Wetdeck Slamming Research Articles

Experimental and numerical study of wet-deck slamming characteristic for a trimaran in regular head waves

Wet deck may suffer from strong slamming effects when sailing under severe sea conditions, which could damage it locally and compromise the structural integrity of trimarans. The wet-deck slamming characteristic for trimaran sailing in head waves is studied in this work using both experimental and numerical simulation methods. We found that the negative pressure occurred at the front of the wet deck, which is caused by the wet deck exit from the wave crest. Regarding the spatial distribution in longitudinal, the slamming behaviour is more severe at the front part of the wet deck, as the relative vertical impact velocity decreased from the front to the back part of it. Simultaneously, the wet-deck slamming is more severe under the short-wave condition. Regarding the spatial distribution in lateral, we found that the water-entry of the main hull could cause the wet-deck slamming pressure near the outrigger larger than that of the main hull. Moreover, the wet deck has been divided into three regions according to their different lateral distribution characteristics of the wet-deck slamming. These results can provide a better understanding of the wet-deck slamming characteristics when trimaran sailing in waves, which can be considered in future designs for the trimaran.

Experimental study of wet deck slamming for a SWATH in regular waves

Small Waterplane Area Twin Hull (SWATH) ships are subjected to severe sea conditions during ocean voyages, which may cause damage to the wet deck structures due to slamming. The flow process of wet deck slamming in catamarans is complex and involves wave breaking and splashing caused by wave impact on the wet deck. The slamming load under the wet deck of vessels is investigated in this paper by experiment. The test measures the slamming pressure of the wet deck in regular waves, and the flow field during slamming is observed using cameras. The experimental data are analyzed to determine the characteristics of the slamming pressure. The influence of wave height and wave length parameters on the magnitude and distribution of the slamming pressure are examined.

Experimental study on the drop test on wet deck slamming for a SWATH segment model

Wet deck slamming is a serious issue for high-speed catamarans if the relative vertical motions are higher. In this study, a drop test on wet deck slamming is conducted for a small waterplane area twin hull segment model, and the vertical water-entry model test is performed under different drop heights and speeds. The slamming pressure under the wet deck and the acceleration are measured. The flow field is obtained through particle image velocimetry measurement. A high-speed camera observation during wet deck slamming shows that air is pressed into water, and a water–gas mixture is formed due to the air compressibility effect. The slamming pressure shows an oscillating property with multiple frequency components, which is due to the air cushion effect caused by the volume variation of the water–gas mixture and the acoustic wave oscillation with the air pocket. The effects of the drop height variation and the constant-drop speed on the slamming load are analyzed. The results show that the pressure peak appears during the demihull's bottom slamming and wet deck slamming. The acceleration time record produces two peaks, one for the demihull's bottom slamming and another for the wet deck slamming. In conclusion, the air cushion effect can reduce the wet deck slamming pressure.

The numerical investigation on the effects of support conditions and flanges during the water impact of a thin elastic plate

The water impact of plates can represent a broad of slamming problems in marine and offshore engineering, such as the wet-deck slamming of a multi-hull vessel, the slamming of the coastal structures like breakwaters, and slamming induced by the sloshing of the LNG ships. This paper has studied the water impact of an elastic plate with/without flanges. The main focus is on the influences of the support conditions and the flanges.For elastic structures, especially thin-walled structures, different support conditions lead to different structural responses. Therefore, it is necessary to investigate the effects of support conditions. The results showed that the stronger the constraint, the higher the vibration frequency of the structure, the greater the force. But the deformation is much smaller.On the other hand, we simulate the water impact of the flat plates with flanges, which is quite different from previous studies in which a rigid plate is considered only. The results of wavelet analysis show that the flanges can reduce the high-frequency components of structure vibration. The amplitudes of the high-frequency components decay over time sharply. Eventually, the responses with a frequency between 40 and 60 Hz remain and are less affected by the flanges.The FSI simulation is carried out with ANSYS commercial software. VOF model is used in the fluid solver, finite element method is used in structure solver, and the partitioned method is used to couple the solvers. Compared with other methods, there is no simplification in the partitioned approach. Therefore, it can reflect the whole slamming process more accurately.

Using Remote Monitoring and Machine Learning to Classify Slam Events of Wave Piercing Catamarans

An onboard monitoring system can measure features such as stress cycles counts and provide warnings due to slamming. Considering current technology trends there is the opportunity of incorporating machine learning methods into monitoring systems. A hull monitoring system has been developed and installed on a 111 m wave piercing catamaran (Hull 091) to remotely monitor the ship kinematics and hull structural responses. Parallel to that, an existing dataset of a similar vessel (Hull 061) was analysed using unsupervised and supervised learning models; these were found to be beneficial for the classification of bow entry events according to key kinematic parameters. A comparison of different algorithms including linear support vector machines, naïve Bayes and decision tree for the bow entry classification were conducted. In addition, using empirical probability distributions, the likelihood of wet-deck slamming was estimated given a vertical bow acceleration threshold of 1 in head seas, clustering the feature space with the approximate probabilities of 0.001, 0.030 and 0.25.

Experimental free-drop test investigation into wet-deck slamming loads on a generic trimaran section considering the influence of main hull profile

The characteristics of wet-deck slamming of a generic trimaran section were investigated through a series of free-drop model tests in this paper. Two models were designed with different main hull profiles. One was the original model which was designed according to a generic trimaran section. The other one was a modified model with a weakened main hull to reduce the disturbance to the flow field during main hull water entry. The displacement, acceleration, slamming pressures were measured directly, and the free surface dynamics were recorded with a high-speed camera. It is found that the main hull profile has a critical influence on the process and mechanism of wet-deck slamming through the comparison between experimental results of these two models. With regard to the original model, the impact on the bottom of the main hull occurred firstly and resulted in violent disturbance of the flow field before wet-deck slamming. While no obvious impact phenomenon occurred on the main hull of modified model. Furthermore, the main hull profile also has an important influence on the magnitude and spatial distribution of wet-deck slamming loads.

Vertical Wedge Drop Experiments as a Model for Slamming

_ A deeper comprehension of hydrodynamic slamming can be achieved by revisiting the wedge water entry problem using flexible structures. In this work, two wedge models that are identical, with the exception of different bottom thicknesses, are vertically dropped into calm water. Pressure, full-field out-of-plane deflection, strain, vertical acceleration, and vertical position are measured. Full-field deflections and strains are measured using stereoscopic-digital image correlation (S-DIC) and strain gauges. A nondimensional number, R, quantifying the relative stiffness of the structure with respect to the fluid load is revisited. An experimental parametric study on the effect of R on the nondimensional hydrodynamic pressure and the maximum strain is presented. It was found there is a sharp change in the trend of pressure and strain when R passes through a critical value. It was also discovered that the structural deformation causes a delay in the peak pressure arrival time and a reduction in the peak pressure magnitude during the wedge water entry. Introduction When high-speed planing craft operating in waves becomes airborne and reenters the water surface, a substantial impact or “slam” between the vessel bottom and the water surface will occur (Faltinsen 2005; Lloyd 1989). The bottom slamming events occur frequently and may injure the passengers, compromise the equipment onboard, or even damage the structure. Slamming is a major cause of speed reduction in small craft where slamming loads are important. Current design criteria are primarily based on empirical measurements with little regard for the fluid–structure interaction (FSI) physics of the slamming phenomenon. This study offers a first step toward better understanding of FSI in slamming for optimal structural design in the future. Since the cross sections of most surface effect ships may be approximated by a V-shaped wedge, the slamming characteristics of these sections may be examined by dropping a wedge model into water (Faltinsen 2005; Lloyd 1989). Studying the wedge water entry problem is also helpful in shedding light on the wet deck slamming of catamaran, sloshing under the chamfered roof of a partially filled tank (Faltinsen 2000), seaplane landing (Wagner 1932), water landing of spacecraft and solid rocket boosters, water landing/ditching of aircraft (Abrate 2013), and animal diving behavior (Chang et al. 2016).

Characteristics of Slamming Pressure and Force for Trimaran Hull

In this paper, the characteristics of the impact pressure and force of a trimaran section was studied by Computational Fluid Dynamics (CFD). The time domain features of the slamming pressure or force showed a strong correlation with the penetration depth regardless of the specific ways of water entry. The effects of velocity and acceleration on the impact pressure and force were analyzed. It was found that the initial impact of the main hull and the wet-deck slamming were predominantly affected by the entry velocities, whilst the acceleration had almost no effect for initial impact. The impact velocity presented a quadratic relation with slamming pressure/forces, and the relation between acceleration and wet-deck slamming pressure/force was linear. These were consistent with the patterns implied by analytical models such as the Wagner or MLM (Modified Logvinovich model) theories.

Hydroelastic Modeling of Wet Deck Slamming on Multihull Vessels

Slamming against the wet deck of a multihull vessel in head sea waves is studied analytically and numerically. The theoretical slamming model is a two-dimensional, asymptotic method valid for small local angles between the undisturbed water surface and the wet deck in the impact region. The disturbance of the water surface as well as the local hydroelastic effects in the slamming area are accounted for. The elastic deflections of the wet deck are expressed in terms of "dry" normal modes. The structural formu­lation accounts for the shear deformations and the rotatory inertia effects in the wet deck. The findings show that the slamming loads on the wet deck and the resulting elastic stresses in the wet deck are strongly influenced by the elasticity of the wet deck structure.

CLASSIFYING BOW ENTRY EVENTS OF WAVE PIERCING CATAMARANS IN RANDOM WAVES USING UNSUPERVISED AND SUPERVISED TECHNIQUES

An onboard monitoring system can measure features such as stress cycles counts and provide warnings due to slamming. Considering current technology trends there is the opportunity of incorporating machine learning methods into monitoring systems. A hull monitoring system has been developed and installed on a 111 m wave piercing catamaran (Hull 091) to remotely monitor the ship kinematics and hull structural responses. Parallel to that, an existing dataset of a geometrically similar vessel (Hull 061) was analysed using unsupervised and supervised learning models; these were found to be beneficial for the classification of bow entry events according to the kinematic parameters. A comparison of different algorithms including linear support vector machines, naïve Bayes and decision tree for the bow entry classification were conducted. In addition, using empirical probability distributions, the likelihood of wet-deck slamming was estimated given vertical bow acceleration thresholds.

The influence of the centre bow and wet-deck geometry on motions of wave-piercing catamarans

The effects of tunnel height and centre bow length on the motions of a 112-m wave-piercer catamaran with an above-water centre bow were investigated through model tests. Five alternative centre bow configurations were considered, and multiple series of model tests were conducted in regular head sea waves. The results showed that both heave and pitch increased over a wide range of wave encounter frequency as the wet-deck height of the catamaran model increased. However, increasing the length of the centre bow showed an increase in the pitch but a decrease in the heave for a limited range of wave encounter frequency near the heave and pitch resonance frequencies of the catamaran model. The positions of minimum vertical displacement were found to be aft of the longitudinal centre of gravity, between 20% and 38% of the overall length from the transom. Increase in the wet-deck height and consequently the archway clearance between the main hulls and centre bow also resulted in an increase in the vertical displacement relative to the undisturbed water surface in the centre bow area. The results also indicated the vulnerability to wet-deck slamming for the different bow and wet-deck designs.

Wetdeck slamming loads on a developed catamaran hullform – experimental investigation

ABSTRACTCatamaran wetdeck slamming has been experimentally investigated using a servo hydraulic slam testing system. A series of controlled-speed water impacts was undertaken on a rigid catamaran bow section with two interchangeable centrebows. Entry into the body of water was at two fixed trim angles: 0° and 5°. The vertical velocity was varied from 3 to 5 m/s in 0.5 m/s increments. This study presents a new dataset of pressure distributions and slam forces on the arched wetdeck structure of catamaran vessels. The relationships between the peak force magnitudes, relative impact angle and vertical velocity are observed, with a small reduction in slam force for an amended centrebow. Limited pressure measurements along the archway were not found to be representative of wetdeck slamming loads.

Cross-Deck Slamming Load Calculation of Trimaran Based on Modified Plate Slamming Theory

Since slamming loads on connecting bridges of trimaran is very important for the structural design. In this paper, trimaran slamming load calculation method is studied. First added mass caused by typical hull section and wet-deck slamming are investigated, then wet-deck slamming peak pressure are gotten by modified flat slamming theory with considered of the effect of air cushion, the regression equation of which is given by statistical process. Finally, the time-spatial distribution of slamming pressure along the wet-deck is given. This method can provide a reference for the preliminary design and operation of trimaran.

Experimental study on water entry of model catamaran sections

Marine vehicles may experience significant slamming loads due to water entry of hull sections in some unsteady regimes. This study focuses on a basic configuration consisting of two V-hulls connected with a flat-bottom platform. Drop tests on water of two-dimensional catamaran model sections with variable platform position and entry velocity have been carried out. Time-dependent acceleration signals and high-speed video records of the body falling into water were obtained. The acceleration peaks were identified and related to the chine wetting, spray hitting the platform bottom, and wetdeck slamming. The obtained results can be applied for validating mathematical models of water impacts and designing catamaran hulls.

CFD simulations of wave–current-body interactions including greenwater and wet deck slamming

A numerical method for the solution of unsteady Navier–Stokes equations has been employed in conjunction with an interface-preserving level-set method for the simulation of greenwater effect on offshore structures and ships. In this method, the free surface flows are modeled as immiscible air–water two-phase flows and the free surface itself is represented by the zero level-set function. The Navier–Stokes equations for both the water and air flows are formulated in moving curvilinear coordinate system and discretized using the finite-analytic method on a non-staggered multi-block grid system. Large eddy simulation (LES) approach is used with Smagorinsky model to account for the effects of turbulence induced by violent free surface motions. A chimera domain decomposition approach is implemented using overlapping, embedding, or matching grids to facilitate the simulation of complex flow around practical configurations. The overset grid system also greatly simplified the simulation of arbitrary translational and rotational motions among various computational blocks. Calculations were performed first for dam-breaking flow and free jet problems involving violent free surface motions. The level-set Navier–Stokes method was then employed for the simulation of slamming of a hemisphere, greenwater on offshore structure and ships, and wet deck slamming of an X-Craft in pitch and heave motions. The numerical results clearly demonstrated the capability of the level-set method to deal with violent free surface flows involving breaking waves, water droplets, trapped air bubbles, and wave–current-body interactions.

Computation of wet deck bow slam loads for catamaran arched cross sections

A computational model for catamaran wet deck slamming is developed on the basis of the variation of added mass as the hulls enter the water. In the case of a wave-piercing catamaran the bow cross section has a double arch cross section and slamming occurs when the arches fill. Residual air is entrained at the top of the arch due to bubble formation by turbulent mixing and this modifies the effect of the water added mass on the hull. The computational model therefore introduces a soft connection between the water added mass associated with the slam and the hull. The method has been evaluated by comparison with two-dimensional model drop tests in terms of the maximum forces and acceleration imposed on the hull, the variation of velocity during the slam event and the depth of penetration into the water. It is concluded that the added mass computation is adequate for slam modelling in global motions and loads calculations since it gives a good representation of the maximum total forces on the section and their duration.

Global Hydroelastic Response of Catamarans Due to Wetdeck Slamming

Global hydroelastic effects due to wetdeck slamming on a high-speed catamaran with forward speed in regular head sea waves are studied experimentally and theoretically. The elastic response is dominated by a two-node bending mode. Predictions of vertical shear forces, bending moments, and slamming force are in fair agreement. Experimental and theoretical error sources are discussed. An important uncertainty is associated with improper measurements of trim angles. The hydrodynamic model of the side hulls needs to be improved by considering hydrodynamic hull interaction.

Slamming in marine applications

Practical slamming problems for ships and ocean structures are briefly described. Theoretical status and future challenges for water entry on an initially calm free surface, wetdeck slamming, green water and sloshing are presented. It is emphasized that slamming should be considered in the framework of structural dynamics response and integrated with the global flow analysis around a ship or ocean structure or with violent fluid motion inside a tank. Two-phase flow can give important loading and needs to be better understood. Slamming on a VLFS with shallow draft is dealt with in detail.

Wave impact on a horizontal elastic plate

The objective is to find slamming-induced local stresses in the steel or aluminum wetdeck of a multihull vessel. This is studied theoretically by representing the wetdeck as a beam model and accounting for dynamic hydroelastic effects. Two numerical methods are used, one being a simplified asymptotic solution. Satisfactory agreement between the two methods is reported. Experimental drop tests of horizontal elastic plates of steel and aluminum are also reported, and the results from the experiments agree well with the numerical computations. This study reveals, both numerically and experimentally, that slamming-induced local stresses are strongly influenced by dynamic hydroelastic effects. The maximum bending stresses are insensitive to where waves hit, the curvature of the waves, and maximum pressures. Measured maximum pressures are very sensitive to external conditions, and cannot be used as a measure of maximum local bending stresses. A simple procedure for local design stresses due to wetdeck slamming is outlined.

The effect of hydroelasticity on ship slamming

Wetdeck slamming is studied theoretically by a hydroelastic beam model. The problem is simplified by introducing an initial structural inertia phase and a subsequent free vibration phase. Forward speed effects are included. The theoretical model is validated by comparing with drop tests of elastic plates on waves. The stresses in the plates have a linear dependence on the impact speed and are neither sensitive to the radius of curvature of the waves nor where the waves initially hit. Hydroelasticity is important. The maximum impact pressures can be extremely high and have a stochastic nature even under deterministic environmental conditions, but they are not important for maximum bending stresses.