Zonal Coupling Analysis Method of Seismic Response of Offshore Monopile Wind Turbine

The seismic safety of offshore wind turbines is an important issue that needs to be solved urgently.Based on a unified computing framework,this paper develops a set of seawater-seabed-wind turbine zoning coupling analysis methods.A 5 MW wind turbine and a site analysis model are established,and a seismic wave is selected to analyze the changes in the seismic response of offshore monopile wind turbines under the change of seawater depth,seabed wave velocity and seismic wave incidence angle.The analysis results show that when the seawater increases to a certain depth,the seismic response of the wind turbine increases.The shear wave velocity of the seabed affects the bending moment and displacement at the bottom of the tower.When the angle of incidence increases,the vertical displacement and the acceleration of the top of the tower increase in varying degrees.

Comparison of Pile-Soil-Structure Interaction Modeling Techniques for A 10-MW Large-Scale Monopile Wind Turbine Model Under Wind and Wave Conditions

Considering the large diameter effect of piles,the influence of different pile-soil analysis methods on the design of monopile foundations for offshore wind turbines has become an urgent problem to be solved.Three different pile-soil models were used to study a large 10 MW monopile wind turbine.By modeling the three models in the SACS software,this paper analyzed the motion response of the overall structure under the conditions of wind and waves.According to the given working conditions,this paper concludes that under the condition of independent wind,the average value of the tower top x-displacement of the rigid connection method is the smalle st,and the standard deviation is the smallest under the condition of independent wave.The results obtained by the p-y curve method are the most conservative.

Assessment of natural frequency of installed offshore wind turbines using nonlinear finite element model considering soil-monopile interaction

A nonlinear finite element model is developed to examine the lateral behaviors of monopiles, which support offshore wind turbines(OWTs) chosen from five different offshore wind farms in Europe. The simulation is using this model to accurately estimate the natural frequency of these slender structures, as a function of the interaction of the foundations with the subsoil. After a brief introduction to the wind power energy as a reliable alternative in comparison to fossil fuel, the paper focuses on concept of natural frequency as a primary indicator in designing the foundations of OWTs. Then the range of natural frequencies is provided for a safe design purpose. Next, an analytical expression of an OWT natural frequency is presented as a function of soil-monopile interaction through monopile head springs characterized by lateral stiffness KL, rotational stiffness KRand cross-coupling stiffness KLRof which the differences are discussed. The nonlinear pseudo three-dimensional finite element vertical slices model has been used to analyze the lateral behaviors of monopiles supporting the OWTs of different wind farm sites considered. Through the monopiles head movements(displacements and rotations), the values of KL, KRand KLRwere obtained and substituted in the analytical expression of natural frequency for comparison. The comparison results between computed and measured natural frequencies showed an excellent agreement for most cases. This confirms the convenience of the finite element model used for the accurate estimation of the monopile head stiffness.

Monopile responses to monotonic and cyclic loading in undrained sand using 3D FE with SANISAND-MSu

Monopile response under undrained conditions in sand is gaining increasing interests owing to the recent development of offshore wind farms in seismic regions.Pore pressure evolution in liquefiable soil can significantly reduce the strength and stiffness of the soil which in turn affects the structural dynamic response.Several numerical models have been developed in the last two decades to enhance understanding of the mechanism of monopile-soil interaction with the existence of pore water pressure.In this study,the effects of geometry and static vertical load on monopile lateral response were studied using three-dimensional finite element methods that consider the existence of lateral cyclic load-induced pore water pressure.To achieve reliable simulation results of pore pressure development and pile displacement accumulation during cyclic loading,the simple anisotropic sand model with memory surface for undrained cyclic behavior of sand was adopted.For piles with the same diameter,a accumulated pile head displacement during lateral cyclic loading decreased linearly with increasing pile embedded length but increased with increasing eccentricity.Static vertical load had minor effects on pile cyclic lateral response.The distributions of mean effective stress and pore water pressure in the soil domain were presented.The pile reaction curve(cyclic soil reaction against pile defection)of the monopile was extracted.The numerical results aim to provide reference for optimized engineering design procedures.

Experimental Study on the Cyclic Behavior of Monopiles in Fine Sandy Beds Under Regular Waves

This paper presents an experimental study on the wave-induced behavior of monopiles. Laboratory experiments were conducted at the constant initial state of the sandy beds in a wave flume with a soil trench. The responses of the pile- head displacement, the pile strain and the pore water pressure on regular waves were investigated. The experimental results show that the monopiles lean along the direction of the wave progression and the inclination increases with the duration of wave actions. The pile-head displacement （consisting of the permanent displacement and cyclic displacement） increases as the wave height increases, especially more significantly for the permanent displacement. The head-fixed pile suffers from larger wave load than that on the head-free pile under the same wave condition. Increasing pile diameter or fixing fins on the monopile is effective in reducing the pore water pressure in the upper part of the bed and the permanent displacement.

Local scour at offshore windfarm monopile foundations: A review

In this article,current research findings of local scour at offshore windfarm monopile foundations are presented.The scour mechanisms and scour depth prediction formulas under different hydrodynamic conditions are summarized,including the current-only condition,wave-only condition,combined wave-current condition,and complex dynamic condition.Furthermore,this article analyzes the influencing factors on the basis of classical equations for predicting the equilibrium scour depth under specific conditions.The weakness of existing researches and future prospects are also discussed.It is suggested that future research shall focus on physical experiments under unsteady tidal currents or other complex loadings.The computational fluid dynamics-discrete element method and artificial intelligence technique are suggested being adopted to study the scour at offshore windfarm foundations.

Cyclic Lateral Responses of Monopiles Considering the Influence of Pile−Soil Relative Stiffness in Sand

The existing studies have primarily focused on the effect of cyclic load characteristics(namely,cyclic load ratio and amplitude ratio)on cyclic lateral response of monopiles in sand,with little attention paid to the effect of pile−soil relative stiffness(K_(R)).This paper presents a series of 1-g cyclic tests aimed at improving understanding of the cyclic lateral responses of monopiles under different pile−soil systems.These systems are arranged by two model piles with different stiffness,including four different slenderness ratios(pile embedded length,L,normalized by diameter,D)under medium dense sand.The K_(R)-values are calculated by a previously proposed method considering the real soil stress level.The test results show that the lateral accumulation displacement increases significantly with the increment of the K_(R)-value,while the cyclic secant stiffness performs inversely.The maximum pile bending moment increases with the cycle number for the rigid pile−soil system,but shows a decreasing trend in the flexible system.For an uppermost concern,an empirical model is proposed to predict the accumulated displacement of arbitrary pile−soil systems by combining the results from this study with those from previous experimental investigations.The validity of the proposed model is demonstrated by 1-g and centrifuge tests.

Simplified Design Procedure of Monopile Foundation for Offshore Wind Turbine in Gujarat, India

OWTs （offshore wind turbines） are currently considered as a reliable source of renewable energy. OWT support structures account for 20%-25% of the capital cost for offshore wind installations. Pre-feasibility studies involving estimation of preliminary dimensions of the wind turbine structure need to be performed for initial costing to arrive at the commercial viability of the project. The main objective of the paper is to obtain preliminary configuration for commercial viability and approximate sizing of the foundation pile. Design equations and nomograms are proposed for quick preliminary design of monopile founded wind turbines located offshore of Gujarat. Parametric studies are carried-out on various configurations of a hollow monopile by varying water depths and properties of sand. A nonlinear static analysis of substructure is performed considering aerodynamic forces and hydrodynamic forces for various structural and soil parameters. The sub-structure design of wind turbine is based on API （American petroleum institute） standards. A simplified design methodology for monopile support structure under extreme loading condition is presented based on multivariable linear regression analysis. The input variables for the regression analysis are hydrodynamic data, angle of internal friction of sand, and the output variables are length and outer diameter of monopile. This simplified methodology is applicable in pre-studies of wind power parks.

Comparative Experimental and Numerical Study of Wave Loads on A Monopile Structure Using Different Turbulence Models

This study numerically and experimentally investigates the effects of wave loads on a monopile-type offshore wind turbine placed on a 1:25 slope at different water depths as well as the effect of choosing different turbulence models on the efficiency of the numerical model.The numerical model adopts a two-phase flow by solving Unsteady Reynolds-Averaged Navier−Stokes(URANS)equations using the Volume Of Fluid(VOF)method and three differentk-ωturbulence models.Typical environmental conditions from the East China Sea are studied.The wave run-up and the wave loads applied on the monopile are investigated and compared with relevant experimental data as well as with mathematical predictions based on relevant theories.The numerical model is well validated against the experimental data at model scale.The use of different turbulence models results in different predictions on the wave height but less differences on the wave period.The baseline k-ωturbulence model and Shear-Stress Transport(SST)k-ωturbulence model exhibit better performance on the prediction of hydrodynamic load,at a model-scale water depth of 0.42 m,while the laminar model provides better results for large water depths.The SST turbulence model performs better in predicting wave run-up for water depth 0.42 m,while the laminar model and standard k-ωmodel perform better at water depth 0.52 m and 0.62 m,respectively.

Experimental Investigation of the Response of Monopiles in Silty Seabed to Regular Wave Action

Excessive displacement responses of monopiles affect the serviceability of offshore structures.Related to complicated pile−seabed−wave interactions,the actual behavior of monopiles in silty seabed under periodic wave action remains unclear,and relevant studies in the literature are limited.A series of experiments were conducted in a wave flume containing single piles in silty seabed with relative density of 0.77 subjected to regular waves.Two stages of wave loading were applied successively,accompanied by data recording which included pore water pressure,water surface elevation,pile head displacement,and pile strain.Development of pile-head displacement and pore pressure in silty seabed was the main focus,but the effects of pile diameter,pile type,and pile stiffness were also investigated.The experimental results indicate that,in silty seabed,piles of large diameter or with fins accelerate soil liquefaction,resulting in strengthened soil which allows a higher upper boundary of pore pressure.Using fins at deeper locations led to a quick failure of the piles,but the opposite result was observed with an increase in fin dimensions.Once pile-head displacement entered its rapid development period,the wave load calculated via the pile moment was an overestimation,especially for the piles of large diameter.

Circular and Faceted Monopile Installation Fatigue Damage

There are currently no models predicting localised stressing induced in monopole foundations resulting from pile driving installation. A scaled down test was conducted for both circular and faceted monopile, during which monopile stressing was measured. From the stress data gathered fatigue damage was estimated. Fatigue damage of the faceted geometry is significantly larger than that of the circular geometry. It is shown that in the worst case the fatigue damage incurred is still negligible compared to the full service life of the foundation. Suggestions for future developments are made, such developments can be helpful in providing greater understanding of the occasional cases where fatigue damage resulting from pile driving is not negligible and has perhaps resulted in failure.

CFD-Based Load Calculation Method for Monopile Support Configuration of Offshore Wind Turbine

An unsteady load calculation method for the support configuration of a monopile-supported offshore wind turbine is developed based on the Fluent software platform.Firstly,the water wave is generated by imposing the inlet boundary conditions according to the exact potential flow solution.Then the wave evolution is simulated by solving the unsteady incompressible Navier-Stokes(N-S)equations coupled with the volume of fluid method.For the small amplitude wave with reasonable wave parameters,the numerical wave result agrees well with that of the given wave model.Finally,a monopile support configuration is introduced and a CFD-based load calculation method is established to accurately calculate the unsteady load under the combined action of wave and wind.The computed unsteady wave load on a small-size monopile support located in the small amplitude wave flow coincides with that of the Morison formula.The load calculations are also performed on a large-size monopile support and a monopile-supported offshore wind turbine under the combined action of small amplitude wave and wind.

地震作用下变截面钢管混凝土单桩动力响应

为了探讨桥梁位于软弱土层时,地震作用下变截面钢管混凝土桩基的动力响应,以翔安大桥为工程背景,利用FLAC3D有限元软件建立地震作用下桩-岩土体-钢管耦合作用的数值模型,研究在不同强度的地震作用下变截面钢管混凝土单桩和变截面普通混凝土单桩桩身加速度、桩身位移、桩身弯矩的动力响应差异.结果表明:变截面钢管混凝土单桩在地震作用下的桩身加速度、加速度放大系数、桩身位移最大值和桩身弯矩变化规律均同变截面普通混凝土单桩类似,但钢管使桩身加速度放大系数降低、桩基抵抗侧向变形能力和抗弯承载力提高.因此,钢管可以显著提升地震作用下桩基的抵抗变形能力和承载力.

人工鱼礁群对单桩基础局部冲刷防护效应的试验研究

为探究人工鱼礁群对单桩基础冲刷的影响,在波流水槽中开展局部冲刷物理模型试验,分析不同波流条件、鱼礁类型、布置方式以及鱼礁群与桩基础距离条件下鱼礁群的自身稳定性及其对桩基的防护效果.结果表明:立方体空心礁相比实心礁具有更好的稳定性,交错布置对桩基础冲刷的防护效果优于均匀布置形式;纯流作用下,人工鱼礁群桩基础冲刷深度的削弱效果随流速的增大而减小;波流作用下,空心礁对桩基础冲刷深度的削弱效果更优;2种礁体的不同平面布置形式对桩基础冲刷范围的削弱程度相近;鱼礁群与桩基础前缘距离的改变影响桩基础冲刷深度和冲刷形态.

礁坪上单桩在孤立波作用下的受力状况研究

文中介绍了孤立波冲击礁坪上单桩的三维水槽试验和OpenFOAM数值模拟研究。礁体模型坡度为1:4,直径0.15m的单桩布置在礁坪上。试验以水深的30%作为入射孤立波的波高,对沿程波面、特定位置处流速以及单桩上波浪冲击荷载进行了采集。选用realizablek-8端流模型进行RANS数值模拟,并与物理试验数据进行验证分析。基于试验数据和RANS模拟结果,研究了岛礁地形上孤立波传播的水动力特性及其对礁坪上单桩的作用力,并探讨了不同坡度和人射波高下孤立波对单桩作用力的影响。研究成果对于桩基础选址和防护有一定工程指导意义。

海上单桩基础风机土体阻尼校核与预测方法研究

对长期承受动荷载作用的海上风机而言,土体阻尼是影响其疲劳寿命的重要因素之一,不同学者对其考虑方式尚未统一,同时土体阻尼的输入和校核结果间存在一定差异.针对这一问题,本文以江苏黄海某风机工程项目为依托,在ABAQUS软件中建立单桩基础式海上风机有限元计算模型,设计了包含风机结构壁厚、基础埋深、结构密度及土体阻尼等因素的系列计算方案,采用自由振动分析对土体阻尼输入与校核间差异性进行系统研究,并设计正交试验采用极差分析方法进行各因素的影响权重分析,在此基础上基于多参数非线性回归分析提出了综合拟合公式,并给出相应土体校核阻尼预测方法.结果表明:风机土体阻尼校核结果远小于其输入值,仅为后者的2%~15%;校核阻尼与风机结构壁厚、结构密度呈正相关关系,与基础埋深呈负相关关系,但当基础埋深达到8倍桩径后,则基本不再影响校核结果;以上各因素并非线性独立,三者共同作用决定土体阻尼校核结果,其中结构壁厚影响最为显著,其次为结构密度,最后为基础埋深;本文提出的以结构壁厚与校核结果间幂函数为基础的拟合经验公式,其拟合精度良好,可有效模拟各因素与校核阻尼间的关系,同时可为同类型风机土体阻尼校核预测提供数学基础.

海上风力发电机组单桩基础侧向位移估计研究

常规海上风力发电机组单桩基础侧向位移估计模型的构建多采用粒子群优化算法。但该算法无法准确计算出海水与单桩基础之间的耦合力,估计准确率较低。对此,提出海上风力发电机组单桩基础侧向位移估计研究。首先,根据海上风力发电机组与连接单桩基础的基本结构,将单桩基础结构进行断面划分,建立单桩基础结构动力学方程。然后,结合海水域、桩基区域和两者交界处这三个场域,求取海水与单桩基础之间的耦合力,并计算单桩基础的受力值。基于此,采用风力和海浪循环荷载作用下单桩基础的累积应变指数型经验式,构建单桩基础侧向位移估计模型。以某海域NREL 5 MW单桩基础海上风力发电机组为算例,对所提模型进行仿真对比试验。试验结果表明,所构建的模型具有更高的估计准确率,即准确率在98%以上。该模型能够更好地监测和评估海上风力发电机组的结构安全性,及时发现潜在的问题并进行修复或维护。

弹簧地基上大直径单桩海上风机的一阶自振频率

自振频率是海上风机前期动力设计的主要难点之一,计算精度要求极高,而桩-土相互作用对自振频率影响显著。目前,针对桩-土相互作用普遍采用弹簧地基简化模型,为对比分析不同弹簧地基简化模型的准确性和精度,本文采用回传射线矩阵法,建立固定端、单弹簧、双弹簧和三弹簧这4种地基简化模型,基于Bernoulli-Euler梁理论和Timoshenko梁理论,求解风机系统的自振频率,并与风机基频实测值作对比,进一步分析三弹簧简化地基模型中地基旋转刚度、地基耦合刚度和地基水平刚度对风机系统基频的影响。研究结果表明,桩-土相互作用不可忽略,三弹簧地基简化模型计算风机系统基频精度最高;地基刚度对风机系统基频的敏感性次序为,地基耦合刚度>地基旋转刚度>地基水平刚度;当地基旋转(水平)刚度较小时,地基耦合刚度与地基旋转(水平)刚度的耦合效应对风机系统的基频影响较大;地基耦合刚度与地基旋转刚度的耦合效应对风机系统基频的影响程度大于地基耦合刚度与地基水平刚度的耦合效应。

嵌岩单桩水平静力加载现场试验及 p-y 曲线研究

为研究风化花岗岩地质条件下嵌岩单桩的受荷响应和p-y曲线规律,本文基于现场试验,开展了水平静力荷载作用下嵌岩单桩的荷载-位移关系、弯矩、桩身变形及p-y曲线响应研究。试验表明:在所研究的相对深度范围内,随着z/D的增加,岩土阻力和相应位移都减小,上部荷载很难传递到深部岩层;桩身最大弯矩在3~4倍桩径之间,极限荷载主要取决于桩身强度而不是岩土体。将p-y曲线分别与一般双曲函数和双曲正切函数进行拟合,评估现有p-y曲线模型在风化花岗岩地基中的适用性,修正了模型在风化岩层中的参数取值。结果表明:一般双曲函数在较浅区域拟合良好,相对深度较大的区域数据离散性较强;双曲正切函数在各深度处拟合优度均较好。分析结果简化了现场p-y曲线的预测程序。

局部冲刷下砂土中桩-盘复合基础V-H组合承载特性

海上风电基础除承受复杂组合荷载作用外,也受波流冲刷作用.为探讨局部冲刷对砂土地基中V-H联合加载下单桩-摩擦盘复合基础承载特性的影响,基于自行设计的室内水槽试验装置,完成了一系列冲刷及模型载荷试验,获得了复合基础冲刷前后的荷载-位移曲线,经无量纲化处理与曲线拟合,得到复合基础承载包络线及其简化计算公式.计算结果表明:局部冲刷作用使复合基础的竖向及横向抗力明显降低,削弱程度分别约12.1%和26.9%.进一步分析表明:存在一个最优预加竖向荷载(V),其对复合基础的极限横向承载力起到最显著的增强效果;此外,局部冲刷作用导致V-H联合受荷单桩-摩擦盘复合基础的承载包络线明显向内收缩,所提出的简化计算公式可供工程应用参考.