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This comprehensive review explores the application and impact of digital twin (DT) technology in bolstering the reliability of Floating Offshore Wind Turbines (FOWTs) and their supporting platforms. Within the burgeoning domain of offshore wind energy, this study contextualises the need for heightened reliability measures in FOWTs and elucidates how DT technology serves as a transformative tool to address these concerns. Analysing the existing scholarly literature, the review encompasses insights into the historical reliability landscape, DT deployment methodologies, and their influence on FOWT structures. Findings underscore the pivotal role of DT technology in enhancing FOWT reliability through real-time monitoring and predictive maintenance strategies, resulting in improved operational efficiency and reduced downtime. Highlighting the significance of DT technology as a potent mechanism for fortifying FOWT reliability, the review emphasises its potential to foster a robust operational framework while acknowledging the necessity for continued research to address technical intricacies and regulatory considerations in its integration within offshore wind energy systems. Challenges and opportunities related to the integration of DT technology in FOWTs are thoroughly analysed, providing valuable insights into the role of DTs in optimising FOWT reliability and performance, thereby offering a foundation for future research and industry implementation. [ABSTRACT FROM AUTHOR]

With the recent trend toward larger floating offshore wind turbines (FOWT), the influence of blade deformation becomes more obvious. Modelling this fully coupled fluid-structure interaction (FSI) system, especially at extreme sea level with large-scale motion of the entire system, is an essential challenge. A hybrid numerical model integrating the potential-viscous flow model (qaleFOAM), the unstable actuator line method (UALM), the Legendre spectral finite element model (BeamDyn), and the Lumped Mass Mooring Model (MoorDyn) is employed in the current research. This model is capable of effortlessly and accurately reflecting the blade aeroelastic effect on the dynamics of a FOWT system under the circumstances of a focused wave and uniform wind. The result reveals that the aeroelastic impacts have been identified to not only increase the fatigue damage on the turbine blades but also affect the wake field distribution. Based on the temporal and spatial distribution of the velocity fields, it is found that the coupled effects of the large wave elevation and the FOWT highfrequency motions will disturb the velocity in the wake region. The slight variation of the wind speed in the wake region will be exacerbated by about 0.21% to 18.6%, and it transforms the shape of the wake region when the blade aeroelastic effect participates in the coupled effect. This phenomenon may further affect the downstream FOWT system features through the upstream FOWT wake field. [ABSTRACT FROM AUTHOR]

The drive for the cost reduction of floating offshore wind turbine (FOWT) systems to the levels of fixed bottom foundation turbine systems can be achieved with creative design and analysis techniques of the platform with free-form curves to save numerical simulation time and minimize the mass of steel (cost of steel) required for design. This study aims to compare four parametric free-form curves (cubic spline, B-spline, Non-Uniform Rational B-Spline and cubic Hermite spline) within a design and optimization framework using the pattern search gradient free optimization algorithm to explore and select an optimal design from the design space. The best performance free-form curve within the framework is determined using the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS). The TOPSIS technique shows the B-spline curve as the best performing free-form curve based on the selection criteria, amongst which are design and analysis computational time, estimated mass of platform and local shape control properties. This study shows that free-form curves like B-spline can be used to expedite the design, analysis and optimization of floating platforms and potentially advance the technology beyond the current level of fixed bottom foundations. [ABSTRACT FROM AUTHOR]

An effective damage diagnosis and prognostic management method can considerably reduce operation and maintenance costs of floating wind turbines. In this research, an intelligent damage diagnosis framework, named "MS-ACNN", has been developed using a multi-scale deep convolution neural network model fused with an attention mechanism. The framework is used to detect, localize, and quantify existing and potential damages on multibody floating wind turbine tendons. The MS-ACNN framework is fitted with two multi-scale extractors, designed to capture multi-scale information from raw wind turbine response signals measured using multi-sensor. The attention mechanism uses weight ratios of extracted damage feature to enhance the MS-ACNN's capability in offering a better generalization in damage diagnosis. The framework's performance is examined under normal and noisy environments and with a diagnosis accuracy of 80%, which is higher than those obtained using most generic industrial grade diagnostic tools (MS–CNN–I, MSCNN-II, CNN, CNN-LSTM and CNN-BiLSTM) by at least 10%. The framework is also fitted with a Majority Weighted Voting rule to reduce false alarms and ensure optimum performance of the multi-sensor during collaborative diagnosis. Further examination shows that the inclusion of a voting rule increases the diagnostic performance's F1 index from 90% for single sensor and 84% for multi-sensor results to 94%. [ABSTRACT FROM AUTHOR]

Numerical simulations are performed within the time domain to investigate the dynamic behaviors of an SPAR-type FOWT under wave group conditions. Towards this goal, the OC3 Hywind SPAR-type FOWT is adopted, and a JONSWAP (Joint North Sea Wave Project)-based wave group is generated by the envelope amplitude approach. The FOWT motion under wave group conditions, as well as the aerodynamic, hydrodynamic, and mooring performances, is simulated by our established in-house code. The rotating blades are modelled by the blade element momentum theory. The wave-body interaction effect is calculated by the three-dimensional potential theory. The mooring dynamics are also taken into consideration. According to the numerical results, the SPAR buoy motions are slightly increased by the wave group, while the heave motion is significantly amplified. Both the aerodynamic performance and the mooring tension are also influenced by the wave group. Furthermore, the low-frequency resonant response could be more easily excited by the wave group. [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 growing energy demand has increased the consumption of hydrocarbons in developing countries such as Mexico, which has contributed to accelerating global warming. Although Mexico has suitable offshore wind energy harvesting sites, technical assessments to identify technologies to be placed in specific locations are scarce. In Mexico, offshore wind resources are found in depths larger than 50 m. There, floating platforms are convenient for harnessing wind energy. This work presents a technical evaluation of offshore wind energy in one of the regions with a higher availability of wind resources on the Mexican coasts, the Tehuantepec Gulf, in the Pacific Ocean. First, a specific location is chosen to evaluate its maritime conditions and theoretical wind energy potential. Next, the performance of a floating offshore wind turbine platform has been investigated numerically using potential flow simulations. The motions of the platform were analyzed for different drafts when subjected to different environmental conditions in the evaluated region. These conditions include wave and wind scenarios with the highest probability of occurrence and the most extreme ones. From the evaluation of the platform dynamics in these scenarios, it was possible to identify the general working conditions of the platform. Results showed that the proposed FOWT could be suitable to be deployed in the region of interest for the most probable environmental conditions, presenting a possible energy production within 3–4 MW, approximately. However, motion stabilization should be required when subjected to extreme environmental conditions as the structure could significantly increase its surge and pitch motions. The simplified approach employed in this work can help perform the following steps in technical or practical resource assessments of wind energy in other places in Mexico or similar developing regions. [ABSTRACT FROM AUTHOR]

This paper addresses a spectral approach for evaluating the fatigue damage in a floating offshore wind turbine (FOWT) structure accurately by taking account of the nonlinear coupling effects between wind and wave loads. It is shown that the spectral moments under the combined loads can be estimated by a sum of those under wind-only cases and wave only-cases, however, with some corrections to account for the nonlinear coupling effects. A formula to estimate the variance of the stress under combined loads as a function of wind speed and wave height is developed based on a mechanical model. The formula is extended to even higher spectral moments. Then, the developed method is applied to the fatigue damage evaluation in a semi-submersible FOWT. The results from the developed method in terms of the fatigue damage are compared with those from the direct approach for validation. An efficient method for long-term fatigue damage evaluation of FOWT is envisaged based on the proposed method. [ABSTRACT FROM AUTHOR]

The wind condition around floating offshore wind turbines (FOWTs) can be largely different from that developed around bottom-mounted wind turbines due to the platform motions. The existing literature identifies four working state of FOWTs, one of them being the vortex ring state (VRS) which may occur as the rotor moves in its own wake. It is potentially a problem that influences the aerodynamic performance and lifetime of FOWTs. It is still unclear when, and to what extent, does the VRS happen to floating offshore wind turbines. The aim of this paper is to quantitatively predict the occurrence of VRS during the operation of FOWTs. Three different criteria are used and compared: the axial induction factor, Wolkovitch's criterion and Peters' criterion. The results show that the VRS phenomena may occur for a large range of operating conditions and can be correlated with the minima in the relative wind speed normal to the rotor plane. Also, the probability of occurrence of VRS is smaller for the floating platforms that exhibit the least motions such as the TLP. Finally, Wolkovitch's criterion seems to be the most suitable one for the VRS prediction, while Peters criterion indicates the initial aerodynamic change and is thus suitable for early warning of VRS. • Quantitatively predicted the occurrence of vortex ring state of floating offshore wind turbines for the first time. • Adopt and adept the vortex ring state prediction methods from helicopter society for the application on floating offshore wind turbines for the first time. • Use aero-hydro-elastic coupled analysis method to simulate particularly the vortex ring state phenomena for floating offshore wind turbines for the first time. •Compared and discussed the different vortex ring state prediction methods. •Output the thrust load on the wind turbines and show significant fluctuation during vortex ring state. [ABSTRACT FROM AUTHOR]

The system reliability of the mooring system for floating offshore wind turbines (FOWT) is analyzed in this paper. Various failure modes such as mooring line breaking, fatigue and floater offset are included and solved as a series system. This work employed a new mooring breaking assessment algorithm, wherein the effect of fatigue damage on breaking strength is simulated by introducing functional dependences into the coupled dynamic analysis. In this research, the extreme design loads are estimated using the environmental contour (EC) approach and a logarithmic function is proposed to describe the relationship between the extreme response and the target return period to simplify the full long-term analysis. The time-domain coupling analysis of the FOWT is performed considering all one-year sea states, the long-term fatigue damage was approximated based on linear criterion. The fatigue damage can contribute significantly to the breaking strength of mooring lines. The results indicate that ignoring the dependence between the residual strength and cumulative fatigue damage would cause under-estimated results. The calculated failure probability of the floating system is larger than that of any single failure mode, while it is mainly dominated by the main single failure mode with the largest failure probability. • The system reliability of FOWTs was evaluated by considering various failure modes. • The functional dependences between mooring fatigue damage and residual strength are considered. • The CDF of long-term response is simulated using a simplified EC-based algorithm. • The critical sea states are identified based on a mesh grid scheme. • The relationship between maximum response and environmental parameters is investigated. [ABSTRACT FROM AUTHOR]

Simulations are conducted in time domain to investigate the dynamic response of a spar-type floating offshore wind turbine (FOWT) under the freak wave scenarios. Toward this end, a coupled aero-hydro-mooring in-house numerical code is adopted to perform the simulations. The methodology includes a blade-element-momentum (BEM) model for simulating the aerodynamic loads, a nonlinear model for simulating the hydrodynamic loads, a nonlinear restoring model of Spar buoy, and a nonlinear algorithm for simulating the mooring cables. The OC3 Hywind spar-type FOWT is adopted as an example to study the dynamic response under the freak wave conditions, meanwhile the time series of freak waves are generated using the random frequency components selection phase modulation method. The motion of platform, the tension applied on the mooring lines, and the power generation performance are documented in several cases. According to the simulations, it is indicated that when a freak wave acts on the FOWT, the transient motion of the FOWT is induced in all degrees-of-freedom, as well as the produced power decreases rapidly. Furthermore, the impact of freak wave parameters on the motion of FOWT is discussed. [ABSTRACT FROM AUTHOR]

The tremendous wind-wave excitations bring about structural vibrations, which would have adverse influences on the power generation efficiency of the spar floating offshore wind turbine (FOWT). Therefore, two tuned mass dampers (TMDs) are installed in the platform and nacelle of the spar FOWT to control the vibration responses of the structure. The aero-hydro-servo-structure-TMDs coupling kinetics model of 16-degree-of-freedom (DOF) is firstly established for the spar FOWT. The correctness of the coupled model is then verified through comparing with OC3 project of FAST developed by the National Renewable Energy Laboratory (NREL). Subsequently, the TMDs stiffness and damping coefficients are optimized in constraints of the TMDs mass and stroke. Furthermore, the vibration reduction effects of TMDs are studied in the free decay state and wind-wave load cases, respectively. The simulation results demonstrate that the platform TMD can effectively reduce the platform pitch (PFPI) movement and low frequency vibration of the tower top fore-aft (TTFA) deflection, while the nacelle TMD is effective for the high frequency vibration of the TTFA deflection. Thus, the TMDs can control the structural vibration responses of spar FOWTs. • Coupled kinetics model with TMDs of the spar FOWT is established. • Generator power of the wind turbine is studied in different wind-wave loads. • Platform TMD could reduce platform motions and tower low frequency vibrations. • Nacelle TMD would improve tower high frequency vibrations. [ABSTRACT FROM AUTHOR]

To obtain fundamental knowledge on the elastic response characteristics of a light-weight floating support structure of a FOWT (floating offshore wind turbine) with guywire supported tower, basic load transmission mechanism was investigated. Static analysis with elastic frame model, numerical analysis and wave tank experiment with elastically and dynamically similar segmented backbone model were conducted to clarify the dynamic elastic response characteristics of the structure. In the numerical analysis, analysis code of a rotor-floater-mooring-control coupled response NK-UTWind developed in University of Tokyo is used. It is clarified that when the rigidity of the frame structural part is low compared with guywire, the load is mainly borne by guywire under pitch motion. It was found that the tension fluctuation of guywire becomes large at wave period of 6 s when the inertial force due to pitch motion is large, and at wave period of 18–20 s when inclination of tower is larger the tension fluctuation also becomes large due to the overturning moment. [ABSTRACT FROM AUTHOR]

This study investigated the transient response due to a mooring line failure on a barge-type floating offshore wind turbine (FOWT). A 1/64 scale model of the FOWT with eight catenary mooring lines was installed in a wave tank to be separately subjected to wind, wave, and current loads. An electromagnet attached to the fairlead of the FOWT was used to disconnect the mooring lines. First, free decay test was performed to ensure the suitability of the scaled model. Then, the individual contributions of wind, wave, and current loads were compared based on the 50-year return period to investigate their influences on the movement of the FOWT after failure. The results showed that the wave effect was the most important factor, followed by current and wind. Then, the mooring loads and rigid body motions of the FOWT under mooring line failure in wind, current, and wave conditions were investigated. Finally, two commercial codes (ANSYS AQWA and OrcaFlex) were used to compare the experimental results in waves. The comparison findings showed that the potential flow-based numerical model overestimates the dynamic response. This result implies the significant damping effect of the barge-type FOWT. • An experimental study of mooring line failure on a barge-type FOWT under wind, waves, and currents were conducted. • Wave forces had a greater influence on the movement of the FOWT after a mooring failure compared to wind and current. • The damping effect on the barge-type FOWT is very important. • The conclusions can be helpful for the mooring design in real-life applications of FOWT. [ABSTRACT FROM AUTHOR]

• Experiment study on an OWC device integrated into a FOWT was carried out. • Optimal opening ratio of 3.0% produces a maximum relative capture width. • The effects of the FOWT geometries on the OWC device are discussed. • The introduction of the OWC device reduces the heave-motion of the FOWT. A multi-functional floating foundation integrating an Oscillating Water Column (OWC) Wave Energy Converter (WEC) with a Floating Offshore Wind Turbine (FOWT) is a promising design for achieving cost reduction, adaptability and stability improvements. Yet, such a multi-functional integration is still in a demonstration phase and additional investigations are required to reach industrial maturity. This work assesses the hydrodynamic performance of such a design by carrying out a series of carefully instrumented scaled model tests. Two distinct sets of experimental models were designed accordingly, i.e., a pure FOWT without the OWC device, and the FOWT incorporating the OWC device. The multi-functional foundation is either fixed or allowed to move in heave mode along four vertical sliding rails. The introduction of the OWC device enhances the stability of the FOWT in heave, with a maximum motion reduction rate of 54.1%. The chamber air orifice size and the foundation geometries were optimized. The OWC with an air chamber opening ratio of 3.0% produces the maximum relative capture width. A larger chamber draft and breadth lead to a lower chamber resonance frequency. The main objective of this paper is to guide the design and application of the OWC device integrated into a floating offshore wind turbine. [ABSTRACT FROM AUTHOR]

It demands many computational resources to model the coupled responses of a floating offshore wind turbine (FOWT), especially for its aero-elastic-hydro-mooring-wake dynamics and their interaction. In this paper, a new hybrid numerical model for FOWT systems is developed, which is based on the hybrid potential-viscous flow model called qaleFOAM. In this model, the aerodynamics of wind turbine are solved by the unsteady actuator line method (UALM); the elastic responses of the turbine blade are calculated by the Legendre spectral finite element model (BeamDyn); the hydrodynamics of the floating platform are dealt with by the combination of the fully nonlinear potential solver and a two-phase Navier–Stokes solver; the mooring dynamics are considered with the Lumped Mass Mooring Model (MoorDyn), and the turbine wake is solved with the large eddy simulation (LES) model. This newly formulated model can deal with wind, wave, mooring dynamics, platform motions, and turbine structural dynamics involved in the FOWT system. To demonstrate the capability of the present model, various cases with different complexities are investigated and compared with the experimental data and other numerical results. Then, the model is applied to simulation of a semi-submersible FOWT system, subjected to a regular wave and a uniform wind. The prediction of the aerodynamic performance, blade tip deflection, platform motion responses, and mooring line tension loads show good agreements with the results from other methods. In addition, the phenomenon of the coupled effects between the dynamic responses of platform, blade deformation and wake flow are captured reasonably well. • A new hybrid numerical model for FOWT systems is developed by combining qaleFOAM with ALM code, BeamDyn and MoorDyn. • The aerodynamic loads, blade tip deflections, mooring dynamics, dynamic responses of the FOWT and its wake are investigated. • A satisfactory computational robustness of the present model is achieved. [ABSTRACT FROM AUTHOR]

• A new model-test method for wind turbine is proposed for achieving similarity of both model- and full-scale thrust and torque. • Both lift and drag forces are considered when determining the chord length of the airfoil of the model blade, which helps achieving the correctly-scaled thrust and toque. • The model blade is redesigned by keeping its twist angle same as that of the full-scale blade, which makes it possible for the model to suite a wide range of wind speeds. • Numerical and experimental results show that the new proposed model-test method can realize the similarity of thrust and torque simultaneously for all the range of wind speeds. For model tests of a floating offshore wind turbine (FOWT) system, a great challenge is how to model the interaction between the wind turbine and floating platform with correctly-scaled aerodynamic and hydrodynamic loads because of a well-known contradiction between Reynolds and Froude scaling. Several approaches have been proposed in the literature to tackle the challenge but none of them can correctly and simultaneously model the scaled thrust and torque, and so the interaction between turbine and platform. This paper will present a new model-test method for achieving similarity of both thrust and torque. This is achieved by redesigning the model blades with keeping the blade twist angle same as that of the full-scale turbine and by adjusting the pitch angle of blades and rotational speed of wind turbine in model tests. Numerical simulations and wind tunnel model tests are carried out to validate the present method. Both numerical results and experimental data show that the present method can realize the similarity of thrust and torque simultaneously, and thus, making it possible to study full interaction between turbine and floating platform by physical testing in wind-wave basins. [ABSTRACT FROM AUTHOR]

This study investigates the aero-hydro-mooring coupled dynamics of the National Renewable Energy Laboratory (NREL) offshore 5-MW baseline wind turbine supported by the Offshore Code Comparison Collaboration Continuation (OC4) DeepCwind semisubmersible floating platform by considering the effects of suddenly broken mooring behaviour. The wind and wave loads can be selected as the Cummins time-domain equation inputs based on the Norwegian Petroleum Directorate wind and Joint North Sea Wave Project wave spectrums. The nonlinear viscous drag was estimated using the quadratic damping matrix rather than Morison's element. Additionally, the quadratic transfer function matrix was used to calculate the slow-drift force. Compared to the multi-segmented quasi-static mooring model, the lump-mass method is a dynamic mooring model that can solve real-time motion on the platform. Once a single or a pair of mooring lines is intentionally disconnected from the floating platform at a certain time, the transient responses of mooring line tensions and turbine performance will be evaluated. The novelty of this study is that the drift trajectories and power performances of OC4 DeepCwind semisubmersible floating offshore wind turbine under different conditions of environmental loads and broken mooring lines can be approximately estimated in the offshore wind farm. The analysis can be used for the environmental impact assessment in harsh sea conditions. • The aero-hydro-mooring coupled dynamics of the floating offshore wind turbine (FOWT) have been investigated by considering broken mooring behaviours. • The drift trajectories and power performances of the FOWT under different conditions of broken mooring lines have been analysed comprehensively. • Once there are two broken mooring lines in the scenario of power production, the equilibrium position of the FOWT cannot be reached due to the unbalanced resultant force. [ABSTRACT FROM AUTHOR]

The development of novel energy technologies to meet net zero carbon emission is essential in the provision of solutions to realize an increasing worldwide demand for renewable energy. Floating Offshore Wind Turbine (FOWT) is one of the emerging technologies to exploit the vast wind resources available in deep waters within the offshore wind sector. However, as a result of the complexity of a FOWT system, bringing FOWT technology up to speed requires a detailed understanding of the different disciplines within the system and the relationship between the FOWT system and the dynamics of the marine environment; hence, the need for Multidisciplinary Design Analysis and Optimization (MDAO) of the system. This paper reviews the MDAO of FOWT substructures/platforms proposed in the literature. This review covers an overview of floating offshore wind turbine substructures' concepts, the design using geometric shape parameterization techniques and the analysis approaches (time and frequency domain) for response assessment of the FOWT system. It also provides a comprehensive review of MDAO frameworks for FOWT substructures. Regarding the optimization aspect, a review of some optimization algorithms used for floating offshore wind turbine substructure is provided, i.e., from the global search heuristic and meta-heuristics algorithms to the local search gradient-based optimization algorithms. This work further identifies the research gaps in MDAO for FOWT substructures. The main proposed future research areas to address these gaps are: increasing design space richness by adopting more advanced parametrization techniques to represent the platform geometry (and other characteristics), utilize surrogate/meta models to replace the most computationally expensive high-fidelity models needed for quick sensitivity studies before detailed analyses on selected models are conducted, and exploring the upscaling of the geometric design parameters of an optimal shape parameterized FOWT platforms derived from existing designs which can be coupled with new generation highly rated and heavier turbines. • Floating Offshore Wind Turbine (FOWT) is an emerging technology that can reduce greenhouse emission using the vast wind resources in deep waters. • FOWT substructure is a complex multidisciplinary system designed with the multidisciplinary design analysis and optimization (MDAO) approach. • Efficient parameterization techniques are required to explore large design space for a FOWT substructure. • Reviews MDAO framework and optimizers used in the design of FOWT substructure. • Research gaps identified in the design and optimization of the support structure discipline of the multidisciplinary FOWT system. [ABSTRACT FROM AUTHOR]

The coupled (Potential theory and Navier–Stokes) solver is applied to simulate the interaction of sea waves with substructure of floating offshore wind turbines (FOWT) platform, notably similar to the OC3 Hywind SPAR structure. The intention is to develop a numerical tool that allows the study of the survivability of floating structures in extreme sea states. In this study, the moorings are modeled in two ways. One is by considering the mooring lines as a linear spring with defined spring stiffness, and another is by coupling the solver (foamStar) with a lumped-mass mooring dynamics model (MoorDyn). MoorDyn represents mooring line behavior subject to axial elasticity, hydrodynamic forces, and vertical contact forces with the seabed. The coupled model has been validated against the experiments carried out as part of the SOFTWIND project. The numerical model results of free surface elevation, floating body motions and mooring tensions are compared with the experiments. For wave cases with mild and moderate amplitudes, mooring in the form of a stiffness matrix is sufficient. However, dynamic mooring simulation (MoorDyn) is required for the extreme sea state conditions. • Strategic coupling between HOS, OpenFOAM, floating body and mooring is developed • Static and dynamic mooring models based on stiffness matrix and MoorDyn, respectively • Numerical motions and mooring tensions of SPAR in waves validated with experiments [ABSTRACT FROM AUTHOR]

Recent studies and developments in the wind industry indicate a trend of floating offshore wind turbine (FOWT) designs with larger power ratings. The fatigue damage and cost of maintaining the hull structure become more critical for the asset integrity as the turbine size grows larger, which exacerbates the urgency to incorporate fatigue life estimation early in the design spiral for proper asset optimisation. Fatigue calculations for FOWTs are complex and computationally more expensive than for most of the other floating systems deployed in use of offshore industry. To enable fatigue prediction in design development, this paper introduces a pseudo-coupled analysis approach to reduce the computational expense by the order of one-hundredth whilst avoiding a significant compromise in the accuracy. The results are benchmarked against the state-of-the-art fully-coupled approach, and a good agreement is observed. A preliminary design spiral for VolturnUS-S FOWT is established through a Monte Carlo simulation, accounting for fatigue along with other design criteria. The results show that fatigue is sensitive to even minor changes in the design parameters. The fatigue damage can be three times more or less for similar size and shape FOWTs which proves that fatigue prediction for FOWTs at initial design development is crucial. • Pseudo-coupled analysis reduces the computational expense by 1/100 whilst avoiding a significant compromise in the accuracy. • The results are benchmarked against the state-of-the-art fully coupled approach, and a satisfactory agreement is observed. • A preliminary design spiral for VolturnUS-S is established through a Monte Carlo simulation where fatigue is also calculated. • The results of the design spiral show that the fatigue life is sensitive to even minor changes in the design parameters. • Sensitivity of the results proves that fatigue assessment for FOWTs at the early stages of the design development is crucial. [ABSTRACT FROM AUTHOR]

In this work an extensive verification and validation study is performed to evaluate the accuracy and credibility for CFD simulations of hydrodynamic response of a Floating Offshore Wind Turbine (FOWT) with semi-submersible platform under regular waves. A dynamic mooring model is coupled with the CFD code to accurately simulate the mooring system. The surge, heave and pitch RAOs at wave frequency and double wave frequency, mean surge offset and zero frequency surge QTF of the semi-submersible platform are investigated. The numerical uncertainties of the above metrics are quantified based on three sources - statistical, iterative and discretization uncertainty. Amongst them, the discretization uncertainty is identified as the dominant source of numerical uncertainty. The validation uncertainty is finally estimated combining the numerical, input and experimental uncertainties. Most of the metrics have achieved the validation against the experimental measurement within the validation uncertainty except for the mean surge offset and zero frequency surge QTF. The utilization of the coupled CFD-FEM code is proved to be accurate for simulations of FOWT under regular waves. Guidelines for best-practices of the procedure for verification and validation study of CFD simulations of floating structures under regular waves are also given. • Identification of transient phase using TST technique and quantify the statistical uncertainty. • Quantification of iterative uncertainty using simulations with different convergence tolerance. • Quantification of the discretization uncertainty using simulations with different grid and time step size setups. • Validation against experimental data with the validation uncertainty bound. • Guideline for CFD simulation of floating marine structures. [ABSTRACT FROM AUTHOR]

Aiming at failure problems associated with floating offshore wind turbine (FOWT) mooring systems, a new reliability assessment method is established based on an intelligent algorithm. Using the NREL 5 MW OC4 semi-submersible wind turbine as the research object, the hydrodynamic is obtained, together with the aerodynamic, and axial tension forces of the mooring chains under different working gust conditions through integrated simulations. The intelligent teaching-learning-based optimization (TLBO) and improved teaching-learning-based optimization (ITLBO) algorithms are proposed to calculate the reliability of the FOWT mooring system. The results show that the reliability index of the windward chain is 4.21 under an extreme working gust, which is less than the code recommended value 4.26 and should be further evaluated. The penalty coefficient, teaching factor, and population number have a great impact on the convergence speed of the TLBO algorithm. Compared with traditional reliability algorithms, such as the first-order second-moment algorithm and Monte Carlo algorithm, the TLBO algorithm has higher efficiency, especially the ITLBO algorithm, which has the fastest convergence speed by constantly adjusting the teaching factor. Therefore, it is recommended to use the TLBO or ITLBO algorithm to evaluate the reliability of FOWT mooring systems. [ABSTRACT FROM AUTHOR]

The accurate prediction o f unsteady aerodynamic performance and loads, for floating offshore wind turbines (FOWTs), is still questionable because several conventional methods widely used for this purpose are applied in ways that violate the theoretical assumptions o f their original formulation. The major objective o f the present study is to investigate the unsteady aerodynamic effects for the rotating blade due to the periodic surge motions of an FOWT. This work was conducted using several numerical approaches, particularly unsteady computational fluid dynamics (CFD) with an overset grid-based approach. The unsteady aerodynamic effects that occur when an FOWT is subjected to the surge motion of its floating support platform is assumed as a sinusoidal function. The present CFD simulation based on an overset grid approach provides a sophisticated numerical model on complex flows around the rotating blades simultaneously having the platform surge motion. In addition, an in-house unsteady Made element momentum (UBEM) and the FAST (fatigue, aerodynamic, structure, and turbulence) codes are also applied as conventional approaches. The unsteady aerodynamic performances and loads o f the rotating blade are shown to be changed considerably depending on the amplitude and frequency of the platform surge motion. The results for the flow interaction phenomena between the oscillating motions of the rotating wind turbine blades and the generated blade-tip vortices are presented and investigated in detail. [ABSTRACT FROM AUTHOR]

Hydrodynamic forces on heave plates for a semi-submersible floating offshore wind turbine are discussed herein. A model of one of the platform columns has been built. This allows for the fitting of either a plain solid plate or the real heave plate prototype design. The latter is equipped with a vertical flap at its edge. The influence of the flap on the hydrodynamic coefficients is investigated through a results comparison with the plain solid one. The model plate diameter is 1 m, thus becoming, to the authors' knowledge, the largest for which results have been published. Results from experiments, in which added mass and damping coefficients have been measured, are presented. This experimental campaign also comprised the direct measurement of dynamic pressures on both heave plates, a fundamental magnitude for the structural design, which, until now, had not been experimentally explored for this type of system. For comparative reasons, numerical simulations were also conducted following common industry standards, both with a wide-spread frequency domain panel method (WADAM) and a RANS CFD commercial code (ANSYS CFX). Finally, results are compared with literature and consistent non-dimensionalizations are sought, with the aim of making these results useful for preliminary design purposes. The authors believe this research could benefit the offshore wind industry by improving the hydrodynamic design of the concept. [ABSTRACT FROM AUTHOR]

The quasi-dynamic mooring line model is a new approach to assess the mooring line tension that considers the static solution of the catenary shape, but updates the distributed vertical force along the mooring line as a function of the external forces, drag and inertial, acting on the line. In this way, an updated apparent weight is applied to the static solution of the catenary shape that improves significantly the solution obtained by the quasi-static approach, with an equivalent computational cost. Then, the model improves the tension solution of the quasi-static models by using that updated weight of the mooring, achieving results much closer to the dynamic models than the standard quasi-static ones. To appraise the improvement of the results, a parametric analysis within a wide range of mooring physical parameters is conducted. The error of the quasi-static tension of one cycle is less than 20% only for the 18% of the simulations, while the quasi-dynamic model produces an error less than 20% for the 84% of simulations. In addition, the model can predict the slack phenomenon. • A new approach for mooring line tension assessment named quasi-dynamic mooring model is presented. • The model assumptions are verified and the capacity of the model is assessed through a parametric study. • The model is validated through the simulation and comparison with the DeepCwind semisubmersible experimental data. • The model presents high accuracy with low computational cost. [ABSTRACT FROM AUTHOR]

In this paper a detailed verification and validation study is performed to evaluate the accuracy of CFD simulations for a semi-submersible floating offshore wind turbine in pitch free-decay motion. The simulations are carried out with both linear and dynamic mooring models. The linear and quadratic damping coefficients of the semi-submersible are derived from the pitch motion data. In the verification study, a least-squares method using the observed order of convergence in combination with a data quality measure for different spatial and temporal refinement is applied to estimate the discretization errors. The numerical uncertainty is quantified by applying a safety factor to the summation of numerical errors. The numerical solutions are validated against the experimental measurements obtained from wave tank test of OC5-DeepCWind semi-submersible. Validation is achieved in the pitch natural period and the linear damping coefficient within the level of validation uncertainty. While the numerical simulation under-predicts the quadratic damping coefficient. A guideline is provided for verification and validation study of a CFD code application for floating marine structures under decay tests. • Evaluation of iterative uncertainty from all sources. • Estimation of spatial and temporal discretization uncertainty in flow solver and mooring solver. • Validation against model test data. • Guideline for CFD simulation of free decay of floating marine structures. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbine technology has seen an increasing and continuous development in recent years. When designing the floating platforms, both experimental and numerical tools are applied, with the latter often using time-domain solvers based on hydro-load estimation from a Morison approach or a boundary element method. Commercial software packages such as OrcaFlex, or open-source software such as OpenFAST, are often used where the floater is modeled as a rigid six degree-of-freedom body with loads applied at the center of gravity. However, for final structural design, it is necessary to have information on the distribution of loads over the entire body and to know local internal loads in each component. This paper uses the TetraSpar floating offshore wind turbine design as a case study to examine new modeling approaches in OrcaFlex and OpenFAST that provide this information. The study proves the possibility of applying the approach and the extraction of internal loads, while also presenting an initial code-to-code verification between OrcaFlex and OpenFAST. As can be expected, comparing the flexible model to a rigid-body model proves how motion and loads are affected by the flexibility of the structure. OrcaFlex and OpenFAST generally agree, but there are some differences in results due to different modeling approaches. Since no experimental data are available in the study, this paper only forms a baseline for future studies but still proves and describes the possibilities of the approach and codes. [ABSTRACT FROM AUTHOR]

Multi-purpose offshore structures are very complex systems to be designed as many different requirements have to be taken into account simultaneously. Experimental tests on scaled models can be very useful to verify structural behavior and to validate numerical models. The definition of the scale can play a key role in performing tests as high scales, closer to full-scale, are generally related to more reliable results. The present paper concerns the design of a wind turbine model for a large-scale model of a Multi-purpose offshore Platform developed within the EU project H2020 Blue Growth Farm. This project aims at developing an offshore farm, based on a modular floating structure that integrates wave energy converters and a wind turbine with aquaculture. The scale model of the complete structure will be deployed at the Natural Ocean Engineering Laboratory (NOEL). The paper will focus on the strategies adopted to scale 1:15 the DTU 10 MW wind turbine in order to represent both the main aeroelastic features and all the functionalities of a real machine. The complete design of the machine and its control will be also provided. • The present work was developed within the EU H2020 project 'The Blue Growth Farm'. • The aim of the project is the design of a multi-purpose platform for offshore applications, equipped with a wind turbine. • A large outdoor scaled model of a wind turbine is completely designed. • Typical scaling procedure were adopted to obtain an aeroelastic model for a natural laboratory. • The outdoor scaled model was verified from structural and mechatronic point of view. • Control strategies for the outdoor scaled model were provided. [ABSTRACT FROM AUTHOR]

In this paper, a new algorithm for the real-time structural health monitoring of TLP (tension-leg platform) tendons is presented. The algorithm is based on bi-axial sensors installed along the tendon and top tension-meter. Then, the tension and bending moment along the line can be estimated by using the developed algorithm. For bending moment, a generalized-coordinate-based FEA (Finite Element Analysis) formulas with cubic interpolation are derived. For tension, analytical formulas with small angle assumption are derived. Hull-mooring-riser fully coupled software is used as a simulation tool to generate the corresponding numerical-sensor signals. To validate the developed algorithms, several case studies are conducted, considering variations in sensor spacing, line-failure detection, different boundary conditions, and ocean-storm conditions. The results show that the proposed algorithms can monitor the actual variations of tensions and bending moments (or axial and bending stresses) along the tendon with high accuracy, which means that the real-time monitoring of accumulated fatigue damage at hot-spots is also possible. The developed algorithm can also detect various line-failure scenarios by observing the change of stress signals. Its capability of real-time stress tracing in 10-yr and 100-yr storm conditions is also demonstrated. [ABSTRACT FROM AUTHOR]

In this study, a floating barge moon pool platform model equipped with an NREL 5 MW wind turbine and mooring systems was numerically and experimentally tested under wave and wind loads to analyze the hydrodynamic performance and stability of the barge platform and the mooring system in a 50 m water depth condition. The numerical model was based on ANSYS AQWA, and this hydrodynamic model included nonlinear hydrostatic and Froude–Krylov forces, diffraction/radiation forces obtained from the linear potential theory, and Morison forces to consider viscous effects on the heave plates. The experiment model was a 1:64 scaled barge platform with an NREL 5 MW wind turbine scaled in the same ratio. Both full coupling platform were simulated under three conditions, namely free decay test, regular wave test, and irregular wave test, including a typhoon extreme case, with wind operation and parking. First, the hydrodynamic model was calibrated against the free decay test results. Consequently, a good agreement was achieved by calibrating the mooring properties of the experiment model. A comparison of the regular wave cases was then performed, with the same trend of results observed for the surge, heave, and pitch motions of the floating barge system. Finally, irregular model tests, which aimed to measure the dynamics of the barge platform and the mooring system in a 50-year sea state in Taiwan offshore, were conducted. The motion responses of the barge platform and the mooring tensions were recorded and compared with full coupled dynamic analysis results. The DNV GL standards were used to check the reliability of the mooring system and the stability of the barge platform. • The dynamic responses of a floating barge platform model with NREL 5 MW turbine were studied numerically and experimentally. • Two key technologies were applied in this study: the scaled wind turbine model, and the shallow water mooring system design. • The study covered three types of simulation: free decay test, regular wave test, irregular wave test with wind operation, and parking. • In irregular tests, to focus on the dynamics of barge platform and mooring system tension in Taiwan offshore condition. [ABSTRACT FROM AUTHOR]

The studies on floating offshore wind turbines have been increasing over recent decades due to the growing investments of industries in marine renewable energy. Compared with horizontal-axis wind turbines, research on vertical-axis wind turbines is relatively few, though the latter may have advantages over the former in some aspects, for example, potentially lower cost of energy. In this paper, the characteristics of the motion responses of a barge-type floating system with four moonpools and a vertical-axis wind turbine are experimentally and numerically investigated. The focus is on the gyroscopic effects of turbine rotations on the motion responses of the floating system under different wave conditions. Physical model tests are conducted in a wave tank, where a 2-MW-class turbine is modelled in the 1:100 scale. It is found that the first-order sway and roll motion amplitudes near the resonant frequencies are reduced by the turbine rotations, whereas the first-order heave motion of the floating system is hardly influenced. The second-order motion responses and mooring tether tensions may be significantly amplified with the increasing rotating speed of turbine. The maximum ratio of the second-order response amplitude of roll motion to the first-order roll amplitude is up to 17% in the experiments of this study. A radiation and diffraction code based on linear potential flow theory is used to predict the motion responses and mooring forces. The comparisons with experimental data suggest that the viscous damping of water resonance in moonpools and the gyroscopic effects of turbine rotations should be taken into account in predictions. • Motion characteristics of a barge-type floating vertical-axis wind turbine are studied through 1:100 scaled model tests. • Gyroscopic effects of turbine rotations on the first- and second- order motion responses are demonstrated. • Feasibility of linear potential flow theory in predicting motion responses of a floating VAWT with moonpools is evaluated. [ABSTRACT FROM AUTHOR]

This paper aims at investigating the effect of water depth increase on the global performance of a floating offshore wind turbine, with a special focus on the environmental loading effects and turbine operating status. An integrated aero-hydro-servo-elastic (AHSE) analysis was simulated in the time domain. The model was first validated against published results in terms of mooring system restoring force and platform natural frequencies. The considered water depth is between 200 and 300 m, which is the deep-water range used in the current floating offshore wind turbine (FOWT) industry. In this study, both normal operating and failure conditions were considered. Key conclusions from case studies indicated that, based on the current water depth range, platform heave motion with slack mooring configurations and mooring line top tension are more sensitive to water depth. Water depth increase influences the tower base bending force when the turbine has a high-speed shaft brake due to grid loss, but the effects are restricted to the high-frequency response range (>2 Hz) and less obvious than the influences on mooring lines. • Water depth effects for wind turbines are quantified considering both slack and taut moorings. • The developed model has been validated and verified against test data and numerical results. • Water depth increase effects are more apparent for the heave motion of slack moorings. • Slack mooring lines have smaller damage equivalent loads than taut mooring lines. [ABSTRACT FROM AUTHOR]

Accurate estimation of the dynamic behavior of Floating Offshore Wind Turbines (FOWTs) under typhoon environment is essential to design and install FOWTs in a prone area of typhoons such as around Japan. Up to now, extensive efforts for development of design tools for FOWTs have been made, and nowadays several design tools are available. Needless to say, it is of utmost importance that the engineering design tools have been verified and validated for the real physical phenomena before being applied to real designs/projects with confidence. Much efforts have thus been made as code-to-code comparisons and by validations using model test data. Validations of the design tools using full-scale field data for FOWTs have also been made, but available open literatures are quite limited, particularly for design-driving extreme cases. In this paper, we describe the analysis of the dynamic response of a 2-MW spar-type FOWT at the time of typhoon attack in the actual sea area. The central atmospheric pressure of the typhoon at the closest time was 965 hPa, the maximum instantaneous wind speed at the hub height was 52.2 m/s, and the maximum significant wave height was 6.9 m. The dynamic responses under the typhoon environment are numerically simulated by using the time-series records of the wind speed, the wind direction, the wave height, the wave direction, the current speed, and the current direction which were acquired during the typhoon passing through close to the FOWT. Then the simulated motion responses are compared with the measured motion responses for the same durations. By the comparisons, the numerical simulation tool which was used for the design of the FOWT has been partially validated. It has also been confirmed that the spatial coherence of the wind speed has a significant effect for the platform motions, particularly for yaw motion. • Dynamic response of a spar-type floating wind turbine during typhoon attack is presented here. • In-site measured values are compared with numerical simulations. • Basically, good agreement is observed except for yaw motion. • The spatial coherence of the wind speeds has a significant effect for the platform motions, particularly for yaw motion. [ABSTRACT FROM AUTHOR]

In this paper, the dynamic responses of a Spar-type floating offshore wind turbine (FOWT) are simulated and analyzed under the scenarios with freak wave superimposed a uniform current. The wave-current interaction is considered in the process of freak wave generation. An improved phase modulation algorithm is used to generate the free surface profile of the freak wave. Compared with the deterministic wave modeling method, this method can realize the randomness of the sea state and also achieve high efficiency. The effect of the presence of current on the wave energy spectra is firstly investigated. Then the OC3 Hywind Spar-type FOWT in the parked condition is chosen as an example to study the dynamic responses under the extreme wave scenarios. An in-house MATLAB code based on a nonlinear aero-hydro-mooring coupled model is adopted to perform these numerical investigations. The simulations are conducted in the time domain and then the results of wave forces and platform motions in the surge, heave and pitch are summarized and analyzed considering the presence of current with different velocities. The results of mooring lines tension and the acceleration at nacelle are also investigated. Through the simulation results, we can have a clear understanding of the mechanism of the wave-current interaction phenomenon for extremely high waves in Gaussian seas. Furthermore, it also allows identifying the severest wave conditions which are important for the analysis of the floating structure ultimate design conditions. [ABSTRACT FROM AUTHOR]

As offshore wind turbines are moving to deeper water depths, mooring systems are becoming more and more significant for floating offshore wind turbines (FOWTs). Mooring line failures could affect power generations of FOWTs and ultimately incur risk to nearby structures. Among different failure mechanics, an excessive mooring line tension is one of the most essential factors contributing to mooring failure. Even advanced sensing offers an effective way of failure detections, but it is still difficult to comprehend why failures happened. Unlike traditional parametric studies that are computational and time-intensive, this paper applies deep learning to investigate the major driven force on the mooring line tension. A number of environmental conditions are considered, ranging from cut in to cut out wind speeds. Before formatting input data into the deep learning model, a FOWT model of dynamics was simulated under pre-defined environmental conditions. Both taut and slack mooring configurations were considered in the current study. Results showed that the most loaded mooring line tension was mainly determined by the surge motion, regardless of mooring line configurations, while the blade and the tower elasticity were less significant in predicting mooring line tension. [ABSTRACT FROM AUTHOR]

The mooring loading of floating offshore wind turbines are mainly induced by the platform motions through the fairleads. The dynamic analysis usually only considers the hydrodynamic forces over the mooring by the motion of the line disregarding the wave loads. Using the FloaWDyn aero-servo-hydro-elastic model, the influence of the wave hydrodynamic loads over the catenary mooring system of the DeepCWind and the OC3 Hywind platforms is assessed by comparing simulations for different sea states with and without the wave forces. The results show a variation in the tensions of the fairleads by an increment of the maximum, a decrement of the minimum and an increment of the standard deviation for the analyzed cases. The widening of the tension range of the line affects the fatigue resistance of the line by increasing 5% the damage in the most loaded line in a sever sea state. • Wave hydrodynamic forces over mooring lines on Floating Offshore Wind Turbines: • Standard deviation of line tension is 5.5% higher after including wave forces on lines. • The wave hydrodynamics on mooring lines increases the tension range. • The increase of the tension range reduces the fatigue life of the lines. [ABSTRACT FROM AUTHOR]

The semi-submersible floating offshore wind turbine has been studied in detail due to its good stability. However, the occurrence of typhoons are very frequent in China's offshore area, putting forward a higher requirement for the stability of the floating wind turbine system. By changing the connection mode of the mooring line as well as the structural form of the platform based on the original OC4 model, two groups of models were examined by an in-house developed code named as the Analysis Tool of Floating Wind Turbine (AFWT). The influence of the arrangement of the mooring lines and the inclination angle of the upper columns on the motion response were clarified. It was found that the surge motion of the platform would be obviously decreased by decreasing the length of the upper segments of the mooring lines, while the heave motion of the platform would be significantly decreased as increasing the inclined angle of the columns. Therefore, a new model integrating the optimized multi-segmented mooring lines and the optimized inclined columns was proposed. The examinations showed that compared with the response motions of the original OC4 semi-submersible model, the proposed model could reduce both the surge and heave motions of the platform effectively. [ABSTRACT FROM AUTHOR]