Abstract:
Methods and arrangements for controlling the tension of tensioning cables in precompressed towers are disclosed. The towers may comprise a tower section ( 5 ), a pair of flanges ( 15, 15 ′), a plurality of tensioning cables ( 10 A- 10 D) and at least one tensioner ( 30 AB,  30 CD). The pair of flanges may be arranged around an upper and a lower part of the tower section. The at least one tensioner may be arranged between two of the plurality of tensioning cables ( 10 A- 10 D). The tensioner may pull the tensioning cables in response to a load signal to increase the tension.

Description:
This application claims the benefit of European Patent Application EP13382103 filed 21 Mar. 2013. 
     The present disclosure relates to fixed constructions and more specifically to methods and arrangements for controlling the tension of tensioning cables in precompressed tower sections. 
     BACKGROUND ART 
     Most existing concrete towers, are pre-compressed (or “pre-stressed”) to account for extreme loads, such as winds that may affect the integrity of their structure. Typically these towers have a reinforced concrete column fitted with tensioning cables, such as steel cables. Towers for wind turbines may be steel, concrete or hybrid towers. Hybrid towers may have a lower concrete section and an upper steel section. 
       FIG. 1  shows a typical wind turbine tower. A concrete tower section has been indicated.  FIG. 2  is a cross-section of the tower section of  FIG. 1  with a typical arrangement of tensioning cables. The tensioning cables exert a compression on the tower section to to avoid or reduce the possibilities of the concrete section being submitted to tension under the influence of a load, such as a wind load. As the cables must account for extreme events, such as ripples of high wind, the towers are precompressed to withstand loads caused by these extreme events and the cables are accordingly tensioned. 
       FIG. 3  shows the negative and positive stress distribution in the base of a tower under a wind load. The point suffering the highest negative stress is point A in  FIG. 3 . This is the windward point at the base of the tower. A tensioning cable at point A must be pre-tensioned to counteract the negative stress caused by wind load W. The tensioning cables may generally be equally pre-tensioned around the base as wind loads may be expected from all sides. As a consequence, when a windward tensioning cable counteracts a wind load, a leeward tensioning cable simply adds compression to the leeward point (point B in  FIG. 3 ) that is already under compression by the wind load. This means that the tower has to be dimensioned to withstand compression that is at least double the compression exerted by the tensioning cables. Consequently, the cross-section of the tower is calculated accordingly. Therefore, large amounts of concrete are required to account for this additional compression. This has a direct impact on the cost of construction of a tower. 
     SUMMARY OF THE INVENTION 
     There is a need for a new tower and a new tensioning method that at least partially resolves some of the above mentioned problems. It is an object of the present invention to fulfill such a need. 
     In a first aspect of the invention a tower is disclosed that may comprise a tower section, a pair of flanges, a plurality of tensioning cables and at least one tensioner. The tower section may have a wall surrounding an inner space. The pair of flanges may extend from the wall and may be arranged around an upper and a lower part of the tower section. Each flange may be arranged with a plurality of cable support elements. The plurality of tensioning cables may extend along the tower section. Each tensioning cable may be attached at one end to a cable support element arranged with the upper flange and at the other end to a cable support element arranged with the lower flange. The at least one tensioner may be arranged between two of the plurality of tensioning cables. 
     The term “flange” in this respect may be used to denote a tower portion where cables are attached or embedded. Such tower portion may or may not be connecting the tower section with the foundation or with another tower section. 
     The cable support elements may form part of the flange or may be attached to the flange. An example of a cable support element is a cable terminator. However, any type of element that may support the cable with the flange may be used. 
     The at least one tensioner may pull the two cables towards each other, thus increasing the tension exerted by each cable. As the tension increases, so does the compression of the respective area of the tower. 
     In some embodiments, each cable may be coupled to one tensioner. For an even number 2*n of cables, n tensioners are required so that the tension of each pair of cables can be individually set. 
     In some embodiments, the at least one tensioner may be arranged half-way along the length of each pair of tensioning cables. This arrangement distributes the stress induced to the tensioning cables more evenly between the upper cable support element and the lower cable support element. 
     In some embodiments, each tensioning cable may be coupled to more than one tensioner. By coupling each tensioning cable to more than one tensioner, the same tension may be achieved with smaller or less potent tensioners. 
     In some embodiments, the tensioning cables may be arranged in consecutive pairs and the cables of each pair may be coupled to the same tensioners. The resulting tension is then a product of the sum of pulling forces from the plurality of tensors arranged between each pair of tensioning cables. This arrangement may be beneficial if the space between two consecutive cables is limited. 
     In some embodiments each tensioning cable may be coupled to a first tensioner and to a second tensioner. The first tensioner may be arranged between the tensioning cable and a first neighboring tensioning cable. The second tensioner may be arranged between the tensioning cable and a second neighboring tensioning cable. This arrangement allows a more uniform distribution of tensions between consecutive cables, as the tension of each cable is related to the tension of both neighboring cables. 
     In some embodiments, the tower may further comprise a controller, coupled to each tensioner, for detecting a load and instructing each tensioner to pull the tensioning cables. The controller may be connected to sensors for detecting a load, such as a wind load caused by a wind ripple. Detecting a load may comprise detecting force and direction of the load. Detecting the direction of the load may determine the principal tensioner, or a principal group of tensioners that needs to be actuated. Detecting the force of the load may determine the pulling force of the principal tensioner or group. A principal tensioner may be defined as the tensioner at the point of the most negative stress due to the detected load. For example, if the load is a bending load caused by a wind ripple, the principal tensioner shall be defined as the tensioner closer to the windward part of the tower section where the most tension in the tower would be expected due to the wind ripple. By contrast, the hindmost tensioner shall be defined as the tensioner closer to the leeward part of the tower section, where the least tension is expected and the most compression will take place due to the wind. 
     In some embodiments each tensioner may comprise a first cable grip, for gripping the first cable of each pair of cables, a second cable grip for gripping the second cable of each pair of cables, and a tensioning module, attached to said first and second cable grips, for setting the tension of each tensioning cable by pulling the cable grips towards each other. The cable grips may be in the form of sleeves or jackets each firmly surrounding a portion of its respective tensioning cable. One skilled in the art may appreciate that any suitable type of grip for tensioning cables may be used. The grip shall surround the tensioning cable in such a way that it would not slip along the tensioning cable during or after a pulling action by the tensioning module. The tensioning modules may be pistons. However, any type of actuator that can exert a pulling force may be used as a tensioning module without departing from the scope of the invention. 
     When the tower is a wind turbine tower, then the expected load is a wind load. However, the arrangement of the tensioners may also account for the loads caused by the rotation of the blades or by the rotation of the nacelle. 
     In another aspect of the invention, a method of setting the tension of tensioning cables in a tower is disclosed. The method may comprise the steps of detecting a load, calculating a desired tension of a pair of consecutive cables for counteracting the load, calculating a pulling force between the consecutive cables for setting the desired tension, and pulling the consecutive cables until the tension is the desired one. The first step may be undertaken by sensors arranged around the tower or even external to the tower. The second and third steps may be undertaken by a controller. The controller may be part of the tower or may be external to the tower. The fourth step may be undertaken by a tensioner. The cables may be pretensioned with a safety tension corresponding to a safety precompression of the tower. This pretensioning may be provided by cable support elements or by tensioners. In the latter case, a minimum pulling force may be applied to the cables by the tensioners to provide the required minimum pretensioning. Finally, when the tower is a wind turbine tower, the load may be a bending load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Particular embodiments of the present invention will be described in the following by way of non-limiting examples, with reference to the appended drawings, in which: 
         FIG. 1  shows a wind turbine tower. 
         FIG. 2  is a cross-section of a tower section with a typical arrangement of tensioning cables. 
         FIG. 3  is an illustration of the positive and negative stresses around the base of a tower under a wind load. 
         FIG. 4  shows a tensioning cable arrangement according to an embodiment in a relaxed state. 
         FIG. 4A  shows the tensioning cable arrangement of  FIG. 4  in an excited state. 
         FIG. 5  shows a tensioning cable arrangement according to another embodiment in a relaxed state. 
         FIG. 5A  shows the tensioning cable arrangement of  FIG. 5  in an excited state. 
         FIG. 6  shows a tensioning cable arrangement according to yet another embodiment in a relaxed state. 
         FIG. 6A  shows the tensioning cable arrangement of  FIG. 6  in an excited state. 
         FIG. 7  is a comparative tension diagram. 
         FIG. 8  is a comparative compression diagram. 
         FIG. 9  is a flow diagram of a method according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 4  shows a tensioning cable arrangement according to an embodiment in a relaxed state. A portion of a concrete tower section  5  may include a portion of an upper flange  15  and of a lower flange  15 ′. A plurality of tensioning cables  10 A,  10 B,  10 C,  10 D may be arranged in parallel in the portion of the tower section  5 . Each tensioning cable extends along the tower section  5 . 
     Each tensioning cable  10 A- 10 D may be attached at one end to a cable support element  20 A- 20 D of the upper flange  15  and at the other end to a cable support element  20 A′- 20 D′ of the lower flange  15 ′. A first tensioner  30 AB is arranged between cables  10 A and  10 B. A second tensioner  30 CD is arranged between cables  10 C and  10 D. Each of the tensioners  30 AB,  30 CD comprises a tensioning module  32 AB,  32 CD and a pair of grips ( 35 A,  35 B) and ( 35 C,  35 D). During the relaxed state of  FIG. 4 , the tensioners  30 AB and  30 CD do not pull cables  10 A- 10 D and the tension the cables exert to tower section  5  is a minimum safety tension. The grips may be in the form of sleeves or jacket, elastically gripping the cables so that they may not slip when the cables are in the relaxed state. Alternatively, the tensioners may exert a limited safety tension during the relaxed state so that the tower is under compression. 
       FIG. 4A  shows the tensioning cable arrangement of  FIG. 4  in an excited state. When a load is detected, the tensioners  30 AB and  30 CD may be instructed to pull the cables  10 A- 10 D so that the tension in the tower section  5  is increased. As shown in  FIG. 4A , the tensioning module  32 AB is contracted and the distance between grips  35 A,  35 B is reduced. As a consequence the cables  10 A,  10 B are pulled closer and the compression they exert on the tower section  5  increases. Accordingly, the tensioning module  32 CD is contracted and the distance between grips  35 C,  35 D is reduced. As a consequence the cables  100 ,  10 D are pulled closer and the compression they exert on the tower section  5  increases. 
     One skilled in the art may appreciate that a relatively small horizontal pulling force of the tensioners may translate in a high vertical tensioning force at the cables. The arrangement of  FIG. 4A  shows that the pairs  10 A,  10 B and  10 C,  10 D are equally pulled. One skilled in the art may appreciate that this would typically be the case if the windward point was in the middle between cables  10 B and  10 C, indicated with dashed line A-A′. In other cases, the tension required from each pair may be individually adapted and as a consequence, the distance between the cables of each pair would not be the same. 
     Furthermore, the distribution of tensioning between pairs of cables may be at the discretion of the tower operator. Therefore, in some cases a higher tension may be desired by a principal tensioner and a lower tension by neighboring tensioners for a certain load, while in other cases a more distributed tensioning between a principal and neighboring tensioners may be desirable. 
       FIG. 5  shows a tensioning cable arrangement according to another embodiment in a relaxed state. In this embodiment two tensioners are arranged between tensioning cables belonging to a pair. A first tensioner  130 AB and a second tensioner  130 AB′ are arranged between cables  110 A and  110 B. A third tensioner  130 CD and a fourth tensioner  130 CD′ are arranged between cables  110 C and  110 D. Each of the tensioners  130 AB,  130 AB′,  130 CD,  130 CD′, may comprise a tensioning module  132 AB,  132 AB′,  132 CD,  132 CD′, respectively, and a pair of grips ( 135 A,  135 B), ( 135 A′,  135 B′), ( 135 C,  135 D) and ( 135 C′,  135 D′), respectively. During the relaxed state of  FIG. 4 , the tensioners do not pull cables  110 A- 110 D and the tension the cables exert to tower section  105  is a minimum safety tension. 
       FIG. 5A  shows the tensioning cable arrangement of  FIG. 5  in an excited state. When a load is detected, the tensioners  130 AB,  130 AB′,  130 CD,  130 CD′ are instructed to pull the cables  110 A- 110 D so that the tension in the tower section  105  is increased. As shown in  FIG. 5A , the tensioning modules  132 AB,  132 AB′ are contracted and the distance between grips  135 A,  135 B and  135 A′,  135 B′ is reduced. As a consequence the cables  110 A,  110 B are pulled closer and the compression they exert on the tower section  105  increases. Accordingly, the tensioning modules  132 CD,  132 CD′ are contracted and the distance between grips  135 C,  135 D and  135 C′,  135 D′ is reduced. As a consequence the cables  110 C,  110 D are pulled closer and the compression they exert on the tower section  105  increases. The arrangement of  FIG. 5A  shows again that the pairs  110 A,  110 B and  110 C,  110 D are equally pulled. Similarly to  FIG. 4A , this would ideally be the case if the windward point was in the middle between cables  110 B and  110 C, indicated with dashed line A-A′. In other cases, the tension required from each pair may be different and as a consequence, the distance between the cables of each pair would not be the same. 
     Comparing the embodiments of  FIG. 4A  and  FIG. 5A , it may be seen that the contraction of the tensioning modules is the same. However, in  FIG. 5A  the cables are under higher tension as the angle  59  of each cable to the flange is higher than the corresponding angle  49  of  FIG. 4A . As a consequence, with the arrangement of  FIGS. 5 and 5A , and using the same type of tensioners, it is possible to have the same tension with smaller pulling force at each tensioner, compared to the arrangement of  FIGS. 4 and 4A . Accordingly, it is possible to have a higher tension with the same pulling force. Therefore, the arrangement of  FIG. 5, 5A  allows the use of smaller or less potent tensioners for achieving the same tension as the one achieved with the arrangement of  FIG. 4, 4A . 
       FIG. 6  shows a tensioning cable arrangement according to yet another embodiment in a relaxed state. In this embodiment each tensioning cable is coupled to a first tensioner and to a second tensioner. The tensioner  230 AB is arranged between tensioning cables  210 A,  210 B. The tensioner  230 BC is arranged between tensioning cables  210 B,  210 C. The tensioner  230 CD is arranged between tensioning cables  210 C,  210 D. Further tensioners are partially shown arranged between tensioning cable  210 A and another cable (not shown) and between tensioning cable  210 D and another tensioning cable (not shown). During the relaxed state of  FIG. 6 , the tensioners do not pull cables  210 A- 210 D and the tension the cables exert to tower section  205  is a minimum safety tension. 
       FIG. 6A  shows the tensioning cable arrangement of  FIG. 6  in an excited state. When the tensioning module  232 AB is contracted the distance between the respective grips  235 A,  235 B is reduced. As a consequence the cables  210 A,  210 B are pulled closer along the line formed by grips  235 A,  235 B and the compression they exert on the tower section  205  increases. Accordingly, when the tensioning module  232 BC is contracted the distance between grips  235 B,  235 C is reduced. As a consequence the cables  210 B,  210 C are pulled closer along the line formed by grips  235 B,  235 C and the compression they exert on the tower section  205  further increases. Finally, when the tensioning module  232 CD is contracted the distance between grips  235 C,  235 D is reduced. As a consequence the cables  210 C,  210 D are pulled closer along the line formed by grips  235 C,  235 D and the compression they exert to the tower section  205  increases even further. In  FIG. 6A , the contraction of tensioning module  232 BC is shown higher than the contraction of tensioning modules  232 AB,  232 CD which is shown equal among the two. This would be the case if the windward point was in the center between the cables  210 B and  210 C, indicated with dashed line A-A′. 
     The arrangement of  FIGS. 6, and 6A  allows for a more uniform and fine-tuned distribution of tension between the cables, as the tension of each cable may be set by two tensioners, each allowed to exert a different pulling force. 
       FIG. 7  is a comparative tension diagram. It illustrates that in examples of the present invention, less tension is required during a relaxed state of a tower, while the appropriate tension is exerted when a load is present. The X axis of the diagram represents the distance from a point of the tower to the most windward point of the tower. 
     The Y axis represents the tension value. Conventionally, the tensioning cables would exert the tension shown with the dashed line L 1 . That is, conventionally, all the cables always exert the same tension to the tower as the tension is not controllable and must remain maximum at all times to account for winds in all directions. 
     In contrast, according to the various examples disclosed herein, in a relaxed state, only a minimum safety tension Ts is required, as depicted with line L 2 . 
     Lines L 1  and L 2  have a tension difference equal to “A” as shown in  FIG. 7 . During the relaxed state, the tensioners are not pulling any cables. When a load is detected, some of the tensioners around the tower are activated. Those tensioners closer to the windward point exert a higher pulling force leading to a higher tension in the respective cables. Those closer to the leeward point do not exert any pulling force or exert a lower pulling force leading to a lower tension. This is represented by the inclined line L 3 . 
     Although the line L 3  is shown straight, this is only for illustration purposes. The shape of line L 3  may actually vary and be crooked or stepped, based on the number of cables in the tower and their arrangement within the tower, and the tension exerted to each cable or pair of cables by the corresponding tensioners. In general, the starting point may always be higher than the ending point, as illustrated by line L 3 , i.e. the tension of the cables and the compression they exert on the tower may be higher at the windward side of the tower than at the leeward part. The area R shown in  FIG. 7  represents the area of allowable range of cable tension. 
       FIG. 8  is a comparative compression diagram under load. Again, the X axis of the diagram represents the distance from a point of the tower to the most windward point of the tower. The Y axis represents the compression value. The compression of the tower is equal to the sum of cable tension plus compression due to a load. 
     Closer to the windward point, the compression due to the load is negative, i.e. the tower portion is submitted to tension due to the wind load. In a typical tower without tensioners between cables, the compression is equal to Cs (safety compression). The safety compression which is the sum of the tension T in the cables and the tension due to the wind load-Cw. The tension T in the cables directly determines the compression in the tower section. At the windward point, as a result of the load, the compression of the tower section is reduced to a minimum safety compression. 
     The tension T of the cables at the windward point must always be above an anticipated maximum-Cw so that a concrete tower section is always under compression. 
     In a tower in accordance with examples of the present invention, under a certain load W, the total compression is again equal to Cs at the windward point, as the cable tension is lowered from the minimum safety tension Ts to the value Tw (then tension corresponding to a design wind load). Tw may be equal to the value T used in towers without tensioners so that a minimum safety compression Cs remains the same at the windward point. 
     However, at the leeward point, in a typical tower without tensioners between cables, the compression is equal to Cmax, which is the sum of the tension T of the cables (leading to a compression of equal amount in the concrete tower section) plus compression C L . C L  is the amount of compression at the leeward point due to the load W. In a tower with tensioners according to examples of the present invention, the value of tension T of the cables remains equal to the safety tension Ts at the leeward point (no tensioners are activated). The total compression is then, at the leeward point, equal to C L  plus Ts. 
     In some implementations, the safety tension Ts may even be reduced, possible even to zero, under a load W, if the minimum safety tension is provided by the tensioners and not by the cable support elements. The minimum compression Cs required for keeping the tower under compression is then provided by the positive stress C L  under a load situation. As may be seen by  FIG. 8 , the maximum compression is equal to Cmax-A. Therefore the maximum compression at a point around the tower may be reduced by at least a value A when using the tensioners disclosed. Consequently, towers with significantly less concrete may be constructed. Similarly, existing towers can be retrofitted to withstand higher loads than what they were constructed for, or to extend their lifetime by reducing loads. 
       FIG. 9  is a flow diagram of a method according to an embodiment. In a first step  910 , a load is detected by a sensor. In a next step  920 , a desired tension of a tensioning cable is calculated. The desired tension for each cable may be calculated according to the direction of the load and the intensity of the load. In a next step  930 , a pulling force of a tensioner is calculated so that the corresponding cable(s) can exert the desired tension. Finally, in step  940 , a pair of cables is pulled by a tensioner based on the calculated pulling force. Accordingly, all the required cables are pulled based on the respective pulling force calculated during the previous step for each cable or pair of cables. 
     The cables may be pretensioned by cable terminators in a relaxed stated, i.e. without a load present, to provide a minimum compression to the tower section. Alternatively, the cables may be pretensioned by tensioners arranged between the cables, pulling the cables to provide the minimum tension required for the minimum compression. In this case, under a load, the tensioners closer to the leeward point may be relaxed, as the minimum compression is provided by the load. 
     Although only a number of particular embodiments and examples of the invention have been disclosed herein, it will be understood by those skilled in the art that other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof are possible. Furthermore, the present invention covers all possible combinations of the particular embodiments described. Thus, the scope of the present invention should not be limited by particular embodiments, but should be determined only by a fair reading of the claims that follow.