A variational principle is presented for solving the problem how we determine an unknown wetted surface of a planing ship in fixed condition. The functional of variational principle is not represented by the pressure distribution but loading function, circulation distribution around longitudinal sections, and the unknown wetted length distribution in order to avoid troublesome behavior of the pressure at the spray root line which is the boundary of the wetted surface. The extremal of the functional satisfies the lifting-surface integral equation and the elevated water surface condition along the spray root line, both are sufficient to determine the wetted surface. And also, an approximate functional actually determines well, regardless of adoption of trial function, the unknown wetted surface distribution of high aspect ratio planing plate.

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To predict hydrodynamic characteristics of high-speed crafts, a lifting-body theory is needed, because lifting bodies such as hydrofoils, fins and so on, are generally attached to recent high-speed crafts to improve the hydrodynamic performance like wave-making resistance, lift and so on. For this purpose, PAFS (Panel method Applied to Free Surface flow) based on the potential theory is suggested by the authors, in which Morino's panel method is extended to the problem of free surface flow based on Kelvin's linearized free surface condition. In order to get more accurate results based on PAFS, however, it is expected that the nonlinear effects of free surface conditions should be taken into consideration for the free surface flows around nonlifting/lifting bodies. In the present study, the second-order effect of nonlinear free surface conditions is formulated by using perturbation technique. Numerical examples are shown for a few nonlifting/lifting models to evaluate the effectiveness of PAFS with second-order effect of nonlinear free surface conditions.

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Safety, comfort and high-efficiency in marine systems are becoming strongly needed in recent years. For the design of the functional systems such as smoke control of marine fire and purging of gases in a tank, it is often required to solve the problem of two horizontal flows with different densities. In the case of the lower stream with the greater density, the flow is stably stratified under some conditions. If this phenomenon occurs, the efficiency of the gas purging in an enclosed space may be seriously reduced. Acquiring an extensive knowledge of such stratified flow is therefore necessary.
In this paper, the stability of the stratified horizontal shear layer between two uniform streams is investigated by numerical simulation. The numerical method is a control volume method in which the SIMPLE procedure is applied and the QUICK scheme is used for convection term. The development of the Kelvin-Helmholtz (K-H) instability is simulated to show that the shear layer becomes unstable, rolls up into billows and collapses finally. The secondary Kelvin-Helmholtz instability is also observed in the simulation results in a case of high Reynolds number flow. The effects of Richardson number and Reynolds number on the stratified shear flow are investigated and it is found that Richardson number is the dominant parameter for Kelvin-Helmholtz instability. Finally, the unstable region of the stratified flow, which is governed by Richardson number and Reynolds number, is presented from the numerical simulation results.

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Flow mechanism of contractive and dilative motion is numerically investigated to obtain a propulsive force in highly viscous fluid. The computing code for the analysis of the motions is developed with unstructured grid.
After the developed code is validated with the problem of oscillation of a plane below a viscous fluid, it is applied to two cases. The first is the analysis of the contractive and dilative motion of surface in shallow water whose corresponding experiments have been carried out by Ninomiya and Mizutani. The computed results are compared with the experimental ones. It is found that a propulsive force can be obtained in shallow water by pressure force which can be derived by contractive and dilative motion of surface.
The second one is the analysis of the contractive and dilative motion of body. The computations are carried out both in unbounded fluid and in narrow channel. It is found that the trochoidal motion of the moving surface of body produce propulsive force in unbounded fluid by frictional force rather than pressure force. As the Reynolds number becomes smaller, more propulsive force can be obtained by the frictional force with the trochoidal motion of surface. This tendency is almost the same as in narrow channel.
By the present simulation, it can be made clear that propulsive force can be obtained in highly viscous fluid by the trochoidal motion of surface, which is expected to be applied to the propulsor of a micro-hydro machine.

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The model test and its analysis to obtain the basic data for performance prediction of full-scale ships equipped with contrarotating propellers are shown. Particular emphasis is given on a method for determining the interference component of effective wake fraction induced by fore and aft propellers.The experimental results show that the present method is reasonable.
Secondly, a performance prediction method is described to estimate the delivered horsepower DHP of the full-scale ships equipped with contrarotating propellers by the extrapolation of model tests results. Then, the estimations of DHP of the two ships by the present method are made. The main findings are :
(1) The DHP and interference components of effective wake fraction are significantly influenced by the change of propeller load factor between model and full-scale ships.
(2) On the other hand, the DHP is hardly influenced by the difference of share of the rudder component of effective wake fraction between fore and aft propellers.
(3) The DHP differs about 2%from the value obtained by treating one whole contrarotating propeller as a single propeller, which is due to the influence of imbalance of load distribution between fore and aft propellers.

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This paper proposes a new concept for the frictional drag reduction technique. The new technique makes use of a specific coating surface (Super-Water-Repellent Surface) which has a highly repellent effect and an ability to form a thin air film over it under water. When supplying a small amount of air to the specific coating surface from the outside continuously, the supplied air (secondary air) is absorbed in and joined with the primary air film and spread to form a filmy air flow along the surface. This technique reduces the frictional drag because of this phenomenon.
A frictional drag test in the rectangular pipe flow and a resistance test of horizontal flat plate were carried out in order to verify the validity of this technique. As a result of these tests, it was confirmed that drag reductions of about 80% and about 55% were obtained for flow velocities of 4 m/s and 8 m/ s, respectively.

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This paper describes the effects of drag reduction by microbubbles injection on very long ship models with flat bottom. The local skin friction and the total drag of 20 m and 40 m long slender ship models were measured in a towing tank with injecting microbubbles from porous plates on the flat bottom. Three cases of injecting position, bow, midship and both, were tested. Microbubbles were observed by CCD camera.
The drag reductions by injecting microbubbles were obtained in all experimental cases, and the maximum reduction rate was about 14% in total drag, and about 60% in frictional drag on the part of flat bottom covered by microbubbles. It became clear that the reduction of local skin friction persists for about 40m downstream from the injection point, although the reduction rate decreases with distance. The relation between drag reduction rate and air flow rate, velocity, injection position, etc., are illustrated, and the possibility of energy saving by microbubbles injection is discussed.

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On the Estimation of wind loads on offshore structures which have the deck supported by substructures, it is clear through previous studies that lift force of deck is caused by asymmetry of upper and lower surfaces of deck and that it may be much larger than drag force in case of large waves. Therefore, wind-wave tank experiments on such structures under progressive wave conditions were conducted and the effect of progressive wave on lift force was investigated. Lift forces were measured as a function of substructure, unit number, and wave height. The Experimental results showed that the lift force is significant, especially on leading edge of deck. The study also showed that the fluctuating lift force can be comparable in magnitude to the mean lift force in case of large waves. Further estimation method is provided to help define the nature of lift forces on such structures.

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Various reports have been given on the study of wind forces and moments acting on ships by using wind tunnel experiments over a long time. Utilizing these results, Yamano et al. and Isherwood proposed estimation methods of wind force coefficients by linear multiple regression model in the 1970's. After that, many kinds of ships have been built all over the world, for example VLCC, PCC, LNG, and so on. The estimation methods lack reliability when wind forces and moment of such kinds of ships are predicted, since these new types of ships aren't considered in the methods.
Recently Yoneta et al. proposed a estimation method consisting of several force components, which is called a physical-mathematical model. However, the applicability of this method isn't enough discussed.
In the present paper, we wish to report on a new and simpler method of estimating wind force and moment coefficients acing on ships by using stepwise method which comes under linear multiple regression analysis. This estimation method was obtained after considering various ships more than previous estimation methods and the effect of wind velocity profile that some reports have already pointed out its importance in the experimental condition. As there is no way to estimate heel moment caused by strong wind, a estimation method of it is also presented in order to make use of strict simulation of maneuverability for the first time.
This method has been found to possess high practical value compared with previous estimation methods.

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This paper proposes an artificial neural network system for regressive estimation of wave making resistance, which is significantly important in preliminary design of high-speed ship. The neural network can explicitly realize nonlinear mapping between hull form and wave making resistance.
The system is composed of two kinds of neural networks ; Estimating Net and Descriptive Net. The Estimating Net learns the relation between hull form parameters and wave making resistance coefficients from a number of model-resistance test data. Consequently, when Froude Number, principal particular ratios, and area curve parameters of a hull form are given, the Estimating Net provides the wave making resistance coefficient. The Descriptive Net learns the density distribution of the learned data points in the hull form parameters' space. It provides the information about the density of the learned data at the input point in the above.parameters' space.
In this paper, the test data of 62 models : Series 60, are used for the construction of the system. The learning is successful and the results of playback calculation show good agreement with the original test data. Some applicable cases for non-learned hull forms are also explained. It is shown that the accuracy of the estimation is in accordance with the output of the Descriptive Net.
When other model test data are available, it is easy to modify the constructed system, taking advantage of learning ability of neural networks.

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The searching for speed is resulting in very fast yachts but a maximum limit for speed seemed to be achieved. In order to provide an additional velocity, a better understanding of added resistance in waves is necessary. This facts point out the necessity of a deep research in the field of ship motion and added resistance in waves. The free running technique was already developed by the authors to determine motion characteristics and the upright and heeled resistance in still water and in waves for a sailing yacht with similar characteristics of a typical IACC. In this paper, an improved way of pulling the model was applied using rigid poles or lines to simulate the action of wind as well as springs to allow the free motion when the yacht was performing in waves. This provided free of motion and little interference between the linkage and the model itself. The forces and motions measured during the experiment are presented and they show consistence and a good trend. Also some theoretical calculation obtained through the Strip Method were used for comparison, showing the inherent limitations of this method and necessity of improvement.

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This paper describes the generation of the irregular wave specified by the spectrum of the wave envelope using a wave maker in a test basin. An effective generation algorithm for the irregular wave specified by the wave envelope spectrum is proposed.
The design for mooring systems of a vessel in a port as well as a floating offshore structure is important because the long period wave happens to cause the break of mooring lines in spite of relatively calm ocean conditions. In very large floating structures, such as the mega-float case, especially, this problem will become a severe one due to no past experience in real design. The long period wave can be induced by the irregular wave specified by the spectrum of the wave envelope.
The proposed algorithm is verified by the computer simulation. The experiment results using a pontoon model are also presented.

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It is important to collect many wave data in order to know long term distribution of a ship response caused by wave loads. Japanese Meteorological Agency (JMA) has been collecting observed wave data from many ships since 1976. The amount of data is enormous and sufficient to exaimine the statistical characteristics of wave parameters.
Wave data collected from merchant ships consists of some items, which are direction, period, and height of significant wave for wind wave and swell. There are some human errors in observed data because many data are obtained by visual observation. And the accuracy of some collected wave data is not so good.
This paper examines the characteristics of visually observed data obtained from merchant ships by using two methods compared with weather ships of JMA. One is the comparison method between probability distributions in the same region and the other is the comparison method between near ships at the same time.

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Instead of deploying direct sea-state monitoring systems, the authors propose a method for indirect measurement of sea-state condition using information obtained by satellite remote sensing.
Using remotely sensed visible (VIS) and infrared (IR) images from “GMS” (Geostationary Meteorological Satellite), cloud-systems are extracted. Considering that two main cloud-systems, the Stratus (St) and Cumulus (Cu), are closely related to the height, thickness, edge-slope and complexity of cloud-system, they develop a method to discriminate these two cloud-systems.
The area of cloud-system is obtained by determining threshold brightness temperature using VIS and IR images. The height, thickness and edge-slope are analyzed using the temperature and picture element. The complexity of fringe of cloud-system is analyzed by introducing the fractal dimension. The fuzzy integral method with fuzzy measures is applied for the discrimination of cloud-systems.
They show that the proposed method is quite useful to discriminate cloud-systems, because that it can indicate the possibility of Stratus and Cumulus for each cloud-system.

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Two laboratory investigations of natural and forced breaking waves in a two-dimensional wave tank are reported. They are considered to be simple representations of breaking waves that exist in a real ocean, in moderate and rough sea states. The first type (Crescent Waves) is usually observed on a sea surface in relatively calm weather. Initially, for a nonlinear initial wave train of wave steepness ak=0.34, a three-dimensional instability dominates the side band instability, and stable Crescent Waves are observed. The direction angle of radiated oblique waves as a most unstable mode, and associated growth rate are found to be in excellent agreement with Mclean's theory. The second type (strongly overturning breaking waves) was generated on the surface of essentially deep water using a wave energy focusing technique. Time evolution of the transient wave was observed using a high speed video image, and the inherent complicated unsteady flow fields was measured by means of a two-dimensional Laser Doppler Anemometer. We found that, 41 % of the wave momentum was lost by the extreme wave plunging breaker. The maximum horizontal velocity at the overturning top of the crest was found to be equal to 1.4 times the phase velocity of the wave, in its most developed stage. The twodimensional velocity field reveals that the flow is essentially irrotational up to the top of the crest. Surface wave height measurements before and after both an isolated plunging and spilling breaking wave, reveal a frequency downshift phenomenon in the corresponding spectral shapes which seems not to have been previously seen.

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When a ship is required to keep the specified position with high accuracy under influence of external disturbances, dynamic positioning system (DPS) is often used in order to keep its position. It will be able to construct a control system which has no interaction among the controlled variables by using the inverse linear quadratic (ILQ) optimal servo theory. It will be also able to express the optimal feedback gain in terms of the system matrices and the design parameters. In ILQ optimal servo theory, time constant T_i as the design parameters have much influence on the performance of the control system, and the suitable values of T_i should be selected as small as possible corresponding to external disturbances. However, it is troublesome to select suitable T_i because we have to carry out numerical simulations repeatedly.
In this paper, we applied the ILQ optimal servo theory to design the control system of DPS for a selfpropulsive barge. About time constant T_i, we designed the system with T_i defined automatically in response to the dimensions of controlled variables. Applying this method, it is possible to adjust the suitable values of T_i automatically. From the numerical simulations, it is found that design process taking the system with T_i defined automatically would be more useful and easier than taking usual ILQ optimal servo theory.

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In the previous paper, the authors proposed the convenient design method of the ship manoeuvring control system with the learning feed-forward control technique, and showed by computer simulation that the system was effective to both the follow-up control system and the disturbance compensation system for a ship.
Now, the authors have performed experiments to confirm the effectiveness of the systems for an actual ship and to put it to practical use. The experiments are performed mainly about the follow-up control of the ship with the bow thruster and with the rudder, respectively. The experimental results are discussed about the characteristics and the problems of the learning feed-forward control system, and also examined about the stability of the system for the actual ship with the root locus method.
It is proved that the ship manoeuvring control systems with the learning feed-forward control technique are useful as a control system of an actual ship.

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This paper presents results of a feasibility study on a concept of anti-rolling tank with a liquid mass-air spring system. The study is made by a theoretical approach with the following three phases. Coupled motion equations are developed at first with the use of Lagrange's Equation. Then a design method of the tank is developed on the basis of coupled motion equations referring to the theory of dynamic absorber. Next extensive computations with respect to response characteristics of roll and tank motion are made for tanks designed for three kinds of small-sized ships. Through discussions for roll reduction and tank configuration, it is concluded that a concept of the anti-rolling tank is certainly feasible.

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This paper describes a new simulation technique for diffusion phenomena over the sea surface using Cellular Automata (CA), which is capable of making prediction of oil dispersion from tankers come into collision or stranded on rocks. Automaton is known as an abstract expression of sequencial machine in the field of information science, and CA is recognized as one of the model of finite state automata with sectional analysis domain called “cell”. CA has also been applied to demonstrate various engineering phenomena, and one of the most promising application field is flow simulation of viscous fluid, percoration, multi-phase fluid, reaction diffusion problems and so on. In this field, CA is called Lattice Gas Automata (LGA) and the solutions by LGA has been proved to coinside those by Navier-Stokes equation. In the procedure of CA, the analytical area is devided into cells and all the state variables defined on each cell are made transition by local neighbor rule and transition rule along the discrete time steps without solving the governing equation.
In the present paper, CA is applied to solve a diffusion problem in which the simulation results are compared to the analytical solution of diffusion equation, and possibility is shown in predicting the oil dispersion in the accident of tanker occured in Japan Sea in January, 1997, as an example. It is shown that the simulation by CA shows good agreement with the analytical solution of diffusion equation, and that the prediction of oil dispersion in Japan Sea can be made in a short time by a personal computer.

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This paper deals with progressive collapse behavior of mooring system of very large floating structure. The floating structure considered in this paper is assumed to be installed in calm sea protected by breakwater and subjected to strong wind. The effect of in-plane elastic response which has been dealt as rigid response on progressive collapse behavior is investigated. It is shown that elastic response slightly reduce the strength of mooring system against progressive collapse. Concerning the modeling of wind, it has been accepted that scale of space fluctuation (scale of turbulence) is smaller than the scale of floating structure. But recent research results indicate that the scale of turbulence becomes much larger in typhoon, which is significantly large compared with the typical size of the structure. It become clear that the effect of higher correlation (largeness of scale of turbulence) in typhoon significantly increase the possibility of progressive collapse. We suppose to use dolphin mooring, and modeled it as nonlinear spring with hysterisis effect. It can be concluded that due to redistribution effect through the hysterisis, the mooring system becomes significantly strong against progressive collapse compared without consideration of hysterisis effect.

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A prediction method of hydroelastic behaviours of a very large floating structure (VLFS) that may be used for such purposes as an international airport or an offshore city is presented. In the present analysis, the structure is divided into small substructures and the continuous deformation of the structure is represented by the succession of the discrete displacement of each substructure.
The present one is our 3rd report under the same title and shows two major new developments in the hydroelastic analysis of a VLFS. The first one is the evaluation of the structural constraints on the motions of substructures. They are evaluated by a finite difference scheme instead of the differentiation of mode functions, which had been adapted in the previous two reports. In this way the force-free as well as the moment-free conditions at the rims of a VLFS can be rationally satisfied in the analysis. Besides, as will be shown in the paper, an approximation that exploits the fact that the structure is very large can be incorporated in the analysis fairly easily. The other development is a new mathematical technique for the evaluation of hydrodynamic interactions among substructures. The diffraction/ radiation characteristics of a certain number of substructures, which hereafter is called a sub-group, are first determined by the hydrodynamic interaction theory of Kagemoto and Yue (1986). Then the diffraction/radiation characteristics of a group of these sub-groups are determined by the same interaction theory while treating each of the sub-groups as a single floating structure. After repeating this procedure several times, interaction characteristics of a quite large number of substructures can be evaluated with reasonable computational effort. Moreover, exploiting the periodicity of the substructures, this procedure is further simplified because the same diffraction/radiation characteristics can be used for most of the sub-groups.
The efficaciousness of the presented new techniques are discussed through comparisons with experimental results. Several example calculations are also carried out for the hydrodynamic analyses of a VLFS supported on more than 10, 000 legs, for which, to the knowledge of the authors, no numerical results have been successfully obtained because the application of conventional numerical techniques is practically prohibitive due to the required huge computational burden.

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Recently, efficient numerical analysis codes have been developed for pontoon type VLFS with the capability to analyze hydroelastic responses in waves of relatively short periods. In this paper, these numerical results for a 5 km class VLFS are investigated in detail using an analogy with optics laws to clarify the frequency and direction dependent structural response characteristics.
When the Airy waves are incident to the pontoon edges at an angle, transmitted flexural waves in the pontoon are shown to be refracted as a consequence of the difference in the wave propagation velocities. For incident angles greater than a critical value, the situation is similar to total reflection in optics. The angles of refraction and the critical angles were calculated using Snell's law and compared with the numerical analysis results. Excellent correlation was found for the refraction angles. Abrupt changes in the magnitude of response amplitude are observed at the predicted critical angles.
A formula for the amplitude transmission coefficients is derived based on the analogy with Fresnel's law in optics. Predicted response levels by the formula are not exactly the same as those calculated by the numerical analysis code but the tendency of frequency and directional dependence of the hydroelastic responses are in good agreement with the numerical analysis results.
Resonance characteristics are also investigated based on the one dimensional wave propagation theory. It is shown that the damping due to the wave radiation, which is independent of the order of the vibration mode, is considerably higher comparing to the ordinary structural damping in the low frequency regions.

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A new modal analysis method, the B-spline element method, has been developed in this study for estimating the hydroelastic responses of a very large floating structure in waves. A motion equation for the elastic structure has been proposed in this method according to the energy-conservation principle. The unknown deflection and load, the hydrodynamic pressure, of the structure are both discretized with bi-cubic B-spline function. The B-spline expressions of the dry-eigenmodes of the structure are obtained by solving the eigenvalue problem of the proposed motion equation. With the resultant eigenmodes, hydrodynamic pressures due to the structure motion and incident waves are calculated by applying the pressure distribution method. Differing from the conventional Finite Element Method, B-spline element method ensures the continuity for not only the structure deflection but also the strains and stresses, and therefore it may have better accuracy. The usage of B-spline discretization for the pressures makes it possible to predict the hydroelastic responses in very short wavelength case. Comparisons between the calculations of the proposed method and experimental data have been carried out, and it shows that they are in a very good agreement.

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The research of VLFS (Very Large Floating Structure) for various applications such as floating airports is making a steady progress by the Technological Research Association of Mega-Float and others.
The pontoon type structure with the length of 5 km, breadth of 1 km and draft of 1 m can be expected in the concept. This structure will be surrounded by the breakwaters for decreasing the influence of waves. Therefore the floating units of several hundred meters long should be connected together on offshore site instead of the construction in the dockyard.
This paper deals with the wave induced connecting loads and the elastic behaviors of the box-shaped floating unit divided into three parts longitudinally. The 300 m long prototype structure used for atsea test by the Technological Research Association of Mega-Float has been modeled for the experiment. Three kinds of connecting conditions such as free condition without connection, pin-connection and beam connection have been considered.
The numerical analysis code has been developed to estimate the elastic motions and the connecting loads. The analysis is based on FEM and the pressure distribution method using the zero-draft Green function. The authors have also carried out the tank tests using a 1/30 scale model for the validation of it. The test model has been designed on the basis of law of similitude concerning the stiffness and the mass of the structure.
The calculated results show a quite satisfactory agreement with the experiment. It has been confirmed that the method has a sufficient accuracy for the practical use.

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A new method to analyze the response of a thin elastic plate of large horizontal size floating in waves is presented. Huge horizontal size and small thickness of it are typical of the recent design of floating airport. A benefit of this new method is that we need not the modal analysis of the body deflection, and the solution of a hydrodynamic boundary value problem and the solution for the body motion including the elastic deflection are simultaneously obtained. Some results of numerical implementation of the method are presented.

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The attitude control of a very large floating structure (VLFS) could be very important when the structure is to be used as a floating airport. Especially when a semisubmersible type VLFS is in assembly or unit replacing condition at sea, an appropriate method for the recovery and attitude control is indispensable. The authors have proposed a pneumatic type active control system to achieve such purpose, the basic simulation results were shown in the first report. For the practical use, in this report, the characteristics of air actuator are investigated and their effects on behavior of structure are taken into account in calculation. Furthermore, the influences of bending rigidity are studied and comparisons of simulations with experiments are made.

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When the water quality of a bay is considered, the concept of exchange of sea water is useful. In recent years, there are some projects of constructing very large floating structures such as ocean airports. The exchange ratio of sea water can be an important index in evaluating influences of very large floating structures over marine environment. The main purpose of the present study is to investigate the influences of large-scale ocean structures on exchange of sea water by means of numerical experiments.
At first, the results of numerical calculations were compared with field observation and hydraulic model experiments. There have been a lot of studies about the numerical simulation of coastal currents, but many of them examined about only the characteristic current pattern under constant boundary conditions. From the viewpoint of the reliability of the numerical simulations, it is necessary to investigate more whether the model can reproduce real phenomena in a certain period. In the present study, numerical experiments of exchange of sea water in Tokyo Bay were carried out using real changing atmospheric conditions such as wind and atmospheric temperature as external forcing. The tidal exchange ratio obtained by the calculation was compared with that by the field observation. At the same time, the comparison was also made about current and salinity distribution in the scene area. It showed that the exchange ratio of sea water by the numerical calculation agreed well with the observation, and the calculation could also approximately reproduce the observed current and salinity distribution. The numerical model was also applied to the calculations of tidal exchange in a bay with or without a breakwater. It is found that the influences of the breakwater by the numerical model have the same tendency as the results of the hydraulic model experiments. Therefore, it is concluded that the exchange ratio of sea water can be evaluated properly by the numerical model used in the present study.
By using the same numerical model, some numerical experiments were carried out in order to investigate the influence of a very large floating structure on tidal exchange of sea water. The results showed that the tidal exchange of sea water is promoted by the circulation caused by the structure, while it is suppressed because of the reduction of section area of the bay when a large-scale ocean structure is installed.

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In summer of 1996 and 1997, anomalous sudden fall of sea water temperature and rise of salinity were observed several times, simultaneously off Oppama and off Negishi in Tokyo Bay. These phenomena occurred in a common situation that typhoons stayed for a while in the western part of Japan or passed through it, and consequently, relatively strong blow of southwesterly wind lasted for a few days in Tokyo Bay. On the occasion of the typhoon No. 12 in the middle of August 1996, for example, a salinity increase of about 3 psu and a temperature drop of about 5°C were observed two and a half days after southwester started to blow. These unusual temperature fall and salinity rise were assumed to be attributed to coastal up welling caused by continuous blow of southwesterer in Tokyo Bay. In order to verify this reasoning, use was made of the so-called multilevel model to simulate sea water movement. Results of numerical simulations carried out under the condition of southwester of 10 m/ s indicates that wind-driven up--welling is generated mainly in the western coast of Tokyo Bay, and at the same time temperature fall and salinity rise observed in August 1996 are due to the coastal up - welling. At this point, however, it should be noted that to the contrary the water temperature in the head of Tokyo Bay, for example, off Ichikawa and off Chiba, rises definitely as far as results of numerical simulations are concerned.
A huge floating offshore structure (HFOS) is expected to have some effects on oceanophysical environment of its surrounding sea area. Hence, an additional numerical simulation was carried out to reveal such effects under an assumption that an imaginary HFOS of 5 km in length and 1 km in breadth is placed on the sea surface off Oppama. As a result, it is shown that predicted change of sea water temperature and salinity in Tokyo Bay are completely identical to those predicted by taking no account of the imaginary HFOS, although sea water movement in the close vicinity of the HFOS is slightly affected by the presence of the HFOS.

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A research and development on “Density Current Generator” which is a new concept machine to agitate and upwell the stratified water area widely and calmly by using a density current is carried out. The machine draws water from top and bottom layer of the water area, and the vertical pipes of the machine lead both top and bottom waters to the pump casing located on middle layer of the water area. Two different waters are mixed up and blown off horizontally via the ring-nozzle by the motor-driven impeller installed in the pump casing. The mixed water is diffused toward every direction on the horizontal plane as a density current which is driven by gravity.
A model test and study using the law of similarity was carried out to predict the performance of the full scale prototype machine, which is working in Gokasho Bay, Mie Prefecture Japan since June 1997. The result of performance measurement is that the density current of 120, 000 m³/day is generated by electric power of only 12 kw, and the water area measurement is that the significant changes of sea water qualities are observed. Although further study should be needed to verify that these changes are caused by the machine, it is clear that the machine widely influences on the environment of the water area. These results lead us to believe that the Density Current Generator has excellent potential in terms of agitating and purifying stratified water of very large area.

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A behavior of CO₂ in the deep ocean should be clarified for the study of its storage in the deep ocean. The first thing to be considered is the formation of hydrate on the surface of liquid CO₂ (LCO2) and its growth in the direction of depth. Secondly an attention should be paid to the behavior of LCO2 which is closed and separated from a sea water with/without a barrier of hydrate. A stability of CO₂ in the ocean may be discussed taking these phenomena into considerations quantitatively. In this paper CO₂ hydrate formation was investigated experimentally at two levels of pressure, that is a medium pressure level of about 5 MPa and a high pressure level of 30 MPa corresponding to 3000 m class deep ocean complying with the pressure-temperature equilibrium diagram. Two methods of hydrate formation were applied, one is the forced mixing of LCO2 and water, and the other is the natural contact of two liquids statically and in both cases hydrate was confirmed to be formed in the pressure vessels. Dissociation of formed hydrate was also tried by relieving a pressure in the vessels. Molecular diffusions of CO₂ from the bulk CO₂ through the hydrate or directly from the CO₂ was studied by measuring the electric conductivity of CO₂ solution by using the characteristics of conductivity change by the dissolved ion in diluted CO₂ solution. From these data, three quantitative values of molecular diffusion coefficient in water, mass transfer coefficient at the interface of water and hydrate and density change of CO₂ solution were derived from the analysis of two-dimensional convection and diffusion flow by using the code which is developed for this purpose. The relative magnitude of density of LCO2 to water changes at about a pressure of 300 kg/cm², therefore two kinds of experimental conditions for direction of flow and diffusion could be represented.

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For storage and transport of LNG or LH₂ (liquefied hydrogen gas) it is of great importance to control the loss of boil off gas, and the thermal insulation and the effective utilization of the boil off gas are here playing an important roll. Regarding the reduction of the evaporation quantity of the cargo, increasing the insulation thickness is probably the most common solutions chosen. But due to the fact that there might be limited space available in hold space, other method could be preferable. The authors, however, have directed their focus of engineering on a recovery of cold energy imposed on the evaporated vapor and tried to take advantage of the boil off gas which is a result of heat passing through the protective thermal insulation from the surrounding atmosphere. This theory of “Vapor insulation system” was applied to the spherical tanks of LNG carrier and LH₂ carrier. Cold nitrogen gas which was heat-exchanged with the boiled off LNG or LH₂ vapor was introduced into the annular space around the tank, in which the gas flows upward along the tank surface bringing the heat absorbed from the hold space out of the cargo containment system before reaching the valuable liquid cargo. In order to optimize the Vapor insulation system, the effect of several important parameters such as gas flow quantity, gas temperature and location of gas passage and others were analyzed carefully and realistic examples of design were proposed.

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The lithium adsorption tests using the granulated absorbent have been done on the bench scale model. On the basis of the adsorption tests, a conceptual design on extraction system of lithium from sea water is presented in this study. The extraction system consists of a barge with absorbent bed tanks and a pusher boat. In the proposed system, sea water flows into the absorbent bed tanks from suction valves on the inner bottom of the barge, then passes through the piled absorbent and is finally discharged out of the tank by drain pumps.
In addition, numerical simulations for the extraction system are carried out under the assuming operation circumstances. A feasible assessment on the extraction system is also made in this paper.

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In this paper, the Genetic Algorithm is used to optimize the engine room structure under the static and dynamic constraints. A penalty function method is proposed to handle the complicated constraint conditions which include stress, natural frequency, and allowable magnitude of design variables. The new operator called objective elitism as well as fitness elitism is introduced to improve the efficiency of the method. Varied probability of mutation is proposed to improve the convergence of the algorithm without the risk of premature convergence to a poor solution. To handle the discrete problems, we introduced the serial numbers of cross-sections whose shape data are stored in database beforehand. Successful application of the Genetic Algorithm in the engine room optimization suggests its use to more complex engineering design problems.

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An elastic finite element method to predict the welding deformation and the residual stress in multipass narrow gap welding is proposed. The proposed method employs the inherent strain as the source of the welding deformation and the residual stress. The inherent strain in welding joint is mainly governed by the maximum heating temperature and the constraint at each material point. In addition to these, the residual stress created during the preceding welding passes is also an influential factor in case of multipass welding. Considering these three factors, the general procedure to compute the inherent strain is shown. Further, the effectiveness of the proposed method is demonstrated through an numerical example.

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The authors have accomplished to extend the logic of the Finite Element Method to the principle of Thermal Forming or Line Heating so as to facilitate creating curvature effectively on a hull steel plate by using fully automated, revolutionized line heating system in shipbuilding process. This paper writes of the elaboration on the development of the line heating unit. The elastic calculation based on the theory of inherent strain makes design of heating process plans available in reasonable time, though the forming by line heating consists of thermal conduction and thermal-elastic-plastic phenomena.
The result of experiments on precise forming for saddle shape by stepping heat source travel was presented in the previous report. Succeedingly, this paper states the followings :
(1) a calculation method for designing heating process plans with use of continuous traveling heating which is more practical in view of operation.
(2) the results of quantitative analysis on the factors which affect thermal deformation, e. g. residual stress.
(3) what actually happened at experiments with experimental apparatus in consideration of the above study results.
Further, the realization of the first, fully automated line heating unit for actual steel forming work process in shipbuilding, being considered the above, ought to be completed in the near future.

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A method of a posteriori error estimation is proposed in this paper where the spatial and time discretization errors are estimated separately. There arise two kinds of discretization errors for dynamic finite element analyses, one is for space domain, the other is for time domain. A posteriori spatial error estimation for dynamic plate bending problems in each time step has been proposed by authors, while a posteriori time error estimation is under our consideration in this paper. A posteriori error estimation both for spatial and time discretization error is developed for dynamic plate bending problems. Two numerical examples are solved in order to demonstrate the efficiency and validity of the proposed method.

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In the present paper, post-ultimate strength behaviour of a rectangular panel subjected to uni-axial thrust is discussed based on the results of elastoplastic large deflection analysis by the FEM. It has been found that :
(1) When a panel is accompanied by initial deflection of a buckling mode, deflection of a sinusoidal mode gradually changes to that of a periodical roof mode in the post-ultimate strength range except when the panel is very thick.
(2) When a panel is accompanied by initial deflection of a hungry-horse mode, plastic deformation is concentrated at a certain part of deflection mode, and unloading takes place in the remaining part. Because of this, capacity after the ultimate strength rapidly decreases.
A simple plate element is proposed to simulate the behaviour mentioned in (2), and the calculated results applying the proposed elements show good agreements with those by the FEM. At the end, fundamental idea is described to develop new Idealized Structural Units (ISUM element).

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This paper discussed the transfer of local stress fields in microstructures to fracture property evaluation of structural steels. Initiation behavior of brittle microcrack in dual-phase steels was mainly focused. The Weibull stress employed in the local approach is available for the evaluation of fracture driving force of each microstructure in dual-phase steels. The idea in this procedure is the integration of local stress in the microstructure responsible for fracture initiation. The local stress in dual-phase steels can be different considerably from a macroscopic stress field obtained from the global analysis of specimen. Generally, the high-strength phase bears larger stress than the low-strength phase, which means that fracture property evaluation based on the global stress field is not always reliable. The local stress in the microstructure is affected by the morphology of second phase and matrix. Increase in strength and slenderness of hard phase leads to elevation of fracture driving force of the hard phase. The stresses in low-strength phase can be relaxed to a large extent, provided that the low-strength phase is enclosed by the high-strength phase. These microscopic characteristics of stress fields are important in the fracture control design of structural steels. The Weibull stress is expected as a measure to link between microscopic and macroscopic aspects of fracture.

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The antifouling system for ship hull and ocean structures by sea water electrolysis has been developed. In this system, the ClO^- ions, which are effective for antifouling, is generated by the sea water electrolysis reaction caused by electric current supplied from electro-conductive coatings on the surface of ship hull or the ocean structure. In order to design a reasonable antifouling system, therefore, the technique to estimate the polarizing potential distribution and the current density distribution on the interface between sea water and electro-conductive coatings is expected.
In report (1), based on the Interface Electro-double Layer theory, a finite element method for the numerical analysis of the potential field caused by the antifouling system was developed, and the validity and usefulness of which was confirmed by its application to a test tank model.
For the analysis of the potential field caused by the antifouling system of a ship hull, however, it is very difficult to apply the finite element method to the semi-infinite sea water domain under water surface. Therefore, in this paper a new numerical analyzing technique by using both of finite element method (for electro-conductive coatings and Ti foil potential filed) and boundary element method (for sea water potential field) jointly is developed and the validity of which is verified by its applications to a small-size passenger boat and a steel sea water pipe, for which the antifouling systems were undertaken and the antifouling efficiency tests have been performed.

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In order to evaluate precisely the life and residual-life time of ship structures, it is essential to examine the S/N relationships of their fatigue strengths. The long term corrosion fatigue behaviors, however, have not been fully investigated yet.
In the present study, we have measured the corrosion rates and the fatigue crack growth rates of a 500 MPa class steel plate in synthetic seawater, and discussed the long term corrosion fatigue strength obtained under a cyclic frequency of 0.17 Hz within stress ranges less than the fatigue limit tested in air. The results obtained are as follows.
(1) The corrosion rate, R_c, in an immersion test was 0.07 and 0.09mm/year for a polished and shotblasted surfaces of the plate, respectively. The effect of the stress increase in corrosion wastage cannot be neglect in the corrosion fatigue tests exceeding five years under the cyclic frequency of 0.17 Hz.
(2) The crack growth rate measured in seawater was two to three times larger than that in air in the range where Paris equation may hold. The effect of the environment on a crack growth rate is significant in the range where the crack intensity factor is relatively low.
(3) The range of the threshold stress intensity factor, ΔK_th, is 2.5 and 1.2 MPa m^1/2 in air and in seawater, respectively.
(4) The long term corrosion fatigue strength can be calculated roughly from the empirical equation.ΔK= f (R_c, y) C_cNⁿ_c, where f (R_c, y) is the corrosion wastage rate estimated from the corrosion rate of material and year, N_c is crack initiation life, n is the index given by crack growth rate, and C_c is a constant obtained from the corrosion fatigue tests of about 10⁶ cycles.
(5) For the long term corrosion fatigue, according to the results of Komai and coauthor, no fatigue cracks initiate from the corrosion pits after 14 to 24 years in the plate specimen area of 320 mm² with R_c=0.09 mm/year, and the pits move to the progression of corrosion wastage in corrosion fatigues less than ΔK_CF.
(6) In order to decide the f (R_c, y) during corrosion fatigue, the tests through five years are necessary.

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The corrosion of internal structures of ships is a complex issue, influenced by many variables. Many guidances for example have been published to prevent members from corrosion and wear. These guidances, which are controllable within the acceptable criteria, can go a long way in reducing corrosion.
For more rational and precise corrosion life assessment, many research plans for example have been made and resarches are under way.
The target of this paper are to offer electrochemical properties in dry/wet alternation tests for unified understanding of various kinds of corrosion tests and to consider actual ship hull corrosion connection with dry/wet alternation test results.
Acceleration rate of dry/wet test in comparison with actual hull corrosion, electrochemical properties of dry/wet test, effects of some paint specifications on corrosion progress and so on are clarified.

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Optical fiber sensors have attracted much attention as a new type of sensor that could be used under environmental conditions where conventional electric ones such as the strain gauge are difficult to use. There are many attempts to apply optical fiber sensors to the permanent monitoring of time dependent structural damages as is the case of fatigue cracking. In this study, the possibility of strain measurement and crack detection by interferometric optical fiber sensors is discussed based on experimental data. The MachZehnder and the Michelson interferometer types were employed and fatigue crack detection tests were carried out. Regarding strain measurements, there is still considerable difficulty in determining whether strain is increasing or decreasing. However, the present work indicates that the use of optical fiber sensors is viable provided the circuit and signal processing methods are improved. As the fatigue crack detection, the results show that such an application of optical fiber sensors shall prove to be successful.

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A method for reliability updating of a group of similar structures is presented. The sources for variation among inspection results of structural details are divided into the effect of the particular structure a detail is imbedded in, the effect of the detail type, and the effect of a modeling error term. By using the proposed method, a deeper understanding of the causes of the differences among inspection results is possible, and as a consequence, a better inspection planning can be made.
Furthermore, since data of several structures and details are investigated at the same time, faster convergence rates to the true values are obtained than if just one individual structure is considered, making it possible for an owner or operator to keep the reliability of his fleet above a specified minimum level more cost effectively.

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The objective of this study is to reveal the hydroelastic behavior of the cross deck structure of a high speed catamaran subjected to slamming load. An analytical method using flat plate approximation is proposed to solve the slamming problem of the two dimensional structures where the hydroelastic effects as well as the pile up water on the free surface are accounted for.
Dynamic responses of simply supported seven-span panels penetrating the wave crest are analyzed. It is found that the maximum deflection and bending moment at the location of initial hit tend to be proportional to the impact velocity as far as the wave profile is relatively flat and the impact velocity is high enough. The results are similar to those obtained by Faltinsen et al. A simple equation to estimate the maximum deflection of the panel is obtained by considering the transformation of fluid kinetic energy into the strain energy.

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In this paper, the authors propose a new automatic technique, learning Tracking Controller (LTC), to install structures whose flexibility can not be ignored any more in unknown current. This method also control elastic response of the equipment to be installed and applicable much larger structures. The proposed LTC can install underwater structures not only in excellent accuracy in any water depth but also control the elastic responses of structures simultaneously.
The presented LTC, which consists of both feedback controller and feedforward controller, can be operated without any information about current which is difficult and costly to be collected before operation. By learning unknown disturbances translated from the errors between the realized trajectory and objective trajectory, LTC improves its feedforward control force and makes the structure track the objective trajectory accurately in the final stage. The convergence condition and robustness of LTC is shown.
Two types of experimental models, a plane flexible structure and a cubic rigid structure, are designed for basin tests which are carried out under unknown current, in order to confirm the capability and effectiveness of LTC. Both of two models were successfully made track the given trajectory and docked to their targets, by their own LTC respectively in the experiments. To obtain docking accuracy of ± 5 mm the models tracked the trajectory six to seven times for learning. The maximum speed of unknown steady current is 0. 07 m/s. No elastic responses of the flexible model were stimulated.

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MS Nakhodka broke down into halves all of sudden on January 2 of 1997 in the Japan sea giving rise of serious and disastrous oil spill to the coastline of Japan. During the inquiry of the cause of the accident, one of the main tasks of the inquiry committee was to identify external loads which made the ship structure collapse. Among several scenarios for the accident, the wave loads in the heavy weather at the accident was taken up as the most plausible cause after careful examination. This paper describes how the wave loads was estimated on the Nakhodka at the moment of the accident. First description is given to details of the accident including the location of the accident, sea condition, loading condition, ship's heading and speed and so forth. Then the static loads due to the distribution of the cargo weight and buoyancy is given to show the static bending moment at the accident was exceedingly on sagging side. Next the wave loads in the irregular waves calculated by nonlinear time domain simulation program SRSLAM is presented. It is shown that the bending moment in the wave reaches up to 1, 087, 800 KN*m at maximum at SS 6.9 where the hull break down presumably took place.
It is cocluded that the ship broke down because the bending moment exceeded the hull strength which had been reduced due to the corrosion, by comparing with the results derived in the paper dealing with structral aspect of the ship.
Furthermore discussions are given to the stochastic significance of the estimated value and nonlinear nature of the peak distribution. Also the effect of the ship speed and wave heading to the estimation is discussed.

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In the early morning of January 2, 1997, a Russian tanker, Nakhodka, had broken into two in the Sea of Japan, and the coast of Japan was seriously polluted by the spilled heavy oil. Following this disaster, the committee for the investigation on causes of the casualty of Nakhodka was established. In this paper, the characteristics of structural strength of Nakhodka at the casualty are described, which could have been known through the research activities and discussions in the committee.
At the beginning, structural characteristics of Nakhodka are described, and the thickness reduction of structural members are estimated based on the measured results. Then, the ultimate longitudinal strength of Nakhodka at the casualty is evaluated applying the so-called Smith's method, and the possibility of overall collapse is discussed. The mechanism of fracture at the bottom plating is also discussed according to the scenario based on the observed fracture surface of the cross-section. At the end, a numerical simulation of the break-up of the hull girder is performed. It is shown that buckling/ plastic collapse took place at the deck plating near Fr. 153, which was followed by the successive collapse of side shell plating. At nearly the same time, the bottom plating fractured just in front of the transverse bulkhead at Fr. 153.

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This paper proposes a vision based guidance system for Underwater Vehicles (UVs) to follow moving targets by collaborating with a pan and tilt camera mechanism. The proposed system can be divided into three main sections. The vision section provides the position information of the UV with respect to the object being tracked. A commercially available CCD camera mounted on the pan and tilt mechanism attached to the UV provides the environmental information as a two-dimensional image and the proposed vision algorithm detects the position of the interested target in that image. By the pan and tilt calibration, a transformation matrix is derived for the image pixel to pan/tilt angle relationship. The second section consisting of the pan and tilt mechanism, brings the interested object in the image to a predefined pixel position. The pan action is derived from the horizontal pixel movement and the tilt action from the vertical pixel movement. As a result of this action, the camera unit will be looking in a direction other than the UVs forward direction. The third section consisting of the UV controller, aligns the camera unit in the forward direction of the UV by controlling the vehicle. An experiment is conducted to demonstrate the performance of the proposed algorithm using the testbed UV available at the University of Tokyo called the Twin-Burger 2. The vision algorithm was implemented on the MaxVideo 250 image processing board. On these hardware, the proposed system successfully managed to track objects moving with a maximum speed of 4 cm/sec.

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Many approaches, which to modeling data structure and data behavior for the total shipbuilding, have been made so far. Especially Object-Oriented product modeling approaches have led to some successful results.
Data in one product model for shipbuilding is inevitably referenced and manipulated during each phase of design or production activities, and data or manipulation status naturally varies from the original with the advance of each activities. For Object-Oriented approach, it is hard to identify object classes dealing with those variations, and even if once a product model developed, it might be getting much harder to modify it to cope with a new additional phase of activities.
This paper proposes an approach, temporarily named Concept-Relationship Modeling Approach, which handles data structure and behavior independently of each other in order to resolve the difficulties above.

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This paper deals with the dynamic response of Very Large Floating Structures (VLFS) in regular waves considering the deflection wave propagation (VLFS-wave) using a simple beam or plate modeling.
Firstly, two kinds of dispersion relations of the VLFS-wave are derived based on the analogy of linear water wave theory, and it is shown that the eigen functions of the VLFS-wave are independent, but not orthogonal. Next, the function separation method-the method for obtaining the relationships among progressive waves-which are the incident wave, reflecting wave, VLFS-wave, and so on, is proposed. Finally, using this method, the stationary wave, reflection and refraction waves of the VLFS are considered. The law of reflection and refraction between the incident wave, reflecting wave and the VLFS-wave are derived, and the simplified dynamic response formulae are proposed for the stationary wave of a 2-dimensional beam, and for a semi-infinite large plate in which case the stationary wave is neglected.

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