Aero-Hydrodynamic Coupled Dynamic Characteristics of Semi-Submersible Floating Offshore Wind Turbines Under Inflow Turbulence

In this study,the frequency characteristics of the turbulent wind and the effects of wind-wave coupling on the low-and high-frequency responses of semi-submersible floating offshore wind turbines(FOWT)are investigated.Various wave load components,such as first-order wave loads,combined first-and second-order difference-frequency wave loads,combined first-and second-order sum-frequency wave loads,and first-and complete second-order wave loads are taken into consideration,while different turbulent environments are considered in aerodynamic loads.The com-parison is based on time histories and frequency spectra of platform motions and structural load responses and statistical values.The findings indicate that the second-order difference-frequency wave loads will significantly increase the natural frequency of low-frequency motion in the responses of the platform motion and structure load of the semi-submersible platform,which will cause structural fatigue damage.Under the action of turbulent wind,the influences of second-order wave loads on the platform motion and structural load response cannot be ignored,especially under extreme sea conditions.Therefore,in order to evaluate the dynamic responses of semi-submersible FOWT more accurately,the actual environment should be simulated more realistically.

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.

Evaluating effectiveness of multiple tuned mass dampers for vibration control of jacket offshore wind turbines under onshore and seafloor earthquakes

The dynamic characteristics and structural responses of operation and grid loss offshore wind turbines(OWTs)under onshore and seafloor earthquakes are analyzed based on the established coupled seismic analysis model.In addition to the remarkable influence of the rotor system on the responses of the operation OWT under earthquakes,interactions among the natural modes of the grid loss OWT in the fore-aft and side-to-side directions are revealed.By comparing with the onshore earthquakes,the more significant differences of structural response are observed under the selected seafloor earthquakes,due to the longer duration and more abundant energy distribution around the natural frequencies of OWT.Concurrently,a multiple tuned mass damper(MTMD)is designed and applied to the operation and grid loss OWTs.Then,the comparisons of the mitigation effects under onshore and seafloor ground motions are carried out,and the necessity of applying seafloor ground motions to OWTs are proved.Moreover,in comparison to the operation OWT,more effective reductions are observed for the grid loss OWT under onshore and seafloor earthquakes using the designed MTMD.Therefore,the combined shutdown procedures and MTMD vibration control strategy is suggested for OWTs under earthquakes.

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.

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.

Review of Experimental-Numerical Methodologies and Challenges for Floating Offshore Wind Turbines

Due to the dissimilar scaling issues,the conventional experimental method of FOWTs can hardly be used directly to validate the full-scale global dynamic responses accurately.Therefore,it is of absolute necessity to find a more accurate,economic and efficient approach,which can be utilized to predict the full-scale global dynamic responses of FOWTs.In this paper,a literature review of experimental-numerical methodologies and challenges for FOWTs is made.Several key challenges in the conventional basin experiment issues are discussed,including scaling issues;coupling effects between aero-hydro and structural dynamic responses;blade pitch control strategies;experimental facilities and calibration methods.Several basin experiments,industrial projects and numerical codes are summarized to demonstrate the progress of hybrid experimental methods.Besides,time delay in hardware-in-the-loop challenges is concluded to emphasize their significant role in real-time hybrid approaches.It is of great use to comprehend these methodologies and challenges,which can help some future researchers to make a footstone for proposing a more efficient and functional hybrid basin experimental and numerical method.

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.

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.

Review on Research about Wake Effects of Offshore Wind Turbines

In recent years,the construction of offshore wind farms is developing rapidly.As the wake effect of the upstream wind turbines seriously affect the performance of the downstream wind turbines,the wake effect of offshore wind turbines has become one of the research hotspots.First,this article reviews the research methods of wake effects,including CFD numerical simulation method,wind turbine wake model based on roughness and engineering wake models.However,there is no general model that can be used directly.Then it puts forward some factors that affect the wake of offshore wind turbines.The turbulence intensity in offshore wind fields is lower than that in onshore wind fields.This makes the wake recovery length of offshore wind turbines longer than that of onshore wind turbines.Floating offshore wind turbines are simultaneously disturbed by wind loads and wave loads.Unsteady movement of the platform caused by wave loads.It affects the development and changes of the wake of wind turbines.In this regard,the focus of research on the wake effects of offshore wind farms will be the proposal of accurate prediction models for the wake effects of sea wind farms.

Current Status and Future Trends for Mooring Systems of Floating Offshore Wind Turbines

With the increasing demand of energy and the limitation of bottom-fixed wind turbines in moderate and deep waters,floating offshore wind turbines are doomed to be the right technical choice and they are bound to enter a new era of rapid development.The mooring system is a vital system of a floating wind turbine for station-keeping under harsh environmental con­ditions.In terms of existing floating wind turbine projects,this paper is devoted to discussing the current status of mooring systems and mooring equipment.This paper also presents the mooring analysis methods and points out the technical difficulties and challenges in mooring design,in­stallation,operation and maintenance stages.Finally,the developing trends of the mooring system are summarized,aiming to provide a reference for future mooring research.

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.

Numerical Study of a Novel Procedure for Installing the Tower and Rotor Nacelle Assembly of Offshore Wind Turbines Based on the Inverted Pendulum Principle

Current installation costs of offshore wind turbines(OWTs) are high and profit margins in the offshore wind energy sector are low, it is thus necessary to develop installation methods that are more efficient and practical. This paper presents a numerical study(based on a global response analysis of marine operations) of a novel procedure for installing the tower and Rotor Nacelle Assemblies(RNAs) on bottom-fixed foundations of OWTs. The installation procedure is based on the inverted pendulum principle. A cargo barge is used to transport the OWT assembly in a horizontal position to the site, and a medium-size Heavy Lift Vessel(HLV) is then employed to lift and up-end the OWT assembly using a special upending frame. The main advantage of this novel procedure is that the need for a huge HLV(in terms of lifting height and capacity) is eliminated. This novel method requires that the cargo barge is in the leeward side of the HLV(which can be positioned with the best heading) during the entire installation. This is to benefit from shielding effects of the HLV on the motions of the cargo barge, so the foundations need to be installed with a specific heading based on wave direction statistics of the site and a typical installation season. Following a systematic approach based on numerical simulations of actual operations, potential critical installation activities, corresponding critical events, and limiting(response) parameters are identified. In addition, operational limits for some of the limiting parameters are established in terms of allowable limits of sea states. Following a preliminary assessment of these operational limits, the duration of the entire operation, the equipment used, and weather-and water depth-sensitivity, this novel procedure is demonstrated to be viable.

Contribution of tuned liquid column gas dampers to the performance of offshore wind turbines under wind, wave, and seismic excitations

The main intention of the present study is to reduce wind, wave, and seismic induced vibrations of jacket- type offshore wind turbines （JOWTs） through a newly developed vibration absorber, called tuned liquid column gas damper （TLCGD）. Using a Simulink-based model, an analytical model is developed to simulate global behavior of JOWTs under different dynamic excitations. The study is followed by a parametric study to explore efficiency of the TLCGD in terms of nacelle acceleration reduction under wind, wave, and earthquake loads. Study results indicate that optimum frequency of the TLCGD is rather insensitive to excitation type. In addition, while the gain in vibration control from TLCGDs with higher mass ratios is generally more pronounced, heavy TLCGDs are more sensitive to their tuned frequency such that ill-regulated TLCGD with high mass ratio can lead to destructive results. It is revealed that a well regulated TLCGD has noticeable contribution to the dynamic response of the JOWT under any excitation.

Prediction of Short-Term Distributions of Load Extremes of Offshore Wind Turbines

This paper proposes a new methodology to select an optimal threshold level to be used in the peak over threshold （POT） method for the prediction of short-term distributions of load extremes of offshore wind turbines. Such an optimal threshold level is found based on the estimation of the variance-to-mean ratio for the occurrence of peak values, which characterizes the Poisson assumption. A generalized Pareto distribution is then fitted to the extracted peaks over the optimal threshold level and the distribution parameters are estimated by the method of the maximum spacing estimation. This methodology is applied to estimate the short-term distributions of load extremes of the blade bending moment and the tower base bending moment at the mudline of a monopile-supported 5MW offshore wind turbine as an example. The accuracy of the POT method using the optimal threshold level is shown to be better, in terms of the distribution fitting, than that of the POT methods using empirical threshold levels. The comparisons among the short-term extreme response values predicted by using the POT method with the optimal threshold levels and with the empirical threshold levels and by using direct simulation results further substantiate the validity of the proposed new methodology.

Analysis of Key Disciplinary Parameters in Floating Offshore Wind Turbines with An AI-Based SADA Method

Floating offshore wind turbines(FOWTs)are a promising offshore renewable energy harvesting facility but requesting multiple-disciplinary analysis for their dynamic performance predictions.However,engineering-fidelity level tools and the empirical parameters pose challenges due to the strong nonlinear coupling effects of FOWTs.A novel method,named SADA,was proposed by Chen and Hu(2021)for optimizing the design and dynamic performance prediction of FOWTs in combination with AI technology.In the SADA method,the concept of Key Disciplinary Parameters(KDPs)is also proposed,and it is of crucial importance in the SADA method.The purpose of this paper is to make an in-depth investigation of the characters of KDPs and the internal correlations between different KDPs in the dynamic performance prediction of FOWTs.Firstly,a brief description of SADA is given,and the basin experimental data are used to conduct the training process of SADA.Secondly,categories and boundary conditions of KDPs are introduced.Three types of KDPs are given,and different boundary conditions are used to analyze KDPs.The results show that the wind and current in Environmental KDPs are strongly correlated with the percentage difference of dynamic response rather than that by wave parameters.In general,the optimization results of SADA consider the specific basin environment and the coupling results between different KDPs help the designers further understand the factors that have a more significant impact on the FOWTs system in a specific domain.

Long-term dynamic behavior of monopile supported offshore wind turbines in sand

The complexity of the loads acting on the offshore wind turbines （OWl＇s） structures and the significance of investigation on structure dynamics are explained. Test results obtained from a scaled wind turbine model are also summarized. The model is supported on monopile, subjected to different types of dynamic loading using an innovative out of balance mass system to apply cyclic/dynamic loads. The test results show the natural frequency of the wind turbine structure increases with the number of cycles, but with a reduced rate of increase with the accumulation of soil strain level. The change is found to be dependent on the shear strain level in the soil next to the pile which matches with the expectations from the element tests of the soil. The test results were plotted in a non-dimensional manner in order to be scaled to predict the orototvoe conseouences usin~ element tests of a soil usin~ resonant column aoDararus.

Study on Gyroscopic Effect of Floating Offshore Wind Turbines

Compared with bottom-fixed wind turbines,the supporting platform of a floating offshore wind turbine has a larger range of motion,so the gyroscopic effects of the system will be more obvious.In this paper,the mathematical analytic expression of the gyroscopic moment of a floating offshore wind turbine is derived firstly.Then,FAST software is utilized to perform a numerical analysis on the model of a spar-type horizontal axis floating offshore wind turbine,OC3-Hywind,so as to verify the correctness of the theoretical analytical formula and take an investigation on the characteristics of gyroscopic effect.It is found that the gyroscopic moment of the horizontal axis floating offshore wind turbine is essentially caused by the vector change of the rotating rotor,which may be due to the pitch or yaw motion of the floating platform or the yawing motion of the nacelle.When the rotor is rotating,the pitch motion of the platform mainly excites the gyroscopic moment in the rotor’s yaw direction,and the yaw motion of the platform largely excites the rotor’s gyroscopic moment in pitch direction,accordingly.The results show that the gyroscopic moment of the FOWT is roughly linearly related to the rotor’s inertia,the rotor speed,and the angular velocity of the platform motion.

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.

Scour Effect on Dynamic Characteristics and Responses of Offshore Wind Turbines

The monopile foundation is the main form of offshore wind turbine foundation,and its surrounding scouring pit will reduce the constraints of the soil on the piles,which makes wind turbine foundation instability a key issue affecting the structural safety of offshore wind turbines.In previous studies,the rotating rotor and control system are neglected when studying the influence of scour on the offshore wind turbine structure.In this paper,the numerical model of the blade-tower-monopile integrated offshore wind turbine is established,and the influence of scour on the dynamic characteristics of wind turbine is obtained considering parameters,such as blade azimuth,pitch angle,rotor speed,and soil stiffness.After calculating wind load by using the modified Blade Element Momentum theory,the impact of scour on the wind-induced response of a monopile wind turbine is achieved,considering control system and aeroelastic coupling.Therefore,a reference method is proposed for estimating scour depth according to the monopile foundation offshore wind turbine wind-induced response.

Fault Diagnosis Based on Interpretable Convolutional Temporal-spatial Attention Network for Offshore Wind Turbines

Fault diagnosis(FD)for offshore wind turbines(WTs)are instrumental to their operation and maintenance(O&M).To improve the FD effect in the very early stage,a condition monitoring based sample set mining method from supervisory control and data acquisition(SCADA)time-series data is proposed.Then,based on the convolutional neural network(CNN)and attention mechanism,an interpretable convolutional temporal-spatial attention network(CTSAN)model is proposed.The proposed CTSAN model can extract deep temporal-spatial features from SCADA time-series data sequentially by:(1)a convolution feature extraction module to extract features based on time intervals;(2)a spatial attention module to extract spatial features considering the weights of different features;and(3)a temporal attention module to extract temporal features considering the weights of intervals.The proposed CTSAN model has the superiority of interpretability by exposing the deep temporal-spatial features extracted in a human-understandable form of the temporal-spatial attention weights.The effectiveness and superiority of the proposed CTSAN model are verified by real offshore wind farms in China.