore structures, it is a challenge of the vessel to overcome its motion due to wind, waves and current. Different vessels and methods have been suggested for this purpose.

European patent EP 1 220 775 B discloses a vessel for transporting and mounting windmills on previously built structures on the seabed. The vessel comprises at least four vertically elevational support legs mounted in consoles connected to the hull for the support of the vessel on the seabed.

German patent DE 100 21 163 B discloses a vessel comprising four vertically elevational support legs, two by two connected to lift tanks. The lift tanks can be elevated to contact with the seabed whereby the vessel can be lifted above the waterline with its lift tanks resting on the seabed. A disadvantage of the described vessels is that they are sensible to seabed conditions due to seabed penetration when jacked up. Furthermore rigid vertically elevational support legs are used which in its construction is a relatively complicated set-up.

International patent application WO 2004/046467 A discloses a composite converted vessel for performing drilling operations while supported on a bottom of a water body. The vessel comprises a floating drilling rig component and a support barge component. The barge component comprises a ballast system including pumps that is able to ballast the vessel to rest on the bottom of a water body. Furthermore the barge component comprises a substantially planar bottom.

A disadvantage for the described converted vessel is that it is a complicated and non- flexible setup regarding the construction of the support barge component that must be specific designed to the drilling rig component to which it supports.

It is an object of the invention to provide technique without the above mentioned disadvantages and to simplify the construction of the vessel and hereby increase its operational reliability and capability and flexibility. Furthermore it is an object to preserve the ability of a vessel according to the invention to maintain it in a substantially fixed position during loading/unloading and/or during mounting of offshore structures.

The invention

The invention relates to a mooring system for stabilizing a vessel for handling offshore structures where said system comprise at least one mooring unit, connection means for connecting said mooring unit to said vessel, and tractive means for stabilizing said vessel by establishing permanent tension in said connection means. Furthermore said tractive means comprises means for lowering said connected vessel's position in relation to the water level.

By vessel is meant any types of boat, ship, craft or the like that is suitable and/or can be arranged for the purpose of handling offshore structures.

By mooring unit is meant a unit that is not a part of the vessel and is free to operate in relation to the hull of said vessel. For various embodiments the mooring unit can be mobile or established at a fixed position.

By tractive means is meant means that is able to establish tension in attached connection means. Tractive means also comprises means that is able to regulate and/or dynamically control the established tension in the connection means.

By the terms permanent tension is meant that the tension in said connection means is applied without interruption when said mooring system is in use but the tension force can be altered, controlled, regulated and/or adjusted during use i.e. the tension force is not constant in relation to time.

By stabilizing said vessel in relation to the present invention is meant that the vessel can be held in positions in such a way that the influence on the movement of the vessel in all directions due to environmental parameters such as waves, tidal, current and/or wind is minimized.

By lowering the vessel's position in relation to the water level it is ensured that especially the impact of environmental parameters such as waves on the vessel movement in e.g. vertical direction is reduced.

Hereby it is ensured that the window in which it is possible to operate from the vessel due to the influence of said environmental parameters can be extended. Furthermore it is ensured that loading and unloading of said structures can be done in a safe and reliable matter when working in said window.

It is furthermore ensured that the relative movement to a substantially steady installation such as an offshore wind turbine installation or a drilling rig is minimized. Hereby co-operations such as loading/unloading cargo or offshore structures between the vessel and said installation can be ensured.

Even further it is ensured that an easy and fast preparation prior to initiating the work on said vessel is achieved ensuring a relatively short mobilization and demobilization time.

By the present invention it is furthermore ensured that the mentioned disadvantages by the prior art are eliminated or significantly reduced.

In another aspect of the invention said mooring system comprises means for changing the buoyancy of said at least one mooring unit. Hereby it is ensured that the mooring unit or units can form a sufficient and desired counterweight as to establish a sufficient permanent tension in the connection means. Furthermore it is ensured that the buoyancy of the mooring unit or units can be altered as to adapt to different situations such as installation on vessel, transportation to offshore site, operation on site etc.

In another aspect of the invention said means for changing the buoyancy comprises means for changing a filler of said mooring unit. By changing a filler of the mooring unit the total weight of the mooring unit can be changed whereby it is ensured that the buoyancy also is changed and the buoyancy properties of the mooring unit can thereby be adapted to specific tasks. In another aspect of the invention said means for changing a filler of said mooring unit comprises remotely operated valves and pumps. Hereby it is ensured that the operation of changing a filler can be done from physical locations at a distance away from the mooring unit such as from said vessel, from an offshore installation such as an offshore wind turbine installation or a drilling rig. Furthermore it is ensured that the valves and pumps can be operated manually or by electrical control means.

In another aspect of the invention said mooring unit comprises one or more cavities that can be fully or partly filled and drained with a filler such as air, water or sand. Hereby it is ensured that the volume that can be filled and drained with a filler is known whereby the properties of the mooring unit is known, can be predicted and/or can be calculated. Furthermore it is ensured that a filler material that is available can be used such as air, water and/or sand. It is even further ensured that different properties of the mooring unit can be obtained by controlling the combination of partial filling with said filling materials.

In another aspect of the invention said mooring unit is demonstrating a floating behavior when it is substantially fully filled with air. Hereby it is ensured that operations such as transportation of the unit can be done without requiring extra resources as to lift the weight of the unit. It is furthermore ensured that the mooring unit can be stored in the water when it is not used without resting on the seabed. It is even further ensured that the mooring unit can be stabilized in a preferred position below the water level e.g. during installation of a mooring system on a vessel.

In another aspect of the invention said at least one mooring unit comprises at least one suction bucket system. Hereby it is ensured that the mooring unit is supported in all directions enhancing the capability of the mooring system to stabilize a vessel under influence of said environmental parameters. Due to the effectiveness of suction buckets it is furthermore ensured that the dimensions of a mooring unit can be reduced. In another aspect of the invention said connection means for connecting said mooring unit to said vessel comprise at least two separate connection units. Hereby it is ensured that the vessel can be better stabilized in more directions.

In another aspect of the invention said at least two connection units are flexible pulling means such as wires, chains, ropes, cordage or the like, suitable for winding on a pulley, winch drum or the like.

In another aspect of the invention said tractive means comprises one or more winches, pulleys and/or cranes. Hereby it is ensured that said permanent tension can be established. Furthermore it is ensured that a relative easily installed and relatively low cost installation is obtained.

In another aspect of the invention said mooring unit is adapted to be positioned substantially at the bottom section of said vessel or in a hollow part of the vessel bottom section during transport.

In another aspect of the invention the physical dimensions of said mooring system is designed as to optimize its hydrodynamic properties such as propulsion resistance.

In another aspect of the invention said mooring system can be mounted and dismounted on different vessels. Hereby it is ensured that the mooring system is versatile and can be used for a plurality of purposes.

In another aspect of the invention said mooring unit is established and permanently located on the seabed. Hereby it is ensured that no transportation of mooring unit or units is necessary which in turn extends the number and models of vessels suitable for operating with said mooring system. In another aspect of the invention said mooring unit is a mooring pole mounted on a monopole. Hereby it is ensured that the mooring system is non-dependent on placing a mooring unit on the seabed, and hereby non-dependent on seabed conditions at the place of mooring. Furthermore it is ensured that no transportation of mooring unit or units is necessary which in turn extends the number and models of vessels suitable for operating with said mooring system.

Furthermore the invention relates to a vessel for handling offshore structures comprising a mooring system according to any of the preceding claims. Hereby it is ensured that a standard vessel can be converted to a vessel dedicated for handling offshore components such as wind turbine components for relative shorter time intervals. Hereby the use of the vessel can be optimized as it can be used to alternating marine tasks.

The invention also provides a method for stabilizing a vessel for handling offshore structures, said method comprises the steps of:

- establishing said vessel at the site of operation,

- establishing an mooring unit substantially on the seabed, said mooring unit being connected to said vessel by connection means, and

- lowering said vessel's position in relation to the water level by pulling said connection means.

The invention also relates to the use of a mooring system for a vessel.

Hereby an advantageous method and use is obtained. Figures

The invention will be described in the following with reference to the figures in which

fig. Ia illustrates schematically a mooring system according one embodiment of the invention, comprising a mooring unit elevated to a transporting position,

fig. Ib illustrates a mooring system according another embodiment of the invention, comprising a mooring unit elevated to a transporting position,

fig. Ic illustrates a mooring system according a further embodiment of the invention, comprising a mooring unit elevated to a different transporting position,

fig. 2 illustrates schematically a mooring system comprising one or more cavities,

fig. 3 illustrates schematically a mooring system according to one embodiment of the invention, comprising a mooring unit submerged to rest on the seabed,

fig. 4 illustrates schematically a mooring system according to one embodiment of the invention during operation with a vessel in a lowered position,

fig. 5 illustrates schematically a top view of one embodiment of the invention, fig. 6 illustrates connection means positioned in a non vertical manner,

fig. 7a illustrates a top view of a mooring system comprising more than one mooring units,

fig. 7b illustrates a mooring system comprising more than one mooring units submerged on the seabed,

fig. 8 illustrates a mooring system according to an embodiment of the invention,

fig. 9 illustrates a mooring system comprising suction buckets,

fig. 10a illustrates a mooring system according to an embodiment of the invention,

fig. 10b illustrates a mooring system according to another embodiment of the invention,

fig. 11a illustrates a side view of a loading of a mooring system on a vessel,

fig. l ib illustrates a top view of a loading of a mooring system on a vessel,

fig. 12 illustrates a mooring system comprising an offshore structure with a pole,

fig. 13 illustrates a mooring system comprising solid stationary blocks with a hook, and fig. 14 illustrates connection means can be marked with floating buoys.

Detailed description

Fig. Ia illustrates according to one embodiment of the invention, a side view of a vessel 1 comprising a hull 2, a mooring system comprising tractive means 3 and a mooring unit 4 connected to the tractive means 3 by connection means 5.

On the deck of the vessel 1 lifting means such as one or more cranes (not illustrated) can be positioned for the purpose of e.g. loading and unloading of cargo such as wind turbine tower segments, nacelle, hub, rotor blades etc. The cranes have properties such as loading capacity, working range etc. suited for wind turbine installation operation. Furthermore the vessel can comprise means for stowing of cargo.

For one embodiment of the invention during non-operational use such as transportation, anchoring, towing etc. the mooring unit 4 is elevated by tractive means 3 and connection means 5 to be located in close proximity to the vessel bottom section as illustrated in the figure.

In one embodiment of the invention the physical dimensions of the mooring system comprising at least one mooring unit 4 is designed as to minimize its hydrodynamic properties such as to lower its propulsion resistance during moving (transportation) along any direction of the vessel 1.

In a preferred embodiment of the invention, the mooring system is detachable. This means that the mooring system can be mounted and operated on numerous suited vessels. This also allows the vessel 1 to operate in other trades, i.e. the vessel 1 is further capable of performing conventional marine trades instead of being dedicated to e.g. offshore wind turbine tasks exclusively. For preferred embodiments of the invention said connection means comprises wires, chains, ropes, cordage or the like, suitable for winding on a pulley, winch drum or the like.

Fig. Ib illustrates for another embodiment of the invention the hull 2 of a vessel 1 comprising a hollow part 13 in the vessel bottom section 10 where said mooring unit 4 can be located during non-operational use such as transportation, anchoring, towing etc. as to further minimize the propulsion resistance.

In yet another embodiment of the invention said mooring unit 4 is elevated by tractive means 3 and connection means 5 and transported when it is elevated and located at a distance below the vessel bottom section 13 but above the seabed 7 as illustrated in figure Ic.

Fig. 2 illustrates for one embodiment of the invention, part of the mooring system a mooring unit 4 comprising one or more cavities 9.

A mooring control system comprising valves controls the filling and draining of water and air, to and from one or more cavities 9 comprised in the mooring unit 4. By controlling the filling and draining of the cavities 9, the weight and buoyancy of the mooring unit and thereby the tension in the connection means 5 can be controlled.

For one embodiment of the invention the mooring system comprises one or more mooring units 4 where the cavities 9 is part of a closed filling and draining system where the cavities can be filled with a liquid with a density greater than seawater.

The physical outlining of said mooring system may for any embodiments vary as to suit the various needs. In a preferred embodiment of the invention, where the cavities 9 in the mooring unit 4 is substantially fully filled with s suitable filler such as air, the mooring system demonstrates a floating behavior.

Fig. 3 illustrates schematically a vessel 1 according to one embodiment of the invention, comprising a mooring system on a working location and where a mooring unit 4 is submerged to rest on the seabed 7. The length of the connection means 5 is increased to be substantially the distance between tractive means 3 comprised in the mooring system and the seabed 7.

This is an initial working situation where the vessel 1 is still non-stabilized by the mooring system and where cavities 9 in the mooring unit 4 is partly filled with a filler such as water whereby the weight of the total mooring unit is increased. Hereby the vertical buoyancy of the mooring unit 4 is decreased as to be substantially the same or lower than the sum of other vertical forces acting on said mooring unit 4.

At this initial point the tension in the connection means 5 is non-significant and the waterline 8 on vessel 1 in non-stabilized condition is the same as the water level 6.

Fig. 4 illustrates schematically for one embodiment of the invention a vessel 1 during stabilized positioning and operation. The mooring unit 4 is submerged to the seabed 7 and the cavities 9 in the said mooring unit 4 are substantially fully filled with a filler such as water whereby the buoyancy of the mooring unit 4 is minimized.

A mooring control system controls the tension in the connection means 5 e.g. by operating tractive means 3. By increasing the tension in the connection means 5 the vessel 1 is substantially dragged vertical towards the seabed 7. Hereby the waterline on the vessel 1 in non-stabilized condition 8 is displaced to occur below the water level 6 whereby a new waterline 12 on the vessel in stabilized condition is introduced.

The difference that can be obtained between the waterline in non-stabilized condition 8 and in stabilized condition 12 i.e. 11 - 12 in a working condition depends on e.g. the weight and buoyancy of the mooring unit 4, the tension in connection means 5 and on various environmental parameters such as wind, waves, current and tidal.

For one embodiment the obtained difference can be in the range from 0.2 - 2.0 meters preferably in the range from 0.5 to 1.5 meters.

Due to the tension in the connection means 5 the vessel 1 is stabilized and hold in position and the impact on the vessel's from said environmental parameters is minimized. In one embodiment of the invention said mooring unit 4 is elaborated in a way that it enhances its properties regarding anchoring to the seabed 7 i.e. it is more difficult to move when it is exposed to dragging forces in the seabed plane directions.

Fig. 5 illustrates schematically a top view of one embodiment of a vessel 1 comprising a mooring unit 4 connected to the hull 2 by tractive means 3 and connection means 5. The size of said mooring unit 4 depends on numerous factors such as size of vessel 1 , buoyancy of vessel 1 , seabed 7 condition, level of desired stabilization, weight of mooring unit 4, elaboration of mooring system etc.

In one embodiment of the invention the connection means 5 are connected to the mooring system in a non-vertical manner as indicated in fig. 6 where it is shown that the distance between tractive means 3 connected to the hull 2 of a vessel 1 is smaller than the distance between attachment points of connection means 5 to a mooring unit

4. For other embodiments of the invention the opposite may be implemented i.e. the distance between attachment points of connection means 5 to a mooring unit 4 is smaller than the distance between tractive means 3 connected to the hull 2. For further embodiments tractive means may even cross in either or both of the vessel 1 longitudinal and transversal directions.

In another embodiment of the invention the connection means 5 are connected to tractive means 3 comprising hydraulic systems.

In another embodiment of the invention the permanent tension in the connection means 5 established by tractive means 3 is controlled dynamically as to compensate for changing environmental conditions such as tidal, current, waves, wind or the like or for other alternating demands such as desired water line level, desired tension in said connection means etc. Said tension can be controlled manually and/or automatically by control means such as microprocessor based or PLC based. Furthermore said control means can control on the basis of measuring said environmental parameters and using them as feedback to the control means. In other embodiments control is performed on the basis of lookup-tables, GPS measurements, data-communication with other control systems, manual loaded data, weather forecasts, predicted parameters etc.

Figures 7a and 7b illustrates for one embodiment of the invention, a mooring system comprising a mooring unit 4 connected to the hull 2 of a vessel 1 by tractive means 3 and connection means 5. This embodiment shows a mooring unit 4 that is divided into a plurality of sub-segments 4a. Each sub-segment 4a can be individually controlled regarding changing of filler and/ or tension in connection means 5.

Figure 7a illustrates this embodiment seen from a top view, whereas fig. 7b illustrates the embodiment from a side view.

Figure 8 illustrates an embodiment of the present invention where two sub-segments 4a are placed on the seabed 7 and tension in connection means 5 is controlled by tractive means 3. The connection means 5 extends in a non- vertical direction. Figure 9 illustrates another embodiment of the present invention where a mooring system comprises a suction bucket system 14 located on the seabed 7. A suction bucket 14 can be located on the seabed 7 with the "open end" downwards and when suction is applied to the bucket 14 it tend to penetrate the seabed 7. When the suction bucket 14 has penetrated the seabed 7 to the desired depth suction is removed. Means for applying suction to the bucket 14 is not illustrated but for one embodiment one or more hoses from one or more suction pumps located e.g. on the vessel 1 running to one or more suction buckets 14 are substantially integrated with one or more connection means 5 as to form an umbilical between the involved components.

For further embodiments of the present invention, one or more suction buckets 14 are used in combination with a mooring unit 4 and/or sub-segments 4a as to form a mooring system.

Storage of said mooring system when it is not in use can be done by lowering the mooring unit 4 to rest on the seabed 7 by fully or partly filling the cavities 9 in the mooring unit 4 with a filler. As for a preferred embodiment of the present invention where the connection means 5 can be disconnected from the tractive means 3 the mooring unit 4 and connection means 5 can be stored in the water by holding the disconnected distal ends of the connection means 5 substantially near the surface of the water by one or more buoys. Hereby the connection means 5 can be picked up and re-attached to tractive means 3 on the next session. The buoys also have the function of indicating the location of storage of the mooring unit 4.

Figure 10a and figure 10b illustrates schematically in top views and rear views of embodiments of the present invention how a mooring system can be loaded and/or connected to a vessel 1. Figure 10a illustrates a preferred embodiment where tractive means 3 are attached directly to/on the hull 2 of the vessel 1. Figure 10b illustrates another embodiment where tractive means 3 are attached on one or more booms 15 that are placed transversely over the entire vessel 1.

Figures 11a and l ib illustrates schematically for one embodiment of the invention how said mooring system can be loaded to a vessel 1.

During storage the mooring system comprising one or more mooring unit 4, tractive means 3, booms 15 and connection means 3 can be held in position in a storage area. When the vessel 1 is loading the stabilizing system, the booms 15 are raised above the water level 6 and the connection means 5 and mooring unit 4 is lowered below the water level 6 to such a position that the vessel 1 can enter the gap between said booms 15 and said mooring unit 4 as schematically illustrated in figure 11a. Figure 1 Ib illustrates schematically a top-view and a side view of the same situation where a vessel 1 is entering said gap between booms 15 and said mooring unit 4.

When the vessel 1 is located on the correct position relative to said mooring system, the booms 15 are lowered and attached to the hull 2 of the vessel 1.

For one preferred embodiment of the invention said mooring system is floating during storage and/or loading.

Figure 11a and l ib furthermore illustrates another embodiment of the invention where said mooring system comprises a mooring unit 4 comprising piles 16 and hydraulic cylinders 17 that allow the booms 15 to be raised to a suitable height above water level 6 whereby it is possible for a vessel 1 to enter the gap between the booms 15 and the mooring unit 4.

For a further preferred embodiment during loading the booms 15 can be jacked-up to a height above water level and the mooring unit 4 can be lowered below the water level 6 to such a position that the vessel 1 can enter the gap between said booms 15 and said mooring unit 4 while the mooring system is floating substantially by it self.

For all embodiments regarding the present invention, the number of connection means 5 connected to said mooring system can be more than two dependent of the specific task.

The present invention furthermore comprises a control system comprising means for filling/draining of cavities 9 in said mooring unit 4, controlling tension in the connection means 5, controlling hydraulic cylinders 17 etc.

Fig. 12 illustrates schematically for yet another embodiment of the invention where said mooring unit is a mooring pole 19 established e.g. on a monopile 18 such as the tower of an offshore wind turbine. Fig. 12 shows a vessel 1 during stabilized positioning and operation.

For various embodiments of the invention the connection means 5 is attached to a mooring pole 19 established on a monopile 18. The mooring pole 19 is adapted as to receive connection means 5 by e.g. a hook, anchor, sling, loop or the like. The tension is operated by tractive means 3.

The monopole 18 has dimensions as to withstand the forces that act on it when a vessel 1 is moored and can for various embodiments comprise supporting structures between the mooring pole 19 and e.g. the monopole 18, the seabed 7, the foundation of a tower of an offshore wind turbine etc.

Fig. 13 illustrates for another embodiment of the invention said mooring system comprising mooring blocks 20 placed on seabed 7 at sites of operation. Each mooring block 20 comprises a mooring hook 22 mounted on a solid block 21. For various embodiments of the invention the connection means 5 is attached to mooring hook 22 by e.g. a hook, anchor, sling, loop or the like. The tension is operated by tractive means 3.

The mooring blocks 20 are dimensioned as heavy as to remain steady on the seabed 7 when a vessel 1 is moored by said mooring blocks 20 by connection means 5 and increases the tension of said connection means 5 by operation of tractive means 3 according to the invention.

The mooring blocks 20 can be established at sites of operation e.g. at the time of an offshore wind turbine installation.

Fig. 14 illustrates for yet another embodiment of the invention, connection means 5 fixed to the mooring unit 4 such as mooring blocks 20 located on the seabed 7. Said connection means can be marked with floating buoys 23 so that the connection means 5 can be located from the vessel 1 and be connected to the tractive means 3.

List

1. Vessel

2. Hull

3. Tractive means

4. Mooring unit

4a. Mooring unit sub-segments

5. Connection means

6. Water level

7. Seabed

8. Waterline on vessel in non-stabilized condition

9. Cavities in mooring unit

10. Vessel bottom section

11. Height of mooring unit

12. Waterline on vessel in stabilized condition

13. Hollow part of vessel bottom section

14. Suction bucket

15. Boom

16. Piles

17. Hydraulic cylinders

18. Monopile

19. Mooring pole

20. Mooring block

21. Solid block

22. Mooring hook

23. Buoy

Claims

1. A mooring system for stabilizing a vessel for handling offshore structures, said system comprising

at least one mooring unit,

connection means for connecting said mooring unit to said vessel,

tractive means for stabilizing said vessel by establishing permanent tension in said connection means, and

wherein said tractive means comprises means for lowering said connected vessel's position in relation to the water level.

2. A mooring system according to claim 1, wherein said mooring system comprises means for changing the buoyancy of said at least one mooring unit.

3. A mooring system according to claim 2, wherein said means for changing the buoyancy comprises means for changing a filler of said mooring unit.

4. A mooring system according to claim 3 wherein said means for changing a filler of said mooring unit comprises remotely operated valves and pumps.

5. A mooring system according to any of the preceding claims, wherein said mooring unit comprises one or more cavities that can be fully or partly filled and drained with a filler such as air, water or sand.

6. A mooring system according to claim 5, wherein said mooring unit is demonstrating a floating behavior when it is substantially fully filled with air.

7. A mooring system according to any of the preceding claims, wherein said at least one mooring unit comprises at least one suction bucket system.

8. A mooring system according to any of the preceding claims, wherein said connection means for connecting said mooring unit to said vessel comprise at least two separate connection units.

9. A mooring system according to claim 8, wherein said at least two connection units are flexible pulling means such as wires, chains, ropes, cordage or the like, suitable for winding on a pulley, winch drum or the like.

10. A mooring system according to claim 1, wherein said tractive means comprises one or more winches, pulleys and/or cranes.

11. A mooring system according to any of the preceding claims, wherein permanent tension in said connection means is controlled automatically by control means.

12. A mooring system according to any of the preceding claims, wherein said mooring unit is adapted to be positioned substantially at the bottom section of said vessel or in a hollow part of the vessel bottom section during transport.

13. A mooring system according to any of the preceding claims, wherein the physical dimensions of said mooring system is designed as to optimize its hydrodynamic properties such as propulsion resistance.

14. A mooring system according to any of the preceding claims, wherein said mooring system can be mounted and dismounted on different vessels.

15. A mooring system according to any of the preceding claims, wherein said mooring unit is established and permanently located on the seabed.

16. A mooring system according to any of the preceding claims, wherein said mooring unit is a mooring pole mounted on a monopole.

17. A mooring system according to any of the preceding claims, wherein floating buoys are established together with said connection units.

18. A vessel for handling offshore structures comprising a mooring system according to any of claims 1 to 17.

19. Method for stabilizing a vessel for handling offshore structures, said method comprises the steps of:

- establishing said vessel at the site of operation,

- establishing a mooring unit substantially on the seabed, said mooring unit being connected to said vessel by connection means, and

20. Method for stabilizing a vessel according to claim 19, wherein said mooring unit is established by changing the buoyancy of said mooring unit.

21. Method for stabilizing a vessel according to claim 20, wherein said buoyancy is changed by changing a filler of said mooring unit.

22. Method for stabilizing a vessel according to any of claims 19 to21, wherein said establishing an mooring unit substantially on the seabed, further comprises the step of applying suction to one or more suction buckets.

23. Method for stabilizing a vessel according to any of claims 19 to 22, wherein said pulling is performed by winding said connecting means on pulley means.

24. Method for stabilizing a vessel according to claim 23, wherein said pulley means comprises one or more winches, pulleys and/or cranes.

25. Use of a mooring system according to claims 1 to 17 for a vessel for handling offshore wind turbine components.