Patent Publication Number: US-2012027523-A1

Title: Device and method for assembling a structure at sea

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on Belgian Patent Application No. 2009/0330, filed May 28, 2009, which is hereby incorporated by reference in its entirety. Priority is not being claimed. 
     FIELD OF THE INVENTION 
     The invention relates to a device for assembling a structure at sea. The invention also relates to a method for assembling a structure at sea. The invention relates particularly to a device and method for assembling a wind turbine at sea. 
     BACKGROUND OF THE INVENTION 
     A growing number of structures must be constructed, maintained and repaired at sea. A typical example is an offshore wind turbine which is constructed from a gondola or nacelle which is supported by a mast which can be over 100 metres high. The mast comprises different mast segments generally placed successively one onto another. The gondola is the housing on the mast in which a large part of the necessary equipment is located and which alone can already have a weight of more than 350 tons. The wind turbine is further provided with a hub on which a number of rotor blades are arranged. The hub forms the connection between the rotor blades of the wind turbine and the shaft thereof. The rotor blades, which can have a length of more than 70 metres, convert the kinetic energy of the wind into a rotating movement of the shaft. The currently very common wind turbines are equipped with three rotor blades, wherein the rotor blades can rotate over their longitudinal axis. They can hereby take up the optimal position at any wind speed, thereby achieving an optimum production. They can in this way also be rotated out of the wind at high wind speeds by means of a so-called pitch installation. A typical wind turbine is further provided with a transmission or gearbox to adjust the generator rotation speed (50 Hz) to the rotation speed of the rotors. The generator converts the movement of the shaft into electricity and supplies the generated power to the public grid. 
     During the assembly, maintenance and/or repair of such large structures at sea it is usual to transport components of the structure to an offshore jack-up platform moored in the vicinity of the location where the structure is situated or must be constructed. For the construction of for instance a wind turbine the mast components and an assembly of rotor blades arranged on a hub are navigated to the location of construction using a suitable vessel. The jack-up platform is provided with a lifting device which can lift the supplied components from the transport vessel and can carry them to the structure to be constructed. 
     During the lifting and placing of components it is usual for these to have to be placed at least temporarily in a position other than the transport position. Components must more particularly be manipulated in order to move them from their transport position (the position in which the relevant component was transported) to their assembly position (the position in which the relevant component is added onto or to an already existing construction). During lifting and placing of wind turbine rotor blades onto an already existing or constructed mast the jack-up platform is thus provided with an auxiliary crane (a so-called tailing crane) which ensures that the assembly of hub and (for instance three) rotor blades is erected at least partially in vertical direction from the horizontal transport position in order to enable arranging of this assembly on the mast. An outer end of a rotor blade is here secured to the tailing crane. 
     The known device and method are time-consuming. The tailing crane moreover takes up a great deal of space on the platform, whereby valuable space is lost. In addition, working with a tailing crane requires good coordination and skill, a lack of which may result in danger. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a device with which (components of) large structures, and particularly wind turbines, can be assembled at sea. 
     This object is achieved by providing a device for assembling a large structure at sea, particularly a wind turbine, which device comprises a support structure for a component of the structure which can be arranged on a transport vessel, wherein a part of the support structure is movable from a transport position, in which the component is transported, to an assembly position in which the component is rotated and/or tilted relative to the transport position, wherein the movable part is also provided with fixing means for fixing of the component. 
     The invention likewise relates to a method for assembling a large structure at sea, particularly a wind turbine, making use of the device according to the invention. The method comprises of providing a device according to the invention on a transport vessel, arranging a component of the structure on the movable part of the device using the fixing means, transporting the component to the location at sea where the component must be assembled using the transport vessel, wherein the component is situated in a transport position during the transport, optionally rotating and/or tilting the movable part of the support structure from the transport position to an assembly position, wherein the component is rotated and/or tilted relative to the transport position, and arranging the component on the structure in the assembly position. Moving the movable part of the support structure from the transport position to an assembly position preferably takes place after mooring of the vessel in the immediate vicinity of the structure to be assembled, and particularly against a jack-up offshore platform moored in the vicinity of the construction location, the platform being provided if desired with a lifting device which can pick up the supplied components from the transport vessel and carry them to the structure to be constructed. It is however also possible for the transport vessel to also serve as mooring platform. 
     In a further embodiment of the method the transport position can correspond to the assembly position. The component is thus transported in the assembly position, whereby rotation or tilting from the transport position to the assembly position is not necessary. 
     The component to be transported, rotated and/or tilted can in principle be any component of the structure. The method is particularly suitable for wind turbines wherein the components to be transported, rotated and/or tilted can for instance comprise a gondola or nacelle, and/or a hub, and/or one or more rotor blades. 
     The use of a tailing crane is avoided by providing a support structure with a movable part, on which part the component can be arranged. A further advantage is that the component can also be placed in any desired angular position relative to the deck of the transport vessel during transport. This improves convenience and safety during transport. 
     The movable part can in principle be rotatable and/or tiltable around any axis. It is thus possible to provide a movable part which is rotatable and/or tiltable around a vertical axis. This allows placing of the component in an optimal position for the transport and/or for the mounting during the assembly of the structure. 
     A preferred embodiment of the device according to the invention is characterized in that the movable part is tiltable around a horizontal axis. This is particularly useful for the assembly of a wind turbine at sea, wherein an assembly of wind turbine rotor blades must be moved from a transport position, in which the rotor blades extend substantially parallel to the deck of the transport vessel, to an assembly position in which the rotor blades extend substantially perpendicularly of the deck of the transport vessel. 
     A further preferred embodiment of the device according to the invention is characterized in that the movable part is rotatable around a horizontal axis. This is particularly useful for the assembly of a wind turbine at sea, wherein an assembly of wind turbine rotor blades must be moved from a transport position, in which the rotor blades extend substantially perpendicularly of the deck of the transport vessel, to an assembly position in which the rotor blades still extend substantially perpendicularly of the deck of the transport vessel but are rotated relative to the transport position. 
     In another preferred embodiment the support structure of the device according to the invention comprises a tower, wherein the rotatable part is situated at the top of the tower. Such an embodiment allows the hub of an assembly of rotor blades to be placed at a height such that, when rotor blades are placed in the assembly position, they do not come into contact with the deck of the transport vessel or with other components such as the jack-up platform. The tower can in principle be embodied in any manner. The tower can thus be static or movable. It is advantageous to embody the tower as a static structure, for instance in the form of a framework, or as a beam structure which if desired comprises different rotatable beam segments pivotable relative to each other. Such an embodiment is stable, strong and relatively inexpensive. A framework is moreover light and can be erected as desired in simple manner. 
     A preferred embodiment of the device according to the invention comprises a tower which is height-adjustable. Not only can components of different dimensions hereby be transported and rotated, the height can moreover be adjusted subject to the conditions on site, such as for instance the desired transport and/or assembly position. 
     The jack-up offshore platform and/or the transport vessel generally comprise a work deck which can bear a substantial load and anchor piles which support the work deck. Each anchor pile extends substantially vertically and is movable in this direction from a high position during transport to a low position in the anchored position, in which position the piles support on the seabed. The height position of the work deck relative to the water level can be changed by shifting the work deck relative to the piles by means of hydraulic pistons. In the anchored position the work deck can be jacked up to a position above the water level. It is however also possible to only stabilize the platform. Such a platform is only partially jacked up to a level at which the piles support on the seabed but the work deck still partially makes contact with the water. A platform stabilized in this manner can take a lighter form because part of the load is supported by the water. 
     A component of almost any dimension can in principle be manipulated with the device according to the invention. A preferred embodiment relates to a tower with a maximum height of at least 20 metres above the work deck of the transport vessel, more preferably at least 30 metres, still more preferably at least 40 metres. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be elucidated in more detail with reference to the accompanying figures, without otherwise being limited thereto. In the figures: 
         FIG. 1A  is a schematic side view of an embodiment of the device according to the invention during transport; 
         FIG. 1B  is a schematic side view of a second embodiment of the device according to the invention during transport; 
         FIG. 2A  is a schematic side view of the embodiment shown in  FIG. 1A  during assembly; and 
         FIG. 2B  is a schematic side view of the embodiment shown in  FIG. 1B  during assembly; and 
         FIG. 3  is a schematic top view of the embodiment of the device according to the invention shown in  FIG. 2A  during assembly. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1A , a device is shown for assembling a wind turbine  50  at sea. The device comprises a support structure  20  arranged on a transport vessel in the form of a jack-up platform  10 , wherein a part  24  of support structure  20  is tiltable around a horizontal axis in the indicated direction R 1  by means of a hinge connection  25 . Rotatable part  25  is provided with fixing means (not shown), such as for instance bolts, for the fixing a component of wind turbine  50 , and in particular rotor  55 . Rotor  55  comprises an assembly of a hub  56 , on which three rotor blades ( 56   a ,  56   b ,  56   c ) are mounted. It is also possible for hub  56  to be provided with only one, two or, conversely, more than three rotor blades. Rotatable part  24  can be moved from a transport position as shown in  FIG. 1A , in which rotor  55  with the rotor blades is transported in horizontal position, to an assembly position as shown in  FIG. 2A  by being tilted around hinge  25 . The necessary force can for instance be provided by hydraulic piston  26 , although any other suitable method of force transmission is also possible. In the shown embodiment the transport position corresponds to a position in which the rotor blades ( 57   a ,  57   b ,  57   c ) extend substantially parallel to deck  27  of platform  10 , while the assembly position corresponds to a position in which the rotor blades ( 57   a ,  57   b ,  57   c ) extend substantially perpendicularly of deck  27  of platform  10 . 
     Referring to  FIG. 1B , the device as already shown in  FIG. 1A  is shown. In this embodiment however, part  24  of support structure  20  is placed in the upright tilted position, whereby the transport position corresponds to a position in which the rotor blades ( 57   a ,  57   b ,  57   c ) extend substantially perpendicularly of deck  27  of platform  10 . Rotor  55  is therefore transported into vertical position. According to the present embodiment, part  24  is likewise rotatable around a horizontal axis so that rotor  55  can be anchored in different rotation positions (in the direction R 2 ). The transport position shown in  FIG. 2B  corresponds to a position rotated in the direction R 2 , in which one rotor blade  57   c  is placed substantially vertically and the two remaining rotor blades ( 57   a ,  57   b ) are placed at the same angle to the water surface. In this position the distance from hub  56  of rotor  55  to the top of the two rotor blades ( 57   a ,  57   b ) is minimal, whereby hub  56  need not be placed very high above the water surface. 
     Rotor  55  can be moved from the vertical transport position shown in  FIG. 1B , in which one rotor blade  57   c  is placed substantially vertically, to the assembly position shown in  FIG. 2B  by rotating the part  24  in the direction R 2 . In the shown embodiment the transport position corresponds to a position in which rotor blade  57   a  extends substantially vertically and protrudes over the edge of platform  10 . Rotor blades ( 57   b ,  57   c ) extend at substantially the same angle to the water surface. This assembly position is advantageous for the manipulation of rotor  55  with a lifting device  30 . 
     The support structure of the device according to the invention comprises a tower in the form of a framework  20  provided with uprights  22  and transverse beams  23 , the cross-sections of which are large enough to be able to support the necessary loads. In the shown embodiment movable part  24  is situated on top of tower framework  20 . This latter is preferably height-adjustable, wherein the height  21  of tower  20  with rotor  55  will generally be lower in the transport position ( FIG. 1 ) than the height  21  of tower  20  with rotor  55  in the assembly position ( FIG. 2 ). The maximum height of tower  20  can be freely chosen. In view of the dimensions of rotor blades, the maximum height of tower  20  is preferably at least 20 metres above work deck  27  of transport vessel  10 , more preferably at least 30 metres, and most preferably at least 35 metres above work deck  27  of transport vessel  10 . 
     The jack-up offshore platform  10  comprises a work deck  27  which can bear a substantial load, typically of more than 1000 tons, and is provided with anchor piles  3  which support work deck  27 . Each anchor pile  3  extends substantially vertically and is movable in this direction from a high position during transport (shown in  FIG. 1 ) to a low position in the anchored position (shown in  FIG. 2 ), in which position piles  3  support on seabed  200 . The height position of work deck  27  relative to water level  100  can be changed by shifting work deck  27  relative to piles  3  by means of hydraulic pistons  4 . 
     Transport vessel or platform  10  provided with the device according to the invention is generally moored in the immediate vicinity of a second jack-up platform  101  as shown in  FIG. 2 . Parts of platform  101  comparable to parts of platform  10  are designated in the figures with the same numerals as those of platform  10  and not further described here. Reference is made to the description thereof in respect of platform  10 . 
     Platform  101  is anchored in the immediate vicinity of the wind turbine  50  to be assembled and/or maintained, which is shown in semi-finished state and which is anchored in seabed  200  by foundation  51 . In the shown semi-finished state wind turbine  50  comprises, in addition to foundation  51 , two mast parts ( 52 ,  53 ) and a gondola  54 . Mast parts ( 52 ,  53 ) and gondola  54  have already been put in place by a lifting device  30  provided on platform  101 . Any suitable lifting device  30  can in principle be applied. In the shown embodiment lifting device  30  substantially comprises two parts. The first part comprises a structure which is connected pivotally to point of rotation  13   a  and which comprises at least a main boom  14  for manipulating rotor  55  of wind turbine  50 . A turntable  11  is provided which can rotate main boom  14  around a vertical axis  12  relative to platform  101 . Lifting device  30  further comprises a frame  13  which supports main boom  14  in rotatable manner and a second boom  15 . Additional booms  16  can likewise rotate around point of rotation  14   a  of main boom  14 . The booms are mutually connected by means of cables. Rotor  55  can be secured to the lifting device by means of gripper means  17  and then be manipulated, for instance lifted, rotated and so on. Lifting device  101  also comprises a counterweight  32  which is connected to frame  13  for movement from a position toward and away from point of rotation  13   a  of main boom  14 . Optionally incorporated in the cables are force sensors (not shown) which are connected to a control system for regulating the cable tension. 
     According to the embodiment of the invention shown in the figures, a method is provided for assembly of a wind turbine  50  at sea. The method comprises of providing a transport vessel  10  as shown in  FIG. 1A , arranging rotor  50  of wind turbine  50  on movable part  24  of the device using the fixing means (not shown), transporting rotor  50  using transport vessel  10 , wherein rotor  50  is situated in a preferably horizontal transport position (substantially parallel to work deck  27 ), tilting movable part  24  around a horizontal axis R from the transport position shown in  FIG. 1A  to the assembly position shown in  FIG. 2A , wherein rotor  50  is rotated relative to the transport position and preferably extends substantially perpendicularly of work deck  27  of platform  10 . An alternative method comprises of providing a transport vessel  10  as shown in  FIG. 1B , arranging rotor  50  of wind turbine  50  on movable part  24  of the device using the fixing means (not shown), transporting rotor  50  using transport vessel  10 , wherein rotor  50  is situated in a vertical transport position (substantially perpendicularly of work deck  27 ), rotating movable part  24  around a horizontal axis R 2  from the transport position shown in  FIG. 1B  to the assembly position shown in  FIG. 2B , wherein rotor  50  is rotated relative to the transport position. 
     By moving rotor  50  from the transport position shown in  FIGS. 1A and 1B  to the assembly position (this position is shown in  FIGS. 2A and 2B ) it can be transferred easily to the semi-finished wind turbine  50  by means of lifting device  30  and arranged thereon in the assembly position (the assembly position during arranging is shown schematically in  FIGS. 2A and 2B  by the rotor in broken lines). During transfer of rotor  50  the lifting device will be moved from a position  30 - 1  to a position  30 - 2  ( FIG. 3 ). In position  30 - 1  rotor  55  is already in the assembly position. The method and device according to the invention avoid the use of tailing cranes present on platform  10  and/or  101  in the existing method. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments and those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “to comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage in the figures, and many variants thereof are possible within the scope of protection of the appended claims.