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Review

Review of Study on the Coupled Dynamic Performance of Floating Offshore Wind Turbines

by
Yehong Dong
1,2,†,
Yewen Chen
3,4,*,†,
Hao Liu
5,
Shuni Zhou
2,
Yuanxiang Ni
1,2,
Chang Cai
3,
Teng Zhou
3 and
Qing’an Li
3,4,*
1
Southern Marine Science and Engineering Guangdong Laboratory (Zhanjiang), Zhanjiang 524013, China
2
CSSC Haizhuang Windpower Equipment Co., Ltd., Chongqing 401122, China
3
CAS Laboratory of Wind Energy Utilization, Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China
4
University of Chinese Academy of Sciences, Beijing 100049, China
5
CSSC Windpower Development Co., Ltd., Beijing 100097, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Energies 2022, 15(11), 3970; https://doi.org/10.3390/en15113970
Submission received: 18 April 2022 / Revised: 17 May 2022 / Accepted: 20 May 2022 / Published: 27 May 2022
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Abstract

:
Floating offshore wind turbines (FOWT) have attracted more and more attention in recent years. However, environmental loads on FOWTs have higher complexity than those on the traditional onshore or fixed-bottom offshore wind turbines. In addition to aerodynamic loads on turbine blades, hydrodynamic loads also act on the support platform. A review on the aerodynamic analysis of blades, hydrodynamic simulation of the supporting platform, and coupled aero- and hydro-dynamic study on FOWTs, is presented in this paper. At present, the primary coupling method is based on the combination of BEM theory and potential flow theory, which can simulate the performance of the FOWT system under normal operating conditions but has certain limitations in solving the complex problem of coupled FOWTs. The more accurate and reliable CFD method used in the research of coupling problems is still in its infancy. In the future, multidisciplinary theories should be used sufficiently to research the coupled dynamics of hydrodynamics and aerodynamics from a global perspective, which is significant for the design and large-scale utilization of FOWT.

1. Introduction

Over the last few decades, global climate change and energy supply have become ever more significant for society. The interest in the exploitation of renewable energies is increasing significantly worldwide. As a clear alternative to fossil fuels, wind power shows excellent potential in the energy structure of the future decarbonized world.
The development and utilization of wind energy has a long history. From the use of sails for marine navigation in about 3000 BC to the modern use of large-scale offshore wind turbines for wind power generation, wind energy development technology has become increasingly sophisticated. The onshore wind power industry has developed rapidly since 2000 and has gradually matured with the support of governments and the development of technology. As a result, onshore wind resources that can be used have become scarcer, and high-quality wind farms have become fewer and fewer. Compared with onshore, offshore wind energy reserves are larger, have broader distribution, and wind speed is more stable. Therefore, wind turbines have gradually moved from land to ocean and from shallow to deep sea.
With offshore wind turbines moving further from the shore, the economic cost of the traditional bottom-fixed type increases significantly. The concept of floating offshore wind turbines (FOWTs) was proposed in 1972 [1]. FOWT costs are anticipated to follow a similar downward trajectory as onshore sectors and are expected to decrease by 37–49% by 2050 [2]. With the development of technology and the significant increase in turbine sizes, FOWTs may gradually become the main force in the development and utilization of wind energy offshore in the future (Figure 1).
Since the concept of floating wind turbine was first proposed, many different forms of the platform concepts have been developed.
In 2004, Withee [3] proposed a combined foundation of a tension-leg platform (TLP), spar and barge platform and adopted the radial mooring of a spar platform. In 2009, Equinor ASA built the world’s first officially operational FOWT named Hywind (Figure 2a). Based on the single-pillar floating platform, Hywind works at a depth of 200 m with three anchor chains. Blue H installs a TLP FOWT (Figure 2b). In 2009, the National Renewable Energy Laboratory (NREL) [4] integrated WindPACT, RECOFF and DOWEC projects to develop a 5 MW benchmark wind turbine. In 2010, Jonkman [5,6] modified the parameters of Hywind and designed a semi-submersible platform foundation in the OC4 project based on the 5 MW OC3-Hywind FOWT project. In 2011, Principle Power installed a 2 MW three-pillar semi-submersible FOWT called WindFloat with an operating water depth of 45 m and four catenaries for mooring (Figure 2c). In 2013, Japan installed two FOWTs, including a semi-submersible FOWT named Fukushima (Figure 2d) and a single-pillar FOWT named GOTO (Figure 2e). In addition to typical models mentioned above, the company Danish Sway also proposed a Spar/TLP hybrid structure (Figure 2f).
The FOWT system generally includes four parts: wind turbine (including blades, hubs, nacelles, etc.), tower, floating support platform, and mooring system. Movements of one part can affect other parts of the structure. Because of the FOWT system structure and the complicated offshore environment, environmental loads on FOWTs are more complicated than those on onshore wind turbines (Figure 3). In addition to aerodynamic loads on turbine blades, there are also hydrodynamic loads acting on the support platform and mooring loads acting on the mooring system. In addition, FOWTs working in special environments may bear more complicated environmental loads. For example, when FOWTs operate in ice areas, the impact of ice on the structure and hydrodynamic performance must be considered. It is worth noting that environmental loads acting on FOWTs are not independent but coupled with each other. Therefore, in the field of FOWT research, the focus has always been on analyzing the coupled dynamic performance accurately under the premise of complex environmental loads.
This paper reviews the development of coupling analysis of FOWTs, including aerodynamic performance analysis of blades, hydrodynamic performance analysis of floating supporting platforms, coupling dynamic analysis of the FOWT system, and related experiments, which helps future researchers to make a footstone for proposing a more efficient and functional coupling dynamic analysis method and basin experimental method.

2. Aerodynamic Analysis Theories

Synthetically considering multiple factors including power generation and cost, FOWTs are usually required for a design capacity of 5 to 20 MW [12,13]. High-power wind turbines will result in longer blades and heavier loads, which is a great challenge for accurate prediction and analysis of aerodynamic load on FOWTs. At present, blade element momentum (BEM) theory, generalized dynamic wake (GDW) and computational fluid dynamics (CFD) are the main methods of aerodynamic load analysis on wind turbines [14].

2.1. Blade Element Momentum (BEM)

BEM combines the blade element theory and the momentum theory to obtain the locally induced velocity of blade [15]. This theory simplifies the flow around three-dimensional blades to flow around two-dimensional sections and assumes that the different sections of the blade are independent. BEM assumes that wake and induced velocity field change rapidly with the loads on blades. In fact, the change must pass a delay time to reach a stable state. In order to make up for the loss of accuracy caused by the simplification problem and obtain accurate blade aerodynamic performance, the results need to be corrected, including the tip loss correction considering the three-dimensional effect of the tip vortex [16], the dynamic stall correction considering the rapid change of the blade angle of attack, the hub loss correction and the yaw wake correction, etc.
Due to the simple principle and small amount of calculation, BEM has been widely used in engineering calculation software, such as FAST, Bladed, etc.

2.2. Generalized Dynamic Wake (GDW)

The theory of GDW is based on the potential flow solution of Laplace equation for inviscid and incompressible gas flow, which was firstly proposed by Peter and He [17] to study the aerodynamic performance of helicopter rotors and later modified by Suzuki [18] to study the aerodynamic performance of wind turbines. Compared with BEM, GDW allows a wider airflow pressure distribution on the blades. However, under conditions of low wind speed, atmospheric bomb deviation or large impeller cone angle, results may show large deviations or evenly instability.

2.3. Computational Fluid Dynamics (CFD) in Aerodynamic Analysis

The method of CFD directly solves the Navier–Stokes (N-S) equations describing the conservation of momentum of viscous incompressible fluids. It is more accurate calculation method, but it is suitable for analyzing simple load conditions due to the large amount of calculation.
The floating support platform will move under the action of environmental loads and mooring force. The wind turbine will produce the same additional movement as platform, which will have a certain impact on the aerodynamic performance of the wind turbine. The additional wind speed component caused by the platform motion will destroy the basic momentum balance assumption in BEM. Therefore, traditional codes (such as FAST and HAWC2) that use BEM plus some semi-empirical correction formulas to calculate the aerodynamic loads cannot realize the numerical simulation of unsteady aerodynamic performance. Matha [19] used CFD to analyze the aerodynamic performance of a rigid blade under a given motion. Wu et al. [20] proposed a CFD model considering that the aerodynamic performance of turbine is affected by the motion of floating platform, in which the turbine motion is realized through arbitrary mesh interface. The results are in good agreement with the results of FAST developed by NREL. CFD will play an increasingly important role in the aerodynamic performance analysis of wind turbines with the continuous development of computers and calculation methods.

3. Hydrodynamic Analysis

As the foundation of the whole FOWT system, the floating support platform is subject to the dynamic loads from the combined action of wind, waves and currents and the restraining force of the mooring system, which will affect the aerodynamic performance of the upper blade. Therefore, for FOWT systems, it is of great significance to accurately predict the hydrodynamic loads and responses of the floating support platform. Since the floating foundation of FOWT is mainly developed from some typical types of offshore floating platforms, research methods of traditional floating platforms can be used for reference, mainly including Morison’s equation, potential flow theory and CFD method.

3.1. Morison’s Equation

Morison’s equation is an empirical equation widely used in the field of ocean engineering, which was proposed in 1950 by Morison [21].
F = C m ρ π D 2 4 ∂ u ∂ t + C d ρ 2 D u | u | ,
where F is the hydrodynamic force, D is the diameter of the pile, Cm is the inertia force coefficient, u is the velocity of water particle, and Cd is the drag coefficient.
Morison’s equation assumes that the existence of the cylinder has no significant effect on the wave motion and the effect of the wave on the cylinder is mainly viscous effect and additional mass effect. So far, Morison’s equation is still widely used in the calculation of wave force of slender cylinder with relatively small scale (diameter to wavelength ratio less than 0.2). To correctly calculate Morison hydrodynamic force, a suitable wave theory should be used, and reasonable drag force coefficient and mass coefficient should be selected.
Morison’s equation is widely used to analyze bottom-fixed offshore wind turbines. Cheng [22] used Morison’s equation to calculate hydrodynamic loads under extreme response of floating platform. Without considering the effect of ocean currents, the accuracy of the Morison’s equation can meet the requirements. Morison’s equation is also directly used in the hydrodynamic calculation of FOWTs. Chakrabarti et al. [23] used the simplified Morison’s equation to analyze the motion response of a semi-submersible platform with a truss pontoon. They compared the calculation results with test results to verify the reliability of the simplified Morison’s equation.
However, the traditional Morison’s equation method cannot solve the memory effect of the free potential surface and the radiation of multi-mode coupled motion, so its application in the analysis of FOWTs is limited.

3.2. Potential Flow Theory

Compared with small-scale structures, the interference of the existence of large-scale structures and their motion on the flow field cannot be ignored. In this condition, the influence of additional mass effect and diffraction effect are more significant than that of the viscous effect. Therefore, the three-dimensional potential flow theory can be used to analyze the hydrodynamic performance of the floating wind turbine support platform. Due to its simplicity and high efficiency, the potential flow theory is currently widely used to calculate hydrodynamic loads of floating platforms. Some widely used commercial software is also based on the three-dimensional potential flow theory, such as SESAM, AQWA and so on. From the perspective of solution domain, potential flow theory is divided into frequency domain method and time domain method.
The frequency domain method only solves the constant term that does not change with time by separating the time term of each physical quantity from the control equation. Wayman [24,25] conducted pre-research on a wind turbine platform that is used to support 5 MW wind turbines and suitable for water depths from 30 m to 300 m by developing a set of programs that can be used to calculate the structure, hydrodynamic and aerodynamic coupled response of FOWT in the frequency domain. Brommundt [26] conducted an optimization study on the catenary chain system of a semi-submersible FOWT in the frequency domain. Without considering the second-order wave drift force, Matlab programming was used to obtain the optimal length, angle and arrangement direction of the chain. Korsmeyer et al. [27] used Green’s function to analyze the motion response of tension leg platform on regular waves. Nakos and Sclavounos [28] used Rankine source to calculate the steady velocity potential and unsteady velocity potential of the flow field.
Time domain method has great advantages in solving transient and nonlinear problems. Finkelstein [29] systematically derived two-dimensional and three-dimensional time-domain Green’s functions under finite and infinite water depth. Based on the impulse response function method, Cummins [30] decomposes the perturbation potential into two parts: instantaneous effect and memory effect, so as to separate the geometry and motion of the object. Ferrant [31] used the time domain Rankine source to calculate the three-dimensional nonlinear wave motion.
The structural size of the floating platform is large, and the analysis method based on the potential flow theory can meet the accuracy requirements of engineering applications to a large extent. Therefore, in the hydrodynamic research of floating wind turbine supporting platform, the analysis method of potential flow theory has been favored by many scholars.

3.3. Computational Fluid Dynamics (CFD) in Hydrodynamic Analysis

In recent years, with the increase of calculation speed and the development of calculation methods, CFD method has been used more and more on solving N-S equation. Compared with the potential flow theory, the CFD method can take the influence of real fluid viscosity into account and capture more flow field details. Therefore, the CFD method has higher calculation accuracy and is widely used in the calculation of the hydrodynamics of floating platforms.
Zhao [32] and Cheng [33] compared and analyzed hydrodynamic characteristics of the platform movement under conditions of the wind turbine stalling and regular operation based on OpenFOAM. The result shows that the pitch and sway movement of the floating platform is affected by aerodynamic loads of the turbine. Yan et al. [34] applied finite element method and equal geometry analysis method to solve the hydrodynamic force on the floating platform in CFD method. Bredmose [35,36] used the open-source CFD software OpenFOAM and free surface processing technology volume of fluid (VOF) to study the impact of breaking waves on offshore wind turbine platforms. Tran [37] used the potential flow method and the CFD method, respectively, to study the hydrodynamic performance of the DeepCWind semi-submersible wind turbine platform and analyzed the influence of different turbulence models on the calculation results. Dunbar [38] developed a tightly coupled six-degree-of-freedom solver using OpenFOAM for high-fidelity simulation of offshore floating wind turbine platforms. The solver is then used in simulations of the DeepCwind semi-submersible platform and compared well with FAST. Yang [39] examined the wave type and wave steepness impacts onto the FOWT. The research indicated that the reconstructed focused wave can be an alternative of the irregular wave for extreme wave studies.
CFD method is impractical in engineering design because its computational cost is too high to calculate the hydrodynamic force of offshore wind turbine floating platform. However, these results are helpful to give the influence degree of nonlinear wave. In some cases, it must be deeply studied by CFD method.
As mentioned above, Morison’s equation method and potential flow theory method have high computational efficiency and can meet basic engineering requirements. In comparison, the CFD method has higher requirements on grid accuracy and computing resources. So CFD methods have been limited for a long time in the field of theoretical research. With the development of computer technology, the limitation of computing resources is no longer significant. Therefore, in hydrodynamic analysis on floating support platforms, the CFD method is also favored by more and more scholars.

4. Coupled Dynamic Analysis

Compared with onshore wind turbines, floating wind turbines work in a more complex marine environment: the upper wind turbine bears wind loads, and the floating support platform and mooring system are subject to the combined action of waves and ocean currents. Moreover, mutual coupling will occur between the various parts of the structure. On one hand, the aerodynamic load received by the upper wind turbine will be transmitted to the platform through the tower. The movement of the platform under this action is no longer a simple response under waves especially as the size of the wind turbine continues to increase. On the other hand, the motion response of the platform under the combined action of the wave and mooring system will be transmitted to the upper wind turbine through the tower, which will change the aerodynamic loads and output power of the wind turbine.
The numerical simulation of the aerodynamic performance of wind turbine and the hydrodynamic performance of floating platform provides a certain reference for the design and research of FOWT system. In order to simulate the performance of FOWT system more accurately under normal operating conditions, it is necessary to simulate the aerodynamic-hydrodynamic fully coupled numerical simulation of FOWTs.
With the further development of the research on aerodynamic and hydrodynamic performance analysis of FOWT and the advancement of computing technology, the full coupling analysis of the aerodynamic and hydrodynamic loads of FOWT is now possible. More and more scholars are focusing on coupled analysis of FOWT system.
At present, some software for coupled analysis of FOWTs is based on the existing onshore wind turbine simulation program, adding the simulation of floating platform and mooring system. Others add a wind turbine aerodynamic simulation module on the basis of original floating platform hydrodynamic simulation code. In order to analyze and compare the accuracy of different numerical calculation procedures, the International Energy Agency organized “wind energy task 30” [40] and used 24 different numerical calculation procedures to analyze the dynamic response of 5 MW semi-submersible FOWT in OC4 project, which has played a guiding role in the development of numerical calculation program of FOWT. Table 1 [40] shows several typical theoretical modeling codes and methods.
National Renewable Energy Laboratory (NREL) has carried out a lot of research work on FOWTs and made great contributions to the research. In 2006, Jonkman and Sclavounos [41] of MIT improved the calculation software FAST developed by NREL, enhancing its preprocessor Adams function and integrating it with swim-motion-lines software and WAMIT software developed by MIT to make it suitable for the numerical calculation of dynamic response of FOWTs. In 2007, Jonkman [42] introduced HydroDyn module and mooring module into FAST, making it a numerical full coupling analysis program of aerodynamic analysis, hydrodynamic analysis, servo control and elastic analysis.
Norwegian University of Science and Technology has also done a lot of works on the development of numerical calculation program for FOWT. Linde [43] used Simo to analyze the dynamic response of 5 MW FOWT in OC3 project. Thomas [44] integrated HydroD, DeepC and TDHMILL3D to conduct fully coupled numerical calculation for TLP, Spar and semi-submersible FOWTs.
The fully coupled numerical simulation of FOWTs is still in its infancy. Most studies are based on the above-mentioned methods of aerodynamic and hydrodynamic simulation. However, some traditional aerodynamic or hydrodynamic analysis methods have been found unreasonable to be directly used in coupled analysis. Roddier [45] pointed out that the empirical characteristics of Morison’s equation and potential flow theory do not help the design of a new supporting platform. Sebastian and Lackner [46] pointed out that traditional BEM plus some corrections (such as dynamic inflow, yaw model, etc.) cannot accurately describe the interaction between blades and wake. Moreover, the commonly used stall models such as the semi-empirical B-L model and their applicability to FOWTs have not been fully studied. Therefore, it is necessary to develop more advanced FOWT modeling methods and coupled analysis models.
Due to the complexity of the aerodynamic-hydrodynamic coupled analysis, the full-model and real-scale CFD numerical simulation will become the most ideal tool. For large-scale FOWTs, there are not only complicated structures but also multi-scale effects, which brings great challenges to the study of the fully coupled analysis of FOWTs using CFD methods. Nematbakhsh [47] used the CFD method to conduct a preliminary study on the coupled effect of FOWT. They simplified loads on blades to constant thrust and used the single-phase flow Navier–Stokes equation to study the dynamic response of a TLP FOWT system. Quallen [48] conducted a two-phase flow CFD simulation of FOWT system. They used overset technology to deal with the grid movement around the platform and blades.
In general, the current research based on CFD methods is still in its infancy. There are still many unresolved difficulties in CFD methods, such as the division of the calculation domain of the water and gas flow field, the selection of the time step, the time calculation of the random wave action and so on.

5. Model Tests

Model tests are a very important research method in the field of ship and ocean engineering. Due to limitations of various calculation theories and methods, even in the development of floating structures in the offshore oil and gas industry, model experiments are widely used to predict and verify their motion performance. In addition to numerical methods, model tests are also important methods to study the overall performance of FOWTs. A model test method is more intuitive and can provide reference for numerical analysis.
The basin model test is an important method for studying FOWTs. However, there are also certain difficulties in the experimental design, such as the selection of similar criteria. The basin model tests conducted in the general ocean engineering field need to focus on simulating gravity and inertial force, so it is necessary to give priority to Froude number, but blades are affected by aerodynamics, which is closely related to the viscosity and should meet the Reynolds number similarity. The Reynolds number (Re) represents the ratio between the magnitude of the inertial and viscous forces, whereas Froude number (Fr) represents the ratio between the magnitude of the inertial and gravitational forces. These two numbers can be expressed as follows:
F r = u g l ,
R e = ρ u l μ ,
where u is the velocity of the fluid, l is the characteristic length of the body in the fluid field, g is the gravitational acceleration, ρ is the density of the fluid, and μ is the dynamic coefficient of viscosity.
According to Equations (2) and (3), the Reynolds number is directly proportional to the characteristic length, whereas the Froude number is inversely proportional to the characteristic length. Therefore, it is difficult to unify these two numbers. At present, FOWT models used in the experiments satisfy Froude scaling law. Due to the contradiction of scaling law, only the aerodynamic thrust load is treated equivalently. Aerodynamic torque and blade inertia force are ignored. This method simulates the motion response of FOWTs to a certain extent, but inevitably introduces errors.
In response to this problem, different scholars have proposed different solutions. Martin [49] increased the experimental wind speed by 80% and increased the edge roughness of experimental blades in the DeepCwind experimental project. In addition to increasing the experimental wind speed and the edge roughness, the DeepCwind experimental group also proposed a scheme for designing experimental blades in a low Reynolds number experimental environment in the experimental report [50]. Some scholars have done some work in this field [51]. In addition, the real-time hybrid model is used in the experiment. The model uses numerical simulation to replace the real aerodynamic (or hydrodynamic) load to solve the problem of scale law conflict, which provides a new experimental scheme for FOWTs [52].

5.1. Physical Model Experiments

Because of high requirements for experiments, the research on the model test of FOWT system is very limited so far. This paper introduces some typical experiments of FOWT in order to provide a certain reference and verification basis for numerical simulation. The statistics of the floating wind turbine model experiments are shown in Table 2 [53].

5.1.1. Model with Actuator Disc

In early pool experiments, some experiments simplified the wind turbine blade as an actuator disc in order to simulate aerodynamic thrust under Froude scale law.
In 2010, Principle Power conducted a 1:105 model experiment of a semi-submersible platform at the University of California, Berkeley [45]. In this experiment, aerodynamic loads on blades are replaced by an actuator disc and a rotating mass is used to generate the rotational force.
In general, the use of brake discs is an effective method to achieve pneumatic thrust equivalence. However, due to the lack of blades, the thrust disc cannot simulate other aerodynamic loads, such as aerodynamic torque. In addition, the shape of the disk leads to a more stable wind field compared with the actual situation. Therefore, the thrust disc can simulate the aerodynamic load to some extent, although there are many limitations.

5.1.2. Froude-Scaled Model

In 2005, a 1:47 Spar model experiment [54] was conducted in the wave basin at MARINTEK, as is shown in Figure 4a. In this experiment, the motion characteristics of FOWTs under the influence of wave and wind loads were studied.
In 2011, the DeepCWind team led by the University of Maine [49] carried out a series of model experiments in the MARIN pool in the Netherlands, as is shown in Figure 4b, which included a semi-submersible platform experiment, a TLP experiment and a Spar platform experiment. The experiment adopts a scale ratio of 1:50. The motion response was studied under different experimental conditions, including free attenuation, regular and irregular wave. Since the model established under the similarity of Froude number cannot guarantee the consistency of the Reynolds number, the expected result was not obtained in the end.
In general, the Froude criterion experimental model can measure but not accurately simulate the aerodynamic torque. In addition, the contradiction of scaling law still cannot be solved. The aerodynamic performance of the model rotor does not match that of the full-scale rotor, so the active blade pitch control cannot be applied to the experiment. Therefore, the use of the Froude-scaled full rotor is a major progress in the model test of FOWTs, although there are still many limitations to be overcome.

5.1.3. Performance Scaled Model

The Reynolds number of Froude-scaled model has an undeniable error, which directly leads to the significant aerodynamic performance deviation between the model scale rotor and full-scale rotor. In order to solve the contradiction of two scaling laws, a reasonable scheme on designing the experimental blade in the low Reynolds number experimental environment was proposed. A series of performance-scaled blade experiments were carried out.
In 2013, the DeepCWind team led by the University of Maine redesigned the blades and conducted another test in the Marin basin using the previous semi-submersible platform [56], as is shown in Figure 5.
In general, using performance matched blades is one of the most effective methods to obtain the aerodynamic equivalence of FOWT physical model test. According to the research, the performance matched blade can provide relatively satisfactory aerodynamic load under Froude’s law, although the mass distribution of the redesigned performance matched blade does not match that of full-scale turbine.

5.2. Real-Time Hybrid Model Experiments

The real-time hybrid model experimental method divides the model into physical substructure and numerical substructure. The physical substructure belongs to the scale model test, while the numerical substructure belongs to the numerical simulation. The load acting on the numerical substructure is measured by the model experiment in the physical substructure and fed back to the physical substructure in real time. The real-time hybrid model experiment involves data measurement, filtering, force estimation, motion observing and force actuation. The main challenge is to fit all of those items into one time step [57].
There are two main methods in the real-time hybrid model experiment of FOWT. One is to select the wind turbine as the numerical substructure and floating platform and mooring system under Froude scale law as the physical substructure. The experiment is carried out in a wave basin. Another method is to select floating platforms and mooring systems as the numerical structures and tower and wind turbine as the physical substructure. The experiment is carried out in a wind tunnel.
In 2014, a model experiment of a set of non-pillar semi-submersible platforms (Concrete Star Wind Floater) was done in the pool of Central University of Technology in Nantes [58]. In order to avoid problems caused by the mismatch of Reynolds number under the condition where model was established according to Froude number, the aerodynamic loads of blades are replaced with a ducted fan model with feedback control. Meanwhile, experimental data were compared with results calculated by FAST.
In 2015, the 1:60 model [59] experiment of the OC4 DeepCWind semi-submersible platform used for the 10 MW wind turbine was also conducted in the pool of the Central University of Technology in Nantes, France. Similar as the previous model experiment of Concrete Star Wind Floater, the ducted fan model with feedback control was used to simulate the action of aerodynamic loads. In addition to free attenuation experiments and response experiments in regular and irregular waves, response experiments were also conducted under extreme sea conditions.
In 2015, some experiments were conducted on the semi-submersible platform model in MARINTEK pool by Norwegian Marine Technology Research Institute [60]. In the experiment, a series of tensioned wires connected to the actuators to replace the duct fans and provide aerodynamic loads were used. They also compared the experimental results with those calculated by FAST software.
In addition, the method selecting floating platforms and mooring systems as the numerical structures has been adopted in one typical experimental project [61], which was conducted in a wind tunnel at Politecnico di Milano, as shown in Figure 6.
In general, the real-time hybrid model can solve the contradiction of scaling law. However, the real-time hybrid model requires high computational accuracy and speed, which is a major challenge in the experiment. Although the real-time hybrid model has not been verified in practical engineering and has not been widely used, it is a method with development potential. The research of FOWT has just started, the experimental research methods are also in the stage of continuous development. As a result, there is a lack of the mature experimental data.

6. Conclusions

FOWTs are important devices for the development and utilization of offshore wind energy resources. Compared with onshore wind turbines, floating wind turbines work in a more complex marine environment: the upper wind turbine bears wind loads, and the floating support platform and mooring system are subject to the combined action of waves and ocean currents. The coupled environmental loads bring great challenges to the design and development of FOWTs. Therefore, the accurate prediction of various environmental loads on FOWTs is of great significance. This paper introduces the analysis methods and current research status of various environmental loads on FOWTs, including aerodynamic performance analysis of blades, hydrodynamic performance analysis of floating supporting platforms, coupled aero- and hydro-dynamic analysis of FOWT system and related experiments.
In the process of aerodynamic analysis on FOWTs, theoretical methods of aerodynamic analysis on onshore wind turbines are mainly used for references such as BEM, potential flow theory and CFD method. Similarly, in the field of marine engineering, theoretical methods of predicting the hydrodynamic performance of floating platforms have also been used for reference in the analysis of hydrodynamic performance of floating offshore wind turbine supporting platforms.
The more incredible difficulty lies in the overall coupled aerodynamics and hydrodynamics of the FOWT system. Scholars have also achieved preliminary results on the aero-hydrodynamic coupling problem by combining different blade aerodynamic analysis methods and platform hydrodynamic analysis methods. At present, the primary coupling method is still based on the combination of BEM and potential flow theory. There are certain limitations in solving the complex problem of coupled FOWTs. The more accurate and reliable CFD method used in the research of coupling problems is still in its infancy. There is still a lot of works need to be done in the future on the coupling analysis of FOWTs, which is also a significant challenge on the development of FOWTs.
In addition to numerical analysis methods, model experiments are also critical in the coupling analysis of FOWTs. Due to high requirements for experiments, successful experimental experience is relatively scarce. The model of FOWTs is far more complicated than onshore wind turbine models and traditional floating platform models. Moreover, the scale effect problem is challenging to solve in the model experiment. Therefore, there are not many model experiments available for reference, and the mature experimental data is still relatively lacking.
In the future, multidisciplinary theories should be used sufficiently, and load calculation and dynamics modeling methods should be studied, in order to research the coupled dynamics of hydrodynamics and aerodynamics from a global perspective. In terms of simulation research, the existing commercial software has inevitable errors in the coupling analysis of FOWT. Considering the complex 6-DOF motion of FOWT, it is necessary to analyze the unsteady aerodynamic performance. BEM and potential flow theoretical equations need to be further improved. Moreover, problems, including the division of the calculation domain of the water and gas flow field, the selection of the time step, and the time calculation of the random wave action, need to be clarified in the CFD method. In experiments of FOWT, the contradiction of scaling laws is still the main challenge which can be preliminarily solved by the real-time hybrid model. The real-time hybrid model is a method with development potential, especially the method selecting floating platforms and mooring systems as the numerical structures. A fast and accurate numerical algorithm needs to be developed, and the motion of the 6-DOF robot needs to be more accurate.

Author Contributions

Investigation, S.Z. and Y.N.; formal analysis, H.L.; writing—original draft preparation, Y.D. and Y.C.; writing—review and editing, C.C. and T.Z.; supervision, Q.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Fund of Southern Marine Science and Engineering Guangdong Laboratory (Zhanjiang) (ZJW-2019-02).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Nomenclature

FOWTfloating offshore wind turbine
TLPtension-leg platform
NRELNational Renewable Energy Laboratory
BEMblade element momentum
GDWgeneralized dynamic wake
CFDcomputational fluid dynamics
DSdynamic stall
PFpotential flow
MEMorison’s equation
QDquadratic drag
IWLinstantaneous water level
QSquasi-static
Dyndynamic
FEfinite element

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Figure 1. Types of offshore wind turbines.
Figure 1. Types of offshore wind turbines.
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Figure 2. Typical Floating Offshore Wind Turbines. (a) Hywind [7]; (b) Blue H [8]; (c) WindFloat [7]; (d) Fukushima [9]; (e) GOTO [10]; (f) Sway [11].
Figure 2. Typical Floating Offshore Wind Turbines. (a) Hywind [7]; (b) Blue H [8]; (c) WindFloat [7]; (d) Fukushima [9]; (e) GOTO [10]; (f) Sway [11].
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Figure 3. Sketch of environmental loads on a floating offshore wind turbine.
Figure 3. Sketch of environmental loads on a floating offshore wind turbine.
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Figure 4. Several typical Froude-scaled model experiments. (a) HyWind project (Reprinted/adapted with permission from Ref. [54]. 2006, Skaare, B); (b) DeepCWind project (Reprinted/adapted with permission from Ref. [55]. 2014, Koo, B.J.).
Figure 4. Several typical Froude-scaled model experiments. (a) HyWind project (Reprinted/adapted with permission from Ref. [54]. 2006, Skaare, B); (b) DeepCWind project (Reprinted/adapted with permission from Ref. [55]. 2014, Koo, B.J.).
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Figure 5. DeepCWind project (Reprinted/adapted with permission from Ref. [56]. 2014, Goupee, A.J.); (a) DeepCWind project in 2011; (b) DeepCWind project in 2013.
Figure 5. DeepCWind project (Reprinted/adapted with permission from Ref. [56]. 2014, Goupee, A.J.); (a) DeepCWind project in 2011; (b) DeepCWind project in 2013.
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Figure 6. Experimental project selecting floating platforms and mooring systems as the numerical structures (Reprinted/adapted with permission from Ref. [61]. 2020, Belloli, M.).
Figure 6. Experimental project selecting floating platforms and mooring systems as the numerical structures (Reprinted/adapted with permission from Ref. [61]. 2020, Belloli, M.).
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Table
Table 1. List of codes for fully coupled aero-hydrodynamic analysis of floating offshore wind turbines.
Table 1. List of codes for fully coupled aero-hydrodynamic analysis of floating offshore wind turbines.
CodeCode DeveloperAerodynamicsHydrodynamicsMooring Model
FAST v8NREL(BEM or GDW) + DSPF + MEQS
SIMPACK + HydroDynSIMPACKBEM or GDWPF + QDQS
Bladed (Advanced Hydro beta)DNV GL(BEM or GDW) + DSPF + ME + (IWL)QS
Sino, Riflex + AerodynMARINTEK, NREL(BEM or GDW) + DSPF + MEFE/Dyn
HAWC2DTU Wind(BEM or GDW) + DSMEFE/Dyn
GDW: generalized dynamic wake; DS: dynamic stall; PF: potential flow; ME: Morison’s equation; QD: quadratic drag; IWL: instantaneous water level; QS: quasi-static; Dyn: dynamic; FE: finite element.
Table
Table 2. Floating Offshore Wind Turbine Experiment Comparisons.
Table 2. Floating Offshore Wind Turbine Experiment Comparisons.
Experiment NameScaleTesting LocationPlatform TypeAerodynamic Setup
WindFloat (2010)1:105UC BerkeleySemi-submersibleActuator Disc + Rotating Mass
DeepCWind (2011)1:50MARINSemi-submersible, Spar, TLPFull Rotor
DeepCWind (2013)1:50MARINSemi-submersibleFull Rotor
Concrete Star (2014)1:40ECNBraceless Semi-submersibleDucted-fan
MARINTEK Braceless (2015)1:30MARINTEKBraceless Semi-submersibleNovel Actuator
INNWIND.eu Model Test (2015)1:60ECNSemi-submersibleDucted-fan and Froude-scaled Rotor
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Dong, Y.; Chen, Y.; Liu, H.; Zhou, S.; Ni, Y.; Cai, C.; Zhou, T.; Li, Q. Review of Study on the Coupled Dynamic Performance of Floating Offshore Wind Turbines. Energies 2022, 15, 3970. https://doi.org/10.3390/en15113970

AMA Style

Dong Y, Chen Y, Liu H, Zhou S, Ni Y, Cai C, Zhou T, Li Q. Review of Study on the Coupled Dynamic Performance of Floating Offshore Wind Turbines. Energies. 2022; 15(11):3970. https://doi.org/10.3390/en15113970

Chicago/Turabian Style

Dong, Yehong, Yewen Chen, Hao Liu, Shuni Zhou, Yuanxiang Ni, Chang Cai, Teng Zhou, and Qing’an Li. 2022. "Review of Study on the Coupled Dynamic Performance of Floating Offshore Wind Turbines" Energies 15, no. 11: 3970. https://doi.org/10.3390/en15113970

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