Tower Segment Research Articles

Fatigue assessment for tower bolts of floating offshore wind turbine considering preload loss due to ambient temperature creep

A framework for fatigue assessment for tower bolts of floating offshore wind turbine (FOWT) taking into account the impact of preload loss due to ambient temperature creep and plastic deformation is proposed. The ambient temperature creep of high strength steel is modeled in terms of Norton’s theory. The residual preload of tower bolt is predicted by a detailed finite element (FE) model of the threaded bolt connection. Subsequently, the residual preload is applied to the FE model of the entire tower segment to achieve the stress range of the bolts. Fatigue life prediction is finally conducted using the Palmgren-Miner’s (PM) rule based on S-N curves, with corrections by effects of mean stress and bolt sizes. A fatigue assessment for tower bolts of a 5MW FOWT is then performed to demonstrate the capability of the proposed framework. Moreover, impact of initial preload, thread friction coefficients and flange gap on preload loss and fatigue life of tower bolts are discussed.

On the imperfection sensitivity and design of buckling critical wind turbine towers

Wind turbine towers pose major challenges for design engineers due to their complex geometry, nonlinear material behavior and imperfection sensitivity. In service, these thin-walled shells are burdened by a combination of complex load cases and prone to buckling. In fact, one of the main design drivers of wind turbine towers is stability failure for which often the design recommendation of the EN-1993–1–6 are used.Recently an international shell buckling exercise was caried out by the team behind the EN-1993–1–6 design standard. Within this exercise 29 teams from academia and industry were asked to perform a series of linear and non-linear finite element simulations of an 8-MW multi-strake steel wind turbine support tower segment. In general, the linear and nonlinear analyzes posed no challenge for the shell buckling experts from around the world. However, the imperfection sensitivity analysis results scattered significantly among the participants. In addition, there was little consensus as to whether the given tower design is actually safe.The authors, whose background is aerospace engineering, participated in this exercise and show in this article how they overcome the challenges of this typical civil engineering problem. Among linear and non-linear analyzes the authors show the results of state-of-the-art shell buckling concepts which were developed for aerospace shells like interstage tanks and adapters but are also applicable to wind turbine towers.

Seismic Response and Collapse Analysis of a Transmission Tower Structure: Assessing the Impact of the Damage Accumulation Effect

This paper delves into the impact of the damage accumulation effect, which leads to the degradation of material strength and stiffness, on the seismic resistance of transmission towers. Building upon the elastic–plastic finite element theory, a mixed hardening constitutive model is derived for circular steel tubes, standard elements in transmission towers, incorporating the damage accumulation effect. A user material subroutine, UMAT, is created within the LS–DYNA framework. The program’s validity and reliability are established through axial constant–amplitude loading tests on single steel tubes. The subroutine is employed to conduct the incremental dynamic analysis (IDA) of an individual transmission tower and to contrast it with the structure utilizing the Plastic Kinematic material model, assessing the discrepancies in tower top displacements and segment damage indices (SDIs) at both macroscopic and microscopic scales. The results shows that the Plastic Kinematic model inflates the seismic performance of the transmission tower. When considering the damage accumulation effect in structural failure, the damage index of the members increases, leading to a reduction in both the structural strength and stiffness. The dynamic response in the plastic phase becomes more pronounced, and the onset of structural failure is accelerated. Consequently, structural analysis under seismic conditions should account for the damage accumulation process. Through the delineation of member and segment damage, the extent of damage to transmission tower segments can be quantitatively assessed. Subsequently, the ultimate load–bearing capacity and the most vulnerable location of the transmission tower can be ascertained. Finally, this paper provides a detailed analysis of the transmission tower collapse process under seismic action and summarizes the mechanism of collapse for the structure.

Numerical investigation of vortex-induced forces on a wind turbine tower segment at very high Reynolds numbers

The vortex-induced forces on an extruded cylinder with a span of two diameters representative of a finite segment of a non-tapered wind turbine tower at a very high Reynolds number (Re = 8.0×106) are numerically investigated using an incompressible Navier-Stokes flow solver with an Improved Delayed Detached Eddy Simulation (IDDES) turbulence model and correlation-based boundary layer transition modelling. The solution shows spanwise correlated structured vortex shedding with the Strouhal number St = 0.48. The boundary layer transition is found to occur at θ transition = 70 ◦ , and boundary layer separation is found to occur at θ separation = 120 ◦ . Results from the grid dependency study strongly imply that when using IDDES, the Strouhal number converges to higher values than previously reported by the literature as the grid is refined, with results ranging from St ∼ 0.44 using a grid with 4.2 × 106 cells, to St = 0.48 for the finest considered grid with 33 × 106 cells. This behaviour is not seen for URANS, where St = 0.33 for the finest grid.

Buckling behavior of tubes subjected to combined bending and torsion

AbstractWind turbine towers are highly slender and made of thin cylindrical shells with large diameter‐to‐thickness ratios, which means stability is an essential concern. The high diameter‐to‐thickness ratios of these cylinders make them highly imperfection sensitive, meaning their failure loads and failure modes are highly dependent on initial geometries. Combined bending and torsion is commonly a controlling load case in the upper tower segments, but this load case has not seen significant study to date. To address the knowledge gap, an experimental study was conducted on the stability of cylinders under combined bending and torsion. A total of 48 cylinders were tested with varying diameter‐to‐thickness ratios and bending‐to‐torsion combinations seen in wind turbine towers. To better understand how imperfections affect the buckling modes of these thin‐walled cylinders, a 3D laser scanner was used to determine geometric imperfections of each test specimen prior to testing. This paper details the test setup and results of the experiments.

Structural Behavior of L-Type Flange Joint with Various Flange Flatness Conditions

The L-type flange joint is widely used to attach steel tower segments to each other. However, tolerances on the flange surface flatness may occur during its fabrication, leading to a negative impact on the bolt stress distribution. This study evaluates the influences of the flange surface flatness on the behavior of L-type flange joints through numerical simulations. First, the finite element model of a 5 MW L-type flange joint is established, and its accuracy is verified based on comparison with an experimental test. Using the same loading conditions and material properties, the influences of geometrical imperfections (i.e., flange-sided gap, tower-sided gap) on the structural response are investigated. Furthermore, the impact of the flange gap opening length is reported. The results show that the flange-sided gap outperforms the tower-sided gap, resulting in reduced stress concentration in the bolt. In addition, the stresses in flange-sided gapping joints increase with an increase in the opening length.

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.

Static Test Verification on Retrofitting Method for On-service Transmission Tower

Retrofitting methods for transmission tower on-service conditions should avoid punching and welding. This paper presents a non-destructive retrofitting method, the T-shaped composite angles method, to meet the demand. The original angle was combined with retrofitting components by V-shaped clamps. Both numerical analysis and static loading test were carried out to verify the effectiveness of this retrofitting method. The test results show that compared with the lateral stiffness of the original transmission tower segment, the stiffness of the reinforced one increased. The load-bearing capacity of the reinforced transmission tower segment almost doubled that before the reinforcement. The proposed retrofitting method is non-destructive and effective.

Health monitoring of ultra high fiber performance reinforced concrete communication tower using machine learning algorithms

Within the last decades, the needed for communication towers has accelerated with the requirements for effective communication, especially for radio, radar, and television. The complexity configuration of the tower and limit access to the structure body especially inner part of the tower with hollow section is led the health monitoring of tower as the main challenging issue to maintenance during its function. The change of natural frequencies can be considered as one of the prevalent damage detection methods in structural assessment procedures. Therefore, the main aim of present research is to develop health monitoring system for Ultra High Fiber Performance Reinforced Concrete (UHPFRC) communication tower based on frequency domain response. Since the frequency data of tower is mostly noisy and interpreting of frequency in different modes in variant case of tower damage. The hybrid algorithm based on the Adaboost, Bagging and RUSBoost algorithms are implemented to identify the damage in the UHPFRC communication tower using frequency domain data. The training samples for the algorithm are obtained from a finite element simulation and full-scale experiment testing is also performed to generate the testing samples. The finite element simulation dynamic frequency results are verified through conducting a full-scale experimental test on 30 m height UHPFRC communication tower. For this propose, frequency Response Functions (FRF’s), for healthy and damaged structures were obtained by exciting of tower by an impact hammer and the acceleration response recorded by three accelerometers sensors attached in suitable positions. The developed hybrid algorithm to identifying the damage is tested and verified by considering the part of tower segments 2–3 and conducting experimental testing on the healthy structure as well as a damaged structure which caused using dynamic actuator. The testing results proved the accuracy of the developed optimized hybrid algorithm to identify damage in the tower structure in variant condition.

Study on failure pattern evaluation method of transmission tower under wind load

To investigate the association between the failure pattern and the proportion of external load in the tower segment, the capacity experiment of the scaled segment model is designed in this paper, and the FE model of the experiment of the segment is build and compared with experiment results. The pattern of the tower segment failed is evaluated by the relationship of PH(horizontal load) and MP (bending moment). The critical points of P-MP curve were analyzed by selecting the capacity of diagonal and main elements. And the failure assessment theory for the tower segment was established. The results indicate that the FE model of the experiment segment can reflect the experiment loading process effectively. The capacity of the diagonal element has a significant impact on the tower’s PH, and the main element’s bearing capacity is an important factor affecting the flexural capacity. Using the proposed method, the weak positions and elements of the structure can be judged.

8-MW wind turbine tower computational shell buckling benchmark. Part 1: An international ‘round-robin’ exercise

An assessment of the elastic-plastic buckling limit state for multi-strake wind turbine support towers poses a particular challenge for the modern finite element analyst, who must competently navigate numerous modelling choices related to the tug-of-war between meshing and computational cost, the use of solvers that are robust to highly nonlinear behaviour, the potential for multiple near-simultaneously critical failure locations, the complex issue of imperfection sensitivity and finally the interpretation of the data into a safe and economic design.This paper reports on an international ‘round-robin’ exercise conducted in 2022 aiming to take stock of the computational shell buckling expertise around the world which attracted 29 submissions. Participants were asked to perform analyses of increasing complexity on a standardised benchmark of an 8-MW multi-strake steel wind turbine support tower segment, from a linear elastic stress analysis to a linear bifurcation analysis to a geometrically and materially nonlinear buckling analysis with imperfections. The results are a showcase of the significant shell buckling expertise now available in both industry and academia.This paper is the first of a pair. The second paper presents a detailed reference solution to the benchmark, including an illustration of the Eurocode-compliant calibration of two important imperfection forms. Full details are here: https://doi.org/10.1016/j.engfailanal.2023.107133.

New Test Bed for Corroded Large Size Bolted Connections Under Fatigue and Pretension

AbstractIn this paper, a newly developed test method to evaluate the influence of corrosion on pretension and fatigue of bolted connections is presented, suitable from size M36 to M72. A first preliminary series of M30 property class 10.9 bolts were tested to validate the individual test setup steps and to detect challenges and improvement. Pretension load tests, static load tests and fatigue load tests were performed while monitoring stresses on a small number of bolts. The method presented is used to compare different coatings for bolts, like Hot‐dip galvanized coatings and zinc flake coatings, as well as nano‐coatings. To create a realistic approach on offshore corrosion and loading conditions, the bolts will be preloaded in a clamping device resembling the ring flange of an offshore wind tower segment. In a next step, the bolts are corroded in a climate chamber, using the salt spray test or condensation test. Following the corrosion process, the bolts are tested for fatigue without loosening the nut and damaging the thread. Before corrosion and after the fatigue test, 3D surface scans are made to document the corrosion process and to create numerical models of the corroded surface. The method presented offers a new way to evaluate fatigue, pretension and corrosion effects, which is validated on bolts M30 property class 10.9. In close future, the test method is used for larger series of corroded M36 property class 10.9 bolts, having different coating systems applied.

Piezoelectric Impedance-Based Structural Health Monitoring of Wind Turbine Structures: Current Status and Future Perspectives

As an innovative technology, the impedance-based technique has been extensively studied for the structural health monitoring (SHM) of various civil structures. The technique’s advantages include cost-effectiveness, ease of implementation on a complex structure, robustness to early-stage failures, and real-time damage assessment capabilities. Nonetheless, very few studies have taken those advantages for monitoring the health status and the structural condition of wind turbine structures. Thus, this paper is motivated to give the reader a general outlook of how the impedance-based SHM technology has been implemented to secure the safety and serviceability of the wind turbine structures. Firstly, possible structural failures in wind turbine systems are reviewed. Next, physical principles, hardware systems, damage quantification, and environmental compensation algorithms are outlined for the impedance-based technique. Afterwards, the current status of the application of this advanced technology for health monitoring and damage identification of wind turbine structural components such as blades, tower joints, tower segments, substructure, and the foundation are discussed. In the end, the future perspectives that can contribute to developing efficient SHM systems in the green energy field are proposed.

A member-based porous method for predicting flow distortions around a lattice tower

In the wind field measurement, the readings of an anemometer mounted on a lattice tower will be influenced by the tower. Therefore, evaluation of the flow distortions generated by a lattice tower is essential for correcting the wind speed data or guiding the arrangement of anemometers. Wall-refined CFD is a powerful tool to simulate the flow field but is quite time consuming for lattice structures. As an alternative approach for improving the computational efficiency of CFD, a member-based porous method was proposed by this article. It employs porous zones to substitute the real members of a lattice tower, so that the computational resources can be considerably saved. Wind tunnel test of two lattice tower segments were conducted to validate the member-based porous method. Models of wall-refined CFD were also established. Results of the member-based porous method are in acceptable agreement with those of test and wall-refined CFD. The method is proved to be an effective way to predict flow distortions of a lattice tower. Both the test and numerical simulations (wall-refined CFD and member-based porous) reveal that disparities exist between the flow field of different measurement planes. The results highlight the need to consider the impacts of member arrangement in predictions of the flow distortion. At last, some crucial issues on the flow distortion effects of a lattice tower were also discussed, results of the classic theoretical methods were compared with those of the member-based porous method.

Automatic Detection of Transmission Line Dangerous Points Based on Point Cloud Tower Section Classification Mode

In the actual laser power inspection data processing process, the classification error of point cloud is inevitable. Some classification errors often need to be located with the help of dangerous points which is detected by mistake. Therefore, in the process of laser power inspection point cloud data processing, the detection of Dangerous points often needs multiple calculations. However, the dangerous point detection is often based on the whole line, and involves the data calculation of hundreds of tower section at the same time. The data magnitude is large and the calculation speed is slow, which seriously affects the efficiency of point cloud category correction. Aiming at the above problems, this paper divides the point cloud into tower segments, transforms the calculation unit of dangerous point detection from a whole line to a single tower segment, and records the modification of the point category in the tower segment. In the process of correcting the classification error and repeatedly calculating the dangerous points, only the tower segment with the modified category is recalculated. The practical application shows that this method can effectively speed up the data processing efficiency of laser power inspection.

Aerodynamic forces on a high-voltage delta-configuration lattice transmission tower segment

This paper presents a study on aerodynamic properties of the upper segment of a high-voltage delta-configuration (cat-head) lattice transmission tower. These segments have been damaged in recent strong-wind events. A representative 1:100 scale specimen of the segment was subjected to wind tunnel tests. A computational fluid dynamics-based model was developed to simulate experimental observations. These models were extended to consider segments with different properties, and force coefficients in transverse and longitudinal directions were calculated. Further, provisions outlined in five design standards were used to estimate force coefficients for these segments. Wind forces for a segment can be determined by multiplying that computed using ASCE 74 or IEC 60826 by a factor, which is a function of yaw angle and solidity ratio. This factor varies between 1.0 and 1.6 (1.0 and 2.0) for the transverse (longitudinal) direction. Additional analyses revealed that an approximately 100% increase in the gap between adjacent longitudinal faces leads to an approximately 10% increase in the force coefficient, and that the influence of Reynolds number and turbulence parameters is small. Further, introduction of cross-arm led to a less than 10% reduction in the force coefficient for transverse direction; the influence was negligible in the longitudinal direction.

Mechanical Behavior of a Multisegmented Tower Anchorage Structure with an Exposed Steel Anchor Box

A tower anchorage structure with an exposed steel anchor box is commonly used for cable‐stayed bridges. Many researchers have conducted studies on this structure by considering a single segment. However, in practical engineering, the stress of multisegmented tower anchorage structure is not completely similar to that of single segment, and the forces between segments affect each other. Hence, in this study, the mechanical behavior of a multisegment anchorage structure with an exposed steel anchor box was investigated via finite element analysis. Furthermore, the load transfer path and stress distribution characteristics of the structure were investigated. The results indicate that the horizontal component of the cable force is borne by the side plate of the steel anchor box, the diaphragm, and the side wall of the concrete tower column, while the vertical component is transmitted by the steel anchor box and concrete tower column. Under the action of this cable force, the horizontal component of the cable force borne by the middle segment increases, while the components at the two end segments decrease. The vertical force is greater on the lower tower segments. The stress levels on the side plate and on the diaphragm of the steel anchor box in the middle section are high. Under the cable force load, the frame formed by the end plate and side plate of the steel anchor box expands outward. The end plate is mainly under a tensile load, and the tensile stress level on the lower section exceeds that on the upper section. A high‐stress area for the concrete tower is observed in the steel‐concrete joint. The stud group of the anchorage structure is subjected to horizontal and vertical shear forces, and no “saddle‐shaped” distribution of the stud shear is found. An optimal arrangement method for the stud group was proposed to optimize its mechanical performance.

Feasibility of segmented concrete in wind turbine tower: Numerical studies on its mechanical performance

Segmented concrete slices have been widely used in the construction of wind turbine tower due to easy transportation and cost savings. Factors such as the size and form of the concrete slice and the connector may affect the stiffness and reliability of the structure. The objective of this study is to investigate the influence of the above factors on mechanical performance of a wind turbine tower. A finite element model of 40 m segmented concrete tower was developed for the modal analysis, and a local refinement model was established for the static analysis. Results show that the slice form and connectors have impact on the rigidity of the tower structure. Dividing the tower into two pieces per layer, increasing the height of the tower segment, using the grout connection of the longitudinal and staggering longitudinal seams can increase the stiffness of segmented concrete tower. In addition, this study analyzes the role of rubber pads in improving the mechanical performance of the tower, results show that the peak stress at the joints can be reduced by setting rubber pads with low elastic modulus at the longitudinal seams.

Experimental and numerical investigations on adhesively bonded tubular connections for moulded wooden tubes

Moulded wooden tubes combine the advantages of the sustainable building material wood with the material efficiency that can be found in the field of steel and composite construction. They are particularly suitable for lightweight constructions and cantilevered structures under dynamic loads. The objective of ongoing research is the design of a joining solution that is appropriate for the material and structure of the tubes. This paper describes an adhesive approach for connecting thin-walled timber elements to tubular steel connectors. The presented solution will be applied for the fabrication of connections within a truss tower segment used in small wind power turbines. In addition to experimental investigations, numerical approaches to determine the load carrying capacity of such joints are presented. The experimental investigations include both compression and tensile tests on specimens of different sizes. For the numerical determination of the joint capacity a probabilistic approach is used which takes into account specific properties of brittle failure modes and allows for the determination of a characteristic strength value used by engineers in the design of structures. The numerical results are verified by the experimental data and show good accordance for both load cases.

PERENCANAAN ULANG BANGUNAN PENGAMBILAN BERTIPE MORNING GLORY MENGGUNAKAN PONDASI BORED PILE PADA BENDUNGAN LOGUNG

The existing square tower of intake has 1,397.57m3 volume and using shallow foundation Logung Dam with its weight stability will cause inefficient structures and potential to earthquake force effect. It is necessary to have an alternative planning of the intake tower. The authors intends to redesign the intake with morning glory type and used bored pile; to find out the position of intake tower, dimension, reinforcement, duration, cost estimate, and to compare the redesign.The required data were of topographic map, irrigation, and raw water discharge, bearing capacity, and work unit price analysis of project 2016. Manning Method was applied to find out the dimension; Shell Slab Method with Column Approach to calculate the structure of the intake tower, and Skempton Method to calculate bearing capacity. The redesign results in the position of ∅ 1.75-m intake tower on conduit channel with 728,08 m3 volume; the dominant load of operational-earthquake combination with different values; D22-200 steel bar for y-direction, D19-150 for x-direction on conduit channel, D19-150 for x,y-direction on intake tower segment 1, D16-150 for x,y-direction on intake tower segment 2, and D13-150 for x,y-direction on intake tower segment 3; on 105 workdays; at a total cost of IDR 2,327,806,700 with 40.64 % efficiency cost.