Regular Wave Conditions Research Articles

On the mooring methodology of heaving point absorber arrays

This paper aims at developing a mooring methodology for floating point absorber arrays. To date, the literature gives no clear indications of the mooring influence on the potential energy conversion of a floating wave energy converter. Hence, this study targets to provide general insights into the dynamics of a floating wave energy converter under the influence of moorings. These are gained by conducting an experimental study on the motion response of a wave energy converter array in regular wave conditions using three different mooring systems: a taut line mooring, a vertical tension leg mooring, and a conventional slack catenary mooring. Results show that the potential energy conversion is strongly influenced by the choice of the mooring system and the incident wavelength. In intermediate wavelengths, a taut mooring system leads to a 8% increase in energy conversion potential. In contrast, the tension-leg moored array exceeds the potentially converted energy of the catenary moored array in wavelengths significantly longer than the wave energy converter’s length by over 80%. The results of this study provide an insight into the site-specific design of point absorber arrays and the best choice of the mooring system to increase the potential power output.

Design and model test of a soft-connected lattice-structured floating solar photovoltaic concept for harsh offshore conditions

Various types of floating solar photovoltaic (FPV) devices have been previously proposed, designed and constructed with applications primarily limited to onshore water bodies or near-shore regions with benign environmental conditions. This paper proposes a novel FPV concept which can survive harsh environmental conditions with extreme wave heights above 10 m. This concept uses standardised lightweight semi-submersible floats made of circular materials as individual modules. The floating modules are soft connected with ropes to form an FPV array. We first present the conceptual design of the floats and the connection systems, including hydrostatic, hydrodynamic, and structural assessments of the floats. To verify the motion response performance, we carried out 1:60 scaled model tests for a 2 by 3 array under regular and irregular wave conditions. From the time series and response spectra, the motion characteristics of the array and the mooring responses are analysed in detail. The proposed concept exhibits excellent performances in terms of modular motions with limited wave overtopping and no contact is observed between adjacent modules under the extreme wave conditions. The findings of this study can serve as a valuable reference to developing reliable and cost-effective FPV technologies for offshore conditions.

Predictive Control of a Heaving Compensation System Based on Machine Learning Prediction Algorithm

Floating structures have become a major part of offshore structure communities as offshore engineering moves from shallow waters to deeper ones. Floating installation ships or platforms are widely used in these engineering operations. Unexpected wave-induced motions affect floating structures, especially in harsh sea conditions. Horizontal motions on the sea surface can be offset by a dynamic positioning system, and heave motions can be controlled by a heave compensation system. Active heave compensation (AHC) systems are applied to control vertical heave motions and improve safety and efficiency. Predictive control based on machine learning prediction algorithms further improves the performance of active heave compensation control systems. This study proposes a predictive control strategy for an active heave compensation system with a machine learning prediction algorithm to minimise the heave motion of crane payload. A predictive active compensation model is presented to verify the proposed predictive control strategy, and proportion–integration–differentiation control with predictive control is adopted. The reliability of back propagation neural network (BPNN) and long short-term memory recurrent neural network (LSTM RNN) prediction algorithms is proven. The influence of the predictive error on compensation performance is analysed by comparing predictive feedforward cases with actual-data feedforward cases. Predictive feedforward control with regular and irregular wave conditions is discussed, and the possible strategies are examined. After implementing the proposed predictive control strategy based on a machine learning algorithm in an active heave compensation system, the heave motion of the payload is reduced considerably. This investigation is expected to contribute to the motion control strategy of floating structures.

Numerical investigation on the hydrodynamic performance of a 2D U-shaped Oscillating Water Column wave energy converter

The U-Oscillating Water Column (U-OWC) is a wave energy harvester exploiting the working principle of oscillating water columns for capturing and converting energy from sea waves. U-OWC devices can be integrated into a breakwater to enable wave energy extraction and provide shelter for port activities. In this work, a coupled Smoothed Particle Hydrodynamics (SPH) model was developed and applied to investigate the hydrodynamics of a U-OWC breakwater. The numerical model is validated against the experimental results over a range of regular wave conditions. An extensive campaign of computational tests is then carried out, studying the effects of geometrical parameters on the hydrodynamic performance and wave loading over the U-OWC breakwater. It shows that the geometrical parameters of the U-shape have a significant effect on the air pressure inside the chamber and the load phase difference between the two sides of the lip wall. The minimum load and maximum capture efficiency designs for U-OWC breakwaters cannot be satisfied geometrically at the same time. This demonstrates that it is necessary to consider comprehensively the structural reliability and hydrodynamic performance in the design and construction of a U-OWC breakwater.

Dynamic responses of serially connected truss pontoon-MOB – A numerical investigation

Very large floating structures (VLFS) are typically made up of several modules of offshore platforms, such as semi-submersibles, connected by mechanical connectors. The proposed Truss Pontoon Mobile Offshore Base (TP-MOB) is a new type of offshore platform based on the truss pontoon semi-submersible design. The design of the connectors is crucial for the functionality of the structure. In this study, the flexible connectors are modeled with a 3D source distribution method and the wave forces are estimated with 3D linear potential theory. The open-source finite element-based numerical model, HYDRAN-XR, is used to analyze the structural responses and connector forces in both regular and irregular wave conditions. The results indicate significant differences in responses between single and multi-module cases, especially in extreme waves. Loads on the connector change greatly for three- and five-module cases, with maximum values at specific wave angles.

An experimental study of a rectangular floating breakwater with vertical plates as wave-dissipating components

Floating breakwaters have received growing attention in recent years from academic and engineering sectors alike. The wave-dissipating components are commonly attached to the bottom of floating breakwaters for better wave protection performance. Vertical plates, which have been proven effective in dissipating waves, were attached to a rectangular floating breakwater in this study. A series of experiments were conducted under regular wave conditions to analyze the wave protection and motion response performances, and the effects of mounting position, mounting rows, plate porosity, and protruding plate height were examined. Our experimental results elucidated that the pitch motion plays a critical role in dissipating energy. Although variation in the settings, such as mounting with more than one row of plates, smaller plate porosity and longer protruding plate height do not cause significant changes in the mass and moment of inertia of floating breakwater, they all exert stronger confining effects on the water body beneath. The water body confined by plates significantly changes the dynamic characteristics of floating breakwaters, and increase the natural period of pitch motion towards longer waves. As a result, the violent energy dissipation and corresponding better wave protection occur at longer waves. It has been demonstrated that the primary working function of vertical plates as wave-dissipating components is due to water body confinement. This finding can be utilized to tailor the design of the vertical plates as wave-dissipating components in accordance with the seasonal weather and sea conditions at specific engineering sites.

Hydrodynamic responses of a reversed L type floating breakwater integrated with a wave energy converter impacted by extreme waves

This research studies the hydrodynamic responses of a reversed L type floating breakwater integrated with a wave energy converter moored by a vertical pile in extreme sea states and intermediate water depth in a numerical wave tank based Navier-Stokes equations. Three wave-type representations (solitary, regular, and focus waves) of the same wave state are considered. The surface wave elevation, heave motion, wave force, impulse, wave moment on the mooring pile, moment of impulse, pressure distribution and flow field are investigated and compared. The comparison shows that different waves yield to diverse peak hydrodynamic responses. Largest wave force and moment are found in the regular wave condition, while largest heave motion, impulse and moment of impulse are obtained for the solitary wave. The pressure distribution on the bottom of the floater under the solitary wave shows a distinct character from the two others. The effect of the PTO damping parameter with the focus on the maximum wave force and moment is discussed and it is revealed that there exists an optimal PTO damping force for which the minimum wave force on the floater is obtained. This study therefore gains an insight into the hydrodynamic responses under different extreme waves.

Performance characteristics and parameter analysis of a multi-DOF wave energy converter with hybrid power take-off systems

A multi-degree-of-freedom (multi-DOF) wave energy converter (WEC) with hybrid power take-off (PTO) systems may capture more wave energy than a traditional single-DOF WEC, which usually obtains energy from pitch or heave. Compared with the existing WECs with a single PTO, this paper presents the concept design of a multi-DOF WEC with hybrid PTOs named PUWEC, which has prismatic pairs in the hydraulic cylinder and universal pairs in the gear transmission system. Two corresponding PTOs are matched to the motion of the buoy in heave and pitch to enhance wave energy conversion. This work focuses on establishing a time-domain numerical model to simulate the coupled dynamics and power capture performance of the proposed PUWEC. The model tests are carried out in a wave tank to verify the validity of the numerical model. The results show that the numerical model is efficient in predicting the dynamic performance of the PUWEC. Regular and irregular wave conditions are considered in the numerical simulations to explore the performance characteristics of the devices. It is found that the maximum improvement of the captured power of PUWEC is 49.8 % compared with single-DOF PWEC under the regular wave frequency 0.42 Hz, and the capture width ratio of PUWEC is close to 30 % under three wave conditions. In addition, the motion trajectory of the center of gravity of the buoy is analyzed. With the increase in U-PTO damping, the trajectory of the float changes from horizontal bowl-shaped irregular path to inclined regular hollow elliptical orbit. The trajectory coefficient is introduced to provide insights into how the PUWEC improves the energy conversion. The actual working space of the PUWEC is then calculated, which is less than the design working space, indicating that the actual performance of the PUWEC is fully developed. The proposed PUWEC and constructed numerical model which considers the coupled dynamic performance of dual PTOs, can provide valid reference for the design of other multi-DOF WECs with hybrid PTOs.

Numerical analysis of the dynamic behavior of a Floating Wind Platform in regular waves

The goal of this study is to analyze the hydrodynamic behavior of a Floating Wind Platform (FOWT) under regular wave conditions. The analysis considers three degrees of freedom: Heave, Pitch and Surge. The study was car-ried out using a CFD (Computational Fluid Dynamics) tool, based on the Navier-Stokes equations and the Finite Volume Method, and results were compared against experimental tests performed at the Wave/Towing Tank at Universidad Austral de Chile (CEH-UACh). For this analysis, a generic scales semi-submersible platform was used under different regular wave conditions, obtain-ing wave height data at different points of the control volume and translation and rotation movements to obtain the RAO's (Response Amplitude Operator) in each of the considered degrees of freedom. For the CFD simulations, commercial software STAR CCM+ was used, where the flow characteristics were defined through Volume of Fluid (VOF) and their respective boundary conditions. Final-ly, the results of both methodologies were compared, showing an adequate de-gree of correlation between them.

Wave-induced cross-shore distribution of different densities, shapes, and sizes of plastic debris in coastal environments: A laboratory experiment

Plastic debris is a significant threat to marine and coastal ecosystems. Previous research found that waves, wind, as well as density, size, and shape of microplastics, drive their transport and dispersion. In this paper, a set of laboratory experiments on the effect of waves and wave-induced currents on the input rate and cross-shore transport and dispersion of different types of plastic debris, including the macro and mesosizes, in addition to microplastics is presented. 15 plastic-debris types characterized by different sizes, shapes, and densities, including facemasks, were analyzed under regular and irregular wave conditions. The results show that input and transport rates of plastics depend on their terminal velocities and wave steepness. Plastics with higher settling velocities under less-steep wave conditions are likely to escape coastal entrapment and end up in the breaking zone. However, plastics with greater buoyancy rates under steeper waves show a predominant accumulation closer to the shoreline.

Imaging of Hydrodynamic Field Around Submerged Objects Regular Wave and Tsunami Conditions

Hydrodynamic particle movement under regular waves and tsunami wave are rarely studied due to its complicated sensors. This research is aimed at investigating flow fields around submerged structures due to regular waves and tsunami wave. A series of experiments were performed at Tsunami Flume Workshop Facility at Tsunami and Disaster Mitigation Research Center (TDMRC) of Universitas Syiah Kuala. The flume has 60 m in length, 2.5 m in width and 1.7 m in height. To model the both waves, a set of electrical paddle and sensors were placed at one end of the flume. This set of equipment is able to mimic any waveform model, in this case a regular wave with three scenarios and the 2004 Indian Ocean Tsunami wave scenario. A submerged structure was placed on the bed of the flume to model underwater structures. To capture the flow fields, we use a Laser Particle Image Velocimetry (PIV) Camera made in Seika. During the experiment, the results showed different vortex field for both simulations, regular wave and tsunami, generated in front of the submerged structure. Regular wave flow has maximum velocities of 0.5 m/s, 0.7 m/s, 0.9 m/s in each paddle amplitude change scenario. The flow field produces an anticlockwise vortex around the pipe with differents pattern. Tsunami wave flow velocity during the maximum amplitude phase is about 0.25 m/s with a complex vortex field around the pipe, a combination of clockwise and anticlockwise. This could be interpreted as more chaotic hydrodynamic fields around the submerged structures under regular wave and tsunami conditions.

Numerical study on a hybrid WEC of the Backward Bent Duct Buoy and Point Absorber

This study aims to investigate a wave energy converter that utilizes heave motion as the primary driver for energy harvesting. The main objective is to examine the effect of applying a new form of hybrid Backward Bent Duct Buoy (BBDB) with Point Absorber (PA) at various wave conditions. The effects of single BBDB, PA, and a new hybrid form of BBDB-PA on heave Response Amplitude Operator (RAO) and power absorption were examined within various ranges of wave periods under regular wave conditions using ANSYS-AQWA. The results reveal that the characteristics of excitation force, radiation damping coefficient, and added mass of the BBDB and PA are significantly influenced by the new hybrid form. This observation is due to the advantage of the hybrid form that would have more contact with the wet surface area. Moreover, the hybrid BBDB-PA yields higher heave RAO and power absorption compared to the non-hybrid counterpart. The results offer recommendations for optimizing the design of hybrid WECs and imply the possibility of synergy between BBDB and PA to fully utilize ocean space for energy.

Efficiency and Wave Run-Up of Porous Breakwater with Sloping Deck

In order to protect fragile shoreline and coastal assets during extreme storms, a combined floating breakwater-windbreak has been proposed to reduce both wind and wave energies in the sheltered area. The 1 km-long breakwater has a porous hull with internal tubes to allow free passage of water; thereby further dissipating wave energy. The deck of the structure is designed to have a slope of 25 degrees facing the upstream side, and arrays of cylindrical tubes are placed on the sloping deck to form a windbreak. A reduced-scale (1:50) model test was carried out in a wave flume to examine wave sheltering performance under significant wave heights Hs = 3.0 m to 7.5 m and peak wave periods Tp = 9.4 s to 14 s sea states. Both regular and random wave conditions with different wave heights were considered. It is found that transmission coefficients ranging from 0.4 to 0.6 can be achieved under tested wave conditions. Porous breakwater hull increases the wave dissipation coefficients and is effective in reducing the wave reflection at the upstream side. The wave run-up length is dependent on the Iribarren number if the reduction induced by vertical freeboard is considered. Based on experimental data, empirical formulae have been proposed to predict the wave run-up responses in regular waves, probability of non-zero wave run-up occurrence, modified Weibull distribution of the wave run-up peaks and extreme wave run-up in random waves.

The impact of piston and sloshing motions on added resistance from moonpool configurations

Moonpools are vertical watertight openings employed in the decks and lower structures of marine vessels and offshore platforms to allow drilling pipes and risers to be lowered into the sea. In the present work, the resistance performance of a drillship with varied moonpool configurations under calm water and regular wave conditions is investigated using a shear-stress transport (SST) k-ω model based on OpenFOAM. It is confirmed by comparing experimental and numerical results that this method can accurately predict the resistance of a ship with a moonpool. The numerical results suggest that the added resistance caused by a moonpool can be effectively reduced by optimally arranging the notch on the recess. A significant variation in the local flow field of the moonpool accounts for this. The implementation of the notch affects the development of vortices structures, thus transforms the free surface evolutions as well as the fluid resonance. Finally, the variation in pressure distribution contributes to a decline in the added resistance associated with the moonpool.

Experimental Investigation of Mooring Performance and Energy-Harvesting Performance of Eccentric Rotor Wave Energy Converter

To obtain the optimal mooring mode and the best-matching wave condition of an eccentric rotor wave energy converter (ERWEC), a physical model of the ERWEC was developed. Ten mooring modes and eight wave conditions were set up. Several experiments were carried out to analyze the influence of mooring modes and wave conditions on the mooring and energy-harvesting performances of the ERWEC. The results showed that the mooring and energy-harvesting performances changed significantly for the same mooring mode under various regular wave conditions, but the opposite situation was found under irregular wave conditions. The wave-facing direction of the buoy was a critical factor affecting the mooring and energy-harvesting performances, while the number of anchor lines had little effect on them. In addition, a method to evaluate the motion response of the buoy based on the number of effective excitations and a method to evaluate the comprehensive performance based on the cloud chart are proposed. The mooring mode and wave condition combination that obtained the optimal mooring and energy-harvesting performances for the ERWEC was determined. This paper provides a novel perspective on how to balance the efficiency and reliability of wave energy converters.

A Fully Coupled CFD-DMB Approach on the Ship Hydroelasticity of a Containership in Extreme Wave Conditions

In this paper, we present a fully coupled computational fluid dynamic (CFD) and discrete module beam (DMB) method for the numerical prediction of nonlinear hydroelastic responses of a ship advancing in regular and focused wave conditions. A two-way data communication scheme is applied between two solvers, whereby the external fluid pressure exported from the CFD simulation is used to derive the structural responses in the DMB solver, and the structural deformations are fed back into the CFD solver to deform the mesh. We first conduct a series of verification and validation studies by using the present CFD–DMB method to investigate the global ship motion, vertical bending moments (VBMs), and green water phenomenon of the ship in different regular wave conditions. The numerical results agreed favourably with the CFD–FEA model and experimental measurements. Then, the extreme ship motions are studied in focused wave conditions to represent extreme sea conditions that a ship may experience in a real sea state. According to the conclusion drawn from the numerical simulations, it is founded that the focused wave case will lead to the increase of the longitudinal responses of the hull compared to regular wave condition, i.e., the heave, pitch, and total VBMs rise about 25%, 20% and 9%, respectively. In focused wave conditions, intensive ship responses and severe waves cause stronger slamming phenomena. It is found that the instantaneous impact pressure from the focused wave is higher and sharper compared to the regular waves and comes along with the obvious green-water-on-deck phenomena.

Experimental Investigation of the Dynamic Behavior of Submerged Floating Tunnels under Regular Wave Conditions

Submerged floating tunnels (SFTs) are an innovative traffic structure for transportation in deep and long-distance ocean environments. SFTs have the risk of being subjected to wave action in the complex ocean environment. Therefore, in order to ensure the safety and stability of SFTs during their service, the dynamic behavior of an SFT subjected to the action of waves is experimentally investigated in this study. Based on the wave–current flume, a physical scale-model experiment of an SFT for studying the dynamic behavior of an SFT subjected to regular waves interactions is deeply and thoroughly explored. The results show that the wave outputs from the wave-making system are verified to be reliable. The influence of major load parameters on the dynamic behavior of the SFT are deeply and thoroughly explored. The effects of wave height and wave period on acceleration, wave pressure, anchor cable force and displacement in an SFT are discussed. This experimental study has theoretical and practical significant for SFT design and safety.

Analysis of the influence of wave parameters on helicopter floating characteristics

Foreign countries have accumulated rich experience in the research of helicopter floating characteristics. From the model test in the 1950s to the current numerical simulation and extrapolation evaluation, its technical maturity is significantly higher than that in domestic related fields. In order to fully study the influence of wave parameters on helicopter floating characteristics, sort out and refine the model test methods suitable for the study of helicopter floating characteristics, this paper takes a helicopter model as the research object, and carries out the research on the helicopter model test technology with emergency buoys, focusing on the influence range of wave parameters and wave direction on helicopter test results. The results show that the motion response of the helicopter under regular wave conditions presents typical periodic changes. When the helicopter encounters the influence of transverse wave, the rolling amplitude decreases with the increase of wavelength, and the acceleration amplitude of the nose and in the aircraft first increases and then decreases with the increase of wavelength; Under other wave direction conditions, the rolling angle, the acceleration amplitude of the nose and in the machine first increase and then decrease with the increase of the wavelength.

Higher-order gap resonance and heave response of two side-by-side barges under Stokes and cnoidal waves

Coupled piston-mode fluid response and the heave motion of two identical barges in side-by-side configuration is studied under finite-depth and shallow-water waves using a two-dimensional fully nonlinear numerical wave tank. To understand possible critical responses of the gap flow and the floating barge, regular-wave conditions which are able to excite up to 5th-order nonlinear gap resonance and also the resonant heave motion of the barge are considered. In shallow-water waves, high-frequency oscillations, featured by secondary peaks in the time histories, are observed for both wave elevation in the gap and the heave motion of the barge. The shallow-water wave-induced 4th- or 5th-order gap resonance can be equally crucial as the 1st- and 2nd-order resonances due to finite-depth waves. At higher-order gap resonance, the higher-harmonic heave motion of the barge is negligibly small, in contrast to the gap-flow response. Compared with fixed barges, the free-heave motion of an upstream barge tends to increase the wave elevation in the gap in most of the resonant conditions, except at 1st-order gap resonance where the gap response is greatly reduced. When the resonant heave motion of a floating barge, either located upstream or downstream, is excited, significant barge motion is observed. However, the relative motion between the gap flow and the floating barge is seen to be very small, ascribed by small phase difference between the two. The present study suggests that the effects of heave motion and water-depth should be carefully considered in the design of side-by-side marine operations, and hiding the small bunkering ships behind the large receiving ships is regarded as a preferred arrangement during the bunkering operations in offshore and coastal environments.

Experimental Modelling of an Isolated WECfarm Real-Time Controllable Heaving Point Absorber Wave Energy Converter

To offer point absorber wave energy converters (WECs) as a bankable product on the marine renewable energy market, multiple WECs will be installed together in an array configuration. The wave energy community (research and industrial) has identified the urgent need for available realistic and reliable data on WEC array tests in order to perform a better WEC array optimization approach and in order to validate recently developed (non-linear) numerical models. The ‘WECfarm’ project is initiated to cover this scientific gap on necessary experimental data. The ‘WECfarm’ experimental setup consists 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 realized with a Speedgoat Performance real-time target machine, offering the possibility to implement advanced WEC array control strategies in the MATLAB-Simulink environment. The presented article describes the experimental setup, the performed tests and the results of the test campaign using a single, isolated ‘WECfarm’ WEC in April 2021 at the wave basin of Aalborg University (AAU), Denmark. A Coulomb and viscous friction model is determined to partly compensate for the drivetrain (motor, gearbox, rack and pinion) friction. A system identification (SID) approach is adopted considering the WEC system to be composed of two single input single output (SISO) models, the radiation and the excitation model. Radiation tests yield the intrinsic impedance. Excitation tests yield the excitation frequency response function. Adopting an impedance matching approach, the control parameters for the resistive and reactive controller are determined from the complex conjugate of the intrinsic impedance. Both controllers are tested for a selection of regular wave conditions. The performed experimental test campaign using an isolated ‘WECfarm’ WEC allows a full evaluation of the WEC design prior to extending the setup to five WECs. Within the ‘WECfarm’ project, an experimental campaign with a five-WEC array in the Coastal and Ocean Basin (COB) in Ostend, Belgium, is under preparation.