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This paper presents a comprehensive review of advanced control methods specifically designed for floating offshore wind turbines (FOWTs) above the rated wind speed. Focusing on primary control objectives, including power regulation at rated values, platform pitch mitigation, and structural load reduction, this paper begins by outlining the requirements and challenges inherent in FOWT control systems. It delves into the fundamental aspects of the FOWT system control framework, thereby highlighting challenges, control objectives, and conventional methods derived from bottom-fixed wind turbines. Our review then categorizes advanced control methods above the rated wind speed into three distinct approaches: model-based control, data-driven model-based control, and data-driven model-free control. Each approach is examined in terms of its specific strengths and weaknesses in practical application. The insights provided in this review contribute to a deeper understanding of the dynamic landscape of control strategies for FOWTs, thus offering guidance for researchers and practitioners in the field. [ABSTRACT FROM AUTHOR]

The ever-increasing consumption of electrical energy due to increasing requirements arising in parallel with the developing technology forces manufactures and users to seek new and environmentally compatible energy resources. Due to it endless energy potential and continuity, wind energy, which has been used by humanity for various purposes for centuries, continues its journey, which started with windmills that transform one form of mechanical energy into another, with offshore wind energy turbines. In this study, installation site selection, which is the first step for floating offshore wind turbine installation, is carried out. First, a Fuzzy Analytic Hierarchy Process (F-AHP) is applied to obtain the weights of criteria. Transformed alternatives and criteria in a fuzzy decision matrix through triangular fuzzy numbers are used to obtain a database via a Geographic Information System (GIS). Finally, the installation site and study area are determined. This area is located in the Northeast Aegean Sea, between Çanakkale and Gökçeada, 12 km to the northeast of Gökçeada, and 12 square kilometers. The minimum distance of the area to ship routes is 3000 m, and its minimum distance to fault lines is 2000 m. The average sea depth is 110 m, and the average wind speed is 9.26 m/s (18 knots). With this potential, enough energy can be generated for settlements on Gökçeada and the coastlines. [ABSTRACT FROM AUTHOR]

The design and optimization of substructure of a floating offshore wind turbines (FOWT) is an extremely challengeable work. Before this work, it is necessary to investigate the correlation of design variables of the FOWT substructure. The correlation analysis can help to reduce the dimensions of design parameters and output results, and provide insight into the nature of the design space. The subsequent design and optimization workload can therefore be reduced. This paper aims to provide a framework of parametric modeling and correlation analysis of the substructure design variables for a single-column FOWT with a slack catenary mooring system. The NREL offshore 5 MW baseline wind turbine is selected in this study, and various floating foundation configurations are designed to adapt to this wind turbine. In order to evaluate the main dynamic performances of the FOWT well and quickly, a reduced-degree-of-freedom model of the FOWT is constructed and verified. Then, the substructure design parameters are parameterized to facilitate modeling, and simulations based on design of experiments are performed to save evaluation time and cost. In simulations, three sets of steady wind speeds are applied to eliminate the influence of controller dynamics. The results showed that the correlation of dynamic performances of the FOWT is significant, indicating that it is enough to consider several outputs to represent the dynamic behavior of the system. Regarding the designing parameters, the draft generates the most significant influence on the FOWT dynamic performances. The column radius has different influences for the cases with and without heave plates. Additionally, whether or not the heave plate is added, the mooring line length mainly influences the 95th percentile of platform horizontal surge displacement and standard deviation of fairlead tension of the downwind mooring line. [ABSTRACT FROM AUTHOR]

Operations and maintenance (O&M) of floating offshore wind turbines (FOWTs) require regular inspection activities to predict, detect, and troubleshoot faults at high altitudes and in harsh environments such as strong winds, waves, and tides. Their costs typically account for more than 30% of the lifetime cost due to high labor costs and long downtime. Different inspection methods, including manual inspection, permanent sensors, climbing robots, remotely operated vehicles (ROVs), and unmanned aerial vehicles (UAVs), can be employed to fulfill O&M missions. The UAVs, as an enabling technology, can deal with time and space constraints easily and complete tasks in a cost-effective and efficient manner, which have been widely used in different industries in recent years. This study provides valuable insights into the existing applications of UAVs in FOWT inspection, highlighting their potential to reduce the inspection cost and thereby reduce the cost of energy production. The article introduces the rationale for applying UAVs to FOWT inspection and examines the current technical status, research gaps, and future directions in this field by conducting a comprehensive literature review over the past 10 years. This paper will also include a review of UAVs' applications in other infrastructure inspections, such as onshore wind turbines, bridges, power lines, solar power plants, and offshore oil and gas fields, since FOWTs are still in the early stages of development. Finally, the trends of UAV technology and its application in FOWTs inspection are discussed, leading to our future research direction. [ABSTRACT FROM AUTHOR]

As the industry transitions toward Floating Offshore Wind Turbines (FOWT) in greater depths, conventional chain mooring lines become impractical, prompting the adoption of synthetic fiber ropes. Despite their advantages, these mooring lines present challenges in inspection due to their exterior jacket, which prevents visual assessment. The current study focuses on vibration-based Structural Health Monitoring (SHM) in FOWT synthetic mooring lines under uncertainty arising from varying Environmental and Operational Conditions (EOCs). Six damage detection methods are assessed, utilizing either multiple models or a single functional model. The methods are based on Vector Autoregressive (VAR) or Transmittance Function Autoregressive with exogenous input (TF-ARX) models. All methods are evaluated through a Monte Carlo study involving 1100 simulations, utilizing acceleration signals generated from a finite element model of the OO-Star Wind Floater Semi 10 MW wind turbine. With signals from only two measuring positions, the methods demonstrate excellent results, detecting the stiffness reduction of a mooring line at levels 10% through 50%. The methods are also tested for healthy cases, with those utilizing TF-ARX models achieving zero false alarms, even for EOCs not encountered in the training data. [ABSTRACT FROM AUTHOR]

The effect of watertightness criteria on structural assessment was investigated in this study through the design of a semi-submersible floater for a 10 MW wind turbine. A structural assessment was carried out for the ultimate, fatigue, and serviceability limit states, where six cases of combinations of the loading conditions and structural requirements were assumed for the assessment of watertightness. The different assessment conditions for watertightness resulted in different designs of prestressing tension, which also caused differences in the design margins for the ultimate limit states as well as the SN diagram used for the assessment of the fatigue limit states. The maximum stress acting on the member of the floating wind turbine differed by about 7.6 to 16.2% for the 50-year and 1-year environmental conditions. The small difference between the stresses for the two return periods can be attributed to the large component of the wind turbine thrust in the variable loads. When the partial load factor of 1.0 was used, the structural requirement in the prestressing design caused larger differences compared to the return period of the loading conditions in the watertightness assessment. The use of the partial load factor of 0.5 for the prestressing design also caused a large difference in the results of the structural assessment, indicating the necessity of further investigation of the determination of the partial load factor for floating offshore wind turbines. [ABSTRACT FROM AUTHOR]

There has been a great effort towards development of renewable energy systems to combat global warming with significant interest towards research and development of floating offshore wind turbines (FOWTs). With commercial projects such as Hywind Scotland, Hywind Tampen and others, there is a shift of industry attention from bottom-fixed offshore turbines to FOWTs. In this work, we focus on comparing industry standard Potential Flow (PF) methods versus Computational Fluid Dynamics (CFD) solvers for a scaled version of the IEA 15 MW turbine and associated FOWT system. The results from the two solvers are compared/validated using experimental thrust values for the fixed turbine. The motions and the thrust for the FOWT system are then compared for the two solvers along with hydrodynamic properties of the floater hull. The wake features downstream of the turbine are analyzed for the fixed and floating turbine using the CFD solver. The wake from the CFD solver is also compared with a simplified PF model. Finally, a simplified cost-benefit analysis is presented for the two solvers to compare the usefulness and utility of a CFD solver as compared to presently used industry-standard PF methods. [ABSTRACT FROM AUTHOR]

Green renewable energy is produced by floating offshore wind turbines (FOWT), a crucial component of the modern offshore wind energy industry. It is a safety concern to accurately evaluate excessive weights while the FOWT operates in adverse weather conditions. Under certain water conditions, dangerous structural bending moments may result in operational concerns. Using commercial FAST software, the study's hydrodynamic ambient wave loads were calculated and converted into FOWT structural loads. This article suggests a Monte Carlo‐based engineering technique that, depending on simulations or observations, is computationally effective for predicting extreme statistics of either the load or the response process. The innovative deconvolution technique has been thoroughly explained. The suggested approach effectively uses the entire set of data to produce a clear but accurate estimate for severe response values and fatigue life. In this study, estimated extreme values obtained using a novel deconvolution approach were compared to identical values produced using the modified Weibull technique. It is expected that the enhanced new de‐convolution methodology may offer a dependable and correct forecast of severe structural loads based on the overall performance of the advised de‐convolution approach due to environmental wave loading. [ABSTRACT FROM AUTHOR]

A novel failure analysis method named D‐vine copula Bayesian Network is proposed, aimed to extract fault correlation information from various condition monitoring channels of floating offshore wind turbine components, quantify risk probability and consequence, and obtain risk priority of components considering condition correlation. First, a copula Bayesian model is established based on the Bayesian network and D‐vine copula theory. Then, a risk consequence calculation method considering relative loss is developed. Finally, critical failure items are identified by a risk matrix. The proposed technique is expected to: (i) Release the limitation that parent nodes with condition monitoring input are processed as independent in Bayesian analysis. (ii) Provide an alternative way for presenting relationships between nodes instead of conditional probability tables. (iii) Simplify the calculation of high‐dimensional copula. This study screened out high‐risk level components of floating offshore wind turbines, and operation recommendations avoiding potential failure risk are put forward. The comparative results demonstrate the feasibility and reliability of the proposed method. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines (FOWTs) are slender structures that are sensitive to hazardous conditions, mainly to winds. Wind action can cause undesirable vibrations that may affect safety and operability. These vibrations can be controlled in numerous ways through passive, active, semiactive or hybrid control. An example of passive control is a tuned mass damper (TMD) that dissipates mechanical energy from the main system when well-tuned to the structure. An inverted pendulum is a device that can be used as a TMD that vibrates out of phase in relation to the main system, reducing the vibrations in comparison to a classical TMD. This work describes a procedure for controlling a FOWT, which is subjected to wind actions, using a proposed tuned mass damper by means of an inverted pendulum (TMD-IP). TMD-IP is evaluated by two specific metrics: tower peak rotation and tower variance rotation. Optimization of the parameters of the inverted pendulum achieved reductions of approximately 95% for the rotation of the tower and the translation of the barge when a harmonic excitation is applied. Using the Monte Carlo method, random wind loading is applied to the FOWT; the reduction in the tower rotation is approximately 70%, and the reduction in the barge translation is approximately 80%. In addition, it is verified that changes in blade stiffness and damping alter the response of the structure very little when the FOWT is under extreme wind conditions. Finally, the results obtained through time and frequency domain analysis are closer to those. [ABSTRACT FROM AUTHOR]

Studying the dynamic characteristics of the platform under unsteady loads is of great significance to the development of floating offshore wind turbines (FOWTs). In this work, a 15 MW semi-submersible FOWT is extensively simulated using OPENFAST software, and the influence of turbulent wind and irregular waves on the platform's motion is analyzed in detail. Results show that the surge motion of the platform of a complete FOWT system is mainly affected by wind speed, and the influence of turbulence intensity (TI) is slightly greater than the wave height. The heave motion is most significantly affected by the wave height. When the wave height is large, the influence of wind speed on the platform gradually decreases, and the effect of TI becomes more evident. The pitch and yaw motions are greatly affected by the wind speed and TI. The research results can serve as a reference for the platform design of large FOWTs in the future. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines (FOWTs) are an opportunity for floating production storage and offloading units (FPSOs) to reduce emissions. To avoid long connecting power cables with long transmission distances between a FOWT and an FPSO, the novel concept of a suspended power cable in a large water depth of 1000 m is investigated in this study. The power cable is kept floating between the sea surface and the seabed without touching either of them. A catenary configuration and two configurations with subsea buoys attached at different locations along the cable are investigated. The OC3-Hywind 5 MW reference FOWT is set up with a deepwater mooring system, and a spread-moored FPSO is modeled with characteristics similar to existing FPSOs. Steady-state and dynamic simulations are carried out in the numerical software OrcaFlex. The different configurations are first evaluated in steady-state analyses. The largest tensions are observed for the catenary configuration, whereas it shows the lowest horizontal cable excursions. Buoys attached along the center section of the cable lift it into regions with strong currents. This results in a large horizontal excursion of the cable and large tensions. The suspended configuration with buoys attached evenly over the cable results in significantly lower tensions than the other two configurations. It is studied further with dynamic analyses. The tensions at the floater hang-offs increase by a maximum of 24% compared to steady-state results indicating that dynamic analysis is crucial for the design of suspended cable configurations. [ABSTRACT FROM AUTHOR]

The SPH (smoothed particle hydrodynamics) method coupled with a mooring analysis program is adopted to investigate the accuracy of the SPH method in simulating the hydrodynamics of FOWTs (floating offshore wind turbines). Both experimental and numerical studies were conducted for a floating box and a scaled FOWT model interacting with multiple wave conditions. The SPH method is proven to be accurate in solving the motion of the floaters under multiple wave conditions, showing good consistency with the experimental data. In addition, the snapshots of the wave profile and the motions of the box and FOWT model are compared between SPH results and experiments. Finally, the application of the SPH method is extended to real-scale FOWT simulations. The motion response and mooring line force in the simulations are analyzed and show that the SPH method has the potential to be applied to real-scale FOWT hydrodynamics problems. [ABSTRACT FROM AUTHOR]

The unsteady aerodynamic characteristics and interference effects of floating offshore wind turbine (FOWT) are mainly affected by the pitch motion of the ocean platform. Based on computational fluid dynamics (CFD) and advanced overset grid technology, the aerodynamic performance and wake characteristics of a fully configured wind turbine with rotating blades, nacelle , and tower are studied in this paper. The effects of the amplitude and frequency of pitch motion on the wind turbine aerodynamic loads and flow field are investigated herein in detail. The power and thrust between numerical simulation and experiment are compared. The results show that the grid and simulation parameters used in this study can accurately capture the aerodynamic characteristics and the flow field around wind turbines. The influence of the pitch amplitude and frequency on the performance of wind turbines is discussed. The complex flow interaction between the tip vortex, tower shedding vortex, and the turbulent wake was observed. The present results indicate that the pitch motion amplitude and frequency have a great influence on the power, thrust, and wake characteristics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

With wind energy utilization shifting to the deep sea, floating offshore wind turbines (FOWTs) have huge potential to improve the competitiveness of offshore wind energy generation. However, traditional FOWT systems are often in force imbalance due to the unique swaying characteristics caused by the unfixed foundation and high center of mass of the platform. Therefore, a novel bionic semi-submersible floating platform based on Victoria Amazonia (with random fractal structure) is proposed to increase the hydrodynamic stability of FOWT. A regular fractal structure with similar number of perforations and perforated area is also implemented to verify the effectiveness of the bionic FOWT. In this study, the unsteady computational fluid dynamic approach is adopted to simulate the aerodynamic and hydrodynamic response. More specifically, the dynamic fluid-body interaction module integrated in STAR-CCM+ is employed to establish a FOWT numerical model with rotating blades, tower, and mooring line system. Furthermore, the volume of fluid model in conjunction with the 6-DOF solver was used to efficiently solve the fluid-induced dynamic motion of FOWT in a multi-phase flow composed of air and water. Finally, the fully coupled calculation of the fractal structure FOWT was performed using the established reliable numerical model. The maximum average thrust and power of 736.43 kN and 5294.04 kW are obtained by random fractal FOWT in aerodynamic responses, while in hydrodynamic amplitude responses, the maximum decreases of 19.16% are obtained by both random fractal and regular fractal structures. As for amplitude responses in heave and surge, the random fractal FOWT of 12.91% and 5.05% decreases performs relatively better than the regular one (11.48% and 4.09% decreases). In addition, the visualization of unsteady flow fields and vortices in fractal structures is investigated in detail. Compared with the regular fractal structure, vortices within random fractal structure interact adequately with the pontoon walls, resulting in higher energy absorption effect and stability. • The present study firstly establishes a reliable full-coupled system for FOWT through the CFD method. • The hydrodynamic performance of FOWT is improved via a novel bionic fractal (random fractal structure) floating platform. • A regular fractal structure is tested to verify the effectiveness of random fractal structure. • Three-dimensional visualization is presented to further analyze the influence of fractal structures on FOWT. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper, a site-specific optimization procedure aimed at finding the optimal substructures of a 10 MW Floating Offshore Wind Turbine (FOWT) is presented. An in-house developed Frequency Domain (FD) model is adopted for the simulation of the coupled system. Two Mediterranean sites have been chosen for the characterization of the metocean environments. 20-year databases have been used to obtain the joint distributions of wind speed, significant wave height and peak spectral period. The optimizations, performed adopting a Genetic Algorithm (GA), are aimed at reducing the costs of the floating substructure, controlling the maximum system response under both extreme and fatigue wind-wave loads. Results show that the optimized solution significantly reduces the system cost with an acceptable increase of the loads, opening interesting perspectives for the reduction of the Levelized Cost of Energy (LCOE) in sites characterized by mild sea states and low wind resource. • A site-specific optimization procedure for a 10 MW semisubmersible wind turbine is proposed. • Simulations are performed using a 7-DoF coupled frequency-domain model. • Optimizations are performed considering two installation sites in the Mediterranean Sea. • Objective functions are: maximum tower load, manufacturing cost and fatigue damage. • Results show the existence of improved solutions compared to an upscaled configuration. [ABSTRACT FROM AUTHOR]

Floating wind turbine is one of the promising technology due to its consistent and powerful wind sources in deep-sea zone resulting in high power generation. In the present study a 5 MW baseline spar-type offshore wind turbine is considered for modelling and modal analysis. Sub-systems viz., rotor, nacelle, supporting tower, platform and mooring lines are developed using Ashes software. Commercial software which is specially dedicated for the analysis on onshore and offshore wind turbine. Structural steel is considered for the material properties of the sub-systems. Considering the wind speed and wave effects on the live conditions, modal analysis is carried out. The natural frequencies up to six modes and respective mode shapes are established and reported. [ABSTRACT FROM AUTHOR]

With the deployment of floating offshore wind turbines, the effect of their wind- and wave-induced platform motion on the turbine's performance and life is of concern and requires low-order models to develop appropriate control strategies. This work focusses on platform surge as one of the main additional degrees of freedom. The aerodynamic behaviour during the rotor's transition into propeller state is explored by assessing the spanwise and rotor-integrated aerodynamic forces on the rotor of the NREL 5 MW turbine through a 3D URANS CFD simulation. The CFD findings are compared with a modified actuator disk (AD) theory and 2D Blade-Element Momentum (BEM) approach. In addition to propeller state, where both rotor torque and thrust are negative, two other states were observed with opposite signs of torque and thrust: a braking state, with negative torque but positive thrust, and the second a quasi-windmill state with thrust negative yet torque positive. The results demonstrate that BEM coupled with AD can reliably predict the transitions to these states during such high levels of platform surge and give hints to how the underpinning force balances are affected by temporal perturbations. • Surging floating wind turbines can enter a succession of power and thrust states. • A new quasi-windmill state has been found beyond propeller state. • High-resolution CFD simulations have been used to develop a low-order model. • The model is based on a modified actuator disk and blade element momentum theory. • The model can be used to predict likelihood of states and magnitude of their impact. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Hybrid platforms composed of a floating offshore wind turbine (FOWT) and wave energy converters (WECs) may possibly reduce the levelised cost of energy of both energy harvesting devices and improve the platform dynamics. This paper investigates the dynamics of a hybrid platform composed of a semi-submersible FOWT combined with three spherical heaving point absorbers (PAs), either floating or submerged, under turbulent wind and irregular waves conditions. Since the WECs are attached to the floating structure, complex hydrodynamic couplings occur between the floating bodies. In this regard, a parametric investigation of the power-take-off system is performed to understand the wave power absorption and motion characteristics, which are compared against their respective stand-alone configurations. At sea states with short wave peak period, both hybrid platforms (with floating or submerged PAs) were able to operate under maximum wave power absorption condition and extract nearly the same amount of wave energy as the PAs attached to the ground (stand-alone), while providing the additional benefit of reducing the horizontal nacelle accelerations of the FOWT. On the other hand, when operating at sea states with long wave periods, the hybrid platforms may need to restrict the wave power absorption capability to prevent additional nacelle horizontal acceleration. • Analyses the dynamics of two hybrid platforms under random wind and wave loads. • Provides insights on the performance and nacelle acceleration of hybrid platforms. • Presents a critical comparison of power and motion compared to standalone platforms. • Presents a sensitive analysis of the power take-off coefficients of hybrid platforms. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper investigates the suppression of in-plane vibrations of a floating offshore wind turbine (FOWT) using the electromagnetic shunt tuned mass damper (EMSTMD), where the dashpot is replaced by a shunted electromagnetic (EM) transducer for improving performance and durability of the damper. A series resistive–capacitative–inductive (R C L) network is designed as the supplemental shunt for the EM transducer, which results in an additional resonance for performance improvement. A 17-degree-of-freedom (17-DOF) aero-hydro-servo-elastic model is established for the FOWT, which is used for performance evaluation of the EMSTMD in the time domain. A reduced-order 6-DOF model is then developed for the FOWT in-plane vibrations with EMSTMD mounted, accounting for rotor mass moment of inertia, stiffness from buoyancy and mooring lines, as well as hydrodynamic added mass. This model facilitates an efficient frequency-domain optimization procedure for the EMSTMD, taking into account uncertainties in the environmental conditions. The performance of the optimized EMSTMD is compared with that of the classic TMD in both frequency and time domains. It is shown that both EMSTMD and TMD effectively reduce tower side–side vibrations of FOWT, and also have lightly positive effect on the spar roll and blade edgewise vibrations. EMSTMD consistently outperforms TMD for all environmental conditions, at the cost of slightly increased absorber stroke. • A novel electromagnetic shunt TMD proposed for vibration damping of FOWT tower. • A series RCL network designed as the supplemental shunt for the EM transducer. • A 6-DOF frequency-domain model developed for the EMSTMD-controlled FOWT in-plane vibrations. • EMSTMD outperforms TMD in terms of damping ratios and both frequency- and time-domain responses. [ABSTRACT FROM AUTHOR]

In this paper, a scaled model experiment of a spar-type floating offshore wind turbine (FOWT) is carried out. The scaled wind turbine model is designed based on the OC3-Hywind FOWT, and the pitch control mechanism is considered. An optimal arrangement scheme of anemometers deployment on a disc plane is proposed to improve the efficiency of wind field measurement. Three categories of load case tests corresponding to the wind-only, wave-only as well as wind-wave concurrent loads are carried out. The rigid-body motion of the FOWT model and the deformation of the tower as well as the mooring tension are captured. It is revealed that the yaw is greatly affected by the rotor, and the trend and amplitude of the structural rigid-body motion are dominated by aerodynamic loads and hydrodynamic loads, respectively. Moreover, the coupling effects of multi-degree-of-freedom (MDOF) rigid-body motions of the structure are quite obvious. As for the flexible tower, although its deformation is quite small, the resonant response caused by doubling frequency coupling effects is significant. Besides, large amplitude of stress can be induced in the flexible tower under wave-contained conditions. The present experiment provides a benchmark for the validation of numerical simulation tools of FOWTs. • A fully-physical scaled model test of spar-type FOWT is carried out. • An innovative wind field measurement scheme is proposed. • The elastic deformation of the tower of FOWT is captured. • Rigid-body motion responses of FOWT are dominated by winds and waves, respectively. • The resonant response of the elastic tower caused by n P effects cannot be neglected. [ABSTRACT FROM AUTHOR]

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. [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. [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]

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. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines (FOWTs) are subject to undesirable platform motion and a significant increases in fatigue loads compared to their onshore counterparts. We have recently proposed using the fishing line artificial muscle (FLAM) actuators to realize active mooring line force control (AMLFC) for platform stabilization and thus load reduction, which features a compact design and no need for turbine redesign. However, as for the thermally activated FLAM actuators, a major control challenge lies in the asymmetric dynamics for the heating and the cooling half cycle of operation. In this paper, for a tension-leg platform (TLP) based FOWT with FLAM actuator based AMLFC, a hybrid dynamic model is obtained with platform pitch and roll degrees-of-freedom included. Then a hybrid model predictive control (HMPC) strategy is proposed for platform motion stabilization, with preview information on incoming wind and wave. A move blocking scheme is used to achieve reasonable computational efficiency. Fatigue, aerodynamics, structures, and turbulence (FAST) based simulation study is performed using the National Renewable Energy Laboratory (NREL) 5 MW wind turbine model. Under different combinations of wind speed, wave height and wind directions, simulation results show that the proposed control strategy can significantly reduce the platform roll and tower-base side-to-side bending moment, with a mild level of actuator power consumption. [ABSTRACT FROM AUTHOR]

The pitch controller of a floating offshore wind power system has an important influence on the power generation and movement of the floating body. It drives the turbine blade pitch using a hydraulic actuator, whose inherent characteristics cause a delay in response, which increases with the system capacity. As a result, the power generation is reduced, and the pitch motion of the floating body is increased. This paper proposes an advanced pitch controller designed to compensate for the delay in the hydraulic actuator response. The proposed pitch controller applies an artificial-intelligence-based deep learning algorithm to predict the delay time in the hydraulic actuator. This delay is compensated for by preferentially predicting the blade pitch control angle even if a delay occurs in the hydraulic actuator. The performance of the proposed pitch controller was analyzed using the Fatigue, Aerodynamics, Structures, and Turbulence (FAST) v8 model developed by the US National Renewable Energy Laboratory and was compared against that of the ideal pitch controller and the pitch controller that reflects the response delay. Compared with the latter, the proposed method increased the average power generation by approximately 5% and reduced the standard deviation of the floating body's pitch motion by approximately 50%. [ABSTRACT FROM AUTHOR]

Wind energy is expected to play a significant role in meeting emission targets over the next 20 years. Offshore wind turbines in deep water (>150 m) must be developed due to resource quality, environmental, and activity constraints. Floating offshore wind turbines (FOWT) will be the best technology for reaching these targets. The dynamic submarine electrical cable (DSEC) is a key component of FOWT. Its electric insulation system is intended to withstand a maximum conductor temperature of 90 °C. However, biofouling growth, particularly mussels, can modify the heat transfer around the cable and thus its maximum conductor temperature, as well as temperature fluctuation, affecting the fatigue lifetime. In our work we estimate the effective thermal conductivity of mussels of various ages, as well as the heat transfer coefficient of the water around them. The results revealed that the effective thermal conductivity of juvenile mussels is lower than that of mix (both juvenile and adult) and only adult mussels. This variation in effective thermal conductivity with mussel age is related to the water porosity of the mussel's layer. Then, the thermal effect of the resulting global thermal resistance can lead the DSEC conductor wire to either overheat (colonized by juvenile and mixed mussels) or cool down (colonized by adult mussels). Numerical simulations are used to quantify this effect. [ABSTRACT FROM AUTHOR]

For the design of floating offshore wind turbines (FOWT), all the load cases need to be calculated by using a coupled model of wind turbine and platform, while the uncoupled approach will help to reduce the number of simulations for the design and optimization of floating offshore wind turbines. In this study, the effects of the wind and wave actions on the tower loading of a FOWT were investigated and an uncoupled approach for the load calculation was proposed and verified by comparing with the result of coupled simulation. First, the effect of elastic platform was considered by tuning the Young's modulus of the tower material when rigid platform model was used in the analysis. The effect of wind and wave actions on the loading of the tower was then investigated. It was found that the difference of the wind load between fixed and floating wind turbines is observed only in the mean component and can be predicted by considering the hydrostatic stiffness of the platform and mooring stiffness. The standard deviation of the fore-aft tower moment increased as the significant wave heights increased when the mean wind speeds and peak wave periods were fixed. This is caused by the increase of the inertia force induced by the pitch and surge motions of the platform and the increase of the fluctuating pitch angle. On the other hand, the standard deviation of the fore-aft tower moment decreased as the peak periods increased when the mean wind speeds and significant wave heights were fixed. The increase of the peak period caused the decrease of the pitch and surge accelerations of the platform and results in the decrease of the inertia force. Finally, the tower loading in extreme sea states during power production was carried out by using the proposed uncoupled approach and the results showed good agreement with those by the coupled approach, and the simulation time was reduced to 1/40. [ABSTRACT FROM AUTHOR]

Currently, floating offshore wind is experiencing rapid development towards a commercial scale. However, the research to design new control strategies requires numerical models of low computational cost accounting for the most relevant dynamics. In this paper, a reduced linear time-domain model is presented and validated. The model represents the main floating offshore wind turbine dynamics with four planar degrees of freedom: surge, heave, pitch, first tower fore-aft deflection, and rotor speed to account for rotor dynamics. The model relies on multibody and modal theories to develop the equation of motion. Aerodynamic loads are calculated using the wind turbine power performance curves obtained in a preprocessing step. Hydrodynamic loads are precomputed using a panel code solver and the mooring forces are obtained using a look-up table for different system displacements. Without any adjustment, the model accurately predicts the system motions for coupled stochastic wind–wave conditions when it is compared against OpenFAST, with errors below 10% for all the considered load cases. The largest errors occur due to the transient effects during the simulation runtime. The model aims to be used in the early design stages as a dynamic simulation tool in time and frequency domains to validate preliminary designs. Moreover, it could also be used as a control design model due to its simplicity and low modeling order. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines are easily affected by typhoons in the deep sea, which may cause serious damage to their structure. Therefore, it is necessary to study further the dynamic response of wind turbine structures under typhoons. This paper took the 5MW floating offshore wind turbine developed by the National Renewable Energy Laboratory (NREL) as the research object. Based on the motion theory of platforms in waves, a physical model with a scale ratio of 1:120 was established, and a hydraulic cradle was used to simulate the effect of waves on the turbines. The dynamic response characteristics of offshore wind turbines under typhoons are systematically studied. The research results clarified that the turbine structure is mainly affected by wave loads under typhoons, and its motion response reaches its maximum value under the action of extreme wave loads. The research results of this paper can provide reference value for the design of offshore wind turbine structures under typhoons. [ABSTRACT FROM AUTHOR]

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]

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]

The global installed capacity of floating offshore wind turbines is projected to increase by at least 100 times over the next decades. Station-keeping of floating offshore renewable energy devices is achieved through the use of mooring systems. Mooring systems are exposed to a variety of environmental and operational conditions that cause corrosion, abrasion, and fatigue. Regular physical in-service inspections of mooring systems are the golden standard for monitoring their health status. This approach is often expensive, inefficient, and unsafe, and for this reason, researchers are focusing on developing tools for digital solutions for real-time monitoring. Floating offshore renewable energy devices are usually equipped with a wide range of sensors, some low-cost, low/zero maintenance, and easily deployable (e.g. , accelerometers on the tower), contrary to others (e.g. , direct tension mooring line measurements), producing real-time data streams. In this paper, we propose exploiting the data coming from the first type of sensors for mooring systems health status nowcasting. In particular, we will first rely on state-of-the-art supervised shallow and deep learning models for predicting the health status of the different sections of the mooring lines. Then, since these supervised models require types and amount of data that are seldom available, we will propose new shallow and deep weekly supervised models that require a very small amount of data regarding worn mooring lines. Results will show that these last models can potentially have practical applicability and impact for real-time monitoring of mooring systems in the near future. In order to support our statements, we will make use of data generated with a state-of-the-art digital twin of the mooring system, OrcaFlex 1 1 www.orcina.com. , for a floating offshore wind turbine reproducing the physical mechanism of the mooring degradation under different loads and environmental conditions. Results will show errors around 1% in the simplest scenario and errors around 4% in the most challenging one, confirming the potentiality of the proposed approaches. [ABSTRACT FROM AUTHOR]

Accurate load data of wind turbines operating in real sea conditions is crucial for real-time control and offline structural optimization. However, the sheer size of these structures and the complexities of their working environments challenge the direct load measurement through strain gauges and fiber Bragg grating sensors. To address these problems, this paper introduces an indirect measurement method to acquire the thrust and pitch moment loads of the turbine. First, load identification techniques based on the frequency domain separation and aerodynamic damping were applied to decouple the wind and wave loads and establish the inverse load calculation model. Then, numerical simulations were carried out to calibrate and verify this model under a range of steady and unsteady conditions. Preliminary conclusions indicated the strong viability of this method, revealing a mean relative error of 2.32% for thrust and a corresponding equivalent high-frequency error of 24.66% for pitch moment under the designed operating conditions. Finally, a hybrid scaled model was constructed, combining a virtual wind turbine with a physical platform. Experimental results in the wave basin demonstrated the effectiveness of this indirect method in measuring thrust loads of wind turbines, achieving a mean relative error of less than 10%. • An indirect load measurement method for floating offshore wind turbines is proposed. • An online load measurement system has been developed and calibrated. • A hybrid model of the floating offshore wind turbine based on software in the loop is built. • Scaled model tests are carried out in the wave basin. [ABSTRACT FROM AUTHOR]

The mooring system plays a crucial role in the seakeeping ability of floating offshore wind turbines (FOWTs) but complicates the overall dynamics of the system. Many quasi-static (QS) and dynamic models for mooring analysis exist but a comprehensive comparison among open-source or semi open-source codes in various scenarios is rare. In the present study, six mooring models including three dynamic and three QS ones are assessed. The three dynamic models are the local discontinuous Galerkin (LDG) finite element method (FEM), the lumped mass (LM) method and the finite different method (FDM), and the three QS models are the piecewise extrapolation method (PEM), the catenary and the QS of MAP. Three groups of tests involving catenary line under fairlead excitation, hanging line with axial excitation and FOWT in waves are conducted. The first two test groups focus on single line dynamics to assess among the three dynamic models while the third test group focuses on coupled hydro-mooring modeling with the FEM, PEM, catenary and QS coupled to OpenFOAM. The test results show that there are substantial differences among the models in various scenarios. And the LDG FEM and LM models are recommended to be applied in realistic FOWT hydrodynamics problems. • Three dynamic mooring line models were evaluated for catenary line analysis. • Three dynamic mooring line models were evaluated for vertically hanging line analysis. • Four hydro-mooring models were built and were evaluated for hydrodynamic analysis of floating offshore wind turbines. [ABSTRACT FROM AUTHOR]

The work presented in this paper investigates the possibility of using statistical linearization approach to predict the stochastic dynamics of nonlinear floating offshore wind turbines (FOWT) system under coupled nonlinear wave excitation. OC3-Hywind Spar floating platform equipped with a 5 MW reference wind turbine offered by NREL is taken as the discussed FOWT of the study. A coupled nonlinear analytical model subjected to coupled irregular wave excitation is established to describe the dynamic characteristics of the FOWT, and further, the governing equation of the model is transformed into the equivalent linear one through the statistical linearization approach. The error caused in the linearization process is minimized through an iterative process. The accuracy, adaptability and efficiency of the model and the proposed method built in this paper are verified by numerical examples and comparison with other methods, such as Monte Carlo method and FAST etc. The research shows that the proposed method based on statistical linearization process to predict the stochastic dynamic response of nonlinear FOWT system under coupled nonlinear wave excitation has not only high accuracy but also high computational efficiency. It is about 4–5 orders of magnitude faster than the deterministic analysis method, and is of great significance for the design and research of the FOWT. An application example conducted in this paper indicates that this method is effective and can be used extensively, such as structural design optimization, reliability analysis, determination of the optimal parameters of vibration control and fatigue analysis of components, etc. • Coupled nonlinear analytical model to describe the dynamic characteristics of the floating offshore wind turbine. • Consideration of both system nonlinearity and hydrodynamic nonlinearity. • Application of the statistical linearization, and determination of an equivalent linear system. • Solution of the stochastic response via frequency domain method. • A sensitivity study is performed to investigate the second-order motion under different sea conditions. • Nonlinear model shows better agreement with FAST compare to linear model, especially in extreme sea conditions. [ABSTRACT FROM AUTHOR]