Abstract:
An offshore wind turbine foundation comprising a platform carrying a support for the wind turbine tower in its central region, and a plurality of leg guides in its peripheral region; and a plurality of legs, each of which may be movable between a raised position for transport and lowered positions for resting on the seabed. Each leg is capable of freely sliding in its guide.

Description:
FIELD OF THE INVENTION 
     The present invention relates to offshore wind turbines and, in particular, an offshore wind turbine foundation. 
     BACKGROUND OF THE INVENTION 
     The document US-7 163 355 B2 describes a foundation of this type, in which each leg and its guide are equipped with a device for relative movement with multiple cylinders and racks. 
     When the foundation, or complete wind turbine reaches its site of operation, these movement devices make it possible to move each leg down until it makes contact with the seabed, and then raise the platform to the desired height above sea level. 
     This design therefore requires legs with complex structures and numerous cylinders, which are only used during installation on site and then left in place. 
     SUMMARY OF THE INVENTION 
     The invention aims to provide a solution that is more economical and easier to construct. 
     To this end, the invention aims to provide an offshore wind turbine foundation of the aforementioned type, characterised in that each leg is freely sliding in its guide. 
     In one embodiment, the foundation for an offshore wind turbine comprises:
         a platform carrying a support for a wind turbine tower in its central region, and a plurality of leg guides in its peripheral region,   a plurality of legs each of which may be moved between a raised position for transport and lowered positions for resting on the seabed,       

     wherein each leg is freely sliding in its guide and wherein the foundation comprises a lock for locking each leg in position in its guide. 
     The invention also relates to an offshore wind turbine comprising:
         a foundation as defined above,   a wind turbine tower mounted on the tower support; and   a nacelle rotor assembly mounted in the upper region of the tower.       

     In one embodiment, each leg is locked in place in its guide by a ring made of a curable material, such as concrete, cast in an annular space located between the leg and the guide. 
     In another embodiment, at least one of the facing cylindrical surfaces of the leg and of the guide is equipped with at least one relief projecting out towards the other of these surfaces. 
     In another embodiment, each of the cylindrical surfaces is provided with at least one circular ring constituting said relief, the rings of the two surfaces being offset vertically relative to each other. 
     One embodiment relates to a method for the on site installation of an off-shore wind turbine comprising: 
     (a) transporting to the site of operation a foundation comprising:
         a platform carrying a support for a wind turbine tower in its central region, and a plurality of leg guides in its peripheral region,   a plurality of legs, each of which may be moved between a raised position for transport and lowered positions for resting on the seabed, each leg being freely sliding in its guide, or a wind turbine that comprises:   a foundation comprising:   a platform carrying a support for a wind turbine tower in its central region, and a plurality of leg guides in its peripheral region,   a plurality of legs each of which may be moved between a raised position for transport and lowered positions for resting on the seabed, each leg being freely sliding in its guide,   a wind turbine tower mounted on the support of the tower, and   a nacelle—rotor assembly mounted in the upper region of the tower, with the legs in the raised position;       

     (b) transporting and positioning on the site of at least one work barge which carries:
         a crane; and   for each leg, at least one device for relative movement of the leg in relation to the platform;       

     (c) lowering each leg successively with the crane until it is resting on the seabed; 
     (d) positioning of each relative movement device in a manner such that it cooperates with the associated leg and with the platform; 
     (e) moving vertically the platform to its desired operational-service position by simultaneously actuating the relative movement devices; 
     (f) locking the platform into position in relation to the legs; and 
     (g) removing the relative movement devices from the foundation and the legs, and 
     (h) placing the devices placed back on the work barge. 
     In one embodiment of the method, a work barge provided with a supply of curable material, such as concrete, and a pump for this material is used, and, after step (c), each leg is locked in place relative to the platform by injecting the material into an annular space located between the leg and the associated guide. 
     In another embodiment of the method, a work barge provided with a water pump is used, and, after step (c), the legs are packed by ensuring that the ballasts provided on the platform are filled up with seawater and then emptied of it in a repetitive manner. 
     In another embodiment of the method, the wind turbine has a submerged platform, wherein the method makes use of a work barge provided with a gravel pump, and, after step (e), the foundation is ballasted by filling the ballasts of the platform with gravel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of embodiments of the invention shall now be described with reference made to the accompanying drawings, in which: 
         FIG. 1  schematically presents a perspective view of an off shore wind turbine according to the invention for which the platform is positioned above the water surface; 
         FIG. 1A  is a similar view of a variant having a submerged platform; 
         FIG. 2  schematically presents on a larger scale, a perspective view from above, of the foundation of the wind turbine in  FIG. 1 ; 
         FIGS. 3A and 3B  present on a larger scale, in a side view, two possible configurations of the lower end of the legs of the foundation shown in  FIG. 2 ; 
         FIG. 4  schematically presents a perspective view of a barge for transporting the wind turbine; 
         FIG. 5  schematically presents a side view of an associated work barge; 
         FIG. 6  schematically presents a side view of a relative movement device suitable for lifting the platform; 
         FIG. 7  schematically illustrates by means of a perspective view, the locking into position of the platform relative to the legs; 
         FIG. 8  presents on a larger scale, in a semi meridian cross sectional view, the detail VIII in  FIG. 7 ; and 
         FIG. 9  schematically presents a side view of a variant of the relative movement device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The offshore wind turbine  1  shown in  FIG. 1  consists of a foundation  2  and a superstructure  3 . All the structural components of the wind turbine are constructed out of steel. 
     The foundation  2  comprises a platform  4  and three legs  5  for support on the seabed  6 . In the example shown, the platform is designed to be positioned above the surface  7  of the sea  8 , at a height that is greater than that reached by the highest waves expected in the region considered. 
     The platform  4  depicted in  FIG. 2 , has a regular star shaped form with three arms  9 . Each arm is constructed out of metal sections and delineates a ballast cavity  10  whose functional role will become apparent in the following section/s. 
     Each arm  9  includes, in the vicinity of its free end, a vertical sleeve  11  which passes through the platform over its entire height and projects out upwards, above the upper surface of the latter. The lower end of the sleeve  11  is flush with the underside of the platform, as is shown in  FIG. 6 . 
     At its center, the platform  4  bears a support  50  comprising a tube  13  projecting out over its upper surface. This tube, is equipped with three buttresses  14 , and receives and positions the base of the tower  15  of the superstructure  3 . 
     Each leg  5  is constituted by a tube whose length is such that when it is resting on the seabed and if the platform is in its operational—service position, the upper end of the leg extends above the sleeve  11  as shown in  FIG. 1 . The outer radius R e  of the tube ( FIG. 8 ) is slightly smaller than the inner radius R i  of the sleeve  11 . Thus, the tube  5  is able to freely slide vertically in the sleeve, with an annular space  16  between them. 
     Each tube  5  is closed at its base by a foot  17  which may have various different configurations depending on the nature of the seabed. For example, the foot  17  may be conical and having the same diameter as the leg ( FIG. 3A ), or may be in the form of a sole enlarged at the central tip facing downwards ( FIG. 3B ). 
     As is known per se, the superstructure  3  comprises, in addition to the tower  15 , a nacelle  18 -rotor  19  assembly fixed to the upper end of the tower and equipped with an electric generator. 
     Indeed the wind turbine  1  obviously includes all of the plant and equipment necessary for accessing the platform, for processing the electricity generated, for control, handling, etc., that are common in the field of offshore wind turbines. These plant and equipment are not represented and shall not be described in greater detail. 
     The installation of the wind turbine thus described is carried out by means of a transport barge  20  ( FIG. 4 ) and a work barge  21  ( FIG. 5 ). 
     The transport barge  20  is a flat deck barge, for example self propelled and capable of transporting two complete wind turbines  1  right to their site of operation. As a variant, each wind turbine may be set afloat by means of a floating assembly of buoyancy chambers, and towed up to the site of operation. 
     The work barge  21  is a barge having smaller dimensions than the barge  20  and is for example self propelled and having a rectangular form. The barge  21  is provided at each corner with a sleeve  22  projecting upwards and passing through the barge. In this sleeve slides a tubular positioning leg  23  closed at its base by a foot. The barge  21  is provided with, for each leg, a mechanism for raising/lowering of the leg, for example of the rack and pinion type. 
     The barge  21  has a flat deck on which have been provided all of the various plant, apparatus and equipment required for the installation of the wind turbine: a crane  25 , three lifting devices  26 , a sea water pump  27 , and a concrete pump  29  connected to a reservoir for concrete  30 . As described here below, for the installation of a wind turbine with submerged platform, the barge  21  is also equipped with a gravel pump  28 , while the devices  26  are replaced with lowering devices. 
     Each lifting device  26  is a linear winch which consists essentially of an assembly of vertical cylinders  31  and a cable assembly. The bodies of the cylinders are fixed to a lower horizontal plate  32 , while the piston rods of the two cylinders are fixed to an upper horizontal plate  33 . The plates  32  and  33  comprise unidirectional clamping means for clamping the cables. The alternative control for the cylinders makes it possible for the device to climb along the cables when they are hooked up to a fixed high point. 
     Such devices are well known in the field of building and public works and are, for example cable pull cylinders marketed by the company ENERPAC. 
     The barge  21  also has various actuation and control devices, which facilitate and enable the implementation of the steps for installing the wind turbine which shall now be described. 
     (1) The wind turbine is transported to its operating site by the barge  20 , with the platform resting on the deck  34  of the barge and the legs  5  in the raised position. In this position, the feet  17  are located just below the level of the deck  34  and are therefore submerged. The legs are held in this raised position by appropriate temporary wedges that are not shown. 
     (2) The barges  20  and  21  are brought on site, the legs  23  of the barge  21  being in the raised position. 
     (3) The barge  21  is manoeuvred in a manner so as to position the boom  35  of the crane  25  on top of a first leg  5 , and then the barge  21  is positioned by lowering its four legs  23  and is supported by the latter on the seabed. 
     (4) By means of its lifting hook  36 , the crane seizes a suspension member  37  ( FIG. 2 ) provided at the top of the leg  5 , the wedges of this leg are removed therefrom, and the crane causes the lowering of the leg until it is resting supported on its foot  17  on the seabed. During this descent, the leg fills up with water through the openings provided over the length (height) of the leg. 
     (5) The crane then proceeds to seize a first movement device  26  and deposits it on the platform  4 , in the immediate vicinity of the sleeve  11 . A team attaches one end of the cables  38  ( FIG. 6 ) to the member  37 , passes them over a semi circular guide  39  fixed to the top of the leg ( FIGS. 2 and 6 ), and passes them through the device  26 . The bottom plate  32  of the device  26  is attached to the platform, in the proximity of the sleeve  11 . 
     (6) The barge  21  is manoeuvred in order to bring the boom of the crane over a second leg, and is then positioned by means of its legs  23  and the steps ( 4 ) and ( 5 ) are repeated. 
     (7) The barge  21  to cause the operation of the crane boom above the third leg  5  and is positioned by means of its legs  23 , and the steps ( 4 ) and ( 5 ) are repeated. 
     (8) The three linear winches  26  are actuated simultaneously in a manner such that they climb up slightly along the cables  38 . This operation slightly raises the platform  4 , which releases itself from the deck  34  of the barge  20 . It is then removed. 
     (9) The legs  5 , resting on the seabed, are packed with alternating stresses by the filling of seawater in the ballast  10  situated near each leg and contained in the platform, alternated with an emptying of the ballast. 
     For this purpose, by means of a pipe, the pump  27  of the barge  21  alternately fills and empties the ballasts  10  of the platform. 
     (10) The three linear winches  26  are actuated simultaneously in order to continue their ascent by rising along the cables  38 . In doing so, they pull the platform  4 , upwards until it reaches the desired altitude. 
     (11) The platform being thus in its final position, this position is locked in the following manner, as illustrated in  FIGS. 7 and 8  for one of the legs  5 . 
     The sleeve  11  is fitted in advance on the interior surface thereof, with a number of circular welded rings  40 . The leg  5  is fitted in advance, on its exterior surface situated opposite to the sleeve  11 , with a number of circular welded rings  41 . The rings  41  are offset by about one half pitch in relation to the rings  40 . 
     By means of a pipe  42 , the pump  29  injects into the annular space  16  separating the leg  5  from the sleeve  11  the concrete withdrawn from the reservoir  30 . 
     The same process is repeated for each leg. The setting of the concrete locks the platform in its position, in an extremely secure manner that is further enhanced by the presence of the rings  40  and  41 . 
     As a variant, this locking into position of the platform can be achieved by way of using mechanical wedges. 
     (12) The three linear winches  26  are dismantled and removed, and the crane  25  carries them back on to the barge  21 . 
     It should be noted that when this is necessary, the legs  23  of the barge  21  can be retracted in order for the barge to be rendered mobile again and to thus enable it to move from one leg  5  to the other and from one ballast  10  to the other. 
     The installation of the wind turbine  1  is thus completed. The barge  21  can be moved to the installation site of a new wind turbine, for example, of a similar wind turbine also carried by the barge  20 . 
     The installation process described here above makes it possible to undertake construction on dry land of the entire wind turbine, and to reduce the cost thereof. In fact, the legs  5  are simple tubes that are able to slide freely in a sleeve of the platform, and the lifting devices  26 , which are used only once for each wind turbine during its installation, are recovered and reused for the installation of all successive wind turbines. 
     When installing a wind turbine  101  with a submerged platform ( FIG. 1A ), as illustrated in  FIG. 9 , the linear winches  26  are replaced by simple drum  44  winches  43 . 
     These winches, carried initially by the barge  21  are set in place and secured to the top of each leg after the descent of the latter, and then their cable  45  is fastened to a hook  46  provided on the platform in the vicinity of the sleeve  11 . 
     Such winches  43  are used to enable the lowering of the platform  4  down to a position of service located below sea level. 
     Thus, in order to install the wind turbine, the steps (1) to (7) described here above are carried out. Then, instead of step (8), the barge  20  is to be ballasted in order for the wind turbine to be released therefrom and to allow for the barge  20  to be moved away. Step (10) is replaced by the simultaneous operation of the winches  43  in order to enable the lowering of the platform  4  down to the desired depth. 
     Finally, after step (12) of removal of the winches  43 , an additional step is to be carried out as follows: 
     (13) By means of the gravel pump  28 , gravel, that is contained either in a reservoir carried by the barge  21 , or in an auxiliary barge, not shown, is sent by means of a pipe not shown into the ballast tanks  10  of the platform, so as to complete the ballasting of the wind turbine. The ballasts  10  may in this case be opened upwards. 
     Of course, in each case a number of relative movement devices  26 ,  43  may be associated with each leg  5 .