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To support large wind turbines in deeper waters (30-60 m) jacket structures are currently being considered. As offshore wind turbines (OWT's) are effectively a slender tower carrying a heavy rotating mass subjected to cyclic/dynamic loads, dynamic performance plays an important role in the overall design of the system. Dynamic performance dictates...
Citations
... Ding HY et al. [17,18] carried out extensive tests on three-and four-bucket BJFs and compared their dynamic performance under seismic load conditions; the results prove the inhibition effects of the three-bucket BJFs on the seismic responses of soils. Jalbi et al. [19] developed analytical solutions to predict the natural frequencies of the BJF wind turbines, which may impact the choice of foundations for jackets. ...
A composite bucket foundation (CBF) is a new type of supporting structure in offshore wind engineering. Its huge transition part is the key difference compared to other offshore foundations. Firstly, the vibration measurement system of a wind turbine with the CBF is introduced. A finite element method (FEM) was developed, and the rigid deformation performance of the transition part was characterized. Then, to clarify the influence of the transition part brings to wind turbines with CBFs, a three-DOF theoretical model was established by simplifying the transition part as a rigid body. Horizontal and rotational foundation stiffness were considered to present the constraint effect below the mudline. Sensitivity studies were conducted on the parameters (including mass, moment of inertia and mass center height) of the transition part. Further, the vibration properties of the CBF structures under different operation load conditions were compared through the theoretical model and the in situ data. The results show that the relative errors between the theoretical model and FEM model are 3.78% to 5.03%, satisfying the accuracy requirements. The parameters of the transition part have varying degrees of influence on the natural frequency, foundation stiffness and vibration response of the wind turbines with CBFs. Compared to wind and 1P loads, the 3P load has a greater influence if the 3P frequency is close to the natural frequency of the wind turbine.
... Little research has examined offshore multiple foundation design. [4][5][6][7][8][9] Single shallow foundations have been studied extensively, with findings suggesting the soil-foundation plastic yielding models, [10][11][12][13][14][15][16][17][18][19][20][21] rather than treating the three-dimensional (3D) stress-distribution problem with conventional horizontal reaction concepts developed for piles. 22,23 The behaviors of pile-supported and jacket-supported wind turbines differ clearly, and the placement of suction-bucket foundation elements underneath tripod or jacket structures is a new concept in the offshore wind energy sector. ...
With the shift of the offshore wind energy sector to deeper waters, demand for the development of more complex foundation solutions, particularly suction bucket–supported tripod/tetrapod and jacket foundations, has increased.
This paper is divided into two main sections. The first part comprises a comprehensive review of the performance of circular surface and shallow foundations under combined loading (VHM), and how it can principally be understood in a theoretical framework of plasticity theory. Examination of the considered data suggested that the general assumption of over-estimated non-association degree with constant failure surface parameters and increasing vertical load may require further investigation. This may be attributed to the complex interplay of multiple properties such as stress level, soil strength profile and foundation geometry. The existing data in the literature were also used to provide practical guidance for a successful implementation of the elasto-plastic constitutive relationships in offshore foundation design.
In second part of the paper, the suitability of the non-associated plasticity formulation for a baseline multi-pod system in H-M load space was investigated using three-dimensional finite element (FE) analyses and not verified. Furthermore, the failure envelopes and hardening law for caissons with different embedment ratios differed from those recommended in the literature were established. Parametric studies of multi-caisson foundations revealed that the failure mechanism of multi-bucket foundations under horizontal loading depended greatly on the bucket spacing. The horizontal bearing capacities increased with the bucket spacing until they reached a threshold. Meanwhile, analyses of the multi-bucket foundation under moment loading confirmed the occurrence of a push-pull failure mechanism.
... To meet this requirement, researchers computed the hub-height aerodynamic loads using aero-elastic software and then established a finite element model using the hydro-geotech code USFOS [22,23], where the RNA was also simplified as a concentrated mass. This model has been utilized to investigate the dynamic behavior of OWTs under various loads, such as extreme wind and wave conditions [24], earthquakes [25,26], normal ocean states [27], etc. In summary, rigid blade models have been proposed and improved by researchers. ...
At present, monopole-supported offshore wind turbines (MOWTs) are widely used in offshore wind farms. The influence of blade flexibility on the dynamic behaviors of MOWTs excited by waves and earthquakes was investigated in this study. Numerical analysis models were established for 5 MW and 10 MW MOWTs, incorporating flexible and rigid blade configurations. The modes and natural frequencies of the full system were compared between these two numerical models, and their dynamic responses were evaluated under wave-only and earthquake-only excitations. It was revealed that the influence of blade flexibility on the first- and second-order modes of the system can be neglected. The dynamic response of these MOWTs under wave excitation can be predicted by the rigid blade model, where the maximum relative difference is less than 5%. However, higher-order modes of the system are significantly affected by the blade flexibility. Under high-frequency excitations, these higher-order modes of the system are remarkably stimulated. Additionally, a large relative difference, exceeding 50%, is detected when the rigid blade model is used to predict the seismic response of the two MOWTs. Consequently, the blade flexibility should be adequately modeled when predicting the dynamic response of OWTs.
... Jacket-type foundations (see Fig. 1) display great potential for future offshore wind turbines (OWTs) farms characterized by large-capacity (heavier turbines) and deeper waters (30-60 m) [3,24,29,40,53,62]. The considerable overturning moments generated by wind and wave loads on the jacket are primarily resisted by a ''push-pull'' action, leading to opposite and equal vertical loads upon mudline foundation (see Fig. 1). ...
... Suction caisson foundation is a hollow steel cylinder enclosed at the top and opened at the bottom, which can mobilize considerable suction to augment its capacity and stiffness against pull-out loads. This unique feature makes the suction caisson jacket an emerging and promising foundation concept for OWTs [2,3,20,24,26,27,34,35,44,60,61]. Accordingly, the response of individual suction caisson to compressive and tensile loads is critical to the overall behavior of the OWT jacket structures. ...
Suction caisson jackets are promising foundation solutions for offshore wind turbines (OWTs) in deep water. The resistance of such a foundation against overturning actions depends on the uplift response of individual caisson. Winkler models (i.e., foundation displacement versus soil reaction relationships) have been shown powerful and efficient in modeling general soil–foundation interactions, whereas those targeting suction caisson subjected to tensile loading are relatively underdeveloped. The goal of this study was to construct a soil reaction model capable of accounting for site-specific soil stress–strain relations and project-specific foundation geometries. This objective is pursued via the concept of “inferred Winkler model” and by constructing soil reaction curves based on the outcomes of rigorous numerical modeling. First, finite element analyses (FEAs), in combination with a well-established hyperbolic soil model, are utilized to evaluate the soil reaction responses associated with vertically loaded caisson in undrained clays. The FEA then establishes the interrelationships between the key characteristics of soil reaction behavior, soil stress–strain relations, and foundation geometries, leading to an inferred Winkler model capable of directly utilizing soil model parameters. Lastly, the proposed soil reaction model is assessed against centrifuge test results and shown capable of reasonably representing test observations and delivering solutions comparable to FEA but at a much lower computational cost.
... Experiments were also conducted on structural ultimate and fatigue damage loads, and the new hybrid platform was found to have superior performance. Jalbi et al. (2018Jalbi et al. ( , 2019 considered SLS, FLS, and inherent frequency to optimize the caisson foundations for FOWT and found that the optimized caissons performed well. Thöns et al. (2012) considered parameters such as hot spot stresses and natural frequency in a model in order to study the FLS and SLS of offshore wind turbine converter support structures. ...
As the world's need for renewable energy has grown in recent years, the possibility of creating and collecting deep-sea wind energy has become a research hotspot. Floating wind turbines need damping devices to provide a stable working state and structural safety. Damping systems are often used for offshore floating constructions based on various operating principles and locations. Damping technology of various sorts is continually being researched for various demands, such as floating body size, form, and operating circumstances. To react to complex and changeable external circumstances, new perspectives on damping method categorization and selection are required. The conclusion was reached by classifying and comparing, tuned liquid column dampers are often employed in operational conditions. Dampers with power sources perform well in extreme conditions, such as Magnetorheological dampers. Rotational inertia dampers can greatly decrease torque but have yet to be widely employed in floating wind turbines. The purpose of this study is to review the latest improvements in offshore damping technology. The research results will provide characteristics and design references for future vibration damping of floating offshore wind turbines.
... One is the global finite element modeling to analyze from macroscopic scale [6,7]. The joints of global model are established using beam elements, ignoring the influence of nodal effects, such as the flexibility, geometry and weld configuration of its substructure joints [8,9]. The other is local modeling via small-scale analysis, and the local joint flexibilities of tubular K/DK-joints [10,11], T/Y-joints [12][13][14], or X-joints [15,16] are established individually using solid or shell elements for analysis. ...
The fatigue damage of a local joint is the key factor accounting for the structural failure of a jacket-type offshore wind turbine. Meanwhile, the structure experiences a complex multiaxial stress state under wind and wave random loading. This paper aims to develop a multi-scale modeling method for a jacket-type offshore wind turbine, in which local joints of the jacket are modeled in a detail by using solid elements, and other components are modeled via the common beam element. Considering the multiaxial stress state of the local joint, multi-axial fatigue damage analysis based on the multiaxial S–N curve is performed using equivalent Mises and Lemaitre methods. The uniaxial fatigue damage data of the jacket model calculated using the multi-scale finite element model are compared with those of the conventional beam model. The results show that the tubular joint of jacket leg and brace connections can be modeled using the multi-scale method, since the uniaxial fatigue damage degree can reach a 15% difference. The comparison of uniaxial and multiaxial fatigue results obtained using the multi-scale finite element model shows that the difference can be about 15% larger. It is suggested that the multi-scale finite element model should be used for better accuracy in the multiaxial fatigue analysis of the jacket-type offshore wind turbine under wind and wave random loading.
... Not all attributes can be accurately scaled, such as wall thickness and flexural stiffness, but these parameters do not significantly affect the test results. In previous studies, model tests have been proved to reflect the basic characteristics of the hybrid foundation under in situ conditions (Jalbi et al. 2019;Shi et al. 2022). ...
The monopile-friction wheel hybrid foundation withstands various loads in the marine environment , such as the vertical load (V) transmitted from the superstructure, horizontal loads (H) caused by wind or wave, torsional loads (T) caused by rotating structures. In this article, 1 g model tests are used to investigate the behavior of the monopile-friction wheel hybrid foundation under independent horizontal load (H) or torsional load (T), and combined loads (H-T) in sand soil, respectively. The failure envelopes of H-T loading plane are obtained from the measured load-displacement data with the simplified calculation equations presented as well. Finally, we also discuss quantitatively the influences of the pre-vertical loads, foundation geometry and loading eccentricity on the bearing capacity of the hybrid foundation via the three-dimensional finite element method. The results indicate that the displacement response of the hybrid foundation under independent loading is significantly different from that subjected to combined loads (H-T). The torsional bearing capacity of the hybrid foundation can be significantly improved by 6.6-33.34% under pre-horizontal load. The horizontal bearing capacity of the hybrid foundation decreases sharply after the pre-torsional load reaches a certain value, which decreases by about 20%. The presence of a friction wheel improves the torsional/bending moment distribution of pile shaft. ARTICLE HISTORY
... In recent years, a large number of marine engineering projects have been constructed in China, such as transmission line towers, drilling platforms, cross-sea bridges, port terminals, wind turbines, etc., among which the development of offshore wind power projects is particularly rapid [1][2][3][4][5]. To tackle the challenges posed by the intricate offshore environment, these structures often rely on the pile group, as the preferred choice for foundation design [6,7]. ...
... As shown in Figure 10, the development of cracks within the soil around the pile under horizontal cyclic loading followed the following process: (1) At the beginning of loading, a micro-crack was generated in the soil behind the pile, and with the increase of cycles, the micro-crack started to link up, and formed a circular crack around the pile side; then, the crack width widened, and the depth continuously extended under the continuous action of horizontal loading. (2) The water in the upper layer of soil around the pile continuously drained along the crack and interacted with the soil around the pile, causing the mud to flow into the crack, which then softened the soil surrounding the pile. (3) The pile foundation underwent repeated movements under cyclic loading, which resulted in the compression of the soil in front of the pile. ...
In order to investigate the cumulative deformation of the pile group in saturated clay under horizontal cyclic loading, a series of 1g model tests were conducted using the self-made loading equipment in this paper. Firstly, the loading equipment and testing procedure are introduced. Then, the cumulative deformation of the pile group, the dynamic response of the soil, and the bending moment of the pile shaft under horizontal cyclic loading are studied. Finally, the horizontal cyclic stiffness of the pile group is analyzed based on the experimental results. It can be found that the cumulative displacement, the rotation angle of the bearing platform, the pile shaft bending moment, and the pore water pressure can attain 90% of the peak values within the first 1000 cycles, and the growth rate slows down in subsequent loading cycles. Moreover, the bending moment of each pile increases with the burial depth and gradually decreases after the peak values. Notably, the horizontal cyclic stiffness of the pile group grows with the cycle loading times and decreases with the loading amplitude.
... The tripod bucket jacket foundation is one of the shallow foundations, with an embedment smaller than that of the traditional pile foundation, leading to a relatively low vertical stiffness of the bucket foundation. Jalbi et al. [41,42] proposed that the free vibration mode of the jacket foundation supported on multi-buckets under lateral excitation may be represented as the rocking mode, and the suction bucket below the ground surface can be simplified as a spring with a rotational stiffness K r , as illustrated in Figure 20. Therefore, the rotational stiffness directly affects the dynamic characteristics of the tripod jacket foundation. ...
... The tripod bucket jacket foundation is one of the shallow foundations, with an embedment smaller than that of the traditional pile foundation, leading to a relatively low vertical stiffness of the bucket foundation. Jalbi et al. [41,42] proposed that the free vibration mode of the jacket foundation supported on multi-buckets under lateral excitation Mar. Sci. ...
The tripod bucket jacket foundation is proven to be a practicable solution for offshore wind turbines (OWTs) to withstand huge environmental loads in deep water. This paper presents model tests for a scaled tripod bucket jacket foundation with reference to a prototype applied in China to obtain its lateral load bearing behavior in medium-dense sands. Extended finite element analyses were conducted by ABAQUS to compare anti-overturning responses for the tripod bucket foundation in both sand and soft clay, and the influences of loading direction and aspect ratio were also taken into account. The results showed that the failure modes of the laterally loaded tripod bucket foundation are the pull-out of the windward bucket in sand and the settlement of the leeward bucket in soft clay, respectively. Thus, the unfavorable loading direction of the foundation changes with soil type. It is also shown that the bearing capacity for the foundation in soft clay will be enhanced more effectively as the bucket diameter increases. Instead of the rotational soil resistance resulting from the rotation of the bucket, the vertical soil resistance governs the anti-overturning bearing capacity of a tripod bucket foundation. As the tilt created by the overturning moment rises, the rotational stiffness of the foundation dramatically declines.
... Recently, little research has been conducted in the field of design of offshore multiple foundations [2][3][4]; however, soil-structure interaction (SSI) problems arising from mono-caisson foundations have been investigated extensively [5][6][7][8][9][10][11][12][13][14][15][16][17][18]. ...
This study explored the issue of cyclic loading on a tripod suction bucket foundation system in silty sand through three-dimensional (3D) finite element (FE) models by developing a cyclic stability diagram, in which the response is presented in terms of the average load ratio (ALR) and the cyclic load ratio (CLR), and three regions are identified: (1) no cyclic load effects, (2) cyclic load effects, and (3) severe load effects after approximately 1000 one-way load cycles. The simulations, based on dynamic FE analysis, enabled the identification of the ‘’self-healing’’ phenomenon due to enhanced stiffness degradation of a foundation in tension by incorporating the correct estimate of the soil-foundation interface and geometric nonlinearities. In addition, an approximating expression for the calculation of the accumulated rotation of offshore wind turbine (OWT) supported on multiple foundations is provided as a function of ALR and CLR utilizing modified least-squares method (MLSM). The proposed expression leads to notable improvements in the prediction of induced rotation of OWTs such that the coefficient of determination, R^2, was obtained ranging from minimum value of 0.89 to a maximum value of 0.99. This study suggests that the salient features of multiple foundations must be considered in the design process of offshore wind turbines.
