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This paper proposes novel methods for the modeling and control of spar-type floating offshore wind turbines (FOWTs) by focusing on the dependency of the equilibrium and perturbed dynamics on the rotor azimuth angle. In addition, three new reduced models for controller design are derived using trajectory linearization by accounting for the dependency of the equilibrium on the azimuth angle. A thorough simulation study shows that the proposed models reproduce the important dynamic characteristics of FOWTs more accurately than the conventional models. Then, nonlinear model predictive controllers (NMPCs) minimizing the nonquadratic cost functions are developed for the proposed models, which include nonlinear terms for the rotor azimuth angle. These NMPCs suppress the variation in the forces applied to the blades better than the conventional linear MPCs while maintaining a low computational cost. The best NMPC for the models is one that accounts for the dependency of both the equilibrium and perturbed dynamics on the rotor azimuth angle. This NMPC suppresses the platform yaw and forces added on the blades. The performance of such an NMPC can be further improved using the inflow wind disturbance data predicted using a light detection and ranging wind sensor. [ABSTRACT FROM AUTHOR]

Currently, there is a strong interest to develop offshore wind energy because of the great impact of greenhouse gases and the energy crisis. Extraordinary endeavors have been dedicated to creating floating offshore wind turbine (FOWT) innovations such as the DeepCwind semi-submersible FOWT that can be relied upon to harness wind energy in deep water. However, DeepCwind structure faces a significant surge motion limiting the operational time. Therefore, this paper presents three different configurations for the DeepCwind semi-submersible FOWT by varying the number of offset columns from three to five columns to improve the wave resistance ability. Hydrodynamic analysis is carried out to figure and compare the performance of these platforms via the ANSYS-AQWA tool. The free-decay test has shown that the natural frequency is affected by the number of offset columns. The results of statistics on regular waves and irregular waves have shown that the surge response is inversely proportional to the number of offset columns; the statistics decrease with the increase of the offset columns number, while the impacts of increasing offset columns are negligible for the heave and pitch motions. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbine foundations are based on platforms operated by the oil and gas industry. However, they are designed and optimized to meet the wind turbines' operating criteria. Although Malaysia is considered a low-wind-speed country, there are some locations facing the South China Sea that are found to be feasible for wind energy harnessing. The average daily wind speed may reach up to 15 m/s. Therefore, designing a cost-effective platform that can operate in Malaysian waters which has less severe environmental conditions compared to the North Sea would be a prudent undertaking. In this study, a new design of a multi-purpose floating offshore wind turbine platform (Mocha-TLP) is presented. In addition, the dynamic response of the platform to wave loads was investigated using the Navier–Stokes code STAR CCM+ developed by CD-adapco. Moreover, free-oscillation tests were performed to determine the natural periods of the platform. Three approaching wave cases and two wave conditions (WC) were considered. The results show that the natural periods of the platforms were within the recommended range for pitch, roll, yaw, heave, sway and surge motions. The platform was stable in rotational motion within the three cases. However, it experienced a noticeable surge motion which was more critical with wave condition one (WC1) since the wavelength equalled the length of the structure. The dynamic response of the platform to wave loads wase minimal and within the operational requirements for wind turbines. [ABSTRACT FROM AUTHOR]

The number of installed offshore wind turbines is continuously growing worldwide in recent years. Offshore wind farms are generally located near the coast close to traffic lanes and are exposed to the risk of collisions from visiting and passing ships. Potential consequences of collisions may vary from local structural damage to the detachment of turbine nacelles and rotors, and even tower collapse and capsizing of the turbine platform, causing significant economic loss and fatalities. This paper investigates ship collision responses of a semi-submersible floating offshore wind turbine (FOWT), i.e. the OO-STAR floater with the DTU 10 MW blades, using the nonlinear finite element code USFOS. The OO-STAR floater is made of post-tensioned concrete designed by Dr. techn. Olav Olsen. The striking ships are selected to be a modern supply vessel of 7500 tons and a shuttle tanker of 150,000 tons, representing respectively service/coastal merchant vessels and large passing vessels. Modelling of the FOWT in USFOS is described in detail including the OO-STAR floater, the DTU 10 MW turbine blade, the turbine tower, and the mooring system. The modelled hydrodynamic loads include buoyancy loads and motion induced radiation loads using the Morrison equation. The effects of external waves and currents are assumed to be small and ignored in all directions. Eigenmode analysis of the turbine model is performed to verify the established model. Global collision response analyses of the FOWT were performed in both parked and operating conditions. The ship resistance is modelled as nonlinear springs in USFOS containing force displacement curves simulated in LS-DYNA. In operating conditions, wind loads are introduced including wind thrust loads and wind induced torque to rotate the turbine blades. The changes of upstream wind speeds and rotor/wake interactions during collisions are neglected. The results are discussed with respect to energy absorption of the ship and the FOWT, and structural responses of the FOWT including global motions, nacelle accelerations, tower clearance, tower vibrations, and responses of the mooring system. • A semi-submersible floating turbine subjected to ship collisions is analyzed. • Modelling of the turbine in USFOS is described and verified by eigenmode analysis. • The striking ships are service/coastal merchant vessels and large passing vessels. • Global response analyses were performed in parked and operating conditions. • Results are discussed with respect to energy absorption and motions. [ABSTRACT FROM AUTHOR]

[Introduction] Floating offshore wind turbines technology is developing rapidly in Europe, America and Japan, and other developed countries, because of its advantages, such as non-restriction of water depth, easy installation, maintenance, transport, etc.This paper reviews the key technologies and engineering cases of floating offshore wind turbines, in order to provide references for research works and projects in this field. [Method] This paper introduced the background of floating wind turbines technology,and then provided detailed introduction on its key technologies, for instance, stability checking, mooring sysytem, dynamic cable,hydrodynamics, aerodynamics, integral calculation, model basin tests, manufacture, installation, engineering challenges and cases.Finally, the paper summarized potential market and applications of floating wind turbines technologies. [Result] The paper summarizes the state of art of the key technologies and future development of floating wind turbines. [Conclusion] This review presents the key technologies and engineering cases of floating wind turbines. There are quite a few technology challenges and problems which worried industry. Hence, the solution to reduce project cost and to facilitated the growth of the industry is multipath, for instance, the more active policy support, making technological breakthrough, and cultivation of the industrial chain.

Floating Offshore Wind Turbines (FOWT) installed in the deep sea regions where stable and strong wind flows are abundant would have significantly improved energy production capacity. When designing FOWT, it is essential to understand the stability and motion performance of the floater. Water tank model tests are required to evaluate these aspects of performance. This paper describes a model test and numerical simulation for a 750-kW semi-submersible platform wind turbine model-II. In the previous model test, the 750-kW FOWT model-I suffered slamming phenomena from extreme wave conditions. Because of that, the platform freeboard of model-II was increased to mitigate the slamming load on the platform deck structure in extreme conditions. Also, the model-I pitch Response Amplitude Operators (RAO) of simulation had strong responses to the natural frequency region. Thus, the hub height of model-II was decreased to reduce the pitch resonance responses from the low-frequency response of the system. Like the model-I, 750-kW FOWT model-II was built with a 1/40 scale ratio. Furthermore, the experiments to evaluate the performance characteristics of the model-II wind turbine were executed at the same location and in the same environment conditions as were those of model-I. These tests included a free decay test, and tests of regular and irregular wave conditions. Both the experimental and simulation conditions considered the blade rotating effect due to the wind. The results of the model tests were compared with the numerical simulations of the FOWT using FAST (Fatigue, Aerodynamics, Structures, and Turbulence) code from the National Renewable Energy Laboratory (NREL).

This paper presents an integrated analysis about dynamic performance of a Floating Offshore Wind Turbine (FOWT) OC4 DeepCwind with semi-submersible platform under real sea environment. The emphasis of this paper is to investigate how the wave mean drift force and slow-drift wave excitation load (Quadratic transfer function, namely QTF) influence the platform motions, mooring line tension and tower base bending moments. Second order potential theory is being used for computing linear and nonlinear wave effects, including first order wave force, mean drift force and slow-drift excitation loads. Morison model is utilized to account the viscous effect from fluid. This approach considers floating wind turbine as an integrated coupled system. Two time-domain solvers, SIMA (SIMO/RIFLEX/AERODYN) and FAST are being chosen to analyze the global response of the integrated coupled system under small, moderate and severe sea condition. Results show that second order mean drift force and slow-drift force will drift the floater away along wave propagation direction. At the same time, slow-drift force has larger effect than mean drift force. Also tension of the mooring line at fairlead and tower base loads are increased accordingly in all sea conditions under investigation.

Due to the existence of rotor and tower, the floating offshore wind turbine (FOWT) is subjected to greater wind heeling moment than the conventional floating platform, which would cause significant pitch motion and bring great challenge to the structural safety and power generating efficiency. Moreover, the dynamic cable used for power transmission is sensitive to the horizontal motion of floating platform. Thus, it is essential to inhibit the pitch and horizontal motions of FOWT. The FOWT is connected to the seabed by a mooring system to resist the motions, while the conventional mooring system mainly resists the horizontal motion. In this article, an innovative type of mooring system was proposed to resist the pitch and horizontal motions simultaneously, by dividing the fairleads into two groups at different depths. The motion responses and structural loads of FOWTs were calculated and compared under the effects of the conventional and innovative mooring systems. In addition, the restoring forces and moments of two mooring systems were also given to better verify the motion-inhibiting performances. According to the results, the innovative mooring system was able to significantly reduce the pitch and surge motions of FOWT.

Offshore wind energy is a renewable energy source that is developing fast. It is considered to be the most promising energy source in the next decade. Besides, the expanding trend for this technology requires the consideration of diversified seabeds. In deep seabeds, floating offshore wind technology (FOWT) is needed. For this latter technology, such as for conventional WT, we need to consider aspects related to performance, aerodynamic force, and forces during operation. In this paper, a two-bladed downwind wind turbine model is utilized to conduct experiments. The collective pitch and cyclic pitch angle are adjusted using swashplated equipment. The fluid forces and moments acting on the rotor surface are measured by a six-component balancing system. By changing the pitch angle of the wind turbine blades, attempts are made to manage the fluid forces generated on the rotor surface. Under varied uniform wind velocities of 7, 8, 9, and 10 m/s, the effect of collective pitch control and cyclic pitch control on the power coefficient and thrust coefficient of FOWT is then discussed. Furthermore, at a wind speed of 10 m/s, both the power coefficient and loads are investigated as the pitch angle and yaw angle change. Experimental results indicate that the combined moment magnitude can be controlled by changing the pitch-angle amplitude. The power coefficient is adjusted by the cyclic pitch-angle controller when the pitch-angle phase changes. In addition, the thrust coefficient fluctuated when the pitch angle changed in the oblique inflow wind condition.

The studies on floating offshore wind turbines (FOWTs) have been increasing over recent decades due to the growing interest in offshore renewable energy. The present paper proposes a barge platform with four moonpools to support the Technical University of Denmark 10 MW wind turbine for a designed water depth of 60 m. A 4 × 2 mooring system with eight mooring lines is also proposed for the barge platform. The main dimensions of the barge platform are optimally selected with respect to its preliminary hydrodynamic properties and potential financial benefit. The proposed barge-type FOWT is then demonstrated to be aligned with the DNV standard requirements in terms of its intact and damage stability. Furthermore, coupled time-domain simulations are conducted for the proposed barge FOWT with mooring under the selected environmental and operational conditions by using Simo-Riflex-AeroDyn (SRA). Through decay test simulations, the natural periods of the barge-type FOWT are demonstrated to be within the DNV recommended ranges. The proposed mooring system is also benchmarked with the 3 × 3 mooring concept that was used for a 3 MW barge-type FOWT installed in Kitakyushu. The response magnitudes of the barge platform and mooring line tension are similar to both mooring systems, and thus the 4 × 2 mooring system is preferred due to its lower cost. In addition, the proposed barge platform is preliminarily demonstrated to be able to survive for the 50-year extreme environmental conditions under parked wind turbine status, as well as the normal environmental conditions under the operating status.

Vortex-induced motion (VIM) is a critical issue for floating structures made of one or more columns, due to its significant impacts on their operational stability. Supported by column-type floating platforms, floating offshore wind turbines (FOWTs) may also experience large-amplitude VIM responses in current flow. Existing research on FOWTs has mostly focused on their wind/wave induced responses, yet less attention has been paid to their responses in current flow. In this paper, the VIM of the OC4 semi-submersible FOWT platform is studied numerically over a wide range of flow velocity. Three incidence angles, i.e., 0°, 90°, and 180°, are considered and the effect of current incidence on platform VIM is analysed. Results show that the so-called lock-in phenomenon is present and that a large transverse response amplitude of more than 0.3D persists until Vr = 30, with its maximum reaching over 0.8D at Vr = 8. Meanwhile, the transverse response amplitude for cases with the incidence angle of 180° is generally smaller, with a narrower lock-in regime, than those under the other two incidence scenarios. Flow field visualisation reveals that upstream vortices continuously interact with the downstream side column when the incidence angle turns to 180°, impacting the vortex shedding process and consequently fluid forces of the downstream column.

Floating offshore wind turbines (FOWTs) are generally located in the harsh deep-sea environment and are highly susceptible to extreme loads. In order to ensure the normal operation of FOWTs, this article takes the semi-submersible FOWT as an example, proposes a new double-rope mooring system, and studies the dynamic performance of the FOWT with the double-rope mooring system and its effectiveness in reducing the dynamic response of the wind turbine. At the same time, the tuned mass damper (TMD) is installed in the nacelle of the wind turbine, and the TMD parameters are optimized considering the space limitation of the nacelle by limiting the TMD’s stroke, which further reduces the dynamic response of the FOWT and improves its stability. Numerical simulation and analytical studies show that the new double-rope mooring system can reduce the dynamic response of the wind turbine to a greater extent than the traditional single-rope mooring system. Considering the stroke restriction, the control performance of TMD will be slightly weakened, but it is more in line with the actual engineering requirements. Compared with the original FOWT, the proposed new type of FOWT has better dynamic stability and has the prospect of extending to real engineering applications.

The structural health monitoring of a Floating Offshore Wind Turbine (FOWT) tendons, taking into account the comprehensive damage diagnosis problem of damage detection, damaged tendon identification and damage precise quantification under varying environmental and operating conditions (EOCs), is investigated for the first time. The study examines a new concept of a 10 MW multibody FOWT whose tower is supported by a platform consisting of two rigid-body tanks connected by 12 tendons. Normal and the most severe EOCs from a site located in the northern coast of Scotland, are selected for the simulation of the FOWT structure under constant current but varying wind and wave conditions. Dynamic responses of the platform under different damage states are obtained based on the simulated FOWT. The damage scenarios are modelled via stiffness reduction (%) at the tendon's connection point to the platform's upper tank. Damage diagnosis is achieved via an advanced method, the Functional Model Based Method, that is formulated to operate using a single response signal and stochastic Functional Models representing the structural dynamics under the effects of varying EOCs and any magnitude of the considered damages. Due to the robustness and high number of the existing tendons, the effects of considered damages on the FOWT dynamics are minor and overlapped by the effects of the varying EOCs, indicating a highly challenging damage diagnosis problem. Very good damage detection results are obtained with the damage detection almost faultless and with no false alarms. Accurate damaged tendon identification is achieved for the 95% of the considered test cases, while the mean error in damage quantification is approximately equal to 4% using measurements from just a single accelerometer within a very limited frequency bandwidth of [0–5] Hz. • Investigated damaged tendon diagnosis in a new multibody Floating Wind Turbine. • Proposed a damaged tendon diagnosis method under varying conditions. • Diagnosis uses single accelerometer within a limited low frequency bandwidth. • Representation of structural dynamics under varying conditions by Functional Models. [ABSTRACT FROM AUTHOR]

In order to mitigate dynamic responses of floating offshore wind turbines (FOWTs) under combined wind-wave-current loading, employment of structural control techniques is often considered. In this paper, a novel concept of a passive FOWT structure is proposed to overcome the previous limitations on space and mass of tuned mass dampers (TMDs). The conceptual design is examined on the basis of a finite element model. The dynamic responses, mainly roll and pitch rotations, are compared between the controlled and uncontrolled FOWTs. Subsequently, a parametric study is conducted and the main parameters in the model are optimised accordingly. Finally, the optimal performance of the proposed FOWT structure demonstrates the effectiveness of the proposed design, and also its superiority over the existing concepts. [ABSTRACT FROM AUTHOR]

Innovative floating offshore wind turbine (FOWT) platforms that deviate from the conventional semi-submersible, spar, and tension leg platforms (TLP) have become increasingly common due to the need to tap into the high wind energy potential located in deeper waters. One example is the hanging-mass concept, in which a suspended counterweight stabilizes a positively buoyant floater. This work presents a two-dimensional, nonlinear, multi-body model used to assess the influence of the counterweight mass and the suspension line stiffness on the system's global performance, using linear stability analysis and time-domain simulations to conduct a parametric study. For example, the counterweight mass has a strong influence on the amplitude of rotational degrees-of-freedom. Corresponding natural periods may occur within the linear wave energy range for suitable counterweight sizes due to this strong influence leading to undesirable motions. High-frequency multi-body modes are also dependent on both the line stiffness and counterweight mass, which may result in high relative motion amplitudes and slack lines in certain conditions. Finally, the parametric study results contribute to preliminary hanging-mass FOWT design recommendations. [ABSTRACT FROM AUTHOR]

In this study dynamic responses of a 10 MW offshore wind turbine supported by a multi-body floating platform that consists of a wide cylindrical platform and a cylindrical ballast body suspended by six tendons are analyzed and predicted for different tendon breakage scenarios. A newly-developed and validated fully coupled numerical tool (F2A) based on AQWA and FAST is used to perform aero-hydro-servo-elastic analysis of the floating offshore wind turbine (FOWT). The results indicate that the dynamic behavior of the platform is heavily influenced by the state of tendons health. Roll and yaw motions of the platform under a tendon breakage are found to experience 6 times magnitude amplification of the typical responses, depending on the specific environmental conditions considered. Moreover, the peak tension in the tendon adjacent to the broken tendon experienced an increase of 165% in magnitude. The collective-pitch mode of the platform and wave excitation that are the main contributors to the surge and pitch fluctuations are slightly affected by tendon breakages. The influence of tendon breakages is found to be only significant on the local-pitch and coupled-pitch modes of the platform. In addition, multifractal spectra of the platform accelerations under different tendon failure scenarios show distinct fractal characteristics that can effectively identify and diagnose tendon failures, which is essential to the development of a structural health monitoring system of FOWTs. • Coupled responses of a 10 MW floating wind turbine under damaged scenarios are investigated. • Dynamic responses under intact and damaged tendon conditions are analyzed and predicted. • Transient effects of tendon failures on the dynamic behavior are assessed. • Multifractal spectrum analysis method is used to identify tendon damage features. [ABSTRACT FROM AUTHOR]

A Multi-Scale Convolutional Neural Network with Self Attention-based Auto Encoder–Decoder (MSCSA-AED), is a novel high-performance framework, presented here for the quantification of damage on a multibody floating offshore wind turbine (FOWT) structure. The model is equipped with similarity measurement to enhance its capability to accurately quantify damage effects from different scales of coded features using raw platform responses and without human intervention. Case studies using different damage magnitudes on tendons of a 10 MW multibody FOWT were used to examine the accuracy and reliability of the proposed model. The results showed that addition of Square Euclidean (SE) distance enhanced the MSCSA-AED model’s capability to suitably estimate the damage in structures operating in complex environments using only raw responses. Comparison of the model’s performance with other variants (DCN-AED and MSCNN-AED) used in the industry to extract the coded features from FOWT responses further demonstrated the superiority of MSCSA-AED in complex operating conditions, especially in low magnitude damage quantification, which is the hardest to quantify.

The WIND-bos spar Floating Offshore Wind Turbine is studied both experimentally and numerically. The experimental model of the moored WIND-bos platform is presented, and the different numerical models that have been developed to analyze the hydrodynamics of the platform are described. The results provide a detailed comparison of numerical and experimental motion responses of the floating structure in regular and irregular waves. The numerical study includes frequency domain results from spectral analysis, weakly nonlinear time-domain results from a validated in-house code, and coupled time-domain results from commercial software. The importance of damping calibration is put in evidence, whereas damping ratios are calculated iteratively in the coupled time-domain simulations, and nonlinear damping force is considered within the developed numerical scheme. The results compare well and also show that the novel concept has a good motion performance in general.

The tension legs are the essential parts of the tension legs platform-type (TLP-type) floating offshore wind turbine (FOWT) against the extra buoyancy of FOWT. Therefore, the TLP-type FOWT will face the risk of tension leg failure. However, there are seldom analyses on the hydrodynamic response and tension leg failure performance of FOWT with inclined tension legs. In this paper, a hydrodynamic model was established using three-dimensional hydrodynamic theory and applied in the motion response and tension analyses of FOWT with conventional and new tension leg arrangements on Moses. The influence of draft and tension leg arrangement on the performance of FOWT with inclined tension legs were studied. The optimum draft was the height of the column and lower tensions were obtained for the new tension leg arrangement. Moreover, the tension leg failure performance of FOWT with inclined tension legs was evaluated under different failure conditions. The results illustrated that the FOWT with the new tension leg arrangement can still operate safely after one tension leg fails.

This paper proposes an improved failure mode and effect analysis method for a comprehensive failure analysis that provides a holistic perspective of actions on the potential failures of floating offshore wind turbines. A new way of constructing risk priority numbers was developed by considering the background knowledge of the specialists involved in the failure analysis. The failure analysis was conducted based on an extensive dataset from multiple specialists that covers five floating offshore wind turbine systems, 15 main components, 42 failure modes, and 104 failure causes. Consequently, 21 recommendations are suggested for designers and operators to prevent and mitigate the risk of unexpected failures of floating offshore wind turbines. Furthermore, a comparison analysis was conducted to illustrate the similarities and differences between the proposed failure mode and effect analysis and the conventional method.

To enable the fast growth of the floating offshore wind industry, simulation models must be validated with experimental data. Floating wind model-scale experiments in wind–wave facilities have been performed over the last two decades with varying levels of fidelity and limitations. However, the turbine controls in these experiments have considered only limited control strategies and implementations. To allow for control co-design, this research focuses on implementing and experimentally validating more advanced turbine control actions and strategies in a wind–wave basin for a 1:70-scale model of the International Energy Agency’s wind 15 MW reference wind turbine. The control strategies analyzed include torque control, collective pitch control, and transition region control (setpoint smoothing). Our experimental and numerical results include the effects of varying rotor speeds, blade pitches, and wind environments on the turbine thrust and torque. Numerical models from three different software tools are presented and compared to the experimental results. Their ability to effectively represent the aero-dynamic response of the wind turbine to the control actions is successfully validated. Finally, turbine controller tuning parameters based on the derivatives of thrust and torque are derived to allow for improved offshore wind turbine dynamics and to validate the ability of modeling tools to model the dynamics of floating offshore wind turbines with control co-design.

Intra-array wake meandering increases fatigue loading in downstream turbines and decreases farm total power output. In the case of floating offshore wind turbines (FOWTs), the motions of the floating substructure could have a non-neglectable contribution to wake meandering dynamics. This research experientially analyses the influence of imposed motions on the far-wake meandering of a FOWT. The study considers a 1:500 scaled porous disc representation of the 2 MW FLOATGEN system (BW Ideol) located off the coast of Le Croisic, France. A representative marine neutral atmospheric boundary layer is generated in a wind tunnel whilst monochromic and multi-frequency content three degrees of freedom (surge, heave, pitch) motion is imposed on the model tower. The stereoscopic particle image velocimetry (S-PIV) is then utilised to measure velocity vectors at a cross-section located at 8.125 D downstream of the model. No significant effect on the far-wake recovery in the velocity, turbulence and turbulent kinetic energy distribution is observed. However, the frequency characteristics of the imposed motions were observed in the far-wake meandering spectral content and streamwise characteristics of far-wake, such as normalised available power. While the frequency spectrum of the vertical oscillations showed more sensitivity to the three degrees of freedom (3DoF) imposed motion in all frequency ranges, the lateral oscillation was sensitive for the reduced frequency above 0.15. The monochromic motions with a reduced frequency of less than 0.15 also did not influence the far-wake centre distribution in both lateral and vertical directions. Regardless of reduced frequency, imposed motions show a strong effect on average power, in which the harmonic signature can distinguish in far-wake memory. This study provides an investigation, which its result could be beneficial to developing and examining wake models for offshore wind turbines, with a particular focus on the influence of FOWTs motions.

To maximize the utilization of ocean resources, shorten the return period of investment and directly supply energy to the fishing cage, this paper performs a preliminary study for a state-of-the-art concept integrating a floating offshore wind turbine with a fishing cage. An octagonal semisubmersible rigid fishing cage with a slack catenary mooring system is designed to match the NREL 5 MW offshore baseline wind turbine. Combined with the blade pitch controller, fully coupled aero-hydro-elastic-servo-mooring simulations are performed through FAST and AQWA to explore the dynamic performance of the integrated system. Free decay conditions, uniform wind with irregular and regular waves, and turbulent wind with irregular waves are tested. The results showed that the integrated system works normally at the operating conditions and exhibits different dynamic characteristics for various scenarios. Additionally, the study on the influence of mooring line length indicates that the increasing line length can significantly affect the cage surge motion and the maximum and mean values of the upwind line tension at fairlead. Specifically, the maximum surge motion with a 924-m-long line is 404.8% larger than that with an 880-m-long line. When the line length increases by 5%, the maximum and mean line tensions decrease by 45.7% and 47.7%, respectively, while when the line length increases by 10%, the maximum and mean line tension decrease by 52.9% and 54.2%, respectively. It should be noted that the main purpose of this work is to conduct a preliminary study on this integrated system, aiming to provide an idea for the conceptual design, modeling and simulation analysis of this integrated system.

Wind turbines are installed offshore with the assistance of a floating platform to help meet the world’s increasing energy needs. However, the incident wind and extra incident wave disturbances have an impact on the performance and operation of the floating offshore wind turbine (FOWT) in comparison to bottom-fixed wind turbines. In this paper, model predictive control (MPC) is utilized to overcome the limitation caused by platform motion. Due to the ease of control synthesis, the MPC is developed using a simplified model instead of high fidelity simulation model. The performance of the controller is verified in the presence of realistic wind and wave disturbances. The study demonstrates the effectiveness of MPC in reducing platform motions and rotor/generator speed regulation of FOWTs.

Individual pitch control (IPC) is an effective and widely used strategy to mitigate blade loads in wind turbines. However, conventional IPC fails to cope with blade and actuator faults, and this situation may lead to an emergency shutdown and increased maintenance costs. In this paper, a fault‐tolerant individual pitch control (FTIPC) scheme is developed to accommodate these faults in floating offshore wind turbines (FOWTs), based on a Subspace Predictive Repetitive Control (SPRC) approach. To fulfill this goal, an online subspace identification paradigm is implemented to derive a linear approximation of the FOWT system dynamics. Then, a repetitive control law is formulated to attain load mitigation under operating conditions, both in healthy and faulty conditions. Since the excitation noise used for the online subspace identification may interfere with the nominal power generation of the wind turbine, a novel excitation technique is developed to restrict excitation at specific frequencies. Results show that significant load reductions are achieved by FTIPC, while effectively accommodating blade and actuator faults and while restricting the energy of the persistently exciting control action. [ABSTRACT FROM AUTHOR]

Offshore wind energy is one of the most important sources of renewable energy to make sure supplying enough electricity for the world in the future. The floating offshore wind turbine (FOWT) operates under many influences such as both the aerodynamic force, the hydrodynamic force and coupled motion between the two. Therefore, in this paper about the effect of collective pitch control and cyclic pitch control on power and loads of FOWT are discussed under the uniform wind velocity of 10 m/s with wind tunnel and numerical investigation. FAST tool (Fatigue, Aerodynamics, Structures, and Turbulence) is used to simulate FOWT under the same wind conditions as wind tunnel experiments. A two-bladed downwind turbine model is used for this experiment to estimate the fluctuations of the power coefficient, thrust coefficient and moments. The pitch moment and the yaw moment are measured at the top of the tower and the fluctuation of loads is compared. The phase angle of the pitch angle is controlled by three actuators through a swash plate equipment. From the simulated results, the effect of the pitch angle change on the power coefficient is higher than the yaw angle change. For the yawed wind condition, the load of the wind turbine at the phase angle of ξ = 60° was equal to the optimum operating point. Thence, the load of the wind turbine was decreased when the cyclic pitch control method was applied. • The phase angle of the pitch angle is adjusted by a swash plate. • Aerodynamic force is simulated by FAST under the uniform wind condition. • Power and thrust coefficients fluctuated when the pitch angle and the yaw angle change. • Loads of wind turbine were decreased by the cyclic pitch control method. • The simulated results were in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

High‐order sliding mode control laws with gain adaptation algorithms are applied, in Region III, on a floating offshore wind turbine equipped by permanent magnet synchronous generator (PMSG). These adaptive control methods are especially efficient for systems with uncertainties and external perturbations and are well adapted to wind turbines systems. Such controllers can be implemented with very reduced knowledge of system model (only the relative degree is necessary) and strongly reduce the controller gains tuning effort. Simulations are made on Fatigue, Aerodynamics, Structures and Turbulence (FAST) software and compared with standard gain‐scheduled proportional–integral (PI) controllers. [ABSTRACT FROM AUTHOR]

The present paper aims to investigate the fatigue behavior of suspended inter-array power cables connecting two floating offshore wind turbines (FOWTs). Two spar FOWTs interconnected with a dynamic power cable are chosen as the base configuration for the present investigation. The base configuration is modeled in the dynamic analysis software OrcaFlex. The relationship between the axial tensions and curvatures of the power cable and its stresses is obtained by cross-section analysis in UFLEX (special purpose Finite Element program for nonlinear stress analysis of complex umbilical and cable cross-sections). Several different environmental conditions and load directions are applied. A fatigue life of 85 years is determined for the base configuration, with curvature-induced stresses being identified as the main contributor to the fatigue damage. Effects of marine growth on the fatigue life of the power cable are investigated. It is observed that marine growth negatively affects the fatigue life of the power cable. A parametric study on the effect of the number of buoys attached to the cable on the fatigue life is performed. Decreasing the number of buoys below the optimal value leads to an increase in fatigue life due to lower curvatures. In addition to the base configuration, an alternative configuration using semi-submersible floaters is investigated under the same metocean conditions and cable geometry. The power cable fatigue life is higher for semi-submersible floaters compared to the spar floaters. The present findings reveal that the main reason is the reduced number of large amplitude curvature cycles. The critical component prone to fatigue failure for all evaluated cases is the armor layer, and the largest fatigue damage always occurs at buoy-cable connection points. The critical direction of environmental load resulting in the highest fatigue damage is found to be the load case where waves, wind and current approach the suspended cable in a crossflow direction. • The fatigue behaviour of suspended inter-array power cables connecting two floating offshore wind turbines (FOWTs) is investigated. • Two spar and two semi-submersible FOWTs interconnected with dynamic power cables are chosen, respectively, for the present investigation. • A parametric study on the effect of the number of buoys attached to the cable on the fatigue life is performed. • The power cable fatigue life is higher for the semi-submersible floaters compared to the spar floaters under the same met-ocean conditions. • The critical component prone to fatigue failure for all evaluated cases is the armour layer. The largest fatigue damage always occurs at buoy-cable connection points. [ABSTRACT FROM AUTHOR]

• The use of suspended inter-array power cables in floating offshore wind farms is explored. • Several different cable configurations connecting two spar FOWTs are assessed based on Fitness Factors using static and dynamic analyses. • The number of buoys is found to be the main parameter affecting the maximum effective tension and the minimum bending radii of the cables. • The optimum cable configuration found for the two spar FOWTs is employed to connect two semi-submersible FOWTs. • The influence of the floater type on the suspended power cable tension and curvature is found to be negligible. • Analysis of power cable behavior in different offshore wind farm layouts shows that a two-turbine setup with different environmental load directions is representative of the suspended power cables design in entire wind farms. The aim of the present study is to propose an engineering approach of designing suspended inter-array power cable configuration for offshore wind farms. To achieve this, a spar type and a semi-submersible type floating offshore wind turbines (FOWTs) are modeled in the dynamic analysis software OrcaFlex. The accuracy of these models is validated against reference studies. Several different cable configurations connecting two spar FOWTs are assessed based on Fitness Factors using static and dynamic analyses. The non-linear bending behavior and the marine growth effects are considered in the analyses. The number of buoys is found to be the main parameter affecting the maximum effective tension and the minimum bending radii of the cables. The buoy spacing determines whether the cable touches the seabed, floats fully submerged, or floats at the sea surface. The influence of the cable length and the marine growth on the observed cable tensions and curvatures is small for the investigated configurations. The optimum cable configuration found for the two spar FOWTs is employed to connect two semi-submersible FOWTs. The influence of the floater type on the suspended power cable tension and curvature is found to be negligible. Analysis of power cable behavior in an offshore wind farm layout shows that a two-turbine setup with different environmental load directions can be used as a representative case for the suspended power cables design of the entire wind farm. [ABSTRACT FROM AUTHOR]

This paper considers the dynamic response of a novel lightweight FOWT concept being developed by T-Omega Wind Ltd, that is able to float over even steep high waves, and be economical in deep water. The study aims to understand the response to waves during marine operations (installation, or maintenance) as part of optimizing its design. For this purpose real-time 6 degrees-of-freedom (6 DOF) simulations are computed for the system under operational and extreme sea wave scenarios in the state-of-the-art Multiphysics Marine Simulator at the National Decommissioning Centre (NDC). RAOs for heave and pitch displacements are evaluated across varying wave heights and periods of excitation to identify system behaviour including resonant frequencies. The model is calibrated by adjusting system damping parameters for each wave frequency to match experimental tests on a 1:60 scaled prototype at the Kelvin Hydrodynamics Laboratory, resulting in an ad hoc damping expression to produce appropriate system dynamic behaviour for "High" and "Low" Sea States. The study concludes by identifying ranges of wave parameters that limit peak motions, proposes analytical expressions for RAO responses and provides damping parameters that validate the Marine Simulator as a suitable tool to predict FOWT dynamic responses with reduced computation time. [Display omitted] • Dynamic response simulations of a novel FOWT concept under periodic sea wave excitation. • Virtual model of a novel low-mass FOWT concept developed for evaluation of marine operations. • Analytical RAO expressions for a novel FOWT with fitted hydrodynamic damping. • Wave parameter values providing better performance of a novel FOWT concept. • Frequency-dependent hydrodynamic damping of a novel FOWT to tune heave and pitch RAOs. [ABSTRACT FROM AUTHOR]

The integration of wave energy converters (WEC) into floating offshore wind turbines (FOWT) is regarded as a promising approach for comprehensively harnessing deep-sea energy and reducing the levelized cost of energy. This study aims to investigate the WEC power take-off (PTO) control effects on the dynamic performance of a floating wind-wave combined system, wherein three heaving-type WECs are integrated into a semi-submersible FOWT. In particular, the hydraulic PTO is modelled as a Coulomb damping system to enable a more realistic analysis. The aero-hydro-servo-elastic-mooring coupled numerical simulations and 1:50 wave basin experimental data demonstrate good agreement. It is observed that wave power production varies significantly with different control settings, potentially reaching 24.0 % of the overall hybrid energy production with proper control parameters. Furthermore, platform pitch oscillation generally shows a decreasing trend with increasing damping forces under below-rated and rated conditions, while showing minimal influence when above-rated. Conversely, tower base damage equivalent load (DEL) tends to initially decrease and subsequently increase across all examined conditions. This consistent convexity indicates the potential use of DEL as a performance index for optimising PTO control. In summary, power increment, motion reduction, and load mitigation could be achieved concurrently with appropriate control design. For instance, an extra 0.48 MW wave power could be produced under the rated condition, while a 10.4 % reduction in tower base DEL and a 6.57 % mitigation in platform pitch oscillation could also be achieved at the same time. [ABSTRACT FROM AUTHOR]

The influence of atmospheric turbulence inflow on coupled dynamic behaviors and wakes of floating offshore wind turbine (FOWT) is pronounced, especially as blade size increases. In this study, we explore the effects of atmospheric inflow on aero-hydrodynamics and wake characteristics of a spar-type FOWT. A well-validated in-house computational fluid dynamic (CFD) solver, FOWT-UALM-SJTU, is utilized to conduct the numerical simulations. To generate the realistic atmospheric inflow condition, the large eddy simulation (LES) with long-duration is employed. Two other cases involving uniform inflow and shear inflow are conducted to provide some comparable results. Compared to uniform inflow and shear inflow, the power and thrust of FOWT under atmospheric inflow display greater instability, with their power spectrums exhibiting higher intensity in high frequency region. The variation of yaw moment in atmospheric scenario is significantly drastic, leading to a remarkable response in platform yaw motion. Notably, the dominant frequency in this scenario is the blade passage frequency, as opposed to the incident wave frequency observed in the uniform and shear scenarios. The atmospheric inflow promotes the manifestation of wake breakdown and wake meandering, resulting in the faster wake recovery of FOWT. Besides, the meandering in the far wake is more significant compared to uniform and shear scenarios. The absence of vortex rings in atmospheric scenario is observed, along with the presence of more complex and smaller vortices in far wake. Our findings highlight the remarkable impacts of atmospheric inflow on dynamic responses and wake evolution of FOWT, and it is recommended that the atmospheric inflow be taken into account when assessing wake interactions between multiple FOWTs. • The LES is used to generate the physically realistic atmospheric turbulence inflow. • The fully coupled aero-hydrodynamics of FOWT are considered. • Wake characteristics of FOWT are investigated, extending as far as 9D downstream. [ABSTRACT FROM AUTHOR]

The floating offshore wind turbines (FOWTs) have a higher failure rate compared to onshore wind turbines, which could lead to aerodynamic imbalance on the floating systems. In addition, yaw error presents an inevitable challenge for FOWTs. This study aims to investigate the effects of the yaw error and the fault conditions on the dynamic properties of the IEA 15 MW semi-submersible FOWT using OpenFAST. The fault conditions involve blade seize without shutdown, blade seize with shutdown and grid loss. The platform motions and structure loads under different yaw errors (−20°, −8°, 0°, 8°, 20°) for different fault conditions were investigated. The dynamic response resulting from the fault was compared with the normal condition. Moreover, the optimal yaw angle under fault conditions was investigated in this paper. The results revealed that the fault shutdown conditions significantly impact the motions and loads of the wind turbine, with substantial reverse tower top and base bending moments. The yaw error, conversely, has less effect on the motions of the wind turbine, but can ameliorate the transient effect in the fault shutdown conditions. Additionally, it is recommended to operate the wind turbine at a yaw angle of 25–30° for wind speeds greater than 12 m/s. • The IEA 15 MW semi-submersible floating offshore wind turbine is used as the reference structure. • The aero-hydro-servo-elastic coupling analysis is performed in the nonlinear time and frequency domain using OpenFAST. • The combined impact of fault conditions and yaw error on system dynamic is investigated. • The fault conditions are considered as the control cases for designing the components of IEA 15 MW FOWT. • The optimum yaw angle is presented to ameliorate the transient effects of the structure loads under fault shutdown conditions. [ABSTRACT FROM AUTHOR]

This study proposes a coupling strategy between OpenFAST and WEC-Sim (OWS) to provide a general solution for the design of offshore floating wind turbines (FOWTs). The fully-coupled calculation is implemented by transferring the tower-base loads from OpenFAST to WEC-Sim, which in turn transfers the platform displacements, velocities and accelerations back to the former. The cross-platform data interface is achieved via S-function in Simulink, and a parallel route in OWS ensures a comparable calculation speed compared with OpenFAST. The model validation is conducted based on the IEA-15-MW reference wind turbine (RWT) and UMaine-VolturnUS-S semisubmersible platform with the reference site located at the northern North Sea. Through code-to-code comparisons against OpenFAST, the newly developed OWS model is tested in various environmental conditions to ensure its correctness and reliability. Furthermore, the dynamic responses of the FOWT are comprehensively investigated using spectral analysis. The time lag of the platform displacements with respect to the incident wave is quantitatively evaluated based on correlation analysis. Generally, there is little time lag between the heave displacement and the wave, but the time lag in the pitch direction (6–9 s) is approximately twice that (3–4 s) in the surge direction. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines, as large equipment for deep-sea wind energy utilization, must be carefully studied for their aerodynamic performances under platform motions. In experiments, the model wind turbine designed by the traditional thrust coefficient similarity is usually insufficient for unsteady aerodynamic reproductions of the prototype wind turbine. In this paper, a dynamic aerodynamic similarity criterion based on the mapping of the optimal tip speed ratio is introduced for wind tunnel model tests, and a model wind turbine is designed using the NREL 5-MW prototype wind turbine. In this case, to avoid the over-height model tower aroused in pitch motions, a method of equivalent surge motions instead of pitch motions is proposed and validated for feasibility. Then dynamic aerodynamic similarities under different pitch amplitudes and frequencies are validated by the Unsteady Blade Element Momentum theory. Results indicate that temporal and spatial characteristics and hysteresis effects of thrust and power coefficients all maintain better similarity with those of the prototype wind turbine than those of the model wind turbine designed by the traditional criterion. This paper provides a criterion with equivalent pitch motions for unsteady aerodynamic studies in wind tunnel model tests, especially the high-fidelity Hardware/Software-In-the-Loop test. • A new similarity criterion is proposed for FOWT wind tunnel model tests. • The feasibility of equivalent surge motions instead of pitch motions is verified. • The problem of over-height model tower is avoided by equivalent pitch motions. • Spatiotemporal and hysteresis characteristics of unsteady aerodynamics are analyzed. • Unsteady aerodynamic performances of the prototype can be reflected more accurately. [ABSTRACT FROM AUTHOR]

The design requirements for offshore engineering stipulate that floating structures should maintain their overall performance even in the event of a single mooring line failure. However, it is crucial to ensure that the platform does not drift or capsize in the case of two mooring line failures. Therefore, the investigation into the dynamic response of wind turbines after mooring line failures is of great significance. In this study, the aerodynamic-structural simulation capability of FAST was coupled with the hydrodynamic analysis software AQWA by modifying the dynamic link library. The dynamic response of a Barge-type floating offshore wind turbine (FOWT) and the variations in mooring line tensions were computed under different sea conditions after the successive failures of two mooring lines with varying time intervals. The findings reveal that in rated sea conditions, there is a significant increase in surge motion, reaching a maximum value 2.08 times that of the original, following the failure of two mooring lines. The tension in mooring line #3 reaches 1.57 times the pre-failure value. In extreme sea conditions, the simultaneous failure of two mooring lines at the same corner triggers a cascading failure phenomenon within the mooring system, and a shorter interval between failures amplifies the dynamic response of the platform. Therefore, it is not advisable to deploy the Barge platform in harsh environmental conditions. • Coupled FAST - AQWA simulation for FOWT dynamic response analysis. • Study on the impact of dual mooring line failures on FOWT dynamics. • Time-domain response under varied sea conditions and failure intervals are investigated. • Clustered mooring failures and rapid succession intensity the dynamic response. [ABSTRACT FROM AUTHOR]

In this study, experiments with a floater using flapping foils were performed to develop a new station keeping system that can maintain its position in waves without mooring lines. The foils applied to this system generate thrust using wave energy. In this experiment, the motion of the floater was analyzed in three different wave periods. Sixteen foils were attached to the cylindrical floater. The thrust of each foil was controlled by changing its azimuth angle, and three cases were compared. Based on the previous data, we made more precise measurements and found an optimal model for stationkeeping under each wave condition. We verified the potential of this new stationkeeping system using flapping foils, and conclusions were drawn from the results.

This study addresses the planning procedures for the installation of the mooring systems that support the floating offshore wind turbines in a wind farm. It considers the logistics of the installation process and discusses the important role of the weather windows in the planning of those operations at a preliminary stage of the project. The case study is based on a wind farm array of 47 Telwind floating wind turbine platforms, to be located in Lannion (France), with a potential of 470 MW. The study includes the transport and logistics requirements of different mooring components, such as chains, connectors and drag anchors; the description of the installation operations considering the typology of vessels that are necessary in these manoeuvres; as well as the planning and costs associated with the transport and installation. Given the diversity of elements and operations involved in the installation procedure, it is demonstrated that the research results of duration and costs of this type of operations are only possible to obtain using a simulation tool.

This study investigates the elastic response characteristics of a floating wind turbine (FOWT) with two vertical-axis wind turbines (VAWTs), called double-rotor VAWTs. The model consists of two VAWTs mounted on a single semi-submersible floating structure and employs a single point mooring, which allows the FOWT to always self-align with the wind. Usually, a coupled analysis of the wind turbine and floating structure is used in the design of FOWTs; however, there is no coupled analysis available for VAWTs. In this study, we attempted to combine the wind turbine design software “QBlade” and the coupled wind turbine/floating body analysis code “UTWind” as one of the methods of coupled analysis of a VAWT and a floating body. Numerical simulation results were compared with experimental results using an elastic model scaled down to 1/100 of its actual model to determine the motion response and cross-sectional bending moments. The experimental results showed that the thrust of the VAWT had a particular influence on the cross-sectional forces and motion response between the two VAWTs. For cross-sectional forces, all results showed similar trends. Overall, the results of UTWind for double-rotor VAWTs are reasonable. It was also found that the pitch motion must be accurately reproduced to improve the accuracy.

The full exploitation of offshore wind resources can essentially satisfy the massive energy demand. The realization and application of ultra-high-power offshore wind turbines are crucial to achieving full use of deep-sea wind energy and reducing the cost of wind power. For the VolturnUS-S semi-submersible floating foundation of a 15 megawatt (MW) offshore wind turbine, the effect of structural parameters on hydrodynamic performance was investigated by controlling the variables described in this paper. Accordingly, the floating foundation was optimized and coupled to the 15 MW offshore wind turbine. The dynamic performance of the integrated 15 MW offshore wind turbine was analyzed under different operating conditions, by applying the aero-hydro-servo-elastic coupled method. The results show that for a wave in a 0-degree direction, a 5% increase of column spacing will reduce the peak value of the pitch transfer function by 33.61%, and that a 5% decrease of the outer column diameter will further reduce the peak value by 26.27%. The standard deviation of the time-domain surge responses was reduced by 19.78% for the optimized offshore wind turbine, and the maximum value of the mooring line tension was reduced by 13.55% under normal operating conditions.

This article proposes a review of the modeling approaches for floating offshore wind turbines (FOWTs) for nonlinear control design. The aerodynamic, hydrodynamic and mooring line dynamic modules for the FOWT have been reviewed to provide an overview of several modeling approaches with their respective features. Next, three control-oriented models from the literature are revisited by presenting their methodological approaches to modeling and identification. These three models cover the three most popular types of FOWTs. Then, the performances of these models are validated with the open fatigue, aerodynamics, structures, and turbulence (OpenFAST) code, and their performances are evaluated according to several criteria. Finally, one of the three models is used to illustrate a nonlinear second-order sliding mode control based on the twisting algorithm to optimize the performance of the FOWT in terms of energy extraction and reduction in the platform pitch oscillation.

In this article, a new strategy for switching control has been proposed with the aim of reducing oscillations in floating offshore wind turbines. Such oscillations lead to a shortage in the system's efficiency, lifespan and harvesting capability of wind and wave energies. In order to study the decreasing of undesired oscillations in the system, particularly in pitch and top tower fore-aft movements, a square-shaped platform barge equipped with four symmetric oscillating water columns has been considered. The oscillating water columns' air flux valves allow to operate the air columns so that to control the barge movements caused by oscillatory motion of the waves. In order to design the control scheme, response amplitude operators have been used to evaluate the performance of the system for a range of wave frequency profiles. These response amplitude operators analysis makes it possible to implement a switching control strategy to adequately regulate the valves opening/closing transition. The obtained results show that the proposed controlled oscillating water column-based barge present a better performance compared to the traditional barge one. In the case study with the period of 10 s, the results indicate the significant oscillation reduction for the controlled oscillating water column-based system compared to the standard barge system by 30.8% in pitch angle and 25% in fore-aft displacement. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbine (FOWT) towers are dynamically sensitive to wind and wave excitations. Since the wave tends to be harsher with the increase of wind speed, FOWT towers are likely to experience the most severe vibration operating at the cut-out wind speed. In the present study, the short-term dynamic reliability of a spar-type FOWT is evaluated based on the probability density evolution method (PDEM). For this purpose, an integrated coupled dynamics model for the FOWT is firstly established by incorporating the multibody dynamics with the finite element (FE) method. Next, the conditional joint probability distribution of the significant wave height and peak spectral wave period at the cut-out wind speed is constructed based on the copula model. Then, the stochastic dynamic response and reliability of the FOWT can be analyzed via PDEM. The numerical example of reliability analysis of a 5-MW spar-type FOWT operating at the cut-out wind speed is carried out, in which the long-term met-ocean data at a South China Sea site is utilized. Simulation results show that the reliability of the FOWT for normal operation is less than 0.2 when the acceleration at the tower top is adopted as the failure criterion. • An integrated coupled dynamics model for floating offshore wind turbines. • Data-based conditional joint distribution of wave parameters at given wind speed. • A PDEM-based framework for dynamic reliability evaluation of FOWT. • Reliability evaluation of FOWT at cut-out wind speed based on observed data. • Multiple failure criteria emphasized from the operational reliability perspective. [ABSTRACT FROM AUTHOR]

Design of floating offshore wind turbines (FOWTs) needs reliable and innovative technologies to overcome the challenges on how to better predict the dynamic responses in terms of aero-hydro-servo-elastic disciplines. This paper aims to demonstrate the optimized prediction of the dynamic response of FOWTs by Simulation annealing diagnosis algorithm (SADA). SADA is an Artificial Intelligence technology-based method, which utilizes the advantages of numerical simulation, basin experiment and machine learning algorithms. The actor network in deep deterministic policy gradient (DDPG) is adopted to take actions to adjust the Key disciplinary parameters (KDPs) in each loop according to the feedback of 6DOF motions of platform in dynamic response analysis. The results demonstrated that the mean values of the platform's motions and rotor axial thrust force could be predicted with higher accuracy. On this basis, other physical quantities that designers are more concerned about but cannot be obtained from experiments and actual measurements will be predicted by SADA with more credibility. This SADA method differs from traditional supervised learning applications in renewable energy, which do not need to be provided physical quantities with strong direct correlation. All targets can be artificially set for SADA to obtain a better self-learning performance. In general, designers can use SADA to get a more accurate and optimized prediction of the dynamic response of FOWTs, especially those physical quantities that cannot be directly obtained through the basin experiments. [ABSTRACT FROM AUTHOR]

Compared with bottom-fixed wind turbines, the supporting platform of a floating offshore wind turbine has a larger range of motion, so the gyroscopic effects of the system will be more obvious. In this paper, the mathematical analytic expression of the gyroscopic moment of a floating offshore wind turbine is derived firstly. Then, FAST software is utilized to perform a numerical analysis on the model of a spar-type horizontal axis floating offshore wind turbine, OC3-Hywind, so as to verify the correctness of the theoretical analytical formula and take an investigation on the characteristics of gyroscopic effect. It is found that the gyroscopic moment of the horizontal axis floating offshore wind turbine is essentially caused by the vector change of the rotating rotor, which may be due to the pitch or yaw motion of the floating platform or the yawing motion of the nacelle. When the rotor is rotating, the pitch motion of the platform mainly excites the gyroscopic moment in the rotor's yaw direction, and the yaw motion of the platform largely excites the rotor's gyroscopic moment in pitch direction, accordingly. The results show that the gyroscopic moment of the FOWT is roughly linearly related to the rotor's inertia, the rotor speed, and the angular velocity of the platform motion. [ABSTRACT FROM AUTHOR]

There is a need for new numerical tools to capture the physics of floating offshore wind turbines (FOWTs) more accurately to refine engineering designs and reduce costs. The conventional measurement apparatuses in tank tests, including wave probes, velocity and current profilers, and Doppler sensors, are unable to provide a full 3D picture of velocity, pressure, turbulence, and vorticity profile. In tank tests, use of the underwater stereoscopic particle image velocimetry (SPIV) method to fully characterise the 3D flow field around floating wind platforms can overcome some of the limitations associated with classical measurement techniques and provide a rich source of validation data to advance high-fidelity numerical tools. The underwater SPIV method has been widely used for marine and offshore applications, including ship and propeller wakes, wave dynamics, and tidal stream turbines; however, to date, this technology has not seen widespread use for the hydrodynamic study of FOWTs. This paper provides a critical review of the suitability of underwater SPIV for analysing the hydrodynamics of FOWTs, reviews the challenges of using the method for FOWT tank test applications, and discusses the contributions the method can make to mitigating current research gaps in FOWT tank tests.

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.

Wind energy in the deep-sea area is more abundant and the capacity of wind turbines can be made larger. Therefore, the research on deep-sea floating offshore wind turbines will be the primary strategy for wind energy exploitation in the future. The spar-type platform depends on the characteristics of a small water plane, deep draft, and good stability, which has been applied to the commercial development of deep-sea wind energy. In the next ten years, the 10-MW wind turbine will become the mainstream class installed in the floating offshore wind turbine farm. Thus, it is very necessary to conduct a comprehensive and in-depth study on the 10-MW spar type floating offshore wind turbine. The direct-drive 10-MW offshore wind turbine was proposed by the International Energy Agency (IEA) in Wind Task 37 in 2019. In this paper, a spar-type platform is designed to support the IEA 10-MW reference wind turbines, and a nonlinear aero-hydro-servo-elastic numerical model is established using the FAST tool (which is developed by the National Renewable Energy Laboratory, NREL). Then, the accuracy of the wind turbine and the sensitivity of the controller are verified, and the natural periods of the floating offshore wind turbine are obtained by free-decay tests. The natural periods of the platform in six degrees-of-freedom are found to be within the range recommended by the design standard. The measured wind and wave data of the target site close to Fujian Province of China are used to evaluate the performance of the floating offshore wind turbine under the 100-, 50-, 5-, and 2-year-return stochastic weather conditions. The results indicate that the design of the spar platform is reasonable and has excellent hydrodynamic performance.