Recently many kinds of high speed boats have been developed. The hydrofoil type is one of them. On considering the hydrofoil type, one important problem is interaction, for example, interaction between the main hull and the hydrofoil, the hydrofoil and the free surface etc. Here we consider the interaction between the free surface and a submerged lifting body by numerical simulation. The submerged lifting body is composed of a submerged body and a hydrofoil adapted to it. The numerical method is the Boundary Element Method (BEM) that was originally developed at Hiroshima University by Mori and Qi. We have modified the original method and have applied it to this case.
We measured the lifting force and the pressure distribution on the hydrofoil in the NKK Tsu Ship Model Basin, and confirmed that BEM is an effective method for the interaction problem between the free surface and a submerged lifting body. By using this BEM, we considered the depth effect of the lifting force of the submerged lifting body, the optimum location of the adapted hydrofoil and the optimum submerged body shape.

View full abstract

A finite-volume method is applied to a problem of tandem hydrofoil advancing beneath the free surface. The curvilinear grid system is fitted both to the free surface and to the hydrofoil surfaces which moves by the wave motion and ship motion respectively. It is demonstrated that such simulation technique is useful for understanding of hydrodynamical properties of system with moving boundaries and that it can be practically used for the design of hydrofoils with flaps and associated control system.

View full abstract

It is intended to solve a problem to determine the unknown wetted surface of free-running planing plate. Efforts from the view point of the perturbation method are paid to analyze both the near and far field of flow around a restrained planing plate with comparably large breadth such a wave-dozer, disregarding gravitational effects. Matching process of each height of water surface leads to a non-linear system of integro-differential equations to determine the distributions of both the wetted length and the apparent circulation around the plate. Serious solution of reduced single integral equation, still holding 3-dimensional characteristics but the so-called downwash effects, diverges from the assumed high aspect ratio solution because of the logarithmic non-linearity. The equation gains an insight into static stability in performance and the limitation in height to restrict the plate in exposure above still water. The obtained configuration of wetted surface draws well the behaviour of the spray root line crossing the hard chine, however it is unfortunately rather wide to coincide with the experimental one. Free-running condition is estimated.

View full abstract

An investigation through numerical experiments was conducted to determine the aerodynamic characteristics of a power-augmented ram wing in ground effect (PAR-WIG). The Navier-Stokes solver used was a MUSCL-type third-order accurate upwind differencing, finite-volume, pseudo-compressibility code based on a multi-block grid approach. In order to understand the mechanism of the power-augmentation effect, two boundary conditions on the ground were considered : (1) a moving belt ground plate condition ; and (2) a fixed ground plate condition corresponding to the wind-tunnel tests. Thrust was represented using prescribed body-force distributions. The flow around a rectangular wing with end-plates and propellers which were placed forward of the wing and blew under the wing, were computed by the solver with different trailing edge heights. Results were compared with experimental data and the aerodynamic characteristics were discussed.

View full abstract

A new numerical simulation code is developed for elucidating the behavior of a bubble being based on the Navier-Stokes equation. The capturing of the interface between liquid and gas is implemented by the modified density function method significantly suppressing numerical diffusion in the rectangular coordinate system. The density function method, which eliminates the efforts of generating grids fitted to the interface, is shown to capture the complex 3-dimensional deformation of a bubble with sufficient degree of accuracy. Computational results of a rising bubble at high Reynolds number greater than 1000 showed the unsteady motions such as those with zigzag or spiral trajectory and the various deformation of bubble shape like a spherical cap. It is also clarified that the unsteady motion is caused by the asymmetric formulation of longitudinal vortex behind the bubble. This code can be applied to the case of bubbles in a boundary layer to investigate the effects of bubbles on the boundary layer properties.

View full abstract

The Distributed Mass/Discrete Floe model, DMDF model, is a new numerical model for pack ice motion computation. This model expresses the ice floe collision by dividing the pack ice into many ice floe bunches in which ice floes are distributed uniformly : thus it can express the discrete nature of force transmission between the floes and also can treat a larger number of floes than a discrete element approach. This paper applies the DMDF model to the computation of one-week pack ice motion in the Okhotsk Sea of February 1994, transferring a large amount of data from the Japan Meteorological Agency, and then discusses the characteristics of the model and ice motion in this particular sea area.
The main conclusions are :
(1) The DMDF model showed rigorous ice motion against the external forces such as wind and current.
(2) The ice motion is determined mainly by the wind, but the local behavior is strongly affected by the current.
(3) The interaction between the wind, ice and current is important, thus the coupled computation with the current is required to predict the ice motion more precisely.
(4) Since the direction of shearing force due to wind is different from the wind direction by the influence of Coriolis force due to the earth rotation, the computed ice motion agrees better with the observation when the shearing force direction is deviated in clockwise direction.
(5) The computed ice motion is compared with the buoy observation. The result showed fairly good agreement. It is recognized, however, that more accurate and detailed current data is necessary.

View full abstract

This paper discusses the reconstruction on the algorithm of the long-term prediction by using a stochastic model. The model is a rational long-term stochastic model for calculating the long-term statistics of sea waves which was proposed by G. A. Athanassoulis et al. On the basis of the same framework of treating the wave climate as a stochastic model, the algorithm for the long-term stochastic prediction of ship responses in ocean waves is examined and improved for the purpose of using the combination of some long-term wave frequency data which presented with some differences in data format. The long-term predictions of vertical acceleration induced on the bow of a container ship are executed for a few patterns of combination of wave data, and further investigations for reconstruction of the algorithm are discussed.

View full abstract

A method to estimate the statistical properties of non-linear short-crested irregular waves without any limitation regarding the directional spreading or the spectral band width is presented which is based on the secondary interaction theory of surface waves. It is shown that the statistical problem can be reduced to that of finding the eigenvalues and the eigenvectors of two real symmetric matrices and the probability density functions of surface elevation can be obtained using the so-called Saddle Points Method. Numerical investigations regarding the effect of shortcrestedness on the statistics of wave amplitudes are performed. And the method is also used to analyze full-scale data measured in stormy sea states and is shown to be a powerful tool for the estimation of the statistical properties of the directional sea.

View full abstract

Capsizing experiments were carried out for both models of container ship and purse seiner running in regular and irregular astern seas. Dangerous situations in ship speed and heading angle of ship to waves are experimentally and analytically investigated for the phenomena which are the so-called harmonic and parametric resonance, pure loss of stability, surf-riding and broaching-to. An analytical approach was attempted to investigate profoundly the dynamics of ship motions and capsizing in severe astern seas. The results of the analytical approach by making use of computer program are in a good agreement with experiments.

View full abstract

This paper deals with non-linear periodic motions of a ship running in following and quartering seas with low encounter frequency. It aims at providing a methodology for more comprehensive understanding of broaching. The motions discussed here include surge, sway, yaw and roll with an auto pilot. A manoeuvring mathematical model in waves is transformed with a mean-velocity inertia axis system and then an averaging method is applied to the transformed model. As a result, local stability and outstructure of periodic motions are discussed with numerical results derived using a purse seiner. These cannot be directly assessed by conventional linear models or purely simulation based approaches. Comparisons with experimental results are also shown.

View full abstract

RO-RO passenger vessel has a wide non-separated car deck. Once free flooded water is piled up on it, the large heel moment could be the cause of capsize because of this feature.
The stability standard of RO-RO passenger vessels was deliberated at IMO commission from 1994 to 1995 in order to prevent capsizing disaster like the one of ESTONIA in 1994. The authors, one of which was a member of the IMO expert panel, conducted an experiment on the stability of this type of ship because few papers have been published on this problem. The experiment was carried out in beam seas using a model ship with a side damage hole which is prescribed by the SOLAS regulation.
After the duration time of the experiment (30 minutes in ship scale) in irregular waves the ship survived with a constant mean heel angle φ₀ and a mean water volume on deck w in most test conditions, but capsized in a few ones. The variation of these constant values with CG height, existence of center casing, height of freeboard and initial heel was discussed. It was clarified that φ₀ tends to decrease and w tends to increase as GM_d (GM in damaged condition) have a larger value, and that ship can survive even with w of 40% of intact ship displacement if she has a large (not extraordinary) GM_d value.
The effect of resonance of roll motion on this problem was also studied based on the test result in regular waves. φ₀, w and some other data often have a peak near the resonant frequency, so the stability test of RO-RO passenger vessel should be carried out including resonant conditions.
The height of water on deck above the calm sea surface H_d was proposed as an index which settles the balancing condition. It was clarified that Hd keeps a certain value above calm outer surface when wave hight is not so low and mean heel angle is not so large to lee side. The equilibrium curve for each GM_d can be calculated from GZ-curves with a constant volume of water on deck, fully static calculation, and figured on H_d-φ₀ diagram. It is concluded that the possibility of capsize can be judged by this equilibrium curve.

View full abstract

A new image analysis system for a motion measurement of a free-running model ship is developed.
In the present system, a target is traced through template matching procedure, and error identification is distinguished by using some characteristic parameters. The tracking procedure by a human judgment is supported in the system.
The change in the mutual correlation coefficient of the image and the contrast of the image are used as characteristic parameters. The characteristic parameters are projected on a characteristic vector field and the error identification is judged on the characteristic vector field.
For the verification of the present system, a target on a rotating disk with a constant speed was traced. The error of the rotating radius remained less than 2% and the mean tangential velocity remained less than 0.5% respectively.
The measurement of a free running model ship succeeded with satisfactory results. Moreover, it was confirmed that the judging error identification of the present system was very effective.

View full abstract

A continuous effort has been made to investigate flow field around two full ships in maneuvring motion by means of numerical and experimental methods. Following the previous reports a finitevolume simulation method based on the Navier-Stokes equation is applied to the ships in steady turning motion, and the flow field is thoroughly studied and compared with measured results in the present paper. The degree of accuracy in predicting hydrodynamic forces and moments is improved by revising the numerical method. Detailed measurement of the flow field on the vertical plane at A. P. is carried out in the steady turning motion and the results agree well with numerical solutions. The typical characteristics of the flow field in steady turning condition is revealed by the numerical simulation and the relations between hull form, flow field and forces are clarified.

View full abstract

One of the authors presented the component-type mathematical model of hydrodynamic forces in steering motion on the basis of kinematic forces acting upon the both ends of the hull. The mathematical model can describe well the forces X, Y and N with large drift angles and tough turning motion. The rotative coefficients in the model are estimated from the static coefficients at oblique motion. The rotative coefficients thus derived are, however, somewhat different from the ones delivered from the experimental data of turning motion.
In order to improve this point, the authors will now break up the hydrodynamic forces into the following components, i. e., ideal flow force, viscous force, induced drag, cross flow drag, cross flow lift and frictional resistance in the normal sense. Then a simplified vortex theory is called upon to evaluate the ideal flow force, viscous lift and induced drag.
As the results, the present mathematical model can describe the hydrodynamic forces acting upon a hull in turning/drifting motion with a fair accuracy. The model also provides a sensible insight on the mechanism of hydrodynamic forces appearing in steering motion.

View full abstract

The hydroelastic responses of a semisubmersible type floating airport, which consists of multiple removable units supported by column-footings in head sea are discussed. The emphases are placed on the response characteristics in short wave length range and the influence of lacking of units in this third report. Active inclination control by using air pressure in underwater columns is attempted for the case of unit lacking supposing replacement of the corroded or damaged units. Numerical analyses are performed based on the mode superposition method which combining the 3-D analyses of hydrodynamic forces (source distribution method) and mode analyses (FEM).
In this report, the calculated added mass and wave exciting force of each column are shown for understanding the interactions among columns especially in high frequency range. The calculated deflections and bending moments including higher elastic deformation modes are compared with the experimental results obtained from high frequency excitation tests. Also the responses in regular waves are calculated and compared with experiments for different conditions including unit lacking, with and without air pressure control. Finally, the influence of unit lacking on response and the effect of air pressure control are investigated consequently.

View full abstract

Kansai international airport was newly constructed and opened in 1994 at the site of offshore in Osaka bay. This airport was built on the newly reclaimed land. Although similar airport concepts are recently discussed in Japan, yet newly reclaimed land in the deeper sea is very difficult from the points of view of necessity of huge volume of sand for land filling, long period for construction and environmental impacts.
On the contrary, concepts of a floating land for the airport are recently discussed again. These concepts were discussed more than a decade ago, even though that was not realized because of no experience of the construction of such huge floating body for the mankind before. After these concepts were proposed, huge floating oil storage bases with million ton capacities were realized in Japan. Therefore, a floating airport should be discussed more and deeper to realize it.
The authors wish to show one of the concept designs of floating airport and to discuss the estimates of the environmental forces.

View full abstract

The floating structures that may be used for such purposes as an international airport or an offshore city are expected to be as large as several kilometers long and wide. For the estimation of hydrodynamic forces due to waves or body motions that will act on such huge structures, a direct application of conventional numerical methods is practically prohibitive, because the required computational burden is enormous. In order to get rid of this difficulty, an approximate numerical method is proposed in which computational time is drastically reduced without appreciable loss of accuracy. Although a direct application of conventional methods is difficult for the reason that the corresponding structure is so large, the method proposed in this paper exploits the very fact that a structure is huge to simplify the calculation. The effectiveness of the new method is demonstrated in comparison with results obtained without any approximations.

View full abstract

The paper develops a numerical method for predicting wave drift damping of three dimensional bodies. The first order potentials are calculated by a new integral equation method, in which the second derivatives of the steady potential has been removed from the integration on the free surface. The second order mean potential is calculated by an integral equation method which is firstly proposed and implemented. Comparison is made with analytic solution for a uniform cylinder. Numerical examinations are made on the convergence with radius of the mesh on the free surface, and magnitude of each component of wave drift damping. Timman-Newman relation is also used to check the correction of the first order potentials. Comparison with experimental results is made on an array of four cylinders which are restrained and freely moving respectively. It was found that good agreements exist between the present calculation and experimental results and negative wave damping may occur at some wave frequencies.

View full abstract

A pitch, roll and depth controllable towed vehicle has been developed and named as “FLYING FISH”. The towed vehicle equips with an acoustic Doppler current profiler (ADCP), CO₂ analyzer and sensors for measuring temperature, salinity, dissolved oxygen, PH, turbidity, chlorophyll.
Flying Fish enables us to obtain the space continuous data of physical and chemical properties in the ocean upper mixed layer efficiently. Its maximum submerged depth is 200 m. The length is 3.84 m, breadth 2.26 m, height 1.4 m, weight in air 1400 kg and weight in water is about 0 kg.
Numerical simulations were carried out in order to design mechanical parts of the control system and estimate the accuracy of motion control of Flying Fish. The simulations are based on the six degree freedom motion equations for underwater vehicle and lumped mass method for the towing cable. Field experiments were conducted to confirm the performance of Flying Fish and accuracy of numerical simulations. Results of field experiments and numerical simulations are compared.

View full abstract

Free-fall lifeboats provide significant advances in the maritime lifesaving systems. Much of the danger associated with conventional lifeboat systems can be eliminated by this new evacuation method if the boat is launched selecting suitable launching parameters. A primary consideration in the freefall lifeboat system is the acceleration field to which the occupants are subjected during water entry. The international regulation, therefore, requires that a lifeboat for free-fall launching shall be capable of ensuring protection against harmful accelerations when it is launched with its full complement of persons and equipment from at least the maximum designed height.
When the lifeboat enters the water, the acceleration forces exerted upon the boat due to impact are very high. According to the basic study upon the human body response, tolerance level for acceleration is different for each axis of the human body. Therefore, the safe seats in a free-fall lifeboat are usually reclined relative to the axes of the lifeboat to reduce the effect of high accelerations. In this paper, the authors introduce a new and probably the simplest concept to evaluate the acceleration field of the free -fall lifeboat and apply it to the analysis of the safe seat orientation for the occupants. The results have been compared with those of the SRSS acceleration criteria and the dynamic response criteria, both of which are recommended by the IMO, and good agreement has been found.

View full abstract

A hull form definition system consisting of a hull geometry acquisition program and two hull design tools for fishing crafts was developed. One tool for definition involved hull variation where the longitudinal positions of the stations were changed and mapped to common parameters. In the other tool, hull forms were generated by selecting simple shapes representing boundary curves of hull surfaces. Different hull surfaces were produced by defining families of curves based on the edges. Analysis modules such as hydrostatics and reference charts for powering requirements were included for evaluation purposes. The results were also referred to a related knowledge based advisory system that gave necessary advice and guidance.

View full abstract

It is often said that Concurrent Engineering (CE) is one of the most important technics in various industries today. In this paper, authors indicate methodology of computer aided CE for hull structure design and piping design.
Authors have already reported the prototype system for CIM (Computer Integrated Manufacturing). This system is SODAS (System Of Design and Assembling for Shipbuilding). In SODAS, design system for hull structure and design system for piping are integrated. And these systems are integrated by the concept of Product Model. So authors develop the computer aided CE system based on SODAS in this study.
Keeping consistency of product information and management of supposed information are important in CE. Relation of product information is very important to keep consistency of product information. So authors define “Constraint Relation Information” and anthors make it possible to generate “Constraint Network” from “Constraint Relation Information”. By using them, keeping consistency of product information and management of supposed information are made possible.

View full abstract

The classical PID control law is still used for many marine control systems. Gain tuning in the controller is, however, a formidable task for mariners. This paper provides a simple and a safe automatic P, I, D gains tuning method using relay control, for typical marine controllers with PID control law.
In this method, a limit cycle with small amplitude is occurred, at the beginning, through a proper actuator such as a rudder, a thruster and so on, and then the amplitude and the period of the limit cycle are observed. Since the two quantities obtained under such limit cycle equivalently correspond to those by the ultimate method (Ziegler Nichols scheme), these are utilizable as the parameters for obtaining the optimal gains recommended by Ziegler Nichols.
The on-board computer control system using the above method is applied to typical three types of ship's controller ; (1) autopilot system, (2) yaw control system through bow thruster and (3) diesel engine governor system, and the results of the full scale experiments using a small training ship are discussed.

View full abstract

The marine structure using under hot environment should be designed with careful thought of defending the workers against heat stress. An adequate heat stress index of human body is necessary for classifying the severity of working environment and taking some counter-measures.
In order to grasp the relation between the thermal state of human body and the thermal environmental factors such as operative temperature, metabolic heat and clothes, the experiment is carried out during the excise by ergo-meter in the constant temperature and humidity room and under the sunshine environment. Being based on the results of experiment, the heat balance model of human body is established with a view of calculating the quantity of heat storage as a heat stress index.
Further, the storage of body heat defined in this report is clarified to be useful for various thermal conditions.

View full abstract

It is known that the effect of fluid upon structural vibration is very large with the estimation of vibrational performance on floating structures, therefore fluid-structure coupled analysis method which combine the added mass matrix into FEM has been used. Thence pretty accurate eigenvalue of structural vibration can be estimated for many cases of ship structure. But the vibration level can not be estimated so well, because the damping of structure and fluid viscosity is not known. Therefore it is necessary for the estimation of vibration level to investigate the damping caused by fluid viscosity.
In this paper, a new theory of added damping matrix based on dissipation energy caused by fluid viscosity is described. The distribution of flow velocity in the vibrational boundary layer is determined by assuming laminar flow in spite of existence of turbulence, then shearing stress is decided using virtual kinematic viscosity coefficient. Dissipation energy caused by fluid viscosity is formulated as the function of the nodal amplitude of the BEM mesh model. The added damping matrix can be obtained by comparing that with dissipation energy of usual damping matrix of FEM.
The difference of dissipation energy between in deep water and in shallow water is clarified by using the added damping matrix. A new idea of reduced added damping matrix is defined, then combination analysis of the experimental modal analysis and BEM is executed using the reduced added mass & damping matrix. The validity of the added damping matrix is verified by comparing with some experiments in the deep water and the shallow water.

View full abstract

It is known that the effect of fluid upon structural vibration is very large with the estimation of vibrational performance on floating structures, therefore fluid-structure coupled analysis method which combine the added mass matrix into FEM has been used since many years ago. Thence pretty accurate eigenvalue of structural vibration can be estimated in many cases of ship structure, but the vibration level can not be estimated so. Then a new method of combination analysis using the experimental modal analysis and BEM was presented by authors. To use this new method, the effect of arrangement of the measurement points upon the solution of the modal-BEM combination analysis must be explained.
In this paper, the effect is explained by some concrete parametric calculations on an ellipsoid of revolution. As the result of analysis on the calculations, 21 points distribution at regular intervals is enough for the number of the observation points to apply the method to the vertical bending vibration mode even below 4 node. Then the effect of rotation vector is analyzed, it is explained that the accuracy of the solution without considering rotation vector can not be better on higher vibration mode, if many observation points are set. This is verified by comparing with some experiments in the deep water and the shallow water.

View full abstract

The slamming problems of the three-dimensional bodies are investigated analytically. The similar method of flat plate approximation developed by Wagner to solve the two-dimensional slamming is applied to the water entry problems of arbitrary axisymmetric bodies.
The theoretical approach is extended to the more general case where the shape of the impact surface is elliptic. An analytical expression for the three-dimensional pressure distribution is derived. It is shown that the maximum impact pressure is approximately proportional to the square of the expanding velocity of the major diameter of the elliptic wet surface.

View full abstract

The load resistance mechanism of thin shells, such as the submerged shell structures in underwater marine constructions, are in principle of a nonconservative nature since the associated loads are the hydrostatic pressure and drag forces of the follower type.
The general governing equations of equilibrium for thin shells are available from the literature in various details. In this research we consider shells defined in a monoclinically convected coordinate system subjected to follower loads and undergoing large deformations. The feasibility and significance of the theoretical formulations have already been substantiated through various numerical simulation results.
In the simultaneous equations governing the equilibrium of shells, the presence of terms related to the shell curvature and other consequential terms such as the Christoffel symbols, clearly make them a substantially different class from the plate theory. This distinction, which may otherwise be denoted as the 'Form Effect' in more common terminology, renders the shell theory a lot more complicated and one that needs in-depth analytical effort to unravel the full significance of all the possible implications.
The present paper elaborates on the mechanism of large deformation by studying the share of different stiffness factors on the total load resistance equilibrium picture of shells from the very shallow to the deep curvature range going through the subsequently increasing loading stages. This study concentrates mainly on the equilibrium in the normal direction. The resisting mechanism due to the extensional and the bending stiffness parts are considered as the two main entities and their subdivisions and other separable factors are studied in unison to draw out an overall picture. The partial cylindrical and spherical shells are considered and the results bring out the clear distinction between resistance mechanism of the two shells, especially in the deep curvature region.
These results in their total details are believed to be helpful in understanding the deformation mechanism and formulate some design philosophy for shells and thereby serve as a guideline for appropriate formulations of the governing equations in particular cases.

View full abstract

A discrete optimization method using genetic algorithms is developed for the optimization of ship structures. In this method, the constrained minimization problem is first transformed into an unconstrained one by a penalty term depending on the degree of constraint violation. Since the search procedure of the genetic algorithm is done based on the evaluation of fitness function, the unconstrained minimization problem is further converted into a maximization of the fitness function. The discrete design variables are coded into a binary string of finite length. The search procedure from generation to generation is carried out by a simple genetic algorithm with the genetic operators of reproduction, crossover and mutation. A cargo ship with large hatch opening is taken as the numerical example for the illustration purpose. The influences of penalty coefficient, population size, crossover probability and mutation probability on the optimum design are investigated. The comparison between the genetic algorithm and the multiplier method is also made. It demonstrates that the present method can handle the optimization of ship structures with discrete design variables well.

View full abstract

The methods for a posteriori error estimation in element-wise analysis and improvement of finite element solution are developed recently by the authors for two-dimensional elasto-plastic problems. Based on the distributions of the error energy norm, the paper discusses the effects of mesh shape and size on the error energy norm and the relationship between total error energy norm and the distribution of error on each element. The minimum total error energy norm of the model is obtained and the stress solution of element-wise method is improved with the adaptive mesh techniques. Two adaptive mesh methods of r-version and h-version procedures are used for two elasto-plastic structural models respectively in this paper. Good results show that the minimum error principle and r-version and hversion adaptive procedures can be used in elasto-plastic problems effectively and accurately.

View full abstract

In the previous paper, we had shown the methodology of FES, Finite Element by Source method, developed by the authors and its advantages by some examples for static elastic structural problem using 2 D plane stress FES and 3 D solid FES.
This paper describes a fundamental formulation and some examples about new finite element by source method for plate bending problems. As well as the FES for static elastic problems written in the previous paper, we can perform numerical analysis with further less mesh and we can obtain more accurate results than the traditional Finite Element Method. Especially, in plate bending problems, semi-theoritical method FES has advantages because the number of derivative is large with regard to its differential equation.

View full abstract

Cavitation erosion tests of high tensile stainless steels for the Techno-Superliner (TSL-F) hulls were carried out. The TSL-F is a new generation, high-speed, hybrid hydrofoil lift/buoyancy type vessel. The hull structure consists of an upper hull, lower hull, hydrofoils and struts which connect the upper hull to the lower hull and hydrofoils. A 13 Cr-5 Ni martensite, a 24 Cr-13 Ni austenite which is produced by a new thermo-mechanical controlled rolling process (TMCP) and a 22 Cr-5 Ni dualphase stainless steel were examined, whose tensile strength ranged from 1078 MPa to 710 MPa, by three methods ; i. e., a vibratory method, a high speed fluid channel method and the test using a 1/6 scale sea going test ship.
It became clear that these materials have better anti-cavitation erosion properties compared to other conventionally used structural materials such as a mild/high-tensile steel and have the same properties as 15-5 PH (precipitation hardening) stainless steel which is well known as a high tensile stainless steel and that the vibratory method and the high speed fluid channel method gave the same characteris-tics qualitatively. Finally, a rough estimation of the total life cavitation erosion of an actual TSL-F was done by using these three test results.

View full abstract

To analyze the dynamic load of lifting load for floating crane moving in waves is one of the important factors for making the operation manual of the floating crane. In addition, it is necessary to consider the strength of floating crane structure since the inertia load caused by the motion of barge in waves is very large. In this study, the dynamic load of lifting load due to the motion of barge in waves is evaluated by means of elastic response analysis on the interaction between the crane structure and the barge hull body. The tank experiment of acrylic fiber model to one hundredth (1/100) for the floating crane with lifting capacity 3600 tons is made to verify the reasonability of the elastic response analysis. On the other hand, the numerical results of movement between the crane as a rigid structure and the one as an elastic structure are compared for the above-mentioned floating crane. It has been found that
(1) They have a good agreement if λ/Lthe wave length to the ship length, is 0.5, but the difference between them become large as λ/L is over 1.0.
(2) When the crane is simplified as the elastic structure moving in headseas, the dynamic load of lifting load is about 5% of the lifting load in the case with λ/L=1.5 and the wave height =2 m. However, the dynamic load is 13% of the lifting load under the same conditions if the crane is taken as the rigid structure.

View full abstract

This paper presents two topics related to the design of large floating structure. One is a proposal of a new type of large floating structure in which the dynamic response characteristics of the edge structure is modified from the other remaining uniform part. According to the understanding on the general response of the uniform large floating structure clarified in the previous report, the response of the edge structure becomes significant. This response will not be acceptable when the structure is used for runway of an airport from the view point of noraml function and structural strength. The dynamic response of the whole structure is improved by this modification.
Another topics is stability of the large floating structure considering elastic deformation of the structure. When some buildings and other structures are built on the floating structure, local elastic deformation must be considered. By using analytical solutions of continuous plate on elastic foundation, the deflection and inclination of the large floating structure under concentarted force and moment are calculated.

View full abstract

The stiffness equation of a beam finite element using the dynamic shape function of Bernoulli-Euler beam on an elastic foundation is derived and applied to the dynamic structural response analysis of pontoon-type very large floating structures in waves. The influence of virtual mass distribution, bending stiffness and the decay of amplitude of progressive waves on structural responses is discussed. It has been found that :
(1) The reduction of virtual mass of the end part of structures is an effective way to avoid resonance in waves for pontoon-type very large floating structures.
(2) The increase of bending stiffness is also effective to reduce structural responses.
(3) The quasi-static response at the weather-side end of the structure is reduced by a decrease of amplitude of progressive waves, while passing the stucture, whereas the resonance response is hardly affected by it.

View full abstract

The present study concentrates on the development of the Bayesian reliability analysis for estimating appropriate values of several uncertain parameters in probability distributions and generating appropriate non-periodic structural inspection schedules using small sample data gathered during in-service inspections. A pressurized fuselage structure of transport-type aircraft with multiple fatigue-critical elements is accepted as a structural model in this analysis. This element is a multiple component of two-bay fail-safe structure, designed by the damage tolerance principle, which consists of three frames and a skin panel subjected to cyclic stress due to differential pressure. Fatigue cracks are initiated and propagated from a hole for a rivet to connect the skin panel to the center frame. Probabilistic factors considered in this model are fatigue crack initiation and propagation, failure rates before and after crack initiation, yield stress, fracture toughness, cyclic stress range and crack detection capability of detailed visual inspection. Uncertain parameters estimated from inspection data such as the number of detected cracks, crack sizes and whether or not failures were detected are a scale parameter in a probability density function of fatigue crack initiation and a parameter in a simplified fatigue crack propagation equation. Both parameters can not be a priori estimated because of the paucity of pertinent data. Monte Carlo simulation techniques are utilized in order to generate a failure process in the structural element and to evaluate the validity of the proposed Bayesian reliability analysis.

View full abstract

Thin walled structures such as ships and offshore structures are composed of many stiffened plate panels. To analyze these structures efficiently, “Idealized Structural Unit Method (ISUM)” was proposed by one of the authors. In the previous reports, eigen-function for large deflection and sectional yield conditions at each integration points for plasticity were introduced to calculate a plate panel, which exhibits complex behavior especially among some ISUM elements. This method can efficiently and accurately analyze the behavior of rectangular plates with initial imperfection such as initial deflection and welding residual stress.
Recently researches on structural reliability of ships and offshore structures are actively performed such as reliability design of ship structure, quantifying randomness and uncertainties on response and strength of ship structure, and reliability design assessment of offshore structure. Reliability evaluation of structures is important.
In this study reliability analysis of the ultimate strength of ship structural models is performed using this developed element and method. Firstly it is shown that the developed element can be applied very efficiently to ultimate strength analysis in contrast with finite element method. Secondly reliability evaluation for ultimate strength of structural models is performed using the present method of ultimate strength analysis. Local failure modes of the models change as the load increases until ultimate strength state. Numerical analysis such as the present method is only possible to analyze the complex behavior. Since analysis of the reliability of ultimate strength needs analysis of ultimate strength repeatedly, finite element method isn't an effective method to use for reliability analysis of thin walled structures. This method is very effective not only for the analysis of ultimate strength but also for problems of reliability evaluation.

View full abstract

In this study, an equivalent CTOD concept was proposed on the basis of the local approach. This concept was born to consider the transferability of 3-point bend CTOD results to fracture performance evaluation of structural components. The equivalent CTOD was defined as the CTOD at which the 3 -point bend specimen and the structural component provide the compatible Weibull stress. The fracture performance of a wide plate with a surface notch predicted by the equivalent CTOD concept was consistent with the experimental result. On the other hand, the analysis based on the conventional CTOD concept gave very conservative estimation of the fracture performance of the wide plate.
A new procedure for fracture toughness requirement was presented based on the equivalent CTOD concept. It was pointed out that the equivalent bend CTODδ^R_3P to meet the required deformability of the wide plate was much smaller than the required CTOD δ^R_WPfor the wide plate. The equivalent CTOD concept was effective to quantify the strength mis-match effect between the base and weld metals on the required fracture toughness of the weld metals. The required weld metal CTOD δδ^R_WP for the wide plate increased with decreasing the yield strength mis-match ratio σ^W_Y/σ^B_Y between the base and weld metals. By contrast, the required bend CTOD δ^R_3Pof the weld metal was not always affected very much by the mis-match condition in welds.

View full abstract

Dynamic crack-growth model, which is based on the crack-tip local fracture stress criterion and takes account of the effect of unbroken side-ligaments near the plate surfaces and crack-bowing inside the plate, is extended for simulating crack-growth and arrest behavior in the duplex-type test. Different local fracture stresses for starter and arrester plate are assumed. Depending on the applied stress and temperature of the arrester plate, a crack continues to propagate or arrests in the arrester plate. Discrepancy of arrest toughness which was observed between temperature gradient-type and duplex-type tests is reproduced by the present model and its reason is explained by the different thickness and length of the unbroken side-ligament between the two types of the tests.

View full abstract

In crack propagation analyses of ship side outer platings subjected to collision load, effect of prestrain on crack propagation behavior should be clarified because not a few plastic deformation is usually generated on outer platings before crack initiation. And crack propagation simulation methods using a coarse mesh FEM model would be required in ship collision analysis owing to a huge numerical simulation model.
In this paper, pre-strain effect on crack propagation behavior was investigated with tensile tests of center notched plates whose pre-strains were 0.0, 0.15 and 0.3. In the experiment crack tip opening angles measured from grid line deformation drawn on the specimens were obtained for each pre-strain level and were verified to be a useful parameter for crack propagation simulation by performing FEM analyses of the experiment. Then a practical FEM simulation method for crack propagation, which includes the finite element size effect, was proposed.

View full abstract

Crack growth of a single crack or a periodical array of cracks initiated at the stiffeners in a stiffened panel has been investigated. Using stiffened panel specimens, fatigue crack propagation tests have been carried out with cyclic stress of constant amplitude and frequency. A procedure for simulation of crack propagation for multiple cracks was introduced. The stress intensity factors have been calculated by a FEM program. From experimental a-N data of flat plate specimens, the fatigue crack propagation rate was obtained, and the Paris' constants have been determined. By means of the material constants and the stress intensity factors, the fatigue crack growth has been simulated for all test specimens. The experiment and numerical simulation have shown that in case of a stiffened panel with three cracks, the crack growth rate is higher compared with a stiffened panel having a single crack, because in case of three cracks the net sectional area is reduced, and also the adjacent cracks interact with each other. For an actual deck of a ship structure, the ratios of crack growth lives for multiple site damage and a single crack damage have been investigated. In case of multiple site damage it is found that the crack propagation rate is several times higher than the single site damage.

View full abstract

Fatigue behavior of boxing welded joints of JIS A 5083 P-O Al-Mg alloy was examined. For a specimen, a 20 mm thick stiffener was attached to a 20 mm thick main plate by a MIG boxing welding. Previous to fatigue tests, residual stress measurement, a static loading test and an elastic finite element analysis (FEA) were carried out. The residual stress in the fatigue stress direction was 135 MPa at a distance of 5 mm from a boxing weld toe, and the FEA gave a fairly good estimation for the elastic strain distribution near the boxing weld toe.
Then fatigue tests were performed under both constant amplitude and random loads by 3-point bending. For a load wave in the random loading fatigue tests, a direct current component, a zero-mean narrow band random process, and a high frequency component were variously and selectively combined and superimposed.
As a result, a reference stress 65, which was determined as the stress at a distance of 5 mm from a boxing weld toe, was proved effective in reducing the amount of scattering in the fatigue test results. And it is also found that the crack initiation lives determined by the 5% drop method largely scattered because of the determination error brought by the 5% drop method itself, showing to be unsuitable for putting the fatigue test results in order as in a S-N diagram.
With regard to the effects of high frequency components, it was found that a simple handling of fatigue data using the RMS value of stress is not available while arrangement using a representative frequency is effective, and that neither the range count method nor the peak to peak count method are suitable to those superimposed wide band stress waves.
Finally the life estimation procedure proposed in the former report, which comprises a 2-dimensional rainflow method, the modified Goodman's relation and the modified Miner rule, was shown to be valid also in this study.

View full abstract

There has been proposed several methods to evaluate hot spot stress (HSS), which is an effective parameter to assess fatigue strength of various fillet welded structures. One is 2-points extrapolating method in which HSS is evaluated by linear extrapolation of stress at two specified points distant from weld toe and the other is 1-point representative method in which HSS is defined as stress at a specified point distant from weld toe.
In these methods, specified point (s) to estimate HSS is defined as a function of plate thickness t. But, generally, the specified point (s) should be defined as a function of not only plate thickness t but also weld leg length l.
First, in order to investigate a universal method to evaluate HSS for various combinations of t and l in fillet welded joints, stress analysis in the vicinity of weld toe has been carried out for 2-dimensional fillet welded joint models, by means of boundary element method (BEM). As a result, the authors propose the following new equation to estimate HSS σ_hs for various fillet welded joints.
σ_hs = σ (X_hss)
This means that HSS is defined as stress at x_hss the specified point distant from weld toe. And X_hss/t is the function of l/t and is expressed as follows.
X_hss/t=f (l/t) =c·a· (l/t) +b· (l/t) ²/a+ (l/t) ²
where a, b, c are constants.
Second, in order to verify the validity of the proposed HSS evaluating method, the fatigue tests of fillet welded joints in thick plate structures have been carried out and other fatigue test data has been refered. As a result, each S-N data based on HSS range evaluated from the new proposed method has gathered within a narrow band respectively, independent of structural stress concentration factors. And, it has been confirmed that the new proposed method to evaluate HSS is applicable for unified fatigue strength assessment of fillet welded structures.

View full abstract