Green Water Phenomenon Research Articles

ANN-based prediction models for green water events around a FPSO in irregular waves

The green water problem has been a critical concern for the safety of offshore structures in extreme environmental conditions. However, accurately predicting the green water events using the conventional diffraction analysis method is challenging, primarily due to the highly nonlinear physics of the green water phenomenon. In this study, a novel methodology for the prediction green water events is proposed by integrating the linear diffraction method integrated with deep learning techniques. The proposed methodology is designed as a screening tool to predict the occurrence of green water events in terms of relative wave motions around the hull. In the proposed model, a linear prediction model is employed to account for the dominant physics of ship hydrodynamics and wave mechanics, while the ANN model is introduced to complement its nonlinear effects. First, the prediction performance was compared between the linear model and the ANN-based models for the peak value of relative wave motion. The ANN-based prediction model significantly reduced the prediction error of the linear prediction model. Moreover, the ANN model based on the time series itself shows a higher predictive performance than the ANN model using features extracted from the time series. Next, the predictive performance of the ANN model was analyzed when applied to different wave conditions. It was found that the prediction error greatly decreased even for different wave conditions. Finally, the screening performances of the prediction models are investigated. The ANN-based prediction model was found to be a more effective screening tool than the linear prediction model in terms of screening quality index.

청수현상 추정을 위한 댐 붕괴 흐름의 유체동역학적 특성에 관한 실험적 연구

In this study, hydrodynamic characteristics of dam break flow were investigated by a series of experiments. The experiments were performed in a 2-D rectangular flume with obtaining instantaneous images of dam break flow to capture the free surface elevation, and pressure distributions on vertical wall and bottom of the flume. The initial water depth of the dam break flow was changed into 3 different heights, and the gate opening speed was changed during the experiments to study the effect of the gate speed in the dam break flow. Generation of dam break phenomena could be classified into three stages, i.e., very initial, relatively stable, and wall impact stages. The wall impact stage could be separated into 4 generation phases of wall impinge, run-up, overturning, and touchdown phases based on the deformation of the free surface. The free surface elevation were investigated with various initial water depth and compared with the analytic solutions by Ritter (1892). The pressures acting on the vertical wall and bottom were provided for the whole period of dam break flow varying the initial water depth and gate open speed. The measurement results of the dam break flow was compared with the hydrodynamic characteristics of green water phenomena, and it showed that the dam break flow could overestimate the green water loading based on the estimation suggested by Buchner (2002).

The CFD Investigation of General FPSO Green Water

The main research is to investigate the seakeeping effect resulting from general FPSO hull bow flare. CFD-FV (Computational Fluid Dynamics-Finite Volume) method is applied with fluent software to simulate in the 3D time domain. Simulation objects are two different bow flare models exposed to the green water phenomenon, which is under the sea state of the Brazilian sea 20 years of return extreme sea state. Seakeeping analysis includes green water. The results indicated that a 3.5 m vertical bow flare would reduce about 35.09% level of green water on deck. A comprehensive comparison indicated that bow flare improved the global seakeeping performance, which can be used as a reference to the design of general FPSO hull bow flare.

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

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

Investigations on the second-order transient gap resonance induced by focused wave groups

Two or more marine structures deployed side by side may encounter strong water-body resonances within narrow gaps between them. It may cause significant wave loads on structures and the green water phenomenon on the deck. In this article, the transient fluid motion within a narrow gap formed by two fixed boxes suffered from incident focused wave groups is investigated using a two-dimensional viscous flow numerical wave flume. The focused wave groups adopted have the spectral peak frequency equal to half the fluid resonant frequency inside the gap. The wave fields both inside the gap and around the two-box system, the response/damping time of the transient wave surfaces inside the gap, the maximum wave forces and the ratios of the 2nd-order to the corresponding 1st-order wave surfaces/forces are investigated. It is revealed that the most dangerous place to green water is always the front edge of the two-box system. The damping time of the 2nd-order wave surface is significantly larger than that of the 1st-order one. As the incident wave amplitude rises, the ratios of the 2nd-order to the first-order wave surfaces/forces becomes increases gradually and can even exceed 100% for the wave surface, the horizontal wave force and the moment.

Experimental study of green water on rectangular structure with varying flare angle

In this study, an experimental study was performed with various flare angles to investigate the generation behavior and pressure distribution of green water on a structure. The green water phenomenon was reproduced in a 2D wave tank. The experiments were conducted under regular wave conditions and with a fixed, simplified structure. The structure was a 1:125 scaled model, in terms of its dimensions, of the BW Pioneer floating production storage and offloading (FPSO). The Bubble Image Velocimetry (BIV) measurement technique was used to obtain the bubbly flow velocity in order to investigate the flow kinematics of the green water on the deck with various flare angles. The generation behaviors of green water were measured quantitatively for each flare angles and compared. The velocity profiles of the bubbly flow were obtained and compared with the solution of the dam-break theory. The pressure distributions on the structure with various flare angles were obtained by analyzing the maximum peak pressure, rise time, pressure oscillations, and impulsiveness.

Numerical analysis on three-dimensional green water events induced by freak waves

ABSTRACT Freak waves are a type of extreme waves with great wave heights and concentrated energies. When they encounter ships, large amounts of water flow onto deck and lead to strong impact pressures, threatening the structural safety significantly. Many efforts concerned about green water phenomena, while few of them focused on that induced by freak waves. This paper investigates the green water events induced by freak waves in three-dimensional numerical wave tank, which is validated well against experimental data under regular waves. The Peregrine breather solution model is applied to simulate freak wave and a 3D hull structure is modelled to head it. In this study, the water motion scenario is observed and several characteristic stages are classified. The impact pressures on deck and superstructure are analysed and the three-dimensional effect is discussed. At last, the features and dangerous moments of this type of green water events are concluded.

Experimental study on flow kinematics and pressure distribution of green water on a rectangular structure

In this study, flow kinematics and pressure distribution of green water on a fixed rectangular structure were investigated with a series of experiments performed in a two-dimensional wave flume. The experiments were conducted under regular wave conditions which were scaled down by a ratio of 1:125 from the BW Pioneer Floating Production Storage and Offloading (FPSO) operated in the Gulf of Mexico. PIV and BIV measurements were applied to obtain the water and bubbly flow in order to study the flow kinematics of multi-phase flow induced by green water. The mass flow rate and the kinetic energy, which were calculated from the water velocity profiles obtained by the PIV measurements were compared with and without the structure to investigate the quantitative flow kinematics of the water flow at the weather side of the structure. The characteristics of the bubbly flow in the water shipping phase on the deck were investigated based on the velocities of the bubbly flow obtained by the BIV measurements, and the velocities of the bubbly flow were compared with the solutions of dam break theory. Pressure measurements were taken along the structure to estimate the local loading of the green water phenomena and they provided the maximum peak pressure, rise time, impulsiveness of the pressures, and the peak frequencies of the pressure oscillation due to the air pocket generation on the wall of the structure during the green water.

Numerical Investigation of Heave and Pitch Motion Effects on Green Water Loading for a Floating Body

In this paper, the influence of heave and pitch motions on green water impact on the deck is numerically investigated. The vessel motions are determined using a potential theory based method and provided as input to finite volume based CFD computations of green water phenomenon. A dynamic mesh approach is adopted to determine instantaneous body positioning in the fluid domain. Detailed validation studies with published experimental results for 2D and 3D fixed vessel cases are initially performed to validate the present numerical approach before studying the moving vessel problem. The results show that inclusion of heave and pitch motion changes the disturbed wave field near the bow which influences the free surface as well as the impact loading due to green water. The effect of wave steepness on green water impact is also investigated and it is seen that the present numerical method is capable of capturing green water load. It is observed that the effects of vessel motions on green water load are not negligible and one should consider this effect too. The incorporation of vessel motions in the vertical plane affects the green water loading on the deck.

Experimental Study on Variations in Behavior of Green Water and Flow Kinematics on Deck with Various Flare Angles

In this study, a series of experiments were performed to investigate the variations in the behavior of green water generation and the flow kinematics of bubbly flow on deck with various flare angles. The experiments were conducted in a 2-D wave flume using a simplified model of a BW Pioneer FPSO operating in the Gulf of Mexico, with a 100-year return period wave condition. The green water phenomena were captured with a high speed CCD camera. The variations in the behavior of the green water generation were investigated with various flare angles, and the horizontal mean velocity profiles of bubbly flow on deck obtained using bubble image velocimetry (BIV) were provided. The differences in flow kinematics of bubbly flow on deck were analyzed with various flare angles.

Structural response of deck structures on the green water event caused by freak waves

Freak waves are huge and unexpected surface waves in the ocean which usually lead to severe destructions to ships and offshore structures. Green water is one of the most commonly observed phenomena when ships and marine structures encounter with freak waves. As a result, freak waves will likely to create significant wave impact on the deck structures. However, the existing researches on the interaction between freak waves and marine structures are still inadequate. In order to investigate the green water phenomenon caused by freak waves and the structural response of the deck, a 2-D numerical wave flume is built in which a nonlinear freak wave based on the Peregrine breather solution is generated. By setting an elastic bare deck horizontally in the flume, the fluid-structure interaction (FSI) is considered during the green water event. To approximate the complicated real decks, a deck model containing intermediate elastic supports are used. The simulation results include the snapshots of the entire green water process, the pressures of the fluid on the deck and the displacements of the elastic decks analyzed with the fast Fourier transformation (FFT) and the wavelet transformation method. By comparing the results of the rigid deck, the elastic bare deck and the elastic deck with intermediate elastic supports, advisable conclusions are drawn.

사각형 단면 구조물에 대한 그린워터의 생성 특성 연구

In this study, the green water phenomena on rectangular shaped structure is numerically simulated by STAR-CCM+ to investigate the flow pattern including the velocity profiles in bubbly water flow. 5 phases of the formation of green water in front of and over the rectangular shaped structure is simulated at the design condition which is scaled down by 1:125 from FPSO operating in GOM. All numerical results are compared with the experimental results performed in a two dimensional wave flume. The water deformation due to the green water are obtained by the high speed CCD camera with employing the shadow graphy technique, which is allowed to take the bubbly water flow into images. A series of image taken by shadow graphy technique is analyzed with MQD method to calculate the velocity in bubbly water flow.

A combined wave-dam-breaking model for rogue wave overtopping

Although great efforts have been devoted to revealing the formation mechanisms of rogue waves, the study of wave–structure interaction such as the green water phenomenon is still not sufficient. Since the traditional Ritter׳s solution of dam-breaking model gives improper estimations on the rogue wave overtopping, a combined wave-dam-breaking (CWDB) model is proposed. The CWDB model is deduced from the Ritter׳s solution, by including the influence of the rogue wave propagation as well as the delayed effect of the dam-breaking problem. Various rogue waves, which are based on the Peregrine breather solution of nonlinear Schrödinger (NLS) equation, are generated in a numerical wave flume. The rogue waves impinge onto a horizontal deck placed at the center part of the tank, creating green water on the deck surface. The CWDB model is validated by making comparisons with both numerical results and existing experimental measurements. Great improvement can be revealed by contrasting the results of the CWDB model with those of the Ritter׳s solution.

규칙파 중 갑판침입수 충격하중 감소를 위한 선수형상 설계

In this research, the relationship between the bow shape and green water phenomenon on the bow deck of an FPSO was studied using an experimental method. A 140,000 DWT FPSO was used as the objective hull form in the present research. The incident waves were regular types. The heights were 1.0 and 1.5 times the freeboard, and the length was equal in size to LBP. The wave heights and pressures on the deck were measured in experiments. Model tests were performed to determine the effects of bow flare angles, bow shapes, and a forecastle deck. The free heave and pitch conditions were applied to the models in these experiments. From the results of the model tests, an optimized bow shape was designed, which was found to decrease the green water impact loading. The results of this research could be used as fundamental data in the design of a bow shape.

규칙파 중 갑판침입수에 미치는 선수 플레어 각도의 영향에 관한 실험적 연구

It is very important to investigate and understand the motion of a FPSO on waves because green water phenomenon occurs owing to the relative motions between incident waves and a ship on them. In this research, both experimental and some numerical approaches have been performed in head sea conditions with regular waves. As an object model of this research, a FPSO model is set free to heave and pitch during the experiments. Also, the motions of the FPSO model which are the results of the experiments are used for the corresponding numerical computations. The purpose of this study is to clarify the effect of bow flare on green water load. In this research, it is found that the amount of green water entered from the side of bow is decreased by the increase of bow flare angle. Moreover, the relation between the green water on the bow upper deck and the impact load on the vertical wall located at turrethead is investigated. The results of this research could be used as one of the fundamental data to design bow flares. Also, an optimized bow flare angle is proposed in this study.

선수부 갑판침입수의 수치시뮬레이션에 대한 기초연구

Green water load is an important parameter to be considered in designing a modern ship or offshore structures like FPSO and FSRU. In this research, a numerical simulation method for green water phenomenon is introduced. The Navier-Stokes equations and the continuity equation are used as governing equations. The equations are calculated using Finite Difference Method(FDM) in rectangular staggered grid system. To increase the numerical accuracy near the body, the Cartesian cut cell method is employed. The nonlinear free-surface during green water incident is defined by Marker-density method. The green waters on a box in regular waves are simulated. The simulation results are compared with other experimental and computational results for verification. To check the applicability to moving ship, the green water of the ship which is towed by uniform force in regular wave, is simulated. The ship is set free to heave and to surge.

Finite-difference simulation of green water impact on fixed and moving bodies

A numerical method was developed to predict the water impact pressure caused by green water phenomena. The density function method was employed in the framework of a locally refined overlapping grid system. The simple problem of a rectangular body placed in regular waves was considered and the simulation results were compared with tank experiments. Good agreement with experiment was shown for wave–body interactions and for the pressure values at three different positions on the body. The case of a rectangular body with a vertical wall on the deck was also considered and the comparison showed satisfactory agreement. It was demonstrated that this method could be extended to a moving body problem, in which the body was free to undergo heave, pitch, and surge motions.

Evaluation of green water loads on high-speed containership using CFD

Green water phenomenon and the need for an effective simplified model of this problem to assist design process are introduced. Relevant models are revised and used as building blocks to develop a more efficient model. Green water experiments are described and the use of the results to derive defining parameters for the new hydrodynamic model to represent green water is explained. The model of dam-break with initial velocity is developed and simulated using CFD. Loading effects in both horizontal and vertical directions are analysed and compared with experimental results. For horizontal loading, results of the simulation of a conventional dam-break model are also included in the comparison. The comparison shows that a new model of dam-break with initial velocity is adequate to represent green water flow when investigation into green water loading effects is attempted. Improvement on the prediction of vertical deck pressure is possible if deck acceleration and its associated deck pressure component can be included.

Basic Study of Glimm's Algorithm for Green Water Simulation

Experiments revealed that green water phenomena resemble dam-break, in which flow over deck edge forms a vertical wall of water and suddenly falls down into deck. In this paper the dam breaking problems were formulated using Glimm's algorithm, so-rolled, Random Choice method and, several validations were presented.