Experimental Study on the Sinking and Leveling of A Large-Capacity Offshore Wind Turbine Five-Bucket Foundation in Sand

As offshore wind farms expand into deeper and farther ocean regions and the unit capacity of offshore wind turbines(OWTs)increases,there is a pressing need for a new foundation structure that can accommodate deep-sea conditions and support large capacities while maintaining economical and safe.To meet this goal of integrated transportation and one-step installation,a novel five-bucket jacket foundation(FBJF),with its suction installation and leveling methods in sand,has been proposed,analyzed and experimentally studied.First,seepage failure experiments of the FBJF at various depths were conducted,and a formula for calculating the critical suction of seepage failure suitable for the FBJF in sand was chosen and recommended for use with a range of values for the permeability coefficient ratio.Second,through leveling experiments of the FBJF at different depths,the maximum adjustable leveling angle during the sinking process was defined using seepage failure and the adjustable leveling angle of the foundation as control criteria.Various leveling control strategies were proposed and verified.Finally,an automatic sinking and leveling control system for the FBJF was developed and experimentally verified for feasibility.

Hydrodynamic Characteristics of Three-Bucket Jacket Foundation for Offshore Wind Turbines During the Lowering Process

The three-bucket jacket foundation is a new type of foundation for offshore wind turbine that has the advantages of fast construction speed and suitability for deep water. The study of the hoisting and launching process is of great significance to ensure construction safety in actual projects. In this paper, a new launching technology is proposed that is based on the foundation of the three-bucket jacket for offshore wind turbine. A complete time domain simulation of the launching process of three-bucket jacket foundation is carried out by a theoretical analysis combined with hydrodynamic software Moses. At the same time, the effects of different initial air storage and sea conditions on the motion response of the structure and the hoisting cable tension are studied. The results show that the motion response of the structure is the highest when it is lowered to 1.5 times the bucket height. The natural period of each degree of freedom of the structure increases with the increase of the lowering depth. The structural motion response and the hoisting cable tension vary greatly in the early phases of Stages Ⅰ and Ⅲ, smaller in Stage Ⅱ, and gradually stabilize in the middle and late phases of Stage Ⅲ.

Design of Large-Scale Prestressing Bucket Foundation for Offshore Wind Turbines

The key in the force transmission between the tower and the foundation for offshore wind turbines is to transfer the large moment and horizontal loads. The finite element model of a large-scale prestressing bucket founda- tion for offshore wind turbines is set up and the structural characteristics of the arc transition structure of the founda- tion are analyzed for 40-60 channels(20-30 rows) arranged with prestressing steel strand under the same ultimate load and boundary conditions. The mechanical characteristics of the key parts of the foundation structures are illus- trated by the peak of the principal tensile stress, the peak of the principal compressive stress and the distribution areas where the principal tensile stress is larger than 2.00 MPa. It can be concluded that the maximum principal tensile stress of the arc transition decreases with the increasing number of channels, and the amplitude does not change signifi- cantly; the maximum principal compressive stress increases with the increasing number of channels and the amplitude changes significantly; however, for the distribution areas where the principal tensile stress is larger than 2.00 MPa, with different channel numbers, the phenomenon is not obvious. Furthermore, the principal tensile stress at the top of the foundation beams fluctuantly increases with the increasing number of channels and for the top cover of the bucket, the principal tensile stress decreases with the increasing number of channels.

Towing characteristics of large-scale composite bucket foundation for offshore wind turbines

In order to study the towing dynamic properties of the large-scale composite bucket foundation the hydrodynamic software MOSES is used to simulate the dynamic motion of the foundation towed to the construction site.The MOSES model with the prototype size is established as the water draft of 5 and 6 m under the environmental conditions on site.The related factors such as towing force displacement towing accelerations in six degrees of freedom of the bucket foundation and air pressures inside the bucket are analyzed in detail.In addition the towing point and wave conditions are set as the critical factors to simulate the limit conditions of the stable dynamic characteristics.The results show that the large-scale composite bucket foundation with reasonable subdivisions inside the bucket has the satisfying floating stability.During the towing process the air pressures inside the bucket obviously change little and it is found that the towing point at the waterline is the most optimal choice.The characteristics of the foundation with the self-floating towing technique are competitive for saving lots of cost with few of the expensive types of equipment required during the towing transportation.

Bearing Capacity and Technical Advantages of Composite Bucket Foundation of Offshore Wind Turbines

Based on mechanical characteristics such as large vertical load, large horizontal load, large bending moment and complex geological conditions, a large scale composite bucket foundation （CBF） is put forward. Both the theoretical analysis and numerical simulation are employed to study the bearing capacity of CBF and the relationship between loads and ground deformation. Furthermore, monopile, high-rise pile cap, tripod and CBF designs are compared to analyze the bearing capacity and ground deformation, with a 3-MW wind generator as an example. The resuits indicate that CBF can effectively bear horizontal load and large bending moment resulting from upper structures and environmental load.

Experimental Investigation of Local Scour Around A New Pile-Group Foundation for Offshore Wind Turbines in Bi-Directional Current

The local scour around a new pile-group foundation of offshore wind turbine subjected to a bi-directional current was physically modeled with a bi-directional flow flume. In a series of experiments, the flow velocity and topography of the seabed were measured based on a system composed of plane positioning equipment and an ADV.Experimental results indicate that the development of the scour hole was fast at the beginning, but then the scour rate decreased until reaching equilibrium. Erosion would occur around each pile of the foundation. In most cases, the scour pits were connected in pairs and the outside widths of the scour holes were larger than the inner widths. The maximum scour depth occurred at the side pile of the foundation for each test. In addition, a preliminary investigation shows that the larger the flow velocity, the larger the scour hole dimensions but the shorter equilibrium time. The field maximum scour depth around the foundation was obtained based on the physical experiments with the geometric length scales of 1:27.0, 1:42.5 and 1:68.0, and it agrees with the scour depth estimated by the HEC-18 equation.

Failure Envelopes of Wide-Shallow Composite Bucket Foundation for Offshore Wind Turbines in Silty Sand

The wide-shallow composite bucket foundation(WSCBF) is a new type of offshore wind power foundation that can be built on land and rapidly installed offshore, there by effectively reducing the construction time and costs of offshore wind power foundation. In this study, the horizontal bearing capacity is calculated by finite element simulation and compared with test results to verify the validity of results. In this process, the vertical load and bending load are respectively calculated by the finite element simulation. Under the vertical load effect, the bucket foundation and the soil inside are regarded as a whole, and the corresponding buckling failure mode is obtained. The ultimate vertical bearing capacity is calculated using empirical and theoretical formulas; the theoretical formula is also revised by finite element results. Under bending load, the rotational center of the composite bucket foundation(in a region close to the bucket bottom) gradually moves from the left of the central axis(reverse to loading direction) to the nearby compartment boards along the loading direction. The H–M envelope line shows a linear relationship, and it is determined that the vertical and bending ultimate bearing capacities can be improved by an appropriate vertical load.

Load Transferring Mechanism on Transition Section of Composite Bucket Foundation for Offshore Wind Turbines

The composite bucket foundation (CBF) is a new kind of foundation which has been applied in the offshore wind industry. A reasonable connection pattern between the tower and the CBF top cover is crucial for load transmissions from the superstructure. Therefore, it is essential to choose an optimum structure type for the transition section. The line type and the arc transition section models were established by ABAQUS, and the internal forces of cross section were extracted along the height direction. Specifically, the force transfer mechanism for different types of the transition sections was investigated comparatively with monotonic as well as composite loadings. The results show that the curved transition structure exhibits the better mechanical characteristics under the monotonic and composite loadings, and the reason can be illustrated that its specific arc-shape structure can effectively convert the tremendous bending moment from the turbine tower into the limited tensile and compressive stresses downwards, without the occurrence of force concentration. © 2017, Tianjin University and Springer-Verlag Berlin Heidelberg.

Application of Converted Displacement for Modal Parameter Identification of Offshore Wind Turbines with High-Pile Foundation

In the actual measurement of offshore wind turbines(OWTs),the measured accelerations usually contain a large amount of noise due to the complex and harsh marine environment,which is not conducive to the identification of structural modal parameters.For OWTs with remarkably low structural modal frequencies,displacements can effectively suppress the high-frequency vibration noise and amplify the low-frequency vibration of the structure.However,finding a reference point to measure structural displacements at sea is difficult.Therefore,only a few studies on the use of dynamic displacements to identify the modal parameters of OWTs with high-pile foundations are available.Hence,this paper develops a displacement conversion strategy to study the modal parameter identification of OWTs with high-pile foundations.The developed strategy can be divided into the following three parts:zero-order correction of measured acceleration,high-pass filtering by the Butterworth polynomial,and modal parameter identification using the calculated displacement.The superiority of the proposed strategy is verified by analyzing a numerical OWT with a high-pile foundation and the measured accelerations from an OWT with a high-pile foundation.The results show that for OWTs with high-pile foundations dominated by low frequencies,the developed strategy of converting accelerations into displacements and then performing modal parameter identification is advantageous to the identification of modal parameters,and the results have high accuracy.

Concept Design and Coupled Dynamic Response Analysis on 6-MW Spar-Type Floating Offshore Wind Turbine

Tower, Spar platform and mooring system are designed in the project based on a given 6-MW wind turbine. Under wind-induced only, wave-induced only and combined wind and wave induced loads, dynamic response is analyzed for a 6-MW Spar-type floating offshore wind turbine （FOWT） under operating conditions and parked conditions respectively. Comparison with a platform-fixed system （land-based system） ofa 6-MW wind turbine is carried out as well. Results demonstrate that the maximal out-of-plane deflection of the blade of a Spar-type system is 3.1% larger than that of a land-based system; the maximum response value of the nacelle acceleration is 215% larger for all the designed load cases being considered; the ultimate tower base fore-aft bending moment of the Spar-type system is 92% larger than that of the land-based system in all of the Design Load Cases （DLCs） being considered; the fluctuations of the mooring tension is mainly wave-induced, and the safety factor of the mooring tension is adequate for the 6-MW FOWT. The results can provide relevant modifications to the initial design for the Spar-type system, the detailed design and model basin test of the 6-MW Spar-type system.

A simplified method for analyzing the fundamental frequency of monopile supported offshore wind turbine system design

Preliminary design of offshore wind turbines requires high precision simplified methods for the analysis of the system fundamental frequency. Based on the Rayleigh method and Lagrange’s Equation, this study establishes a simple formula for the analysis of system fundamental frequency in the preliminary design of an offshore wind turbine with a monopile foundation. This method takes into consideration the variation of cross-section geometry of the wind turbine tower along its length, with the inertia moment and distributed mass both changing with diameter. Also the rotational flexibility of the monopile foundation is mainly considered. The rigid pile and elastic middle long pile are calculated separately. The method is validated against both FEM analysis cases and field measurements, showing good agreement. The method is then used in a parametric study, showing that the tower length Lt, tower base diameter d0, tower wall thickness δt, pile diameter db and pile length Lb are the major factors influencing the fundamental frequency of the offshore wind turbine system. In the design of offshore wind turbine systems, these five parameters should be adjusted comprehensively. The seabed soil condition also needs to be carefully considered for soft clay and loose sand.

Influential Factors of Bucket Foundation for Offshore Wind Turbine

In this paper, the influential design thctors of wide-shallow composite bucket foundation for 3 MW off- shore wind turbine are systematically studied by numerical simulation. The results show that the bucket diameter is larger than 27 m in generak and the range of 7--12 m is appropriate for cylinder height. In particular the bucket foun- dation with diameter of 30 m and cylinder height of 10 m is suitable for most soils. Under ultimate loads, the bucket diameter and elasticity modulus of soil have major effects on the deibrmability of bucket foundation, while the influ- ence of friction coefficient between the bucket and soil is relatively slight.

Experimental Study on Tilt Adjustment Technique of Tripod Bucket Jacket Foundation for Offshore Wind Turbine in Sand

For the tripod bucket jacket foundations used in offshore wind turbines, the probable critical tilt angles should be avoidedduring tilt adjustment operation. Thus, these critical values must be identified by engineers, and remedial techniques mustbe established prior to the occurrence of the problem. Model tests were carried out for typical tilting conditions of tripodbucket foundations, which were allowed to tilt freely at various penetration depths without interruption by manualoperation. After the foundation ceased its tilting, some measures, such as water pumping, water injection, air injection, or acombination of the above methods, were enabled for adjustment. The research results showed two critical values in thetilting state of the tripod bucket jacket foundation, namely the terminal and allowable angles. In the installation condition,the terminal angle was negatively correlated with the initial penetration depth, but the opposite was observed with theremoval condition. The allowable angle was less than or equal to the terminal angle. The allowable angle in the installationwas related to the terminal angle. The critical angles all varied linearly with the initial penetration depth. When tiltingduring installation, adjustment measures can be used in the order of high drum pumping, low drum water injection, highdrum pumping and low drum water injection, air injection, and exhaust. When tilting during removal, the sequential use oflow drum water injection, air, and exhaust was applied. For buckets that were sensitive to angle changes, adjustmentmeasures of the “point injection” mode can be selected.

Preliminary Design of a Submerged Support Structure for Floating Wind Turbines

Cost-effective floating wind turbines with efficient installations are highly desired in deep waters(>50 m).This paper presents a submerged floating offshore wind turbines(SFOWT)concept for intermediate water depths(50-200 m).The performance of SFOWTs can be improved through a judicious choice of configuration,pretension,and mooring line layout.Four SFOWTs with different configurations and a similar mass,named Cyl-4,Cub-4,Cyl-3,and Hex-3,were designed and analyzed.The responses of the four SFOWTs were predicted under operational condition and extreme condition.The results show that the four SFOWTs exhibited good performance under both conditions.The effect of platform configurations on power output was negligible under the operational condition.Under the extreme condition,among the four SFOWTs,the mean bending moments at the tower base were very close,while the maximum values differed by up to 21.5%,due to the configurations.The effect of wind-wave misalignment under the extreme condition was further analyzed.In general,the motion performances of the four-pontoon SFOWTs,Cyl-4 and Cub-4,were superior to those of the three-pontoon SFOWTs,Cyl-3 and Hex-3.Optimization studies of the mooring system were carried out on Cub-4 with different mooring line pretensions and four mooring layouts.The optimized Cub-4 could reduce the maximum motion responses in the surge,heave,and yaw by 97.7%,91.5%,and 98.7%,respectively.

Motion Characteristics of Novel Floating Foundation for Offshore Wind Turbine

A novel floating foundation to support the NREL offshore 5 MW wind turbine was designed conceptually by combining the characteristics of barge and Spar. The main focus was structural design and hydrodynamic modelling. Based on this novel floating foundation, the hydrodynamic performance was investigated in the frequency domain and time domain by using the wave analysis software Hydro D and Deep C from Det Norske Veritas. The frequency domain analysis was conducted to investigate the effects of the incident wave angle and water depth. The time-domain analysis was carried out to evaluate the response of the floating foundation under a selected operational condition. The hydrodynamic performances of this floating foundation with respect to time series and response spectra were also investigated in this study.

Criterion of aerodynamic performance of large-scale offshore horizontal axis wind turbines

With the background of offshore wind energy projects, this paper studies aerodynamic performance and geometric characteristics of large capacity wind turbine rotors （1 to 10 MW）, and the main characteristic parameters such as the rated wind speed, blade tip speed, and rotor solidity. We show that the essential criterion of a high- performance wind turbine is a highest possible annual usable energy pattern factor and a smallest possible dimension, capturing the maximum wind energy and producing the maximum annual power. The influence of the above-mentioned three parameters on the pattern factor and rotor geometry of wind turbine operated in China＇s offshore meteoro- logical environment is investigated. The variation patterns of aerodynamic and geometric parameters are obtained, analyzed, and compared with each other. The present method for aerodynamic analysis and its results can form a basis for evaluating aerodynamic performance of large-scale offshore wind turbine rotors.

Seismic Centrifuge Modelling of Offshore Wind Turbine with Tripod Foundation

The development of offshore wind energy is fast as it is clean, safe and of high efficiency. The harsh marine environment raises high demand on the foundation design of offshore wind turbine. Earthquake loading is one of the most significant factors which should be considered in the design phase. In this paper, a group of earthquake centrifuge tests were conducted on a physical wind turbine model with tripod foundation. The seismic responses of both wind turbine model and foundation soil were analyzed in terms of the recorded accelerations, pore water pressures, lateral displacements and settlements. The results were also compared with those measured in the previous research on mono-pile foundation. It is demonstrated that the tripod foundation can provide better resistance in the lateral displacement and structural settlement under earthquake loading.

Seismic Behavior of Offshore Wind Turbine with Gravity Foundation

The development of offshore wind energy becomes very fast in recent years due to its clean, safe, and high efficiency. However, the issue that quite a few offshore wind farms have been built in seismic active areas raises a great engineering challenge for the selection, design, and seismic evaluation of offshore wind turbine foundations. Earthquake is one of the most critical hazards for offshore wind turbines. The softening of soil due to pore water pressure buildup can sharply reduce the bearing capacity of the foundation, and consequently result in stability failure. The induced strong structural vibration has adverse impact on the normal operation of wind turbine as well as on the efficiency of power generation. In this study, a group of earthquake centrifuge tests was performed on a physical model of a wind turbine with gravity foundation. The seismic behavior of both the structure and the foundation soil was analyzed based on the recorded accelerations, pore water pressures, lateral displacements and settlements. The emphasis was on the interaction between foundation and soil. The results showed that gravity foundation can effectively resist the overturning moment induced by the superstructure. However, it was quite sensitive to the subsoil conditions. The large settlement and tilt in the offshore foundation might affect the performance of a wind turbine.

Dynamic responses research of offshore wind turbine tripod foundation and upper structure

In order to obtain the performance of the offshore wind turbine tripod foundation, a tripod foundation model was built by ANSYS. The static analysis, modal analysis and the transient dynamic analysis were run. Different parameters such as displacement, velocity, acceleration, stress were obtained and by analyzing these data, it is reasonable to draw a conclusion that the tripod foundation has a good performance used on the offshore wind turbine.

Experiment Study of Dynamics Response for Wind Turbine System of Floating Foundation

The floating foundation is designed to support a 1.5 MW wind turbine in 30 m water depth. With consideration of the viscous damping of foundation and heave plates, the amplitude-frequency response characteristics of the foundation are studied. By taking into account the elastic effect of blades and tower, the classic quasi-steady blade-element/momentum（BEM） theory is used to calculate the aerodynamic elastic loads. A coupled dynamic model of the turbine-foundationmooring lines is established to calculate the motion response of floating foundation under Kaimal wind spectrum and regular wave by using the FAST codes. The model experiment is carried out to test damping characteristics and natural motion behaviors of the wind turbine system. The dynamics response is tested by considering only waves and the joint action of wind and waves. It is shown that the wind turbine system can avoid resonances under the action of wind and waves. In addition, the heave motion of the floating foundation is induced by waves and the surge motion is induced by wind. The action of wind and waves is of significance for pitch.