Surge Motion Research Articles

A comprehensive numerical model for aero-hydro-mooring analysis of a floating offshore wind turbine

This paper presents a comprehensive study of a Floating Offshore Wind Turbine (FOWT), requiring multidisciplinary expertise in floating platform hydrodynamics, mooring system dynamics, and wind turbine aerodynamics. We introduce a fully coupled numerical model, focusing specifically on the NREL's (National Renewable Energy Laboratory's) 5 MW OC4 FOWT. The model is validated through both numerical simulations using the Computational Fluid Dynamics (CFD) based software OpenFOAM and experimental results. Key findings demonstrate the model's accuracy in forecasting the aerodynamic behaviors of the turbine, the platform's response to motions, and the behavior of the mooring system across diverse wind and sea state scenarios. Furthermore, the study enhances the understanding of FOWT's stability and efficiency by examining the influence of different Center of gravity (COG) heights. Results show that reduction in COG height has a minor effect on heave and surge motion but significantly decreases pitch motion and mooring line tension, thereby improving static stability and reducing the impact of wave loads on dynamic responses. Additionally, the results show that this reduction in COG height enhances the aerodynamic power output, suggesting that optimized FOWT designs could achieve improved energy capture efficiency. These insights optimize FOWT design and efficiency, enhancing renewable energy performance.

Dynamic Formulation of the Double Multiple Stream Tube Model of Offshore Vertical Axis Wind Turbines

Abstract The paper proposes a novel algorithm to simulate Vertical Axis Wind Turbines in floating motion based on a low-fidelity approach. The conventional Double Multiple Stream Tube Model is formulated as a steady state model to deal with the inherent unsteady aerodynamics of VAWTs. The Double Multiple Stream Tube Model makes use of an average contribution of the blade passages over a revolution to solve the Blade Element Momentum equation. The model reduces the dependence of the solution (the induction field) to a space variable. Floating Vertical Axis Wind Turbines undergoes a variation of aerodynamic quantities according to wave periods, also different from the rotor period. This paper provides a new formulation of the Double Multiple Stream Tube Model to solve the turbine rotation in time and space, introducing the variation of the platform velocity and a revised approach to the downstream actuator disc. The most impactful parameter is found to be the wave frequency: wave periods at full-scale are lower than the ones of the turbine rotor, featuring optimal operating points at low tip speed ratios. The increase of the wave frequency highlights the differences of the average torque prediction for a single blade up to 10% between the unsteady and the standard formulation. In this context, the development of fast low-fidelity computational tools play a key role in the assessment of the preliminary designs of Floating Offshore Vertical Axis Wind Turbines and it will be used to optimise the aerodynamic design of the laboratory scaled model under surge motion.

Predictive Modeling of Semi-Submersible Floater Motion Using Bi-LSTM Model

Abstract Floating offshore wind turbines (FOWTs) are emerging as a promising renewable energy solution, tapping into the vast wind resources in deep-sea locations. Accurate predictions of heave, pitch, sway, roll, yaw, and surge are essential to ensuring the structural integrity and power generation efficiency of FOWTs. Traditional methods relying on potential flow theory to predict FOWTs’ hydrodynamic responses involve simplifications that may not capture real-world complexities. Although computational fluid dynamics (CFD) simulations offer accuracy, they are computationally expensive. This study explores the use of Bi-directional long-short-term memory (Bi-LSTM) neural networks as an alternative to address computational challenges in predicting FOWTs’ hydrodynamic responses. These networks excel at handling time series data, capturing temporal correlations crucial for understanding FOWT floater motion dynamics. They also effectively capture bidirectional dependencies, enhancing their suitability for this application. The model considers various input parameters, including wave amplitudes, periods, steepness, directions, and floater design characteristics such as stifness, pre-tension, and the free length of mooring lines. The Bi-LSTM model is trained using a subset of the CFD dataset generated for the scaled OC5 semi-submersible platform with mooring, reserving the remainder for validation and testing. The comparison between simulated and predicted motion demonstrated a strong correlation in surge but noticeable discrepancies in heave and pitch. FFT analysis highlighted consistent frequency components but variations in magnitude, suggesting areas for model sensitivity improvement and further investigation into wave interaction dynamics. The correlation coefficients revealed a strong positive correlation for the surge motion, while the heave and pitch motions showed slightly lower correlation coefficients, suggesting better performance in predicting the surge.

Hydro–Aero Coupling Analysis and Surge Motion Effects on Aerodynamic Performance in Floating Offshore Wind Turbines: A Review and Synthesis of Literature

Hydro–Aero Coupling Analysis and Surge Motion Effects on Aerodynamic Performance in Floating Offshore Wind Turbines: A Review and Synthesis of Literature

Effects of Aerodynamic Damping and Gyroscopic Moments on Dynamic Responses of a Semi-Submersible Floating Vertical Axis Wind Turbine: An Experimental Study

Abstract Floating vertical-axis wind turbines (VAWTs) offer certain advantages over floating horizontal-axis wind turbines (HAWTs), particularly in terms of the potential to lower the cost of energy. In this study, a 5 MW floating VAWT concept with three straight blades and a semi-submersible hull deployed in a water depth of 42 m was presented. In addition, the experimental setup is introduced, and calibration tests are also performed to validate the physical model system. Subsequently, the aerodynamic damping and gyroscopic moment effects were investigated by wind/wave basin model tests with a scale ratio of 1/50. Results indicate that aerodynamic damping can suppress the fluctuations of the platform's surge and pitch motion at their respective resonance frequencies and tends to increase with wind speed at below-rated wind speed. Additionally, surge-induced and pitch-induced aerodynamic damping hardly affect the wave frequency response. Meanwhile, the surge natural frequency is substantially altered due to the wind loads. The rotating rotor and pitch motion of the platform together excite significant gyroscopic moments, leading to noticeable oscillations in roll motion. Additionally, there is an increasing trend in the gyroscopic moment effect with rotational speed. During normal operation of the floating VAWT, aerodynamic damping and gyroscopic moment together influence hull roll/pitch motions. Overall, this study contributes to providing valuable insights into the motion characteristics of floating VAWTs.

Fully Coupled Analysis of a 10 MW Floating Wind Turbine Integrated with Multiple Wave Energy Converters for Joint Wind and Wave Utilization

The study focuses on a semi-submersible wind-wave integrated power-generation platform, which consists of an OO-Star semi-submersible platform equipped with a DTU 10 MW wind turbine and a set of wave energy converters. A hydrodynamic model was established using ANSYS-AQWA (2023 R1), and by incorporating upper wind loads and utilizing the open-source program F2A, a fully coupled time-domain model of the integrated power-generation platform was constructed. The primary objective is to explore the interaction mechanisms between the upper wind turbine and the lower wave energy devices under the combined effects of irregular waves and turbulent wind through a series of operational conditions. Additionally, the safety of the mooring system was assessed. The results indicate that, compared to the wave period, the power generation of the lower wave energy devices is more significantly affected by wave height. Overall, the integrated power-generation platform demonstrates optimal performance under the third operational condition. In survival conditions, the introduction of oscillating buoys can improve the motion responses of the platform in terms of sway, roll, pitch, and yaw to a certain extent, but it also increases the surge and heave motion responses and the associated mooring loads. The mooring system can ensure the safety of the integrated power-generation platform under extreme sea conditions.

DMATCH: A novel integrated testing method for floating wind turbine dynamics – Potential error and its impact

With floating offshore wind turbines (FOWT) steadily advancing towards large-scale deployments in deep water, the traditional wave basin model testing method shows its shortage to fulfill the testing requirement. To enhance the effectiveness of model tests for FOWT, this paper introduces a distributed model testing method, dMATCH (distributed-Mooring-Aerodynamics-Testing-Control-Hydrodynamics). The dMATCH method separates the experiment into aerodynamic testing in the wind tunnel and hydrodynamic testing in the wave basin, achieving integrated coupling through real-time data transmission using a data exchange system. Errors in sensor acquisition and actuator actuation during data communication may impact the accuracy of experiments. To investigate the potential error and its impact within dMATCH, an integrated model testing system in the wave basin was constructed. Four load cases were established, introducing varying magnitudes of normally distributed random errors to simulate acquisition and actuation inaccuracies. The investigation revealed a positive correlation between errors in experimental results and those in acquisition and actuation. As error increased, result fluctuations became more pronounced. Within the current experimental framework, acquisition error had a greater influence on results compared to actuation error, with aerodynamic thrust being more sensitive to error than the pitch and surge motions of platform. When acquisition and actuation errors were controlled below 5%, the relative RMSE of the results remained below 5%. The dMATCH method proposed in this paper offers a new approach for integrated model testing of FOWT, and the study of potential error impact is crucial to ensuring the reliability of this innovative testing method.

Hydrodynamic performance characteristics of small-scale in-situ buoys with critical motion requirements

As the accuracy of high-precision oceanographic measurement instruments increases, the operational requirements for buoy motion become critical. This study focuses on the small-scale in-situ buoy, optimizing buoy configuration to ensure stability and performance. Aiming at the special structure which consists of a large floating column and several small tube connectors, a method for evaluating the hydrodynamic performance is proposed which combines potential flow theory with Morison equation. And the method validity is confirmed using experimental data. The performances analysis of different type buoy with catenary or taut mooring system are developed. According to the critical motion requirements for the long-term stability, the cylindrical type buoy equipped with a catenary mooring system is regarded as the most stable and economical choice, exhibiting a surge motion response of 4.07 m, a heave motion response of 2.89 m, and a pitch motion response of 9.37°. Furthermore, the anchor chain acts as an elastic damper which can mitigate the transient impact of the environmental loads, greatly limiting the amplitude of the longitudinal and heaving motions. And the maximum mooring line tension is 249.28 kN, which is significantly lower than other schemes. The evaluation method offers a technical support for engineering applications.

Decomposition and prediction of hydrodynamic loads on a floating counter-rotating tidal turbine under surge motion

Decomposition and prediction of hydrodynamic loads on a floating counter-rotating tidal turbine under surge motion

A model of racing kayak surge motion a numerical approach with realistic oscillating propulsion forces

This study presents a novel forward dynamics model of racing kayak surge motion. The model, driven by adjustable oscillating propulsion forces, replicates true paddle stroke forces. It is based on Newton’s second law of motion, with the net force comprising of propulsion force, skin friction drag, wave-making drag, form drag, and air drag. The total mass includes the kayak, paddler, and added mass. The fourth-order Runge–Kutta method is used to solve for speed and distance. The model incorporates a unique hull formulation for wetted surface area estimation, Michell’s integral for wave-making drag, and a log wind profile with allometric scaling for air drag. It accounts for varying stature height, body mass, kayak dimensions, wind speed, and stroke propulsion profiles. The model is a significant advancement in its domain, with broad applicability for the future. It aligns with existing data and analyses, but validation data is limited.

Aerodynamic Study of a Horizontal Axis Wind Turbine in Surge Motion Under Angular Speed and Blade Pitch Controls

Aerodynamic Study of a Horizontal Axis Wind Turbine in Surge Motion Under Angular Speed and Blade Pitch Controls

Flow memory effects on the nonlinear hydrodynamic load of an ROV in translational maneuvering

This paper describes experimental and numerical investigations of the flow memory effect on the nonlinear hydrodynamic load response of a work-class remotely operated vehicle (ROV) under surge, sway, and heave impulse motions. The hysteresis loops of the dynamic drag coefficients during impulse motion tests are analyzed by comparing them with those obtained in steady drag tests. The areas of the loops and differences between the maximum and minimum dynamic and steady coefficients are used to quantify the flow memory effect. A novel unsteady hydrodynamic model for ROVs in translational maneuvering is established. This model considers the flow memory effect in terms of the unsteady hydrodynamic load response obtained from impulse motion response tests. The results given by the unsteady model are found to be in good agreement with those from CFD simulations. The unsteady hydrodynamic characteristics (including the asymmetric forces, force magnitude, and phase delay), motion amplitude, and frequency effect on the hydrodynamic forces of the ROV are analyzed by comparing the unsteady model results with those obtained from the steady model, revealing the rules governing the flow memory effect.

Effects of coupled platform motions on the aerodynamic performance and wake characteristics of a floating horizontal-axis wind turbine

ABSTRACT The aerodynamics of floating horizontal-axis wind turbines (FHAWTs) are complex due to platform motions in real sea states. The impacts of coupled platform motions on the aerodynamic loads, wake structure, and wake recovery of FHAWTs were investigated through computational fluid dynamics simulations. Results show that the increases in the maximum rotor power of FHAWTs (compared with the fixed-base ones) under sinusoidal pitch-surge motions (3892 kW) were close to the sum of those under pitch (2121 kW) and surge (1622 kW) motions. The wake tilt angles under pitch-surge motions agreed with those under pitch motions, causing similar wake recovery. The increase in wave periods (Tw ) amplified the resonance between waves and platforms under the wind-wave coupling, increasing the maximum rotor power and thrust. Compared with Tw = 16 s, the relative decreases in the averaged wake velocity, which was 7D (where D is the rotor diameter) downwind of the FHAWTs, were 5.93% and 2.64% at Tw = 8 s and 12 s, respectively. Long Tw increased pitch periods, yielding fast wake recovery by enhancing the interaction between blade-tip vortices and hub ones. The output power of a floating wind farm was higher than the fixed-base counterpart due to fast wake recovery.

Numerical study on hydrodynamic interaction between two parallel surge-released ships advancing in head regular waves based on the hybrid method

This study examines the hydrodynamic interaction between two parallel surge-released ships in head regular waves. A hybrid approach combining potential flow theory and functional-decomposition URANS is used for an efficient and accurate simulation. The methodology involves a surge-released module, 4DOF motion equations, multiple coordinate systems, and a dynamic structured grid. Two ship models are used for validation, comparing numerical and experimental results in ship motions and wave loads. The analysis shows good agreement, with differences of less than 5 % in heave and pitch motions, less than 8 % in roll motions, and less than 15 % in total resistance and sway interference forces. Present study also observes consistent trajectories of the wave system between the two ships. The influence of surge motion and wave height on wave-ship-ship interaction is investigated, emphasizing the importance of considering surge motion in seakeeping performance and longitudinal separation. The occurrence of parametric roll in Ship B is accurately resolved by the hybrid method and the parametric roll amplitude is smaller under ship-to-ship interaction conditions. Wave-induced moments play a limited role in this phenomenon, while the dominating roll restoring moment exhibits a hardening effect. Accordingly, roll motion and moments remain relatively unchanged with increasing wave height, indicating the strong nonlinear nature of parametric roll.

Motion and Mooring Load Responses of a Novel 12-MW Semi-Submersible Floating Wind Turbine: An Experimental Study

Abstract Due to the complexity of the integrated Floating Wind Turbine (FWT) system, obtaining reliable results necessitates extensive experiments. This paper conducts a comprehensive study on the motion performance and mooring load responses of a novel 12-MW semi-submersible FWT through model tests carried out in a wave basin. A multi-blade large-scale wind-generation system, equipped with a rectifier network, was enhanced and constructed to provide a dependable wind field. And a flexible tower was designed and fabricated, achieving an accurate simulation of the tower's stiffness characteristic and its impact on the overall dynamic response. The marine environmental conditions encompass various combinations of wind, waves, and currents. Rigorous calibration and identification tests were undertaken to validate the environmental conditions and the model system. The findings reveal that, under mild wave parameters, the mooring load is primarily influenced by the resonance response with platform motions, particularly surge resonance. The load effect of wind and current induces mean surge and pitch motions, while their damping effect reduces the standard deviation of responses, notably suppressing the pitch response peak at its natural motion frequency. Wave loads predominantly dictate the vibration range of motion responses. When the current velocity reaches a sufficient magnitude, the coupling effect between current and wave in the wave–frequency region significantly amplifies the mooring response. Notably, motions and mooring loads in the 60-deg and 90-deg directions surpass those in the 0-deg direction, with the maximum responses occurring at 60 deg.

Coupled analysis of floating offshore wind turbines with new mooring systems by CFD method

This paper presents a fully coupled aero-hydro-mooring numerical model of the National Renewable Energy Laboratory's (NREL) 5 MW OC4 semi-submersible Floating Offshore Wind Turbine (FOWT). The model's accuracy was validated through comparisons with existing experimental and numerical data, utilizing OpenFOAM, a Computational Fluid Dynamics (CFD) software. The key contributions of this study include demonstrating the model's precision in predicting aerodynamic performance, motion responses, and mooring system dynamics under wind and wave conditions. Additionally, a comparative analysis is conducted between a Conventional mooring system (CMS) and a New mooring system (NMS). The results show that the NMS offers improved stability in surge and heave motions, crucial for operational effectiveness and resilience. Furthermore, the NMS demonstrates a more efficient stress distribution and also reduced tensions in the mooring lines, contributing to the long-term durability and safety of the structure. In terms of aerodynamic performance, the differences between the two mooring systems are limited due to the combined surge and pitch motion phase difference. Future research will focus on optimizing these phase differences to enhance performance and refine the cost-efficiency of the mooring system.

Performance and mechanism of adaptive pitch control in a floating vertical axis wind turbine with a surge motion

Floating vertical axis wind turbines are bearing complex aerodynamic fatigue loads due to the movement of the platform and the reciprocating variations in the angle of attack and the blade relative wind speed. To explore the effect of the surge motion of the platform and verify the performance and acting mechanism of adaptive pitch control on reducing the fatigue loads and improving the power coefficient, a floating vertical axis wind turbine with three motion forms (non-surge, pure surge, and surge-adaptive pitch) is investigated in the condition of a low tip speed ratio with a high incoming wind speed. The results show that the surge motion enhances the fatigue loads, with the power coefficient less affected. The adaptive pitch control reduces the enhanced fatigue loads and improves the power coefficient by nearly twice. Vorticity and pressure distributions verify that the fluctuation ranges of the angle of attack and the blade relative wind speed are expanded by the surge motion, and the angle of attack is reduced by the adaptive pitch control especially in the upwind section, with the flow separation mitigated. However, the effects of the surge motion and the pitch variation are also influenced by the shedding vortex and the flow separation reattachment. For the upwind part of the blade aerodynamic forces, with the surge amplitude increase or the surge period decrease, the global amplitudes are increased gradually by the surge motion and then reduced by the pitch control, with the load reduction performance enhanced gradually.

Study on the mooring systems attaching clump weights and heavy chains for improving the typhoon resistance of floating offshore wind turbines

The southeastern coastal region of China is the most favorable area for offshore wind resources in the country, but typhoon impacts constrain the development of floating wind turbines in this region. Therefore, ensuring reliable and efficient mooring performance is crucial due to significant safety concerns presented by dynamic platform motions and mooring line tensions encountered during typhoon conditions. In this study, two mooring models were proposed to investigate the impact of clump weights or heavy chains on enhancing the typhoon resistance of floating offshore wind turbines. The 10-MW OO-Star semi-submersible platform was used as the reference structure. The aero-hydro-servo-elastic coupled simulations using OpenFAST were conducted for the selected environmental condition. The surge motion and fairlead tension were compared in both time-domain and frequency-domain for different mooring configurations. The results demonstrate that adding clump weights and heavy chains effectively limits the surge motion and tension fluctuations. Additionally, five installation positions were proposed and compared. Furthermore, a comparison between clump weights with heavy chains was conducted. The performance of a mooring system can be improved in the typhoon condition by incorporating optimal clump weights or heavy chains.

Theoretical and experimental study of the high-frequency nonlinear dynamic response of a 10 MW semi-submersible floating offshore wind turbine

To further investigate the hydrodynamic performance of a novel 10 MW semi-submersible floating offshore wind turbine (FOWT) OUCwind and the dynamic response of the FOWT, especially the high-frequency dynamic response induced by the high-order high-frequency hydrodynamic loads, an experimental study for OUCwind is carried out. A novel regular wave condition design method is proposed, i.e., making incidence wave periods that exactly meet an integer multiple of the natural period of the wind turbine, to stimulate the structural resonance of the tower. To determine the natural period of the wind turbine with an elastic boundary, an irregular wave test is carried out and the natural period of the wind turbine with the elastic boundary is proved to be 3 s. Different parts of the FOWT model are verified to satisfy the corresponding scale similarity through a series of validation tests. The hydrodynamic performance of OUCwind is investigated. Linear response is obtained by applying the band-pass filter to the total response. The pitch motions have apparent high-frequency components aligned with the natural frequency of the wind turbine under the environmental conditions with a wave periods of 6 s. For surge and heave motion, the linear component dominates these two responses and the effect of the high-frequency component on these two responses is negligible. The second-order doubling and third-order tripling wave effects have a tremendous impact on the tower-top shear force and mooring line tension. The experimental data providing high-frequency dynamic responses up to the quadruple order can be used for the validation and correction of mid-fidelity and high-fidelity numerical models. Finally, the mechanisms of the generation of the high-frequency dynamic response of the FOWT are also concluded in an illustrative form.

A novel vortex-based velocity sampling method for the actuator-line modeling of floating offshore wind turbines in windmill state

The fluid-dynamic simulation of wind turbine aerodynamics is typically tackled by applying multi-fidelity computational tools. In this context, the so-called actuator line model combines a low-fidelity treatment of the rotor with a high-fidelity resolution of the wake. In this paper, a novel formulation of the actuator line model proposes a vortex-based method to sample the flow around the rotor to rigorously assign the forces imparted by the blades. This new technique is implemented into an in-house code developed within the OpenFOAM environment, and it is validated against wind-tunnel experiments on a laboratory-scale horizontal-axis wind turbine operated in fixed-bottom and floating conditions. The calculations are also compared against multi-fidelity simulations performed, on the same test case, in the frame of the OC6 Phase III project. The simulation results, obtained after a systematic analysis and selection of the model parameters, exhibit a remarkable agreement with the available experiments and place the present code in the proper ranking of fidelity levels, in-between momentum-balance methods and blade-resolved CFD models. Finally, the calculations for surge and pitch platform motions demonstrate the capability of the proposed technique to reliably predict the aerodynamics of turbine rotors in dynamic operation at affordable computational cost.