Abstract:
A bolt connection of a tower for a wind turbine is provided, the bolt connection including a tower base module with a tower flange, wherein the tower flange forms the bottom end of the tower, a base flange for connecting the tower base module with a foundation, wherein the base flange includes a traverse bore, and a bolt which extends through the traverse bore of the base flange, wherein the bolt includes a bolt head which is arranged at the bottom side of the base flange facing the foundation. The bolt connection further includes a clamping element which at least partially covers the bolt head. The clamping element presses the bolt head against the bottom side of the base flange.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to European application No. 16181486.8 having a filing date of Jul. 27, 2016, the entire contents of which are hereby incorporated by reference. 
       FIELD OF TECHNOLOGY 
       [0002]    The following relates to a bolt connection of a tower for a wind turbine. Furthermore, the disclosed relates to a wind turbine comprising a tower and a foundation, wherein the tower is attached to the foundation by a bolt connection. 
       BACKGROUND 
       [0003]    Due to the considerable loads that a tower of a wind turbine experiences during its lifetime, the tower needs to be firmly attached to the foundation of the wind turbine. This connection between the tower and the foundation is typically realized by a plurality of bolts. A flange is often situated on top of the foundation. The bolt connection then connects this flange with the bottom end of the tower. 
         [0004]    Conventionally, the bottom end of the tower also comprises a flange, which corresponds to the flange which is present on the foundation. The foundation may be made of concrete and the flange which connects the foundation with the bottom end of the tower is placed in liquid concrete. In other words, the concrete has not yet solidified. 
         [0005]    In the case that the flange connects the bottom end of the tower via bolts these bolts are typically arranged such that the bolt head is at the bottom, i.e. facing the foundation. After solidification of the concrete, a nut or a similar fastening element tightens the bolt at the side which is facing the bottom end of the tower. 
         [0006]    In the described approach of connecting the tower with the foundation, the bolt head is surrounded by liquid concrete during a certain time period. This leads to the problem that concrete or grout could enter into the space between the bolt head and the flange. 
         [0007]    This problem has been addressed in the international patent application WO 2011/047723 A1. Therein, it is proposed to provide a disc or cover to a nut cage, wherein the nut cage surrounds the nut which is at the bottom side of the flange. Such a covered nut cage claims to be able to reduce the risk for grout or concrete entering the space between the bolt head and the base flange. 
         [0008]    This solution, however, has the drawback that it requires a relatively complex and costly provision of a covered nut cage. 
         [0009]    Therefore, there exists the desire to provide a more simple and inexpensive way of reducing the risk that impurities such as grout or concrete enters the space between a bolt head and a flange of a bolt connection. 
       SUMMARY 
       [0010]    An aspect relates to a bolt connection of a tower for a wind turbine, wherein the bolt connection comprises a tower base module with a tower flange, a base flange for connecting the tower base module with the foundation, and a bolt which extends through the traverse bore of the base flange. The tower flange forms the bottom end of the tower. The base flange comprises the traverse bore. The bolt comprises a bolt head which is arranged at the bottom side of the base flange facing the foundation. Furthermore, the bolt connection comprises a clamping element which at least partially covers the bolt head. The clamping element presses the bolt head against the bottom side of the base flange. 
         [0011]    A key aspect of the embodiments of the present invention is that a simple and efficient concept to prevent impurities entering the space between the bolt head and the base flange is provided herewith. In contrast to the known art this is achieved in a simple and cost efficient manner by providing a clamping element which is able to exert a force on the bolt head. In other words, the bolt head is pressed, i.e. pushed against or towards the bottom side of the base flange. 
         [0012]    A further advantage of the clamping element as described in this patent application is the prevention of rotation of the bolt head when the nut of the bolt connection is tightened. This effect may also be described as a prevention or minimization of any torsional movement of the bolt head. This advantage is particularly realized in the case that at least some of the lateral surfaces of the clamping element cover at least some of the lateral surfaces of the bolt head. 
         [0013]    Another advantage of the clamping element in connection with the bolt connection of embodiments of this invention is that the bolt head is fixed with regard to any displacement of the bolt head in the direction of the longitudinal axis of the bolt. This may also be described as a prevention or minimization of any vertical displacement of the bolt. 
         [0014]    In an embodiment of the invention, the bolt head comprises an end face and one or more lateral surfaces, wherein the end face faces a way from the bottom side of the base flange. 
         [0015]    In contrast to the lateral surfaces, i.e. the side surfaces, the end face characterizes the part of the bolt head which connect the lateral surfaces. In the set up according to embodiments of the invention, the bolt which extends through the traverse bore of the base flange is arranged such with regard to the base flange and the tower base module that the bolt head of the bolt is at the bottom. 
         [0016]    In other words, the bolt head as a whole is facing the foundation of the wind turbine. Concentrating on the bolt head alone, the bolt head is arranged such with regard to the base flange and the foundation that the end face is facing the foundation. In other words, the end face of the bolt head faces away from the bottom side of the base flange. An advantage of a bolt head which comprises distinct side surfaces and a distinct end face is that the clamping device can be arranged and adapted to the bolt head in an easy manner. 
         [0017]    The clamping element may comprise an attachment section. The attachment section has the function of providing means or a device for attaching the clamping element to the bottom side of the base flange. The clamping element may furthermore comprise one or more lateral surfaces for covering the one or more lateral surfaces of the bolt head. Additionally, the clamping element may comprise an end face for covering the end face of the bolt head. 
         [0018]    Descriptively speaking, the clamping element is advantageously designed and configured such that its shape is adapted to the shape and design of the bolt head. Consequently, the clamping element also features side surfaces, i.e. lateral surfaces, and/or features a front face, i.e. an end face. 
         [0019]    In order to most efficiently apply a force which is pressing the bolt head against the bottom side of the base flange, the end face of the clamping element may actually cover at least a part of the end face of the bolt head. 
         [0020]    Note that, in general, only some, but not necessarily all lateral surfaces of the bolt head may be at least partially covered by the clamping element. 
         [0021]    Alternatively, all lateral surfaces of the bolt head may be at least partially covered by the clamping element. Covering the bolt head completely has the advantage that the bolt head is firmly fixed and any rotational movement of the bolt head is efficiently prevented. Such a prevention of any rotational movement of the bolt head with regard to the base flange may, however, also be achieved by only covering one, two or more lateral surfaces. 
         [0022]    In another embodiment of the invention, the end face of the clamping element comprises an indentation for exerting compressive forces on the bolt head. In other words, the clamping element may comprise a kink in the end face for applying forces to press, i.e. to push the bolt head to the bottom side of the base flange. 
         [0023]    Note that, advantageously, the indentation of the end face of the clamping element is designed such that a certain pre-tensioning or pre-compression is present when the clamping element is attached to the base flange and the bolt head. 
         [0024]    Note that the provision of an indentation in the end face of the clamping element is only one option to realize the clamping function of the clamping element. Any other ways of realizing the clamping mechanism may also be used and implemented. 
         [0025]    In another embodiment of the invention, the at least one lateral surface of the clamping element comprises an angled inclination. 
         [0026]    Such an angled inclination has the advantage that space for a washer or similar is provided, wherein the washer is provided and foreseen to be arranged between the bolt head and the bottom side of the base flange. In the case of a washer which has a diameter which is not greater than the diameter of the bolt head such an angled inclination may not be necessary. However, in the more typical case that the diameter of the washer is greater than the diameter of the bolt head, the clamping element fits better with the bolt head and the whole arrangement if there is enough space for the washer between the bolt head and the bottom side of the base flange. 
         [0027]    Such an inclined lateral surface of the clamping element may comprise an angle of 45° for example. Furthermore, the angled inclination may cover the half of the lateral surface but may alternatively also just cover a small part close to the attachment section of the clamping element. 
         [0028]    In a particular embodiment of the invention, the bolt head has six lateral surfaces, resulting in a substantially hexagonal cross section as viewed in a plane perpendicular to the longitudinal axis of the bolt. In this example, advantageously at least two of the lateral surfaces are covered with the clamping element. 
         [0029]    Such a hexagonal bolt head is a standard type of a bolt head and the use of such a standard bolt head has the advantage that standard bolts with standard bolt heads can be used. In this case at least two, but also four or, for example, six lateral surface may be covered by the clamping element. 
         [0030]    The clamping element may for example be configured as a bracket. That means that the lateral surfaces and the end face of the clamping element are relatively thin and are configured and designed as plates. This is advantageous in order to save material and space and also to form the clamping element such that compressive forces can be applied to the bolt head. 
         [0031]    Advantageously, the bolt connection further comprises attachment bolts for attaching the clamping element to the base flange via the attachment section of the clamping element. These attachment bolts obviously may be designed much smaller and less robust than the bolt of which the bolt head needs to be pressed or pushed against the base flange. By using appropriate attachment bores, the attachments bolts may easily be attached and screwed into the base flange. 
         [0032]    In practice, it is advantageous to provide a plurality of bolts and a plurality of corresponding traverse bores, wherein each of the bolts and the traverse bores realize a bolt connection according to the embodiments described above. This plurality of bolts may, for example, be arranged in an annular manner, in particular in the typical case that the base flange is designed as a ring. An amount of 50 or even 100 bolts is not uncommon for large wind turbine towers to be connected with the foundation of the wind turbine. 
         [0033]    The following is further directed towards a wind turbine comprising a tower and a foundation wherein the tower is attached to the foundation by means of a bolt connection, in particular a plurality of bolt connections, as described above. 
     
    
     
       BRIEF DESCRIPTION 
         [0034]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
           [0035]      FIG. 1  shows a wind turbine; 
           [0036]      FIG. 2  shows a bolt connection with a clamping element according to embodiments of the invention; 
           [0037]      FIG. 3  shows an example of such a clamping element; and 
           [0038]      FIG. 4  shows a view onto the bottom side of the base flange with bolt connections according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    The illustration in the drawing is in schematic form. It is noted that in different figures similar or identical elements may be provided with the same reference signs. 
         [0040]      FIG. 1  shows a wind turbine  10 . The wind turbine  10  comprises a tower  11  with a bottom end  111  and a top end  112 . At the bottom end  111  of the tower  11 , there is provided a base flange  30  which is connected to the tower  11 . The base flange  30  is also connected to a foundation  16 . Therefore, it can also be stated that the tower  11  of the wind turbine  10  is attached or connected to the foundation  16  of the wind turbine  10  via the base flange  30 . 
         [0041]    At the top end  112  of the tower  11 , a yaw bearing  12  rotatable connects the tower  11  with a nacelle  13  of the wind turbine  10 . The nacelle  13  accommodates the generator and parts of the rotor of the wind turbine  10 . The yaw bearing  12  enables a rotational movement about an axis which is substantially parallel to the longitudinal axis of the tower  11 . This axis is commonly referred to as the yaw axis. 
         [0042]    The nacelle  13  is connected with a hub  14 . The hub  14  is rotatable with regard to the nacelle  13 . The hub  14  may be seen as a part of the rotor. A plurality of rotor blades  15  are mounted to the hub  14 . In the example as shown in  FIG. 1 , the wind turbine  10  comprises three rotor blades  15  of which two rotor blades  15  are visible in  FIG. 1 . 
         [0043]      FIG. 2  focuses on the connection of the tower  11  with the foundation  16 . Note that the foundation  16  itself is not shown in  FIG. 2 , for the sake of clarity. 
         [0044]    It can be seen that the tower  11  is represented by a tower base module  20  which comprises an “extension” at its bottom side. This extension is referred to as the tower flange  21 . The tower base module  20  may be one of several modules or segments of the tower  11 . The provision of a tower  11  comprising several segments is preferred in the case of large towers exceeding a length of fifty meters. The tower base module  20  may, for example, have a length of five meters to ten meters. The tower flange  21  comprises a traverse bore  211 . The traverse bore  211  of the tower flange  21  is arranged and prepared for receiving the bolt  40  of the bolt connection between the tower base module  20  and the base flange  30 . For this purpose, the base flange  30  also comprises a traverse bore which is referenced by the reference sign  31  in the figures. The traverse bore  31  of the base flange  30  is arranged and prepared to receive the bolt  40 . Preferably, the traverse bolt  31  is aligned with a traverse bore  211  of the tower flange  21 . 
         [0045]    The right side of  FIG. 2  illustrates a bolt  40  as it could exemplarily be used in the inventive bolt connection. As illustrated, the bolt  40  features a longitudinal axis  43 , a bolt head  41  at one end, and a threaded part at the other end. The bolt head  41  of the bolt  40  has exemplarily six lateral surfaces  412 , two of which are visible in  FIG. 2 . There can also be seen the end face  411  of the bolt head  41 . 
         [0046]    At the left side of  FIG. 2 , the bolt  40  is inserted into the traverse bores  31 ,  211  of the base flange  30  and the tower flange  21 , respectively. The bolt  40  is tightened with a nut  42 . Furthermore, there are provided a first washer  61  between the nut  42  and the tower flange  21 , and a second washer  62  between the bolt head  41  and the bottom side  301  of the base flange  30 . These washers  61 ,  62  facilitate a durable tightening of the bolt  40 . 
         [0047]      FIG. 2  also illustrates a clamping element  50  which covers the bolt head  41  of the bolt  40 . The clamping element  50  comprises two attachment sections  51 , one on the left side and one on the right side. Furthermore, it comprises two lateral surfaces  52 , again one on the left side and one on the right side. Furthermore, the clamping element  50  comprises an end face  52  with an indentation  54  in the center area of the end face  53 . This indentation enables a compressive force of which the direction is illustrated by the arrow  55 . In other words, the clamping element  50  applies a compressive force which pushes, i.e. presses the bolt head  41  towards the bottom side  301  of the base flange  30 . This has the technical effect that entering of grout or other impurities is minimized or even completely eliminated once that the bolt connection is surrounded by liquid concrete for example. 
         [0048]    Finally note the further traverse bores  31  of which two of them are illustrated in  Figure 2  and which serve as bores for inserting bolts which connect the base flange  30  with the foundation  16 . 
         [0049]      FIG. 3  shows an exemplary clamping element  50 . In particular,  FIG. 3  shows a cross sectional view of such a clamping element  50 . The clamping element  50  comprises two attachment sections  51 , one depicted on the left side of  FIG. 3 , one depicted on the right side of  FIG. 3 . Each of the attachment sections  51  comprises an attachment bore  57 . The attachment bores  57  are destined for providing a hole for inserting corresponding attachment bolts (not shown). 
         [0050]    Furthermore, the clamping element  50  comprises two lateral surfaces  52 , wherein the lateral surfaces  52  are connected with the attachment sections  51  via an inclination  56 . The inclination  56  is included for facilitating the insertion of a washer between the bolt head of the bolt and the surface of the base flange  30 . In the example of  FIG. 3 , the inclination  56  comprises an inclination angle  561  of approximately forty-five degree. 
         [0051]    Furthermore, the clamping element  50  comprises an end face  53 , connecting both lateral surfaces  52  with each other. The end face  53  comprises an indentation  54  for applying a clamping force, in particular a compressive force of the clamping element  50  against the bolt head of the bolt. In the example of  FIG. 3 , the indentation  54  is approximately located at the center of the end face  53 . Such an indentation can also be referred to as a kink in the end face  53 . 
         [0052]    The particular shape, in particular the depth of the indentation  54 , is chosen according to the dimensions of the clamping element  50  and the corresponding bolt head. For instance, an end face angle  531 , which is defined as the angle between the lateral surface  52  and the end face  53 , would be smaller than ninety degree. In the example as illustrated in  FIG. 3 , the end face angle  531  is approximately eighty degree. 
         [0053]    Note that, in general, other mechanisms for enabling a clamping effect of the clamping element with regard to the bolt head of the bolt are possible. Also note that the size and angle of the inclination may also be varied according to the particular specification and dimension of the washer to be used in connection with the bolt connection. 
         [0054]      FIG. 4  illustrates a perspective view of several bolt connections. In particular, a view on the bottom side  301  of the base flange  30  of a wind turbine is shown. It can be seen that the base flange  30  comprises a plurality of radially inner bores, which are referred to as to further traverse bores  32 . Similar to the radially inner ring of further traverse bores  32 , there are provided a radially outer ring of another set of further traverse bores  32 . The further traverse bores  32  are arranged and prepared for a connection between the base flange  30  and a foundation of a wind turbine. Such a connection is preferably also realized by a set of bolts. However, this connection between the base flange  30  and the foundation of the wind turbine is not the content of embodiments of this invention. 
         [0055]    Approximately at the center part of the ring shaped base flange  30 , there are provided the bolts of which the respective bolt heads are visible. The bolt heads are referred to by the reference sign  41 . In a particular embodiment as illustrated in  FIG. 4 , each bolt head  41  comprises six lateral surfaces  412 . Two of the six lateral surfaces  412  are covered by the clamping element  50 . The clamping element substantially comprises the characteristics as shown in the cross sectional view of  FIG. 3 . Thus, it comprises attachment sections  51 , lateral surfaces  52 , an end face  53  and an indentation  54 , the latter one only faintly be visible in  FIG. 4 . Additionally, the clamping element  50  comprises an inclination  56  in order to facilitate accommodation of a washer, in particular the second washer  62 . 
         [0056]    In  FIG. 4  there is furthermore depicted attachment bolts  60  of which two of them connect one clamping element  50  with the base flange  30 . In the perspective view of  FIG. 4  it can be well discerned that the indentation  54  is able to exert compressive forces onto the bolt head  41 . 
         [0057]    In another embodiment of the invention, the base flange  30  may be segmented in several base flange segments. This has the advantage that transportation and also manufacturing of the base flange is facilitated. 
         [0058]    Although the present invention has been described in detail with reference to the preferred embodiment, it is to be understood that the present invention is not limited by the disclosed examples, and that numerous additional modifications and variations could be made thereto by a person skilled in the art without departing from the scope of the invention.