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This paper aims to analyze the levelized cost of energy (LCOE) of floating offshore wind farms from a probabilistic point of view. Understanding and addressing the uncertainty associated with the main parameters that influence the wind energy generated by a wind farm during its lifetime is crucial for the economic evaluation of offshore wind energy in the broader energy landscape. The methodology for probabilistic assessment of LCOE is introduced, and the uncertainty in input parameters are discussed. In a base case study, an assumed Floating Offshore Wind Farm (FOWT) consisting of 250 5-MW wind turbines is considered. The use of bias and randomness in key random variables is discussed and studied in detail. Results indicate that LCOE estimates of 15 EURc/kWh for offshore wind turbines are achievable with reasonable confidence, while estimates of 5 EURc/kWh require careful consideration of uncertainty in the wind farm's parameters. The feasibility analysis showed that techno-economic parameters are more influenced by wind characteristics and efficient use of wind turbines than by the cost of the wind farm. This paper provides general guidance on how to carry out early-stage techno-economic analysis of FOWFs. [ABSTRACT FROM AUTHOR]

This paper describes a Long Short-Term Memory (LSTM) neural network model used to simulate the dynamics of the OC3 reference design of a Floating Offshore Wind Turbine (FOWT) spar unit. It crafts an advanced neural network with an encoder–decoder architecture capable of predicting the spar's motion and fairlead tensions time series. These predictions are based on wind and wave excitations across various operational and extreme conditions. The LSTM network, trained on an extensive dataset from over 300 fully coupled simulation scenarios using OpenFAST, ensures a robust framework that captures the complex dynamics of a floating platform under diverse environmental scenarios. This framework's effectiveness is further verified by thoroughly evaluating the model's performance, leveraging comparative statistics and accuracy assessments to highlight its reliability. This methodology contributes to substantial reductions in computational time. While this research provides insights that facilitate the design process of offshore wind turbines, its primary aim is to introduce a new predictive approach, marking a step forward in the quest for more efficient and dependable renewable energy solutions. [ABSTRACT FROM AUTHOR]

Abstract This paper aims to analyze the levelized cost of energy (LCOE) of floating offshore wind farms from a probabilistic point of view. Understanding and addressing the uncertainty associated with the main parameters that influence the wind energy generated by a wind farm during its lifetime is crucial for the economic evaluation of offshore wind energy in the broader energy landscape. The methodology for probabilistic assessment of LCOE is introduced, and the uncertainty in input parameters are discussed. In a base case study, an assumed Floating Offshore Wind Farm (FOWT) consisting of 250 5-MW wind turbines is considered. The use of bias and randomness in key random variables is discussed and studied in detail. Results indicate that LCOE estimates of 15 EURc/kWh for offshore wind turbines are achievable with reasonable confidence, while estimates of 5 EURc/kWh require careful consideration of uncertainty in the wind farm’s parameters. The feasibility analysis showed that techno-economic parameters are more influenced by wind characteristics and efficient use of wind turbines than by the cost of the wind farm. This paper provides general guidance on how to carry out early-stage techno-economic analysis of FOWFs.

Among the extreme sea scenarios, freak waves pose a serious threat to offshore structures, potentially leading to structural failure, such as mooring line failure, floater capsizing, or structural damage. In this study, we conducted a numerical investigation on the transient performance of a semi-submersible floating offshore wind turbine (FOWT) equipped with a redundant mooring system under the influence of freak waves and mooring failure. Firstly, we analyzed the dynamic responses of an intact-mooring-system FOWT under a freak wave. Next, we examined the effect of mooring failure on the transient responses. The results indicate that floater motions exhibit significant differences in the interval of freak wave crests. The impact of freak waves increases the blade tip deformation and tower root bending moment, while also affecting the tension of the mooring line and the aerodynamic performance of the wind turbine. Consecutive fracture with an interval of 20 s significantly increases surge motion and reduces output power. When mooring lines break separately with an interval of 400 s, the amplification in the responses is noticeably lower compared to consecutive fracture cases. [ABSTRACT FROM AUTHOR]

Extreme waves pose one of the major threats to marine structures. Furthermore, their non-stationary nature makes proper stochastic analysis of their responses a challenging problem. To address this issue, this paper proposes a method based on a novel linear filter wave model incorporated into the dimension-reduced probability density evolution equation (DR-PDEE). The linear filter system is capable of simulating random background waves conforming with the Joint North Sea Wave Project (JONSWAP) spectrum of any arbitrary sea state by adjusting the parameters of filters directly related to the parameters of the JONSWAP spectrum without reidentification. In particular, by conducting the digital filtering, wave kinematics at different depths below the sea surface can be reproduced conveniently, and therefore only one filter is adequate for the depthwise wave kinematics field. Extreme ocean waves are treated as the superposition of background waves and extreme crests according to the constrained quasi-determinism method, with randomness from both parts. Incorporating the filter into the equation of motion of the offshore structure of interest leads to an augmented high-dimensional stochastic system with multiple random variables. The DR-PDEE then is employed to reduce the dimensions of the equation governing the evolution of probability density of responses of the original complex system to two. Solving the DR-PDEE using the path integral method yields the probability function of the response at each time step. A numerical example involving the response of a National Renewable Energy Laboratory (NREL) 5-MW spar-type floating offshore wind turbine (FOWT) subjected to extreme waves was studied to assess the reliability of the proposed method. The method provides an effective tool for the determination of the stochastic extreme response of offshore structures, and provides a foundation for further dynamic analyses. [ABSTRACT FROM AUTHOR]

A numerical investigation of floating-type substructures for wind turbine generators was conducted by using time-domain simulation. A Tri-Star floater for 8–10 MW generators, which was developed by Samsung Heavy Industries (SHI), was chosen as the floating substructure. To make the anchor system, catenary mooring lines, considering redundancy, were installed on the floater. The main sources of external force on the wind turbine generator are wind, waves, and currents. To consider severe environmental conditions, Design Load Cases (DLCs) 1.6 and 6.1 of the IEC guidelines (IEC 61400-3-1) were chosen. From the measured environmental data for the installation site, the main parameters for the simulation conditions were obtained. The tilt angle and horizontal movement of the floater and the mooring tension for the different mooring systems were checked. The response of the floater during the failure of the mooring was also studied, and the critical failure of the mooring was confirmed. During the failure of the mooring, the redundancy system worked well, in which the movement of the floater was constrained within the criteria for all scenarios. [ABSTRACT FROM AUTHOR]

This paper describes a Long Short-Term Memory (LSTM) neural network model used to simulate the dynamics of the OC3 reference design of a Floating Offshore Wind Turbine (FOWT) spar unit. It crafts an advanced neural network with an encoder–decoder architecture capable of predicting the spar’s motion and fairlead tensions time series. These predictions are based on wind and wave excitations across various operational and extreme conditions. The LSTM network, trained on an extensive dataset from over 300 fully coupled simulation scenarios using OpenFAST, ensures a robust framework that captures the complex dynamics of a floating platform under diverse environmental scenarios. This framework’s effectiveness is further verified by thoroughly evaluating the model’s performance, leveraging comparative statistics and accuracy assessments to highlight its reliability. This methodology contributes to substantial reductions in computational time. While this research provides insights that facilitate the design process of offshore wind turbines, its primary aim is to introduce a new predictive approach, marking a step forward in the quest for more efficient and dependable renewable energy solutions.

Among the extreme sea scenarios, freak waves pose a serious threat to offshore structures, potentially leading to structural failure, such as mooring line failure, floater capsizing, or structural damage. In this study, we conducted a numerical investigation on the transient performance of a semi-submersible floating offshore wind turbine (FOWT) equipped with a redundant mooring system under the influence of freak waves and mooring failure. Firstly, we analyzed the dynamic responses of an intact-mooring-system FOWT under a freak wave. Next, we examined the effect of mooring failure on the transient responses. The results indicate that floater motions exhibit significant differences in the interval of freak wave crests. The impact of freak waves increases the blade tip deformation and tower root bending moment, while also affecting the tension of the mooring line and the aerodynamic performance of the wind turbine. Consecutive fracture with an interval of 20 s significantly increases surge motion and reduces output power. When mooring lines break separately with an interval of 400 s, the amplification in the responses is noticeably lower compared to consecutive fracture cases.

The development of floating offshore wind turbines (FOWTs) is gradually moving into deeper offshore areas with more harsh environmental loads, and the corresponding structure response should be paid attention to. Safety assessments need to be conducted based on the evaluation of the long-term extreme response under operating conditions. However, the full long-term analysis method (FLTA) recommended by the design code for evaluating extreme response statistics requires significant computational costs. In the present study, a power response prediction method for FOWT based on an artificial neural network algorithm is proposed. FOWT size, structure, and training algorithms from various artificial neural network models to determine optimal network parameters are investigated. A publicly available, high-quality operational dataset is used and processed by the Inverse First Order Reliability Method (IFORM), which significantly reduces simulation time by selecting operating conditions and directly yielding extreme response statistics. Then sensitivity analysis is done regarding the number of neurons and validation check values. Finally, the alternative dataset is used to validate the model. Results show that the proposed neural network model is able to accurately predict the extreme response statistics of FOWT under realistic in situ operating conditions. A proper balance was achieved between prediction accuracy, computational costs, and the robustness of the model. [ABSTRACT FROM AUTHOR]

Vertical-axis wind turbines (VAWTs) have a long history, with a wide variety of turbine archetypes that have been designed and tested since the 1970s. While few utility-scale VAWTs currently exist, the placement of the generator near the turbine base could make VAWTs advantageous over tradition horizontal-axis wind turbines for floating offshore wind applications via reduced platform costs and improved scaling potential. However, there are currently few numerical design and analysis tools available for VAWTs. One existing engineering toolset for aero-hydro-servo-elastic simulation of VAWTs is the Offshore Wind ENergy Simulator (OWENS), but its current modeling capability for floating systems is non-standard and not ideal. This article describes how OWENS has been coupled to several OpenFAST modules to update and improve modeling of floating offshore VAWTs and discusses the verification of these new capabilities and features. The results of the coupled OWENS verification test agree well with a parallel OpenFAST simulation, validating the new modeling and simulation capabilities in OWENS for floating VAWT applications. These developments will enable the design and optimization of floating offshore VAWTs in the future. [ABSTRACT FROM AUTHOR]

A numerical investigation of floating-type substructures for wind turbine generators was conducted by using time-domain simulation. A Tri-Star floater for 8–10 MW generators, which was developed by Samsung Heavy Industries (SHI), was chosen as the floating substructure. To make the anchor system, catenary mooring lines, considering redundancy, were installed on the floater. The main sources of external force on the wind turbine generator are wind, waves, and currents. To consider severe environmental conditions, Design Load Cases (DLCs) 1.6 and 6.1 of the IEC guidelines (IEC 61400-3-1) were chosen. From the measured environmental data for the installation site, the main parameters for the simulation conditions were obtained. The tilt angle and horizontal movement of the floater and the mooring tension for the different mooring systems were checked. The response of the floater during the failure of the mooring was also studied, and the critical failure of the mooring was confirmed. During the failure of the mooring, the redundancy system worked well, in which the movement of the floater was constrained within the criteria for all scenarios.

The potential sites for floating offshore wind turbines (FOWTs) in Taiwan are primarily located in water depths ranging from 50 m to 100 m, where the mooring system design is the major challenge that needs to be overcome. In this study, a planned test site at a water depth of 70 m is selected as the case study. The aim is to evaluate the engineering feasibility and investigate possible conflicts in navigational safety due to mooring failure. To address this, fully coupled Aero-Servo-Hydro-Mooring simulations for a 15 MW FOWT under operating and parked conditions have been performed. Two common types of semi-taut mooring configurations, including chain-polyester-chain and chain-nylon-chain, are investigated, and the numerical results are compared against those of full chain-catenary mooring. The study includes several scenarios: setting environmental conditions with limited observation data, validating simulation tools, examining catenary and semi-taut mooring configurations, ensuring compliance with offshore standards, analyzing the effects of mooring line failure, and conducting a sensitivity analysis of rope length in semi-taut moorings. The findings show that the mooring failure scenario can result in a significant drift distance and a substantial drop in power generation. This can pose issues for both ocean space utilization and the stability of the power conversion system. Overall, the process of this study can be useful in the site selection of evaluating a potential site for the FOWT at different water depths. • Fully-coupled Aero-Servo-Hydro-Mooring simulations for a 15 MW FOWT at an intermediate water depth of 70 m have been performed. • Two common types of semi-taut mooring configurations, including chain-polyester-chain and chain-nylon-chain, are investigated. • The environmental conditions for the FOWT under operating and parked conditions are considered. • The mooring load, motion response, and power generation for different mooring configurations are studied. • The effects of one-line failure and rope length of synthetic rope on the FOWT are discussed. [ABSTRACT FROM AUTHOR]

A robust numerical tool is presented for simulating the hydrodynamic responses of a semi-submersible floating offshore wind turbine (FOWT) platform under highly nonlinear extreme/irregular wave conditions. The investigation covers both short-duration extreme event and long-duration 3-hour irregular wave scenarios, utilizing a coupled CFD-FEM method integrated with an iterative wave adjustment procedure. The discretization uncertainties of the numerical scheme are rigorously quantified under extreme wave conditions. Comprehensive analysis is conducted on both short and long-duration cases. This study underscores the significant influence of waves on platform hydrodynamic responses, which emphasizes the necessity of accurately replicating the target wave in the numerical wave basin. The importance of the wave calibration method is demonstrated, particularly for capturing low-frequency pitch motion of the platform. • Utilize a coupled CFD-FEM method to predict the response of a moored FOWT platform under short-duration extreme and long-duration irregular wave conditions. • Employ an iterative wave adjustment method in the CFD wave tank to calibrate the incident wave spectrum to the target wave, facilitating accurate predictions of floater motion. • Quantify discretization uncertainty in time and grid refinement under extreme wave conditions, enhancing the robustness of the numerical simulations. • Validate the coupled CFD-FEM solver against model test measurements. • Evaluate the solver's performance under highly nonlinear wave conditions by comparing short-duration and long-duration simulations. [ABSTRACT FROM AUTHOR]

There are various kind of waves in the actual ocean. Changes on freak wave profiles affect the motion of floating offshore wind turbines (FOWTs) and threaten the operation safety. To investigate this issue, the phase modulation method is used to simulate freak waves in our work. The original freak wave is first generated, then the wave profile is modulated to obtain five other types of freak waves, such as the reversed wave, the high-crested wave, the deep-troughed wave, the fluctuated wave, the elevated wave. A coupled time-domain model of a Spar-buoy FOWT is established to simulate the motion under steady wind and various freak waves. The dynamic responses are analyzed in the time domain, including the three-degree-of-freedom (3-DOF) motions of the floating foundation, the tension of mooring line and the nacelle acceleration. The frequency-domain properties under various freak waves are investigated using wavelet analysis. The effects of various wave profiles are compared with the original freak wave. Freak waves in low sea states are also generated to research their effects on pitch motion in different sea states. Results show that the response peaks increase in the impact region under freak waves. The reversed wave increases the maximum value of the 3-DOF motions, especially in pitch motion, and causes the high-frequency energy core of wavelet plots to move. The dynamic responses under the fluctuated wave combines the properties of increased response peaks at high-crested wave and increased amplitude of low-frequency motions at deep-troughed wave. The elevated wave greatly increases the heave motion of the FOWT. The increase in sea state level causes a nonlinear amplification in the transient oscillation. [Display omitted] • Motion characteristics on a Spar-buoy FOWT under various freak waves are studied and compared with original freak wave. • Various freak waves include reversed wave, high-crested wave, deep-troughed wave, fluctuated wave, and elevated wave. • The reversed wave increases the 3-DOF motions and causes the high-frequency energy core of wavelet plots to move. • The dynamic responses under the fluctuated wave combines the properties of high-crested wave and deep-troughed wave. • The increase in sea state level causes a nonlinear amplification in the transient oscillation. [ABSTRACT FROM AUTHOR]

The floating platform-mooring system-pile-soil interactions under wave loadings represent a complex yet understudied aspect of floating offshore wind turbines (FOWTs). This paper presents the development and application of an Incompressible Material Point Method-Finite Element Method-Material Point Method (IMPM-FEM-MPM) model to simulate these interactions for a triangle-shaped semi-submersible platform (Tri-SSP), which serves as the foundation of China's first deep-sea FOWT, named 'Fuyao.' The proposed model is validated by studying the hydrodynamic responses of the OC4-DeepCwind platform. Under regular waves, increasing wave heights lead to heightened motion amplitudes, with the Tri-SSP exhibiting distinct phase differences between surge, pitch, and heave motions. Anchor Group 1 experiences significant peak tensions, ranging from 2.26 MN to 4.47 MN, which influence soil displacement, shear forces and bending moments along the pile shaft, emphasizing the impact of wave loading on pile-soil interactions. Under 1-h irregular waves, tension in Anchor Group 1 reaches extreme levels, with tensions up to 8 MN observed, underscoring the critical role of mooring systems in mitigating wave-induced forces. The calculated maximum stress within the pile indicates the structural integrity of the DH36 steel pile under these conditions. Comparisons of hydrodynamic responses, both with and without pile-soil interactions, show minimal discrepancies in motion amplitudes, indicating a minor influence on the dynamics of the platform and mooring system. These findings contribute to the understanding of complex interactions in deep-sea FOWTs, informing design and operational strategies for enhanced stability and performance. • The IMPM-FEM-MPM model properly evaluates platform and mooring hydrodynamics. • Hydrodynamic responses of a triangle-shaped semi-submersible platform are examined. • The wave-floating platform-mooring system-pile-soil interactions are analyzed. • The deflection and stability of piles are greatly influenced by the mooring system. • Impacts of pile-soil interactions on platform and mooring hydrodynamics are minor. [ABSTRACT FROM AUTHOR]

The development of floating offshore wind turbines (FOWTs) is gradually moving into deeper offshore areas with more harsh environmental loads, and the corresponding structure response should be paid attention to. Safety assessments need to be conducted based on the evaluation of the long-term extreme response under operating conditions. However, the full long-term analysis method (FLTA) recommended by the design code for evaluating extreme response statistics requires significant computational costs. In the present study, a power response prediction method for FOWT based on an artificial neural network algorithm is proposed. FOWT size, structure, and training algorithms from various artificial neural network models to determine optimal network parameters are investigated. A publicly available, high-quality operational dataset is used and processed by the Inverse First Order Reliability Method (IFORM), which significantly reduces simulation time by selecting operating conditions and directly yielding extreme response statistics. Then sensitivity analysis is done regarding the number of neurons and validation check values. Finally, the alternative dataset is used to validate the model. Results show that the proposed neural network model is able to accurately predict the extreme response statistics of FOWT under realistic in situ operating conditions. A proper balance was achieved between prediction accuracy, computational costs, and the robustness of the model.

Three models with different footing types were used to clarify the effect of heave plates on the hydrodynamic behavior of the elastic response of a flexible multicolumn floating offshore wind turbine (FOWT). The models were tested under regular waves, whose added mass, damping, and motion response results were compared with numerical simulations by NK-UTWind and WAMIT codes. As a whole, the attachment of heave plates was responsible for increasing the added mass and damping levels, consequently modifying the RAO of the models. Regarding the response in a sea condition, a decrease of 33% and 66% of the significant motion height (heave and pitch) was observed. Thus, the heave plate can be a good feature for the future design of FOWT. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines (FOWTs) work in a complex natural environment. Under the coupling effect of wind and waves, the platform experiences a six-degree-of-freedom motion, which affects the performance of the wind turbine. In this paper, a computational fluid dynamic method is used to investigate the effect of platform pitch and surge motion coupled at the same frequency as well as at different frequencies with an initial phase difference on the aerodynamic characteristics of FOWTs. The results demonstrate that the platform pitch and surge motion coupling makes the wind turbine operation more unstable. The power and thrust fluctuations are the largest when the two motions are coupled in the same phase, which leads to a dramatic change in the aerodynamic performance of the wind turbine during operation, and can easily cause hazards such as blade fatigue damage. When the initial phase difference does not affect the coupling motion frequency, the effect on the instantaneous power is more significant than that on the instantaneous thrust. However, the effect on the average power and thrust values is weaker. When the initial phase difference leads to reverse coupling of the platform pitch and surge motions, the fluctuation of power and thrust is reduced, and the wind turbine operation is more stable and safer. [ABSTRACT FROM AUTHOR]

The rotor of floating offshore wind turbines with platform motions may undergo different working states during its operation, e.g. from windmill working state to vortex ring and propeller working state. In this paper, an aerodynamic model based on a free wake vortex method is used to simulate the rotor undergoing surge motion. The associated change of working states of the rotor is evaluated quantitatively and visually. The results show that during a full cycle of the surge motion of the floating platform, the rotor experiences alternative onset of the windmill state, vortex ring state, and propeller state, while the later two occur only during the downwind motion of the rotor. The aerodynamic load change corresponding to different working states of the rotor indicates that the vortex ring state is the most unstable phase of the three. [ABSTRACT FROM AUTHOR]

The motivation for this study is the fast development of floating offshore wind energy and the immature methodology and engineering practice related to predictions of vortex-induced motions (VIM). Benefiting from the oil and gas industry, in the past several decades, extensive knowledge and experience on vortex-induced vibrations (VIV) on slender marine structures has been gained. As the learnings from these efforts should be transferred and adapted to the renewable energy industry, a state-of-the-art review on influential VIM research has been carried out in this paper, focusing on: (1) engineering practice, (2) model tests, (3) numerical calculation, and (4) field measurement. Engineering gaps and potential research topics are identified as future work. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines (FOWT) generate green renewable energy and are a vital part of the modern offshore wind energy industry. Robust predicting extreme offshore loads during FOWT operations is an important safety concern. Excessive structural bending moments may occur during certain sea conditions, posing an operational risk of structural damage. This paper uses the FAST code to analyze offshore wind turbine structural loads due to environmental loads acting on a specific FOWT under actual local environmental conditions. The work proposes a unique Gaidai-Fu-Xing structural reliability approach that is probably best suited for multi-dimensional structural responses that have been simulated or measured over a long period to produce relatively large ergodic time series. In the context of numerical simulation, unlike existing reliability approaches, the novel methodology does not need to re-start simulation again each time the system fails. As shown in this work, an accurate forecast of the probability of system failure can be made using measured structural response. Furthermore, traditional reliability techniques cannot effectively deal with large dimensionality systems and cross-correction across multiple dimensions. The paper aims to establish a state-of-the-art method for extracting essential information concerning extreme responses of the FOWT through simulated time-history data. Three key components of structural loads are analyzed, including the blade-root out-of-plane bending moment, tower fore-aft bending moment, and mooring line tension. The approach suggested in this study allows predicting failure probability efficiently for a non-linear multi-dimensional dynamic system as a whole.

The offshore wind power exploitation has experienced rapid development in recent years and has gradually moved into deeper waters with the floating wind turbine technology getting mature. Due to the strong concurrence of the wind and wave power in offshore sites, the idea of combined utilization of wind and wave power by one integrated device has attracted tremendous interests worldwide and a number of concepts and designs have been proposed. This article describes a novel integrated floating wind-wave generation platform (FWWP) consisting of a DeepCwind semi-submersible floating offshore wind turbine (FOWT) and a point absorber wave energy convertor (PAWEC). Three models including the single PAWEC, single FOWT, and FWWP are considered to investigate the feasibility of the FWWP and its advantages over the single device. Hydrodynamic analyses are first conducted using the potential flow code AQWA with the viscous correction to investigate the hydrodynamic interactions effect of the integrated model. Then, a fully coupled model for the FWWP is established by calling OpenFAST in AQWA using the F2A method. The accuracy of the established coupled model is firstly validated with OpenFAST for analysing the dynamics of the single FOWT. Finally, fully coupled analyses of the FWWP are carried out for both regular and irregular waves in the operational sea-states. The coupled dynamics and wind and wave power generation of the FWWP are compared with those of the single PAWEC and FOWT for both the regular and irregular waves.

Floating offshore wind turbines (FOWTs) offer a significant opportunity for harnessing wind energy and achieving clean energy goals. This paper investigates the effects of mooring line failure in semisubmersible FOWTs under normal and extreme environmental conditions. The OC4 DeepCwind type semisubmersible with an NREL 5 MW wind turbine is considered for investigating the behavior of FOWT under intact and failed mooring lines. This study covers a wide range of environmental conditions, operational conditions of wind turbines, and mooring line failure scenarios. This paper investigates the mooring line failure effects on platform responses, mooring line fairlead tensions, and tower responses. The results show that the platform surge, heave, pitch responses, tower fore-aft displacement, base shear force in X -direction, and base moment in Y -direction are increased for upwind side mooring line failure during the operational condition of FOWT. Subsequently, the platform sway, roll, yaw responses, tower side-to-side displacement, base shear force in Y -direction, and base moment in X -direction are more during downwind side mooring line failure in the same environment. On the other hand, the impact of upwind side mooring line failure affects all 6 Degrees of Freedom (DOF) platform responses significantly during the parked state of FOWT. The maximum fairlead tensions observed in the remaining mooring lines remain below the mooring line breaking strength even after a mooring line failure during both operational and parked states of FOWT. It is also observed that the platform responses (excluding heave), tower responses, and fairlead tensions exhibit greater values when an extreme turbulence wind model is employed. The platform surge, heave, pitch responses, maximum fairlead tension, tower fore-aft displacement, and tower base moment in Y -direction show higher values under severe sea state condition. • Effects of mooring line failure under normal and extreme environmental conditions • Simulation of intact and mooring line failure scenarios using coupled analysis • Covering wide range of environmental conditions and mooring line failure scenarios • Effects of failure on platform responses, fairlead tensions, and tower responses [ABSTRACT FROM AUTHOR]

This study focuses on analyzing the load response prediction error of a potential-based tool for a Floating Offshore Wind Turbine (FOWT) with a rectangular pontoon connected to the lower part of the column. The system response was reviewed under extreme environmental loads, and CFD analysis results were compared to analyze the response prediction error contributed by each load component. Based on the CFD free-decay simulation results, we performed potential-based tool calibration by applying damping coefficient and Morison drag coefficient in a composite manner. Through coefficient calibration, the response in the free-decay analysis matched well with the CFD, but there was a significant difference from CFD in the integrated load analysis. Through this study, it was clearly shown that applying an appropriate drag coefficient is very important to implement drag force, especially wave drag force, for FOWT where columns are connected by pontoons. It was confirmed that initial calibration of the potential-based tool using only free-decay analysis is not an appropriate method. However, by applying damping coefficient and Morison drag coefficient in a composite manner when calibrating the potential-based tool based on the free-decay analysis results, it was possible to improve the error by reflecting the current load, and it is expected that the error in the response prediction of the potential-based tool can be further improved through appropriate composite application. • Identifying significant discrepancies in integrated load analysis between CFD and potential-based tools. • Key to FOWT accuracy: applying correct drag coefficients for wave forces."Highlighting the limitations of calibrating potential-based tools solely with free-decay analysis results.A method combining damping and Morison coefficients enhances FOWT tool accuracy. • Highlighting the limitations of calibrating potential-based tools solely with free-decay analysis results. • A method combining damping and Morison coefficients enhances FOWT tool accuracy. [ABSTRACT FROM AUTHOR]

Vertical-axis wind turbines (VAWTs) have a long history, with a wide variety of turbine archetypes that have been designed and tested since the 1970s. While few utility-scale VAWTs currently exist, the placement of the generator near the turbine base could make VAWTs advantageous over tradition horizontal-axis wind turbines for floating offshore wind applications via reduced platform costs and improved scaling potential. However, there are currently few numerical design and analysis tools available for VAWTs. One existing engineering toolset for aero-hydro-servo-elastic simulation of VAWTs is the Offshore Wind ENergy Simulator (OWENS), but its current modeling capability for floating systems is non-standard and not ideal. This article describes how OWENS has been coupled to several OpenFAST modules to update and improve modeling of floating offshore VAWTs and discusses the verification of these new capabilities and features. The results of the coupled OWENS verification test agree well with a parallel OpenFAST simulation, validating the new modeling and simulation capabilities in OWENS for floating VAWT applications. These developments will enable the design and optimization of floating offshore VAWTs in the future.

In our previous study, the vortex ring state (VRS) prediction criteria were introduced from helicopter's realm and applied to floating offshore wind turbines (FOWTs). The existence of the VRS on FOWTs was also successfully predicted. However, the prediction criteria we used have not been verified by comparing them with similar studies because of the lack of reference publications — until recently. In this paper, a comparative analysis of the VRS phenomenon of an FOWT is done and aerodynamic performance of the FOWT is evaluated. We compare the VRS results predicted based on the criteria we proposed with a new study about the VRS by means of a computational fluid dynamics (CFD) method. The aerodynamic performance of an FOWT undergoing surge motions is simulated with an in-house code based on a free wake vortex method. Similarities and differences of the two studies are compared and discussed. The propeller state of the rotor is further analyzed to gain a deeper understanding of the working state change of FOWTs as well as to strengthen the research in this area. • The CFD and Vortex-based research methods about the vortex ring state and propeller state of floating offshore wind turbines are compared. • The change of the aerodynamic parameters of the rotor are compared with the literature. • The identification of the 'propeller state even' is compared with the literature. • The identification of the vortex ring state is compared with the literature and some fundamental differences are discussed. • Peters' criterion is extended to the prediction of the boundary between the vortex ring state and the propeller state. [ABSTRACT FROM AUTHOR]

The present study is aimed at investigating the effect of turbulent wind and shear wind on the floating offshore wind turbine (FOWT) structure by using a high-fidelity computational fluid dynamics (CFD) method. This method is believed to resolve the wind field around the turbine blades, wake and the near air-wave free-surface regime, allowing us to have a more in-depth examination into both aerodynamic and hydrodynamic of the FOWT. In the present study, the modelling of a coupled aero-hydro-mooring FOWT system is focused on a temporal and spatial variable turbulent wind field by using a time-varying spectrum, which has not been examined for a floating wind turbine. The turbulent wind in the study is generated with Mann's wind turbulence model, while the Von Karman wind spectrum is used to represent wind turbulence. In addition, different wind shears were also examined. We can conclude from this study that, when turbulent wind is present, there are fluctuations in both the rotor thrust and power outputs associated with the non-uniform wake region although the time-mean magnitude is almost the same. In addition, turbulence wind lead to a quicker wake diffusion than time-independent inflow wind. Furthermore, the existence of wind shear results in an even larger decrease in the local minimum thrust/power about 2–6% when the turbine blade is passing in front of the tower. Despite this, under the present wind inflow conditions, the inline surge force, dynamic motion, and the mooring tension of the floater are not significantly affected by either the turbulence wind or the wind shear. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines are subjected to higher tower fatigue loads than their fixed-to-seabed counterparts, which could lead to reductions in turbine life. The worst increases are generally seen in the tower axial fatigue, associated with the tower fore-aft bending moment. For a spar type platform, this has been shown to increase by up to 2.5 times and for a semi-submersible platform, by up to 1.8 times. Reducing these loads would be beneficial, as the alternative of strengthening the towers leads to increase in cost. Here, two offshore floating wind turbine systems, of the spar type, are analyzed and selected responses and tower fatigue are compared: one incorporates a variable-speed, variable-pitch-to-stall blade control system and a back twisted blade, and the other a conventional pitch-to-feather control. The results are then compared with those obtained in an earlier study, where the same turbine configurations were coupled to a semi-submersible platform. A weighted wind frequency analysis at three mean turbulent wind speeds of 8, 13, and 18 m/s highlights that the impact of the back twist angle magnitude and initiation point on tower axial fatigue life extension was the same for both platform types. Compared with their respective feather base models, an increase in the tower axial fatigue life of 18.8% was seen with a spar platform and 10.2% with a semi-submersible platform, when a back twist angle to the tip of -6 deg was imposed along with the variable-speed, variable-pitch-to-stall control. [ABSTRACT FROM AUTHOR]

Three models with different footing types were used to clarify the effect of heave plates on the hydrodynamic behavior of the elastic response of a flexible multicolumn floating offshore wind turbine (FOWT). The models were tested under regular waves, whose added mass, damping, and motion response results were compared with numerical simulations by NK-UTWind and WAMIT codes. As a whole, the attachment of heave plates was responsible for increasing the added mass and damping levels, consequently modifying the RAO of the models. Regarding the response in a sea condition, a decrease of 33% and 66% of the significant motion height (heave and pitch) was observed. Thus, the heave plate can be a good feature for the future design of FOWT.

The current design of vertical axis wind turbines (VAWTs) suffers from inevitable change in tip speed ratio, λ, in variant wind conditions due to fixed rotor speed. At relatively high wind speeds, which are promising due to high wind power potential, VAWTs operate at low λ with poor power coefficient. Morphing airfoils can be a potential solution by modifying the airfoil shape to optimal at each λ. The optimal airfoil shape for VAWTs at low λ, where dynamic stall is present, has not yet been studied in the literature, therefore, the present study addresses this gap by focusing on this regime to serve as a step towards designing morphing airfoils for VAWTs by identifying the optimal airfoil shape at low λ. The present study performs a combined analysis of three shape defining parameters, namely the airfoil maximum thickness and its position as well as the leading-edge radius, to reveal the overall design space. The analysis is based on 252 high-fidelity transient CFD simulations of 126 identical airfoil shapes. The simulations are verified and validated with three experiments. The results show that the three shape defining parameters have a fully coupled impact on the turbine power and thrust coefficients. When λ reduces from 3.0 to 2.5, the optimal airfoil changes from NACA0018–4.5/2.75 to NACA0024–4.5/3.5, that is increasing the maximum thickness from 18%c to 24%c and shifting its position from 27.5%c to 35%c, while the leading-edge radius index, I, remains 4.5. In general, reducing I from the default value of 6.0 to 4.5 is found to increase the turbine C P. [Display omitted] [ABSTRACT FROM AUTHOR]

The study aims to investigate the wave motion characteristics of the Optiflow concept, which is a low-rigidity FOWT supported by guy-wires and a tower, and was accomplished through wave tank experiments and numerical calculations using the UTWind code. The study examined the impact of wave height, wave direction, and mooring line configuration on the Optiflow's motion behavior in waves. The research revealed nonlinear motion behavior due to varying levels of viscous damping caused by wave height effects. The single-point mooring system played a significant role, resulting in a coupling of vertical motions, mainly as heave and pitch, which varied with the wave incidence angle. Unexpectedly, mooring line configurations affected first-order motions in waves, making the impact of the mooring line particularly pronounced for very-light FOWT like the Optiflow. Furthermore, all experiments noted a typical semi-submersible type floater behavior where the cancellation point (or waveless) response was observed. Numerical UTWind code simulations showed satisfactory agreement with the experiment's results, except for the resonance period, where the viscous effect had a significant impact on motions that the numerical code did not accurately consider. • A very-light FOWT with a guy-wired-supported tower was investigated. • Regular wave tests were performed experimentally and numerically. • The wave response showed to be non-linear due to the wave height. • The wave response was affected by the wave incidence angle. • The coupling between pitch and heave showed to be significant due to the single-point mooring system. [ABSTRACT FROM AUTHOR]

The motivation for this study is the fast development of floating offshore wind energy and the immature methodology and engineering practice related to predictions of vortex-induced motions (VIM). Benefiting from the oil and gas industry, in the past several decades, extensive knowledge and experience on vortex-induced vibrations (VIV) on slender marine structures has been gained. As the learnings from these efforts should be transferred and adapted to the renewable energy industry, a state-of-the-art review on influential VIM research has been carried out in this paper, focusing on: (1) engineering practice, (2) model tests, (3) numerical calculation, and (4) field measurement. Engineering gaps and potential research topics are identified as future work.

The aerodynamic performance of floating offshore wind turbines is extremely complex due to the motions of the floating platform. The surge and pitch motions are the most influential motions among the six degrees-of-freedom motions. In view of this, the aim of this study is to investigate the unsteady aerodynamic characteristics of a floating offshore wind turbine under single (surge or pitch) and combined motions using computational fluid dynamics simulations, In addition, the coupling technique of dynamic mesh and sliding mesh is employed, as well as the unsteady Reynolds averaged Navier-Stokes method. The numerical simulation method in this paper is first validated by comparison to the results of the blade element momentum method and the vortex method. Then, the aerodynamic characteristics of the floating offshore wind turbine under harmonic platform motions with different periods and amplitudes are investigated. The results show that the increase of amplitude and frequency aggravate the fluctuation of the overall aerodynamic performance of the wind turbine. In addition, the combined surge-pitch motion reduces the average power generation indicating that complex platform motions adversely affect the power generation of floating offshore wind turbines. • The effects of wind load were analyzed by comparing the platform motions under different amplitudes and frequencies. • Two representative platform motions and the combined motion were considered for analysis. • Analyze the aerodynamic characteristics of different airfoil sections to reveal the variations of overall performance. • The detailed analysis of the wake effects showed the three-dimensional flow under the platform motions. [ABSTRACT FROM AUTHOR]

For a turbine mounted on a floating platform, extreme induced loads can be increased by up to 1.6 times those experienced by a turbine situated on a fixed base. If these loads cannot be reduced, towers must be strengthened which will result in increased costs and weight. These tower loads would be additionally exasperated for a pitch-to-feather controlled turbine by a phenomenon generally referred to as "negative damping," if it were not avoided. Preventing negative damping from occurring on a pitch-to-feather controlled floating platform negatively affects rotor speed control and regulated power performance. However, minimizing the blade bending moment response can result in a reduction in the tower fore-aft moment response, which can increase the tower life. A variable-speed, variable pitch-to-stall (VSVP-S) floating semi-submersible wind turbine, which does not suffer from the negative damping and hence provides a more regulated power output, is presented. This incorporates a back twist blade profile such that the blade twist, starting at the root, initially twists toward stall and, at some pre-determined "initiation" point, changes direction to twist back toward feather until the tip. Wind frequency weighting was applied to the tower axial fatigue life trends of different blade profiles and a preferred blade back twist profile was identified. This had a back twist angle of -3 deg and started at 87.5% along the blade length and achieved a 5.1% increase in the tower fatigue life. [ABSTRACT FROM AUTHOR]

Tension leg platform (TLP) is a cost-effective and high-performance support structure for floating offshore wind turbine (FOWT) because of its small responses in heave, pitch, and roll with the constraint of the tendons. China, as the largest market of offshore wind energy, has shown a demand for developing reliable, viable floating platform support structures, especially aiming at the intermediate water depth. The present paper described a newly proposed 10-MW Braceless-TLP FOWT designed for a moderate water depth of 60 m. The numerical simulations of the FOWT are carried out using the coupled aero-hydro-servo-elastic-mooring calculation tool FAST. The measured wind and wave data of the target site close to the Fujian Province of China were used to evaluate the performance of the FOWT under the 100-, 50-, 5-, and 2-year-return stochastic weather conditions. The natural periods of the platform in surge, sway, heave, pitch, roll, and yaw were found to be within the range recommended by the design standard DNV-RP-0286 Coupled Analysis of Floating Wind Turbines. The largest surge of the water depth ratio among all the load cases was 15%, which was smaller than the admissible ratio of 23%. The tower top displacements remained between −1 m and 1 m, which were at a similar order to those of a 10-MW monopile-supported offshore wind turbine. The six tendons remained tensioned during the simulation, even under the operational and extreme (parked) environmental conditions. The Braceless-TLP FOWT showed an overall satisfying performance in terms of the structural stability and illustrates the feasibility of this type of FOWT at such a moderate water depth.

Considering stability and fabrication cost, 3–4 columns are usually adopted for semi-submersible platform designs. Although increasing the number of columns provides more stability for both floating platform and system as a whole, it is generally not economically viable. In this respect, the present work provides a high-fidelity analysis of semi-submersible platform stability and hydrodynamic response for different design concepts. The number of columns was considered as the main design parameter and was varied from 3–6 columns. The semi-submersible weight was kept constant during the simulation period by changing the column diameter and amount of ballast water. The investigation was carried out using the potential code Orcawave, the results of which were input directly to the engineering tool OrcaFlex. Four different types of semi-submersible platforms with a varying number of columns were tested and compared under extreme environmental conditions in order to ensure their stability and hydrodynamic response. The simulation findings revealed that platform stability was more affected by the geometrical features of the floater than by the number of columns. Furthermore, the number of columns did not have a significant impact on hydrodynamic behavior for the same platform geometry.

Offshore wind turbines are becoming increasingly popular due to their higher wind energy harnessing capabilities and lower visual pollution. Researchers around the globe have been reporting significant scientific advancements in offshore wind turbines technology, addressing key issues, such as aerodynamic characteristics of turbine blades, dynamic response of the turbine, structural integrity of the turbine foundation, design of the mooring cables, ground scouring and cost modelling for commercial viability. These investigations range from component-level design and analysis to system-level response and optimization using a multitude of analytical, empirical and numerical techniques. With such wide-ranging studies available in the public domain, there is a need to carry out an extensive yet critical literature review on the recent advancements in offshore wind turbine technology. Offshore wind turbine blades’ aerodynamics and the structural integrity of offshore wind turbines are of particular importance, which can lead towards system’s optimal design and operation, leading to reduced maintenance costs. Thus, in this study, our focus is to highlight key knowledge gaps in the scientific investigations on offshore wind turbines’ aerodynamic and structural response. It is envisaged that this study will pave the way for future concentrated efforts in better understanding the complex behavior of these machines.