Irregular Waves Research Articles

Artificial neural network framework for prediction of hydroelastic response of very large floating structure

The response of a very large floating structure (VLFS) must take into consideration the elastic deformation of the structure (commonly termed hydroelastic response) under wave action. Conventionally, the hydroelastic response could be computed by using the coupled finite element-boundary element (FE-BE) method, where the mat-like structure is modelled using plate theory and the water modelled using the potential theory. The FE-BE method requires the structure to be discretised into finer elements and the wetted surface boundary to be represented by smaller panels to accurately capture the hydroelastic response of the structure. Thus, the coupled FE-BE method could be computationally expensive when the structure gets larger or when subjected to waves of smaller wavelengths. To accelerate the computational time in predicting the hydroelastic response of the VLFS, a surrogate model trained using the feed-forward neural network is proposed. The hydroelastic responses under different wavelengths, structural stiffnesses and wave directions are first generated where these data are split into three groups for training, validation, and testing (prediction) purposes. The accuracy of the prediction in terms of correlation coefficient R is compared for the different train datasets, the number of neurons and hidden layers as well as the optimisation techniques. The finding shows that an accuracy of close to 99% to the ground truth could be achieved with only 80% of the train dataset. The hydroelastic response under irregular wave conditions predicted using the feed-forward neural network framework is also presented.

Multivariable design optimization for adaptive resonance of a surface riding wave energy converter

Since the ocean waves feature irregular waves varying over time, a wave energy converter with adaptive resonance, which feasibly changes the natural frequency for different sea states, is considered. To find its optimum design, multi-dimensional parametric optimization is formed in sequence of setting ranges of submerged geometry for hydrodynamic performance, finding design specifications including the generator for maximum resonance peaks, and locating the natural frequency for individual sea states to achieve the adaptive resonance that converts the maximum annual mechanical power for the given geometry. The multivariable design optimization is performed for a kilowatt-scale Surface-Riding Wave Energy Converter, which controls the pitch natural frequency through relocating a mass vertically. The optimized design presents that the adaptive resonance improves the mechanical power conversion up to 11 times compared to the conventional fixed resonance design. Furthermore, the adaptive resonance for diverse wave spectra indicates that the optimum natural frequencies for individual sea states, maximizing the annual mechanical power conversion, do not coincide with the waves’ peak or energy period. The second-order wave loads entails further change of the optimum natural frequency resulting in subsequent 10 times performance improvement for severe sea states with high peak periods.

Development of a small-sized tanker with reduced greenhouse gas emission under in-service condition based on CFD simulation

In the present study, the hull form and appendage of a 6.5 k DWT tanker was improved to reduce greenhouse gas emissions in the in-service navigation condition. The conventional hull form of the 6.5 k DWT tanker has been designed in terms of performance in the ideal condition of a calm-sea. On the contrary, the performance of a ship in the in-service condition should take the added resistance due to wind and waves in the real sea into consideration. In order to reduce the added resistance due to waves, the bow hull form was modified to have a sharper entrance with increased length between perpendiculars. This was intended to ensure that the waves follow the curved bow surface more smoothly, thereby reducing the added resistance due to waves. The stern hull form was also modified to have V-shaped section shapes to reduce viscous pressure resistance. In addition, the flow control fins were optimized in terms of position and angle of attack. The added resistance of the resulting hull form was then analyzed numerically in regular wave with a varying wavelength ratio λ/Lpp=0.5∼2.0. The added resistance in regular waves was subsequently integrated with respect to the wavelength to obtain the added resistance in irregular waves. Finally, by combining it with the sea state occurrence probability, the daily fuel oil consumption (DFOC) was able to be calculated for the actual in-service routes for the tanker. The entire process of hull form modification and evaluation was conducted with free-surface CFD simulation in the presence of regular waves. A series of model tests was undertaken to confirm the improvements of the optimal hull form over the original in terms of performance in waves as well as calm-sea performance. The resulting DFOC and daily CO2 emissions for the optimal hull form in the in-service conditions was found to be reduced by 14.8%. Furthermore, the calm-sea DFOC and CO2 emissions were also improved by 15.6%.

Hydrodynamic and energy-harvesting performance of a BBDB-OWC device in irregular waves: An experimental study

The back-bent duct buoy (BBDB) can be used as a floating oscillating water column (OWC) wave energy converter, showing its potential and reliability in most previous prototype sea trials. In this study, an experimental test on a generic BBDB OWC model was conducted in a wave flume using irregular wave scenarios. By using a specific mechanism, the degree-of-freedom (DOF) of the model can be preset in four modes: the ZDOF (the model is fixed with zero DOF) mode, the TDOFs mode (the model conducts the pitching and heaving motions with two DOFs), the SDOF-P mode (the model conducts the pitching motion with the single DOF), and the SDOF-H mode (the model conducts the heaving motion with the single DOF). The kinematic, hydrodynamic, and aerodynamic responses and energy-harvesting performances of the BBDB model were presented and analyzed. It was found that the pitching and heaving motions made positive and negative contributions to wave power capturing of the model, resulting in a moderate energy-harvesting performance for the TDOFs mode. Furthermore, the analysis indicated that strengthening the pitching motion and restraining the heaving motion will benefit the wave energy conversion of the BBDB device.

Design of an electrical energy storage system for hybrid diesel electric ship propulsion aimed at load levelling in irregular wave conditions

This paper focuses on the design stage of an electrical energy storage system which is intended to be used to level the power required by ships for propulsion when sailing in irregular seas. Particularly, a preliminary analysis has been carried out aimed at choosing, between two storage technologies namely battery and ultracapacitor, the more adequate storage system for the levelling action with respect to the period of the sea waves. A sizing procedure based upon a Hilbert transform decomposition of load demand is used. More specifically, a specified threshold frequency has been identified such that, power fluctuations related to irregular seas which are characterized by frequencies higher than the specified threshold, indicate the ultracapacitor as the more appropriate device for load levelling. For sizing the storage device, the load demand has been decomposed into two signals without overlapping frequencies. An optimal control strategy is proposed, by exploiting the decomposition of the load power, which allows simulating ultracapacitor real-time operation within the previously determined size constraints. Numerical results have been carried out referring to a Ro-Pax ferry powered by an electrical motor.

Numerical Simulation of a Floating Offshore Wind Turbine Incorporating an Electromagnetic Inerter-Based Device for Vibration Suppression and Wave Energy Conversion

Offshore wind turbines (OWTs) are considered vital to the promotion of the development of renewable energy. Especially, floating OWTs can be deployed over a larger area than bottom-fixed OWTs. The floating OWTs, however, are vulnerable to vibration induced by disturbances and require a backup power supply in the case of power outage. On the one hand, various kinds of inerter-based devices have been proposed especially for vibration suppression of civil structures subjected to earthquake loadings. Recently, combined with electromagnetic devices, the inerter technologies have also been applied in the field of vibration energy harvesting such as point absorber wave energy converters. Thus, this paper proposes a novel floating OWT consisting of two bodies combined with inerter-based power take-off (PTO) devices which accomplishes vibration suppression and wave energy conversion at the same time. To investigate the vibration suppression and energy conversion capabilities of the proposed floating OWT with a variety of inerter-based PTO devices for ocean waves, numerical simulation studies employing WEC-Sim are conducted, and the performance of each system is compared for regular and irregular waves. Results show that the proposed floating OWT with the appropriately designed inerter-based PTO devices for the incident wave period has great potential for both vibration suppression and wave energy conversion in a specific frequency range.

Experimental hydrodynamic assessment of a cylindrical-type floating solar system exposed to waves

In this paper, an experimental investigation on the wave loads and structural motions of two semi-fixed semi-immersed horizontal cylinders type rafts in the free surface zone is conducted. The physical models are tested at the 1:4.5 scale and exposed to a range of regular and irregular waves in a wave flume at Queen's University Belfast. The physical models and experimental setup are discussed alongside an investigation of the hydrodynamic phenomena, surge forces, and dynamic responses that each structure exhibits in the coastal wave climates. Furthermore, an investigation into the wave attenuation by both models is carried out. The results show that the surge forces have a positive correlation with wave steepness for both models. Hydrodynamic phenomena such as wave runup and overtopping, radiative damping and reflected waves, constructive interference, diffraction and flow separation were identified during the experiments. A negative mean heave displacement is observed during the monochromatic sea states which could result in impact loading and submergence of the superstructure components and photovoltaic panels at full-scale. The results presented in this paper may be used to calibrate and verify numerical models that calculate the global responses and hydrodynamic forces. It may also benefit the design processes of geometrically similar floating solar technologies by providing data on surge loads, motion responses and hydrodynamic observations.

Analysis on the split absorber integrated with taut-moored floating turbine

A new wave energy converter is proposed in this paper, consisting of three split heave point absorbers, combined with a taut-moored floating turbine. It is adapted to the waves in the China Sea area, which are characterized by short periods and small amplitudes. Based on a series of physical model tests in regular, irregular, and extreme waves, the hydrodynamic performance of the integrated device is systematically investigated under different damping forces and incident wave directions. The experimental results reveal that the split point absorber presents new hydrodynamic characteristics and that the wave energy capture efficiency of the new device is greatly improved for the short-period waves in low sea states. Moreover, due to the out-of-phase heave motion, as well as the induced shallow water effect, the submerged platform makes a contribution to improving the energy capture efficiency of the split floater, particularly pronounced in the case of a high damping force in the power takeoff system. Under the condition of incident wave direction being coincident with the horizontal projection of mooring lines, the energies of pitch motion and mooring force of the integrated system are increased as a result of the high-frequency oscillation, which needs to be solved by further optimizing the taut mooring system.

A deep learning method for the prediction of focused waves in a wave flume

Rogue waves pose a significant risk to marine safety, emphasizing the need to accurately predict their occurrence in the open ocean. However, the complexity of their evolution, which may involve nonlinear physical phenomena such as wave-wave interaction and modulation instability, makes this task challenging. Currently the reconstruction of rogue waves involves generating focused waves through the superposition of different spectral components of irregular waves that are in phase at the focusing point. Despite its effectiveness, this approach is limited. The paper introduces a deep learning method based on Long short-term memory (LSTM) to predict focused waves generated in a Computational Fluid Dynamics (CFD) flume in the time domain. The model is trained on 60% of the generated wave time series, with the remaining 40% used for both validation and testing. The results demonstrate that the proposed method can assist with the prediction of focused waves at various observation points, indicating its potential as a promising approach for predicting rogue wave behaviour in the ocean.

Time-domain floater stress analysis for a floating wind turbine

There are increasing focuses on developing cost-effective floating wind turbines, for which efficient stress analysis methods are needed for floater structural design. Most of the today's studies focus on global analysis methods in which the floater is assumed as a rigid body or multiple rigid bodies and the stress distributions in the floater cannot be directly obtained. As part of the COWI Fonden funded EMULF project, a summary about the methodology, the numerical modeling procedure and the verification for stress response analysis of a semi-submersible floater for a 15MW wind turbine is presented. This analysis procedure includes the regeneration of the hydrodynamic pressure loads on the external wet surface of the floater due to wave diffraction, radiation and hydrostatic pressure change, and the application of these pressure loads, together with the time-varying gravity due motions, the inertial loads and the forces/moments at the boundaries (i.e. tower bottom and mooring line fairleads) of the floater to obtain the deformation and the stresses of the floater in the time domain. The analysis procedure is implemented in a developed MATLAB code and the DNV software package. The importance of the different hydrodynamic pressure components was discussed considering representative sea states. A verification of the obtained stress time series and statistics using this method against the regeneration from a linear frequency-domain approach was made considering irregular wave actions only, and a very good agreement was obtained. The developed methodology can provide an efficient solution for structural design analysis of floating wind turbines.

Design and Hydrodynamic Performance Analysis of a Two-module Wave-resistant Floating Photovoltaic Device

This study presents a two-module wave-resistant floating photovoltaic device, featuring a photovoltaic installation capacity of 0.5 MW and triangular configurations for both modules. Flexible connections are employed between the two modules, and semi-tensioned mooring cables are used in the mooring system. The system is analyzed for a specific sea area in Zhejiang, China. Based on three-dimensional potential flow theory, frequency and time domain analysis are conducted to obtain the motion response of each module under given sea conditions, as well as the forces acting on the connecting structures and mooring cables. The hydrodynamic performance of the floating photovoltaic device is compared under different wave directions. The results show that the designed floating photovoltaic device can resist irregular waves with a significant wave height of 4.5 m and a spectral peak period of 10 s. Compared with other conditions, the motion response of the floating photovoltaic device is relatively stable, the maximum force of each connecting structure is smaller, and the overall hydrodynamic performance is the best under the 0°wave condition.

Validation and practical application of nonlinear wave decomposition methods for irregular waves

Validation and practical application of nonlinear wave decomposition methods for irregular waves

Design and performance evaluation of novel magnetic tristable wave energy converter

To realize efficient broadband wave-energy conversion, this study proposes a point-absorber wave energy converter with a novel magnetic tristable mechanism (NMT-PAWEC). The tristable mechanism consists of coaxial inner and outer magnetic rings. The outer magnetic ring features two types of sector magnets with opposite poles. The motion control equations of the NMT-PAWEC are established using the Cummins equation and the equivalent magnetic charge method. The preferred parameter region of the novel tristable mechanism was investigated, and the energy-conversion performance of the NMT-PAWEC under regular waves was analyzed. The results show that the NMT-PAWEC with shallower potential wells can easily achieve inter-well oscillation under low-amplitude waves, with higher energy harvesting efficiency, and a wider frequency and work-damping band. Through a nonlinear bifurcation dynamics analysis, the causes of fluctuations and sudden changes in the output power under low-amplitude waves were revealed. Furthermore, the capture width ratio and annual average power of the South China Sea were considered. The influence of optimized PTO parameters on the energy-conversion performance of the NMT-PAWEC in regular and irregular waves was further discussed. In general, the efficient broadband power output capabilities demonstrate the excellent applicability of NMT-PAWEC.

Experimental Study on the Hydrodynamic Characteristics of a Fixed Comb-Type Floating Breakwater

A comb-type floating breakwater is a new wave dissipation structure with particular force and dissipation performance advantages due to the two wave-reflecting surfaces. In this article, physical model experiments are used to study the hydrodynamic characteristics of a fixed floating comb breakwater and two structural optimization-based measures under the combined action of regular waves, irregular waves, and wave currents. The effects of factors such as the relative width, relative wave height, water flow velocity, and irregular waves on the transmission coefficient of the breakwater are analyzed. In addition, the characteristics of the transmission wave waveform are analyzed based on the time and frequency domains. The results show that (1) the wave transmission coefficient of a comb-type floating breakwater is lower than that of a rectangular floating box for long-period waves, while the transmission coefficient is larger than that of a rectangular floating box for short-period waves. (2) Under combined current and waves, the superimposition of bidirectional currents can increase the transmission coefficient, and the transmission coefficient increases with increasing current speed. The superimposition of the anti-directional current can decrease the transmission coefficient. (3) Moreover, with the same wave parameters, the transmission coefficient for irregular waves is larger than that of regular waves. (4) Finally, extending the bottom plate and adding lower baffles can effectively enhance the wave dissipation effect of the comb-type floating breakwater while also stabilizing the transmitted wave waveform.

Numerical Study on Wave Dissipation Performance of OWC-Perforated Floating Breakwater under Irregular Waves

This paper utilizes ANSYS-AQWA hydrodynamic simulation analysis software (2022 R2) to investigate the wave dissipation performance of an Oscillating Water Column (OWC) perforated floating breakwater under irregular wave conditions. The study examines the effect of spacing, width of the OWC opening, water depth, incident wave angle, and significant wave height on the wave dissipation performance of the floating breakwater. The results indicate that the wave dissipation performance of the OWC-perforated floating breakwater surpasses that of similar structures. The transmission coefficient is significantly influenced by spacing and water depth when subjected to irregular waves. The width of the OWC opening also affects the wave dissipation to some extent, with wider openings demonstrating improved performance in the case of long-period waves. The incident wave angle of 0 degrees yields enhanced wave dissipation performance. Although the meaningful wave height has minimal impact on wave dissipation, it increases proportionally with the rise in meaningful wave height. This study offers valuable insights for the design and implementation of floating breakwaters and holds significant practical implications for the research on integrated devices combining floating breakwaters and wave power generation.

Analysis of FPSO Motion Response under Different Wave Spectra

A variety of floating structures at sea play a vital role in the exploitation and utilization of marine resources. The study about interactions between waves and structures is necessary for the impact of the harsh marine environment on the motion and service life of structures. Currently, most studies about the seakeeping of structures are based on simplified regular waves. Because the regular waves do not truly restore the actual wave conditions at sea, the simulation of irregular waves has great practical importance to the study of interactions between waves and structures. Based on the potential flow theory and high-order boundary element method (HOBEM), a Fortran code is developed in this paper and named as SWBI (Solver for Wave–Body Interactions). This program consists of the following two parts: a time–domain numerical model about interactions between waves and 3D structures is based on weakly nonlinear method, and a numerical model about simulation of the nonlinear regular waves, the long-crested irregular waves, and the short-crested irregular waves. This Fortran code is used to simulate the motion of Floating Production Storage and Offloading (FPSO) under three different ocean wave spectra (including ITTC two-parameters spectrum, JONSWAP spectrum and the most likely spectral form of Ochi-Hubble) and found that: To a certain extent, the difference in the motion of FPSO under different wave spectra have a connection with different type of wave, sea conditions and incident angle. The difference in roll of FPSO is quite significant in short-crested irregular waves. The range of FPOS’s roll under the JONSWAP spectrum is the largest when the incident angle is 30°, and range of FPOS’s roll under the most likely spectral form of Ochi-Hubble is the largest when the incident angle is 90°.

Hydroelastic effects on the vertical bending moment of a container ship in head and oblique seas

Large-sized ships like Ultra large container ships (ULCS) experience severe hydroelastic vibrations like springing and whipping. This study numerically investigates the performance of a time domain method to assess the effect of structural vibration in calculating of vertical bending load of an ULCS. The proposed 2-D body nonlinear time-domain numerical method comprises a seakeeping solver based on potential flow theory and a structural solver based on Timoshenko beam theory. The simulations are carried out in the head and oblique waves for various forward speed conditions. The solver captures the effects of high-order springing and whipping in vertical bending. The numerical method identifies the whipping effect based on the slamming associated with the relative motion of the ship. The vertical bending moment is calculated in both regular and irregular waves, and the results are compared with the experimental results. The statistical evaluations of available results for irregular waves are carried out and presented. The numerical results are in good agreement with the experimental ones for most of the analyzed cases.

Reconstruction and prediction of global whipping responses on a large cruise ship based on LSTM neural networks

This study presents a model based on Long Short-Term Memory (LSTM) neural networks for the reconstruction and prediction of global whipping responses using ship motion data. The model is fine-tuned and trained based on a dataset of heave, pitch, surge motion data, vertical acceleration data, and vertical bending moments (VBM) from a large cruise ship model experiment by utilizing 5-fold cross-validation. The established model is tested with a pre-split test dataset and used to carry out reconstruction and multi-step prediction of the VBM amidship in following and head seas. The reconstructed results demonstrate a high degree of fit with the measured VBM amidship under both regular and irregular wave cases, indicating the model's ability to capture whipping responses. The findings of multi-step prediction indicate that the prediction accuracy and the model's ability to capture whipping responses decrease as the horizon increases, and the model is more accurate in predicting whipping responses when they are stronger.

Experimental study on an isolated oscillating water column wave energy converting device in oblique waves

The oscillating water column is the most widely used wave energy conversion technology for long-term operations, which has the potential to play a critical role in massive applications of wave energy. In this study, an isolated OWC model employing a generic cubic air chamber and an impulse turbine model was tested in a wave tank using regular and irregular wave scenarios. The wave distribution characteristics surrounding the isolated OWC model in oblique waves with various incident angles and other testing conditions were reported. As the incident waves were forward, the symmetrical wave-height distributions could be observed with enhancement and reduction zones in front of the skirt wall and in the sheltered area of the model. Furthermore, the wave distributions were asymmetrical and more complicated in oblique waves. In addition, the effects of the incident wave angle, height, period, and rotation speed of the turbine model on the staged and overall energy-harvesting performances were analyzed. The OWC model obtained the peak energy-harvesting performance, which reduced as the incident angle decreased. As the incident angle decreased to 60° and zero, the overall capture width ratio reduced to approximately two-thirds and a quarter of the value for the incident angle of 90°. Based on experimental results, the annual energy production of a prototype isolated OWC plant in a typical sea state in North China was predicted. The annual energy production of the plant in the actual sea condition was approximately two-thirds of that only in the forward wave incidence.

Real-time control of ship's roll motion with gyrostabilisers

The safety of ship manoeuvring operations is greatly affected by large roll motions under severe wave conditions. While existing studies have focused on optimizing hydrodynamic performance devices, these approaches may not be effective under realistic sea conditions where wave frequency and direction are irregular. Thus, predicting ship motion tendencies and implementing a reasonable control approach is crucial for improving ship safety. This study proposes a novel and practical control strategy that can effectively suppress roll motion of ships under realistic random wave conditions. The approach utilizes a real-time controller based on model predictive control (MPC) that is actuated by a dual gyrostabilizer. The ship's roll motion response in beam waves is calculated using one degree-of-freedom hydrodynamic modeling, and the wave memory effect is taken into consideration by employing system identification. A synthetic mathematical model of MPC and the dual gyrostabilizer method is introduced in state-space representation to compensate for wave excitation moment in real-time. The proposed method's efficacy is validated by comparing with a roll stabilization scenario of a particular ship in the literature, and control performance of a standard ship under regular and irregular wave conditions is separately discussed. Results indicate that the proposed method can significantly reduce roll motion amplitude and is feasible for nonlinear ship motion control problems. This study sheds light on the multi-degree-of-freedom motion control of ships under beam wave conditions.