Wave Tank Experiments Research Articles

On the survivability of WECs through submergence and passive controllers

The survivability of WECs during extreme seas and heavy storms has proven to be challenging during the deployment of devices as they often fail during extreme storms. The survival mechanism is often inherited with the device design and mode of operation. It is not economically and logistically viable for devices to be taken back onshore in case of storms. Some devices lock their PTOs during heavy storms and others lock all the moving parts all together, in the case of CETO, its design has an advantage where it can be submergepad. CETO is a buoyant actuator WEC composed of a buoy submerged close to the surface. Three tethers connect CETO to the sea-bottom through rotary PTOs, allowing the device to be wound down, submerged, and therefore, less exposed to the extreme loads at the sea surface during large storms. This paper will study the survivability of the device during extreme sea-states and will examine the required depth to bring its response back to operational conditions. This work will also look at the alteration of some passive controllers, such as a conventional spring to minimize the response of the device instead of maximising the power capture. With the PTO objective altered in extreme sea-states to minimize the response instead of capturing power, the possibility of harvesting power during extreme sea-states with the device submerged will be checked. Finally, the survivability strategy of CETO through submergence will be showcased with wave tank experiments conducted at IHC as part of the Europewave program.

Towards accurate stereo-video based free-surface reconstruction for wave tank experiments

Measuring accurately the shape of the free surface in medium-scale hydrodynamic testing facilities is challenging with conventional wave gauges. To overcome this problem, it is necessary to go towards higher resolution measurement systems. For this purpose, we investigate the ability of stereo-video acquisition systems to reconstruct free surface waves in laboratory conditions. This technique has been proven to give satisfactory results in open sea conditions, although it is dependent on the environmental conditions (weather, solar incidence angle). At sea, the stereo-video reconstruction algorithm makes profit of the short waves generated by the wind in order to correlate the points from the left and right images. The main challenge of adapting this technique to laboratory conditions, i.e. in absence of sun and wind, is to overcome the absence of texture on the free surface. This paper presents recent work aiming at developing a stereo-vision measurement system for laboratory conditions. Our efforts have been devoted to finding an adapted lighting system and generating a texture suitable for the reconstruction algorithm. Different lighting configurations have been tested in order to understand its impact on the reconstructions. For the texture generation, we have investigated different means of generating short surface waves (water droplets, air blowing, circulation current, underwater acoustic emissions) in different experimental facilities. Our conclusions show that it is possible to reconstruct the shape of surface gravity waves in the presence of short surface waves and that the effects of lighting and texture on the quality of the reconstruction are strongly coupled.

Phase-resolved real-time forecasting of three-dimensional ocean waves via machine learning and wave tank experiments

Accurate prediction of ocean waves plays an essential role in many ocean engineering applications, such as the control of wave energy converters and floating wind turbines. However, existing studies on phase-resolved wave prediction using machine learning mainly focus on two-dimensional wave data, while ocean waves are usually three-dimensional. In this work, we investigate, for the first time, the phase-resolved real-time prediction of three-dimensional waves using machine learning methods. Specifically, the wave prediction is modeled as a supervised learning task aiming at learning mapping relationships between the input historical wave data and the output future wave elevations. Four frequently-used machine learning methods are employed to tackle this task and a novel Dual-Branch Network (DBNet) is proposed for performance improvement. A group of wave basin experiments with nine directional wave spectra under three sea states are first conducted to collect the data of 3D waves. Then the wave data are used for verifying the effectiveness of the machine learning methods. The results demonstrate that the upstream wave data measured by the gauge array can be used for control-oriented wave forecasting with a forecasting horizon of more than 20 s, where the directional information provided by the upstream gauge array is vital for accurately predicting the downstream wave elevations. In addition, further investigations show that by using only local wave data (which can be easily obtained), the very short-term phase-resolved prediction (less than 5 s) can be achieved.

Experimental assessment of a nonlinear, deterministic sea wave prediction method using instantaneous velocity profiles

This paper presents the experimental proof-of-concept of a nonlinear, deterministic wave prediction method. The method is based on the adapted version of the HOS-NWT wave model and uses wave-induced velocity profiles as input information on the sea state. Unlike most HOS approaches, it does not need any optimization procedure or data assimilation step to initialize the simulation. Wave tank experiments were conducted to validate the method for irregular, long-crested waves with a low significant steepness. Data was collected using wave probes, an ADV and the fifth beams of two ADCP sensors set-up with a High-Resolution mode. Despite challenging experimental conditions for the ADCPs, the method proved able to reconstruct reliable instantaneous horizontal velocity profiles from the acoustic sensor measurements. These profiles were used as boundary conditions in the wave prediction model. The sea surface elevation predicted was compared to wave probe measurements and showed good agreement all over the theoretical prediction area.

Modeling of a hinged-raft wave energy converter via deep operator learning and wave tank experiments

Model identification for a hinged-raft wave energy converter (WEC) is investigated in this paper, based on wave tank experiments and deep operator learning. Different from previous works which all formulated this issue as a function approximation task, this work, for the first time, formulates it as an operator approximation task (which learns the mapping from a function space to another function space). As such, a continuous-time WEC model is identified from data, greatly expanding the horizon of data-based WEC modeling because previous works were limited to discrete-time model identification. The error accumulation for multi-step predictions in the discrete-time formulation is thus also addressed. The model is developed by first carrying out a set of wave tank experiments to generate the training data, and then the deep operator learning model, i.e. the DeepONet, is constructed and trained based on the experimental data. The validation study shows that the model captures the WEC dynamics accurately. A new set of experimental runs are further carried out and the results show that after training, the model can be used as a digital wave tank, an alternative to the expensive numerical and physical wave tanks, for accurate and real-time simulations of the WEC dynamics.

A bio-inspired foldable-wing wave energy converter for ocean robots

In remote and harsh sea areas, ocean robots are practical tools to replace humans to execute ocean operations. However, limited power supply constrains the performance of ocean robots, especially in terms of duration time and operation range. Inspired by flying fish, a foldable-wing wave energy converter (FW-WEC) is proposed for ocean robots to extract energy from wave. Equipped with the wave-energy wing, the FW-WEC is capable of switching from the WEC mode that possesses large energy capturing surface to the robot mode with the characteristic of compact structure, which allows for the installation on robots. To satisfy the requirements of the wave-energy wing, several foldable structures are considered and evaluated based on the defined performance index. With the serial four-bar linkage selected and adopted, a prototype of FW-WEC is designed, constructed and tested. The hydrodynamic model is established to analysis the motion response of the prototype under regular wave, of which the accuracy is verified compared with experimental results. According to the wave tank experiment carried out under different wave conditions, the maximum power output of FW-WEC reaches up to 2.6 W and the corresponding capture width ratio is calculated to be 8.0 %. The proposed FW-WEC offers a promising solution for the enhancement of endurance capability of ocean robots.

Representative linearised models for a wave energy converter using various levels of force excitation

In guiding the progression, development, and operation of wave energy converters (WECs) in a more efficient way, mathematical analysis and understanding of the dynamic process is essential. Mathematical WEC models, obtained either by numerical analysis or physical modelling, form the basis of most (model-based) energy maximising control strategies available in the literature, where experimental design and system identification methodology directly impact the resulting model. This study, using an experimental-based WEC model (which can be used for linear control design), investigates the dynamic behaviour of a WEC by analysing the dominant poles of the system, generated using fully nonlinear computational fluid dynamics (CFD)-based numerical wave tank (NWT) experiments. The aim is to effectively track the dominant dynamics of the WEC, using different force-input amplitude levels in the NWT setup, and perform a comparison with the classical linear boundary-element-methods (BEM) equivalent methodology. Thus, the presented case studies are shown to agree with previously proposed model assessment of linear WEC models, based on a free-decay NWT setup. In addition, the representative WEC models determined as part of this study can be used for WEC controller design, either singly, or using a form of model/controller gain scheduling.

Data-driven, high resolution ocean wave forecasting and extreme wave predictions

Reliable ocean wave forecasts are critical for reducing the risk posed by extreme waves for ships and offshore infrastructure. Despite insights gained from theoretical investigations and constantly improving operational wave forecasting systems, the emergence of extreme waves remain unpredictable. In this article, the authors propose a data-driven modelling approach to generate high resolution wave forecasts for capturing individual waves, including rogue waves. To this end, a decomposition of the sea surface into a set of rapid oscillations and slowly varying amplitudes is utilized. The slow amplitude variations are subsequently forecasted by using universal, data-driven methods. In this approach, the extrapolation range of the data-driven techniques is extended by the slowness of the amplitude variations. The method’s capabilities are demonstrated by using available measurements from an experimental wave tank and field data from ocean buoys.

Extreme wave excitation from localized phase-shift perturbations.

The modulation instability is a focusing mechanism responsible for the formation of strong wave localizations not only on the water surface, but also in a variety of nonlinear dispersive media. Such dynamics is initiated from the injection of sidebands, which translate into an amplitude modulation of the wave field. The nonlinear stage of unstable wave evolution can be described by exact solutions of the nonlinear Schrödinger equation(NLSE). In that case, the amplitude modulation of such coherent extreme wave structures is connected to a particular phase-shift seed in the carrier wave. In this Letter, we show that phase-shift localization applied to the background, excluding any amplitude modulation excitation, can indeed trigger extreme events. Such rogue waves can be for instance generated by considering the parametrization of fundamental breathers, and thus by seeding only the local phase-shift information to the regular carrier wave. Our wave tank experiments show an excellent agreement with the expected NLSE hydrodynamics and confirm that even though delayed in their evolution, breather-type extreme waves can be generated from a purely regular wave train. Such a focusing mechanism awaits experimental confirmation in other nonlinear media, such optics, plasma, and Bose-Einstein condensates.

Performance analysis of the SJTU-WEC based on the wave-to-wire model and the wave tank experiment

The development of wave energy has attracted wide attention. This paper proposes a conceptual design of a station featuring direct drive 3-degree-of-freedom (3-DoF) wave energy converter (SJTU-WEC), of which the numerical and experimental performances are studied. This article first introduces the power generation principle of the SJTU-WEC, of which the wave-to-wire (W2W) model is simplified and established as a 2-DoF model, including frequency domain and time domain. Then a 1:12.5 prototype is manufactured and tested in wave tank to study its performance. By comparing numerical and experimental results, the accuracy of the W2W model is verified. The experimental results show that the energy conversion efficiency reaches up to 28% under the specific wave condition. Finally, based on the W2W model, the power generation performance of the full-scale SJTU-WEC is obtained. In Lianyungang Sea areas, the average annual power generation of a single SJTU-WEC is calculated to be 108W.

Environmental design load for the line force of a point-absorber wave energy converter

To ensure a reliable operation over the life time of wave energy converters (WECs), a number of load cases need to be considered according to international standards for marine structures to determine an optimal design. This paper outlines the procedure of obtaining an environmental design load for the line force of a 1:30 scaled point-absorber WEC using an environmental contour with a 50-year return period for the Dowsing site in the North Sea. To obtain the response of the WEC during extreme conditions, a numerical WEC-Sim model is developed and calibrated with experimental wave tank tests to augment the data required for such design load analysis. The design load for the line force is estimated based on the full long-term extreme response computed from the full sea state approach by considering 180, 360, and 720 sea state samples as well as the contour approach for the sea state that gives the most extreme response. Further, a probabilistic approach is used to quantify the probability of failure for a critical mechanical component of the system such as shackle. The result shows that the numerical WEC-Sim model is able to sufficiently replicate the real response of the system during extreme irregular waves. The Bayesian theory with Monte-Carlo algorithm is found to be an excellent tool for identifying the degree of belief in the statistical models used for the short-term extreme response analysis. Considering the ultimate limit state, the design load for the 1:30 scaled system is calculated as 670.95 N (i.e. 18.11 MN for a full-scale system) after applying the partial load safety factor of 1.35 on the full long-term extreme response of the system for the 9.1 years return period (i.e. 50 years in a full-scale model).

Oscillating Water Column (OWC) Wave Energy Converter Part 1: Fixed OWC

This study reviews the recent development and research results of a fixed oscillating water column (OWC) wave energy converter (WEC). The OWC WEC can be divided into fixed and floating types based on the installation location and movement of the structure. In this article, the study on a stationary OWC WEC, which is close to commercialization through the accumulation of long-term research achievements, is divided into five research categories with a focus on primary energy conversion research. These research categories include potential-flow-based numerical analysis, wave tank experiments, computational fluid dynamics analyses toward investigation of fluid viscous effects, U-shaped OWC studies that can amplify water surface displacement in the OWC chamber, and studies on OWC prototypes that have been installed and operated in real sea environments. This review will provide an overview of recent research on the stationary OWC WEC and basic information for further detailed studies on the OWC.

Development of a CFD-based numerical wave tank of a novel multipurpose wave energy converter

The development of new wave energy converters usually involves small-scale experiments in physical wave tanks. The jump to physical models at larger scales is an expensive and time-consuming process that can be supported by computational fluid dynamics (CFD) models. Using the CFD approach, it is possible to numerically simulate complex flows with high accuracy, once validation with experimental data has been carried out. This article describes an approach based on guidelines taken from the literature and adopted to develop and explore the capabilities of a CFD-based numerical wave tank for a novel multipurpose wave energy converter, REEFS. An incremental validation procedure, using experimental data collected with a piston-type wave tank, was adopted; the procedure began with wave-only tests which were followed by wave-structure interaction tests. Snapshots of numerical and experimental approaches were used to analyse fluid flow in the envelope of the REEFS converter. The results demonstrate that the CFD-based numerical wave tank model can adequately simulate the global wave surface profile, as well as local complex phenomena, such as the Venturi aspiration effect, that typically occur near the exterior stay vanes of the device. The results encourage the adoption of this model for future REEFS analysis.

Phase-resolved real-time ocean wave prediction with quantified uncertainty based on variational Bayesian machine learning

Phase-resolved wave prediction is of vital importance for the real-time control of wave energy converters. In this paper, a novel wave prediction method is proposed, which, to the authors’ knowledge, achieves the real-time nonlinear wave prediction with quantified uncertainty (including both aleatory and model uncertainties) for the first time. Moreover, the proposed method achieves the prediction of the predictable zone without assuming linear sea states, while all previous works on predictable zone determination were based on linear wave theory (which produces overly conservative estimations). The proposed method is developed based on the Bayesian machine learning approach, which can take advantage of machine learning model’s ability in tackling complex nonlinear problems, while taking various forms of uncertainties into account via the Bayesian framework. A set of wave tank experiments are carried out for evaluation of the method. The results show that the wave elevations at the location of interest are predicted accurately based on the measurements at sensor location, and the prediction uncertainty and its variations across the time horizon are well captured. The comparison with other wave prediction methods shows that the proposed method outperforms them in terms of both prediction accuracy and the length of the predictable zone. Particularly, for short-term wave forecasting, the prediction error by the proposed method is as much as 55.4% and 11.7% lower than the linear wave theory and deterministic machine learning approaches, and the predictable zone is expanded by the proposed method by as much as 74.6%.

LainePoiss®—A Lightweight and Ice-Resistant Wave Buoy

Abstract Wave buoys are a popular choice for measuring sea surface waves, and there is also an increasing interest for wave information from ice-covered water bodies. Such measurements require cost-effective, easily deployable, and robust devices. We have developed LainePoiss (LP)—an ice-resistant and lightweight wave buoy. It calculates the surface elevation by double integrating the data from the inertial sensors of the microelectromechanical system (MEMS), and transmits wave parameters and spectra in real time over cellular or satellite networks. LP was validated through 1) sensor tests, 2) wave tank experiments, 3) a field validation against a Directional Waverider, 4) an intercomparison of several buoys in the field, and 5) field measurements in the Baltic Sea marginal ice zone. These extensive field and laboratory tests confirmed that LP performed well (e.g., the bias of Hm0 in the field was 0.01 m, with a correlation of 0.99 and a scatter index of 8%; the mean absolute deviation of mean wave direction was 7°). LP was also deployed with an unmanned aerial vehicle and we present our experience of such operations. One issue that requires further development is the presence of low-frequency artifacts caused by the dynamic noise of the gyroscope. For now, a correction method is presented to deal with the noise. Significance Statement Operational wave buoys are large and therefore expensive and inconvenient to deploy. Many commercially available devices cannot measure short waves and are not tested in ice. Our purpose was to develop an affordable wave buoy that is lightweight, ice resistant, capable of measuring short waves, and also has a longer operating life than existing research buoys. The buoy is easily deployed with a small boat or even an industrial drone, thus reducing operating costs. The buoy is accurate, and captures waves that are too short for operational wave buoys. This is relevant for coastal planning in, e.g., archipelagos and narrow fjords. We measured waves in ice in the Baltic Sea, and are planning to extend these measurements to Antarctica.

Non-homogeneous analysis of rogue wave probability evolution over a shoal

Non-equilibrium evolution of wave fields, as occurring over sudden bathymetry variations, can produce rogue seas with anomalous wave statistics. We handle this process by modifying the Rayleigh distribution through the energetics of second-order theory and a non-homogeneous reformulation of the Khintchine theorem. The resulting probability model reproduces the enhanced tail of the probability distribution of unidirectional wave tank experiments. It also describes why the peak of rogue wave probability appears atop the shoal, and explains the influence of depth on variations in peak intensity. Furthermore, we interpret rogue wave likelihoods in finite depth through the $H$ – $\sigma$ diagram, allowing a quick prediction for the effects of rapid depth change apart from the probability distribution.

Assessment and improvement of the wave generation accuracy using a wave separation method

Free-wave contamination is a non-desirable but frequent characteristic in many wave tank experiments. Higher-order wave generation is not widely implemented yet, whereas the effectiveness of active wave absorption systems is frequency-dependent, and usually unknown. The resulting free-wave contamination usually remains unnoticed, but may have important consequences especially at low frequencies. We present a Generation–Separation–Compensation (GSC) procedure to identify, quantify and attenuate the free-wave contamination. The resulting wave field behaves as expected from a higher-order wave generation with no re-reflections at the wave paddle. The attenuation ratio of the unwanted energy content is relatively high (between 60% to 90%).The GSC procedure uses the general framework for wave separation proposed by Padilla and Alsina (2020) with an array of in-line probes. The application of this technique over experimental data is presented with excellent results. The robustness of the qualitative analysis is guaranteed for experimental settings fulfilling a minimum distance between the first and the last probe and a maximum spacing between probes. The stability of the quantitative analysis is guaranteed when P, number of probes forming the array, is above five, and Δxsep/λ, ratio between the spacing of the probes forming the array and the wavelength, is within the recommended range 0.1–0.45.

Experimental investigation of a point-absorber wave energy converter response in different wave-type representations of extreme sea states

The experimental results of a 1:30 scaled wave tank experiment of a point-absorber in extreme sea states and intermediate water depth are studied. The effect of the PTO damping parameter and different non-linear phenomena in extreme wave conditions such as wave breaking and overtopping are investigated with the focus on the maximum line (mooring) force in the presence of an upper end-stop. In the comparison of different wave-type representations, i.e. irregular, regular, and focused waves, of the same sea state, not all the wave types necessarily yield to the same peak line force. Moreover, there exists an optimum damping value for each sea state in which the smallest peak force is achieved. Both end-stop spring compression and wave breaking slamming result in peak line forces which may be compensated by overtopping water pressure. Large surge motion is obtained for waves with a long wavelength which can contribute to higher and more damaging line forces.

Effects of Oil and Sediment Properties and Mixing States on the Formation and Settling of Oil‐Particle Aggregates

AbstractUnderstanding the interaction of oil droplets with suspended particulate matter is important not only for sunken oil modeling but also for emergency response. In this study, the effects of mixing time and energy, oil type, and sediment concentration and size on the oil content, morphology, and density of oil‐particle aggregates (OPAs) were investigated through a series of wave tank experiments and sedimentation tests. The results showed that loads of oil and particles in OPAs are high under high mixing energy. Under turbulent hydrodynamics, moderately viscous oils form OPAs more easily and sink rapidly to the seabed. Moreover, an increase in the density and oil capacity of OPAs is observed at a higher sediment concentration. More oil is captured by finer sediments. However, the corresponding oil‐sediment aggregates generally show lower density compared with those formed by larger sediments. The oil‐seawater interfacial tension (IFTow) diminishes with the addition of sediments, further promoting oil dispersion. The increasing duration and mixing energy produce more flake OPAs and multiple‐droplet OPAs. Oil droplets tend to be surrounded by several small particles but attach to the surface of large particles. Regardless of oil type and sediment concentration, the structure of OPAs does not change significantly.

Varying ocean wave statistics emerging from a single energy spectrum in an experimental wave tank

As it strongly impacts the design of offshore structures, the realistic reproduction of sea states in experimental and numerical wave tanks is of great interest to the ocean engineering community. The vast majority of wave qualification procedures rely on the accurate control of i) the wave energy spectrum and ii) the wave crest statistics at a target location in the domain. However the control of the wave field is strongly challenged by nonlinear phenomena such as breaking and high-order nonlinearities, which are at the origin of significant variations of the wave properties along the tank. Considering this issue, the most common industry methodologies focus on reproducing the wave energy spectrum at the target position. The wave crest statistics are compared to reference distributions. The present study aims to explore the limitations of such a practice, investigating in detail the wave field properties for various target locations over a long domain. We address the problem within the framework of deep water long-crested irregular waves. In this respect, using the Ecole Centrale de Nantes (ECN) experimental facility, a specific sea state is consecutively generated at three positions of the wave tank using a dedicated procedure based on wave maker motion iterative corrections. For such nonlinear wave conditions the wave crest statistics are known to be enhanced along the tank by high-order nonlinearities. As a result, configurations characterized by identical wave spectra lead to the generation of different wave crest distributions, revealing an increasing number of extreme events as the spectrum is generated farther from the wave maker. The data yielded by the study provide convincing evidence that controlling the wave field at a target location by correcting the sole energy spectrum is insufficient. Particular attention must be paid to the nonlinear spatial dynamics of the wave field in order to control the wave crest statistics.