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1.
The substructures of offshore wind turbines are subjected to extreme breaking irregular wave forces. The present study is focused on investigating breaking irregular wave forces on a monopile using a computational fluid dynamics (CFD) based numerical model. The breaking irregular wave forces on a monopile mounted on a slope are investigated with a numerical wave tank. The experimental and numerical irregular free surface elevations are compared in the frequency-domain for the different locations in the vicinity of the cylinder. A numerical analysis is performed for different wave steepness cases to understand the influence of wave steepness on the breaking irregular wave loads. The wave height transformation and energy level evolution during the wave shoaling and wave breaking processes is investigated. The higher-frequency components generated during the wave breaking process are observed to play a significant role in initiating the secondary force peaks. The free surface elevation skewness and spectral bandwidth during the wave transformation process are analysed and an investigation is performed to establish a correlation of these parameters with the breaking irregular wave forces. The role of the horizontal wave-induced water particle velocity at the free surface and free surface pressure in determining the breaking wave loads is highlighted. The higher-frequency components in the velocity and pressure spectrum are observed to be significant in influencing the secondary peaks in the breaking wave force spectrum.  相似文献   
2.
A hybrid foundation for offshore wind turbines (OWT) is studied, combining a monopile of diameter d and length L with a lightweight circular footing of diameter D. The footing is composed of steel plates and stiffeners forming compartments, backfilled to increase the vertical load. A special pile–footing connection is outlined, allowing transfer of lateral loads and moments, but not of vertical loads. The efficiency of the hybrid foundation is explored through 3D finite element modelling. Hybrid foundations of L=15 m are comparatively assessed to an L=30 m reference monopile. A detailed comparison is performed focusing on a 3.5 MW OWT. While the moment capacity of the monopile is larger, the hybrid foundation exhibits stiffer response, outperforming the monopile in the operational loading range. Under cyclic loading, the hybrid foundation experiences less stiffness degradation and rotation accumulation. Besides installation, the cost savings depend on the design of the footing and buckling can be crucial. The rubble fill is shown to provide lateral restraint to the stiffeners, being beneficial for buckling prevention. Although seismic shaking is not critical in terms of capacity, it may lead to substantial accumulation of rotation and settlement. Combined with cyclic environmental loading, the latter may challenge the serviceability of the OWT, potentially leading to a reduction of its service life. To derive insights on the effect of seismic loading, two scenarios are investigated: (a) seismic loading; and (b) combined environmental and seismic loading. In the first case, even a D=15 m hybrid foundation may outperform the reference monopile. This is not the case for combined environmental and seismic loading, where a D=20 m hybrid system would be required to outperform the reference monopile.  相似文献   
3.
The objective of this work is to study the uncertainties involved in the modelling of the soil-pile interaction concerning their influence on the prediction of the dynamic structural response of monopile offshore wind turbine support structures. Two main issues are identified and addressed: the adequacy of the method used to deal with the soil-pile interactions and the adequacy of the soil properties reflecting the behaviour of the soil. The present study develops an approach that defines the penetrated pile length depending on the soil profile. Also, a parameter is defined to avoid the excessive usage of steel for the penetrated pile structure. The uncertainties are included in the probabilistic free vibration analysis and the contribution of each random variable to the scatter of the response is estimated by performing a sensitivity analysis. The results indicate that the uncertainty involved in the modelling of the soil profile has a significant effect on the coefficient of variance of the natural frequency, which is a serious issue to be considered in the fatigue life assessment of offshore wind turbine support structures.  相似文献   
4.
Large diameter monopiles are typical foundation solutions for offshore wind turbines. In design of the monopile foundations in sand, it is necessary to understand the drainage conditions of the foundation soil under the design loading conditions as the soil performance (strength and stiffness) is highly dependent on the drainage conditions. This paper presents a numerical investigation into this issue, with a purpose to develop a simple design criterion for assessing the soil drainage conditions around a monopile in sand. It is found that for typical monopile foundations in sand, the drainage condition during a single load cycle is generally expected to be undrained. However, the current state-of-practice uses p-y springs derived for drained soil responses for monopile design. The impact of this discrepancy on monopile foundation design was evaluated and found to be insignificant due to the relatively low level of loading as compared to the capacity of the soil.  相似文献   
5.
The eigenfrequency of offshore wind turbine structures is a crucial design parameter, since it determines the dynamic behavior of the structure and with that the fatigue loads for the structural design. For offshore wind turbines founded on monopiles, the rotational stiffness of the monopile-soil system for un- and reloading states strongly affects the eigenfrequency. A numerical model for the calculation of the monopile’s behavior under un- and reloading is established and validated by back-calculation of model and field tests. With this model, a parametric study is conducted in which pile geometry, soil parameters and load conditions are varied. It is shown that of course the rotational stiffness varies with mean load and magnitude of the considered un- and reloading span, but that for most relevant load situations the initial rotational stiffness of the monopile system, i.e. the initial slope of the moment-rotation curve for monotonic loading, gives a good estimate of the actual stiffness. Comparisons of different p–y approaches show that the ordinary API approach considerably underestimates the initial stiffness, whereas the recently developed ‘Thieken’ approach and also the ‘Kallehave’ approach give a much better prediction and thus might be used in the design of monopiles in sand.  相似文献   
6.
In storm conditions, nonlinear wave loads on monopile offshore wind turbines can induce resonant ringing-type responses. Efficient, validated methods which capture such events in irregular waves in intermediate or shallow water depth conditions are needed for design. Dedicated experiments and numerical studies were performed toward this goal. The extensive experimental campaign at 1:48 scale was carried out for Statoil related to the development of the Dudgeon wind farm, and included both a rigid model and a flexible, pitching-type, single degree-of-freedom model. Twenty 3-hour duration realizations for 4 sea states and 2 water depths were tested for each model. A high level of repeatability in ringing events was observed. Uncertainties in the experimental results were critically examined. The stochastic variation in the 3-hour maximum bending moment at the sea bed was significantly larger than the random variation in repetition tests, and highlighted the need for a good statistical basis in design. Numerical simulations using a beam element model with a modified Morison wave load model and second order wave kinematics gave reasonable prediction of the ringing response of the flexible model, and of the measured excitation forces on the rigid model in the absence of slamming. The numerical model was also used to investigate the sensitivity of the responses with respect to damping and natural period. A simple single degree-of-freedom model was shown to behave similarly to a fully flexible model when considering changes in natural frequency and damping.  相似文献   
7.
Very large diameter steel tubular piles (up to 10 m in diameter, termed as XL or XXL monopiles) and caissons are currently used as foundations to support offshore Wind Turbine Generators (WTG) despite limited guidance in codes of practice. The current codes of practice such as API/DnV suggest methods to analysis long flexible piles which are being used (often without any modification) to analyse large diameter monopiles giving unsatisfactory results. As a result, there is an interest in the analysis of deep foundation for a wide range of length to diameter (L/D) ratio embedded in different types of soil.This paper carries out a theoretical study utilising Hamiltonian principle to analyse deep foundations (L/D2) embedded in three types of ground profiles (homogeneous, inhomogeneous and layered continua) that are of interest to offshore wind turbine industry. Impedance functions (static and dynamic) have been proposed for piles exhibiting rigid and flexible behaviour in all the 3 ground profiles. Through the analysis, it is concluded that the conventional Winkler-based approach (such as py curves or Bean-on-Dynamic Winkler Foundations) may not be applicable for piles or caissons having aspect ratio less than about 10 to 15. The results also show that, for the same dimensionless frequency, damping ratio of large diameter rigid piles is higher than long flexible piles and is approximately 1.2–1.5 times the material damping. It is also shown that Winkler-based approach developed for flexible piles will under predict stiffness of rigid piles, thereby also under predicting natural frequency of the WTG system. Four wind turbine foundations from four different European wind farms have been considered to gain further useful insights.  相似文献   
8.
Large monopiles are used as foundations for offshore wind turbines and are generally designed with a tapered section or conical shape. Some loss of driving energy is expected to occur during installation of these structures due to the submerged section of the tapered monopile. The current literature on this subject is limited and indicates rather large losses compared to field observations.A numerical model of the monopile–water–soil system was set up in the general-purpose finite element package Abaqus. By simulating the hammer impact and the resulting stress wave propagation through the monopile and water, the energy losses to be expected can be calculated accurately. The model was verified against independent finite element analyses and experimental data.A parametric study was performed and the effect of hammer characteristics, submerged monopile length and monopile geometry on the driving energy losses were quantified. The results enable a simple relationship between the energy losses and the monopile geometry to be proposed which increases linearly with pile diameter, taper angle, and submerged length. The losses are typically on the order of 0.15–0.3% per metre submerged length for large tapered monopiles.  相似文献   
9.
Together with new opportunities, offshore wind farms raise new engineering challenges. An important aspect relates to the erosion of bottom material around the foundation of the wind turbines, caused by the local increase of the wave and current induced flow velocities by the pile's presence. Typically, the expected scour has a considerable impact on the stability and dynamic behavior of the wind turbine and a scour protection is placed to avoid erosion of the soil close to the foundation. Although much experience exists on the design of scour protections around bridge piers (which are placed in a current alone situation), at present, little design guidelines exist for the specific case of a scour protection around a monopile foundation subjected to a combined wave and current loading.  相似文献   
10.
Behavior of monopile foundations under cyclic lateral load   总被引:1,自引:0,他引:1  
This paper describes the development and application of design charts for monopile foundations of offshore wind turbines in sandy soil under long-term cyclic lateral load. It outlines a numerical model, working with a numerical concept, which makes the calculation of accumulated displacements based on cyclic triaxial test results possible, and it describes important factors affecting the deformation response of a monopile to cyclic lateral loads. The effects of pile length, diameter and loading state on the accumulation rate of lateral deformation are presented and design charts are given, in which a normalized ultimate lateral resistance of a pile is used. For monopiles with very large diameter, the suitability of the “zero-toe-kick” and “vertical tangent” design critera for determining the required embedded length is discussed.  相似文献   
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