Green Water Loading Research Articles

CFD-FEM simulation of hydroelastic responses and slamming loads of a bow-flare ship advancing in head regular waves

ABSTRACT In this paper, the global motions and wave loads on a large bow-flare ship with the effect of hydro-elasticity, including whipping and springing, are numerically simulated by a CFD-FEM two-way coupled method. The simulation results of ship motions and wave loads by employing the numerical method are demonstrated by verification and validation studies including comparison with tank model experiment results. Additionally, the presented method is applied to study the motions, wave loads and slamming loads of the ship in different wave conditions. The vertical bending moment and vertical shearing force of different frequency components and their distribution along ship length are studied. The characteristics of slamming pressure and green water loads and their spatial distribution are studied. The simulation results indicate that the present numerical method well reproduces ship seakeeping and hydro-elasticity and has potential application values in the design and evaluation of ship wave loads.

Evaluation of the influence of deck slope on green water loads using the wet dam-break approach

Evaluation of the influence of deck slope on green water loads using the wet dam-break approach

Numerical Study of Green Water Loads on a Fixed Structure with Various Gate Release Velocities by the MPS Method

Numerical Study of Green Water Loads on a Fixed Structure with Various Gate Release Velocities by the MPS Method

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.

Modelling green water loads on ships using coupled impulse response function and CFD solution

A 3D coupled method is used to model green water and to study numerically the influence of wavelength, wave steepness, and bow rake angle on the green water occurrence and peak pressure on a ship deck. To do that, the global motions of the vessel are obtained in the time domain using an impulse response function. The local hydrodynamic forces of green water load are computed using a commercial CFD solver together with the Volume of Fluid method. The approach is validated using in-house experimental results. Then a systematic parametric study is performed to understand the effect of several geometric and environmental parameters on green water loads. The results indicate that peak pressure due to green water increases significantly with vessel speed for all the wavelengths investigated and decreases with the increase of bow rake angle.

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.

Development of Ultra High-Performance Fiber reinforced concrete barge for 5 MW wind turbine

Floating wind turbines are gaining more popularity today as one of the effective green energy harvesting systems. In the effort to reduce the cost of construction for floating wind turbines, concrete support structures and concrete barge have been developed. However, due to the concrete low tensile strength and susceptibility to chemical action and freezing temperatures, the concrete barges are designed with very large sections resulting in high energy consumption, high volume of construction materials, weightier structure and more heavy equipment for fabrication and installation. Therefore, in order to overcome these challenges, in this study Ultra High-Performance Fiber Reinforced Concrete (UHPFRC) is proposed for use in casting a barge for a floating offshore wind turbine and compared to a reinforced cement concrete (RCC) barge. The experimental tests conducted on the UHPFRC and RCC barge small sized prototypes, showed less heel on the RCC barge compared to the UHPFRC barge. However, the RCC barge experienced severe green water load which could cause it to capsize. The hydrodynamic analysis results from the finite element analysis showed less pitch motions in the UHPFRC barge in 7 out of the 12 DLCs considered. The roll motions were less than 50 in both barges with insignificant difference between them, while in heave motions, the UHPFRC barge experienced 10% to 20% less motions than the RCC barge in all 12 DLCs. In the structural analysis, the maximum deformation of the UHPFRC barge was 14 mm, which is 129% higher than the deformation of the RCC barge. In overall, the UHPFRC barge proved to be more effective in achieving better hydrodynamic motions for the barge floater in comparison to the RCC barge and can be considered as alternative to the conventional reinforced cement concrete material.

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).

A semi analytic method for the analysis of the symmetric hydroelastic response of a container ship under slamming and green water loads

This paper presents a semi analytic time domain method for the analysis of the symmetric hydroelastic response of a container ship subject to slamming and green water loads. An Impulse Response Function (IRF) is adopted for the calculation of radiation, diffraction and wave excitation forces. Local hydrodynamic forces associated with green water on decks and slamming loads are respectively modelled by the Buchner’s Dam Break Model and a Generalised Wagner Model. The structural responses are captured by Euler-Bernoulli beam theory and solved by the modal superposition method. The Duhamel Integral technique is used to evaluate the dynamic response. A parametric study demonstrates how external forces may affect the global wave induced vertical bending moments and shear forces. Numerical simulations are compared against a hybrid method that combines computational fluid dynamics, boundary element and finite element methods for low to medium frequency induced dynamic response. It is concluded that the proposed semi analytic methodology is fast and accurate and may be useful at concept ship design stage.

Required test durations for converged short-term wave and impact extreme value statistics–Part 2: Deck box dataset

In the assessment of wave-in-deck loads for new and existing maritime structures typically model tests are carried out. To determine the most critical conditions and measure sufficient impact loads, a range of sea states and various seeds (realisations) for each sea state are tested. Based on these measurements, probability distributions can be derived and design loads determined. In air gap model testing usually only few, if any, impact loads occur per 3-hour seed. This can make it challenging to derive reliable probability distributions of the measured loads, especially when only a few seeds are generated. In addition wave impact forces, such as greenwater loading, slamming, or air gap impacts are typically strongly non-linear, resulting in a large variability of the measured loads. This results in the following questions: How many impacts are needed to derive a reliable distribution? How is the repeatability of individual events affecting the overall distribution? To answer these questions wave-in-deck model tests were carried out in 100 x 3-hour realisations of a 10,000 year North Sea sea state. The resulting probability distributions of the undisturbed wave measurements as well as the measured wave-in-deck loads are presented in this paper with focus on deriving the number of seeds and exposure durations required for a reliable estimate of design loads.The presented study is Part 2 of a combined study on guidance for the convergence and variability of wave crests and impact loading extreme values. The data set of Part 1 ([1]) is based on greenwater loads on a sailing ferry and the data set of Part 2 on wave-in-deck loads on a stationary deck box.

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.

Slamming and green water loads on a ship sailing in regular waves predicted by a coupled CFD–FEA approach

A numerical simulation method is presented by integrating Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) solvers to predict ship wave loads and slamming loads taking into account hydroelastic effects. The interest of this study mainly lies in the slamming and green water pressures acting on a flexible ship investigated by the coupled CFD–FEA method. Firstly, verification and sensitivity analysis of the wave loads and slamming pressures on the S175 containership evaluated by the coupled CFD–FEA method is conducted by comparing the results using different mesh sizes and time step schemes. Discussion on the effect of hydroelasticity on impact pressures is also conducted. Then a comprehensive analysis on the global motions, wave loads, slamming and green water pressures of the ship in different regular wave conditions is undertaken. Finally, a simplified bow flare and bottom slamming pressure estimation method based on the seakeeping data of incident wave and ship global motions are proposed, which can reduce the computational burden of the two-way fluid-structure interaction simulations when impact pressure is concerned.

A CFD–FEA two-way coupling method for predicting ship wave loads and hydroelastic responses

This paper presents a co-simulation method for numerical prediction of nonlinear hydroelastic response of ship advancing in severe wave conditions. Two-way coupling of CFD and FEA solvers is applied where the external fluid pressure exported from the CFD simulation is used to derive the structural responses in the FEA solver and the structural deformations are fed back into the CFD solver to deform the mesh. Violent free surface flow such as slamming and green water on deck is well captured by the RANS and VOF techniques in the CFD solver. The nonlinear whipping and springing responses can be accurately simulated by the structural solver through the dynamic 3D FEA of the hull. By conducting co-simulations with a standard S175 containership, the developed fully coupled method was proved to be capable and reliable in accurate simulating almost all the physical characteristics of interests within the scope of ship seakeeping and hydroelasticity, which include large-amplitude motions, accelerations, wave loads, slamming loads, green water loads and whipping and springing responses of the hull in waves.

Design of Breakwaters to Minimize Greenwater Loading on Bow Structures of Fixed Vessels

Greenwater (splashing of water on the deck) loading is a classical problem faced by designers of ship-shaped vessels, which becomes even worse when the vessel operates in harsh weather conditions for an extended period of time. Installation of breakwaters on the deck can play a crucial role in minimizing this impact. However, research on the design and optimization of the breakwater is still in its infancy, and this study aims at shedding further light on this area by proposing and analysing the effectiveness of three breakwater designs on a fixed box-shaped vessel. The commercial CFD software ANSYS Fluent is used for this investigation. The design model (without breakwater) was validated at first against experimental results of greenwater splashing, before performing the actual simulations with the proposed breakwater design. A vertical plate is used as the deck structure, and the greenwater pressure at several locations on that plate is measured to compare the effectiveness of various breakwater designs. Overall, breakwaters with openings (perforations, grillages, etc.) were found to be more effective in minimizing the pressure generated by the greenwater. Nevertheless, there is significant room for improvement on breakwater designs, and some topics for further research are also suggested in this regard.

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.

Numerical Simulation of Green Water on Deck with a Hybrid Eulerian-Lagrangian Method

Green water on the ship deck in rough sea conditions may induce extreme impulsive wave impacts on superstructures and result in severe structural damage. It is of great importance to consider green water loads in ship structure design. However, there are many challenges in predicting green water loads due to the strongly nonlinear wave–ship interactions and the multiphase, multi-scale nature of the wave impact phenomena. In this article, a three-dimensional hybrid Eulerian–Lagrangian approach is proposed for simulating green water loads on the ship deck. It is extended from an efficient and accurate two-dimensional method developed for fluid–structure interaction problems. In this method, the flow field is solved on a fixed regular Cartesian grid system in an Eulerian framework, whereas the solid body motion is tracked with a set of markers immersed in the fluid and solved in a Lagrangian framework. Two benchmark cases, green water on a fixed simplified Floating Production Storage and Offloading (FPSO) model and green water on ship, are simulated. Comparison between experimental data and numerical results shows that our method is a viable choice for predicting green water loads. Introduction In rough sea conditions, ships may undergo large-amplitude motions and suffer green water impacts due to the strongly nonlinear wave–body interaction. Large waves run up to the ship deck and impact on superstructures with huge volume of water impulsively. It can induce extreme impulsive wave impact loads on superstructures and result in severe structural damage. Thus, it is important to predict wave impact loads for ship structure design. However, there are still great challenges in predicting green water impact loads accurately because it is a flow phenomenon of high complexity and randomness due to its multiphase, highly turbulent nature with wave breaking, air entrainment, etc (Temarel et al. 2016).

Experimental and Numerical Investigation of Green Water Occurrence for KRISO Container Ship

The occurrence of green water on the deck of Korea Research Institute of Ships & Ocean Engineering (KRISO) container ship is investigated using model test experiments and a fully coupled impulse response function (IRF)–computational fluid dynamics (CFD)–based numerical approach. In the experimental study, green water pressure over the deck and superstructure is investigated for different regular head wave conditions (wavelength/ship length ratio: .8–1.5) and vessel speeds (Froude number: .055–.166). The impact pressure on the deck is found to be highest at a wavelength/ ship length ratio of 1.2 and increases drastically with the increase in Froude number. The variation of green water pressure with wave steepness is linear for points on the forward deck and quadratic for the superstructure. In the second part, a coupled IRF-CFD–based numerical method is developed in which the global hydrodynamic forces such as radiation–diffraction and Froude–Krylov force are computed using a potential flow solver, whereas the local pressure due to the shipping water impact is computed using CFD and added as an external force. Comparisons of vessel motions and green water pressures with experiments indicate that the coupled IRF-CFD method can be a robust and efficient tool to predict shipping water loads on ships. Introduction The prediction of green water shipped on the deck and the resulting impact on ships and offshore structures have been an important research topic in the last couple of decades because it is directly related to the safety of the structure at sea. Historically, shipping of water on the deck had caused many serious damages to the bow and superstructure, and some of them have even capsized; e.g., the Schiehallion Floating Production Storage and Offloading (FPSO) was damaged severely in November 1998 because of steep waves. These incidents resulted in considerable research on this topic in the past two decades. The primary focus has been on the understanding of critical aspects and the accurate prediction of green water loading which requires proper modeling of green water occurrence. Also, accurate calculation of the green water loads on the deck and superstructure is very important for structural design load calculation. However, this is a nontrivial task because it depends on many physical and geometrical factors such as wave steepness, the wavelength- to-ship length ratio, hull shape around the bow region, and vessel speed. Hence, efforts have been made to develop suitable numerical models that may consider most of these aspects under necessary underlying assumptions.

Green water loads using the wet dam-break method and SPH

This paper presents a systematic numerical approach to investigate green water loads on a fixed structure using Smoothed Particle Hydrodynamics (SPH) simulation and the wet dam-break method to generate rapid flow-structure interactions. The experimental study of Hernández-Fontes et al. (Patterns and vertical loads in water shipping in systematic wet dam-break experiments, Ocean Eng., 2020), investigated vertical green water loads. In this paper, it was extended to evaluate more green water scenarios, study horizontal loads and analyse the performance of three different plate-type deck edge protections in minimizing green water loads. The numerical results were validated. Then, the performance of the protection plates, arranged in different configurations (i.e., horizontal, sloped and vertical), in minimizing and delaying horizontal and vertical green water loads, was investigated. The numerical results allowed different green water load trends to be identified. For most cases, the sloped protection was better in reducing the maximum horizontal and vertical loads, by delaying the occurrence of horizontal loads. This approach can be used as an engineering tool to investigate water shipping loads numerically in a reasonable computational time. It can be extended to other applications that aim to reduce green water loads in a variety of structure configurations.

A Method for the Prediction of Extreme Ship Responses Using Design-Event Theory and Computational Fluid Dynamics

The design of a naval vessel requires accurate estimation of the extreme loads and motions that it will experience during its lifetime. Operation in large seaways in which the ship-wave interaction is highly nonlinear and transient leads to design events such as maximum internal loads due to global wave bending, local slamming loads, extreme roll, combinations of the global wave bending and local slamming, and many others. In this article, a method is presented that allows for nonlinear analysis to be used to predict events with user-specified rareness. The core of the method combines probability, frequency, and time-domain analyses to generate short time-window sea environments that lead to extreme dynamical events. The Office of Naval Research Tumblehome geometry is analyzed for the extreme roll angle when advancing in stern quartering irregular seas. 1. Introduction The design of a naval vessel requires accurate estimation of extreme loads and motions that it will experience during its lifetime. Specific quantities of interest are the maximum slamming load during wet-deck impact, maximum acceleration at different locations on the vessel, maximum green-water load on the bow structure or helicopter deck, maximum roll angle, or frequency of occurrence of capsize, to name a few. It is important to recognize that a ship lifetime is decades long, and the exposure time in different severe storms over the lifetime is of the order of weeks, if not months. Furthermore, because of the random nature of the sea and, hence, the dynamical response of the ship, the extreme response is also random and should be characterized statistically. This means that a single lifetime realization in a given seaway by either model tests or numerical simulation only gives one sample of the extreme response, and multiple lifetime realizations are required to characterize the extreme response.