SE1850355A1

SE1850355A1 - A floating wind turbine with integrated foundation

Info

Publication number: SE1850355A1
Application number: SE1850355A
Authority: SE
Inventors: Magnus Rahm
Original assignee: Magnus Rahm Energy Consulting Ab
Priority date: 2018-03-28
Filing date: 2018-03-28
Publication date: 2019-09-29
Also published as: SE542891C2; WO2019190387A1

Abstract

The present invention relates to a floating vertical axis wind turbine, VAWT, with a central column which at its lower end is connected to a mooring system. The VAWT comprises at least one rigid peripheral power generating unit comprising a blade, a peripheral buoyancy element and a water turbine assembly. The peripheral buoyancy element extends from the lower end of the blade and connects the water turbine assembly to the blade, and wherein the peripheral buoyancy element at least partly supports the rigid peripheral power generating unit by buoyancy. A bearing assembly is rotatably attached to the central column and at least one first strut connects the rigid peripheral power generating unit with the bearing assembly.

Description

A FLOATING WIND TURBINE WITH INTEGRATED FOUNDATION Technical Field The present invention relates to the area of offshore wind power. More specifically, theinvention relates to a ﬂoating vertical axis Wind turbine which comprises one or morerigid peripheral power generating unit comprising a blade, a peripheral buoyancy element and a water turbine assembly.

Background Offshore wind power globally has very large potential to contribute to the transitionfrom a fossil-based society to one where only renewable energy is used. The potentialfor offshore wind power only in Europe has been estimated to 4000 GW which is morethan the global electricity use today. A challenge is that 80% of these 4000 GW islocated at ocean depths of more than 60 m where bottom fixed foundations are noteconomically viable. Also in places like Japan, the US west coast, large parts of theMediterranean, many volcanic islands in the Atlantic and the Pacific oceans, the oceandepth increases rapidly outside the coast. An altemative for these markets are ﬂoating wind turbines which can be anchored at large depths.

Today"s ﬂoating wind turbines are mainly based on four different foundation types allof which offer buoyancy and stability: barge, spar (buoy), semisubmersible and tensionleg platforrns (TLP). The stability requirement is high since the foundations areintended to carry conventional Horisontal Axis Wind Turbines (HAWTs) which canonly withstand small inclination angles. Conventional wind turbines have a rotor withthree blades that is mounted on a relatively heavy nacelle which in tum is placed at thetop of a steel tower. The steel tower is, from the base to the top, designed as a taperedsteel pipe which is welded and bolted in sections. The sections are generally about 20 mlong for ease of handling. The wind acts on the entire rotor disc resulting in a tippingmoment and subsequently a high tower base moment. This is not a problem per se.However, as the wind pressure on the rotor disc increases, the wind turbine willexperience an inclination angle. The tower top weight will then add to the tipping moment on the foundation and to the mechanical tower base moment. The static inclination angle has to be limited in order for the rotor blades to stay clear of the towerduring its rotation. During dynamic motions, especially in the pitch degree of freedomwhich is rotation around an axis perpendicular to the wind direction and parallel withthe sea surface, the inertial loads may be significant especially in terms of blade rootmoments and tower base moments. Large accelerations result in large inertial loadswhich requires more material to reduce stress levels. More material increases the weightwhich in tum requires more material in the foundation to increase the buoyancy to carry the increased weight. A negative weight spiral is attained.

The moment from the wind turbine°s tower base is for semisubmersible typicallyresisted by buoyancy co lumns via stiff construction elements. To secure that this design,that has large distributed masses, is not excited by energy from environmental loads, theload-carrying construction elements (beam sections) between the masses (RNA andbuoyancy co lumns respectively) must be designed stiff enough which requires a lot of material. This is the case also for other known designs of wind turbines.

A general disadvantage of HAWTs is that they need to be poi11ted into the prevailingwind direction. Most Vertical Axis Wind Turbines (VAWTs) converts wind energyfrom any direction equally efficient and is thus less sensitive than HAWTs to a rapidly changing wind direction.

A problem with today°s ﬂoating wind turbine technology is how to exchange majorcomponents such as main bearing, gearbox and generator. The nacelle can be mountedon 80-90 m or more above sea level and a gearbox for a 5 MW wind turbine can weighmore than 60 tonnes. Today, there are only very few crane vessels in the world that canmanage the combination of this lifting height and weight. The operation is also madesignif1cantly more complex by the fact that it is a lift from one ﬂoating vessel toanother, not between a fixed reference point (as on a bottom-mounted offshore wind turbine) and a ﬂoating vessel.

A problem with today"s ﬂoating turbine foundations is the f1rm stability requirement.This is due to that HAWTs must operate within a narrow window of inclination in thepitch (nodding back and forward) degree of freedom. Spar-buoy type foundations solvethe high stability requirement by providing a very deep draft configuration (up to 70-80 m) so that the center of buoyancy is located sufficiently above the center of gravity.

This makes it impossible to toW the ﬂoating turbine in the upright position fromkeyside/dockside to site since most harbours are much too shallow. The Wind turbinesthus have to be mounted on the spar buoy foundation at sea at deeper Water using very large and expensive crane vessels in complex marine operations.

Semisubmersible ﬂo ating foundations solve the high stability criteria through buoyancycolumns Which have sufficiently large Water plane area that is located at a sufficientlylarge distance from the centre of the Wind turbine tower. This requires that theconstruction elements separating them are sufficiently stiff and sturdy in order not tobuckle or fail due to high stress. This requires a lot of material Which causes challengesin handling and logistics and Will often require drydock fabrication Which is limiting and drives high fabrication cost.

Tension-leg ﬂoating foundations are believed to be more material-efficient than otherfoundations, but they are by design inherently unstable Without the tension-legs. Theyare therefore unstable during toW to the installation site and before the mooring tethersare attached and tensioned. The installation sequence is therefore more complex than for spar buoys and semisubmersible platforms.

A problem With VAWTs that are arranged With the blades rotating around a centralturbine shaft to Which an electrical generator is mounted is that if the generatorexperiences a failure, e.g. a short circuit, the central shaft and the struts that secure theblades to the central shaft Will be subj ected to a very high torque transient. The designWill have to Withstand this high stress transient. The shaft also has to be dimensionedfor high torsional stiffness to attain a suitable natural period Which often is chosen to beshorter than occurring environmental loads to avoid harrnful structural excitation. Thisrequires additional material Which increases cost and makes handling during fabricationand installation more onerous. The same measures have to be taken to handle an ESD event, Emergency Shut Down event.

A problem is that the mooring system of a floating VAWT has to provide yaWingstiffness i.e. it has to counteract the torque generated by the turbine if the electricalgenerator is placed on a geo-stationary (and moored) part in relation to the rotatingturbine shaft. If the mooring lines are attached at or near the perimeter of the ﬂoating foundation, if the foundation for example is of the semisubmersible type, the torque resistance will inherently be high due to the large moment arm. But a semisubmersibleplatforrn requires a lot of construction material. If the turbine torque is resisted by ageo-stationary (and moored) central shaft, eg. via a turret assembly, the mooring spreadwill have to be large due to the relatively small diameter of the turret chain table to which the mooring lines are attached.

WO20151 16016 disclo ses a ﬂoating wind turbine solution with one stationary part andone rotating part. The stationary part is the lower part and consists of a circularmonorail resting on a number of vertically oriented semisubmersible co lumns arrangedaround the perimeter of the monorail. The monorail is fixed to the ﬂoatingsemisubmersible co lumns. The rotating, upper, part of the ﬂoating wind turbine solutionis a vertical axis rotor which consists of a central shaft, spokes or struts connecting thecentral shaft to annular rings arranged at the perimeter of the rotor. The annular ringscan be stacked in the vertical direction by interconnecting braces to increase the wind-absorbing area of the rotor. Sails are arranged between the annular rings at the perimeterto convert energy from the wind into rotational motion and kinetic energy in the rotor.The rotation of the rotor is resisted by water turbines mounted either on radiallyarranged arms or struts from the lower part of the central shaft of the rotor or by verticalconso les mounted on the lower part of the rotor rim or spokes altematively struts orcarrying arms between the lowest annular ring and the central shaft. An electrical cableis arranged to exit the central shaft through a rotating connection. The rotor rotates onthe circular monorail by wheels with integrated electromagnetic breaks that can betumed on or off. One disadvantage of the prior art is that the ﬂoating monorail is of verylarge diameter and needs to be structurally very stiff to be able to carry the rotor withthe sails resting on wheels running on the ﬂoating monorail. Either the monorail has tobe very stiff and will therefore be heavy and expensive or, the number of verticalsemisubmersible co lumns will have to be increased to reduce the free-span of themonorail resting on the semisubmersible co lumns to provide a smooth and planarrunning surface for the wheels of the rotating turbine rotor. The wheels will require a lot of maintenance due to being subj ected to salt water spray from the ocean.

A further disadvantage of the ﬂoating wind turbine according to WO2015116016 is thelarge weight of the rotating part being the rim and the central shaft (hub) which is interconnected by spokes (struts). The driving moment from the windforce-absorbing sails is transferred to the central shaft Via the spokes and further from the central shaft tothe consoles on the lower part of the shaft, the consoles carrying water turbines forpower production. The moment is thus transferred from the perimeter sails Via stiffconstruction members to the central shaft and the spokes and its attachments will haveto be dimensioned against transient fault conditions such as loss of grid load orgenerator short circuit. In both these cases, the resisting force of the water turbines willsuddenly be released followed by torsional mode oscillation in the structure. A yetfurther disadvantage is that the electrical cable is connected to the central shaft Via arotating connection. The ﬂoating foundation composed of semisubmersible ﬂoatingcolumns and a monorail is anchored by mooring lines attached at the perimeter. Thefriction in the rotating connection thus has to be resisted by torsional stiffness of the electrical cable.

Summary of the invention An object of the invention is to provide an improved vertical axis wind turbine thatovercomes the drawbacks associated with prior art ﬂoating wind turbines. In particularto provide a structure that is relatively lightweight and easy to transport and still capableof handling the high and varying forces and the harsh environment associated with offshore wind power. This is achieved by the device as defined in claim l.

The ﬂoating vertical axis wind turbine, VAWT, according to the invention has a centralcolumn which at its lower end may be connected to a mooring system which isanchored with at least one anchor. The VAWT comprises at least one rigid peripheralpower generating unit comprising a blade at least one peripheral buoyancy element andat least one water turbine assembly which is adapted to be below a water surface. Theperipheral buoyancy element extends from the lower end of the blade and may alsoconnect the water turbine assembly to the blade. The peripheral buoyancy element doesat least partly supports the rigid peripheral power generating unit by buoyancy. Abearing assembly is rotatably attached to the central column and at least one first strut connects the rigid peripheral power generating unit with the bearing assembly.

According to one aspect of the invention, the rigid peripheral power generating unit ofthe VAWT is provided with at least one hydrofoil assembly. By introducing a hydrofoilassembly the rigid peripheral power generating unit may be supported, i.e. balancingdownward forces being gravitation or forces from the wind during operation, by a staticforce provided by the buoyancy of the peripheral buoyancy element and a dynamicforce of the hydrofoil assembly. The hydrofoil assembly may comprise adjusting meansadapted for adjusting the upward or downward force exerted by the hydrofoil assemblyduring operation. One way of adjusting the upward or downward force comprisesadjusting the angle of attack of at least one hydrofoil wing by means of a mechanicalrod connected between a surface-sensing ﬂoating body and the hydrofoil assembly.Altematively the hydrofoil assembly comprises at least one hydrofoil wing, anelectronic depth sensor and an electric or hydraulic actuator, wherein the angle of attackof the hydrofoil wing is adjustable by the electric or hydraulic actuator and the electronic depth sensor is in communication with the electric or hydraulic actuator.

According to one aspect of the invention the VAWT, the first strut is provided with at least one hinge joint so that the rigid peripheral power generating unit is connected with the bearing assembly via the at least one hinge joint in so that the rigid peripheral powergenerating unit and the central column may have varying relative inclination. Analternative hinge arrangement comprises a first hinge joint provided between thebearing assembly of the central column and the first strut and a second hinge joint is provided between the rigid peripheral power generating unit and the first strut.

According to one aspect of the invention the VAWT comprises a lower strut providedbelow the first strut, the lower strut supporting the rigid peripheral power generatingunit. The lower strut may be arranged to be at least partly below the sea surface during operation. A hydro foil assembly may be provided on the lower strut.

According to one aspect of the invention the VAWT comprises a plurality of rigidperipheral power generating units and at least one connecting member and wherein thelower part of each blade is connected to the blade of another rigid peripheral power generating unit.

One advantage afforded by the present invention, is that the blades are supported bybuoyancy at the perimeter and can mounted to a central hub by hinge joints allowing achange of inclination between the blade and the central column. The structure will thusnot have to be dimensioned for bending moments imposed by change in draft ofstructural elements located on the perimeter compared to structural elements located in the center of rotation of the rotor.

A further advantage is that the largest component is the rigid peripheral powergenerating unit which will be a very light structure in comparison with the rim or themonorail of the prior art and thus will be easy to move around in a fabrication yard or todeploy from keyside by commonly available mobile cranes. The weight of the strut andblade sections will be much less than for example the weight of the nacelle inconventional horizontal axis wind turbines of a comparable size which has a weight of typically between 350 and 450 tonnes.

In the present invention, there is no transfer of wind force or moment from theperimeter to a central shaft. Thereby the risk of torsional oscillation will not requirestiffness-dimensioning of the struts and the attachments of the struts to the rotating part of the central column or to the blades.

Yet a further advantage of the present invention is that the central semisubmersiblecolumn is anchored to the seabed by mooring lines attached to the geo-stationary part ofthe central column. The electrical cable is fixed to the geo-stationary part of the centralcolumn and a standard commercially available electrical swivel is arranged inside thecolumn at the location of the interface between the rotating hub, to which the struts areattached, and the geo-stationary structural parts of the central column. In this way, theelectrical swivel can also be arranged in an electrical room inside the column where it is protected from the weather and also being located above mean sea level.

Brief Description of the drawings A more complete understanding of the above mentioned and other features andadvantages of the present invention will be apparent from the following detaileddescription of preferred embodiments in conjunction with the appended drawings, wherein: Figures l a-c are schematic illustrations of the ﬂoating vertical axis wind turbine aaccording to the invention, in a slightly elevated side view (a) and a top view (b) of one embodiment and a side view (c) of another embodiment; Figures 2 a-b are schematic illustrations of the details of the ﬂoating vertical axis wind turbine according to an embodiment of the invention; Figure 3 is a schematic illustration of the ﬂoating vertical axis wind turbine according to the invention during operation; Figures 4 a-j are schematic illustrations of the different embodiments of the strut and blade arrangement of the according to embodiments of the invention; and Figures 5 a-b are schematic illustrations of the different embodiments of the invention.

Detailed description The ﬂoating Vertical axis wind turbine 1, VAWT, according to the invention isschematically illustrated in figures 1 a-c, wherein la is a slightly elevated side view, lbis a view from the top and 1c is an altemative embodiment. The invention is a ﬂoatingVAWT 1. A 5 MW version of such a VAWT may be typically 110 m to 130 m indiameter and having a total height above mean sea level of about 120 m to 150 m,although the invention is not limited to these, or any other size ranges given herein.According to one embodiment of the invention, schematically illustrated in figures 1a-b,a ﬂoating central column 3 is, when the VAWT is installed and during use, moored tothe seabed 12 with a support structure comprising a mooring system 10 connected toone or several anchors 11. For the 5 MW version the central column 3 typically has adiameter of 6 m to 8 m. Different types of mooring systems are known in the art. Atleast one vertical blade 5a is arranged at the perimeter of the turbine. The blade 5a ispart of a rigid peripheral power generating unit 5b also comprising at least oneperipheral buoyancy element 13 and at least one water turbine assembly 8. At least onebut altematively two or more struts 4a, 4b are arranged to connect each rigid peripheralpower generating unit 5b to the central column 3. The strut may be connected to thecentral buoyancy column by a hinge joint 6 allowing both the central column 3 and rigidperipheral power generating unit 5b to change their inclinations. The upper part of thecentral column 3 carrying the hinges 6 is allowed to rotate by a bearing assembly 7forming an integral part of the central column. This could typically be a combination ofa two-bearing assembly such that on a turret bearing assembly used on FloatingProduction Storage and Ofﬂoading vessels, FPSOs, allowing the FPSO to weathervane.Such bearing assemblies are custom-designed and are manufactured on a regular basisfor the oil and gas industry. An interior part or a lower part of the VAWT°s centralcolumn may thus be geo-stationary and connected to a mooring system. The blade 5a ofthe VAWT 1 is extended downwards by the peripheral buoyancy element 13 so that itpenetrates the water surface and float in the body of water and thereby providesbuoyancy to the blade 5a and other parts. The aerodynamically active part of the bladeis located primarily above the lower 4b of the two supporting struts if two struts areused. Below the lower strut 4b, the blade 5a is primarily optimized for structuralstrength to resist wave loads. The section interrnittently being wetted by waves is apart from structural strength optimized for low hydrodynamic drag. The peripheral buoyancy element 13 is in figure la depicted as direct extension downwards from the blade 5a.The peripheral buoyancy element 13 may also be positioned a distance in radialdirection from the blade 5a, for example on the lower strut 4b. A further altemative isthat the peripheral buoyancy element 13 is connected both to the blade 5b and the lowerstrut 4b. The skilled person would have the knowledge to balance the design to give suff1cient buoyancy and rigidity, among other design criteria.

Schematically illustrated in figure 2a is an embodiment wherein the peripheralbuoyancy element 13 comprises one part extending essentially downwards 13b from theblade 5a and one part forrning a stay 13c extending inwards in a radial direction to joinwith the lower strut 4b, in this case the lower strut 4b is rigidly joint to the blade 5a.Altematively no stay 13c is utilized, for example if the lower strut 4b is connected to the blade 5a with a hinge joint.

At or near the lower end of each blade a water turbine assembly 8 is arranged and isconnected to e.g. an electrical generator 8b for production of electricity. The blade 5a,the peripheral buoyancy element 13, and the water turbine 8 forms a rigid peripheralpower generating unit 5b. The water turbine assembly 8 may be connected to theperipheral buoyancy element 13, which would serve a double function as the buoyancyelement and the connector between the blade 5a and the water turbine assembly 8.Altematively the water turbine assembly 8 is connected to the blade 5a and/or the lower strut 4b with a separate support (not shown).

Depicted is a VAWT with three rigid peripheral power generating units 5b, this isadvantageous for certain aspects, which will be further discussed below. However, theVAWT according to the invention may have one, two or more rigid peripheral powergenerating units 5b. The modular design gives an ability to adapt to power production need, economical factors and environmental factors such as tidal currents and waves.

A suitable water turbine assembly 8 is schematically illustrated in figure 2b and may besimilar to a tidal energy high ﬂow speed free-ﬂow turbine and is commercially availablefrom companies such as Schottel Hydro or Tocardo. The turbine may also be ducted(schrouded) to alter the axial ﬂow pattem, in the figure 2 exemplif1ed with duct 8c Anelectrical generator 8d can be directly connected to the water turbine assembly 8.

Typically the water turbine assembly 8 also comprises supports 8d. The arrangement solves the problem of dimensioning the struts and shaft of a VAWT for ESD events orgenerator failure due to that the driving aerodynamic force on the wind turbine blade isresisted directly at the lower end of each blade. There is thus no transfer of thetangential driving force on the wind turbine blade via the struts to a central shaft of theVAWT. This saves material. The rigid peripheral power generating unit 5b does for thesame reason neither need to satisfy the same firrn global structural stiffnessdimensioning criteria as a VAWT that transfers the driving force to a generator on acentral shaft. The problem of structural excitation in torsional mode is thus heavilyreduced. This also saves material. The water turbine assembly 8 will also run at high rotational speed which makes it possible to use a high-speed electrical generator which is much cheaper than a low-speed and high-torque generator mounted on a central shaft.

A hydrofoil assembly 9 may be provided on the lower end of peripheral buoyancyelement 13 as schematically depicted in figures 2a-b and integrated with, or inconnection with, the water turbine assembly 8. Altematively the hydrofoil assembly isprovided on the strut 4a or the lower strut 4b, if more than one strut is utilized, forexample a distance from the perimeter of the VAWT l. A further altemative is toprovide the hydrofoil assembly 9 on the peripheral buoyancy element 13 above orbelow the water turbine assembly 8. The hydrofoil assembly 9 may have two wings asillustrated, but may also be realized as a single plane or even a more complex structure.

The person skilled in hydromechanics will be able to select an appropriate design.

According to one embodiment of the invention the hydrofoil assembly 9 comprisesadjusting means adapted for adjusting the upward or downward force exerted by thehydrofoil assembly 9 when in operation. The adjusting means may be mechanical, forexample the hydrofoil assembly 9 comprising at least one hydrofoil wing which angleof attack is adjustable by means of a mechanical rod connected between a surface-sensing ﬂoating body and the hydrofoil assembly 9. Altematively the hydrofoilassembly 9 an electronic depth sensor and an electric or hydraulic actuator, wherein theangle of attack of the hydrofoil wing is adjustable by means of an actuator for examplean electric, hydraulic or pneumatic actuator. The angle of attack is based on signalscommunicated from the electronic depth sensor is in communication with the actuator.

Further ways of adjusting the upward and downward force from the hydrofoil assembly 11 9, includes, but is not limited to, altering the size of the wings/plane, altering the profile of the wings/plane and fo lding/unfo lding a further wing/plane.

According to one embodiment of the invention the central column 3 is adapted to bemounted on a fixed support structure which reaches to the seabed. Figure lcschematically illustrates an embodiment of the invention comprising a fixed supportstructure in the forrn of a monopile foundation 15 with the central column resting on topof the monopile. Altematively, the fixed support structure includes, but is not limited toa jacket foundation and a tripod foundation. Depending of the nature of the seabed, forexample if solid rock seabed is not available, the fixed support structure may becomplemented with a gravity base foundation. The term ﬂoating vertical axis windturbine, VAWT, does for the purpose of this application, include also theseembodiments, wherein the central column may be supported by a fixed support structureand still major part, the rigid peripheral power generating unit(s), are supported bybuoyancy. For the purpose of manufacturing and transportation, the central column maystill be made as a ﬂo ating member, or possible to equip with ﬂoatation devices,allowing the VAWT to be towed to its final position in the same manner as the embodiments described above.

Figure 3 illustrates the VAWT during operation. The hinge joint 6 in the connectionbetween the rigid peripheral power generating unit 5b and the central column 3 allowseach rigid peripheral power generating unit 5b to rotate around the hinge centre axis ofrotation. This degree of freedom allows adjusting at which draft the water turbine assembly 8 operates. The draft is controlled by the adjustment of the hydrofoil wing.

The invention separately so lves resisting the tipping moment and resisting the usefuldriving aerodynamic tangential force on the blade. The useful tangential aerodynamicforce is resisted by the water turbine assembly 8 rotor disc at or near the lower end ofthe water-penetrating part of the blade. The rotation of the water turbine is resisted bythe torque e. g. of an electrical generator. The normal force on the blade, which is themain contribution to the tipping moment on the turbine, is resisted by buoyancy whenthe turbine is not operating. When the turbine is in operation, the normal force ishandled by the counterforce of an adjustable hydrofoil assembly at or near the water turbine assembly at the lower end of each blade. 12 When the turbine is in operation, the hydrofoil contributes with lifting force on the rigidperipheral power generating unit 5b if adjusted so as to have a positive angle of attack.Assuming a counter-clockwise revolution of the turbine, this is useful when the blade isin the downwind part of its revolution around the central column 3. In this area, thenorrnal force on the blade is directed radially outward and will thus cause a tippingmoment on the rigid peripheral power generating unit 5b around the hinge centre ofrotation according to the right hand rule with the thumb pointing in the tangentialdirection of revolution. The hydrofoil contributes with a downward force on the rigidperipheral power generating unit 5b if adjusted to have a negative angle of attack. Thisis utilized when the blade rotates in the upwind part of its path around the centralcolumn 3. In this area, the norrnal force on the blade is directed radially inward. Thiscauses a reversed and negative moment around the hinge centre of rotation according tothe right hand rule convention. The hydrofoil has an integrated mechanism for adjustingthe hydrofoil angle of attack that can be active or passive. A passive system can bearranged e. g. by a surface-sensing ﬂoating body connected by a stiff wing profiled strutfixed at an angle to the hydrofoil. When the rigid peripheral power generating unit 5band thus the hydrofoil is further submerged under the action of the aerodynamic loads,the ﬂoating surface-sensing body will mechanically and directly adjust the angle ofattack accordingly until the set design depth of water turbine operation is achieved. Anactive system can e.g. be based on the use of an electronic depth-sensor measuring thedistance from the water turbine assembly to the seabed or to the water surface. Thesignal is used e. g. by electric motors to adjust the hydrofoil angle of attack until thedesired depth of operation of the water turbine is achieved. In case of utilizing an active system, this constitutes a method of operating the VAWT according to the invention.

Various embodiments of the invention with regards to the strut(s) 4a, 4b and how thestruts 4a, 4b are joined to the central column 3 and the blade 5a of the rigid peripheral power generating unit 5b are schematically illustrated in figures 4 a-j.

Figures 4 a-b schematically illustrate a VAWT with one, two or more struts connectingcentral column 3, to each blade. The central column 3can rotate, the geo-stationary part of it being an interior part. 13 Figure 4 c schematically illustrates that the central column 3, can be divided in an upperpart that is rotating and a lower part that is geo-stationary and where the strut(s) are mounted on the upper rotating part of the central column 3.

Figure 4 d schematically illustrates that strut(s) can be mounted to the blade with onlyone hinge joint and arranging a second hydrofoil to control the inclination of the bladeor be rigidly attached to the blade, as illustrated in figure 4 e. The strut(s) can bemounted to the central column 3 by hinge joint(s) 6, which can be placed at differentpositions on central column 3, as schematically illustrated in figure 4 e-g or be rigidlyattached to the central column 3. If two struts are used, they can be attached to the central column 3 in the same place, i.e. to one hinge joint.

Altematively, as schematically illustrated in figure 4 h the struts are placed at differentplaces on the central column 3, e.g. that an upper strut is mounted on the upper part ofthe central column 3, and the lower strut is mounted on the lower part of the centralcolumn 3. This configuration requires hinge j oints for both the upper and lower strut 4a,4b at both the central column 3 and at the rigid peripheral power generating unit 5b, and a parallel arrangement of the upper and lower strut 4a, 4b.

Figure 4 schematically illustrates the VAWT l rotating around a mainly vertical axis ofrotation wherein the central column can tilt during operation if strut(s) are attached to it by hinge joints 6.

Figures 4c, e, f, g and i depict the upper strut 4a and lower strut 4b as being joined at thehinge joint 6. Altematively the two struts may be j oined slightly before the hinge joint6. Such minor design Variations are apparent for the skilled person. In a similar manner,as depicted in for example figure 4c by the dashed strut, rigid joints may be provided with support struts or other means to increase the strength of the construction.

The blade(s) do not need to be straight even though this is preferred from a cost offabrication perspective. From an operational efficiency perspective, the blades arepreferably curved, which is schematically illustrated in figure 4 j. From an operationalefficiency perspective, the blades are preferably curved to attain a rotor with a radiusthat increases with height above mean sea level. This is schematically illustrated infigure 4 j. Such a VAWT turbine rotor design enables equal and optimized local TSR, tip speed ratio, in relation to a power law profile. 14 According to one embodiment, schematically illustrated in figure 5 a (perspective) and5 b (top view), the blades of the VAWT are rigidly attached to the struts and the strutsare rigidly attached to the central column. Further, the blades are interconnected by at least one connecting member such as a beam, a steel wire, line or a rod to the adj acent blade.

A cable for transmission of electrical power from the generator can be arranged insidethe water-penetrating part of the blade and through the lower or upper of the blade-supporting struts. An electrical swivel, also referred to as a marine slip ring, is arrangedin the central column to transfer power from the outer, rotating, side to the geo-stationary, inner, side of the bearing assembly. Such electrical swivels are available onthe market for example from Moo g Focal which offer medium voltage marinerenewable swivels up to a Voltage of 36 kV. SBM Offshore is another large provider ofsuch electrical swivels. Such swivels are today in use around the world in the oil andgas industry for example in turrets on FPSO vessels. A turret is a large bearing andswivel arrangement. Such swivels are used for transferring for example power, sensorsignals and ﬂuids between a rotating and a stationary side. In on-shore applications sliprings are common on e. g. gantry cranes in synchronous electrical generators for turbo or hydropower applications.

An electrical step-up transforrner can be arranged inside the column, either between theelectrical swivel and the connection to the export cable if a low-voltage swivel is used or between the generators and electrical swivel if a medium-voltage swivel is used.

A dynamic electrical export cable is arranged inside the column, is made to exit at thelower end of the column and is led in a suitable manner to the seabed where is cantranscend to or be j oined to a static export cable, whichever is the most practical andeconomical. Such dynamic cables are available on the market and are provided e.g. by companies such as JDR, Prysmian or NKT Cables.

A suitable mooring system is arranged to provide station-keeping for the ﬂoatingVAWT. The mooring system is chosen according to what is most suitable for the depth,bathymetry and soil conditions at the site of installation. Since the useful aerodynamicforce is directly used for production of electricity at the lower end of each blade, the general problem of ﬂo ating VAWTs that the mooring system need to resist the turbine torque is Solved. The mooring system of this invention only needs to provide station- keeping.

The invention so lves the stability problem requirement through that the central columnhas positive buoyancy and supports its own weight and the weight of the rigidperipheral power generating unit 5b resting its inner end on the hinge on the rotatingupper part of the column. The rigid peripheral power generating unit 5b is self-supporting by its own own buoyancy. The ﬂoating VAWT can thereby be ﬂoated outdirectly from a fabrication yard on shore and through shallow harbours as opposed tospar buoy type foundations which have very large draft. The hinge arrangement and theintegral buoyancy support of the surface-penetrating blade sections solve the problem ofglobal stiffness dimensioning without having to use large, heavy and material-intensivebeam sections connecting the buoyancy columns of e. g. a semisubmersible type ﬂoating foundation.

The invention also solves the problem of large component replacement. Since eachblade is self-supported by buoyancy at the perimeter and the bearing assembly is restingon the central co lumn, a crane vessel can, thanks to the arrangement with a hinge at thecentral attachment, lift the blade section out of the water to exchange or perform serviceor maintenance on the water turbine, hydrofoil assembly or the generator. The blade canbe lifted and the lower end can then conveniently be set down on e. g. a ﬂoating barge for safe and easy access while performing maintenance at sea.

The central column can be fabricated e.g. in mild steel as a conventionalsemisubmersible buoyancy column. The struts, blades, water surface-penetratinghydrodynamical struts as well as the hydrofoil wings and the generator enclosure can befabricated in composite plastic, e. g. in glass-f1bre reinforced plastic with reinforcementsin carbon f1bre or e.g. aramid (Kevlar). The water-penetrating peripheral buoyancyelement, hydrofoil wing, as well as the water turbine assembly are due to theirsubmersion in water not as weight sensitive as the struts and blades so they can befabricated e. g. in mild steel and corrosion protected by paint. Submersed parts couldalso be fabricated in stainless steel or therrnoplastic material which would be benef1cial from a corrosion protection perspective. 16 Terminology and acronyms Column Cylindrically or rectangularly shaped vertical construction elementproviding buoyancy Draft Alt. Draught, submerged distance of structure below sea level ESD Emergency Shut-Down, a term defined in the IEC standard for design ofwind turbines.

FAWT Floating Axis Wind Turbine FPSO Floating Production Storage and Ofﬂoading vessel (is ship-shaped andweathervaning).

HAWT Horisontal Axis Wind Turbine Nacelle The housing with machinery (main shaft, gearbox, generator) at top of aHAWT tower RNA Rotor Nacelle Assembly Rotor The blades and the central hub of a HAWT Semisub Semisubmersible, a class of partly submerged ﬂo ating foundationsutilizing distributed buoyancy co lumns with connection members betweenthem Spar buoy A cylinder-shaped, elongated deep-draft ﬂoating foundation with smalldiameter Strut Blade-supporting construction element for a Vertical Axis Wind Turbine Tether Typically a tensioned mooring line TLP Tension-Leg Platform, a class of ﬂoating foundations stabilized through aset of vertical tensioned mooring lines to e.g. a gravity foundation typefoundation on the seabed TSR Tip Speed Ratio, ratio of blade tangential speed divided by the free windspeed.

Turret Rotating bearing assembly with rotating transfer of electrical power and/orﬂuids VAWT Vertical Axis Wind Turbine Elements in ﬁgures: 1)2)3)4) 6) Floating Vertical Axis Wind TurbineSea levelCentral columnStruts a. Strut b. Lower strutPeripheral parts a. Blade b. Rigid peripheral power generating unitHinge joint 17 7) Bearing assembly8) Water turbine assemblya. Water turbineb. Electrical generatorc. Ductingd. Supports9) Hydrofoil assemblyl0) Mooring linesl l) Anchor12) Seabed13) Peripheral buoyancy elementl4) Connecting member 18

Claims

1. Claims . A ﬂoating Vertical axis wind turbine (1) comprising a central column (3) connected to a support structure (10, 11; 15), the ﬂoating vertical axis windturbine (1), characterized by -at least one rigid peripheral power generating unit (5b) comprising at least oneblade (5 a), at least one peripheral buoyancy element (13) and at least one waterturbine assembly (8), the water turbine assembly (8) adapted to be below a watersurface and the peripheral buoyancy element (13), wherein the peripheralbuoyancy element (13) at least partly supports the rigid peripheral powergenerating unit (5) by buoyancy; - a bearing assembly (7) rotatably attached to the central column (3); -at least one first strut (4a) connecting the rigid peripheral power generating unit (5) with the bearing assembly (7). The ﬂoating Vertical axis wind turbine (1) according to claim 1, wherein at leastone hydrofoil assembly (9) is provided in connection to the rigid peripheral power generating unit (5b). The ﬂoating Vertical axis wind turbine (1) according to claim 2, wherein thehydrofoil assembly (9) comprises adjusting means adapted for adjusting theupward or downward force exerted by the hydrofoil assembly (9) during operation. The ﬂoating Vertical axis wind turbine (1) according to claim 3, wherein thehydrofoil assembly (9) comprises at least one hydrofoil wing which angle ofattack is adjustable by means of a mechanical rod connected between a surface- sensing ﬂoating body and the hydrofoil assembly (9). The ﬂoating Vertical axis wind turbine (1) according to claim 3, wherein thehydrofoil assembly (9) comprises at least one hydrofoil wing, an electronicdepth sensor and an actuator, wherein the angle of attack of the hydrofoil wing is adjustable by the electric or hydraulic actuator and the electronic depth sensor is 19 10. 11 12. in communication with the electric or hydraulic actuator. The ﬂoating vertical axis wind turbine (1) according to any of claims 1 to 5,wherein the ﬁrst strut (4a) is provided with at least one hinge joint (6) so that therigid peripheral power generating unit (5b) is connected with the bearingassembly (7) via the at least one hinge joint (6) in so that the rigid peripheralpower generating unit (5b) and the central column (3) may have varying relative inclination. . The ﬂoating vertical axis wind turbine (1) according to claim 6, wherein a first hinge joint (6) is provided between the bearing assembly (7) of the centralcolumn (3) and the first strut (4a) and a second hinge joint (6) is providedbetween the rigid peripheral power generating unit (5b) and the first strut (4a). The ﬂoating vertical axis wind turbine (1) according to any of claims 1 to 6,further comprising a lower strut (4a) provided below the first strut (4a), the lower strut (4b) supporting the rigid peripheral power generating unit (5b). The ﬂoating vertical axis wind turbine (1) according to claim 8, wherein thelower strut (4b) is arranged to be at least partly below the sea surface during operation. The ﬂoating vertical axis wind turbine (1) according to claim 9, wherein the hydrofoil assembly (9) is provided on the lower strut (4b). . The ﬂoating vertical axis wind turbine (1) according to any of the preceding claims, further comprising a plurality of rigid peripheral power generating units(5b) wherein the lower part of each blade (5a) of each rigid peripheral powergenerating unit (5b) is connected to the blade (5 a) of another rigid peripheral power generating unit (5b) by a connecting member (14). The ﬂoating vertical axis wind turbine (1) according to any of the preceding claims, wherein the water turbine assembly is an axial ﬂow upstream turbine, an 13. 14. 15. 16. 17. axial ﬂow downstream turbine or a cross ﬂow turbine. The ﬂoating Vertical axis wind turbine (1) according to any of the precedingclaims, further comprising an electrical cable arranged inside at least one first(4a) or lower strut (4b) for transmitting power from the water turbine assembly (8) to the central column (3). The ﬂoating Vertical axis wind turbine (1) according to any of the precedingclaims, wherein the rigid peripheral power generating unit (5b) is connected to the central column only by one first strut (4a). The ﬂoating Vertical axis wind turbine (1) according to any of the precedingclaims, wherein the central column (3) is rotating along with the first strut (4a)and rigid peripheral power generating unit (y) and the interior of the central column (3) is geo-stationary.The ﬂoating Vertical axis wind turbine (1) according to any of the precedingclaims, wherein the central column (3) is a buoyancy column and thereby the ﬂoating Vertical axis wind turbine (1) is completely ﬂoating. The ﬂoating Vertical axis wind turbine (1) according to claim 16, wherein the support structure is a mooring system (10). 21