Patent Publication Number: US-2015082720-A1

Title: Wind turbine foundation

Description:
This application claims the benefit of European Patent Application EP 12382171.2 filed May 9, 2012 and U.S. Provisional Patent Application Ser. No. 61/669,473 filed Jun. 9, 2012. 
     A foundation for a wind turbine is disclosed herein. The present wind turbine foundation is particularly suitable for, but not limited to, wind turbine pile foundations in offshore structures. A wind turbine comprising such a foundation is also disclosed herein. 
    
    
     BACKGROUND 
     Several types of foundations are known in the art for wind turbine applications. One common type of foundation, which is particularly well known in many offshore structures, is a piled foundation. Piled foundations are widely used for transferring structural loads to the ground. 
     Piled foundations may consist of one pile, referred to as mono-pile foundations, or a number of piles arranged in different ways according to the requirements. 
     Piles consist of steel elongated pieces intended to be driven into a surface, such as the ground or the seabed. Piles are shaped for receiving corresponding pins. 
     Pins are elongated connecting members extending downwards from a wind turbine foundation. 
     A jacket is a lattice substructure that comprises a number of tubes connected to each other by means of bracings and tubular joints. A transition piece connects and supports the lower part of the wind turbine tower to the substructure. 
     When the pins are inserted into the corresponding piles a high performance concrete-like mass such as grout is injected. Grout injection serves the purpose of establishing a firm connection between the piles and the pins. Grout injection also helps to avoid undesirable horizontal deflections and inhibit corrosion. Grout provides an increased energy absorption capacity to the structure. Grout is injected typically through the bottom of a chamber formed between the pile and the pin thus forming a grouted joint. The width of the chamber or annulus between the pile and the pin can be maintained constant through the use of centralizers. 
     The above grouted joints are well known in offshore oil and gas structures. Such fields are however dynamically totally different from the field of wind turbine foundations. 
     The piles in a wind turbine foundation are subjected to highly cyclic tension and compressive loading due to the wind and waves action. The substructure formed by the transition piece and the foundation is under high hydrodynamic and aerodynamic loads. Such loads are transferred from the tower, the nacelle and the rotor structure of the wind turbine to the seabed. Demands for higher power output and the decrease in the number of sites with high wind availability and good access cause wind farms to have larger turbines that must be mounted higher and thus be more inaccessible. Wind turbine substructures must be thus oversized in order to withstand high loads, making their manufacturing and installation processes undesirably costly. 
     For increasing load capacity and stiffness of the pile-pin connection the use of shear keys are known and used in the art. Shear keys consist of spaced weld beads or steel bars that are welded on the inner surface of the piles and on the outer surface of the pins. In use, the shear keys are intended to be in contact with the grout once injected within the chamber. This solution is currently used in known offshore wind turbine foundations for the transfer of shear forces. It has been proven to be efficient particularly in increasing the sliding resistance between the grout and the pile and the pin. One example of the use of shear keys is disclosed in US2006185279. 
     Shear keys however require welding on the outer surfaces of the pins and on the inner surfaces of the piles. This is involves costly and complex operations and often require several welding passes. 
     A more efficient yet cost effective grout connection would be desirable for wind turbine structures, especially for offshore structures, in order to make wind energy industry projects more economically feasible. 
     SUMMARY 
     A foundation for a wind turbine is disclosed herein. The foundation comprises at least one substantially elongated pin associated with a wind turbine tower and at least one corresponding substantially elongated pile to be inserted into a surface and adapted for receiving at least one portion of a length of the pin when in use, a grouting chamber being defined between the pin and the pile when in use for receiving grout, wherein at least one of the pin and the pile is provided with at least one connecting plate extending inside the grouting chamber, the connecting plate being provided with holes through which the grout passes when in use. 
     The present wind turbine foundation is particularly suitable for, but not limited to, wind turbine pile foundations in offshore structures. A wind turbine comprising such a foundation is also disclosed herein. 
     The present wind turbine foundation is intended for connecting a wind turbine tower to a surface, such as the seabed in an offshore wind turbine. 
     The present wind turbine foundation comprises at least one substantially elongated pin, which is associated with a wind turbine foundation, and at least one corresponding pile to be inserted into the seabed. Definitions are given below. 
     As used herein, a pin refers to an elongated connecting member, made e.g. of steel, extending downwards from a wind turbine foundation substructure. The pins have a connecting end to be attached to the wind turbine tower through said turbine foundation substructure, and an inserting end, opposite the connecting end. In preferred examples the pins may be cylindrical in shape. 
     The wind turbine foundation may comprise a jacket consisting of a lattice substructure that comprises a number of tubes. The tubes are connected to each other by means of bracings, tubular joints and the like. A transition piece connects and supports the lower part of the wind turbine tower to the jacket. 
     As used herein, a pile consists of an elongated connecting member, made e.g. of steel. The piles are driven several tens of meters into a surface such as the ground in the case of onshore wind turbines or such as the seabed in the case of offshore wind turbines. The piles are shaped for receiving at least one portion or section of the length of corresponding pins, that is, the pins can be at least partially inserted, i.e. through their inserting ends, into the corresponding piles when in use. For this purposed, the inserting end of the pins may be guided. In preferred examples the piles may be cylindrical in shape. 
     A grouting chamber in the form of an annular space is defined between the pin and the pile when in use, that is, when the pin is at least partially inserted into the corresponding pile. Such grouting chamber defined between the pin and the pile is suitable for receiving grout. 
     As used herein, the term grout includes any cementitious settable material or mixture of settable materials. Grout is used for the support of the present wind turbine foundation. The grout connection thus formed is subjected to compressive and shear load transfer at the chamber. 
     At least one of the pin and the pile in the present wind turbine foundation is provided with one or more connecting plates made, e.g. of high grade steel, extending inside the grouting chamber. The connecting plates are provided with one or more holes through which the grout passes when in use. 
     The arrangement of the connecting plates is such that their length dimension extends lengthwise within the chamber and their width dimension extend through the gap of the chamber defined between the pile and the corresponding pin at least partially fitted therein. In some cases, at least one portion of such length and width dimensions could extend out of the grouting chamber. 
     One example of connecting plates is the so called Perfobond connectors. Perfobond connectors consist of plates with several openings or holes formed therein. Holes are preferably circular but they can assume other shapes such as oval or polygonal as required. Perfobond connectors are mainly defined by the number and size of the openings or holes, the number of plates and their length and width, and the spacing of the openings or holes. These parameters can be conveniently varied according to the wind turbine foundation characteristics. For example, in a typical offshore wind farm it is preferred that the ratio of the distance between the centers two adjacent holes in a connecting plate to the diameter of the holes ranges from 2.20 to 2.40. For example, the number of connecting plates may range from 2 to 16. For example, the ratio of the distance between two adjacent connecting plates to the perimeter of at least one of the pin and the pile may range from ½ to 1/16. 
     The above wind turbine foundation provides a very strong connection of the wind turbine to the ground with a high load capacity in terms of high shear resistance. Perfobond connectors are highly efficient in high axial loaded structures such as piled foundations in offshore wind turbines. Since the load transition takes place by the grout passing through the holes in the plate, higher compression capacity can be allowed and therefore an increased shear resistance. 
     As stated above, the pins or the piles, or both the pins and the piles, in the present wind turbine foundation may be provided with such connecting plates. In any case, the connecting plates can easily be welded by a single weld pass to the pins or piles (or both) such that the connecting plates extend inside the chamber. The connecting plates may radially extend inside the chamber. 
     In this regard, several implementations are envisaged. Both the pin and the pile may be provided with connecting plates. At least some of the connecting plates may extend along at least one section or portion of the length of the at least one of the pin and the pile within the chamber. At least some of the connecting plates may extend inside the chamber such that they are not aligned with the center of at least one of the pin and the pile. Yet in further examples, at least some of the connecting plates may be radially distributed along the perimeter of the at least one of the pin and the pile. Other implementations of the present wind turbine foundation are also envisaged in which at least some of the connecting plates of the pin are substantially aligned with at least some of the corresponding connecting plates of the pile. The aligned connecting plates of the pin and the pile may extend substantially covering the width of the grouting chamber. Still in further examples, at least some of the connecting plates may be arranged symmetrically around the surface of at least one of the pin and the pile. 
     At least some of the pins may have a guiding end arranged opposite to its connection with the wind turbine and a stopping element extending, e.g. radially, inside the grouting chamber adapted to receive said guiding end such that when the pile is inserted into the pile the guiding end is fitted in the stopping element. In some cases, the guiding end may comprise a protruding element for centralizing the pin when mounting it into a pile. 
     The stopping element may comprise a cup, e.g. made of a high grade polymer, intended to be received into a central seat formed in the stopping element and wherein it further comprises a disc radially extending from the central seat towards the inner side of the pile. 
     The stopping element may be welded to the inner wall of the pile. Other ways for attaching the stopping elements to the piles are not ruled out. The stopping element could be also even formed integrally with the pile. 
     Yet in further implementations, the stopping element may further comprise a disc radially extending from the central seat towards the inner side of the pile. 
     The stopping element may also have a plurality of bars radially extending from the central seat towards the inner side of the pile. Disc segments may be provided in between the bars. 
     With the above defined wind turbine foundation a good fatigue resistance structure is obtained which is particularly useful in offshore wind turbine foundations. The present wind turbine foundation may be however useful in onshore wind turbine foundations. 
     The higher load capacity of the present wind turbine foundation over prior art foundations in which headed stud connectors or shear keys are used renders the present wind turbine foundation very advantageous for wind turbine applications. It has been shown that the connection behaviour until maximum loads are reached is essentially elastic as opposed to what happens in prior art solutions where important plastic slip is developed when maximum loads have been attained. 
     A more effective shear transfer is thus obtained by using the present robust, reliable, and easy to manufacture and install wind turbine foundation. This is mainly due to the improved load path transmission between the piles and pins and the grout within the chamber. The present wind turbine foundation provides a stiff and ductile connection of the wind turbine foundation to the seabed, which connection has excellent fatigue resistance. 
     Additional objects, advantages and features of implementations of the present wind turbine foundation will become apparent to those skilled in the art upon examination of the description, or may be learned by practice thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Particular implementations of the present wind turbine foundation will be described in the following by way of several non-limiting examples, with reference to the appended drawings, in which: 
         FIG. 1  is a general elevational view of an offshore wind turbine; 
         FIG. 2  is a diagrammatic part view of one example of the present wind turbine foundation; 
         FIG. 3   a  is an elevational sectional view of one example of the present wind turbine foundation in which one pin is shown fitted in one corresponding pile, with the pin fitted with connecting plates; 
         FIG. 3   b  is a elevational sectional view of an alternative example to the wind turbine foundation in  FIG. 3   a  in which both the pin and the pile is fitted with connecting plates; 
         FIG. 3   c  is a diagrammatic enlarged elevational view of a connecting plate of the Perfobond connector type; 
         FIGS. 4   a - 4   e  are top plan views of pin-pile arrangements showing different examples of the wind turbine foundation according to different connecting plate arrangements within the grouting chamber; 
         FIG. 5  is an enlarged elevational sectional view of the example shown in  FIG. 3   a  or  3   b;    
         FIG. 6  is a top plan view of a further example of the present stopping element; and 
         FIG. 7  is a perspective sectional part view of the example of the present stopping element in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Several examples of the wind turbine foundation are shown in the figures which will be disclosed herein as non-limiting examples. Like reference numerals refer to like parts throughout the description of the figures. 
       FIG. 1  generally shows an offshore wind turbine  100  installed on the sea S. The offshore wind turbine  100  mainly comprises a nacelle  110  that is fitted at the upper end of a tower  120 . The nacelle  110  of the offshore wind turbine  100  is provided with a rotor hub  130  provided with blades  140  for capturing the action of the wind for electricity production. 
     In the particular and non-limiting example shown in the figures, the wind turbine foundation comprises a jacket substructure  150  provided at a platform  156  in the bottom portion of a transition piece  125 . The jacket substructure  150  is formed of a number of tubes  155  connected to each other. The transition piece  125  is provided for connecting the platform  156  to the bottom portion of the wind turbine tower  120  as shown in  FIG. 1  of the drawings. 
     Apart from the above configuration for the wind turbine foundation, comprising (from top to bottom) a transition piece  125 , a platform  156  and a jacket substructure  150 , other configurations could be also possible, such as those comprising (from top to bottom) a transition piece  125  and a jacket substructure  150 , and a small platform attached to the jacket or the transition piece which is only for operations of maintenance and access purposes. 
     The wind turbine foundation  200  connects the wind turbine tower  120  through the jacket substructure  150  with the seabed SB. 
     In the present particular non-limiting example, the wind turbine foundation  200  comprises four piles  210 . Piles  210  consist of elongated cylindrical connecting members made of steel. Piles  210  are driven, i.e. inserted, into the seabed SB. 
     The wind turbine foundation  200  further comprises corresponding four substantially elongated cylindrical pins  250 . Each pin  250  has a connecting end and an opposite, inserting end. The pins  250  are attached to and extend downwards from the jacket substructure  150  of the wind turbine  100  through their connecting ends. 
     The piles  210  are shaped and sized for receiving the inserting end of the pins  250  so that at least one portion of the length thereof is accommodated therein. For this purpose the inner diameter of the piles  210  is larger than the outer diameter of the corresponding pins  250 . The inserting end of the pins  250  may be guided and they may also be slightly tapered for ease of insertion into the piles  210 . 
     A grouting chamber  260  is defined. It is formed by the annular space between the pins  250  and the piles  210  when the former are at least partially inserted into the latter. The grouting chamber  260  of the wind turbine foundation  200  is suitable for receiving grout.  FIGS. 3   a - 3   b  and  4   a - 4   e  clearly shows the grouting chamber  260 . 
     A number of connecting plates,  300   a,    300   b  are provided. Examples of connecting plates,  300   a,    300   b  are shown in  FIGS. 3   a - 3   c  and  4   a - 4   e  of the drawings. They can be steel connecting plates  300   a,    300   b  attached to the pins  250  or the piles  210  (or both) such that the plates  300   a,    300   b  radially extend inside the chamber. Attachment of the connecting plates  300   a,    300   b  to the piles  210  and/or pins  250  may be carried our through a single weld pass process. 
     A number of implementations are envisaged for the connecting plates  300   a,    300   b.  Some of said implementations are briefly explained below. The following arrangements can be combined with each other. 
     For example, there could be only connecting plates  300   a  attached to the pins  250 , as shown in  FIG. 3   a,  or to the piles  210 , as shown in  FIG. 4   b.  Alternatively, there could be connecting plates  300   a  attached to the pins  250  and there could be also connecting plates  300   b  attached to the piles  210 , such as shown in  FIGS. 3   b,    4   a,    4   c,    4   d  and  4   e  of the drawings. 
     In any case, a number of connecting plates  300   a,    300   b  in range from 2 to 16 is preferred. For example, each of the pile  210  and the pin  250  are both provided with four connecting plates  300   a,    300   b  in  FIG. 4   a.  Alternatively, in  FIG. 4   b  the pile  210  is provided with four connecting plates  300   b.  In the example in  FIGS. 4   c  and  4   d,  the pile  210  is provided with eight connecting plates  300   b  while the pin  250  is provided with four connecting plates  300   a.  In  FIG. 4   c  four connecting plates  300   a  are provided which are aligned with corresponding four of the eight connecting plates  300   b  of the pile  210 . In  FIG. 4   d  four connecting plates  300   a  are provided which are not aligned with corresponding four of the eight connecting plates  300   b  of the pile  210 . Finally, in  FIG. 4   e  each of the pile  210  and the pin  250  are both provided with eight connecting plates  300   a,    300   b  that are not aligned with each other. 
     At least some of the connecting plates  300   b  of the pin  250  may be substantially aligned with at least some of the corresponding connecting plates  300   a  of the pile  210  as shown in the example in  FIG. 4   c.    
     At least some of the connecting plates  300   a,    300   b  may extend a length L of at least one of the piles  210  and the pins  250  within the chamber  260  as shown in  FIGS. 3   a  and  3   b.  However, at least some of the connecting plates  300   a,    300   b  of at least one of the piles  210  and the pins  250  could project outwards the chamber  360 . 
     At least some of the connecting plates  300   a,    300   b  may extend at least partially across the inside of the grouting chamber  260  as shown in  FIGS. 3   a  and  3   b  and  4   a - 4   e.    
     Although not shown, at least some of the connecting plates  300   a,    300   b  may extend inside the chamber  260  such that they are not aligned with the center C of the pin  250  and/or the pile  210 . 
     At least some of the connecting plates  300   a,    300   b  may be radially distributed along the perimeter of the pin  250  and/or the pile  210  as shown in  FIGS. 3   a  and  3   b  and  4   a - 4   e.    
     At least some of the connecting plates  300   a,    300   b  may be arranged symmetrically around the surface of the pin  250  and/or the pile  210 . The connecting plates  300   a,    300   b  may evenly distributed on the perimeter of the pin  250  and/or the pile  210  although in some cases the connecting plates  300   a,    300   b  may arranged only in some portions of the perimeter of the pin  250  and/or the pile  210 . 
     In any of the above cases, at least some of the connecting plates  300   a,    300   b  are of the Perfobond type. More specifically, they are longitudinal plates provided with a number of holes  310   a,    310   b.  Holes  310   a,    310   b  are sized suitably such that the grout inside the chamber  260  passes when in use, that is when the pins  250  are at least partially inserted into corresponding the piles  210 . In the present examples, the holes  310   a,    310   b  in the Perfobond connecting plates  300   a,    300   b  are circular in shape. Other suitable shapes are however not ruled out. 
     In the example illustrated in  FIG. 3   c,  some design parameters of a Perfobond connecting plate  300   a  are depicted. In the example in  FIG. 3   c  the ratio of the distance d between the centers of two adjacent holes  310   a  in a connecting plate  300   a  to the diameter D of the holes  310   a  in said connecting plate  300   a  is shown. In said example, such ratio lies from 2.20 to 2.40. In some cases, the ratio of the distance d′ (see  FIG. 4   a ) between two adjacent connecting plates to the perimeter of the pin  250  or the pile  210 , or both the pin  250  and the pile  210 , may range from ½ to 1/16. 
     Turning back to  FIGS. 3   a  and  3   b  of the drawings, the inserting end of at least some of the pins  250  may be a guiding end  255 . The guiding end  255  is provided at a lower portion of the pins  250 , opposite to the connection end of the pin  250  where the pin  250  is connected with the tower  120  of the wind turbine  100 . 
     A stopping element  256  may be also provided. The stopping element  256  is radially arranged, welded to an inner wall  211  of the pile  210  within the grouting chamber  260  extending radially therein. Specifically, the stopping element  256  is arranged inside the pile  210  on a plane substantially transversal to a longitudinal axis  252  of the pile  210  as shown in  FIGS. 3   a,    3   b,  and  5 - 7 . 
     The stopping element  256  is adapted to receive the guiding end  255 . As shown in said  FIGS. 3   a,    3   b  and  5 - 7  the stopping element  256  includes a central seat  254  provided with a cup  257  made of a polymeric material such as a high grade polymeric material in which cup  257  the guiding end  255  of the pin  250  is fitted. A disc  258  is also provided extending radially between the central seat  254  and the inner wall  211  of the pile  210 . 
     The cross-section of the guiding end  255  is smaller than that of the rest of the pin  250 . In the intersection of the cross-section of the guiding end  255  and the pin  250 , the pin  250  has a protruding element, e.g. an annular ring or disc  253  for centralizing the pin  250  when it is being inserted into the pile  210 . This can be seen in  FIGS. 3   a,    3   b  and  5 . 
     The central seat  254 ′ in the example of  FIGS. 6 and 7  comprises an upward protrusion  254 ″. This upward protrusion  254 ″ is integrally formed with the central seat  254 ′ such that both, the central seat  254 ′ and the upward protrusion  254 ″, define a cup shaped seat. The cup shaped seat comprises a cup body  257 ′ made of a polymeric material such as a high grade polymeric material. The cup body  257 ′ as shown in the  FIGS. 6 and 7  is concentrically arranged on the inner surface of the upward protrusion  254 ″ (and on the central seat inner surface). 
     In the example shown in said  FIGS. 6 and 7  four bars  259  are provided in a cross configuration between the central seat  254  and an inner wall of the pile  210 . The bars  259  in any case may radially extend between the central seat  254 ,  254 ′ and the inner wall of the pile  210 . Disc segments  258 ′ are also provided in between these bars  259 . 
     Although only a number of particular implementations and examples of the present wind turbine foundation have been disclosed herein, it will be understood by those skilled in the art that other alternative implementations and/or uses and obvious modifications and equivalents thereof are possible. Furthermore, the present disclosure covers all possible combinations of the particular examples described. Thus, the scope of the present disclosure should not be limited by particular examples, but should be determined only by a fair reading of the claims that follow.