Abstract:
A tower part for a wind turbine includes at least one wall segment and at least one aperture segment including at least one aperture. The wall segment and the aperture segment are connected and the aperture segment is of a substantially uniform thickness and is thicker than the thickness of the wall segment. The tower part further includes an aperture cover system for covering at least one aperture in a tower part for a wind turbine, including cover plates and attachments for attaching the system to the tower part, where the attachments do not affect the fatigue limit of the tower part.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a tower part for a wind turbine, an aperture cover system, and a method for manufacturing a tower part and uses hereof. 
     DESCRIPTION OF THE RELATED ARTF 
     A wind turbine known in the art typically comprises a wind turbine tower and a wind turbine nacelle positioned on top of the tower. A wind turbine rotor, comprising three wind turbine blades, is connected to the nacelle through a low speed shaft, which extends out of the nacelle front as illustrated on  FIG. 1 . 
     The wind turbine tower typically comprises a number of round tower parts mounted on top of each other, where each tower part is made of a steel plate rolled into a circular shape and welded to constitute a closed ring. The tower parts are assembled by welding into transportable sections at the manufacturing plant for wind turbines. When the different sections, that constitute the wind turbine tower, have been transported to the site at which the tower is to be erected, the sections are connected by flanged joints. 
     To access the nacelle on top of the wind turbine tower of the fully mounted wind turbine, the tower is usually provided with a ladder on the inside of the tower, and a tower part close to or at the bottom of the tower is provided with an aperture to enable access to the inside of the tower, and a door to cover and seal this aperture. This tower part or other rings in the tower can also be provided with apertures for e.g. cables, lamps or other things. An aperture is usually made by flame cutting a hole in the tower part. 
     A door aperture comprises a thick door frame welded to the edge on the inside of the aperture, to compensate for some of the loss of strength in the tower part caused by the hole. But since welding reduces the tower parts fatigue limit and thereby its strength, the tower part for accommodating a door is also made of a thicker plate than what would be necessary to make a similar tower part of substantially the same strength without a door aperture. As the ring is of uniform thickness, most of the ring is thicker than necessary. 
     Furthermore on wind turbines known in the art most of the equipment attached to the tower part such as door hinges, covers for ventilation holes, flight warning lamps, cable fixating means and other are typically attached by welding. This reduces the tower parts fatigue limit and to compensate for this loss of strength even thicker plates and/or bigger door frames are demanded. 
     Consequently, the known tower part with aperture has the problem of being quite heavy and complicated and thus expensive to manufacture. 
     A way to provide a similar tower part with a door aperture is disclosed in international patent application no. WO-A 03/036085. Here a casted door frame is welded into a door aperture of a standard wind turbine tower. Any connections on the door frame such as door hinges and locks may be casted with the door frame. Since cast iron is more brittle than the steel the wind turbine tower is made of, the cast iron door frame has to be considerably heavier than if it was made of the same steel as the rest of the tower. 
     Furthermore a foundry is considerably more expensive than a rolling mill. The equipment to make such a cast iron door frame would be considerably more expensive in purchase and in use, than the equipment to roll the tower parts. 
     Compared to rolling, casting takes much specialised knowledge both in the design of the cast iron parts and to manufacture them, while rolling is a more commonly used and simple process. The required welding technique in order to connect cast iron and a regular steel plate is also quite demanding. 
     Consequently, the disclosed solution in international patent application no. WO-A 03/036085 also has the problem of being quite heavy and complicated and thus expensive to manufacture. 
     Further, the weight of the wind turbine tower parts is a significant and increasing problem, both in handling and in transport. 
     The invention is intended to minimize the use of material in a wind turbine tower part with aperture such as a door aperture. 
     The invention further simplifies the way the wind turbine tower part with aperture is manufactured in order to reduce production costs. 
     The invention presents a way to cover the apertures in the tower parts without reducing the tower parts fatigue limit. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides for a tower part wherein the aperture segment is of a substantially uniform thickness and is thicker than the thickness of the wall segment. 
     By making the aperture segment of a substantially uniform thickness, it can be made out of an ordinary steel plate. In the quality normally used to make tower parts, steel plates are relatively inexpensive in purchase and relatively easy to machine, roll and weld. 
     By making the aperture segment thicker than the wall segment, strength is provided to the section of the tower part where it is needed. Hereby the wall segment can have the same thickness as if it constituted a full tower part without aperture, which contributes considerably to minimising consumption of material and hereby the weight of the tower part. 
     It shall be emphasised that the term “aperture” refers to any kind of opening such as e.g. a hole, gap, slit or cleft in any form of shape. 
     It shall further be emphasised that the term “segment” refers to an angular portion cut off from a geometrical figure, such as the geometrical figure of a round tower part. 
     Even further it shall be emphasised that the term “thickness” refers to the segments radial extent e.g. in a round tower part. 
     In an aspect of the invention, both the aperture segment and the wall segment are made of steel plates. Steel is a strong, rigid and a relatively inexpensive material with good rolling, machining and welding qualities, which makes it the preferred material for making tower part segments and tower parts in towers for modern wind turbines. Furthermore steel has good magnetic qualities which make it possible to attach accessories such as e.g. ladders, doors, hinges and lamps to the tower parts by means of magnets. 
     In an aspect of the invention, the wall segment and the aperture segments are connected by welding. Hereby it is possible to make a strong and rigid joint between them, and at the same time keep the consumption of material at a minimum. 
     In an aspect of the invention, the tower part is substantially round or has a polygonal shape. By making the tower part round, according to the invention a minimum of material is used. The wind pressure on the wind turbine blades creates a considerable bending moment, which the wind turbine tower has to stand. Since the nacelle can turn in any direction to always face the wind, the tower has to stand bending moments from all sides. In view of this a round tower is the preferred design in modern wind turbine towers. 
     By giving the tower and hereby the tower part a polygonal shape it is possible to shape the tower part and hereby the wall and aperture segments by bending, by pressing or a combination of the two. Pressing or bending could be an advantageous method of manufacturing tower parts. 
     It should be emphasised that the term “round” or “polygonal” not in any way restricts the tower part to being cylindrical. The invention relates to tower parts which can be both cylindrical and of a non-constant diameter shape in the vertical direction such as e.g. conical. 
     In an aspect of the invention, the aperture segment is four sided, which enables the aperture segment to be made of an ordinary plate, preferably a steel plate, which further enables a relatively simple manufacturing process of both the aperture segment and the wall segment. 
     However it should be emphasised that this does not restrict the aperture segment to having a square or rectangular shape. If the tower and hereby the tower part is conical, the aperture segment could also have a substantially conical shape in that the aperture segment could be wider at the bottom than at the top. Or to minimize the use of material it could be advantageous to make the sides of the aperture segment non-linear e.g. with rounded corners or by following the shape of the aperture making the sides for example round or shaped as part of an ellipse. 
     In an aspect of the invention, the at least one wall segment and at least one aperture segment forms segments of a ring and when connected by said connecting means constitute a full 360° ring. 
     Both the aperture segment and the wall segment can be made of rolled plates which reduces production costs, in that both segments can be shaped by the same production method, which reduces investments in production equipment. By rolling the aperture segment can be made of steel of the same quality as the wall segment. This provides the joints between the aperture segment and the wall segment with considerably more strength, than if the aperture segment was made of e.g. cast iron, when joining them by welding. 
     In an aspect of the invention, the aperture segment constitutes between 5° and 180°, preferably 30° and 100° and most preferred 40° and 80° of the full 360° tower part. Hereby is obtained an advantageous relation between strength and weight of a tower part comprising the aperture segment and the wall segment. 
     In an aspect of the invention, the at least one aperture segment has a width which is between 10% and 500%, preferably 50% and 300% and most preferred 100% and 200% wider than the total width of the at least one aperture as seen in a horizontal cross section of the aperture segment where the at least one aperture is widest. 
     With the width ranges of an aperture segment it is possible obtain an efficient use of material in the tower part with a compensation for the loss of strength caused by the aperture. Further, the distance between the aperture and the connections to the wall segment are enough to avoid significant lines of force through the connections. 
     The term “horizontal” refers to an aperture segment in a tower part mounted in an erected wind turbine tower. 
     In an aspect of the invention, the wall segment has an outer diameter of between 1 meter and 10 meters, preferably 2 meters and 6 meters and most preferred 3 meters and 5 meters. The outer diameter of the wall segment has to correspond with the outer diameter of the tower parts above or beneath said wall segment, to make a strong and rigid wind turbine tower. At a given nacelle weight, a given wind turbine height and a given maximum bending moment in the tower, there are primarily two factors that can be adjusted in the construction of a traditional wind turbine tower; namely the diameter of the tower and the thickness of the tower parts. The preferred design of the tower is found by adjusting these two factors to get the strongest tower with a minimum use of material. 
     In an aspect of the invention, at least two edges on said aperture segment are sharpened. By sharpening the edges it is possible to make a smooth junction between the aperture segment and wall segment and to the tower parts above and beneath the aperture segment. A smooth junction is preferred in that it leaves no edges for e.g. water, snow or dirt to gather and it enables an advantageous link tension wise between the different material thicknesses. However it should be emphasised that the sharpening not necessarily have to be linear. To optimize the distribution of stresses in the junction between the different material thicknesses, it could be advantageous to make the sharpening non-linear or partially non-linear. 
     In an aspect of the invention, at least two opposite edges on both the concave and the convex side of said aperture segment are sharpened e.g. substantially to a thickness of said wall segment. By sharpening the edges both on the inside and on the outside of the tower, it is possible to make a smooth junction between the aperture segment and wall segment and to the tower parts above and beneath the aperture segment, both on the inside and the outside of the tower. This also enables substantially identical welding seams on the inside and on the outside of the segment connection, which provides the tower part with very little internal tension caused by the welding. 
     In an aspect of the invention, all four edges on both the concave and the convex side of said aperture segment are sharpened. 
     In an aspect of the invention, the pitch diameter of the aperture segment is substantially the same as the pitch diameter of the wall segment. 
     The term “pitch diameter” is to be interpreted as the diameter of the circle going through the middle of a segment as seen in a top or bottom view, and by the middle is meant where the radial distance to the inside and to the outside, of the rolled plate of which the segment is made, is equal. 
     By “Centering” the aperture segment in the wall segment, it is relatively simple to manufacture and assemble the wall and aperture segment, in that the aperture segment can be sharpened the same way on the inside and the outside, and in that it is easy to determine the correct position of the aperture segment in relation to the wall segment during assembling. 
     In an aspect of the invention, the pitch diameter of the aperture segment is smaller than the pitch diameter of the wall segment. 
     By making the pitch diameter of the aperture segment smaller than the pitch diameter of the wall segment, the aperture segment is positioned a little into the tower part. This is advantageous in that it enables a uniform distribution of the tension forces between the inside and the outside of the tower part. 
     In an aspect of the invention, the wall segment has a thickness of between 5 millimeters and 120 millimeters, preferably 15 millimeters and 90 millimeters and most preferred 20 millimeters and 70 millimeters. 
     The preferred design of the tower and hereby the wall segment is found in a relation between the segments wall thickness and the diameter of the tower to get the strongest tower with a minimum use of material. With the thickness ranges of a wall segment it is possible to obtain an efficient use of material in the tower part. 
     In an aspect of the invention, the aperture segment is between 10% and 500%, preferably 50% and 250% and most preferred 80% and 180% thicker than the thickness of the wall segment. By this thickness range of the aperture segment an advantageous relation between the aperture segment thickness and its strength is achieved. 
     In an aspect of the invention, the height of the wall segment is between 1 meter and 10 meters, preferably 1.5 meters and 5 meters and most preferred 2 meters and 4 meters. This height range provides for an advantageous relation between the wall segment height, the ability to handle it and the cost of producing it. 
     In an aspect of the invention, the aperture segment is of substantially the same height as the wall segment. By this both manufacturing and the process of joining the different segments are made simple. 
     In an aspect of the invention, the at least one aperture in the at least one aperture segment is made by flame cutting. Flame cutting is an inexpensive, fast and relatively simple process of making an aperture in a plate, preferably a steel plate, which traditionally makes flame cutting the preferred method for cutting relatively thick steel plates. 
     It shall be emphasised that the term “flame cutter” refers to any kind of plasma cutter, laser cutter or flame cutter. 
     In an aspect of the invention, the at least one aperture in the at least one aperture segment is a door aperture with a door connected to said aperture segment by means of one or more magnetic hinges. By connecting the hinges of the door to the aperture segment by means of magnetism, the segment is not weakened by internal tension, as e.g. welding would do. Avoiding a weakening around the aperture is especially preferred as the lines of force are concentrated here. 
     In an aspect of the invention, said door aperture is of a substantially oval shape with vertical parallel sides and round endings in the longitudinal direction. In modern wind turbine towers, this is a preferred door aperture design in that it minimises the risk of dangerous stress concentrations, and thereby allows for a minimum use of material. 
     In an aspect of the invention, the at least one aperture segment and/or the at least one wall segment is given a substantially circular shape by rolling, in that rolling is the preferred method for providing workable plates and especially steel plates with a circular shape. 
     In an aspect of the invention, the tower part is a tower ring. By making the tower part as a single tower ring the tower part becomes easy to manufacture. 
     The invention further provides for an aperture cover system for covering at least one aperture in a tower part for a wind turbine, said system comprising one or more cover plates and one or more attachment means for attaching said system to said tower part, wherein said attachment means do not affect the fatigue limit of said tower part. 
     The lines of force are concentrated around the apertures and by attaching covers e.g. by welding, the tower part is weakened where strength is particularly needed. This problem is solved by attaching the aperture cover in a way which does not affect the fatigue limit and hereby the strength of the tower part. 
     In an aspect of the invention, the attachment means are magnetic means and/or adhesive means. Magnets and/or adhesives are a relatively simple, reliable and inexpensive method of attaching accessories such as aperture covers to the tower part. 
     In an aspect of the invention, said aperture cover system being a door with hinges attached to said tower part by means of magnetism. The problem of covering the apertures without reducing the tower parts fatigue limit, is particularly pronounced when the aperture is a door aperture, in that the door aperture is often the biggest aperture in a tower and the door is usually placed at the bottom of the tower where the concentration of stresses are the highest. The effect of using the aperture cover system to connect a door to the tower part by magnetic hinges is therefore particularly pronounced. 
     The invention further provides for a method for manufacturing a wind turbine tower part comprising at least one aperture segment and at least one wall segment, said method comprising the steps of, establishing said at least one wall segment as less than a full 360° ring, establishing a aperture in said aperture segment, fixating one of said segments in a fixating unit, moving the other segment to a position in proximity of the segment in said fixating unit and connecting the segments by connection means. By this method an advantageous procedure for manufacturing the tower part with aperture is achieved. 
     By using a fixating unit for fixating either the wall segment or the aperture segment and then positioning the other segment in a position at where it can be joined with the fixated segment, provides a relatively simple method for stabilising the segments during the connection, which enables advantageous circumstances for making the connection between the segments of good quality, especially if the connection is done by welding. 
     In an aspect of the invention, a method for manufacturing a wind turbine tower part includes the connection of at least one wall segment of a given thickness and at least one aperture segment of a substantially uniform thickness and being thicker than the thickness of said wall segment. By this an advantageous embodiment of the invention is achieved. 
     An aspect of the invention provides for, a method for manufacturing a wind turbine tower part, wherein said at least one aperture segment and said at least one wall segment are made by rolling. 
     An aspect of the invention provides for, a method for manufacturing a wind turbine tower part, wherein at least two edges on said aperture segment are sharpened. 
     An aspect of the invention provides for, a method for manufacturing a wind turbine tower part, wherein at least two opposite edges on the concave and the convex side of said aperture segment are sharpened e.g. substantially to a thickness of said wall segment. 
     An aspect of the invention provides for, a method for manufacturing a wind turbine tower part, wherein the connections between said at least one aperture segment and said at least one wall segment comprises welding. 
     An aspect of the invention provides for, a method for manufacturing a wind turbine tower part, wherein the fixating unit is a rolling mill or part hereof. By using the rolling mill for fixating the wall segment or the aperture segment when joining the segments, provide for a method which does not require a separate fixating unit and the cost thereto. 
     An aspect of the invention provides for, a method for manufacturing a wind turbine tower part, wherein the manufacturing takes place at a manufacturing plant of a wind turbine plant. By manufacturing the wind turbine at the manufacturing plant transportation and handling of the different wind turbine parts are reduced. 
     The invention further provides for use of a tower part for establishing one or more apertures in a wind turbine such as door openings, cable openings, and openings for ventilation or warning lights. By this an advantageous embodiment of the invention is achieved. 
     An aspect of the invention provides for, use of a method for manufacturing a wind turbine tower part for establishing one or more apertures in a wind turbine such as door openings, cable openings, openings for ventilation or warning lights. By this an advantageous embodiment of the invention is achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will be described in the following with reference to the figures in which 
         FIG. 1 . illustrates a large modern wind turbine, 
         FIG. 2  illustrates an embodiment of a tower part of a wind turbine tower comprising a wind turbine tower part with a door aperture as seen in perspective, 
         FIG. 3  illustrates the wind turbine tower part with a door aperture in a vertical cross section through the middle of the door aperture as seen from the side, 
         FIG. 4  illustrates the wind turbine tower part with a door aperture and door in a horizontal cross section through the middle of the door aperture as seen from the top, 
         FIG. 5  illustrates an embodiment of a tower part comprising two wall segments and two aperture segments, 
         FIG. 6  illustrates an embodiment of a tower part comprising two wall segments and one aperture segment, 
         FIG. 7  illustrates an embodiment of a conical tower part, 
         FIG. 8  illustrates in perspective a manufacturing method of a wind turbine tower part where a cart with an aperture segment is moved to a position in proximity of a fixating unit with a wall segment, 
         FIG. 9  illustrates a method step that includes the cart with the aperture segment being in a tilted position, 
         FIG. 10  illustrates a method step where the cart with the aperture segment in a tilted position is brought in position for joining with the wall segment in the fixating unit, and 
         FIG. 11  illustrates a further method step where the wall segment is joined with the aperture segment, 
         FIG. 12  illustrates in perspective another manufacturing method of a wind turbine tower part where an aperture segment is positioned in a fixating unit, and 
         FIG. 13  illustrates a further method step including the lowering of a wall segment to the fixating unit with an aperture segment and connecting the segments, 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a modern wind turbine  1  mounted on the foundation  18  comprising a tower  2  with a door  8  attached to the tower  2  by means of hinges  23 , and a wind turbine nacelle  3  positioned on top of the tower  2 . The wind turbine rotor  4 , comprising three wind turbine blades  5 , is connected to the nacelle  3  through the low speed shaft which extends out of the nacelle  3  front. 
     The different components of a wind turbine  1  are usually transported separately to the site of mounting and assembled there e.g. the different tower sections, the nacelle  3  and the wind turbine blades  5 . 
       FIG. 2  illustrates a lower part of a wind turbine tower  2  comprising a tower part  6  with a door aperture  7  for accommodating a door (not shown). In this embodiment of the invention the tower part  6  comprises a wall segment  9  and an aperture segment  10 , which in this case results in the tower part  6  constituting a tower ring  22 . The tower part  6  with door aperture  7  is joined to the neighboring tower rings  22  by connecting means such as screws, bolts, rivets, gluing or welding. In normal use in a wind turbine tower the tower part  6  is welded to the tower rings  22  above and beneath. In another embodiment the top and/or the bottom of the tower part  6  is provided with a flange for connecting the tower part  6  to the tower ring  22  above and/or the tower ring  22  or foundation beneath by use of fasteners such as screws, bolts, rivets or other. Furthermore the figure illustrates that in this embodiment of the invention the aperture segment  10  is of substantially the same height as the wall segment  9 . The dimension H shows that the term height is to be interpreted as the concerned objects extend in the vertical direction on a fully erected wind turbine tower  2 . 
     In an embodiment of the invention the door aperture  7  can be provided with a door (not shown) connected to the aperture segment  10  e.g. by means of magnetic hinges. 
       FIG. 2  further illustrates a circular embodiment of a part of a wind turbine tower  2 , but in another embodiment the rings and hereby the tower  2  could be conical, with a decreasing diameter upwards. 
     In an embodiment of the present invention, the wall segment  9  is made of a relatively thin plate which is rolled into a circular shape of a diameter determined by the size of the wind turbine  1  in which the tower part  6  and thereby the tower  2  is to be used. When rolled the wall segment  9  does not constitute a full 360° tower part  6 , but has an opening for accommodating at least one aperture segment  10 . 
     The aperture segment  10  is made of a plate thicker than what the wall segment  9  is made of, and rolled into a circular shape with substantially the same diameter as the wall segment  9 . Before or after the aperture segment  10  is rolled, at least one aperture  7  is cut in said segment  10 , by means of a flame cutter or another metal cutting device. 
     If more than one aperture  7  is needed in a tower part  6 , more than one aperture segment  10  could be welded to at least one wall segment  9 , so that the three or more segments together constitute a full tower part  6 . For example two aperture segments positioned horizontally next to each other such as a door segment and a cable segment and both connected to the wall segment. 
     In another embodiment of the invention, the aperture segment  10  could be provided with more than one aperture  7 . This could for instance be the case if a tower part  6  needed to be provided with both a door and an aperture for a main cable, ventilation or other (not shown). 
     A wind turbine tower  2  can also be provided with more than one tower part  6  with aperture  7 . This could be the case if e.g. a tower part  6  close or at the bottom of the tower was provided with e.g. a door aperture  7  and another tower part  6  at or close to the top of the tower  2  was provided with one or more apertures  7  e.g. for airplane warning lamps (not shown). 
     In this embodiment of the invention the figure also illustrates that when the wall segment  9  and the aperture segment  10  are joined, by some sort of connecting means such as screws, bolts, rivets, gluing or preferably welding (not shown), they together constitutes a full 360° substantially round tower part  6 . In another embodiment of the invention the tower part could have a polygonal shape of at least three sides. 
       FIG. 3  illustrates the tower part  6  comprising a wall segment  9  and an aperture segment  10  with door aperture  7  in a vertical cross section through the middle of the door aperture  7 . 
     In an embodiment of the invention all four edges on both the concave and the convex side of the aperture segment  10  are provided with a sharpening  11 . In an embodiment of the invention the sharpening  11  of the top and bottom edges of the aperture segment  10  results in that the aperture segment  10  ends up being of the same thickness as the neighboring tower rings  22  above and beneath. The sharpening  11  on the top and bottom edges could advantageously be made before the aperture segment  10  is rolled into the circular shape. In another embodiment of the invention the sharpening  11  could be non-linear e.g. having the shape of a part of an ellipse and/or the edge between the sharpening  11  and full material thickness of the aperture segment  10  could be rounded. 
       FIG. 4  illustrates the tower part  6  comprising a wall segment  9  and an aperture segment  10  with a door aperture  7  and a door  8  in a horizontal cross section through the middle of the door aperture  7 . 
     In an embodiment of the invention all four edges on both the concave and the convex side of the aperture segment  10  are provided with a sharpening  11 . In an embodiment of the invention the sharpening  11  on the side edges of the aperture segment  10  results in the aperture segment  10  ends up being of the same thickness as wall segment  9 . Because of the way a traditional rolling machine known in the art functions, the sharpening  11  on the side edges of the aperture segment  10  would preferably not be made until after the aperture segment  10  has been rolled into the circular shape. 
     The illustrated aperture segment  10  is for accommodating one door and therefore the aperture segment  10  is provided with only one aperture  7 . In this embodiment of the invention the width of the aperture segment  10  is approximately three times the width of the aperture  7 , where the aperture is positioned symmetrically in the aperture segment  10 . The dimension W shows that the term width is to be interpreted as the peripheral extend of the segment. If the aperture  7  in the aperture segment  10  is a 0.7 meter wide door aperture  7  and the tower part  6  has an outer diameter of e.g. 4 meters, the aperture segment  10  would be approximately 2.1 meters wide, which in this case constitutes about 60° of the full 360° tower part. 
     In this embodiment of the invention  FIG. 4  also illustrates that, the pitch diameter of the wall segment  9  and the aperture segment  10  is coincident. The dimension P shows these coincident pitch diameters. In another embodiment of the invention the aperture segment could be made with a pitch diameter smaller than the wall segments pitch diameter. 
     The width of the aperture segment  10  is determined by the size and the numbers of apertures  7  in the aperture segment  10 . In an embodiment of the invention, if an aperture segment  10  is to be provided with one aperture  7 , the width of the aperture segment  10  would approximately be three times the width of the aperture  7 , in that horizontally the aperture  7  would be positioned substantially in the middle of the aperture segment  10  with a part of the aperture segment  10  of approximately the same width as the aperture  7  on both sides of the aperture. If the aperture segment  10  is provided with more than one aperture  7  the width of the aperture segment  10  would be determined by the sizes of these apertures  7  and how they are positioned on the aperture segment  10 . 
       FIG. 4  further illustrates a door  8  covering the aperture  7 . In this embodiment of the invention the door  8  is connected to the tower part  6  by means of one or more hinges  23 . The hinges  23  could be attached to the tower part by means of e.g. screws, bolts, rivets, welding or other, but more preferred the hinges  23  are connected in a non-destructive way which does not weaken the tower part  6 . Attaching in a non-destructive way could be by magnets, adhesive means such as glue or a combination of the two. 
       FIG. 5  illustrates an embodiment of a tower part  6  comprising two wall segments  9  and two aperture segments  10 . If an aperture  7  is of a size, that it is higher than a tower ring  22 , the aperture  7  could be made by e.g. welding two tower rings  22  together, each comprising an aperture segment  10  provided with a cut-off constituting a part of the total aperture  7 . 
       FIG. 6  illustrates an embodiment of a tower part  6  comprising two wall segments  9  and one aperture segment  10 . If an aperture  7  is of a size that it is higher than a tower ring  22 , the aperture  7  could be made by e.g. welding two wall segments  9  onto one aperture segment  10  having the same height as two wall segments  9 . 
       FIG. 7  illustrates an embodiment of a conical tower part  6 , where the sides of the aperture segment  10  is linear but not parallel. In another embodiment of the invention the corners of the aperture segment  10  could be rounded, or the sides could follow the shape of the aperture  7 , in this case making the sides having the shape a part of an ellipse. The wall segment  9  would have a shape corresponding to the given shape of the aperture segment  10 . 
       FIG. 8  illustrates in perspective a manufacturing method of a wind turbine tower part where a cart with an aperture segment is moved to a position in proximity of a fixating unit with a wall segment. The figure further illustrates a rolling mill  13  with the ready-made wall segment  9  mounted. 
     In this embodiment of the invention the rolling mill  13  functions as a fixating unit  12 . An aperture segment  10  is mounted and fixated on a cart  14  by holding means  15  in the door aperture  7 . In this embodiment of the invention the cart  14  is provided with wheels  16  to make the cart moveable either by pushing or pulling the cart  14  or by providing the cart  14  with driving means such as an electrical motor (not shown) or another kind of motor. 
       FIG. 9  illustrates the cart  14  with the fixated aperture segment  10  in a tilted position. In this embodiment of the invention a part of the cart  14  whereon the aperture segment  10  is mounted is tilted e.g. by means of hydraulics, pneumatics or an electrical motor or another kind of motor (not shown). 
       FIG. 10  illustrate the cart  14  with the aperture segment  10  in a tilted position brought in a position where it is possible to join the wall segment  9  and the aperture segment  10  preferably by welding. 
       FIG. 11  illustrates a wall segment  9  still mounted in the fixating unit  12  joined with the aperture segment  10 , so that they together in this embodiment of the invention constitutes a full 360° tower part  6 . 
       FIG. 12  illustrates in perspective another manufacturing method of a wind turbine tower part where an aperture segment  10  is positioned in a fixating unit  17 . 
     The fixture plates  19 , on which the aperture segment  10  is placed, are provided with an arcuate shaped edge  20  at the top, which corresponds with the outer diameter of a specific tower part  6 . 
       FIG. 13  illustrates a further method step including the lowering of a wall segment to the fixating unit with an aperture segment and connecting the segments. 
     The wall segment  9  is hoisted down, by some kind of hoisting means  21 , in joining position on the fixating unit  17 , where the aperture segment  10  already has been placed. When joined the segments constitutes a full 360° tower part  6 , which can be hoisted off the fixating unit  17  as one part. 
     LIST 
     
         
         
           
               1 . Wind turbine 
               2 . Tower 
               3 . Nacelle 
               4 . Rotor 
               5 . Blades 
               6 . Tower part 
               7 . Aperture 
               8 . Door 
               9 . Wall segment 
               10 . Aperture segment 
               11 . Sharpening 
               12 . Fixating unit 
               13 . Rolling mill 
               14 . Cart 
               15 . Holding means 
               16 . Wheels 
               17 . Fixating unit 
               18 . Foundation 
               19 . Fixture plates 
               20 . Arcuate shaped edge 
               21 . Hoisting means 
               22 . Tower ring 
               23 . Hinge