海上风机单桩基础桩形影响因素分析

  [目的]  随着大直径单桩基础建造与施工能力的日益进步，单桩基础型式的应用正向更大水深发展，需要研究深水海域单桩基础不同桩形的优缺点，为同类工程提供参考。  [方法]  按高中低三种锥段位置，对广东阳江某海上风电场的单桩基础机位参数进行分组统计，分析其中的桩形选用规律。  [结果]  分析表明:高锥段大桩径设计会导致风机疲劳荷载偏高，机位浅层土中砂土和黏土的比例及具体力学指标决定了最佳桩形的锥段位置。  [结论]  通过对比研究:发现降低锥段是降低大桩径机位疲劳荷载的最佳方法，砂土含量高且内摩擦角较大的机位可通过提高锥段减小桩径，而黏土含量高地质偏软的机位则适用较低锥段的桩形。提出用单桩最优解的泥面刚度矩阵行列式大小来衡量机位的地基刚度水平，并分析了地基刚度水平与基础钢料量的关系。     

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.     

风冰联合作用下大型单桩海上风电机组动力特性

考虑风荷载与冰荷载联合作用对大型单桩海上风电机组的影响,基于IEA 15 MW超大型单桩海上风电机组,采用一体化分析软件Openfast建立风冰联合作用下大型单桩耦合数值模型,开展超大型单桩海上风电机组在风冰联合作用下的动力响应分析。探究不同加载时长、冰激振动模型以及疲劳损伤组合方法对大型单桩海上风电机组的动力响应规律。计算结果显示：不同冰载数值计算模型塔基与泥面线载荷的计算结果差别较大,泥面线受冰荷载影响较大,同时泥面线位置较塔基位置承受更大的疲劳损伤,应重点关注。采用不同的荷载组合方向进行泥面线与塔基位置的疲劳损伤估计时,计算结果较风冰联合作用下疲劳损伤相对误差较大。因此,宜采用风冰联合加载的方法进行大型单桩海上风电机组的动力响应模拟,进而开展超大型单桩海上风电机组的疲劳损伤估计。     

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.     

海上风机单桩基础疲劳影响因素分析

  [目的]  为提高评估海上风机单桩基础疲劳寿命准确性,重点探讨了影响海上风机单桩基础疲劳寿命计算的影响因子。  [方法]  首先利用简化梁单元模型与局部有限元模型模拟了某工程的单桩基础结构,其次结合波浪谱疲劳方法对单桩基础由波浪载荷造成的疲劳强度进行了评估,最后分别计算并讨论了传递函数、波浪理论和局部开孔的结构形式对风机基础疲劳损伤的影响。  [结果]  分析结果表明:传递函数和局部开孔的结构形式对钢管桩结构的疲劳强度影响显著。  [结论]  所提计算方法和分析结果是正确并有效的,可为实际应用提供指导。     

A study of modal damping for offshore wind turbines considering soil properties and foundation types

The modal damping ratio for each mode is crucial to characterize the dynamic behavior of offshore wind turbines and widely used by simulation software in wind turbine engineering, such as Bladed and FAST. In this study, modal damping ratios of offshore wind turbines are systematically studied for different soil properties and foundation types. Firstly, the modal damping ratios and modal frequencies for the first and second modes of a gravity foundation–supported offshore wind turbine are studied. An offshore wind turbine supported by a monopile foundation is then investigated to clarify the characteristics of modal damping ratios and modal frequencies for the monopile foundation. The soil parameters are identified by means of genetic algorithm (GA). Predicted modal damping ratios and modal frequencies as well as modal shapes show good agreement with the field measurements for both foundations. Finally, a sensitivity analysis study is carried out to investigate the effects of soil properties and foundation types on modal damping ratios. For the gravity foundation–supported offshore wind turbine, soil properties affect the modal damping ratio of the second mode largely, but affect that of the first mode little, while for the monopile‐supported offshore wind turbine, soil properties affect the modal damping ratios of the first and second modes significantly. Predicted natural periods and modal damping ratios of the first mode for both foundations by a pair of simple models agree well with those by numerical models.     

Load case reduction for offshore wind turbine support structure fatigue assessment by importance sampling with two‐stage filtering

A complete fatigue assessment for operational conditions for offshore wind turbines involves simulating thousands of environmental states. For applications such as optimization, where this assessment needs to be repeated many times, that presents a significant computational problem. Here, we propose a novel way of reducing the number of simulated environmental states (load cases) while maintaining an acceptable accuracy. From one full fatigue analysis of a base design, the OC3 monopile (with the NREL 5MW turbine), the distribution of fatigue damage per load case can be used to estimate the lifetime fatigue damage of a range of modified designs. Using importance sampling and a specially adapted two‐stage filtering procedure, we obtain pseudo‐optimal sets of load cases from which the fatigue damage is estimated. This is applied to seven different designs that have been modified to emulate iterations of an optimization loop. For several of these designs, sampling less than 1% of all load cases can give damage estimates with median errors of less than 2%. Even for the most severe cases, using 3% of the environmental states yields a maximum error of 10%. While further refinement is possible, the method is considered viable for applications within design optimization and preliminary design.     

强非线性波作用下海上风机动力特性模型试验研究

目的  单桩式基础是目前近海海上风电场应用最广泛的支撑结构。中国海洋环境多为浅水和中等水深,受到非线性波浪影响较为明显。与行进波相比,聚焦波可以在短时间内形成一个作用在桩柱上、比常规波浪力大的冲击力,从而影响海上风机的运行性能和疲劳寿命。文章旨在探究强非线性波浪作用下海上风机单桩基础荷载特性,掌握聚焦波与基础相互作用规律,为工程设计提供参考。 方法  文章采用水池模型试验的方法对NREL 5 MW单桩海上风机开展研究,缩尺比例为1∶80。结合我国东部沿海风电场的海洋环境条件,选取了三种典型聚焦波模型,通过浪高仪器和测力天平分别记录了不同工况下单桩周围波浪爬高和底部受力变化情况。 结果  结果表明：桩基所受的水平波浪力具有很强的瞬态性;单桩基础所受水平波浪力在聚焦波的作用下会突然增大,在波谷会受到反向冲击力。 结论  文章揭示了聚焦波引起的海上风机单桩基础荷载变化规律,证实了非线性波浪对于风机基础动力特性的重要影响,相关成果具有较高的理论和工程应用价值。     

海上风机塔架和单桩一体化试验设计方法

  目的  国内海上风电项目的业主在招主机标时,对风机厂商提供的基础顶载荷和塔架质量提出要求,在招设计标时很少对设计院提供的基础质量提出要求;详细设计阶段时风机厂商和设计院分别对塔架和基础进行设计,因此通常难以得到塔架和基础总质量最小的全局最优设计。  方法  提出了海上风机塔架和单桩一体化试验设计方法,通过对某海上风电项目进行试验设计得到多个不同塔架构型、塔底和单桩直径的设计方案,并通过载荷计算、塔架和单桩设计迭代优化得到相应的基础顶载荷、塔架质量和单桩质量等结果。  结果  研究表明:基础顶载荷或塔架质量最小的方案不是塔架和单桩总质量最小的方案。通过试验设计方法可以给出单桩和塔架总质量最小的最优设计。  结论  研究成果表明,在设计中采用一体化试验设计方法寻找塔架和单桩总质量最小的全局最优设计是降低海上风电度电成本的有效方法。业主将制定塔架和基础总造价最小的招标规则,要求采用一体化试验设计方案找到全局最优设计。     

Fatigue assessment of offshore wind turbines on monopile foundations using multi‐band modal expansion

Offshore wind turbines (OWTs) are subjected to both quasi‐static loads originating from variations in the thrust force and dynamic loads linked to turbulence, waves and turbine dynamics. Both types of loads contribute to fatigue life progression and thus define the turbine's age. As a structural health monitoring solution, one could thus directly measure the stress history at fatigue critical locations. However, for OWTs on monopile foundations some fatigue critical locations are located below the seabed. Installing strain sensors at these hotspots is therefore impossible for existing wind turbines. This measurement restriction is overcome by reconstructing the full‐field response of the structure based on the limited number of accelerometers and strain sensors (installed at a few easily accessible locations) and a calibrated finite element model of the system. The system model uses a multi‐band modal expansion approach constituted of the quasi‐static and dynamic contributions. These contributions are superimposed to reconstruct the stress history at all degrees of freedom of the finite element model, and the subsequent assess fatigue life consumption at all fatigue hot spots of the OWT. In this paper, the proposed virtual sensing technique is validated by predicting the stresses in the transition piece with 12 days of consecutive measurements from an operational OWT. The data set contains both variations in environmental and operating conditions as well as extreme events. Finally, a full‐field strain assessment in the tower and foundation system of the OWT is demonstrated. Copyright © 2017 John Wiley & Sons, Ltd.     

Natural frequency degradation and permanent accumulated rotation for offshore wind turbine monopiles in clay

Offshore wind turbine (OWT) monopile foundations are subjected to cyclic loading from wind, waves, and operational loads from rotating blades. Lateral monopile capacity can be significantly affected by cyclic loading, causing failure at cyclic load amplitudes lower than the failure load under monotonic loading. For monopiles in clay, undrained clay behavior under short-term cyclic soil-pile loading (e.g. extreme storm conditions) typically includes plastic soil deformation resulting from reductions in soil modulus and undrained shear strength which occur as a function of pore pressure build-up. These impacts affect the assessment of the ultimate and serviceability limit states of OWTs via natural frequency degradation and accumulated permanent rotation at the mudline, respectively. This paper introduced novel combinations of existing p-y curve design methods and compared the impact of short-term cyclic loading on monopiles in soft, medium, and stiff clay. The results of this paper indicate that short-term cyclic loading from extreme storm conditions are unlikely to significantly affect natural frequency and permanent accumulated rotation for OWT monopiles in stiff clays, but monopiles in soft clay may experience significant degradation. Further consideration is required for medium clays, as load magnitude played a strong role in both natural frequency and permanent rotation estimation.     

Post‐installation adaptation of offshore wind turbine controls

The cost of offshore wind energy can be reduced by incorporating control strategies to reduce the support structures' load effects into the structural design process. While effective in reducing the cost of support structures, load‐reducing controls produce potentially costly side effects in other wind turbine components and subsystems. This paper proposes a methodology to mitigate these side effects at the wind farm level. The interaction between the foundation and the surrounding soil is a major source of uncertainty in estimating the safety margins of support structures. The safety margins are generally closely correlated with the modal properties (natural frequencies, damping ratios). This admits the possibility of using modal identification techniques to reassess the structural safety after installing and commissioning the wind farm. Since design standards require conservative design margins, the post‐installation safety assessment is likely to reveal better than expected structural safety performance. Thus, if load‐reducing controls have been adopted in the structural design process, it is likely permissible to reduce the use of these during actual operation. Here, the probabilistic outcome of such a two‐stage controls adaptation is analyzed. The analysis considers the structural design of a 10 MW monopile offshore wind turbine under uncertainty in the site‐specific soil conditions. Two control strategies are considered in separate analyses: (a) tower feedback control to increase the support structure's fatigue life and (b) peak shaving to increase the support structure's serviceability capacity. The results show that a post‐installation adaptation can reduce the farm‐level side‐effects of load‐reducing controls by up to an order of magnitude.     

10 MW大型单桩式海上风机桩土作用研究

目的  世界范围内已经建成的海上风场大部分位于浅水区域（水深<30 m）,主要以单桩等固定式基础为主。随着风电技术的不断成熟,海上风电逐渐走向机组大型化趋势,而单桩海上风机的基础直径也将随着机组大型化而增加。其所受到的环境载荷和土质条件要求也愈加严苛,对于大直径单桩式海上风机的桩土相互作用的研究成为海上风电技术的关键技术问题之一。方法  拟对浅水水域10 MW大型海上风机,研究不同桩土模型对大型单桩海上风机的动力响应的影响。结果  结果表明,宏单元法考虑了非线性的刚度与塑性,在特征频率附近的功率谱密度总和大,对比其他传统桩土模型时有很大的优势。结论  所做研究对风机的整体安全运行具有深远的理论价值与工程应用前景。     

De‐trending of wind speed variance based on first‐order and second‐order statistical moments only

The lack of efficient methods for de‐trending of wind speed resource data may lead to erroneous wind turbine fatigue and ultimate load predictions. The present paper presents two models, which quantify the effect of an assumed linear trend on wind speed standard deviations as based on available statistical data only. The first model is a pure time series analysis approach, which quantifies the effect of non‐stationary characteristics of ensemble mean wind speeds on the estimated wind speed standard deviations as based on mean wind speed statistics only. This model is applicable to statistics of arbitrary types of time series. The second model uses the full set of information and includes thus additionally observed wind speed standard deviations to estimate the effect of ensemble mean non‐stationarities on wind speed standard deviations. This model takes advantage of a simple physical relationship between first‐order and second‐order statistical moments of wind speeds in the atmospheric boundary layer and is therefore dedicated to wind speed time series but is not applicable to time series in general. The capabilities of the proposed models are discussed by comparing model predictions with conventionally de‐trended characteristics of measured wind speeds using data where high sampled time series are available, and a traditional de‐trending procedure therefore can be applied. This analysis shows that the second model performs significantly better than the first model, and thus in turn that the model constraint, introduced by the physical link between the first and second statistical moments, proves very efficient in the present context. © 2013 The Authors. Wind Energy Published by John Wiley & Sons Ltd.     

Effective turbulence and its implications in wind turbine fatigue assessment

The effective turbulence approximation is widely used in the wind energy industry for site‐specific fatigue assessment of wind turbines with reference to loads. It significantly reduces the amount of aero‐elastic simulations required to document structural integrity by integrating out the directional variation of turbulence. Deriving the effective turbulence involves assumptions related to load effect histories, structural dynamics, and material fatigue strength. These assumptions may lead to low accuracy of fatigue load assessments by the effective turbulence compared with full directional simulations. This paper quantifies the implications of the effective turbulence for a multimegawatt wind turbine during normal operation. Analyses based on wind measurements from almost one hundred international sites document that the effective turbulence provides accurate results compared with full sector‐wise simulations, but only when linear SN ‐curves are assumed. For a more advanced steel tower design approach using a bilinear SN ‐curve, a reduction of the cross‐sectional design parameters by almost 10% is achieved. Additional 10% reduction can be obtained if fatigue damage is estimated utilizing the wind direction information. By applying a probabilistic approach, it is shown that this reduction in the design parameter of the steel tower does not compromise the structural integrity when the current IEC 61400‐1 standard is followed. The results presented may improve decision making in site‐specific fatigue assessments of wind turbines and prevent overconservative design, which results from the use of the effective turbulence, and thereby reduce the cost of wind energy.     

不同海况下近海超大型风力机动力学响应及结构损伤分析

以超大型DTU 10 MW单桩式近海风力机为研究对象,通过p-y曲线和非线性弹簧建立桩-土耦合模型,选取Kaimal风谱模型建立湍流风场,基于P-M谱定义不同频率波浪分布,并利用辐射/绕射理论计算波浪载荷,采用有限元方法对不同海况下单桩式风力机进行动力学响应、疲劳及屈曲分析。结果表明：不同海况波浪载荷作用下塔顶位移响应及等效应力峰值远小于风及风浪联合作用,其中风浪联合作用下风力机塔顶位移响应及等效应力略小于风载荷;波浪载荷对风载荷引起的单桩式风力机动力学响应具有一定抑制作用,此外相较于波浪载荷,风载荷为控制载荷;风载荷与风浪联合作用下风力机等效应力峰值位于塔顶与机舱连接处,波浪载荷风力机等效应力峰值位于支撑结构与桩基连接处;仅以风载荷预估风力机塔架疲劳寿命将导致预估不足;随着波浪载荷的增大,风力机失稳风险加大,波浪载荷不可忽略;不同海况下,风浪联合作用局部屈曲区域位于塔架中下端,在风力机抗风浪设计时,应重点关注此处;变桨效应可大幅降低风力机动力学响应、疲劳损伤及发生屈曲的风险。     

Design clustering of offshore wind turbines using probabilistic fatigue load estimation

In large offshore wind farms fatigue loads on support structures can vary significantly due to differences and uncertainties in site conditions, making it necessary to optimize design clustering. An efficient probabilistic fatigue load estimation method for monopile foundations was implemented using Monte-Carlo simulations. Verification of frequency domain analysis for wave loads and scaling approaches for wind loads with time domain aero-elastic simulations lead to 95% accuracy on equivalent bending moments at mudline and tower bottom. The computational speed is in the order of 100 times faster than typical time domain tools. The model is applied to calculate location specific fatigue loads that can be used in deterministic and probabilistic design clustering. Results for an example wind farm with 150 turbines in 30–40 m water depth show a maximum load difference of 25%. Smart clustering using discrete optimization algorithms leads to a design load reduction of up to 13% compared to designs based on only the highest loaded turbine position. The proposed tool improves industry-standard clustering and provides a basis for design optimization and uncertainty analysis in large wind farms.     

AK‐DA: An efficient method for the fatigue assessment of wind turbine structures

Lifetime damage estimation is a complex and demanding task that needs to be performed during the design of offshore wind turbine structures. A general damage analysis framework is proposed by the certification bodies. Therein the total lifetime of the structure is considered as a series of elementary situations combining structural and environmental states. For a given structural state, the loading environment is described using statistical parameters such as the wind mean speed at hub height or the peak spectral period of the sea. An estimation of the structural response is to be computed for each of the environmental combinations of parameters, therefore leading to tens of thousands simulations. The cost of a single simulation makes this process often unfeasible for engineers who are usually forced to reduce the number of simulations considering industrial feedback with risks of potential lack of representativity of results. This paper aims at presenting a novel method for the reduction of the simulation costs relative to the long‐term damage estimation (relative to a design load case) and based on the so‐called adaptive Kriging approach. From on a reduced set of observations (multiphysics simulator runs), a Kriging metamodel is here used to approximate the damage model response for all the nonsimulated sets of environmental parameters. The latter are subsequently used to assess the long‐term damage as presented in the standards. The statistical measure of the metamodel error of prediction is used into an iterative structure in order to progressively enrich the design of experiments with informative sets of environmental parameters. This allows us minimizing the global uncertainty of the approximation. The proposed algorithm, hereafter called the AK‐DA for “Adaptive Kriging Damage Assessment,” is illustrated with two industrial cases of fatigue analyses for the NREL 5MW reference monopile structure and its direct application     

海上风电单桩基础地基加固技术研究

目的  在以“双碳”为目标的能源政策导向下,海上风力发电作为一种新型的能源技术,得到了广泛的应用,并且在中国迅速发展。单桩基础是海上风电中最经济、应用最广泛的基础。为解决单桩基础的变形及冲刷保护问题,提出了一种地基加固技术,并对该加固技术进行研究分析,给出加固方法的依据。 方法  针对桩基地基加固技术进行方法研究论证,通过针对加固技术路线及工艺研究,选取一种适合于海上施工的地基加固技术,并采用数值分析方法针对加固效果及加固影响范围进行详细分析。 结果  结果表明：在1.15倍荷载下,采用地基加固的单桩基础的水平承载力提升15%以上,通过地基加固可以适用于更大容量的机型。本分析案例中,当地基加固深度、水平范围都达到8 m时,加固深度方向范围的土体比加固平面方向范围的土体获得的收益更大。 结论  利用水泥土加固的技术方案经数值分析验证可靠,对单桩承载能力提升显著,且加固范围合理经济,可为后续海上风电实际工程单桩地基加固技术的应用提供借鉴。     

破碎波作用下单桩式海上风机水动力学数值分析

  目的  我国南汇、东海、大丰等海上风电场的设计波浪十分接近波浪破碎极限,需要探究浅水水域单桩式海上风机基础受破碎波载荷的动力响应。  方法  利用FAST耦合分析软件,考虑使用波浪拉伸的方式计算波浪力载荷,对单桩式风机在东海海况下的耦合动力响应特性进行分析。  结果  分析结果表明:使用波浪拉伸方式计算破碎波载荷更加接近实际海上风机所受到的载荷,且明显高于不使用拉伸方式计算的破碎波载荷。破碎波更容易激发塔筒的一阶固有频率,风浪联合作用下风机结构动力响应更加显著。  结论  研究成果为我国浅水非线性波浪区域单桩式海上风电的开发提供了一定的参考。