Search Results (217)

The high cost of mooring systems, driven by extreme peak tensions during storm conditions, remains a significant barrier to the commercialization of floating offshore wind turbines (FOWTs). This paper proposes an innovative active tension-regulating joint (TRJ) for FOWT mooring lines. The TRJ consists of nested cylinders and an actively controlled accumulator, designed to release additional line length under high tension and to recover it under low tension, thereby reducing extreme dynamic peaks. A finite element scheme is also developed for efficient line dynamics analysis. The TRJ concept is applied to a benchmark IEA 15-MW semi-submersible FOWT in 100 m water depth under 50-year return period environmental conditions. The simulation results demonstrate that the TRJ reduces the maximum mooring line tension by approximately 53% and the maximum suspended line length by over 23%. This active control technique enables the downsizing of mooring components and a significant cost reduction. Full article

Fatigue assessment of floating offshore wind turbines (FOWTs) remains challenging because fatigue-sensitive regions may occur outside conventional predefined hotspots. This study applies a previously numerically verified full-field fatigue-screening workflow combining Unit Load Response, submodeling, and Virtual Test Rig concepts to the TaidaFloat semi-submersible FOWT under Taiwan Strait environmental conditions. Reconstructed nodal stress histories are used to map hull fatigue and evaluate occurrence-weighted contributions from 182 environmental bins, including operational and typhoon conditions. The results identify fatigue-sensitive regions not only at conventional column–bracing and column–pontoon connections but also in the upper main column and along the turbine–hull load path. Upper column fatigue is mainly associated with turbine-induced bending, whereas lower column and waterline-adjacent regions are more sensitive to wave-induced global hull bending. Frequently occurring near-rated operational conditions dominate the occurrence-weighted hull fatigue contribution, while selected typhoon conditions produce high short-term damage but limited long-term contributions within the four-year dataset. Approximately 94.6% of hull fatigue damage is captured by 28% of the bins, and a common hull–mooring set captures 97.0% of both contributions using 62% of the bins. These findings support hotspot screening and environmental-bin prioritization rather than detailed or certification-level fatigue life prediction. Full article

This study investigates the hydrodynamic behavior of the OC4-DeepCwind floating offshore wind turbine, with a specific focus on the influence of wave reflection and hydrodynamic interaction effects between the platform components. The OC4-DeepCwind semi-submersible platform, supporting the NREL 5 MW reference wind turbine, is analyzed using the commercial software ANSYS AQWA 2024 R1 and the in-house codes HAMVAB and SEMISUB. While ANSYS AQWA and HAMVAB account for multiple wave scattering effects within the multi-column configuration, SEMISUB neglects hydrodynamic interactions, enabling a systematic assessment of their influence on the predicted response. Interaction effects are most pronounced in the surge degree of freedom, where neglecting wave reflection distorts the exciting wave force above 0.65 rad/s, and in the surge, heave, and pitch added mass and radiation damping coefficients, with substantial deviations above approximately 0.6 rad/s. The influence of column separation distance on these diffraction loads is also examined. Stochastic-wave simulations of the moored wind turbine under realistic JONSWAP sea states show normalized errors across all examined sea states of 16.6% (heave), 14.6% (surge), and 9.9% (pitch) in platform motions when interactions are neglected, whereas tower-base loads and mooring line tensions are less sensitive, with errors of 11.5% (vertical shear force), 9.4% (horizontal shear force), 9.2% (bending moment), 8.7% (downstream mooring tension), and 5.6% (upstream mooring tension). These results indicate that hydrodynamic interaction effects are critical for predicting platform motions and hydrodynamic coefficients but have a comparatively limited effect on design-governing structural and mooring loads, offering quantitative guidance on when simplified interaction-free models remain adequate for semi-submersible FOWT design. Full article

This study maps the dynamic response of the IEA 15 MW reference wind turbine mounted on the UMaine VolturnUS-S semi-submersible platform across seven wave headings, spanning $\beta$ = 0–180°. A fully coupled aero–hydro–servo–elastic OpenFAST model with lumped-mass catenary mooring is simulated over three environmental groups: below-rated operation, a severe sea state at rated wind, and a parked extreme, combined with seven wave headings and supplemented by a 77-case operational envelope sweep across hub wind speeds of 4–24 m/s. Responses are analyzed through time-domain and frequency-domain statistics, drift kinematics, and exceedance curves. At operating states, the maximum side-to-side moment rises from 80.44 to 273.60 MNm between following and beam seas, while the maximum fore–aft moment reduces from 566.90 MNm to 485.23 MNm, respectively. The parked extreme LC-C maximum pitch response (2.77° at following seas, 2.59° at beam seas) is roughly half that of the severe operating LC-B (6.29° and 5.38°, respectively). This directional coupling is robust across environmental severities. In the parked group, the lateral motions (sway, roll) are amplified, as feathering reduces the rotor’s aerodynamic contribution to lateral damping. The developed wind speed misalignment response atlas condenses the aforementioned data, providing a compact basis for rapid early screening of ultra-large floating offshore wind turbines. Full article

Floating offshore wind turbines (FOWTs) have become an important structural configuration for deep-water offshore wind energy development. However, existing studies have mainly focused on towing experience for conventional offshore structures and static stability assessment, while a systematic understanding of the multi-body coupled dynamic response characteristics and hazardous response factors of large-scale FOWTs under combined wind, wave, and current loads remains limited. To address the insufficient understanding of critical hazardous response indicators in existing studies, a 10 MW semi-submersible floating wind turbine was investigated in this study. Variations in environmental loads, towline constraints, and FOWT responses during towing were incorporated into a multi-body coupled analysis framework, and the key hazardous response indicators governed by different dominant environmental factors were identified. The results indicate that increasing wind speed significantly amplifies the pitch response, with the extreme pitch angle reaching approximately −7.17° under the 24 m/s wind condition. Variations in current velocity have limited influence on response amplitudes. Wave height has the most pronounced effect on heave motion and nacelle acceleration. Under the 6.5 m wave height condition, their extreme values reach approximately −1.37 m and 1.15 m/s², respectively. Under the single-tug towing configuration, the 45° and 90° environmental directions induce pronounced lateral and yaw offsets, indicating insufficient path-keeping capability under unfavorable environmental directions. Comprehensive analysis demonstrates that pitch motion should be regarded as the primary hazardous response indicator under high wind speed conditions, while nacelle acceleration and heave motion require particular attention under high wave height conditions. Full article

This paper analyzes the fatigue loads of wind turbines in a floating offshore wind farm (FOWF) whose layout can be reconfigured. Such wind farm reconfiguration will be useful for wake effect mitigation in varying wind conditions. As an example FOWF, a farm with three 5 MW floating offshore wind turbine (FOWT) models on semi-submersible platforms, developed by the National Laboratory of the Rockies (NLR) (formerly the National Renewable Energy Laboratory (NREL)), is considered. Simulations for the example FOWF are conducted with various realistic turbulent wind and irregular wave conditions in the medium-fidelity wind farm simulator FAST.Farm. Using the simulation data, fatigue analysis is conducted by calculating the damage equivalent loads (DELs) using the computational tool MLife at critical components of the three FOWTs. The analysis results demonstrate the potential of reconfigurable FOWFs in not only increasing power outputs but also reducing fatigue loads for many critical components of turbines. Full article

Wind–wave misalignment is a pervasive environmental phenomenon that significantly affects the structural integrity of floating offshore wind turbines (FOWTs). For a Spar-type FOWT across the full 0°–90° misalignment range, this study systematically conducts dynamic response characterization and parameter sensitivity analysis, quantifying the directional modulation effects on five critical dynamic indicators, including tower-base Fore-Aft (F-A) and side-to-side (S-S) bending moments, maximum Von Mises stress, and fairlead tensions. Results demonstrate that wind–wave misalignment triggers a significant redistribution of structural energy, where side-to-side bending moments and fairlead tensions exhibit distinct peak characteristics at specific non-collinear headings. Rather than merely evaluating structural responses, this study emphasizes the sensitivity of environmental parameters to reveal a dominance-switching mechanism. As the misalignment angle increases, the governing factors of structural response dynamically shift from wind variables to wave variables. This research provides a rigorous mechanical explanation for complex response evolution and offers a scientific basis for the robust design of floating wind turbines in multi-directional sea states. Full article

In this study, a systematic numerical study on design optimization was conducted for a tension leg platform (TLP)-type floating offshore wind turbine (FOWT), aiming to improve hydrodynamic performance, tendon behavior, and cost-effectiveness. Six design variables associated with hull geometry and tendon properties—pontoon length (PL), pontoon width (PW), platform draft (PD), main column diameter (CD), tendon pre-tension (Pre), and axial stiffness (EA)—were considered. For the global performance analysis, hydrodynamic coefficients were first obtained in the frequency domain, and motion and tendon tension responses were subsequently evaluated in the time-domain under a survival condition representative of a Southeast Asian site. A surrogate model based on the response surface method (RSM) was developed to predict platform responses across the design space. Multi-objective optimization was then performed using the non-dominated sorting genetic algorithm II (NSGA-II), yielding Pareto-optimal solutions that reveal trade-offs among competing performance metrics. The proposed framework is intended to provide Pareto-optimal design candidates for preliminary TLP-FOWT design, while the selection of a final design requires project-specific criteria and is beyond the scope of the present conceptual study. The optimization results show that the tendon tension can be effectively reduced while maintaining cost efficiency by increasing the pontoon length and slightly decreasing the tendon axial stiffness. For the tension–surge motion optimization, the Pareto-optimal solutions provide a balanced trade-off, where tendon tension is clearly reduced with only a slight increase in surge motion. In addition, the cost–nacelle acceleration optimization shows that nacelle acceleration can be further reduced by increasing the platform draft and pontoon length, although this is accompanied by a slight increase in the cost index. These findings provide practical insights for balancing global performance and cost in TLP-type FOWT design. Full article

Offshore wind and wave energy have emerged as promising alternatives due to their abundant availability and substantial energy potential. This research explores a V-shaped semi-submersible platform designed to support both wind turbines and wave energy converters (WECs). The V-shaped configuration is selected for its ability to enhance hydrodynamic performance by reducing wave-induced loads and improving motion characteristics, while also providing increased structural stability through a wider effective footprint. In addition, the geometry creates a favourable layout for integrating WECs between the pontoons, enabling efficient wave energy capture without significantly interfering with the aerodynamic performance of the wind turbine. The study assesses the performance of different V-shaped platform configurations, ensuring their motion responses meet the operational limits required for wind turbines. It also examines whether interactions between the platform and coexisting WECs can lead to an improvement in wave energy absorption efficiency. Numerical hydrodynamic diffraction was conducted using the boundary element method in ANSYS AQWA, based on 3D potential flow theory and considering viscous damping effects, to calculate platform motion and the wave power output of WECs with a linear power take-off system. Preliminary analyses revealed that optimising the placement of WECs on a V-shaped semi-submersible can significantly improve energy generation while maintaining acceptable platform motion. This research demonstrates the additional potential of integrated wind-wave energy systems in delivering efficient and sustainable offshore energy solutions. The study also highlights the advantages of a turret mooring system for passive alignment with environmental forces, prolonging platform structure longevity and enhancing energy efficiency. Full article

Spar-type floating offshore wind turbines (FOWTs) operating in deep-sea environments are subjected to coupled wind and wave excitations spanning a wide frequency range, rendering single-frequency passive damping solutions inadequate. A folded-beam nonlinear energy sink (FB-NES) is proposed for broadband vibration suppression of spar-type FOWTs. The device employs pre-buckled elastic beam arms integrated with constrained layer damping patches, and a closed-form analytical relationship between the beam geometric parameters and the nonlinear stiffness coefficients is derived, enabling direct parameter design without iterative calibration. The pre-buckled geometry introduces a negative-stiffness mechanism that substantially lowers the targeted energy transfer (TET) threshold, ensuring device engagement under all normal operational sea states. A 14-degree-of-freedom aero-hydro-elastic model of the NREL 5 MW OC3-Hywind FOWT with the FB-NES is established via the Euler–Lagrange formulation and validated against OpenFAST. Based on the numerical results under operational and extreme parked load cases, the FB-NES achieves substantial broadband vibration reductions that grow monotonically with wave severity, consistently and substantially surpassing both the optimally tuned mass damper (TMD) and a conventional cubic nonlinear energy sink of equal mass. Wavelet analysis confirms that targeted energy transfer, rather than direct viscous damping, is the dominant energy dissipation mechanism. The FB-NES also maintains effective control over a wide frequency detuning range, demonstrating superior robustness compared to the TMD. Full article

As floating offshore wind turbines (FOWTs) scale towards 10 MW+ capacities, suppressing wave-induced rotational resonance becomes critical for system survivability. This study introduces an optimized, highly symmetrical four-float semi-submersible platform, explicitly tailored to support the DTU 10 MW wind turbine and paired with an orthogonal four-point mooring system. Using three-dimensional linear potential flow theory via ANSYS AQWA, comprehensive frequency- and time-domain hydrodynamic evaluations were conducted. To address the inherent limitations of inviscid potential flow assumptions, an empirical added-damping method was implemented. Quantitative results demonstrate a drastic reduction in motion responses: the peak Response Amplitude Operator (RAO) for heave decreased by 68.6% (from 1.945 m/m to 0.610 m/m). Most notably, the peak RAOs for the critical rotational degrees of freedom—pitch and roll—were reduced by over 92% (from 2.080 °/m and 2.216 °/m to ~0.168 °/m, respectively). Ultimately, compared to traditional asymmetric three-float concepts, this novel symmetric omnidirectional layout provides a more uniform restoring stiffness. The resulting suppression of pitch and roll resonance results in a profound reduction in tower-base bending moments and gyroscopic loads, thereby significantly enhancing the dynamic stability, safety margins, and fatigue life of the 10 MW FOWT under extreme survival sea states. Full article

Stable wind resources in far-reaching sea areas are important direction for the development of renewable energy, making floating offshore wind turbine (FOWT) a focus of current research. However, the working environment of FOWT is severe. Under the condition of changeable wind and waves, the floating platform exhibits various motion responses, which may reduce power generation efficiency and even lead to structural damage with unpredictable consequences. In this paper, the National Renewable Energy Laboratory (NREL) 5 MW OC4-DeepCwind semi-submersible wind turbine is considered, and a multi-degree-of-freedom (M-DOF) tuned mass damper (TMD) system is designed to simultaneously suppress its roll and pitch motion responses. A multi-objective optimization problem is formulated to unify the frequency tuning accuracy, damping ratio constraints, and mass ratio limits through penalty functions. Then an improved Particle Swarm Optimization algorithm with time-varying acceleration coefficients (TVAC-PSO) is employed to determine the optimal TMD parameters, which dynamically adjusts exploration and exploitation capabilities to overcome the limitations of standard PSO in handling the strongly coupled parameter space. A high-fidelity aero-hydro-servo-elastic simulation model is established using OpenFAST to verify the vibration suppression performance under various sea state conditions. Simulation results demonstrate that the proposed M-DOF TMD system can effectively reduce the roll and pitch motion responses and significantly suppress the resonant peak energy, substantially improving the dynamic performance of FOWT. Full article

Vertical axis wind turbines (VAWTs) suffer from dynamic stall (DS) at low tip-speed ratios (λ), where cyclic variations in angle of attack (α) dominate the blade aerodynamics, severely undermining aerodynamic performance and power extraction. The coupled influence of airfoil parameters on DS remains unexplored. To address this gap, a fully coupled parametric study using 126 incompressible URANS simulations is conducted, examining three geometric parameters of symmetric airfoils: maximum thickness (t/c), chordwise position of maximum thickness (x_t/c), and leading-edge (LE) radius index (I). The results show that coupled geometric modification fundamentally alters the stall mechanism, shifting it from abrupt, LE-driven separation toward a gradual, trailing-edge (TE)-controlled process as airfoils transition from thin, forward-x_t/c profiles to thicker configurations with aft x_t/c and reduced I. This transition enhances boundary-layer (BL) stability, delays DS onset, weakens dynamic stall vortex (DSV) formation, and mitigates unsteady aerodynamic loading. Within the investigated design space, the best-performing configuration (NACA0024–4.5/3.5) achieves a 73% increase in turbine power coefficient (C_P) relative to the baseline airfoil (NACA0018–6.0/3.0), mainly through passive control of BL separation and vortex development. These findings highlight the limitations of single-parameter optimization and establish a physics-based, coupled-design framework for mitigating DS-induced performance losses in VAWTs. Full article

Unlike traditional marine floating platforms, floating offshore wind turbines (FOWTs) are subjected to larger overturning moments. This study presents a novel floating offshore wind turbine concept—termed the Multi-Body Anti-Pitching Floating Wind Turbine (MAFWT)—designed to mitigate excessive pitching motion of semi-submersible FOWTs. The MAFWT integrates three Wave-star-like appendages arranged in the UMaine VolturnUS-S platform. A fully coupled dynamic model is developed within the FAST-to-AQWA (F2A) simulation framework. Parametric time- and frequency-domain analyses are subsequently conducted under both regular wave/steady wind and irregular wave/turbulent wind conditions to investigate the influence of stiffness parameter K and damping parameter B on system dynamics. Results demonstrate that increasing stiffness enhances the restoring moment, thereby reducing the static pitching offset and overall dynamic response (with the maximum and average values decreasing by 27.6% and 31.9%, respectively). However, it may amplify low-frequency slow-drift motions (with the maximum and average values of surge increasing by 9.4% and 9.5%, respectively). In contrast, damping primarily dissipates kinetic energy, yielding up to a 25.5% reduction in pitch angular velocity and significantly mitigating power output fluctuations (the standard deviation decreased by 16.4%). Furthermore, increases in the stiffness coefficient and damping coefficient result in respective slight increments of 0.12% and 0.18% in the average power output. This work elucidates the distinct physical mechanisms through which stiffness and damping govern pitch suppression. Full article

Tension leg platform (TLP) floating offshore wind turbines (FOWTs) show strong potential for future commercial deployment for the advantages in global performance, cost efficiency, and economic spatial utilization. However, as system sizes expand and multi-source vibrations become more prominent, the integrated design and dynamic responses of the FOWT system grow increasingly complex. This research presents the design of a TLP foundation for a 22 MW FOWT and examines its dynamic response under extreme sea states via a combined numerical and experimental approach. An integrated numerical model of the TLP FOWT is established and subsequently calibrated using data obtained from a 1:64 scale physical model test in a wind-wave flume. By using the calibrated model, the reliability of the TLP FOWT was further validated through an extended Ultimate Limit State (ULS) analysis under a 50-year return period metocean data in the East China Sea. Numerical study demonstrates that the extreme motion responses under 50-year return period data comply with safe operational limits, and the safety factors meet standard specifications. Therefore, this study provides a systematic design scheme along with valuable model test data. These contributions serve as a critical reference for the design and research of future large-megawatt TLP FOWTs. Full article