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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
       [0001]    The present invention concerns a device for indicating correct bolt holt alignment, and possibly also for aligning bolt holes, during assembly of a tower section part of a wind turbine plant. Also claimed is a tower section configured to form part of a wind turbine and having such an indicator device, for indicating correct alignment and possibly also directional/rotational guiding of the structural part during assembly with another structural parts. 
         [0002]    A bolt hole alignment device and a structural part of a wind turbine having such an alignment device is already known, wherein the alignment device is secured to a flange of one of the structural parts. 
         [0003]    To increase personnel safety the alignment and assembly of two wind turbine tower sections should ideally be carried out without any personnel remaining inside the already standing hollow tower section anywhere near the upper end thereof. While this may be possible with the prior art solutions it remains a problem that the alignment devices should also be easy to install and preferably also easy to remove and reuse for the purpose of performing assembly with a next tower section. It is also a problem that the prior art devices for aligning bolt holes are easily damaged requiring extreme care on the side of a crane operator and, hence, adding to the construction time. 
       OBJECT OF THE INVENTION 
       [0004]    The present invention sets out to solve the aforementioned problems by providing a novel bolt hole alignment indicator device which may—in a preferred embodiment—also assist in the actual alignment, as well as a method for assembling sections of a tower of a wind turbine plant using the novel indicator device, wherein the indicator device may be mounted to the inside face of a flange of one of the tower section from the inside of the latter, using—as desired—a dedicated connector in the form of a magnet. The indicator device may be an integral part of a primary alignment device used for aligning the central axis of the two tower section and wherein a leverage action arising from external loads on the primary alignment device during the assembly procedure provides temporary forces required to maintain the alignment device in correct position. 
       SUMMARY OF THE INVENTION 
       [0005]    More specifically the invention proposes use of a specialized indicator device, for indicating proper alignment of bolt-and-nut holes of two abutting flanges of two tower sections of a wind turbine tower structure, the indicator device comprising a connector for connecting the indicator device to an inner face of one of the flanges, and a body having a first body end and an opposite second body end closer to the connector, the indicator device including a resiliently deformable part, a pneumatic system or a hydraulic system allowing for the indicator device to assume a normal configuration wherein the first body end is distant from the connector, and a second configuration wherein the first body end is located closer to the connector, the resiliently deformable part or pneumatic system or hydraulic system urging the first body end from the second configuration towards the first configuration. 
         [0006]    It is foreseen that three or more indicator devices may be used, being preferably equidistantly spaced along the periphery or rim of the flange. 
     
    
     
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
         [0007]    Embodiments of the present invention will now be explained in details with reference to the appended schematic drawings where 
           [0008]      FIG. 1  is a drawing showing two tower sections of a wind turbine being assembled, 
           [0009]      FIGS. 2 and 3  are schematic partial cross-sectional side views showing a first and second embodiment of the inventive indicator device, respectively, integral with a primary alignment device and mounted to a tower flange, and 
           [0010]      FIG. 4  shows a third embodiment of the indicator device, operative also for alignment and in a first configuration. 
       
    
    
       [0011]      FIG. 1  shows a lower or first hollow metal tower section  2  standing on a base and configured to form part of the supporting tower structure of a wind power plant  1 . A second hollow tower section  5  is suspended from a crane  7  and is to be connected with the first tower section  2 . For safety reasons the assembly of two tower sections  2 ,  5  should ideally be carried out without any personnel remaining inside the hollow tower section  2  anywhere near the upper end  2 ′. 
         [0012]    For assisting in aligning the two tower sections a plurality of primary alignment devices schematically illustrated by numeral  10  may be mounted to an inwardly oriented peripheral flange  70  at the upper or first end  2 ′ of the first tower section  2 . The primary alignment devices  10  preferably have a guide portion with a protracted guiding surface part which extends upwardly, i.e. in the general direction away from the opposite second end (not shown) of the first tower section  2 , generally inwardly towards a centre line CL 2  of the first tower section  2 , and preferably also along a portion of the rim of the tower section  2 . In this way, on lowering the second tower section  5  towards the first tower section  2  the inside face of a corresponding peripheral flange  70 ′ of the second tower section  5  will bear against the guiding surface part, and the second tower section  5  will shift laterally towards the desired position in alignment with the centre line CL 2  of the first tower section  2 . This procedure is shown schematically in  FIG. 2 . 
         [0013]    When the centre lines CL 2  and CL 5  are aligned, or essentially aligned, the upper tower section  5  is then lowered such that the peripheral flange  70 ′ thereof sits on the aforementioned peripheral flange  70  of the first tower section  2 , with a multiplicity of bolt holes on the two adjoining flanges  70 ,  70 ′ being aligned. The two tower sections  2 ,  5  are then connected using bolts extending through the aligned bolt holes  72 ,  72 ′ of the two adjoining flanges  70 ,  70 ′. 
         [0014]    To properly align the bolt holes  72 ,  72 ′ during the lowering procedure a secondary alignment by a slight rotation of the upper tower section  5  about its longitudinal centre line CL 5  must normally be done by the operator of the crane  7  right before the two flanges  70 ,  70 ′ touch each other. According to the invention an indicator device  90  is proposed whereby correct alignment of the bolt holes  72 ,  72 ′ is indicated to the operator in that the upper tower section  5  at this instance becomes locked against any further rotation. This secondary alignment may if preferred be assisted by a suitable design of the indicator device  90 , as discussed further below, wherein the indicator device may serve the additional function as a secondary alignment device. 
         [0015]    Shown in  FIGS. 2 and 3  is a first and second embodiment of the invention, respectively, wherein the aforementioned indicator device  90  is designed as an integral part of the aforementioned primary alignment device  10  and where the indicator device  90  and the primary alignment device  10  are installed from below the flange  70  using a common connector indicated by numeral  122  and which may be a magnet. In use, a force F arising from the upper tower section  5  striking the guiding surface part  38  during the primary alignment procedure will result in some deformation by inward bending or deflection of the guiding surface part  38 , the forces being transferred to the flange  70  by a body  30  of the primary alignment device  10  being connected to the flange  70  as discussed further below. 
         [0016]    Shown in  FIG. 2  is a rigid elongated tubular sheath  100 ′ that extends from below the flange  70  into one of the through-going holes  72  used for the final bolt-and-nut connection of the two structural parts  2 ,  5 . The length of the sheath  100 ′ is selected such that the upper free end thereof does not extend above the upper face  75  of the flange  70 , and the hole  72  is preferably slightly oversized compared to the outer dimension of the sheath  100 ′. The sheath  100 ′ is normally not, at the outset, in direct engagement with the flange  70 . Such engagement with the flange  70  arises primarily when the guiding surface part  38  is subjected to the relatively large external force F which tend to deform, elastically or not, the guiding surface part  38  whereby the sheath  100 ′ rigidly connected to the body  30  tilts within the hole  72 , to secure the primary alignment device  10  to the flange  70  by opposite faces of the sheath  100 ′ pressing against the inside of hole  72 , thus effectively providing somewhat of a wedge-effect, preventing undesired release of the sheath  100 ′ from the hole  72 . Preferably, in the embodiments of  FIGS. 2 and 3  a root portion of the guiding surface part  38  is located at a distance z from the outer face of the flange  70 . This spacing accommodates for situations where the upper flange  70 ′ may be slightly oval, requiring a slight off-set of the upper tower section  5  to allow for alignment of the bolt holes  72 ,  72 ′. 
         [0017]    The indicator device  90 , also shown in a third embodiment in  FIG. 4 , generally includes a body  100 , best seen in  FIGS. 3 and 4 , which is extends in the hole  72  and which has a first body end  101  and an opposite second body end  112 , a portion  105  of the body  100  including the first body end  101  projecting from the outer face  75  of the flange  70  in a first configuration of the indicator device  90 . The indicator device  90  includes means  150 , such as the illustrated spring  150  or a pneumatic system or a hydraulic system, that allows for the indicator device  90  to assume a second configuration wherein the first body end  101  is located closer to the outer face  75 . The means  150  are configured to urge the indicator device  90  to assume the first configuration shown in broken lines in  FIG. 3 . 
         [0018]      FIG. 4  shows the indicator device  90  in one basic design in which it is separate from the aforementioned primary alignment device  10 , with magnets  122  for mounting the device  90  against the lower face  71  of the metal flange  70 , with a cylindrical sheath or barrel  100 ′ including a spring  150  and receiving a portion of a displaceable bar  100  configured to extend into a bolt hole  72 , with a projecting portion  105 . The bar  100  is shown in a position corresponding to a first configuration of the device wherein stops prevent the bar  100  from moving further out from the sheath  100 ′. Preferably, the length of the sheath  100 ′ is such that the bar  100  may be moved to a retracted position where a first or upper end  101  of the bar  100  is flush with the upper face  75  of the flange, i.e. where the entire portion  105  is retracted. Although in  FIG. 3  the upper end  101  is flat it is preferred that the bar  100  has a part  110  tapering towards the upper end  101 , as shown in  FIG. 4 . In this way the face of the tapering part  110  may serve as a secondary alignment device which will ride against the entry portion of an adjoining hole  72 ′ of the second tower section  5 , thus providing a force that tends to align hole  72 ′ with the hole  72  of flange  70 , by rotating the second tower section  5  as it is lowered. 
         [0019]    The body  100  may alternatively itself be of an elastic material defining as such the means  150 , and the sheath  100 ′ may be press-fit in the hole  72 . A lock (not shown) may be provided for maintaining the indicator device in the first configuration, after having assumed once the second configuration. 
         [0020]    It will be understood that in order to connect the two tower sections  2 ,  5  the crane operator proceeds by first lowering the upper tower section  5  towards the lower one  2 , until contacting the annular flange  70 ′ of the upper tower section  5  with the first body end  101 , to bring the indicator device  90  towards the second configuration, at which point the crane operator rotates the upper tower section  5  until the indicator device  90  indicates, such as visually or by an electronic signal being sent to the crane operator, the first configuration, wherein the hole  72  having the indicator device  90  is aligned with a hole  72 ′ of the upper tower section  5  such that it is capable of reassuming the first configuration in that the tip or first end  101  of the bar or body  100  is driven into the aligned hole  72 ′ by the means  150 . Preferably, the indicator device  90  is mounted to the lower tower section  2 , but it may be mounted to the upper tower section  5 . Rotation may be induced by configuring the body  100  with a tapering part  110  with a conical (see  FIG. 4 ) or spherical shape defining guiding faces. 
         [0021]    Where the lower tower section  2  has the primary alignment device discussed above and shown in  FIGS. 1-3 , before said rotation, the upper tower section  5  is first brought into contact with the protracted guiding surface part  38 , to shift the upper tower section  5  laterally. After the rotation the indicator device  90  is removed, and a connecting bolt is then passed through all the adjoining holes  72 ,  72 ′ for connecting the two tower sections  2 ,  5 . 
         [0022]    Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. In particular, while the term “bar” as used herein may preferably refer to a member having a circular cross-section, members having other cross-sectional configurations may be used; this also applies to the cross-section of the holes, that preferably, however, have a circular or oval/elliptical cross-section. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms “comprising” or “comprises” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Summary:
Indicator device ( 90 ), for indicating proper alignment of bolt-and-nut holes of two abutting flanges ( 72,72 ′) of two tower sections ( 2,5 ) of a wind turbine tower structure ( 1 ), the indicator device ( 90 ) comprising a connector ( 122 ) for connecting the indicator device ( 90 ) to an inner face ( 71 ) of one of the flanges, and a body ( 100 ) having a first body end ( 101 ) and an opposite second body end ( 112 ) closer to the connector ( 122 ), the indicator device ( 90 ) including a resiliently deformable part ( 150 ), a pneumatic system or a hydraulic system allowing for the indicator device ( 90 ) to assume a normal configuration wherein the first body end ( 101 ) is distant from the connector ( 122 ), and a second configuration wherein the first body end ( 101 ) is located closer to the connector ( 122 ), the resiliently deformable part or pneumatic system or hydraulic system urging the first body end ( 101 ) from the second configuration towards the first configuration.