Showing papers in "Journal of Marine Science and Application in 2018"

An Overview of Submersible Research and Development in China

Abstract: Given the recent success in the development of several submersibles in China, people’s interest in the history of submersible development is increasing. This paper presents the history of submersible development in China, which can be briefly divided into three periods. The first one is the early period of hardship (1971–2000). Many prototype submersibles of HOVs, ROVs, and AUVs were developed at this time, but the main achievement was the establishment of special research organizations and the training of research and development personnel. The second period can be regarded as the quick development period (2001–2015). All currently used submersibles were developed during this period. The most remarkable achievement was the successful development of 7000 m-deep manned submersible “Jiaolong.” The third period aims to develop 11 000 m submersibles for challenging the full ocean depth (2016–2020). In this period, two unmanned submersibles and two manned submersibles will be the significant indicators of achievement. If this 5-year plan can be successfully completed, China can play a significant role in the investigation of the deepest part of the oceans, namely, the hadal trenches (6500–11 000 m).

CFD Investigations of Ship Maneuvering in Waves Using naoe-FOAM-SJTU Solver

Abstract: Ship maneuvering in waves includes the performance of ship resistance, seakeeping, propulsion, and maneuverability. It is a complex hydrodynamic problem with the interaction of many factors. With the purpose of directly predicting the behavior of ship maneuvering in waves, a CFD solver named naoe-FOAM-SJTU is developed by the Computational Marine Hydrodynamics Lab (CMHL) in Shanghai Jiao Tong University. The solver is based on open source platform OpenFOAM and has introduced dynamic overset grid technology to handle complex ship hull-propeller-rudder motion system. Maneuvering control module based on feedback control mechanism is also developed to accurately simulate corresponding motion behavior of free running ship maneuver. Inlet boundary wavemaker and relaxation zone technique is used to generate desired waves. Based on the developed modules, unsteady Reynolds-averaged Navier-Stokes (RANS) computations are carried out for several validation cases of free running ship maneuver in waves including zigzag, turning circle, and course keeping maneuvers. The simulation results are compared with available benchmark data. Ship motions, trajectories, and other maneuvering parameters are consistent with available experimental data, which indicate that the present solver can be suitable and reliable in predicting the performance of ship maneuvering in waves. Flow visualizations, such as free surface elevation, wake flow, vortical structures, are presented to explain the hydrodynamic performance of ship maneuvering in waves. Large flow separation can be observed around propellers and rudders. It is concluded that RANS approach is not accurate enough for predicting ship maneuvering in waves with large flow separations and detached eddy simulation (DES) or large eddy simulation (LES) computations are required to improve the prediction accuracy.

Efficient AUV Path Planning in Time-Variant Underwater Environment Using Differential Evolution Algorithm

Abstract: Robust and efficient AUV path planning is a key element for persistence AUV maneuvering in variable underwater environments. To develop such a path planning system, in this study, differential evolution (DE) algorithm is employed. The performance of the DE-based planner in generating time-efficient paths to direct the AUV from its initial conditions to the target of interest is investigated within a complexed 3D underwater environment incorporated with turbulent current vector fields, coastal area, islands, and static/dynamic obstacles. The results of simulations indicate the inherent efficiency of the DE-based path planner as it is capable of extracting feasible areas of a real map to determine the allowed spaces for the vehicle deployment while coping undesired current disturbances, exploiting desirable currents, and avoiding collision boundaries in directing the vehicle to its destination. The results are implementable for a realistic scenario and on-board real AUV as the DE planner satisfies all vehicular and environmental constraints while minimizing the travel time/distance, in a computationally efficient manner.

Wave and Current Effects on Floating Fish Farms

Abstract: The paper is partly a review on hydrodynamic and structural aspects of fish farms. In addition, new numerical results are presented on the stochastic behavior of bending stresses in the floater of a realistic net cage in extreme wave conditions. The behavior of traditional-type fish farms with net cages and closed fish farms in waves and currents is discussed. Hydroelasticity can play a significant role for net cages and closed membrane-type fish farms. The many meshes in a net cage make CFD and complete structural modeling impracticable. As an example, a hydrodynamic screen model and structural truss elements are instead used to represent the hydrodynamic loading and the structural deformation of the net. In addition, the wake inside the net due to current plays an important role. The described simplified numerical method has been validated by comparing with model tests of mooring loads on a single net cage with two circular elastic floaters and bottom weight ring in waves and currents. It is discussed which parts of the complete system play the most important roles in accurately determining the mooring loads. Many realizations of a sea state are needed to obtain reliable estimates of extreme values in a stochastic sea. In reality, many net cages operate in close vicinity, which raises questions about spatial variations of the current and wave environment as well as hydrodynamic interaction between the net cages. Live fish touching the netting can have a non-negligible influence on the mooring loads. It is demonstrated by numerical calculations in waves and currents that a well boat at a net cage can have a significant influence on the mooring loads and the bending stresses in the floater. The latter results provide a rational way to obtain operational limits for a well boat at a fish farm. Sloshing has to be accounted for in describing the behavior of a closed fish farm when important wave frequencies are in the vicinity of natural sloshing frequencies. The structural flexibility has to be considered in determining the natural sloshing frequencies for a membrane-type closed fish farm. Free-surface non-linearities can matter for sloshing and can, for instance, result in swirling in a certain frequency domain for a closed cage with a vertical symmetry axis.

Numerical Evaluation of Rudder Performance Behind a Propeller in Bollard Pull Condition

Abstract: Correct evaluation of rudder performance is a key issue in assessing ship maneuverability. This paper presents a simplified approach based on a viscous flow solver to address propeller and rudder interactions. Viscous flow solvers have been applied to this type of problems, but the large computational requests limit (or even prevent) their application at a preliminary ship design stage. Based on this idea, a simplified approach to include the propeller effect in front of the rudder is considered to speed up the solution. Based on the concept of body forces, this approach enables sufficiently fast computation for a preliminary ship design stage, thereby maintaining its reliability. To define the limitations of the proposed procedure, an extensive analysis of the simplified method is performed and the results are compared with experimental data presented in the literature. Initially, the reported results show the capability of the body-force approach to represent the inflow field to the rudder without the full description of the propeller, also with regard to the complex bollard pull condition. Consequently, the rudder forces are satisfactorily predicted at least with regard to the lift force. However, the drag force evaluation is more problematic and causes higher discrepancies. Nevertheless, these discrepancies may be accepted due to their lower influence on the overall ship maneuverability performance.

Determining the Scour Dimensions Around Submerged Vanes in a 180° Bend with the Gene Expression Programming Technique

Abstract: Submerged vanes are installed on rivers and channel beds to protect the outer bank bends from scouring. Also, local scouring occurs around the submerged vanes over time, and identifying the effective factors on the scouring phenomena around these submerged vanes is one of the important issues in river engineering. The most important aim of this study is investigation of scour pattern around submerged vanes located in 180° bend experimentally and numerically. Firstly, the effects of various parameters such as the Froude number (Fr), angle of submerged vanes to the flow (α), angle of submerged vane location in the bend (θ), distance between submerged vanes (d), height (H), and length (L) of the vanes on the dimensionless volume of the scour hole were experimentally studied. The submerged vanes were installed on a 180° bend whose central radius and channel width were 2.8 and 0.6 m, respectively. By reducing the Froude number, the scour hole volume decreased. For all Froude numbers, the biggest scour hole formed at θ = 15°. In all models, by increasing the Froude number, the scour hole volume significantly increases. In addition, by increasing the submerged vanes’ length and height, the scour hole dimensions also grow. Secondly, using gene expression programming (GEP), a relationship for determining the scour hole volume around the submerged vanes was provided. For this model, the determination coefficients (R2) for the training and test modes were computed as 0.91 and 0.9, respectively. In addition, this study performed partial derivative sensitivity analysis (PDSA). According to the results, the PDSA was calculated as positive for all input variables.

Cavitating Propeller Performance in Inclined Shaft Conditions with OpenFOAM: PPTC 2015 Test Case

Abstract: In this paper, we present our analysis of the non-cavitating and cavitating unsteady performances of the Potsdam Propeller Test Case (PPTC) in oblique flow. For our calculations, we used the Reynolds-averaged Navier-Stokes equation (RANSE) solver from the open-source OpenFOAM libraries. We selected the homogeneous mixture approach to solve for multiphase flow with phase change, using the volume of fluid (VoF) approach to solve the multiphase flow and modeling the mass transfer between vapor and water with the Schnerr-Sauer model. Comparing the model results with the experimental measurements collected during the Second Workshop on Cavitation and Propeller Performance – SMP’15 enabled our assessment of the reliability of the open-source calculations. Comparisons with the numerical data collected during the workshop enabled further analysis of the reliability of different flow solvers from which we produced an overview of recommended guidelines (mesh arrangements and solver setups) for accurate numerical prediction even in off-design conditions. Lastly, we propose a number of calculations using the boundary element method developed at the University of Genoa for assessing the reliability of this dated but still widely adopted approach for design and optimization in the preliminary stages of very demanding test cases.

Numerical Investigation of Contra Rotating Vertical-Axis Tidal-Current Turbine

Abstract: In this study, the performance of a contra rotating vertical-axis tidal-current turbine was investigated. The incompressible unsteady Reynolds-averaged Navier-Stokes (U-RANS) equations were solved via two-dimensional (2D) numerical simulation using ANSYS Fluent computational fluid dynamics (CFD) code. An algorithm known as SIMPLE from the CFD code was used to calculate the pressure-velocity coupling and second-order finite-volume discretization for all the transport equations. The base turbine model was validated using the available experimental data. Three given scenarios for the contra rotating turbine were modeled. The contra rotating turbine performs better in a low tip speed ratio (TSR) than in a high TSR operation. In a high TSR operation, the contra rotating turbine inefficiently operates, surviving to rotate in the chaotic flow distribution. Thus, it is recommended to use contra rotating turbine as a part of new design to increase the performance of a vertical-axis tidal-current turbine with a lower TSR.

CFD Analysis of Ship-to-Ship Hydrodynamic Interaction

Abstract: A numerical study of ship-to-ship interaction forces is performed using a commercial CFD code, and the results are compared with experimental data and with the results of a panel method analysis. Two ship models have been used in the interaction forces analysis: a tug and a tanker, advancing parallel to each other with different lateral distances and two different values of the fluid depth. Computations are carried out with four different flow models: inviscid and viscous flow with the free surface modeled as a rigid wall and inviscid and viscous flow with the deformable free surface. A fair agreement was obtained with available experimental data and results obtained by panel method. The influence of viscosity in the computations is found to be comparatively weak, while the wavemaking effects may be important, at small magnitude of the horizontal clearance.

Effect of a Submerged Porous Plate on the Hydroelastic Response of a Very Large Floating Structure

Abstract: Scattering of oblique flexural-gravity waves by a submerged porous plate in a finite water depth is investigated under the assumptions of linearized surface waves and small-amplitude structural response. The study is carried out using eigenfunction expansions and the corresponding orthogonal mode-coupling relations associated with flexural-gravity waves in uniform water depth. The characteristics of the roots of the complex dispersion relation are examined using the principle of counting argument and contour plot. Characteristics of the flexural-gravity waves are studied by assuming both the floating elastic plate and the submerged porous plate are infinitely extended in horizontal directions. The effectiveness of the submerged porous structure on the reflection, transmission, and dissipation coefficients is analyzed for various wave and structural parameters.

Fast Ferry Smoothing Motion via Intelligent PD Controller

Abstract: A novel type of control law was adopted to reduce the vertical acceleration of a fast ferry as well as the motion sickness incidence suffered by the passengers onboard by means of a submerged T-foil. Considering the system changing characteristics under high disturbances, a model-free approach was adopted. In addition, an upgraded proportional-derivative (PD) controller with correction terms resulting from a fast-online estimation of the system dynamics was designed. The overall controller, known as intelligent PD (i-PD) controller, was tested, and the obtained results were compared with those of a classic PD controller. The controllers were also tested in a changing environment and at different operating velocities. The results confirmed the effectiveness of the i-PD controller to smooth the motions with low computational cost control schemes. Furthermore, thanks to ability of the i-PD controller to continually update the estimated dynamics of the system, it showed a better reduction in both vertical motions and the seasickness level of the passengers with the needed robustness under external disturbances and system changing parameters.

Analysis of Efficiency of the Ship Propulsion System with Thermochemical Recuperation of Waste Heat

Abstract: One of the basic ways to reduce polluting emissions of ship power plants is application of innovative devices for on-board energy generation by means of secondary energy resources. The combined gas turbine and diesel engine plant with thermochemical recuperation of the heat of secondary energy resources has been considered. It is suggested to conduct the study with the help of mathematical modeling methods. The model takes into account basic physical correlations, material and thermal balances, phase equilibrium, and heat and mass transfer processes. The paper provides the results of mathematical modeling of the processes in a gas turbine and diesel engine power plant with thermochemical recuperation of the gas turbine exhaust gas heat by converting a hydrocarbon fuel. In such a plant, it is possible to reduce the specific fuel consumption of the diesel engine by 20%. The waste heat potential in a gas turbine can provide efficient hydrocarbon fuel conversion at the ratio of powers of the diesel and gas turbine engines being up to 6. When the diesel engine and gas turbine operate simultaneously with the use of the LNG vapor conversion products, the efficiency coefficient of the plant increases by 4%–5%.

Comparison of Microbubble and Air Layer Injection with Porous Media for Drag Reduction on a Self-propelled Barge Ship Model

Abstract: Ship resistance issues are related to fuel economy, speed, and cost efficiency. Air lubrication is a promising technique for lowering hull frictional resistance as it is supposed to modify the energy in the turbulent boundary layer and thereby reduce hull friction. In this paper, the objective is to identify the optimum type of air lubrication using microbubble drag reduction (MBDR) and air layer drag reduction (ALDR) techniques to reduce the resistance of a 56-m Indonesian self-propelled barge (SPB). A model with the following dimensions was constructed: length L = 2000 mm, breadth B = 521.60 mm, and draft T = 52.50 mm. The ship model was towed using standard towing tank experimental parameters. The speed was varied over the Froude number range 0.11–0.31. The air layer flow rate was varied at 80, 85, and 90 standard liters per minute (SLPM) and the microbubble injection coefficient over the range 0.20–0.60. The results show that the ship model using the air layer had the highest drag reduction up to a maximum of 90%. Based on the characteristics of the SPB, which operates at low speed, the optimum air lubrication type to reduce resistance in this instance is ALDR.

A Simplified Model for the Effect of Weld-Induced Residual Stresses on the Axial Ultimate Strength of Stiffened Plates

Abstract: The present work investigates the compressive axial ultimate strength of fillet-welded steel-plated ship structures subjected to uniaxial compression, in which the residual stresses in the welded plates are calculated by a thermo-elasto-plastic finite element analysis that is used to fit an idealized model of residual stress distribution. The numerical results of ultimate strength based on the simplified model of residual stress show good agreement with those of various methods including the International Association of Classification Societies (IACS) Common Structural Rules (CSR), leading to the conclusion that the simplified model can be effectively used to represent the distribution of residual stresses in steel-plated structures in a wide range of engineering applications. It is concluded that the widths of the tension zones in the welded plates have a quasi-linear behavior with respect to the plate slenderness. The effect of residual stress on the axial strength of the stiffened plate is analyzed and discussed.

Three-dimensional Analysis of Prestressed Concrete Offshore Wind Turbine Structure Under Environmental and 5-MW Turbine Loads

Abstract: A concrete gravity base structure may not be suitable for offshore weak soil because of its heavy weight Therefore, a conceptual model for a concrete offshore wind turbine structure suitable for weak soils is proposed The proposed model is composed of a prestressed concrete (PSC) supported by a pile foundation For a three-dimensional analysis of the large concrete structure, wave pressures based on the diffraction wave theory are developed using a three-dimensional solid finite element method Static and dynamic analyses were performed to achieve the conceptual model of a PSC structure subjected to ocean environmental loads and a 5-MW turbine load on southwest coast in Korea From the analysis, the maximum displacement and stresses of the proposed model did not exceed the allowable values from design standard, and the first mode of natural frequency of the structure was in a safe range to avoid resonance The proposed model has enough structural stability to withstand external loads, and it is expected to be used in locations suitable for concrete gravity structures

Optimal Risk-Based Maintenance Planning of Ship Hull Structure

Abstract: Various structures such as marine structures age over time. In order to always maintain safety conditions, maintenance processes including inspection and repair should be implemented on them. Corrosion and fatigue cracks are two main factors that reduce the ultimate strength of the ship’s hull girder over time and thus increase the probability and risk of failure. At the time of inspection, the structural conditions must be checked so that, if necessary, the required repairs can be done on it. The main objective of this paper is to provide optimized maintenance plans of the ship structure based on probabilistic concepts with regard to corrosion and fatigue cracks. Maintenance activities increase the operational costs of ships; therefore, it is advisable to inspect and repair in the optimal times. Optimal maintenance planning of the ship structure can be conducted by formulating and solving a multi-objective optimization problem. The use of risk as a structural performance indicator has become more common in recent years. The objective functions of the optimization problem include minimizing the structure’s lifecycle maintenance costs, including inspection and repair costs, and also minimizing the maximum risk of structural failure during the ship’s life. In the following, to achieve better responses, reliability index has been added to the problem as the third objective function. The multi-objective optimization problem is solved using genetic algorithms. The proposed risk-based approach is applied to the hull structure of a tanker ship.

Study on Operation of Arctic Offshore Complex by Means of Multicomponent Process-Based Simulation

Abstract: We developed a detailed simulation model of the Arctic marine transport system (MTS) for oil platform Prirazlomnaya. The model has a multidisciplinary nature and involves: sub-models of various transport and technological processes; stochastic weather generator to obtain time series of 15 environmental parameters; and contextual planning algorithm to build voyage plan considering several types of ships and cargoes. We used a significant amount of real operational data to identify model parameters and to prove its statistical reliability. Our main scientific task is to investigate the interaction of various processes of a different nature, while the practical aim is to find a set of measures to increase the efficiency of MTS. The results of the study reveal many examples of the mutual interaction of various processes that need to be considered at the design stage to avoid technical mistakes. The study formed a basis for making managerial decisions at the top level of Gazprom Neft Shelf Company.

Real-time Monitoring of Subsea Gas Pipelines, Offshore Platforms, and Ship Inspection Scores Using an Automatic Identification System

Abstract: The aim of this research is to develop an algorithm and application that can perform real-time monitoring of the safety operation of offshore platforms and subsea gas pipelines as well as determine the need for ship inspection using data obtained from automatic identification system (AIS). The research also focuses on the integration of shipping database, AIS data, and others to develop a prototype for designing a real-time monitoring system of offshore platforms and pipelines. A simple concept is used in the development of this prototype, which is achieved by using an overlaying map that outlines the coordinates of the offshore platform and subsea gas pipeline with the ship’s coordinates (longitude/latitude) as detected by AIS. Using such information, we can then build an early warning system (EWS) relayed through short message service (SMS), email, or other means when the ship enters the restricted and exclusion zone of platforms and pipelines. The ship inspection system is developed by combining several attributes. Then, decision analysis software is employed to prioritize the vessel’s four attributes, including ship age, ship type, classification, and flag state. Results show that the EWS can increase the safety level of offshore platforms and pipelines, as well as the efficient use of patrol boats in monitoring the safety of the facilities. Meanwhile, ship inspection enables the port to prioritize the ship to be inspected in accordance with the priority ranking inspection score.

Radiation and Exciting Forces of Axisymmetric Structures with a Moonpool in Waves

Abstract: A highly efficient “hybrid integral-equation method” for computing hydrodynamic added-mass, wave-damping, and wave-exciting force of general body geometries with a vertical axis of symmetry is presented. The hybrid method utilizes a numerical inner domain and a semi-infinite analytical outer domain separated by a vertical cylindrical matching boundary. Eigenfunction representation of velocity potential is used in the outer domain; the three-dimensional potential in the inner domain is solved using a “two-dimensional” boundary element method with ring sources and ring dipoles to exploit the body symmetry for efficiency. With proper solution matching at the common boundary, both radiation and diffraction potentials can be solved efficiently while satisfying the far-field radiation condition exactly. This method is applied to compute the hydrodynamic properties of two different body geometries: a vertical-walled moonpool with a bottom plate that restricts the opening and a spar-like structure with a diverging bottom opening inspired by designs of floating Oscillating Water Columns. The effects of the size of the bottom opening on the hydrodynamic properties of the body are investigated for both geometries. The heave motion of the floater as well as the motion of the internal free surface under incident wave excitation are computed and studied for the spar-like structure.

Experimental Investigation of the Characteristics of an Artificial Cavity During the Water-Exit of a Slender Body

Abstract: Experimental studies are carried out with slender bodies vertically exiting out of the water using a high-speed camera. The mechanisms for the formation, development, and collapse of the cavity around the slender body are explored. The dynamic characteristics of the shoulder cavity and the trail cavity during the water-exit of low-speed bodies are analyzed for various water depths and initial velocities. The results show that the initial velocity has a great influence on the formation, development, and collapse of the cavity. The length and the thickness of the shoulder cavity vary non-linearly with the depth.

Comparison of Scour and Flow Characteristics Around Circular and Oblong Bridge Piers in Seepage Affected Alluvial Channels

Abstract: The present study examines scour geometry and turbulent flow characteristics around circular and oblong piers in alluvial channel with downward seepage. Experiments were conducted in plane sand bed of non-uniform sand under no seepage, 10% seepage and 15% seepage conditions. Scour depth at oblong pier is significantly lesser than the scour depth at circular one. However, the scour depth at both piers reduces with downward seepage. The measurements show that the velocity and Reynolds stresses are negative near the bed at upstream of piers where the strong reversal occurs. At downstream of oblong pier near the free surface, velocity and Reynolds stresses are less positive; whereas, they are negative at downstream of circular pier. The streamline shape of oblong pier leads to reduce the strength of wake vortices and consequently reversal flow at downstream of pier. With application of downward seepage turbulent kinetic energy is decreasing. The results show that the wake vortices at oblong pier are weaker than the wake vortices at circular pier. The strength of wake vortices diminishes with downward seepage. The Strouhal number is lesser for oblong pier and decreases with downward seepage for both oblong and circular piers.

Viscous-Flow-Based Analysis of Wave Near-Trapped in a Four-Cylinder Structure

Abstract: The hydrodynamic analysis of multi-floating bodies is important and widely used in marine engineering. In this study, we systematically simulated the wave diffraction problem of a fixed vertical four-cylinder structure in regular waves in the time domain in a viscous numerical wave tank. The hydrodynamic interaction of waves with a bottom-mounted structure consisting of four vertical cylinders arranged at the corners of a square shows a complicated interference phenomenon. In this paper, we illustrate and analyze the run-up around the structure and the corresponding wave forces. To investigate the viscous effect on the near-trapping phenomenon, we pay particular attention to investigating the waves near-trapped inside the four-cylinder structure, and make a comparative study of the viscous- and inviscid-flow solutions with the experimental measurements. The results show that the maximum wave elevation occurs on the inner side of the leeside cylinder, and that the wave elevations on the outer side of the cylinders are lower than those on the inner side. We can conclude that viscosity has an obvious damping effect on wave elevations inside the structure. The cylinders show a tendency to drift apart from each other when the near-trapping phenomenon occurs.

Sloshing-Coupled Ship Motion Algorithm for Estimation of Slosh-Induced Pressures: Keynote Contribution for the International Workshop on Wave Loads and Motions of Ships and Offshore Structures, Harbin, China, 5-7 November, 2017

Abstract: The effect of coupling between sloshing and ship motions in the evaluation of slosh-induced interior pressures is studied. The coupling between sloshing loads and ship motions is modelled through a hybrid algorithm which combines a potential flow solution based on transient Green function for the external ship hydrodynamics with a viscous flow solution based on a multiphase interface capturing volume of fluid (VOF) technique for the interior sloshing motion. The coupled algorithm accounts for full nonlinear slosh forces while the external forces on the hull are determined through a blended scheme of linear radiation-diffraction with nonlinear Froude-Krylov and restoring forces. Consideration of this level of nonlinearities in ship motions is found to have non-negligible effects on the slosh-coupled responses and slosh-induced loads. A scheme is devised to evaluate the statistical measure of the pressures through long-duration simulation studies in extreme irregular waves. It is found that coupling significantly influences the tank interior pressures, and the differences in the pressures between coupled and uncoupled cases can be as much as 100% or more. To determine the RAO over the frequency range needed for the simulation studies in irregular waves, two alternative schemes are proposed, both of which require far less computational time compared to the conventional method of finding RAO at each frequency, and the merits of these are discussed.

Spectral Wave Modeling in Very Shallow Water at Southern Coast of Caspian Sea

Abstract: This study evaluates the capability of the Simulating WAves Nearshore (SWAN) wave model (version 41.01) in predicting significant wave height and spectral peak energy content for swell waves in very shallow water of surf zone during depth-induced wave breaking and dissipation. The model results were compared with field measurements at five nearshore stations. The results demonstrated that some breaker index formulations were successful for significant wave height prediction in surf zones. However, an incorrect shape of the energy spectrum and overestimated near spectral peak energy content at shallow water stations were obtained using all of the embedded depth-induced wave breaking formulations in SWAN. The dependent breaker index on relative depth (Kpd) formulation, which was successful in predicting near spectral peak energy content, resulted in an average error of 30%. Finally, this formulation was modified to enhance the model performance in reproducing the spectral peak energy content.

Wave-Induced Vertical Motions and Bending Moments in Damaged Ships

Abstract: The wave-induced vertical ship motions and bending moments of a double hull-oil tanker in realistic flooding conditions are studied. The scenarios investigated are represented by water ingress into the starboard ballast tanks for collision damage cases and both starboard and portside ballast tanks for grounding situations. Seakeeping computations are performed for eight damage scenarios and for the intact condition, each corresponding to different changes in displacement, trim, and heel. For each of the damage conditions, transfer functions of vertical motions and loads are calculated using a potential linear 3D panel hydrodynamic code in the frequency domain that includes effect of the motion of the water in flooded tanks. A MATLAB code is developed to facilitate automated hydrodynamic simulation of many damage scenarios. Verification of seakeeping results is performed by comparing transfer functions with results of the previous study. Wave-induced vertical responses of damaged ship are then compared to those of intact ship using two spectral-based methods originating from uncertainty analysis of wave loads, which are convenient tools to assess consequences of damage on short-term ship responses. Generally, observed trend is that vertical wave-induced responses of damaged ship converge toward those of intact ship with increasing wave period. Fairly small differences between responses of asymmetrically damaged ship with respect to the symmetrical incoming wave directions are found. The results of the study are an efficient method for seakeeping assessment of damaged oil tankers and the framework for evaluating consequences of damage scenarios, heading angles, and sea conditions on seakeeping responses of damaged ships. The results can be used to decide if the intact ship model can be used instead of the damaged one for the emergency response procedure or for the risk assessment studies when modeling and computational time represent important limitations.

Hydrodynamic Coefficients for a 3-D Uniform Flexible Barge Using Weakly Compressible Smoothed Particle Hydrodynamics

Abstract: The numerical modelling of the interactions between water waves and floating structures is significant for different areas of the marine sector, especially seakeeping and prediction of wave-induced loads. Seakeeping analysis involving severe flow fluctuations is still quite challenging even for the conventional RANS method. Particle method has been viewed as alternative for such analysis especially those involving deformable boundary, wave breaking and fluid fragmentation around hull shapes. In this paper, the weakly compressible smoothed particle hydrodynamics (WCSPH), a fully Lagrangian particle method, is applied to simulate the symmetric radiation problem for a stationary barge treated as a flexible body. This is carried out by imposing prescribed forced simple harmonic oscillations in heave, pitch and the two- and three-node distortion modes. The resultant, radiation force predictions, namely added mass and fluid damping coefficients, are compared with results from 3-D potential flow boundary element method and 3-D RANS CFD predictions, in order to verify the adopted modelling techniques for WCSPH. WCSPH were found to be in agreement with most results and could predict the fluid actions equally well in most cases.

Internal Resonances for the Heave Roll and Pitch Modes of a Spar Platform Considering Wave and Vortex Exciting Loads in Heave Main Resonance

Abstract: The objective of this paper is to study the nonlinear coupling internal resonance of the heave, roll, and pitch response performance of a spar platform when their frequencies are in the ratio of 2:1:1 under wave and vortex exciting loads. The three degree-of-freedom (DOF) nonlinear coupled equations are established by considering a time-varying wet surface with a first-order wave force in heave and pitch and a vortex-induced force in roll. The first-order steady-state response is solved using the multi-scale method in heave main resonance. The multiple solutions of the motion equations are discussed using the analytic method and a numerical simulation. A sensitivity analysis is conducted to test the influence of the damping and internal detuning parameter. The regions of multiple solutions are found, and the jump phenomenon exists with the changes of the wave excitation. The regions of multiple solutions depend on the values of damping and detuning parameter.

Numerical Simulation of Float-Over Installation for Offshore Platform

Abstract: In this paper, a numerical investigation of a float-over installation for an offshore platform is presented to verify the feasibility of the actual installation. The hydrodynamic performance of a T-barge is investigated in the frequency domain, and the coupled motions are analyzed in the time domain. We then compare with those of the model test and determine that the response amplitude operator and the time series agree quite well. The barge exhibits favorable hydrodynamic behavior in the considered sea state, and the equipment loads are allowable. The mooring system and sway fender forces are within the permissible range. Based on these results, we can verify that the actual installation of the offshore platform is feasible. We accurately simulated many important factors and effectively reduced the risk associated with the offshore installation, which is of great importance. As such, we demonstrate that the numerical simulation of the float-over installation for offshore platforms has practical engineering significance.

Numerical Investigation of Green Water Loading on Flexible Structures Using Three-Step CFD-BEM-FEM Approach

Abstract: In this paper, the effect of green water impact on a flexible structure is studied based on three-step computational fluid dynamics (CFD)–boundary element method (BEM)–finite element method (FEM) approach The impact due to shipping of water on the deck of the vessel is computed using commercial CFD software and used as an external force in coupled BEM-FEM solver Other hydrodynamic forces such as radiation, diffraction, and Froude-Krylov forces acting on the structure are evaluated using 3D time domain panel method To capture the structural responses such as bending moment and shear force, 1D finite element method is developed Moreover, a direct integration scheme based on the Newmark–Beta method is employed to get the structural velocity, displacement, etc, at each time step To check the effect of the green water impact on the structure, a rectangular barge without forward speed is taken for the analysis The influence is studied in terms of bending moment, shear force, etc Results show that the effect of green water impact on the bow region can be severe in extreme seas and lead to various structural damages Similarly, it is also verified that vessel motion affects green water loading significantly and therefore one must consider its effect while designing a vessel

Experimental and Numerical Studies of the Wave-Induced Responses of a River-to-Sea Ship

Abstract: The ship motions and wave-induced loads of a new type of river-to-sea ship are investigated experimentally and numerically. A river-to-sea ship is an unconventional type of container ship characterized by high breadth to draft ratio and low length to breadth ratio, which makes it more prone to hydroelasticity than conventional ships of the same size. A segmented model was tested under two loading conditions, namely, ballast and loaded conditions, to determine the vertical motions and wave-induced loads under each condition. Results are compared with numerical simulations in the frequency domain. The wave-induced responses are calculated by a nonlinear time domain code at each time step. The response amplitude operators of vertical ship responses in regular waves are analyzed, and the wave-induced responses are consistent with the experimental results.