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The ocean surface offers enormous potential for generating renewable energy, but cost-effective, reliable, and robust systems are needed. Developing floating solar farms (FSF) can contribute to the process of reducing carbon emissions. However, the rational methodology for hydrodynamic analysis of floating solar farms is still not well established. This paper aims to identify a suitable methodology for the analysis of floating solar farms for mild and moderate environments in nearshore, and eventually deeper offshore deployments. This paper reviews the various type of FSFs developed in recent years and the methodologies applied to evaluate their hydrodynamic performance. Following that, the paper focuses on assessing three potential methodologies for the evaluation of the hydrodynamic performance of articulated FSFs in a nearshore region. The three methods are the two-step approach with multi-body radiation and diffraction analysis, hydroelastic/generalized mode method, and empirical approach utilizing Morison's equation. To evaluate these methods, a simplified small-scale FSF which is comprised of 16 articulated box-type modules, is established. A soft mooring system is introduced to constraint the horizontal motion of the farm. The small-scale farm is representative of a typical large FSF in the nearshore region of Singapore. Numerical models of the farm based on the three methods are established separately, and the dynamic responses of the farm are simulated and analyzed. The motion response operators of the modules of the FSF covering the typical wave periods in nearshore conditions are obtained, and the results from the three methods are evaluated in terms of their efficiency and accuracy. It is found that the three methods show consistent results of the dynamic responses of the solar farm in long waves. However, some discrepancies are present in short waves, mainly due to the increasing importance of hydrodynamic interactions which are neglected in one of these methods. The results could be a useful reference for the design and hydrodynamic analysis of similar FSFs.

Ship structures are subjected to various deteriorating mechanisms throughout their service life. Continuous awareness of the vessel’s structural health is a critical aspect of the overall situational awareness. Model tests are often used to validate software which predicts the vessels’ hydrodynamic loading and dynamic structural response. Generally, two different methods can be used to model the flexibility of the ship. The most common method is to sub-divide the hull into a number of rigid segments that are interconnected by a flexible backbone beam. The elasticity of the model is represented by the elastic beam to which rigid segments are connected. However, the segmented model limits the measurements to prescribed locations between segments. The other method is to fabricate the model using a continuous elastic material. In this paper, a new method for fabricating a fully elastic model is introduced as part of a structural health monitoring system. Since the model is built from continuous materials with known elastic properties, it can be instrumented to measure strain at a larger number of locations. A suitable material for construction of the elastic model has been identified. Material tests are conducted to better understand the static and dynamic behavior of the elastic material. The material shows linear stress-strain relationship and stable mechanical properties within the loading range. Due to the low elastic modulus of the material, the strain gauge stiffening effect is obvious and has been taken into account in the calibration process. Using the elastic material, a fully elastic model of the S175 midship section is designed. As a first step trial, the middle part of the model representing a three-cargo hold is manufactured. Static bending tests are conducted to examine the elastic characteristics of the fabricated model. Wave experiments are carried out. The results from these experiments are compared to numerical simulations.

In this paper, an end-to-end nonlinear model reduction methodology is presented based on the convolutional recurrent autoencoder networks. The methodology is developed in the context of the overall data-driven reduced-order model framework proposed in the paper. The basic idea behind the methodology is to obtain the low dimensional representations via convolutional neural networks and evolve these low dimensional features via recurrent neural networks in the time domain. The high dimensional representations are constructed from the evolved low dimensional features via transpose convolutional neural networks. With an unsupervised training strategy, the model serves as an end to end tool which can evolve the flow state of the nonlinear dynamical system. The convolutional recurrent autoencoder network model is applied to the problem of flow past bluff bodies for the first time. To demonstrate the effectiveness of the methodology, two canonical problems namely the flow past a plain cylinder and the flow past side-by-side cylinders are explored in this paper. Pressure and velocity fields of the unsteady flow are predicted in future via the convolutional recurrent autoencoder model. The performance of the model is satisfactory for both the problems. Specifically, the multiscale nature and the gap flow dynamics of the side-by-side cylinders are captured by the proposed data-driven model reduction methodology. The error metrics, the normalized squared error, and the normalized reconstruction error are considered for the assessment of the data-driven framework., arXiv admin note: text overlap with arXiv:1808.01346 by other authors

This paper describes the development of data-driven models for the prediction of mooring line tensions by separating the low- and wave-frequency components of the tensions, such that the former is approximated as quasi-static tensions while the latter is predicted using ANN models. A bilinear model is used to interpolate the low-frequency quasi-static tensions between known values in a look-up table while a feed forward neural network model that utilizes the fairlead motions as input is used to predict the tension dynamics at the fairlead. The ANN models are trained using the results from numerical simulations, such as those generated during the engineering design and construction stages of the floating structure. The predicted line tensions are compared with those obtained using coupled numerical simulations, including test cases for different wave realizations that are not included in the training dataset. Models trained for single and multiple directions of the environment are also assessed for prediction accuracy. Initial comparisons show that good predictions of line tensions can be obtained for certain environments using the proposed approach, thus demonstrating the potential for use in applications where real-time predictions are required to enhance the safety and / or reliability of mooring systems.

Autonomous tugs may play an important role in future ports, due to the shortage of qualified mariners. A digital twin (mathematical model incorporating a vessel’s hydrodynamic behavior and response, suitable for real-time control) would be needed for autonomous operations. Yet, partly because tugs are generally high-powered and very maneuverable compared to conventional vessels, there is little published data on the hydrodynamic performance of such vessels. As a first step in the development of the tug’s digital twin, the present work studies the maneuvering and seakeeping performance of a generic tug at model scale. Numerical simulations are performed for an approximately 1:10 scale model for standard resistance, static and dynamic captive and seakeeping cases. Reynolds-Averaged Navier-Stokes (RANS) k-ω model is employed for the simulations including the free surface through the Volume of Fluid approach. The hydrodynamic forces and moments on the tug model in the simulations of the standard resistance and the static and dynamic captive cases, as well as the tug model’s motions and the added resistance in headseas, are investigated. The simulation results provide data to build a mathematical maneuvering model for the tug based on 4-DoF MMG manoeuvring model, which serves as the digital twin in this case.

The development of an accurate digital performance twin of a tug requires a complete understanding of its propulsive capacity and hull-thruster interactions. In this study, the propulsion characteristics of an Azimuth Stern Drive (ASD) tug is investigated using model-scale Reynolds-averaged Navier-Stokes (RANS) simulations. The propulsion plant consists of two counter-rotating thruster units, with each having a Ka4-70 series propeller and 19A duct profile. Comparisons in propulsive performances using the steady-state moving reference frame (MRF) approach and the transient rigid body motion (RBM) models are shown, and validated against data from openwater experiments. The MRF method gives sufficiently accurate predictions of thrust and torque in forward flow and moderate angles-of-attack, while the RBM method performs better at larger inflow angles. The effects of thruster-hull and thruster-thruster interactions on wake characteristics and propulsion performance are also investigated over a range of advance and inflow/azimuth angles. Convergence and mesh independence studies are conducted to determine the optimal spatial and temporal simulation parameters. Results from this study identify flow regimes where hull and thruster interactions are significant.

An innovative floating hydrocarbon storage facility (FHSF) has recently been proposed. The facility consists of several modular floating hydrocarbon storage tanks (FHSTs) surrounded by floating barges with compliant mooring system. Hydrodynamic loads on the FHST and its dynamic response are important for the global design. To validate the concept of the single FHST, model tests at 1:48.7 scale is performed in the coastal basin in National University of Singapore. Two models of single FHST with different bottom slab shapes are tested, one of which has bottom skirts. The mooring fenders to support the model were mimicked by a flexible beam system. Fresh water is used to fill the tank at different filling ratios. Decay tests, regular wave tests and irregular wave tests under design sea states are performed. The rigid body motions in six degrees of freedom and the sum fender reaction forces were measured. Numerical model of the single FHST is established and analyzed in frequency domain and time domain. The effect of the bottom skirt, internal liquid, and the fender system were considered. Viscous damping is calculated based on the empirical formula and adjusted by comparing with measured results. Numerical results are comparative to the experimental results. This paper shows the motions of the tested FHSTs are mild even in the 100-year storm, and the bottom skirts effectively reduce the hydrodynamic responses.

This paper presents model scale computations of self-propelled turning circle and zig-zag manoeuvres for a benchmark combatant DTMB 5415M adopting the unsteady Reynolds-Averaged Navier-Stokes (URANS) equations. A hierarchy of overset grids is utilised to allow rudder deflections while the ship undertakes 6 DOF dynamic motions. With both manoeuvres executed at approach speed corresponds to Fr = 0.41, two different propulsion techniques are applied to drive the free running vessel; the body force propeller model (BFM) and the discretised propeller model (DPM). A verification and validation study is also performed for estimating the numerical uncertainties within the URANS simulated manoeuvres. Comparisons of the results are made between computations adopting the two propulsion approaches and against experimental data from literature. The BFM propulsion method in this study is shown to under-predict the magnitude of propeller induced wake passing the rudders compared to the DPM approach which is able to resolve high fidelity flow physics behind the propellers. In general, the comparison between experimental and numerical results agree mostly within about 10% for the studied turning circle and zig-zag manoeuvres.

The present work investigates the effect of different outermost connector types on the hydrodynamic responses of a modular multi-purpose floating structure (MMFS) system. The MMFS system is preliminarily designed for a mild sea zone, which is composed of a number of standardized floating modules. In this study, the MMFS system has been simplified as a seven-module connected system. Considering the structural deformation of the MMFS system mainly occurs in the connectors among adjacent modules, each module is viewed as a rigid body. Both the hydrodynamic interaction effect and mechanical coupling effect among modules have been taken into consideration in the time-domain analysis. Different outermost connector designs have been proposed and investigated for the MMFS system. The results indicate that the hydrodynamic responses of the MMFS system are sensitive to the effects of the connector types, the wave phase, and the wavelength. The hinge-type design of the outermost connector can significantly reduce the extreme responses of both the bending moment and the shear force of the connectors. The additional PTO damper design for the outermost hinge-type connector can effectively reduce the motion of the outermost module, and also produce considerable wave energy.

Two dimensional wave-current-structure interaction problems are investigated in a sea environment with a flat or sloping seabed using linear boundary conditions. This problem is separated into steady and unsteady potential components. The former accounts for the current flowing over the seabed or fixed body whereas the latter describes the wave interaction with the current through modification of the free surface boundary conditions. Both steady and unsteady problems are solved using a boundary element model incorporating a continuous Rankine source method. The developed numerical model is validated by comparison with published data, where possible, and the investigation extended to examine the hydrodynamic responses of a forced oscillating body subject to the combined interactions arising from the wave, current and seabed.

Ocean-going vessels frequently experience severe weather conditions. Occasionally, the actual environmental loads are much larger than design values, resulting in deviations from design conditions that may cause serious structural damage. A structural digital twin can enhance predictability and reliability of marine structures, and thus provide vessel operators, owners, and stakeholders with a more accurate overview of a vessel’s condition. It can also help reduce lifecycle costs through proposing rational and optimal inspection, maintenance, repair programs, and assist on maximizing service life and mission availability. This paper uses a benchmark vessel to investigate structural behaviors of the vessel through dynamic loading approach. The correlation between structure members and critical responses are established, and high stress concentration areas have been identified. Subsequently, this paper develops a conceptual digital twin architecture. The proposed digital twin predicts responses of the entire structure to measured or predicted environmental conditions with a hybrid model combining numerical simulations with measurements. While the traditional structural monitoring system focuses on the health condition of typical hot spots, the proposed digital twin may improve structural health awareness of the entire hull structure based on a limited number of sensors.

A hydrodynamic digital twin of vessel can be used to replicate the behaviour and response of the vessel in a virtual environment. In this paper, a real-time simulation model (RTSM) for an azimuth stern-drive (ASD) tug has been developed for simulating the hydrodynamic performance of the vessel under a range of environmental conditions. Based on the framework of a 4-DoF MMG manoeuvring model, the RTSM comprises manoeuvring, propulsion and environmental loads which are parameterised using numerical results from a combination of computational fluid dynamics (CFD) modelling work, including virtual planar motion mechanism (vPMM), seakeeping analysis, wind drag prediction and propulsion modelling. The RTSM is used to demonstrate the manoeuvrability of the vessel in calm water and under external loads from waves, winds and currents.

This paper presents verification of a deep water FPSO with a semi-taut mooring system using model tests and numerical modelling commonly referred to as the hybrid method. The vessel under investigation is a FPSO of 310m in length and 47m in beam with an internal turret mooring system of 12 lines in 2000m water depth. Two configurations of the mooring systems i.e. inline and bisecting are investigated for sea-states up to 1000yr return period. A full depth mooring system has been developed for the FPSO and model tests will be carried out to verify the model. Due to limitations to the size of the model basins, the model tests will be carried out for a truncated mooring setup. Non-linear horizontal stiffness of a single mooring line and the complete mooring system with truncation is compared to that of the existing full depth mooring system. Discrepancies in the vertical forces due to truncation of line length will be discussed in the paper. A numerical model of the truncated set-up will be calibrated using model test results.

A flotel is often used to house personnel and equipment for on-site maintenance of ageing FPSOs. Although tankers are designed to be dry-docked, operating FPSOs (which may be reconverted tankers) must be maintained in the field. The advantage to industry of on-site maintenance and inspection rather than dry-docking of FPSOs is crucial to cost-effectiveness. For multibody systems like FPSO and flotel in a side-by-side configuration, one of the primary questions is the time for which the gangway can remain safely connected between the two bodies. Operators always seek for a maximum possible uptime to operate the gangway, so that FPSO maintenance works are not halted for long durations during the harsh environments. The main factors limiting the operation of a gangway are its extension and rotation due to the relative motions between FPSO and flotel. Therefore, it is firstly important to accurately predict the hydrodynamic interaction between the two bodies for estimating their motions. In the present work, the variation of current drag on the flotel due to the presence of a turret-moored FPSO is investigated. The current drag variation on the flotel is studied by modelling the multibody system in OpenFOAM. Steady state simulations are performed using simpleFoam algorithm to compute the hydrodynamic coefficients of pontoons and columns. SimpleFoam is a steady-state solver for incompressible, turbulent flow, using the SIMPLE (Semi-Implicit Method for Pressure Linked Equations) algorithm. The meshes for FPSO, flotel and fluid domain are created in ICEM-CFD and imported into OpenFOAM. A mesh convergence study is conducted. The simulations are performed for various current velocities and directions in combination with different drafts of FSPO. Finally, a lookup table comprising of current drag coefficients for several cases is prepared for future reference in designing the flotel. A non-collinear environment is the worst scenario for turret-moored FPSO. In most cases, the wind-driven current direction is within ±45° relative to the wave heading. The drag load experienced by the flotel on the columns and pontoons varies as a function of the current direction, the draft and the heading of FPSO relative to the flotel and the location of the flotel (upstream or downstream of FPSO). As FPSO changes yaw angle, the flotel experiences current loads which sometimes vary abruptly due to the shielding effects. The paper presents novel results and explains how the hydrodynamic interactions affect the important current loads experienced by the flotel under various scenarios. The choice of orientation of the flotel relative to FPSO (pontoons parallel or perpendicular to FPSO) is influenced by and must be decided by considering the effect on the current loads. The effects of varying shielding need to be considered by the dynamic positioning system in order to develop robust control methods to maximize uptime for a given installed thruster power. Several studies have been performed to investigate the hydrodynamic interaction between two bodies like cylinders, square columns, rectangular structures, etc. However, very little work has been on the hydrodynamic interaction between FPSO and flotel (semi-submersible platform), especially when this multibody system is subjected to current forces. The present study aims to fill this gap and aid the industry in the preliminary design and configuration of the flotel.

Sustainable use of the ocean for food and energy production is an emerging area of research in different countries around the world. This goal is pursued by the Australian aquaculture, offshore engineering and renewable energy industries, research organisations and the government through the “Blue Economy Cooperative Research Centre”. To address the challenges of offshore food and energy production, leveraging the benefits of co-location, vertical integration, infrastructure and shared services, will be enabled through the development of novel Multi-Purpose Offshore-Platforms (MPOP). The structural integrity of the designed systems when being deployed in the harsh offshore environment is one of the main challenges in developing the MPOPs. Employing structural reliability analysis methods for assessing the structural safety of the novel aquaculture-MPOPs comes with different limitations. This review aims at shedding light on these limitations and discusses the current status and future directions for structural reliability analysis of a novel aquaculture-MPOP considering Australia’s unique environment. To achieve this aim, challenges which exist at different stages of reliability assessment, from data collection and uncertainty quantification to load and structural modelling and reliability analysis implementation, are discussed. Furthermore, several solutions to these challenges are proposed based on the existing knowledge in other sectors, and particularly from the offshore oil and gas industry. Based on the identified gaps in the review process, potential areas for future research are introduced to enable a safer and more reliable operation of the MPOPs.

Maritime autonomous surface ships (MASS) require accurate future state projection to initiate collision-avoidance manoeuvres. Forecasts of the vessels’ trajectories and motions are fundamentally based on the mathematical manoeuvring model, which is an essential component of their hydrodynamic digital twin nowadays. Using the benchmark container ship KCS as an object of study, this paper adopts a 4-DOF modular-type manoeuvring (MMG) model to predict the vessel trajectories in calm water and under the presence of steady current and regular waves. The current effects are treated as additional ship over water speed, while the wave effects are considered by superimposing the second-order mean wave drift loads to the calm water hull hydrodynamics. The wave drift loads are solved using the potential flow solver WASIM, which is based on Rankine panel method. The computed vessel trajectories and motions are compared with available literature results and show good correlation.

Experimental and numerical studies are performed to investigate drillship moonpool gap resonance in both stationary and transit conditions in a wave flume. This study contains an assessment of the influence of size and depth of the moonpool on the gap resonance phenomenon. An openfoam-based computational fluid dynamics (CFD) model was established, and the numerical data show good agreement with measurements from the model tests. Both piston and sloshing mode gap resonances are clearly observed. This study shows that the gap resonance frequency and wave elevation response amplitude operator (RAO) inside the moonpool are dependent on its dimensions, and the transit speed of the drillship and wave direction significantly influences the characteristics of gap resonances. It is noticed that the nearness of the wave flume sidewalls significantly influences the piston and sloshing wave elevation RAO at certain frequencies regardless of moonpool length and draft.

In this paper, we present a stability analysis of passive suppression devices for the vortex-induced vibration (VIV) in the laminar flow condition. A data-driven model reduction approach based on the eigensystem realization algorithm is used to construct a reduced-order model in a state-space format. From the stability analysis of the coupled system, two modes are found to be dominant in the phenomenon of self-sustained VIV: namely, the wake mode, with frequency close to that of the wake flow behind a stationary cylinder; and the structure mode, with frequency close to the natural frequency of the elastically mounted cylinder. The present study illustrates that VIV can be suppressed by altering the structure mode via shifting of the eigenvalues from the unstable to the stable region. This finding is realized through the simulations of passive control devices, such as fairings and connected-C devices, wherein the presence of appendages breaks the self-sustenance of the wake–body interaction cycle. A detailed proper orthogonal decomposition analysis is employed to quantify the effect of a fairing on the complex interaction between the wake features. From the assessment of the stability characteristics of appendages, the behaviour of a connected-C device is found to be similar to that of a fairing, and the trajectories of the eigenspectrum are nearly identical, while the eigenspectrum of the cylinder–splitter arrangement indicates a galloping behaviour at higher reduced velocities. Finally, we introduce a stability function to characterize the influence of geometric parameters on VIV suppression.

The development of supercomputing technologies has enabled a shift towards high-fidelity simulations that is used to complement physical modelling. At the Technology Centre for Offshore and Marine, Singapore (TCOMS), such simulations are used for high-resolution investigations into particular aspects of fluid-structure interactions in order to better understand and thereby predict the generation of important flow features or the complex hydrodynamic interactions between components onboard ships and floating structures. In addition, by building on the outputs of such simulations, data-driven models of actual physical systems are being developed, which in turn can be used as digital twins for real-time predictions of the behaviour and responses when subjected to complex real-world environmental loads. In this paper, examples of the high-resolution investigations, as well as the development of digital twins, are described and discussed.

A modular floating concept for oil storage is proposed. The concept is composed of modular floating tanks and barges. The floating tanks are self-stable and designed for storage of oil and hydrocarbon material. The barges are connected together and moored by mooring dolphins. Mooring system is designed between barges and the tanks enclosed by the barges. An innovative mooring system that consists of several mooring ropes besides the fender system is proposed and investigated in this study. The mooring ropes are stretchy and can carry the mooring loads in all the loading and environmental conditions. Single floating tank is investigated under different filling levels, i.e. empty, full and partially filled conditions. It is assumed in this study that the barges are fixed, and multi-body hydrodynamic interaction is neglected. Linear and nonlinear mooring rope properties are investigated. Failure condition of one of the mooring rope is also considered. Wind, wave and current load effects under 100-year storm are also investigated, as well as the uncertainty analysis in terms of viscous effects.

Future state projection is an important prerequisite for the maritime autonomous surface ships (MASS) to initiate a collision-avoidance manoeuvre. Forecasts of MASS′ trajectories and motions are fundamentally based on the vessel's mathematical manoeuvring model, which is also referred to as the hydrodynamic digital twin nowadays. Using the benchmark container ship KCS as an object of study, this paper adopts a 3-DOF modular-type manoeuvring (MMG) model to predict the vessel trajectories in calm water and under the presence of steady current and regular waves. The current effects are treated as additional ship over water speed, while the wave effects are considered by superimposing the mean second-order wave drift loads to the calm water hull hydrodynamics. Concurrently, selected manoeuvring cases including turning circle and zig-zag are solved using the unsteady Reynolds-averaged Navier-Stokes (uRANS) equations based CFD method, where a hierarchy of overset grids is utilised to allow self-propulsion and active rudder steering. The computed vessel trajectories and velocities are compared with that predicted by the MMG model and show reasonable agreement. The wave loads experienced by the vessel when turning in waves are also analysed and discussed.

This paper investigates the effectiveness of floating breakwaters consisting of two barges (L-shape), three barges (U-shape), four barges (barge frame), and conventional single floating breakwater. The floating breakwaters of different spatial layouts have sheltered internal gaps/moonpools when compared to their conventional counterparts. The motions of these floating breakwaters and their effectiveness in wave transmission and motion reduction of the protected floating bodies are evaluated. The study is conducted based on a subsystem of a floating hydrocarbon storage facility that combines a floating breakwater with two floating tanks, studied previously. Numerical models based on linear potential flow theory are built for these floating breakwaters with and without the floating tanks. The numerical models of the barge frame are validated through laboratory experiments, and the dipole damping lids to reduce the resonant fluid motions in the gaps/moonpools are calibrated with experimental results. The L-shape floating breakwater is found effective in reducing wave transmissions without the presence of the floating tanks, while the barge frame is the most effective in the motion reduction of the floating tanks. In addition, significant fluid resonant motions are identified for all investigated floating breakwaters, including the conventional one. Orienting the structure obliquely can improve the performance of the floating breakwaters of special configurations, especially the barge frame. The results and findings show the importance of considering the protected floating structures in the analysis when designing floating breakwaters for many types of marine structures, including floating fish farms and floating cities in coastal waters.

An innovative floating hydrocarbon storage facility (FHSF) has been proposed to utilize the shielded near-shore area for countries with large demand on the land space such as Singapore and Japan. The concept comprises 14 floating hydrocarbon storage tanks (FHST) and several surrounding floating barges. All the modular designed FHSTs are loosely connected to the barges through a soft mooring system so as to reduce the loads, and the entire system is free to float to reduce the tidal influence. The single FSHT has been proven to have moderate hydrodynamic responses in previous studies, but there still exist concerns on the influence of potential resonances in the narrow gaps and the strong hydrodynamic interactions. The loads on the specially designed soft mooring system have to be checked. The complete system is complex and difficult to analyze. So, experimental studies were performed on both a simplified system and the complete system to ensure the quality and reduce the uncertainty in the experiments. The simplified system consists of two FHSTs and a surrounding floating barge frame. The experiments were performed in the ocean basin in SINTEF Ocean. A series of random, wide-band and realistic random wave tests were carried out to generate benchmark data to verify numerical analysis tools. This paper will focus on this simplified system that represents the complete system’s behavior. A frequency domain numerical model of the simplified system was established based on potential theory. Empirical coefficients were used to account for viscous damping. The numerical results are comparable to the experimental results in general. The statistical responses of the FHST in the design sea states are also within the acceptable range even with the hydrodynamic interactions. However, further improvement on the system such as a better design of the floating barge is necessary.

System identification is crucial to predict the maneuverability of the ship. In this work, ε-support vector regression (ε-SVR) is implemented to identify hydrodynamic derivatives of Abkowitz maneuver model. A proposed technique, batch learning, is implemented with the addition of Gaussian white noise to reconstruct the samples and alleviate the parameter drift in the system identification of the ship maneuvering model. The predicted results are compared with results obtained from Planar Motion Mechanism (PMM) test. Standard maneuvers, 35° turning circle, 10°/10° and 20°/20° zigzags, are simulated and compared with the predicted model by ε-SVR. The presented results show that the proposed batch learning technique with Gaussian white noise is an effective technique, which improves the accuracy and robustness of ε-SVR in system identification. The results obtained from the predicted model match well with the those obtained from PMM results, which shows its excellent generalization performance. The developed model is applied to understand control requirements for vessels under different conditions.

In this paper, we present a data-driven approach to construct a reduced-order model (ROM) for the unsteady flow field and fluid-structure interaction. This proposed approach relies on (i) a projection of the high-dimensional data from the Navier-Stokes equations to a low-dimensional subspace using the proper orthogonal decomposition (POD) and (ii) integration of the low-dimensional model with the recurrent neural networks. For the hybrid ROM formulation, we consider long short term memory networks with encoder-decoder architecture, which is a special variant of recurrent neural networks. The mathematical structure of recurrent neural networks embodies a non-linear state space form of the underlying dynamical behavior. This particular attribute of an RNN makes it suitable for non-linear unsteady flow problems. In the proposed hybrid RNN method, the spatial and temporal features of the unsteady flow system are captured separately. Time-invariant modes obtained by low-order projection embodies the spatial features of the flow field, while the temporal behavior of the corresponding modal coefficients is learned via recurrent neural networks. The effectiveness of the proposed method is first demonstrated on a canonical problem of flow past a cylinder at low Reynolds number. With regard to a practical marine/offshore engineering demonstration, we have applied and examined the reliability of the proposed data-driven framework for the predictions of vortex-induced vibrations of a flexible offshore riser at high Reynolds number.

The present work investigates the effect of different outermost module designs on the hydrodynamic responses of a modular multi-purpose floating structure (MMFS) system. The MMFS system is initially designed for a mild sea zone. As the entire system consists of more than 20 bodies, a simplified system with seven interconnected standardized modules is proposed for numerical and experimental study. In this simplified system, each module is assumed as a rigid body. Both hydrodynamic interactions and mechanical coupling among modules are taken into consideration in the time-domain numerical analysis. The structural deformation of the MMFS system mainly occurs in the connectors among adjacent modules. The maximum deformation appears at the connectors between outermost modules with the internal modules. To reduce the deformation and improve the concept, two special design, outermost module of deeper draft and outermost module with additional heave plate are proposed and investigated for the MMFS system. The numerical results indicate that the two proposed designs for the outermost module can significantly reduce the hydrodynamic responses of the MMFS system, especially the motion of the outermost module and the vertical bending moment on the connector. The extreme responses of the MMFS system with different outermost module designs are also studied and compared.

Side-by-side operation of multiple floaters in the ocean environment is very challenging and the operators always prefer a maximum operable weather window, in order to minimise the cost incurred from the downtime. The safety of the gangway connecting the floaters is very crucial and its dynamic response in the ocean environment raises concerns during operations. Therefore, an efficient dynamic positioning system is essential to maintain the floater and ultimately, the gangway response within the desired limits. In this work, a novel dynamic positioning system for floater operating aside another vessel is presented. The system includes an adaptive controller combined an optimised thruster allocation law and with a sea state detector. The adaptive control is achieved by using the barrier Lyapunov function and a predictor-based method in combination with the neural network scheme. The limitations include the saturation of inputs and the forbidden zones due to thruster-thruster or thruster-hull interaction. An optimised allocation for lower fuel consumption, wear and tear of the thruster equipment and to ensure the resultant command in the respective direction of the azimuth thrusters is designed. The optimisation here is a non-convex problem and a locally convex reformulation of second order is implemented. The presence of unknown shielding effect due to nearby vessel in a side-by-side configuration and input time delay is also considered in the development of this thruster allocation law. In order to overcome these effects, a novel sea state detector is designed. The sea state detector can effectively monitor the variation of drift wave-induced force on the vessel and activate the neural network compensator in the controller when a large wave drift force is identified. Simulation studies are conducted to verify the efficiency of this dynamic position system and a demonstration of flotel in side-by-side configuration with a turret moored FPSO is presented for the non-collinear ocean environment.

The paper presents model tests with a floating hydrocarbon storage facility performed in SINTEF Ocean’s basin. The system may be considered a very large floating structure with a footprint of 300m × 310m. Hydrocarbons are stored into independent tanks, which are soft moored to a modular floating frame which is kept in station by dolphins. The system is composed of 14 tanks and 21 frame barge modules, therefore 35 connected floating bodies. The objective of the model tests was twofold. First, to verify the feasibility of the global system, namely in terms of tank and frame motions, connection loads, station keeping loads and relative wave elevations. Second, to acquire data for tuning and validation of numerical models, i.e. to investigate effects that cannot be reliably established by simulations alone. The model tests were performed in wave, current and wind conditions corresponding to 1 year and to 100 years return periods, in addition to a set a simplified wave conditions. The paper describes the model test setup, the experimental program, the procedures for analysis of test data, it presents representative results and discusses the main observations.

A compliant modular floating hydrocarbon storage facility (FHSF) has been proposed for ocean space utilization. The FHSF consists of many floating hydrocarbon storage tanks (FHST) and several surrounding barges. These modules are connected in proximity through soft mooring system which reduces the environmental loads significantly. However, there are concerns on the potential resonance in the narrow gaps and the strong hydrodynamic interactions. This paper focuses on the hydrodynamic properties of a simplified subsystem which qualitatively represents part of the behavior of the entire FHSF but with reduced complexity. This subsystem consists of two FHSTs and a barge frame and is suitable for evaluating numerical analysis tools. Experimental studies on the subsystem and the complete system were performed in the ocean basin in SINTEF Ocean. A series of random, wide-band and realistic random wave tests were carried out to generate benchmark data to verify numerical analysis tools. Empty, partially loaded and fully loaded conditions were tested. The free surface elevations in the narrow gap and in the internal tank were obtained. A frequency domain numerical model of the subsystem was established in WAMIT based on linear potential theory. Higher order boundary element method (HOBEM) has been utilized to improve the accuracy and convergence. The influence of internal liquid (sloshing) inside the tanks is also considered in the linear range. The mooring system is simplified as linear springs. Linearized damping is introduced. The response amplitude operators (RAOs) of 6 degrees of freedom (D.O.F) and free surface elevations from numerical and experimental studies under the head sea conditions are compared. Good agreement on the motion RAOs are found, but significant differences are found on the RAOs of the elevation of the free surface. Finally, the influence of significant gap resonances is used to generate insight into how these problems might be mitigated.

In this paper, we present two deep learning-based hybrid data-driven reduced-order models for prediction of unsteady fluid flows. These hybrid models rely on recurrent neural networks (RNNs) to evolve low-dimensional states of unsteady fluid flow. The first model projects the high-fidelity time series data from a finite element Navier–Stokes solver to a low-dimensional subspace via proper orthogonal decomposition (POD). The time-dependent coefficients in the POD subspace are propagated by the recurrent net (closed-loop encoder–decoder updates) and mapped to a high-dimensional state via the mean flow field and the POD basis vectors. This model is referred to as POD-RNN. The second model, referred to as the convolution recurrent autoencoder network (CRAN), employs convolutional neural networks (instead of POD) as layers of linear kernels with nonlinear activations, to extract low-dimensional features from flow field snapshots. The flattened features are advanced using a recurrent (closed-loop manner) net and up-sampled (transpose convoluted) gradually to high-dimensional snapshots. Two benchmark problems of the flow past a cylinder and the flow past side-by-side cylinders are selected as the unsteady flow problems to assess the efficacy of these models. For the problem of the flow past a single cylinder, the performance of both the models is satisfactory and the CRAN model is found to be overkill. However, the CRAN model completely outperforms the POD-RNN model for a more complicated problem of the flow past side-by-side cylinders involving the complex effects of vortex-to-vortex and gap flow interactions. Owing to the scalability of the CRAN model, we introduce an observer-corrector method for calculation of integrated pressure force coefficients on the fluid–solid boundary on a reference grid. This reference grid, typically a structured and uniform grid, is used to interpolate scattered high-dimensional field data as snapshot images. These input images are convenient in training the CRAN model, which motivates us to further explore the application of the CRAN-based models for prediction of fluid flows.

Large-scale cultivation of seaweed has become one of the most important aquaculture activities in Malaysia which may help increase farmers’ incomes as well as seaweed itself can be processed into many beneficial end products. The present location of seaweed farming selected by farmers is situated close proximity to the coastline which is between 100 and 200 m from the seashore. The unfavourable condition of sea during rough sea with high wave and high speed of current is always a problem to the farmers since this environmental condition destroys their seaweed planting lines. To avoid the above problem, especially in monsoon prone area, a thorough analysis needs to be done in order to prevent environmental load from destroying seaweed platform on its mooring line when subjected to greater stress. The main objective of this study is to perform a simulation study which will allow analysis of the best mooring system for multi-body floating seaweed farm, together with understanding of the reliability a...

A subsystem of a large compliant floating hydrocarbon storage facility (FHSF) is established as a simplified model to validate its hydrodynamic performance. The subsystem is composed of two floating hydrocarbon storage tanks (floating tanks) placed in a large moonpool of a rigid barge frame. The barge frame is expected to serve as a breakwater as well as a protection to the floating tanks. The wave-induced responses of the subsystem with the existence of moonpool/gaps are investigated through experimental and numerical studies. The experimental studies followed a Froude scaling ratio 1:45 were conducted in an ocean basin under various wave conditions corresponding to nearshore environments. Both free surface elevations in the moonpool/gaps and the hydrodynamic responses of the three floating bodies were obtained. Besides, a numerical model based on linear potential flow theory is developed, and a dipole lid is introduced to incorporate the viscous effect on the resonant free surface motions. Comparable experimental and numerical results show that the free surface elevations in the moonpool/gaps can be reasonably predicted. Next, four different configurations of the subsystem were investigated, and different hydrodynamic coupling behaviors between the fluid resonant motions, the motions of the barge frame, and the floating tanks are revealed. It is demonstrated that the motions of the floating bodies and the resonant fluid motions in the narrow gaps are the smallest in the oblique sea. The present study provides a useful reference for the design and hydrodynamic analysis of floating multi-body systems for nearshore applications.

The ASD tug is considered the modern habour tug due to its versatility in performing a wide variety of tugging/towing operations. With the advent of autonomous systems and desire to improve operations, there are benefits to automating the tug operations. Under different operating environments, there could be uncertainties in the model and the disturbance from the environment significant enough to affect the performance and control of the tug. In this paper, we consider the problem of tracking a desired trajectory for an autonomous ASD tug in the presence of uncertainties and unknown disturbances. The numerical modelling of an ASD tug based on a modified 4-DoF MMG model is first presented. The modified Maneuvering Modeling Group (MMG) model is then used to design a model based backstepping control. Thereafter, a adaptive neural network approximator is introduced which has the capability to account for uncertainties and unknown disturbance. The combination of approximation-based and backstepping design techniques allows us to handle time-varying model uncertainties. Stability analysis is carried out for the control design via Lyapunov analysis. Simulation are carried out for the tracking of maneuvering motion paths to demonstrate the performance of proposed approach.

A novel Floating Hydrocarbon Storage and Bunker Facility (FHSBF) is designed for use of nearshore area around Singapore. The system will be composed of floating hydrocarbon storage tanks (FHST) and supporting barges. The FHST will be freely floating vertically while restrained horizontally by fenders installed on the barges. Two concepts for the FHST are designed different bottom shapes, namely with and without bottom skirt. The function of bottom skirt is to serve as a heave plate and it is expected to influence the hydrodynamic properties including hydrodynamic damping and natural periods of a single tank. To investigate the bottom shape effect on hydrodynamic responses of single FHST, model tests were carried out in the wave basin at National University of Singapore for various wave conditions. Two tank models together with the model test setup are introduced and the decay test results of surge, pitch, heave and yaw motions for two different tanks are compared. Results show that with bottom skirt, heave natural period is significantly increased, and the skirt will introduce additional viscous damping that helps reduce both heave, pitch and surge motions.

In this paper, a general data-driven approach to construct a reduced-order model (ROM) for the coupled fluid-structure interaction (FSI) problem of a transversely vibrating bluff body in an incompressible flow is presented. The proposed data-driven approach relies on the Eigensystem Realization Algorithm (ERA) to design ROM models in a state-space format. The stability boundaries of the coupled FSI system are obtained by examining the eigenvalue trajectories of the ERA-based ROM. These stability boundaries provide us valuable quantitative insights into the lock-in phenomenon of the bluff-body vibration. We demonstrate the present ERA-based ROM technique for various configurations of bluff bodies such as an isolated single cylinder, the side-by-side and the tandem cylinder arrangements. A comparative study on the effect of different appendages to suppress the VIV of a cylinder is also presented using the ERA-based stability analysis. The validity of the proposed method for the FSI stability analysis on such variety of configurations has not been presented before and is the novel contribution of this paper. Overall, the proposed data-driven framework is found to be much more effective in terms of computational costs and the predicted lock-in regions are comparable to high-fidelity full-order simulations. This work has a potential for a profound impact on the design optimization and control of bluff body structures used in offshore industry.

For better utilization of ocean and coastal space, hydrocarbon products can be stored in the floating tanks, which can be enclosed by barge system. The barge system can be moored through pile foundations. The tanks are moored through marine fenders connected to barges. In the system, hydrodynamic and mechanical interaction problems are involved. Different scenarios including two barge, three barge and four barge systems are investigated. In addition, one tank plus four barge system are also studied. Hydrodynamic interactions between different bodies are firstly studied to investigate the significance of interaction. Different barge configurations are then considered in terms of mechanical interaction significance. Tank dynamic responses with and without hydrodynamic interaction are evaluated.

Experimental and numerical studies are carried out to examine the moonpool gap resonance for a drillship at both stationary position and forward speed conditions. The moonpool size and draft are also changed to study their effects for the gap resonance phenomenon. An OpenFOAM based CFD model is developed and the numerical results show good agreement with model tests. Both piston and sloshing modes gap resonances are clearly observed. The study shows that the resonance frequency and RAO of the wave elevation inside the moonpool are subject to the effects of moonpool length, drill ship draft and ship forward speed. The model test shows that moonpool elevation RAO generally significantly increases in head seas and noticeably decreases in following seas condition. It is interesting to notice that the wave flume sidewall significantly depresses the moonpool elevation RAO at a certain frequency regardless of moonpool length and draft. Further study shows that the presence of the flume sidewall results in a trapped mode that coincides with the moonpool piston mode resonance at zero speed. This depresses the peak of the moonpool resonance, which occurs at the same frequency.

A modular floating concept for oil storage is proposed. The concept, which is advantageous for space-constrained locations, is composed of several modular floating tanks and barges. The floating multi-body hydrodynamic interaction of barges are studied in this paper. In the analysis, a single barge and then two-barge system are firstly analyzed to investigate the hydrodynamic interaction effect. More bodies are involved in the subsequent investigation. This study provides physical principles for different basic scenarios, and practical insights into how the hydrodynamic interaction problem affect the multi-body floating system.

For deepwater developments involving floating platforms in harsh environmental conditions which lie beyond well-established industry practices, model tests are usually required. The paper describes the different types of model tests and their respective objectives, scopes, and requirements. Different types of testing facilities are also described. The objective is to give an overview of various aspects of model testing, so the reader will come away with a better understanding of how and why model tests are performed and how they can best be used in projects. The timing of tests and their interaction with the project schedule is also described. For example, model tests may be carried out early in the design for concept selection studies, or sometimes at the end of FEED for a specific concept application. Project tests are often performed at the beginning of detailed engineering to resolve any outstanding design issues or sometimes near the end of the project to confirm the effects of certain changes. These different test types have different methods, requirements, procedures and results. Model tests may address installations, operations or performance of deepwater systems such as mooring and dynamic riser interactions or may generate benchmarking data for CFD. Depending on the type of tests, certain key aspects must be carefully controlled. Required test data and the cost-benefit trade-offs of different test objectives are discussed. Results and observations are given for several model testing applications of deepwater developments in Asia Pacific region and elsewhere. Examples of different types of tests are used to draw conclusions about the role of tests. Looking ahead to the future, several deepwater testing basins are under development in different parts of the world. Limitations such as scale effects and basin boundary truncations are discussed. Ongoing research into novel model testing methods currently being carried out and their potential to improve the accuracy and reliability of full-scale predictions are pointed out. CFD or so-called numerical wave tank is a relatively mature tool which is gaining use in offshore projects and promises to overcome some of these limitations. Fortunately, improvements in available computing power and software are continually reducing the required computation time and cost. Both physical tests and complementary CFD simulations are required to obtain a complete picture of the full-scale performance of deepwater platforms. The need for full-scale measurements and design feedback is often over-looked but benefits future projects by closing the design loop to reduce future conservatism.

A modular floating concept for oil storage is proposed. The concept is composed of several modular floating tanks and barges. The floating tanks are self-stable and designed for storage of oil and hydrocarbon material. The barges provide platforms for equipment and workers quarters, and enclose the floating tanks through mooring fender systems. The whole concept is then moored by mooring dolphins. This concept is designed for finite water depth application, however, potential application for deep water condition is also investigated. In this study, only single floating tank is investigated with both empty and full tank conditions considered. Different mooring fender configuration designs are the focus. It is assumed that the barges are fixed in the study of single tank dynamic properties, and multi-body hydrodynamic is neglected. Different regular wave conditions are used for the study of the dynamic performance of the tank under various fender mooring system parameters. Irregular wave together with wind and current for 100 year return period are also applied for the study of the tank dynamic performance. At last, sensitivity study of the viscous effects is performed.

This study concerns a new concept of floating oil storage facility, to be deployed in coastal waters, in which separate oil storage tanks float in an array, separated by a mooring fender system. In this paper, hydrodynamic properties of a single module are investigated numerically. The effects of different mooring fender parameters including fender stiffness and fender position on the coupled motions are studied. Design criteria and a design approach for the marine fender selection are proposed. Next, time-domain simulations under random waves are performed. Finite water depth effects are taken into consideration. Then a brief parametric study on sloshing phenomenon in fender-supported tanks is conducted. Results show that a carefully designed marine fender will help reduce the roll and pitch motions of the storage tank, and thus function as a stabilizer. This analysis is the basis of a global hydrodynamic response analysis for multiple tanks, including the effects of multibody hydrodynamic interactions between tanks in the future.

In this paper, a floating bridge concept is proposed. This bridge concept comprises a two oppositely curves in plan, which enables the cancellation of the axial forces at the bridge as one arch will be under compression while the other arch is in tension due to environmental forces acting in one direction. The road deck is carried by truss structures that are kept above the water by several elliptical cylindrical pontoons. To reduce drag load, the cross sectional area facing the current is reduced as much as possible, while the buoyancy is kept the same based on the initial weight estimation. Initial design consideration and methodology of a double curved floating bridge is presented, and a numerical model is established for analyzing this concept. Hydrodynamic and structural dynamic aspects are included in the numerical model. Parametric study of the bridge structural rigidity is performed to investigate the effect to the responses. White noise, regular and irregular wave simulations are carried out to investigate the dynamic responses of the floating bridge under different conditions. Copyright © 2017 by ASME

When floating structure with internal fluid compartment is close to other structures, the multibody interaction problem needs to be addressed in addition to the internal fluid influence. Furthermore, shallow water effects become important, especially when the gap between the floating structure and the sea floor is small. These issues are encountered when designing a novel floating oil storage facilities in nearshore area. To investigate these issues, floating models under 1:50 scale are built to perform model tests. The test set-up uses a set of flexible constraints working as fenders placed on frames to restrain the motions of the models in the horizontal plane. Various tests in waves are carried out to measure motion responses of single model in waves with different filling levels and stiffness of “fenders”. The reaction forces on the “fenders” are also measured. Several regular wave conditions are selected to perform tests on double model system to investigate multibody interactions under the influence of internal fluid and effects of waves between the tanks. The drag forces for both single model and double models are measured by performing model tests under constant current from different directions, to check the shielding effects. The tests are performed in shallow-water wave basin, and the constant currents tests are performed by towing the models in a flume tank. Both facilities are located at National University of Singapore (NUS). This paper presents the detailed setting of the model tests. The single model’s RAOs with 20% filling level of internal fluid are given to demonstrate the influence of internal fluid on the motions. The performances of a single tank, including six DOF motions are shown. The results will be used for validation of numerical analysis results in the near future.

Multiple-channel inlet design is commonly used in artificial ocean basins for improving the uniformity of current flows in horizontal direction, as well as for the generation of uniform or sheared currents in vertical direction. However, boundary layers developing along the walls between the channels result in lower velocities after the fluid leaves the ducts and enters the basin, which is undesired. To reduce the effects of the boundary layers and increase the flow uniformity at the basin inlet, the present work aims to improve the inlet design. Experimental study are performed in a wind tunnel at wind velocity of 20 m/s. To simulate the walls, a Perspex plate with thickness of 20mm is fixed at the center of wind tunnel test section. A triangle end tip with tip angle of 7° is attached to the training edge of the plate. Four configurations of honeycombs are applied to study the effects of honeycombs on the flow uniformity. Among the four configurations, honeycomb with thickness t = 50 mm and cell size dg = 5 mm is used as bench mark case. In the second and third configurations, the thickness of vertical central 80mm region is reduced to be 25 mm. In the fourth configuration, the central region is then replaced by honeycomb with thickness of 50mm, but with cell size of 10mm. The experimental results show the possibility to eliminate the lower velocity region by using shaped honeycomb or honeycomb with various cell sizes. With the experimental results as validation, the honeycomb configuration is then optimized using numerical simulation with OpenFoam.

A general formulation applicable to multi-segment mooring systems is derived from first principles using the concept of lumped mass model and presented in this paper. The problem of seabed contact is addressed using an elastic-dissipative model of seabed. The contribution of seabed force along with the tension force is considered in creating global tangent stiffness matrix of the system. An implicit generalized-α method is used for time integration and it is modified for the current problem by developing an incremental iterative version of the method with corresponding predictor and corrector terms. An in house code named LM3D is developed based on the derived formulation. The code is verified with orcaflex results and also validated with experimental results. The main purpose of this paper is to showcase the reduction in tension observed in a simple branched mooring system when compared to a single line. Three configurations of branched system were considered and compared against a single line arrangement with same material properties. The dynamic analysis of this branched system was carried out with the developed in-house code.Copyright © 2017 by ASME

Liquid inside a floating structure influences both hydrostatic and hydrodynamic response of the floating structure. Some examples of floating structure with internal liquid are: vessels with roll damping tanks, floating hydrocarbon storage facility, LNG tankers and etc. A floating oil storage tank is considered in this study, which has a cuboid-shaped external wall, cylinder-shaped internal wall and simple structure configurations for its roof and bottom. Influence of internal liquid on the hydrostatic response of floating structures is well established and must be taken into consideration. The internal liquid reduces the stability of floating structures. The focus of this study is the influence of internal liquid on the hydrodynamic response of floating tank. Frequency domain analysis is performed with WAMIT, for partially filled tank with both solid mass and liquid mass. By comparing the different cases, the force induced by the internal liquid on the floating tank is illustrated. Based on the WAMIT calculated radiation damping force for the external flow, Impulse Response Function (IRF) connecting frequency domain and time domain solution is constructed and the force and moment induced by internal liquid is considered as an excitation force. By assuming linear tank motion, the internal liquid induced force is related to the incoming wave by a set of force transfer functions and it is moved to the right hand side of the tank motion equation. In practice, this set of force transfer function due to internal liquid has to be combined with the wave excitation force transfer function and it is the total force transfer function (internal liquid plus wave excitation) to be imported to SIMO. SIMO time domain simulation is performed in regular waves. Motion transfer functions from WAMIT frequency domain and SIMO time domain calculations are compared and reasonable agreement is achieved.

Integration of an air-spring device into an offshore platform can be an effective solution to modify the system properties, such as the restoring stiffness. In this study, the dynamic stiffness due to the linear vibration of the air and fluid entrapped inside an air-spring type vibration absorber is investigated. The device is modelled as a hollow column with a closed top and floats vertically in a water of constant depth. At the initial time, the free surfaces inside and outside the device are at the same level and there is an amount of air entrapped above the inner water surface. Within the device, the variation to the air pressure is assumed to occur adiabatically and the free-surface boundary condition is derived based on the ideal gas state equation. Energy dissipation is introduced into the region within the device by adding a resistance force on the inner free surface. The wave radiation due to the heave motion of the device is then concerned and an analytical model is developed to evaluate the associated hydrodynamic coefficients and the induced dynamic stiffness by using the method of separation of variables in conjunction with the matching technology. An alternative solution of the dynamic stiffness has also been developed based on a decomposition of the total velocity potential into two parts which depend purely on the oscillation of the device and the dynamic air pressure applied on the inner free surface, respectively. Based on the developed model, detailed numerical analysis is performed. The relative phases between the body motion and the dynamic stiffness are discussed for different wave conditions and geometric parameters.

Velocity deficits resulted from boundary layers developing along walls in multiple-channel inlet of artificial ocean basins disturb the flow uniformity in the basin and are undesirable. Experimental tests on a uniform and a simple shaped honeycomb at Reynolds number R e h = 3.8 × 105 in a wind tunnel show the possibility of eliminating the velocity deficit by using a shape-optimized honeycomb with gradual change of thickness. Inspired by the results, the present study develops a systematic and efficient method to design the shaped honeycomb to improve the flow uniformity. The method comprises of optimization process based multi-fidelity numerical simulations and precise experiments. Firstly, with the test results as validation, a numerical algorithm based on the “porousSimpleFoam” solver of OpenFOAM® is developed to optimize the shape of the honeycomb using low-fidelity 2D simulations. High-fidelity 3D simulations are also performed to verify the optimized solution. The optimized honeycomb is then fabricated using 3D printing technology and validated experimentally in the wind tunnel. By applying the optimized honeycomb, the velocity deficit is significantly mitigated. The coefficient of variation, i.e. ratio of the standard variation to the mean velocity, is reduced from 6.61% to 1.55% at R e h = 3.8 × 105. The optimized honeycomb performs well at lower Reynolds numbers too. At R e h = 1.9 × 105 and 9.5 × 104, the coefficients of variation decrease from 7.13% and 8.25% without the honeycomb to 1.87% and 2.11% with the honeycomb, respectively.