Pitch Resonance Research Articles

A wind semi-sub platform with hinged floats for omnidirectional swell wave energy conversion

AbstractThe capacity of wind turbines on offshore wind platforms is presently much greater than that for wave energy conversion. However, wind availability with speed greater than 5 m/s, just above cut in, is typically 30–40% requiring storage to provide uniformity of supply, but this may be improved by adding swell wave energy conversion with typical availability of 90%. A hybrid platform is considered with three effectively rigid cylindrical floats connected by beams at right angles to support a wind turbine with its base on the central float, and two wave energy floats, opposite the wind floats, with beams at 90° and hinges with dampers for mechanical energy absorption on the central float. With swell periods over 10 s, pitch resonance may be achieved with the fore and aft floats about half a wavelength apart with anti-phase forcing causing a moment on hinges above water level. The NREL 5 MW wind turbine is incorporated and average swell wave power absorption in a typical significant wave height of 2 m is over 200 kW. The analysis is by time domain linear diffraction–radiation modelling validated for other multi-float configurations. Significant wave energy conversion is omnidirectional over a wide range of heading angles. An added benefit is that in larger waves associated with strong winds, when the wave energy conversion would be disengaged, the wave float rotation on free hinges reduces the hub accelerations below that for rigid floats, enabling a longer time for wind power generation.

The role of acoustic features of maternal infant-directed singing in enhancing infant sensorimotor, language and socioemotional development

The quality of infant-directed speech (IDS) and infant-directed singing (IDSi) are considered vital to children, but empirical studies on protomusical qualities of the IDSi influencing infant development are rare. The current prospective study examines the role of IDSi acoustic features, such as pitch variability, shape and movement, and vocal amplitude vibration, timbre, and resonance, in associating with infant sensorimotor, language, and socioemotional development at six and 18 months. The sample consists of 236 Palestinian mothers from Gaza Strip singing to their six-month-olds a song by their own choice. Maternal IDSi was recorded and analyzed by the OpenSMILE- tool to depict main acoustic features of pitch frequencies, variations, and contours, vocal intensity, resonance formants, and power. The results are based on completed 219 maternal IDSi. Mothers reported about their infants’ sensorimotor, language-vocalization, and socioemotional skills at six months, and psychologists tested these skills by Bayley Scales for Infant Development at 18 months. Results show that maternal IDSi characterized by wide pitch variability and rich and high vocal amplitude and vibration were associated with infants’ optimal sensorimotor, language vocalization, and socioemotional skills at six months, and rich and high vocal amplitude and vibration predicted these optimal developmental skills also at 18 months. High resonance and rhythmicity formants were associated with optimal language and vocalization skills at six months. To conclude, the IDSi is considered important in enhancing newborn and risk infants’ wellbeing, and the current findings argue that favorable acoustic singing qualities are crucial for optimal multidomain development across infancy.

CFD Study of the Non-Linear Physical Phenomena of the TLP of a 15-MW-Class FOWT under Extreme Waves

In this study, a numerical simulation was conducted to investigate the non-linear physical phenomena of a tension leg platform (TLP) of a 15-MW-class floating offshore wind turbine (FOWT). Computational fluid dynamics was employed as the numerical tool, and a deforming mesh technique was used to describe the moving body. To examine the non-linear physical phenomena, an irregular wave was generated with a focus on head sea conditions. The springing and ringing responses were calculated from the numerical simulation results, and the relations between the motions and dynamic tensions of the 15-MW-class FOWT TLP were investigated. From the irregular wave impact simulation, it was found that the springing response via the wave sum frequencies and the ringing response occurred at approximately three times the wave peak frequency. Additionally, whipping simulations were conducted under a focused wave. The results show that the response in pitch resonance frequency was caused by the wave impact. The numerical results of this study could be used as fundamental data for FOWT TLP design.

Experimental Validation of Models of a Hull-Based Tuned Mass Damper System for a Semisubmersible Floating Offshore Wind Turbine Platform

Floating offshore wind turbine designs can be further optimized if the controller and remaining systems are designed together, known as control co-design. To effectively perform control co-design, modeling tools predicting the influence of the control systems on the response of a system must be validated. This article presents an experimental validation that utilizes a scale model of a semisubmersible platform for an offshore wind turbine that is fitted with adjustable tuned mass dampers. These dampers can be tuned to attenuate either the hull-pitch resonance or the tower-bending resonance. The data from the experiment are used to validate state-of-the-art modeling tools. It is shown that the models capture the overall effects of the tuned mass dampers; however, some models overpredict the reduction in platform pitch motion when the dampers are tuned to the pitch resonance. The relative reduction in the tower-base bending moment is more consistently captured by the models when the dampers are tuned to the tower-bending resonance. However, there are significant differences in the absolute level of tower-base bending moment between the models and the experiment. Much of the differences observed are a consequence of the challenge with accurately predicting the baseline platform resonance motion and the tower-bending moment, which should be addressed in future modeling efforts.

Numerical and experimental studies of the effects of WEC motion on a combined wind-wave energy platform

The development of eco-friendly carbon-free marine energy is actively underway to alleviate the impact of global climate change due to excessive emission of carbon dioxide. To reduce the variability of energy production according to marine environment conditions and maximize energy production, the need for research on a suitable ocean energy platform is increasing. Therefore, various studies on motion control of ocean energy platforms are being conducted.
 In this study, the motion response of a combined wind-wave energy platform equipped with multiple wave energy converters (WEC) capable of controlling the motion of a floating offshore wind turbine (FOWT) was numerically analysed and verified through a wave tank experiment. The combined energy platform with multiple WECs attached to the FOWT not only produces stable energy by controlling the motion response of the FOWT with the movement of the WEC, but also produces wave energy by the relative motion of the WEC.
 The FOWT is a spar platform with taut moorings, and WEC is a moving cylinder hinged to the FOWT. The motion reduction effect of the FOWT by the movement of the WEC was investigated. Numerical analysis used the ANSYS AQWA program, which is a potential flow-based hydrodynamic program. To consider the restraining force of the FOWT due to the motion of the WEC, the multi-body dynamics theory was applied. The accuracy of the numerical model was improved by applying mooring dynamics, weakly nonlinear Froude-Krylov force, and nonlinear hydrostatic force. The experimental model was fabricated on a scale of 1/100 of the numerical model and was performed in a two-dimensional mini wave tank. To prevent the sidewall effect of the tank, the experiment was conducted only for the incident waves with various periods under the head sea conditions.
 The numerical analysis results were in good agreement with the experimental results. In particular, the pitch of FOWT decreased significantly near the pitch resonance period because of the motion of WEC. The change in motion characteristics of the combined energy platform according to the movement of the WEC under various wave conditions was analysed.

Causality-Free Modeling of a Floating Wind Turbine Semisubmersible Platform with Validation Results

This paper presents an acausal modeling approach for developing a hydrodynamic module in the Control-oriented, Reconfigurable, and Acausal Floating Turbine Simulator (CRAFTS), a wind turbine simulation tool being developed by the authors, to facilitate control co-design of floating offshore wind turbines (FOWTs). The available data of the semi-submersible VolturnUS-S FOWT is incorporated into the modeling, simulation, and control demonstrations. The model is verified and validated using numerical data from industrial-standard simulation platform OpenFAST and experimental data from the Floating Offshore-wind and Controls Advanced Laboratory (FOCAL) Project. Numerical results demonstrate that the hydrodynamic module in CRAFTS can qualitatively capture loads and responses from the free decay tests, as well as the stabilizing effects of the tuned mass dampers (TMDs), which are tuned to attenuate the platform pitch resonance or the tower bending resonance.

Validation of CFD simulations of the moored DeepCwind offshore wind semisubmersible in irregular waves

This article examines the use of computational fluid dynamics (CFD) simulations to predict the response of a floating wind platform to irregular-wave excitation. This work was conducted as part of the Reproducible CFD Joint Industry Project for Floating Offshore Wind Applications and involved verification across several participants and modeling tools and validation against experiments. The authors pay special attention to the uncertainties in both CFD results and experimental measurements. We perform detailed comparisons of the incident waves and the motion of a moored structure. The nonlinear, low-frequency resonance motion is of particular interest because it potentially drives the mooring and tower-base loads. The verification and validation study is partially successful in that the CFD simulations capture the low-frequency slow-drift motion well but underpredict the low-frequency pitch resonance. This underprediction can be attributed in part to the CFD incident waves, which showed some discrepancies with the experimental waves, especially around extreme events. The effects of the wave discrepancies are also estimated and investigated using a mid-fidelity OpenFAST model. Overall, the present study increases our confidence in using CFD simulations to predict the global performance of offshore wind platforms in irregular waves and to produce data for the calibration of lower-fidelity models.

Coexisting attractors in floating body dynamics undergoing parametric resonance

This study pertains to analysing the dynamical behaviour of a floating body undergoing parametric resonances. A simple vertical cylinder, representing a classical spar-buoy, is considered, limiting its motion to heave and pitch degrees of freedom. Its geometry and mass distribution are chosen such that a 2:1 ratio of heave to pitch/roll natural frequency makes the spar-buoy prone to parametric resonance. The system is then studied by the shooting method, combined with a pseudo-arclength continuation, and the harmonic balance procedure. Results show that an extensive bistable region exists, where stable parametric resonance coexists with a regular resonance response. The analysis also unveiled the existence of stable quasiperiodic motions existing in correspondence of both pitch and heave resonance. Results are qualitatively validated using a model based on the explicit nonlinear Froude–Krylov force calculation.

On the aerodynamic loading effect of a model Spar-type floating wind turbine: An experimental study

Aerodynamic loading is one of the most dominating environmental excitations of Floating Wind Turbines (FWTs) and plays an important role in the FWT dynamics. In this study, we developed a model Spar-type FWT and then constructed a dedicated experiment apparatus to reveal the aerodynamic loading effects. As for the floater motion, the wind loading serves as an external exciting force, as well as potential damping source and equivalent added mass item. To take all these roles into account, we proposed a concept of aerodynamic loading effect. The presence of aerodynamic loading effect is validated by free decay tests and white noise wave tests. Results show that the aerodynamic loading effect alters the natural frequencies and damping ratios of the FWT system. We suggest the FWT designers refer to the altered natural frequencies when designing the floater and the FWT controllers. We experimentally observed that the increased aerodynamic loading seems to suppress the pitch resonance vibration while amplifies the resonance vibration at surge frequency. Besides, the nacelle motions, blade loads, and the tower dynamics, are all significantly impacted by the aerodynamic loading effect. The presented results are potentially helpful for optimizing FWTs and developing advanced FWT controllers.

OC6 Phase Ib: Validation of the CFD predictions of difference-frequency wave excitation on a FOWT semisubmersible

During the previous OC511 project, state-of-the-art mid-fidelity engineering tools for floating wind systems were found to consistently underpredict the nonlinear, low-frequency responses of semisubmersible offshore wind platforms, leading to substantial errors in the structural loads because of low-frequency surge and pitch resonance. To examine this underprediction, a coordinated investigation with computational fluid dynamics (CFD) simulations and model-basin experiments was carried out. Both investigations involved a fixed and simplified OC5-DeepCwind semisubmersible in bichromatic waves. The wave excitations—especially the nonlinear, difference-frequency excitation—on the structure from the CFD simulations were compared to the experimental measurements for validation, with uncertainty analyses for both the experimental and the CFD results. Further, the wave excitations on each column of the semisubmersible were measured separately in the experiment, allowing the validation of the CFD results to be done on a per-column basis. Overall, the CFD predictions of the difference-frequency excitations agree with the experimental measurements, suggesting the CFD solutions can be used as a reference for tuning and improving the engineering-level tools and can provide a means to better understand the underprediction at low frequencies.

Experimental investigation on the influence of hybrid mooring system configuration and mooring material on the hydrodynamic performance of a point absorber

The performance of the three hybrid mooring systems was studied by conducting 1:40 model tests of a moored point absorber subjected to regular and irregular waves. The compact hybrid mooring adopts two segments design, i.e., a top synthetic fibre rope (polyester or nylon) and a bottom chain. A submerged buoy is installed on the joint between these two segments. Another design that installs two clumped weights at the end part of the chain in the nylon mooring system is also investigated. Seventeen regular and three irregular wave tests were conducted. Heave and pitch resonance wave conditions were tested to study the system dynamics in resonance modes. Results show that the nylon rope is a favourable material for mooring ropes of wave energy converters in terms of energy conversion efficiency. Compared with nylon rope, polyester restricts WEC heave motion responses effectively and reduces the energy conversion efficiency of the floater studied in this work. The compact compliant mooring system design that consists of nylon rope, buoy, chain and clumped weights is the best mooring concept for the specified floater in this work. It demonstrates that when pretension is not large enough during the tests, the mooring will experience slack configuration during the random waves.

On the impact of motion-thrust coupling in floating tidal energy applications

Floating systems provide an opportunity to expand the available tidal stream energy resource and reduce the levelised cost of energy by increasing the number of viable deployment sites; simplifying the installation, maintenance and decommissioning; and by accessing greater flow speeds near the free surface. However, the inevitable exposure to free surface conditions raises questions over both the power delivery and the survivability of these systems, both due to the presence of waves and the associated excitation of the floating structures. Without addressing these concerns through scale modelling in laboratories and prototype deployments, the risk to investors is too high to gain significant support for the industry. Therefore, this paper presents physical modelling and analysis of a 1:12 scale model of the Modular Tide Generators floating tidal platform concept, a pseudo-generic design consisting of a catamaran-style platform, catenary mooring system and a submerged horizontal axis tidal turbine. The behaviour of the full system is explored in a range of wave, current, and following wave–current conditions; with and without the turbine; and in rigidly fixed and moored configurations. The results imply that a linear superposition of waves and currents is an adequate modelling approach for determining the mean and cyclic values for the mooring loads and motion of the system. However, the loading on, and power generated by, the turbine are more complex and have implications for power delivery quality and the lifespan of the turbine. In particular, the magnitude of the cyclic loading increases with the current speed, as well as the surge and pitch motion which is observed to be most influential near harmonics of the pitch resonance frequency.

Operational and extreme responses of a new concept of 10MW semi-submersible wind turbine in intermediate water depth: An experimental study

A floating wind turbine concept, named as SPIC, is composed of the DTU 10MW reference wind turbine and a newly designed semi-submersible platform featured with three partially inclined side columns. The dynamic responses of the SPIC concept considering an intermediate water depth of 60 m are investigated through the basin model test approach. After calibrating the wind and wave conditions and identifying the model wind turbine system, the operational and short-term extreme responses of the floating wind turbine concept were studied. This innovative concept meets reasonable natural period requirements and achieves great stability under design environmental conditions. For operational responses, low-frequency turbulent wind increases the surge resonance response but provides aerodynamic damping to reduce pitch resonance and tower vibration. The wind also excites the responses at 1P, 2P, and 3P frequencies, but the current slightly suppress these responses. The extreme surge, mooring line tension and nacelle acceleration occur under extreme environments, while the extreme pitch, tower-top shear force and tower-base bending moment should be examined under operational conditions with rated wind as well. These extreme responses are proved to be smaller than the limited values recommended by standards, demonstrating the reasonability of the SPIC concept in intermediate water depth.

Effects of bracings and motion coupling on resonance features of semi-submersible platform under irregular wave conditions

Resonance is a critical consideration in the design of offshore floating structures. This paper aims at analysing the nonlinear effects of bracings and motion coupling on the resonance features of a semi-submersible platform. An improved mathematical model based on potential theory is proposed to simulate the motion response of a semi-submersible platform under irregular wave conditions, considering both the variations of hydrostatic and hydrodynamic forces induced by the bracings entering and exiting the water and the nonlinear coupling induced by the platform motions. For comparison purposes, numerical simulations are also performed using a mathematical model without considering the aforementioned effects. Validated by results of wave basin tests and numerical simulations, the proposed model performs much better in capturing the characteristic resonance features of pitch motion in low-frequency region. The nonlinear hydrostatic effect of bracings leads to the increase of resonance frequency as the motion amplitude increases, while the hydrodynamic force on the bracings and the nonlinear motion coupling only influence the amplitude of resonance spectral peak. In addition, factors influencing the nonlinear effects such as the vertical position and diameter of bracings and the pitch restoring coefficient are further investigated. It is revealed that the deviation of pitch resonance frequency has evident dependence on the ratio between nonlinear and linear volumetric variations, and an empirical formula estimating the resonance frequency is proposed using the observed dependence. Theoretically, both smaller bracing radius and larger pitch restoring coefficient are beneficial for suppressing the resonance induced by the nonlinear effects. The proposed model can be an effective tool for predicting the motion response, and the understanding of the resonance features is helpful for the design of semi-submersibles.

Validation of a 750 kW semi-submersible floating offshore wind turbine numerical model with model test data, part II: Model-II

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

The influence of the centre bow and wet-deck geometry on motions of wave-piercing catamarans

The effects of tunnel height and centre bow length on the motions of a 112-m wave-piercer catamaran with an above-water centre bow were investigated through model tests. Five alternative centre bow configurations were considered, and multiple series of model tests were conducted in regular head sea waves. The results showed that both heave and pitch increased over a wide range of wave encounter frequency as the wet-deck height of the catamaran model increased. However, increasing the length of the centre bow showed an increase in the pitch but a decrease in the heave for a limited range of wave encounter frequency near the heave and pitch resonance frequencies of the catamaran model. The positions of minimum vertical displacement were found to be aft of the longitudinal centre of gravity, between 20% and 38% of the overall length from the transom. Increase in the wet-deck height and consequently the archway clearance between the main hulls and centre bow also resulted in an increase in the vertical displacement relative to the undisturbed water surface in the centre bow area. The results also indicated the vulnerability to wet-deck slamming for the different bow and wet-deck designs.

Fatigue damage characteristics of a semisubmersible-type floating offshore wind turbine at tower base

Fully coupled analysis of a semisubmersible-type floating offshore wind turbine is carried out for the fatigue damage investigation at tower base by using the code FAST. By combining the axial force and the bending moment, the stress around the circumference can be calculated. This study investigates the effect of second order wave force, wind load, mooring model, and tower elasticity on the stress response at tower base. There are three peaks in the stress amplitude spectrum, corresponding to pitch resonance, wave frequency, and first-order natural mode of the tower, respectively. The first and third peaks are mainly induced by wind load. The effect of the second order wave force and mooring model on the stress seems to be neglectable based on the numerical results. By using the rainflow counting method, the fatigue damage is calculated. The obtained fatigue damage under several environmental conditions indicates that the misalignment and coupling effect of wind and wave loads may significantly affect the total fatigue damage. The wind and wave induced responses are calculated separately (uncoupled method). Next, two superposition methods for fatigue damage calculation are analyzed: fatigue damage superposition and stress superposition. Compared with the fully coupled analysis considering wind and wave together, the former gives lower prediction but the latter seems to provide acceptable results. Finally, considering the long-term wind and wave distribution, the fatigue damage under different environmental conditions is calculated, and the fatigue damage obtained from stress superposition method shows reasonable agreement with that obtained from a fully coupled analysis.

Dynamic response and viscous effect analysis of a TLP-type floating wind turbine using a coupled aero-hydro-mooring dynamic code

This paper presents a coupled dynamic motion response analysis of a floating wind turbine using an in-house code, CRAFT (Coupled Response Analysis of Floating wind Turbine). Viscous drag forces on horizontal pontoons are carefully calculated, and a nonlinear spectral method is applied to efficiently solve the coupled tendon dynamics. Viscous drag forces and tendon dynamics are two important factors when assessing a tension-leg platform (TLP)-type floating wind turbine in a time-domain simulator. The analysis object is the NREL 5 MW Wind Turbine, which is supported by a three-leg mini-TLP platform. Simulations of the free decay and response amplitude operator (RAO) tests are conducted using CRAFT as well as FAST, another commonly used code. The obtained results are compared with experimental results to verify the capability of CRAFT. Viscous drag force induces higher harmonic pitch resonance, which is most prominent when the wave period is three times the natural period of the pitch and the wave height reaches a threshold. Springing motion is identified and found to be caused by this resonant pitch motion. Time-domain statistics show that extreme increases in tendon loads caused by springing as well as pitch and tendon tension probability distributions are non-Gaussian in random sea states. In addition, the resonant pitch motion is significantly reduced by aerodynamic damping.

Experimental Study on the Suppression of a Vertical Circular Cylinder with a Heave Plate

Studies of floating structures are becoming increasingly important due to their increased application. The hydrodynamic motion response of floating structures must be controlled within a certain range, especially in the resonance region. In this study, a scale model of a circular cylinder having a height of 300 mm, outer diameter 60 mm, natural heave period 0.78 s and a natural pitch period 1.12 s, was designed and analyzed for hydrodynamic load. We compared this model with other models of cylinders with various heave plates systems. In order to find the size effect of the heave plate, two heave plates of 60 and 80 mm in diameter with a thickness of 5 mm. At first, a free decay experiment was carried out to examine the damped heave and pitch period, as well as the hydrodynamic coefficients in the resonance frequency. The motion response was also analyzed to obtain the heave and pitch motion response in a series of regular waves having periods between 0.51 s and 1.24 s. It was observed that the heave plate significantly enhanced the damped heave period, whereas it had a negligible effect on the damped pitch period. Moreover, the peak of heave Response Amplitude Operator (RAO) was observed to be reduced for the heave plate in the resonance region and the peak of heave RAO was shifted to a long period. Furthermore, a comparatively small peak heave RAO was observed in the experiments. It was indicated that pitch motion has an effect on heave motion in the pitch resonance frequency.

Voicing the real Self

Voicing the real Self