海上风机吸力式桶形基础竖向承载力特性研究

吸力式桶形基础作为一种新型的海上风机基础,正逐渐以单桶或者多桶组合形式被应用于海上风机支撑基础设计中。然而目前对应用于海上风机基础的桶形基础的极限承载力的研究仍存在研究不全面和结果不统一的问题。本文以宽浅型单桶基础为例,采用有限元软件Abaqus对海上风机吸力式桶形基础在饱和黏土地基中的竖向承载特性进行三维有限元分析。考虑桶土接触面分离条件对极限承载力和土体破坏模式的影响,并且对桶形基础长径比、土体的有效重度以及土体不排水抗剪强度分布对桶形基础竖向极限承载特性的影响进行分析。研究成果可以为海上风机吸力式桶形基础设计提供参考。     

Static and dynamic loading behavior of a hybrid foundation for offshore wind turbines

Considering the deficiencies of the traditional monopile foundation for offshore wind turbines (OWTs) in severe marine environments, an innovative hybrid foundation is developed in the present study. The hybrid foundation consists of a traditional monopile and a wide–shallow bucket. A series of numerical analyses are conducted to investigate its behavior under the static and dynamic loading, considering various loading eccentricities. A traditional monopile with the same steel volume is used as a benchmark. Although the monopile outperforms the hybrid foundation in terms of the ultimate moment capacity under each loading eccentricity, the latter can achieve superior or the same performance with nearly half of the pile length in the design loading range. Moreover, the horizontal load and moment are mainly resisted by the bucket and the single pile in the hybrid foundation respectively. The failure mechanism of both the hybrid foundation and the monopile is excessive rotation. In the rotation angle of 0.05 rad, the rotation center is located at the depth of approximately 0.6–0.75 times and 0.65–0.75 times the pile length for the hybrid foundation and the monopile respectively. The increasing loading eccentricities can lead to increasing moment bearing capacity, increasing initial stiffness and upward movement of the rotation center of the two foundations, while decreasing load sharing ratio of the single pile in the hybrid foundation. Three scenarios are considered in investigating the dynamic loading behavior of the hybrid foundation. Dynamic response results reveal that addition of the bucket to the foundation can restrain the rotation and lateral displacement effectively. The superiority of the hybrid foundation is more obvious under the combined wave and current loading.     

Research on the calculation method of penetration resistance of bucket foundation for offshore wind turbines

The cost of foundations for offshore wind turbines constitutes approximately 35% of the total cost of an offshore wind farm. The bucket foundations show significant potential due to their superior transportation and installation efficiencies compared to pile foundations, leading to potential cost savings for the foundation of up to 30%. For a bucket foundation to be installed successfully, the penetration resistance must be predicted. However, there is currently a lack of clarity on how to select a suitable calculation method for penetration resistance based on known geological parameters to guide construction. In order to evaluate the current methods of calculation for bucket foundation penetration resistance, this study combines theoretical calculation methods with two sets of practical measurement data from the field. The calculation methods of penetration resistance for bucket foundation are first reviewed and categorized. The applicability range of each method and the parameters needed for calculation are given. Next, according to the measured data in the process of penetration of bucket foundation on site, the evolution of compartment pressure, tilt angle, resistance and required suction in the process of penetration are analyzed. Finally, the reviewed methods are compared to the results of two practical projects in order to analyze the differences between them and make recommendations for the calculation technique. The findings can be used as a guide for calculating the bucket foundation's penetration resistance in complex geological conditions.     

Equivalent linear pseudostatic and dynamic modelling of vertically vibrating monopile

To optimize offshore wind turbine (OWT) design, an engineering tool has been developed allowing for a detailed investigation of the effects of nonlinear soil stiffness and damping on foundation dynamics. We have studied the response of a vertically oscillating offshore wind monopile foundation in a realistic soil profile subjected to loads between 1 and 200 MN in the frequency range 0–10 Hz with pseudo-static and equivalent linear dynamic model. The non-linear soil behaviour is modelled with an equivalent linear method with shear modulus reduction and damping curves as input. The tool is verified and validated by comparison with elasto-dynamic model and experiments. With increasing load amplitudes foundation stiffness increases and damping decreases. For large load amplitudes the lower part of the pile foundation contributes more to foundation damping. The results indicate the nonlinear foundation stiffness and damping can be modelled rationally by combining stiffness and hysteretic damping from nonlinear static tools with apparent mass and radiation damping from elasto-dynamic analysis. The tool can be used to compute soil springs and dampers based on laboratory-based soil stiffness and damping.     

Estimation of natural frequencies and damping using dynamic field data from an offshore wind turbine

The dynamic characteristics of offshore wind turbines are heavily affected by environmental loads from wave and wind action and nonlinear soil behaviour. In the design of the monopile structures, the fatigue load due to wind and wave loading is one of the most important problems to consider. Since the fatigue damage is sensitive to the foundation stiffness and damping, increasing the accuracy of analysis tools used in the design and optimization process can improve the reliability of the structure and reduce conservatism, thereby leading to a more cost-efficient design. In this context, analysis of field data is important for calibrating and verifying purposes. This paper presents analysis of measured accelerations and strains from a wind farm in the North Sea with monopile foundations. Field data during idling conditions, collected over long periods of operation, are analysed and the natural frequencies are determined, and damping is estimated. The measured natural frequencies are compared to calculated values using an aero-servo-hydro-elastic code, showing a good agreement in the frequency range below 2 Hz. Variation of the natural frequencies with intensity of loading may indicate effect of soil nonlinearity on the overall OWT response. Since the first natural bending modes have the largest potential to mobilize soil reactions, they are of primary interest in this context. The effect of load (wave, wind and dynamic bending moment) on the first natural frequency is investigated using different analysis techniques in the frequency domain and time domain. A clear correlation between load level and first natural frequency is demonstrated. A simple nonlinear SSI model of the tower/soil system is employed to numerically investigate the observed changes in the measured first natural frequency with the level of loading and increased overall damping. The simulated results reproduce the general trends in the observed reduction in the first natural frequency and increased damping ratio with the load level. However, the effect of the load level is less than that observed in the measurements, indicating contribution also from other factors than soil nonlinearity.     

海上风力发电机组支撑结构动力响应特性研究

支撑结构设计是大型海上风电机组设计的重要部分。文章分析了海上风电机组的各种环境载荷,并以3MW风力机组为例计算其所受环境载荷,包括作用在支撑结构顶端的由风机叶轮转动引起的水平轴向力、作用在塔筒上的风载荷以及作用在基础上的海流、海浪载荷,并采用非线性弹簧来模拟基础与海底土层之间的相互作用。在考虑风轮影响情况下,利用有限元法对支撑结构进行了模态分析。最后,分析了环境载荷作用下支撑结构的动态响应。计算结果表明,在对海上风力发电机组进行动态响应计算时,环境载荷之间的相互耦合作用不能忽略。     

Bucket Group Effect of the Composite Multi-bucket Structure

As a novel type of foundation in beach and shallow sea,the bucket structure is especially suitable for complex conditionssuch as soft clay ground and the worse types of sea environments.In this paper, the bearing capacity of a multi-bucket structure isstudied by experiments with a single bucket and four-bucketfoundation in a saturated sand layer. Based on the experimentaldata and numerical analysis results, the bearing capacity behaviorand the bucket group effect are compared and analyzed.Furthermore, some influential factors, such as the soil type, theratio of length to diameter L/D, the ratio of the bucket spacing tothe bucket diameter S/D, and the bucket number are introduced andtheir effects on the multi-bucket structural capacity are investigatedThe vertical static capacity adjustment factor is introduced toevaluate the bucket group effects of the multi-bucket foundation.     

筒式基础结构分仓形式对抗倾承载力的影响

使用ABAQUS建立了4种不同分仓形式的筒式基础结构与土相互作用的非线性弹塑性有限元分析模型,得到了结构在水平荷载作用下的P-S曲线及极限状态的土压力。重点分析筒仓内壁土压力在不同区域的分布和变化趋势,根据结构的P-S曲线评估分仓形式的改变对结构抗倾能力的影响。结果表明,经过分仓优化的结构可更好地发挥其抗倾潜力,其结构承载力可提升15％～25％。     

Temporal and spectral characteristics of seismic ground motions: Offshore versus onshore

To distinguish offshore and onshore seismic ground motions, conventional analyses in terms of peak ground acceleration (PGA) and earthquake response spectrum (ERS) have been carried out in a recent work by authors and other papers in literature. In the present study, distinct temporal and spectral characteristics between onshore and offshore earthquake ground motions are further investigated in time-domain and frequency-domain. The data used is 69 pairs of concurrent onshore and offshore ground motions collected from the Kyoshin Network (K-NET). Each pair of data are of approximately identical epicenter distances. Comparisons are made on zero-up-crossing period (T_z), peak-to-trough acceleration range (A_pt) and period (T_pt), duration of ground motion (T_d), predominant frequency (f_p) and the spectral bandwidth parameter (ε). The results indicate that for offshore horizontal and vertical seismic signals, statistics of T_z, T_pt, T_d and predominant period T_p tend to be larger than the onshore counterparts. Meanwhile, ε of the offshore vertical ground motions is also greater. Through the proposed energy ratio (ER) analysis, the spectral energy of offshore ground motion is found to shift to moderate and low frequency bands. The time-frequency analysis conducted by Hilbert-Huang transform (HHT) shows that the Hilbert spectra of offshore accelerations contain larger spectral energy than the onshore counterpart but the corresponding instantaneous frequencies at peak energy are smaller, especially for horizontal recordings. Therefore, larger dynamic response of offshore structures is prone to be induced by the offshore ground excitation. This is further validated through the dynamic analysis of a marine pipeline in case study.     

裙式吸力基础水平承载力影响因素的数值分析

近年来,吸力基础已在各种悬浮结构、海洋平台锚固装置和离岸风电场工程中得到成功应用.重点研究离岸风电场新型裙式吸力基础的水平承载力,分析设置接触面对结构水平位移的影响、裙式基础的尺寸比变化等因素对水平承载力的影响.数值分析结果表明,通过增加裙式基础的高度能有效提高基础的水平承载力.     

Hysteretic damping model for laterally loaded piles

Fatigue assessment is a critical design aspect for many offshore structures. Soil-foundation interaction has a direct impact on the system dynamic response of these structures. While the stiffness of the soil-foundation interaction influences the system's natural frequency, the damping influences the amplification of the structural response to environmental excitations. This paper presents a simplified model for estimating the soil damping due to nonlinear soil response for pile foundations, which have wide applications in the offshore industry, such as for supporting jacket platforms, wind turbines and wellhead facilities. The proposed model is fundamentally linked to the damping response of the soil measured at element level therefore it offers design engineers an efficient and accurate way to estimate soil-pile interaction damping based on site-specific soil data. Approaches to include the suggested model for structural analysis are also proposed.     

Numerical analysis of lateral – Moment capacity of bucket foundations for offshore wind turbine in sand

This paper reports the lateral – moment bearing capacity of bucket foundations under lateral loading in sand. The Modified Mohr-Coulomb (MMC) model is adopted to capture the hardening – softening behaviour in medium dense and dense sands within a finite element (FE) modelling framework. The FE model performance is assessed against available field test data as well as analytical solutions showing a relatively good agreement. A series of parametric study is conducted to investigate the effects of bucket aspect ratio, bucket diameter, load eccentricity, vertical load and relative density of sand on the lateral - moment bearing capacity of the bucket. Comparisons are drawn between the conventional Mohr-Coulomb (MC) model and the stress dependent MMC model highlighting the role of sand dilatancy in mobilising the lateral moment capacity. Based on the FE results, a simple stepwise calculation framework is proposed for two scenarios: (i) to predict the lateral - moment bearing capacity of the bucket if the bucket dimensions are known, and (ii) to design the bucket dimensions for a known required bucket capacity.     

Processing method and governing parameters for horizontal wave impact loads on a semi-submersible

Semi-submersible platform has been widely used in offshore oil exploitation due to its excellent performance, but can be attacked by wave impact loads in extreme ocean environments. Determining wave impact loads accurately is of great significance to the design and operation of offshore structures. An experimental study was carried out to investigate the critical governing parameters for the horizontal wave impact loads on a semi-submersible. The wavelet denoising technique and the frequency response function method are employed successfully to remove the effect of noise and dynamic contamination from the experimental data. The strongly nonlinear characteristics of the wave impact load are demonstrated. The results show that wave impact events are governed by the upwell height and upwell velocity. Most major wave impact events occur where both the two parameters are large, and the upwell velocity is more dominant in the wave impact process. In general, larger parameters tend to result in larger peak pressures and higher probabilities of wave impacts. The motion behaviors of the platform are benefit to reduce the occurrence probabilities of wave impact events and maximum impact pressures, owing to the escape velocities following the wave direction and the rotations leading to the above-water structure away from the waves. The insights given in this study provide a motivation and foundation for developing a sophisticated prediction model of the wave impact load on floating platforms.     

冰区海上风电基础的抗冰性能分析

海上风电基础属于典型的柔性结构。由于冰与柔性抗冰结构相互作用的复杂性,长期以来尚未形成基于动冰力响应分析的结构设计。结构抗冰设计中大都是从极端荷载出发,只考虑最大静冰力或最大倾覆力矩。基于对渤海辽东湾柔性抗冰结构的多年监测,发现强烈的冰激振动引起柔性结构的风险性要远大于极端静冰荷载下结构的整体安全问题。为了明确冰区风电基础结构的抗冰性能及抗冰设计的合理性,文章结合基于多年现场冰与结构作用观测及冰荷载的研究成果,明确该类柔性结构与海冰作用形式及其动力特性;提出了柔性抗冰结构设计中应考虑的主要失效模式及评价方法。最后,以渤海某典型风电基础为例,对其抗冰性能进行评价。该文的研究可为寒区风电基础的抗冰设计及安全保障提供合理依据。     

Global wave loads on intact and damaged Ro-Ro ships in regular oblique waves

A nonlinear time-domain simulation method is presented for the prediction of dynamic global wave loads on a Ro-Ro ship at zero speed in regular oblique waves in an intact and a damaged condition. Numerical computations and model tests have been carried to investigate the structural responses of Ro-Ro ship Dextra to various wave amplitudes at three different wave headings (DTR-4.1-NEW-12.98, DEXTREMEL project BE97-4375, 1998; DTR-4.2-NEW-11.99, DEXTREMEL project BE97-4375, 1999). The results of numerical and experimental investigations for stern quartering waves are reviewed. Comparisons between predictions and measurements for global wave loads at the midship section of the intact and the damaged Ro-Ro ship show that the agreement between the theory and experiment for dynamic horizontal and vertical bending moments is excellent. On the other hand, correlation between the predictions and measurements for dynamic vertical shear force is better than that for dynamic horizontal shear force. Nevertheless, the calculated torsion moment values are higher than the measured values. As the wave amplitude is not small, the positive and negative peaks of global wave loads are no longer equal to each other as found in both the calculations and experiments. The dynamic vertical global wave loads in the damaged condition are larger than that in the intact condition.     

软弱地基条件下大型矿石堆场斗轮机基础的设计与研究

通过建立矿石堆场地基的二维有限元模型,对矿石堆场下软弱地基的应力变形进行模拟计算,计算中采用了土体的线弹性模型和剑桥模型进行对比分析。重点研究了各种工况下堆场地基的水平、竖向变形问题,对荷载作用下斗轮机基础的长桩基、短桩基以及片筏基础设计方案进行计算,并与无基础方案进行对比分析,得出各种工况下不同基础的变位情况。计算研究成果直接为工程地基处理方案的确定及斗轮机基础的详细设计提供了依据。     

MTMD to increase fatigue life for OWT jacket structures using Powell's method

The Powell's method was developed to determine the optimal stiffness and damping of multi-tuned mass dampers (MTMD) in offshore wind turbine (OWT) support structures under fatigue loads. Numerical examples indicated that the Powell's method results are always better than those using MTMD formulations. With the exception of the blade passing (3P) frequency, it was found in this work that a positive integer (n) multiple of the 3P frequency will also result in a large wind-induced vibration, which can be excited by the frequency of the first structural vertical rotation mode and will cause significant fatigue damage. The first translation mode TMD installed at the tower top is efficient to increase fatigue life at the tower and brace connections, but it cannot reduce fatigue damage at the column and brace connections below the platform. The second translation mode TMD can reduce fatigue damage resulting from large wave loads and thus increase the fatigue life of the braces and columns. The mode-3 TMD with a reduction in the 3(3P) vertical rotation can effectively increase the fatigue life of the braces and columns. Thus, the appropriate use of these TMDs can be effective for the fatigue problem of OWT support structures.     

筒形基础安装失败案例分析

结合现场筒形基础安装全过程,有针对性地记录安装失败的关键环节,利用筒形基础设计及理论知识对贯入力、负压贯入过程和土塞效应影响进行全面地剖析,带有建设性意见地提出了导致筒形基础安装失败的原因,对日后该形式海工结构物海上安装具有一定的工程借鉴意义。     

Capacity and failure mechanism of monopile-wheel hybrid foundation in clay-overlaying-sand deposits under combined V–H-M loadings

Innovative monopile-wheel hybrid foundations are proposed to enhance the lateral load and moment capacities of monopile for offshore wind systems. This paper presents a comprehensive numerical study on the bearing capacities of this hybrid foundation in clay-overlaying-sand soil conditions under combined V–H-M (vertical-horizontal-moment) loadings. Numerical models are generated and validated by comparing with laboratory experiment results and available centrifuge testing data on similar foundation systems. Parametric analysis is then carried out to quantify the effects of potential influencing factors on the failure mechanisms and bearing capacities of hybrid foundations, including the hybrid foundation geometry, soil properties, upper clay thickness, height of the lateral loading and pre-vertical load. It is found that in clay-overlaying-sand deposits, the hybrid system manifests totally different failure mechanism compared with that in uniform soil deposits. The thickness of the upper clay layer (T_c/L), within the practical range of T_c/L = 0.1–0.7, has a significant influence on the failure patterns and the bearing capacities of the hybrid system, and the proportion of bearing capacity provided by the pile and wheel is determined by the ratios of D_w/L and L/D_p. In addition, the failure envelopes in the V–H-M space manifests that the failure envelopes are shrank with the increase of the normalized vertical resistance, V/V_ult, which is highly related to the clay layer thickness (T_c/L).     

自施工式桶形基础平台着底稳性研究*

自施工桶形基础平台在海上受风、浪、流、冰等载荷作用,平台应有足够的着底稳性,以保证平台的作业安全。根据研究对象和环境参数,确定了工作水深17 m 的典型自施工式桶基平台主体结构形式,阐述了平台的抗滑稳性、垂向稳性和抗倾稳性计算方法,计算结果表明平台具有良好的着底稳性,平台在着底状态是安全的。