Regular Wave Tests Research Articles

Experimental and numerical study on the hydrodynamic responses of a novel offshore floating photovoltaic system

Over the past decade, floating photovoltaics (FPVs) have experienced rapid growth. To effectively reduce costs and actively promote the engineering application of floating photovoltaic systems, an innovative FPV design characterized by low cost, high stability, and excellent seakeeping performance is proposed in this paper. A series of floating pontoons serve as buoyant elements within the system, offering the necessary buoyancy, and solar panels are installed on a support structure which is made up of a truss frame. A series of regular and irregular wave model tests were conducted at the wave basin of Jiangsu University of Science and Technology with model scale determined as 1:20 to assess hydrodynamic characteristics of the FPV. Twenty-One regular wave tests were conducted to establish the Response Amplitude Operator (RAO) of the FPV. Furthermore, irregular wave tests were performed to predict hydrodynamic performance of the FPV under more realistic scenarios. It was found that the FPV system encounters the highest level of danger during beam sea conditions, as compared to when it faces heading sea and quartering sea conditions. Additionally, a comparative numerical model which has been validated by experimental data was applied to study hydrodynamics of the FPV and mooring tensions under extreme sea conditions, thus providing design considerations and profound insights for the future development of this FPV system.

An Experimental and Numerical Study of Motion Responses of Multi-Body Arrays with Hinge Connections

Hinged multi-body systems are gaining popularity in the field of ocean engineering. Their performance is commonly evaluated using numerical simulations, but comparisons with experimental data are required to ensure the accuracy of the computational tools. However, there is a dearth of experimental studies on the motion performance of hinged multi-body systems, particularly those involving more than two hinged floating bodies. This study aims to fill this gap in experimental data for hinged multi-body systems beyond two bodies. The rectangular box was chosen as the test model due to its stable hydrodynamic properties and ease of numerical modelling. Five identical boxes were prefabricated and subsequently tested in the pool in a sequence ranging from one to five boxes to capture the motion performance. Additionally, a numerical programme based on potential flow theory was developed for mutual validation with the experimental data. Firstly, the physical properties of each box were determined through equations calculation and a free decay test, enabling the acquisition of all parameters for conducting numerical simulations. Then, the response amplitude operator (RAO) curves for the heave and pitch motion of a single box were depicted, and the results indicated that the resonant frequency in pitch direction obtained from the regular wave test was consistent with that obtained from the free decay test. Finally, the motion RAO curves of hinged multi-body systems were presented and analysed. The agreement between the measured and computed results confirms the suitability of the experimental data presented in this study as benchmark data for validating numerical simulations.

Verification and validation of a coupled CFD–FEM approach with overset structured grid through free decay and regular wave tests

This paper presents a comprehensive verification and validation study applied to the hydrodynamic analysis of a semi-submersible Floating Offshore Wind Turbine (FOWT) platform. The numerical simulation is conducted using a coupled Computational Fluid Dynamics-Finite Element Method (CFD–FEM) approach. The CFD program incorporates an overset grid interpolation method, facilitating a more systematic grid refinement in the estimation of discretization uncertainties. A mooring analysis program, based on the FEM scheme, is integrated with the CFD program to simulate the dynamic responses of mooring system. The necessity of the nonlinear mooring model is demonstrated by comparing solutions with a comparative test using a linear mooring model. The verification study, based on a least-squares-based extrapolation method, quantifies the spatial and temporal discretization errors of the numerical model. The numerical solutions based on the baseline model are further validated against the model test measurements. A pitch free decay model test and a regular wave model test conducted within the OC5 project are utilized for the verification and validation study. Results from both tests demonstrate the reliability and accuracy of the coupled method.

Numerical and experimental studies on a 2-module VLFS with hinged connector

The design of Very Large Floating Structures (VLFSs) often involves multiple modules with connectors. Consequently, Fluid Structure Interaction (FSI) has been attracted more and more attention in the design of VLFSs due to their considerable dimensions. This paper presented results of numerical and experimental studies on a 2-module VLFS with hinged connectors. A Flexible Module and Flexible Connector (FMFC) method was established in this paper, which allows for considering the influence of elastic deformation on each module and connectors. Model test was designed and conducted in wave basin, including wet modal tests in still water in, a white noise wave test and regular wave tests. The present method was verified by the model test, and good agreements were observed between the numerical and experimental results. Subsequently, the characteristic of wave loads in different wave directions was investigated by numerical calculation, and the design wave parameters were obtained by short-term analysis. Several useful conclusions were summarized, which can provide a reference for the analysis of dynamic responses, design, and construction of the VLFSs.

Numerical and experimental investigations on dynamic response of twin-barge floatover system in standby phase

The twin-barge floatover installation method is an efficient method for the mega topsides installation for the offshore platform because of the large capacity and the gap-free for the substructures. This paper develops the dedicated software TBF-TJU (Twin-Barge Floatover by TJU) for dynamic response analysis of the twin-barge floatover installation system. A numerical model of the elastically-connected twin-barge floatover installation system was developed to analyze the coupled dynamic response of topsides and barges. The motion equations of the twin-barge and the topsides with consideration of the mechanical connections and the hydrodynamic interactions are derived and then solved in the time domain. The loads on mating units DSU(Deck Support Unit) in head and beam seas are numerically and experimentally investigated in detail. The numerical results are compared to the results of the corresponding tests in regular and random waves. Time-series and statistical comparisons of the motions and loads indicate that the numerical model can provide a reasonable estimation of the dynamic response of the twin barges and the topsides. The motion of the upwind barge is significantly larger than that of the leeward barge in beam sea due to the shielding effect and the hydrodynamic interactions.

Intelligent prediction of wave loads based on multi-source data-driven methods

ABSTRACT High-fidelity wave-load tests is pivotal in the design and development of modern ship structures. Nonetheless, the prohibitive cost of conducting such tests limits their applicability. A wave-load data fusion method was implemented using the model test data from a typical displacement-type ship to generalise the small sample test, and a multi-source data characterisation and model parameter optimisation method were established to achieve efficient wave-load (wave frequency and slamming superposition) prediction. Using multi-source data, a multi-fidelity wave-load prediction (MFWLP) model was constructed by combining numerical computation, regular wave tests (RWT), and irregular wave test (IWT) data based on transfer learning with a secondary correction wave load prediction under irregular wave conditions. The model was effectively employed in a ship wave-load engineering case. This study reveals that the adopted MFWLP model demonstrates adequate extrapolation and prediction capabilities, even with limited high-fidelity test samples. The discrepancy between the nonlinear wave load prediction and model test was found to be within 15%. Therefore, the proposed method can serve as a foundation for rapidly assessing ship wave loads across various sea conditions, especially under high sea states.

Coupling a nonlinear finite element mooring model with an overset CFD solver for dynamic analysis of floating structures in waves

An accurate prediction of motion responses of a moored floating structure requires high-fidelity coupled analysis between the structure and its mooring system in waves, which involves both mooring line dynamics and fluid dynamics. In this paper, we develop a nonlinear finite element dynamic mooring analysis module that allows large axial stretching of mooring lines under the open source CFD framework OpenFOAM, and couple it with an Overset CFD solver to establish a fully coupled finite element mooring-CFD analysis model for floating structures. The coupled model is advantageous over existing models in that it is able to deal with large axial mooring stretching and complex mesh motions induced by significant structure responses under severe operating conditions. The developed dynamic mooring analysis method is firstly validated against publicly available data. In order to validate the coupled model, a series of experimental tests were carried out for a semi-submersible platform moored by eight catenary lines under different operating conditions, including free decay, regular and irregular wave tests. The dynamic responses of the floating structure and mooring tension are analyzed and compared with experimental data. Results show that numerically predicted structure responses and mooring tension are generally in good agreement with experimental measurements. Meanwhile, the strongly nonlinear large-amplitude resonant motions generated by the interaction of irregular waves with the moored floating structure are also investigated. Comparisons reveal that the coupled model can accurately predict the resonant spectral peaks of the structure responses, indicating that the it is capable of simulating large-amplitude motions of moored floating structures. The coupled model developed in this paper can be further applied to study the survivability of moored floating structures under harsh sea conditions.

Experimental investigation on gap resonance between two floating vessels

This paper explains the influence of gap resonance phenomena in floating vessels having unequal draughts. Two separate model configurations are considered here: one is a vessel having a shallow draught at the weather side (SW), and the other is a deep draught vessel that comes at the weather side. An experimental investigation was carried out in beam sea conditions, where the vessel could only move freely in the heave and roll directions. By beam sea, we mean the incident waves come at a right angle to the broadside of the vessels. Regular wave tests reveal that even though only a slight difference in the gap resonance frequency is observed for the two configurations, the wave amplification at resonance is 57% higher for SW. The reason for this behavior is explained using the instantaneous velocity and vorticity obtained from the particle image velocimetry analysis for a selected test case. Furthermore, the turbulent characteristics of the flow inside the gap for the configurations are compared at different time instances. The horizontal force acting on the vessels and motions (heave and roll) of the vessels in the beam sea condition are also reported in the study.

Experimental study of tendon failure analysis for a TLP floating offshore wind turbine

This paper describes an experimental study conducted on a multi-column tension leg platform (TLP) floating offshore wind turbine (FOWT). A prototype model of the TLP FOWT supporting the NREL 5-MW wind turbine with a scale ratio of 1:50 is tested under various wind and wave conditions at the State Key Laboratory of Coastal and Offshore Engineering at Dalian University of Technology, China. This work has particularly focused on the tendon failure and its impact on the dynamic response of the FOWT. Free decay tests, regular wave tests, wind-wave combined tests and tendon failure tests are conducted using different environmental parameters.The results suggest that natural periods, dynamic responses of the platform, and forces in the tendons satisfy the design requirements. The analysis indicates that the impact of tendon failure on the platform surge, heave and pitch responses are found to be insignificant. When one of the tendons is broken, the adjacent tendons experience a significant increase in tensile force; and, the maximum tensile force in the remaining tendon is found to increase by about 130%. The overstepping of the minimum breaking load as recommended by the design standard DNV GL is not reached, and this indicates that the safety of the system is ensured even during the harshest failure condition.

Sloshing Response of an Aquaculture Vessel: An Experimental Study

The sloshing response is crucial to the design and operation of aquaculture vessels and affects the safety of the culture equipment and the efficiency of the culture operation. A 1/50 scaled model was utilized to investigate the coupled sloshing response characteristics of a novel aquaculture vessel in a wave basin. Two wave directions (beam and head wave) and two filling levels (81.5% and 47.4%) are taken into account. The time-domain and frequency-domain characteristics of the sloshing response under the linear regular wave and extreme operational sea state were investigated using regular wave tests and irregular wave tests, respectively. The sloshing mechanism in the aquaculture tanks is complicated, due to the coupling effect between external waves, ship motion, and internal sloshing. In linear regular waves, the wave frequency mode dominates the sloshing response, which is larger under beam wave conditions than under head wave conditions and larger under half load conditions than full load conditions. The irregular wave test results confirmed the regular wave test conclusions, but the sloshing response has stronger nonlinearity, higher natural modes appeared, and the amplitude of the higher natural modes is also relatively larger.

Comparison of prediction methods for added power and propeller revolution for a 1,800 TEU container ship using resistance tests in regular and irregular waves

This paper predicts the added power and propeller revolution due to irregular head waves for a 1,800 TEU container ship. Resistance tests were conducted with 11 regular waves and irregular waves at SS = 5. Time series (TS) and autocovariance analysis methods were applied to estimate statistical variables and uncertainties. The paper investigated the statistical convergence times of measured data. The wave elevation, heave and pitch motions obtained from the TS agree well with those from spectral analysis in regular and irregular waves. The added resistance from the TS is larger than that from spectral analysis using a regular wave test by 6.1%. The characteristics of response operators and spectra of the added (or reduced) propulsion performances and overload factors are discussed. The power and propeller revolution were evaluated with a variety of methods: SA(LVM-W), LVM-IR, DPM, and RTIM.

Power and dynamic performance of a floating multi-functional platform: An experimental study

Hybrid platforms which integrate a floating offshore wind turbine and a wave energy converter are a promising technology as the wave energy converter can lead to a more stable platform and potentially lower costs. In the present work, a semi-submersible floating offshore wind turbine platform integrating with an array of wave energy converters (WECs) in the form of an oscillating water column (OWC) is presented and tested experimentally. The hydrodynamic properties of the multi-functional platform were investigated under a series of regular and irregular wave tests considering both operational and survival sea states. The influence of various parameters (including incident wave directions and chamber opening ratios) on the dynamic responses and the wave energy capturing were systematically studied. The introduction of the oscillating water column can not only capture wave energy but also reduce the heave, roll and pitch responses, especially for long waves kh ≤ 2.0, kh ≤ 1.4 and kh ≤ 1.76 respectively. The maximum reduction in heave, roll and pitch are 35 %, 55.4 % and 13.8 %, respectively. The chamber air orifice size was analyzed and optimized. From the survival sea state tests, it is found that the introduction of the OWC WECs decreases the maximum amplitudes of the platform motions in all five freedoms and the nacelle acceleration.

Stereo vision-based measurement of wave evolution around square column in laboratory

The spatial-temporal measurement of complex wave evolution is significant in studying wave-structure interactions. Current methods, such as that using wave probes, have shown limitations in measuring the wave evolution around structures in laboratories. In this study, an improved stereo imaging method is proposed for measuring the wave evolution around a fixed structure. Regular wave tests were conducted on a fixed surface-piercing square column in a wave flume to validate the reliability and accuracy of the proposed method. A flexible marker-net made of foam particles was arranged around the column to provide Lambertian features for the water surface. Two synchronized stereo imaging systems covered all the surrounding areas of the column and provided stereo pair sequences for wave evolution. Subsequently, image segmentation techniques were used to mask the low-confidence disparities in stereo matching, and finally, three-dimensional (3D) wave surfaces were reconstructed in the time sequence. The time histories of the wave elevations at particular locations were extracted and agreed well with the measurements of wave probes with an average bias of 2.4 %. Subsequently, the reconstructed 3D wave field was sliced, exhibiting the instantaneous profiles that agreed with the measurements of wave probes. Moreover, the wave run-up height ratios were consistent with those of a previous study, thereby verifying the method's accuracy from the perspective of spatial evolution. The results demonstrated that the proposed method was capable of precisely measuring the spatial-temporal evolution of the wave field around the square column and displayed potential for application in more studies on wave-structure interactions.

Investigation of Low-Frequency Pitch Motion Characteristics for KRISO Standard Offshore Structure (K-Semi) Moored with a Truncation Mooring System

In this study, a model test was carried out to evaluate the station-keeping performance of Korea Research Institute of the Ships and Ocean Engineering (KRISO) standard semi-submersible, K-Semi in the Deep Ocean Engineering Basin (DOEB). The test was performed using a 1/50 scaled model with 12 mooring lines. The water depth was set to 3.2 m using a moveable bottom structure and truncated mooring lines. The dynamic behavior of the K-Semi was examined using a free decay test and regular wave as well as irregular wave tests of 1800. Large surge, heave and pitch due to drift motion from second-order effect were observed. The reason for the excessive low-frequency pitch motion is attributed to the restoring force and moment related to the mooring system as excessive surge occurred. The numerical model of K-Semi with the truncated mooring system was tuned and calibrated using model test results such as free decay and regular wave tests. Through numerical analysis, the motion characteristics of the K-Semi were compared with irregular model test results. In the case of a semi-submersible using a mooring system, it is observed that excessive pitch motion due to vertical restoring force and moment may occur when large horizontal displacement occurs. The effect of low-frequency pitch motion related to surge motion is an important factor in upwell estimation as well as second-order motion in referred DNV-GL OTG13.

Effects of wave direction on ship turning in regular waves

In this study, we focused on the drift motion of a ship during turning in regular waves and investigated the effects of wave direction on ship turning and drifting. First, turning tests in regular waves were conducted using a bulk carrier model by systematically changing the wave direction and wavelength to capture drifting characteristics. Subsequently, long-duration turning simulations were performed to capture the drifting characteristics with a large heading angle change over 2000° in waves using the two-time scale method (Yasukawa, 2006b; Skejic and Faltinsen, 2008). The simulation results indicated that the ship constantly drifted in a particular direction with respect to the incident wave direction during turning when sufficient time had passed. This phenomenon can be understood analytically using new formulas derived for the drifting indices (drifting distance and drifting direction). According to the formulas, the ship turning trajectory is deformed in waves drifting in proportional to time. The drifting velocity and drifting direction were determined by combining the average velocity components (ship surge velocity, lateral velocity, and yaw rate) and the first-order velocity fluctuation components with turning circular frequency. The formulas for the drifting indices were verified through comparison with the simulation results.

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.

Experimental validation of Orcaflex-based numerical models for the PEWEC device

To design a Wave Energy Converter mooring system that ensures maintainability, resistance, and low costs, without affecting productivity, it is necessary to foster reliable numerical models. In the present paper, the validation of Orcaflex© simulations against the experiment results of the Pendulum Wave Energy Converter (PEWEC) 1:25 scale model, conducted at the University of Naples Federico II, is presented. The experiments consist of free-decay and static pull-out tests to assess the inertial properties of the model and mooring system; tests in operative and extreme regular and irregular waves to fully characterize the mooring system and the device dynamics. The same wave records measured in the towing tank have been used in numerical simulations. PEWEC motion results from numerical simulations and experiments are given in terms of Response Amplitude Operators (RAOs) for regular waves, as statistical values in tabular form, and in Taylor's diagrams for irregular waves. The most probable fairleads tensions for the load dimensioning have been obtained from the Generalized Extreme Value (GEV) distribution of experimental and numerical data. The obtained differences show that the numerical model accurately predicts the experimental data and correctly estimates the dimensioning load of the mooring lines.

Experimental study on hydrodynamic behavior and energy conversion of multiple oscillating-water-column chamber in regular waves

An oscillating water column (OWC) device is a promising wave energy converter that uses ocean wave energy resources. The OWC chamber absorbs the energy of ocean waves and significantly affects energy conversion performance. In this study, the hydrodynamic performance of a multiple OWC chamber was investigated experimentally, emphasizing its hydrodynamic behavior and energy conversion characteristics. A series of regular wave tests were conducted for single-, triple-, and asymmetric triple OWC chambers in a two-dimensional wave tank. The same orifice ratio was applied to the single and triple chambers to induce an equivalent turbine-chamber interaction owing to the pressure drop. It was found that the triple OWC chamber contributes to an increase in the converted pneumatic power compared to the single OWC chamber due to reducing the sloshing motion of the internal fluid. Moreover, the temporal fluctuation of the overall output power was reduced because of the water column motion of each triple OWC chamber with a phase difference. The rear chamber of the asymmetric triple chamber traps the wave energy and contributes to not only increasing the output power but also the variability. Therefore, it can be observed that multiple OWC chambers improve energy conversion performance by increasing the mean power output and reducing temporal fluctuation.

Prediction of wave force on netting under strong nonlinear wave action

Under the strong nonlinear wave environment, accurate simulation of wave force for aquaculture netting is an effective guarantee for cage design and safety. In this paper, the horizontal wave forces of a nylon square-mesh netting panel were obtained through a series of strong nonlinear regular wave tests, and their nonlinearity was analyzed by amplitude spectrum. Moreover, the Morison equation based on fifth-order Stokes wave theory was used to reasonably predict the wave force on the netting. The results showed that both wave and wave force have strong nonlinearity, especially the latter. The frequency domain characteristics of the test wave and wave force are similar, while the higher frequency components of the test force are more apparent. The predicted wave forces are in good agreement with the test values in time and frequency domain, and zero or higher frequency components of predicted force are more prominent with the increase of wave steepness. When the range of the Keulegan-Carpenter number is 35-120, the average drag and inertia coefficient of the predicted force are 2.4 and 2.1, respectively. The results can provide a more accurate assessment of the nonlinear wave force on aquaculture facilities.

Experimental Study on the Effect of Extreme Waves on a LNG Carrier

This paper presents a comprehensive experimental study on the effect of extreme waves on a LNG carrier. The LNG carrier model was equipped with a variety of sensors to measure motions, green water height on deck as well as local and global loads. Experiments in transient wave packets provided the general performance in waves in terms of response amplitude operators and were accompanied by tests in regular waves with two different wave steepness. These tests allowed detailed insights into the nonlinear behavior of the vertical wave bending moment in steep waves showing that green water on deck can contribute to a decrease of vertical wave bending moment. Afterwards, systematic model tests in irregular waves were performed to provide the basis for statistical analysis. It is shown that the generalized extreme value distribution model is suitable for the estimation of the extreme peak values of motions and loads. Finally, model tests in tailored extreme wave sequences were conducted comparing the results with the statistical analysis. For this purpose, analytical breather solutions of the nonlinear Schrödinger equation were applied to generate tailored extreme waves of certain critical wave lengths in terms of ship response. Besides these design extreme waves, the LGN carrier was also investigated in the model scale reproduction of the real-world Draupner wave. By comparing the motions, vertical wave bending moment, green water column and slamming pressures it is concluded that the breather solutions are a powerful and efficient tool for the generation of design extreme waves of certain critical wave lengths for wave/structure investigations on different subjects.