Steel shell tower in two sections and ring flange. 

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Context 1

... welded steel shell tower today dominates the wind turbine market. It consists of cylinders made of steel plate bent to a circular shape and welded longitudinally ( Figure 2). Transversal welds connect several such cylinders to form a tower section. Each section ends with a steel flange in each end. The sections are bolted to each other. The bottom flange is connected to the foundation and the top one to the nacelle. A tower is primarily dimensioned against tension and buckling in the extreme load cases. Ideally the margin should be the same for both criteria, since increasing the diameter, with a corresponding reduction of plate thickness, increases the tension strength but reduces the buckling margin. Finally the tower has to be checked against fatigue. According to BSK and Eurocode connecting welds (transversal and longitudinal) and dimension changes (flanges) affects the strength in a negative way. Thus it is the welds and the geometry that primarily determine the fatigue strength rather than the quality of the steel. Therefore wind turbine towers mostly use ordinary qualities of steel. In this report use of S355J2G3 (earlier known as SS2134, tensile yield limit 355 MPa) is assumed for both the welded and friction joint towers [2]. In the dimensioning load case, the tower is affected by the thrust from the rotor. This thrust will create a bending moment, which increases with the distance from the turbine shaft, i.e. inversely proportional to the height above the ground. To cope with this increasing bending moment it is favourable to make the tower conical in shape, to the limit of buckling. However, land transportation even with a special permit is not possible for diameters exceeding 4,5 m in many countries (Figure 3). Ring flange connections result in high fabrication cost and long delivery times. Another disadvantage of such connections is their low fatigue resistance which is approximately 50 MPa. The idea behind building a hybrid concrete/steel tower (Figure 4) is to use concrete in the wide lower part and steel in the upper part, where a conventional welded steel shell tower section may be designed without any risk of conflict with the transportation limitations. In reality it also makes it easier to design the concrete part and to get the eigen-frequencies right [2]. Lattice tower applied as wind turbine support structure does not come until recently. In fact, during the earliest period of onshore wind turbine development, lattice tower was already adopted. This type of structure is simple to construct and stiff in function. A lot of such structures can be seen on the early onshore wind turbines (Figure 5). As the size of wind turbine increased, lattice tower was gradually displaced by tubular tower. In the early experimental stage of wind energy and especially when the size of wind turbine was still moderate, the emphasis was not placed on cost reduction of the tower, which is why the tubular tower was very widely and popularly used. However, as commercialization of wind energy is urged and the size of wind turbine grows, after cost reduction measurements on mechanical components like the gearbox and generator are achieved, cost minimization associated with the turbine support structure is again attracting interests and this is why these years the lattice structure is receiving more and more attentions, especially on large scale offshore wind turbines [2]. The visual qualities are controversial, especially due to the resemblance to towers for high- voltage power lines, generally claimed to be ugly. An open design, like a lattice tower, is more prone to icing than a tubular tower. The possible impact on the dynamic properties may be the most severe consequence, which may endanger the wind turbine in an extreme case. It may also be a problem for maintenance personnel, even if their elevator runs on heated rails. Increased risk of falling ice is also another important danger for such towers. Use of hybrid towers is possible to achieve greater heights for the turbine shaft. This type of towers is composed of 3 parts, the lower lattice part fixed to the foundation and assembled at the installation site, a piece of tubular tower consisting of several parts bolted together, as happens in most tubular towers, and a transition piece which ensures the connection and transmission of efforts between the two main parts. A tower of this kind was installed at the wind farm in Gujarat, India some years ago (Figure 6). It is expected that this new type of towers produces about 10 to 12% more energy, because gains against the normal towers more than 40 meters in total height, with a combined height of 120 meters against the 80 meters of most tubular towers, therefore an ideal bet for low wind areas, due to its superior performance, with a potential to be installed in all parts of the world, without having to look for places where the wind speed is high. These towers can be climbed from the inside, and have platforms inside the tubular part for maintenance and repair work, but also to maintain all the equipment necessary for its operation. Different types of towers for wind energy converters are given above. A cost comparison for currently existing tower types are also given in Figure 7 below ...

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... 1. Summary of specific investment cost for 3 and 5 MW wind turbines furnished with slip formed concrete towers ............................................................................................. 4 Figure 2. Steel shell tower in two sections and ring flange ...................................................... 5 Figure 3. Transportation of steel tubular tower segments ........................................................ 6 Figure 4. Concrete-steel hybrid tower. .................................................................................... 6 Figure 5. Steel lattice tower ..................................................................................................... 7 Figure 6. Hybrid lattice-tubular tower. .................................................................................... 8 Figure 7. Tower costs for the alternative designs. Turbine power 3 MW, hub height 125 m ... 9 Figure 8. Fatigue life stages [6]. ............................................................................................ 13 Figure 9. Crack occurrence in cyclic loading [5]. .................................................................. 15 Figure 10. Fatigue crack growth [6]. ..................................................................................... 16 Figure 11. Typical S-N curve of a medium-strength steel. .................................................... 18 Figure 12. Stress definitions. ................................................................................................. 19 Figure 13. Strain-life curves showing total, elastic, and plastic strain components. .............. 23 Figure 14. Mean stress relaxation under strain-controlled cycling with a mean strain [3]. .... 24 Figure 15. Crack growth: (a) tensile crack growth according to SWT criterion and (b) effect of normal stress on shear crack growth according to FS criterion. ........................................ 29 Figure 16. Fatigue crack propagation regimes [6]. ................................................................ 33 Figure 17. Schematic explanation of fatigue analysis procedure. .......................................... 36 Figure 18. Strain ‐ life curves for the S355 steel, Rε= ‐ 1. ......................................................... 38 Figure 19. Experimental fatigue crack propagation data of the S355 steel for distinct stress ratios: experimental results [41, 42] ......................................................................................... 39 Figure 20. Preload validation (356.1 kN). ............................................................................. 41 Figure 21. Stiffness model. .................................................................................................... 42 Figure 22. Stiffness calculation. ............................................................................................ 43 Figure 23. Elastic joint stiffness. ........................................................................................... 43 Figure 24. Connector element used for stiffness. ................................................................... 44 Figure 25. Global model. ....................................................................................................... 45 Figure 26. Max. and min. axial forces in members. ............................................................... 47  ...