The total costs per produced kilowatt-hour for wind turbines depend significantly on the investment costs. Thereby, the tower is a relevant cost component, which depends on the chosen supporting structure, the material, and especially on the erection process. Here, an innovative erection process is presented in order to minimize the wind turbine installation, which leads to excluding the extra tall cranes for installing the wind turbines with hub heights over 180 m. In order to propose the innovative erection process, a new hybrid lattice/tubular supporting structure for the onshore wind turbines is designed. The connection component between the tubular part and lattice structure is proposed considering the support functionality for the new erection process. Furthermore, the building steps of the complete erection process are explained. The operational and the lifting loads on wind turbine supporting structure are estimated, and consequently, the erection process stages were analyzed. Finally, the finite element simulation are performed to specify the critical stresses in subcomponents of the supporting structure in each lifting stage and to show the feasibility of the erection process. Moreover, the most critical points and the stages are investigated and stress level in the supporting structure components is computed.
Steel tubular structures are somewhat entrenched for the wind turbine towers. Recently, steel hybrid lattice/tubular towers are being investigated as a conceivable answer for taller onshore wind turbines for which convectional steel tubular towers are less competitive. Hybrid lattice/tubular towers require a transition piece which serves as a connection between lattice and tubular part. As the transition piece is supposed to transfer all the dynamic and self-weight loads to the lattice and foundation, these structural elements present unique features and are critical components to design and ought to resist strong cyclic bending moments, shear forces, and axial loads. Well-designed transition pieces with optimized ultimate state and fatigue capacities for manufacturing contribute to the structural soundness, reliability, and practicability of new onshore wind turbines hybrid towers. This research focuses on the investigation of the transition piece for an onshore wind turbine hybrid tower. The 5-MW reference wind turbine and a hybrid lattice/ tubular tower were simulated in the servo-hydro aero-elastic analysis tool (by ASHES software) from which the loads and dynamic response of the supporting structure were obtained. Cross-sectional forces at the transition piece elevation were calculated and the connection with the lattice structure is designed. The transition piece was designed by finite element model considering ultimate limit load and fatigue load, using nonlinear analysis and multiaxial fatigue for lifetime prediction, respectively. Multiaxial fatigue life was calculated based on Brown-Miller and Smith-Watson-Topper methods. In comparison, Smith-Watson-Topper method comes out to be more conservative. Potential of using high-strength steel S690 was investigated.
Nowadays the wind turbines with the high power capacity are installed for the onshore wind parks. In order to achieve higher wind speeds and more stable wind, wind turbine towers are designed for higher altitude. Moreover, higher altitude reduces turbulence and wind shear induced vibrations. The convectional tubular towers are assembled using welded ring flanged joints. Moreover, preloaded bolts are used to connect the segments' flanges together. Therefore, the fatigue problem is predominant in the tall towers and fatigue failure due to welded connection and/or due to the bolts in tension may be the governing design situation. To tackle the fatigue problem in the bolted joints and discard the welded connections, a new friction connection is designed for tubular segment assembly. The tower segments are designed with long open slotted holes. The preloaded bolts are used to provide contact between the slotted holes and the intermediate auxiliary plate. Therefore, the connection will resist only by the friction in between the plates. In order to provide necessary friction between the plates, sufficient pre-load should be applied on the bolts. Moreover, it should be guaranteed that the load loss in the bolts is trivial. Therefore, Bobtail® bolt as free maintenance preloaded bolts are used in the friction connections. This paper deals with the identification of a function for bolts load loss behavior in the friction connections and provide an estimation for lifetime loss. A monitoring setup has been settle to measure the load in the bolts and the environmental temperature simultaneously for almost a year. Consequently, the Hammerstein-Weiner method, was applied to create the nonlinear dynamic model of the friction connection preloading behavior regarding the environmental temperature variation. Furthermore, the identified function output for the measured temperature was compared with the measured force and then the error was calculated. Based on the model, the load loss for constant temperature was calculated for the period of the measurement and was estimated for 20 years of life time span of the structure. The findings demonstrated acceptable amount of load loss in a year and estimation showed that bolts loads retain constant pre-load after a two years period. Moreover, it is obtained the initial fastening conditions were important to achieve free-maintenance bolts.
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