8 th International Congress on Computational Mechanics Volos, 12 July 15 July 2015 NUMERICAL SIMULATION OF FLANGE-BOLT INTERACTION IN WIND TUBRINE TOWER CONNECTIONS Aikaterini I. Ntaifoti 1, Konstantina Koulatsou 2 and Charis J. Gantes 3 1 Institute of Steel Structures National Technical University of Athens Athens, GR-15780, Greece e-mail: ntaifotik@gmail.com 2 Institute of Steel Structures National Technical University of Athens Athens, GR-15780, Greece e-mail: konkoulatsou@gmail.com 3 Institute of Steel Structures National Technical University of Athens Athens, GR-15780, Greece e-mail: chgantes@central.ntua.gr, web page: http://users.ntua.gr/chgantes Keywords: wind turbines, tower connections, bolted ring flanges, numerical analyses, contact nonlinearity, ultimate load Abstract. Wind turbines constitute the most cost-effective way for the exploitation of the available wind potential, thus investigation of the behavior of such structures is of particular interest. Since fatigue is one of the most common types of structural failure of wind turbines, due to the dynamic nature of wind loads acting on their blades, consideration of connections between adjacent parts of wind turbine towers is very significant and demanding. In the present paper, a wide range of numerical analyses are carried out, using appropriate finite element software, in order to simulate one such typical connection. A detailed numerical model is presented, including only a part of the L-shaped connection i.e. one bolt, as well as the connected ring flanges, using solid elements. Contact elements are appropriately taken into account to introduce the connection s nonlinear behavior caused by the interaction between flanges and bolts. Objective of this work is to understand in depth the connection s static behavior and to determine its ultimate load under static loading. For that purpose, parametric analyses are performed in order to evaluate the influence of different parameters on the connection s load transfer mechanism and on its strength. 1 INTRODUCTION Nowadays, the need for better exploitation of the available wind resources constantly grows and wind energy is gaining increased attention, leading to an unprecedented expansion of the utilization of wind turbines. This triggers an effort for deeper investigation of the design, construction and operation of the mechanical and structural parts of wind turbines. The dimensions of modern wind turbines are growing, as well as the loads acting on them in order to take better advantage of the available wind potential. Due to the dynamic nature of wind loads [1], which are the prevailing loads on wind turbines, fatigue of connections between adjacent tower parts is one of the most common types of structural failure. Thus, further investigation of the structural behavior of the tower connections, as well as of the way in which the applied actions are transferred through the tower, is of particular interest. The most common type of wind turbine in use today is that with free-standing steel tubular towers, with the tower having a conical shape. Due to the large length of the tower of modern wind turbines, it is divided into different shorter sections. At both ends of these tower sections ring flanges are pre-welded, as shown in Figure 1. During the erection of the wind turbine these ring flanges are bolted together with closely spaced fully preloaded bolts. Such connections have been investigated in the past by means of approximate analytical models [2], as well as numerical analysis and experimental tests [3, 4]. In the present research a detailed numerical model is presented as part of the investigation of the nonlinear response of such connections up to collapse. For that purpose a typical 3-bladed wind turbine with 1.5mW rated power is considered, having a 82.39m long conical tubular tower. The examined connection is located at the upper 85% of the tower height and in the presented
numerical model only one of the bolts and its effective part of the flange are simulated, resulting in an L-shaped connection. The connection is simulated using 3D solid elements for the ring flange, the tower shell and the bolt, as well as appropriate contact elements to account for the interaction between flanges and bolts and between nuts and flanges. Figure 1. Wind turbine tower segment ring flange Purpose of the present paper is to determine the ultimate load of this part of the connection under static loading and to investigate how it is affected by different parameters. Both material and geometric nonlinearity is taken into consideration in the analyses. Geometrical nonlinearity exists due to the fact that the imposed load on the connection is resisted through the redistribution of the contact forces over the contact surfaces. In a parallel investigation, the entire ring flange connection has been simulated and investigated via a less refined numerical model [5]. The two models, the present one focusing on local bolt behavior, and the other one examining global connection behavior, will be employed for fatigue investigations in future research work. 2 ULTIMATE STRENGTH OF THE L-SHAPED CONNECTION 2.1 Description of numerical model The dimensions of the aforementioned simulated connection are presented in Figure 2. The inner and outer diameters of the wind turbine tower, at the position of the connection, are equal to 3.04m and 3.3m, respectively. The ring flanges are bolted together with fully preloaded bolts M36-10.9 and their thickness is 60mm. The tower s shell thickness adjacent to this connection is 11mm. Figure 2. Vertical section of the connection with dimensions in mm Due to symmetry, the numerical model includes only the upper flange and the half bolt, in order to reduce the model s size and computational cost. The top surface of the lower flange is also simulated as fully fixed, in order to employ the appropriate contact boundary conditions to the bottom surface of the upper ring flange. Tolerance gaps between the two ring flanges, as well as the upper flange and the bolt head are taken equal to 0.2mm. The ring flanges holes are considered 3mm larger than the bolt s diameter, according to pertinent EC3 recommendations for normal bolts [6]. The bolt head is designed according to DIN6914 [7] and the bolt s pretension force is taken equal to the maximum allowable value (70% of the bolt tensile strength) [6]. Successive
mesh refinement is employed, using FEMAP software [8], to achieve a proper balance between accuracy of the results and computational cost. The nonlinear analyses are performed using finite element program ADINA [9]. Wind loads acting on the rotating blades of the wind turbine are transferred to the tower as well as to the connection as a bending moment with direction perpendicular to the wind, which constitutes the prevailing action, as well as an accompanying shear force. Moreover, an axial force due to weight of electromechanical equipment and weight of the upper part of the tower is also present. In Figure 3 the ADINA numerical model is presented, where the combined effect of bending moment and axial force is transferred as a distributed upwards vertical load acting on the tower shell, while the shear force is neglected. For both the ring flanges and the tower, as well as for the bolt, a bilinear elastoplastic material with hardening up to rupture is applied, assuming steel qualities for the ring flanges and the pylon and 10.9 for the bolt. M Figure 3. Numerical model created in ADINA 2.2 Results of numerical analyses The results extracted from the aforementioned numerical analyses are given in the present section. Namely, in Figure 4 the equilibrium path of the L-shaped connection is shown for a maximum applied load at the tower shell equal to 22kN per node. In the horizontal axis the y-displacement of point M (shown in Figure 3) at the top of the modeled part of the tower shell is illustrated. It is observed that the connection s equilibrium path is curved even for small values of the imposed load with a gradual stiffness decrease. This is due to the nonlinear nature of the contact. Figure 4. Connection s equilibrium path In Figure 5 the stress distribution on the upper flange and the bolt at the two characteristic points 1 and 2 of the equilibrium path, illustrated in Figure 4, is shown. In general terms, stress concentration focuses around the bolt s hole and the bolt s shaft, while the major part of the ring flange remains elastic. It is noted that the bolt shaft enters plasticity first at its bottom, due to combined tension and bending. Material yielding occurs also at
the bottom edge of the upper flange, where the connection s two ring flanges are in contact. For the maximum value of imposed load material failure of the bolt s shaft as well as of the ring flange is at advanced state. Point 1 Point 2 Figure 5. Von Mises contours of the upper flange and the bolt at the two characteristic points In order to better appreciate the qualitative features of the connection s behavior the axial force of the bolt, as well as the vertical contact forces between the two ring flanges and the maximum equivalent stress at the welding between flange and tower shell are also presented as functions of the imposed load. The graph of the bolt s axial tensile force is illustrated in Figure 6 and the resultant of contact stresses along z axis developed at the interface between the two ring flanges in Figure 7. At the beginning of load application the tolerance gaps are totally closed and gradually the two ring flanges are moving apart. From these figures the redistribution of contact forces occurring during load transfer through the connection is evident, following three successive phases. Firstly, the imposed force is received through the redistribution of the vertical stresses on the ring flange, while the bolt force remains constant, equal to the initial pretension force. Then, the bolt force increases and stress distribution continues. In the last phase the redistribution ends and the remaining load is received exclusively from the bolt, with its axial force continuously increasing. Figure 6. Axial tensile bolt force as function of imposed load
Figure 7. Contact force z at the interface between the two ring flanges Ιn Figure 8 images of the distribution of the contact reaction at the lower surface of the upper flange are illustrated for different values of the imposed force. Blue color denotes loss of contact, while red and purple show areas of high reactions. Initially, before load application, the distribution is extended through the whole surface, and as the imposed load increases it is gradually transferred towards the edge. The maximum value of stress developing at the position of welding between ring flange and tower shell remains low and in any case below the material s yield limit, even for the connection s ultimate load. Moreover, the pertinent diagram is constantly linear, as shown in Figure 9. Figure 8. Illustration of the redistribution of the contact reaction at the interface of the two ring flanges for different values of the imposed force Figure 9. Maximum equivalent stress at the position of welding
3 PARAMETRIC INVESTIGATION OF THE L-SHAPED CONNECTION In the present section a wide range of parametric analyses are presented in order to evaluate the influence of different parameters on the connection s stiffness and ultimate strength, as well as on the way the load is transferred through the connection. The parameters investigated are the bolt s pretension force and quality, as well as the thickness of the ring flanges and the bolt s diameter. These parametric analyses are carried out retaining all other factors of the numerical model same as described in section 2 and modifying only the specific parameter examined. Regarding the bolt s pretension force, the initial value, assumed as equal to 70% of the bolt s tensile strength, is reduced to values between 60% and 10%. The bolt quality is decreased from 10.9 in the initial numerical model to 8.8, while lower bolt quality is not considered, taking into account that these are preloaded bolts. Concerning the thickness of the connection s ring flanges, it is reduced from 60mm to 40mm and the bolt s diameter is changed from M36 to M20, M24, M27 and M30. 3.1 Influence of different parameters on the connection s ultimate strength In Figures 10 to 13 the equilibrium paths of the examined numerical models for the four investigated parameters are presented. It is observed that the connection s ultimate strength is affected by all these parameters. The bolt s pretension force and the thickness of the ring flanges affect not only the ultimate load of the connection, but also its stiffness. As shown in Figures 10 and 12, the initial slope of the equilibrium path is reduced for smaller values of pretension and thinner ring flanges. On the other hand, the quality and the diameter of the bolt do not affect the connection s stiffness, according to Figures 11 and 13. Figure 10. Connection s equilibrium paths for different values of the bolt s pretension force Figure 11. Connection s equilibrium path for different bolt qualities
Figure 12. Connection s equilibrium paths for different values of the flange thickness Figure 13. Connection s equilibrium paths for different bolt diameters 3.2 Influence of different parameters on load transfer mechanism Next, the influence of the load transfer mechanism by the aforementioned investigated parameters is presented. Regarding the bolt pretention force, it is noted that the load resisted by the bolt gradually increases for smaller values of pretension, thus the stress range acting on it becomes significant (Figure 14). As a result, the connection s resistance against fatigue is expected to be significantly reduced. Figure 14. Equilibrium paths of the bolt s axial force and of the vertical contact forces for different pretension forces Ιn Figure 15 the respective results regarding the bolt quality are presented. It is observed that this parameter does not affect load transfer, but only the value of the imposed force at which material failure occurs. The part of the load for which the bolt force remains constant is almost the same for both bolt qualities.
Figure 15. Equilibrium paths of the bolt s axial force and of the vertical contact forces for different bolt qualities Similarly, it is noted that the influence of the thickness of the ring flanges on the connection s behavior is almost negligible. As illustrated in Figure 16, the redistribution taking place is only slightly different for each value of thickness. Figure 16. Equilibrium paths of the bolt s axial force and of the vertical contact forces for different values of the flange s thickness Finally, it is observed that the bolt diameter has similar effect on the load transfer mechanism as the pretension force. More specifically, for smaller diameters the stress redistribution of the contact surfaces taking place is constantly reduced. Figure 17. Equilibrium paths of the bolt s axial force and of the vertical contact forces for different bolt diameters 4 CONCLUSIONS The response of a typical connection between adjacent parts of shell-type wind turbine towers has been investigated by means of a detailed numerical model and nonlinear analyses up to collapse. Specifically, a connection of an 80m tall tubular conical steel tower of a wind turbine with rated power 1.5mW is considered. Such connections consist of ring flanges that are pre-welded on the shell parts and are the bolted together with preloaded bolts. Only a typical L-shaped part of such connection has been simulated using 3D solid elements for the ring flange, the tower shell and the bolt, as well as appropriate contact elements to account for the interaction between flanges and bolts and between nuts and flanges, in order to evaluate the local behavior. In accompanying work, a simpler model of the entire connection is analyzed. Numerical results lead to useful
conclusions about the connection s ultimate strength and its stiffness. Due to flange-bolt interaction, the connection s equilibrium path is softening already at low load levels, in spite of the elastic stress distribution. Furthermore, parametric analyses offered a wide range of information about the influence of different parameters either on the ultimate load, or other significant characteristics of the connection. The ultimate strength of the examined model was decreased due to reduction of the connection s dimensions and pretension. Parameters, such as the thickness of the ring flanges and the bolt s pretension force influenced significantly not only the ultimate load but the initial stiffness, as well. Generally, when it comes to the way the load is resisted, all parameters except for the bolt s quality, influence the stress range acting on the bolt and the redistribution of stresses taking place between the two ring flanges. As a result, fatigue strength of these connections is expected to decrease abruptly when dimensions of its components are reduced. Detailed fatigue investigation will take place in the next phase of this research. ACKNOWLEDGEMENT This research has been co-financed by the European Union (European Social Fund - ESF) and Hellenic national funds through the Operational Program "Competitiveness and Entrepreneurship" of the National Strategic Reference Framework (NSRF 2007-2013) - Research Funding Program: Bilateral R&D Cooperation between Greece and China 2012-2014, under project SEAWIND with code 12CHN184. REFERENCES [1] Schmidt, H., Jakubowski, A. (2001), Ermüdungssicherheit imperfekter vorgespannter Ringflanschstösse in windbeanspruchten turmartigen Stahlbauten, Universität Essen. [2] Petersen, C. (1993), Stahlbau: Grundlagen der Berechnung und baulichen Ausbildung von Stahlbauten, Vieweg. [3] Wang, Y.Q., Zong, L., Shi, Y.J. (2013), Bending behavior and design model of bolted flange-plate connection, Journal of Constructional Steel Research, Vol. 84, pp. 1-16. [4] Hoang, V.-L., Jaspart, J.-P., Demonceau, J.-F. (2013), Behaviour of bolted flange joints in tubular structures under monotonic, repeated and fatigue loadings I: Experimental tests, Journal of Constructional Steel Research, Vol. 85, pp. 1-11. [5] Ntaifoti, A.I. (2014), Investigation of the behavior of wind turbine tower connections by numerical simulations, Diploma Thesis, Department of Civil Engineering, National Technical University of Athens (in Greek). [6] EN 1993 1 8: 2005, Eurocode 3: Design of steel structures Part 1 8: Design of joints, European Committee for Standardization, 2005. [7] DIN 6914, High Strength hexagon head bolts, Deutsche Norm, 1989. [8] FEMAP 101, Introduction to Femap with NX Nastran Student Guide, Version 10.1.1, Siemens PLM Software, 2010. [9] ADINA System 8.3, Release Notes. ADINA R & D Inc., 71 Elton Avenue, Watertown, MA 02472; USA. 2005.