Slamming Loads Research Articles

Assessment of three-dimensional effects on slamming load predictions using OpenFoam

Three-dimensional effects on slamming loads predictions are investigated numerically using the unsteady incompressible Reynolds-Average Navier-Stokes (RANS) equations and volume of fluid (VOF) method, which are implemented in the open-source library OpenFoam. Three cases of free-falling water entry problems are considered. The numerical solutions are validated through the comparisons of slamming loads and motions between the CFD simulations and the available experimental values. The total slamming force and slamming pressures on the 3D wedge cylinder, ship section and slender circular cylinder are compared with the results on the 2D sections, respectively. The results show that the 3D predictions on the maximum slamming force coefficient are around 21%∼23% lower for the wedge with a deadrise angle of 30º and are around 16% lower for the slender circular cylinder, relative to the 2D calculations.

Numerical prediction of ship motion and slamming load characteristics in cross wave

When waves from two weather systems converge at a certain sea area, cross waves can be produced. It is important to have a better understanding of the motion characteristics of ship in cross sea to ensure its navigation safety. In this paper, the motion responses of a S175 container ship hull in cross sea is first analyzed based on CFD technique using RANS method. VOF method is used to model and capture the free surface and overset grid technique is adopted to simulate ship motions in waves. Verifications on ship motion by the numerical method have been conducted for time step- and grid-independent studies. According to the motion results by numerical simulation, typical conditions are chosen for further investigation of slamming pressure on bow area and green water on deck. It is found that vertical motion responses of ship induced by cross waves can be quite large and the ship experienced obvious roll–pitch-coupled motion. Severe bow slamming and green water under cross sea state are also observed. The bow flare slamming pressure on the baseline can be quite large in symmetric cross wave. Asymmetrical slamming impacts occurred when the ship sails in an asymmetric cross wave field.

Operation of Ride Control Systems to Minimise Slamming of Wave-Piercing Catamarans

Wave slam produces dynamic loads on the centre bow of wave piercing catamarans that are related to the relative vertical motion of the bow to the encountered wave surface. Rapid slam forces arise when the arch sections between centre bow and main hulls fill with rising water. In this paper time domain solutions for high speed ship motion in waves, including the action of active motion controls, are used to compute the slam forces. Slamming occurs at specific immersions of the bow whilst the peak slam force is characterised by the maximum relative vertical velocity of the bow during bow entry. Vertical motions of bow and encountered wave are in antiphase at encounter frequencies where slamming is most severe. The range of encounter frequencies where slamming occurs increases with wave height. Wave slam loads reduce ship motions, the heave motion being most reduced. Deployment of a fixed, inactive T-foil can reduce slamming loads by up to 65 %. With active controls peak slamming loads on the bow can be reduced by up to 73% and 79% in 4 m and 3 m seas, local control feedback being marginally the most effective mode of control for reduction of slamming.

Simulation of Flexural Dynamics of a Slender Ship undergoing Heave and Pitch

A simulation model for the flexural dynamics of a slender ship undergoing heave and pitch in regular waves is presented. The heave and pitch motions are found from the strip theory of Gerritsma and Beukelman [1] which is known to be experimentally validated. It is assumed that the ship structure is sufficiently stiff so that bending deformations do not significantly alter the wave field and hence the hydrodynamic loads determined from the strip theory may be used as inputs to the flexural model. The flexural dynamics is modeled by applying the hydrodynamic loads to a lumped-mass beam model and the equations of motion are formulated using the methods of Kane and Levinson [2]. The flexural model can capture large deformation behavior and is tested by demonstrating convergence to a known analytic solution. The model is easily implemented and may be used to evaluate time histories of bending moment responses to waves as well as to slamming events if the slamming load is known or estimated. Some typical results are presented.

Numerical study on the dynamic characteristics of water entry of cavity body using two-phase SPH method

The air usually has a major influence on the water entry of a typical cavity body (cavity body is a hollow, cylindrical, semi-closed structure), which not only lowers the slamming load but also affects the dynamic characteristics of water entry. In this paper, a two-phase smoothed particle hydrodynamics (SPH) model for simulating the water entry of cavity body is presented. The SPH model combined with Riemann solver is improved to deal with the two-phase flows with the discontinuous quantities across the interface. One-sided Riemann problem is used to impose the fluid–structure interaction and a switch-function-based Riemann solver dissipation is formulated to improve the interfacial instability owing to the strong impact. The motion equations of rigid body are incorporated into two-phase SPH model to describe the motion of cavity body. The proposed model is validated by research on the test cases in the published literature. Finally, this work presents a study of water entry of cavity body by experiment and this two-phase SPH method. The dynamics phenomena in the coupling process between cavity body and two-phase flow are investigated. And the effects of air, mass, the sizes and incline angles of cavity body on the dynamic characteristics of cavity body and two-phase flows are shown. An improved two-phase SPH model was used to simulate the water entry of cavity body. The cavity body impacts the water surface and the cavity body is closed by the flooding water. It comes into being open air cavity and water jets. The jet walls protrude and the air cavity is closed. With the increase of the water entry depth, the complex hydrodynamic phenomena can be seen again. The sinking depth shows the law of fluctuation up and down with time.

Experimental investigation of the entrapped air on the hydroelasticity induced by liquid slamming under shallow-water condition

The liquid impact induced by slamming in the membrane-type tank is an important issue for the design of Floating Liquefied Natural Gas (FLNG) facilities. The free surface entrapping air may easily happen under shallow-water condition in the tank, which could result in structural local failure. Studying the effect of entrapped air on slamming load is essential to understand the air-free surface-elastic structure interaction. Thus, a series of experiments are designed and conducted in an elastic rectangular tank under shallow-water condition. The evolution of free surface and the development of entrapped air are analyzed quantitatively. Furthermore, both impact load and structural response are measured and studied in the spatial and temporal distribution under shallow-water condition. The results show that the air cavity plays a cushion role between free surface and elastic wall. Therefore, the impact mode is changed from liquid-direct impact to liquid-indirect impact. The existence of the air cavity leads to a significant reduction of the maximum slamming load and structural response but an increase in the pressure impulse in spatial distribution. It is suggested to consider the effect of the entrapped air on the determination of the slamming load for the design of super-large LNG tanks in the offshore engineering.

Experimental study for characteristics of slamming loads on bow of a ship-type FPSO under breaking and irregular wave conditions

An experimental investigation of the characteristics of the slamming impact loads on the bow of a ship-type Floating Production Storage and Offloading (FPSO) under breaking and irregular wave conditions was conducted. The breaking waves were generated by the focusing wave method and three different heading angles were applied. To examine the repeatability of the slamming loads, the model experiments were repeated 20 times for each heading condition. The slamming impact loads were measured using 15 force sensors attached to the bow of the FPSO unit model, while force panels were used to measure the local impact loads. The locations of the largest slamming impact loads were observed to vary with the heading angle. Two types of slamming impact loads were identified and damping was also observed in the slamming impact load with two peaks. Irregular wave tests conducted under oblique wave conditions were further used to investigate the characteristics of the slamming impact loads, and the velocities of the directional sway and roll motions were found to be significant determinants of the slamming impact load.

Experimental and numerical investigations of water entry in subsea modules with porous structures

Slamming impacts on subsea modules used for oil and gas exploration and production have become an increasingly important structural safety issue. There are many aspects of the subsea module water entry problem that have not yet been sufficiently explored, and these include the shape of the module and its impact velocity. In this study, a large-scale model and computational fluid dynamics (CFD) simulations were used to explore these aspects, with a focus on their role in porous plates. The numerical simulation results for vertical forces showed good agreement with the experimental results. The CFD simulations, numerical methods, and computing grids can be used to accurately and efficiently determine the slamming loads both in the time domain and at specified points. The critical vertical speed snap load was 0.093 m/s for the studied module. The air layer under the porous plates plays an important role in the slamming impact. The maximum delay in slamming caused by the porous plates was 79% at a vertical speed of 0.1 m/s.

Study on Nonlinear Stochastic Process of Deck Slamming on Floating Offshore Platform

In this paper, a semi-analytic method is introduced to predict the deck-slamming probability and corresponding loads. This method is based on a nonlinear statistical approach that takes into account the linear and second-order components of the relative wave elevation up to the second order. The linear and second-order wave elevation is assumed to be a two-term Volterra series. The joint probability density function of the relative wave elevation and velocity are formulated using the Hermite-moment method, and the probability distributions of the wave crest and relative wave velocity are calculated. These probability distributions are verified using the data sampled from the linear and second-order relative wave elevation. Based on this formulation, the probabilities of deck slamming and slamming-induced loads are estimated. This method is applied to a tension leg platform (TLP) model, and the effects of the second-order component of the relative wave elevation on the deck slamming are investigated.

Investigation of hydroelasticity in water entry of flexible wedges with flow detachment

Prediction of structural response during water entry is of great significance for the design of naval architecture and aircraft. In order to study fluid-structural interaction (FSI) of flexible wedge during water entry, coupling algorithms based on boundary element method (BEM)/Wagner theory and modal superposition method (MSM) are utilized. The first solution is a numerical method by coupling BEM and MSM based on domain decomposition technique. The second solution is a semi-analytical method by coupling Wagner theory and MSM. Non-linear pressure composition in the Bernoulli equation is used to improve the accuracy for wedge with moderate deadrise angles. The implicitly Newmark-β scheme is employed for time marching of structural dynamic equation. Effects of hydroelasticity on fluid pressure and structural response are studied by the comparison between coupling and decoupling solutions. The results show that the structural response is significantly affected by the presence of fluid added mass and damping. Structural response is lagged due to the fluid added mass and damping. Besides, the latter one causes the decrease of amplitudes of slamming load and structural response. This paper highlights the effect of hydroelasticity on slamming load and structural response during water entry of flexible structure.

Three-dimensional simulation of ship bow slamming

The bow flare slamming load was studied by using the software Ls-dyna. A coupling finite element model including air, water and 3d bow was established. Flare slamming pressure was picked up from the finite element model in order to discuss the relation between flare slamming pressure and the velocity as well as the distribution rule of slamming pressure in different velocity and different water entry angle along the length and height of the ship.

Experimental and Numerical Study of the Hydroelastic Response of a River-Sea-Going Container Ship

The hydroelastic behaviour of a river-sea-going ship hull is analysed experimentally and numerically. A segmented ship model connected by a steel backbone is tested in regular waves, and its high-frequency vibrations such as springing and whipping responses are identified. The hydroelastic response of the ship is numerically calculated using a hydroelastic time domain method based on strip theory, which is extended to include an improved model of the slamming load. The slamming forces in the bow section are determined using the Modified Longvinovich Model (MLM) instead of the Von Karman model. The vertical motions and wave-induced loads are calculated and compared with the experimental results. The response amplitude operators of the vertical loads and the high-order harmonics are analysed under different speeds, showing good agreement with the experiments. The slamming loads on the bow section of a river-to-sea ship are predicted utilizing the MLM model and compared with the Arbitrary Lagrangian Eulerian algorithm by LS-DYNA and with the measured results.

Slamming load and hydroelastic structural response of bow flare areas of aluminium fast displacement crafts

The paper evaluates the response of an aluminium stiffened panel of a fast displacement craft under bow flare slamming loads by using three finite element methods: (1) a linear static analysis to calculate the structural elastic deformation and stress under the design load; (2) an independent analysis of the slamming load and the elastic structural response (a decoupled method); (3) a method that couples their interaction (a fully coupled method). These methods are compared to analyse the hydroelastic effect when predicting the structural loading and response. Similar results are obtained by the three methods and the linear static analysis based on design load evaluates with accuracy the local structural deformation and stress. The primary objective of the present work is to investigate the strength and safety of bow-flared structures when designing high-speed and fast displacement aluminium crafts subjected to slamming pressure loads. The influence of spray rail on the bow-flared slamming pressures and the effect of boundary conditions on the elastic structural responses are discussed.

Investigation of trimaran slamming under different conditions

In this paper, the slamming pressure and load characteristics of a trimaran cross-section under various conditions were investigated by CFD (Computational Fluid Dynamics) simulations. The computation model was first validated against experimental results. After that, the simulation of different types of motion, i.e. free fall with different mass and entry velocity, entry with constant velocity and harmonic motion with different entry velocities were analysed. It was found that the characteristics of the dynamics during slamming strongly correlates to penetration depth, regardless of the types of entry. Distinguishing features during the wet-deck slam were observed for free fall motion under different conditions, including pressure drop under the main hull during wet-deck slam. The maximum acceleration would start to decrease after the mass has reached a certain value, and the second acceleration peak (corresponding to the wet-deck slam) would be smaller than the first one if the falling height is large enough.

Prediction of slamming pressure considering fluid-structure interaction. Part I: numerical simulations

ABSTRACT Marine structures are subjected to slamming loads characterised by high hydrodynamic pressure within short time durations. Such loads can cause local and global damages to structures. This paper, which is Part I in a series, reports a numerical investigation of slamming loads acting on flat stiffened plates and their dynamic response. The nonlinear explicit finite element code LS-Dyna with the Multi-Material Arbitrary Lagrangian-Eulerian (MMALE) solver was adopted to simulate the slamming impact on marine structures. The numerical method was validated by the relevant experimental data from the open literature in a comparison of slamming pressure and deflection data, as well as existing formulations. The Eulerian formulation was applied to describe the fluid flow, and a Lagrange formulation was employed to model flat stiffened plates. A penalty coupling algorithm was utilised to realise the fluid-structure interaction (FSI) between the plate and the fluid. Bilinear strain hardening with no strain-rate hardening and effect of the heat-affected zone (HAZ) were considered for the numerical simulation of the aluminum model, while nonlinear strain hardening and strain-rate hardening were adopted to verify the steel models. Effects of key influenced parameters such as water impact velocity, material behaviour, and air cushion on the slamming response were addressed accordingly. Additionally, a simulation of water hitting structure was proposed to investigate the slamming load characteristics acting on the bottom decks of offshore structures. As the next paper in this continuing series, Part II will present empirically derived formulations for prediction of slamming loads based on extensive parametric studies of actual scantlings of offshore structures, using the simulation methodology of water hitting proposed herein in Part I.

Hydroelasticity effects on water-structure impacts

Local and global loadings, which may cause the local damage and/or global failure and collapse of offshore structures and ships, are experimentally investigated in this study. The research question is how the elasticity of the structural section affects loading during severe environmental conditions. Two different experiments were undertaken in this study to try to answer this question: (i) vertical slamming impacts of a square flat plate, which represents a plate section of the bottom or bow of a ship structure, onto water surface with zero degree deadrise angle; (ii) wave impacts on a truncated vertical wall in water, where the wall represents a plate section of a hull. The plate and wall are constructed such that they can be either rigid or elastic by virtue of a specially designed spring system. The experiments were carried out in the University of Plymouth’s COAST Laboratory. For the cases considered here, elasticity of the impact plate and/or wall has an effect on the slamming and wave impact loads. Here the slamming impact loads (both pressure and force) were considerably reduced for the elastic plate compared to the rigid one, though only at high impact velocities. The total impact force on the elastic wall was found to reduce for the high aeration, flip-through and slightly breaking wave impacts. However, the impact pressure decreased on the elastic wall only under flip-through wave impact. Due to the elasticity of the plates, the impulse of the first positive phase of pressure and force decreases significantly for the vertical slamming impact tests. This significant effect of hydroelasticity is also found for the total force impulse on the vertical wall under wave impacts.Graphic abstractHydroelasticity effects on water-structure impacts: a impact pressures on dropped plates; b impact forces on dropped plates; c, d, e, f wave impact pressures on the vertical walls; g wave impact forces on the vertical walls; h wave force impulses on the vertical walls: elastic wall 1 vs. rigid wall (filled markers); elastic wall 2 vs. rigid wall (empty markers)

Numerical and experimental investigation on the oblique water entry of cylinder.

The study of water entry cavity and the analysis of load characteristics are hot topics in water entry research. The coupled Euler-Lagrange method is used to carry out simulation research on the water entry process of a cylinder. Aiming at the water vapor mixing phenomenon caused by structure slamming on the water at the initial time of water entry, the slamming load is further studied by correcting the sound speed in the water. The differences between the calculated results obtained by adjusting the number of units in the numerical simulation prove the convergence of the numerical method. Water entry experiments of a cylinder were carried out, and the results are in good agreement with the simulation data. The motion state simulation and analysis are carried out for the process of water entry with different initial speeds and angles. The changes in the structure's positions, air cavities, and slamming loads are obtained. The rule of slamming pressure with the water entry angle and the relationship between pressure and acceleration are determined.

Numerical prediction of asymmetrical ship slamming loads based on a hybrid two-step method

Slamming load is one of fundamental importance in the design of ship structures. However, the prediction of ship slamming load largely focuses on the head sea due to the more complex physics mechanism in oblique waves. In this paper, asymmetrical slamming loads of ship in oblique waves had been numerically predicted, where one hybrid two-step solution was applied through combining seakeeping theory and CFD method. Firstly, asymmetrical ship motions (vertical, horizontal and roll motions) were obtained through the seakeeping analysis. Then, slamming loads of two typical bow-flare sections were predicted through applying the CFD method, where the complicated flow forms (flow separation and air pocket) were included. The characteristics of impact hydrodynamics including free surface, pressure distribution and pressure time history were analyzed systematically showing that the slamming load in oblique wave has obvious asymmetry and is much larger than the head sea. These findings clarify the complicated impact phenomenon in oblique wave cases, which could improve our understanding on the asymmetrical impact of bow-flare sections.

CFD Simulation of Ship Seakeeping Performance and Slamming Loads in Bi-Directional Cross Wave

Accurate prediction of ship seakeeping performance in complex ocean environment is a fundamental requirement for ship design and actual operation in seaways. In this paper, an unsteady Reynolds-averaged Navier–Stokes (RANS) computational fluid dynamics (CFD) solver with overset grid technique was applied to estimate the seakeeping performance of an S175 containership operating in bi-directional cross waves. The cross wave is reproduced by linear superposition of two orthogonal regular waves in a rectangle numerical wave tank. The ship nonlinear motion responses, bow slamming loads, and green water on deck induced by cross wave with different control parameters such as wave length and wave heading angle are systemically analyzed. The results demonstrate that both vertical and transverse motion responses, as well as slamming pressure of ship induced by cross wave, can be quite large, and they are quite different from those in regular wave. Therefore, ship navigational safety when suffering cross waves should be further concerned.

Numerical study on the dynamic response of a truncated ship-hull structure under asymmetrical slamming

Dynamic response of ship-hull structure under slamming has tracked widespread attention in the marine structural design. However, our understanding on the dynamic characteristics largely relies on the symmetrical slamming cases. This paper presented a preliminary numerical investigation on the dynamic response of a truncated ship-hull structure under asymmetrical slamming based on the uncoupled CFD-FE method. Asymmetrical slamming loads were predicted through combining the seakeeping analysis and CFD method. In there, three kinds of motions (vertical, horizontal and roll motions) of 2D ship sections were obtained through the seakeeping analysis and then the slamming pressure was predicted through simulating the water entry with various motions based on CFD method. The dynamic response was analyzed through finite element method. Numerical predictions including ship motions, slamming loads and dynamic analysis were validated against published experimental data and numerical calculations. The characteristics of asymmetrical slamming loads were analyzed showing obvious asymmetry in space, and the dynamic characteristic of the ship bow structure was further clarified through discussing the deformation and stress distribution. These results are useful for readers for better understanding the dynamic characteristics of the bow structure under slamming.