The finite difference method TUMMAC-IV developed for the simulation of ship waves is applied to practical hull forms. Waves and pressure distributions of the hull forms of bulk carriers are computed by the two versions of TUMMAC-IV, one with a mesh system of equal spacing and the other with a variable mesh system in the vertical direction. The comparison with the measured results indicates that the computed waves are in good agreement and that the method is useful for the determination of better hull form with smaller wave resistance.

View full abstract

A vast amount of numerical and experimental results are presented in this final report. Numerical simulations are performed by the TUMMAC-Vot program to know the effect of Froude number on the breaking bow waves, as well as the effect of shear flow near the free-surface. The existence of a critical Froude number for the occurrence of large-scale breakers and the variation of the breaking behavior, from spilling to plunging type, due to the increase of Froude number are demonstrated. The shear flow is proved to be influential to wave-making, breaking and vortex generation. Experimental analyses are made using grid-projection method, flow visualization and turbulence measurement techniques. The experimental results partly verify the usefulness of the numerical simulation and partly indicate further complexity of the wave dynamics with turbulence.

View full abstract

This paper describes a numerical method to solve steady potential flow around ships based on the non-linear free surface boundary condition. One of the authors previously examined an iterative method for solving the velocity potential by using Rankine source, taking into account the non-linear free surface conditions in two-dimensional aspect and showed the usefulness of the method through the comparison between the computations and the experiments.
In this paper, the authors expand this method to three-dimensional problem and apply to the computation of free surface flow around ships. Starting with a solution based on the double model linearized free surface condition, boundary conditions at the free surface in its non-linear form are satisfied iteratively. The method of numerical solution is examined on the steady waves and non-linearity of the waves is discussed.
Further, the pressure distribution on the hull surface and wave resistance are computed for two types of hull form, that is a slender from and a blunt form. It is found that the computed results show better agreement with the experiments than the results of linear theories.

View full abstract

The present paper describes a method for calculating viscous resistance of ships based on a higher order boundary layer theory, which is solved by the integral method. Higher order terms in the direction normal to the hull surface were thoroughly examined on the basis of the flow characteristics measured near the stern for an 8 meter long ship model. Viscous-inviscid interaction is taken into account by an iterative calculation procedure with underrelaxation method. The frictional and viscous pressure resistance components are calculated separately by integrating wall shear stress and pressure over the hull surface to obtain explicitly the correlation between hull form and viscous resistance components. Furthermore, in order to change the ship hull form systematically and easily, the ship form is represented by use of the exponential splines.
From comparison between calculations and measurements, it is found that the present method predicts the viscous resistance of full form ships with allowable accuracy and will become a useful tool for systematic investigations of various ship forms in the course of hull form design.

View full abstract

Turbulence measurements in the boundary layer of a double model are made by a hot film probe. Turbulence intensity, turbulent kinetic energy, Reynolds shear stress, turbulent energy production and kinematic eddy viscosity obtained from three turbulence components are investigated. Measuring positions are selected as followings, which are along a potential flow streamline, across a separation line, on the separation line, just past a bilge and beneath the waterline. It is shown that the turbulence quantities at each position have peculiar distributions throughout the layer since the quantities are influenced by the hull surface curvature, pressure gradient, separation and longitudinal vortex.

View full abstract

Following the first report, the author tries to represent the oscillating velocity field of Navier-Stokes equation following Oseen's scheme in two-dimension by introducing a linear oscillating kernel function and the reciprocity theorem.
The reciprocity theorem might serve as a tool for an approximate method to solve the boundary value problem and the eigen-value problem.
For two examples on the hydrodynamic stability of laminar boundary layer and Karmanvortex street in the wake of a cylinder, the proposed method of approximation has made fit well the upper curve of the famous neutral stability of the laminar boundary layer and succeed to predict Strouhal number of a circular cylinder.

View full abstract

In recent years, the propeller's fatigue fracture at the blade's root of motorcar carriers and refrigerated cargo carriers has become an important problem awaiting to be solved. On the other hand, the use of the highly skewed propeller for the reduction of ship vibration and noise leads to the strength problem of propeller. With such technical background, an accurate method for the calculation of the pressure distribution acting on the surface of the propeller blade has been needed.
This paper presents the direct formulation of boundary element method for the analysis of three-dimensional flow around the marine propeller which is expected to be a more accurate method than the lifting surface theory. The present method is applied to the analysis of flowfields around the rectangular wing, conventional propeller and highly skewed propeller. The result is compared with that of the lifting surface theory, and the usefulness of the present method is verified.

View full abstract

A numerical method to estimate propeller open-water characteristics based on Quasi-Continuous Method (QCM) is presented. QCM was originally developed by Lan for solving planar thin wing problems. QCM has both advantages of continuous loading method and discrete loading method; loading distribution is assumed to be continuous in chordwise direction and stepwise constant in spanwise direction. Simplicity and flexibility of discrete loading method are also retained.
In the present paper, explanations are made first on the essential features of QCM for planar wings and then its application to marine propellers is described in detail. Some numerical results on the propeller open-water characteristics for conventional and skewed propellers showed good agreement with those of experiments.

View full abstract

It is known that the cavitation is dependent upon the boundary layer characteristics. Therefore, it is considered that cavitation can be controlled by controlling the boundary layer.
This paper deals with a SLAT-FOIL combination, main foil and upstream slat. The slat is one of the boundary layer control methods for foil sections.
Firstly, the authors designed the SLAT-FOILs to increase the pressure at the laminar separation point as high as possible, and verified the cavitation suppression effect by cavitation tunnel tests.
Secondly, the authors studied the interaction between the wake of the slat and the boundary layer on the main foil, and found that when the wake of the slat merged into the boundary layer on the main foil, it accelerated turbulent transition. If the pressure of the turbulent transition point was high enough, the cavitation was remarkably suppressed.
In conclusion, two methods of the cavitation control (suppression) can be suggested, those are as follows.
1. A slat can change the pressure distribution on the foil. As the result of increase of the pressure at the laminar separation point, the incipient cavitation number of sheet type cavitation can be decreased.
2. When a slat is near the foil, the wake of the slat is mixed into the boundary layer on the main foil, which accelerates turbulent transition. If the pressure on the transition point is high enough, the incipient cavitation number is remarkably decreased.

View full abstract

This report presents the longitudinal dynamics of underwater towed body system.
Firstly, the longitudinal equations of motion of underwater towed body system were derived by treating the cable as lumped parameter system of discrete masses.
Secondary, hydrodynamic characteristics of the hypothetical towed body, which is necessary for the analysis, were obtained experimentally. Utilizing above results, the longitudinal equations of motion of underwater towed body system under external disturbances were solved numerically.
As the result, dynamic characteristics of underwater towed body system, mainly towed body response to the heaving towing point, were made clear.

View full abstract

To make clear hydrodynamic characteristics of Surface Effect Ship (SES) bow seals, comparative model tests were conducted in calm water and in irregular waves on 12 bow seal models whose types were 4 basically, namely finger seal, bag and finger seal, planing seal and transversely supported membrane (TSM) seal.
As the result of tests, it was found that the drag characteristics of the tested seal models were almost identical and lift forces acting on planing seals were greater than those on other types of seals slightly.

View full abstract

The cavitation minimization of the waterjet inlet is an important matter in designing the inlet configuration for high speed SES (Surface Effect Ship). For this purpose it is necessary to know the pressures on the inlet surface. However it takes much effort to calculate the pressure distribution in 3-D flow even when it is inviscid. On the other hand, the 2-D inviscid flow calculation is expected to be an useful design tool.
From the point of view, a new calculation method of the 2-D potential flow is developed. It can estimate the pressure on arbitrary point precisely, by using the spline function. The calculation result is compared to an analytic solution and shows a fairly good agreement.
To examine the availability of the 2-D calculation to the 3-D inlet flow, the pressure measurement is carried out by an inlet model in the wind tunnel. The differences between the measured and calculated pressures are discussed.

View full abstract

A three-dimensional finite-difference method for ocean wave problems called TUMMAC-IVw is developed. The method can cope with the interaction of ocean waves with floating or submerged bodies of an arbitrary shape. The treatment of free surface condition is especially modified in order to suit to the oscillatory motion of ocean waves. Simulated results are presented and discussed for the two problems, namely, the wave diffraction at the bow of a blunt ship and the wave deformation over a submerged mountainous obstacle.

View full abstract

In the previous paper, the authors presented a method of predicting wave loads acting on a ship fixed in large amplitude waves, and verified its validity in computing the peak values of wave loads as well as in clarifying their time dependent nonlinear behavior.
In this paper, 2 nd report, prediction of ship motion and wave loads acting on a ship freely floating in large amplitude waves are performed using the method in its extensioned form. All the results computed by the present method, in which time-variation of the submerged portion of ship's hull and coupling effect between vertical and horizontal hydrodynamic forces are taken into account, shows quite good agreement with experimental ones.
As a result, it is found that time-varying relative vertical displacement of a ship's body does a dominant role in nonlinear behaviors of wave loads, and that torsional moment can be affected by the coupling phenomena between vertical and horizontal hydrodynamic forces to a greater extent in relatively short waves of beam-sea condition.

View full abstract

This paper deals with the wave-free forms of a two-dimensional body in swaying oscillation. The wave-free body generates no radiating waves when oscillating horizontally on the free surface of a fluid at a wave-free frequency. By Haskind relations, the body experiences no sway exciting force in incident waves of the frequency.
A method of the tracing of stream lines of the wave-free potentials which satisfy the conditions of the free surface and no radiating waves at infinity is used to investigate the features of the wave-free forms. The numerical approach gives two kinds of wave-free forms. One is a floating body, the sides of which are not vertical and inclined inwardly beneath the free water surface, and another is a submerged body. Numerical calculations of hydrodynamic forces are carried out for the floating bodies derived from the stream line tracing. Hydrodynamic characteristics of the bodies are discussed.
Measurements of wave exciting forces are performed on the models of floating bodies and a submerged body with the various amplitudes of incident waves. The results reveal that the effect of wave height on sway exciting forces is significant. A tendency is observed that the measured exciting forces for smaller wave height are closer to the calculated ones, so that the existence of wave-free frequency in swaying oscillation can be verified experimentally.

View full abstract

The authors have made extensive investigations on the stability of the moored semisubmersible platforms for past several years and have pointed out that the existing intact stability criteria considering only unmoored platforms have not been sufficient to prevent the dangerous situations of the platforms in violent sea states.
In the case that the platform is moored, the various kinds of environmental loadings such as waves and currents should be generally taken into consideration in addition to the wind loadings. Besides those, the steady tilt of the platforms caused by the wave induced vertical steady force on the lower hulls has been found to be important to introduce the capasizing phenomena. These were already published in the first report.
The second report shows the study on the combined environmental loadings for the semi-submersible platforms by using the time-domain simulation program. As the results of the study, it was concluded that the currents and waves induce the same order effects on the moored semi-submersible platforms as the wind's effects.
In the present report, the authors summarized some problems on the existing intact stability criteria by using the results of platform behaviors obtained from the time-domain simulation. Finally the new computational approach for determining the required minimum GM (GM_r) is proposed to avoid the significant larger inclination. This GM_r consists of the standard GM (GM₀) and the several correction terms (ΔGM) which come from the effects of various kinds of environmental lodings and mooring tensions. The present approach is believed to be useful for designers as it is easily applied to many semi-submersible platforms for determining the adequate GM. This approach also has a great potential of application for finding the alternative solution of platform's configuration.

View full abstract

In recent years, ocean exploration and development activities have moved into deeper and more exposed seas, with progress of offshore technology. In these offshore construction works, it is very important to minimize the downtime of work vessels which have special functions, such as crane, pipe laying, diving support, etc.. Therefore, the reduction of motions of such vessels and floating structures has much significance on progress of offshore projects. In order to reduce the motions of floating structures, the effectiveness of new motion suppression system is dealt with based on modified Bell's type tanks which are composed of air chamber, water columns opened to sea and air duct between tanks.
In this paper, as fundamental study, hydrodynamic forces acting on rectangular barge and its water columns in tanks are calculated by two-dimensional singular distribution method. Then, after modelization of air flow in chamber and duct, motion equations are derived. Results of calculation are compared with the experimental data. The results of motion calculation agree well with experimental ones.
The results of this study are following.
(1) The main factor of effects of this motion suppression system is shift of resonant period of structure to longer due to reduced GM.
(2) The magnitude of rolling amplitude of structure with motion suppression system at its resonant period becomes abt.1/5 compared with amplitude without suppression system.
(3) By handling of valve in air duct, very high performance of motion characteristic of structure on overall range of frequency is obtained.

View full abstract

The tension mooring of the floating ocean structures seems promising to expand the ocean space exploitation. This kind of mooring method becomes relatively inexpensive in deep ocean, and has many advantages such that it can suppress the motion of the floating structures and needs only minimum mooring space.
This paper presents the results of the fundamental study on the tension mooring. A semisubmersible model with 4 footing columns is tautly moored by the rubber-chain cable in the regular waves. The results of the study are following.
(1) The rubber-chain cable has a nonlinear load-elongation characteristics. The static characteristics before stress relaxation is applicable to the dynamic analysis.
(2) The heaving and pitching motions of the floating structures are negligible, but the surging motion increases as the wave length does.
(3) The frequency responses of the tension fluctuations of the cable do not depend on the material in case of the large initial tension. The axial rigidity of the rubber-chain cable is, however, much less than that of the wire cable so that in designing the rubber-chain tension mooring system, we must carefully set the natural frequency of the system sufficiently out of the wave spectrum.
(4) The snap load easily occurs when the initial tension is small.
(5) The analysis model is applicable to the design of the tension mooring system. It predicts, however, much larger snap load. The authors expect that the analysis model will be improved by including the nonlinear hydrodynamic forces.

View full abstract

The system simulation for a fixed O. W. C. type wave absorber is proposed, which takes into consideration not only the interaction between the O. W. C. and the turbine but also the effects of air compressibility in the air-chamber. Linear calculations for steady high velocity revolution in frequency domain are also shown.
It is shown that the reactance due to the air compressibility is as large as restoring force due to static pressure of the water column in case of real sea trial.

View full abstract

It can be said that the mechanism and the characteristic of wave energy absorption etc. of attenuator type devices have not yet been clarified experimentally and theoretically. Therefore in this paper, the authors deal with an attenuator type device which consists of five air chambers with and without a bottom plate. The authors carried out the experiments of a motion test in transient waves, a wave excitation test in transient waves and a transient forced oscillation test in which the load damping force is varied systematically by making use of the authors' experience of terminator type devices. The authors obtain the corresponding reliable experimental data of responses such as characteristics of wave energy absorption which will be used to develop the practical prediction method of characteristics of this kind of primary energy converters. Next, with regard to the numerical simulation, the authors compare the numerical results by the strip method, in which the equivalent floating body approximation is used considering the load damping of each air chambers, with the corresponding experimental results and they show the applicability of the strip method to the problem of this kind of wave energy absorption devices and its limitation.

View full abstract

The previous papers, the firt and the second reports, described a new method to calculate the hydrodynamic sway force and yaw moment acting on a ship with forward-speed oscillating on the free surface, and verified its validity by comparing the hydrodynamic derivatives predicted by the present method with the experimental ones in both cases of a flat plate and a Wigley model.
In this paper, the method is applied furthermore to predict the hydrodynamic derivatives of ships with realistic hull shapes, namely, a tanker ship, a Series 60 model, and a container ship. In case of a container ship, in particular, the hydrodynamic derivatives related to roll as well as sway and yaw were computed, and then compared with those obtained by model experiments. As the result, it is shown that the present method predicts very well the effects of ship's forward-speed, which have never been successfully explained by the so-called Strip Method, on the hydrodynamic derivatives.
In the previous papers, Sommerfeld's radiation condition, that is to say, ∂φ/∂t+C∂φ/∂l=0 was imposed at the open boundary which is placed at a distance sufficiently far from the ship in order to limit the fluid region to be treated in the numerical calculation. In this paper, however, the condition is replaced by a more relaxed condition ∂φ/∂t+C∂φ/∂l=E in which E denotes the numerical error of the left-hand term of the above-stated Sommerfeld's radiation condition.
By making use of the relaxed condition instead of the exact radiation condition, it becomes possible to make the waves, which are generated on the free surface and propagate toward the outside region, pass smoothly through the open boundary.

View full abstract

For a traffic control of ships passing the control region arbitrarily, optimization techniques can be utilized to get smooth and safe courses avoiding dangerous encounters where miss distance and destination error for each ship are evaluated as a criterion function. To realize such optimal course selection, however, reliable communication systems are indispensable, because ships may take the nearest courses just beyond the miss distance.
In the paper, a communication system using conventional marine radars is proposed to realize mutual communication between ships and/or the control station. In the proposed system, control information such as position or velocity of each ship is broadcast to surrounding ships by PPM (pulse position modulation) of radar pulse with dual-k convolutional coding. The information is broadcast, divided into 4 subframes corresponding to each scan of the radar antenna, and those subframes of different radars are identified by detecting the preamble uniquely specified to each radar. The data of 80 bit length can be broadcast within 20 seconds using conventional radars.
Use of the coding technique as well as the PPM ensures reliability of the communication, enabling each receiver to separately extract correct information of other ships in heavy radio interference environment. The applicability of the system is evaluated for the situation investigated in the previous papers. From the investigation, it is shown that the overall error rate of the communication system, taking account of false preamble error and decoding error, is less than 10^-5 over 99.99999% of the total time, which is regarded sufficient for such purposes.

View full abstract

In congested waters, the traffic control is required for improving the safety and the traffic capacity.
In the previous report, the methods of the traffic control on the crossing points were considered, and then the effectiveness was verified.
Even if the traffic control is optimum for the each crossing point, it is not always suitable for all over.
Whether the control on the double crossing is to be systematic or not is decided by the distance between the crossings and traffic density, etc.
Therefore in this report, the authors investigated how to divide the control area, using the results of the traffic flow simulation on the single and double crossing. (Fig.1) Traffic control on the continuous crossing is available to be individual each other on condition that the distance between the crossings (D_c) is larger than D_min : D_min is defined as the minimum distance in which all ships are able to adjust the passing time. if D_c is less than D_min, the traffic capacity is improved by systematic control.
As the large ship is difficult to adjust the passing time in the short distance, it is useful to apply the systematic control for large ship only. This combined control system decreases D_min, to 80%.
In the many congested waters, the traffic flow will be improved by these control systems.

View full abstract

In order to study the fracture behavior at the side shell to annular fillet weld toe in the flat bottom oil storage tank, the burst test of model tanks has been carried out by hydraulic internal pressure. It is clarified from the results that the initiation of the ductile crack is the critical event in the fracture of the tanks, and the ductile fracture toughness of materials and the geometry of the weld toe have the significant effect on the fracture.
So far, L-shape bend test has been performed and the effect of the geometry of the weld toe and mechanical properties was formulated. In the present study, the fracture criterion of fillet weld toe in the tank is derived by modifying that formula based on the crack initiation characteristics.
Finally, the reliability analysis has been carried out by means of Monte-Carlo simulation. The probability distribution for the weld toe geometries is assumed to be normal and Weibull function based on the data measured in the real tanks. The ductile fracture toughness δ_i is evaluated using two and three parameters Weibull distribution functions. The reliability of tanks are disscussed from these results.

View full abstract

The statistics of hull failures of ships chassed with Nippon Kaiji Kyokai obtained from annual survey reports were analysed. From their databank, an approximate method to predict number of annual failures has been proposed for ships of several ages and types, mainly in case of corrosive damages in previous reports.
This report presents a more exact theory to predict the expected number of fatigue failures by using a fatigue model which is characterized by uncertainties of crack initiations and applied stress levels of Gamma distribution type as well as Paris' law of propagation.
An expected theoretical distribution of crack length which would be detected at annual survey is discussed in a group of transverse hull members.
Two examples by the present method are shown; one is the case of fatigue cracks in bottom transverses of cargo ships due to vibration under load of approximately constant amplitude and the other is the case of fatigue cracks in transverse bulkheads of vehicle carriers under irregular wave loads.

View full abstract

Fleet leader and rotational sampling inspection programs for aged aircraft structures are performed to detect fatigue damage and to extend their service lives. Although the rotational inspection has recently been recommended instead of the fleet leader inspection on account of higher detectability of fatigue damage and operators' advantage, there are almost no discussions on the comparison between effects of both inspections. It is considered that the two inspections should be evaluated from two kinds of capability, that is, reduction of number of failed aircraft and estimation of unknown parameters. By the reliability analysis on the statistical model developed appropriately, the first report concludes that, as compared with the fleet leader inspection, the rotational inspection can detect more fatigue cracks and reduce the number of failed aircraft.
The present report intends to evaluate the two inspections with emphasis on capability of estimating parameters determined apriori with low confidence. Thus, Bayesian method whose effectiveness has already been demonstrated is applied to the present reliability analysis. The numerical example indicates that the parameters estimated from the results of the rotational inspection are more reliable than those from the fleet leader inspection.

View full abstract

Many off-shore structures of fixed or mobile platforms, are mainly composed of tubes of various diameters. The structures have many tubular joints called nodes, where two tubes or more connected together with welding in the same space. Due to the weld, there are residual stresses in tubular joints. It is said that the residual stresses have bad effects on static and fatigue strength of the joints. However, few investigations have been carried out for the problem of residual stresses in welded tubular joints. So, it has been almost imposible to take residual stresses into consideration when designing the joints.
This paper deals with the residual stresses in welded tubular T-joint, that is the most simple and popular type of the tubular joints. Experimental and analytical results are shown. Residual stresses of two specimens are measured. One specimen has 700 mm diameter chord and 400 mm diameter brace, another has 700 mm diameter chord and 300 mm diameter brace. FEM with shell elements is used for the full model analysis. Then, plane strain elements are used to perform the analysis of weld toes. Both experimental and analytical results are thoroughtly agreed for the most part.

View full abstract

50 kgf/mm²-class high-tensile steel plate produced by newly developed TMCP (thermo-mechanical control process) is superior to conventional rolled steel plate in weldability (resistance to low-temperature cracking) as well as in toughness of HAZ of the high heat-input welded joint.
However, HAZ of high heat-input welded joint softens because of the low carbon equivalent.
The authors investigated the tensile strength of the welded joint with soft HAZ in ship hull and other heavy steel structures.
As a result, it was found that even with the maximum heat-input welding process prevalently in use in most of the domestic ship-yards, the tensile strength of the welded joint thereby produced in wide plate would be 50 kgf/mm² or more and approximate that of the base metal when Ceq. _≥0.23%.

View full abstract

A paper-surveying on fatigue tests of tubular and foundamental joints for offshore structure design was carried out and tested data were collected. The collected data were arranged as two sets of data base : the data base on tubular joint tests and the data base on foundamental joint tests including cruciform joints, butt-welded plates and so on. Based on these data bases, the regression analyses were conducted in order to evaluate effects of factors on the fatigue strength of these joints. Plate thickness, material, load ratio, random loading, corrosive environments, grinding, TIG-dressing etc. are included in these factors.
Correlations between the effect of each of these factors on the fatigue strength of foundamental and tubular joints were discussed and it was, generally speaking, found that the correlations are good. Finally, the current fatigue design curves were compared with the collected data and the extent of safety of the curves were confirmed.

View full abstract

In the previous papers, the authors proposed a realistic crack arrest concept to assess the structural integrity of LNG storage tank and the surface-notched double tension test to evaluate the crack arrest properties of 9% Ni steels and their weldments. This paper deals with dynamic analysis of the run/arrest events confirmed in the surface-notched double tension tests conducted on 9% Ni steels by three-dimensional linear elastic finite element method.
Prior to the analysis, dynamic deformation behaviors of the specimen and crack velocities were measured under the potentially extensive propagation and arrest of brittle crack during testing. These experiments verified that a brittle crack propagated through the specimen and/ or was arrested at the notch tip under the fixed grip conditions. These experimental data were used as input data in the analysis by general purpose program ADINA. Crack propagation was simulated in such manners that the nodal points on crack propagation line were constrained by truss elements which were eliminated at the time interval corresponding to crack velocity. Excellent agreement was obtained between experimental and FEM results as to the dynamic deformation behavior of specimen. Variation of dynamic stress intensity factor with crack propagation was obtained by energy balance theory.
Dynamic analysis shows that K_D decreases as the notch depth becomes shallow and that K_D indicates minimum value at the notch end. In linear elastic fracture mechanics, the arrest of propagating crack is to occur when dynamic stress intensity factor K_D of the crack becomes lower than crack arrest toughness K_a of the material. Present results of experiments and analyses do not necessarily support the K_a concept stated above because it happened that K_D of the propagationg crack showed lower value than that of the arrested crack.
Crack arrest at the notch end occurs if sufficient plastic deformation is allowed there when triaxiality at the crack tip is low and crack velocity is slower than plastic wave velocity. Since the surface defects existed in the welded structure are generally small, there hardly exists the possibility of crack propagation failure of 9% Ni steel and its weldment in LNG storage tank.

View full abstract

As a model of the stress concentrators in the welded structures, a wide plate test with stiffener was carried out, and the brittle fracture characteristics from a surface notch at the toe of the fillet weld was studied. The results of the investigation are summarized as follows.
CTOD values obtained from the wide plate test are agreed well with those from COD test with a/w=0.5, even for the very shallow surface notch in the wide plate test. It is confirmed from this results that CTOD criterion can be applied to elastic-plastic fracture from the surface crack in the highly deformed region.
The brittle fracture strength from the surface notch at the fillet weld toe is evaluated using COD design curve, which is obtained by modifying WES 2805. The strain at the toe used in the evaluation is estimated for the average value in the cracked area. The strain concentration factor at the toe is caluculated from the elastic stress concentration factor by means of the modified Neuber's relation. Using such strain and COD design curve, the brittle fracture strength is well estimated.

View full abstract

The steel for ice breaking vessels is required to have an excellent notch toughness at low temperatures and a good weldability and to meet the strict requirements in terms of workability and cost.
The first report discussed the base-metal properties and joint performance of the 40 mm thick steel plate newly developed for ice breaking vessels.
The present report discusses 75 mm thick steel plate also newly developed for the bow part, which has to meet severer requirements. The developed material is the 50 kgf/mm² high tensile strength steel of 36 kgf/mm² yield point manufactured by the CLC process. The test results showed that this new heavy plate has good mechanical properties, weldability and workability. It was also indicated that an ice breaking vessel to be made of this steel would be sufficiently safe against brittel fracture.

View full abstract

Following “Shinkai 2000”, as Japanese deep research project, a 6000 m class deep submergence research vehicle is under planning. For the research vehicle of this class, from the maneuverability and operationability, of utmost importance is the reduction of weight. As for a pressure hull, which houses crew members, the authors have carried out a series of research for the application of titanium alloy (Ti-6 Al-4 V ELI) considering the strength-weight ratio, and anti-corrosion capability against sea water. In the 1 st report, the authors have made a full scale model of the pressure hull of Ti-6 Al-4 V ELI for the 6000 m class submersible to establish the manufacturing procedure. In this report, by disassembling the model, the authors have investigated the material characteristics for the application of a deep submergence research vehicle. The research covered the fields of mechanical properties, fracture toughness, fatigue strength, stress corrosion cracking, hardness distribution, micro-structure, and residual stress. As a result, 6 A1-4 V titanium alloy was confirmed to be one of the most effective material for the pressure hull of a deep submergence research vehicle.

View full abstract

Recently, various kinds of arctic offshore structures have been constructed corresponding to the promising oil industry.
In designing such structures, the designers have to pay particular attention how to overcome the ice loads.
One of solutions for strength against ice loads is the application of composite steel-concrete structure.
In this paper, selecting five feasible composite models where concrete is placed inbetween steel plates, the elastic-plastic behaviour and their ultimate strength are investigated by both theoretical and experimental approaches.
The theoretical analysis has been performed by finite element method which is newly developed to incorporate non-linearity of concrete and interaction between steel and concrete.
In addition, thermal fatigue, i. e. recurrence of freezing and thawing of concrete, is experimentally investigated anticipating real environmental condition in arctic area.
As the results of our investigations, followings can be concluded.
1) Our newly developed computer program can predict the elastic and plastic behaviour and ultimate strength of composite structure with due accuracty.
2) Most benefitial combination of steel and concrete can be found.
3) The recurrence of freezing and thawing do not give an appreciable influence on the strength of structure.

View full abstract

Artificial gravel or sand islands were constructed in shallow water of the Arctic Sea to drill explorative or productive wells. However as increasing the water depth of the site, it became difficult and expensive to complete an island in one season. So the mobile islands such as Mobile Arctic Caisson (MAC) have been developped in place of artificial gravel or sand islands.
In this paper, the authors studied the design criteria of a mobile caisson island, and investigated the structural strength analysis with its application to MAC, which was constructed in our yard.
In the structural analysis of a mobile caisson island, the effect of sand shall be the most important. Usually the sand shall be treated as simple lumped springs distributed on soil face and bottom of the caisson. But in this method, it is difficult to obtain the accurate results, because the calculation of the spring constant is very difficult, and it is impossible to be taken into account of the interaction of each spring.
So in this paper, to obtain the accurate results, the authors analyzed that interation and contact problem of the sand and the steel structure with finite element model.

View full abstract

It was reported that even a full-bodied bulk carrier in the fully laden condition suffered serious bottom slamming in an unhappy superposition of swells and waves and that buckling of the upper deck of the bow was caused by the subsequent whipping of the hull. Damage analysis of such a disaster requires the accurate estimation of internal force distribution in the fore part of a hull and also of nonlinear impact force due to slamming. In the present paper the finite element method is proposed for the structural response of ship hulls in rough seas, which considers shear deformation and higher frequency vibration than 2 nodes and 3 nodes modes. The appropriate added mass and wave damping coefficients for ship hull motion and impact force are obtained by the Close-Fit method in this procedure. Good correlation is observed between the present calculated results and the experimental ones.

View full abstract

The present paper investigates theoretically the maximum pressure occurred in the water impact of the rigid wedge model. It is well known that Wagner's theory predicts the maximum pressure much higher than the experimental results by Chuang in case of small dead rise angle 4°, where air cushioning effects can be ignored. One of the reasons mentioned of such a discrepancy is error in measurement by a pressure gage of a finite diameter. It is not proved, however, that all the discrepancy can be attributed to the measurement error.
The authors determine the wetted width by considering the splash width derived from the condition of energy balance and of the free surface under the assumption of self-similarity. The smaller value of the wetted width than Wagner's prediction is obtained and it shows a good agreement with the experiment by Bisplinghoff. &. Doherty.
The present theory gives the lower maximum pressure than that predicted by Wagner, since the maximum pressure is approximately in proportion to the square of the wetted width ratio c/c' for small deadrise angle.
It is concluded, therefore, that the maximum pressure is essentially lower than Wagner's prediction under the assumption of self-similarity in free surface.

View full abstract

Dynamic behaviour of a thin-walled beam due to impact are investigated through experiments, and a particular attention is paid to lateral buckling of the beam : T-shaped beam is chosen for tests and impact force is applied by a pendulum-type impact testing machine, of which weight itself is a load-cell and the impact force is measured. Strains of the beam are also measured by strain gages. On the other hand, static tests for lateral buckling of beam are carried out to examine the dynamic effects on the ultimate strength. These experimental results are compared with the calculated ones by the finite element method. The dynamic behaviours of the beam-component of frame structures are clarified through the investigation.

View full abstract

The mean crushing load for axially compressed circular cylindrical shells is studied in the present paper. The theoretical rigid-plastic solutions for the axisymmetric and the nonaxisymmetric collapse mode are compared with the experimental data given in the published literatures and their validities are investigated. The obtained results are summarized as follows :
(1) For the mean crushing load in case of axisymmeric collapse the modified Alexander's solution (eq. (10)) in which the effect of stroke to length ratio is taken into account is extremely in good agreement with the experimental results with regard to the crushing wavelength as well as the crushing load.
(2) In the range of large R/t ratios where the crushing mode is mainly due to the non-axisymmetric elastic buckling the modified Johnson's theory (eq. (33)) in which the inextensional collpse mode is assumed gives reasonable mean crushing stresses.
(3) In the range of small R/t ratios where the crushing mode is mainly due to the nonaxisymmetric plastic buckling the generalized Pugsley's theory (eq. (29)) in which the internal plastic work by the membrane shearing force as well as the bending moment is taken into account agrees relatively well with the experimental results.
(4) When the axisymmetric and the non-axisymmetric collapse are both considered, the modified Alexander's theory and the modified Johnson's theory can be regarded as the upper and the lower bound solution for mean crushing stresses respectively, however, no rigidplastic solution is sufficiently accurate for non-axisymmetric collapse mode. The detailed numerical analysis such as the finite element analysis will be needed in order to estimate the mean crushing load for the non-axisymmetric collapse mode with sufficient accuracy.

View full abstract

In this paper, a new elastic buckling interaction equation for simply supported rectangular plates is proposed, when the plate is subjected to five load components, such as compressions and bendings in two directions and shear.
In order to construct this interaction, buckling strengths are calculated in addition to the existing data, and buckling interaction relationships for simply supported rectangular plates subjected to two load components are developed. The accuracy of these relationships is discussed.
The accuracy of the newly proposed interaction equation is checked against the above mentioned interaction relationships for two load components. Further assessment of accuracy is carried out when a rectangular plate is subjected to more than two load components. It is found that the proposed equation yields results with accuracy sufficient for practical purposes.
A comparison is also made with interaction equations proposed by Lloyd's Register and Det Norske Veritas. The result indicates that the new interaction equation proposed in this paper yields better accuracy in safety side.

View full abstract

An analytical model of dented tubular members is constructed. Using this model, an ultimate strength interaction relationship between axial force and biaxial bending moments is derived. The implementation of this interaction relationship to take account of denting and bending damage in the analysis of nonlinear behavior of tubular space frames by ISUM (the Idealized Structural Unit Method) is described. An experimental study of the behavior of dented members subjected to pure bending is reported. Comparison with theoretical predictions is presented.

View full abstract

In this study, expected total cost, which is composed of structural cost and expected maintenance cost, is introduced as a reasonable utility function for the multi-objective optimization design problem such as maximum safety and minimum cost. The optimum design procedure is proposed for offshore platforms minimizing the expected total cost. The design variables are the geometrical dimensions of the structure while its configuration and the materials to be used are given. Partial failure due to formation of plastic hinges and/or fatigue cracks of the critical sections during design life is considered. The design is performed for the platforms subjected to wave loads. In addition, the effect of periodic inspection is investigated. The optimum design problem contains many parameters which are difficult to estimate exactly, and thus the optimum solution may not be absolute. However, the failure probability corresponding to the optimum solution is used as an index for specifying the allowable failure probability in probabilistic optimum design problems. Numerical examples for a jacket-type offshore platform are provided to illustrate the applicability of the proposed method.

View full abstract

To improve seakeeping quality some advanced high speed crafts have been developed and in service. However, in view of total performance including building cost, operation and maintenance cost, they are not satisfactory in commercial use. Under this circumstance, a new type of high speed craft having a submerged torpedo-shaped body under the bow bottom of conventional monohull high speed craft was developed. This new type of craft with Semi-Submerged Bow (SSB) has improved seakeeping quality with comparable total performance of conventional monohull type high speed craft. In this present paper, procedure in developing this new type of craft are explained.
First, as a preliminary study, theoretical calculations were made before tank tests to confirm effect of a submerged body for the reduction of ship motion in waves.
Next, preliminary tank tests were carried out in waves with two small models of 1. 4 m in length (SSB and Conventional monohull) towing at the same time in parallel position to compare directly the seakeeping quality of SSB and conventional monohull and hull resistance of two models were also measured to evaluate propulsive performance.
Based on this preliminary study, an initial design of a large high speed passenger craft of SSB hull form was made.
Then, applicability of this type of hull form to actual ship was confirmed with some additional experiments by use of small models.
Finally, tank tests were carried out by use of a 3. 8 m self-running model to evaluate quantitatively both seakeeping quality and propulsive performance.
It was found from the test results that the bow acceleration in waves was reduced considerably and the effective horse power was almost comparable to conventional monohull in the speed range of conventional use.

View full abstract

The paper describes a study of procedures on risk assessment of liquefied gas tankers. The study consists of : a) a proposal of a simple analysis method for the assessment, b) assumption of essential failures and human errors, and their probability involved, and c) an example of the assessment applied the proposal to a 70, 000 m³ Ref. LPG tanker.
Concludedly, following formula is proposed for safe design of liquefied gas tankers;
P_i≤A. C_i^-n or C_i≤ (Pi/A) ^-1/n
P_i is probability of accident (i) per year. C_i is coefficient of accident effect and takes 100 for the maximum accident assumed, such as collision in a harbour, and reasonably decreases by risk level in each accident. A is 5 × 10^-5 to 10^-6 and n is 0.7 for Ref. LPG, LNG, Ethylene tankers.

View full abstract

A water-jet flagellator, having the maximum jet speed of 100 kt, was made to evaluate vibration fatigue life of materials for the SES flexible seal. With this device, the fatigue life was investigated on ten conventional Fabric Reinforced Rubber materials.
This flagellation test worked very well as a ranking method of the candidate materials, and it was revealed that two materials, NBR (PVC) on plain weave nylon and NR/SBR blend on twill nylon, gave good fatigue life among the materials supplied to the test.

View full abstract

The flexible bow fingers of Surface Effect Ships (SES) and hovercraft wear away as a result of violent vibration called flagellation when the finger tips of the skirt contact with the water surface. Finger wear is one of the important problems for SES and hovercraft. A water-jet flagellator was made which can simulate the conditions of the actual fingers contacting with the water surface in order to study the durability of flexible fingers for SES bow seals. Two kinds of experiments were carried out using actual size fingers of the Mitsui hovercraft MV-PP 5. One was a flagellation test carried out over many hours from which the wear rate of test fingers was estimated. The other was an acceleration measurement test at the vicinity of the finger tips carried out over a few minutes. In these tests some parameters having influence on finger wear were changed, especially the wetted length of fingers and the water-jet velocity. The essential results are as follows.
(1) It seems that this water-jet flagellator can simulate the actual finger motion contacting with the water surface because the wear patterns of test fingers resembel those of the actual hovercraft bow fingers.
(2) The finger wear rate increases with wetted length, but remains almost constant above a certain wetted length.
(3) The finger wear rate increases very rapidly with water-jet velocity.
(4) It seems that there is some relationship between wear rates and amplitudes of finger acceleration.

View full abstract

The early stage deterioration of anti-corrosive epoxy coating for offshore structure use was discussed theoretically and experimentally. The degree of deterioration was determined by the critical strain measured at the three-point bending of coated test specimen exposed to the marine environment. The relation between this critical strain ε_c and the detached area S and the deflection at cracking δ_c of the coating at the three-point bending was theoretically calculated and it showed fairly good agreement with experimental data.
Based on the assumption about 6, mentioned above and also on the assumption that the deterioration velocity of coating obeyed to the usual thermo-chemical reaction velocity law, the accelerating effect of several kinds of accelerated deterioration test methods were evaluated quantitatively.
Finally the deterioration phenomena of epoxy coating in an early stage was schematically shown for the changes of critical strain, volume resistivity and alternating current resistance against the exposed period.

View full abstract

The result of theoretical and experimental studies on the longitudinal vibration of the four cylinder longstroke engine installed in the 34000 dwt bulk carrier is shown in this paper as a continuation of 1st report. In these studies, it is shown that the longitudinal engine vibration has such a characteristics of soft spring system that the frequency decreases with the increased amplitude and the engine top bracing to restrict the vibration amplitude has a possibility to excite the vibration of the ship structure.
In addition the longitudinal engine vibration is considered to be excited by the fluctuating thrust force originated from the longitudinal shaft vibration coupled with the torsional shaft vibration.

View full abstract