Abstract:
A number of pre-casted elements are stacked vertically to build the tower, while parts of the elements are forming the tower wall. Each element of the tower is fixed on its position and is connected with a tower foundation by a number of assigned post-tensioned cables. These cables are running inside the tower and they are pulled through the tower without embedding in dedicated channels. The post-tensioned-cables are fixed at certain points with the tower wall via damper-means to prevent their oscillation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2008/060807, filed Aug. 18, 2008 and claims the benefit thereof. The International Application claims the benefits of U.S. provisional application No. 61/080,812 filed Jul. 15, 2008. All of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF INVENTION 
     The invention relates to a method for the assembly of a tower and to the tower. In a preferred embodiment the tower is used for a wind-turbine. 
     BACKGROUND OF INVENTION 
     Wind-turbines are conventionally mounted on top of steel-towers. The towers consist usually of a number of modules. 
     As the price of steel is increasing more than the price of concrete it is advantageous to build wind-turbine-towers of concrete. 
     For large experimental wind-turbines it is known to build and use concrete towers, which are built by using a so called “slip-form pouring method”. One example of this kind of tower was built 1977 for the Tvind-turbine in Denmark. 
     This method has the disadvantage that the concrete has to be filled into a mould, which is located at the top of the tower. At the end of the construction procedure the concrete has to be filled into the mould at the final height of the tower. In dependency of this height the efforts for the fill-in increases. Furthermore personnel are required to fill-in in the concrete into the mould at this final-height, so their work is limited by the time of the day, by health-regulations and by safety-requirements due to the height. 
     The WO 07025947 A1 discloses a method whereby a concrete tower is extruded vertically. This method has the disadvantage that it requires a very substantial technical arrangement, since high pressure is required for large-dimension components in order to push up the tower during casting. Large pressures at large diameters require very large technical arrangements. 
     It is also known to build concrete towers by the use of pre-casted segments. Such segments show dimensions which might anticipate the transport of the segments via roads or bridges. So additional effort need to be done to solve the problems of transportation. 
     It is known to build concrete towers by stacking of complete cylindrical elements. These elements are connected together by a number of post-tension cables. After the stacking of the elements a number of post-tension cables are inserted into channels in the tower walls. The channels transit the tower from the top to its bottom, while each post-tension cable is without discontinuation so the cable might reach a great effective length in dependency to the tower height. After cable insertion the channels are filled with a slurry material. 
     This arrangement has the disadvantage that for a high tower a reliable injection of the slurry needs special precautions. 
     Furthermore it may be difficult to insert the cables in the channels, particularly for a high tower. 
     The U.S. Pat. No. 7,114,295 discloses an improved method to solve these problems. A funnel-shaped apparatus is used for guiding the tension-cables and for establishing a seal to produce a pressure-tight transition between two tower segments. However despite these arrangements the problem remains to insert the post-tension-cables and to inject slurry into the channel for greater tower heights. 
     The U.S. Pat. No. 7,106,085 discloses a tower consisting of segments where no post-tension-cables are needed. This arrangement has the disadvantage that numerous mounting operations are required and that a high number of fasteners are needed. 
     The US 2008 004 0983 A1 discloses a tower consisting of segments. The segments do not require tensioning-cables, because they are pre-assembled on ground. This arrangement has the disadvantage that numerous mounting operations are required and that a high number of fasteners are needed. 
     The WO 08031912 A1 discloses a wind-turbine-tower, which is mounted with pre-fabricated elements. The tower has longitudinal ribs, which form longitudinal joints. These joints comprise metal elements and high resistance mortar. This leads to the disadvantage that numerous mounting operations are required and that a high number of fasteners are needed. Additionally high-strength mortar is needed. 
     SUMMARY OF INVENTION 
     It is the aim of the invention to provide an improved method for the assembly of a tower for a wind-turbine, and to provide an improved tower. 
     This aim is solved by the features of the independent claims. 
     Preferred embodiments are object of the dependant claims. 
     According to the invention a number of pre-casted elements are stacked vertically to build the tower. Parts of the elements are forming the tower wall. Each element of the tower is fixed on its position and is connected with a tower foundation by a number of assigned post-tensioned cables, which are running inside the tower. 
     The post-tensioned-cables of the elements are pulled through the tower without embedding in dedicated channels in the tower walls. The post-tensioned-cables are fixed at certain points with the tower wall via damper-means to prevent or to minimize their oscillation. 
     The invention combines
         a stacking of pre-casted elements,   the elements being fixed with post-tensioned cables that do not require to be inserted into special channels, and   the post-tensioned cables being damped at certain points to minimize their vibrations.       

     According to the invention a concrete tower is constructed by the stacking of cylindrical or tapered concrete pipes on top of each other. The pipes are joined to form a structural entity with post-tension cables which do not run inside cavities in the tower walls. The cables are hindered from oscillation through the application of suitable damper-means. 
     In a preferred embodiment the concrete tower is built by a number of cylindrical or tapered pre-cast elements as modules, each forming a complete annular element. 
     Some or all of these elements are fitted with structural elements that support dampers for attachment to the post-tensioning cables. 
     The tower is constructed by a stacking of the pre-cast modules on top of each other, until the complete tower is formed. After this stacking the post-tensioning cables are fitted and tensioned. During or after the cable installation suitable damper means are attached to the cables in order to prevent oscillation. 
     In a preferred embodiment one or more of the pre-casted elements or modules are casted on a planned site. A bottom module is cast directly on the foundation. Supplementary modules are cast adjacent to the turbine-location or in another suitable location on or near a wind-farm site. Other modules are supplied as precast or prefabricated elements, maybe from elsewhere. Such other modules may be made of concrete or steel. 
     Modules which are cast on a site can preferably be made with a module height that does not exceed the height at which an ordinary portable concrete pump for common contracting purposes can reach. 
     A module or element can be cast in a form or mould consisting of a bottom part, an inner part, an outer part and a top part. The top part and/or the bottom part are integrated in a preferred embodiment into either the outer part or into the inner part. For example the bottom part may be integrated with the inner part and the top part may be integrated with the outer part. 
     Due to the effect of installed post-tensioning cables longitudinal reinforcement of individual modules may not be needed to carry tensile stresses. The longitudinal reinforcement may be limited to the amount needed for handling purposes. Circumferential and shear reinforcement may be limited to the amount needed to ensure integrity under load and to transfer shear forces and torque. 
     In a preferred embodiment fibre-reinforced concrete is used, classical reinforcement with rebars is avoided. Fibers could be steel- or glass-fibers. 
     When the stacking of the modules is completed a number of cables are pulled partly and/or completely through the completed tower. The cables are fixed at a first end, thereafter they are fixed at the other end and tensioned. 
     The tensioning-cables are fitted with suitable damper means. The damper means may be tuned absorbers or dampers achieving their effect by viscous means. 
     In a preferred embodiment the damping is obtained by connecting the cables at regular intervals to a tower wall with a bracket or similar structures. The joint between cable and bracket and/or bracket and tower is fitted with a viscous damping element, e.g. a rubber or a tar compound 
     In a preferred embodiment the lowest tower module is cast directly onto a foundation-base-plate, so the preparation of a tower plinth is avoided. 
     In another preferred embodiment the lowest tower module is cast directly on rocky ground and the foundation is limited to simple rock-anchors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is shown in more detail by help of the following figures, where: 
         FIG. 1  shows a wind-turbine using the tower according to the invention, 
         FIG. 2  shows the concrete tower according to the invention, referring to  FIG. 1 , 
         FIG. 3  shows the tower according to the invention in more detail, referring to  FIG. 2 , 
         FIG. 4  shows a transversal section through the tower  3 , referring to  FIG. 3 , 
         FIG. 5  shows a longitudinal section through the concrete tower according to the invention. 
         FIG. 6  shows a transversal section through the tower  3 , referring to  FIG. 5 , 
         FIG. 7  shows four variants of a joint to connect tower modules, and 
         FIG. 8  shows further variants of the joint between adjacent tower modules and of cable arrangements. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  shows a wind-turbine using the tower according to the invention. The wind-turbine comprises a rotor  1 , which is supported by a nacelle  2 . The nacelle  2  is mounted on a tower  3 , which is supported by a foundation  4 . 
       FIG. 2  shows the concrete tower  3  according to the invention, referring to  FIG. 1 . 
     The concrete tower  3  is constructed with elements as modules  5 , which are stacked on top of each other. In a preferred embodiment a last module  6 , which is located on top of the tower  3 , is substantially shorter than its preceding module  5 . 
       FIG. 3  shows the tower according to the invention in more detail, referring to  FIG. 2 . 
     In this embodiment each tower module  5  (except the tower module  6  on the top) shows a cable-supporting protrusion  7  at its top. 
     On the right side of the tower  3  centerlines of post-tensioning cables  8  are shown. Some of them run through the entire length of the tower  3 , from the top module  6  down to the foundation  4 , crossing all the modules  5 . 
     Other post-tensioning cables  8  transit only through a number of modules  5 , so they run from the top of a dedicated module  5  through all the modules  5 , which are located below the dedicated module  5 . 
     In this figure the post-tensioning cables  8  are shown descending vertically. 
       FIG. 4  shows a transversal section through the tower  3 , referring to  FIG. 3 . 
     In this example each of the tower modules  5  and  6  has four post-tensioning cables, which connects the modules  5  and  6  to the foundation  4 . 
     The cables from the tower modules  5 ,  6  are located in an offset-circumferentially manner, so they do not interfere with each other. 
     A tower wall  9  encloses the cables. 
     As the cables are descending vertically in this example, four cables  10  from the top module  6  are closest to a centre CT of the tower. 
     Four cables  11  are assigned to a module  5 - 1 , while four cables  12  are assigned to a module  5 - 2  and four cables  13  are assigned to a module  5 - 3 , counted down from the top of the mast  3  to the foundation  4 . 
     The cables  11 ,  12  and  13  are located progressively closer to the tower wall  9 . 
       FIG. 5  shows a longitudinal section through the concrete tower  3  according to the invention. 
     Differing to  FIG. 3  the post-tensioning cables  8  descend parallel to the tower wall  9 . 
       FIG. 6  shows a transversal section through the tower  3 , referring to  FIG. 5 . 
     In this example each of the tower modules  5  and  6  show four post-tensioned cables, which connect the modules  5  and  6  to the foundation  4 . 
     The cables from the tower modules are located in an offset-circumferentially-manner, so they do not interfere with each other. 
     A tower wall  9  encloses the cables. Because the cables descend in parallel to the tower wall  9 , the four cables  10  from the top module  6 , the four cables  11  from a module  5 - 1 , the four cables  12  from a module  5 - 2  and the four cables from a module  5 - 3  show an equally spacing from the tower wall  9 . 
       FIG. 7  shows four variants of a joint to connect the tower modules. 
     Referring to  FIG. 7A  the tower module  5 - 1  has a cable-supporting protrusion  7  that either serves as anchor point for a post-tensioning cable  8  or that serves as support for the damping of a cable from a higher module—e.g. by a channel  14  that may be filled with a tar-based or a rubber-based compound once the cable  8  is already inserted. 
     Referring to  FIG. 7B  adjacent modules  5 - 1  and  5 - 2  are centered using a finger- and groove-arrangement  15 . 
     Referring to  FIG. 7C  adjacent modules  5 - 1  and  5 - 2  are centered using an overlap. 
     Here the cable-supporting protrusion  7  is extended inwards to serve as a platform, only leaving a hole  16  for power cables, for a ladder or a lift. 
     An upper module  5 - 1  has a recess  17  that centers the upper module  5 - 1  when it is mounted onto the lower module  5 - 2 . 
     Referring to  FIG. 7D  adjacent modules  5 - 1  and  5 - 2  are centered using an overlap. 
     Here the cable-supporting protrusion  7  is extended upwards to provide a centering recess  18  for an upper module  5 - 1 . The upper module  5 - 1  centers on this recess  18  when it is placed onto a lower module  5 - 2 . 
       FIG. 8  shows further variants of the joint between adjacent tower modules and of cable arrangements. 
     Referring to  FIG. 8A  the tower module  5 - 1  and  5 - 2  does not have a cable supporting protrusion as described above. 
     Instead of this a centering piece  19  is placed between two adjacent modules  5 - 1  and  5 - 2 . The centering piece  19  has holes  14 , which are used for the cables  8 . 
     Referring to  FIG. 8B  the centering piece  19  has only a small hole  20  for power cables, for a lift or ladder and thereby it is used as a platform. 
     Referring to  FIG. 8C  an attachment of the post-tensioning cables  8  at a centering piece  19  is shown. 
     The cable  8  projects through a hole  14  in the centering piece  19 . On top of a load distributing washer  20  or ring  20  the cable  8  is tensioned using a nut  21 . 
     Referring to  FIG. 8D  a damping of a post-tensioning cable  8  attached at a higher level is shown. 
     The cable  8  passes through a hole  14  in the centering piece  19 . 
     Once the cable  8  is tensioned, a suitable damping compound  22  is applied to be filled into the hole  14 .