Application of 2D hydroelasticity theory to investigate the failure of a containership

This paper focuses on the investigation carried out on the failure of the MSC Napoli using two-dimensional (2D) symmetric (i.e. vertical bending) hydroelasticity analysis. The aim of the investigation was to assess the influence of whipping-induced loads on the structural strength of this containership. Relevant structural, hydrostatic and operational data were provided. The calculations were carried out in head regular and long-crested irregular waves. Both cases included the effect of bottom slamming only. Global wave-induced loads were evaluated along the hull, focusing in particular in the vicinity of the engine room. The investigation showed that whipping, due to bottom slamming, is only important for severe seas. The investigation also showed that the keel stresses, in way of the engine room, can be as large as the keel stresses at amidships

A hybrid stabilization technique for simulating water wave - Structure interaction by incompressible Smoothed Particle Hydrodynamics (ISPH) method

The Smoothed Particle Hydrodynamics (SPH) method is emerging as a potential tool for studying water wave related problems, especially for violent free surface flow and large deformation problems. The incompressible SPH (ISPH) computations have been found not to be able to maintain the stability in certain situations and there exist some spurious oscillations in the pressure time history, which is similar to the weakly compressible SPH (WCSPH). One main cause of this problem is related to the non-uniform and clustered distribution of the moving particles. In order to improve the model performance, the paper proposed an efficient hybrid numerical technique aiming to correct the ill particle distributions. The correction approach is realized through the combination of particle shifting and pressure gradient improvement. The advantages of the proposed hybrid technique in improving ISPH calculations are demonstrated through several applications that include solitary wave impact on a slope or overtopping a seawall, and regular wave slamming on the subface of open-piled structure

Numerical modelling of aeration and hydroelasticity in slamming loads and responses of marine structures

Slamming plays a significant role in the ultimate and fatigue limit state design of marine structures. Despite a relatively long history of investigations, there are still gaps in knowledge and open questions in understanding the slamming phenomenon and the approach it needs in the design phase due to its complex nature and limitations of research tools. The so-called hydroelasticity effect, which is the coupled interaction of structural responses with the body of fluid on both global and local scales, is one of the main complex aspects of slamming. Variation of fluid compressibility due to the mixing of air bubbles with the fluid, called aeration, alters the slamming loads and could also affect the hydroelastic coupling. The possible interaction of the two mentioned processes affecting the slamming physics and how to approach it in the analysis of slamming is still not well understood and is the focus of investigation in this thesis. The research methodology of this work is based on studying the details of pressure and the flow field around the slamming area and the evolution of slamming force and structural response employing numerical modeling. A numerical tool for this purpose was developed and validated against benchmark experimental data available in the literature. In the study of hydroelasticity, local shell deformations, as well as global deformations of the structure, were studied. The interlinked effects of local hydroelasticity and aeration were investigated by performing two sets of numerical simulation campaigns on the water entry of elastic flat plates and cylindrical shell sections. Both studies revealed that local flexibility has a noticeable reducing effect on peak values of slamming pressure and forces. This reduction effect of flexibility disappears for plates in the presence of aeration, which shows a significant indication of interdependence in the roles of aeration and hydroelasticity in slamming dynamics. In plate entry and cylinder entry simulations, aeration shows a damping effect on the response strain oscillations, strengthening with increasing aeration. Both water entry studies present new insights with valuable details on slamming load's major characteristics and local structural response of plates and cylinders. Noticeable differences between plate and cylinder entries were observed; for instance, aeration causes a substantial extension of slamming load duration in plate entries, but no meaningful change is observed in cylinder entries. Extensive parameter studies led to new functional relations to determine peak slamming pressure/force in pure and aerated water entries in terms of relatively simple power-law approximations, which have been derived for plates and cylinders. The study shows that hydroelasticity may not be an essential issue for locally stiff structures, but considering air entrainment and entrapment processes is important to determine local loading characteristics. This thesis also presents a novel simplified model of wave slamming on an SDOF cylindrical structure. The model could reproduce the experimental slamming force and pressure time series of the large-scale wave slamming on a vertical monopile with a reasonable accuracy level. The validation study shows that the introduced simplified model could present valuable data on the physics of the interaction of a flexible cylindrical structure with impacting body of water. The simplified model was applied in a parameter study to investigate the effect of global structural vibration characteristics and aeration on wave slamming loads and structural response characteristics. The parameter study indicates that processes related to compressibility, such as aeration and air entrapment, are far more important than the structure's global flexibility. Since wave impact events in natural conditions may incorporate variable aeration levels in the water, which is shown to alter the structural response and duration of vibration, in both deterministic and stochastic studies of wave impact dynamics, the compressibility parameter is important and should be considered in the analysis

An experimental investigation into the constant velocity water entry of wedge-shaped sections

Constant velocity water entry is important in understanding planing and slamming of marine vessels. A test rig has been developed that drives a wedge section with end plates down guides to enter the water vertically at near constant velocity. Entry force and velocity are measured. Analysis of the test data shows that the wetting factor is about 1.6 at low deadrise angles and reduces nearly linearly to 1.3 at 451 deadrise angle. The added mass increases quadratically with immersed depth until the chines become wetted. It then continues to increase at a reducing rate, reaching a maximum value between 20% and 80% greater than at chine immersion. The flow momentum drag coefficient is estimated from the results to be 0.78 at 51 deadrise angle reducing to 0.41 at 451 deadrise angles. Constant velocity exit tests show that the momentum of the added mass is expended in driving the water above the surface level and that exit forces are low and equivalent to a drag coefficient of about 1.0-1.3. Considerable dynamic noise limits the accuracy of the results, particularly after chine immersion

On the three-dimensional effects of the water entry of wedges

Three-dimensional (3D) effects on the total and spanwise slamming coefficients and the pressure reductions acting on the wedge surface during water entry are modeled in a unified manner. First, the water entry of wedges with a constant speed is numerically studied using the finite volume method (FVM) combined with the volume of fluid (VOF) method. The wedge is assumed to enter water with high speeds such that the compressibility, viscosity, gravity and surface tension effects of the fluid can be neglected. The numerical method is validated against an experimental measurement of a freefall water entry of a wedge with a deadrise angle of 30°. Then, the water entry of 3D wedges with deadrise angles of 30, 35, 40 and 45° and beam-span ratios between the width and length varying from 0 to 1 are simulated. The total and spanwise slamming coefficients and the reduction of the pressure distribution are analyzed. The total slamming coefficient is derived with a 3D effect coefficient using a dimensional analysis in the slamming stage and its expression for the transition stage is proposed based on a two-dimensional (2D) transition stage model of the water entry. The spanwise slamming coefficients on the different spanwise sections are modeled with the pressure coefficient distribution of a supercavitating flow around a 2D flat plate. The reductions of the pressure acting on the wedge surface are found to be approximately constant along the wetted length for all spanwise sections, which can be derived using the expressions of the total and spanwise slamming coefficients. Finally, using the proposed model, the predictions of the abovementioned variables are in good agreement with the numerical simulation results in the slamming and transition stages during the water entry of the wedges with different deadrise angles and beam-span ratios.publishedVersio

Development Of A Semi-Swath Craft For Malaysian Waters

Small Waterplane Area Twin Hull (SWATH) and Catamaran vessels are known to have more stable platform as compared to mono-hulls. A further advantage of SWATH as compared to Catamaran is its smaller waterplane area that provides better seakeeping qualities. However, the significant drawback of the SWATH vessel is when encountering head-sea at high forward speed. Due to its low stiffness, it has a tendency for large pitch motions. Consequently, this may lead to excessive trim or even deck wetness. This phenomenon will not only degrade the comfortability but also results in structural damage with greater safety risks. In this research a modified SWATH design is proposed. The proposed design concept represents a combination of Catamaran and SWATH vessel hull features that will lead to reduce in bow-diving but still maintains good seakeeping capabilities. This is then called the Semi- SWATH vessel. In addition, the full-design of this vessel has been equipped by fixed fore fins and controllable aft fins attached on each lower hull. In the development of controllable aft fins, the PID controller system was applied to obtain an optimal vessel’s ride performance at speeds of 15 (medium) and 20 (high) knots. In this research work, the seakeeping performance of Semi-SWATH vessel was evaluated using time-domain simulation approach. The effect of fin stabilizer on the bare hull performance is considered. The validity of numerical evaluation was then compared with model experiments carried out in the Towing Tank at Marine Technology Laboratory, UTM. It is shown that the Semi-SWATH vessel with controllable fin stabilizer can have significantly reduction by about 42.57% of heave motion and 48.80% of pitch motion

Comparison of experimental and numerical sloshing loads in partially filled tanks

Sloshing phenomenon consists in the movement of liquids inside partially filled tanks, whichgenerates dynamic loads on the tank structure. Resulting impact pressures are of great importance in assessingstructural strength, and their correct evaluation still represents a challenge for the designer due to the highnonlinearities involved, with complex free surface deformations, violent impact phenomena and influence of airtrapping. In the present paper a set of two-dimensional cases for which experimental results are available areconsidered to assess merits and shortcomings of different numerical methods for sloshing evaluation, namely twocommercial RANS solvers (FLOW-3D and LS-DYNA), and two own developed methods (Smoothed ParticleHydrodynamics and RANS). Impact pressures at different critical locations and global moment induced by watermotion for a partially filled tank with rectangular section having a rolling motion have been evaluated and resultsare compared with experiments

A Solution to the Galactic Foreground Problem for LISA

Low frequency gravitational wave detectors, such as the Laser Interferometer Space Antenna (LISA), will have to contend with large foregrounds produced by millions of compact galactic binaries in our galaxy. While these galactic signals are interesting in their own right, the unresolved component can obscure other sources. The science yield for the LISA mission can be improved if the brighter and more isolated foreground sources can be identified and regressed from the data. Since the signals overlap with one another we are faced with a ``cocktail party'' problem of picking out individual conversations in a crowded room. Here we present and implement an end-to-end solution to the galactic foreground problem that is able to resolve tens of thousands of sources from across the LISA band. Our algorithm employs a variant of the Markov Chain Monte Carlo (MCMC) method, which we call the Blocked Annealed Metropolis-Hastings (BAM) algorithm. Following a description of the algorithm and its implementation, we give several examples ranging from searches for a single source to searches for hundreds of overlapping sources. Our examples include data sets from the first round of Mock LISA Data Challenges.Comment: 19 pages, 27 figure