Wind-wave System Research Articles

Modelling of a Floating-Type Hybrid Wind-Wave System with Oscillating Water Column Wave Energy Converters: A Study Towards Floater Motion Reduction

The integration of floating offshore wind turbines with wave energy converters is regarded as a promising solution for offshore renewable energy development. Given the early stage of wave energy conversion technologies and the substantial influence of control methods on overall system dynamics, a faithful aero-hydro-thermo-elastic-servo-mooring coupled model, along with an engineering environment offering high flexibility for control implementations, is essential. To address the requirement, a numerical modeling framework is developed in this study based on Simulink, known for its superiority in control design and implementation, and OpenFAST, which offers a reliable floating wind turbine model. The model incorporates the thermodynamics of the air in chambers, power take-off dynamics, and oscillating water column dynamics. Furthermore, bypass valves are utilized for the wave energy converters to adjust chamber pressure and reduce floater motion, with a control law proposed to regulate the valve opening ratio. A case study is conducted under harsh ocean conditions to validate the model. The numerical results not only demonstrate the feasibility of the model but also underscore the effectiveness of the control law in improving floater motion performance.

Introducing the Azimuth Cutoff as an Independent Measure for Characterizing Sea-State Dynamics in SAR Altimetry

This study presents the first azimuth cutoff analysis in Synthetic Aperture Radar (SAR) altimetry, aiming to assess its applicability in characterizing sea-state dynamics. In SAR imaging, the azimuth cutoff serves as a proxy for the shortest waves, in terms of wavelength, that can be detected by the satellite under certain wind and wave conditions. The magnitude of this parameter is closely related to the wave orbital velocity variance, a key parameter for characterizing wind-wave systems. We exploit wave modulations exhibited in the tail of fully-focused SAR waveforms and extract the azimuth cutoff from the radar signal through the analysis of its along-track autocorrelation function. We showcase the capability of Sentinel-6A in deriving these two parameters based on analyses in the spatial and wavenumber domains, accompanied by a discussion of the limitations. We use Level-1A high-resolution Sentinel-6A data from one repeat cycle (10 days) globally to verify our findings against wave modeled data. In the spatial domain analysis, the estimation of azimuth cutoff involves fitting a Gaussian function to the along-track autocorrelation function. Results reveal pronounced dependencies on wind speed and significant wave height, factors primarily determining the magnitude of the velocity variance. In extreme sea states, the parameters are underestimated by the altimeter, while in relatively calm sea states and in the presence of swells, a substantial overestimation trend is observed. We introduce an alternative approach to extract the azimuth cutoff by identifying the fall-off wavenumber in the wavenumber domain. Results indicate effective mitigation of swell-induced errors, with some additional sensitivity to extreme sea states compared to the spatial domain approach.

Experimental investigation of a co-located wind and wave energy system in regular waves

The integration of wave energy converters (WECs) into offshore wind farms offers a promising solution to potentially reduce the cost of both technologies. In a co-located wind-wave system, in addition to generating electricity, WECs can be used to reduce wave forces acting on the support structure of a wind turbine. This work aims to investigate the interactions between a fixed-bottom wind turbine substructure and a heaving WEC with a primary focus on wind turbine wave loads. For this purpose, physical experiments are developed and carried out using a 1:13 scale model of the WEC and the support structure of a wind turbine (monopile) in regular waves. The results demonstrate that active control of WEC motion can lead to a significant reduction in the horizontal forces acting on the wind turbine substructure.

Hydrodynamic analysis of a novel multi-buoy wind-wave energy system

Hybrid wind-wave systems combining the wave energy converters (WECs) with offshore wind turbines (OWTs) is a promising way to enhance the power production and improve the sea space utilization. In this paper, a novel hybrid wind-wave conceptual system, in which a multi-buoy WEC is integrated with a fixed monopile OWT, is proposed. This is the first concept utilizing multi-buoy WECs and is distinguished from existing hybrid wind-wave systems with a fixed monopile OWT, which integrate a single oscillating water column or a heaving point absorber. To characterize the hydrodynamics associated with the proposed system in operational wave conditions with different directionalities, a potential flow solver with an appropriate power take-off (PTO) model is applied. The results demonstrate a significant buoy-buoy and buoy-monopile hydrodynamic interaction, suggesting that the existing hydrodynamic characteristics for the wind-wave system with a single buoy WEC may not be applicable to the new system. More importantly, the power performance of the present system is proven to be better than the corresponding single-buoy wind-wave system, as being quantitatively assessed by the newly-defined evaluation index within the range of the consideration of this paper.

Study on hydrodynamic characteristics of a hybrid wind-wave energy system combing a composite bucket foundation and wave energy converter

In this paper, a new type of combined wind-wave system is proposed, that is, composite bucket foundation-oscillating buoy (CBF-OB) combined device. A three-dimensional numerical wave flume is established by using the renormalization group k–ε model. The hydrodynamic characteristics around the wave and the combined power generation device are studied. The relationship between wave parameters and wave run-up and wave pressure is analyzed. The absorption efficiency performance of the combined power generation device is evaluated. The results show that in the combined power generation system, the wave run-up and wave pressure at 0°–135° around the CBF are smaller than those in the presence of only CBF, but near 180° the ones are larger than those in the presence of only CBF. In the rear side of the combined power generation system, the smaller the scattering parameters, the more obvious the phenomenon of the second wave peak, and the stronger the nonlinearity of the wave and CBF-OB combined power generation system. The proposed CBF-OB combined power generation device can significantly improve the absorption efficiency of the buoy, which can be increased by about 1.5–4.0 times compared with the absorption efficiency under the action of only the buoy alone. There is an optimal power takeoff parameter, that is, when the damping parameter is 500 N·s/m, to maximize the absorption efficiency of the combined power generation device.

Spectral wave modeling of bimodal sea states at laboratory and coastal scales

A downshift of the wind wave peak frequency was observed in a wind wave tunnel when irregular long paddle-waves (i.e. generated mechanically with a plane wave-maker) are added in the sea state. The 3rd generation spectral wave model, TOMAWAC, is used to assess the extent at which this peak frequency downshift can take place at prototype scale in bimodal sea-state conditions involving swell and wind wave systems. Several parameterizations of the modeled physical processes are selected to numerically reproduce the laboratory experiments in the first place. Then, the model performances are further inquired in reproducing coastal observations during a specific event combining a wind wave and a swell system. Overall, a good agreement is obtained between the simulations and the observations both at laboratory and coastal scale. In particular, a set of parameterizations combining one of the latest developments in spectral wave models for the whitecapping dissipation and the nonlinear 4-wave interactions reveals high performances in reproducing the observations. Lastly, based on the performances of this latter set of parameterizations, a generic numerical domain with typical coastal scale dimensions is created to inquire the occurrence of the downshift at prototype scale. This last study reveals a wind wave peak period shift from 5 s without swell to more than 6.5 s with a 2 m high swell.

Characterization of wave energy resource hotspots and dominant wave energy systems in South Korean coastal waters

As wave climates around the South Korean coasts are not predominated by a particular wave system, but rather supplied by multiple wind-wave systems, the assessment of the total wave power, an integration of the wave power over all frequencies and directions, is not helpful to wave energy converter technologies, which ideally operate within a narrow frequency-directional band. This study develops a new methodology for discerning wave energy hotspots for potential energy project sites and identifies dominant wave systems contributing large energy to the total wave power at each hotspot by analyzing frequency-directionally-temporally resolved joint wave power distributions. The joint distributions were computed using bulk wave parameters obtained from the most recent wave measurements ranging from 2008 to 2019. Three wind-wave systems, i.e., year-round northeasterly, southeasterly in summer, and northwesterly in winter, are the main energy suppliers, and determine resource attributes (e.g., frequency-directional spreading and seasonal variability) within the hotspots. This study demonstrates that the assessment of the dominant wave systems differs from that of the total wave power. While previous studies have highlighted the waters near Southern Jeju as a potential project site, having the largest total wave power, we observe that the wave system with a 6.0–7.0 s peak period from the NNE direction near Western Ulleung exhibits a higher energy density and a lower seasonal variability, which is a more desirable resource. The present study supports the scoping of energy projects and the optimization of wave energy conversion design targeting the dominant wave system and desirable resource conditions.

Simulation-based study of wind—wave interactions under various sea conditions

Despite their importance in the ocean environment, the key physical processes in wind-wave interactions are poorly understood. Using a solver developed for undulatory boundaries, we perform numerical simulations of the wind-wave system under various sea conditions, including wind over monochromatic waves, early wind-wave generation, and wind over a broadband wave field. Our results show that the wave direction and wave age can significantly change the distribution of streamwise vorticity in the wind field. Different wave patterns are observed in the process of wind-wave generation. In a broadband wave field, the wave growth rate due to wind input is found to depend on the wave steepness.

Monopile-mounted wave energy converter for a hybrid wind-wave system

Multipurpose platforms are innovative solutions to combine the sustainable exploitation of multiple marine resources. Among them, hybrid wind-wave systems stand out due to the multiple synergies between these two forms of marine renewable energy. The objective of this work is to develop a hybrid system for monopile substructures, which are currently the prevailing type of substructure for offshore wind turbines, and more specifically to focus on the wave energy converter sub-system, which consists in an oscillating water column. For this purpose, an in-depth experimental campaign was carried out using a 1:40 scale model of the wave energy converter sub-system and the monopile substructure, considering regular and irregular waves. Based on the experimental results the performance of the device and its interaction with the wave field were characterised – a fundamental step to fully understand the benefits and limitations of this hybrid wind-wave system, which sets the basis for its future development. Regarding the performance, the best efficiency was obtained with the turbine damping corresponding to a 0.5% orifice size, and two resonance peaks were identified (T = 9 and 6 s). As for the interaction of the hybrid system with the wave field, between 5% and 66% of the incident wave power is reflected and between 3% and 45%, transmitted. The wave period was found to be the parameter that most influenced wave run-up on the substructure. This characterisation of the behaviour of the hybrid system shows that it is indeed a promising option for further development.

Hydrodynamic response of the WEC sub-system of a novel hybrid wind-wave energy converter

Multiple marine resources are usually available in the same area and synergies between the different users of these resources exist. Multipurpose platforms, which combine more than one of these renewable resources, have been proposed as a sustainable approach. One type of multipurpose platforms is the hybrid wind-wave systems, in which a single platform combines the exploitation of offshore wind and wave energy. In this paper a novel hybrid system that integrates an oscillating water column (OWC) wave energy converter (WEC) with an offshore wind turbine on a monopile substructure is considered. The main objective of this paper is to define and test a simplified version of the WEC sub-system of this hybrid energy converter. An experimental campaign was carried out to characterise the hydrodynamic response of a 1:37.5 scale model of the WEC sub-system under regular and irregular waves. On the basis of the data from the experimental campaign, the hydrodynamic response of the WEC sub-system is characterised in four steps: (i) through an incident and reflected wave analysis (IRWA), to characterise the interaction between the device and the waves; (ii) through the capture width ratio, to study the performance of the device; (iii) through response amplitude operators (RAOs) of the free surface elevation and pneumatic pressure inside the OWC chamber, to study the effects of the incident waves on the device response; and (iv) through the wave run-up on the device. The results from this multifaceted analysis lead to the proof of concept of this novel hybrid system, supporting its feasibility to be combined with offshore wind substructures; but also to characterise its behaviour and interaction with the wave field, essential to full understanding of the benefits of hybrid systems.

A Novel Hybrid Wind-Wave Energy Converter for Jacket-Frame Substructures

The growth of the offshore wind industry in the last couple of decades has made this technology a key player in the maritime sector. The sustainable development of the offshore wind sector is crucial for this to consolidate within a global scenario of climate change and increasing threats to the marine environment. In this context, multipurpose platforms have been proposed as a sustainable approach to harnessing different marine resources and combining their use under the same platform. Hybrid wind-wave systems are a type of multipurpose platform where a single platform combines the exploitation of offshore wind and wave energy. In particular, this paper deals with a novel hybrid wind-wave system that integrates an oscillating water column wave energy converter with an offshore wind turbine on a jacket-frame substructure. The main objective of this paper is to characterise the hydrodynamic response of the WEC sub-system of this hybrid energy converter. A 1:50 scale model was tested under regular and irregular waves to characterise the hydrodynamic response of the WEC sub-system. The results from this analysis lead to the proof of concept of this novel hybrid system; but additionally, to characterising its behaviour and interaction with the wave field, which is a requirement for fully understanding the benefits of hybrid systems.

Effective Fetch and Duration of Tropical Cyclone Wind Fields Estimated from Simultaneous Wind and Wave Measurements: Surface Wave and Air–Sea Exchange Computation

AbstractSimultaneous wind and wave measurements have been obtained inside tropical cyclones in several hurricane hunter missions. Analyses of these datasets show that the surface wave development inside hurricanes follows essentially the same duration- and fetch-limited growth functions established in steady wind forcing conditions. This paper explores the application of several parameterization functions of wind-wave systems to quantify the energy and momentum exchanges inside hurricanes from an initially limited input of the environmental parameters, such as the wind field alone. A critical prerequisite to applying the wind-wave growth functions is the knowledge of fetch and duration for the hurricane wind field. Four sets of simultaneous wind and wave measurements from hurricane hunter missions are analyzed to derive a fetch and duration scaling model. Time series of 2D hurricane wind fields can then be used to investigate the detailed spatial distribution and temporal evolution of the sea state parameters and the associated air–sea energy and momentum exchanges following the hurricane development.

A fully coupled atmosphere–ocean wave modeling system for the Mediterranean Sea: interactions and sensitivity to the resolved scales and mechanisms

Abstract. It is commonly accepted that there is a need for a better understanding of the factors that contribute to air–sea interactions and their feedbacks. In this context it is important to develop advanced numerical prediction systems that treat the atmosphere and the ocean as a unified system. The realistic description and understanding of the exchange processes near the ocean surface requires knowledge of the sea state and its evolution. This can be achieved by considering the sea surface and the atmosphere as a continuously cross-talking dynamic system. Following and adapting concepts already developed and implemented in large-scale numerical weather models and in hurricane simulations, this study aims to present the effort towards developing a new, high-resolution, two-way fully coupled atmosphere–ocean wave model in order to support both operational and research activities. A specific issue that is emphasized is the determination and parameterization of the air–sea momentum fluxes in conditions of extremely high and time-varying winds. Software considerations, data exchange as well as computational and scientific performance of the coupled system, the so-called WEW (worketa-wam), are also discussed. In a case study of a high-impact weather and sea-state event, the wind–wave parameterization scheme reduces the resulted wind speed and the significant wave height as a response to the increased aerodynamic drag over rough sea surfaces. Overall, WEW offers a more realistic representation of the momentum exchanges in the ocean wind–wave system and includes the effects of the resolved wave spectrum on the drag coefficient and its feedback on the momentum flux.

Evaluation of a Wind-Wave System for Ensemble Tropical Cyclone Wave Forecasting. Part II: Waves

Abstract A wind-wave forecast system, designed with the intention of generating unbiased ensemble wave forecasts for extreme wind events, is assessed. Wave hindcasts for 12 tropical cyclones (TCs) are forced using a wind analysis produced from a combination of the North American Regional Reanalysis (NARR) and a parametric wind model. The default drag parameterization is replaced by one that is more in line with recent studies where a cap at weak-to-moderate wind speeds is applied. Quadrant-based significant wave height (Hs) statistics are composited in a storm-relative reference frame and stratified by the radius of maximum wind, storm speed, and storm intensity. Improvements in Hs are gleaned from both downscaling the NARR winds and tuning the wave model. However, the paradigm whereby the drag coefficient depends solely on the wind speed is limiting. Results indicate that Hs is biased low in the right quadrants (for all statistical subcategories). Conversely, Hs is high biased in the left-rear quadrant even though the analysis wind field is underforecast there. At radii less than 100 nautical miles, the model peak wave direction is offset from the observed, with the model (buoy) peak more in line with (to the left of) the direction of the tropical cyclone motion. As a result, the predominant storm-relative wind direction, which is northwesterly in the left-rear quadrant, opposes that of the buoy peak wave direction, while the model peak is more crosswise with respect to the wind. This will likely reduce the magnitude of the wind stress in the model.

Evaluation of a Wind-Wave System for Ensemble Tropical Cyclone Wave Forecasting. Part I: Winds

Abstract A computationally efficient method of producing tropical cyclone (TC) wind analyses is developed and tested, using a hindcast methodology, for 12 Gulf of Mexico storms. The analyses are created by blending synthetic data, generated from a simple parametric model constructed using extended best-track data and climatology, with a first-guess field obtained from the NCEP–NCAR North American Regional Reanalysis (NARR). Tests are performed whereby parameters in the wind analysis and vortex model are varied in an attempt to best represent the TC wind fields. A comparison between nonlinear and climatological estimates of the TC size parameter indicates that the former yields a much improved correlation with the best-track radius of maximum wind rm. The analysis, augmented by a pseudoerror term that controls the degree of blending between the NARR and parametric winds, is tuned using buoy observations to calculate wind speed root-mean-square deviation (RMSD), scatter index (SI), and bias. The bias is minimized when the parametric winds are confined to the inner-core region. Analysis wind statistics are stratified within a storm-relative reference frame and by radial distance from storm center, storm intensity, radius of maximum wind, and storm translation speed. The analysis decreases the bias and RMSD in all quadrants for both moderate and strong storms and is most improved for storms with an rm of less than 20 n mi. The largest SI reductions occur for strong storms and storms with an rm of less than 20 n mi. The NARR impacts the analysis bias: when the bias in the former is relatively large, it remains so in the latter.

Ocean wave mapping from ERS SAR images in the presence of swell and wind-waves

We present a method for estimating the significant wave height of ocean waves (SWH) in coastal zones from satellite SAR images and using the value of the width of the spectral azimuthal cutoff. The method deals with the case of an energetic swell propagating over a wind wave system. It requires the preliminary retrieval of swell parameters using the wave-SAR spectral quasi-linear relationship. The method is applied to two ERS-1 SAR images in coastal zones.

On the Transversal Instability of a Coupled Wind-Wave System at the Initial Stage of Wave Growth

Abstract The evolution of the growing surface waves having a nonuniform transversal structure is discussed using the semiempirical equation for wind velocity profile in two dimensions and the expression suggested by Plant and Wright for the initial growth rate of waves. Positive feedback has been found in the coupled wind-wave system, leading to an algebraic growth of a weak transversal modulation of the wave field.

A new model for nonlinear wind waves. Part 1. Physical model and experimental evidence

A new interpretation of a nonlinear wind-wave system is proposed. It is proposed that, for steady wind blowing in one direction, a nonlinear wind-wave system can be completely characterized, to a good first approximation, by a single nonlinear wave train having a carrier frequency equal to that of the dominant frequency in the wind-wave spectrum. In this model, the spectral components of the wind-wave system are not considered a random collection of free waves, each obeying the usual dispersion relation, but are effectively non-dispersive bound-wave components of a single dominant wave, travelling at the speed of the dominant wave. To first order, the nonlinear wind-wave system is considered to be a coherent bound-wave system which propagates energy only at the group velocity of the dominant wave and is governed by nonlinear self-interactions of the type found in amplitude-modulated wave trains. The role of short free waves in the system is discussed. Results of laboratory experiments performed by the authors and by Ramamonjiarisoa & Coantic (1976) are found to provide evidence supporting the applicability of such a model to wind waves under virtually all laboratory conditions. Preliminary consideration is given to possible application of the model to oceanic wind waves and conditions are identified for which the model would be most likely to apply.

Growth and equilibrium of short gravity waves in a wind-wave tank

Temporal and spatial development of short gravity waves in a linear wind-wave tank has been measured for wind speeds up to 15 m/s using microwave Doppler spectrometry. Surface waves of wavelength 4·1 cm, 9·8 cm, 16·5 cm and 36 cm were observed as a function of fetch, wind speed and wind duration. The waves grew exponentially from inception until they were about 10 dB smaller than their maximum height, and the temporal growth and spectral transport (spatial growth) rates were about equal when the wave amplitude was sufficiently small. The amplitude of a short gravity wave of fixed wavelength was found to decrease substantially at winds, fetches or durations greater than those at which the short gravity wave was approximately the dominant wave; such phenomena are sometimes referred to as overshoot. The dominant short gravity wave was observed to reach a maximum amplitude which depended only on wavelength, showing that wave breaking induced by an augmented wind drift cannot be the primary limitation to the wave height. Waves travelling against the wind were observed for wavelengths of 9·8 cm, 16·5 cm and 36 cm and were shown to be generated by the air flow at low wind speeds.Measured initial growth rates for 16·5 cm and 36 cm waves were greater than expected, suggesting the existence of a growth mechanism in addition to direct transfer from the wind via linear instability of the boundary-layer flow. Initial temporal growth rates and spectral transport rates were compared to yield an experimental determination of the magnitude of the sum of nonlinear interactions and dissipation in short gravity waves. If the steady-state energy input in the neighbourhood of the dominant wave occurs at the measured initial temporal growth rates, then most of the energy input is locally dissipated; relatively little is advected away. Calculated gravitycapillary nonlinear energy transfer rates match those determined from initial growth rates for 9·8 cm waves and the gravity–capillary wave interaction continues to be significant for waves as long as 16·5 cm. For longer waves the gravity–capillary interaction is too small to bring the short gravity wave to a steady state when it is the dominant wave of the wind-wave system.