Sail forces were measured in a full-scale sailing boat with the use of a sail force dynamometer. This apparatus consisted of an aluminum frame fixed to the hull by way of several load cells. The sailing boat was modified so that the dynamometer frame could be installed inside the hull. The mast, stays, winches, and other sailing rig were fixed on the frame so as to transmit all the forces acting on sail to the frame. By transforming the measured forces, the lift force, drag force, thrust, side force, or the center of effort of the sail force could be obtained. The sailing conditions of the boat, such as the boat speed, heel angle, wind speed, wind angle, and so on, were also measured.
Sail shapes of the boat in the up-wind condition were also measured with the use of CCD cameras installed in the boat. The sail shape images taken by the cameras were transformed to bit-map files, and then processed by an SSA-2 D, a sail shape analyzing software. With the use of this software, sail shape parameters were obtained. The relationship between the measured sail forces and the sail shape parameters is discussed in this paper. Moreover, the measured sail shapes were used as the input data for the numerical calculations.
Numerical calculations were performed to estimate the sail forces of the boat. In the calculations, two sails, a mainsail and a jib, were modeled in the form of a vortex lattice. The vortex lattice method was adopted as the numerical calculation method. Step by step calculations were conducted up to attaining the steady state of the sail in steady wind. Calculated sail forces were compared with the measured forces, and the validity of the numerical method was studied.

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A new system of performance prediction simulation is developed for the design of sailing boats. The finite-volume method within the framework of the boundary-fitted, moving coordinate system is combined with the equations of motion. The forces and moments derived from the CFD computation are given to the equation of motion so that the acceleration and movement of the boat is calculated. Then the movement is treated as deformation of the grid system. This system is applied to the design procedure for a IACC class sailing boat for which steady sailing attitude plays an important role for the sailing performance.

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The three-dimensional viscous flow around a jib and main sails of a sailing yacht is simulated by using of finite-volume method and zonal boundary technique. For the generation of grid system of the complicated three-dimensional geometry of sails, an interface boundary technique is introduced at the narrow space between two sails. This simulation is applied to the sail system of a IACC class boat and studies are made on the detailed flow around jib and main sails, the influence of interaction on the performance, the effect of the wind angle and so on. It is demonstrated that this method can be used as a tool for sail design and performance prediction of sails.

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A new simulation method is developed for simulating unsteady motions of a sailing boat on the free surface. The aim of this method is to predict hydrodynamic performances of a prototype sailing boat by CFD simulations. Fluid dynamic forces acting on elements of the boat are calculated in a CFD system and combined with the equation of motion of the boat. Unsteady motion of the free surface and body boundary is treated in a moving grid system that is regenerated in every time step. Some calculations are presented for the steady advancing, rolling and maneuvering motions.

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A finite volume method in unstructured grid system is developed for the study on complicated twodimensional low Reynolds number flows. The numerical method is first validated by a cavity flow problem, and then applied to some problems such as a surface flow accelerator and a rotating body problem which would be difficult to solve by a structured grid.
A moving grid system is introduced and applied to an unsteady problem. The developed moving grid system makes it possible to change the grid fast to fit a moving body at every time step without any regeneration of grid. The present computations show the possibilities of applying the developed technique to the analysis of flows around multiple moving bodies.

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This paper deals with numerical computation on viscous flows with free surface past a full ship hull such as a VLCC by using an improved Reynolds-Averaged Navier-Stokes solver with global conservation.
In the previous report, we investigated the detailed computation of viscous flows with free surface by the modified Baldwin-Lomax turbulence model which 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. The numerical computations were applied to a series 60 ship model. The numerical results were compared with measurement data and we showed the usefulness of this numerical scheme.
In the present paper, this scheme is applied to the simulation of the viscous flows with free surface around SR 196 A, B and C of a large full ship type having slightly different frame lines in the aft half. We could show the difference of viscous flows around the aft half by the differences of frame lines of each ship model.

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Flows around two full hull forms in oblique towing or steady turning motion have been computed using a CFD code NICE with the Baldwin-Lomax turbulence model. The computed flow in oblique towing agreed well with experiments in terms of the sway force distribution in the longitudinal direction, as well as the integrated sway force and yaw moment, and the computed hull surface pressure distribution explained well the difference in the sway force distribution on two hull forms. The modified Baldwin-Lomax model gave better results than the original model. Using the same code, flows in steady turning were also computed. Although the agreement with experiments were slightly less satisfactory than those in oblique towing, the modified turbulence model gave better results again.

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In the 1st report, it was shown that the icebreaking resistance at low ship speed is proportional to the total crack length independently of the bow shape, and is determined by the tangent of hull stem angle at the hull/ice contact point. The effect of bow shape on the icebreaking resistance is investigated in detail by analyzing the results of extensive model experiment with three different bow forms. The icebreaking resistance is obtained by subtracting the resitance in pre-sawn ice from that in level ice.
This paper studies the speed dependency of icebreaking resistance, i.e. the increase of icebreaking resistance with ship speed, comparing it with the vertical motion of ship bow. The major results are as follows :
(1) As the ship speed increases, icebreaking resistance increases as a linear function of bow vertical motion. A spoon bow shows the lowest speed dependency, a wedge-shaped bow is the second, and a concave bow is the highest. The icebreaking resistance nondimensionalized by ice flexural strength, ice thickness and tangent of stem angle at the hull/ice contact point increases linearly with the heave amplitude at bow nondimensionalized by ice thickness, independently of the bow shape.
(2) Frequency analyses of resistance and bow heave have shown three origins of icebreaking resistance. They are (a) resistance due to hull motion, (b) resistance due to icebreaking at the shoulder, and (c) resistance due to icebreaking at the bow center. The former two components are large in case of the wedge-shaped and concave bows. The third component is large in case of the spoon bow.
(3) The rotation of broken ice piece is closely related to ship motion and broken ice piece size and shape. Such ice/ship motion coupled resistance can not be well removed by a pre-sawn ice test.
(4) The resistance increase due to broken ice piece rotation is determined by the rotation speed and broken ice piece shape. The rotation speed depends on the hull inclination angle at hull/ice contact point. The broken ice piece shape depends on the bow waterline angle and curvature.
(5) Because of the above-mentioned relationship between bow shape and broken ice piece, the wedge-shaped and concave bows produce very large and long ice pieces at the shoulder and the ice piece rotation speed is high, while the spoon bow gives wider but shorter (less slender shaped) ice pieces whose rotation speed is low. This is the reason why the spoon bow shows the lowest speed dependency of ice resistance.

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A numerical method to solve integral equation using Green's function for harmonically oscillating three dimensional flow field around an advancing ship is presented. The method named as “The integrated Green's Function Method” is based on idea that the singular behavior of the kernel of Fourier integral type can be removed by integrating spatially before making inverse transform. In doing so, trapeziod plane panel with constant singularity strength is assumed and the Green's function is integrated analytically over the panel to give well behaved coefficients of linear equations for the integral equation. The analytical properties of the derived coefficient are examined and are shown to be good enough to give stable numerical estimation for all cases of panel arrangement. Moreover, the method proves to be capable of making estimation of effect of the line integral term.

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This paper describes a real time, delay time control method for lower frequency waves in a test basin. In doing wave tolerance experiments by irregular waves in a test basin, we have a problem : higher frequency waves can't reach a test target even when lower frequency waves approach it. Wave boards generate a number of frequency component waves simultaneously, however higher and lower frequency waves propagate separately due to the dispersion relation. A given wave spectrum must affect a test body from the beginning of a test to obtain valid test results.
The authors have developed a real time control algorithm to solve this problem. The developed algorithm is based on an idea : delay time control for lower frequency waves. Irregular waves are realized by the summation of a finite number of sinusoidal waves with single frequencies to generate them. The representative frequencies of generated spectrum are known variables. Therefore the time to reach a test target for each frequency component wave can be calculated. The start time for generation of each frequency component wave is set at the time which is conversely calculated from the time to reach a test target. Consequently all frequency component waves can reach a target at the same time.
The authors have obtained good experimental results of this algorithm. Moreover, using the short -time Fourier transform technique, the validity of the proposed method is verified.

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In the first report, we have developed a new modal analysis method for estimating wave-induced motions of a mat-like very large floating structure (VLFS), and verified it by comparing the numerical results with the experimental ones with respect to the motion amplitudes of the elastic mode in head sea condition. Furthermore, we applied it to a large floating module, and investigated the motion performances of it and disturbance waves around it in head and quartering seas. Finally we showed the response amplitude operators of the first 10 modes in all heading waves with various wave lengths.
In this paper, we deals with motion performances of two adjacent mat-like structures in waves which are assumed to be under construction of VLFS. We have applied our method to a Γ-shaped structure and a small rectangular-shaped structure which will be connected to construct VLFS in open seas. First, several lower eigenmodes of the Γ-shaped structure with a length of 600 m and a breadth of 120 m are evaluated. Then its motion responses and wave profiles around it are calculated. In the next place, we investigate the motion performances of the Γ-shaped structure and the small rectangularshaped structure which will be connected and wave profiles around them. Furthermore, we discuss the variations of vertical gap between them and rigidity effects of the structures on the motion performances of these structures in waves.

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Recently large floating structures which may be used as a floating airport have been studied actively. National project of developing very large floating structures has been in progress in Japan as well. This very large structure is relatively flexisible compared with real floating structures like large ships. When we estimate dynamic responses of these structure, fluid-structure interation cannot be ignored because elastic deformation as well as rigid motion of the structure produces hydrodynamic forces. In the present study, we have developed a numerical code for dynamic response analysis of a flexible floating structure of mat-type in regular waves. The floating structure is treated as a rectangular flat plate by use of finite elements, and boundary elements are used in the formulation of the sea region. The present numerical results agree well with numerical or experimental results reported by SRI. In order to know the characteristics of the dynamic response of the larger floating structures, effects of bending rigidity of the structure and wave direction on dynamic response of the strucuture are studied by use of numerical calculation. And it is also shown that the dynamic response for larger L/λ is proved to be very similar to that caused by concentrated exciting force on an edge.

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A prediction method of the hydroelastic behaviors of a very large floating structure (VLFS) that may be used for such purposes as an international airport or an offshore city is prensented. In the present analysis the structure is divided into a number of small structures and the continuous deformation of the structure is represented by the succession of the discrete displacement of each small structure. Each structure is treated as if it were an independent floating body while the structural constraints are taken into account as additional restoring forces on its motions. The hydrodynamic interactions among the divided structures are accounted for in the evaluation of hydrodynamic forces by the method of Kagemoto & Yue (1986) once their complex motion amplitudes are known. The motion amplitudes are, in turn, determined from the equation of motions which are excited by the hydrodynamic forces. Therefore by solving the equations of hydrodynamic interactions and those of motions simultaneously, the hydroelastic interactions are properly accounted for and the motions of each small structure, which represent the local behaviours of the total VLFS, are determined in one computation.
The effect of the present method is discussed by comparing predicted dynamic behaviors of elastic floating structures with their experimental results, and the method can be used to carry out the hydroelastic analysis of a VLFS through several example calculations.

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This paper describes mathematical analysis of ejection dynamics as a fundamental study of subsystem for underwater vehicles. The main objective of this paper is to give a practical method to estimate maximum reaction forces during ejection process of a vehicle.
A cylinder is adopted as a basic structural object of the vehicle for formulation and calculation. This paper treats an ejection process of the cylinder, which is ejected from a region in the launcher immersed in still water, to an outer region in uniform cross flow. In such a case, conventional method incorporating static equations of balance gives infinite value as reaction forces during ejection, which results in bad evaluation of the forces. Simple method of structural response analysis is adopted to evaluate approximation that the object is rigid. Under the approximation, the motion equations incorporating hydrodynamic forces are derived to perform sample computations.
There are two main conclusions. The first point is that the calculated reaction forces were so reasonable that the subsystem could be realizable in comparison with that of conventional static analysis The second point is that this approach brings out the dominant parameters to the forces. The calculation results show that ejecting speed is dominant parameter affecting.

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Requiring highly to create the safe and comfortable environments in the marine systems, the phenomena on the functional design of atmosphere control system in enclosed spaces have to be clarified by an accurate analysis. For settling this problem, it is necessary to establish a suitable method on the turbulent diffusion state of heat and atmospheric gas in which it has the tendency to form thermal layers and heat / density pockets in the absence of properly balanced heat and gas concentration based on turbulent gas flow.
A numerical method is presented in this paper to give a precise prediction on turbulent heat diffusion of high Rayleigh number which is governed almost by natural convection. This method is a control volume method in which the SIMPLE procedure is applied and the turbulent flow is solved by the k-E two equations model. The natural convection and heat transfer in an enclosed space is calculated to validate the present method and the effect of the low Reynolds number is also investigated. Furthermore, the characteristic analysis of two-linked cabins with one opening is executed to examine the applicability for marine fire spread phenomenon.

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A preliminary study on an ecohydrodynamic model for the environmental assessment of a Very Large Floating Structure (VLFS) or Mega-Float in a bay is presented. The model consists of two parts, a hydrodynamic model and a marine ecosystem. Tidal currents and steady currents by the density anomaly are calculated in a bay with/without Mega-Float in the hydrodynamic model. A simplified marine ecosystem composed of nutrient, marine plankton and organic matter floating freely in the ocean is newly developed and combined with the hydrodynamic model.
Numerical calculations with/without a Mega-Float in Tokyo Bay are demonstrated and the results are discussed by comparison with the existing computations and measurements.

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This study deals with the influences of probability density distribution of exceeding lasting-time on deck wetness for ocean-going ships. Using the envelope theory for the random functions, a general formula for approximation calculation of density function between zero level is induced and expanded to the problems above a given non-zero level. A supplementary algorithm of estimating the time lasting period for deck wetness is also induced in this paper. Comparing results in the distribution range between the 1^st quarter number and 3^rd quarter number of probability density, the reliability of the proposed method to estimating the deck wetness for ocean-going ships is also confirmed. Based on wave statistics data, GWS, the deck wetness related to freeboard sea zones of ILLC 1966 is appraised by discussing exceeding lasting-time during which the amplitude of relative motion between deck and wave surface remains above a level of freeboard. A series of numerical calculations are executed for an ocean-going container ship based on the 5 freedom ship motions theory (the Ordinary Strip Method, OSM), and it is certified that the lasting time affects the trend of deck wetness.

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Recently, it was reported that a fishing boat (displacement is about 180 ton) had capsized in head seas. Comparing to the case in following waves, capsizing in head irregular waves are not investigated enough in the past. So, in this paper, we investigated capsizing process both by experiments in directional spectrum waves, and by numerical simulation in long crested regular waves including the effect of maneuvering.

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Motions of a free-fall lifeboat have been classified into four categories by observations of model experiments. In this paper, these categories are examined quantitatively using a numerical simulation method that was developed by the authors. Furthermore, four parameters, i. e., angle of launching skid, sliding distance of a boat on the skid, fall height and guide rail length, are chosen as launching parameters, and the relationships of these parameters to the motion categories are investigated. As the results of numerical studies, zero horizontal velocity of the boat at water exit is found to be an effective criterion that identifies unfavourable motions of the free-fall lifeboat after water entry. Finally, using the CAR index proposed by IMO, acceleration exerted on the occupants of the boat during water entry is evaluated, and its relationships with the above mentioned velocity criterion, the launching parameters and the angle of safe seat for the occupants are discussed.

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Recently, the powerful high-lift type rudder such as a flapped rudder is widely used in order to improve a steering quality or to achieve the reduction of labour cost in ship operations. For the prediction of manoeuvrability with these ships, it has been done by means of employing some empirical amplification factor in the calculation of rudder normal force. This procedure, however, makes some problems. The empirical amplification factor differs depending upon rudder angle especially when the flap angle is not proportional to the rudder angle. It is not also proper when a ship is swaying and yawing, as the inflow angle to the rudder changes depending on the ship motion, but the flap angle is kept unchanged. These facts may cause the mis-prediction of the manoeuvrability.
In this paper, a mathematical model is proposed to predict the manoeuvrability for a flapped rudder equipped ship. It has been validated by free-running model tests. Based on the proposed mathematical model, the suitable flap angle is investigated by the numerical simulations. Then, it is found that the smaller flap angle is better for the stable ship in course-keeping, and that the larger flap angle for the unstable ship, though the maximum flap angle is the same in each other.

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This paper is concerned with the maneuvering motion of a ship with VecTwin Rudder system. Firstly the forces on a couple of rudders and the interaction between both rudders steered with rudder angle are investigated analytically and experimentally. Secondly the free running model experiments are conducted for the turning, zig-zag and stopping maneuvers which are compared with the results simulated by mathematical model to describe the maneuvering motion. Finally an expression of force on VecTwin Rudder is proposed for the MMG model.

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A small cruising-type test-bed vehicle named “Manta-Ceresia” is developed to ensure performance of various control architectures for Autonomous Underwater Vehicles (AUVs). Although it is so compact (13 kg in weight) that one researcher can carry and handle the vehicle, it is equipped with sensors and actuators required for prototype AUVs. Pairs of elevators and thrusters make it possible to move in 3 dimensional space. It is realized to swim along walls of a pool keeping constant distance automatically making use of 6 channels of range finder. Taking advantage of these characteristics, adaptive controller which requires comparatively long swimming for adjustment can be examined in a small pool.
The performance of the constant-altitude-controller utilizing neural-network, which can accumulate experience by adaptation, is examined with this robot. Switching structure of neural networks is introduced to keep experience which is apt to be forgotten through additional learning. It is demonstrated by the developed test-bed vehicle that introduced switching system represents underwater terrain more precisely and the controller is adjusted appropriately.

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This paper proposes and demonstrates a cable tracking system based on a visual feedback navigation as an application for the environmental survey using AUVs (Autonomous Underwater Vehicles). The proposed cable tracking system consists of three levels of controllers : (1) The higher level controller decides the AUV's steering mode which includes a mode to search the cable in case that the vehicle loses it. (2) The middle level controller generates target values to realize the steering determined in (1) by fusing vision data and other sensing data. (3) The lower level controller generates control commands for the actuators according to the target values. This paper also addresses the vision processing techniques including Hough transformation and the transformation from two dimensions to three dimensions which are necessary for the steering decision in (1) and the target generation in (2).
The proposed system is implemented on an actual AUV named “Twin-Burger 2”. The performance of the system was examined though trials carried out at the Lake Biwa on October 1996. The vehicle navigated for about fifteen minutes without losing the cable although the cable was sporadically veiled in waterweed and transparency of water was not good. It is shown that the vehicle implemented the proposed cable tracking system can be a practical and general platform for environmental survey in the predetermined area.

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In preliminary designs of high speed craft, it is often that main engines, reduction gears, and propellers are specified referring to accumulated trial data of actual craft, which is usually converted to design charts. These stern components should satisfy the requirements of both propulsive performance and practicable stern arrangement. The design knowhow of compromise between these requirements builds up by collecting the trial data.
This paper proposes a trial database system for high speed craft, which consists of a collection of trial data, mapping neural networks (what we call a memory model), and a descriptive neural network. The memory model, whose input is a design condition (length, displacement, target speed), and outputs are the required horse power and the standard propeller specification, is generated by learning of the trial data. The descriptive neural network, which denotes the frequency of actual craft with the similar design condition, indicates the designer how conservative the given design condition is.
The proposed database system is based on the trial data of 36 craft. The constructed neural networks set standard for the stern components, quickly estimate the propulsive performance, and reduce the number of iteration of the design spiral. When additional trial data is available, it is easy to modify the constructed networks, taking advantage of learning ability of neural network.

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Recently, demands for increased safe operation and ocean environment protection, especially preventing oil pollution in the sea are remarkable as the international tendency. On the other hand, the trend of the shipping world indicates clearly a strong demands to reduce the cost for operations and maintenance of ships. From the viewpoint of survey and inspection of ship hull structure, the improvement of reliability of inspection, safety of work and reduction of costs for inspections are one of the most required subjects today. In order to correspond to the social requirements for surveys and inspections of hull structures, the authors are studying and developing the new monitoring system by the underwater RTV robot (MITSUI RTV-SHIP). The advantages of the RTV-SHIP are as follows : 1) All the tank walls can be inspected and easily recorded on video tapes. 2) All the operations can be controlled on the upper deck by minimum operators. 3) All the surveys can be performed on voyage. The basic tests of RTV-SHIP with regard to positioning in the tank, measurement of plate thickness and large deflection of panels were carryed out and satisfactory results were obtained. The utility of the RTV-SHIP was proved in this study and further tests for the actual ships are now planned with the aim of realizing this system. The RTV-SHIP is expected to make a major contribution to the safe operations and ocean environment protection.

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Current Jetfoil control has been made by platform and contour mode selection and by manual input of fore-foil depth according to sea conditions by human operator, which cause overload and may be difficult for operators. This requires the further automation in the control system.
In this paper, authors propose unified automatic control system (ACS) which is based on optimal servo theory and eliminates human operation in ACS. The reference input signal in the servo system was modified to the sea conditions to achieve unification of platforming and contouring. First, optimal servo system (OSS) was designed to keep the fore and aft foil depths in waves to find that the sole OSS was still sluggish. Therefore, authors added proportional and differential element in inner feedback loop in addition to the modification of reference input.
The MOSS+MRI, which represents modified optimal servo system (MOSS) and modification of reference input (MRI), was confirmed the good control performance in the wide range of waves through simulation.

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Accuracy management of products is one of the most important issues in industries. There are two significant ways for accuracy management. One way is accuracy control by measuring products in assembling stage. The other way is “accuracy planning” that is accuracy management in production design stage. Authors have already reported about the former, and the latter is discussed in this paper.
Authors have been engaged in development of SODAS for shipbuilding CIM. Therefore authors developed accuracy planning system considered weld deformation based on SODAS.
For accuracy planning, it is important to estimate deformation. Authors practice modeling of weld deformation of products and make it possible to estimate easily welding deformation. By using them, the accuracy management system by estimating of weld deformations is implemented.

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The present paper aims at developing a computer aided design and manufacturing system for advanced composite marine structures, that is, shell structures in which stiffness and strength for outplane pressure and light-weight are achieved by making efficient use of advanced composite materials. In the previous reports, some specialized prototypes of CAD/CAE system for laminated materials were developed by means of object oriented language. It showed that the developed system is effective in designing and analyzing laminated panels.
In this report, specifications of the system are discussed in the point of developing an effective system. Based on the specifications, two points are important in the systems aided for manufacturing, one point is making tool mold with easiness and accuracy, and the other is layup simulation on a curved surface.
The system aided for making tool mold was developed by means of generating surfaces which are apart thickness from the base surface. By using a numerical cutting machine with cutting paths on the offset surfaces, it is available to obtain an accurate male mold, which is achieved smooth outer surfaces.
In order to simulate of prepreg layup process on a curved surface, three deformation mechanisms of laminated materials are investigated in the point of fitability, plastic deformation, shear deformation, and apparently deformation induced to out-plane deformation. According to observations, unidirectional fiber yarn is discrepancy from the curved surfaces without some points, and it induced that fiber wrinkles in a vacuuming process. Then a criterion of fitability is proposed by means of calculating apparently in-plane deformations.
Some examples are demonstrated, and it shows that the developed system is effective in manufacturing simulations of laminated panels.

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Research and development of deepwater risers for scientific drilling and offshore oil development are now under way in Japan. Depending on the purpose of riser, target water depth varies from 2000 m of offshore oil development to 4000 m of scientific drilling. Water depth of 4000 m is far deeper than the world record of deepwater drilling with riser, and the technology to be developed in the research and development will be one of the most advanced one. This technology will be also applicable for other fields such as sequestration system of carbon dioxide into deepwater and so on.
This paper discusses overview of dynamic elastic response of a deepwater riser and design of the response. It becomes clear that a lighter and more rigid riser is generally desirable for deepwater from a viewpoint of elastic response. It also becomes clear that the distribution of buoyancy material must be designed so that the initial tension distribution is optimized and compressive axial force becomes hard to excite under dynamic tension fluctuation in hung-off condition. Two types of coupled response of riser, riser-mud coupled response and longitudinal-lateral coupled response of riser, which might have significant effects on the design and operation of a deepwater riser are chosen and examined.
From a series of calculation, it is understood that dynamic response of mud has significant effects on the dynamic tension induced in the riser. Natural frequency of mud column is around 12 seconds for 4000 m riser case and a sharp peak is observed in the frequency response of riser tension. The peak height is still significant even when highly viscous mud is used. In a hung-off condition LMRP will need to be opened under severe weather condition.
It is shown that the lateral response, so called parametric oscillation induced by tension fluctuation under longitudinal vibration, will not be a serious problems for real 4000 m riser case. Deepwater risers are designed so that the compressive axial force will not be induced even in hung-off condition. Under this condition, some amount of lateral drag force which is usually expected for normal configuration of riser will suppress the lateral response. But it is also shown that if a large longitudinal response is applied or lateral drag is very small, lateral large response can be induced.

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Very large floating structures have been proposed for various applications, e. g. floating airports, cities and plants. Among them are semi-submersible type structures. This paper presents a computational method for analyzing responses of very large semi-submersibles in waves.
When designing usual semi-submersibles, we need to model and analyze them as three-dimensional frame structure. The output data from this analysis is used as input at the next stage. This will be also true even in very large semi-submersible case.
Because of their enormous size, some techniques are needed for such frame analyses. We have extended the formerly developed method by the authors applying sub-structure method. This is rational because the structure is expected to be composed of repetitions of simular structures. We newly introduce the concept of group body by which several columns are treated as one body in terms of hydrodynamics. The unknowns concerning both hydrodynamic and structural equations can be reduced greatly in number. In this process, both hydro-elasticity and hydrodynamic interactions among floating bodies are considered.
The results obtained by the present method are compared with the results of the formerly developed method for validity check. We show the 3, 000 (m) structure case. The results are shown to be applicable to zoom-up FEM analyses of local structure as input data.

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A number of studies on the dynamic structural analysis of large scale floating structures with length or breadth to the order of several thousands of meters have been done by several authors, as reviewed by MIYAJIMA et al.. Most of the studies deal with the basic characteristics of the vertical vibrations of the structure modeled as a beam or plate on an elastic foundation. Especially SUZUKI and YOSHIDA have given relatively precise prediction for the deflection and stress of the structure at the lower frequency zone by deriving the analytical solution of such models.
This paper firstly deals with the dynamic response and strength of a very large floating structure (about 4560m × 1000m) using the simple beam modeling by the analytical method. Effects of boundary conditions and structural parameters on characteristics of dynamic behavior of the structure are discussed including the characteristic wave number derived by SUZUKI and YOSHIDA.
Finally, the dynamic behavior for the same structure replaced with a plane framework is also investigated by applying FEM. The modes and natural circular frequencies obtained are compared with those for the structure modeled as a beam on an elastic foundation. It is also pointed out that not only vertical bending vibrations but horizontal bending vibrations and longitudinal vibrations should be considered at the lower frequency zone.

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Recently many studies on the elastic response behavior of very large floating structures have been done. Such a very large structure is relatively flexible compared with other existing floating structures like large ships. For estimating the dynamic response behavior of structure, it is important to consider the deflection wave propagation based on fluid-structure interaction analysis.
This paper deals with the dynamic response and strength of VLFS except peripheral part considering deflection wave propagation for the simple beam or plate model in regular waves.
From analytical results considering fluid-structure interaction, effects of wave length and direction of incident waves on the response and strength are examined. Moreover, the dispersion relation of the deflection wave is also calculated.

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To clarify the buckling/plastic collapse behaviour and strength of ship bottom plating, a series of elastoplastic large deflection FEM analyses is performed on stiffened plates subjected to combined biaxial thrust and lateral pressure. A part of continuous stiffened plate is considered for analysis taking into account of symmetry conditions. A series of elastic large deflection FEM analyses is also performed on continuous plating without stiffeners to examine the influence of lateral pressure on the buckling behaviour of plating.
It has been found that :
(1) When initial deflection is in a hungry horse mode, bifurcation buckling takes place. The buckling strength increases with the increase in applied lateral pressure.
(2) The buckling strength is increased also by the stiffeners, which constrain the rotation of panel along its edge. An analytical formula is derived to evaluate the local buckling strength of stiffened plate considering the influence of stiffeners.
(3) With the increase in applied lateral pressure, the boundary condition of the panel between stiffeners changes from simply supported condition to clamped condition. This change increases the buckling/plastic collapse strength of stiffened plate.
(4) With larger lateral pressure, yielding starts earlier. This reduces the buckling/plastic collapse strength of stiffened plate.
(5) Owing to the opposite effects described above, the buckling/plastic collapse strength of stiffened plate takes its maximum value at a certain magnitude of lateral pressure, especially when transverse compression is dominant.
(6) The formulae by classification societies give conservative buckling strength under bi-axial compression, and the bottom plating has much reserve up to the ultimate strength even when lateral pressure is acting.

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Many parts of marine structures are constituted of thin shell components, which by way of structural design or due to functional requirements may possess a negative Gaussian curvature. The load resisting mechanism of such shells are expectedly different from shells with positive Gaussian curvatures, and this aspect is studied in this research using the circular hyperbolic shell as a typical example.
The Thin shell is defined here in the monoclinically convected coordinate system and considered to be subjected to a uniform follower load. The numerical accuracy and practical significance of all the theoretical formulations have already been substantiated through various simulations. In the simultaneous equations governing the equilibrium of shells, the presence of terms related to curvatures and their consequential factors create the distinctive aspect of 'Form Effect', which would obviously be rendered more complicated by a negative Gaussian curvature. The resisting mechanisms of zero and positive Gaussian curvature shells have been examined earlier in detail through a similar study using the partial cylindrical and spherical shells. The present paper elaborates on the initial resisting mechanisms of circular hyperbolic shells by studying the share of different component terms associated with the extensional and bending stiffness factors, separately and in various details, to draw out an overall picture of the total load resisting equilibrium picture.
All the possible combinations of circular hyperbolic curvatures are analyzed in a schematic division using a simultaneous measure based on the curvature invariants of shells. Compatible curvature ranges of hyperbolic shells are compared with the partial cylindrical and spherical shells to bring out some clear distinctions between the resisting mechanisms of each. These results are believed to be helpful in understanding the resisting mechanisms of hyperbolic shells in a clear perspective of both qualitative and quantitative terms, which would serve as a guideline for design and analysis.

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It is important that ships are prevented from fatal failures such as the hull girder collapse under severe sea conditions. On the other hand it is also important to know the reduction of the strength of a ship with lightening caused by the constant demand for lighter ship hulls. Severe shear force acts in the side shell near the bow of the ship during slamming conditions. Thus it becomes important to investigate not only compressive behavior of the deck panels but also shear collapse behavior of the side shell panels to determine the collapse behavior of the ship's hull girder.
Simply supported and uniform shear load boundary conditions are suitable to examine the plate behaviour until the shear collapse load. But strength evaluation considering the continuity between side shell panels is needed to analyze the load carrying capacity accurately during the post shear collapse.
In this paper it is intended to solve only one panel considering the inplane and rotational continuity between panels as the boundary condition instead of solving many panels to investigate the panel behavior of the post ultimate strength region, such as the load carrying capacity under shear.

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A posteriori error estimation method for finite element solutions of natural frequencies and natural mode shapes of structures is proposed in this paper. The governing equations of a posteriori error for free vibration problems are introduced. A method used for solving the error equations is presented and an efficient solution strategy is developed since it is difficult to solve the error equations directly. A technique of eliminating the degrees of freedom at the mid-node of element is developed in order to reduce the size of matrix in error equations. The finite element solutions of eigenvalues and eigenvectors are improved by adding the estimated error onto the original solutions.

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In order to investigate the structural integrity of large scale structures, the prediction of the crack path is very important. For this reason, a step-by-step finite element approach has been proposed by one of the authors, Sumi, in which a cracked domain must be remeshed by an automatic mesh generator as the crack propagates in thedomainlm. In this approach, the modified quadtree algorithm is used for automatic mesh generation. One of the problem of this method is that it is difficult to generate mesh near the boundary and therefore reliable mesh generation is sometimes not possible. In this paper, the automatic quadrilateral mesh generation based on the pavingmethod is developed for the crack propagation analysis. By the developed mesh generator, completely automatic simulation and reliable analysis are possible. Also, a method for automatic choice of the increment size of the crack propagation in each step is proposed in this paper. In this method, proper increment size is first selected based on the numerical data from the computational experiences by authors. Furthermore, if the computed error ofK_IIK_I exceeds the allowable limit specified by the user, re-analysis of the previous step is automaticaly carried out by decreasing the increment size so that the crack path prediction can be performed accurately.

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The purpose of this study is to perform a Bayesian reliability analysis for developing optimal nonperiodic inspection schedules and estimating values of uncertain parameters from field data collected during in-service inspections for practical aircraft structural elements. It is suitable that information obtained from inspection results through the long service term is utilized in order to examine the effectiveness of this reliability analysis. However, it is close impossible that such actual data is obtained to the actuality. Then, the previous report adopted the damage-tolerant structural element in typical fuselage structures modeled by referring to various design data and fracture processes. Furthermore, the failure rate function which uses for this reliability analysis was also examined, and a sufficient knowledge was obtained. Transport wing lower surface structures with a number of fatigue -critical elements are used as a realistic structural model for the present analysis. Each element consisting of a skin panel and stringers, is subjected to flight-by-flight loading and is designed by the damage tolerance criterion. Probabilistic factors considered in this analysis are fatigue crack initiation and propagation, crack detection capability and failure rate before and after crack initiation. Monte Carlo simulations are conducted to demonstrate the efficiency of the Bayesian reliability analysis for development of inspection schedule and for the estimation of uncertain model parameters.

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Sensitivity analysis on fatigue reliability and inspection is carried out for a typical structural member of bulk carrier. Fatigue properties of the member and probability of crack detection at the field inspection are obtained by the questionnaire asked to the naval engineers. Changing stress level (fatigue life) of the member, crack growth curve and inspection interval as parameters around a standard condition, failure probabilities of member during 20 years' service are calculated by the Markov Chain Model. The result of analysis can suggest the solutions for design requirements such as improvement of reliability level, reduction of structural weight and the simplification of inspection.
In the latter part of the paper, effectiveness of combined inspection method and sampling inspection method is discussed through the sensitivity analysis assuming member sets having different number of initiated cracks. It is found that combined inspection has more advantage than sampling inspection, when the number of members having initiated crack is a few in the inspecting member set.

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Corrosion fatigue tests were carried out in synthetic seawater, in order to clarify the temperature effect on the corrosion fatigue strength of coated ship steel. To simulate the shipbuilding construction process, KA 32 TMCP steel in 10 mm thickness was sprinkled with water in outdoor twice a day for 20 days. After the exposure, the surface preparation was conducted, and the tar epoxy resin was painted with 50, 100, 200, and 300 μm in thickness. These specimens were subjected to cyclic axial stress at 0.17 Hz in 25, 40 and 60°C synthetic seawater. For the comparison, corrosion fatigue tests of uncoated specimens were also carried out.
In the low cycle region, the corrosion fatigue strength of the coated specimen was almost equivalent to that of uncoated. On the contrary, it was improved in the high cycle region. The temperature effect of the corrosion fatigue strength was assessed by introducing corrosion factor K_c, which implies the reduction rate of the corrosion fatigue strength against that at 25°C. The corrosion factor K, indicated 1.031.13 at 4060°C.
In addition the maximum depth of the corrosion pits was found to be 3040 μm after 20-day exposure. In the long-life corrosion fatigue, cracks were initiated from such corrosion pits under the coating film before it was destroyed. Finally, effect of corrosion pits on the fatigue crack initiation behavior was considered under the conditions of surface preparation of specimens.

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