Spar-type Floating Offshore Wind Turbine Research Articles

Hydrodynamic Responses of a 6 MW Spar-Type Floating Offshore Wind Turbine in Regular Waves and Uniform Current

In order to improve the understanding of hydrodynamic performances of spar-type Floating Offshore Wind Turbines (FOWTs), in particular the effect of wave-current-structure interaction, a moored 6MW spar-type FOWT in regular waves and uniform current is considered. The wind loads are not considered at this stage. We apply the potential-flow theory and perturbation method to solve the weakly-nonlinear problem up to the second order. Unlike the conventional formulations in the inertial frame of reference, which involve higher derivatives on the body surface, the present method based on the perturbation method in the non-inertial body-fixed coordinate system can potentially avoid theoretical inconsistency at sharp edges and associated numerical difficulties. A cubic Boundary Element Method (BEM) is employed to solve the resulting boundary-value problems (BVPs) in the time domain. The convective terms in the free-surface conditions are dealt with using a newly developed conditionally stable explicit scheme, which is an approximation of the implicit Crank–Nicolson scheme. The numerical model is firstly verified against three reference cases, where benchmark results are available, showing excellent agreement. Numerical results are also compared with a recent model test, with a fairly good agreement though differences are witnessed. Drag loads based on Morison’s equation and relative velocities are also applied to quantify the influence of the viscous loads. To account for nonlinear restoring forces from the mooring system, a catenary line model is implemented and coupled with the time-domain hydrodynamic solver. For the considered spar-type FOWT in regular-wave and current conditions, the current has non-negligible effects on the motions at low frequencies, and a strong influence on the mean wave-drift forces. The second-order sum-frequency responses are found to be negligibly small compared with their corresponding linear components. The viscous drag loads do not show a strong influence on the motions responses, while their contribution to the wave-drift forces being notable, which increases with increasing wave steepness.

A Numerical Prediction on the Transient Response of a Spar-Type Floating Offshore Wind Turbine in Freak Waves

Abstract Simulations are conducted in time domain to investigate the dynamic response of a spar-type floating offshore wind turbine (FOWT) under the freak wave scenarios. Toward this end, a coupled aero-hydro-mooring in-house numerical code is adopted to perform the simulations. The methodology includes a blade-element-momentum (BEM) model for simulating the aerodynamic loads, a nonlinear model for simulating the hydrodynamic loads, a nonlinear restoring model of Spar buoy, and a nonlinear algorithm for simulating the mooring cables. The OC3 Hywind spar-type FOWT is adopted as an example to study the dynamic response under the freak wave conditions, meanwhile the time series of freak waves are generated using the random frequency components selection phase modulation method. The motion of platform, the tension applied on the mooring lines, and the power generation performance are documented in several cases. According to the simulations, it is indicated that when a freak wave acts on the FOWT, the transient motion of the FOWT is induced in all degrees-of-freedom, as well as the produced power decreases rapidly. Furthermore, the impact of freak wave parameters on the motion of FOWT is discussed.

Dynamic response of spar-type floating offshore wind turbine in freak wave considering the wave-current interaction effect

In this paper, the dynamic responses of a Spar-type floating offshore wind turbine (FOWT) are simulated and analyzed under the scenarios with freak wave superimposed a uniform current. The wave-current interaction is considered in the process of freak wave generation. An improved phase modulation algorithm is used to generate the free surface profile of the freak wave. Compared with the deterministic wave modeling method, this method can realize the randomness of the sea state and also achieve high efficiency. The effect of the presence of current on the wave energy spectra is firstly investigated. Then the OC3 Hywind Spar-type FOWT in the parked condition is chosen as an example to study the dynamic responses under the extreme wave scenarios. An in-house MATLAB code based on a nonlinear aero-hydro-mooring coupled model is adopted to perform these numerical investigations. The simulations are conducted in the time domain and then the results of wave forces and platform motions in the surge, heave and pitch are summarized and analyzed considering the presence of current with different velocities. The results of mooring lines tension and the acceleration at nacelle are also investigated. Through the simulation results, we can have a clear understanding of the mechanism of the wave-current interaction phenomenon for extremely high waves in Gaussian seas. Furthermore, it also allows identifying the severest wave conditions which are important for the analysis of the floating structure ultimate design conditions.

A Novel Dynamics Analysis Method for Spar-Type Floating Offshore Wind Turbine

The dynamic behavior of floating offshore wind turbine (FOWT) is crucial for its design and optimization. A novel dynamics analysis method for the spar-type FOWT system is proposed in this paper based on the theorem of moment of momentum and the Newton's second law. The full nonlinearity of the equations of motion (EOMs) and the full nonlinear coupling between external loads and the motions are preserved in this method. Compared with the conventional methods, this method is more transparent and it can be applied directly to the large-amplitude rotation cases. An in-house code is developed to implement this method. The capability of in-house code is verified by comparing its simulation results with those predicted by FAST. Based on the in-house code, the dynamic responses of a spar-type FOWT system are investigated under various conditions.

Investigation of Interference Effects Between Wind Turbine and Spar-Type Floating Platform Under Combined Wind-Wave Excitation

In order to further understand the coupled aero-hydrodynamic performance of the floating offshore wind turbine (FOWT) in realistic ocean environment, it is necessary to investigate the interference effects between the unsteady aerodynamics of the wind turbine and different degree-of-freedom (DOF) platform motions under combined wind-wave excitation. In this paper, a validated CFD analysis tool FOWT-UALM-SJTU with modified actuator line model is applied for the coupled aero-hydrodynamic simulations of a spar-type FOWT system. The aero-hydrodynamic characteristics of the FOWT with various platform motion modes and different wind turbine states are compared and analyzed to explore the influence of the interference effects between the wind turbine and the floating platform on the performance of the FOWT. The dynamic responses of local relative wind speed and local attack angle at the blade section and wind-wave forces acting on the floating platform are discussed in detail to reveal the interaction mechanism between the aerodynamic loads and different DOF platform motions. It is shown that the surge motion and the pitch motion of the floating platform both significantly alter the local attack angle, while only the platform pitch motion have significant impacts on the local relative wind speed experienced by the rotating blades. Besides, the shaft tilt and the pro-cone angle of the wind turbine and the height-dependent wind speed all contribute to the variation of the local attack angle. The coupling between the platform motions along different DOFs is obviously amplified by the aerodynamic forces derived from the wind turbine. In addition, the wake deflection phenomenon is clearly observed in the near wake region when platform pitch motion is considered. The dynamic pitch motion of the floating platform also contributes to the severe wake velocity deficit and the increased wake width.

Long-term assessment of a floating offshore wind turbine under environmental conditions with multivariate dependence structures

Floating offshore wind turbines (FOWT) have been increasingly deployed to harvest the sustained high-speed offshore wind in recent years. Under the complex offshore environments, FOWT could experience significantly different environmental conditions with various occurrence frequencies during their lifetimes. In the present study, the dependence structure of six wind and wave-related environmental parameters (wind direction, wind speed, mean wave direction, significant wave height, peak spectral wave period, and wave directional spread) is established using a C-vine (canonical vine) copula model. Different families of bivariate copula are used as the building blocks to model the interdependence for each pair of environmental parameters. Thereafter, the environmental contour method and Rosenblatt transformation, according to a prescribed 50-year return period, are used to determine the long-term design loads at critical locations of a spar-type FOWT. Fractile level and multiplication factor are obtained for five different structural responses to provide guidance in parameter selections for future designs.

Numerical Analysis of a Floating Offshore Wind Turbine by Coupled Aero-Hydrodynamic Simulation

The exploration for renewable and clean energies has become crucial due to environmental issues such as global warming and the energy crisis. In recent years, floating offshore wind turbines (FOWTs) have attracted a considerable amount of attention as a means to exploit steady and strong wind sources available in deep-sea areas. In this study, the coupled aero-hydrodynamic characteristics of a spar-type 5-MW wind turbine are analyzed. An unsteady actuator line model (UALM) coupled with a two-phase computational fluid dynamics solver naoe-FOAM-SJTU is applied to solve three-dimensional Reynolds-averaged Navier–Stokes equations. Simulations with different complexities are performed. First, the wind turbine is parked. Second, the impact of the wind turbine is simplified into equivalent forces and moments. Third, fully coupled dynamic analysis with wind and wave excitation is conducted by utilizing the UALM. From the simulation, aerodynamic forces, including the unsteady aerodynamic power and thrust, can be obtained, and hydrodynamic responses such as the six-degrees-of-freedom motions of the floating platform and the mooring tensions are also available. The coupled responses of the FOWT for cases of different complexities are analyzed based on the simulation results. Findings indicate that the coupling effects between the aerodynamics of the wind turbine and the hydrodynamics of the floating platform are obvious. The aerodynamic loads have a significant effect on the dynamic responses of the floating platform, and the aerodynamic performance of the wind turbine has highly unsteady characteristics due to the motions of the floating platform. A spar-type FOWT consisting of NREL-5-MW baseline wind turbine and OC3-Hywind platform system is investigated. The aerodynamic forces can be obtained by the UALM. The 6DoF motions and mooring tensions are predicted by the naoe-FOAM-SJTU. To research the coupling effects between the aerodynamics of the wind turbine and the hydrodynamics of the floating platform, simulations with different complexities are performed. Fully coupled aero-hydrodynamic characteristics of FOWTs, including aerodynamic loads, wake vortex, motion responses, and mooring tensions, are compared and analyzed.

Linear Quadratic Optimal Control of a Spar-Type Floating Offshore Wind Turbine in the Presence of Turbulent Wind and Different Sea States

This paper presents the design of a linear quadratic (LQ) optimal controller for a spar-type floating offshore wind turbine (FOWT). The FOWT is exposed to different sea states and constant wind turbulence intensity above rated wind speed. A new LQ control objective is specified for the floater-turbine coupled control, in accordance with standard requirements, to reduce both rotor speed fluctuations and floater pitch motion in each relevant sea state compared with a baseline proportional-integral (PI) controller. The LQ weighting matrices are selected using time series of the wind/wave disturbances generated for the relevant sea states. A linearized state-space model is developed, including the floater surge/pitch motions, rotor speed, collective blade pitch actuation, and unmeasured environmental disturbances. The wind disturbance modeling is based on the Kaimal spectrum and aerodynamic thrust/torque coefficients. The wave disturbance modeling is based on the Pierson–Moskowitz spectrum and linearized Morison equation. A high-fidelity FOWT simulator is used to verify the control-oriented model. The simulation results for the OC3-Hywind FOWT subjected to turbulent wind show that a single LQ controller can yield both rotor speed fluctuation reduction of 32–72% and floater pitch motion reduction of 22–44% in moderate to very rough sea states compared with the baseline PI controller.

Validation of a FAST spar-type floating wind turbine numerical model with basin test data

Global development of the offshore wind energy is devoted to applying into deep water sea areas. Spar-type floating offshore wind turbine (FOWT) has been demonstrated as the most mature FOWT concept, which has been studied for the longest time by researchers among all of the FOWT concepts in the world. Due to its excellent hydrodynamic performance, spar-type FOWT is considered as the most suitable type applied in deep water and harsh sea environment conditions, which has a huge market application foreground in the future. As a consequent, investigation of the offshore wind energy requires the hydro-aero-elastic-servo simulation tools to predict the coupled complex behavior of the FOWT system. However, little validation work has been done by now in the public domain. Thus, the purpose of this paper is to validate the commonly used simulation tool, FAST (Fatigue, Aerodynamics, Structures, and Turbulence), by referring to published basin test data. Through in-depth analysis of its advantages and deficiencies, it provides a good reference for the further improvement of this floating wind turbine’s numerical simulation tool in the future.

Research on Dynamic Response Characteristics of 6MW Spar-Type Floating Offshore Wind Turbine

A 6MW spar-type floating offshore wind turbine (FOWT) model is put forward and a fully coupled aero-hydro-servo-elastic time domain model is established in the fatigue, aerodynamics, structures and turbulence (FAST) code. Influence rules of wind load and wave load on the characteristics of 6MW spar-type FOWT are investigated. Firstly, validation of the model is carried out and a satisfactory result is obtained. The maximal deviations of rotor thrust and power between simulation results and reference values are 4.54% and −2.74%, respectively. Then the characteristics, including rotor thrust, rotor power, out-of-plane blade deflection, tower base fore-aft bending moment, and mooring line tension, are researched. The results illustrate that the mean value of dynamic response characteristics is mainly controlled by the wind-induced action. For characteristics of tower base fore-aft bending moment and platform pitch motion, the oscillation is dominated by the wave-induced action during all conditions considered. For characteristics of out-of-plane blade tip deflection and mooring line tension, the oscillation is commanded by combination effect of wave and wind loads when the wind speed is lower than the rated wind speed (hereinafter referred to as below rated wind speed) and is controlled by the wave-induced action when the wind speed is higher than the rated wind speed (hereinafter referred to as above rated wind speed). As to the rotor thrust and power, the oscillation is dominated by the wind induced action at below rated wind speed and by the combination action of wind and wave loads at above rated wind speed. The results should be useful to the detailed design and model basin test of the 6MW spar-type FOWT.

Fatigue load estimation of a spar-type floating offshore wind turbine considering wave-current interactions

This paper considers the effects of current and wave-current interactions in fatigue analysis of floating offshore wind turbines (FOWTs). Surface water waves experience frequency shifts and wave shape modification when traveling on underlying currents. The wave-current interactions are known to be important for the responses of offshore structures, however, they have not been considered in FOWT fatigue analysis. To include such interactions, a nonlinear mooring hydrodynamics model is presented which is able to consider the cable geometric nonlinearity, seabed contact, and the current effect. The mooring model is then coupled with a spar-type FOWT model which simulates the structural dynamics of turbine blades and tower; aerodynamics of the wind-blade interaction and wave-current actions on the spar. Analytical wave-current interaction models based on Airy’s theory considering the current effect are applied for generating the flow field. Based on a spar-type FOWT and the wave-current interaction model, numerical simulations have been performed for three cases with only waves, wave and current without and with interactions. The comparison of the structural responses shows that the current and the wave-current interaction can have significant influences on FOWT tower and cable responses. Furthermore, cable fatigue life is estimated for two particular cases when the cable tension is decreased and increased respectively due to the presence of current. It is found that if the current tends to increase the cable tension, neglecting the current and wave-current interactions leads to overestimate of the cable fatigue life.

The influence of different mooring line models on the stochastic dynamic responses of floating wind turbines

Mooring line modelling plays an important role in predicting the dynamic response of the floating offshore wind turbines (FOWTs), especially under extreme load conditions. This paper investigates the influence of different mooring line models on the stochastic dynamic responses of a spar-type FOWT. A 16-degree-of-freedom (16-DOF) aero-hydro-servo-elastic model for the spar-type FOWT is first established using Euler-Lagrangian approach, taking into consideration the full coupling of the blade-drivetrain-tower-spar vibrations, a collective pitch controller and a generator controller. Three different mooring line models have been established and incorporated into the 16-DOF model, namely the linear spring model, the quasi-static model and the lumped-mass model, the last of which include the hydrodynamic loads, inertial force and damping force of the mooring cable. Stochastic dynamic analysis of the coupled FOWT-mooring line system is carried out using 3 different turbulent wind conditions and 4 different sea states and a total of 2160 10-min simulations. The mean value, the standard deviation and the extrapolated extreme value of the structural responses (blades, tower and mooring cable) as well as the fatigue equivalent loads are compared for the three different mooring line models.

A coupled finite difference mooring dynamics model for floating offshore wind turbine analysis

This study develops a coupled nonlinear hydrodynamic model of a mooring system consisting of multiple cables for analyses of floating offshore wind turbines (FOWTs). The model is based on a finite difference model of submerged cables which considers cable-seabed interaction, current effect, cable bending and torsional stiffness. The implementation of the model proposes a parallelization scheme for solving the cable responses to improve the computational efficiency. The developed program is then coupled with a spar type FOWT and verified using an experimentally validated open source mooring simulation program. Furthermore, the model is used to study the impact of nonlinear dynamics of the mooring system on FOWT responses in the presence of current. Both static responses of a spar FOWT under current load and dynamic responses of the spar FOWT under wind, wave and current loads are investigated. Responses are compared where varied mooring models are used including the linear model, quasi-static model and nonlinear mooring models without and with current effect on cables considered. The results show that the current effect on cables can have a considerable impact on the restoring effect of the mooring system and hence the spar and cable responses. The current effect on mooring cables needs to be properly considered in the FOWT analysis.

Influence of Vortex-Induced Loads on the Motion of SPAR-Type Wind Turbine: A Coupled Aero-Hydro-Vortex-Mooring Investigation

The nonlinear coupling effect between degree-of-freedom (DOFs) and the influence of vortex-induced loads on the motion of SPAR-type floating offshore wind turbine (FOWT) are studied based on an aero-hydro-vortex-mooring coupled model. Both the first- and second-order wave loads are calculated based on the three-dimensional (3D) potential theory. The aerodynamic loads on the rotor are acquired with the blade element momentum (BEM) theory. The vortex-induced loads are simulated with computational fluid dynamics (CFD) approach. The mooring forces are solved by the catenary theory and the nonlinear stiffness provided by the SPAR buoy is also considered. The coupled model is set up and a numerical code is developed for calculating the dynamic response of a Hywind SPAR-type FOWT under the combined sea states of wind, wave, and current. It shows that the amplitudes of sway and roll are dominated by lift loads induced by vortex shedding, and the oscillations in roll reach the same level of pitch in some scenarios. The mean value of surge is changed under the drag loads, but the mean position in pitch, as well as the oscillations in surge and pitch, is little affected by the current. Due to the coupling effects, the heave motion is also influenced by vortex-induced forces. When vortex-shedding frequency is close to the natural frequency in roll, the motions are increased. Due to nonlinear stiffness, super-harmonic response occurs in heave, which may lead to internal resonance.

Research and practice of offshore wind turbine model test platform and technology

The abundance of offshore wind energy resources has promoted the development of offshore wind turbine. With the increase of wind turbine power, the fixed foundation for shallow sea area no longer meets the design requirements, and the floating foundation for deep sea area has been developed rapidly. In this paper, the development process, types and particularity are introduced, and it demonstrates that the model experiment is indispensable in the development of floating offshore wind turbine (FOWT). Firstly, key technologies of basin model test are concluded, such as similarity criteria, scale ratio determination technology, model blade design technology, mooring system processing technology and environmental conditions simulation technology. Then object of 6 MW SPAR-type FOWT proposed by Shanghai JiaoTong University is researched, and production process of model is summarized, including model blade, model nacelle and model tower. Finally, the construction of the deep water test tank of marine engineering of Shanghai JiaoTong University is introduced, and the test results of the new multi-column TLP FOWT model test in the test tank are shown. The test results have great agreement with software simulation results. The results show that the deep water test tank of marine engineering of Shanghai JiaoTong University can be well developed as basin model test of FOWT. This paper supplies some guidance for the following model test and provides a theoretical basis for the development of the FOWT.

Investigation of the VIMs of a spar-type FOWT using a model test method

This article presents the research findings of an extensive model test investigation of the dynamic response to vortex-induced motions (VIMs) of the OC3 spar-type floating offshore wind turbine (FOWT). Particulars of this research are to investigate the unique and crucial effects of wind load on the spar-type FOWT, which differs considerably from other traditional spar-type floating systems utilized in the gas and oil industry, on the VIMs. The model test was performed with a sequence of current, wind, and irregular wave sea states to analyze the nature of the coupled dynamic response behavior of VIMs. Many unique characteristics were found and analyzed. The lock-in phenomenon of sway in the cross flow (CF) direction was found to occur first, followed by the lock-in phenomenon of surge in the in-line (IL) direction. For the current-only case, the remaining responses, including the other 4-degree-of-freedom motions, mooring tensions, and turbine bearing loads, were found to be coupled by sway/surge VIMs. Moreover, the oscillation amplitudes of all of the responses increased significantly with increasing current velocity, particularly after locking-in. Furthermore, the wind load had a clear suppression effect on the CF and IL VIM responses. The experimental measurements demonstrated that the wave load excites wave-induced oscillations for some CF responses and all IL responses, and it further restrains the VIM oscillation amplitudes on the foundation of the wind suppression effect. In particular, when a wind load is involved, the influence of current or wave is limited to the yaw and IL tower-top bending moment.

Semi-active control of vibrations of spar type floating offshore wind turbines

A semi-active algorithm for edgewise vibration control of the spar-type floating offshore wind turbine (SFOWT) blades, nacelle and spar platform is developed in this paper. A tuned mass damper (TMD) is placed in each blade, in the nacelle and on the spar to control the vibrations for these components. A Short Time Fourier Transform algorithm is used for semi-active control of the TMDs. The mathematical formulation of the integrated SFOWT-TMDs system is derived by using Euler-Lagrangian equations. The theoretical model derived is a time-varying system considering the aerodynamic properties of the blade, variable mass and stiffness per unit length, gravity, the interactions among the blades, nacelle, spar, mooring system and the TMDs, the hydrodynamic effects, the restoring moment and the buoyancy force. The aerodynamic loads on the nacelle and the spar due to their coupling with the blades are also considered. The effectiveness of the semi-active TMDs is investigated in the numerical examples where the mooring cable tension, rotor speed and the blade stiffness are varying over time. Except for excessively large strokes of the nacelle TMD, the semi-active algorithm is considerably more effective than the passive one in all cases and its effectiveness is restricted by the low-frequency nature of the nacelle and the spar responses.

A parametric study of spar-type floating offshore wind turbines (FOWTs) by numerical and experimental investigations

The use of spar platforms as a substructure for floating offshore wind turbines (FOWTs) is a new concept that is developing quickly in the offshore wind industry owning to the excellent stability and adaptability to different water depths. However, the lack of studies about the dynamic response and design guidelines of spar-type FOWTs is a barrier to further development of the offshore wind industry. Therefore, the goal of this study is to carry out dynamic response analysis and to develop design guidelines for spar-type FOWTs. To achieve this goal, the dynamic responses of full-scale spar-type FOWT models with different values of three design variables (spar diameter, depth, and concrete ratio) were first numerically obtained in the time domain and experimentally validated by considering all environmental conditions such as wind, regular wave, and constant current loads, as well as the mooring line loads. Then, regression and perturbation analyses, which were also validated by the analysis of variance method, were performed to analyse the effects of the design variables and to propose design guidelines of spar-type FOWTs.

Study of motion of spar-type floating wind turbines in waves with effect of gyro moment at inclination

Recently, a number of research groups have paid much attention to the study of Floating Offshore Wind Turbines (FOWTs). Similar to other offshore structures, the FOWTs are subjected to irregular waves and wind loads which cause a dynamic response in the structures. Under marine environmental conditions, they face many forces which prevent them from floating in the upright condition; they incline as a result of the winds, strong currents, typhoons, cyclones, storms etc. The motion of the FOWT might be changed by a change in gyroscopic effect which depends on the angular velocity and moment of inertia of the blade. Therefore, to investigate the effect of the gyro moment on the motion of the FOWT, two types of experiment were carried out in a water tank using a 1/360 scale model of a prototype FOWT. Firstly, the interaction between the rotary motion of the wind turbine blade and the dynamic motion of the SPAR-type FOWT was studied at small angles of inclination in regular waves. Secondly, the interaction between the change of rotational speed as well as moment of inertia of the blade and the motion of the FOWT was studied. In this paper, numerical calculations have been carried out using potential theory based on the 3D panel method. Finally, the experimental results are compared with the results of numerical simulation and findings are discussed. DOI: http://dx.doi.org/10.3329/jname.v9i1.10732 Journal of Naval Architecture and Marine Engineering 9(2012) 67-79