Patent Publication Number: US-2011074155-A1

Title: Floating offshore wind farm, a floating offshore wind turbine and a method for positioning a floating offshore wind turbine

Description:
BACKGROUND OF THE INVENTION 
     The subject matter described herein relates generally to floating offshore wind farms, floating offshore wind turbines and methods therefore, and, more particularly, to methods for reducing the load and/or increasing the electric power production of floating offshore wind farms, and to related floating offshore wind turbines and offshore wind farms. 
     At least some known wind turbines include a tower and a nacelle mounted on the tower. A rotor is rotatably mounted to the nacelle and is coupled to a generator by a shaft. A plurality of rotor blades extend from the rotor. The blades are oriented such that wind passing over the blades turns the rotor and rotates the shaft, thereby driving the generator to generate electricity. 
     The wind energy resources off the coasts are vast. Therefore, offshore wind energy is an energy resource that may help countries to meet their renewable energy objectives. Offshore wind energy may be harvested in coastal areas using traditional fixed-foundation wind turbine technologies, and in deep-water areas using floating wind turbines. So far, fixed foundation offshore wind farms have only been used commercially in water depth up to about 30 m. Thus, fixed foundation offshore wind farms can only harvest a small percentage of the globally available offshore wind energy. Further, offshore wind farms are typically only accepted when acoustic and visual impact is small. Floating wind turbines may be used in deep water further away from the shore, and from shipping and fishing lanes. Still, floating offshore wind farms may be installed close to heavily developed coastal cities. In addition to lower impact on coastal areas, deep water wind farms may benefit from stronger and steadier wind conditions due to the absence of topographic wind obstructing features. 
     However, the higher distance and the stronger wind conditions may yield higher expenditures for maintenance. Accordingly, there is ongoing need to improve floating wind turbines and floating wind farms, in particular with respect to power production and/or load reduction. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a floating offshore wind turbine is provided. The floating offshore wind turbine includes a floating platform anchored to an underwater ground, a wind turbine mounted on the floating platform, and a drive adapted to move the floating platform in at least one horizontal direction. 
     In another aspect, a floating offshore wind farm is provided. The floating offshore wind farm includes a first wind turbine mounted on a first floating structure, a second wind turbine, a positioning system adapted to determine a horizontal position of the first wind turbine relative to the second wind turbine, and a drive system which is adapted to horizontally move the first wind turbine relative to the second wind turbine. 
     In yet another aspect, a method for positioning a floating offshore wind turbine is provided. The floating offshore wind turbine includes a wind turbine mounted on a floating structure which is anchored to an underwater ground. The method includes providing the floating offshore wind turbine, determining a first horizontal position of the floating offshore wind turbine, determining a second horizontal position, and moving the floating offshore wind turbine to the second horizontal position. 
     Further aspects, advantages and features of the present invention are apparent from the dependent claims, the description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures wherein: 
         FIG. 1  is a perspective view of an exemplary floating offshore wind turbine; 
         FIG. 2  is a schematic side view of a floating offshore wind turbine according to an embodiment; 
         FIG. 3  is schematic front view of the floating offshore wind turbine shown in  FIG. 2  according to another embodiment; 
         FIG. 4  is a schematic sectional view of a floating offshore wind turbine according to an embodiment; 
         FIG. 5  is a schematic sectional view of a floating offshore wind turbine according to another embodiment; 
         FIG. 6  is a schematic sectional view of a floating offshore wind turbine according to yet another embodiment; 
         FIG. 7  is a schematic illustration of a force acting on the floating offshore wind turbine of  FIG. 6 ; 
         FIGS. 8 to 10  illustrate a floating offshore wind farm and its operation under changing wind condition according to embodiments; 
         FIG. 11  illustrate a floating offshore wind farm and its operation according to further embodiments; 
         FIG. 12  illustrates a method for horizontally positioning a floating offshore wind turbine according to an embodiment; 
         FIG. 13  illustrates a method for horizontally positioning a floating offshore wind turbine according to another embodiment; 
         FIG. 14  illustrates a method for horizontally positioning a floating offshore wind turbine according to yet another embodiment; 
         FIG. 15  illustrates a method for horizontally positioning a floating offshore wind turbine according to an embodiment; and 
         FIG. 16  illustrates a method for horizontally positioning a floating offshore wind turbine according to yet another embodiment; 
         FIG. 17  illustrates a method for a floating offshore wind turbine according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the various embodiments, one or more examples of which are illustrated in each figure. Each example is provided by way of explanation and is not meant as a limitation. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet further embodiments. It is intended that the present disclosure includes such modifications and variations. 
     The embodiments described herein include floating offshore wind farms, floating offshore wind turbines and methods therefore. The floating offshore wind turbines and operating methods facilitate a lower load of the wind turbines and/or a higher energy yield of floating offshore wind farms. Further, the energy production and/or load of the wind turbines in a floating offshore wind farm may be better balanced. Thus, the maintenance cost per produced energy unit may be reduced, the lifetime of floating wind turbines increased, and the acoustic impact on the marine fauna may be reduced. Furthermore, the floating wind turbines may avoid a floating obstacle such as a damaged ship. Thus, the safety of a floating offshore wind farm may be increased. 
     As used herein, the term “wind turbine” is intended to be representative of any device that generates rotational energy from wind energy, and more specifically, converts kinetic energy of wind into mechanical energy. The term “wind turbine” as used herein shall particularly embrace devices that generate electrical power from rotational energy generated from wind energy. 
     As used herein, the terms “offshore wind turbine” and “offshore wind farm” refer to the installation of a wind turbine and a wind farm, respectively, in bodies of water such as the sea. Unlike the typical usage of the term “offshore” in the marine industry, the terms “offshore wind turbine” and “offshore wind farm” as used herein embrace respective installations in inshore areas such as lakes, fjords and sheltered coastal areas. As used herein, the terms “floating offshore wind turbine” and “floating offshore wind farm” refer to an offshore wind turbine mounted on a floating structure and a wind farm of such wind turbines, respectively. 
       FIG. 1  shows a perspective view of an exemplary floating offshore wind turbine  99 . The exemplary floating offshore wind turbine  99  includes a wind turbine  10  and a floating platform  69  to which the wind turbine  10  is mounted. Platform  69  floats in a waters  90 , typically in a sea  90  or in a lake  90 , and carries wind turbine  10 . As illustrate by the reference numerals  79 ,  80 , platform  69  is anchored to an underwater ground to limit the freedom of movement of the floating offshore wind turbine  99  in horizontal directions, i.e. in directions which are substantially parallel to the surface  95  of the waters  90 . However, floating offshore foundations have typically no fixed vertical positions to be able to compensate sea level variations. 
     Floating platform  69  may be anchored to the underwater ground by one or more flexible coupling members  79 ,  80  such as anchoring ropes, anchor cables and anchor chains. Long-term stability of floating platforms and their mooring in deep waters has been successfully demonstrated by the marine and offshore oil industries over many decades. In the exemplary embodiment, the body of floating platform  69  is anchored by two flexible coupling members  79 ,  80  such that an upper part of platform  69  is above the water. Floating platform  69  may, however, also be immersed in the waters  90 . In the exemplary embodiment, floating platform  69  is of the single pontoon type. Floating platform  69  may, for example, be a box-shaped or a disc-shaped tank  69  with a large horizontal extension and a relatively short vertical extension. In other embodiments, floating platform  69  is of the spar-buoy type. Spar-buoys consist of a single long cylindrical tank and achieve stability by moving the center of mass as low as possible. In still other embodiments, floating platform  69  is a more complex structure and includes three or more buoyant columns to support wind turbine  10 . In the following, the terms “floating platform” and “floating structure” are used synonymously. 
     In the exemplary embodiment, wind turbine  10  is a horizontal-axis wind turbine. Alternatively, wind turbine  10  may be a vertical-axis wind turbine. In the exemplary embodiment, wind turbine  10  includes a tower  12  that extends from the floating platform  69 , a nacelle  16  mounted on tower  12 , and a rotor  18  that is coupled to nacelle  16 . 
     Rotor  18  includes a rotatable hub  20  and at least one rotor blade  22  coupled to and extending outward from hub  20 . More specifically, hub  20  is rotatably coupled to an electric generator  42  positioned within nacelle  16 . Electric generator  42  is typically coupled via a transformer (not shown) and an underwater power cable  75  to a grid. 
     In the exemplary embodiment, rotor  18  has three rotor blades  22 . In an alternative embodiment, rotor  18  includes more or less than three rotor blades  22 . In the exemplary embodiment, tower  12  is fabricated from tubular steel to define a cavity (not shown in  FIG. 1 ) between a support system (not shown in  FIG. 1 ) on floating platform  69  and nacelle  16 . In an alternative embodiment, tower  12  is any suitable type of tower having any suitable height. 
     Rotor blades  22  are spaced about hub  20  to facilitate rotating rotor  18  to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. Rotor blades  22  are mated to hub  20  by coupling a blade root portion  221  to hub  20  at a plurality of load transfer regions  26 . Load transfer regions  26  have a hub load transfer region and a blade load transfer region (both not shown in  FIG. 1 ). Loads induced to rotor blades  22  are transferred to hub  20  via load transfer regions  26 . 
     In one embodiment, rotor blades  22  have a length ranging from about 15 meters (m) to about 90 m. Alternatively, rotor blades  22  may have any suitable length that enables wind turbine  10  to function as described herein. For example, other non-limiting examples of blade lengths include 10 m or less, 20 m, 37 m, or a length that is greater than 90 m. The exemplary embodiment of  FIG. 1  illustrates an upwind wind turbine  99  in which the rotor  18  faces the wind. The rotor  18  may, however, also be arranged downwind, i.e. on the lee side of tower  12 . As wind strikes rotor blades  22  from a direction  28 , rotor  18  is rotated about an axis of rotation  130 . Further, in the exemplary embodiment, as direction  28  changes, a yaw direction of nacelle  16  may be controlled about a yaw axis  38  to position rotor blades  22  with respect to direction  28 . 
     Moreover, a pitch angle or blade pitch of rotor blades  22 , i.e., an angle that determines a perspective of rotor blades  22  with respect to direction  28  of the wind, may be changed by a pitch adjustment system  32  to control the load and power generated by wind turbine  10  by adjusting an angular position of at least one rotor blade  22  relative to wind vectors. Pitch axes  34  for rotor blades  22  are shown. During operation of wind turbine  10 , pitch adjustment system  32  may change a blade pitch of rotor blades  22  such that rotor blades  22  are moved to a feathered position, such that the perspective of at least one rotor blade  22  relative to wind vectors provides a minimal surface area of rotor blade  22  to be oriented towards the wind vectors, which facilitates reducing a rotational speed of rotor  18  and/or facilitates a stall of rotor  18 . In the exemplary embodiment, a blade pitch of each rotor blade  22  is controlled individually by a control system  36 . Alternatively, the blade pitch for all rotor blades  22  may be controlled simultaneously by control system  36 . Further, in the exemplary embodiment, as direction  28  changes, a yaw direction of nacelle  16  may be controlled about a yaw axis  38  to position rotor blades  22  with respect to direction  28 . 
     In the exemplary embodiment, control system  36  is shown as being centralized within nacelle  16 , however, control system  36  may be a distributed system throughout wind turbine  10 , on or in the floating platform  69 , within a floating wind farm, and/or at a remote control center. Control system  36  includes a processor  40  configured to perform the methods and/or steps described herein. Further, many of the other components described herein include a processor. As used herein, the term “processor” is not limited to integrated circuits referred to in the art as a computer, but broadly refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. It should be understood that a processor and/or a control system can also include memory, input channels, and/or output channels. 
     In the embodiments described herein, memory may include, without limitation, a computer-readable medium, such as a random access memory (RAM), and a computer-readable non-volatile medium, such as flash memory. Alternatively, a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc (DVD) may also be used. Also, in the embodiments described herein, input channels include, without limitation, sensors and/or computer peripherals associated with an operator interface, such as a mouse and a keyboard. Further, in the exemplary embodiment, output channels may include, without limitation, a control device, an operator interface monitor and/or a display. 
     Processors described herein process information transmitted from a plurality of electrical and electronic devices that may include, without limitation, sensors, actuators, compressors, control systems, and/or monitoring devices. Such processors may be physically located in, for example, a control system, a sensor, a monitoring device, a desktop computer, a laptop computer, a programmable logic controller (PLC) cabinet, and/or a distributed control system (DCS) cabinet. RAM and storage devices store and transfer information and instructions to be executed by the processor(s). RAM and storage devices can also be used to store and provide temporary variables, static (i.e., non-changing) information and instructions, or other intermediate information to the processors during execution of instructions by the processor(s). Instructions that are executed may include, without limitation, wind turbine control system control commands. The execution of sequences of instructions is not limited to any specific combination of hardware circuitry and software instructions. 
     In order to deliver the required electrical output power in accordance to external request and stably under variable wind conditions a control system is required. Usually, the control system  36  can operate as a central control system which controls floating wind turbine  99  via special hardware components, such as for example a Single-Point-Operational status (SPS) controller and bus connections such as an Ethernet LAN, a Controller Area Network (CAN) bus, a FlexRay bus or the like. Typically, control system  36  operates as primary controller which supervises at least a part of the functions of the wind turbine  10 . This may include controlling of other controllers of the wind turbine  10 , communication with other wind turbines and/or a wind farm management system as well as error handling and operational optimization. Further, a SCADA (Supervisory, Control and Data Acquisition) program may be executed on the hardware of control system  36 . Control system  36  typically receives information from sensors and other components of the wind turbine  10  such as the generator  42 , load sensors of the rotor blades  22  and an anemometer  39 . In the exemplary embodiment, control system  36  further receives the position of the floating wind turbine  99  from a satellite-based positioning system, e.g. from a GPS. For safety reasons, floating wind turbine  99  is typically also equipped with a radar system for transmitting its position to passing ships. The radar system may also be controlled by the control system  36  and used to measure distances to other floating wind turbines and passing ships. Alternatively, a laser range finder or the like may be controlled by the control system  36  for this purpose. 
       FIG. 2  illustrates schematically an embodiment of a floating offshore wind turbine  101 . Floating offshore wind turbine  101  of  FIG. 2  is similar to the floating offshore wind turbine  99  of  FIG. 1 . However, the floating platform  70  of the floating offshore wind turbine  101  is anchored to the seabed  96  by at least two anchor cables  81 ,  82  such that it is completely immersed in the sea  90 . Floating platform  70  includes a main body  71  and unit for attachment  72  in a lower part. Main body  71  provides the lifting force for carrying wind turbine  10  and is typically partly floodable to adjust lifting force. In the exemplary embodiment, main body  71  is mounted to a cantilever  72  with two distal ends  72   a ,  72   b . The anchor cables  81 ,  82  are fed through the respective distal ends  72   a ,  72   b  and connected with respective anchors  91 ,  92  in the seabed  96 . Buoyant force keeps anchor cables  81 ,  82  substantially taut. The mooring is therefore also known as taut-leg mooring. Instead of cables  81 ,  82 , other flexible coupling members such as chains or ropes may be used. 
     According to embodiments of the invention, floating offshore wind turbine  101  includes a drive  50  which enables movements of the floating offshore wind turbine  101  in a horizontal direction as indicated by the double arrow. In the exemplary embodiment, drive  50  is mounted centrally beneath main body  71  and includes two motor driven cable winches (not shown) to change the length of the anchor cables  81 ,  82 . In other embodiments, a motor driven winch is mounted on or close to each distal end  72   a ,  72   b  or is included in each anchor  91 ,  92 . When the length of cable  81  between distal end  72   a  and anchor  91  is shortened and the length of cable  82  between distal end  72   a  and anchor  91  is increased, or vice versa, floating offshore wind turbine  101  is moved horizontally in x-direction, i.e. horizontally towards anchor  92  and  91 , respectively. 
     Horizontal movements may be used to move the floating offshore wind turbine  101  out of the way of a ship or boat on collision course. Accordingly, the safety on sea may be improved. Horizontal movements may also be used to optimize the horizontal position of wind turbine  10  within an offshore wind farm. For example, shadowing effects between different wind turbines may be at least partially compensated. Accordingly, mechanical load and energy production may be better balanced. 
     When the total length of the cables  81 ,  82  between the anchors  91 ,  92  and the distal ends  721 ,  72   b  remains constant, the vertical position of the floating offshore wind turbine  101  is typically not changed during horizontally moving the floating offshore wind turbine  101 . Otherwise, the vertical position is typically also changed. Typically, the horizontal and/or vertical movement of the floating offshore wind turbine  101  is controlled by control system  36 . 
     In another embodiment, reference numerals  81  and  82  correspond to parts of a continuous flexible coupling means. The length of the parts is adjusted by a motor driven winch  50  to move floating offshore wind turbine  101 . Accordingly, mainly horizontal movements of floating offshore wind turbine  101  are achieved in this embodiment. 
     In the embodiment of  FIG. 2 , wind direction  28  coincides with x-direction. This is however only for clarity reasons as wind turbine  10  may adjust rotor  18  and wind direction  28  using a yaw drive mechanism  37 . Moreover, floating offshore wind turbine  101  is typically also movable in all horizontal directions. This is explained in more detail with respect to  FIG. 3   
       FIG. 3  is another schematic view of the floating offshore wind turbine shown in  FIG. 2 .  FIG. 3  is a front view which is orthogonal to the view of  FIG. 2 . In this view, a further cantilever  73  with two distal ends  73   a ,  73   b  is visible. Cantilever  73  is orientated orthogonal to cantilever  72  shown in  FIG. 2 . Main body  71  is also mounted to cantilever  73 . Anchor cables  83 ,  84  are fed through respective distal ends  73   a ,  73   b  and connect a further drive  51  of the floating platform  70  with respective anchors  93 ,  94  in the seabed  96 . Typically, drive  51  includes one or two winches to change the length of the anchor cables  83 ,  84  for horizontally moving floating offshore wind turbine  101  in elongation direction of the cantilever  73 , i.e. in y-direction. This is indicated by the double arrow. As explained with reference to  FIG. 2  for drive  50 , the two winches of drive  51  may also be arranged at or close to the distal ends  37   a ,  37   b  or integrated in the anchors  93 ,  94 . 
     As the cantilever  73  is substantially orthogonal to cantilever  72  shown in  FIG. 2 , floating offshore wind turbine  101  may be moved by the drives  50  and  51  horizontally in any direction. Drives  50  and  51  may also be realized as a single device or drive system. 
     In other words, the floating platform of a floating wind turbine is typically anchored to an underwater ground, typically to a sea bed, with a mooring system which includes a drive system and several anchors. According to embodiments of the invention, the drive system is adapted to change the length of the connection between the anchors and the floating platform such that the floating platform is moved in one or more horizontal directions. 
       FIG. 4  illustrates schematically an embodiment of a floating offshore wind turbine  102 . Floating offshore wind turbine  102  is similar to the floating wind turbine  101  of  FIGS. 2 and 3 . However, instead of taut-leg mooring, a catenary mooring systems is used for anchoring floating platform  70 . Accordingly, each of the anchoring ropes, anchor cables or anchor chains  85 ,  86  that may be used as flexible coupling members to connect the floating platform  70  with anchors  93 ,  94  sags and typically forms a catenary curve. Compared to taut-leg moorings, catenary mooring systems typically allow larger vertical and horizontal movements. 
     According to embodiments of the invention, floating offshore wind turbine  102  includes a drive  51  which allows movement of the floating offshore wind turbine  102  at least in one horizontal direction by changing the length of the flexible coupling members  85 ,  86  between the floating platform  70  and the anchors  93 ,  94 . Typically, floating offshore wind turbine  102  includes a further drive (not shown) to move the floating platform  70  in another horizontal direction by changing respective flexible coupling members. This has been explained above with respect to  FIGS. 2 and 3  for taut-leg mooring. Drive  51  and the further drive may also be realized as a single device. Furthermore, drive  51  and the further drive may also be configured to force vertical movements of floating platform  70  by shortening the total length of the flexible coupling members. 
       FIG. 5  illustrates schematically an embodiment of a floating offshore wind turbine  103 . Floating offshore wind turbine  103  is similar to the floating wind turbine  102  of  FIG. 4 . However, the length of the flexible coupling members  85 ,  86  between the floating platform  70  and the anchors  93 ,  94  is fixed in  FIG. 5 . 
     According to an embodiment, a propeller  52 , typically an underwater propeller, is connected to floating platform  70  to move platform  70  in a horizontal direction as indicated by the double arrow. Typically, the propeller is rotatably mounted so that it can rotate about axis  38 . Accordingly, platform  70  may be moved in any horizontal direction. Alternatively or in addition, two or more propellers may be used. Furthermore, instead of the one or more propellers one or more pump jet may be used to horizontally move floating platform  70 . 
       FIG. 6  illustrates schematically an embodiment of a floating offshore wind turbine  104 . Floating offshore wind turbine  104  is similar to the floating offshore wind turbine  99  of  FIG. 1 . Floating offshore wind turbine  104  includes a yaw drive  37  to adjust rotor axis  130  relative to the wind direction  28  by rotating nacelle  16  about yaw axis  38 . Typically, rotor axis  130  is maintained parallel to wind direction  28  for maximum power production. In addition to causing rotation of rotor  18 , the wind exerts a translatory force on wind turbine  10 , and particular on rotor blades  22  which rotate in rotor plane  180 . The magnitude and direction of this force depend on the yaw angle between rotor axis  130  and wind direction  28 . Accordingly, floating offshore wind turbine  104  is typically pushed or pulled in a horizontal direction x′ when the yaw angle is changed as indicated by the dashed arrows. Thus, floating offshore wind turbine  104  may “sail” in a direction which depends on the yaw angle. This is illustrates schematically in  FIG. 7 . 
       FIG. 7  shows rotor plane  180  and rotor axis  130  of the wind turbine  10  of  FIG. 6 . Depending on yaw angle θ between rotor axis  130  and wind direction  28 , the incoming air is locally deflected in direction  28 ″ by the rotor blades in rotor plane  180 . Accordingly, a force F is exerted on the wind turbine. For a given rotor, the magnitude of force F depends on yaw angle θ, wind velocity, rotor speed and pitch angle. Typically, force F points in direction of rotor axis  130 . Thus, the floating wind turbine may be horizontally moved by changing yaw angle θ. 
     According to an embodiment, control system  36  of wind turbine  10  is adapted to calculate a yaw angle between wind direction  28  and rotor axis  130  such that the floating offshore wind turbine  104  moves towards a given new horizontal position. The new horizontal position may be a new horizontal position in a floating offshore wind park or a position that avoids a possible collision. In other words, control system  36  is adapted to determine for a given new horizontal position and given wind condition, a yaw angle or a sequence of yaw angles such that the floating offshore wind turbine  104  sails towards the new horizontal position. For this purpose, the control system  36  typically analyses the signals of an anemometer, for example wind direction and wind speed. Furthermore, control system  36  typically takes into account the restoring forces of the used mooring system. 
     It goes without saying, that the different drive mechanism to horizontally move floating offshore wind turbines explained with reference to  FIGS. 2 to 7  may also be combined. For example, the above explained “sailing” of the floating wind turbine may be used in addition to a mooring system with a drive for changing the length of flexible coupling members between the floating platform and respective anchors. Accordingly, the floating offshore wind turbine may be moved more quickly into the new horizontal position. 
       FIGS. 8 to 10  illustrate embodiments of a floating offshore wind farm  500  and its operation under changing wind condition. For clarity, only two offshore wind turbines  100 ,  200  of floating offshore wind farm  500  are shown. Floating offshore wind farm  500  may however include any number of wind turbines. The number of wind turbines may, for example, depend on power requirements of the grid, rated power of wind turbines and on geographic conditions. 
     According to embodiments of the invention, floating offshore wind farm  500  includes a first wind turbine mounted  100  mounted on a first floating structure, a second wind turbine, a positioning system and a drive system for moving the wind turbines  100  and  200  relative to each other in at least one horizontal direction. For this purpose, the positioning system is typically adapted to determine a horizontal position P 1  of the first wind turbine  100  relative to a position P 2  of the second wind turbine  200 . 
     In the embodiment of  FIG. 8 , the positioning system includes two satellite-based positioning systems such as the shown two global positioning systems (GPS)  135  and  235 . Alternatively or in addition, a radar system or a laser range finder may be used to determine relative horizontal position P 1 . 
     Further, the relative horizontal position of wind turbines  100  and  200  may be given in polar coordinates, e.g. as distance and an angle relative to the wind direction  28  measured by one or both anemometers  139 ,  239 . This information is sufficient to determine or at least estimate if an aerodynamic influence or interaction of the wind turbines  100 ,  200  is within an acceptable range. The aerodynamic influence may, for example, be a shadowing or wake influence which reduces the energy production. Further, a turbulence caused by one wind turbine may cause increased or fluctuating mechanical loads on the other wind turbine. The dashed rectangles in  FIGS. 8 to 10  represent exemplary aerodynamic influence boundaries of the wind turbines  100 ,  200 . If the dashed rectangles do not overlap, the aerodynamic influence between the wind turbines  100 ,  200  is in an acceptable range. The rotor axes  130 ,  230  of both wind turbines  100 ,  200  are orientated parallel to the wind direction  28 . The wind direction  28  in  FIG. 8  may correspond to the main wind direction. Accordingly, positions P 1  and P 2  of the offshore wind turbines  100  and  200  are chosen such that the dashed rectangles do not overlap when the wind blows in main direction  28 . The shape of the aerodynamic influence boundaries typically depends on wind speed, shape of floating wind turbine, rotor speed, pitch angles, yaw angle and wind direction. 
     When the incoming direction changes to a new wind direction  28 ′, the yaw systems of the offshore wind turbines  100 ,  200  will typically reorientate the respective rotor axes  130 ,  230  accordingly. Neglecting the influence of the not shown floating platforms, the aerodynamic influence boundaries will also rotate. This may, however, be accompanied by an increased aerodynamic influence of the wind turbines  100 ,  200 , when the positions P 1  and P 2  are not changed. This is illustrated in  FIG. 9  by the overlapping dashed rectangles. Although absolute positions P 1  and P 2  and absolute distance between the positions P 1 , P 2  of the wind turbines  100 ,  200  is not changed, the angle between to the wind direction  28 ′ and the vector between the positions P 1  and P 2  is changed. Accordingly, wind turbines  100  and  200  may start aerodynamically influencing each other in an unfavorable manner. 
     This can be avoided in an offshore wind farm by a large enough fixed horizontal distances between offshore wind turbines. This would, however, require a larger area which is not always possible and less economic. According to an embodiment, the area per floating offshore wind turbine in a wind farm which is required to ensure low aerodynamic influence between the offshore wind turbines under changing wind conditions is reduced by horizontally rearranging the floating offshore wind turbines according to the wind condition. 
     Due to the fact, that the drive system of floating offshore wind farm  500  is able to horizontally move the first offshore wind turbine  100  relative to the second offshore wind turbine  200 , the aerodynamic influence between the wind turbines  100 ,  200  can be reduced or completely avoided when the wind direction changes. This is illustrated in  FIG. 10  in which wind turbine  100  is moved to a new horizontal position P 1 ′. Accordingly, the electric power production of the floating offshore wind farm  500  may be increased. 
     According to an embodiment, the drive system of wind farm  500  is formed by a drive of at least one of the offshore wind turbines  100 ,  200 . Accordingly, the offshore wind turbines  100 ,  200  can be moved relative to each other in horizontal directions. In the exemplary embodiment explained with respect to  FIGS. 8 to 10 , at least offshore wind turbines  100  is a floating offshore wind turbine  100  with a drive to horizontally move floating offshore wind turbine  100 . This drive is used to move wind turbine  100  to a new horizontal position P 1 ′. Typically, both offshore wind turbines  100 ,  200  of wind farm  500  include individual drives coupled to respective floating structures. Thus, both offshore wind turbines  100 ,  200  are independently movable in one or more horizontal directions. Accordingly, the freedom of horizontal movement is typically increased compared to an offshore wind farm having floating offshore wind turbines and fixed foundation offshore wind turbines. 
     The drive of the offshore wind turbines  100 ,  200  may include a propeller or a pump jet which is connected to the respective floating platform, and a drive which is adapted to adjust the length of a flexible coupling member between the respective floating platform and an anchor as explained with respect to  FIGS. 2 to 5 . 
     The drive may also be formed or supported by a yaw drive of the offshore wind turbines  100 ,  200  as explained with respect to  FIGS. 6 and 7 . In this embodiment, the control system of the respective wind turbine  100 ,  200  is adapted to determine at given wind direction a yaw angle between the respective wind turbine rotor axis and the wind direction such that the respective floating offshore wind turbine  100 ,  200  is moved in a desired horizontal direction when the respective yaw drive rotates the rotator axis such that the angle between the rotor axis and the wind direction is set to the yaw angle. 
     Typically, offshore wind farm  500  includes a control system to coordinate positioning of floating wind turbines  100 ,  200 . According to an embodiment, control system is adapted to determine the horizontal position P 1  of wind turbine  100 , for example using a GPS, a radar system and/or a laser rangefinder. The control system is further adapted to calculate a new horizontal position P 1 ′ of wind turbine  100 , and to control the drive system to move wind turbine  100  towards the new horizontal position P 1 ′. The control system may, e.g., be the control system of one of the offshore wind turbines  100 ,  200 . 
     According to an embodiment, the control system of wind farm  500  is adapted to calculate or estimate for given wind conditions the horizontal position P 1 ′ of floating offshore wind turbine  100  such that the electric power production of floating wind farm  500  is increased when floating offshore wind turbine  100  is moved to the new horizontal position P 1 ′. Typically, the control system of wind farm  500  monitors one or more anemometers  135 ,  235  to determine the actual wind condition. If wind condition, for example wind direction changes, new positions of the floating wind turbines  100  and/or  200  are typically calculated such that an aerodynamic influence is minimized or at least reduced when the floating wind turbines  100  and/or  200  move to their new positions. 
     Alternatively or in addition, the control system of wind farm  500  is typically adapted to calculate at given wind condition new horizontal position P 1 ′ of the first wind turbine  100  such that the mechanical load of at least one the offshore wind turbines  100 ,  200  is reduced when the first wind turbine  100  is moved to the new horizontal position P 1 ′. For this purpose, the control system of wind farm  500  typically monitors an operational status of offshore wind turbines  100  and an operational status of offshore wind turbines  200 . The operational status of the offshore wind turbines  100 ,  200  may include power production, voltage of the generator, rotary speed of the rotor, mechanical load of the rotor blades, yaw angle relative to wind direction, and the like. For example, turbulence behind one offshore wind turbine may cause fluctuating and/or increased load on the rotor blades of another offshore wind turbine. The control system of wind farm  500  is typically adapted to detect such a situation by monitoring at least the mechanical load of the rotor blades. Thereafter, new horizontal positions of the offshore wind turbines are calculated or estimated by the control system such that the aerodynamic influence is reduced. In doing so, mechanical load peaks may be avoided and thus maintenance cost reduced and/or life time of offshore wind turbines increased. Furthermore, the increased noise which may be associated with an aerodynamic influence or turbulence may be avoided. Thus, the acoustic impact on the environment may be reduced. In another example, control system of wind farm  500  uses monitored values of power production to determine new horizontal positions such that power production is optimized and/or more equally distributed between the offshore wind turbines when the offshore wind turbines are in their new horizontal positions. 
       FIG. 11  illustrate a further embodiment of a floating offshore wind farm  500 . For clarity, only two offshore wind turbines  100 ,  200  of floating offshore wind farm  500  are shown. 
     According to an embodiment, floating offshore wind farm  500  includes a first wind turbine  100  mounted on a first floating structure  170 , a second wind turbine  200 , mounted on a second floating structure  270 , and a drive system which is adapted to exert substantially horizontal forces between the two floating structures  170 ,  270 . In the exemplary embodiment, the drive system includes two winches  190 ,  290  which are adapted to change the length of a rope, cable or chain  87 . Accordingly, the wind turbines  100  and  200  can be moved relative to each other in at least one horizontal direction. Thus, repositioning and/or stabilizing the positions of the floating offshore wind turbines  100 ,  200  in the offshore wind farm  500  can be assisted. In another embodiment, only one winch is used to change the length of a flexible coupling member between two floating offshore wind turbines. 
       FIG. 12  illustrates a method  1000  for horizontally positioning a floating offshore wind turbine according to an embodiment. Method  1000  includes a block  1100  for providing a floating wind turbine. The floating wind turbine includes a wind turbine mounted on a floating structure which is anchored to an underwater ground, typically a sea bed or a lake bed. Typically, a floating offshore wind turbine having a drive to horizontally move the floating offshore wind turbine as explained with reference to  FIGS. 2 to 11  is provided in block  1100 . Method  1000  further includes a block  1200  for determining a first horizontal position of the floating wind turbine, a block  1400  for determining a second horizontal position of the floating wind turbine, and a block  1500  for moving the floating wind turbine to the second horizontal position. The first horizontal position may be an absolute position. The absolute position may be measured by a satellite-based positioning system such as a GPS in block  1200 . The first horizontal position may also be a relative position to another wind turbine in an offshore wind farm or to a passing ship. In this case, the first position may be calculated from absolute positions or measured by a radar system or a laser range finder system. The first and second horizontal positions may correspond to a horizontal position and a new horizontal position of a floating offshore wind turbine in an offshore wind farm as explained with reference to  FIG. 10 . The first and second horizontal positions may also correspond to a normal position of a floating offshore wind turbine and a deflected position in which a collision with a ship is avoided. 
     The floating wind turbine typically includes an anemometer for measuring a wind condition, a rotor axis and a yaw drive to horizontally orientate the rotor axis relative to the wind direction. According to a further embodiment, block  1500  includes operating the yaw drive such that wind turbine is moved towards the second horizontal position. In other words, a yaw angle between rotor axis and wind direction is changed in block  1500  such that the floating wind turbine sails towards the second horizontal position. The floating structure of the floating wind turbine is typically anchored to the underwater ground with a mooring system which allows horizontal movement of the floating structure within boundaries. Typically, restoring forces and/or boundaries of the mooring system are taken into account in block  1400 . 
       FIG. 13  illustrates a method  1001  for horizontally positioning a floating offshore wind turbine according to another embodiment. Method  1001  is similar to method  1000 . Method  1001  additionally includes a block  1150  for providing a further floating wind turbine, a block  1250  for determining a further horizontal position, i.e. a horizontal position of the further floating wind turbine, and a block  1300  for measuring a wind condition such as a wind speed and/or wind direction. The further floating wind turbine is typically also a floating offshore wind turbine having a drive to horizontally move the floating offshore wind turbine as explained with reference to  FIGS. 2 to 11 . According to an embodiment, method  1001  takes into account the wind condition and the horizontal position of the further wind turbine for determining the second horizontal position of the floating wind turbine in block  1400 . Accordingly, method  1001  may be used to change the positions of floating wind turbines in an offshore wind farm when the wind condition changes. 
       FIG. 14  illustrates a method  1002  for horizontally positioning a floating offshore wind turbine according to yet another embodiment. Method  1002  is similar to method  1001 . Method  1002  additionally includes a block  1320  for measuring a wind turbine operational status, i.e. an operational status of the floating wind turbine and/or an operational status of the further floating wind turbine. According to an embodiment, method  1002  takes into account the wind condition and the operational status of at least one floating wind turbine in block  1400 . Accordingly, method  1002  may be used to rearrange the positions of floating wind turbines in an offshore wind farm depending on wind condition and operational status of one or more floating wind turbines. For example, the floating wind turbines may be rearranged to distribute the energy production more uniformly or to maximize energy production of an offshore wind farm. In this example, the power production of a floating wind turbine may be used as wind turbine operational status. 
       FIG. 15  illustrates a method  1003  for horizontally positioning a floating offshore wind turbine according to still another embodiment. Method  1003  is similar to method  1002 . Method  1003  includes the more specific block  1323  for estimating a wind turbine load in stead of the more general block  1320  for measuring a wind turbine operational status. Accordingly, method  1003  may be used to rearrange the positions of floating wind turbines in an offshore wind farm depending on wind condition such that load peaks of individual floating wind turbines in an offshore wind farm are avoided or reduced. Thus, maintenance costs may be reduced and/or the life time of the floating wind turbines increased. 
       FIG. 16  illustrates a method  1004  for horizontally positioning a floating offshore wind turbine according to an embodiment. Method  1004  is similar to method  1001 . Method  1004  additionally includes a block  1340  for estimating a wake influence between the floating wind turbine and the further floating wind turbine. Typically, the second horizontal position is calculated or estimated in block  1400  such that the wake influence is reduced when the floating wind turbine is moved to the second horizontal position in block  1500 . Accordingly, the mechanical load of the floating wind turbine and/or the further floating wind turbine may be reduced. Further, the overall power production of the wind turbines in an offshore wind farm may be increased. 
       FIG. 17  illustrates a method  1005  for horizontally positioning a floating offshore wind turbine according to yet another embodiment. Method  1004  is similar to method  1001 . Method  1005  additionally includes a block  1600  for horizontally moving the further wind turbine. Accordingly, the freedom to position the wind turbines in a floating offshore wind farm is increased. Thus, the positioning of wind turbines in the floating offshore wind farm may be more flexible with respect to changing wind conditions. It goes without saying that block  1600  may also be added to the methods  1001  to  1004  and that the sequence of blocks  1500  and  1600  may be changed. 
     The above-described floating offshore wind turbines and methods facilitate a flexible horizontal positioning of wind turbines in an offshore wind farm. This enables optimizing the positioning of wind turbines with respect to overall power production, distribution of power production, and/or mechanical load. Furthermore, floating offshore wind turbines are enabled to avoid an obstacle such as a ship on colliding course. Accordingly, safety may be increased. 
     Exemplary embodiments of floating offshore wind turbines, floating offshore wind farms and methods for their operation are described above in detail. The systems and methods are not limited to the specific embodiments described herein, but rather, components of the systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. The embodiments are not limited to practice with respect to wind turbines and wind farms installed in deep sea. Rather, the exemplary embodiment can be implemented and utilized in connection with many other applications of floating wind turbines. For example, wind farms installed close to the coast, e.g. in a fjord, or in a lake may use the floating wind turbines and the methods disclosed herein. Moving wind turbines of a wind farm relative to each other in horizontal directions facilitate increase of electric power production and/or reducing the mechanical loads of the wind turbines. Thus, maintenance cost of floating wind turbines may be reduced and/or lifetime of floating wind turbines increased. 
     Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. While various specific embodiments have been disclosed in the foregoing, those skilled in the art will recognize that the spirit and scope of the claims allows for equally effective modifications. Especially, mutually non-exclusive features of the embodiments described above may be combined with each other. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.