Green Water Events Research Articles

The Effect of Surge on Extreme Wave Impacts and an Insight into Clustering

_ The original goal of the present research is to investigate the influence of surge on green water and slamming. Long-running experiments with forward velocity and irregular waves were repeated with and without surge. Surge is found to increase the probability of green water events, but the impact pressures on deck and the probability of a green water event reaching the deck box decreases when the ship is free to surge. Green water and slamming events turned out to not occur independently as both event types cluster for large probabilities of occurrence. Clusters are caused by large pitch motions. Larger pressures on deck are found for clustered events. Keywords green water; slamming; surge; clustering; probability

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.

Modeling Green Water Load on a Deck Mounted Circular Cylinder

Abstract This article uses scaled physical and numerical modeling to investigate an idealized but complicated problem in which green water impacts a circular cylindrical structure located on top of a fixed box representative of a vessel. A focused wave group was used to overtop the box and generate the green water event, which resembled a plunging wave with air entrainment. The plunger collapsed and ran across the deck before impacting and then scattering from the cylinder. To test the adequacy of the physical modeling, nominally identical experiments were conducted in two different laboratories, in different countries. The numerical modeling comprised computational fluid dynamics (CFD) simulations performed using openfoam. The flow features, the force on the cylinder, and the surface elevation on top of the box are compared in detail across the two physical models and the CFD. Consistent load measurements were obtained from the two physical model tests, with force impulse results differing by less than 10%, underscoring the validity of the results, even accounting for the complexity of flow–structure interactions. A comparison with numerical model results reveals some sensitivity to experimental precision in the flow measurements on top of the box and the green water load. Nonetheless, the overall force impulse discrepancy between experiments and numerical models is within 15%, highlighting that the robustness of the methods was used despite these sensitivities. The sensitivity to CFD mesh and iterating the incident wave to match CFD and experiment are also explored. The agreement between experiment and CFD serves as an example of the utility of CFD for modeling green water loads.

Numerical study of green water loads on a fixed structure with elastic sidewalls by MPS-FEM coupled method

In rough seas, large masses of water will exceed the freeboard and cause violent slamming on the deck, known as green water. Due to its high destructive power, many scientists have devoted themselves to the study of green water. Given the complexity, various simplified methods have been used to study the mechanism and stresses of green water. In this paper, the loads and patterns of green water events on a fixed structure with rigid or flexible sidewalls were investigated using the solver MPSFSI-SJTU based on the Moving Particle Semi-Implicit (MPS) method coupled with the Finite Element Method (FEM). The rapid flow-structure interaction is generated by the wet dam-break method. Firstly, the numerical simulation of the water shipping events with the rigid structure was carried out, which was in good agreement with the existing numerical simulation results. Secondly, numerical simulations were carried out for different cases of fixed structures with elastic side wall and elastic upper wall. The influence of elastic modulus on wave loads and waveform was analyzed.

Numerical Study on the Green-Water Loads and Structural Responses of Ship Bow Structures Caused by Freak Waves

In recent decades, freak waves, characterized by their unusual high amplitude, sharp crest, and concentrated energy, have attracted researchers’ attention due to their potential threat to marine structures. Green-water loads caused by freak waves can be significant and may lead to local damage to the ship structures. Therefore, this paper focuses on the study of green-water loads and examines the structural responses of ship bow structures under the influence of the green-water loads caused by freak waves. Firstly, a three-dimensional numerical wave tank is established in which the superposition model is used to generate freak waves. Validations on the freak-wave generation, ship motion response and the wave loading are carried out to verify the present solvers. The simulation on the interaction between the freak wave and the ship are conducted to obtain the interaction process and green-water loads. Secondly, a finite element (FEM) model of the ship bow is built, on which the green-water loads are applied to calculate the structural responses. Finally, the displacement and stress of the deck and breakwater structures are analyzed. It is found that green water events caused by freak waves can generate enormous impact forces on the bow deck and breakwater, resulting in severe structural responses and even possible damage to the structures. The local strength of structures under freak waves needs to be considered in practical engineering applications.

Three-dimensional study on the interaction between a container ship and freak waves in beam sea

Freak wave occurs unexpectedly in the ocean with an extreme wave crest and focused wave energy, resulting in many marine accidents. The interaction between a container ship and freak waves in beam sea is studied in this paper to better understand the influence of freak waves on ships. A three-dimensional in-house solver is developed and validated for the freak wave generation and the wave-ship interaction. Characteristics of the interaction process, motion responses of the ship and the green water loadings induced by the freak waves are obtained and analyzed. Comparisons are carried out to reveal the influences of the freak wave crest and sequence on the roll, heave and impact pressures. Relations between motion responses of the ship and the green water event are discussed. Influences of ship speeds on the wave-ship interaction are addressed.

Probability and distribution of green water events and pressures

This article proposes a method to quantify, first, the probability of occurrence of green water and, second, the expected maximum pressures during green water events using their statistical distribution for ships at forward speed. A large green water data set which represents 1945 hours of continuous sailing on full scale with different sea states, forward speeds and drafts was obtained with model test experiments in a wave–current tank. The data of the experiment are available as open data through https://doi.org/10.4121/21031981. With the large data set obtained, the distribution of the time between green water occurrences is identified as exponential, indicating that when green water occurs is independent of the time since the last occurrence. Two methods were compared to estimate the probability of green water occurrence. One method is based on the probability of water exceeding the deck and one on a ship’s freeboard and the significant wave height, the former being in better agreement with the data, the latter being more practical for designers. The maximum pressures caused by green water are distributed according to the Fréchet distribution, also called extreme value distribution II. With the newly identified distributions, finally, an equation to calculate the probability of a pressure limit being exceeded for a ship in operation is formulated.

Flow kinematics in the generation of different types of green water events with incident wave trains

Various types of green water events, generated with incident wave trains, have been identified in experimental research, but experimental analysis of their kinematics is still required to improve comparisons of analytical and numerical models. This paper presents an experimental investigation of the kinematics of consecutive green water events generated with incident wave trains, particularly during their formation and interaction with the bow of a fixed structure. These events include Plunging-Dam-Break (PDB) and Hammer-Fist (HF) types of green water. The kinematics of the latter has not been explored experimentally before. The experiments were carried out in a wave flume, considering incident waves trains of different steepness. Particle Image Velocimetry (PIV) methods were used to study the flow kinematics. The role of flow velocity during the formation of a representative PDB and a HF green water event was investigated in detail. Then, the averaged velocities of the initial stages of green water were compared for all the wave trains considered. For most the events, the shipping water tends to accelerate horizontally over the deck since it crosses the deck, where horizontal velocities close to zero are observed. Results suggest that PDB events have lower run-up velocities than HF events, but higher water-on-deck velocities. The analysis presented here may be useful for engineering purposes, in the validation of analytical and numerical methods. A database with results of the events investigated is available for interested readers.

The three-dimensional green-water event study on a fixed simplified wall-sided ship under freak waves

As non-linear extreme waves, freak waves have huge wave crests, large wave heights and concentrated energies. The freak waves usually lead to severe impacts on ships and marine structures in green water events. Nevertheless, the general mechanisms, water motion scenarios and impact loads on bow deck and superstructure in these events remain unclear. In the study, the freak waves based on Peregrine breather solution of NLS equation are generated and the green water events of a fixed ship model are numerically investigated in three-dimensional wave flume by CFD method. First, the accuracy of green water simulation method is validated with experimental data. Then, the propagation and evolution of freak waves in front of ship bow are observed. After that, the water motion scenarios, velocity fields and impact pressures on ship bow in different stages are investigated. The necessity of three-dimensional simulation is verified. The effects of wave heights, wave periods and breaking waves are analyzed. The results indicate the water elevations and largest peak pressures increase nonlinearly with freak wave heights and the short-period freak waves cause more severe impacts than long-period ones.

On the Evolution of Different Types of Green Water Events—Part II: Applicability of a Convolution Approach

Recent research related to the evolution of different types of green water events, generated in wave flume experiments, has shown that some events, such as plunging-dam-break (PDB) and hammer-fist (HF) types, can present multiple-valued water surface elevations during formation at the bow of the structure. However, the applicability of analytical models to capture the evolution (i.e., the spatio-temporal variation of water elevations) of these events has not been tested thoroughly. This could be useful when estimating green water loads in the preliminary design stage of marine structures. The present work extends the research by Fontes et al. (On the evolution of different types of green water events, Water, 13, 1148, 2021) to examine the applicability of an analytical convolution approach to represent the variation in time of single-valued water elevations of different types of green water events generated by incident wave trains, particularly PDB and HF types. Detailed experimental measurements using high-speed video in wave flume experiments were used to verify the applicability of the model for single and consecutive green water events of type PDB and HF. The present work is a tentative attempt to compare an analytical approach for HF evolution. Results were also compared with the classic analytical dam-break approach. It was found that the convolution model allows the variation of water elevations in time to be captured better in comparison with the dam-break approach. The convolution model described the trend of water elevations well, particularly at the bow of the structure. The model captured the peak times well in single and consecutive events with multiple-valued water surfaces. Results suggest that this conservative and simplified approach could be a useful engineering tool, if improved and extended, to include the evolution of green water events in time domain simulations. This could be useful in the design stages of marine structures subject to green water events.

Screening wave conditions for the occurrence of green water events on sailing ships

Design loads for extreme wave events on ships, such as slamming and green water, are hard to define. These events depend on details in the incoming waves, ship motions and structure layout, which requires high-fidelity tools such as CFD or experiments to obtain the correct loads. These tools (presently) do not have the capability to fully resolve the long-term statistics of rare events in all metocean conditions over the ship’s lifetime. The idea of ‘screening’ is to use lower-fidelity numerical methods to identify the occurrence of extreme load events based on an indicator. A good indicator has a strong correlation to the design load, but is easier to calculate. A high-fidelity tool can then be used to find the loads in these events. The low-fidelity statistics and the high-fidelity loads can be combined to define a design load and its probability. The present study compares different numerical screening indicators for green water loads on a containership against experiments. The quality of the identification of the critical events and the required computational time served as comparison metrics. This showed that screening both with potential flow tools and with coarse mesh CFD tools is feasible, provided the indicator, grid, time step and wave input settings are well chosen. The results from coarse mesh CFD are slightly better than from potential flow, but the computational costs are much higher. The results also show that the peaks and steepness of the relative wave elevation around the bow are suitable green water load indicators, as well as the undisturbed wave crests at the bow. Fine mesh CFD calculations were done for the identified events based on an example indicator, which resulted in a green water load distribution very close to that of the experiments. This study shows that screening could potentially reduce the required high-fidelity modelling time with up to ∼90% compared to common practice.

On the Evolution of Different Types of Green Water Events

Green water events may present different features in the initial stage of interaction with the deck of a structure. It is therefore important to investigate the evolution of different types of green water, since not all the events interact with the deck at the same time. In this paper, the evolution of three types of green water events (dam-break, plunging-dam-break, and hammer-fist) are studied. The water surface elevations and volumes over the deck in consecutive green water events, generated by incident [wave trains in a wave flume, were analyzed using image-based methods. The results show multiple-valued water surface elevations in the early stage of plunging-dam-break and hammer-fist type events. Detailed experimental measurements of this stage are shown for the first time. The effect of wave steepness on the duration of the events, maximum freeboard exceedance, and volumes were also investigated. Although the hammer-fist type showed high freeboard exceedances, the plunging-dam-break type presented the largest volumes over the deck. Some challenges for further assessments of green water propagation are reported.

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.

A Detailed Description of Flow-Deck Interaction in Consecutive Green Water Events

Abstract This paper gives a detailed description of the main hydrodynamic characteristics of different types of green water events occurring on a fixed structure in a series of experiments. High-speed video, at 250 fps, was used to capture the flow details from regular waves produced inside a wave flume. The green water events were classified according to their type and the results of the evaluation from the apparent and effective interaction features. The air cavities in the plunging-dam-break (PDB)-type events were almost spherical for longer waves. PDB with large, elliptical cavities, and hammer-fist (HF)-type events occurred with steeper waves. The highest wave front and bow run-up velocities were found in PDB and HF events, respectively. The size of the cavities in PDB events increases and becomes more elliptical for higher bow run-up velocities. The results of these experiments show that not all events can be treated in the same way as the dam-break (DB)-type events when designing structures or implementing models, as not all interactions with the deck happen when the shipping water crosses the bow edge.

Identification of the advection-diffusion equation for predicting green water propagation

This paper presents an approach that involves the advection-diffusion equation together with a parameter identification approach to analyze the evolution of consecutive green water events. The goal of the parameter identification approach is the estimation of the AD coefficients in real-time by using a method based on a bank of state estimators, which in turn are based on a spatial-discrete version of the AD equation. In order to test the approach with real data, laboratory experiments of consecutive green water events on a fixed structure were performed. Such events were generated with regular waves of different steepness in a wave flume by employing high-speed video to capture flow evolution. Time series of water elevations were obtained at several positions over the deck to perform the parameter identification and to validate the present approach. Results suggested that the present approach allowed capturing acceptably the evolution of consecutive events even when considering constant AD parameters. Maximum elevations in each event as well as their time of occurrence were well approximated, which is an advantage against traditional analytical methods.

Green water evolution on a fixed structure induced by incoming wave trains

Marine structures, such as ships and oil platforms, are often subject to green water propagating over their decks and causing overloading. Understanding the spatial and temporal evolution of these events allows estimation of water loads and thus an analysis of the structural dynamics. From this information the design of marine structures can be improved. This article investigates the evolution of green water events on a fixed structure, produced by incoming wave trains in a wave flume. Wave trains were generated with different wavelengths and heights. High-speed video was employed to capture details of the green water flow and image-based methods were used to extract water elevation data for the spatial and temporal analysis. A preliminary evaluation of an engineering convolution approach for capturing the water-on-deck elevations generated by the wave trains is also included. The experimental results were systematically carried out to allow the study of the effect of the wave trains on freeboard exceedance, the distribution of water on the deck, and the green water evolution. The convolution approach reproduced adequately the water elevations in time and space for all the study cases. It also captured the trends of amplitudes and approximated the time at which the peak values occurred, showing potential for further green water analysis.

Lagrangian finite-difference method for predicting green water loadings

A meshless CFD method is proposed for numerical simulation of incompressible flows and estimation of hydrodynamic loads during green water events. In this paper, governing equations are described in Finite Differences context, where spatial operators are derived using Weighted-Least Squares. Volume-conservative Lagrangian advection is handled by solving a set of geometrical constraints, while adjusting to moving boundaries represented by triangulated meshes. The validation of the method is performed by simulating four green-water experiments on fixed and moving structures, and comparing the kinematics of flow and hydrodynamic loads during impacts. It is shown that the method accurately reproduces patterns of waves that are wetting the deck, as well as the pressure distribution along the deck. The resulting method intrinsically handles violent fluid–structure interaction with free surface fragmentation, while providing second-order accurate pressure field.

3D Numerical Simulations of Green Water Impact on Forward-Speed Wigley Hull Using Open Source Codes

A series of CFD RANS simulations are presented for Wigley hulls of two freeboard heights progressing with forward speed in waves. Free surface effects are captured using the Volume of Fluid (VOF) method embedded in open source software OpenFOAM. Comparisons of heave, pitch motions and added resistance of the first Wigley model against the experiments of Kashiwagi (2013) confirm the numerical validity of the hydrodynamic modelling approach. Further simulations for the lower-freeboard Wigley model reveal that the highest green water impact on decks appears in way of λ / L = 1.3 and at the highest instantaneous pitch amplitude where the water propagates far downstream and across the deck. The simulations also demonstrate that the green water events are associated with air bubble entrapment.

Green Water on A Fixed Structure Due to Incident Bores: Guidelines and Database for Model Validations Regarding Flow Evolution

This paper presents a two-dimensional experimental study of the interaction of wet dam-break bores with a fixed structure, regarding the evolution of the incident flows and the resultant green water events on the deck. The study employs image-based techniques to analyse flow propagation from videos taken by high-speed cameras, considering five different shipping water cases. The features of small air-cavities formed in some green water events of the plunging-dam-break type were analysed. Then, the spatial and temporal distribution of water elevations of the incident bores and green water were investigated, providing a database to be used for model validations. Some guidelines for the selection of the freeboard exceedance, which is of relevance for green water simulations, were provided. Finally, the relationship between the incident bore and water-on-deck kinematics was discussed. The proposed study can be used as a reference for performing simplified and systematic analyses of green water in a different two-dimensional setup, giving high-resolution data that visually capture the flow patterns and allow model validations to be performed.

Wet dam-break simulation using the SPS-LES turbulent contribution on the WCMPS method to evaluate green water events

The paper analyzes the contribution of the sub-particle-scale large-eddy simulation (SPS-LES) turbulent model on the dynamic behavior of the weakly compressible moving-particle semi-implicit (WCMPS) method. To search a robust WCMPS method configuration, the next models are tested: two pressure gradient models, two Laplacian models and the constrained stabilization technique through the control adjustment of the particle velocity by Xu and Jin (Comput Fluids 137:1–14, 2016. https://doi.org/10.1016/j.compfluid.2016.07.014). Then, for these combinations the influence of the SPS-LES turbulent dynamic contributions on the WCMPS method is analyzed. The WCMPS resultant configurations are applied to represent the physics of two different isolated green water events, produced by incoming bores that were generated with the two-dimensional wet dam-break approach. Numerical results captured well the evolution of the green water events, indicating that the WCMPS method can be suitable tool to represent the complex physics of these phenomena, including the breaking features of flow. Moreover, it is shown that even though turbulence is a three-dimensional phenomenon, turbulent contribution can be observable on the velocity field in two dimensions, having a considerable contribution on the adequate representation of green water amounts.