SE2000206A1

SE2000206A1 - Floating wind power plant

Info

Publication number: SE2000206A1
Application number: SE2000206A
Authority: SE
Inventors: Bertil Moritz
Original assignee: Josok Ab
Priority date: 2020-11-04
Filing date: 2020-11-04
Publication date: 2022-05-05
Also published as: WO2022098286A1; WO2022098286A9; EP4240965A4; EP4240965A1; SE546025C2

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

Floatinq wind power plant TECHNICAL FIELD The present invention concerns a floating power plant for oonverting wind energyto electrical energy. Especially the invention concerns a floating platform includinga wind generator for being moored at sea in a stable position and orientation. ln 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 energyfrom the wind into mechanical energy that is then converted to electric energy.Wind turbines have been developed for land-based installations as well asoffshore installations. The land-based wind turbines are fixed to the ground andlocated in windy areas. Most common wind turbines have the main rotor shaftarranged horizontally. They have a horizontal rotor shaft that is pointed into thewind. Horizontal axis wind turbines generally have a tower and an electricalgenerator coupled to the top of the tower. The generator may be coupled directlyor via a gearbox to the hub assembly and turbine blades.

Wind turbines have also been used for offshore applications. Single long toweroffshore 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 expectedto be economically feasible. To fully exploit vvind energy Offshore it is necessary tofind economical solutions for deep xrvater. Shallotrv "vvater resources are limited andrepresent only a fraction of the offshore wind resources. VVind turbines close toshore may also block the shore view and create navigational obstructions and potential hazards for water vessels and aircrafts. 2 There are known a plurality of concepts for offshore floating wind turbineplatforms. Generally, these fall into three main categories: Spars; Tension LegPlatforms (TLP); and semi-submersible systems.

Spars comprise elongated structures that are balanced with significant ballast atthe bottom of the structure and buoyant tanks near the waterline. For stabilitypurposes, the centre of gravity must be lower than the centre of buoyancy. Thiswill insure that the spar will float upright. The spar is moored to the sea bottomwith a plurality of anchored lines that hold the spar in position. ln general termsspar type structures have good heave performance due to reduced response tovertical wave exciting forces. They require substantial depth, especially when windturbine 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 thatconnect the floater directly to the bottom of the sea. There is no requirement for alow centre of gravity for stability. Only during the installation phase buoyancymodules may be temporarily added to provide sufficient stability. The TLP havevery good heave and angular motions. Due to complexity of structure and themooring installation the costs may escalate. Also the change in tendon tensiondue to tidal Variations and the structural frequency coupling between the towerand 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 buoyancyelements penetrating the sea surface. When comparing different types of offshorewind turbine structtiresïxfi/ave and wind induced motions are not the only elementsof performance to consider. Economics play a significant role. lt is thereforeimportant to carefully study the fabrication, installation, commissioning costs andease of access for maintenance methodologies. Semi-submersible concepts witha shallow draught and good stability in operational and transit conditions are significantly cheaper to tow out, install and commission. f\Jf ri 3 From EP 278984851 (Komatsu) is previously known a floating body wind powergenerating device and method for mooring the floating body wind powergeneration device. An objective of the wind power generation device is to providea floating body wind power generating device with which it is possible to moor thefloating body stably with respect to drift force or rotational moment acting on thefloating body. The wind power generating device comprises a wind powergenerator and a floating body. Further the device comprises a first column whichis located on the upwind side of a primary wind direction and whereupon the windpower generator is installed. A second column and a third column which arelocated on the further downwind side of the primary wind direction than the firstcolumn are connected to the first column with two rower hulls to the first column. Aplurality of mooring cables connects the floating body to anchors. At least two ofthe plurality of mooring cables is connected to the first column. At least one of thepluralities of mooring cables is respectively connected to the second column andthe 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 windturbines is previously known. The floating wind turbine platform includes afloatation frame that includes three columns that are coupled to each other withhorizontal main beams. A wind turbine tower is mounted above a tower supportcolumn 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 thatpumps water between the columns to keep the tower in a vertical alignment gregardless of the vvincl speed. Water-entrapmertt plates are mounted to the wbottoms of the columns to minimize the rotational movement of the floatationframe 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 floatingwind power plant. A second object of the invention is to provide a light weightfloating 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 powerplatform characterized by the features in the independent claim 1, or by a methodcharacterized by the steps in the independent claim 9, 10 or 11, or by a windpower plant characterized by he features in the independent claim 13. Preferredembodiments are described in the dependent claims.

According to the invention each stabiiizing arm of the floating wind power platformcomprises a lightweight construction consisting of two kinds of building elementsonly. The first kind is a tensile resisting element designed to resist compression inits longitudinal direction. The second kind is a pressure resisting element designedto resist pressure forces in its longitudinal direction. Examples of the first kind arestrut, brace, stick, beam, framework construction etc. ln the text hereinafter a strutelement 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 designedwhere part of the tower comprises one of the triangle legs. The other legs are abeam 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 bevvithstood. 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 beamand 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 wirepart of the triangle. This outer force is obtained by balancing the buoyancy of thebuoy and the tower. ln an embodiment the beam is connected between the buoy 'll I\J'Il and a main position of the tower. The wire is connected between the buoy and afirst position of the tower. ln an embodiment the first position is located under themain position. ln this embodiment the buoyancy of the buoy must be greater thanthe buoyancy of the tower. ln an embodiment the first position is located abovethe main position. ln this embodiment the buoyancy of the tower must carry thewhole platform and the buoyancy of the buoy only be used for balancingpurposes. ln an embodiment according to the invention a stabilizing arm consists of a beamand a first and second wires positioned on opposite sides of the beam. The beamis connected between the buoy and a main position of the tower. The first wire isconnected between the buoy and a first position of the tower positioned below themain position. The second wire is connected between the buoy and a secondposition of the tower positioned above the main position. The construction mayresemble a mast on a sailing boat where the mast is supported by two prestressedstays or shrouds. However in the present embodiment the mast is alignedhorizontally and the wires are prestressed against the tower. ln an embodiment ofthe 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 offreedom (ZRDOF). Thus the beam is freely rotatable up and down as well assideways but cannot rotate around its own axis. This design ensures that onlya> connected with a connection wire. The distance between the second and third ill ill 'j-C -u 6 connection points on the tower is chosen from a cost-benefit evaluation of stresslimitation 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 towerfrom rotation relative to the arms the tower must be locked to one of the arms. lnan embodiment one wire is connected to the tower in a way to lock this relativerotation. This means that one of the stabilizing arms will prevent the tower torotate in relation to the other arms. The locking means comprises according to anembodiment of the invention a console beam protruding from the tower where oneof the upper wires of one of the arms is connected. ln an embodiment a bracketmeans such as a shackle is used. ln an embodiment the second wire is attach tothe tower with a wire span which is transversally attached to each sides of thetower. ln an embodiment of the invention the floating wind power platform comprises atower and a plurality of stabilizing arms. The tower comprises a hollow structurecarrying a pivotal nacelle and includes a main float at its lower end. ln anembodiment the tower is partly a framework structure. Each arm comprises asecondary float connected to its outer end. The main float is preferably designedto carry the tower and its equipment as well as the generator and rotor. Animmersed position of the platform may be achieved by pumping water into themain float. There-by each secondary float needs only to carry its own weight andpart 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 itmust also withstand forces from the waves. lt is therefore favourable to design thebeams with a minimum exposed area, such as a framework construction withmoderate diameters of tube elements. Most suitable the beam is made of metalsuch as steel and protected against oxidation and fouling by a protective paint. lnan 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 IxflCT! 7 reinforcement material may be coal fibres, synthetic fibres such as for instancearomatic 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 floatand three stabilising arms having secondary floats. The secondary float comprisesa hollow column of arbitrary cross section. ln an embodiment each secondary floatcomprises an elongated cylindric body having a small cross section to decreasethe movement in the sea. By pumping out ballast water from the floats theplatform will float in a high position during transport. This ensures the possibilityfor the platform to be moored to a quay and transported in shallow water. On thesite of operation the platform is docked to an existing mooring system. By partlyfilling the floats with ballast water the platform will immerse and take its operationposition. ln an embodiment the beams in this operating position will be locatedunder 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 anoperating level. At this operating level the assembled cross-sectional areas of thesecondary 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 toreach 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 thedisplaced water. When moving upwards it will experience a downwardly directedforce equal to the volurne of the non-displaced water. Since the three arms aresymmetrically spread around the tower there vvill always be an equal amount ofstabilizing forces on opposite sides of the tower. Hence when the tower tends tolean caused by wind forces the floats on the leeward side will exert an uprisingforce and simultaneously the floats on the upwind side will exert a traction forceThus 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. ,__ ¿\_ 8 The necessary stabilizing force for keeping the tower in an upright position is thusprovided by the length of the beam and the cross-sectional area of the secondaryfloat. Cross-sectional area is the imprint of the secondary float in the sea. A longerarm and a greater cross-sectional area of the floats will increase the uprisingforces. A big cross-sectional area will however make the float more affected by thewave motion. Thus a small cross-sectional area is desirable. When decreasing thecross-sectional area of the secondary float the arm needs to be longer. However alonger arm will increase in weight and make the platform heavier. According to theinvention a fair compromise is to make the arm approximately as long as the toweris 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 lattercase the cross-sectional area will increase by the immersion of the float and thusresulting in a non-linear increasing fore. Such design will effectively act asdamping.

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 platformsuitable for docking a quay. lt is a feature of the invention that the tower can bebrought very close to the quay which facilitate lifting, mounting and replacement ofthe 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 tomake 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 shortthe 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 atsea. The twin buoys are separated from each other by a distance wire to keep adesired distance between them. The distance between the twin buoys ispreferably the same as the distance between two adjacent arms of a floating windpower plant. The twin buoys are anchored by four anchors to receive a geographicstable position and orientation. Preferably suction anchors and prestressed anchoring lines are used to anchor the buoys. The anchors are spread equally on 9 the bottom in a square pattern. By providing a mooring system with only twodistinctive mooring points any floating platform may be moored in a secure way toreceive a desired position and a desired orientation. The twin buoy concept maybe applicable to any self-floating and stabilised platforms.

According to an embodiment of the invention the twin buoys are anchored withfour anchor lines and four anchors in square pattern. ln this embodiment the twinbuoys are oriented parallel to a side of the anchor square. Each of the twin buoysare anchored by two anchors and kept in position by the distance wire to the othertwin buoy which in turn is anchored by two anchors. ln an embodiment the twinbuoys are anchored with six anchor lines to four anchors. ln this embodiment eachtwin buoy is anchored with three anchor lines. However, on opposite sides of thetwin buoys separated by the distance wire one anchor line from each buoy may beanchored at the same anchor. Thus two of the anchors are arranged to receiveone anchor line from each individual of the twins. Still the distance between thetwin buoys are defined by the distance wire. ln an embodiment of the invention two sets of twin buoys are anchored by fouranchors. The first and second sets of twin buoys are arranged in parallel with eachother. ln this embodiment each set of twin buoys are connected to a corresponding set of twin buoys by a separating means. ln an embodiment theseparating means comprise a single cable connected by a span arrangement toeach set of tvvin buoys. ln an einbodintent the separating means comprlse twocables crossing each other. Each crossing cable is connected between diagonallyopposite twin buoy of thefirstqaitd second seten twin buoy's...lr| an entbodimeint-- ïeach twin buoy of the first set is connected to 'a corresponding twin buoy of the ksecond 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 startwith one platform using four anchors. A second platform may be anchored in thevicinity of the first platform. A third and a forth platform may be anchored sidewaysof the first and second platform. Each of these platforms need four anchoringpoints 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 firstplatform. And the second line of platforms including the third and fourth platformneeds only three new anchors since three anchors are common to the first line ofplatforms. ln this embodiment four platforms need nine anchors. By organizing avast number of platforms side by side in lines and columns will make use of aplurality of anchors being used in common by a plurality of platforms. The limit ofthe number of anchors needed in a facility with a great number of platforms tendsto one anchor for each platform. Hence each anchor receives anchor lines fromfour platforms.

According to an embodiment of the invention a second facility of platforms areorganised diagonally to the first facility of platforms. The position of the secondsuch platform will be just on top of an already existing anchor. Hence by addingthe second set of platforms which all uses already existing anchors the efficiencyof the use of anchors may be drastically improved. For a wind power plantcomprising a vast number of both facilities of anchored platforms each anchorreceives 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 dockingmeans involving the twin buoys and two of the secondary floats. Each buoycomprises an upper hollow body and a lower hollow body sharing a commonwater 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 waterinto 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 twinbuoys and a second part containing a pair of secondary floats connected to theplatform to be docked. The docking is performed by a relative movement in thevertical plane of the parts of the docking means. According to the invention a method of docking comprises immersing a twin buoy, orienting the secondary float 11 on top of the twin buoy, raising the twin buoy by emptying the ballast water tointeract with the float and lock the two parts together.

When the first docking means has docked the platform is rotated by tugs so thatthe second docking means are aligned. Then the second docking is made thesame way as the first docking. When fully docked the platform is immersed to itssemi-submersed operational position. The advantage of letting the upper buoystay above water is that wave forces and motions are avoided to better facilitatethe 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 anybuoyant body are high. Even though the body is partly submersed the forces aresevere. Each buoyant body and the platform have their own characteristic naturalfrequencies in the heave of the sea. To minimize the hazardous portion of dockinga set of principles are used to design the platform. One such principle is tominimize the cross-sectional area of sea protruding elements. Another principle isto decrease the natural frequencies of bodies which are arranged to mate. Thenatural frequencies in different directions may be decreased by increasing themass of the body, which is achieved by balancing ballast water. According to theinvention the two buoys which are permanently moored have small cross-sectionalareas. 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 atower carrying a wind generator housed in a nacelle and a plurality of aims. thetower comprises a main float andjeach arm comprises a secondary float tostabilize the tower, wherein each arm consists of a tensile resisting element and afirst tension element forming with part of the tower a triangular construction, thestrut element being attached between the secondary float and the tower in a mainposition, and the first tensile element being attached between the outer end of thestrut element and the tower in a first position, whereby the strut element willexperience pressure forces only and the tensile element will experience tensionforces only. llll 12 ln an embodiment the floating wind power platform further consists of a secondtensiie element attached between the outer end of the strut element and the towerin a second position on opposite side of the strut element. ln an embodiment thestrut element comprises a framework beam and each of the tensiie elementcomprises a wire. In an embodiment each secondary float comprises a centre partcontaining an elongated cylinder having a small cross-sectional area to decreasethe natural frequency of the platform. ln an embodiment the arms are arrangedfoldable around the tower and connected to each other with a detachableconnecting wire to stabilize the arms. In an embodiment one arm comprisesmeans for preventing a rotation of the tower around the main axle, ln anembodiment two of the secondary floats comprises means for docking with a firstand a second dockable buoy of a mooring system. ln a second aspect the object is achieved by a method for designing a floatingwind power platform having a tower carrying a wind generator housed in a nacelleand a plurality of arms, the tower comprising a main float and each armcomprising a secondary float to stabilizing the tower, wherein a strut element, afirst tension element, and a second tension element is provided between the towerand 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 towerin a main position. wherein the first tensiie element is attached between the outerend of the strut element and the tower in a first position, wherein the secondtensile 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 commonaxis whereby the arm is foldable attached to the tower.

BRIEF DESCRIPTION OF THE DRAVVINGS l'\.7(_71 13 Other features and advantages of the present invention will become moreapparent to a person skilled in the art from the following detailed description inconjunction 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 thetower 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 mainconnection point 10, ln an alternative embodiment (not shown) of the invention thebeam is connected to the first connection point 14 and the first wire is connectedto the main connection point 10. il Ii _ f. According to the plan view in fig of thíe secondary floats 12 are moored to a -_ 14 ln the embodiment shown in fig 1 the secondary floats 12 comprise the elongatedbody 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 partof the secondary float 31 comprises an elongated cylinder having a small crosssection. ln the embodiment shown the upper end of the secondary float 12 maycomprise a funnel shaped body 32. This funnel shaped body exerts a dampingeffect when moving in sea heave. The lower end of the secondary float comprisesa cylindric body 33. This body also exert a damping effect but also a convenientcontainer to fill or pump out water for balancing purposes. According to theembodiment shown the secondary float 12 is moored to an anchor or a buoy (notshown) by cable 24 which is attached to the float by a span arrangement 23.

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 wherethe stabilizlng forces must be transferred. This stability is achieved with onepressure 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 buoyantforce may be balanced between the main float 5 and the three secondary floats12. By increasing an air portion in the secondary float the arm will exert a liftingforce that will increase the stress of the wire. ln a semi-submerged operatingposition the platform is immersed by increasing the water balance of the floats. lnthe 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 buoycomprises a lower body 22 with a water ballast compartment and an upper body21 comprising docking means. The twin buoys are anchored with a plurality ofanchor lines 24 connected to the buoy. The tvi/in buoys are held together by adistance wire 25. The secondary float 12 comprises a second docking means 20 suitable 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 constructionconsists of a horizontally aligned strut element 7, a first tensile element 9 and asecond 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 andthe 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 directionwhere the stabilizing forces must be transferred. This stability is achieved by justone pressure force resisting element, the beam, and two tension force resistingelements, the wires. ln this embodiment the beam is attached to the tower in amain connection point 10. A secondary float 12 is attached to the outer end 11 ofthe beam. The first wire 9 is connected to the outer end 11 of the beam 7 and tothe tower in a first connection point 14 positioned below the main connection point10. The second wire is connected to the outer end 11 of the beam 7 and to thetower in a second connection point 13 positioned above the main connection point10. ln order to achieve an equal stress in the first and second wire the buoyantforce may be balanced between the main float 5 and the three secondary floats12. ln the semi-submerged state the framework beam and the second wire will bepositioned under the water surface A. Only part of the first wire and part of thesecondary 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 waterfrom the floats. ln a transporting position all floats will be filled with air and theplatform will rise to a level indicated by a dashed line B in fig 3. To stabilise theplatform during transport secondary beams or secondary floats may be attachedto 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. {\.7f 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 mooring 16 ldeally 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 suchconnections can be made with a swivel construction well known to a person skiliedin the art. ln an embodiment of the invention the three arms are connected withconnecting 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 shownthe tower is rotationally fixed to one arm. ln the embodiment shown in fig 3 there isa bracket means 16 in the connection point 13 which prevent the tower fromrotating. ln an embodiment shown in fig 2 the lower wire of arm 6a is split into afirst lower wire 9a and a second lower wire 9b connected crossways on either sideof the first float 5. The tower rotating preventing arrangement may also comprise aspan means between the end of the arm and the first float. The connection wires15 can be detached and adjusted to facilitate a temporary angular rotation of twoarms. ln an embodiment tvvo adjacent arms would resume a straight line whichmakes possible the tower coming close to a quay. ln an embodiment two armsmay be folded to form a preferably perpendicular angle with the first arm which willallow 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 constructioncan be made very big. According to the invention the diameter of the propellermay 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 adistance wire 25. A first buoy 19a and a second buoy 19b are anchored at sea byan anchor system 30 consisting of four anchors 26 in a square pattern. The tvvobuoys are acting as twins since they are permanently held apart by a distance wire25. 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 . _- .__ 17 anchored with the other two anchors. Thus the distance wire is a commonanchoring means to both twin buoys. ln order to distribute the forces from theanchor lines the connection to the buoys comprises a span arrangement 23. Byuse of a common distance wire each of the twin buoys are anchored with threeanchor lines. Thus the anchor arrangement will keep both twin buoys at apredeterminated location and orientation. ln the embodiment shown the distance wire 25 and the anchor lines 24 are cut toindicate that they may comprise considerably longer lines at an actual site. Theanchor lines may comprise hundreds of meters depending on the sea bedcondition and the dept. Preferably suction anchors are used. The distance wiremay be in the range of 50 to 100 meters. Each of the twin buoys comprises anupper body 21 and a lower body 22 which are structurally connected to form acommon ballast compartment. By pumping water in or out of the compartment thetwin 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 alower body 22. The upper body and the lower body comprise a common ballastcompartment. ln an embodiment the twin buoys comprise docking means 20arranged to hook or mate with a dockable object. By pumping water into and outfrom these compartments the floating height of each of the twin buoys is balancedin the sea. Thus each of the twin buoys may be lowered to be able to dock with adockable 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 methodstable position and orientation by the mooring system comprising twin buys only.

According to the invention there are twogembodiments of anchoring the mooringsystem. ln fig 5 is shown a first einbodinient 18a of a mooring system asdescribed above. Thus the first embodiment of the mooring system comprises thetwin buoys 19 and the distance wire 25 arranged as a mooring unit 27. The twinbuoys are anchored in parallel with the square pattern of anchors. A first anchorline 24a and a second anchor line 24b are anchoring the first twin buoy at thelefthand 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 a 18 wire 25 is common to both twin buoys and thus comprising the third anchoringmeans to hold each of the twin buoys firmly anchored. The length of the distancewire is preferably the same as the distance between two adjacent arms of afloating platform.

A second embodiment of the mooring system 18b according to the invention isalso shown in fig 5. ln this embodiment the mooring unit 27 comprlses a secondpair of twin buoys 19c and 19d, each having a distance wire 25 between them anda separating wire 29. The second pair is arranged in parallel with the first pair oftwin buoys 19a and 19b. The first and second pair of twin buoys are connectedwith the separating wire 29. Thus the mooring unit 27 of the second embodiment18b of the mooring system according to the invention comprises two pairs of twinbuoys 19 with distance wires 25 and a separating wire 29. ln the embodimentshown the separating wire 29 is connected to opposite pairs of twin buoys with aspan. The aim of the separating wire is to keep the two pairs of twin buoys in astable location at all weather conditions. Hens there may be a plurality ofpossibilities 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 inventionis shown in fig 6. Since every mooring system 18 only needs four anchors 26 theanchors are organized in a square pattern. As shown in the upper part of the fig afirst 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 ftirtlierlimit of one anchor for every mooring system.

Having an anchor pattern like the top view a further set of platforms may beanchored to the already existing anchors. A new set of mooring systems may berotated diagonally and end up in a pattern like the mid view. Thus when the firstset of mooring systems 18a is added to the second set of mooring systems 8b theresult is shown in the lower view of fig 6. ln this view every new mooring system ispositioned over an existing anchor. The effect of the capability to anchor thesecond set of platforms results in the limit of one anchor for every two mooringsystems. 19 Although favourable the scope of the invention must not be limited by theembodiments presented but also contain embodiments obvious to a person skilledin the art. For instance there could be more than 3 stabilizing arms. The wires maycomprise any kind of material with good tensile properties. The secondary floatsmay comprise a landing pad for a helicopter. The framework beam may comprisea 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, HVDCequipment and/or other electrical equipment.

Claims

1. CLAIMS

2. Semi-submersible wind power platform (17) having a tower (1) carrying awind generator housed in a nacelle (2) and a plurality of arms (6), the towercomprising 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) consistsof, a first tensile resisting element (9), a second tensile resisting element (8)and a pressure resisting element (7) arranged between the first and secondtensile resisting element, the pressure resisting element (7) beingconnected between a main position (10) of the tower and the secondaryfloat (12), the first tensile resisting element (9) being connected between afirst position (14) of the tower (1) and to the secondary float (12), and thesecond tensile resisting element (8) being connected between a secondposition (13) of the tower (1) and the secondary float (12).

3. Semi-submersible wind power platform according to claim 1, wherein eachsecondary float (12) comprises an elongated water fillable containercomprising a centre part (31) having a cross-section in the ratio of 10-15 %of the length of the secondary float (12) to decrease the heave frequency ofthe platform.

4. Semi-submersible wind power platform according to claim 1 or 2, whereinthe first position (14), the main position (10) and the second position (13) ofthe tower (1) are aligned along a common axis thus making the armswingable.

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

6. Semi-submersible wind power platform according to any of the precedingclaims, wherein one arm (6a) comprises first and second wire (9a, 9b) forpreventing a rotation of the tower (1) relative to the first arm (6a).Semi-submersible wind power platform according to any of the precedingclaims, wherein the pressure resisting element (7) comprises a frameworkbeam and each of the tensile element (8, 9) comprises a wire.

7. Semi-submersible wind power platform according to any of the precedingclaims, wherein the floats (12) comprises docking means (20) for docking toa mooring system having a pair of buoys (19) anchored (24) at sea.

8. Method of building a semi-submersible wind power platform (17) having atower (1) carrying a wind generator housed in a nacelle (2) and a plurality ofarms (6), the tower comprising a main float (5) and each arm comprising asecondary float (12) to stabilize the tower, c h a r a c t e r i z e d b yproviding between the tower (1) and the secondary float (12) a first tensileresisting element (9), a second tensile resisting element (8), and betweenthe first and second tensile resisting element a pressure resisting element(7), to form with part of the tower (1) an arm (6).-

9. l\/lethod according to claim 8, wherein the method further comprisesattaching the pressure resisting element (7) between the secondary float(5) and the tower (1) in a main position (10), attaching the first tensileresisting element (9) between the outer end (1 1) of the pressure resistingelement (7) and the tower in a first position (14), attaching the secondtensile resisting element (8) between the outer end (1 1) of the pressureresisting element (7) and the tower in a second position (13), and aligningthe main connection point (10), the first connection point (14) and thesecond connection point (13) on a common axis (C), whereby the armbeing swingable attached to the tower.

10. 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 db 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 harbourfacility, adjusting theconnection wire (15) to swing the arms to resume a straight line, andmooring the platform to the quay whereby the tower come close to thequay.