SE2000207A1 - Mooring system - Google Patents

Abstract

A mooring system (18) for a floating wind power platform (6) has a plurality of stabilizing arms (6) with secondary floats (12), the system (18) comprises a plurality of buoys (19) anchored by anchor lines (24) at sea. 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).Figure (1)

Description

Mooring system November 2, 2020 TECHNICAL FIELD The present invention concerns a floating wind power plant and a system foranchoring a platform to the sea bed. Especially the invention concerns a mooringsystem comprising a plurality of anchors for mooring a wind power platform. lnparticular the invention concerns a mooring system for a semi-submergedplatform.

BACKGROUND OF THE lNVENTlON A mooring system in deep sea comprises an anchor system containing a pluralityof anchors. The anchors are organized in special patterns to make the mooredobject in a desired geographic position at all wind and weather conditions. Theanchors 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 platformcontaining a plurality of buoyancy element. The platform carries a single or aplurality of rotor towers. The buoyancy elements comprise an inner cavity intowhich 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 anacelle for turning the rotor tovvtards the wind the platform needs to be stablyoriented 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 offoundation elements is previously known. The foundation elements are arrangedso 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 a 2 hexagon is connected to the foundation elements which form the hexagon. Thefloating offshore wind turbines are connected to the foundation elements bymeans of connection means designed as a chain and/or a cable or a combinationof a chain and a cable. The connection means have a length which allows theoffshore 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 ispreciously known. The object is to provide an offshore wind power generationfacility which can be placed on the ocean easily even in a deep sea area, whichkeeps respective floating bodies moderately apart from one another even undersevere weather and ocean conditions, and which prevents the power generatingcapacity from being reduced when the relative positional relationship betweenwind power generation equipment is changed. The facility comprises a plurality offloating bodies which respectively support the wind power generation equipmentor supports control equipment. The floating bodies are connected to one anotherby means of mooring chains each having an intermediate sinker in the middle ofthe chain. The floating bodies positioned at the outermost location are furtherconnected to mooring anchors at their one ends by means of mooring chains eachhaving an intermediate sinker in the middle of the chain. The floating bodies andthe 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 floatationframe that includes three columns vvhich are coupled to each other with horizontalmain beams. A wind turbine tower is mounted above a tower support column tosimplify the system construction and improve the structural strength. The turbineblades are coupled to a nacelle that rotates on top of the tower. The floatationframe includes a water ballasting system that pumps water between the columnsto 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 therotational 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 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 rotationai 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 is 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 are connected to the first column. At least one ofthe plurality 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 plan view.

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

One of the main advantages of floating-platforntis that it may be fully fabricated ata shipyard and thus reduce the installation cost. lt is then transported to a desiredlocation and permanently moored to an anchor system. lt is, however, desirable tobe 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. Alreadywhen planning and deploying a wind farm with a plurality of floating platforms, it isdesirable to first establish an infrastructure of electric cabling and a mooringsystem. 'lt rig 4 When several buoys or other floating elements are used to make the platformstable, it is associated with considerable cost to install the mooring system eachtime the platform need to be repaired and return to shipyard. Thus, there has beena 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 aplatform 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 floatingwind power plant including the mooring of a floating wind power platform. Asecond objective of the invention is to provide a permanent mooring system and amethod of mooring the platform.

These objects are achieved according to the invention by a mooring systemcharacterized by the features in the independent claim 1 or by a mooring facilitycharacterized by the features in the independent claim 6, or by a methodcharacterized by the steps in the independent claim 8 and 9, or by a wind powerplant 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 niooringbuoys anchored at sea. The twin buoys are seruarated from each other by adistance vvire to keep a desired distance between them. The distance between thetwin buoys is preferably the sarne as the fjistance between two adjacent arms of afloating wind power plant. The twin buoys are anchored by four anchors to receivea geographic stable position and orientation. Preferably suction anchors andprestressed anchoring lines are used to anchor the buoys. The anchors arespread 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 ijl secure way to receive a desired position and a desired orientation. The twin buoyconcept may be appiicable 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 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 diagonallyopposite 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 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 secondplatform 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 illl 6 needs 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 upper body comprises an elongated hollow structurehaving 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 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 amethod of docking comprises immersing a twin buoy, orienting the secondary floaton top of the twin buoy, raising the twin buoy by emptying the ballast water tointeract with the float and lock the two parts together. fx)'Jï .. platform comprises a lightweight cottslriictiob consistiitg of two kinds of building 7 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 isrelatively 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 resistcompression in its longitudinal direction. The second kind is a pressure resistingelement designed to resist pressure forces in its longitudinal direction. Examplesof the first kind are strut, brace, stick, beam, framevvork construction etc. ln thetext 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 designedwhere 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 75. _» 8 the plane of the triangle. By the use of two such triangles where one leg is a beamcommon to both triangles great stability is achieved and great forces may bewithstood. Besides the lightvveight elements may be used to build bigconstructions yet stable enough to withstand big forces. ln an embodiment according to the invention a stabilizing 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 buoyand 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 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 ofthe 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 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 onlyaxial 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 three 9 connection points are aligned on the same axis the arm construction offers1RDOF. Thus the arm is very stable in the vertical plane but weak in the horizontalplane. The arm construction may be seen as a hinged door that is stable in onedirection and swingable in its transversai direction. The plurality of arms is equallyconnected to the tower and symmetrically spread around the tower by aconnection wire. By this symmetry all pressure forces and all tension forces fromthe arms are neutralized in the centre of the tower. Due to the pre-stressing of thewires and pre-compression of the beam elements there are no net force on thetower. To keep the arms equally spread all adjacent pair of arm ends areconnected with a connection wire. The distance between the second and thirdconnection 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 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 tfvltitfh is transversally attached to each sides of the tower. ln an embodiment of the invention the floatmgrvvirtd power platform comprises atower and a plurality of stabilizing arrns, The tower contprises 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. 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 thelower wire at a normal operating position of the platform. ln an embodiment the beam comprises a Iattice girder or a frameworkconstruction. 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 suitablymade of metal such as steel but may also be made of synthetic fibres. Suitablereinforcement 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 clocked to an existing mooring system. By partlyfilling the floats with ballast water the platform will immerse and take its operationtïlositiort. ln an embodirnertt the beamsJgLthis operatingposition will be .locatedftrnder" 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 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 water surface. The function of the ÖF k; 11 secondary float makes use of Archimedes principle. Thus when movingdownwards it will experience an upwardly directed force equal to the volume of thedisplaced water. When moving upwards it will experience a downwardly directedforce equal to the volume of the non-displaced water. Since the three arms aresymmetrically spread around the tower there will 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.

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 noirlineai' increasing fore. Such design will effectively act as dlaminiiwg.

For erection, transport and service of the floating wind power platform one or twoof 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 wiremay 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 sectionof 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 12 the quay will the be equal to the length of an arm. When transported temporaryfloats and beams may be attached to the platform. ln a first aspect the object is achieved by a mooring system for a floating windpower platform having a plurality of stabilizing arms with secondary floats, thesystem comprising a plurality of buoys anchored by anchor lines at sea. Themooring 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 anchorline to anchor the first twin buoy, and a third anchor and a third anchor line toanchor the second twin buoy. ln an embodiment the anchor system comprises asecond 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 bodyhaving a common ballast container for adjusting the immerse level of the buoy atsea. ln an embodiment each twin buoy comprises a docking means for dockingwith arbitrary self floating and stabilizing platform.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the present invention will become moreapparent 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 ofmooring systems, fig 4. Is a side view of semi-submersed platform suitable to moor at themooring system, and fig 5. is a plan view of the semi-submersed platform suitable to moor at the mooring system. ri; 13 DESCRIPTION OF PREFERRED EIVIBODIMENTS A mooring system 18 according to the invention is shown in fig 1. The mooringsystem 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 twobuoys are acting as twins since they are permanently held apart by a distance wire25. The mooring system is anchored with four anchor lines 24. Each twin buoy isanchored with two anchor lines. By the distance wire 25 each twin buoy is alsoanchored 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 vi-fhiclw are structurally connected to form acoininon 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 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 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\JÉl 14 may 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 mooringsystem. ln fig 2 is shown a first embodiment 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 areanchoring the second twin buoy at the righthand side of the figure. The distancewire 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 2. ln this embodiment the mooring unit 27 comprises 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 embodimentiSb 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 astable 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 inventionis shown in fig 3. 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 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 alimit 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 3. 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.

A suitable wind power platform to be moored to the mooring system is shown in fig4. ln the embodiment of the invention shown the arm construction consists of ahorizontally aligned strut element 7, a first tensile element 9 and a second tensileelement 8. ln the embodiment shown the strut element comprises a frameworkbeam and the tensile element comprises a wire. The first wire 9 and the secondwire 8 are pre-stressed to achieve a play free arm construction. According to theinvention the arm design is very stable in the vertical direction where thestabilizing forces must be transferred. This stability is achieved by just onepressure 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 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 thetower in a second connection point 13 positioned above the main connection point10. lnorder 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. ln the embodiment shown in fig 4 the secondary floats 12 cornprise the elongatedbody structure. To keep the natural frequency of the platform low the cross- TOC71 16 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 embodlment 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. The secondary float 12may be moored to an anchor or a buoy (not shown) by cable 24 which is attachedto 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 stabilizing 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.

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 ofanchor lines 24 connected to the buoy. The twin 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.

The bigger a structure the more exposed is the structure for wave forces. Thusminimizing the exposure surface of the structure in the region were waves occurwould be good design practice for a floating wind power plant. A submergedplatform where only necessary parts penetrate the water surface is therefore beneficial to reduce slamming forces caused by waves. ln an embodlment of the 17 invention the first float and the major part of the arms are positioned under waterand only the tower and the three secondary floats break the water surface. Bykeeping all these protruding structures small in horizontal cross section the wholeplatform will act calmly in the sea. The framework structure of the strut elementreduces slamming forces caused by waves when the strut element temporary is atwater 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 4. 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 eitherbe 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 suchconnections can be made with a swivel construction well known to a person skilledin the art. ln an embodiment of the invention the three arms are connected withconnecting 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 shownthe tower is rotationally fixed to one arm. ln the embodiment shown in fig 3 there isa laracket means 16 in the connection point 13 which prevent the tower fromrotating. ln an embodiment shown in fig 5 the lower wire of arm Ga is split into afirst lower wire Qa and a second lower vvire 9b connected crossways on either sideofthe met float 5. Thetovtfei' rotating preventing arrangeinengmay atso contprtse aspan means betweenftlte 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 two adjacent arms would resume a straight line whichmakes possible the tovtfer coming close to a quay. ln an entbodiment 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 securemooring 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 propeller 18 may be 150 m. The total height of the tower including the first float may be 130 m.The length of the arm may be in the range of 100-130 m. Hence the ratio betweenthe arm and the tower would almost one. According to the invention the transportposition of the platform is about 19 m higher that the submerged position. Thedraught of the platform under transport may be less than 9 meters.

Although favourable the scope of the invention must not be limited by theembodiments presented but contain also embodiments obvious to a person skilledin the art. The anchor lines may comprise any kind of material with good tensileproperties. 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.

1. CLAIMS looring system (18) for a floating platform, comprising a plurality of buoys(19) anchored by anchor lines (24) at sea, c h a r a c t e r i ze d in thatthe mooring system (18) comprises a plurality of mooring unit (27a, 27b)each comprising a plurality of mooring buoys (19a, 19b, 19c, 19d), and eachmooring unit being anchored by an anchor system (30) consisting of fouranchor lines (24a, 24b, 24c, 24d) each connected to an anchor (26). Mooring system according to claim 1, wherein a first mooring unit (27a)consists of a first mooring buoy (19a), a second mooring buoy (19b), and afirst distance wire (25a) connecting the first and second mooring buoys, thefirst and second mooring buoy forming with the first distance wire a first pair of mooring buoys. Mooring system according to claim 2, wherein a second mooring unit (27b)further consists of a third mooring buoy (19c), a forth mooring buoy (19d)and a second distance wire (25b) connecting the pair of buoys, the third andforth mooring buoy forming with the second distance wire a second pair ofmooring buoys, and wherein the second mooring unit further comprises aseparating wire (29) connecting the first and second pair of mooring buoys. Mooring system according to any of the preceding claims, wherein eachbuoy (19) comprises an upper body (21) and a lower body (22) having acommon ballast compartment fillable with water or air for adjusting theimmersed level of the buoy at sea. Mooring system according to any of the preceding claims, wherein eachbuoy (19) comprises a docking means (21 ) arranged to hook or mate with adockable floating object (28). Mooring facility comprising a large number of mooring systems (18)according to any of the preceding claims, c h a r a c t e ri z e d in that thelarge number of mooring systems (18) consisting of four anchor lines (24)are anchored adjacently in a column and line like pattern of anchors (26), allusing adjacent anchors in common, whereby the limit of necessary anchorstends to one anchor per platform.Mooring facility according to claim 6, wherein a large number of second setof mooring systems (18) consisting of four anchor lines (24) is anchoreddiagonally to the existing square pattern of anchors, whereby most anchorswill receive eight anchor lines (24) whereby the limit of necessary anchorstends to one anchor for every pair of mooring systems. Method for providing a mooring system (18) for a floating platform (28)comprising a plurality of buoys (19) anchored by anchor lines (24) at sea, c h a r a c t e ri z e d b y providing a mooring unit (27a) comprising a first(19a) and a second (19b) buoy, and a first distance wire (25a) connectingthe first (19a) and a second (19b) buoy, anchoring the first buoy (19a) with afirst anchor line (24a) to a first anchor (26a) and with a second anchor line(24b) to a second anchor (26b), anchoring the second buoy (19b) with athird anchor line (24c) to a third anchor (26c) and with a fourth anchor line(24d) to a forth anchor (26d). Method of mooring a floating platform (28) to a mooring system (18)comprising a plurality of buoys (19) anchored by anchor lines (24) at sea, c h a r a c t e ri z e d b y providing a mooring unit (27) comprising a first(19a) and a second (19b) buoy connected by a distance wire (25), providingin each buoy a common ballast compartment (21, 22), providing each buoyto include docking means (20) arranged to hook or mate with the platform,lowering the buoys by filling water into the ballast compartment, aligning theplatform, raising the buoys by emptying water from the ballast compartmentwhereby the docking means will interact with an equivalent mooring meansof the floating platform.