Flexible Towers Research Articles

Active vibration control for flexible towers based on displacement observation and reduced-order controller in modal space: Theory and experiment

Active vibration control is not only suitable but also urgently needed in order to mitigate excessive vibration for dynamic systems such as wind turbine towers. However, there is a challenge of directly observing displacement and developing an efficient reduced-order control strategy. With this in view, a reduced-order controller based on displacement observation is developed for the vibration mitigation of flexible high towers, where the problems of low signal-to-noise ratio (SNR) and intricate acceleration-based control algorithms are eliminated completely. The displacement can be measured by laser or modern videometrics technology. A reduced-order model with an active mass damper (AMD) at the top of the tower is created using modal truncation approach based on modal participation factor (MPF), which successfully preserves the dominant modes of the original structure, and reduces the influence of high-frequency uncertainties. Considering structure uncertainties caused by environmental factors and aging process, an unscented Kalman filter (UKF) is introduced to identify the modal stiffness of the reduced-order model. The present study, using a 5 MW onshore wind turbine as an example, investigates active vibration control under wind and seismic loads and compares it with traditional tuned mass damper (TMD). From a dynamic perspective, the simplified model of wind turbine tower takes the form of a multi-degree-of-freedom series model. Consequently, a six-story steel frame platform has been constructed to experimentally validate the efficacy of the reduced-order controller. According to numerical simulation and laboratory testing, the control effect of the reduced-order controller with a few displacement states as feedback can be utilized as an active control strategy for high-rise flexible towers.

Influence of the Flexible Tower on Aeroelastic Loads of the Wind Turbine

The finite element discretization of a tower system based on the two-node Euler-Bernoulli beam is carried out by taking the cubic Hermite polynomial as the form function of the beam unit, calculating the structural characteristic matrix of the tower system, and establishing the wind turbine-nacelle-tower multi-degree-of-freedom finite element numerical model. The equation for calculating the aerodynamic load for any nacelle attitude angle is derived. The effect of the flexible tower vibration feedback on the aerodynamic load of the wind turbine is studied. The results show that, when the stiffness of the tower is large, the effect of having tower vibration feedback or not on the aeroelastic load of the wind turbine is small. For the more flexible tower system, wind-induced vibration time-varying feedback will cause larger aeroelastic load variations, especially the top of the tower overturning moment, thus causing a larger impact on the dynamic behavior of the tower downwind and crosswind. As the flexibility of the tower system increases, the interaction between tower vibration and pneumatic load is also gradually increasing. Taking into account the influence of flexible towers on the aeroelastic load of a wind turbine can help predict the pneumatic load of a wind turbine more accurately and improve the efficiency of wind energy utilization on the one hand and analyze the dynamic behavior of the flexible structure of a wind turbine more accurately on the other hand, which is extremely beneficial to the structural optimization of wind turbine.

Designing Video Measuring Device for Monitoring Ice Deposits on Overhead Power Lines

The diagnosis of icing of overhead power lines (OHPLs) by the optical technique involves controlling the linear gap according to video surveillance data. During operation, the wire is exposed to mechanical and thermal strains. This article reveals the need for controlling the wire heating process and suggests an algorithm for calculating the procedure error caused by deflections of modern flexible towers. A conclusion is made that it is important to monitor tower deflections at wind loads. The procedure for correcting the resulting error is considered.

Health Monitoring and Safety Evaluation of the Offshore Wind Turbine Structure: A Review and Discussion of Future Development

With the depletion of fossil energy, offshore wind power has become an irreplaceable energy source for most countries in the world. In recent years, offshore wind power generation has presented the gradual development trend of larger capacity, taller towers, and longer blades. The more flexible towers and blades have led to the structural operational safety of the offshore wind turbine (OWT) receiving increasing worldwide attention. From this perspective, health monitoring systems and operational safety evaluation techniques of the offshore wind turbine structure, including the monitoring system category, data acquisition and transmission, feature information extraction and identification, safety evaluation and reliability analysis, and the intelligent operation and maintenance, were systematically investigated and summarized in this paper. Furthermore, a review of the current status, advantages, disadvantages, and the future development trend of existing systems and techniques was also carried out. Particularly, the offshore wind power industry will continue to develop into deep ocean areas in the next 30 years in China. Practical and reliable health monitoring systems and safety evaluation techniques are increasingly critical for offshore wind farms. Simultaneously, they have great significance for strengthening operation management, making efficient decisions, and reducing failure risks, and are also the key link in ensuring safe energy compositions and achieving energy development targets in China. The aims of this article are to inform more scholars and experts about the status of the health monitoring and safety evaluation of the offshore wind turbine structure, and to contribute toward improving the efficiency of the corresponding systems and techniques.

Seismic vulnerability and failure modes simulation of ancient masonry towers by validated virtual finite element models

Seismic protection of ancient masonry towers is a topic of great concern among the scientific community. A methodology for the seismic vulnerability assessment of all types of towers and slender unreinforced masonry structures (e.g., light houses and minarets) is presented. The approach is developed by four validated 3D FEM models representative of European towers. The models are subjected to linear elastic investigations to establish load carrying capacity and dynamic properties for validation against similar towers. Seismic simulations are developed through intensive nonlinear static pushover analyses. From the assessments, the failure modes and overall seismic response of the towers are obtained. Low tensile strength of masonry and large openings at belfries have significant influence on the seismic behavior, resulting in a quasi-brittle failure. All the towers presented an imminent high vulnerability to seismic actions. The few investigations reported in literature on the seismic behavior of towers are focused on in-plane behavior, disregarding out-of-plane behavior and toe crushing, both aspects are investigated in this paper. The more flexible towers are close to present toe crushing in both planes. The failure mechanisms are validated with reported post-earthquake observations on real damaged towers.

Results from a Full-Scale Study on the Condition Assessment of Pendulum Tuned Mass Dampers

AbstractPendulum tuned mass dampers (PTMDs) are one of the most popular vibration control devices in use today for flexible towers, bridges, and buildings. Their most attractive feature is the design simplicity; the natural period can be controlled simply by adjusting the suspended length and viscous dampers can be easily integrated into the design. While this simplicity has resulted in their widespread adoption, the installed performance of PTMDs has not been investigated in much detail. This paper presents a methodology for conducting condition assessment of in-service PTMDs. Results from a full-scale study of a PTMD-equipped structure is used as a test bed to demonstrate the approach. The condition of an in-service PTMD is assessed using two criteria: the frequency and damping tuning ratios, and equivalent damping provided by the PTMD. These criteria are estimated while the PTMD is in service, without arresting the PTMD motion, using an extended Kalman filter. Practical considerations typical in full-s...

Optimisation studies for a deep-sea cubic-kilometre neutrino telescope based on flexible towers

The design and optimisation of the KM3NeT telescope has been a major activity over the last years. Some of the results for a detector design based on flexible towers are presented.

The seismic response of elevated liquid storage tanks isolated by lead-rubber bearings

The seismic response of elevated liquid storage tanks isolated by the lead-rubber bearing is investigated under real earthquake ground motion. Two types of isolated tank models are considered in which the bearings are placed at the base and top of the tower structure. The tank liquid is modelled as lumped masses referred as convective mass, impulsive mass and rigid mass. The corresponding stiffness associated with these lumped masses has been worked out using the properties of the tank wall and liquid mass based on simple single-degree-of-freedom concepts. The mass of the tower structure is lumped equally at top and bottom. The assembled equations of motion are solved by Newmark's step-by-step method with iteration. The seismic response of two types of tanks, namely slender and broad tanks is obtained and a parametric study is carried out to study the effects of important system parameters on the effectiveness of seismic isolation. The various important parameters considered are the tank aspect ratio, the time period of the tower structure, damping and the lime period of the isolation system. It has been observed that the earthquake response of the isolated tank is reduced significantly. Further, it is observed that the isolation is more effective for the tank with a stiff tower structure in comparison to flexible towers. In addition, a simplified analysis is also presented to evaluate the response of the elevated tanks using a two-degrees-of-freedom model and two single degree-of-freedom models. It is observed that the proposed methods predict accurately the seismic response of elevated liquid storage tanks with less computational efforts.

Earthquake response of isolated elevated liquid storage steel tanks

Earthquake response of elevated liquid storage steel tanks isolated by the linear elastomeric bearings is investigated under real earthquake ground motion. Two types of isolated tank models are considered in which the bearings are placed at the base and top of the steel tower structure. The continuous liquid mass of the tank is modelled as lumped masses known as sloshing mass, impulsive mass and rigid mass. The corresponding stiffness constants associated with these lumped masses have been worked out depending upon the properties of the tank wall and liquid mass. The mass of steel tower structure is lumped equally at top and bottom. Since the damping matrix of the isolated tank system is non-classical in nature, the seismic response is obtained by the Newmark’s step-by-step method. The response of two types of tanks, namely slender and broad tanks, is obtained and a parametric study is carried out to study the effects of important system parameters on the effectiveness of seismic isolation. The various important parameters considered are the tank aspect ratio, the time period of tower structure, damping and the time period of isolation system. It has been shown that the earthquake response of the isolated tank is significantly reduced. Further, it is also observed that the isolation is more effective for the tank with stiff steel tower structure in comparison to flexible towers. In addition, a simplified analysis is also presented to evaluate the response of the elevated steel tanks using two-degrees-of-freedom model and two single-degree-of-freedom models. It is observed that the proposed analysis predicts the seismic response of elevated steel tanks accurately with significantly less computational efforts.

Closure to “ Analysis of Cable‐Stayed Bridges Supported by Flexible Towers ” by Fakhry Aboul‐ella (December, 1989, Vol. 114, No. 12)

Closure to “ <i>Analysis of Cable‐Stayed Bridges Supported by Flexible Towers</i> ” by Fakhry Aboul‐ella (December, 1989, Vol. 114, No. 12)

Discussion of “ Analysis of Cable‐Stayed Bridges Supported by Flexible Towers ” by Fakhry Aboul‐ella (December, 1989, Vol. 114, No. 12)

Discussion of “ <i>Analysis of Cable‐Stayed Bridges Supported by Flexible Towers</i> ” by Fakhry Aboul‐ella (December, 1989, Vol. 114, No. 12)

Stochastic response of suspension bridges to earthquake forces

AbstractThe purpose of the paper is to study the applicability of stochastic methods for determining the response in the vertical plane of long‐span bridges to earthquakes. The bridges are of the modern type, with flexible towers, box‐decks and inclined hangers, Humber and Bosporus being used as the main examples, although some useful information is obtained by studying a small suspension footbridge. The input ground acceleration is that of the Pacoima Dam record of the 1971 San Fernando event, but use is also made of filtered white noise.Because the stochastic approach is essentially an attempt to abstract usable information which could otherwise be obtained from a lengthy time‐history approach, the procedure adopted here is to compare the same parameters obtained in these two ways, as far as possible. In particular, the maximum values of displacements and bending moments are considered.The essential question to be answered is whether the short length of earthquake records, coupled with the large natural periods of vibration of long‐span bridges, allows sufficient response information to be generated to make statistical parameters meaningful. The short‐span (50 m) footbridge gives no cause for concern here, but the detailed comparison with time‐history solutions shows that the stochastic approach for both Bosporus and Humber has to be cautiously assessed, particularly if some trailing zeros are not added to the earthquake record. However, general conclusions are not made, because only one earthquake record has been used.

Analysis of Cable‐Stayed Bridges Supported by Flexible Towers

An analytical iterative method for the static analysis of continuous cable-stayed bridges is presented. The towers can have any degree of flexibility and the interaction with their supporting media (caissons or piles) is considered. The system is divided into two substructures: the tower and the girder plus the cables. The continuous girder is solved by means of the method of three-moment equations, while the deflections are calculated from Fourier series. It was found that a small number of iteration cycles and Fourier coefficients are sufficient for convergence and a few simultaneous equations are required for solving the entire system. Therefore, large bridge systems with a large number of cables can be analyzed under different loading conditions using the PC computers. A comparison between the numerical results of the present method and those obtained from the nonlinear stiffness method shows the advantages of the present method and also shows that nonlinearity effects are not important for such types of bridges.

Secondary Stresses on Transmission Tower Structures

The power industry is moving toward the use of high‐voltage transmission lines on tall, flexible towers. These towers are designed for heavier loadings than were previous towers, but the secondary effects of tower displacement tend to be ignored in the present day tower analysis. This paper examines the secondary effects from the large displacement of flexible towers, reviews the secondary stresses arising from joint rigidity and member continuity, and examines the reliability of tower strength predicted by the ASCE design guidelines. More realistic tower test acceptance criteria and a more reliable tower design methodology are suggested.