SE545893C2 - Mooring system for a floating platform - Google Patents

Abstract

A mooring system (18) for a floating platform, comprising a plurality of buoys (19) anchored by anchor lines (24) at sea. The mooring system (18) comprises a plurality of mooring unit (27a, 27b) each comprising a plurality of mooring buoys (19a, 19b, 19c, 19d), and each mooring unit being anchored by an anchor system (30) consisting of four anchor lines (24a, 24b, 24c, 24d) each connected to an anchor (26).

Description

Mooring system November 2,TECHNICAL FIELD The present invention concerns a floating wind power plant and a system for anchoring a platform to the sea bed. Especially the invention concerns a mooring system comprising a plurality of anchors for mooring a wind power platform. ln particular the invention concerns a mooring system for a semi-submerged platform.

BACKGROUND OF THE lNVENTlON A mooring system in deep sea comprises an anchor system containing a plurality of anchors. The anchors are organized in special patterns to make the moored object in a desired geographic position at all wind and weather conditions. The anchors may be traditionally digging anchors, gravity anchors or suction anchors. The anchor lines may comprise catenary lines or taut leg lines.

A floating semi-submerged wind power plant comprises a structural platform containing a plurality of buoyancy element. The platform carries a single or a plurality of rotor towers. The buoyancy elements comprise an inner cavity into which water is pumped to immerse the platform. ln its operational state the plant is immersed to a level where only the buoyancy elements penetrate the water stirface. Thus in its operational state only the tower and the top of the buoyancy elements can be seen above seaçstrrfaceffwßx plant havingva tower with a nacelle for turning the rotor tovvtards the wind the platform needs to be stably oriented by the mooring system. A great plurality of different types of semi- submersible platforms for generating wind power have been suggested. l\/lost of them have one single turbine on top of a tower.

From US2019024635 (Siegfriedsen) an offshore wind farm with a plurality of foundation elements is previously known. The foundation elements are arranged so as to form the corners of a plurality of parquetted hexagons and with a plurality of floating offshore wind turbines. Each floating offshore wind turbine within ahexagon is connected to the foundation elements which form the hexagon. The floating offshore wind turbines are connected to the foundation elements by means of connection means designed as a chain and/or a cable or a combination of a chain and a cable. The connection means have a length which allows the offshore wind turbines to drift within a circular area with a radius of up to 10% of the hexagon circumradii about the respective hexagon centre.

From .JP 2004176626 (Takada) an offshore wind power generation facility is preciously known. The object is to provide an offshore wind power generation facility which can be placed on the ocean easily even in a deep sea area, which keeps respective floating bodies moderately apart from one another even under severe weather and ocean conditions, and which prevents the power generating capacity from being reduced when the relative positional relationship between wind power generation equipment is changed. The facility comprises a plurality of floating bodies which respectively support the wind power generation equipment or supports control equipment. The floating bodies are connected to one another by means of mooring chains each having an intermediate sinker in the middle of the chain. The floating bodies positioned at the outermost location are further connected to mooring anchors at their one ends by means of mooring chains each having an intermediate sinker in the middle of the chain. The floating bodies and the mooring anchors are arranged to connect a plurality of element structures of plan view equilateral triangles.

From US 8471396 (Roddierfi a column-stabilized offshore platform with water- entrapment plates and asï/minfïtric mo:oring systern for support of a single wind . turbine is previously known. The floating wind turbine platform includes a floatation frame that includes three columns vvhich are coupled to each other with horizontal main beams. A wind turbine tower is mounted above a tower support column to simplify the system construction and improve the structural strength. The turbine blades are coupled to a nacelle that rotates on top of the tower. The floatation frame includes a water ballasting system that pumps water between the columns to keep the tower in a vertical alignment regardless of the wind speed. Water- entrapment plates are mounted to the bottoms of the columns to minimize the rotational movement of the floatation frame due to waves. The platform is connected to seabed by anchor lines from each column. rltl 13,; ._ p).

From EP 278984881 (Komatsu) is previously known a floating body wind power generating device and method for mooring the floating body wind power generation device. An objective of the wind power generation device is to provide a floating body wind power generating device with which it is possible to moor the floating body stably with respect to drift force or rotationai moment acting on the floating body. The wind power generating device comprises a wind power generator and a floating body. Further the device comprises a first column which is located on the upwind side of a primary wind direction and whereupon the wind power generator is installed. A second column and a third column which are located on the further downwind side of the primary wind direction than the first column is connected to the first column with two rower hulls to the first column. A plurality of mooring cables connects the floating body to anchors. At least two of the plurality of mooring cables are connected to the first column. At least one of the plurality of mooring cables is respectively connected to the second column and the third column. Each of the plurality of mooring cables is positioned extending in radiating directions from the floating body so as not to intersect in plan view.

The long-term accuracy of positioning the wind power units is important. lf drag anchors are used, the accuracy is not sufficient. The anchoring lines are pulled until the anchor catch sufficient grip to the sea bed. Hence the seabed conditions may cause the anchor and thereby the platform to change its position hundreds of meters from a desired position. Therefore, position reliability requires the use of suction anchors and pre-stressed mooring lines.

One of the main advantages of floating-platforntis that it may be fully fabricated at a shipyard and thus reduce the installation cost. lt is then transported to a desired location and permanently moored to an anchor system. lt is, however, desirable to be able to tug the platform back to the shipyard for major repair and maintenance. lt is therefore preferable to avoid installing a mooring system each time. Already when planning and deploying a wind farm with a plurality of floating platforms, it is desirable to first establish an infrastructure of electric cabling and a mooring system. 'lt rigWhen several buoys or other floating elements are used to make the platform stable, it is associated with considerable cost to install the mooring system each time the platform need to be repaired and return to shipyard. Thus, there has been a Iongtime need within the floating wind power industry to provide a permanent anchoring system and yet providing an easy facility to moor the wind power plant.

Since the water depth is limited at a shipyard and the optimal design of a semi- submersible platform requires deeper water, there is also a need to provide a platform capable of easily changing the draft of the platform.

SUMMARY OF THE INVENTION A primary objective of the present invention is to seek ways to improve a floating wind power plant including the mooring of a floating wind power platform. A second objective of the invention is to provide a permanent mooring system and a method of mooring the platform.

These objects are achieved according to the invention by a mooring system characterized by the features in the independent claim 1 or by a mooring facility characterized by the features in the independent claim 6, or by a method characterized by the steps in the independent claim 8 and 9, or by a wind power plant characterized by he features in the independent claim 13. Preferred embodiments are described in the dependent claims.

According to the irtverttion. the inoorirwg system comprises of a parr of t-wfin niooring buoys anchored at sea. The twin buoys are seruarated from each other by a distance vvire to keep a desired distance between them. The distance between the twin buoys is preferably the sarne as the fjistance between two adjacent arms of a floating wind power plant. The twin buoys are anchored by four anchors to receive a geographic stable position and orientation. Preferably suction anchors and prestressed anchoring lines are used to anchor the buoys. The anchors are spread equally on the bottom in a square pattern. By providing a mooring system with only two distinctive mooring points any floating platform may be moored in a ijlsecure way to receive a desired position and a desired orientation. The twin buoy concept may be appiicable to any self-floating and stabilised platforms.

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

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

According to an embodiment of the invention a second facility of platforms are organised diagonally to the first facility of platforms. The position of the second such platform will be just on top of an already existing anchor. Hence by adding the second set of platforms which all uses already existing anchors the efficiency of the use of anchors may be drastically improved. For a wind power plant comprising a vast number of both facilities of anchored platforms each anchor receives anchor lines from eight platforms. The limit of the number of anchors needed of such a facility tends to one anchor for every pair of platforms. ln an embodiment of the invention the mooring system comprises a docking means involving the twin buoys and two of the secondary floats. Each buoy comprises an upper hollow body and a lower hollow body sharing a common water ballast container. The upper body comprises an elongated hollow structure having a small cross-sectional area. A small cross-sectional area impedes the movement of the buoy in the sea. The lower body is preferable a wider structure than the upper body. Each tvvin buoy also comprises a pump facility to pump water into and outfrom the ballast contaiilïlr. Bl: pumpingjttfatei* into the lsuoyf the ballast increases and the buoy linrnerse in the sea and vice versa. ln an embodiment the docking means comprises a first part containing the twin buoys and a second part containing a pair of secondary floats connected to the platform to be docked. The docking is performed by a relative movement in the vertical plane of the parts of the docking means. According to the invention a method of docking comprises immersing a twin buoy, orienting the secondary float on top of the twin buoy, raising the twin buoy by emptying the ballast water to interact with the float and lock the two parts together. fx) 'Jï .. platform comprises a lightweight cottslriictiob consistiitg of two kinds of buildingWhen the first docking means has docked the platform is rotated by tugs so that the second docking means are aligned. Then the second docking is made the same way as the first docking. When fully docked the platform is immersed to its semi-submersed operational position. The advantage of letting the upper buoy stay above water is that wave forces and motions are avoided to better facilitate the docking.

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

Platform A suitable platform to be moored by the mooring system comprises a tower and a plurality of stabilizing arms, Each stabilizing arm of the floating wind power elements only. The first kind is a tensile resisting element designed to resist compression in its longitudinal direction. The second kind is a pressure resisting element designed to resist pressure forces in its longitudinal direction. Examples of the first kind are strut, brace, stick, beam, framevvork construction etc. ln the text hereinafter a strut element is denoted a beam. Examples of the second kind are wire, rope, rod etc. ln the text hereinafter a tensile element is denoted a wire.

By the use of such lightweight elements triangular constructions may be designed where part of the tower comprises one of the triangle legs. The other legs are a beam and a wire. Such constructions are capable of withstanding severe forces in 'll _»the plane of the triangle. By the use of two such triangles where one leg is a beam common to both triangles great stability is achieved and great forces may be withstood. Besides the lightvveight elements may be used to build big constructions yet stable enough to withstand big forces. ln an embodiment according to the invention a stabilizing arm comprises a beam and a wire. These two elements form together with part of the tower a triangle. Since one side of this triangle is a wire an outer force is needed to stretch the wire part of the triangle. This outer force is obtained by balancing the buoyancy of the buoy and the tower. ln an embodiment the beam is connected between the buoy and a main position of the tower. The wire is connected between the buoy and a first position of the tower. ln an embodiment the first position is located under the main position. ln this embodiment the buoyancy of the buoy must be greater than the buoyancy of the tower. ln an embodiment the first position is located above the main position. ln this embodiment the buoyancy of the tower must carry the whole platform and the buoyancy of the buoy only be used for balancing purposes. ln an embodiment according to the invention a stabilizing arm consists of a beam and a first and second wires positioned on opposite sides of the beam. The beam is connected between the buoy and a main position of the tower. The first wire is connected between the buoy and a first position of the tower positioned below the main position. The second wire is connected between the buoy and a second position of the tower positioned above the main position. The construction may , gresernble a mast on a sailing boat where theinast isystipported by two prestressed stays or shrouds. However in the present ernbodiitwent the mast is aligned horizontally and the wires are prestressed against the tower. ln an embodiment of the invention the beam comprises a framework construction which results in the arm construction being an extremely' lightweight construction. ln an embodiment all connection points comprise two rotational degrees of freedom (ZRDOF). Thus the beam is freely rotatable up and down as well as sideways but cannot rotate around its own axis. This design ensures that only axial forces and no bending forces may be transferred from the beam to the tower.

The second and third connection points also provide ZRDOF. When all threeconnection points are aligned on the same axis the arm construction offers 1RDOF. Thus the arm is very stable in the vertical plane but weak in the horizontal plane. The arm construction may be seen as a hinged door that is stable in one direction and swingable in its transversai direction. The plurality of arms is equally connected to the tower and symmetrically spread around the tower by a connection wire. By this symmetry all pressure forces and all tension forces from the arms are neutralized in the centre of the tower. Due to the pre-stressing of the wires and pre-compression of the beam elements there are no net force on the tower. To keep the arms equally spread all adjacent pair of arm ends are connected with a connection wire. The distance between the second and third connection points on the tower is chosen from a cost-benefit evaluation of stress limitation requirements. Preferably the beam is horizontally aligned.

All three arms are one-dimensionally connected to the tower like hinged doors. This means that the arms can freely rotate around the tower. To prevent the tower from rotation relative to the arms the tower must be locked to one of the arms. ln an embodiment one wire is connected to the tower in a way to lock this relative rotation. This means that one of the stabilizing arms will prevent the tower to rotate in relation to the other arms. The locking means comprises according to an embodiment of the invention a console beam protruding from the tower where one of the upper wires of one of the arms is connected. ln an embodiment a bracket means such as a shackle is used. ln an embodiment the second wire is attach to the tower with a wire span tfvltitfh is transversally attached to each sides of the tower. ln an embodiment of the invention the floatmgrvvirtd power platform comprises a tower and a plurality of stabilizing arrns, The tower contprises a hollow structure carrying a pivotal nacelle and includes a main float at its lower end. ln an embodiment the tower is partly a framework structure. Each arm comprises a secondary float connected to its outer end. The main float is preferably designed to carry the tower and its equipment as well as the generator and rotor. An immersed position of the platform may be achieved by pumping water into the main float. Thereby each secondary float needs only to carry its own weight and part of the stabilizing arm. By pumping water into and from the secondary float the il arm may be balanced to achieve the same tension forces in the upper and the lower wire at a normal operating position of the platform. ln an embodiment the beam comprises a Iattice girder or a framework construction. Although the main task of the beam is to withstand pressure forces it must also withstand forces from the waves. lt is therefore favourable to design the beams with a minimum exposed area, such as a framework construction with moderate diameters of tube elements. Most suitable the beam is made of metal such as steel and protected against oxidation and fouling by a protective paint. ln an embodiment the beam is made of a tubular structure. The wires are suitably made of metal such as steel but may also be made of synthetic fibres. Suitable reinforcement material may be coal fibres, synthetic fibres such as for instance aromatic polyamide, etc. ln an embodiment of the invention the wind power platform comprises a semi- submersible platform. The platform comprises the tower including the main float and three stabilising arms having secondary floats. The secondary float comprises a hollow column of arbitrary cross section. ln an embodiment each secondary float comprises an elongated cylindric body having a small cross section to decrease the movement in the sea. By pumping out ballast water from the floats the platform will float in a high position during transport. This ensures the possibility for the platform to be moored to a quay and transported in shallow water. On the site of operation the platform is clocked to an existing mooring system. By partly filling the floats with ballast water the platform will immerse and take its operation tïlositiort. ln an embodirnertt the beamsJgLthis operatingposition will be .located ftrnder" the sea surface and only the tower and the upperend of the three secondary floats may be seen.

To reach its operation position the platform must immerse in the sea to an operating level. At this operating level the assembled cross-sectional areas of the secondary floats must be minimized in order to lower the heave natural frequency. This is accomplished by filling water into the main float and the secondary floats. The volume of the secondary floats is such that it only needs to be partly filled to reach the operational position. However it needs to be elongated enough in the vertical direction to protrude through the water surface. The function of the ÖF k;secondary float makes use of Archimedes principle. Thus when moving downwards it will experience an upwardly directed force equal to the volume of the displaced water. When moving upwards it will experience a downwardly directed force equal to the volume of the non-displaced water. Since the three arms are symmetrically spread around the tower there will always be an equal amount of stabilizing forces on opposite sides of the tower. Hence when the tower tends to lean caused by wind forces the floats on the leeward side will exert an uprising force and simultaneously the floats on the upwind side will exert a traction force Thus at any given moment the floats on each side of the tower will exert opposite forces resulting in a turning effect which will put the tower in an upright position.

The necessary stabilizing force for keeping the tower in an upright position is thus provided by the length of the beam and the cross-sectional area of the secondary float. Cross-sectional area is the imprint of the secondary float in the sea. A longer arm and a greater cross-sectional area of the floats will increase the uprising forces. A big cross-sectional area will however make the float more affected by the wave motion. Thus a small cross-sectional area is desirable. When decreasing the cross-sectional area of the secondary float the arm needs to be longer. However a longer arm will increase in weight and make the platform heavier. According to the invention a fair compromise is to make the arm approximately as long as the tower is high. ln an embodiment of the invention the float comprises a cylindrical shape. ln an embodiment the float comprises a conical or a funnel shape. ln the latter case the cross-sectional area will increase by the immersion of the float and thus resulting in a noirlineai' increasing fore. Such design will effectively act as dlaminiiwg.

For erection, transport and service of the floating wind power platform one or two of the stabilizing arms may be folded in the horizontal plane to make the platform suitable for docking a quay. lt is a feature of the invention that the tower can be brought very close to the quay which facilitate lifting, mounting and replacement of the heavy tower top and nacelle from land-based services. One connecting wire may be loosened whereby one of the arms can be rotated or folded horizontally to make two arms in 180 degrees with each other and thus permit the tower section of the platform to get close to the quay while still stably floating. lf the quay is short the folded arm may be folded further than 180 degrees. The necessary length ofthe quay will the be equal to the length of an arm. When transported temporary floats and beams may be attached to the platform. ln a first aspect the object is achieved by a mooring system for a floating wind power platform having a plurality of stabilizing arms with secondary floats, the system comprising a plurality of buoys anchored by anchor lines at sea. The mooring system comprises a first pair of twin buoys, a distance wire connecting the pair of twin buoys, and an anchor system consisting of four anchors. ln an embodiment the anchor system comprises a first anchor and a first anchor line to anchor the first twin buoy, and a third anchor and a third anchor line to anchor the second twin buoy. ln an embodiment the anchor system comprises a second anchor and a second anchor line to further anchor the first twin buoy, and a forth anchor and a forth anchor line to further anchor the second twin buoy. ln an embodiment each twin buoy comprises an upper body and a lower body having a common ballast container for adjusting the immerse level of the buoy at sea. ln an embodiment each twin buoy comprises a docking means for docking with arbitrary self floating and stabilizing platform.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the present invention will become more apparent to a person skllled in the art from the following detailed description in coïtjtinctioit tvith the aploended diravringsritvtæfltich; r; ¿~ fig t. is a perspective viev-.f of a inooriitg system. fig 2. is plan view with two embodiments of the mooring system, fig 3. is a plan view of inooriitg facility comprising a large number of mooring systems, fig 4. Is a side view of semi-submersed platform suitable to moor at the mooring system, and fig 5. is a plan view of the semi-submersed platform suitable to moor at the mooring system. ri;DESCRIPTION OF PREFERRED EIVIBODIMENTS A mooring system 18 according to the invention is shown in fig 1. The mooring system comprises a mooring unit 27 including a pair of twin buoys 19 and a distance wire 25. A first buoy 19a and a second buoy 19b are anchored at sea by an anchor system 30 consisting of four anchors 26 in a square pattern. The two buoys are acting as twins since they are permanently held apart by a distance wire 25. The mooring system 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 anchored with the other two anchors. Thus the distance wire is a common anchoring means to both twin buoys. ln order to distribute the forces from the anchor lines the connection to the buoys comprises a span arrangement 23. By use of a common distance wire each of the twin buoys are anchored with three anchor lines. Thus the anchor arrangement will keep both twin buoys at a predeterminated location and orientation. ln the embodiment shown the distance wire 25 and the anchor lines 24 are cut to indicate that they may comprise considerably longer lines at an actual site. The anchor lines may comprise hundreds of meters depending on the sea bed condition and the dept. Preferably suction anchors are used. The distance wire may be in the range of 50 to 100 meters. Each of the twin buoys comprises an upper body 21 and a lower body 22 vi-fhiclw are structurally connected to form a coininon ballast conjpartrttent. By ptirnpingttyater lg or otitíoí the compartment the tvvilt buoy may be adjusted in the sea to keep a precleterinined float position.

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

According to the invention there are two embodiments of anchoring the mooring system. ln fig 2 is shown a first embodiment 18a of a mooring system as described above. Thus the first embodiment of the mooring system comprises the twin buoys 19 and the distance wire 25 arranged as a mooring unit 27. The twin buoys are anchored in parallel with the square pattern of anchors. A first anchor line 24a and a second anchor line 24b are anchoring the first twin buoy at the lefthand side of the figure. A third anchor line 24a and a forth anchor line 24b are anchoring the second twin buoy at the righthand side of the figure. The distance wire 25 is common to both twin buoys and thus comprising the third anchoring means to hold each of the twin buoys firmly anchored, The length of the distance wire is preferably the same as the distance between two adjacent arms of a floating platform.

A second embodiment of the mooring system 18b according to the invention is also shown in fig 2. ln this embodiment the mooring unit 27 comprises a second pair of twin buoys 19c and 19d, each having a distance wire 25 between them and a separating wire 29. The second pair is arranged in parallel with the first pair of twin buoys 19a and 19b. The first and second pair of twin buoys are connected with the separating wire 29. Thus the mooring unit 27 of the second embodiment iSb of the mooring systern according to the invention coinprises two pairs of twin buoys 19 with distance vvires 25 and a separating vvire 29. ln the embodiment .slloyvn theåeparating wire 29 is connected to opposite of twíin btio;-fs..\.f\fitl1 a span. The aim of separating wire is to keep the two pairs of twin buoys in a stable location at all weather conditions. Hens there may be a plurality of possibilities of designing such a connection known to a person skilled in the art.

A vast facility of a large number of mooring systems 18 according to the invention is shown in fig 3. Since every mooring system 18 only needs four anchors 26 the anchors are organized in a square pattern. As shown in the upper part of the fig a first mooring system needs four anchors. An adjacent mooring system anchored nearby needs only two extra anchors since the other two may be shared with the first mooring system. The next three also needs two extra anchors but further systems need only one new anchor. Building up a large facility thus ends up in a limit of one anchor for every mooring system.

Having an anchor pattern like the top view a further set of platforms may be anchored to the already existing anchors. A new set of mooring systems may be rotated diagonally and end up in a pattern like the mid view. Thus when the first set of mooring systems 18a is added to the second set of mooring systems 8b the result is shown in the lower view of fig 3. ln this view every new mooring system is positioned over an existing anchor. The effect of the capability to anchor the second set of platforms results in the limit of one anchor for every two mooring systems.

A suitable wind power platform to be moored to the mooring system is shown in fig 4. ln the embodiment of the invention shown the arm construction consists of a horizontally aligned strut element 7, a first tensile element 9 and a second tensile element 8. ln the embodiment shown the strut element comprises a framework beam and the tensile element comprises a wire. The first wire 9 and the second wire 8 are pre-stressed to achieve a play free arm construction. According to the invention the arm design is very stable in the vertical direction where the stabilizing forces must be transferred. This stability is achieved by just one pressure force resisting element, the beam, and two tension force resisting elements, the wires. ln this embodiment the beam is attached to the tower in a main connection point 10. A secondary float 12 is attached to the outer end 11 of the beam. The first vvire 9 is connected to the outer end 11 of the beam 7 and to -thetoçlftfer inía first connection poinL14 positioned below'fileslnaincçinnecttett point . The second wire is connected to the outer end 11 of the beam 7 and to the tower in a second connection point 13 positioned above the main connection point 10. lnorder to achieve an equal stress in the first and second wire the buoyant force may be balanced between the main float 5 and the three secondary floats 12. ln the semi-submerged state the framework beam and the second wire will be positioned under the water surface A. Only part of the first wire and part of the secondary float will be seen above the water surface. ln the embodiment shown in fig 4 the secondary floats 12 cornprise the elongated body structure. To keep the natural frequency of the platform low the cross- TO Csectional area of the secondary float 12 must be kept small. Thus the centre part of the secondary float 31 comprises an elongated cylinder having a small cross section. ln the embodlment shown the upper end of the secondary float 12 may comprise a funnel shaped body 32. This funnel shaped body exerts a damping effect when moving in sea heave. The lower end of the secondary float comprises a cylindric body 33. This body also exert a damping effect but also a convenient container to fill or pump out water for balancing purposes. The secondary float 12 may be moored to an anchor or a buoy (not shown) by cable 24 which is attached to the float by a span arrangement The three arms 6a, 6b and 6c are aligned symmetrically around the tower 1. According to the invention the arm design is stable in the vertical direction where the stabilizing forces must be transferred. This stability is achieved with one pressure force resisting element, the beam, and a tension force resisting element, the wires. ln order to achieve a sufficient stress in the second wire the buoyant force may be balanced between the main float 5 and the three secondary floats 12. By increasing an air portion in the secondary float the arm will exert a lifting force that will increase the stress of the wire. ln a semi-submerged operating position the platform is immersed by increasing the water balance of the floats. ln the semi-submerged state part of the framework beam and the second wire will be positioned under the water surface A.

According to the plan view in fig 5 two of the secondary floats 12 are moored to a pair of twin buoys 19 associated to a docking system. The dockable buoy . conlpflsesíaåoxvei* lgodïv' 22 vtflth a water ballast cornpartrnetgnd ag upperíbody 21 comprising docking means. The twin buoys are anchored with a plurality of anchor lines 24 connected to the buoy. The twin buoys are held together by a distance wire 25. The secondary float 12 comprises a second docking meanssuitable to mate with the first docking means to for a unity float.

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 embodlment of theinvention the first float and the major part of the arms are positioned under water and only the tower and the three secondary floats break the water surface. By keeping all these protruding structures small in horizontal cross section the whole platform will act calmly in the sea. The framework structure of the strut element reduces slamming forces caused by waves when the strut element temporary is at water surface in heavy sea states.

For transport the platform is raised to a float position by emptying ballast water from the floats. ln a transporting position all floats will be filled with air and the platform will rise to a level indicated by a dashed line B in fig 4. To stabilise the platform during transport secondary beams or secondary floats may be attached to the platform. Being transported to the site of operation the platform can either be anchored in a traditional way or being moored to a set of prepositioned buoys. 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 such connections can be made with a swivel construction well known to a person skilled in the art. ln an embodiment of the invention the three arms are connected with connecting wires 15 that holds the three arms equally spread around the tower. However the tower can still rotate relatively to the arms. ln the embodiment shown the tower is rotationally fixed to one arm. ln the embodiment shown in fig 3 there is a laracket means 16 in the connection point 13 which prevent the tower from rotating. ln an embodiment shown in fig 5 the lower wire of arm Ga is split into a first lower wire Qa and a second lower vvire 9b connected crossways on either side ofthe met float 5. Thetovtfei' rotating preventing arrangeinengmay atso contprtse a span means betweenftlte end of the arm and the first float. The connection wires 15 can be detached and adjusted to facilitate a temporary angular rotation of two arms. ln: an embodiment two adjacent arms would resume a straight line which makes possible the tovtfer coming close to a quay. ln an entbodiment two arms may be folded to form a preferably perpendicular angle with the first arm which will allow a shorter quay. Hence the tower is enabled to approach the quay for secure mooring and maintenance.

By the lightweight construction of the floating wind power platform the construction can be made very big. According to the invention the diameter of the 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 the arm and the tower would almost one. According to the invention the transport 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.

Although favourable the scope of the invention must not be limited by the embodiments presented but contain also embodiments obvious to a person skilled in the art. The anchor lines may comprise any kind of material with good tensile properties. The anchor lines may comprise chains at one or two of its ends. The buoys may comprise stationary or temporary means for pre-tensioning the lines.

Claims (6)

1. Claims Mooring system (18) for a floating wind power platform (6) having a plurality of stabilizing arms (6) with secondary floats(12), the system (18) comprising a plurality of buoys (19) anchored by anchor lines (24) at sea, c h a r a c t e r i z e d in that the mooring system (18) comprises a first pair of twin buoys (19a, 19b), a distance wire (25) connecting the pair of twin buoys, and an anchor system (30) consisting of four anchors (26). l\/looring system according to claim 1, wherein the anchor system (30) comprises a first anchor (26a) and a first anchor line (24a) to anchor the first twin buoy (19a), and a third anchor (26c) and a third anchor line (24c) to anchor the second twin buoy (19b). Mooring system according to claim 1 or 2, wherein the anchor system (30) comprises a second anchor (26b) and a second anchor line (24b) to further anchor the first twin buoy (19a), and a forth anchor (26d) and a forth anchor line (24d) to further anchor the second twin buoy (19b). l\/looring system according to any of the preceding claims, wherein each twin buoy (19) comprises an upper body (21) and a lower body (22) having a common ballast container for adjusting the immerse level of the buoy at sea. l\/looring system according to any of the preceding claims, wherein each twin buoy (19) comprises a docking means (21) for docking with arbitrary self floating and stabilizlng platform, according to any of the preceding claims, c h a r a c t e r i z e d in that the large nuinber of mooring systems (18) consisting of four anchors (26) are anchored adjacentlyf in a column and line pattern of anchors (26), all using adjacent anchors in common, vvhereby the limit of necessary anchors tends to one anchor per platform. lVlooring facility according to claim 6, wherein a large number of second set of mooring systems (18) consisting of four anchors (26) is anchored diagonally to the existing square anchor pattern, whereby most anchors will receive eight anchor lines (24) and the limit of necessary anchors tends to one anchor for every pair of mooring systems. Method for providing a mooring system (18) for a floating wind power platform (6) oomprising a plurality of buoys (19) anchored by anchor lines (24) at sea, c h a r a c t e r i z e d b y providing a mooring unit (27) oomprising a first (19a) and a second (19b) twin buoys connected with a distance wire (25), anchoring the first twin buoy (19a) with a first anchor line (24a) to a first anchor (26a) and a third anchor line (24c) to a third anchor (26c), anchoring the second twin buoy (19a) with a second anchor line (24b) to a second anchor (26b) and a fourth anchor line (24d) to a forth anchor (26d). l\/lethod of docking a floating platform (17) having two secondary floats (12) with a mooring system according to claim 5, c h a r a c t e r i z e d b y transporting the platform in float position (B) to the mooring site, immersing the first twin buoy (19a) of the mooring system by pumping ballast water into a buoyancy compartment (22) of the first twin buoy, positioning a first secondary float (12a) of the platform (17) over the first twin buoy, raising the first twin buoy by pumping out ballast water, thereby mating and locking the first twin buoy (19a) to the secondary float (12a) of the platform (17). Vx/ind power plant oomprising a rnooring system according to any of the claims 1-5, further corrfprising a floating wind power platform (17) having a tower (1) carrying a wind generator housed in a nacelle (2) and a plurality of errnlgfš). the tovvêr,.ír_ornprising a main float (ö) and each artigt) corngrisrng Faq secondary; float (12ïr4to stabilizirfg the tower, c h a r a c t e r i z e d; i n that each arm (6) comprises a first triangular construction consisting of a pressure resisting element (7) a first tensile resisting element (9) and part of thetower (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).