SE546025C2 - Semi-submersible wind power platform - Google Patents

Abstract

A floating wind power platform (17) has a tower (1) carrying a wind generator housed in a nacelle (2) and a plurality of arms (6), the tower comprises a main float (5) and each arm comprises a secondary float (12) to stabilizing the tower. Each arm (6) comprises a first triangular construction consisting of a pressure resisting element (7), a first tensile resisting element (9) and part of the tower (1), the pressure resisting element (7) being attached between the secondary float (5) and the tower in a main position (10), and the first tensile resisting element (9) being attached between the outer end (11) of the beam and the tower (1) in a first position (14).

Description

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 shallovv vvater depths up to 30 nfeters. By tising a wider base, such as a extended but only rnarginallyf. ln deeper ttfater' only floating systems are expected to be economically feasible. To fully exploit vvind energy Offshore it is necessary to find economical solutions for deep xrvater. Shallotrv “vvater resources are limited and represent only a fraction of the offshore wind resources. VVind 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. ln 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. Also the change in tendon tension due to tidal Variations and the structural frequency coupling between the tower and the mooring systern are major hurdles. TLPs have low stability before tendon connection and a very expensive anchor arrangement.

A semi-stibifwersiblesçteitt cloinprisesa wind generator carrying tower one; 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 structtiresïxfi/ave and wind induced motions are not the only elements of performance to consider. Economics play a significant role. lt 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. f\J f riFrom EP 278984851 (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 are 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 water- entrapment 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 turbines gearbox geneiïetoi* aitd otherïetectrical gear can be mounted eitherg; ri traditionally in the nacelle. or lower in the tower or in the top of the tower- supporting column. The floatation franie includes a water ballasting system that pumps water between the columns to keep the tower in a vertical alignment g regardless of the vvincl speed. Water-entrapmertt plates are mounted to the w 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 buoy, for stabiiizing the tower.

This object is achieved according to the invention by a floating wind power platform characterized by the features in the independent claim 1, or by a method characterized by the steps in the independent claim 9, 10 or 11, or by a wind power plant characterized by he features in the independent claim 13. Preferred embodiments are described in the dependent claims.

According to the invention each stabiiizing 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 compression in its longitudinal direction. The second kind is a pressure resisting element designed to resist pressure forces in its longitudinal direction. Examples of the first kind are strut, brace, stick, beam, framework construction etc. ln the text hereinafter a strut element is denoted a beam. Examples of the second kind are wire, rope, rod etc. ln the text hereinafter a tensile element is denoted a wire.

By the use of such lightweight elements triangular constructions may be designed where part of the tower comprises one of the triangle legs. The other legs are a beam and a wire. Such constructions are capable of withstartcling severe forces in the plane of the trianglefäy theäzse ofïtvm such triangles »where one leg isfa steam a '^ 'll common to both triangles great stability is achieved and great forces may be vvithstood. Besides the lightvveigltt elements may be used to build big constructions yet stable ertouglt to t-afithstand big forces. ln an embodiment according to the invention a stabiiizing arm comprises a beam and a wire. These two elements form together with part of the tower a triangle. Since one side of this triangle is a wire an outer force is needed to stretch the wire part of the triangle. This outer force is obtained by balancing the buoyancy of the buoy and the tower. ln an embodiment the beam is connected between the buoy 'll I\J 'Iland a main position of the tower. The wire is connected between the buoy and a first position of the tower. ln an embodiment the first position is located under the main position. ln this embodiment the buoyancy of the buoy must be greater than the buoyancy of the tower. ln an embodiment the first position is located above the main position. ln this embodiment the buoyancy of the tower must carry the whole platform and the buoyancy of the buoy only be used for balancing purposes. ln an embodiment according to the invention a stabilizing arm consists of a beam and a first and second wires positioned on opposite sides of the beam. The beam is connected between the buoy and a main position of the tower. The first wire is connected between the buoy and a first position of the tower positioned below the main position. The second wire is connected between the buoy and a second position of the tower positioned above the main 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. ln an embodiment of the invention the beam comprises a framework construction which results in the arm construction being an extremely Iightweight construction. ln an embodiment all connection points comprise two rotational degrees of freedom (ZRDOF). Thus the beam is freely rotatable up and down as well as sideways but cannot rotate around its own axis. This design ensures that only a> connected with a connection wire. The distance between the second and third ill ill 'j-C -uconnection points on the tower is chosen from a cost-benefit evaluation of stress limitation requirements. Preferably the beam 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 must be locked to one of the arms. ln an embodiment one wire is connected to the tower in a way to lock this relative rotation. This means that one of the stabilizing arms will prevent the tower to rotate in relation to the other arms. The locking means comprises according to an embodiment of the invention a console beam protruding from the tower where one of the upper wires of one of the arms is connected. ln an embodiment a bracket means such as a shackle is used. ln an embodiment the second wire is attach to the tower with a wire span which is transversally attached to each sides of the tower. ln 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. ln an embodiment the tower is partly a framework structure. Each arm comprises a secondary float connected to its outer end. The main float is preferably designed to carry the tower and its equipment as well as the generator and rotor. An immersed position of the platform may be achieved by pumping water into the main float. There-by 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.toaiçbiexfelhe sangïeateiwsion forceain the tipper and the lower wire at a normal Operating position of the platform. ln an embodiment the beam comprises a lattice girder or a framework _ construction. Although the main task of the beam is to withstand pressure forces it must also withstand forces from the waves. lt is therefore favourable to design the beams with a minimum exposed area, such as a framework construction with moderate diameters of tube elements. Most suitable the beam is made of metal such as steel and protected against oxidation and fouling by a protective paint. ln an embodiment the beam is made of a tubular structure. The wires are suitably made of metal such as steel but may also be made of synthetic fibres. Suitable ill lll Ixfl CT!reinforcement material may be coal fibres, synthetic fibres such as for instance aromatic polyamide, etc. ln an embodiment of the invention the wind power platform comprises a semi- submersible platform. The platform comprises the tower including the main float and three stabilising arms having secondary floats. The secondary float comprises a hollow column of arbitrary cross section. ln 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. ln an embodiment the beams in this operating position will be located under the sea surface and only the tower and the upper end of the three secondary floats may be seen.

To reach its operation position the platform must immerse in the sea to an operating level. At this operating level the assembled cross-sectional areas of the secondary floats must be minimized in order to lower the heave natural frequency. 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 vvatei' surface. The function of the secondary float niakestiselgïf Arclitmetlesflpriitciple. Thus vvhen nioviiwg _- __* downwards it will experience an upvvardly cflrectecl force equal to the volume of the displaced water. When moving upwards it will experience a downwardly directed force equal to the volurne of the non-displaced water. Since the three arms are symmetrically spread around the tower there vvill always 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 turning effect which will put the tower in an upright position. til. ,__ ¿\_The necessary stabilizing force for keeping the tower in an upright position is thus provided by the length of the beam 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. 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. ln an embodiment of the invention the float comprises a cylindrical shape. ln an embodiment the float comprises a conical or a funnel shape. ln 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. lt 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 connecting 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. lf the quay is short the folded arm may be folded further than 180 degrees. The necessary length of the quayf will the be equal length of anarm. When transported tentporaryr.. floats and beams may be attached to the platform.

Mooring system i _ A suitable mooring system comprises of a pair of twin mooring buoys anchored at sea. The twin buoys are separated from each other by a distance wire to keep a desired distance between them. The distance between the twin buoys is preferably the same as the distance between two adjacent arms of a floating wind power plant. The twin buoys are anchored by four anchors to receive a geographic stable position and orientation. Preferably suction anchors and prestressed anchoring lines are used to anchor the buoys. The anchors are spread equally onthe bottom in a square pattern. By providing a mooring system with only two distinctive mooring points any floating platform may be moored in a secure way to receive a desired position and a desired orientation. The twin buoy concept may be applicable to any self-floating and stabilised platforms.

According to an embodiment of the invention the twin buoys are anchored with four anchor lines and four anchors in square pattern. ln this embodiment the twin buoys are oriented parallel to a side of the anchor square. Each of the twin buoys are anchored by two anchors and kept in position by the distance wire to the other twin buoy which in turn is anchored by two anchors. ln an embodiment the twin buoys are anchored with six anchor lines to four anchors. ln this embodiment each twin buoy is anchored with three anchor lines. However, on opposite sides of the twin buoys separated by the distance wire one anchor line from each buoy may be anchored at the same anchor. Thus two of the anchors are arranged to receive one anchor line from each individual of the twins. Still the distance between the twin buoys are defined by the distance wire. ln an embodiment of the invention two sets of twin buoys are anchored by four anchors. The first and second sets of twin buoys are arranged in parallel with each other. ln this embodiment each set of twin buoys are connected to a corresponding set of twin buoys by a separating means. ln an embodiment the separating means comprise a single cable connected by a span arrangement to each set of tvvin buoys. ln an einbodintent the separating means comprlse two cables crossing each other. Each crossing cable is connected between diagonally opposite twin buoy of thefirstqaitd second seten twin buoy's...lr| an entbodimeint-- ï each twin buoy of the first set is connected to 'a corresponding twin buoy of the k second set by parallel lines Thus the separating means comprises two parallel lines. With long enough length of the separating means two floating wind power platforms may be moored to the anchoring systern with a predefined distance.

Building up a facility of a large numbers of wind power platforms at sea may start with one platform using four anchors. A second platform may be anchored in the vicinity of the first platform. A third and a forth platform may be anchored sideways of the first and second platform. Each of these platforms need four anchoring points thus four platforms would need 16 anchoring points. However the second o; , platform needs only two new anchors since two anchors are shared with the first platform. And the second line of platforms including the third and fourth platform needs only three new anchors since three anchors are common to the first line of platforms. ln this embodiment four platforms need nine anchors. By organizing a vast number of platforms side by side in lines and columns will make use of a plurality of anchors being used in common by a plurality of platforms. The limit of the number of anchors needed in a facility with a great number of platforms tends to one anchor for each platform. Hence each anchor receives anchor lines from four platforms.

According to an embodiment of the invention a second facility of platforms are organised diagonally to the first facility of platforms. The position of the second such platform will be just on top of an already existing anchor. Hence by adding the second set of platforms which all uses already existing anchors the efficiency of the use of anchors may be drastically improved. For a wind power plant comprising a vast number of both facilities of anchored platforms each anchor receives anchor lines from eight platforms. The limit of the number of anchors needed of such a facility tends to one anchor for every pair of platforms. ln an embodiment of the invention the mooring system comprises a docking means involving the twin buoys and two of the secondary floats. Each buoy comprises an upper hollow body and a lower hollow body sharing a common water ballast container. The tipper body comprises an elongated hollow structure having a small cross-sectional area. A small cross-sectional area impedes the movement of the buoy in thessea. Thedovifei' boíii/ is preferable a vxficier structure __ than the upper body. Each twin buoy also coinprises a pump facility to pump water into and out from the ballast container. By purnping water into the buoy the ballast increases and the buoy imrnerse in the sea and vice versa. in an embodiment the docking means comprises a first part containing the twin buoys and a second part containing a pair of secondary floats connected to the platform to be docked. The docking is performed by a relative movement in the vertical plane of the parts of the docking means. According to the invention a method of docking comprises immersing a twin buoy, orienting the secondary floaton top of the twin buoy, raising the twin buoy by emptying the ballast water to interact with the float and lock the two parts together.

When the first docking means has docked the platform is rotated by tugs so that the second docking means are aligned. Then the second docking is made the same way as the first docking. When fully docked the platform is immersed to its semi-submersed operational position. The advantage of letting the upper buoy stay above water is that wave forces and motions are avoided to better facilitate the docking.

Docking two objects or bodies at sea may be cumbersome if the water is not calm. This is due to the fact that waves are irregular and the forces acting on any buoyant body are high. Even though the body is partly submersed the forces are severe. Each buoyant body and the platform have their own characteristic natural frequencies in the heave of the sea. To minimize the hazardous portion of docking a set of principles are used to design the platform. One such principle is to minimize the cross-sectional area of sea protruding elements. Another principle is to decrease the natural frequencies of bodies which are arranged to mate. The natural frequencies in different directions may be decreased by increasing the mass of the body, which is achieved by balancing ballast water. According to the invention the two buoys which are permanently moored have small cross-sectional areas. When in the position before docking the motion in horizontal direction is relatively well restricted by ballast water and by the pre-tensioned anchor lines.

Aspects- _ g; ._ ,. ln a first aspect the object is achieved bky a floating wind power platform having a tower carrying a wind generator housed in a nacelle and a plurality of aims. the tower comprises a main float andjeach arm comprises a secondary float to stabilize the tower, wherein each arm consists of a tensile resisting element and a first tension element forming with part of the tower a triangular construction, the strut element being attached between the secondary float and the tower in a main position, and the first tensile element being attached between the outer end of the strut element and the tower in a first position, whereby the strut element will experience pressure forces only and the tensile element will experience tension forces only. l lllln an embodiment the floating wind power platform further consists of a second tensiie element attached between the outer end of the strut element and the tower in a second position on opposite side of the strut element. ln an embodiment the strut element comprises a framework beam and each of the tensiie element comprises a wire. In an embodiment each secondary float comprises a centre part containing an elongated cylinder having a small cross-sectional area to decrease the natural frequency of the platform. ln an embodiment the arms are arranged foldable around the tower and connected to each other with a detachable connecting wire to stabilize the arms. In an embodiment one arm comprises means for preventing a rotation of the tower around the main axle, ln an embodiment two of the secondary floats comprises means for docking with a first and a second dockable buoy of a mooring system. ln a second aspect the object is achieved by a method for designing a floating 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 stabilizing the tower, wherein a strut element, a first tension element, and a second tension element is provided between the tower and the secondary float to form with part of the tower a triangular construction, wherein the strut element is attached between the secondary float and the tower in a main position. wherein the first tensiie element is attached between the outer end of the strut element and the tower in a first position, wherein the second tensile element is attached between the outer end of the strut element and the tower in asecond position. andaivtlereintlje firstñtension element and the second __ tension element is prestressed to achieve stability of the platform. ln an embodiment the method further comprises aligning the main connection point, the first connection point and the second connection point on a common axis whereby the arm is foldable attached to the tower.

BRIEF DESCRIPTION OF THE DRAVVINGS l'\.7 (_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 the invenflon, fig 2. is a plan view of the floating wind power platform, fig 3. is a side view of a fioating wind power platform according to an embodiment of the invention, fig 4. is an example of a suitable mooring system, fig 5. is a further embodiment of the mooring system, and fig 6. is an anchoring facility according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS A fioating 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. The tower comprises a main float 5. ln the embodiment shown there are three arms. Each arm consists of a strut element 7 and a first tensile element Qfln the embodimentslíuítirf theïstrut elêfršïent coinprisešia frarnevvorlft TT' beam and the tensile element coinprises a vvire. The beam is attached to the tower in a main connection point 10. A secondary float 12 is attached to the outer end 11 of the beam. The first vifire íifis connected to the outer end 11 of the beam i Tf' and to the tower in a first connection point 14 positiohed beneath the main connection point 10, ln an alternative embodiment (not shown) of the invention the beam is connected to the first connection point 14 and the first wire is connected to the main connection point il Ii _ f. According to the plan view in fig of thíe secondary floats 12 are moored to a -_ln the embodiment shown in fig 1 the secondary floats 12 comprise the elongated body structure. To keep the natural frequency of the platform low the cross- sectional area of the secondary float 12 must be kept small. Thus the centre part of the secondary float 31 comprises an elongated cylinder having a small cross section. ln 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. According to the embodiment shown the secondary float 12 is moored to an anchor or a buoy (not shown) by cable 24 which is attached to the float by a span arrangement 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 stabilizlng forces must be transferred. This stability is achieved with one pressure force resisting element, the beam, and a tension force resisting element, the wires. ln order to achieve a sufficient stress in the second wire 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 wire. ln a semi-submerged operating position the platform is immersed by increasing the water balance of the floats. ln the semi-submerged state part of the framework beam and the second wire will be positioned under the water surface A. pair of twin 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 twin buoys are anchored with a plurality of anchor lines 24 connected to the buoy. The tvi/in buoys are held together by a distance wire 25. The secondary float 12 comprises a second docking meanssuitable to mate with the first docking means to for a unity float.

According to an embodiment of the invention shown in fig 3 the arm construction consists of a horizontally aligned strut element 7, a first tensile element 9 and a second tensile element 8. ln the embodiment shown the strut element comprises fx) f TI a framework beam and the tensile element comprises a wire. The first wire 9 and the second wire 8 are pre-stressed to achieve a play free arm construction. According to the invention the arm design is very stable in the vertical direction where the stabilizing forces must be transferred. This stability is achieved by just one pressure force resisting element, the beam, and two tension force resisting elements, the wires. ln this embodiment the beam is attached to the tower in a main connection point 10. A secondary float 12 is attached to the outer end 11 of the beam. The first wire 9 is connected to the outer end 11 of the beam 7 and to the tower in a first connection point 14 positioned below the main connection point 10. The second wire is connected to the outer end 11 of the beam 7 and to the tower in a second connection point 13 positioned above the main connection point 10. ln order to achieve an equal stress in the first and second wire the buoyant force may be balanced between the main float 5 and the three secondary floats 12. ln the semi-submerged state the framework beam and the second wire will be positioned under the water surface A. Only part of the first wire and part of the secondary float will be seen above the water surface.

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. ln 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 structutie; srnattin horigotttal 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. ln a transporting position all floats will be filled with air and the platform will rise to a level indicated by a dashed line B in fig 3. To stabilise the platform during transport secondary beams or secondary 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 prepositioned buoys. {\.7 f TT :ine arm and the ton/er would almostolae. Acclordingïtoihe invention the transport . a. a; A suitable mooring system 18 for the platform is shown in fig 4. The mooringldeally 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 skilied in the art. ln an embodiment of the invention the three arms are connected with connecting wires 15 that holds the three arms equaily spread around the tower. However the tower can still rotate relatively to the arms. ln the embodiment shown the tower is rotationally fixed to one arm. ln the embodiment shown in fig 3 there is a bracket means 16 in the connection point 13 which prevent the tower from rotating. ln an embodiment shown in fig 2 the lower wire of arm 6a is split into a first lower wire 9a and a second lower wire 9b connected crossways 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 two arms. ln an embodiment tvvo adjacent arms would resume a straight line which makes possible the tower coming close to a quay. ln an embodiment two arms may be folded to form a preferably perpendicular angle with the first arm which 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 propeller may be 150 m. The total height of the tower including the first float may be 130 m The length of the arm may be in the range of 100-130 m Hence the ratio between position of the platform is about 19 m higher that the submerged position. The draught of the platform under transport may be less than 9 meters. system comprises a mooring unit 27 including a pair of twin buoys 19 and a distance wire 25. A first buoy 19a and a second buoy 19b are anchored at sea by an anchor system 30 consisting of four anchors 26 in a square pattern. The tvvo buoys are acting as twins since they are permanently held apart by a distance wire 25. The mooring systern is anchored with four anchor lines 24. Each twin buoy is anchored with two anchor lines. By the distance wire 25 each twin buoy is also i .tnay be used. According to the inventiopostichia rnooried, platform will be held in a . _- .__anchored with the other two anchors. Thus the distance wire is a common anchoring means to both twin buoys. ln order to distribute the forces from the anchor lines the connection to the buoys comprises a span arrangement 23. By use of a common distance wire each of the twin buoys are anchored with three anchor lines. Thus the anchor arrangement will keep both twin buoys at a predeterminated location and orientation. ln the embodiment shown the distance wire 25 and the anchor lines 24 are cut to indicate that they may comprise considerably longer lines at an actual site. The anchor lines may comprise hundreds of meters depending on the sea bed condition and the dept. Preferably suction anchors are used. The distance wire may be in the range of 50 to 100 meters. Each of the twin buoys comprises an upper body 21 and a lower body 22 which are structurally connected to form a common ballast compartment. By pumping water in or out of the compartment the twin buoy may be adjusted in the sea to keep a predetermined float position.

Each of twin buoys of the mooring system 18 comprises an upper body 21 and a lower body 22. The upper body and the lower body comprise a common ballast compartment. ln an embodiment the twin buoys comprise docking means 20 arranged to hook or mate with a dockable object. By pumping water into and out from these compartments the floating height of each of the twin buoys is balanced in the sea. Thus each of the twin buoys may be lowered to be able to dock with a dockable platform. ln the ernbodiinent shown there is a structure line 28 of a floating platform to be moored to the mooring system. Any kind of mooring method stable position and orientation by the mooring system comprising twin buys only.

According to the invention there are twogembodiments of anchoring the mooring system. ln fig 5 is shown a first einbodinient 18a of a mooring system as described above. Thus the first embodiment of the mooring system comprises the twin buoys 19 and the distance wire 25 arranged as a mooring unit 27. The twin buoys are anchored in parallel with the square pattern of anchors. A first anchor line 24a and a second anchor line 24b are anchoring the first twin buoy at the lefthand side of the figure. A third anchor line 24a and a forth anchor line 24b are anchoring the second twin buoy at the righthand side of the figure. The distance __systeiiis need only one new anchor; Building Liga largveqiacility thus ends up in awire 25 is common to both twin buoys and thus comprising the third anchoring means to hold each of the twin buoys firmly anchored. The length of the distance wire is preferably the same as the distance between two adjacent arms of a floating platform.

A second embodiment of the mooring system 18b according to the invention is also shown in fig 5. ln this embodiment the mooring unit 27 comprlses a second pair of twin buoys 19c and 19d, each having a distance wire 25 between them and a separating wire 29. The second pair is arranged in parallel with the first pair of twin buoys 19a and 19b. The first and second pair of twin buoys are connected with the separating wire 29. Thus the mooring unit 27 of the second embodiment 18b of the mooring system according to the invention comprises two pairs of twin buoys 19 with distance wires 25 and a separating wire 29. ln the embodiment shown the separating wire 29 is connected to opposite pairs of twin buoys with a span. The aim of the separating wire is to keep the two pairs of twin buoys in a stable location at all weather conditions. Hens there may be a plurality of possibilities of designing such a connection known to a person skilled in the art.

A vast facility of a large number of mooring systems 18 according to the invention is shown in fig 6. Since every mooring system 18 only needs four anchors 26 the anchors are organized in a square pattern. As shown in the upper part of the fig a first mooring system needs four anchors. An adjacent mooring system anchored nearby needs only two extra anchors since the other two may be shared with the first mooring systern. The next three also needs two extra anchors but ftirtlier limit of one anchor for every mooring system.

Having an anchor pattern like the top view a further set of platforms may be anchored to the already existing anchors. A new set of mooring systems may be rotated diagonally and end up in a pattern like the mid view. Thus when the first set of mooring systems 18a is added to the second set of mooring systems 8b the result is shown in the lower view of fig 6. ln this view every new mooring system is positioned over an existing anchor. The effect of the capability to anchor the second set of platforms results in the limit of one anchor for every two mooring systems.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 beam may comprise a footbridge. The platform may arbitrary be moored in a traditionally way by a pluraiity of anchors and anchor lines. The tower may contain a transformer, HVDC equipment and/or other electrical equipment.

Claims (10)

1. Claims 'Floating 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 stabilizing the tower, c h a r a c t e r i z e d in that each arm (6) comprises a first triangular construction consisting of a pressure resisting element (7), a first tensile resisting element (9) and part of the tower (1), the pressure resisting element (7) being attached between the secondary float (5) and the tower in a main position (10), and the first tensile resisting element (9) being attached between the outer end (11) of the beam and the tower (1) in a first position (14). Floating wind power platform according to claim 1, wherein each arm (6) further comprises a second triangular construction consisting of the pressure resisting element (7), a second tensile resisting element (8) and part of the tower (1), the second tensile resisting element (8) being attached between the outer end (11) of the pressure resisting element (7) and the tower (1) in a second position (13) on opposite side of the pressure resisting element (7), whereby the pressure resisting element (7) comprises a common pressure resisting element to both the first and the second triangular construction. Floating wind power platform according to claim 1 or 2. wherein the pressure resisting element (7) comprises a framework beam and each of the tensile element (8, 9) contprises a vrire. Floating wind power platform according to claim 1 or 2, wherein the strut element (7) comprises a framemfork beam and each of the tensile element (8. 9) comprises a wire Floating wind power platform according to any of the preceding claims, wherein each secondary float (12) comprises a centre part (31) containing an elongated cylinder having a small cross-sectional area to decrease the natural frequency of the platform.Floating wind power platform according to any of the preceding claims, wherein the arms are arranged foldable around the tower and connected to each other with a detachable connecting wire (15) to stabilize the arms. Floating wind power platform according to any of the preceding claims, wherein one arm (6a) comprises means (16, 9a, 9b) for preventing a rotation of the tower (1) around the main axle (C). Floating wind power platform according to any of the preceding claims, wherein two of the secondary floats (12) comprises means for docking with a first (19a) and a second (19b) dockable buoy of a mooring system (18). Method for designing a floating 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 stabilizing 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 strut element (7), a first tension element (9), and a second tension element (8) to form with part of the tower (1) a triangular construction, attaching the strut element (7) between the secondary float (5) and the tower (1) in a main position (10), attaching the first tensile element (9) between the outer end (11) of the strut element (7) and the tower in a first position (14), attaching the second tensile element (8) between the outer end (_11) of the strut element (7) and the tower in a second position (13), prestressing the first tension element and the second tension element to achieve stability of the platform; 19, Method according to claim 7, tf-vhereiiw the rnethod ftirther comprises aligifiitg the main connection point (10), the first connection point (14) and the second connection point (13) on aiconwrnon axis (C), vvhereby the arrn being foldable attached to the tower. 11. Method of docking a floating platform according to any of the claims 1-7 to a mooring system (18) comprising a pair of twin buoys (19) anchored by pre-stressed taut leg anchor lines (24) at sea, c h a r a c t e r i z e d b y transporting the platform in float position (B) to the mooring site, immersinga first twin buoy (19a) of the mooring system by pumping ballast water into a buoyancy compartment (22) of the buoy, positioning a first of the secondary floats (12a) of the platform (17) over the first twin buoy, raising the first twin buoy by pumping out ballast water, having the first twin buoy (19a) to mate with the secondary float (12a) by exercising a relative movement between the buoy and the secondary float. 12. Method of mooring a floating platform according to any of the claims 1 to 7 to a quay, c h a r a c t e r i z e d b y transporting by a tugboat the platform in float position (B) to shallow location in the vicinity of the quay, detaching and adjusting the connecting line (15) between the first arm (6a) and the second arm (6b) to form a preferable perpendicular angle, detaching the connection line between the first and third (6c) arm and folding the third arm close to the second arm, mooring the platform with the first arm alongside the quay thereby achieving a short quay length and availability for the tower to be maintained by land-based lifting devices. 13. Wind power plant comprising a floating wind power platform according to any of the preoeding claims 1-8, further comprising a mooring system (18) for a floating wind power platform (6) having a plurality of stabilizing arms (6) with secondary floats(12), the system (18) comprising a plurality of buoys (19) anchored by anchor lines (24) at sea, c h a r a c t e r i z e d i n that the mooring system ('18) coinprises a first pair of twin buoys (l9a, 19b), a distance wire (25) connecting the pair of twin buoys, and an anchor system (30) consistiitg of four anchors (26),