In order to establish an experimental method to evaluate resistance and running attitudes of planing craft using a very small-scale model, the scale effects on hydrodynamic forces (drag, lift and trim moment) acting on two model ships, a 0.45m model and a 0.9m model, are experimentally investigated by fully captured towing tests. The results demonstrate that the lift and the moment acting on the smaller model are smaller than those on the larger model. The predicted results of running attitudes using the modified hydrodynamic forces obtained for the smaller model, in which the scale effects on the lift and the moment are taken in to account, are in fairly good agreement with the measured running attitudes of the larger model. The residual resistances predicted for the two models based on Froude's hypothesis using Blasius and Plandtl-Schlichting formulae in the prediction of frictional resistance are in fairly good agreement with each other.

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Unsteady viscous flow field around a fish-like advancing NACA wing is numerically studied. The main objective is to study the abilities of a flexible fish-like body to produce thrust, achieve higher propulsive efficiency as well as to catch the basic flow characteristics in viscous flow field. The Reynolds numbers based on the oncoming velocity and the body length range from 10⁶ to 3×10⁶. The flow is simulated by solving two-dimensional unsteady Reynolds averaged Navier-Stokes equation in a primitive value formulation, while the turbulence transportation is described by updated algebraic eddy-viscosity model and transitional zone is computed by empirical formula. The numerical scheme is based on the MAC method with a body fitted coordinate system. The simulation shows that the high propulsive efficiency of fish-like locomotion comes from relaminarization and effective vorticity control. Relaminarization reduces the frictional force at higher Reynolds number. Fish-like locomotion can recapture energy contained in the eddies by skillfully controlling vorticity generation, travel and shed through the body undulation.

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Propulsive performances of fish-like motion have been investigated using waving wing model. It has been said that the fish-like motion achieves high propulsive performances compared with the artificial propulsion systems such as motor and propeller. Present paper describes the analyzed results of hydrodynamic-force and flow-field measurement using waving wing model. The waving wing model was designed and made so as to analyze the basic characteristics of fish-like propulsion in two-dimensional case. The authors have proposed the definition of dynamic drag coefficient, which enables to know the resistance characteristics of waving-motion object. The basic data was collected in the simple cases, and that has been used to estimate the thrust forces under self-propulsive condition. The results of the analysis of hydrodynamic force have also been considered in connection with the flow field measured by particle image velocimetry (PIV) system.

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The fully-nonlinekr multi-directional waves are reproduced using by a 3D viscous Numerical Wave Tank (NWT) simulation technique. The governing equations, Navier-Stokes (N-S) and continuity equations, are discretized by a finite-difference method in the framework of a rectangular coordinate system, and the boundary values are updated at each time step in a time-marching procedure. The fully-nonlinear kinematic free-surface condition is satisfied by the marker-density function technique. The directed incident waves are generated by multi-segmented wavemaker on the basis of the so-called “snake-principle”, and the outgoing waves are numerically dissipated inside an artificial damping zones located at the opposite side of wavemaker.
In this study, we present results for the generation of regular waves including numerical convergence tests, irregular waves, and multi-directional random waves including the Bull's eye wave.

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The project to create the fishing ground in the open ocean by rising Deep Ocean Water (DOW) has been carried out since April 2000, sponsored by Japanese Government Fisheries Agency and Marino-Forum 21. DOW is discharged into the euphotic layer as a density current where photosynthesis can be done by DOW's rich nutrient salt. The name of this machine is called Ocean Nutrient Enhancer (ONE for short).
The outline concept of the ONE is ;
“Rising DOW of 500 m depth and 500, 000 m³/day with the special impeller, which also sucks the surface water in order to make a mixed water of proper density.”
“Discharging the nutrient rich mixed water continuously and making density current at the euphotic region.”
“Increasing the phytoplankton (primary production) on the layer of the density current.”
“Increasing fishes in the above sea area.”
“Power for driving the impeller is supplied by OTEC (Ocean Thermal Energy Conversion) using a cold characteristic of DOW.”
To confirm the characteristics of the density current in Sagami Bay where the real sea experiment is scheduled, the scale model experiment of 1/200 in the continuously stratified salty water tank was carried out. And the suitable discharge velocity to maintain the rich nutrient in the density current layer was researched in the way of 2 and 3 Dimensional flow experiments.
In conclusion, the thickness of the layer of density current in Sagami Bay in Summer season was estimated about 12 m, and the thinning ratio of DOW in case of using 3 Dimensional ring nozzle is from 3 to 4 times, according to the analysis of a low of similarity. These results are very valuable for designing the ONE that is scheduled to be set-up in Sagami Bay in the year 2003.

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An optical measurement device of microbubble void fraction was developed especially aiming at the full-scale test. The strut with a side window was installed on the hull surface of M.S. Seiun-maru.
A CCD camera in the strut took pictures of microbubbles through the side window under a stroboscopic light. The image of microbubbles was processed by a PC using a picture processing software.
The void fraction distribution has a peak at the distance of 5-10mm from the hull surface. The void fraction also changed much according to the ship speed. Such the data is invaluable for the research on the turbulent frictional drag reduction of a ship by microbubbles.

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In the ocean engineering field, the information of ocean environment is important for planning, design and operation. In those information, the wave information is very important. High precision wave data is also important for considering environmental problem like warming of earth. In this context, we already developed a new wave field detecting system using marine radar as wave probe. The most important point is that this system detect wave field directly, instead of estimating that by measuring the point like wave buoy.

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A wave observation buoy has been developed, which uses a system where incident wave height in the ocean can be obtained from the heave motion of the buoy measured using a gyro system and the heave response function determined through theoretical calculations and model experiments.
The accuracy of the theoretical calculation of the response function of the buoy motion was first confirmed by model experiments in a wave tank. Next, the validity of the method was verified by comparing the time series of irregular wave profiles measured with a wave height meter fixed in the wave tank and a reproduced series using the heave motion of the buoy model measured with a motion sensor and the response function.
A real buoy was designed and constructed based on these studies and has been moored in the sea near Ishigaki Island since March of 2001.
Finally, the wave data obtained using the above method were compared with those directly obtained from motions measured with a gyro and the relative wave height measured with a wave sensor fixed to the buoy, and the results were discussed.

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When regular waves are incident to a long array of equally-spaced cylindrical columns, very large water-surface elevations among the columns and very large horizontal forces on the columns are expected in a certain critical wave-frequency range according to the linear potential theory. This is a kind of resonant phenomenon and closely related to the 'trapped waves' that may occur around a vertical cylinder placed in a narrow wave tank. Experimental facts suggest, however, actual water-surface elevations or horizontal forces are much smaller than the theoretical predictions. It has been found that the apparent contradictions between the linear potential theory and the experimental results can be attributed to the small energy dissipations induced in the viscous boundary layers that develop along the column surfaces.
In fact, inclusion of tiny energy dissipations in the theory result in drastic change of the final results in the critical wave-frequency range, which agree quite well with the experimental results. This fact may have very important practical implications for the design of column-supported VLFSs (very large floating structures) in that the dynamic responses of the VLFSs could be quite different from those predicted by the linear potential theory, which is the subject of the present work.

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Ships disabled from marine disasters are generally towed by tug-boat. Issues concerning tug-towed ship systems are factors such as the length of the towing rope, the location of towing points and the conditions of the disabled ships. Up to the present time, these important factors have been entrusted to the knowledge and the experience of towing operators, and under their supervision, many accidents may occur because there isn't an effective method to tow disabled ships, towing operators are forced to use their own method, creating inconsistency and errors in methodology.
In view of the development of the optimum towing method, we estimated the course stability of the tug-towed ship systems including various towing conditions. Furthermore, we examined the manoeuvring performance of the tug and the towed ship systems by using the numerical towing simulations considering wind and wave effects. The towing simulations systematically need to be done which take into account various towing conditions, and which will ultimately lead to guidelines for the safe towing operations.

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An overage tanker “ERIKA” was wrecked in 1999 at rough sea off the coast of Brest in France. In consequence of the marine disaster, a large quantity of heavy oil spilled into the sea from the wrecked tanker and all over the coast of France was heavily polluted. In such a case, she should be towed immediately to a safe place to avoid a secondary calamity and minimize the marine pollution. We had a similar marine disaster in the case of Russian tanker “NAKHODKA” which was wrecked in 1997 at the Japan Sea. However, there has been no study of methods for stable towing or of maneuvering motion during the tow of wrecked tanker's fore part. Whenever we encountered such a marine disaster, we have been relying upon our experience and intuition in the towage of wrecked tanker.
In this paper, unstable motion and course keeping ability of tow and towed ships at rough sea have been examined as a first step by using measured hydrodynamic forces acting on a tanker model of which fore part is wrecked.

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In this study, large amplitude rolling motion of a ship without bilge keels in heavy regular beam seas is experimentally investigated, and a new non-linear feature of lateral ship motions in high waves is revealed.
The experiments show that, in high beam waves, large amplitude rolling motion occurs not only in the frequency region of the roll resonance but also in the frequency region higher than the resonance frequency, and that jump phenomena of roll amplitudes appears near the heave natural frequency. These behaviors obtained by the experiments are investigated, and the reasons of these non-linear features are clarified as follows; the former is caused by changing encounter frequencies by large drifting velocity in high waves to make the roll-resonance region wide. The latter is caused by different drifting velocities for the same regular wave near at the heave natural frequency.
A non-linear feature of heaving motion is also experimentally confirmed near heave natural frequency.

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It is well known that VIV (Vortex-Induced Vibration) occurs in a riser. Effects of the top tension and the surface roughness of a riser have not been clear for a behavior of a riser. In this study, the model test in which geometrical similarity models are used is carried out in order to understand the behavior of a riser including VIV phenomenon. The model risers correspond to the production riser of 120m in a real scale, and the corresponding model tests are carried out in scales of 1/50. The behaviors in uniform flows, waves and under the forced oscillation are measured three dimensionally by the real time graphic data processing system. This model test focuses on the effect of top tension and the surface roughness.
The present paper shows the vertical distributions of a maximum horizontal displacement and the time histories at arbitral point of the corresponding riser model. The results show that the behavior characteristics of the risers differ against the top tension. Then VIV is remarkably influenced by the top tension. In addition, the maximum horizontal displacement is different about 20% due to the surface roughness.

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Model test of the Near Surface Disconnecable Drilling Riser (NSDDR) which has an air can system near the top for making free-standing condition was conducted by use of a riser model corresponding to 750m length in fullscale. Purpose of the test is to grasp the characteristics of NSDDR in irregular waves with constant current. In the test, we observed remarkably riser's horizontal motions due to vortex induced vibration (VIV), and found that the bending moment variation induced by VIV is the same order as the variation induced to the flow direction of the riser. In the drilling mode, bending moment variation of NSDDR is partially about 25% larger than that of the conventional riser without air can. Bending moment variation in free-standing mode is about 20% of that in hang-off mode at the same significant wave height except the riser bottom. At the riser bottom, both static and significant values of the bending moment become largest, so we should pay attention to the strength.

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The response of the thrusters installed in “SHINKAI 6500” at present is very slow and those thrusters are individually manipulated manually because reduction of radiation noise and course keeping stability were considered most important when those thrusters are designed, so that it is not possible to operate the complicated motion.
So, this study was carried out in order to develop the maneuvering system that enables advanced vehicle movements using rapid reaction thrusters and Doppler sonar.
In this paper, the result of real sea test at which SHINKAI6500 was equipped with the test thrusters experimentally is described. It was proved that the test thruster has enough power and rapidity of the response and that it enables the ship to do the stationkeeping in surge direction at the very high accuracy by automatic control using optimal control and Ossman's adaptive control algorithm.

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In the previous papers, it is shown that not only the position of a bottom transponder but also the underwater acoustic velocity distribution can be determined, if the position of surface transponders and ranges between the surface and bottom transponders are known. Also, a theoretical possibility was shown that the position of the bottom transponder can be obtained in the order of centimeters by using equipments available now. They were extended to three-dimensions in the third paper. More realistic results were obtained there. In the present paper, the theory is extended to include the effects of current. The basic nonlinear equations are obtained by a variational principle. Some two-dimensional numerical calculations are conducted, and the convergence of the iteration calculations is verified. The numerical results show that the effects of current can be obtained and can't be neglected for precise positioning.

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The microwave scatterometer is one of the useful remote sensors for the sea surface measurement. The basic principle of the measurement is a scattering at sea surface. Therefore, it is necessary to understand the characteristic of the microwave scattering at sea surface in order to measure the sea surface condition. The purpose of this study is to investigate characteristics of microwave scattering under the various conditions in the experimental tank and also the consideration of measurement technique and data analysis method. Bragg scattering model is applied to the experimental condition, and the tendency in the measuring results of scattering strength is explained in this model. For the quantitative evaluation, however, it is necessary to more accurately grasp water surface profiles. The Doppler shift of scattering microwave has related to the speed of the scatterer.

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The standard ocean wave spectrum is important for short-term prediction and long-term prediction of ships or floating structures in waves. The standard wave spectra used until today are basically based on P-M (Pierson-Moskowitz) type spectrum, that has ω^-5 shape in high frequency or equilibrium range.
Recently, with the observed data, it has been suggested that the shape of spectrum is ω^-4 instead of ω^-5 shape at high frequency range. Technical committee of ITTC recommended to investigate the effect of this type of spectrum on ship responses.
In this work, after introducing ω^-4 type spectrum, the resultant investigations are reported for the differences in short-term and long-term predictions of vertical bending moment and vertical acceleration of a typical ship.

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With a recent tend to take high interest in the environmental problem and the acquisition of ISO 14001 standard, the time has come to reconsider the present working environment. Especially, air pollution by metallic fume from the progress of cutting and welling is an unavoidable issue for labor health. For settling this matter, it is necessary to design a highly efficient ventilation system that is generally composed of large-scale equipments due to big capacity of the fabrication shop. However, the physical property and movement characteristic of the fume, which is composed by minute particles in different sizes, have not been grasped accurately even now.
In this paper, for establishing efficient ventilation system to control the fume flow, the transportation characteristic of fumes is studied by assuming the fume as a turbulent buoyant gas-phase flow. Three-dimensional numerical simulation on a part of a fabrication shop in some cases are carried out using k-ε model and SIMPLE algorithm to investigate the tendency of forming stagnant fume areas where advection effect on turbulent gas flow is weak. Furthermore, laboratory experiments on fume diffusion, in which pigmented salt waters are used in quasi-two dimensional models of a fabrication shop under an appropriate similarity law, are executed. A two-dimensional numerical simulation is also made and the predicted fume flow shows considerably good agreement with the experimental observation.

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Though the physical burdens by welding works and manual material handling works in the shipbuilding have been studied before, the grinding work will be one of the most severe work for the human body. In the grinding work, undesired sound, hand-arm vibration and reaction force from the tool are big, and the adequate countermeasures for the body work seem to be necessary, because the aging of the workers and embrittlement of the youth will become remarkable in near future. Therefore, in this paper, the human body analysis at grinding work has been carried out, using general-purpose human simulation software “Jack”; static strength prediction of human body, lumbar force analysis, and energy consumption analysis, same as previous reports.
This time, “Kansei Engineering” technique has been introduced for the evaluation of the results by biomechanical analysis. SD (Semantic Differential) evaluation has been carried out concerning the body feeling of the grinding workers, and then multiple factor analysis, one of multivariate analysis, has been executed, according to the process of “Kansei Engineering”. The results have been compared with the analytical results by the digital human model, and it was concluded that both were well coincided especially for the lower human body.

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We show a new method which can simulate the quantities of the domestic unit-load freight in Japan by trains, trucks and ships. The method is working with “sacrifice model” and “Dijkstra method”. We found that the method showed a sensible simulation for the actual domestic freight flow and it could be useful to estimate a proper plan for a future car ferry route etc.

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The growth of 3D CAD system in ship hullform design makes curves and surfaces data use prevalent. For transmitting those data on networks in design process, the data size becomes critical. This paper describes a compression method of free-form surface for ship hullform design. It consists of surface interpolation over boundary curves and surface modification using differences between original surfaces and interpolating ones. The differences data is compressed using the discrete cosine transform (DCT). We achieved to balance data size and tolerance by the method. It enables to select effective compression ratio according to objectives of data usage.

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Various design loads used in the strength evaluation of ship structures have been introduced currently by classification societies. As most of such design loads have been determined as the standard loads for the sake of convenience, the relationships between the design loads and the sea states actually encountered by ships seem to be poor. Accordingly, it may be difficult to refer and utilize them as the fundamental design concepts for ship designers or the operational guidelines for ship operators. Under these circumstances, authors propose the practical estimation method of the design loads having transparent and consistent backgrounds to the actual loads acting on primary structural members of tankers through the 1st report and 2nd report. In the 1st report, the design sea states that closely resemble the actual sea states, which are considered as the most severe for hull structures are proposed. Furthermore, the practical estimation methods of the design sea states are proposed by parametric studies using the results of series calculation of representative tankers.

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Various design loads used in the strength evaluation of ship structures have been introduced currently by classification societies. As most of such design loads have been determined as the standard loads for the sake of convenience, the relationships between the design loads and the sea states actually encountered by ships seem to be poor. Accordingly, it may be difficult to refer and utilize them as the fundamental design concepts for ship designers or the operational guidelines for ship operators. Under these circumstances, authors proposed the practical estimation methods of the design sea states in the 1st report. In this 2nd report, the practical estimation methods of design regular waves resulting in the same level of stresses with that induced in irregular waves under the design sea states are proposed. Furthermore, the practical estimation methods of the design loads such as ship motions, accelerations, hull-girder bending moments and hydrodynamic pressures that are induced under design regular waves are proposed.

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In the last two papers, the author proposed a new decision process of design values related to wave-induced responses. The process is composed of the two stages, namely, the selection of most severe sailing conditions and the estimation of short-term responses including non-linear effects in the selected conditions. The author considers that the preparation studies were finished by the last two papers.
The present paper demonstrates the effectiveness of the new process taking the large container ship for which full-scale measurements were carried out in the North Pacific Ocean in winter, as a practical example. In view of bending strength and deck wetness from the bow, most severe sailing conditions are selected and non-linear simulations are carried out around them. The results seem to be reasonable, when compared with the results of the full-scale measurements and the previous study on non-linear responses of a ship with large bow flare.

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In this study, fatigue crack propagation properties in sea water are investigated, when considering the usage for hull structural steels. The rate of the optimum ferrite-bainite (F/B) steels shows superior resistance even in a sea water.
Corrosion fatigue properties in sea water are investigated for both conventional ferrite-pearlite (F/P) steels and the F/B steels. It is clarified that fatigue strength of the F/B steels is higher than that of the F/P steels. As to corrosion fatigue crack propagation rates, the F/B steel originally has much superior resistance in normal conditions, and acceleration factors of corrosion crack propagation rate in sea water to that of air is almost constant regardless of microstructure in steels. These two characters brings superior propagation resistance in sea water. As to corrosion fatigue strength, the F/B steel has the advantage of not only a long fatigue crack propagation life, but also long fatigue crack initiation life. This extended initiation life in the F/B steel comes from the small number of corrosion pits, which might be corrosion fatigue crack initiation sites. This difference of pit number may depend on homogeneity to general corrosion. In case of the F/P steels, the microstructure is not so homogeneous, therefore, general corrosion is not uniform. In case of the F/B steels, the general corrosion is uniform because of fine and homogeneous microstructures.

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This paper deals with precautions needed for an accurate numerical analysis of welding residual stress and deformation. By comparison of numerical results with experimental data, the precautions of the thermal-elastio-plastic FEM are investigated. The bead-on-plate welding was selected as the first object of study, because the boundary conditions does not change with the moving of heat souce. The obtained precautions are as follows : The element division is determined by the size on the welding direction of mechanical melting point and the size of inherent area calculated by the fix-bar model. The yield strength in the area of inherent strain and the coefficient of linear expansion in the vicinity of mechanical melting point are needed.

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A catamaran is often subjected to pitch connecting moment (PCM) when navigating in oblique sea. PCM is one of the important loads for the structural safety of a catamaran. With the wide spread of finite element analysis (FEA) system, most of the designer uses FEA for calculating structural strength of ships. However, in case of early stage of the ship design, finite element analysis is time consuming and costly expensive. So, simplified analysis is necessary in order to make repeated design analysis efficient, and to avoid errors that may occur in refined and complex analysis. This paper proposes the simplified formula to calculate the strength of cross-deck structures. In this method superstructure is not accounted for, and twin hull is treated as rigid. The simplified method was applied to two types of models. One is of 16m real catamaran and the other is 32m box-type simple catamaran. To validate the accuracy of the simplified method, three dimensional finite element analysis was conducted with FE-models of both ships. The results of simplified analysis and that of FEA was compared and it shows fair agreement. Moreover, to investigate the effect of load distribution, sinusoidal loads and concentrated four points-loads was loaded to the box-type catamaran. By comparing these results, it is found that sinusoidal load is recommended for PCM analysis to avoid the stress concentration.

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Buckling collapse test was carried out on 1/10-scale hull girder models of an existing Chip Carrier applying pure bending load. Two models were provided with and without carlings at the upper part of the side shell plating. The test results indicate that the buckling collapse of the deck plating as a stiffened panel and the local buckling of the upper side shell plating lead to the overall collapse of a hull girder under the sagging condition. Theoretical analysis was performed on test models applying the Smith's method, and it was confirmed that the calculated results show similar characteristics with the measured results.

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Series of elastic eigenvalue analyses and elastoplastic large deflection analyses were performed to investigate into the influence of perforation on the buckling/ultimate strength of rectangular plate subjected to uni-axial and bi-axial thrust. It has been found that :
(1) Beyond the ultimate strength, plastic deformation is concentrated around the opening, and elastic unloading takes place in the remaining part in case of long plate subjected to longitudinal thrust.
(2) Beyond the ultimate strength, plastic mechanism of a roof mode developes in case of long plate subjected to transverse thrust.
(3) Ultimate strength interaction relationship is derived for bi-axial compression, and it was found that general yielding at the perforated cross-section affects the shape of the interaction curve.
Simple formulas are proposed to evaluate the elastic buckling strength of a perforated plate under uni-axial thrust. These formulas are used to evaluate the ultimate strength, which showed good correlations with the FEM results.

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In case of optimizing the structures of ships, designers often select one of the responses such as weight, stress, cost, etc., as an objective function, and consider other responses to be constraints. The reason for this is that the multi-objective optimization considering several responses simultaneously is very difficult to perform. Thus, we applied response surface methodology (RSM) to this problem together with the concept of Pareto set. RSM is composed of design of experiment (DOE) and the least square method. One of the principal features of RSM is that we can obtain response function with minimizing the number of response evaluations by using DOE. Another advantage of RSM is the capability of displaying the performance of designs as a response surface that is represented by the obtained response function. In multi-objective optimization problems, the concept of Pareto set is also very useful for trade-off between candidate designs. In this paper, we propose a method of multi-objective optimization using the above mentioned techniques and the concept, and the effectiveness of the method is verified by the results of optimization of oil tankers' transverse structures.

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In order to prevent harmful vibration in ships at the early stage of design, anti-vibration design is required on the basis of accurate estimation methods. To investigate vibration response of superstructure, very large scale of 3-dimensional finite element analysis including hull structure is sometimes conducted, however, significant improvement of accuracy is not yet achieved due to the difficulty in estimating damping and exciting forces.
In the 1^st report, a new identification method of damping factor is proposed by calculating dynamic response directly on the basis of Rayleigh's damping; [C] = α [M] β [K]. Then the capability of this method is verified by the measured data of frame structure model.
In this 2^nd report, this method is applied to the measured data of a 280, 000DWT VLCC's superstructure and its ability is confirmed in the following procedures :
1) Precise transfer functions of superstructure vibration were obtained by the sweep exciter test of the superstructure. This test was performed with the aid of the active mass damper controlled by electromagnetic force recently developed), which enabled to oscillate the superstructure with fine frequency increment.
2) 3-dimensional FE model of superstructure including hull structure is analyzed to investigate the influence of FE modeling manner on the calculated natural frequency, in particular the influence of FE model realization of local structures such as radar mast, funnel and etc. Consequently the guideline of FE modeling of superstructure is proposed to estimate the accurate natural frequency of superstructure vibration.
3) By means of the above accurate superstructure model with hull structure, the new identification method described in the 1^st report was adapted to identify the measured response of the VLCC's superstructure. As a result of this identification, calculated response showed a good agreement with the measured data in both ballast condition and full load condition.
4) Obtained damping ratio is ζ=0.00896 and 0.01025 in ballast condition, ζ=0.01297 and 0.01626 in full load condition. These values almost agree to the assumed lower limit of the published damping ratio.

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When marine organisms become entrapped in the ballast water of ships, they are carried to regions of the sea that are different from where they originated. Once released, those marine organisms may destroy the ecosystem in the area to which they are transferred, with adverse consequences likely. This represents a new maritime pollution problem. A strategy to control this problem is under examination at the IMO (International Maritime Organization). In addition, in numerous countries there is a growing movement to enact municipal laws requiring the management of ballast water in ships entering a domestic sea area. Such laws require ballast water to be exchanged in the ocean a certain distance from land. However, ballast exchange in the ocean can, under certain circumstances, endanger ship hulls.
There are several candidate operations to exchange ballast water at sea : the Sequential method, by which all of the ballast water is pumped out from tanks one by one; the Flow-through method, by which the ballast water is pumped up from the tank to overflow across the deck and is replaced gradually by new sea water; and so on. The Sequential method has the advantage of speed over the Flow-through method. However, some issues with the Sequential ballast exchange method remain to be resolved. One of these problems is the sloshing of ballast water in the tank. In the Sequential method, the volume of ballast water in the tank varies greatly, from full to empty, during the ballast exchange operation. Therefore, there is a danger of generating resonant motion of water in the tank; intensive sloshing may occur during the ballast-water exchange operation. For this reason, it is necessary to carefully examine whether or not an exchange operation of ballast water in the ocean is possible and which restrictions to the exchange operation may or may not be acceptable.
In this paper, the ballast water exchange in irregular seaways is simulated by using a numerical-simulation of sloshing that the authors have developed. The obtained characteristics of the sloshing of ballast water and of the safety of the tank structures are discussed.

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One of the critical points in the numerical assessment of liquid sloshing is accurate and stable descriptions of the hydrodynamic impact pressures. Due to discrete approximations adopted by the available numerical methods, unnatural violently-oscillating time histories of the impulsive pressure, caused by unstable or even non-converging numerical computation, are generally obtained. The aim of this paper is to investigate the unstable nature of the impulsive pressure computation. Also, a new numerical treatment of the boundary condition for accurate and stable assessment of the sloshing impact pressure based on rectangular grid system is proposed. The comparisons of the computed results with the experimental ones confirmed the accuracy of the proposed technique. The improvements achieved over the existing approaches are also shown herein.

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To realize the semi submersible type mega-float (SSMF), it is very important to reduce the cost, which is higher than one of pontoon mega-float. Because the building cost leads to much of SSMF's initial cost, to reduce the structural weight is well meaning. In this study, we tried to optimize SSMF in order to minimize the structural weight, under the condition that the maximum bending stress is less than the allowable stress. We inspected the results by VODAC and got the knowledge between the structural weight and deck depth D, column radius R and column number N.

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A Very Large Floating Structure (VLFS) is expected as a new institution on the ocean. Rigidity of VLFS is, however, relatively small, so we cannot disregard an hydroelastic response of VLFS in waves. Many researchers studied about the elastic response of a pontoon type VLFS, and now we have knowledge about it. In addition, recent studies of column support type VLFS are progressing. The VLFS supported by columns shows the smaller elastic response than that of the pontoon type VLFS. Especially a column with footing has a wave-free frequency on wave exciting force, so we hope more restraining effects on the elastic response.
Thus, we investigated on the hydroelastic responses of VLFS supported by footing-columns. In the analyses, the hydrodynamic forces of the single column are calculated by using 3-dimensional source sink method, and the hydrodynamic characteristics of VLFS are obtained by using hydro interaction theory. Then we discuss about the characteristics of the hydroelastic responses of VLFS in this paper.

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Researches and developments of VLFSs for airport use and so on have been carried out for recent years. There is a VLFS which has a pontoon type floating structure supported with many dolphins and it is important to investigate sufficiently the behaviors of such VLFS under earthquake loading.
In this paper, a floating structure supported with 49 dolphins was modeled as both a horizontally elastic plate and rigid plate supported with springs and dashpots through gap elements. The time history response analyses of the structure when the gaps between fenders and dolphins were equal to zero were performed with varying the period and the velocity of the applied sinusoidal waves as an earthquake loading and the following conclusions have been obtained.
1. The floating structure oscillates transversely with their natural periods of sway and yaw irrespective of the periods of sinusoidal earthquake wave applied, though the transverse oscillation is overlapped with an oscillation, of which period is the same as that of sinusoidal earthquake wave applied.
2. The floating structure has resonance with the sinusoidal earthquake wave not only in the modes of sway and yaw but also in the modes of two and more nodes elastic vibration due to the horizontal elasticity of the structure. The fender, dolphin and ground also have resonance.
3. When the sinusoidal earthquake wave velocity is relatively low, the resonance in the mode of yaw appears and the fender and dolphin reaction forces in the mode of yaw become larger than those in the mode of sway.
4. An equation is proposed to estimate the sinusoidal earthquake wave velocity that the strong resonance of the floating structure with the sinusoidal earthquake wave in the mode of yaw and two and more nodes elastic vibration of the floating structure.
5. Resonance periods of the fender, dolphin and ground are obtained accurately from the vibration equation on spring and dashpot and the resonance period of floating structure in the mode of sway is obtained as equal to that of fender.
6. Within the low period region where dolphin and ground have resonance with the sinusoidal earthquake wave, the fender and dolphin reaction forces are not affected by the horizontal elasticity of the floating structure and the sinusoidal earthquake wave velocity.

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