Wind Load Cases Research Articles

Controller influence on the fatigue of a floating wind turbine and load case impact assessment

Abstract The assessment of fatigue in Floating Offshore Wind Turbines (FOWT) is a complex and widely debated topic within the industry. The dynamic behaviour of FOWT is heavily influenced by sea and wind conditions, which can significantly challenge the achievement of structural integrity and performance objectives. Additionally, the interaction of blade aerodynamics and control strategies further complicates this assessment. This study evaluates the impact of different control strategies and blade formulations on both the expected real performance and the estimated fatigue of FOWT. While control strategies can influence both the expected real performance and fatigue estimations, the blade formulation primarily affects the accuracy of performance estimations without altering the actual expected performance. However, the blade formulation can significantly modify the fatigue estimation of the FOWT. Damage Equivalent Loads (DEL) calculation is a commonly used method for assessing fatigue. However, while individual DEL calculations are useful, they do not show the contribution of each load case to the overall fatigue, since they do not consider the probability of occurrence. Therefore, this paper proposes a new method of normalising DEL, allowing for comparison of loading cases and identifying operational areas that pose greater fatigue risks. The findings underscore the substantial impact that different control strategies and blade designs can have on fatigue and overall performance. Essentially, there is a trade-off between maximising power output and minimising fatigue damage.

Design load provisions for simulating the critical effect of downbursts on wind turbines

Wind turbines are among the rapidly growing sources of sustainable energy. The review of design codes for this type of structure shows a lack of procedures for estimating the wind loads on wind turbines due to high-intensity wind events such as downbursts. There is a challenge in the analysis and design of wind turbines under downbursts because of the sudden and localized nature of such events, which makes the forces acting on the tower and blades dependent on the characteristics of the event. As such, the objective of the current study is to develop design load provisions that can simulate the critical effect of downbursts on wind turbines. These provisions are developed in two phases. Identifying the peak effects of the moving mean component of the downburst and determining the associated critical profiles are the aim of the first phase. To achieve this task, a comprehensive parametric study is conducted using the previously developed numerical model, HIW-TUR, on a wide range of wind turbine sizes and airfoil types in order to assess the critical response of wind turbines under the mean component of downbursts. The study takes into account the variations in the downburst size and its location relative to the tower center. Then, dynamic analyses under turbulence are conducted in the second phase using the open-source code, FAST, to determine a downburst gust response factor to magnify the effect of the mean component. The downburst gust response factor is then superimposed with the mean component profiles to present a complete set of design provisions. An example is presented to demonstrate the application of the developed loading provisions on a real wind turbine and to compare with the extreme synoptic wind load cases specified by the International Electro-technical Commission Loading Code.

Analysis of Mechanical Properties of Air-Ribbed Skeleton Membrane Structure

The existing large inflatable membrane structure is complex in structure and takes a long time to be extended. In this paper, an air-ribbed skeleton membrane structure is proposed to meet the requirement of camp and emergency tents needing to be set up quickly and easily. Ten single-arch air ribs are arranged side by side to support the tarpaulin, and wind ropes are added to the end and side faces of the tarpaulin to remain stable. The finite element model of the air-ribbed skeleton membrane structure is established to analyze the stress and the displacement of the membrane structure under combined wind and snow loads. The maximum displacement (439.4 mm) and maximum stress (29.94 MPa) are both within the safe standard. The stress and the displacement of the membrane structure in the wind load case is affected by the angle of the wind. The value of maximum stress and displacement at the wind angle of 90° are both lower than those at 0° and 45° in the wind load case. It is advisable to align the site layout of the membrane structure at the wind angle of 90°. The effect of the angle of wind ropes on the stress and displacement of the membrane structure is also studied. The maximum stress and displacement in the case when the angle of wind rope is 30° is smaller than those in the case when the angle of wind ropes is 45°. It is recommended that the wind rope should be laid at 30° to reinforce the membrane structure.

Estimation of reduction factors for combined wind and ice loading of power transmission lines using multivariate scenario sampling

In this paper, a multivariate scenario sampling method was used for estimation of reduction factors for loading of structures in power transmission lines in the combined wind and ice load case. The values of wind speed and ice thickness that must be used in combined load cases are obtained through multiplying the reduction factors by the design extreme (reference) values of each variable. The reduction factors were estimated according to the criteria of IEC 60826 standard for the mean recurrence intervals (MRI) of 50, 150, and 500 years. The reduction factors have been established through bivariate (ice and concurrent wind) hazard curves that extracted via two approaches; one based on a statistical analysis of the wind force applied to an ice-covered conductor (force-based approach) and the other based on a joint probability distribution of wind speed and ice thickness (joint probability-based approaches). In order to extract the hazard curves, an annual scenario sampling approach was utilized based on historical data recorded in 12 selected meteorological stations located in the cold regions of western Iran. Due to the lack of historical data, the thickness of ice formed around the conductor of transmission lines has been evaluated via simulations of ice accretion using a numerical model for cylindrical growth of wet snow sleeves. The results obtained for most of the meteorological stations show that the combined wind and ice reduction factors presented in IEC 60826 are conservative. It was also shown that two main reduction factors, (which specify the reduced value of a variable that must be combined with the extreme value of another) can be calculated merely by the force-based approach, and are not sensitive to MRI. In the same way, the MRI effect on hazard curves constructed by joint probability-based approaches is not significant, except for the relatively smaller ice thickness.

Simplified expressions for reliability assessments in code calibration

First Order Reliability Methods (FORM) have been used by specification committees in the reliability analyses required for the calibration of resistance and safety factors for the past 40 years. However, these methods are iterative, require input information that may not be readily available, and make comparisons between different approaches or design frameworks difficult. This paper presents a set of simplified equations to estimate reliability indices β, resistance factors ϕ and partial safety factors γM based on simpler First Order Second Moment (FOSM) considerations for the US and Eurocode frameworks, which are particularized for different load cases, and on the semi-probabilistic approach prescribed in the Eurocode 0. The equations provide direct relationships between the reliability calibration results corresponding to different design frameworks, and can be used to estimate resistance factors as simple cross-checks for the US framework based on the partial safety factors derived for the Eurocode (or vice versa) from basic statistical input information and given target reliability, including when the data available in the literature is insufficient to perform FORM analyses. The accuracy of the proposed equations is assessed against reliability results derived using FORM techniques for an extensive database of steel and stainless steel frames subjected to gravity and combined gravity plus wind load cases collected from the literature, and limitations for their applicability are recommended. The results demonstrate that the set of equations proposed in this paper provides accurate estimations of the reliability index β, resistance factors ϕ and partial safety factors γM and can assist specification committees in the process of calibrating suitable ϕ and γM-factors.

Analysis of Load Reduction of Floating Wind Turbines Using Passive Tuned Mass Dampers

Abstract: An efficient method for restraining the large vibration displacements and loads of offshore floating wind turbines under harsh marine environment is proposed by putting tuned mass dampers in the cabin. A dynamics model for a barge-type offshore floating wind turbine with a fore–aft tuned mass damper is established based on Lagrange’s equations; the nonlinear least squares Leven berg–Marquardt algorithm is employed to identify the parameters of the wind turbine; different parameter optimization methods are adopted to optimize tuned mass damper parameters by considering the standard deviation of the tower top longitudinal displacement as the objective function. Aiming at five typical combined wind and wave load cases under normal running state of the wind turbine, the dynamic responses of the wind turbine with/without tuned mass damper are simulated and the suppression effect of the tuned mass damper is investigated over the wide range of load cases. The results show that when the wind turbine vibrates in the state of damped free vibration, the standard deviation of the tower top longitudinal displacement is decreased approximately 60% in 100 s by the optimized tuned mass damper with the optimum tuned mass damper mass ratio 1.8%. The standard deviation suppression rates of the longitudinal displacements and loads in the tower and blades increase with the tuned mass damper mass ratio when the wind turbine vibrates under the combined wind and wave load cases. When the mass ratio changes from 0.5% to 2%, the maximum suppression rates vary from 20% to 50% correspondingly, which effectively reduce vibration responses of the offshore floating wind turbine. The results of this article preliminarily verify the feasibilities of using a tuned mass damper for restraining vibration of the barge-type offshore floating wind turbine

Effect of unsteady aerodynamic loads on driving safety and comfort of trains running on bridges

In order to investigate the effects of unsteady aerodynamic loads on the driving safety and comfort of trains running on bridges, a three-dimensional and multi-body system model of train–track–bridge was established and the dynamic responses of the coupling system were calculated by combining the finite element software ANSYS with the multi-body dynamics software SIMPACK. The driving safety and comfort of a train running on a bridge under steady and unsteady aerodynamic loads were compared and analyzed. The effects of different crosswind speeds on the driving safety of the train running on the bridge under unsteady aerodynamic loads were studied. It is found that the index values of the driving safety and comfort of the train at the speed of 200–300 km/h without the wind loads are smaller (meaning safer) than those of the train under the wind loads. When the average speed of crosswind is 20 m/s, the driving safety assessment results of the train are better and its comfort assessment results are more conservative with considering the unsteady aerodynamic loads than the steady wind load case. When the average speed of crosswind is smaller than 10 m/s and the train speed is 250 km/h, the driving safety and comfort of the train on the bridge meet the requirements, and the level of stability can reach “good” or above. Through the analysis of driving safety of the train on the bridge under different crosswind speeds, the threshold values of safe driving were obtained, which can provide a better basis for the safe operation of trains on bridges.

Verification of the Structural Performance of Textile Reinforced Reactive Powder Concrete Sandwich Facade Elements

As a part of the SESBE (Smart Elements for Sustainable Building Envelopes) project, non-load bearing sandwich elements were developed with Textile Reinforced Reactive Powder Concrete (TRRPC) for outer and inner facings, Foam Concrete (FC) for the insulating core and Glass Fiber Reinforced Polymer (GFRP) continuous connectors. The structural performance of the developed elements was verified at various levels by means of a thorough experimental program coupled with numerical analysis. Experiments were conducted on individual materials (i.e., tensile and compressive tests), composites (i.e., uniaxial tensile, flexural and pull-out tests), as well as components (i.e., local anchorage failure, shear, flexural and wind loading tests). The experimentally yielded material properties were used as input for the developed models to verify the findings of various component tests and to allow for further material development. In this paper, the component tests related to local anchorage failure and wind loading are presented and coupled to a structural model of the sandwich element. The validated structural model provided a greater understanding of the physical mechanisms governing the element’s structural behavior and its structural performance under various dead and wind load cases. Lastly, the performance of the sandwich elements, in terms of composite action, was shown to be greatly correlated to the properties of the GFRP connectors, such as stiffness and strength.

A framework for isogeometric‐analysis‐based optimization of wind turbine blade structures

AbstractEarly‐stage wind turbine blade design usually relies heavily on low‐fidelity structural models; high‐fidelity, finite‐element‐based structural analyses are reserved for later design stages because of their complex workflows and high computational expense. Yet, high‐fidelity structural analyses often provide design‐governing feedback such as buckling load factors. Mitigation of the issues of workflow complexity and computational expense would allow designers to utilize high‐fidelity feedback earlier, more easily, and more often in the design process. Thus, a blade analysis framework that employs isogeometric analysis (IGA), a simulation method that overcomes many of the aforementioned drawbacks associated with traditional finite element analysis (FEA), is presented. IGA directly utilizes the mathematical models generated by computer‐aided design (CAD) software, requiring less user interaction and no conversion of parametric geometries to finite element meshes. Furthermore, IGA tends to have superior per‐degree‐of‐freedom accuracy compared with traditional FEA. Issues unique to IGA in the context of wind turbine blade design, such as coupling of thin‐shell components, are addressed, and a design approach that combines reduced‐order aeroelastic analysis with IGA is outlined. Aeroelastic analysis is used to efficiently provide dynamic kinematic data for a wide range of wind load cases, while IGA is used to perform buckling analysis. The value of incorporating high‐fidelity analysis feedback into blade design is demonstrated through optimization of the NREL/SNL 5 MW wind turbine blade. A variety of potential designs are produced with reduced blade mass and material cost, and IGA‐based buckling analysis is shown to provide design‐governing constraint information.

Vibration Mitigation of the Barge-Type Offshore Wind Turbine with a Tuned Mass Damper on Floating Platform

This paper evaluates the application of a passive control technique with a tuned mass damper on platform for the barge-type offshore wind turbine. First of all, the three degrees of freedom mathematical model for the floating wind turbine is established based on Lagrange's equations, and the Levenberg-Marquardt algorithm is adopted to estimate the parameters of the wind turbine. Then, the method of frequency tuning which is utilized in engineering projects and genetic algorithm are employed respectively to simulate the optimum parameters of the tuned mass damper. The vibration mechanism about the phase-angle difference between tuned mass damper and floating platform is analyzed. Finally, the dynamic responses of floating wind turbine with/without tuned mass damper are calculated under five typical wind and wave load cases, and the vibration mitigation effects are researched in marine environment. Partial ballast is substituted by the equal mass of tuned mass damper due to the mass of floating platform with tuned mass damper would increase obviously, which would change the design of the wind turbine, and the vibration mitigation is also simulated in five typical load cases. The results show that the suppression rate of standard deviation of platform pitch is up to 47.95%, after substituting the partial mass of ballast, the suppression rate is 50%. Therefore, the dynamic responses of the barge-type floating wind turbine would be reduced significantly when the ballast is replaced by the equal mass of the tuned mass damper on floating platform.

18.03: Comparative analysis of steel telecommunication tower: By using V shaped & L shaped hot rolled steel profiles under design & fabrication stage

ABSTRACTThe purpose of this paper is to minimise the cost of steel telecommunication lattice self‐supported towers by using V shaped hot rolled steel profiles. Comparative analyses were carried out for different tower heights using different configuration of V shaped and L shaped hot rolled steel profiles. An elastic 3D truss finite element model was used to determine the axial forces and stresses in each member under wind and various antenna load cases. Strength and serviceability limit states were controlled according to ANSI/EIA/TIA‐222‐G (addendum 2) during design stage. For the cost optimisation, both production, manufacturing, installation and material costs were addressed. During manufacturing stage, there are many obstacles to manufacture V shaped steel profiles, but the results indicate that the minimum cost solution corresponds to V shaped steel profiles.

Drag Coefficients for Construction-Stage Stability Analysis of Bridge Girders under Wind Loading

AbstractLateral wind loads often control the stability of bridge girders during construction stages in which the deck is not yet structurally effective. In this study, the key objectives were to experimentally quantify aerodynamic wind-load coefficients (drag, lift, and torque) for common bridge girder shapes and to quantify shielding effects arising from aerodynamic interference between adjacent girders. Wind tunnel tests were performed on reduced-scale models of built-up steel plate girders and prestressed concrete Florida I-beams (FIBs). Aerodynamic properties were measured for individual girder cross-sectional shapes and for systems of multiple adjacent girders (tested both with and without bridge deck formwork in place). Results from the wind tunnel tests, which revealed negative drag coefficients at selected downstream girder positions, were synthesized into simplified wind-loading cases suitable for use in bridge design. Wind-load cases were separately developed for assessing overall multigirder sy...

Optimization design of tuned mass damper for vibration suppression of a barge-type offshore floating wind turbine

An efficient method for restraining the large vibration displacements and loads of offshore floating wind turbines under harsh marine environment is proposed by putting tuned mass dampers in the cabin. A dynamics model for a barge-type offshore floating wind turbine with a fore–aft tuned mass damper is established based on Lagrange’s equations; the nonlinear least squares Levenberg–Marquardt algorithm is employed to identify the parameters of the wind turbine; different parameter optimization methods are adopted to optimize tuned mass damper parameters by considering the standard deviation of the tower top longitudinal displacement as the objective function. Aiming at five typical combined wind and wave load cases under normal running state of the wind turbine, the dynamic responses of the wind turbine with/without tuned mass damper are simulated and the suppression effect of the tuned mass damper is investigated over the wide range of load cases. The results show that when the wind turbine vibrates in the state of damped free vibration, the standard deviation of the tower top longitudinal displacement is decreased approximately 60% in 100 s by the optimized tuned mass damper with the optimum tuned mass damper mass ratio 1.8%. The standard deviation suppression rates of the longitudinal displacements and loads in the tower and blades increase with the tuned mass damper mass ratio when the wind turbine vibrates under the combined wind and wave load cases. When the mass ratio changes from 0.5% to 2%, the maximum suppression rates vary from 20% to 50% correspondingly, which effectively reduce vibration responses of the offshore floating wind turbine. The results of this article preliminarily verify the feasibilities of using a tuned mass damper for restraining vibration of the barge-type offshore floating wind turbine.

Coupled dynamic response analysis of a multi-column tension-leg-type floating wind turbine

This paper presents a coupled dynamic response analysis of a multi-column tension-leg-type floating wind turbine (WindStar TLP system) under normal operation and parked conditions. Wind-only load cases, wave-only load cases and combined wind and wave load cases were analyzed separately for the WindStar TLP system to identify the dominant excitation loads. Comparisons between an NREL offshore 5-MW baseline wind turbine installed on land and the WindStar TLP system were performed. Statistics of selected response variables in specified design load cases (DLCs) were obtained and analyzed. It is found that the proposed WindStar TLP system has small dynamic responses to environmental loads and it thus has almost the same mean generator power output under operating conditions as the land-based system. The tension mooring system has a sufficient safety factor, and the minimum tendon tension is always positive in all selected DLCs. The ratio of ultimate load of the tower base fore-aft bending moment for the WindStar TLP system versus the land-based system can be as high as 1.9 in all of the DLCs considered. These results will help elucidate the dynamic characteristics of the proposed WindStar TLP system, identify the difference in load effect between it and land-based systems, and thus make relevant modifications to the initial design for the WindStar TLP system.

Low Cycle Fatigue Failure of a Sitka Spruce Tree in Hurricane Winds

Pine plantations are prone to stem breakage due to high cyclic stress levels associated with hurricane force winds. Stress analytical and finite element simulation models were constructed of a representative profile of a (Sitka) Picea sitchensis tree. The profile surface stress (S) was determined due to the combined load of tree self-weight and hurricane wind speed. The results were complemented by reference to two other studies by other researchers that investigated the impact of fatigue cycles on failure (N) of pine wood and tree sway cycles to present a stem fatigue life prediction. The position of maximum surface profile stress and trunk fracture initiation location was ascertained from a non-uniform stress response. No stress uniformity along the trunk profile was observed for any wind-load case examined. The analytical model and finite element analysis of the P. sitchensis tree trunk profile revealed a statically adequate strength reserve factor of 1.4, which suggested another mode of failure was responsible. Fatigue life failure prediction was examined under cyclic and same-stress amplitude related to the hurricane wind speed of 33 m s-1. Predicted trunk fracture occurred in 2.6 hours, which dramatically reduced to two minutes with an increase in wind speed of only 1 m s-1. The calculated exposure time was similar to that recorded during Hurricane Hugo’s transit in 1989. The time-to-failure prediction obtained by the method of analysis provided in this study seemed plausible, and that the profile associated with the P. sitchensis tree would suffer trunk breakage by low cycle fatigue failure.

Displacement Control Technology on High-Rise Steel Structure about Main Power House of Thermal Power Plant

With the background of optimal design study on main powerhouse of Power Plant, the finite element software ANSYS is adopted to control the overall structure's displacement. After the choice of structural form and loads bearing system, the static and dynamic characteristics of this boiler steel frame are analyzed in detail. And the structure's displacement response of this boiler steel frame will also be discussed under permanent load, wind load and seismic action. The structural displacement response causing by four wind load cases (front wind, back wind, left wind and right wind) is emphatically discussed and the dynamic characteristics and response spectrum analysis are also be studied. Results show that this main powerhouse owns good seismic performance and excellent lateral displacement resistance capacity because of the high structure stiffness. It shows that the horizontal displacement of the whole structure under all load cases can meet the requirements of the limit value, which demonstrates the feasibility of this structural program.

Design and Fatigue Performance of Large Utility-Scale Wind Turbine Blades

Aeroelastic design and fatigue analysis of large utility-scale wind turbine blades have been performed to investigate the applicability of different types of materials in a fatigue environment. The blade designs used in the study are developed according to an iterative numerical design process for realistic wind turbine blades, and the software tool FAST is used for advanced aero-servo-elastic simulations. Elementary beam theory is used to calculate strain time series from these simulations, and the material fatigue is evaluated using established methods. Following wind turbine design standards, the fatigue evaluation is based on a turbulent wind load case. Fatigue damage is estimated based on 100% availability and a site-specific annual wind distribution. Rainflow cycle counting and Miner's sum for cumulative damage prediction is used together with constant life diagrams tailored to actual material S-N data. Material properties are based on 95% survival probability, 95% confidence level, and additional material safety factors to maintain conservative results. Fatigue performance is first evaluated for a baseline blade design of the 10 MW NOWITECH reference wind turbine. Results show that blade damage is dominated by tensile stresses due to poorer tensile fatigue characteristics of the shell glass fiber material. The interaction between turbulent wind and gravitational fluctuations is demonstrated to greatly influence the damage. The need for relevant S-N data to reliably predict fatigue damage accumulation and to avoid nonconservative conclusions is demonstrated. State-of-art wind turbine blade trends are discussed and different design varieties of the baseline blade are analyzed in a parametric study focusing on fatigue performance and material costs. It is observed that higher performance material is more favorable in the spar-cap construction of large blades which are designed for lower wind speeds.

Wind pressure on a solar updraft tower in a simulated stationary thunderstorm downburst

Thunderstorm downbursts are responsible for numerous structural failures around the world. The wind characteristics in thunderstorm downbursts containing vortex rings differ with those in 'traditional' boundary layer winds (BLW). This paper initially performs an unsteady-state simulation of the flow structure in a downburst (modelled as a impinging jet with its diameter being <TEX>$D_{jet}$</TEX>) using a computational fluid dynamics (CFD) method, and then analyses the pressure distribution on a solar updraft tower (SUT) in the downburst. The pressure field shows agreement with other previous studies. An additional pair of low-pressure region and high-pressure region is observed due to a second vortex ring, besides a foregoing pair caused by a primary vortex ring. The evolutions of pressure coefficients at five orientations of two representative heights of the SUT in the downburst with time are investigated. Results show that pressure distribution changes over a wide range when the vortices are close to the SUT. Furthermore, the fluctuations of external static pressure distribution for the SUT case 1 (i.e., radial distance from a location to jet center x=<TEX>$D_{jet}$</TEX>) with height are more intense due to the down striking of the vortex flow compared to those for the SUT case 2 (x=<TEX>$2D_{jet}$</TEX>). The static wind loads at heights z/H higher than 0.3 will be negligible when the vortex ring is far away from the SUT. The inverted wind load cases will occur when vortex is passing through the SUT except on the side faces. This can induce complex dynamic response of the SUT.

풍력발전기 증속기에 전달되는 풍하중 변동특성 연구

본 논문은 정상풍속과 돌발풍속을 수학적으로 모델링하고 풍향에 따라 전달되는 메인축에서의 전달모멘트를 조사하여 기어박스에 전달되는 풍하중의 특성을 파악하였다. 정상풍속은 지상에서 고도가 높아짐에 따라 속도가 증가하게 설정을 하였다. 풍하중에 의해서 메인축으로 전달되는 모멘트의 평균값과 하모닉값을 풍향 입사각을 -45° ~ 45°로 변화를 주며 특성을 파악하였다. 또한 기어 트레인의 미스얼라인먼트를 유발시키는 굽힘 모멘트의 특성을 파악하였다. 정상풍속모델에서는 블레이드의 3배수 주파수(3X)로 하는 토크의 가진이 생기며, 바람의 방향이 +22.5° 일 때 수평방향의 굽힘 모멘트가 주축으로 들어가는 토크의 50%수준으로 발생하는데 이는 수평방향으로의 탄성 축 휘임을 유발하여 치가 모서리에서 물림이 발생하게 하는 원인을 제공함을 알 수 있었다. 돌발풍속의 경우, 3X, 6X, 9X를 가진주파수로 하는 토크의 가진이 바람의 방향이 +방향으로 커질수록 하모닉항의 상대 비율이 증가하였다.

풍력발전시스템의 유연체 다물체 동역학 시뮬레이션 프로그램 개발

A wind turbine simulation program for the coupled dynamics of aerodynamics, elasticity, multi-body dynamics and controls of turbine is newly developed by combining an aero-elastic code and a multi-body dynamics code. The aero-elastic code, based on the blade momentum theory and generalized dynamic wake theory, is developed by NREL(National Renewable Energy Laboratory, USA). The multi-body dynamics code is commercial one which is capable of accounting for geometric nonlinearity and twist deflection. A turbulent wind load case is simulated for the NREL 5-MW baseline wind turbine model by the developed program and FAST. As a result, the two results agree well enough to verify the reliability of the developed program.