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This study investigates the sectional loads on an elastic semi-submersible platform for a 2 MW FOWT (floating offshore wind turbine) used in the Fukushima demonstration project. A water tank test is firstly carried out with an elastic model to study the dynamic responses and sectional loads of the platform in regular and irregular waves. Numerical simulations are then performed using multiple hydrodynamic bodies connected by elastic beams. The dynamic responses of the elastic model are compared to those of a rigid model to clarify the influence of the structural stiffness on the platform motion and mooring tension. The predicted sectional loads on the deck, brace and pontoon by the proposed nonlinear hydrodynamic models show good agreement with the experimental data obtained from the water tank test and a simplified formula is proposed to evaluate the distribution of the moments on the platform. Finally, the structural optimization of the elastic semi-submersible platform is conducted. The sectional moments and fatigue loadings on the pontoons are significantly reduced using the strut between the pontoons since the horizontal wave loads on the side column are dominant and the vertical wave loads acting on the platform are relatively small due to the deep draft.

Numerical investigations of floating platforms with different outer column inclined angles under two operating conditions of regular wave and irregular wave are presented in this paper. A coupled aero-hydrodynamic computational fluid dynamics in-house solver FOWT-UALM-SJTU is applied for the calculation. First, the validation for wave and wind generation are conducted to determine mesh distribution strategy. Based on these, the hydrodynamic motion response, aerodynamic performance and wake flow are analyzed to explore the impact of inclined angle. Conduct spectral analysis on the motion response under wave action, discuss the aerodynamic attack angle and inflow wind velocity along the blade spanwise direction in detail, reveal different trends in wake development and recovery. The results show that for the regular wave condition with the increase of inclined angles, the equilibrium position of surge motion is constantly rising, while pitch is decreasing. The maximum root mean square (rms) value occurs at angle = 30°, compared with the original OC4 FOWT, the rms in power and thrust increase 0.35%, 0.71%. And there are two low regions of attack angle and high regions of axial inflow velocity, corresponding to aerodynamic loads. The spectral analysis indicates that the natural frequency of pitch motion will increase with inclined angle. Besides, from the middle to far region of wake flow, the velocity recovery of FOWT with inclined angle will become faster, which is beneficial for downstream turbines to enhance more wind energy.

This paper presents a multi-objective design optimization approach for floating wind turbines with a design space that spans three stability classes of floating wind turbine support structures. A single design parameterization scheme was used to define the geometries of tension-leg, spar buoy, and semi-submersible candidate designs in terms of nine design variables. The seakeeping analysis of any particular platform configuration was completed using a simplified frequency-domain dynamic model applying linearized dynamics for the floating platform, mooring system, and a reference 5 MW wind turbine that were derived using existing functionality in FAST and WAMIT. Evaluation and comparison of different platforms was performed using a Pareto front pursuing multi-objective Genetic Algorithm (GA) optimization method to find the locus of platform cost minima and wind turbine performance maxima for a given environmental condition and sea state spectrum. Optimization results for the single-body platforms indicated a dominance of tension-leg platforms in this subset of the design space. Results for multi-body platforms showed that semi-submersible platforms with four floats demonstrated better stability and were more cost effective than other semi-submersible designs. In general, the full exploration of the design space demonstrated that four float semi-submersible platforms with angled taut mooring systems are a promising concept that can be used as a foundation for a detailed design and costing study. The results generated here are subject to the specifics of the targeted environmental conditions, cost model, linearized dynamics and choice of performance metric. As these elements evolve, the optimization framework presented here should be reapplied to track how the Pareto fronts for the different classes of platforms respond.

In offshore crane systems, the floating platform motion has a significant impact on the dynamics of the cart motion. Nevertheless, previous studies have ignored the dynamic coupling interaction between the crane and the floating platform induced by changes in hydrostatic, hydrodynamic, and mooring loads affecting the offshore platform-crane system response. To address this problem, this study presents, firstly, a comprehensive model of the crane-platform dynamic coupling under realistic surge-roll-heave motions of the floating platform induced by ocean waves. While the payload motion can be known, the surge-roll-heave motions of the floating platform are considered unknown. Therefore, secondly, we propose an output feedback control approach that combines a state feedback controller and an extended high-gain observer to primarily achieve desired trajectories of the cart motion under unknown payload mass, dynamic friction, dynamic coupling, and external disturbances. The extended high-gain observer uses the measured displacement of the cart to estimate the dynamic states and external disturbances, providing the state feedback controller with the necessary information and increasing the robustness of the control system. The effectiveness of the proposed model-based control approach under unknown dynamic and wave motion disturbances is verified through simulation.

Floating offshore wind platform (FOWP) has become the economically favored option for supporting wind turbines in deep waters. It is urgent to propose new concept designs for FOWPs that can be effectively deployed. Additionally, the extensive use of steel in such platforms significantly escalates costs, necessitating the optimization of steel utilization. Motivated by these challenges, a V-shaped floating semi-submersible platform equipped with NREL 5 MW wind turbine is designed and analyzed based on the potential flow theory and the blade element momentum theory. Fully coupled time-domain simulations are conducted using the F2A program, which couples NREL FAST and ANSYS AQWA via a Dynamic Link Library (DLL), to compare the hydrodynamic performance and stability of the V-shaped floating platform with the original triangle-shaped model of “Fuyao”. Various sea conditions have been considered, including combined wind-wave action and wind-wave-current action at different incidence angles. The results show that the V-shaped floating platform has better economic and hydrodynamic performance (e.g., a reduction of 40.4% and 12.9%, respectively, in pitch and yaw motions, and a 17.4% reduction in maximum mooring tension), but lower stability than its triangle-shaped counterpart.

Rayleigh lidar equipped on airborne floating platforms has received increasing attention in recent years due to the demand for exploring the middle atmosphere. However, the inevitable attitude fluctuation of the platform affects the measurement accuracy of the photon profile, which greatly affects temperature retrieval. Here, an extensive theoretical analysis model of geometrical transformations between the actual altitude and detection distance under attitude fluctuations was constructed by taking pitch, roll, and observation angles into consideration. Based on this model and measured attitude angles, the influence of platform fluctuation on lidar measurement was analyzed by calculating the deviations between temperature retrieval results and the NRLMSISE-00 model at different observation angles, which demonstrated that the altitude displacement from the variation of pitch angle is a crucial factor in causing temperature retrieval error, especially at large observation angles. Then, an attitude compensation method was designed to eliminate the impact of fluctuations, incorporating the merits of good robustness. Under the observation angle of 45° and average pitch angle of around 4°, the maximum temperature deviation after attitude compensation was reduced from 21.29 K to 0.366 K, a reduction of around two orders of magnitude, indicating that the method can significantly improve the measurement accuracy of Rayleigh lidar.

The working condition of the floating platform will be affected by wind and waves in the marine environment. Therefore, it is of great importance to carry out real-time prediction research on the mooring load for ensuring the normal operation of the floating platform. Current researches have focused on the real-time prediction of mooring load using the machine learning method, but most of the studies are about the application and generalization analysis of different models. There are few studies on the influence of data distribution characteristics on prediction accuracy. In view of the above problems, this paper investigates the effect of data skewness on the prediction performance for the deep learning model. The long short-term memory (LSTM) neural network is applied to construct the mooring load prediction model. The numerical simulation datasets of the deep water semi-submersible platform are employed in model training and data analysis. The prediction performance of the model is preliminarily verified based on the simulation results. Meanwhile, the distribution characteristics of mooring load data under different sea states are analyzed and a skewness processing method based on the Box-Cox Transformation (BCT) is proposed. The effect of data skewness on prediction accuracy is further investigated. The comparison results indicate that reducing the mooring load data skewness can effectively improve the prediction accuracy of LSTM model.

In this article, the effects of the changes in the mass of the floating wind turbine (as a multi-body system) on its nonlinear vertical vibrations are investigated. The fluctuations of the hydrodynamic added mass of the floating platform and the mass of the vibration absorbers, which added to the structure to mitigate the lateral vibrations, change the mass and consequently the dynamics of the vertical vibrations. In this regard, first, the governing equations of the vertical vibrations of the floating wind turbine are derived. The FAST code is used to validate the proposed model of the dynamics of the vertical vibrations through numerical simulations. Then, derived equations are solved approximately by the perturbation method. According to the approximate solutions, the fluctuations of the added mass of the floating platform and the masses of the vibration absorbers increase the frequency and amplitude of the vertical vibrations, which increases the fatigue loads on the tower of the wind turbine as well as moorings of the floating platform.

Floating offshore wind turbines (FOWTs) still face many challenges in improving platform stability. A fully submersible FOWT platform with inclined side columns is designed to tackle the current technical bottleneck of the FOWT platform, combining the structural characteristics of the semi-submersible and Spar platform. An integrated numerical model of FOWT is established considering the fully coupled effect, and the hydrodynamic performance of the novel FOWT, the semi-submersible FOWT, and the Spar FOWT are compared and analyzed under different wave incidence angles and wave frequencies, as well as the blade and tower dynamic response of the three FOWTs under the coupling effect of wind, wave, and current. The results show that the novel floating platform can significantly optimize the hydrodynamic performance and has a better recovery ability after being subjected to external loads. The novel floating platform can significantly reduce the heave peak and its corresponding wave frequency compared to the semi-submersible platform, reducing the possibility of heave resonance. FOWT operation should ensure positive wave inflow as far as possible to avoid excessive wave forces in the lateral direction. Both blade and tower dynamic response are affected by rotor rotation and tower vibration to varying degrees, while tower dynamic response is mainly affected by platform motion. This study suggests that the application of the novel FOWT concept is feasible and can be an alternative in offshore wind exploitation in deep water.

The growing prospect for large farms of floating offshore wind turbines requires a better understanding of wake effects for floating turbines. In this work, large eddy simulations with an actuator line model are used to study the wake of the NREL 5 MW reference turbine mounted on the OC3-UMaine spar and OC4-DeepCwind semi-submersible platforms. The simulations are carried out in the Simulator fOr Wind Farm Applications (SOWFA) coupled with OpenFAST for the platform and turbine motion. The wake location, deficit, and turbulence levels are compared for the two floating platforms and equivalent fixed-turbine cases. The effects of neutral versus stable atmospheric conditions are also compared. Most notably, floating-turbine wakes are deflected upwards compared to fixed-turbine wakes, because of mean platform pitch. The spar wake deflects upwards more than the semi-submersible, while the stable atmosphere increases this vertical deflection compared to the neutral. The time-varying rotor motions do not significantly affect the mid-to-far wake, though the stable atmosphere shows larger fixed-floating differences in horizontal wake fluctuations.