Steel Tubular Tower Research Articles

Investigation of Nonlinear Effects in the Tower Structure of Wind Turbines and Their Influence on System Behavior

In multibody simulation (MBS) software, large structural components such as the tower are often modeled as flexible bodies using the Craig-Bampton method. While being very computationally efficient, nonlinear effects cannot be modelled in this way. The flange connections between the segments of tubular steel towers are one such source of nonlinearity. The magnitude of this nonlinearity increases with damages at the flanges as flange gaping becomes more common. The nonlinearities in the flanges lead to a changing stiffness and hence changing eigenfrequencies of the structure as a function of its deformation. In this paper, a nonlinear model of a tower with and without damages is being developed and validated based on an FEM model. The model is assembled from linear flexible bodies linked with nonlinear rotatory springs. The damages considered are broken bolts as well as axially symmetric angular flange gaps. It is shown that the MBS model closely emulates the FEM model including all nonlinear effects. The nonlinear tower model is incorporated into an existing MBS model of a commercial wind turbine in the 4 MW range. The eigenfrequencies of the full turbine model are being calculated using the build-in eigenvalue solver in Simpack. The results show a small but measurable decrease in eigenfrequencies compared to the linearized model, which scales with the extend of the damage.

Considerations for the structural design of wind turbine towers: A practical application

AbstractAiming to achieve efficient structural performance, this article presents a methodology for the design of the shell structure and dimensioning of the connections of an S355J2 tubular steel tower with a height of 80 m, compatible with a SWT‐2.3‐93 wind turbine. The tower is made up of three segments, interconnected by flanged connections made of high‐strength steel. The analysis considers various load cases, taking into account stress and resistance in different directions, as well as designing connections using Petersen's theory, according to maximum strength and fatigue criteria. The results indicate that the circumferential stresses are nearly negligible compared with the resistant stresses, while the shear stress is significantly higher at the base due to the torsional moment. Meridional stress determines the stability of the structure, requiring consideration of internal pressure for safety. Maximum stress values range from 135.00 to 168.47 MPa, depending on the location along the tower height. Flanged connections meet the strength and fatigue criteria, with the first flange enduring 63.0% of the fatigue effect and the second, 39.7%. Therefore, the results provide reliable information and methodologies for tower design, contributing to the practical and efficient development of these structures.

Hybrid FRP-concrete-steel prestressed double-skin wind turbine towers: Concept, design considerations and research needs

The past decade has seen rapid development of offshore wind energy around the world. Furthermore, to improve the efficiency of power generation, wind turbine development has been trending towards increasingly large and tall turbines. These developments call for innovations in the form of wind turbine towers to address the challenges faced by existing tower forms (e.g., steel tubular towers and prestressed concrete towers) in structural adequacy, construction efficiency and/or maintenance. This paper presents a new form of hybrid wind turbine towers which possesses many important advantages over the existing tower forms and are particularly suitable for large offshore wind turbines. The new hybrid towers, termed herein hybrid FRP-concrete-steel prestressed double-skin wind turbine towers or PDSWTs, are prefabricated in segments and then assembled on site. The PDSWT segments are a variation of hybrid FRP-concrete-steel double-skin tubular members (DSTMs), and they consist of an outer confining tube made of fibre-reinforced polymer (FRP), a thin steel inner tube with welded shear studs, prestressed concrete between the two tubes, and flanges welded at the ends of the steel tube. The onsite assembly of the tower involves mainly connecting the steel flanges of two adjacent segments using high-strength bolts, and installing prestressed tendons through the whole tower. In this paper, the rationale behind the development of PDSWTs is first explained, followed by a discussion of the design considerations and future research needs to facilitate the practical applications of the new tower form.

Nonlinear Finite Element Analysis of Tubular Steel Wind Turbine Towers near Man Door and Ventilation Openings to Optimize Design against Buckling

The safe and cost-effective design of wind turbine towers is a critical and challenging aspect of the future development of the wind energy sector. This process should consider the continuous growth of towers in height and blades in length. Among potential failure modes of tubular steel towers, shell local buckling due to static axial compressive stresses from the rotor, blades, and tower weight, as well as dynamic flexural compressive stresses from wind actions on the rotating blades and the tower itself, are dominant as thickness is optimized to reduce weight. As man door and ventilation openings are necessary for the towers’ operation, the local weakening of the tower shell in those areas leads to increased buckling danger. This is compensated for by tower manufacturers by the provision of stiffening frames around the openings. However, the cold-forming and welding of these frames are among the most time-consuming aspects of tower fabrication. Working towards the optimization of this design aspect, the buckling response of tubular steel towers near such openings is investigated by means of nonlinear finite element analysis, accounting for geometrical and material nonlinearity and imperfections (GMNIA), and also considering several wind directions with respect to the openings. The alternatives of stiffened and unstiffened openings are investigated, revealing that a thicker shell section around the opening may be sufficient to restore lost stiffness and strength, while the stiffener frame may also be eliminated, offering substantial benefits in terms of manufacturing effort, time and cost.

Failure mechanism of a Q690 steel tubular transmission Tower: Full-Scale experiment and numerical simulation

The failure mechanism of a Q690 steel tubular transmission tower is investigated through full-scale experiment. The tower is mainly subject to the influence of the wind and the loads from the transmission lines. The displacements and the strains of the key positions as well as the ultimate bearing capacity of the tower are thus obtained. The final collapse is induced by the buckling instability of the compressed main member of a tower leg. A finite element model is established, whose validity is verified by the experimental data. Then, the numerical model is used for further analysis on the structural responses of the tower. It is found that once plasticity occurs at a certain point on the main member, its lateral displacement would be much accelerated, thus amplifying the second-order effect thereof. The horizontal auxiliary members, connected to the bottom of the compressed main member, are initially under tension. While the deformation of the main member is aggravated, they gradually undergo the transition from tension to compression, constituting a lateral support to the main member. This support then enforces the plasticity to develop towards the middle section of the main member, resulting in the final failure at the same location as observed in the full-scale experiment. These findings are crucial for a progressive understanding of the failure mechanism of this type of high-strength steel tubular towers, and they have contributed to the optimization of the design method to prevent the collapse of transmission tower under similar circumstances.

Nonlinear coupling vibration analyses of large-scale wind turbine system

With the development of wind energy, the unit capacity of wind turbine is becoming larger and the height of the support tower is getting higher. Limited by the transport conditions, the diameter of the tubular steel tower cannot further increase with the increase of the tower height, which results in the lower natural frequencies of the tower. The higher and more slender wind turbine towers are more sensitive to the external excitation and many tower collapses have occurred in recent years because of the interplay with the longer rotor blades. A 2 MW wind turbine system with a 120 m full steel tubular tower is taken as an example in this study to research the nonlinear coupling vibration mechanism under the normal operation case and the emergency shut-down case. An integrated model of large-scale wind turbine system including the tower, blades, hub, and nacelle is established by the open-source software FAST to study its nonlinear interaction vibration mechanism. Under the normal operation load case, the acceleration response of the steel wind turbine tower is greatly affected by the rotational frequency of the rotor. The maximum accelerations in the fore-aft and side-side directions both appear around the middle-upper height of the tower. Under the emergency shut-down load case, the maximum fore-aft acceleration response appears at the top of the tower, but the maximum side-side response still appears around the middle-upper height of the tower. The first-order vibration mode plays a dominant role in the top fore-aft acceleration and the second-order vibration mode has great contribution to the side-side acceleration under the emergency shut-down load case. The numerical results of this study can provide insight into the vibration control mechanism of the full steel tower.

Life cycle assessment of ultra-tall wind turbine towers comparing concrete additive manufacturing to conventional manufacturing

Wind power is a quickly growing renewable energy resource within the continental United States and is expected to continue increasing as more wind farms are installed onshore and offshore. As a part of this growth, larger turbines benefit from economies of scale from taller towers. However, the development of ultra-tall wind turbine towers is hindered by transportation restrictions which limit the diameter and weight of the prefabricated tower sections. One proposed solution to this problem is to employ concrete additive manufacturing to build ultra-tall wind turbine towers on-site. To evaluate the potential environmental impacts of this approach, this study performed a life cycle assessment comparing four prototype 7.5-MW wind turbine towers designed with a height of 140 m: a conventional tubular steel tower assembled using bolted connections, two 3D printed concrete towers additively manufactured on-site with normal-strength (35 MPa) or high-strength (78 MPa) concrete, and a 3D cast concrete tower with normal strength (35 MPa) concrete cast into concrete formworks additively manufactured with high-strength (78 MPa) concrete. The 3D cast concrete tower segments are manufactured off-site and assembled on-site. The life cycle assessment examined the impacts of differences in materials inventory, structural designs, manufacturing methods, maintenance schedules, and end-of-life options for the four towers. The results indicate that the material production stage dominates, contributing over 92% of the total CO2 emissions and 67–93% of the energy consumption of the four towers. Compared with the steel tower, the normal-strength 3D printed concrete tower has 23% lower total life cycle CO2 emissions but 29% higher energy consumption; the high-strength 3D printed concrete tower has 16% higher life cycle CO2 emissions and 64% higher energy consumption. Parametric studies were also conducted to examine the effects of cement content by weight, distance from the concrete plant to the tower construction site, the number of tower sections, rated tower life, and tower end-of-life recycling rate. The results indicate that reducing cement content in 3D printed concrete such as by incorporating waste or recycled materials can significantly reduce the life cycle environmental impacts of ultra-tall turbine towers.

Coupled behaviour and strength prediction of tapered CFDST columns with large hollow ratios for wind turbine towers

Concrete-filled double-skin steel tubular (CFDST) tower has emerged as a promising type of wind turbine support structure with excellent strength and stiffness. As the basic member in CFDST towers, tapered concrete-filled double-skin steel tubular columns with large hollow ratios (TLHR-CFDSTs) are frequently subjected to complex combined compression-bending-torsion loads. This paper presents a comprehensive analytical study on the coupled behaviour of the TLHR-CFDST column under combined loads and the relevant ultimate strength prediction method. A FE model was established to conduct numerical analyses and extensive parameter studies while the data from the previous experimental research was used to provide a verification basis. It is found that the column subjected to combined loads has three typical failure patterns, which are mainly governed by the bending-to-torsion ratio (mt). Columns with moderate mt undergo the bending-torsion failure and show obvious coupled performances. The load-deformation relation, the ultimate characteristics and the effects of important parameters, including axial compressive ratio (n), tapered angle (θ), hollow ratio (χ) and height-to-diameter ratio (λ) were discussed. The ultimate strength correlations of the column were revealed by analysing strength data within the parameter scopes commonly used in wind turbine towers. Lastly, a practical and accurate prediction method for the ultimate strength of the TLHR-CFDST column under combined compression-bending-torsion loads was proposed.

Experimental and analytical investigation of stiffened steel tubes for wind turbine towers under compression-bending load

Steel tubes with large diameter-thickness ratios are widely used in tubular steel towers of wind turbines. These steel tubes are susceptible to local buckling under compression-bending load, thus leading to a failure of the overall structures. In this paper, a new structural form of steel tubes with longitudinal stiffeners was proposed. Six specimens were tested to study the effects of stiffeners forms and diameter-thickness ratios of steel tubes. Finite element models were compared and validated by test results and used to further study the structural mechanisms. Lastly, the ultimate strength of specimens was evaluated by existing design guidelines preliminarily. According to the results, by setting stiffeners, especially the T-type stiffeners under the same consumption of the steel, the local buckling was prevented and the plastic deformability of the steel tubes was increased, resulting in the great enhancement of ultimate strength and ductility. The limits of quantity and width-thickness ratio of stiffeners to obtain a more stable and significant improvement effect were suggested through parametric analysis. The current methods agree well with the test results of unstiffened steel tubes but might be inaccurate for stiffened steel tubes.

관형 철탑 볼트 풀림 진단을 위한 음향방출 신호의 특징 분석

We conducted an acoustic emission test for the loosened bolt diagnosis of tubular steel towers. Signal processing techniques and machine learning were applied to acoustic emission signals to confirm the classification possibility of the bolt fastening strength. Consequently, a clear difference between the fastened condition of the bolt and the loosened condition was observed; however, signals with different bolt fastening strengths were not classified. In this process, it was confirmed that the bolt fastening strength was classified up to 74.3 N·m using a band-pass filter. In conclusion, we confirmed the performance possibility of the loosened bolt diagnosis through acoustic emission signals.

Residual stress measurements and weld characterization in wind turbine support structures

Tensile residual stresses are a byproduct of welding processes with a negative impact on fatigue life and structural integrity. In large welded structures, the residual stress field will be superimposed on the service and structural loads, thus potentiating structural limit conditions. Due to the high manufacturing costs involved and harsh environments in which large length welded joints may operate, it is highly desirable to have an overview of the residual stresses present in the structure.In order to gain further insight on the magnitude and nature of welding residual stresses introduced into tubular steel towers, two specimens that were circumferentially welded were obtained from two towers. The specimens were extracted from door cut-outs that permit access inside the towers. The Contour method was employed for measuring the locked-in residual stress, and in order to characterize the welds, a microstructural analysis was performed, and mechanical properties, such as impact toughness and Vickers hardness, were assessed.

Impact of the ground clearance on the annual energy production and tower cost of an offshore wind turbine

This article analyzes the impact of the ground clearance on the Annual Energy Production (AEP) and tower cost of a 20 MW offshore wind turbine. In addition, the influence of the rated wind speed on the analysis result will be considered. The AEP is computed by considering wind speed variation over the swept area of the rotor blades. The tapered tubular steel tower is considered for mass and cost calculation. The tower is considered as a fixed-free cantilever beam with concentrated mass at the free end. The analysis shows that the ground clearance only has a minor impact on the AEP but it has a remarkable impact on the tower mass. Specifically, when the ground clearance reaches 50 meters, the AEP only increases by roughly 3% while tower mass is nearly doubled compared to the case with no ground clearance. The results also reveal the significant impact of the rated speed on both the AEP and tower mass.

Development of a transition piece for a self‐erecting wind energy plant

AbstractFor the construction of wind towers with large hub heights up to 200m, the FOSTA research project P1392 HYTOWER [1] investigated a concept for a self‐erecting wind energy plant with steel‐steel hybrid tower. A tubular steel tower is lifted inside a lattice tower using a strand lifting system. During this process the tubular steel tower is stabilized by hydraulic cylinders. A specially developed transition piece is used to accommodate the lifting system during the lifting process and to finally connect the two tower parts. In this paper, the erection process as well as the development of the transition piece is presented.

Buckling analysis of tower shells under patch loading for self‐erecting wind energy plants

AbstractTo achieve a higher performance level and lower erection costs for wind turbines in the future, a concept for the automatic erection of a wind turbine with a hybrid tower was investigated in the FOSTA research project P1392. The tubular steel tower is lifted through the lattice tower using a strand lifting system and stabilized during this lifting process with the help of hydraulic cylinders arranged in two levels in the transition piece. This stabilization results in a transverse pressure acting on the tower shell, which does not occur in the load cases during operation, but only in the construction state. In this paper, the transverse forces (patch load) acting on the tower are investigated, whereby different system states are considered.

Numerical analysis of ring flange connection with defined surface area

AbstractRing flange connections with preloaded high strength bolts are most commonly used to connect two segments of steel tubular towers. Behaviour and durability of ring flange connections is influenced by various factors, including stiffness of ring flanges, fabrication and execution imperfections, welding technology and welding details of the ring flange/segment connection, type of bolts, pretension technique and control of accomplished pretension force in bolts. This paper focuses on ring flange connections with defined contacts compared to the “full ring flange connection”. No imperfections are considered. Behaviour of the ring flange connections is investigated by advanced finite element analysis (FEA). The realistic geometry of bolted connection, ductile damage material model and element removal are used in the advanced FE model with explicit numerical solver. In addition, the influence of two types of high strength bolts, fully‐threaded and partial‐threaded bolts, on the mechanical behaviour of connections is considered. The failure modes and tensile stiffness are compared using different analytical models. Finally, recommendations for calculation of the tensile resistance of the connection and the bolt force with defined surface area are provided.

Buckling Verification of Manhole Area of Tubular Steel Wind Turbine Towers via Non‐linear Finite Element Analysis

AbstractAs wind turbine towers grow in height and their blades become longer, actions on the towers are also increased and their safe and cost‐effective design becomes critical for the further development of the wind energy sector. One potential failure mechanism of tubular steel towers is shell local buckling between ring flanges. In the present paper, the buckling behavior of a 120m tall wind turbine tower under realistic wind loads is investigated numerically, focusing on the buckling response near the man‐hole opening. To that effect, nonlinear finite element analyses accounting for geometrical and material nonlinearity and imperfections (GMNIA) is employed. GMNIA is performed taking into account initial geometric imperfections with the most unfavorable shapes resulting from the three primary buckling modes, determining the imperfection size according to EN1993‐1‐6 depending on the assumed fabrication quality class A, B or C. Numerical results are compared to analytical verification according to EN1993‐1‐6. Different wind directions are considered. Moreover, to investigate the influence of the stiffening of the manhole to the tower buckling strength, a comparison between three alternatives of stiffened and an unstiffened manhole is performed.

A technical analysis and comparison of tubular and lattice towers for wind turbines

The advancement of wind energy technology brings about an increase in the size and height of the wind turbine towers. As the upscaling of the conventional steel tubular tower is problematic, alternatives are being investigated. In this paper, several configurations were reviewed and the lattice tower was identified as a possible solution. This study analyses and compares the structural performance of the two tower configurations, tubular steel and lattice, of equivalent height, stiffness and mounted rotor, in two sets of tower heights, 120 m and 150 m. The towers are mounted by the baseline 5 MW turbine developed by National Renewable Energy Laboratory (NREL). The results indicate that lattice towers are a suitable alternative to the tubular towers, providing significant material savings for a similar structural behaviour. Comparable bending moments were recorded in the base of the structures. The most demanding operating condition of the wind turbines was identified to be system braking cause by a sudden increase in wind velocity outside of the operating range. The 150 m lattice tower responds with excessive displacement during this abnormal operating condition; this is a potential issue that can cause generator failure if not accounted for in the rotor design.

관주형 철탑 상태 감시를 위한 음향 방출 신호처리에 따른 특징 분석

In this study, we propose and analyze a machine learning method based on the genetic algorithm (GA) and supporting vector machine (SVM) for the effective classification of faults detected by an acoustic emission test on the welding parts of tubular steel towers. A band-pass filter, an envelope analysis (EA), and an intensified EA (IEA) are employed to generate feature vectors for the machine learning method based on the GA. After signal processing, the signals are applied to GA-based machine learning to derive the representative features of the received signal, and the SVM classifies the fault signals and normal signals from the detected signals. Consequently, it is confirmed that the received signal processed by EA and IEA can classify faults with an accuracy of 93 % or more. Hence, the proposed fault test and classification method is expected to be useful in the development of a system for constant monitoring and early detection of welding faults inside a tubular steel tower.

Seismic response analysis of steel–concrete hybrid wind turbine tower

The steel–concrete hybrid wind turbine tower is characterized by the concrete tubular segment at the lower part and the traditional steel tubular segment at the upper part. Because of the great change of mass and stiffness along the height of the tower at the connection of steel segment and concrete segment, its dynamic responses under seismic ground motions are significantly different from those of the traditional steel tubular wind turbine tower. Two detailed finite element models of a full steel tubular tower and a steel–concrete hybrid tower for 2.0 MW wind turbine built in the same wind farm are, respectively, developed by using the finite element software ABAQUS. The response spectrum method is applied to analyze the seismic action effects of these two towers under three different ground types. Three groups of ground motions corresponding to three ground types are used to analyze the dynamic response of the steel–concrete hybrid tower by the nonlinear time history method. The numerical results show that the seismic action effect by the response spectrum method is lower than those by the nonlinear time history method. And then it can be concluded that the response spectrum method is not suitable for calculating the seismic action effects of the steel–concrete hybrid tower directly and the time history analyses should be a necessary supplement for its seismic design. The first three modes have obvious contributions on the dynamic response of the steel–concrete hybrid tower.

Buckling Analysis for Wind Turbine Tower Design: Thrust Load versus Compression Load Based on Energy Method

Tubular steel towers are the most common design solution for supporting medium-to-high-rise wind turbines. Notwithstanding, historical failure incidence records reveal buckling modes as a common type of failure of shell structures. It is thus necessary to revisit the towers’ performance against bending-compression interactions that could unchain buckling modes. The present investigation scrutinises buckling performances of a cylindrical steel shell under combined load, by means of the energy method. Within the proposed framework, the differential equations to obtain dimensionless expressions showed the energy-displacement relations taking place along the shell surface. Furthermore, shell models integrated with initial imperfection have been embedded into finite element algorithms based on the Riks method. The results show buckling evolution paths largely affected by bending moments lead to section distortions (oval-shaped) that in turn change the strain energy dissipation routine and section curvature. The shell geometrical parameters also show a strong influence on buckling effects seemingly linked to a noticeable reduction of the shell bearing capacity during the combined loading scenarios.