Comparisons of Wave Force Model Effects on the Structural Responses and Fatigue Loads of a Semi-Submersible Floating Wind Turbine

The selection of wave force models will significantly impact the structural responses of floating wind turbines.In this study,comparisons of wave force model effects on the structural responses and fatigue loads of a semi-submersible floating wind turbine(SFWT)were conducted.Simulations were performed by employing the Morison equation(ME)with linear or second-order wave kinematics and potential flow theory(PFT)with first-or second-order wave forces.A comparison of regular waves,irregular waves,and coupled wind/waves analyses with the experimental data showed that many of the simulation results and experimental data are relatively consistent.However,notable discrepancies are found in the response amplitude operators for platform heave,tower base bending moment,and tension in mooring lines.PFT models give more satisfactory results of heave but more significant discrepan-cies in tower base bending moment than the ME models.In irregular wave analyses,low-frequency resonances were captured by PFT models with second-order difference-frequency terms,and high-frequency resonances were captured by the ME models or PFT models with second-order sum-frequency terms.These force models capture the response frequencies but do not reasonably predict the response amplitudes.The coupled wind/waves analyses showed more satisfactory results than the wave-only analyses.However,an important detail to note is that this satisfactory result is based on the overprediction of wind-induced responses.

Dynamic Response Analysis of Semi-Submersible Floating Wind Turbine with Different Wave Conditions

To address the problem of poor wave resistance of existing offshore floating wind turbines,a new type of semisubmersible platform with truncated-cone-type upper pontoons is proposed by combining the characteristics of offshore wind turbine semi-submersible floating platforms.Based on the coupled hydrodynamic,aerodynamic,and mooring force physical fields of FAST,the surge,heave,pitch,and yaw motions responses of the floating wind turbine under different wave heights and periods are obtained,and the mooring line tension responses are also obtained;and compare the dynamic response of the new semi-submersible platform with the OC4-DeepCwind platformat six degrees of freedom.The results show that different wave conditions have obvious effects on the heave and pitch motions of the new floating wind turbine,and fewer effects on the surge and yaw motions;the tensegrity response of the mooring system is more affected by the wave conditions;compared with the OC4-DeepCwind floating wind turbine,the pitch and roll response of the new floating wind turbine has been significantly reduced and has good stability.

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.

Dynamic Analysis of a 10 MW Floating Offshore Wind Turbine Considering the Tower and Platform Flexibility

Recently,semisubmersible floating offshore wind turbine technologies have received considerable attention.For the coupled simulation of semisubmersible floating offshore wind energy,the platform is usually considered a rigid model,which could affect the calculation accuracy of the dynamic responses.The dynamic responses of a TripleSpar floating offshore wind turbine equipped with a 10 MW offshore wind turbine are discussed herein.The simulation of a floating offshore wind turbine under regular waves,white noise waves,and combined wind-wave conditions is conducted.The effects of the tower and platform flexibility on the motion and force responses of the TripleSpar semisubmersible floating offshore wind turbine are investigated.The results show that the flexibility of the tower and platform can influence the dynamic responses of a TripleSpar semisubmersible floating offshore wind turbine.Considering the flexibility of the tower and platform,the tower and platform pitch motions markedly increased compared with the fully rigid model.Moreover,the force responses,particularly for tower base loads,are considerably influenced by the flexibility of the tower and platform.Thus,the flexibility of the tower and platform for the coupled simulation of floating offshore wind turbines must be appropriately examined.

Coupled Time-Domain Investigation on a Vertical Axis Wind Turbine Supported on a Floating Platform

The dynamic responses of a floating vertical axis wind turbine(VAWT)are assessed on the basis of an aero-hydro-mooring coupled model.The aerodynamic loads on the rotor are acquired with double-multiple stream tube method.First-and second-order wave loads are calculated on the basis of 3D potential theory.The mooring loads are simulated by catenary theory.The coupled model is established,and a numerical code is programmed to investigate the dynamic response of the semi-submersible VAWT.A model test is then conducted,and the numerical code is validated considering the hydrodynamic performance of the floating buoy.The responses of the floating VAWT are studied through the numerical simulation under the sea states of wind and regular/irregular waves.The effects of the second-order wave force on the motions are also investigated.Results show that the slow-drift responses in surge and pitch motions are significantly excited by the second-order wave forces.Furthermore,the effect of foundation motion on aerodynamic loads is examined.The normal and tangential forces of the blades demonstrate a slight increase due to the coupling effect between the buoy motion and the aerodynamic loads.

Wake Effects on A Hybrid Semi-Submersible Floating Wind Farm with Multiple Hub Heights

Wind farms generally consist of a single turbine installed with the same hub height. As the scale of turbines increases,wake interference between turbines becomes increasingly significant, especially for floating wind turbines(FWT).Some researchers find that wind farms with multiple hub heights could increase the annual energy production(AEP),while previous studies also indicate that wake meandering could increase fatigue loading. This study investigates the wake interaction within a hybrid floating wind farm with multiple hub heights. In this study, FAST.Farm is employed to simulate a hybrid wind farm which consists of four semi-submersible FWTs(5MW and 15MW) with two different hub heights. Three typical wind speeds(below-rated, rated, and over-rated) are considered in this paper to investigate the wake meandering effects on the dynamics of two FWTs. Damage equivalent loads(DEL) of the turbine critical components are computed and analyzed for several arrangements determined by the different spacing of the four turbines. The result shows that the dynamic wake meandering significantly affects downstream turbines’ global loadings and load effects. Differences in DEL show that blade-root flapwise bending moments and mooring fairlead tensions are sensitive to the spacing of the turbines.

Research on the Influence of Helical Strakes on Dynamic Response of Floating Wind Turbine Platform

The stability of platform structure is the paramount guarantee of the safe operation of the offshore floating wind turbine. The NREL 5MW floating wind turbine is established based on the OC3-Hywind Spar Buoy platform with the supplement of helical strakes for the purpose to analyze the impact of helical strakes on the dynamic response of the floating wind turbine Spar platform. The dynamic response of floating wind turbine Spar platform under wind, wave and current loading from the impact of number, height and pitch ratio of the helical strakes is analysed by the radiation and diffraction theory, the finite element method and orthogonal design method. The result reveals that the helical strakes can effectively inhibit the dynamic response of the platform but enlarge the wave exciting force; the best parameter combination is two pieces of helical strakes with the height of 15%D （D is the diameter of the platform） and the pitch ratio of 5; the height of the helical strake and its pitch ratio have significant influence on pitch response.

Coupled Dynamic Response Analysis of A Multi-Column Tension-Leg-Type Floating Wind Turbine

This paper presents a coupled dynamic response analysis of a multi-column tension-leg-type floating wind turbine（Wind Star TLP system） under normal operation and parked conditions. Wind-only load cases, wave-only load cases and combined wind and wave load cases were analyzed separately for the Wind Star TLP system to identify the dominant excitation loads. Comparisons between an NREL offshore 5-MW baseline wind turbine installed on land and the Wind Star TLP system were performed. Statistics of selected response variables in specified design load cases（DLCs） were obtained and analyzed. It is found that the proposed Wind Star TLP system has small dynamic responses to environmental loads and it thus has almost the same mean generator power output under operating conditions as the land-based system. The tension mooring system has a sufficient safety factor, and the minimum tendon tension is always positive in all selected DLCs. The ratio of ultimate load of the tower base fore-aft bending moment for the Wind Star TLP system versus the land-based system can be as high as 1.9 in all of the DLCs considered. These results will help elucidate the dynamic characteristics of the proposed Wind Star TLP system, identify the difference in load effect between it and land-based systems, and thus make relevant modifications to the initial design for the Wind Star TLP system.

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.

Experimental Study on New Multi-Column Tension-Leg-Type Floating Wind Turbine

Deep-water regions often have winds favorable for offshore wind turbines, and floating turbines currently show the greatest potential to exploit such winds. This work established proper scaling laws for model tests, which were then implemented in the construction of a model wind turbine with optimally designed blades. The aerodynamic,hydrodynamic, and elastic characteristics of the proposed new multi-column tension-leg-type floating wind turbine(Wind Star TLP system) were explored in the wave tank testing of a 1:50 scale model at the State Key Laboratory of Ocean Engineering at Shanghai Jiao Tong University. Tests were conducted under conditions of still water, white noise waves, irregular waves, and combined wind, wave, and current loads. The results established the natural periods of the motion, damping, motion response amplitude operators, and tendon tensions of the Wind Star TLP system under different environmental conditions, and thus could serve as a reference for further research.

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.

Damage Identification of A TLP Floating Wind Turbine by Meta-Heuristic Algorithms

Damage identification of the offshore floating wind turbine by vibration/dynamic signals is one of the important and new research fields in the Structural Health Monitoring（SHM）. In this paper a new damage identification method is proposed based on meta-heuristic algorithms using the dynamic response of the TLP（Tension-Leg Platform） floating wind turbine structure. The Genetic Algorithms（GA）, Artificial Immune System（AIS）, Particle Swarm Optimization（PSO）, and Artificial Bee Colony（ABC） are chosen for minimizing the object function, defined properly for damage identification purpose. In addition to studying the capability of mentioned algorithms in correctly identifying the damage, the effect of the response type on the results of identification is studied. Also, the results of proposed damage identification are investigated with considering possible uncertainties of the structure. Finally, for evaluating the proposed method in real condition, a 1/100 scaled experimental setup of TLP Floating Wind Turbine（TLPFWT） is provided in a laboratory scale and the proposed damage identification method is applied to the scaled turbine.

Effects of incident wind/wave directions on dynamic response of a SPAR-type floating offshore wind turbine system

As a promising renewable energy,offshore wind energy currently is gaining more attention,by which the economic and efficient operation of floating wind turbine systems is a potential research direction.This study is primarily devoted to the analysis of dynamic response of the NREL-5 MW reference wind turbine supported by an OC3-Hywind SPAR-type platform using a recompiled code which combines FAST with WAMIT.To verify the reliability of the recompiled code,the free decay motions of a floating wind turbine system in still water are examined with satisfactory results.After that,thirteen scenarios with different angles between wind and wave from 0°to 90°are investigated.The dynamic responses of the turbine system in various degrees of freedom(DOFs)for different incident wind/wave directions are presented in both time and frequency domains via the fast Fourier transform.

Study on Rigid-Flexible Coupling Effects of Floating Offshore Wind Turbines

In order to account for rigid-flexible coupling effects of floating offshore wind turbines, a nonlinear rigid-flexible coupled dynamic model is proposed in this paper. The proposed nonlinear coupled model takes the higher-order axial displacements into account, which are usually neglected in the conventional linear dynamic model. Subsequently,investigations on the dynamic differences between the proposed nonlinear dynamic model and the linear one are conducted. The results demonstrate that the stiffness of the turbine blades in the proposed nonlinear dynamic model increases with larger overall motions but that in the linear dynamic model declines with larger overall motions.Deformation of the blades in the nonlinear dynamic model is more reasonable than that in the linear model as well.Additionally, more distinct coupling effects are observed in the proposed nonlinear model than those in the linear model. Finally, it shows that the aerodynamic loads, the structural loads and global dynamic responses of floating offshore wind turbines using the nonlinear dynamic model are slightly smaller than those using the linear dynamic model. In summary, compared with the conventional linear dynamic model, the proposed nonlinear coupling dynamic model is a higher-order dynamic model in consideration of the rigid-flexible coupling effects of floating offshore wind turbines, and accord more perfectly with the engineering facts.

Platform motion minimization using model predictive control of a floating offshore wind turbine

Wind turbines are installed offshore with the assistance of a floating platform to help meet the world’s increasing energy needs.However,the incident wind and extra incident wave disturbances have an impact on the performance and operation of the floating offshore wind turbine(FOWT)in comparison to bottom-fixed wind turbines.In this paper,model predictive control(MPC)is utilized to overcome the limitation caused by platform motion.Due to the ease of control synthesis,the MPC is developed using a simplified model instead of high fidelity simulation model.The performance of the controller is verified in the presence of realistic wind and wave disturbances.The study demonstrates the effectiveness of MPC in reducing platform motions and rotor/generator speed regulation of FOWTs.

Dynamic Responses of A Semi-Type Offshore Floating Wind Turbine During Normal State and Emergency Shutdown

This paper addresses joint wind-wave induced dynamic responses of a semi-type offshore floating wind turbine（OFWT） under normal states and fault event conditions. The analysis in this paper is conducted in time domain, using an aero-hydro-servo-elastic simulation code-FAST. Owing to the unique viscous features of the reference system, the original viscous damping model implemented in FAST is replaced with a quadratic one to gain an accurate capture of viscous effects. Simulation cases involve free-decay motion in still water, steady motions in the presence of regular waves and wind as well as dynamic response in operational sea states with and without wind. Simulations also include the cases for transient responses induced by fast blade pitching after emergency shutdown. The features of platform motions, local structural loads and a typical mooring line tension force under a variety of excitations are obtained and investigated.

Dynamic Performance Investigation of A Spar-Type Floating Wind Turbine Under Different Sea Conditions

Both numerical calculation and model test are important techniques to study and forecast the dynamic responses of the floating offshore wind turbine(FOWT). However, both the methods have their own limitations at present. In this study, the dynamic responses of a 5 MW OC3 spar-type floating wind turbine designed for a water depth of 200 m are numerically investigated and validated by a 1:50 scaled model test. Moreover, the discrepancies between the numerical calculations and model tests are obtained and discussed. According to the discussions, it is found that the surge and pitch are coupled with the mooring tensions, but the heave is independent of them. Surge and pitch are mainly induced by wave under wind wave conditions. Wind and current will induce the low-frequency average responses, while wave will induce the fluctuation ranges of the responses. In addition, wave will induce the wavefrequency responses but wind and current will restrain the ranges of the responses.

Characteristic Aerodynamic Loads and Load Effects on the Dynamics of a Floating Vertical Axis Wind Turbine

The development of offshore wind farms in deep water favors floating wind turbine designs, but floating horizontal axis wind turbines are facing the challenge of high cost of energy(CoE). The development of innovative designs to reduce the CoE is thus desirable, such as floating vertical axis wind turbines(VAWTs). This study demonstrates the characteristics of aerodynamic loads and load effects of a two-bladed floating VAWT supported by a semi-submersible platform. Fully coupled simulations are performed using the time-domain aero-hydro-servo-elastic code SIMO-RIFLEX-AC. It is found that thrust, lateral force, and aerodynamic torque vary considerably and periodically with the rotor azimuth angle. However, the variation in the generator torque can be alleviated to some extent by the control strategy applied. Moreover, the variations of platform motions and tensions in the mooring lines are strongly influenced by turbulent winds, whereas those of tower-base bending moments are not. The towerbase bending moments exhibit notable two-per-revolution(2P) response characteristics.

Coupled Aerodynamic and Hydrodynamic Analysis of Floating Offshore Wind Turbine Using CFD Method

To simulate floating offshore wind turbine(FOWT)in coupled wind-wave domain via CFD method,the NREL 5MW wind turbine supported by the OC3-Hywind Spar platform is modeled in the STAR-CCM+ software.Based on the Reynolds-averaged Navier-Stokes(RANS)equations and re-normalisation group(RNG)k-εturbulence model,the rotor aerodynamic simulation for wind turbine is conducted.Numerical results agree well with the NREL data.Taking advantage with the volume of fluid(VOF)method and dynamic fluid body interaction(DFBI)technology,the dynamic responses of the floating system with mooring lines are simulated under the coupled wind-wave sea condition.The free-decay tests for rigid-body degrees of freedom(DOFs)in still water and hydrodynamic tests in a regular wave are performed to validate the numerical model by comparing its result with the results simulated by FAST.Finally,the simulations of the overall FOWT system in the coupled wind-wave flow field are carried out.The relationship between the power output and dynamic motion responses of the platform is investigated.The numerical results show that the dynamic response of wind turbine performance and platform motions all vary in the same frequency as the inlet wave.During platform motion,the power output of wind turbine is more sensitive than the thrust force.This study may provide some reference for further research in the coupled aero-hydro simulation of FOWT.

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.