WO2022098286A1

WO2022098286A1 - Semi-submersible wind power platform and method of docking such platform

Info

Publication number: WO2022098286A1
Application number: PCT/SE2021/051103
Authority: WO
Inventors: Bertil Moritz
Original assignee: Josok Ab
Priority date: 2020-11-04
Filing date: 2021-11-04
Publication date: 2022-05-12
Also published as: EP4240965A1; SE546025C2; WO2022098286A9; SE2000206A1

Abstract

A semi-submersible wind power platform (17) having a tower (1) carrying a wind generator housed in a nacelle (2) and a plurality of arms (6), the tower comprising a main float (5) and each arm comprising a secondary float (12) to stabilize the tower. Each arm (6) consists of a strut element (7) connected between the tower (1) in a main position (10) and the secondary float (12), a first catenary element (8) connected between the secondary float (12) and a first position (13) of the tower (1), and a second catenary element (9) connected between the secondary float (12) and a second position (14) of the tower (1).

Description

SEMI-SUBMERSIBLE WIND POWER PLATFORM AND METHOD OF DOCKING SUCH PLATFORM

TECHNICAL FIELD

The present invention concerns a floating power plant for converting wind energy to electrical energy. Especially the invention concerns a floating platform including a wind generator for being moored at sea in a stable position and orientation. In particular the wind power plant comprises a semi-submersible platform.

BACKGROUND OF THE INVENTION

A wind turbine comprises a rotating machine which converts the kinetic energy from the wind into mechanical energy that is then converted to electric energy. Wind turbines have been developed for land-based installations as well as offshore installations. The land-based wind turbines are fixed to the ground and located in windy areas. Most common wind turbines have the main rotor shaft arranged horizontally. They have a horizontal rotor shaft that is pointed into the wind. Horizontal axis wind turbines generally have a tower and an electrical generator coupled to the top of the tower. The generator may be coupled directly or via a gearbox to the hub assembly and turbine blades.

Wind turbines have also been used for offshore applications. Single long tower offshore systems are mounted into the sea bed. They are normally limited to shallow water depths up to 30 meters. By using a wider base, such as a framework structure for better stability, the shallow depth requirement may be extended but only marginally. In deeper water only floating systems are expected to be economically feasible. To fully exploit wind energy offshore it is necessary to find economical solutions for deep water. Shallow water resources are limited and represent only a fraction of the offshore wind resources. Wind turbines close to shore may also block the shore view and create navigational obstructions and potential hazards for water vessels and aircrafts. There are known a plurality of concepts for offshore floating wind turbine platforms. Generally, these fall into three main categories: Spars; Tension Leg Platforms (TLP); and semi-submersible systems.

Spars comprise elongated structures that are balanced with significant ballast at the bottom of the structure and buoyant tanks near the waterline. For stability purposes, the centre of gravity must be lower than the centre of buoyancy. This will insure that the spar will float upright. The spar is moored to the sea bottom with a plurality of anchored lines that hold the spar in position. In general terms spar type structures have good heave performance due to reduced response to vertical wave exciting forces. They require substantial depth, especially when wind turbine weight increase, in order to lower the centre of gravity. Spars are complicated to install due to their draught.

A Tension Leg Platform (TLP) has vertically tensioned cables or steel pipes that connect the floater directly to the bottom of the sea. There is no requirement for a low centre of gravity for stability. Only during the installation phase buoyancy modules may be temporarily added to provide sufficient stability. The TLP have very good heave and angular motions. Due to complexity of structure and the mooring installation the costs may escalate by size. Also the change in tendon tension due to tidal variations and the structural frequency coupling between the tower and the mooring system are major hurdles. TLPs have low stability before tendon connection and a very expensive anchor arrangement.

A semi-submersible system comprises a wind generator carrying tower on a stabilizing submerged structure which is kept in balance by a plurality of buoyancy elements penetrating the sea surface. When comparing different types of offshore wind turbine structures, wave and wind induced motions are not the only elements of performance to consider. Economics play a significant role. It is therefore important to carefully study the fabrication, installation, commissioning costs and ease of access for maintenance methodologies. Semi-submersible concepts with a shallow draught and good stability in operational and transit conditions are significantly cheaper to tow out, install and commission. From EP 2789848B1 (Komatsu) is previously known a floating body wind power generating device and method for mooring the floating body wind power generation device. An objective of the wind power generation device is to provide a floating body wind power generating device with which it is possible to moor the floating body stably with respect to drift force or rotational moment acting on the floating body. The wind power generating device comprises a wind power generator and a floating body. Further the device comprises a first column which is located on the upwind side of a primary wind direction and whereupon the wind power generator is installed. A second column and a third column which are located on the further downwind side of the primary wind direction than the first column is connected to the first column with two rower hulls to the first column. A plurality of mooring cables connects the floating body to anchors. At least two of the plurality of mooring cables is connected to the first column. At least one of the pluralities of mooring cables is respectively connected to the second column and the third column. Each of the plurality of mooring cables is positioned extending in radiating directions from the floating body so as not to intersect in planar view.

From US 8471396 (Roddier) a column-stabilized offshore platform with waterentrapment plates and asymmetric mooring system for support of offshore wind turbines is previously known. The floating wind turbine platform includes a floatation frame that includes three columns that are coupled to each other with horizontal main beams. A wind turbine tower is mounted above a tower support column to simplify the system construction and improve the structural strength. The turbine blades are coupled to a nacelle that rotates on top of the tower. The turbine's gearbox generator and other electrical gear can be mounted either traditionally in the nacelle, or lower in the tower or in the top of the towersupporting column. The floatation frame includes a water ballasting system that pumps water between the columns to keep the tower in a vertical alignment regardless of the wind speed. Water-entrapment plates are mounted to the bottoms of the columns to minimize the rotational movement of the floatation frame due to waves. The platform is connected to seabed by anchor lines from each column. SUMMARY OF THE INVENTION

A primary object of the present invention is to seek ways to improve a floating wind power plant. A second object of the invention is to provide a light weight floating wind power platform comprising a tower and a plurality of arms, each connected to a float, for stabilizing the tower.

This object is achieved according to the invention by a semi-submersible wind power platform characterized by the features in the independent claim 1 , or by a method characterized by the steps in the independent claim 8 or 10. Preferred embodiments are described in the dependent claims.

According to the invention each stabilizing arm of the floating wind power platform comprises a lightweight construction consisting of two kinds of building elements only. The first kind is a tensile resisting element designed to resist or transmit tensile forces in its longitudinal direction. The second kind is a pressure resisting element designed to resist or transmit pressure forces in its longitudinal direction. A tension resisting element consists of a catenary element. By catenary is understood a curve assumed by a cord of uniform density and cross section that is perfectly flexible but not capable of being stretched and that hangs freely from two fixed points. Thus a catenary element is a slim element that would collapse when exposed to pressure forces. Examples of the first kind are cable, wire, rope, cord etc. In the text hereinafter a tensile resisting element is denoted a catenary element. Examples of the second kind are strut, brace, stick, beam, framework construction etc. In the text hereinafter a pressure resisting element is denoted a strut element. By a strut element is understood a structural piece designed to resist pressure in the direction of its length.

By the use of such lightweight elements triangular constructions may be designed where part of the tower comprises one of the triangle arms. The other arms are a strut element and a catenary element. Such constructions are capable of withstanding severe forces in the plane of the triangle. By the use of two such triangles where one arm is a strut element common to both triangles great stability is achieved and great forces may be withstood. Besides the lightweight elements may be used to build big constructions yet stable enough to withstand big forces. In an embodiment according to the invention a stabilizing arm consists of a first and second catenary element and a strut element positioned between the catenary elements. The strut element is connected between a stabilizing float and a main position of the tower. The first catenary element is connected between the float and a first position of the tower. The second catenary element is connected between the float and a second position of the tower. Also on the other side of the arm the first and second catenary element may be connected to the secondary float in positions on either side of the strut element. The main position is arranged between the first position and the second position. The construction may resemble a mast on a sailing boat where the mast is supported by two prestressed stays or shrouds. However in the present embodiment the mast is aligned horizontally and the wires are prestressed against the tower. In an embodiment of the invention the strut element comprises a framework construction which results in the arm construction being an extremely lightweight construction.

In an embodiment all connection points comprise two rotational degrees of freedom (2RDOF). Thus the strut element is freely rotatable up and down as well as sideways but cannot rotate around its own axis. This design ensures that only axial forces and no bending forces may be transferred from the strut element to the tower. The second and third connection points also provide 2RDOF. When all three connection points are aligned on the same axis the arm construction offers 1 RDOF. Thus the arm is very stable in the vertical plane but weak in the horizontal plane. The arm construction may be seen as a hinged door that is stable in one direction and swingable in its transversal direction. The plurality of arms is equally connected to the tower and symmetrically spread around the tower by a connection wire. The connection wire connects a secondary float of an arm to a secondary float of an adjacent arm. In an embodiment the connection wire comprises two connecting lines arranged in parallel with each other. By this symmetry all pressure forces and all tension forces from the arms are neutralized in the centre of the tower. Due to the pre-stressing of the catenary elements and pre-compression of the strut element there are no net force on the tower. To keep the arms equally spread all adjacent pair of arm ends or floats are connected with a connection wire all around. The distance between the second and third connection points on the tower is chosen from a cost-benefit evaluation of stress limitation requirements. Preferably the strut element is horizontally aligned.

All three arms are one-dimensionally connected to the tower like hinged doors. This means that the arms can freely rotate around the tower. To prevent the tower from rotation relative to the arms the tower is locked to one of the arms. In an embodiment the second catenary element of one arm is split into two catenary elements which are connected transversally to either side of the bottom of the tower. This means that one of the stabilizing arms will prevent the tower to rotate in relation to the other arms. In an embodiment the second catenary element is attach to the tower with a wire span which is transversally attached to each sides of the tower.

In an embodiment of the invention the floating wind power platform comprises a tower and a plurality of stabilizing arms. The tower comprises a hollow structure carrying a pivotal nacelle and includes a main float at its lower end. In an embodiment the tower is partly a framework structure. In an embodiment the tower comprises a narrow cylindric part penetrating the water surface. Each arm comprises a secondary float connected to its distal end. The main float is preferably designed to carry the tower and its equipment as well as the generator and rotor. An immersed operation position of the platform may be achieved by pumping water into the main float. Thereby each secondary float needs only to carry its own weight and part of the stabilizing arm. By pumping water into and from the secondary float the arm may be balanced to achieve the same tension forces in the first and the second catenary element at a normal operating position of the platform.

In an embodiment the strut element comprises a lattice girder or a framework construction. Although the main task of the strut element is to withstand pressure forces it must also withstand forces from the waves. It is therefore favourable to design the strut elements with a minimum exposed area, such as a framework construction with moderate diameters of tube elements. Most suitable the strut element is made of metal such as steel and protected against oxidation and fouling by a protective paint. In an embodiment the strut element is made of a tubular hollow structure. The catenary elements are suitably made of metal such as steel but may also be made of synthetic fibres. Suitable reinforcement material may be coal fibres, synthetic fibres such as for instance aromatic polyamide, etc.

In an embodiment of the invention the wind power platform comprises a semisubmersible platform. The platform comprises the tower including the main float and three stabilising arms having secondary floats. Every float comprises a water fillable container thus making the platform immersible by filling water into the floats. The secondary float comprises a hollow column of arbitrary cross section. In an embodiment each secondary float comprises an elongated cylindric body having a small cross section to decrease the movement in the sea.

By pumping out ballast water from the floats the platform will float in a high position during transport. This ensures the possibility for the platform to be moored to a quay and transported in shallow water. On the site of operation the platform is docked to an existing mooring system. By partly filling the floats with ballast water the platform will immerse and take its operation position. In an embodiment the strut elements in the operating position will be located under the sea surface and only the tower and the upper parts of the three secondary floats may be seen.

To reach its operation position the platform is immersed in the sea to an operating level. This is accomplished by filling water into the main float and the secondary floats. The volume of the secondary floats is such that it only needs to be partly filled to reach the operational position. However it needs to be elongated enough in the vertical direction to protrude through the water surface. The function of the secondary float makes use of Archimedes principle. Thus when moving downwards it will experience an upwardly directed force equal to the volume of the displaced water. When moving upwards it will experience a downwardly directed force equal to the volume of the non-displaced water. Since the three arms are symmetrically spread around the tower there will be an equal amount of stabilizing forces on opposite sides of the tower. Hence when the tower tends to lean caused by wind forces the floats on the leeward side will exert an uprising force and simultaneously the floats on the upwind side will exert a traction force Thus at any given moment the floats on each side of the tower will exert opposite forces resulting in a pivoting effect which will put the tower in an upright position. The necessary stabilizing force for keeping the tower in an upright position is thus provided by the length of the strut element and the cross-sectional area of the secondary float. Cross-sectional area is the imprint of the secondary float in the sea. A longer arm and a greater cross-sectional area of the floats will increase the uprising forces. A big cross-sectional area will however make the float more affected by the wave motion. Thus a small cross-sectional area is desirable. By a small cross section is understood a cross section width being a fraction of the length of the secondary float. In an operating position the assembled cross- sectional areas of the secondary floats are minimized in order to lower the rocking frequency.

Every structure has modes of natural frequencies. When an oscillating force such as wave forces hit the structure close to such a mode of the natural frequency the movement of the structure may become severe. Therefore it is good practice to avoid oscillating forced in the regions of such natural frequency. In the present embodiment the aim is to lower the frequency of the stabilizing forces caused by the waves. This is accomplished by minimizing the cross section area of the secondary floats. A small cross section area has a lower spring constant than a large cross section area. In an embodiment the ratio between the cross section width and the length of the secondary float is in the range of 6-20 %.

When decreasing the cross-sectional area of the secondary float the arm needs to be longer. However a longer arm will increase in weight and make the platform heavier. According to the invention a fair compromise is to make the arm approximately as long as the tower is high. In an embodiment of the invention the float comprises a cylindrical shape. In an embodiment the float comprises a conical or a funnel shape. In the latter case the cross-sectional area will increase by the immersion of the float and thus resulting in a non-linear increasing fore. Such design will effectively act as damping.

For erection, transport and service of the floating wind power platform one or two of the stabilizing arms may be folded in the horizontal plane to make the platform suitable for docking a quay. It is a feature of the invention that the tower can be brought very close to the quay which facilitate lifting, mounting and replacement of the heavy tower top and nacelle from land-based services. One connection wire may be loosened whereby one of the arms can be rotated or folded horizontally to make two arms in 180 degrees with each other and thus permit the tower section of the platform to get close to the quay while still stably floating. If the quay is short the folded arm may be folded further than 180 degrees. In an embodiment all arms are assembled in quadrant. The necessary length of the quay will the be equal to the length of an arm. When transported temporary floats and beams may be attached to the platform.

In a first aspect the object is achieved by a semi-submersible wind power platform having a tower carrying a wind generator housed in a nacelle and a plurality of arms, the tower comprising a main float and each arm comprising a secondary float to stabilize the tower. Each arm consists of a strut element connected between the tower in a main position and the secondary float, a first catenary element connected between the float and a first position of the tower, and a second catenary element connected between the float and a second position of the tower.

In preferred embodiments the secondary floats comprises elongated water fillable containers having small cross section area, that the first, second and main connection point are aligned on a common axis to make the arms swingable like hinged doors, that the tower is fixed to one of the arms, that the tension resisting element comprises a catenary element and that the pressure resisting element comprises a strut element, and that the secondary floats comprises docking mean to be moored at a mooring facility at sea.

In a second aspect the object is achieved by a method of building a semisubmersible wind power platform having a tower carrying a wind generator housed in a nacelle and a plurality of arms, the tower comprising a main float and each arm comprising a secondary float to stabilize the tower. The method comprising providing between the tower and the secondary float a first catenary element, a second catenary element, and between the first and second catenary elements a strut element, to form with part of the tower an arm.

In a preferred embodiment the method further comprises attaching the strut element between the secondary float and the tower in a main position, attaching the first catenary element between the secondary float and the tower in a first position, attaching second catenary element between the secondary float and the tower in a second position, and aligning the main connection point, the first connection point and the second connection point on a common axis, whereby the arm being swingable attached to the tower.

In a third aspect the object is achieved by a method of docking a semi-submersible wind power platform to a quay for maintenance. The method comprising emptying water from the main float and the secondary floats thereby raising the platform to assume a transport level in the sea, transporting the platform by tug boats to a harbour facility, adjusting the connection wire to swing the arms to resume a straight line, and mooring the platform to the quay whereby the tower come close to the quay.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become more apparent to a person skilled in the art from the following detailed description in conjunction with the appended drawings in which: fig 1. is a side view of a floating wind power platform according to an embodiment of the invention, fig 2. is a plan view of the floating wind power platform, and fig 3. Is an embodiment of the docking means.

DESCRIPTION OF PREFERRED EMBODIMENTS

A floating wind power platform 17 according to the invention is shown in fig 1 and 2. The platform comprises a tower 1 carrying a wind generator housed in a pivotal nacelle 2. The generator comprises a hub 3 with a rotor having a plurality of blades 4. The rotor shown has three blades but according to the invention there may be any number of blades. The platform further comprises a plurality of stabilizing arms 6 each connected to a secondary float 12. The tower comprises a main float 5. In the embodiment shown there are three arms. Each arm consists of a strut element 7, a first catenary element 8 and a second catenary element 9. In the embodiment shown the strut element comprises a framework element and the catenary elements comprise a wire. The first catenary element 8 and the second catenary element 9 are pre-stressed to achieve a play-free or slack-free arm construction. According to the invention the arm design is very stable in the vertical direction where the stabilizing forces is transferred but allow the arms to be swingable around the tower like hinged constructions. This stability is achieved by just one strut element, the framework, and two catenary elements, the wires.

The strut element is attached to the tower in a main connection point 10. The secondary float 12 is attached to the distal end 11 of the strut element. The first catenary element 8 is connected between the secondary float 12 and the tower in a first connection point 13. The second catenary element is connected between the secondary float 12 and the tower in a second connection point 14. The main connection point 10 is located on the tower between the first connection point 13 and the second connection point 14. In the embodiment shown the strut element is connected to the secondary float between the connection point of the first and second catenary elements. In order to achieve an equal stress in the first and second catenary element the buoyant force may be balanced between the main float 5 and the three secondary floats 12. In the semi-submerged state the framework element and the second catenary element will be positioned under the water surface A. Only part of the first catenary element and part of the secondary float will be seen above the water surface positioned above the main connection point 10.

In the embodiment shown in fig 1 the secondary floats 12 comprise an elongated body structure. To keep the natural frequency of the platform low the cross- sectional area of the secondary float 12 must be kept to a fraction of the length of the secondary float. Thus the centre part 31 of the secondary float comprises an elongated cylinder having a cross section in the range of 6-20 % of the length of the secondary float. Also the mid portion of the tower may comprise a small cross section area to help lower the rolling frequency of the platform. In the embodiment shown the upper end of the secondary float 12 may comprise a funnel shaped body 32. This funnel shaped body exerts a damping effect when moving in sea heave. The lower end of the secondary float comprises a cylindric body 33. This body also exert a damping effect but also a convenient container to fill or pump out water for balancing purposes. In an embodiment the cylindric body 33 is positioned eccentric to the centre part of the secondary float. By this eccentricity the buoyancy force of the cylindric body 33 may help increasing the upraising stability of the secondary float.

The three arms 6a, 6b and 6c are aligned symmetrically around the tower 1. According to the invention the arm design is stable in the vertical direction where the stabilizing forces must be transferred. This stability is achieved with one pressure force resisting element, the strut element, and two catenary elements, the wires. In order to achieve a sufficient stress in the wires the buoyant force may be balanced between the main float 5 and the three secondary floats 12. By increasing an air portion in the secondary float the arm will exert a lifting force that will increase the stress of the first wire. In a semi-submerged operating position the platform is immersed by increasing the water ballast of the floats. In the semisubmerged state part of the framework strut element and the second catenary element will be positioned under the water surface A.

According to the embodiment shown the secondary float 12 is moored to a mooring buoy 19 which is anchored with a cable 24 to an anchor (not shown). The cable may be connected to the mooring buoy with a span arrangement 23. According to the plan view in fig 2 two of the secondary floats 12 are moored to a pair of buoys 19 associated to a docking system. The dockable buoy comprises a lower body 22 with a water ballast compartment and an upper body 21 comprising docking means. The pair of buoys are anchored with a plurality of anchor lines 24 connected to the pair of buoys. The two buoys are held together by a distance wire 25.

In the embodiment shown in Fig. 3 a secondary float 30 of an arbitrary semisubmerged platform comprises a first docking means 34 suitable to mate with a second docking means 35 of the mooring buoy 19 to form a unity float. In a first docking process the secondary 30 float is being raised by emptying water from an internal cavity of the secondary float. In a second process the secondary float 30 is being aligned in front of the mooring buoy 19. In a third docking process the secondary float is being immersed by filling water into the inner cavity simultaneously as an inner cavity of the mooring buoy is emptied from water. Thereby the first 34 and second docking means 35 are hooked together and form an entity.

The bigger a structure the more exposed is the structure for wave forces. Thus minimizing the exposure surface of the structure in the region were waves occur would be good design practice for a floating wind power plant. A submerged platform where only necessary parts penetrate the water surface is therefore beneficial to reduce slamming forces caused by waves. In an embodiment of the invention the first float and the major part of the arms are positioned under water and only the tower and the three secondary floats break the water surface. By keeping all these protruding structures small in horizontal cross section the whole platform will act calmly in the sea. The framework structure of the strut element reduces slamming forces caused by waves when the strut element temporary is at water surface in heavy sea states.

For transport the platform is raised to a float position by emptying ballast water from the floats. In a transporting position all floats will be filled with air and the platform will rise to a level in the sea indicated by a dashed line B in fig 1. To stabilise the platform during transport temporary beams or temporary floats may be attached to the platform. Being transported to the site of operation the platform can either be anchored in a traditional way or being moored to a set of stationary prepositioned buoys.

Ideally the arms should be connected to a common centre axis C of the tower. Then the arm would be freely rotatable around the tower. Achieving such connections can be made with a swivel construction well known to a person skilled in the art. In an embodiment of the invention the three arms are connected with connection wires 15 that holds the three arms equally spread around the tower. In an embodiment the connection wires comprise a plurality of wires preferably arranged in parallel with each other. However the tower can still rotate relatively to the arms. In the embodiment shown the tower is rotationally fixed to one arm. In an embodiment shown in fig 2 the lower catenary element 9 of arm 6a is split into a first lower catenary element 9a and a second lower catenary element 9b connected transversally on either side of the first float 5. The tower rotating preventing arrangement may also comprise a span means between the end of the arm and the first float.

The connection wires 15 can be detached and adjusted to facilitate a temporary angular rotation of the arms. In an embodiment two adjacent arms would resume a straight line which makes possible the tower coming close to a quay. In an embodiment two arms may be folded further to form a preferably perpendicular angle with the first arm. By this arrangement the docking will allow a shorter quay. Hence the tower is enabled to approach the quay for secure mooring and maintenance.

By the lightweight construction of the floating wind power platform the construction can be made very big. According to the invention the diameter of the rotor may be 220 m. The total height of the tower including the first float may be 170 m. The length of the arm may be in the range of 85-120 m. Hence the ratio between the arm and the tower would almost a half. The length of the secondary floats may be in the range of 30-50 m and the cross section may be in the range of 2-3 m. According to the invention the transport position of the platform is about 20 m higher that the submerged position. The draught of the platform under transport may be less than 10 meters.

Although favourable the scope of the invention must not be limited by the embodiments presented but also contain embodiments obvious to a person skilled in the art. For instance there could be more than 3 stabilizing arms. The wires may comprise any kind of material with good tensile properties. The secondary floats may comprise a landing pad for a helicopter. The framework strut element may comprise a footbridge. The platform may arbitrary be moored in a traditionally way by a plurality of anchors and anchor lines. The tower may contain a transformer, HVDC equipment and/or other electrical equipment.

Claims

1 . Semi-submersible wind power platform (17) having a tower (1 ) carrying a wind generator housed in a nacelle (2) and a plurality of arms (6), the tower comprising a main float (5) and each arm comprising a secondary float (12) to stabilize the tower, c h a r a c t e r i z e d i n that each arm (6) consists of a strut element (7) connected between the tower (1 ) in a main position (10) and the secondary float (12), a first catenary element (8) connected between the secondary float (12) and a first position (13) of the tower (1 ), and a second catenary element (9) connected between the secondary float (12) and a second position (14) of the tower (1 ).

2. Semi-submersible wind power platform according to claim 1 , wherein each secondary float (12) comprises an elongated water fillable container comprising a centre part (31 ) having a cross-sectional area in the range of 6-20 % of the length of the secondary float, to decrease the rocking frequency of the platform.

3. Semi-submersible wind power platform according to claim 1 or 2, wherein the secondary floats (12) comprises docking means (34) for docking to a mooring system having a pair of mooring buoys (19) anchored (24) at sea.

4. Semi-submersible wind power platform according to any of the preceding claims, wherein the first position (14), the main position (10) and the second position (13) of the tower (1 ) are aligned along a common axis making the arm swingable.

5. Semi-submersible wind power platform according to any of the preceding claims, wherein all arms (6b, 6c) but the first arm (6a) are arranged swingable around the tower and connected to each other with a detachable connection wire (15) holding the arms equally spread.

6. Semi-submersible wind power platform according to any of the preceding claims, wherein one arm (6a) comprises first and second catenary element (9a, 9b) connected transversally to the bottom of the tower for preventing a rotation of the tower (1 ) relative to the first arm (6a).

7. Semi-submersible wind power platform according to any of the preceding claims, wherein the strut element (7) comprises a framework element and each of the catenary element (8, 9) comprises a wire. Method of building a semi-submersible wind power platform (17) having a tower (1 ) carrying a wind generator housed in a nacelle (2) and a plurality of arms (6), the tower comprising a main float (5) and each arm comprising a secondary float (12) to stabilize the tower, c h a r a c t e r i z e d b y providing between the tower (1 ) and the secondary float (12) a first catenary element (8), a second catenary element (9), and between the first and second catenary elements a strut element (7), to form with part of the tower (1 ) an arm (6). Method according to claim 8, wherein the method further comprises attaching the pressure resisting element (7) between the secondary float (5) and the tower (1 ) in a main position (10), attaching from the distal end (11 ) of the pressure resisting element (7) the first catenary element(8) to the tower in a first position (13), attaching the second catenary element(9) to the tower in a second position (14), and aligning the main connection point (10), the first connection point (13) and the second connection point (14) on a common axis (C), whereby the arm being swingable attached to the tower. Method of docking a semi-submersible wind power platform (17) according to any of the claims 2-7 to a quay for maintenance, c h a r a c t e r i z e d b y emptying water from the main float (5) and the secondary floats (12) thereby raising the platform to assume a transport level (B) in the sea, transporting the platform by tug boats ta a harbour facility, adjusting the connection wire (15) to swing the arms to resume a straight line, and mooring the platform to the quay whereby the tower come close to the quay.