The amplitude functions measured by wave analysis experiment for a ship model are expanded in Mathieu function series in order to obtain the informations of improving the hull form. The expansion gives sectional area curve of the model as assumed as a source generating the measured wave system in linear wave-making theory. The expansion coefficients correspond with the coefficients of Mathieu expansion of the assumed sectional area curve and give the strengths of wave pattern resistance components. In order to reduce the total wave resistance the sectional area curve components corresponding with the dominant wave resistance components is to be subtracted.
The amplitude functions measured for three ship models are expanded in Mathieu function series and their wave resistance components are compared with each other at various Froude numbers. The direction of improving hull form obtained by the present method corresponds with the design philosophy of the three ship forms.

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In most of the recent ocean going fine ships such as container ships, the transom stern is adopted, which enables to obtain wider waterplane area for necessary stability than the cruiser stern. Wider transom stern end, however, increases disturbance on water surface behind the stern end (hereafter call “stern wave”) and accordingly hull resistance. The method of transom stern design without increasing the stern wave is a key issue in hull form design for such ships. This paper discusses the relationship between the transom stern end form and the stern wave based on the model test and theoretical calculation.

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Hull forms of sailing yachts differ considerably from those of general merchant ships. As compared with general merchant ships, sailing yachts are small in L/B and d/B of the main hull and have a fin keel below the surface of the water, that converts the fluid force generated by the sail into thrust. The fin keel and the main hull form a junction, and the flow around the fin keel is a typical juncture flow on a protruded curved surface. It is known that horseshoe vortices are generated around the junction. Since these vortices increase fluid drag, they should be suppressed as much as possible. The fin keel has much effect also on the free surface flow around the sailing yacht. It is recognized that the wavemaking phenomena by the fin keel can not be ignored in the high speed range.
In the present paper, experimental studies about the flow field around the fin keel are carried out for the KIT-34 model designed by Masuyannan. Based on these experimental results, possibilities of reducing fluid resistance acting on the sailing yacht are discussed for the condition without yaw angle. In order to suppress horseshoe vortices, a fillet and small bulbs fixed on the junction are tested. For the purpose of reducing wave-making resistance, an inverse taper keel is tested.

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Investigation of the free surface flow phenomena around the bow of floating models is extremely important to predict resistance components acting on floating bodies. Especially in the case of small scale model experiments, the free surface flow becomes more complicated, because the surface tension effect plays an important role in such cases. In order to make clear this effect, Maruo and Ikehata suggested the experimental technique which could weaken the surface tension effect by the method of spray of the surface activator upon the free surface in front of the model in the towing tank. In the previous work, one of the authors carried out the similar experimental study for 2-dimensional rectangular floating models in the circulating water channel. Since elimination of the surface activator remained and accumulated in the towing tank becomes difficult, the circulating water channel is more convenient for these experiments.
In the present paper, the surface tension effect is discussed both experimentally and numerically. As experimental studies, 3-dimensional free surface flow around vertical circular cylinders floating in the circulating water channel can be visualized by means of the same experimental techniques as mentioned above, which can be regarded as typical examples of the free surface flow around floating models with the blunt bow. As numerical studies, simulations of the free surface flow with the surface tension effect are carried out by means of Rankine source method. Through this numerical study, not only simulation of the free surface phenomena but also computations of wave making resistance acting on the floating model with the surface tension effect are expected. Numerical examples are given for a floating vertical circular cylinder and a practical full hull form model.

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This paper deals with numerical computational techniques on viscous flows with free surface past a ship hull by using an improved Reynolds-Averaged Navier-Stokes solver with global conservation.
In the first place, we used the two kinds of computational grids in order to improve the computing efficiency. After computing free surface of flows by coarser grid, we changed to detailed computation of viscous flows with free surface by finer grid.
In the second place, the original Baldwin-Lomax turbulence model was modified by introducing the two kinds of turbulence modifications in order to obtain the simulation of stern flow such as the hook shape of axial velocity contours.
These numerical computational techniques are applied to the simulation for viscous flows with free surface by a series 60 ship model. The numerical results are compared with measurement data and we show the usefulness of these numerical techniques and introducing of the modified Baldwin-Lomax turbulence model.

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In order to calculate the viscous flow around a complex geometry, the three-dimensional viscous flow calculation method with overset grid is developed in the first report using grid embedding technique. In this paper, this method is adopted to the calculation of viscous flow around a hull with appendages (Low Viscous Resistance Fin, L. V. Fin). Reduction of ship resistance by attaching the L. V. Fin is simulated by this method. Influences of the load condition (draft) and the fin span on the effect of the L. V. Fin is discussed.

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A Reynolds-averaged Navier-Stokes (RaNS) equation method for calculating ship boundary layers and wakes including wave effects is evaluated through detailed comparisons with extensive experimental data for the practical 3-D geometry of the Series 60 C_B=0.6 ship model at both low (0.16) and high (0.316) Froude numbers. The former case essentially simulates the zero Froude number condition such that the comparisons with the latter case enables the identification of the salient features of the wave-induced effects. Close agreement is demonstrated between the calculations and the experimental data, which supports the conclusion that the present approach can accurately predict ship boundary layers and wakes, including free-surface effects. Additionally, comparisons are made with inviscid-flow results, which in combination enable an evaluation of the wave/boundary-layer and wake interaction.

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A new type of contra-rotating propellers (CRP) system has been developed through the cooperative research work of five shipbuilding companies (HZ, KHI, MES, NKK, SHI). This paper describes a design system for an optimum CRP, which is one of the numerous fruits obtained from the cooperative work.
The optimum design system is composed of three theoretical programs, i. e. (1) Design program of optimum CRP, (2) Steady lifting surface program of CRP, (3) Unsteady lifting surface program of CRP.
In the former part of this paper, these theoretical programs will be discussed, and the design system supported by these theoretical programs will be verified by comparing calculated results with experiments in the latter part.

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The patrol icebreaker “TESHIO” was built and delivered to the Japan Maritime Safety Agency in Autumn 1995. Full-scale trials in ice were conducted at the Sea of Okhotsk in February, 1996. Several kinds of test were performed in order to confirm her performance such as continuous ice-breaking, ramming and stopping performance, turning capability and zigzag course manoeuvrability, and coordinated ice-breaking operation together with the patrol icebreaker “SOYA”.
While her design and construction were in process, series of model test were carried out both in ice and in ice-free water as the joint research project between Ship Research Institute, Ministry of Transport and NKK Corporation. At the ice model basins of the two organizations, comprehensive model tests in ice were conducted which included resistance and self-propulsion tests in various ice conditions, ramming and turning tests in level ice.
This paper describes the results of model and full-scale tests of “TESHIO” and their correlation for both continuous ice-breaking and ramming performance.
In order to predict the required BHP for the continuous ice-breaking in level ice, the propulsion coefficient was decomposed into three parts which denote effects of open water characteristics, overload and propulsor/ice interaction, respectively. The thrust deduction coefficient obtained from towed propulsion tests in ice showed very good agreement with that from overload tests in ice-free water. The predicted BHP showed good agreement with the full-scale results when the effect of propulsor/ice interaction was taken into consideration.
A prediction formula for ramming penetration distance was devised using an energy-based method. Correlation of both the model and full scale data to the prediction results showed that thrust loss due to the propulsor/ice interaction and the automatic overload protection control of the main engine had much influence on the prediction of ramming performance.

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The problem of propeller-ice interaction is very important for ice going vessels, and this phenomenon is not cleared yet. Since the classical works of Jagodkin in 1963, some calculation models have been presented, but none of them can be widely accepted. One of this reason is that quite a few full-scale measurement data used to verify a model can be seen. Besides, the nozzle propeller has been used for propulsion system of icebreaker recently because of its excellent features, but full-scale measurement data of the nozzle propeller are rare.
This paper describes the results of full-scale data of nozzle propeller concerning propeller-ice interaction. These data were obtained in the full-scale trials of the patrol icebreaker “TESHIO”, which were conducted in the sea of Okhotsk in February 1996. Some strain gauges were attached on the port side shaft, so the regular component and the fluctuations component of shaft thrust, shaft torque and shaft revolution were measured while the ship was going in ice sea. The maximum fluctuations component of shaft horsepower (SHP) was almost as same as the regular component of SHP. The interval of interaction was also proportional to the regular component of SHP. The comparison on the interval of interaction between full scale data and model test data were also shown. It shows the same tendency that the interval of interaction was shorter as the ice thickness become thicker.

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A ship, advancing in a level ice field with continuous icebreaking mode, is subject to several components of ice resistance such as resistance due to breaking of ice, ice buoyancy, removing ice and ship motion. Although the third and forth components increase with ship speed, the first component, i. e. icebreaking resistance, is the most significant. This paper studies the icebreaking resistance at low ship speed.
The results of model experiments on icebreaking resistance were analyzed in detail for three different bows. They were a conventional wedge-shaped bow, spoon bow and concave bow. The following results were obtained.
(1) Independently of the bow shape, icebreaking resistance is in proportion to the total crack length, ice flexural strength and ice thickness squared.
(2) Decreasing hull stem angle at ice contact point raises the width of broken ice piece, resultingin shorter crack length. The icebreaking resistance nondimensionalized by the ice flexural strength and ice thickness is determined by the tangent of hull stem angle at the contact point.
(3) Since the spoon bow has smaller stem angle and larger waterline angle, the total crack length becomes shorter compared to the other bows. This is the reason why the spoon bow gives the lowest ice resistance.

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Distributed Mass/Discrete Floe (DMDF) model is a new numerical model to simulate pack ice motion. In this model, interaction force between ice floes is formuated from the momentum conservation of floe collision. So far, this model has been developed for rectangle floes and disk floes. The most significant difference between rectangle and disk floes is the angle between floe motion and collision. This angle is called as “collision angle”. In the case of packed disk floes collision angle becomes 30 deg., while it becomes 0 deg. for rectangle floes. It is considered, therefore, that the collision angle is a major parameter to express the effect of floe shape.
Experiments on the interaction between a structure and floes were performed at an icetank using plastic floe models. The experiments were made for rectangle floes, disk floes and their mixture with various mixture ratios. As the ratio of disk floe increased,
1. lateral motion of floes increased, resulting in less ice accumulated in front of the structure, and
2. force on the structure decreased.
Above results were also obtained from the computations with increasing collision angle. Thus the collsion angle is an effective parameter to express the difference in floe shape.

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In order to deal with the nonlinear problem of sea keeping, the calculation of ship response in time domain is necessary. For these purpose, Constant Coefficient method (C. C. method) and Convolution Integral method (C. I. method), which are based on the Strip Theory, are generally used. However, Almost all of these researches are focused on the ship motion in regular waves.
Firstly, the authors present the concept of numerical simulation of not only ship motions but also wave loads and pressure distribution in irregular directional waves.
Secondly, by Comparing with frequency domain analyzed results and an experiment, the reliability of numerical results of present method is obtained.
Finally, the difference of the numerical results in directional irregular waves between C. C. method and C. I. method is shown.
In this paper, the equation of ship responses is limited in the linear case, however, this algorithm can be easily extended to the non-linear case, which will be reported in the next paper.

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This study presents a new statistical prediction method for deck wetness on ocean-going ships. The deck wetness is one of the most important performance factor of seaworthiness. When the characteristics of deck wetness are investigated in relation to loads on deck due to shipping of water, seakeeping quality etc, the deck wetness generally means the situation in which the relative motion amplitude between deck and wave surface exceeds the freeboard. The excess has two physical natives, that is, the magnitude of the amplitude and the time during which the amplitude remain above a level of freeboard. Until now, many studies have been proposed in relation to the former, but few studies have been done for the later. Therefore, this paper proposes a practical method for estimating statistically the time period (lasting time) related to deck wetness in sea waves. The method is constructed with the basic statistics and the 5 freedom ship motions theory (the Ordinary Strip Method, OSM). A series of numerical calculations are executed for an ocean-going container ship by using the method, and it is certified that the lasting time affects the trend of deck wetness.

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The research of ocean waves has been important, not changing since the Age of Great Voyages, because we can't still get the complete knowledge of them. The objective of this research is to develop the fast calculation process of ocean waves, which includes the customized hardware as well as the software algorithm. The authors have realized the real time simulation of many types of ocean waves by the parallel computing system which consists of a PC (Personal Computer) and DSPs (Digital Signal Processor). The real time wave synthesis executing in DSPs is also developed. The methods are applied to a wave simulator as well as a wave maker.

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With the progress of ocean development, new type of ocean structures are being commenced to be developed. However, it is not known yet how presently existing mooring systems can cope with these new type of ocean structures. Then, investigation and analysis of characters of existing mooring systems are carried out firstly. The range where characteristics of each existing mooring system can expand is investigated, and the future style of it is studied. The possibility that the ranges of existing mooring systems and their expanding ranges can cope with the mooring characteristic demands of new type of ocean structures is discussed.

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Recent tension leg platform (TLP) model tests have revealed strong transient resonant responses during extreme wave frequency responses. It is said that these responses, called ringing, are associated with higher order forces induced by large-crested, near breaking waves. In this paper, as a second step of giving light on the occurrence mechanism of such nonlinear phenomenon, third order wave force measurement test are carried out in regular and irregular waves. A third order spectral analysis method (the so-called Cross tri-spectral analysis) is newly developed to estimate the second and third order transfer functions of wave loads on TLP. And the numerical calculation method based on the third order diffraction theory (Malenica and Molin theory) has been developed for a vertical uniform cylinder. Both the calculation result and the approximate solution for a vertical uniform cylinder, which is derived by Faltinsen-Newman and Vinje (low ka-low KA theory : k ; wave number, a, cylinder radius, A; wave height), are used to evaluate approximately the third order force on TLP under the assumption taking only the phase difference based on the column distance of TLP into account. As a result, the present higher order spectral analysis method is much effective for identifying the higher order transfer functions from wave force data in irregular waves. However, the approximate methods used for estimating the third order force on TLP underpredict against the experimental results. It is necessary to take account of some interaction effect between columns.

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To design a floating production or drilling structure, we have to estimate the interacted responses among a floating offshore structure, risers and mooring lines in external forces. This study has focused a floating structure with riser. First of all, this paper has discussed with behavior of a forced oscillated riser model.
First, model experiments in a water tank have been carried out in forced oscillation conditions to observe the behavior of a riser model.
Second, the behavior of forced oscillated riser model has been estimated by two numerical mothods. These methods are based on Morison's equations, and CFD method, respectively, for calculating hydrodynamic forces around a 2D riser section.
To validate corresponding of numerical estimation, experimental results have been compared with numerical results. From these results, the numerical methods have been confirmed as useful and appropriate.

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The paper investigates the path integral solution method combined with the gamma-distribution approximation for exciting forces, for calculating the response statistics of nonlinear dynamic systems whose equations of motion can be modelled by the use of Ito stochastic differential equations. The state vector process of motions is generally a diffusion process, and the transition probability density function (TPDF) of the process satisfies a Fokker-Planck-Kolmogorov (FPK) equation. The method solve FPK equation for short time TPDF. Naess assumed that second order wave exciting force is described by one term of square of Rayleigh process, but this assumption is not actual. Authors propose the new path integral solution method which express the wave exciting force by gamma process. In this paper the new method is applied to the cases whose exact solution exist, and is verified its effectiveness.

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A study of the ship capsize phenomenon due to parametric resonance is conducted aiming to identify the occurence of a critical condition leading to capsize by considering an analytical solution of an equivalent linearised equation describing the problem in conjunction with results of numerical simulation of the non-linear equation. The main objective of this paper is to formulate the problem in a way that enables an analytical solution to be derived and facilitates an investigation towards identifying the occurence of the critical condition leading to capsize through comparative studies between the analytical solution and the results of numerical simulation.
Finally, a method for judging the possibility of ship capsize due to parametric resonance is proposed based on application of the energy balance method during the critical rolling motion.

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In ship handling at a harbour, captains or pilots face to various ship handling scenes. In order to overcome these, they are required to derive the maximum maneuverability from the ship. The purposes of this paper are put on making clear from the geometrical and ship handling theoretical view-points about the minimum stopping maneuvering methods of two ships (a small training ship with bow and stern thrusters and a container ship under tug controlling), obtained by numerical solutions using the sophisticating non-linear ship maneuvering models and providing the fundamental data for actual maneuverings and future development of automatic ship's berthing system.

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When a ship is moving in lateral direction at berthing in a harbour or when a ship is moving with large drift angle at low speed, it is known that the component of cross flow drag dominates the hydrodynamic force acting on ship hull. It is important to develop prediction method for the cross flow drag from the view points of safe navigation of ships at low velocity in harbour or restricted water.
The authors has proposed the prediction method for the cross flow drag acting on a ship hull based on the vortex shedding model in previous paper. However, the prediction method requires to determine parameters α which reduces strength of the free vortices shed from the ship hull.
In this paper, we improved the vortex shedding model considering damping of the free vortices. It is shown that longitudinal distribution of the cross flow drag along the ship length can be predicted with comparatively high accuracy using the presented prediction method.

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A series of captive model tests was carried out to measure the stability derivatives of an unmanned untethered submersible AUV-HM1 testbed, which has a body of a streamlined section, developed at National Taiwan University. Furthermore, for estimating its' stability derivatives, a method based on the empirical formula proposed for standard torpedoes by Bottaccini is investigated. The estimated values of stability derivatives are examined and compared with those obtained by measurement. As a result of comparison, the validity of the method is confirmed. It was also shown that the maneuverability of the submersible can be evaluated by the method with accuracy enough for practical use.

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It was pointed out that there are some weak points on the transverse stability and turning ability in research on the development of Super High Speed Container Ship, so, in the 1 st report, we introduced vertical active fin under the bottom of the ship and reported its basic effectiveness by numerical simulation and model experiment in directional spectrum waves which was generated in the towing tank of Yokohama National University.
In this report we introduced optimum regulator for the coupled control of rudder and vertical fin to improve stability and manoeuvrability. In order to carry out numerical simulation of the ship motion in directional following waves, we use experimental data of yaw and roll for estimating wave exciting force. Model propeller used for self propelled experiment was contra-rotating propeller in this time.
We also refer to the reason of problem pointed out in the 1 st report that rolling motion became large even in the case of fin was fixed comparing to without fin case.

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Recent offshore oil and gas development work and/or geoscience research work need heavy offshorework such as setting heavy structure on the deep sea bed, piping connection in deep sea etc. In a long period work, heavy structure may be let suspended in the sea from a floater under large floater motion in rough sea because of inability of work.
Heave compensator seems to be an effective measure to adopt to the above kind of offshore works and compensators are already put to practical use with air spring mechanism. Lately, active heave compensatorsare also researched and developed to get high compensation effect.
However, in case of heave compensation while suspending heavy structure under large floater motion, non-linearity and resonance of the airspring and dynamic friction effect at the compensating mechanism tend todecrease the compensation effect or rather increase the danger during suspension. Therefore, in case of activecompensation in rough sea, above mentioned effects shall be counted in the system design to get effective com-pensation.
In this paper, active heave compensator combined with air spring mechanism is studied considering nonlinear effect of air spring, friction at the compensating mechanism and elastic response of suspended structu-re. High performance of the active compensation is verified by the model test in waves corresponding rough sea.

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A method to estimate the statistical properties of the wave groups in non-linear irregular waves is presented which is based on the Auto Regressive Model and the second order random wave theory. It is shown that the statistical problem of wave groups can be reduced to that of finding the Auto Regressive Coefficient and the marginal probability density function of wave amplitudes, from which the joint probability density function of successive wave amplitudes can be estimated. The method is verified based on numerical simulations and a field data measured in stormy sea states. It is shown that that there is good agreement between data and results estimated by the present method.

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One of the dangerous situations for ships navigating in following and quartering seas is to encounter occasionally a group of high waves, i.e. high run, which may be larger than those observed in ocean wave itself. The spectral analyses in frequency domain and the numerical simulation in time domain of encounter waves have been carried out, to investigate the statistical properties of the encounter wave grouping phenomena.
According to Longuet-Higgins' procedure on ocean waves, the band width parameters of the encounter wave spectra have been used to calculate the probability of occurrence of group length and high run as the functions of various operation conditions of ship, such as ship speed/ wave period ratio, V/T, and encounter angle. The results have been confirmed by the statistical analyses of simulated time series of encounter waves that the probability of occurrence of high runs in following sea becomes higher in the range of V/T between 1.0 and 2.0 with the highest value at V/T equal 1.45.
The influences of directional distribution of wave energy, i.e. the effects of short-crestedness of ocean waves have been also investigated and it has been shown that the increment of encounter wave groupiness is less, as the directionality of waves becomes wider.

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In recent years, more accurate wave data base representing ocean wave environment used for planning for construction of ship and floating offshore structure is demanded. For this purpose, it is needed to develop the wave measuring system at navigation.
In this report, we studied about the method of measurement of directional wave spectrum integrating several information on board including ship motion and RADAR image. Next, we extended Maximum Likelihood Method to following sea problem. For the verification of this theory and method, we carried out towing tank test and full scale ship test using same ship consistently.
Finally, it was shown that this integrated method using ship motions and PPI image of the RADAR give fairly good results.

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For long time simulation of floating body motions, an artificial damping zone is applied to satisfy radiation condition at the end of numerical wave tank. This artificial damping zone works as an efficient absorber of fully nonlinear waves at the tank end. Combination of this artificial damping zone and wave maker works as an absorbing wave maker. The implementation, the characteristic and the efficiency of this artificial damping zone are reported.
Using the damping zone as a wave absorber and an absorbing wave maker, a moored two dimensional body motions in regular waves are simulated by the fully nonlinear simulation method based on the velocity and acceleration potential theories. The simulated body motions, wave periodic motions and slow drift motion, are presented in comparison with measured motions in wave basin. The simulated motions show good correspondence with the measured motions in overall.
For the simulation of wave periodic motions of body, a numerical technique to accelerate convergence of the simulation is also given.

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In recent years, the use of very large floating structures in the seafront areas for various infrastructure facilities such as a floating airport is becoming more popular and important. For such a huge floating structure like an airport, the elastic deformations due to the dynamic effects of waves cannot be neglected in comparison with the motion as rigid body, because it has a very small vertical dimension compared to its longitudinal and lateral dimensions.
A linearized analysis of the motions of a very large floating structure with floating breakwater is presented in this paper. The hydroelastic interactions of the elastic deformations and the hydrodynamic forces are discussed. Present predictions are compared with the avail results based on the other method and experimental data. The influences of some parameters (bending rigidity and water depth) to the motion responses have been investigated by the present method.
For the floating airport, it is imperative that investigations on the effectiveness of floating breakwater and rigidity of the floating airport to decide and understand its sea worthiness. In the present mathematical model, the hydrodynamic interactions between the floating airport and the floating breakwater are considered. The effectiveness of the floating breakwater (including the multiple floating breakwater) as the protection of the floating airport is investigated. According to sea condition and design criteria, many parameters related to floating breakwater system can be discussed by the proposed method.

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This paper deals with three-dimensional responses of a very large floating structure in head and quartering seas. First we estimate three-dimensional dry-eigenmodes of a completely free plate by employing a finite element method. Then, hydrodynamic pressures exerted on the plate are evaluated by combining a pressure distribution method and the dry-eigenmodes. Wave-induced motions of the plate are finally determined by solving a set of equations of wave-induced motion in the principal coordinates. Present modal analysis method is verified through comparisons of calculated amplitudes of vertical motions and instantaneous shapes of a plate model in head seas with available measured data. They show reasonable agreements. Next, we apply this method to the large floating module of 300m × 60m, and investigate the motions of it and the disturbance waves around it in head and quartering seas. Finally, the response amplitude operators of the first 10 modes in all heading waves with various wave lengths are shown.

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A pontoon type structure was recently highlighted since The Floating Structures Association of Japan proposed the new concept as the most beneficial one in 1994. The structure is supposed to be mat-like structure with length of 4 to 5 kilometers and depth (draft) of 5 (1) meters. The concept was further developed by The Technological Research Association of Mega-Float which was established in 1995. They constructed 300 meter long prototype structure to carry out at-sea test. The authors carried out tank tests to investigate hydroelastic response of the prototype structure using its scale model. The research was undertaken partly as joint work with T. R. A. of Mega-Float. The scale ratio of the model was decided as 1/30, with length of 9.75 meters, breadth of 1.95 meters and draft of 1.66 centimeters. The model was designed on the basis of the law of similitude and has elastically similar bending rigidity with 300 meter long prototype structure. A series of tank tests had been conducted focusing on the elastic response of the model in regular waves. The vertical motions have been detected through potentiometers and the bending moment through strain gages both distributed on the upper surface of the model. The analysis method based on 2-D structure and 2-D fluid modeling has been established in which the structure is approximated as a rectangular plate and the hydrodynamic characteristics is evaluated using the pressure distribution method based on the zero-draft Green function. The equation of motion, in which the interaction between structure and fluid is taken into account, is solved by the direct procedure instead of by modal analysis. The calculated results show quite satisfactory agreement with the experiment and the study has highlighted some remarkable characteristics concerning hydroelastic behavior.

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The hydroelastic responses of two very large floating structures, semisubmersible type and pontoon type having same displacement and same flexural rigidity, are analyzed and compared in this study. Three dimensional hydrodynamic forces are calculated by using singularity distribution method in which the symmetries of structure were exploited. By combing the mode analysis based on FEM, the deflections are estimated. Some numerical results are compared with experiments and an overall good coincidence has been obtained over wide frequency range.
The distribution of hydrodynamic force, vertical displacement and bending moment at different positions are shown. Furthermore, the analysis are also extended to oblique wave conditions, it is found that the two structures have different response each other due to their different characteristics in deformation mode and interaction with fluid. Finally, some investigations are done as for short crested irregular waves, the influence of principal direction and directional distribution of waves are discussed and the significant and maximum value of deflection are shown for both two models.

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In this study, to clear the behavior of a Mega-float structure in environmental exciting forces of real sea condition such as wave, current and wind load, we made a pontoon type elastic floating model and carried out model experiments on the characteristics of motions, wave drift force and drag forces in current and wind for a large elastic floating body. And the theoretical results of the wave drift forces for the various water depths and the various bending stiffness are illustrated.
When we estimate the behavior and mooring forces of Mega-float structure in real sea condition, we must consider the combined forces acting on the structure and effects of shallow water.

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A full-nonlinear computation technique based on the Mixed Eulerian-Lagrangian method is presented and validated quantitatively for a fundamental 2-D radiation problem. The time marching is performed with the 4-th order Runge-Kutta-Gill algorithm, and at each time step the integral equations for the velocity and acceleration fields are solved by applying a higher-order boundary-element method with quadratic isoparametric elements. Stable simulations over a large number of periods are possible, because an efficient absorbing beach is used to avoid wave reflections from an outer boundary and regridding is performed to avoid the cluster of nodal points on the free surface.
Computed results are compared with linear-theory results of the Green function method in the time and frequency domains, measurements of the time history of waves generated by a wavemaker, and Fourier-analyzed results of hydrodynamic forces acting on an ellipse and a wedge. All of these results are in excellent agreement, confirming the validity of the calculation method.

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A new method is developed to predict wave drift force acting on a large floating platform set on a sloping beach. The platform is supported with a lot of floating element. We assume the structure's dimensions in the horizontal direction are comparable with the topographical scale, while each floating element's size is small.
To predict drift force acts on such kind of platform, refraction, diffraction and wave breaking should be considered. Assuming the bottom slope is comparable small to wave slope, refraction is described by mild slope equation. It is matched with the solution that holds near the floating element. To consider energy absorption due to breaking, new solution is derived. It is composed of Bessel function and Whittaker function that is a kind of confluent hypergeometric function.
Delaunay triangulation method is adopted for pre-process for FEM. This method generates moderated triangle mesh automatically.
Drift force that acts floating body set on slope bottom has irregular resonance. It shows that topographical effect should be incorporated.

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While it is said that a floating structure has less significant effects on marine environment than a reclaimed island, it is important to predict precisely the effects of a floating structure on the surrounding environment, in particular, the surrounding sea water. The main aim of the present paper is to clarify the sea-covering effect of a huge floating structure on the surrounding environment because heat flux through the sea surface may be strikingly obstructed by the presence of the floating structure. By making use of a numerical simulation program based on multilevel modeling approach, numerical calculations were carried out to estimate not only the change of flow field but also the change of temperature and salinity distribution around a huge floating structure. As a result, it is found that induced flow around the floating structure becomes unnegligible amount especially beneath the structure, and that the estimated flow field is largely depenedent to the choice of both coefficients of vertical eddy viscosity and vertical eddy disffusivity.

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This paper presents the effect of earthquake on the floating structure. We separate the effect of earthquake into two phenomenon, tsunami and seaquake, based on the Sells' work which treat the disturbance of slightly compressible gravitating liquid caused by a block instantaneously being jerked, And by the analysis that the “POSEIDON”, the floating structure for test, is assumed to be exposed to tsunami, we propose the treatment of tsunami to design a floating structure. At the view of local strength from the bending strength of the bottom shell plate, we propose the treatment of seaquake to design a floating structure.

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The paper is concerned with a dynamic positioning control of a moored floating platform model. Moorings are used to maintain the position of the platform against quasi-constant external forces induced by current and wind. The control purpose is to suppress the slow drift oscillation due to drifting force in irregular waves using thrusters so that they do not respond to linear exciting force in wave frequency range. Control of motions in wave frequency region has to be ignored because the amplitude of those motions is smaller than those of the slow drift oscillation and enormous thruster power is needed to control them. The problem is formulated in the framework of H_∞ control, and is solved using LMI (Linear Matrix Inequalities) Control Toolbox on Matlab ; some successful results are shown.

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The analysis of thin shell during large deflection has complex problem associated with the geometrical and material nonlinearities, in which the solutions for stress and deflection are desired to obtain the same level of accuracy. One of ordinary and powerful method for this subject is a finite element method that generally has inconvenience to be necessarily extensive calculation due to large number of freedom.
This paper is concerned with the elasto-plastic analysis of thin shell structures, using a modified version of hybrid method by S. N. Atluri, et al. For elasto-plastic materials, when the stress distribution across the panel thickness is expressed as continuous function using the delta-sequence, numerical integration can be done away with. This approach introduces considerable reduction of computer time. The results obtained by this analysis could be discussed in the mechanism of resisting external force in thin shell structures during large deflection.

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A series of elastoplastic large deflection analyses was carried out on thick stiffened plating with flat -bar stiffeners subjected to uni-axial thrust applying the FEM code ULSAS. A part of continuous stiffened plating was analysed considering symmetry conditions. Both stiffener and plates were modeled by shell elements. Primary attention was focused on the influence of overall initial deflection on the collapse behaviour of continuous stiffened plating. It has been found that :
(1) Ultimate strength of stiffened platings considered here is attained when stiffener tripping takes place at the central part of a span. This is followed by a local plate collapse at the central part of the adjoining span.
(2) Symmetrical mode of overall initial deflection having same directions in the adjacent spans tends to increase overall buckling strength (ultimate strength), whereas unsymmetrical mode of overall initial deflection has an adverse effect.
(3) Ultimate strength of stiffened plating with larger aspect ratio of panels between stiffeners is more influenced by the shape of overall initial deflection.
(4) Ultimate strength of local panel flange with initial deflection of hungry-horse mode is higher than that of a simply-supported plate of the same geometry.
(5) Ultimate strength estimated by Perry-Robertson approach assuming stiffener-induced failure mode shows good correlation with FEM results with some safety margin.

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The present designs of ship structures are mainly based on the theories in which deflection of structure is not more than the order of plate thickness. In order to estimate the strength of ship structures when subjected to extraordinary large load, post-yield behavior of ship plates should be investigated. This paper is a theoretical analysis of this problem. Six structural failure modes are identified that are characteristic of the behaviors of ship plates when subjected to very large load. The new formulas of the load-carrying capacity for these failure modes are proposed. It is pointed out that energy dissipation due to fracture is small and therefore this effect is not taken into account. It is shown that geometry of loading object has strong influence on the load-carrying capacity of structure. By using these newly developed prediction formulas, it is possible to construct very simple method to assess the strength of a ship in the event of an accident.

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The product of large scale high speed aluminium vessels has increased. For those vessels, reduction of structural weight becomes an important subject, and structural elements of light weight but high strength is sought for.
For hull structures of aluminium ships, extrusions from A 5083 S-H 112 alloy, pi shaped sections, had been popular. However, from the view point of the lightening above mentioned, extruded hollow sections from A 6 N 01 S-T 5 alloy and pre-ribbed panels of A 5083 P-H 321/A 5083 S-H 112 were developed and are going to be applied to hull structures.
In this paper, the buckling strength of hull structure by these extrusions and pre-ribbed panel is dealt with. Using these elements, test models were manufactured. Axial compression tests were performed on the models. Based on the test results, the efficiency of the elements, in saving structural weight was studied.

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Based on a series of works finished by our research group this paper presents a systematic method of element-wise a posteriori error estimation and adaptive mesh generation in finite element analysis in two-dimensional elasto-plastic problems for an integrated system. In this method, the error is estimated element by element for the arbitrary shaped elements, such as triangle and quadrilateral isoparametric elements. A technique to partition the error force of traction jump between adjacent elements is discussed. The mesh is originally generated according to the geometric condition of the configure model and regenerated automatically according to the error norm and its distribution with the objective that the total error energy norm in the whole domain becomes minimum. The high convergent rates of error energy norm are obtained in two numerical examples. Since the calculation is done with the arbitrary shaped elements and the mesh is regenerated automatically, the method developed in this paper has high abilities and effectiveness for two-dimensional elasto-plastic problems in the real engineering field.

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The Adaptively Shifted Integration (ASI) technique, which produces the highest computational efficiency in the finite element analysis of framed structures including static and dynamic collapse problems, is applied to the explosive demolition analysis and seismic damage analysis of reinforced concrete buildings. By expressing an explosion or a fracture by a plastic hinge located at the exact position with a simultaneous release of resultant forces in the element, strong non-linear and discontin uous problems such as these can be easily analyzed even by the finite element code with the displacement form.
By using the algorithms described in this paper, sufficiently reliable solutions for the practical use have been obtained in the explosive demolition and seismic damage analyses of a 5 stories-5 span reinforced concrete building.
The present technique can be easily implemented with a minimum effort into the existing finite element codes utilizing the linear Timoshenko beam element.

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This report is concerned with a numerical calculation method for the underwater sound radiation by the hull vibration.
Iwasaki reported a numerical calcuation method for coupled structure-fluid interaction problem. As for the method that Iwasaki suggested, the computer model would necessarily have large number of degree freedom and would require an excessive computer cost. And also, there was a problem of Irregular Frequency that are the eigen frequencies of the corresponding interior problem. In this study, we examine the following improved numerical calaulation method.
(1) We employ a modal approach whereby the structure displacement is approximated by a linear combination of the dominant in-vacuo, undamped normal modes of the structure. By this procedure we can shorten calculation time largely.
(2) We combine discretized surface and interior Helmholtz integral equations to obtain an overdeter-mined system. If the interior points do not all happen to fall on a nodal point of an interior normal mode, then the method uniquely solves the Helmholtz integral equation for all wave numbers. We combine the procedures (1) and (2), then we can estimate the characteristics of the under water sound radiation by mechanical vibration of hull appropriately.
We examine the effect of anti-noise and vibration system of engine by using the improved numerical calculation method. The relation between the property of anti-vibration rubber and the radiated under water sound are investigated numerically.

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The outstanding obstacle to estimate vibration level is, it is well known, that the precise damping of fluid and structure can not be estimated because the mechanism of the damping is not explicated. Therefore it is necessary to investigate the phenomena of damping caused by fluid viscosity, and the authors proposed a theory of added damping matrix based on dissipation energy caused by fluid viscosity. To verify the validity of the damping matrix, more experimental data was necessary.
In this paper, some experiments on the dependency of dissipation energy on vibrational displacement are shown, and the important result, which we may approximate the damping of fluid to linear damping matrix in the vibration problem, is verified. Comparing some calculations and experiments, the following are declared. In case of infinite depth of water, using ν (kinematic viscosity coefficient) as νω (virtual kinematic viscosity coefficient), the calculation and experimental results agree well on the dissipation energy, in the result νω is not so important. In case of shallow water, νω must be used, and if νω estimated from the dissipation energy by experiment is used, the frequency response curve can be reproduced.

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In the previous paper, a method for determining the fatigue limit of mild steel SM 41 B under variable-amplitude loading condition has been proposed. It is shown that the fatigue limit coincidences with the stress level where the deformation mechanism of the material changes from visco-elastic to visco-plastic-elastic behavior. The transition point is detected by measuring hysteresis energy of cyclic stress-strain response when cyclic hardening disappears (stress-strain behavior keeps constant). A visco-plastic-elastic constitutive equation is proposed in order to analyze the above behavior. For arbitrary loading history, the transition stress level, that is the same as the fatigue limit, can be calculated by formalizing the developing equations of the coefficients of the constitutive equation.
In this report, the relationship between macroscopic stress-strain response and strength characteristics of Aluminum alloy A 5083 P-O is investigated. However, the above method can not be applied to A 5083 P-O, because this material yields no plastic strain when cyclic hardening disappears. That is, plastic strain increases just after applied stress amplitude increases, then plastic strain decreases gradually and stress-strain behavior keeps completely elastic stress-strain state. This stress-strain behavior is called “strain burst”. This plastic strain is mainly investigated as a parameter instead of stable stress-strain behavior.
Main points are as follows :
1) Plastic strain measured during transition term depends on loading history, such as load sequence, increment of applied stress and number of loading cycle, etc.
2) It is possible to estimate the density of dislocations which is released when strain burst occurs. The density of released dislocation would be related to the possibility that PSB (persistent slip band) is developed.

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Stress concentration at weld toe is one of the major factors influencing fatigue strength of welded structures. The stress concentration factor (SCF) K_t is expressed by K_t =K_w·K_s, where K_w is the local SCF depending upon the local geometry of a weld, and K_s is the structural SCF determined by the joint type of a structural member. The structural SCF is currently taken into account by some of fatigue design guidances. While the local SCF and K_t are considered as unknown factors and to be involved in the basic S-N curves. For a consistent fatigue evaluation of various types of joints, it is necessary to establish a unified method dealing with both local and structural SCFs quantitatively.
In this study, a fatigue evaluation method has been studied based on the following phenomenon ;
1. Since local stress concentration is principally caused by a very small radius of weld toe, a high stress region is quite limited in a small area. Therefore, the local SCF affects fatigue crack initiation and growth of a very small crack. This means that even if K_w is as large as infinity, the fatigue strength is still greater than that for a crack propagation life. Namely, as K_w becomes large, the fatigue strength decreases from that for a small K_w to that for crack propagation life.
2. Whereas the affecting area of structural stress concentration is relatively large, the structural SCF influences not only crack initiation but also crack propagation until the crack becomes large. Thus, the K_s has the same effect as an increase in nominal applied stress.
Eventually, a unified equation in which the local and structural SCFs are integrated has been proposed for fatigue strength estimation of welded structural members.

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In this paper it is mainly presented on corrosion fatigue behavior of the tar epoxy resin coated KA 32 (TMCP) steel for ballast tank based on the corrosion fatigue test results for the notched plate specimen and ultrasonic corrosion fatigue test results for the plane round bar specimen with different tar epoxy resin coating thickness of 50 to 300 μm in an artificial sea water. In higher nominal stress range an effect of tar epoxy resin coating for improvement of corrosion fatigue life was not observed. The lower the stress level, an improving effect for corrosion fatigue life increased. An increase of corrosion fatigue life by tar epoxy resin coating with 200 μm thickness was 2.8 times for that of uncoated specimen at nominal stress range of 199.8 MPa. The thicker the tar epoxy resin coating thickness the longer the fatigue life is.
Electric impedance and capacitance were measured for plate specimen during exposure test and for fatigue test specimen during corrosion fatigue test in an artificial sea water. The degradation of tar epoxy resin coating occurrs at the notched area or the weld toe where the coating absorbed water.
It can be concluded that the improvement of corrosion fatigue strength is not effective when an interception effect disappear due to the breakage or the degradation of the tar epoxy resin coating.

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