Patent Publication Number: US-2021180568-A1

Title: Method for building a metallic tower for a wind turbine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European Application No. 19380034.9, having a filing date of Dec. 11, 2019, the entire contents of which are hereby incorporated by reference. 
     FIELD OF TECHNOLOGY 
     The following concerns a method for building a metallic, steel, tower for a wind turbine. 
     BACKGROUND 
     Wind turbines known in the state of the art usually comprise a nacelle with a hub and blades as major components. To rotate the blades and thus be able to provide kinetic wind energy to a generator housed in the nacelle, these major components are usually mounted, rotatably, onto a wind turbine tower having a certain height chosen to provide optimal wind conditions at a corresponding hub height. Wind speed increases with height above ground. Further, a larger hub height also allows larger blade size, leading to a greater wind-swept area and lower cut-in speeds of the wind turbine. Increased hub height also leads to reduction in variability, as cut-out speeds of the wind turbine are lower for the same energy production. 
     Towers for wind turbines exist mainly in two variants, namely concrete towers and metallic towers. For both variants, it is known to build the tower in so-called tower sections. Tower sections are tubular/hollow cylindric components of the tower having a certain section height. These tower sections are stacked to build the tower. Regarding concrete towers the sections can be cast at the site where the tower is to be erected. Normal reinforced concrete techniques, for example using shutters or molds, may be used to manufacture the components. Machinery and molds can be moved from site to site. 
     In the variant of a metallic, steel, tower, however, the tower sections need to be transported from a manufacturing site, where they are, for example, cast, to the building site. For this transport, logistic restrictions have to be taken into account. Most legislations or countries, respectively, restrict the maximum size of objects to be transported using their road network, for example to about 4.3 to 4.5 m in many countries. These logistic restrictions, regarding tower sections, apply to their diameter. This also restricts the maximally achievable hub size for steel towers to about 70 to 80 m. 
     To increase to achievable tower height and hence hub height for metallic wind turbine towers in an economical manner, the diameter of the tower sections would have to be increased, since other options come with their own disadvantages. If, for example, the metal thickness, steel thickness, would be increased, cost would increase and limitations regarding the steel supplier&#39;s capacity may occur. This would also increase the weight of each tower section, such that the number of sections would have to be increased, leading to increased logistic costs and construction complexity. 
     SUMMARY 
     An aspect relates to a method for building higher metallic towers while avoiding logistic restrictions and also excessively increased cost and complexity. 
     According to embodiments of the invention, this aspect is achieved by providing a method for building a metallic, steel, tower for a wind turbine, comprising the steps of: 
     at a manufacturing site, manufacturing hollow cylindric metallic tower sections, wherein the tower sections can be stacked in a longitudinal direction to form the tower, 
     cutting, at least for tower sections whose diameter exceeds a predefined maximum diameter, the tower sections longitudinally into a first number of at least two, at least three, section segments, 
     transporting the section segments to a building site where the tower is to be erected, 
     joining the section segments by welding to reconstruct the cut tower sections, and 
     stacking the tower sections to build the tower. 
     The predefined maximum diameter corresponds to a legal limit for transportation, wherein the tower sections are cut into section segments, by choosing a suitable first number, such that each section segment fulfils the at least one legal limit. In this manner, each section segment may be transported legally from the manufacturing site to the building site. Usually, the first number will be at least three. A common case will be four section segments, but also more than four segments are conceivable. 
     Embodiments of the invention thus propose to cut the tower sections longitudinally into section segments which are easier to transport, adapted to the logistic restrictions. These section segments are rejoined at the building site by welding. This allows the transportation of tower sections having a higher diameter, exceeding a legal limit for transportation, such that also the tower height and thus the hub height can be increased. Such towers may bear higher loads and allow larger blade sizes and rotor diameters, respectively. The first number and thus size of the section segments may be flexibly chosen. This allows, for example, to transport tower sections having a length of 12 m by a standard truck. 
     Using embodiments of the invention, transport costs may also be saved due to on-site manufacture. Easy and fast installation at the building site is provided, as the required machinery may easily be transported and prepared. The installation time and installation costs do not increase significantly compared to transporting the complete tower sections. 
     Since the section segments are welded at the building site, no bolt connections are required longitudinally, which would lead to bolt maintenance and cause other issues. Between tower sections, the regular, known flange connections can be used, which may employ bolt connections. 
     In embodiments, each tower section may comprise at least one subsection, wherein manufacturing the subsections comprises providing a metal sheet, rolling the metal sheet and welding the rolled metal sheet. In other words, the tower sections may be manufactured as known before the cutting takes place, such that the inventive method requires no complex modifications regarding these steps. Metal steel, may be provided, cut and bevelled and then rolled to the hollow cylindrical form. The rolled metal sheet is then welded to result in the tower section (in the case of one subsection). In the case of multiple subsections, the multiple subsections may be weldingly connected to form the tower section. Generally, manufacturing the at least one subsection may, of course, comprise further steps, machining steps like bevelling. 
     Generally speaking, manufacturing the tower sections may comprise adding a flange to each side of the tower sections and/or at least one machining step, sand blasting, and/or painting the tower section. The flange serves to connect the tower sections when they are stacked. In summary, the complete manufacturing queue for tower sections may be applied as usual before cutting. For example, after the tower section is made by rolling at least one metal sheet and welding, an inspection step, comprising non-destructive testing, may ensue. Thereafter, the tower section may be sandblasted and painted/varnished. 
     In a preferred embodiment, painting the tower sections comprises covering a cutting area where the tower section is cut into the section segments. That is, the cutting area is excluded from the painting process at this time since cutting would destroy the paint/varnish. Narrow strips may, for example, be used along the intended cutting lines, such that only a small fraction, for example less than 5%, of the surface of the tower section remains unpainted. After rejoining the section segments at the building site, these excluded cutting areas may, for example manually, be painted. This does not result in considerable effort, since only a small fraction remained unpainted. Only long seams have to be painted. No shot blasting cabins and/or painting cabins are required at the building site, since manual roughing and painting processes may be used. 
     Cutting is performed using a flame cutting machine or a plasma cutting machine. The flame cutting process is a thermo-chemical process requiring a source of intense heat and pure oxygen. Flame cutting can be used for cutting most metals used for a tower, steel, since the basic requirement that the oxide formed must have a lower melting point than the base material to be cut is fulfilled. The basic plasma cutting process involves creating an electrical channel of superheated, electrically ionized gas, i.e. plasma, from the plasma cutting machine itself, through the work piece to be cut, thus forming a completed electric circuit back to the plasma cutting machine through, for example, a grounding clamp. A compressed gas, for example oxygen or air, may be blown through a focused nozzle at high speed toward the work piece. An electrical arc is then formed within the gas, between an electrode near or integrated into the gas nozzle and the work piece itself. The electrical arc ionizes some of the gas, thereby creating an electrically conductive channel of plasma. As electricity from the cutter torch travels down this plasma it delivers sufficient heat to melt through the work piece. At the same time, much of the high velocity plasma and compressed gas blow the hot molten metal away, thereby separating, i.e. cutting through, the work piece. Plasma cutting is an effective way of cutting thick materials like stainless steel wind turbine tower walls. 
     Advantageous embodiments, after cutting, the cutting edges are bevelled, V-bevelled from the inside. This bevelling process allows both easy welding from the inside of the tower section to be reconstructed as well as accurate gauging from the outside of the tower section. In preferred concrete embodiments, a reconstruction tool is used to position the section segments, wherein gauging may be performed and evaluated from the outside. After positioning is complete and all section segments are fixed in the reconstruction tool, a mobile welding machine may be inserted into the tower section, welding the section segments while automatically moving longitudinally through the tower section. 
     It is noted at this point that using longitudinal cuts in the current embodiments of invention also has the advantage that circular welding, which would be more complex to realise at the building site, is avoided. 
     After beveling, the cutting edges may be treated with a primer to avoid rusting during the logistic process, transport and possibly storage. 
     In embodiments, as already explained, before welding the section segments to reconstruct the tower sections, a reconstruction tool is used to position the section segments. Such an auxiliary tool may be easily transported from building site to building site and may comprise a frame, wherein the section segments may be positioned, and/or adjustable holding devices for the section segments. 
     The welding of the section segments to reconstruct the tower sections may be performed as submerged arc welding. Submerged arc welding (SAW) involves the formation of an arc between a continuously fed electrode and the workpiece. A blanket of powdered flux generates a protective gas shield and a slag which protects the weld zone. The arc is submerged beneath the flux blanket and normally not visible during welding. For SAW, mobile welding machines have been proposed, which, as already discussed above, may be used to weld the section segments together from the inside of the tower section to be reconstructed. 
     After welding the section segments to reconstruct the tower sections, the welding area is machined, and manually mechanically blasted, and/or, if the cutting area had not been painted, painted, manually. For example, mechanical roughness process tools, handheld tools, may be employed, as well as known manual painting tools. 
    
    
     
       BRIEF DESCRIPTION 
       Some of the embodiments will be described in detail, with references to the following Figures, wherein like designations denote like members, wherein: 
         FIG. 1  shows a flowchart of an embodiment of a method according to embodiments of the invention; 
         FIG. 2  shows a manufactured tower section prepared for painting; 
         FIG. 3  shows cut section segments; 
         FIG. 4  shows a schematic side view of a section segment; 
         FIG. 5  shows two adjacent cutting edges of section segments after bevelling; 
         FIG. 6  shows a reconstruction tool at the building site; 
         FIG. 7  shows a longitudinal welding process using a mobile welding machine; and 
         FIG. 8  shows a principle drawing showing the stacking of tower sections to form a wind turbine tower. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the method according to embodiments of the invention provide a simple and cost-efficient process for building a metallic wind turbine tower, in this case made of steel, wherein the tower has tower sections of a large diameter, greater than 5 m, such that greater hub heights are achievable, for example hub heights greater than 80 m. Despite the large diameters, embodiments of the invention allow for easy transportation respecting local logistic restrictions. 
     In the embodiment according to  FIG. 1 , steps S 1  to S 9  are performed at a manufacturing site, while steps S 11  to S 16  are performed at a building site where the tower and thus the wind turbine is to be erected. 
     First, in steps S 1  to S 8 , tower sections are produced essentially as known from the state of the art, apart from a few minor modifications and the fact that the tower sections&#39; diameter exceeds a maximum diameter, in this case defined according to a legal limit. This means that the tower sections may not be transported as such in a respective country/legislation. 
     In step S 1 , metallic material for the tower sections is provided, in this case stainless steel. In step S 2 , the steel is cut into metal sheets, which are bevelled for later welding. In step S 3 , rolling of the metal sheets into a hollow cylindrical form ensues. The welding to form the hollow cylinder takes places in step S 4 . In step S 5 , at least flanges are added at both ends of the tower section. If the tower section is made from several hollow cylindric subsections, each providing a part of the tower section height, these subsections are welded together in step S 5 . 
     In an optional step S 6 , the tower sections are inspected, including non-destructive testing. 
     In a step S 7 , sand blasting of the tower section takes place, for example in a blasting cabin. In step S 8 , at the tower section&#39;s outer surface is painted/varnished. However, before this takes place, for example in a paint cabin, cutting areas around intended cut lines are covered, for example by narrow tapes, such that the cutting areas are excluded from painting. 
     This is shown in  FIG. 2 , which depicts a tower section  1  to be cut into four section segments longitudinally along intended cut lines  2 , shown dashed. Before painting, cutting areas  3  are covered, for example by a narrow tape, such that the fraction of the surface excluded from painting remains small, for example less than 5%. 
     Returning to  FIG. 1 , in a step S 9 , the tower section  1  is cut into the section segments  4 , which are exemplarily shown in  FIG. 3 , using a flame cutting machine or a plasma cutting machine.  FIG. 4  shows a side view of a section segment  4 , illustrating that not the whole outer surface is painted, but only the central area  5 , leaving the cutting edges  6  in the cutting areas  3  free of paint. 
     These cutting edges  6  are, also in step S 9 , bevelled and treated with a primer. As shown in  FIG. 5 , V-bevelling from the inside  7  is performed, resulting the slanted edges  8 . This greatly simplifies welding from the inside  7  as well as gauging from the outside  9  when the section segments  4  are rejoined at the building site, as further explained below. The section segments  4 , which fulfil any logistic restriction, are now ready for transport. 
     In a step S 10 , see  FIG. 1 , the section segments  4  are transported from the manufacturing site to the building site using a standard truck or, depending on the length of the segments  4 , using special trucks or transport means. 
     In a step S 11 , the section segments  4  of a tower section  1  which is to be reconstructed are placed in a reconstruction tool  10 , as indicated in  FIG. 6 . The reconstruction tool  10  may, for example, comprise a frame and/or respective holding devices. The section segments  4  are positioned correctly, exploiting the bevelling technique explained above to gauge from the outside  9 . 
     Returning to  FIG. 1 , in a step S 12 , the section segments  4  are re-joined to recreate the tower section  1  by longitudinal welding. The welding is performed from the inside  7  of the tower section  1 , exploiting the V-bevel. This is illustrated in  FIG. 7 , wherein, in this case, a mobile welding machine  11  is used which is only schematically indicated. Submerged arc welding (SAW) may be used. Beginning at one face side of the tower section  1 , the cutting edges  6  of the section segments  4  are welded together, as indicated by welding seam  12 . The welding machine  11  is moved longitudinally, according to arrow  13 , until the other face side is reached. 
     After welding, in a step S 13  according to  FIG. 1 , mechanical blasting of the longitudinal welding area is performed using a mechanical roughness tool. This may be done manually. In a step S 14 , the longitudinal welding area is painted manually, using respective painting tools. 
     In an optional step S 15 , internal components may be added to the reconstructed tower section  1 , for example platforms, ladders and the like. 
     In step S 16 , finally, the complete tower sections  1  are stacked to build the tower  14  of the wind turbine, as shown in  FIG. 8  and in principle known in the art. A crane  15  and/or another lifting tool may be used. The tower sections  1  may be connected using the flanges. 
     Once the tower  14  is erected, the nacelle with the hub may be added and the blades may be mounted to complete the wind turbine. 
     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. The mention of a “unit” or a “module” does not preclude the use of more than one unit or module.