Research on coupled dynamic characteristics of Floating Wind Turbine under complex environment

• A high-fidelity CFD-FEM model is developed for floating offshore wind turbine analysis. • Blade deformation and platform motion cause 6.84% power loss and strong load fluctuation. • High stress is observed at the tower base, platform braces, and blade root transition zones. • Elastic deformations of the blades intensify the aerodynamic load fluctuations. • Findings support improved wind farm layout, mooring design, and fatigue risk management. Floating offshore wind turbines (FOWTs) offer distinct advantages for improving the competitiveness of offshore wind energy. However, their operation involves complex dynamics characterized by multiple sources of loading, considerable temporal variability, and high nonlinearity. Understanding the multi-physics coupling mechanisms and subsystem interactions governing the behavior of FOWTs is essential for enhancing operational safety, increasing power output, and promoting commercial deployment. To address these challenges, this study develops a high-fidelity, fully coupled aero-elastic-hydro-mooring framework by integrating computational fluid dynamics (CFD) and the finite element method (FEM). The NREL 5 MW horizontal-axis wind turbine (HAWT) mounted on a semi-submersible platform is used as an exemplar to investigate its nonlinear dynamic responses under combined wind and wave loading. The results show that the platform’s six-degree-of-freedom motion leads to continuous changes in the rotor inflow conditions, resulting in a 6.84 % reduction in the average power coefficient compared with its bottom-fixed counterpart, and producing a noticeable increase in power fluctuations. Nevertheless, the wake behind the FOWT exhibits higher turbulence intensity and a faster rate of dissipation. The two-way fluid–structure interaction analysis indicates that the blades undergo flapwise elastic deformation, particularly from the mid-span to the tip, which alters the angle of attack and induces continuous vortex shedding along the trailing edges. The structural stress distribution highlights significant stress concentration at the tower base, the bottom of the main column, and the connections between the braces and the platform. Although blade stress remains relatively low overall, higher stresses are observed near the blade root transition and at the shear web connections. In addition, the contact opening analysis between the mooring lines and the seabed shows that the windward mooring line periodically separates from and recontacts the seabed due to the surge motion of the platform, resulting in varying contact pressure distributions and large fluctuations in the mooring tension.

The safety and reliability margin of offshore floating wind turbines need to be higher than that of onshore wind turbines due to larger environmental loads and higher operational and maintenance costs for offshore wind turbines compared to onshore wind turbines. However rotor cyclic loads coupled with 6 DOFs motions of the substructure, amplifies the fatigue damage in offshore floating wind turbines. In general a lower fatigue design factor is used for offshore wind turbines compared to that of the stationary oil and gas platforms. This is because the consequence of a failure in offshore wind turbines in general is lower than that of the offshore oil and gas platforms. In offshore floating wind turbines a sub-system fault in the electrical system and blade pitch angle controller also induces additional fatigue loading on the wind turbine structure. In this paper effect of selected controller system faults on the fatigue damage of an offshore floating wind turbine is investigated, in a case which fault is not detected by a fault detection system due to a failure in the fault detection system or operator decided to continue operation under fault condition. Two fault cases in the blade pitch angle controller of the NREL 5MW offshore floating wind turbine are modeled and simulated. These faults include: bias error in the blade pitch angle rotary encoder and valve blockage or line disconnection in the blade pitch angle actuator. The short-term fatigue damage due to these faults on the composite blade root, steel low-speed shaft, tower bottom and hub are calculated and compared with the fatigue damage under normal operational conditions considering same environmental conditions for both cases. This comparison shows that how risky is to work under the fault conditions which could be useful for wind turbine operators. The servo-hydro-aeroelastic code HAWC2 is used to simulate the time domain responses of the spar-type offshore floating wind turbine under normal and faulty operational conditions. The rain-flow cycle counting method is used to calculate the load cycles under normal operational and fault conditions. The short term fatigue damage to the composite blade root and steel structures are calculated for 6-hour reference period. The bi-linear Goodman diagram and a linear SN curve are used to estimate the fatigue damage to the composite blade root and the steel structures respectively. Moreover the fatigue damage for different mean wind speeds, sea states and fault amplitudes are calculated to figure out the region of wind speeds operation with the highest risk of damage.

This study compares the time-varying rotor thrust and shaft power characteristics of a yawed floating offshore wind turbine (FOWT) predicted by three different open-source aerodynamic models. These models involve the blade-element-momentum (BEM) and the general dynamic wake (GDW) methods implemented in the design code fast developed by NREL, and a higher fidelity free-wake vortex model (FWVM) that is capable of modeling the unsteady skewed helical wake development of the yawed rotor. The study is based on the NREL 5 MW baseline rotor installed on the MIT tension-leg platform (TLP) operating with different rotor yaw angles and under regular sea wave conditions. Both the undisturbed wind speed and rotor speed are maintained constant throughout the analysis, though different sea wave heights and periods are considered. Initially, the motions of the FOWT under both axial and yawed rotor conditions are estimated in a time domain using fast. These motions are then prescribed to winds, an open-source FWVM developed by the University of Massachusetts Amherst, to determine the aerodynamic rotor thrust and power as a function of time. Both TLP surge and pitch motions are noted to impact the rotor thrust and power characteristics considerably. The three models have consistently shown that the TLP motion exhibits a negligible impact on the time-averaged rotor shaft thrust and power of the yawed rotor. On the other hand, the cyclic component of rotor thrust and power are found to be significantly influenced by the wave state at all yaw angles. Significant discrepancies between the predictions for this cyclic component from the three models are observed, suggesting the need of further research through experimental validation to ensure more reliable aerodynamics models are developed for floating wind turbine design software packages.

Investigation of mooring breakage impact on dynamic responses of a 15 MW floating offshore wind turbine

Preliminary development of a novel catamaran floating offshore wind turbine platform and assessment of dynamic behaviours for intermediate water depth application

Abstract. The numerical study of floating offshore wind turbines (FOWTs) requires accurate integrated simulations which consider the aerodynamic, hydrodynamic, servo and elastic responses of these systems. In addition, the floating system dynamics couplings need to be included to calculate the excitation over the ensemble accurately. In this paper, a new tool has been developed for coupling NREL's aero-servo-elastic tool OpenFAST with the computational fluid dynamics (CFD) toolbox OpenFOAM. OpenFAST is used to model the rotor aerodynamics along with the flexible response of the different components of the wind turbine and the controller at each time step considering the dynamic response of the platform. OpenFOAM is used to simulate the hydrodynamics and the platform's response considering the loads from the wind turbine. The whole simulation environment is called OF2 (OpenFAST and OpenFOAM). The OC4 DeepCWind semi-submersible FOWT together with NREL's 5 MW wind turbine has been simulated using OF2 under two load cases. The purpose of coupling these tools to simulate FOWT is to obtain high-fidelity results for design purposes, thereby reducing the computational time compared with the use of CFD simulations both for the rotor aerodynamics, which usually consider rigid blades, and for the platform's hydrodynamics. The OF2 approach also allows us to include the aero-servo-elastic couplings that exist on the wind turbine along with the hydrodynamic system resolved by CFD. High-complexity situations of floating offshore wind turbines, like storms, yaw drifts, weather vanes or mooring line breaks, which imply high displacements and rotations of the floating platform or relevant non-linear effects, can be resolved using OF2, overcoming the limitation of many state-of-the-art potential hydrodynamic codes that assume small displacements of the platform. In addition, all the necessary information for the FOWT calculation and design processes can be obtained simultaneously, such as the pressure distribution at the platform components and the loads at the tower base, fairleads tension, etc. Moreover, the effect of turbulent winds and/or elastic blades could be taken into account to resolve load cases from the design and certification standards.

This work focuses on examining the dynamic behavior of large floating offshore wind turbine (FOWT) exposed to extreme (wind and wave with 50-year return period) and accidental (mooring line breakage) loadings. The FOWT considered is 15 MW reference turbine supported on semi-submersible platform stationed using catenary moorings. As the mooring configuration greatly affects the response of FOWT, two different mooring configurations namely non-redundant (three-line) and redundant (six-line) systems were studied and compared. The coupled multi-body dynamic system was solved using Openfast. When simulating the mooring line failure, both the operating and extreme loading conditions were considered. Failure of one mooring was considered at a time. The response of the coupled system due to breakage of the mooring indicates high displacements in surge and sway directions in comparison to the intact system especially for the non-redundant mooring system. Furthermore, change in platform yaw angle and increased tension in the other intact mooring lines were observed. The findings from this study will be helpful in accidental limit state design and preventing failure of similar large FOWT systems. In addition, insights into using non-redundant and redundant mooring configurations for such large structures with respect to extreme and accidental loadings were also discussed.

Effects of the yaw error and the fault conditions on the dynamic characteristics of the 15 MW offshore semi-submersible wind turbine

Due to the aerodynamic load, the gravity load and the inertial load, the blades of floating offshore wind turbines (FOWTs) are prone to flexible deformation. However, the prediction of blades deformation is a crucial challenging task as it is very difficult to obtain in basin experiment and actual onsite measurement. This paper addresses a numerical study based on AI technology and an in-house programme DARwind to predict blade tip deformation for FOWTs. The AI module in DARwind employs deep deterministic policy gradient (DDPG) algorithm of Reinforcement Learning, to make the DARwind code intelligent for the objective FOWTs through basin experimental data training. The concept of KDPs is firstly introduced, which includes the crucial parameters in various discipline. The motions data of FOWT supporting platform from the basin experiment are selected as the target parameters to train the AI-based DARwind. The training results show that the mean value of platform motions from the AI-based DARwind match better with the basin experimental data. Through more accurate corrections to the platform motions, the blade deformation can be predicted more accurately in terms of axial, flap-wise and edge-wise.

An equation-based method for fully coupled analyses of floating offshore wind turbine based on Modelica

Shallow-water mooring chains of floating offshore wind turbines pose challenges such as nonlinear tension characteristics, high cost and prominent fatigue issues, which seriously hinder the large-scale use of floating wind power. Based on the hydrological and meteorological data of an offshore wind farm in the South China Sea, a sensitivity analysis of the mooring chain fatigue of a semi-submersible floating offshore wind turbine in shallow water was conducted with different environmental loads and mooring chain design parameters. It was found that mooring fatigue is mainly caused by the wave condition within diagonal distribution of the wave dispersion diagram. The mooring chain fatigue calculated using the main monochromatic wave was more conservative than that calculated using the wave scatter diagram with a difference of about 28%. The mooring fatigue calculated using turbulent wind spectrum was greater than that calculated using steady wind. In contrast, waves are the main source of mooring fatigue, and the second-order wave force has an undeniable impact on the mooring fatigue of a semi-submersible floating offshore wind turbine, which accounts for approximately 26% of wave-induced mooring fatigue. In addition, the current-induced mooring fatigue should not be neglected, although its effect is not as large as that caused by the wave or wind loads. An increase in the cross-sectional diameter and the length of a mooring chain, as well as a reasonable clump weight arranged, all contribute to improving the fatigue life of mooring chains in shallow water, which provide useful technical references for the optimization of mooring lines fatigue in shallow-water areas.

The fluctuation of wind power will affect the grid frequency stability. The motion of the offshore floating wind turbine (OFWT) will increase the power volatility of it. It is necessary to study the output power characteristics for OFWT. Firstly, this paper analyzes the motion response of an OFWT. Then, the influence of the floating foundation motion response is equivalent to the change of wind catching ability by formula derivation. Finally, compared with the output power of fixed offshore wind turbine, the output characteristics of the OFWT are obtained. The conclusion shows that the pitch, yaw and surge motion of OFWT have an important impact on the output power fluctuation of the wind turbine.

Validation of dynamic response of a 2-MW hybrid-spar floating wind turbine during typhoon using full-scale field data

Characteristics of a turbulence wind model control the magnitude and frequency distribution of wind loading on floating offshore wind turbines (FOWTs), and an in-depth understanding of how wind spectral characteristics affect the responses, and ultimately the design cost of system components, is in shortage in the offshore wind industry. Wind spectrum models as well as turbulence intensity curves recommended by the International Electrotechnical Commission (IEC) have characteristics derived from land-based sites, and have been widely adopted in offshore wind projects (in the absence of site-specific offshore data) without sufficient assessment of design implications. In this paper, effects of wind spectra and turbulence intensities on the strength or extreme responses of a 5 MW floating offshore wind turbine are investigated. The impact of different wind spectral parameters on the extreme blade loads, nacelle accelerations, towertop motions, towerbase loads, platform motions and accelerations, and mooring line tensions are presented and discussed. Results highlight the need to consider the appropriateness of a wind spectral model implemented in the strength design of FOWT structures.

As offshore wind capacity could grow substantially in the coming years, floating offshore wind turbines (FOWTs) are particularly expected to make a significant contribution to the anticipated global installed capacity. However, FOWTs are prone to several issues due partly to environmental perturbations and their system configuration which affect their performances and jeopardize their structural integrity. Therefore, advanced control mechanisms are required to ensure good performance and operation of FOWTs. In this study, a model predictive control (MPC) is proposed to regulate FOWTs’ power, reposition their platforms to reach predefined target positions and ensure their structural stability. An efficient nonlinear state space model is used as the internal MPC predictive model. The control strategy is based on the direct manipulation of the thrust force using three control inputs, namely the yaw angle, the collective blade pitch angle, and the generator torque without the necessity of additional actuators. The proposed controller accounts for the environmental perturbations and satisfies the system constraints to ensure good performance and operation of the FOWTs. A realistic scenario for a 5-MW reference wind turbine, modeled using OpenFAST and Simulink, has been provided to demonstrate the robustness of the proposed MPC controller. Furthermore, the comparison of the MPC model and a proportional-integral-derivative (PID) model to satisfy the three predefined objectives indicates the superior performances of the MPC controller.