Patent Publication Number: US-2023151629-A1

Title: Flange

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to PCT Application No. PCT/EP2021/051085, having a filing date of Jan. 19, 2021, which claims priority to EP Application No. 20158766.4, having a filing date of Feb. 21, 2020, the entire contents both of which are hereby incorporated by reference. 
    
    
     FIELD OF TECHNOLOGY 
     The following describes a flange for connection to a complementary flange; and a method of handling a cylindrical tower section equipped with such a flange. 
     BACKGROUND 
     Tall towers such as wind turbine towers are generally constructed by connecting tower sections together. To this end, each tower section is equipped with flanges. A common flange shape is the “L-flange”, and complementary L-flanges are connected together by fasteners such as bolts arranged in a bolt circle. Tower sections may be manufactured to have L-flanges with inner bolt circles (i.e., the flange extends into the tower interior) or outer bolt circles (i.e., the flange extends outward from the tower). 
     Large wind turbines of the types currently in development have very long rotor blades and therefore require taller towers. However, the limited load-bearing capacity of the commonly used L-flange connection places constraints on the tower structure. 
     The strength of a flange connection depends on various parameters, for example on the choice of steel, the wall thicknesses, the bolt diameters, the number of bolts in the bolt circle, the load-path etc. To increase the strength of a flange connection between tower sections, one approach may be to use a “T-flange” instead, which has the shape of an inverted “T” with an inner flange extending into the tower interior and an outer flange extending outward from the tower. A T-flange may have twice the strength of an L-flange, i.e., it can withstand loads twice as great as the loads that could be withstood by a comparable L-flange. However, a major disadvantage of the T-flange is that it requires access from the outside of the tower as well as access from the tower interior. Although T-flanges are seen as a solution in some cases on account of their greater load-bearing capacity, the assembly and lifetime servicing of a multi-section tower using T-flanges is very expensive. 
     SUMMARY 
     An aspect relates to an improved flange connection that overcomes the problems described above. 
     According to embodiments of the invention, the flange is part of a structural component and is realized for connection to a complementary flange that is part of a further structural component. The inventive flange comprises an essentially planar annular connection face that will lie against a complementary annular connection face of the complementary flange; a first body section with a primary bolt circle comprising an annular arrangement of alternately inclined openings to receive a first set of fasteners for connecting the flange to the complementary flange; and a second body section with an at least partial secondary bolt circle comprising an annular arrangement of openings for temporarily connecting the flange to an interim structure. 
     In the following, for the sake of simplicity but without restricting embodiments of the invention in any way, it may be assumed that the structural component is a tower section, for example a wind turbine tower section. A tower section may also be referred to as a “tower shell” in the following. An embodiment of the inventive flange is “part of” the structural component at the time when the structural components are being connected. This shall be understood to mean that a structural component and its flange may be regarded as a single entity. The structural component and its flange may be formed as a single body. Equally, the structural component and its flange may be manufactured separately and then joined, for example a flange may be welded to a steel tower shell, or the upright cylindrical portion of a flange may be embedded in an outer end of a concrete tower shell. 
     The primary bolt circle shall be understood as the circle along which lie the openings at the flange connection face. The diameter of the primary bolt circle may be assumed to be similar or equal to the mean diameter of the tower section or tower shell. The inclined openings extend from the flange connection face into the body of the flange. An advantage of the primary bolt circle is that the joint is effectively moved “into” the tower shell, i.e., the force exerted by a tightened bolt is directed along an inclined path that intersects the mean diameter of the tower shell. This means that loads are much more effectively transferred from one tower shell to the next tower shell. In contrast, the vertical bolts of an conventional L-flange are always at a distance removed from the mean diameter of the tower shell, so that the load path is offset, resulting in greater bending moments. 
     The openings of the secondary bolt circle may be assumed to be vertical in the conventional sense, i.e., perpendicular to the connection face of the flange. The secondary bolt circle is provided exclusively for connection to an interim structure and overcomes the practical difficulties that would be associated with using the primary bolt circle (with its inclined openings) for this purpose. 
     With its primary bolt circle and secondary bolt circle, the inventive flange effectively offers structural strength comparable to that of a T-flange without sacrificing the main advantage of the L-flange, namely access to the primary bolt circle from within the tower interior. 
     An “interim structure” may be understood in the context of embodiments of the invention to be any apparatus such as a holding structure used during transportation of the tower section, a lifting interface used during installation of the tower section, etc. The interim structure may be assumed to not be an element of the tower of which the tower section will be a part. 
     The terms “flange” and “complementary flange” are to be understood in the usual sense to mean flanges that are essentially identical, e.g., mirror images of each other, so that they can be connected together. 
     An advantage of the inventive flange is that it combines two separate aspects, which in combination lead to a compact yet strong flange connection. In a first aspect, the flange connection is made by bolting two instances of the inventive flange together, with a set of fasteners arranged in the inclined openings of the primary bolt circle. Because of the manner in which the primary bolt circle is formed, this set of inclined fasteners is in favorably close proximity to the tower section body. In a second aspect, the tower section can be connected to an interim structure with relative ease, by fasteners extending through the secondary bolt circle. 
     According to embodiments of the invention, the method of handling a cylindrical tower section equipped with such a flange comprises any of forming a temporary connection between the tower section and a holding apparatus by a number of fasteners inserted through the secondary bolt circle of the flange and subsequently releasing the temporary connection by removing the fasteners from the secondary bolt circle; and/or forming a permanent connection between the tower section and a further tower section by a number of fasteners inserted through the primary bolt circle of the flange and through the primary bolt circle of the complementary flange of the further tower section. 
     The tower section may be assumed to have an essentially cylindrical form, for example a straight cylinder. Equally, a tower section may have a frusto-conical form so that, for example, the diameter at its upper end is smaller than the diameter at its lower end. The tower section may be assumed to have an essentially circular cross-sectional shape. 
     The tower section may be assumed to be “solid”, i.e., to have solid side walls, for example of steel or concrete, although the inventive flange connection is not limited to tower sections with solid side walls. 
     The inventive flange essentially comprises a first body section that incorporates the primary bolt circle, and a second body section that incorporates the (partial or complete) secondary bolt circle. As will be explained below, the inventive flange can be realized as a one-piece component or as a two-piece component. 
     In the following, it may be assumed that the flange has the general shape of an “L”, i.e., the second body section of the flange is essentially a lip or collar that extends into the interior space of the tower section. 
     An inclined opening of the primary bolt circle is characterized by the angle of inclination θ subtended between its longitudinal axis and a surface normal of the flange connection face. In other words, the longitudinal axis of a primary bolt circle opening is inclined relative to the horizontal plane. In an embodiment of the invention, this angle of inclination θ is between 15° and 25°. 
     In an embodiment of the invention, the primary bolt circle comprises an alternating arrangement of downward-extending inclined openings and upward-extending inclined openings. In an embodiment of the invention, using an “upper” flange for the purpose of discussion, a downward-extending inclined opening extends through the flange to accommodate the shank of a fastener extending into a complementary inclined opening of the lower flange, and an upward-extending inclined opening extends from the contact face partway into the flange to accommodate the threaded end of a fastener extending into the flange from the “lower” flange. 
     A downward-extending inclined opening is a through-opening formed such that a fastener inserted through the flange extends into an inclined opening of the complementary flange. The downward-extending inclined openings are therefore “through-holes” since they extend all the way through the body of the flange. 
     An upward-extending inclined opening is a threaded opening formed to receive the threaded end of a fastener inserted into the flange via an inclined opening of the complementary flange. The upward-extending inclined openings are therefore “blind holes” since they terminate in the body of the flange. 
     When viewed from the contact surface, the flange therefore shows a ring of openings. Every second opening is an “exit” opening of the inclined through-holes, and the other openings are the “entrance” openings of the oppositely inclined blind holes. To form a flange connection, two flanges are arranged face-to-face, so that each “exit” opening is aligned with its counterpart “entrance” opening. 
     Since the inventive flange combines two types of bolt circle, namely a bolt circle with alternating inclined fasteners and the bolt circle known from the “L-flange”, the inventive flange may be referred to in the following as an “X-L-flange”. The “X-L-flange” has a favorably high load-bearing capacity because of the alternating arrangement of inclined bolts and because the bolts “cross each other” along a ring that coincides with, or is at least very close to, the tower shell diameter. This results in a more efficient load path, so that prying moments of the type that typically arise in L-flange connections are essentially eliminated. The load-bearing capacity of the inventive flange is comparable to the load-bearing capacity of a comparable “T-flange”. However, unlike the conventional art “T-flange”, assembly of a tower using the inventive “X-L-flange” does not require access from the outside of the tower. This is because all fasteners or studs of the primary bolt ring can all be inserted from inside the tower. This aspect is especially important for the assembly of towers at offshore locations, or at locations in which external cranes cannot be deployed to provide access platforms for personnel. 
     While the purpose of the primary bolt circle is to allow two flanges to be permanently bolted together, the purpose of the secondary bolt circle is to allow a flange to be temporarily connected to some interim structure. In an embodiment of the invention, the longitudinal axis of an opening of the secondary bolt circle is collinear with a surface normal of the flange connection face, i.e., the bolt holes extend vertically through the flange and can easily be accessed. 
     The secondary bolt circle can be based on a complete ring, or on arc-sections of a ring. When based on a complete ring, the second body section is similar in shape to an L-flange, extending horizontally from the first body section towards the flange interior. When based on arc-sections of a circle, the second body section can comprise angular segments, for example each segment subtending an angle of 30° to the midpoint of the flange, with four such sections evenly spaced about the inner perimeter of the first body section. Such an embodiment as the advantage of reduced material costs. 
     In an embodiment of the invention, the flange is manufactured as a one-piece component. In such an embodiment, a secondary benefit is the increased stiffness given by the second body section, i.e., the lip or collar that comprises the secondary bolt circle. This body section or structural element assists in minimizing ovalization during transport, i.e., it helps to reduce or eliminate the likelihood of flange deformation during storage or transport, which might otherwise result in the flange developing a slightly oval form. 
     In an embodiment of the invention, the flange is realized as a two-part flange with a first body section comprising the primary bolt circle and a second body section comprising the secondary bolt circle. In an embodiment, the second body section is mounted to the first body section by welding, or by fasteners extending parallel to the flange connection face. 
     From the conventional art, it is known to design the flanges of various tower shell diameters to have a specific bolt circle diameter (BCD). This simplifies flange design and reduces flange manufacturing costs, but can lead to higher costs elsewhere, for example when a certain flange BCD requires an adapter so that the flange can be connected to a transport fitting or lifting apparatus. Since the inventive flange can be realized as a two-part component, it is relatively easy to provide a range of second body sections, each with a different BCD. In this way, a suitable second body section can be chosen on the basis of the BCD of an interim structure which will be used in handling the tower section. 
     When two tower sections are connected together, loads are generally transferred essentially vertically. Therefore, in an embodiment of the invention, the inner diameter of the annular flange connection face exceeds the diameter of the secondary bolt circle. This can be achieved for example by machining a recess in the second body section of a flange in order to limit the contact area between flanges to the regions between the upper tower section body and the lower tower section body. In other words, the thickness of the second body section is reduced. 
     A further advantage of this embodiment is that such a recess also helps to better define the flange contact face. Restricting the contact area to the region relevant to load-transfer makes it is easier to identify performance-relevant imperfections such as gaps between the flange contact faces. Such a gap would compromise the load transfer path. This embodiment of the invention allows quick identification of such gaps, so that these can be remedied by shimming before tightening the fasteners of the primary bolt circle. 
     As mentioned above, some degree of ovalization may develop in a tower section before assembly of the tower. In an embodiment of the invention, the flange comprises an alignment feature that is formed adjacent to the inner diameter of the flange connection face and shaped to engage with an inverse alignment feature of a complementary flange. When one tower section is lowered into place over another tower section, the weight of the upper tower section in conjunction with the alignment features will be sufficient to correct any ovalization. 
     In an embodiment of the invention, the method of handling a tower section during tower assembly comprising a step of inserting a guide pin through an opening of the secondary bolt circle to align the flange of one tower section to the complementary flange of the other tower section. 
    
    
     
       BRIEF DESCRIPTION 
       Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
         FIG.  1    shows embodiments of the flange; 
         FIG.  2    shows embodiments of the flange; 
         FIG.  3    shows embodiments of the flange; 
         FIG.  4    shows embodiments of the flange; 
         FIG.  5    shows embodiments of the flange; 
         FIG.  6    shows an embodiment of the flange connected to an interim structure; 
         FIG.  7    shows a tower comprising stacked tower sections connected according to the conventional art; and 
         FIG.  8    shows a tower comprising stacked tower sections connected according to the conventional art. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1   ,  FIG.  2   , and  FIG.  3    illustrate an embodiment of the inventive flange  1 . A tower section of a wind turbine can have a mean diameter in the order of 6-8 m, and the inventive flange  1  is dimensioned accordingly.  FIG.  1    shows (in cross-section) one instance of an embodiment of the inventive flange  1  connected to a functionally identical instance of the flange  1 . Each flange  1  has a first body section  10  and a second body section  11 . The first body section  10  incorporates a primary bolt circle  1 P comprising an annular arrangement of inclined openings  10 _thru,  10 _part to receive a set of fasteners  10 B for connecting the flange  1  to the complementary flange  1 . An inclined opening  10 _thru,  10 _part of the primary bolt circle  1 P is characterized by the angle of inclination θ subtended between its longitudinal axis  10 A,  10 A′ and a surface normal N of the flange connection face  1 F. In  FIG.  1   , the center of the “exit” opening of the through-hole  10 _thru and the center of the “entrance” opening of the blind hole  10 _part are points along the primary bolt circle  1 P. This is more clearly seen in the perspective view given by  FIG.  3   , which shows the alternating arrangement of through-hole “exit” openings  10 _out and blind hole “entrance” openings  10 _in that form the primary bolt circle  1 P. 
       FIG.  2    shows an embodiment of the inventive flange  1  as part of a tower section  20 A. The diagram indicates the diameter D  1 P of the primary bolt circle  1 P, and the mean diameter D_ 20  of the tower section  20 A. 
     Ideally, the primary bolt circle  1 P has the same diameter D  1 P as the mean diameter D_ 20  of the tower shell  20 A, i.e., the primary bolt circle  1 P is in-line with (i.e., coincides with) the mid-plane of the tower shell  20 A (as indicated in  FIG.  3   ). However, it may be necessary to slightly offset the primary bolt circle  1 P from the tower shell mid-plane, for example to allow non-destructive testing of a weld between flange  1  and tower shell  20 A (a weld joint between flange and tower shell is close to the outer ends of the inclined through-openings  10 _thru). Such an offset between primary bolt circle  1 P and tower shell mid-plane is kept to a minimum, in order to maintain the favorably high load-carrying capacity of the inventive flange  1 . 
     Each second body section  11  incorporates a secondary bolt circle  1 S with an annular arrangement of openings  11  to receive a set of fasteners  11 B for connecting the flange  1  to an interim structure (not shown).  FIG.  3    indicates the secondary bolt circle  1 S defined by the openings  11 , and  FIG.  2    indicates the diameter D_ 1 S of the secondary bolt circle  1 S. The area of the annular connection face  1 F is determined by the outer diameter D_ 1 F_out and the inner diameter D_ 1 F_in of the flange  1 . 
     The structure shown in  FIG.  2    can be one end of a tower part that is made of several stacked cylindrical elements. The two outer ends of the tower part each terminate in an instance of the inventive flange  1 . The joints between the cylindrical elements can be done using conventional L-flanges or using the inventive X-L-flange. 
     As shown in  FIG.  3   , a flange has a connection face  1 F that will lie against the connection face of a complementary flange. The diagram also indicates a possible variant, showing a recess  14  at the lower face of the flange  1 . The inner diameter D_ 1 F_in of the annular flange connection face  1 F therefore exceeds the diameter D_ 1 S of the secondary bolt circle  1 S. In this embodiment, the total area of the connection face  1 F is less than the connection face area of the embodiment shown in  FIG.  1   , but the load from an upper tower section is still effectively transferred into the body of a lower tower section. The recess  14  can facilitate easier connection to an interim apparatus (not shown). 
       FIG.  4    shows a permanent flange connection  10 _perm made by joining two instances of the inventive flange  1 . The connection is “permanent” in the sense that it may endure for the lifetime of the structure. Here, each flange  1  is formed to have an alignment feature  15 A shaped to engage with an inverse alignment feature  15 B of the complementary flange  1 . The alignment features  15 A,  15 B act to correct any slight ovalization that may be present in a flange, when the tower sections are stacked. The drawing shows a fastener  10 B extending through a through-hole  10 _thru in the upper flange  1  and into a blind tapped opening  10 _part of the lower flange  1 . The drawing also indicates another oppositely inclined fastener  10 B extending through a through-hole  10 _thru in the lower flange  1  and into a blind tapped opening  10 _part of the upper flange  1 . 
       FIG.  5    shows a further embodiment of the inventive flange  1 . Here, the flange  1  is realized as a two-part flange with a first body section  10  and a separate second body section  11 . A horizontal opening for a fastener is provided by a through-opening  12 _thru in the second body section and a partial or blind opening  12 _part in the first body section. The blind opening can have an internal thread to receive the threaded end of a metal screw inserted though the second body section. In this exemplary embodiment, the through-opening  12 _thru,  12 _part extends parallel to the flange connection face  1 F. An alternative to such a bolted joint may be to weld the second body section  11  to the first body section  10 . 
       FIG.  6    indicates embodiments of the inventive flange  1  in temporary connections  11 _temp to interim structures. Here, the flange  1  at the upper end  20 A of the tower part  2  is connected to a lifting fitting  31  of a crane  3 , and the flange  1  at the lower end  20 B of the tower part  2  is connected to an upending tool  32  of another crane  3 . The cranes  3  are controlled so that the tower part  2  is “upended”, i.e., moved from a horizontal storage orientation into a vertical installation orientation. These connections  11 _temp are “temporary” in the sense that the interim structures  31 ,  32  will be disconnected again from the flanges  1 . The flange  1  at each end  20 A,  20 B of the tower part  2  will be permanently connected to a complementary flange  1  in a later stage of the tower assembly process, as explained above with the aid of  FIG.  4   . 
     A tower part is generally handled at multiple stages between manufacture and final installation, and the secondary bolt circle  18  is therefore used to connect the flange at either end of the tower part to a cradle or bracket of a support structure, an anti-ovalization tool, an adapter of a transport vehicle, etc. 
       FIG.  7    shows a tower  2  such as a wind turbine tower, comprising tower sections  20  “stacked” on top of each other and connected in a conventional art manner using L-flanges LF. Fasteners are inserted into the bolt circle on the interior, the through-holes LF_H are indicated in the enlarged portion shown in  FIG.  8   . The offset between bolt circle and tower wall means that this type of connection is vulnerable to excessive bending moments. As a result, the overall height of the tower  2  can be constrained by the load-bearing limitations of the flange connections. To overcome these constraints, an alternative conventional art structure uses T-flanges to connect the tower sections  20  with interior and exterior bolt circles, but such a solution is associated with significantly higher costs as explained above. 
     Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
     For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.