Abstract:
A wind power plant includes a tower, the tower including at least one tower section provided with at least one flange and forming at least part of the wall of the tower. The power plant further includes an element provided with at least one flange and adapted to be installed adjacent the tower section. The tower section and the element have abutting flanges, the flanges being releasably interconnected by sets of bolts and nuts. At least one surface associated with each set of bolt and nut and facing away from the flanges is provided with a plurality of recesses for receiving projections of a tightening device. A method of joining a tower section and an adjacent element is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims prior under 35 U.S.C. § 119(a) to DK Application No. PA 2008 01671, filed Nov, 27, 2008. This application also claims the benefit of U.S. Provisional Application No. 61/118,680, filed Dec. 1, 2008. Each of these applications is incorporated by reference herein in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]    The present invention generally relates to a wind power plant and a method for assembling a wind power plant. 
       BACKGROUND 
       [0003]    Towers for wind power plants may be constructed from, for example, steel lattice frameworks, concrete, steel tubes or composite materials. Currently, the towers are predominantly made of a number of tubular steel sections mounted on top of each other and interconnected by using bolted flange joints. The method of connecting objects by means of bolted flange joints has long been known in the art and the method is universally employed. 
         [0004]    The flanges are usually provided with a plurality of through-holes uniformly distributed along the flanges. This arrangement allows for a large number of bolts and corresponding nuts to be used. 
         [0005]    One way to assemble sections of a wind turbine tower by using bolts and nuts is disclosed in US patent application US2006000185. 
         [0006]    However, ever larger and heavier towers are being continuously developed as this is a fairly simple way to increase power output of the wind power plant by taking advantage of the higher wind speed, and thus higher energy content, at higher altitude. As a consequence, the wind loads on the tower have increased significantly and the strength requirements of the wind tower components in general and their flange joints in particular will also increase. Consequently, the required number of bolts and corresponding nuts has also increased significantly. However, this is in conflict with the constant goal of reducing the weight of the tower, the material consumption, the work required to install the wind power plant and thus the total cost. 
       SUMMARY  
       [0007]    In view of the above, one embodiment in accordance with aspects of the invention renders possible the use of an increased number of nuts and bolts in order to interconnect a flange of a tower section with a flange of an adjacent element. 
         [0008]    Another embodiment in accordance with aspects of the invention decreases the size of the flanges of both the tower sections as well as the adjacent element, thereby achieving material savings. 
         [0009]    In yet another embodiment, the structural strength of the tower as a whole is increased. 
         [0010]    A further embodiment in accordance with aspects of the invention provides a method for assembling a tower, the tower being part of the wind power plant. 
         [0011]    Embodiments in accordance with aspects of the invention relate to a wind power plant comprising a tower, the tower comprising at least one tower section provided with at least one flange and forming at least part of the wall of the tower, the power plant further comprising an element provided with at least one flange and adapted to be installed adjacent the tower section, wherein the tower section and the element have abutting flanges, the flanges being releasably interconnected by sets of bolts and nuts, wherein at least one surface associated with each set of bolt and nut and facing away from the flanges is provided with a plurality of recesses for receiving projections of a tightening device. 
         [0012]    By providing a surface of each bolt or nut facing away from the flanges with a plurality of recesses and consequently enabling it to receive projections of the tightening device, a torque may be applied by means of the tightening device without engaging the external lateral surfaces of the bolt or the nut. Instead, the interface surface for transmitting the torque to the bolt or the nut is the surface provided with a plurality of recesses. As a consequence, less space is required in order to tighten individual sets of bolts and nuts. 
         [0013]    This means that sets of bolts and nuts may be positioned closer to the tower wall since less space around the nut is required for tightening the bolt. Accordingly, the flange width can be reduced, thereby reducing the material consumption, production costs and also the weight of the tower. 
         [0014]    Furthermore, the individual sets of bolts and nuts may be positioned closer to each other, ensuring that an increased number of bolts and nuts may be installed along the circumference of the flanges. The increased number in combination with the closer positioning of the bolts and nuts relative the tower wall may increase structural stability of the tower. 
         [0015]    The element adapted to be installed adjacent the tower section can be another tower section, a tower foundation, a nacelle or some other component of the wind power plant. Accordingly, the sets of bolts and nuts can be arranged in any interconnection between the main structural parts of a wind power plant, which structural parts are interconnected by flange joints. 
         [0016]    The nuts can be provided with the recesses, whereby the bolts as such can be standardized and readily available bolts. 
         [0017]    The bolts can each have a bolt head provided with the recesses. Such bolt can either be used with a standardized, readily available nut or together with a nut provided with recesses. 
         [0018]    At least one of the sets of bolts and nuts can comprise a stud bolt and two nuts, the two nuts each provided with the recesses. A stud bolt is known as a shaft being threaded along its full length or along a portion of its two ends. The stud bolt can be a standardized and readily available stud bolt. 
         [0019]    Each bolt can further comprise an at least partially threaded shaft, the bolt being arranged such that its shaft is parallel to the longitudinal axis of the tower, and wherein the distance between a wall of the tower facing the flange and an outer periphery of the shaft is less than 70 mm, more preferred less than 60 mm and most preferred less than 50 mm. 
         [0020]    It is to be understood that the distance to the tower wall depends on the size of the nut and the bolt head. Since the torque required to tension the bolt and nut can be applied by a tightening device without engaging the lateral sides of the bolt head or the nut, the bolt head or the nut can be arranged directly adjacent the tower wall. Accordingly the flange width can be reduced while maintaining the structural stability of the tower. 
         [0021]    Each bolt can be arranged such that a head thereof is orientated downwards. The technician installing the wind tower may thus perform his tasks in an optimal working position. This also facilitates the connecting of the bolt with the corresponding nut, since threading of the nut on the shaft of the bolt may be visually controlled. 
         [0022]    The recesses can be provided with such depth that projections of a tightening device are fully insertable into the recesses. In this way, the contact of the bolt or nut surface provided with recesses and the surface of the tightening device from which the projections extend is ensured. The friction between these two surfaces may provide additional grip while the bolt and nut are being tightened. 
         [0023]    The recesses can be provided with such width that the inner lateral surfaces of the recesses abut outer lateral surfaces of projections of a tightening device when the projections are inserted into the recesses. In this way, a close fit of the tightening device and the bolt or nut is achieved while the bolt and nut are being tightened. This may contribute to an improved torque transfer from the tightening device to the bolt or nut. 
         [0024]    The recesses can be cylindrical blind bores arranged parallel to each other. In this way, a simplified manufacturing process of the bolts or nuts, using relatively simple tools may be achieved. This may reduce the overall production costs. It is to be understood that recesses with the same function can be arranged as through-holes. 
         [0025]    According to another aspect, embodiments in accordance with aspects of the invention relate to a method for joining a tower section to an adjacent element of a wind power plant comprising: arranging the tower section and the element such that a flange of the tower section is brought into contact with a flange of the element, aligning holes arranged in the flange of the tower section with holes arranged in the flange of the element, interconnecting the flanges by sets of bolts and nuts, the bolts extending through at least some of the aligned holes, and tightening each set of bolt and nut with a tightening device having projections adapted to engage recesses provided in at least one surface associated with the set and facing away from the flanges. 
         [0026]    The method allows, as has been discussed above in view of the wind power plant, sets of bolts and nuts to be positioned closer to the tower wall. This may make it possible to reduce the width of the flange, thereby reducing the material consumption and the total weight of the tower. 
         [0027]    Furthermore, the inventive method may make it possible to install an increased number of sets of bolts and nuts along the circumference of the flange, thereby increasing the structural stability of the tower. 
         [0028]    The method can further comprise the step of arranging each bolt such that a head thereof is oriented downwards. The technician installing the tower may thus perform his tasks (e.g. tightening the sets of bolts and nuts) in an optimal working position. This also facilitates the connecting of the bolt with the corresponding nut, since threading of the nut on the shaft of the bolt may be visually controlled. 
         [0029]    Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, from the attached claims as well as from the drawings. 
         [0030]    Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the [element, device, component, means, step, etc]” are to be interpreted openly as referring to at least one instance of the element, device, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements. 
           [0032]      FIG. 1  is a schematic view of a wind power plant; 
           [0033]      FIG. 2  is a cross-sectional view of a flange joint between two adjacent sections of the tower without any bolts and nuts; 
           [0034]      FIG. 3  is a cross-sectional view of a flange joint between two adjacent sections of the tower with a plurality of sets of bolts and nuts; 
           [0035]      FIG. 4  shows a first embodiment of a set of a bolt and a nut; 
           [0036]      FIG. 5  shows a second embodiment of a set of a bolt and nut; and 
           [0037]      FIG. 6  shows one embodiment of a tightening device. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]      FIG. 1  is a schematic view of a wind power plant  1  with a nacelle  3  supported by a tower  2 . The purpose of the tower  2  is to support the weight of a nacelle  3  that is arranged on top of the tower  2 . Further, it serves to position rotor blades  4  to a suitable operational height. 
         [0039]    Modern towers comprise a plurality of tubular tower sections  5 , which may be manufactured in 20-30 meter long segments and subsequently mounted on top of each other to provide the tower with a sufficient height. Typically, the tower sections are made of steel. The shape of the individual tower sections are normally either straight cylindrical or frusto-conical. 
         [0040]      FIG. 2  is a vertical cross-sectional view of a portion of an upper  8  and a lower  9  tower section with corresponding flanges  6 ,  7 . 
         [0041]    Each tower section  8 ,  9  normally comprises a flange positioned at each end of the tower section. A first flange  6  is attached to the upper tower section  8  and a second flange  7  is attached to the lower tower section  9 . In the disclosed embodiment each flange  6 ,  7  runs along the entire circumference of the associated tower section  8 ,  9 . It is to be understood that the flanges may also be divided into a number of flange sections which together form a circumferential continuous or non-continuous flange. 
         [0042]    For practical reasons, each flange  6 ,  7  is normally mounted as a separate part to the individual tower section by using a suitable joining technique, for example welding. 
         [0043]    The first flange  6  has a substantially L-shaped cross-section with a first leg  10  extending horizontally towards the interior of the tower, thus forming a substantially perpendicular angle with the longitudinal axis LA of the tower sections  8  and  9 , and a second leg  11  attached to the upper tower section  8  by means of a welding seam  50  at the lower edge  12  of the upper tower section  8 . 
         [0044]    Analogously, the second flange  7  also has a substantially L-shaped cross-section with a first leg  13  extending horizontally towards the interior of the tower, thus forming a substantially perpendicular angle with the longitudinal axis LA of the tower sections  8 ,  9 , and a second leg  14  attached to the lower tower section  9  by means of a welding seam  50  at the upper edge  15  of the lower tower section  9 . 
         [0045]    The inwards extending horizontal first legs  10 ,  13  of the first and second flanges  6 ,  7  are in direct contact with each other. 
         [0046]    Each flange  6 ,  7  is provided along its circumferential extension with a number of through-holes  16 . The through-holes  16  may be circumferentially equally spaced from one another or arranged in groups. The through-holes  16  of the first flange  6  are aligned with the through-holes of the second flange  7 . 
         [0047]    The flanges  6 ,  7  are used as flat support surfaces when the two adjacent, upper  8  and lower  9 , tower sections are to be interconnected. 
         [0048]    Now turning to  FIG. 3  a portion of the upper and lower tower sections  8 ,  9  with the corresponding flanges  6 ,  7  are shown in an interconnected state. 
         [0049]    As it may be seen, the flanges  6 ,  7  are interconnected by means of a plurality of sets  18  of bolts  19  and nuts  20  engaging the through-holes  16 . The bolts can be provided with washers (not shown). 
         [0050]    Typically, each bolt  19  is arranged such that a bolt head  25  thereof, when installed, is orientated downwards. However, upwards orientated bolt heads  25  are also conceivable. Typically, standardized and readily available bolts are used. The bolt  19  can be threaded along the full length of its shaft  26  or, as shown, be threaded only along its free end. 
         [0051]    As can be seen in  FIGS. 3 and 4 , each nut  20  has an internal thread which engages the threaded section of the bolt shaft  26 . The nut  20  according to the shown embodiment has circular cross-section, although it is to be understood that other cross-sectional shapes are equally possible. This also applies to the bolt head  25 . 
         [0052]    The surface  22  of the nut  20  facing away from the bolt head  25  and flange is provided with a plurality of recesses  21  extending in the longitudinal direction of the shaft  26 . In the disclosed embodiment the recesses  21  are shaped as cylindrical blind bores and arranged parallel to one another and circumferentially uniformly distributed. Still it is to be understood that any pattern and geometry can be used. Also, it is to be understood that the recesses can be arranged as through-holes. Typically, there are 6-12 recesses  21  per nut  20 , depending on the size of the nut. 
         [0053]    Although not shown, it is to be understood that generally the bolt head  25  as such can be provided with recesses corresponding to the recesses of the nut. 
         [0054]    Now turning to  FIG. 5 , as an alternative to a threaded bolt and a nut provided with the recesses, the bolt can be a so called stud bolt  19 ′, i.e., a shaft  26 ′ provided with threads, either along its full length or along its two free ends. Such stud bolt  19 ′ is used together with two nuts  20 ′, each having the above disclosed recesses  21 . 
         [0055]      FIG. 6  shows one embodiment of a tightening device  27 . The tightening device  27  typically comprises an engagement head  30  having a plurality of circumferentially distributed projections  28 . The distribution pattern as well as the size and geometry of the projections  28  correspond to those of the recesses  21  of the nut  20 . A dedicated tightening device  27  may be made available for each nut size. 
         [0056]    The tightening device  27  may, as shown in  FIG. 6 , include a handle  31  adapted to operate in conjunction with the engagement head  30 . The handle  31  can be a fixed handle, an interchangeable handle or even a ratchet wrench. 
         [0057]    Referring back to  FIGS. 2 and 3 , when joining the two tower sections  8 ,  9  by the flange joint, the two tower sections are arranged on top of each other with the two opposing flanges  6 ,  7  lying flat against each other so that their corresponding through-holes  16  are aligned. A bolt  19  is arranged in each through-hole  16  thus formed and a nut  20  is threaded onto the free end of the bolt  19 . Then the engagement head  30  of the tightening device  27  is arranged to the nut  20  with the projections  28  inserted to the recesses  21 . Finally, the bolt  19  is tensioned by applying a sufficient torque to the nut  20 . This procedure is repeated throughout the flange joint. 
         [0058]    By allowing play between the bottom surface of the recess  21  and the front surface of the projection  28 , i.e., the depth of the recesses is exceeding the length of the projections, the projections  28  of the tightening device  27  are fully insertable into the recesses  21 . This means that the contact between the nut surface  22  provided with recesses  21  and the surface of the tightening device  27  from which the projections  28  extend is ensured. Consequently, the friction between the inner lateral surfaces of the recesses and the outer lateral surfaces of the projections provide additional grip while the nut  20  is being tightened. 
         [0059]    By providing recesses  21  with such width that their inner lateral surfaces abut the outer lateral surfaces of the projections  28 , a close fit of the tightening device  27  and the nut  20  is achieved while the nut  20  is being tightened. This may contribute to an improved torque transfer from the tightening device  27  to the nut  20 . 
         [0060]    By the design of the recesses  21  and the projections  28 , the tightening device  27  can engage the nut  20  and tighten it on the bolt  19  without engaging the lateral surfaces of the nut  20 . As a consequence, less space is required in order to tighten individual nuts  20 . 
         [0061]    By the tightening device  27  engaging the nut  20  in the longitudinal direction of the bolt  19  without any circumferential outer engagement, a larger number of sets  18  of bolts  19  and nuts  20  can be used allowing a higher structural strength of the tower. Also, the width of the flanges can be reduced since the sets  18  of bolts  19  and nuts  20  can be arranged closer to the longitudinal wall of the tower section, allowing a reduced weight and material consumption and thus a cheaper tower design. 
         [0062]    The above disclosed design of the flange joint is known as a L-flange joint. Although the flanges are disclosed as being directed towards the interior of the tower, they can within the scope of the invention be directed outwards from the interior of the tower. Also, the inventive concept is equally applicable to a T-flange joint (not shown) wherein each flange has a T-shaped cross-section with a first leg extending horizontally into the interior of the tower and a second leg extending horizontally to the exterior of the tower, wherein both legs are provided with through-holes to receive sets of bolts and nuts. 
         [0063]    The above disclosed flange joint has been described with regard to two adjacent, upper and lower, tower sections that are to be interconnected. However, the disclosed flange joint is also conceivable with regard to other elements of the wind power plant, such as between a tower section and a part of a tower foundation or between the nacelle and an upper most tower section. In case the sets of bolts and nuts are used between the nacelle and the uppermost tower section, the bolts are preferably arranged to extend through a yaw system, which yaw system allows the nacelle to turn in view of the tower. The flange joint is also applicable while connecting other elements such as transition pieces to the tower sections. 
         [0064]    The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible.