Parametric investigation of the soil–structure interaction effects on the dynamic behaviour of a shallow foundation supported wind turbine considering a layered soil

The proposed investigation is concerned with influential factors of soil–structure interaction issues for onshore wind turbines. Indeed, the awareness of these aspects encounters hardly a straightforward application in practical regulations and therefore is often neglected. However, with the rapid recent growth, the wind energy installations are expanding into regions where the soil conditions may be unfavorable. A consciousness raising of the significance of interaction between the wind turbine, its foundation and the underlying soil is lacking. This paper aims to fill this research gap. It involves a three‐blade wind turbine grounded on a layered half space. The layered soil is simplified as a horizontal layer over an homogeneous half space. However, the method can consider multilayered soil and different bottom conditions, such as rigid bedrock or flexible half space. The soil–structure system is modeled by means of a coupling between finite element and boundary element method. The analysis is carried out in frequency domain. At the first stage, the only foundation–soil system is investigated, and subsequently, the focus shifts to the whole turbine‐soil assembly. The effects of different parameters are systematically evaluated, in order to provide a range of values for which the soil–structure interaction has to be accounted for. The investigation highlighted the importance of the relative stiffness of structure and soil. Also, the ratio of the layer stiffness to the half space stiffness plays an important role. Copyright © 2014 John Wiley & Sons, Ltd.     

Soil–structure reliability of offshore wind turbine monopile foundations

An overview of offshore wind turbine (OWT) foundations is presented, focusing primarily on the monopile foundation. The uncertainty in offshore soil conditions as well as random wind and wave loading is currently treated with a deterministic design procedure, though some standards allow engineers to use a probability‐based approach. Laterally loaded monopile foundations are typically designed using the American Petroleum Institute p‐y method, which is problematic for large OWT pile diameters. Probabilistic methods are used to examine the reliability of OWT pile foundations under serviceability limit states using Euler–Bernoulli beam elements in a two‐dimensional pile–spring model, non‐linear with respect to the soil springs. The effects of soil property variation, pile design parameters, loading and large diameters on OWT pile reliability are presented. Copyright © 2014 John Wiley & Sons, Ltd.     

Wind turbines and seismic hazard: a state‐of‐the‐art review

Wind energy is a rapidly growing field of renewable energy, and as such, intensive scientific and societal interest has been already attracted. Research on wind turbine structures has been mostly focused on the structural analysis, design and/or assessment of wind turbines mainly against normal (environmental) exposures while, so far, only marginal attention has been spent on considering extreme natural hazards that threat the reliability of the lifetime‐oriented wind turbine's performance. Especially, recent installations of numerous wind turbines in earthquake prone areas worldwide (e.g., China, USA, India, Southern Europe and East Asia) highlight the necessity for thorough consideration of the seismic implications on these energy harnessing systems. Along these lines, this state‐of‐the‐art paper presents a comparative survey of the published research relevant to the seismic analysis, design and assessment of wind turbines. Based on numerical simulation, either deterministic or probabilistic approaches are reviewed, because they have been adopted to investigate the sensitivity of wind turbines' structural capacity and reliability in earthquake‐induced loading. The relevance of seismic hazard for wind turbines is further enlightened by available experimental studies, being also comprehensively reported through this paper. The main contribution of the study presented herein is to identify the key factors for wind turbines' seismic performance, while important milestones for ongoing and future advancement are emphasized. Copyright © 2016 John Wiley & Sons, Ltd.     

Power system frequency response from fixed speed and doubly fed induction generator‐based wind turbines

Throughout Europe there is an increasing trend of connecting high penetrations of wind turbines to the transmission networks. This has resulted in transmission system operators revising their grid code documents for the connection of large wind farms. These specifications require large MW capacity wind farms to have the ability to assist in some of the power system control services currently carried out by conventional synchronous generation. These services include voltage and frequency control. It is now recognized that much of this new wind generation plant will use either fixed speed induction generator (FSIG)‐ or doubly fed induction generator (DFIG)‐based wind turbines. The addition of a control loop to synthesize inertia in the DFIG wind turbine using the power electronic control system has been described. The possibility of deloading wind turbines for frequency response using blade pitch angle control is discussed. A pitch control scheme to provide frequency response from FSIG and DFIG wind turbines is also described. A case study of an FSIG wind turbine with frequency response capabilities is investigated. Copyright © 2004 John Wiley & Sons, Ltd.     

Analytical and numerical fluid–structure interaction study of a microscale piezoelectric wind energy harvester

In this paper, an analytical approach and two numerical models have been developed to study an energy‐harvesting device for micropower generation. This device uses wind energy to oscillate a cantilevered beam attached to a piezoelectric layer for generating electric energy output. The analytical approach and the first numerical model consider the fluid–structure interaction phenomenon in the harvester performance. The equations governing beam oscillations and airflow have been coupled to a set of four differential equations in the analytical approach. This set of equations has been solved to determine the beam deflection and the air pressure variation with time. The numerical methods have been conducted by employing a commercial software. The results of the analytical method and the first numerical model have been compared in different working conditions, and their credibility has been discussed. In the second numerical model, the electromechanical performance of the piezoelectric material has also been incorporated in the harvester device analysis. This model has been verified against experimental data for the output voltage and power of the device available in the literature. Finally, the effect of different geometrical parameters has been studied on the harvester performance, and suggestions have been made to improve the harvester efficiency.     

Seismic response of floating wind turbines due to seaquakes

Vertical seismic waves, which are primarily due to pressure waves in the ground, can propagate with the same intensity in the seawater and impact floating bodies such as floating wind turbines (FWTs). Part of this wave can further propagate in the tower and generate large vertical accelerations in the nacelle. This paper presents a methodology for computation of the pressure waves generated by vertical earthquake shaking, referred to as seaquake, its impact on submerged bodies, and the induced dynamic response in the structure. A FWT concept with catenary mooring is used for the assessment of the effects of earthquake shaking. The pressure during a seaquake is determined using a 2D acoustic finite element (FE) model in Abaqus. The acoustic model is benchmarked against a 1D analytical solution. The response due to the environmental loads, namely, wind, current, and waves, is also studied and used as a reference for assessment of the relative significance of the seaquake. Considerable vertical accelerations can occur in the nacelle due to amplification of the platform accelerations through the tower. It is shown that this acceleration could exceed a commonly used operational limit range of 0.2 g to 0.3 g even for moderate accelerations at the seabed. This indicates that earthquake loading should be considered in the design of FWTs in seismic regions. The mooring tensile forces, due to motion of the platform during a seaquake, do not exceed the design tension computed for the extreme environmental conditions. However, the leeward mooring lines could experience zero tension, which could cause snap tension.     

A simple frequency‐domain method for stress analysis of stall‐regulated wind turbines

A simple method, based in the frequency domain, was developed for calculating the dynamic response of a stall‐regulated wind turbine. Emphasis is placed on two aspects of the method, which are necessary in order to obtain a reasonable linearization of behavior when the blades are stalled. First, the tangential (in‐plane) component of turbulence is included, in addition to the axial component. Second, the linearized relationship between lift coefficient and angle‐of‐attack is adjusted to account for the effects of dynamic stall: separate linearizations are used for excitation and damping of vibration. A thorough comparison is made between linear and non‐linear dynamic‐stall methods, with the conclusion that the accuracy of the linear method depends upon the frequency and amplitude of oscillation. The linear dynamic‐stall method is accurate at blade vibrational frequencies, but it can be inaccurate at frequencies in the vicinity of 1P or below, when the angle‐of‐attack oscillates with an amplitude of 3° or more. Load spectra of a Nordtank 500 turbine, calculated using the frequency‐domain method, are compared with measurements. The frequency‐domain method provides estimates of load spectra and aerodynamic damping (stability) that are useful for preliminary design and optimization, but the method lacks sufficient accuracy and generality to be used for certification. Copyright © 2011 John Wiley & Sons, Ltd.     

Seismic fragility analysis of 5 MW offshore wind turbine

Considering nonlinear soil–pile interaction, seismic fragility analysis of offshore wind turbine was performed. Interface between ground soils and piles were modeled as nonlinear spring elements. Ground excitation time histories were applied to spring boundaries. Two methods of applying ground motion were compared. Different time histories from free field analysis were applied to each boundary in the first loading plan (A). They were compared with the second loading plan (B) in which the same ground motion is applied to all boundaries. Critical displacement for wind turbine was proposed by using push-over analysis. Both the stress based and the displacement based fragility curves were obtained using dynamic responses for different peak ground accelerations (PGAs). In numerical example, it was shown that seismic responses from loading plan A are bigger than from plan B. It seems that the bigger ground motion at surface can cause less response at wind turbine due to phase difference between ground motions at various soil layers. Finally, it can be concluded that layer by layer ground motions from free field analysis should be used in seismic design of offshore wind turbine.     

Implementation of a non‐linear foundation model for soil‐structure interaction analysis of offshore wind turbines in FAST

Bottom‐fixed offshore wind turbines (OWTs) involve a wide range of engineering fields. Of these, modelling of foundation flexibility has been given little priority. This paper investigates the modelling of bottom‐fixed OWTs in the non‐linear aero‐hydro‐servo‐elastic simulation tool FAST v7. The OWTs considered is supported on a monopile. The objective of this paper was to implement a non‐linear foundation model in this software. The National Renewable Energy Laboratory's idealized 5MW reference turbine was used as a base for the analyses. Default modelling of foundation in FAST v7 is by means of a rigid foundation. This implies that soil stiffness and damping is disregarded. Damping may lead to lower design loads. A softer foundation, on the other hand, will increase the natural periods of the system, shifting them closer to the frequencies of the environmental loads. This may in turn lead to amplified moments at the mudline. Therefore, it is important to include soil stiffness and damping in analyses. In this paper, a non‐linear foundation model is introduced in FAST v7 by means of uncoupled parallel springs. To verify that the implementation is successful, non‐linear load‐displacement curves of the foundation spring are presented. These show the typical hysteresis loops of an inelastic material, which confirms the implementation. Copyright © 2016 John Wiley & Sons, Ltd.     

Wave‐current interaction effects on structural responses of floating offshore wind turbines

This paper proposes a model considering the wave‐current interactions in dynamic analyses of floating offshore wind turbines (FOWTs) and investigates the interaction effects on the FOWT responses. Waves when traveling on current are affected by the current, leading to frequency shift and shape modification. To include such interactions in FOWT analysis, which has not been considered by the researchers till date, a nonlinear hydrodynamic model for multicable mooring systems is presented that is able to consider the cable geometric nonlinearity, seabed contact, and the current effect. The mooring model is then coupled with a spar‐type FOWT model that handles the structural dynamics of turbine blades and tower, aerodynamics of the wind‐blade interaction, and wave‐current effects on the spar. The analytical wave‐current interaction model based on Airy theory considering the current effect is used in the computation of flow velocity and acceleration. Numerical studies are then carried out based on the NREL offshore 5‐MW baseline wind turbine supported on top of the OC3‐Hywind spar buoy. Two cases, (1) when the currents are favorable and (2) when the currents are adverse, are examined. Differences of up to 15% have been observed by comparing the cable fairlead tension obtained excluding and including the wave‐current interactions. In particular, when irregular waves interact with adverse current, a simple superposition treatment of the wave and the current effects seems to underestimate the spar motion and the cable fairlead tension. This indicates that the wave‐current interaction is an important aspect and is needed to be considered in FOWT analysis.     

考虑塔架和叶片动力耦合的随机风载下风机结构动响应分析

考虑叶片和塔架的动力耦合作用,建立了5 MW风机整体结构的有限元模型,计算其在随机风速下的动响应。为分析叶片和塔架的动力耦合对风机结构动响应的影响,计算比较了刚性支撑的叶片、简化的风机和整体风机3种模型在风载下的动响应位移和应力。计算结果表明:由于叶片和塔架的耦合作用,叶片的位移响应最大增加约20%,但是塔架的位移响应最大降低了约60%。文章还分析了叶片旋转过程中方位角对塔架位移响应的影响。在叶片的一个旋转周期内,塔架的响应幅值会有较大的波动,最大响应幅值约为最小响应幅值的3倍。     

Ultimate load design of jacket‐type offshore wind turbines under tropical cyclones

Tropical cyclones are a high risk to offshore wind turbine (OWT) support structures, so the design conditions, including this risk, are necessary for tropical cyclone frequent occurrence zones. This study developed a computer program to carry out a critical ultimate load analysis and determine the optimum design for a Jacket‐type OWT support structure. The total weight of the OWT support structure after the optimal steel design with the yaw operative condition is always considerably smaller than that without for the steel design results under the loads of the GL Tropical Cyclone Technical Note (GL TCTN). This paper studies OWTs under the tropical cyclone classes 1 to 3 and the terrain categories A, to C, where the 1‐minute wind speed at 10‐m height is gradually increased from classes 1 to 3, and the surface roughness decreases from A to C. When the yaw can operate, the total steel weight consumption due to the tropical cyclone 1C, 1B, and 2C loads is lower than that for the IEC 61400‐3 loads. In the case of 1A, the overall steel consumption is only slightly higher than that of the IEC 61400‐3. However, for other conditions, the design should include the GL TCTN loads. For the yaw inoperative condition, the GL TCTN results are always largely dominant in the steel design, so the use of only the IEC 61400‐3 condition will result in extremely high risk to OWT support structures.     

Development of an aeroelastic code based on three‐dimensional viscous–inviscid method for wind turbine computations

Aerodynamic and structural dynamic performance analysis of modern wind turbines are routinely estimated in the wind energy field using computational tools known as aeroelastic codes. Most aeroelastic codes use the blade element momentum (BEM) technique to model the rotor aerodynamics and a modal, multi‐body or the finite‐element approach to model the turbine structural dynamics. The present work describes the development of a novel aeroelastic code that combines a three‐dimensional viscous–inviscid interactive method, method for interactive rotor aerodynamic simulations (MIRAS), with the structural dynamics model used in the aeroelastic code FLEX5. The new code, called MIRAS‐FLEX, is an improvement on standard aeroelastic codes because it uses a more advanced aerodynamic model than BEM. With the new aeroelastic code, more physical aerodynamic predictions than BEM can be obtained as BEM uses empirical relations, such as tip loss corrections, to determine the flow around a rotor. Although more costly than BEM, a small cluster is sufficient to run MIRAS‐FLEX in a fast and easy way. MIRAS‐FLEX is compared against the widely used FLEX5 and FAST, as well as the participant codes from the Offshore Code Comparison Collaboration Project. Simulation tests consist of steady wind inflow conditions with different combinations of yaw error, wind shear, tower shadow and turbine‐elastic modeling. Turbulent inflow created by using a Mann box is also considered. MIRAS‐FLEX results, such as blade tip deflections and root‐bending moments, are generally in good agreement with the other codes. Copyright © 2017 John Wiley & Sons, Ltd.     

An improved BEM model for the power curve prediction of stall‐regulated wind turbines

Blade element momentum (BEM) theory is the standard computational technique for the prediction of power curves of wind turbines; it is based on the two‐dimensional aerodynamic properties of aerofoil blade elements and some corrections accounting for three‐dimensional wing aerodynamics. Although most BEM models yield acceptable results for low‐wind and pitch‐controlled regimes where the local angles of attack are small, no generally accepted model exists up to date that consistently predicts the power curve in the stall regime for a variety of blade properties and operating conditions. In this article we present a modified BEM model which satisfactorily reproduces the power curves of four experimental wind turbines reported in the literature, using no free fit parameters. Since these four experimental cases comprehend a great variety of conditions (wind tunnel vs field experiments, different air densities) and blade parameters (no twist and no taper, no taper but twist, both twist and taper, different aerofoil families), it is believed that our model represents a useful working tool for the aerodynamic design of stall‐regulated wind turbines. Copyright © 2004 John Wiley & Sons, Ltd.     

Hybrid vortex simulations of wind turbines using a three‐dimensional viscous–inviscid panel method

A hybrid filament‐mesh vortex method is proposed and validated to predict the aerodynamic performance of wind turbine rotors and to simulate the resulting wake. Its novelty consists of using a hybrid method to accurately simulate the wake downstream of the wind turbine while reducing the computational time used by the method. The proposed method uses a hybrid approach, where the near wake is resolved by using vortex filaments, which carry the vorticity shed by the trailing edge of the blades. The interaction of the vortex filaments in the near vicinity of the wind turbine is evaluated using a direct calculation, whereas the contribution from the large downstream wake is calculated using a mesh‐based method. The hybrid method is first validated in detail against the well‐known MEXICO experiment, using the direct filament method as a comparison. The second part of the validation includes a study of the influence of the time‐integration scheme used for evolving the wake in time, aeroelastic simulations of the National Renewable Energy Laboratory 5 MW wind turbine and an analysis of the central processing unit time showing the gains of using the hybrid filament‐mesh method. Copyright © 2017 John Wiley & Sons, Ltd.     

Effect of rotor–tower interaction,tilt angle,and yaw misalignment on the aeroelasticity of a large horizontal axis wind turbine with composite blades

This paper presents a detailed analysis of the rotor–tower interaction and the effects of the rotor's tilt angle and yaw misalignment on a large horizontal axis wind turbine. A high‐fidelity aeroelastic model is employed, coupling computational fluid dynamics (CFD) and structural mechanics (CSM). The wind velocity stratification induced by the atmospheric boundary layer (ABL) is modeled. On the CSM side, the complex composite structure of each blade is accurately modeled using shell elements. The rotor–tower interaction is analyzed by comparing results of a rotor‐only simulation and a full‐machine simulation, observing a sudden drop in loads, deformations, and power production of each blade, when passing in front of the tower. Subsequently, a tilt angle is introduced on the rotor, and its effect on blade displacements, loads, and performance is studied, representing a novelty with respect to the available literature. The tilt angle leads to a different contribution of gravity to the blade deformations, sensibly affecting the stresses in the composite material. Lastly, a yaw misalignment is introduced with respect to the incoming wind, and the resulting changes in the blade solicitations are analyzed. In particular, a reduction of the blade axial displacement amplitude during each revolution is observed.     

基于有限元法的漂浮式风力机Spar平台动态响应分析

以美国可再生能源实验室(NREL)的5 MW漂浮式风力机和Spar平台为参考模型,采用有限元分析法分析了Spar平台的波浪载荷频域特性,包括对振幅响应算子、绕射力和F-K力频域动态响应分析,并对比了三种波浪谱(P-M谱、JONSWAP谱和Wen’s谱)海况下的时域响应.结果表明:Spar平台载荷峰值出现在低频波浪作用下,且响应明显,而随着频率的增大,其值逐渐减小;在P-M谱和Wen’s谱作用下的平台动态响应无明显差异,且两者的振幅和周期大致相同,而在JONSWAP谱作用下的平台动态响应最小,但其往复周期小,频率大,使得系泊系统承受较大的周期性张力.     

离网型风力发电机塔架振动问题的模态分析

对离网型风力发电机模型的塔架振动进行了分析，运用瑞雷法计算其基频，以有限元模态分析和试验测量的方法计算分析出塔架的固有频率和振型。根据分析结果讨论有限元建模的合理性，分析引起振动的原因，并依此提出塔架较合理的锥形管结构改进方案。     

Life cycle energy and greenhouse emissions analysis of wind turbines and the effect of size on energy yield

Wind turbines, used to generate renewable energy, are typically considered to take only a number of months to produce as much energy as is required in their manufacture and operation. With a life expectancy of upwards of 20 years, the energy produced by wind turbines over their life can be many times greater than that embodied in their production. Many previous life cycle energy studies of wind turbines are based on methods of assessment now known to be incomplete. These studies may underestimate the energy embodied in wind turbines by more than 50%, potentially overestimating the energy yield of those systems and possibly affecting the comparison of energy generation options. With the increasing trend towards larger scale wind turbines, comes a respective increase in the energy required for their manufacture. It is important to consider whether or not these increases in wind turbine size, and thus embodied energy, can be adequately justified by equivalent increases in the energy yield of such systems. This paper presents the results of a life cycle energy and greenhouse emissions analysis of two wind turbines and considers the effect of wind turbine size on energy yield. The issue of incompleteness associated with many past life cycle energy studies is also addressed. Energy yield ratios of 21 and 23 were found for a small and large scale wind turbine, respectively. The embodied energy component was found to be more significant than in previous studies, emphasised here due to the innovative use of a hybrid embodied energy analysis approach. The life cycle energy requirements were shown to be offset by the energy produced within the first 12 months of operation. The size of wind turbines appears to not be an important factor in optimising their life cycle energy performance.     

一种考虑海上风电支撑结构非零初始条件的动力响应计算方法

提出一种能够考虑海上风电支撑结构非零初始条件的动力响应计算方法。该方法创新之处在于将波浪等环境荷载用一系列极值及留数替代,从而可在Laplace域与非零初始条件影响项进行合并。分别采用海上风电单桩式和导管架式支撑结构进行验证。结果表明:除起始短暂时刻外,所提出新方法与传统时域方法计算结果基本一致。