Real Sea Conditions Research Articles

The improvement of stochastic averaging method to solve the ship rolling response excited by narrow-band waves

The stochastic averaging method of energy envelope can accurately solve the random motion response under the excitation of white noise. The noise had to fulfill the condition of strong mixing which roughly means that the process has to be broad-band. However, the sea conditions in practical engineering conditions are narrow-band spectra. Research on applicability of the stochastic averaging method of energy envelope shows that for narrow-band excitation, the damping of the system and excitation intensity should be small, and the excitation bandwidth should be large. Based on the above background, the improvement of stochastic averaging method of energy envelope under narrow-band waves is studied. The phase angle θ is introduced to characterize the relationship between kinetic energy and potential energy of the system. The drift coefficients and diffusion coefficients are averaged with respect to the period of θ. When the excitation frequency is in the lock-in field, large parametric rolling appears. The stationary probability density function (PDF) of the roll angle obtained by solving the Fokker-Planck-Kolmogorov (FPK) equation compares well with Monte Carlo simulation in the lock-in field. The improved method can relax the limitation of broad-band excitation required by stochastic averaging method. It can also consider both the nonlinear damping and nonlinear restoring force of roll motion. Therefore, the improved method is an efficient way to calculate the nonlinear roll response of ships in real sea conditions.

Peak shaving control in OWC wave energy converters: From concept to implementation in the Mutriku wave power plant

Control algorithms for wave energy conversion technologies with air turbines require contingency tools to prevent the rated power and maximum rotational speed from being exceeded. Survival problems limit energy conversion when the available power is too large compared to the turbo-generator-rated power. Short-term predictions require information from local wave-measuring buoys, which raises reliability concerns. Wave groups in the ocean result in extremely high wave power peaks that cannot be smoothed by low-inertia power-take-off (PTO) systems, leading to survival problems. The most common strategy to cope with this situation is to set the rated power of the PTO several times more than the annual average produced electrical power. The PTO is shut-down when the available power is excessive. These two engineering decisions have a detrimental impact on the capacity factor and the energy cost. The paper shows a new control algorithm for wave energy oscillating-water-column (OWC) devices that can control the shutter position of a fast-acting valve to dissipate the pneumatic energy excess. Hence, the PTO can operate even when the available power is significantly larger than the generator’s rated power. The algorithm was validated with the 30 kW biradial turbine prototype from the OPERA H2020 European project under real sea conditions at Mutriku’s wave power plant. Results show the algorithm’s effectiveness in real situations and during a generator failure simulated during the experimental campaign. This new concept may reduce the energy cost from wave energy and open a new field of design possibilities for OWC wave energy converters.

Research on Laboratory Test Method of Wave Energy Converter Wave-Wire Conversion Ratio in Irregular Waves

The laboratory test of the wave energy converter model is an important means to evaluate the performance of the device. At present, there are few performance tests for complete specifications under the irregular wave. Referring to the test methods and standards at home and abroad, combined with the actual test work experience in the laboratory, using the irregular wave power calculation formula with the effective wave height and the spectral peak period as parameters, then the wave-wire conversion ratio test method of the wave energy converter physical model under irregular waves in the laboratory is proposed. The method is applied to the basin test experiment of the physical model of the horn-shaped backward bent duct buoy (BBDB) wave energy converter. The research results show that the established test method and process of wave-wire conversion performance have achieved good application results in the irregular waves laboratory test, and can better reflect the device operating characteristics in real sea conditions. The test results provide data support for the model design of the wave energy converter in the next test stage, the demonstration test of the prototype, and the prediction of power generation in real sea conditions.

Performance of a hybrid wave energy converter device consisting of a piezoelectric plate and oscillating water column device placed over an undulated seabed

The present study investigates the hydrodynamic performance of a hybrid wave energy converter device consisting of a piezoelectric plate and the oscillating water column wave energy converter device placed over an undulated seabed. Primary emphasis is given to analyze the power extraction of the hybrid wave energy converter device, piezoelectric plate, and the OWC devices for various values of incident wave parameters and shape parameters associated with the hybrid wave energy converter device and undulated seabed. Moreover, the time-domain analysis is carried out by considering the Bretschneider spectrum as the incident wave spectrum for irregular incident waves. A multi-parameter optimization based on the Taguchi method is provided. The study reveals that the power extraction by the hybrid wave energy converter device consisting of a piezoelectric plate and the oscillating water column device is significantly higher than the power extraction by the standalone piezoelectric plate and the OWC device for a wider range of frequencies. Further, the present study shows that the power extraction by the hybrid wave energy converter device is higher when the piezoelectric plate is placed in the close proximity of the OWC device. Moreover, the time-domain simulation reveals that the piezoelectric plate is able to capture the incident wave energy for a longer period of time in real sea conditions.

Seawater-triggered self-renewable amphiphilic coatings with low water swelling and excellent biofilm prevention properties

Amphiphilic antifouling coatings have the potential to revolutionize the field of eco-friendly antifouling coatings, which are still beset by the drawbacks of instability, complex procedures and water swelling. Herein, we construct seawater-triggered amphiphilic coatings with self-renewable capabilities in seawater via the facile casting of a physical mixture of a hydrolyzable polymer (PTMH), hydroxyl-terminated polydimethoxysilane and an isocyanate crosslinker. The hydrophilic chain is generated in situ from PTMH at the coating/seawater interface and thus ultimately mitigates its seawater swelling. The hydrolyzability of PTMH and the degradation of the urethane crosslinking bond endow the coating with self-renewable properties. Owing to the appropriate hydrolysis rate of PTMH, its water absorption is lower than 3 wt% for 30 days of seawater submersion. Notably, the biofilm prevention capability of the coatings is excellent, outperforming that of polydimethoxysilane-based polyurethane and self-polishing polymers in real sea conditions. We envision that this work will provide a new avenue to design microphase separation-based antifouling coatings for practical applications.

Experimental and numerical studies on working parameter selections of a piezoelectric-painted-based ocean energy harvester attached to fish aggregating devices

Fish aggregating devices (FADs) have not only greatly improved the fishing capacity in recent years, but also play an important role in marine monitoring and data collection. In this study, a flexible piezoelectric device (FPED) for ocean energy harvesting was designed to achieve continuous and stable energy supply for FAD equipments and islands. Firstly, we developed FPEDs coated with piezoelectric paint. Painted FPEDs were then attached to the frame of an FAD to examine their characteristics including submerged depth and the support-type. Moreover, under broadband wave conditions, we investigated the dominant parameters influencing electrical performance. Furthermore, we built a theoretical model according to the experiment setup and developed a computational model based on the immersed boundary method to evaluate the FPED-wave interactions. Both the theoretical and computational results were validated by experimental ones, demonstrating that the two models can be considered as alternative tools in FPED working parameter selections. Finally, we conducted a field test under real sea conditions with a remote data monitoring system for the painted FPEDs, which showed good performance under extreme bending, weathering, and fatigue.

On the influence of multidirectional irregular waves on the PeWEC device

Wave energy is a promising renewable resource for its reliability and power density, and many technological milestones have been achieved. Significant efforts are made to design and optimize Wave Energy Converters (WECs); however, analyses are often limited to simplified conditions. Among such restrictive assumptions, waves are frequently described utilizing monodirectional spectra, thus leading to approximate evaluations, also in terms of absorbed power. In real sea conditions, the waves are multidirectional, and the analysis as a 2D superposition of multiple wave components should be investigated. In particular, linear waves can be analyzed as a sum of sine waves characterized by different amplitudes, frequencies, phases and directions. The case study device analyzed in this paper is PeWEC (Pendulum Wave Energy Converter), a rotating mass device that converts energy based on pitch motion, moored through a spread catenary mooring system. The sea states investigated are those of the island of Cyprus. The spectrum is defined as the combination between the JONSWAP frequency spectrum and the cos-2s directional spectrum. To compute the sea elevation components the Deterministic Amplitude Scheme (DAS) method is used. The forcing acting on the device, the mooring loads and the device motions are examined and compared to quantify the error produced by the monodirectional approximations. The time domain solver OrcaFlex is employed to investigate the interaction of the waves with the moored hull. Compared with the multidirectional analysis, the monodirectional approximation generates an overestimation of the pitch by 5% and of the surge by 3%, highlighting the importance of taking spreading into account if the device is directional.

Details study on the kinematic characteristics of manta ray section in flapping motion and exploring its application in wave glider propulsion system

It has always been a human challenge to inspire natural configurations and phenomena and benefit from their merits in improving the performances of man-made proposed aero/hydro vehicles. For example, the manta rays are known for their great swimming performances. To design and fabricate an underwater robot based on the manta ray geometry and its kinematic characteristics, it is important to initially study its hydrodynamic behavior and possibly arrive at some key design parameters, which can remarkably help to figure out an optimum geometry with high swimming performances. The main objective of this study is to focus on the merits of gliding motion inspired by the manta ray fish considering the real sea conditions. In this regard, two wave-glider configurations with specific submersible geometries are selected to practice the bases of this objective. They are the manta-glider, which has a complicated 3D configuration representing the real manta ray swimming behavior, and the wing-glider, which has much simpler configuration constructing from a flat and uniform wing span shape. The 3D unsteady incompressible Navier-Stokes equations are numerically solved to simulate the gliding motion performances of these two gliders. The results show that the maximum efficiency of manta-glider model is ηmax = 50 %, which is much higher than that of the wing-glider with ηmax = 36 %, within an operational range with a relatively constant significant wave height (SWH). Alternatively, the wing-glider efficiency is about 5 % higher than that of the manta-glider in cases with non-constant SWH values. Irrespective of their efficiencies, the interesting outcome of this study is that the manta-glider model shows fewer fluctuations and performs more stable conditions than a simplified glider model in energy-absorbing applications. Considering the achieved outcomes, this work also suggests a simple guideline, which is so beneficial to arrive at a suitable wave glider propulsion design.

Experimental Modelling of Point-Absorber Wave Energy Converter Arrays: A Comprehensive Review, Identification of Research Gaps and Design of the WECfarm Setup

Commercial wave energy exploitation will be realised by placing multiple wave energy converters (WECs) in an array configuration. A point-absorber WEC consists of a floating or submerged body to capture wave energy from different wave directions. This point-absorber WEC acts as an efficient wave absorber that is also an efficient wave generator. Optimising the WEC array layout to obtain constructive interference within the WEC array is theoretically beneficial, whereas for wind farms, it is only important to avoid destructive interference within an array of wind turbines due to wake effects. Moreover, the WEC array layout should be optimised simultaneously with the applied control strategy. This article provides a literature review on the state of the art in physical modelling of point-absorber WEC arrays and the identification of research gaps. To cover the scientific gap of experimental data necessary for the validation of recently developed (nonlinear) numerical models for WEC arrays, Ghent University has introduced the “WECfarm” project. The identified research gaps are translated into design requirements for the “WECfarm” WEC array setup and test matrix. This article presents the design of the “WECfarm” experimental setup, consisting of an array of five generic heaving point-absorber WECs. The WECs are equipped with a permanent magnet synchronous motor (PMSM), addressing the need for WEC array tests with an accurate and actively controllable power take-off (PTO). The WEC array control and data acquisition are realised with a Speedgoat Performance real-time target machine, offering the possibility to implement advanced WEC array control strategies in the MATLAB-Simulink model. Wave basin testing includes long- and short-crested waves and extreme wave conditions, representing real sea conditions. Within the “WECfarm” project, two experimental campaigns were performed at the Aalborg University wave basin: (a) a testing of the first WEC in April 2021 and (b) a testing of a two-WEC array in February 2022. An experimental campaign with a five-WEC array is under preparation at the moment of writing.

Effect of Considering Wave Angles on the Motion Response of Oversized Floating Bodies in Offshore Airports under Irregular Wind and Wave Loads

Most existing studies are based on the hydroelastic response of oversized floating bodies under regular waves, ignoring the effect of wave conditions and incident wave angle on the vibration response of oversized floating bodies in real sea conditions. In this paper, the stability performance of a single module of a mega-floating body at an offshore airport was investigated by using STAR-CCM + numerical simulations based on a specific model, with the introduction of parameters under extreme random wind and wave combined sea conditions. By comparing and analyzing the distribution characteristics of the single module floating body under the action of regular waves and irregular waves, wind, and wave load, and by considering the wave angle along the flow direction, as well as the spreading direction and vertical displacement value under the action of mooring, the overall displacement amplitude of rigid displacement, elastic deformation displacement and considering the influence of both, and the distribution law of motion response along the length of the floating body, we summarize the influence of the wave angle on the dynamic response of an oversized floating body of an offshore airport. The results show that the maximum value of the hydro-elastic response tends to appear at the head and tail of the floating body, the rigid body vertical displacement dominates the role, the amplitude of all displacements of the floating body under the action of the cross wave is larger, and the stress area along the floating body is larger when the wave angle is between 15–30°. The floating body stress value is smaller with angles of 30–65°, and the ability to bear the load is stronger. The hydroelastic response under irregular wave conditions is more sensitive to the wave direction angle, and the elastic deformation has less influence.

A new method for optimal ship design in real sea states using the ship power profile

Typically, ships are designed for calm water and a constant design speed. However, a ship experiences different speeds and sea conditions over her lifetime. Following the significant correlations between environment and ship behavior at sea, efforts are needed to design energy-efficient and safe ships with good performances in realistic sea conditions. This article proposes a method for optimization of the design of a ship with respect to different levels of propulsive power and different operational conditions using a probabilistic approach. The objective of the optimization is the average ship speed, averaged over all propulsion power and environmental condition combinations based on a long-term analysis. The proposed approach is tested on a Post-Panamax container ship operating in the route from Busan to Hamburg port. The decision variables are the ship and propeller main dimensions. Comparisons with traditional methods used in ship design are also presented. Comparing the proposed method with the traditional methods indicates that 7.89% increase in average ship speed is obtained, demonstrating that the proposed method is effective in designing better ships for actual sea states. Better performance in rough sea for the ship designed with the proposed method is also reported.

On the complementarity of the slender-body and Newman’s approximations for difference-frequency second-order wave loads on slender cylinders

As reported in the literature, Newman’s and the slender-body approximations for second-order wave loads (the latter often referred to as Rainey’s equation) present an interesting complementarity for the case of a bottom mounted vertical cylinder, indicating that each one captures different physical aspects of the problem. In light of that result, the present work aims at analyzing this complementarity by inspecting the different components that constitute the second-order wave loads, including the ones due to body motions, and by relating them to the incident and diffracted waves. This is done by analyzing the case of a single surface piercing cylinder under the action of long-crested bichromatic waves, first considered to be bottom mounted and then in floating condition. It is also shown that the second-order terms from Rainey’s formulation are indeed relevant, reminding one that the common practice of analyzing second-order loads by simply applying Morison’s equation with second-order wave kinematics is not strictly correct.In order to include the effects of body motions, Rainey’s equation, which can be seen as an extension of the inertial part of Morison’s equation to include nonlinear terms, is combined with Pinkster’s approach for the low-frequency second-order loads on floating bodies. This combination is made in order to allow the evaluation of the forces considering the mean body position, in opposition to the original approach that considers the instantaneous body position, allowing an IFFT algorithm to efficiently compute second-order wave loads in a real sea condition directly in time domain. As a drawback of the slender-body approximation, end effects due to the extremities of the cylinder and effects due to wave scattering and radiation are lost, as is the case when using Morison’s equation to evaluate first-order loads, and the question is to what extent they can be neglected when dealing with second-order wave loads.

대잠전용 무인수상로봇의 자율운항시스템 설계를 위한 LQR 기반 경로추종 알고리즘 연구

This paper presents the usage of LQR-based intelligent path tracking methods to control the autonomous navigation system of an Unmanned Surface Robot (USR) in sea-clutter environmental fields. Since considerable interest in the Autonomous Navigation System (ANS) of USR in sea warfare has increased, the stability of the Autonomous Navigation System (ANS) control becomes of the utmost importance. When the USR moves in sea environments, the control stability of autonomous navigation systems becomes a major concern. The Path Tracking Controller (PTC) of USRs is one of the high potential subsystems that can be further improved to achieve more accurate, robust, and comfortable tracking performance. The stability of the PTC should be taken into consideration while performing the path tracking task in autonomous navigation. To solve the automatic waypoint tracking stability of independently-developed PTCs, a waypoint tracking controller is designed based on the Linear Quadratic Regulator (LQR) theory with the kinematic model of USRs. Then, a simulation analysis of the designed waypoint tracking controller is performed, and the results are compared at two different speeds. Additionally, complex waypoint tracking tests under realistic sea conditions are performed based on the designed waypoint tracking controller. Results show good tracking performance, which indicates that the designed waypoint tracking controller can adapt to high-speed USRs.

A DMST-based tool to establish the best aspect ratio, solidity and rotational speed for tidal turbines in real sea conditions

A Double Multiple Stream Tube (DMST) routine to predict the performance of cross-flow hydrokinetic turbines in real environments is presented, along with a site assessment application to identify the most efficient turbine aspect ratio, solidity and configuration (single, or paired) for a selected area of the Northern Adriatic Sea. The peculiarity of this DMST tool is its 3D character, since it allows to reproduce the vertical distribution of the torque generated by the turbine. To this end, correlations for fluid dynamic phenomena, based on high-fidelity fully CFD simulations, were implemented. The marine circulation code SHYFEM is adopted to obtain velocity profiles for a half lunar cycle period. The sites with the highest mean kinetic power were identified. The DMST routine is equipped with an iterative process able to establish which rotational speed maximizes the power output. Indeed, a spatially non-uniform velocity profile requires to determine the flow velocity more suitable to obtain the rotational speed via Tip Speed Ratio (TSR) definition. To this end, the section of the blades working at optimal TSR varies from top to bottom, until the maximum power is reached. It works as a virtual Maximum Power Point Tracking system able to adapt the turbine operating conditions for the different turbine geometries, and for changes in flow conditions. The results show that for the case study, the performance curve shape influences the optimal TSR blade section: the latter is often located in the upper part of the turbine for the low solidity, whereas, for high solidity turbines, in the bottom half part.

Development and Control of an Underwater Vehicle–Manipulator System Propelled by Flexible Flippers for Grasping Marine Organisms

Robots have the potential to assist and complement humans in the exploration of hostile environments. For example, underwater vehicle–manipulator systems (UVMSs) have been researched and applied in underwater operations. Motivated by the need for collecting marine organisms cultivated in shallow sea aquafarm, a lightweight and portable UVMS propelled by flexible flippers (named, F-UVMS), rather than typical propellers or jet-based propulsion systems, is investigated. To this end, integrative mechatronic design of F-UVMS is first presented, involving six flipper propulsors for generating thrust in a flapping way, a four-degree-of-freedom arm for grasping, and a certain amount of onboard sensors for environment perception. A thrust–power measurement platform is established to evaluate thrust–power relationship for different sizes and stiffness of flippers. Meanwhile, the locomotion modalities of F-UVMS are analyzed. A framework based on the nonlinear model-predictive control method is implemented to govern autonomous tracking marine organisms. The capabilities of F-UVMS are validated through a series of experiments in laboratory pool and field missions in real sea conditions.

Experimental simulation study on dynamic response of offshore wind power pile foundation under complex marine loadings

Offshore wind power structures are subjected to complex dynamic loads in marine environment during their service. At present, the real sea conditions can be simulated to the maximum extent by the mixed test technology of boundary layer wind tunnels, wave pools and shaking tables. However, due to the high cost of test and the lack of a complete and mature combined simulation laboratory which can be operated for wind, wave and seismic loads, it is difficult to carry out such joint tests. It is advantageous to use self-developed mechanical devices to simulate marine environmental loads within the controllable errors and which is in simple operation. A series of dynamic model tests were carried out on saturated sand foundation in order to explore the feasibility of studying dynamic response of offshore wind power pile foundations through simulating marine environmental loads by simplified methods. Based on Stokes second-order wave theory and harmonic superposition method, the time history of wind, wave and current load is derived and programmed, then the wind, wave and current load on offshore fan model structure is simulated by using self-developed complex dynamic loading system. Finally, the dynamic response of single pile foundation in saturated sand under complex load is analyzed according to the test results. The test results show that the load displacement curve of single pile foundation in saturated sand is of work hardening type. The maximum pile bending moment at the peak of dynamic load increases with the increase of loading times, but the cumulative effect of bending moment caused by the change of load amplitude should be considered. The dynamic load in marine environment will cause the increase of pore pressure around the pile, but the increase of pore pressure on loading side is not obvious in comparison to that on the opposite side. The displacement change of pile body is basically in accordance with the fluctuation trend of test load and increases with load. The cumulative rate of displacement of single pile decreases with the increase of number of cycles under the long-term cyclic wind wave load. In this study, the method combining theoretical calculation of wind, wave and current load with model test of complex loading system can consider the characteristics of dynamic loads of marine environment more accurately, compared with the empirically determined cyclic load waveforms in conventional tests, the obtained response law is more in accord with actual sea conditions.

A CFD-based wave-to-wire model for the oscillating water column wave energy Convertor

The construction of the prototype OWC plant proposed development requirements of the wave-to-wire model for the overall performance predictions, which should balance the computational cost and accuracy. In this study, a CFD-based wave-to-wire model was developed to couple the air chamber and air turbine. The steady numerical model was employed to derive the relationship between the air pressure drop and the incident airflow velocity, which was induced to the air chamber model in the numerical wave tank using the porous zone module. The numerical predictions on the free surface elevation and air pressure variation in the chamber, the torque output of the turbine in regular and irregular wave scenarios, and the efficiencies were validated by the experimental data. By considering the air compressibility, the wave-to-wire model was employed to predict the performance of the 500 kW Yongsoo prototype plant. The interaction mechanism among energy converting processes and the overall performance of the plant under the real-sea condition were carefully analyzed and reported.

SLAM CHARACTERISTICS OF A HIGH-SPEED WAVE PIERCING CATAMARAN IN IRREGULAR WAVES

Slam characteristics of a 112m INCAT wave piercing catamaran in a range of realistic irregular sea conditions are presented in this paper. Towing tank testing of a 2.5 m hydroelastic segmented catamaran model was used to gather a database of slam events in irregular seas. The model was instrumented to measure motions, centrebow surface pressures and forces, encountered wave elevations and wave elevations within the bow area tunnel arches. From these measurements characteristics of the vessel slamming behaviour are examined: in particular relative vertical velocity, centrebow immersion, archway wave elevations and slam load distributions. A total of 2,098 slam events were identified over 22 different conditions, each containing about 80 to 100 slam events.
 The data, although inherently scattered, shows that encounter wave frequency and significant wave height are important parameters with regard to centrebow slamming. Relative vertical velocity was found to be a poor indicator of slam magnitude and slams were found to occur before the centrebow arch tunnel was completely filled, supporting the application of a two-dimensional filling height parameter as a slam indicator.

Research on the statistical characteristic of freak waves based on observed wave data

Freak waves are characterized by high wave, great destructive power, abrupt behavior, and a significant threat to offshore structures. In this study, the observed wave data of multiple sea areas are utilized, the relation between the occurrence of freak waves at real sea states and the statistical characteristics of wave trains are investigated, and the probable reasons for freak wave generation at real sea conditions is explored. It can be found that for the wave trains with higher kurtosis, the occurrence probability of huge wave height is underestimated by the Rayleigh distribution. It can be accurately predicted by the MER distribution and GC distribution. Generally, the occurrence probability of freak wave increases linearly with the increase of kurtosis from the statistical results, and the MER model is capable of describing the relation between occurrence probability of freak wave and kurtosis. Moreover, the generation types of freak waves are analyzed based on the wavelet energy spectrum and time process of wave surface for typical freak waves in the observation data. Freak waves at real sea states are attributed to the modulation instability and wave energy superposition.

Verification of chlorine production through seawater electrolysis using ion-exchange membrane electrolytic bath

Objective To prevent the marine biofouling adhesion and corrosion of ship hulls and pipes, a system is designed which produces chlorine through seawater electrolysis using an ion-exchange membrane electrolytic bath.Thereby, the law and efficency of the electrolytic bath under different conditions is studied Methods First, the effects of temperature, NaCl solution concentration, current density and residence time on the efficiency of the electrolysis process are investigated. Minitab software is then used to optimize the parameters of the electrolysis process using the specific energy consumption rate as the evaluation index. Finally, the seawater pretreatment process and electrolysis process are verified on site under real sea conditions. Results The current density and residence time of the electrolysis parameters were reasonably controlled at 3,000 A/m2 and 46 s respectively, in this case, the current efficiency was higher than 80%, the cell voltage was lower than 6 V, and the surfaces of the anodes, cathodes and ion-exchange membranes after electrolysis were clean. Conclusion The practical tests show that this system can produce chlorine for anti-fouling.