Tower Section Research Articles

Force-partitioning analysis of vortex-induced vibrations of wind turbine tower sections

Force-partitioning analysis of vortex-induced vibrations of wind turbine tower sections

Robustness of LiDAR-assisted controller towards measurement uncertainty

Many studies have concluded that LiDAR-assisted control can be used to increase annual energy production (AEP) as well as reduce extreme loads and fatigue damage on blades, tower, and main bearing components. However, most studies assume perfect LiDAR measurements and do not consider large measurement uncertainties on LiDAR measurements. In this study we evaluate the potential benefits in terms of fatigue load reduction from using a simple robust feedforward controller. In addition, we evaluate the impact of measurement uncertainties exemplified by a time shift between the expected time the wind hits the rotor and the actual time it hits the rotor as well as a mean wind speed measurement bias. Results shows that fatigue loads can be reduced up to 7% on the tower sections and up to 4% on other main components. However, these results only applies when the LiDAR is assumed to give perfect information about what is hitting the rotor plane. The load reduction degrades with the magnitude of a time shift of the arriving LiDAR wind measurements. A delay up to 2.5 seconds still gives reasonable load reductions. Furthermore, a positive wind speed bias does not affect the performance of the LiDAR-assisted controller, whereas a negative bias significantly deteriorates performance.

Study of surface roughness on flow past a wind turbine tower section

The flow around wind turbine towers usually reaches very high Reynolds numbers greater than a million. Understanding the flow around the towers under these conditions is crucial, as it may lead to vibrations due to the vortices formed. Investigating aerodynamic characteristics at such high Reynolds numbers, both numerically and experimentally, is challenging. The current study validates such an experimental study, where a rough surface is employed to increase the effective Reynolds numbers and accelerate the laminar-turbulent transition in the boundary layer. Unsteady Reynolds-Averaged Navier-Stokes (RANS) simulations are carried out using OpenFOAM for a Reynolds number range of 1.36·105 to 6.8·105. The constant (a 1) used to calculate the eddy viscosity is varied to simulate the flow separation during adverse pressure gradients. A force partitioning method is implemented in OpenFOAM and various force contributions are analysed for this Reynolds number range. It is seen that the RANS simulations overpredict the aerodynamic characteristics and the extent of flow separation unless the value of a 1 is varied as a function of the Reynolds number. Furthermore, it is observed that the only force contributor is the vorticity-induced force, as the simulations are performed for a fixed cylinder.

An open-source analysis workflow for geometrically imperfect bolted ring-flanges in wind turbine support structures

Bolted ring-flanges connect structures via two circular flanges, secured with pre-loaded bolts. In offshore wind turbine support structures the flanges connect the monopile and transition piece, the transition piece and tower, and adjacent tower sections. Fatigue analysis of the bolted ring-flanges in offshore wind turbine support structures is a critical step during design. The consideration of geometric imperfections of the flanges can be an influential factor in assessed fatigue damage over some given Fatigue Limit State load case. Schemes to analyse the effect of geometric imperfections using Finite Element Method simulations are available in literature. These schemes rely on the use of proprietary tools for the analysis. In this paper we introduce an open-source workflow to assess load transfer through geometrically imperfect bolted ring-flanges. The workflow relies on four open-source toolsets: the FreeCAD 3D parametric modelling package, the Code Aster / Salome-Meca Finite Element Method simulation environment, the ParaView post-processing and visualisation package, and the Python programming language. We present a case study of load transfer assessment in a monopile to transition piece bolted ring-flange typical of modern offshore wind turbine support structures. We decompose the load transfer across the flange under a number of geometric imperfection scenarios to determine the expected stress ranges for fatigue analysis. The results are compared with analytical methods specified in engineering standards. The workflow is configurable as per user needs, and is maintained as a GitHub repository at https://github.com/jhjorg/OpenBoltRF.

Structural Improvement and Numerical Simulation of a Perforated Distributor in an Adsorption Tower

AbstractA “closed” perforated plate distributor was proposed to improve the gas distribution in an industrial adsorption tower. Compared with the industrial perforated plate distributor, the designed distributor restricts the space for the disordered movement of gas under the perforated plate. The new distributor effectively converts the gas from the initial axial flow on the inlet section to the axial flow on the tower section to improve the gas uniformity. In addition, the gas velocity at the edge of the inlet cross section of the adsorbent bed is increased by opening holes on the bottom plate. The uniformity of gas distribution is further improved. The simulation method in this study and the distribution performance of the “closed” perforated plate distributor have been verified by experiments.

Fatigue degradation of wind turbines considering dynamic wake meandering effects

This paper systematically investigates the effects of six factors, namely wind shear, ambient turbulence, wake meandering, wake-added turbulence, turbulence intensities, and wind speed, on the dynamic responses and fatigue degradation of wind turbines in the wake region. An improved mid-fidelity code, FAST.Farm, is used, which considers wake-added turbulence by adopting an additional wind domain with the Mann model. Different distributions of short-term damage equivalent stresses are found in two selected components, namely figure-8 and elliptical distributions, respectively, for nonturbulent and turbulent inflows at the blade root and a consistent ∞-distribution at the tower section for different inflows. An apparent lifetime reduction is observed at the blade root and tower base after the introduction of ambient turbulence, wake meandering, and wake-added turbulence. This lifetime reduction is more critical at the tower bases of downstream wind turbines, where the fatigue lifetime can be shorter than the designed service lifetime (20 years) owing to the wake effect. Considering a Rayleigh distribution with an average wind speed of 8.5 m/s, the fatigue lifetimes of turbines located 4D and 8D downstream of another turbine are approximately 45.86 % and 13.12 %, respectively, lower than the fatigue lifetime of the upstream turbine, whereas the reductions are 38.80 % and 5.03 %, respectively, for an average wind speed of 10.0 m/s. Given that the wake effect on the fatigue lifetime has already been highlighted in recent design standards, the findings of the present study are expected to provide guidance for fatigue evaluation in practice, which will help improve fatigue estimations, extend turbine lifetimes, and reduce the construction costs.

Flow field characteristics analysis of vortex-induced vibration of bridge tower based on dynamic mode decomposition method

As a high-rise structure, the tower of a suspension bridge has low stiffness due to the lack of cable system constraints when it is self-supporting. Wind-induced vibration is one of the key factors in design and construction. In this paper, the wind tunnel test of the bridge tower aeroelastic model is carried out in two different flow fields, and the wind vibration response under a self-supporting state is systematically studied. Meanwhile, numerical simulation of the unsteady flow field around the tower section is carried out, and the dynamic modal decomposition (DMD) method is used to study the modal frequency of the flow field, the surface pressure of the section, flow field reconstruction error, and the stability of the flow field around tower section is discussed. The results show that in the self-supporting state, the displacement of the tower top presents a quadratic curve relationship with the wind speed, and no galloping phenomenon occurs under the test wind speed. However, in the uniform flow, when the wind attack angle is 90°, the bridge tower has an obvious vortex-induced vibration (VIV) phenomenon. After decomposing the flow field by the DMD method, it is found that there is strong aerodynamic interference between the double rectangular column sections, the first seven orders of modal energy play a major role in the flow field, and when the main modal energy accounts for more than 90%, the reconstruction can realize the accurate restoration of the information such as the surface pressure. For the complex flow field structure, it shows that the method can more accurately identify the coherent structure and background part in the unsteady flow field. This paper provides an idea for future research on the characteristics of the flow field around complex sections and helps to further improve the understanding of the VIV mechanism.

Selecting model features that lead to linear models of bi-product distillation towers

This work introduces a methodology to develop linear data-driven steady-state models of bi-product distillation towers for use in operations monitoring and control. Linear monitoring models derived from measurements that are typically used for composition control (reboiler duty/bottoms flow, reflux flow/distillate flow, and one temperature measurement per tower section) have low accuracy due to the nonlinear behavior of distillation towers. We show that the addition of a judiciously selected second temperature measurement in each section leads to a highly accurate linear model. These temperatures are selected via an iterative procedure based on analyzing the residuals’ coefficient plots of PLS models. Remarkably, the models accurately predict product compositions, even without knowing the feed composition. Prediction of product composition for the multicomponent mixture feeds requires that the product flows to be included in the model. Our results demonstrate that the nonlinearity of the data-driven models can be reduced or eliminated by the proper selection of the model features.

Analysis and Optimum Design of Self Supporting Steel Communication Tower

The present study deals with the optimum design of self supporting steel communication towers. A special technique is used to represent the tower as an equivalent hollow tapered beam with variable cross section. Then this method is employed to find the best layout of the tower among prespecified configurations. The formulation of the problem is applied to four types of tower layoutwith K and X brace, with equal and unequal panels. The objective function is the total weight of the tower. The variables are the base and the top dimensions, the number of panels for the tower and member's cross section areas. The formulations of design constraints are based on the requirements of EIA and ANSI codes for allowable stresses in the members and the allowable displacement atantenna position. The Sequential Unconstrained Minimization Technique (SUMT) is used to perform the process. Direct stiffness method is used for the analysis of the structure, with beam elements. The strain energy is used to derive the stiffness matrix for members of unsymmetrical cross section. A computer program in FORTRAN is developed to represent the tower as an equivalent beam, and generate the tower nodes and members, analysis, design and to find the optimum design. Four types of tower are studied with different load cases. The effects of earthquake and wind loadings are taken in two directions and two positions of antenna are considered in the process to seek the optimum design. The tower type of X-brace with unequal panels has the minimum weightcompared with other types of tower and the optimum design is satisfied when the angle of main leg is equal to (87O).

Modeling guyed towers of transmission lines in the assessment of backflashover occurrence

This paper evaluates the modeling of guyed towers of transmission lines in EMT-type programs to assess the backflashover occurrence of transmission lines and proposes a new modeling approach to be applied for this kind of towers. Two approaches to model the guyed towers by the revised Jordan model were evaluated: (i) the approach of CIGRE Brochure 63 that suggests representing the tower surge impedance as the parallel of the surge impedances of the mast and the guyed wires, ignoring the mutual coupling among them, and (ii) a new approach assuming the surge impedance of all the conductors in the tower section, guyed and mast conductors, including their mutual coupling effect. The evaluations considered the results provided by the Hybrid Electromagnetic Model (HEM) as reference for assessing the quality of the approaches. The approach (i) provided results of lightning overvoltage significantly lower than the reference, HEM model, leading to an underestimation of the probability of backflashover of the line. On the other hand, the approach (ii) was responsible for the results closest to those provided by HEM, indicating its quality, and the recommendation of applying this proposed approach for modeling guyed towers in the evaluations of backflashover occurrence.

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.

Analysis of the stability behavior of cross bracings in transmission towers based on experiments and numerical simulations

Angle steel towers are widely used in various transmission lines owing to their convenient connections, easy availability, simple production processes, and high efficiencies. Cross bracings play an important role in resisting wire tension, wind, and snow loads as the main support components ensuring the stability of an angled steel tower. Thus, an analysis of the buckling resistance of cross bracings is significant in improving the bearing capacity of transmission towers. In this study, full-scale tests of equal-leg and unequal-leg angles in planar tower sections were conducted. The influence of the axial force ratio on the stability of a cross bracing was studied based on numerical simulations and compared with the current codes. The results revealed that the failure mode of an equal-leg angle cross bracing was always out-of-plane buckling, whereas the final failure mode of an unequal-leg angle steel cross bracing could be out-of-plane buckling or both out-of-plane and in-plane co-buckling. With an increase in the axial force ratio, the buckling resistance of the cross bracing decreased rapidly, with over 40% decline in the amplitude. The buckling resistance of the unequal-leg angle was approximately 20% higher than that of the equal-leg angle at all axial force ratios. Compared with the current codes for tower structures, we discovered that the code values and finite-element results were considerably different. Notably, the calculation methods of ASCE 10-15 and EN1993-3-1 neglect the variation in the effective length with the axial force ratio. Although DL/T 5154-2012 considers the effect of the axial force ratio, the values are all greater than the finite-element results. The findings of this study can help elucidate the cross bracing stability in angle steel lattice towers.

Alternative ring flange models for buckling verification of tubular steel wind turbine towers via advanced numerical analyses and comparison to code provisions

The beneficial effect of ring flanges between adjacent sections of tubular steel wind turbine t6owers to their susceptibility to local shell buckling is investigated by means of linear and nonlinear buckling finite element analyses, using alternative numerical models. Models comprising solid vs shell elements for the tower shell and solid vs shell vs beam elements for the ring flanges are compared, leading to similar results. Thus, computational cost is adopted as the deciding factor towards preferring the simpler and lighter models. Then, the behavior of an approximately 120 m tall wind turbine tower under realistic wind and gravity loads is investigated, focusing on the buckling response of successive configurations of sections of the tower below each flange, in order to assess the buckling potential throughout the tower height and compare to analytical buckling verification procedures proposed in EN1993-1-6. Nonlinear finite element analyses considering geometrical and material nonlinearity and imperfections with the shape of the three primary buckling modes are employed. The imperfection size is determined according to EN1993-1-6 for fabrication quality class A, B or C. Moreover, a parametric investigation of the thicknesses of ring flanges between consecutive tower sections indicates significant margins for thickness reduction in terms of the flanges’ ability to act as stiffeners for preventing local buckling.

SEISMIC VULNERABILITY ANALYSIS OF CABLE-STAYED BRIDGE DURING ROTATION CONSTRUCTION

Due to the swivel construction, the structural redundancy of cable-stayed bridge is reduced, and its seismic vulnerability is significantly higher than that of non-swirling construction structure and its own state of formation. Therefore, it is particularly important to study the damage changes of each component and stage system during the swivel construction of cable-stayed bridge under different horizontal earthquakes. Based on the construction of Rotary Cable-stayed Bridge in Haxi Street, the calculation formula of damage exceeding probability is established based on reliability theory, and the damage calibration of cable-stayed bridge components is carried out, and the finite element model of cable-stayed bridge rotating structure is established. The vulnerable parts of the main tower and the stay cable components of the cable-stayed bridge are identified and the incremental dynamic analysis is carried out. Finally, the seismic vulnerability curves of the main tower section, the stay cable and the rotating system are established. The results of the study show that the vulnerable areas of the H-shaped bridge towers are the abrupt changes in the main tower section near the upper and lower beams, and the vulnerable diagonal cables are the long cables anchored to the beam ends and the short cables near the main tower；At the same seismic level, the damage exceedance probability of main tower vulnerable section of cable-stayed bridge under transverse earthquake is greater than that under longitudinal earthquake, the damage exceedance probability of vulnerable stay cables under transverse seismic action is less than that under longitudinal seismic action；On the premise of the same damage probability, the required ground motion intensity of the system can be reduced by 0.35g at most compared with the component；Under the same seismic intensity, the system damage probability is 6.60 % higher than the component damage probability at most. The research results have reference significance for the construction of rotating cable-stayed bridges in areas lacking seismic records.

Investigation of the effect of structural damping on wind turbine wind-induced fatigue loads

This paper presents a quantitative investigation of the effect of structural damping ratios of the tower and blade vibration modes of wind turbines on wind-induced fatigue loads at critical blade and tower sections. Fatigue loads are used as a relative measure of fatigue life in wind turbine design applications. By increasing the structural damping ratio of each one of the critical structural modes which highly contribute to the overall vibration response, short-term and lifetime fatigue loads at critical tower and blade sections are calculated considering parked and operating conditions for different mean wind speeds as well as lifetime operational conditions.

A Straightforward Approach to Site-Wide Assessment of Wind Turbine Tower Lifetime Extension Potential

This contribution presents a novel methodology to evaluate the lifetime extension potential of wind turbines—taking towers as the key component that preserves onshore turbines’ structural integrity—as a consequence of the difference between design and site-specific loads. Specifically, attention is drawn to the site-specific wind direction distribution, which provides an additional source of lifetime extension potential. For this purpose, variants of closed-form solutions (based on the tower section’s normal stress) are developed to enable fatigue damage accumulation due to fore-aft and side-to-side bending moments at any point on the tower circumference without the need for further information on tower section geometry or material properties. Based on the degree of data availability, different scenarios are defined to estimate lifetime extension potential from the accurate tower’s normal stress and approximations using resultant bending moment, fore-aft bending moment, and finally, wind rose data only. The methodology is applied to a wind farm case study using the actual SCADA data with a partially validated turbine’s aeroelastic model to obtain operational loads. The results indicate that this quick and fairly accurate approach can be used as an initial stage in identifying wind turbines across large farms, which have the largest lifetime extension potential.

Numerical and full-scale test case studies on post-elastic performance of transmission towers

Post-elastic capacity of transmission tower is critical for the realistic assessment of tower vulnerability under the extreme loading conditions. This paper first presented a series of numerical simulations of a lattice transmission tower section under torsional (longitudinal) or bending (transverse) loading in both static and dynamic scenarios. From the results of the numerical simulations, four prototypes of lattice transmission towers were constructed and tested for the investigation of the post-elastic performances of the towers under different loading conditions. The tests results showed that there existed significant post-elastic strength reserve of the tower under investigation. For the towers tested in this research, the post-elastic strength reserve was 1.22 for flexure-torsion (i.e. tower under longitudinal loading) governed by diagonals, and 1.37 for bending (i.e. tower in transverse loading) governed by inelastic buckling of the main legs. Diagonal members affected the failure modes of transmission towers and their connection design may be a weak link in the development of their post-elastic capacity.

Design, structural modeling, control, and performance of 20 MW spar floating wind turbines

As floating wind turbines (FWTs) increase in size and power, the relative contribution of wave and wind loads to their global responses differs from what has been observed for 5–10 MW units. In addition, the larger deflections at the platform, increased natural period range for some degrees of freedom, and larger RNA weight at higher heights invite a review on structural modeling methods, design constraints, dynamic analysis, and control systems. This paper explores these topics through the design and structural analysis of three spar-type 20 MW FWTs, with different constraints on the static pitch angle at rated wind speed. Time-domain simulations are performed with a non-linear aero-hydro-servo-elastic software, and sectional fatigue damage and extreme motions and axial stresses for the three designs are compared. The platform is modeled as a flexible body, with hydrodynamic loads evaluated with potential theory and distributed over the hull. A control system with a motion compensation strategy is adopted, ensuring the same controller bandwidth for the three FWTs and showing significant performance improvements compared to detuning the controller gains. In addition to impacting steel and ballast mass, the static pitch angle at rated thrust affects the platform dynamics and fatigue damage/extreme loads significantly. The platforms with larger restoring in pitch present less fatigue damage at the platform, but more at the tower. Extreme stresses are largely affected by gravitational loads, such that the designs with larger pitch at rated thrust have the highest extreme stresses at the platform and most of the tower sections. Load cases associated with the rated wind speed often govern the extreme loads, unlike previous studies with 5 MW and 10 MW FWTs.

Research on Lightweight of Docking Structure Based on ANSYS

The docking structure has broad application prospects in the construction of tower assembly. However, the docking structure has a relatively large quality, which has a greater impact on the operations of high-altitude workers. This paper studies the impact force on docking structure during the docking process. The finite element analysis (FEA) model of the docking structure and the tower section is established and the impact force of the docking structure under various wording conditions is calculated. Based on the calculation results of the impact force the lightweight research of the docking structure is carried out and the optimization constraint conditions are proposed. The simulation model is established by ANSYS and the optimization design of guiding tools and vertical limit tools are completed by adjusting the structural parameters. After the optimization design, the docking structure is trial-produced and tested. Through the method of FEA and experiment, a butting device that meets the requirements of strength and rigidity and is lighter in weight is obtained. Compared with the existing docking structure, the weight of the optimized docking structure is reduced by about 39%. The research results can provide a reference for the design of the docking structure.

Lightning trip-out risk assessment and differential lightning protection of 35 kV transmission lines in mountain wind farm

Lightning trip out of collecting line has become a serious threat to the safe and reliable operation of mountain wind farm. It is urgent to carry out lightning trip out risk assessment and differential protection of 35kV collecting line of mountain wind farm. In this paper, considering the mountain terrain, tower structure, insulation configuration, grounding resistance and other factors, an accurate calculation model of lightning trip out rate is established, and the lightning trip out distribution law of each tower in wind farm is obtained. In this paper, the lightning trip risk of collector lines is classified according to the improved analytic hierarchy process of ”base-by-base tower – tower section – whole line”. Finally, considering the economy of wind farm lightning protection, three kinds of wind farm collector line differential protection schemes are proposed. The results show that the lightning trip-out risk assessment of the line I and line II of a wind farm is carried out by using the improved chromatography analysis method. The lightning trip rates of the line I and line II are 2.299 times/100 km⋅a and 1.740 times/100 km⋅a, respectively. This shows that the wind farm’s circuit is faced with serious lightning trip risk. Considering the effect of lightning protection and economy of lightning protection, scheme 2 is adopted to carry out the lightning protection transformation of line I and line II. The risk of lightning trip out of line I and line II is reduced by 55% and 62.7% respectively, and the technical economy of lightning protection of scheme 2 is the best. Therefore, it is suggested to adopt scheme 2 as the lightning protection transformation measure of the collection line of the wind farm. The above research can provide technical reference for lightning protection of wind farm collector lines in the future.