Showing papers in "Journal of the Society of Naval Architects of Japan in 1977"

New element models in discrete structural analysis

Abstract: A family of new element models in discrete structural analysis is proposed in this paperThese models consist of finite number of small rigid bodies connected with springs distributed over the contact area of two neighbouring bodies In general size of stiffness matrices of these elements are at most (6 × 6) which are equal to or even smaller than 1/2 of those of conventional finite elements so that considerable reduction of computing time can be expectedEffectiveness of these elements in nonlinear structural analysis will be demonstrated by several numerical examples

Propulsive performance of a container ship in waves

Abstract: With a model of single screw high speed container ship, resistance and self-propulsion tests in regular and irregular waves are carried out in the Experimental Tank of Osaka University The experimental results of ship motion in regular head and following waves and mean increase of resistance in regular head waves are compared with the results obtained from the theoretical calculations The effects of wave height and propeller diameter on ship motions, mean increases of resistance, propeller thrust, torque and revolutions, self-propulsion factors and propeller load fluctuations are investigated The significant amplitudes of heave, pitch and the mean increases of resistance, propeller thrust, torque and revolutions measured in irregular waves are compared with the values which are predicted from the response operators obtained from the experiments in regular waves and the wave spectra by applying the linear superpositon method The self-propulsion factors in regular and irregular head waves are analysed by assuming that the mean characteristics of propeller in waves are identical with those in still water Furthermore, to investigate the characteristics of self-propulsion factors in waves, the inflow velocity into the propeller disk in regular waves are measured by using a ring type wake-meter, and the wake fraction, relative rotative efficiency and propeller open-water efficiency in regular waves are calculated by using the measured inflow velocity distribution, the propeller revolutions and the propeller open-water characteristics The measured amplitudes of propeller load fluctuations are compared with the theoretical calculations by using the propeller open-water characteristics in uniform flow

Behavior of Water in Vertical Well with Bottom Opening of Ship, and its Effects on Ship-Motion

Abstract: Recently some ships have been constructed with a vertical well which have the bottom opening for the purpose of raising and lowering the equipment. Some examples are vessel-type drilling rigs, vessels carrying small submersibles and so on.This paper shows the behavior of the water in the well, and its effects on the motion of the ship, when she is running.Experimental studies were performed under two different sets of conditions.(1) The motion of the free water surface in the well was studied, when the ship-model was fixed stationary in the stream.(2) The effect of the motion of the water in the well on the motion of the ship-model was studied, when the model was towed.The following conclusions were obtained.1) When the ship-model with a well is fixed stationary in the stream, it can be observed that the water surface moves regularly in it. This phenomenon is similar in many respects to the motion of a cylinder which is elastically supported by springs in the stream.2) The type of water motion, whether up and down, right and left or forwards and backwards, depends on the velocity of the stream and the shape of the well.3) The water motion can be seen at the dimensionless stream velocity of 0.2<2U/lω0<1.6, where U is the stream velocity, lis the length of the bottom opening and ω0 is the natural circular frequency of the water moving in the well.4) The experimental values agree well with the results of the theoretical calculation on the heaving amplitude of the free water surface in the well, and on the increased drag of the ship-model by the motion of water in it.5) In order to reduce the motion of water in the well, it is very effective to fix short flanges on its vertical walls a little below the water surface.

On Eddy Making Component of Roll Damping Force on Naked Hull

Abstract: From the results of the forced roll tests of two-dimensional cylinders having various cross sections, it is found that the eddy making component of roll damping force is proportional to the square of the frequency and the amplitude of roll motion. In other words, the damping coefficient CR of this component for ship-like sections can be regarded to be independent of the period parameter for practical use.An empirical formula for the eddy making component of the roll damping force for two-dimensional sections is deduced from the theoretical considerations on the basis of these experimental results. For three-dimensional ship forms without bilge keels at zero Froude number, the estimated results of the roll damping force which consist of the eddy making, the wave making and the frictional component, are in fairly good agreement with experimental results of ship models.

A Study on the Collapse of Ship Structure in Collision with Bridge Piers

Abstract: The present paper gives the results of static tests on the fracture of simplified models of the bow and the side of ships, which were carried out for the purpose of examining the extent of fracture developed in a ship hull in collision with a bridge pier and estimating the amount of collision force. The models simulate the structures of somewhat small ships, ranging from 500 tons to 4, 000 tons in gross tonnage. We assume a bow collision in which a ship strikes in a right angle encounter with the flat plane of a bridge pier. It is assumed in a ship-side collision that a ship hits the corner parts of a bridge pier.The test results show that the buckling of hull plate is dominant in the strength of the bow of a ship. The load-penetration curves of the side of a ship show a distinct difference between transversely-framed side structures and longitudinally-framed ones. The collision force and the hull deformation calculated by a simple theoretical expression are graphically presented, for a range of gross tonnage of ships, in an attempt to derive general information on buffer equipments designed for the protection of both ship hulls and bridge piers.

The Drifting Force acting on a Three-dimensional Body in Waves (1 st report)

Abstract: This paper presents an investigation of the drifting force acting on a sphere which is fixed or freely floating in regular waves. The theoretical drifting force coefficient Cd of a fixed sphere increases monotonically with the increasing non-dimensional wave number Ka, and agrees well with experimental data. This makes the author sure to conclude that the theory can predict correctly the drifting force acting on a fixed body.On the other hand, in a freely floating case, the theoretical Cd curve has a peak at Ka= 1.15 and this tendency is also confirmed by the experimental results. It is interesting that the peak value of Cd is greater than that value when the oncoming waves are completely reflected by the sphere. As yet the author cannot be sure to say whether this excess is still due to the part of the waves in the width of the diameter of the sphere, or to any other part. But if the latter is true, a three dimensional body for which Cd is 1.0 at a complete reflection of the oncoming waves, will presumably have a Cd value greater than 1.0, when it is freely floating in the waves.

Calculations of the Hydrodynamic Forces Acting upon Thin Cylinders Oscillating Vertically with Large Amplitude

Abstract: This paper provides numerical and experimental results for the first-, second- and third-order hydrodynamic forces acting upon cylinders oscillating vertically with large amplitude on a free surface. The procedure of calculations is based on the theory in which it is assumed that the thickness of the cylinder and the free surface elevation are small enough but the amplitude of the oscillation is so large that the second-order contribution to the forces could not be neglected.Numerical calculations are made for two elliptic cylinders, a circular cylinder, a stern-section cylinder, whose section is similar to the one of a ship at the stern, and a wedge, and their results are compared with those obtained through forced oscillation tests. It is observed that the calculated values for the second-order hydrodynamic forces, in particular, are in fairly good agreement with the measured ones.

Analysis of Waves Generated by a Ship Oscillating and Running on a Calm Water with Forward Velocity

Abstract: Wave analysis is proposed for deriving amplitude function of elementary waves composing nonstantionary wave pattern generated by a ship which is forced to oscillate and is running on a calm water with forward velocity. The purpose of this wave analysis is especially to verify the validity of theory of resistance increase in waves in predicting the structure of nonstationary wave pattern due to an oscillating ship.Wave analysis method proposed here is similar to that which is used in research on stationary wave making resistance theory and is called Newman-Sharma method. Wave patterns generated by an oscillating ship, however, vary with time in contrast with those in stationary wave making and accordingly a process is devised for eliminating time factor and for deriving wave amplitude and phase distribution in space available for analysis. With this process we can easily extract space distribution information of amplitude and phase of nonstationary waves from wave height data which are obtained with repeating measurements a few times at only a point in model basin.From the results of wave analysis of measured wave pattern compared with theory we can conclude that slender body theory can predict wave amplitude function distribution fairly good. And a little interaction effects between stationary and nonstationary waves are observed.

A study on the instability criterion of the manual steering of ships

Abstract: A proposal to specify the permissible instability in the manual steering of ships was made in this paper.It had been considered that ships should be course stable to be steered by human operators. However, our experience shows this concept is true only in the case of small ships. In the case of large ships, which are usually course unstable, it is not so difficult to steer them. Then, our new problem is to find the criterion how ships can be unstable in order to be steered by human operator without excessive difficulty. In addition, the criterion should hopefully be applied for any size of ship.The concept of “Phase to be Compensated” was employed in this study to specify the instability criterion. This concept was of course deduced from the Nyquist stability criterion in the control theory.According to this concept, the behavior of human operators can be explained reasonably well and the difficulty in steering of any size of ships can be expressed on the same basis.Several important conclusions were obtained by simulator studies :(1) “Phase to be Compensated” should be less than 30 degrees.(2) Human operators select the cut-off frequency of the feed-back loop at the most favorable point when they steer unstable ships.(3) The time constant of steering gears must be small enough for small size vessels.

Wave Resistance of a Ship of Finite Beam Predicted by the Low Speed Theory

Abstract: A theory for the prediction of wave resistance at low Froude numbers is presented. It takes account of the non-linear effect of the free surface condition by means of the distribution of sources over the undisturbed free surface. As a numerical example, the wave resistance of Wigley model is calculated, and the result is compared with experimental values. It is concluded that the computation based on the double model surface sources shows some discrepancies with the measured values but the employment of the solution of Neumann-Kelvin problem seems to improve the agreement to a considerable extent.

New Approach to Hydrodynamic Forces on Oscillating Low Aspect Ratio Wings

Abstract: A new approach to calculating the hydrodynamic forces acting on harmonically oscillating low aspect ratio wings is presented. This approach is based on free streamline theory and lifting surface theory. The hydrodynamic forces can be represented by a linear combination of the inertia force, the first order damping force, that is to say “lift”, and the second order damping force, that is to say “drag”. Each component of these forces estimated by the model of lifting surface with free stream is compared with the experimental results obtained by the forced oscillation of rectangular low aspect ratio wings. And chordwise pressure distribution estimated by the model is compared with that of measurement.As the results of these comparisons, the following conculsions are derived :1) Although the equivalent total drag coefficient, Cdt, of estimation increases monotonously with the inverse of reduced frequency, that of experiment is not so much varied with reduced frequency, k, for fixed period parameter, P. P., and aspect ratio, A. R. In the range of small 1/k the experimental Cdt becomes less than that at the time when 1/k equals to zero.2) The result of the chordwise pressure distribution shows that as 1/k increases drag forces decrease and lift forces increase, so Cdt is not so much varied then.3) The added inertia coefficient, Cm, of experiment is remarkably dependent on k however, that of estimation is close to 1.0 and less dependent on k in low aspect ratio wings. In the range of small 1/k experimental Cm is much greater than 1.0 and it becomes close at estimated Cm as 1/k increases.

On Dynamic Positioning System Design in particular Reference to the Positional Signal Filtering Technique

Abstract: A systematic design procedures of dynamic positioning system are proposed in view of practical design work for higher efficiency and reliability. In order to make an approach to better dynamic positioning control for given environmental conditions, detailed investigations were carried out on the filtering technique for positional signals. Evaluation of the technique was made on the basis of computer simulations by use of ship motion theory and model test results. It is clarified that the technique will substantially provide an efficient way to the better control performance of dynamic positioning systems.

Ultimate strength of composite steel-concrete structure of sandwich system

Abstract: In the near future, various huge off-shore structures will be constructed. It was concluded through preliminary studies that composite steel-concrete structures are best fitted for the strength members of the huge structures, due to the high strength and ductility of steel and due to the low ratio of the cost to the weight and reliability of concrete.In the paper, the authors carried out both experimental and theoretical investigations into the strength of the composite steel-concrete structures where concrete is placed between steel plates. Experiments were carried out using the two-dimensional models of the composite structures under bending, shear and combined bending and shear. The fundamental properties of the composite structures were clarified through the experiments.A new method for the ultimate strength analysis of the composite structure was developed. The calculated results were in good agreements with experimental ones. The effects of the depth of the composite structure, the sectional areas of compression and tension steel plates and the compressive strength of concrete upon the ultimate strength were investigated by the proposed method of analysis. It was clarified that the depth of the structure and the sectional area of the steel plate under tension exert decisive influences upon the ultimate strength.

On Some Characteristics of Sloshing Force

Abstract: Many researches have been carried out on the sloshing forces of liquid in a ship tank. But most of the researches have focused on the magnitude of the pressure at model tests, and there remain some problems regarding the method to estimate from the results of model tests the sloshing load acting on an actual ship tank. The authors present the results of investigation which they carried out in this respect taking into account the scattering of the data, and also the results of their study on the law of similarity.In this report, as a short term distribution of a sloshing load, a new probability function is offered, which is deribed on the assumption that the sloshing data at regular oscillation will scatter in a form of Rayleigh distribution. Next, how to extrapolate the experimental data to the load acting on an actual ship tank, including the case of gas entrapment at the time of impact, is discussed. And the characteristics of the sloshing load with gas entrapment is qualitatively investigated.

Statistical Character of the Demand on Longitudinal Strength (First Report)

Abstract: In the theoretical estimation of the long term distribution of the ship's wave induced variables, it is well known that the estimated extreme values are significantly affected by the supposed wave conditions and the ship operating conditions. However, there are few reports which investigate the statistical characteristics of the wave conditions. Furthermore, there is no report which concern with the varying of ship operating conditions of every each ship, which may appear as so-called the captain's spectra.Authors studied the statistical characteristics of the wave conditions which are observed by 35 voluntary ships in the North Pacific Ocean during the period of 6 years (1976-1981). From this study, it was clarified that meteorological phenomena such as winds and waves in the ocean had wider scatter band than which was estimate by the usual simple population with constant parameter. Authors proposed the probabilistic model of the population of such phenomena which had a complex distribution with random variable population parameters.In this report, authors study the statistical characteristics of the extreme values of wave induced variable of the ship with the supposed wave condition having the population of the complex distribution.The general formulation of the extreme distribution taking number of N samples from the population of the complex distribution is shown, and the asymptotic distribution is proposed. From the analysis of the distribution of the maximum wave height observed by 35 voluntary ships, it is found that the mean value of the distribution is proportional to the logarithmics of the sample size and the standard deviation is nearly constant. The properties of the statis-tical characteristics of the theoretical maximum distribution agree with the observed distribu-tion. On the other hand, the distribution of the usual simple population with constant parameter is found to be inaccurate.The maximum wave height and its probability of each wave period interval have direct effects upon the estimation of the extreme values of the wave induced variables. Authors' study on the distribution of the maximum wave height shows that the theoretical maximum distribution of the population with the complex distribution well agree with the observed maximum distribution of each wave period. From above result, it is clarified that the coefficient of variation of the maximum wave load is 14% to 18% and its value increase proportional to the ship length. This value is larger than the value from the usual supposed wave conditions of the population with constant parameters.

Study on Fast Fracture and Crack Arrest

Abstract: In order to seek a more reasonable dynamic theory for fast fracture and crack arrest, the authors carried out a dynamic fracture mechanics analysis with the use of finite difference method and revealed that the method is a useful tool to analyze the crack propagation and arrest as reported in the previous paper.This report describes the results of an experimental investigation using polymethylmethacrylate (PMMA) and is a preliminary report on the study of dynamic aspects of brittle crack propagation and arrest in structural steels. Using dynamically measured data in the experiment as the boundary condition for finite difference method, mechanism of fast crack propagation and arrest in PMMA was analyzed by energetics considerations. As one of the most interesting results, a particular relation between dynamic fracture toughness and crack velocity was obtained, i. e. crack acceleration and crack deceleration followed two different curves and the crack arrest stress intensity factor was found to be substantially lower than the critical stress intensity factor for crack initiation.Further studies on the crack arrest phenomenon in structural steels using the procedure described in this paper should be conducted in the near future.

Evaluation of Hull Girder Stress on the Open Ship (1st report)

Abstract: As the analyzing system on the total hull girder stress on open ships such as container ships, open bulkers, etc., has been developed and structural behaviours of longitudinal members were investigated on four container ships, the outline of the system and the results of calculations are reported in this paper.This system consists of many programs which compute ship motions, dynamic loads, structural responses and response amplitudes in regular and irregular waves.The following knowledge about the stress on the upper deck in the irregular waves was obtained.(1) Maximum total hull girder stress which is composed of vertical bending stress, horizontal bending stress and warping stress, occurs at the midship or in front of the engine room.(2) At the midship, the total stress is slightly larger than the vertical bending stress.(3) In front of the engine room, the total stress is comparatively larger than the component stresses of that section, and is nearly the same level as the total stress at the midship.The further discussion concerning these results, will be presented together with the simple method to evaluate the total hull girder stress on the open ship, in the next paper.

Collapse Load Analysis of Bending Plates by a New Discrete Model

Abstract: A new discrete model for plate bending is proposed in this paper. This plate model consists of rigid plate with a rotational spring, and a number of displacement parameter at one node is only one (w), namely 1/3 of a well-known triangular plate bending element. Furthermore strain energy is lumped in the spring so that energy integral is not necessary. Consequently considerable reduction of computing time will be expected. By assuming that the rotational springs are ideal plastic and adopting the well-established incremental loading procedure, collapse load analysis of plate bending problems is proposed. A series of results of analysis have duly proved validity of the present method.

Brittle Fracture Initiation Characteristics under Bi-axial Loading (2nd Report)

Abstract: In the previous report, the brittle fracture initiation characteristics of plate subjected to bi-axial tension was investigated on the cruciform specimens of PMMA (Polymethylmethacrylate) and SM 41 (mild steel), each of which contains an inclined crack. The experiments showed that the fracture of the PMMA specimens was perfectly brittle and that of SM 41 specimens was with a small scale yielding.In this report, brittle fracture initiation characteristics with large scale yielding were investigated. A series of experiments was conducted on the cruciform specimens of SM 41 under bi-axial tension, of which load ratios were 0/1, 1/2, and 1/1. Elastic-plastic stress analysis was also carried out using the finite element method.The results obtained by the experiments and the finite element analysis lead to the following conclusions.(1) The direction of fracture initiation is nearly normal to the vector of crack opening displacement (COD) near the notch tip analyzed by FEM.(2) The brittle fracture stress under mixed mode condition may be conservatively predicted by the application of COD-concept in which the critical COD is evaluated as the one for the opening mode.

Studies on the Evaluation of Corrosion Fatigue Crack Initiation Life in Notched Mild Steel Plate

Abstract: In the ballast tank of oil tanker and offshore structures, corrosion damage caused by the sea water and fatigue damage occurring by wave-induced forces are induced simultaneously. So the fatigue cracks are often initiated in the members of such structures, even if stress lower than the fatigue limit in the air acts on those in the sea water. In this study, fatigue crack initiation life N sub c of a notched steel plate subjected to pulsating tensile load in the air or sea water was analyzed theoretically by applying Neuber's rule with stress-strain function D proposed by Topper et al as a parameter of damage. Experiments were carried out with notched plate specimens, of which the elastic stress concentration factor K sub t were varied from 1.0 to 6.0, under pulsating tensile loading fatigue test in order to verify the theoretical analysis. The completely reversed strain controlled fatigue test was conducted with a smooth round-bar specimen in consideration of a reversed stress state induced at the notch root under pulsating tensile loading. Results of pulsating tensile loading fatigue test of the notched specimen were in a good agreement with theoretical values evaluated with stress-strain function from completely reversed strain-controlled fatigue test results. The corrosion effect factor K sub c of notched specimen was expressed as an exponential function of corrosion time T sub c corresponding to N sub c irrespective of K sub t value, and the K sub c of smooth specimen was less than that of notched specimen because of the difference of corrosion action.

Study on Fast Fracture and Crack Arrest The 1 st Report

Abstract: From the viewpoint of energy balance concept, the resistance of material against fast fracture can be measured by the capacity for absorbing energy dissipated due to crack extension. With respect to brittle fracture of steel, energy absorption is associated with the formation of both flat brittle fracture surface and shear lips.In the previous report, stress gradient type double tension tests using mild steel were carried out and it was found that shear lips were able to absorb rather large amount of energy and played an important role to arrest behavior of brittle crack. In order to evaluate the arrestability of steel, it is thus necessary to clarify the mechanism of initiation and growth of shear lips.In this study, experiments are carried out to investigate the factors which may influence the formation of shear lips. Surface-notched double tension specimen using mild steel is adopted. Crack propagates initially in the notched region where shear lips may be restrained, and then propagates into full thickness region where crack may arrest due to fall of driving force and/or shear lip formation.Data obtained are analyzed on the basis of dynamic fracture mechanics. Shear lips contribute very much to crack arrest phenomena and are influenced by crack velocity, temperature, constraint (stress triaxiality) and applied stress level in this case.

Theory for Articulated Double-Flap Wavemakers in Water of Constant Depth

Abstract: Extending the theoretical work of Biesel, a preliminary study is made of dynamic characteristics of an articulated double-flap wavemaker. The underlying motivation of utilizing a double-flap waveboard is based on its effectiveness in reducing the local disturbances in the immediate vicinity of the waveboard, which is demonstrated through decreasing inertia pressures. The additional degree of freedom in the double-flap waveboard movements is shown to be capable of encompassing a wider range of wavelength that can be satisfactorily generated in a given tank. The inertia pressure is reduced significantly by use of double-flap wavemakers, especially when short waves are generated. The usual wavemaker performance curves are computed for double-flap wavemakers. Comparison with the results of a single-flap wavemaker characteristics reveals that considerable improvement in performance curves is feasible by selecting a suitable combination of design parameters.