Local Skin friction reduction with microbubbles was measured in a two dimensional channel flow. Microbubbles are expected to reduce the skin friction which takes up a large part of the total ship drag. To adapt the process to a real ship is difficult because of the very large amount of microbubbles. In order to adapt the process to a real ship, we must search for more effective bubble conditions and the persistence of the effectiveness in streamwise direction.
The local skin friction reduction was measured in the small high speed water tunnel, systematically changing the averages of void ratio, the main flow velocity and the measurement position from the bubble injection point. The results confirmed the persistence of the effect at the 200δ point (boundary layer thickness) downstream air injection. The local void ratio and the velocity profile using the PIV system were measured. The measurements indicated that the local void ratio near the wall was important to the skin friction reduction.

View full abstract

An unstructured grid method for simulating three-dimensional incompressible viscous flows is presented. The governing equations to be solved numerically are the Navier-Stokes equations with artificial compressibility. The spatial discretization is based on a finite volume method for unstructured grid system. Second order accuracy in space is achieved using a flux-difference-splitting scheme with MUSCL approach for inviscid terms and a central difference scheme for viscous terms. Time integration is carried out by the backward Euler method. The linear system derived from the linearization in time is solved by the Gauss-Seidel iteration. For the analysis of high Reynolds number flows, the one equation turbulence model proposed by Spalart and Allmaras is used. The turbulence equation is solved in a similar way as the Navier-Stokes equations. Brief description of the numerical method is given together with some computational results for flows around a 2 D wing section and a ship form.

View full abstract

A new CFD simulation method is developed for a semi-planing boat in unsteady motion. The density function method for the free surface treatment and the moving-grid method for the moving body boundaries are applied. The motion of a boat is simultaneously solved by combining the equation of the motion of the boat with the flow computation based on the finite volume method. Simulations for two types of semi-planing boat are performed in the Froude number range from 0.5 to 1.0 and the simulation results are compared with the experimantal ones. It is shown that the CFD simulation method can simulate the flow around a semi-planing boat with sufficient degree of accuracy and it can be used as an effective tool for the development of hull form.

View full abstract

Experiments and the linear wave theory have suggested that the wave-making resistance of a submerged body can be reduced by downward lifting force. Although much studies have been done on submerged lifting bodies, all of them are focused on those generating the upward lifting force. The free surface phenomenon of downward lifting force is worthy to be studied for practical applications.
In the present study, numerical simulation is carried out to investigate the characteristics of drag of a lifting body submerged near the free surface. The simulation is done by a direct boundary element method where fully non-linear free surface boundary condition for the submerged lifting body is considered. To simulate the flow around a lifting body, a wake sheet is extended from the trailing edge so as to satisfy the Kutta condition.
Through the computations, it is found that the downward lifting force reduces the drag of a submerged lifting body, and the minimum value of drag appears when the lifting force is produced downward. The computations for various submergence depths have revealed that there exists a depth where the drag is minimum.
The method of optimization is introduced in order to obtain the optimum profile with minimum drag. At a result of iterative optimization, the drag is remarkably reduced compared with that of the initial profile.

View full abstract

A numerical method for designing three-dimensional wing from the specified pressure distribution based on surface panel method is presented. A number of hyperboloidal quadrilateral panels are used to approximate the surface of an original wing and the hydrodynamic characteristics of this original wing are calculated by potential based surface panel method employing the pressure Kutta condition. Then the geometry of the original wing is made to change in a small amount and the pressure increment due to this change is calculated by the same method. From them the influence coefficient matrix of wing geometry on pressure distribution is constructed. Utilizing the difference between the specified pressure distribution and the pressure distribution of the original wing, a linear equation system about the geometrical corrector of wing is established. The original wing geometry is modified with this corrector to form a new wing for next iteration. By means of repeating this computation procedure the wing that will have the specified pressure distribution can be obtained.
Several wings have been designed as examples and the results have indicated that the method is effective to design the three-dimensional wing having the specified pressure distribution.

View full abstract

In this study, a trans-cavitating propeller is defined as a propeller whose design condition is between those of conventional propellers and supercavitating propellers. A propeller for high speed vessel of about 30 knots falls in such a design condition, where a suitable propeller design method has not yet been found. For such a condition, it would be realistic to adopt supercavitating blade sections near the tip and noncavitating ones near the root, since ordinary blade sections can lead to harmful violent cloud cavitation. This study investigates foil sections suitable for a trans-cavitating propeller. The purposes of this study are to investigate the noncavitating and supercavitating foil design methods for such intermediate design condition, and to find a criterion to switch the design between noncavitating and supercavitating foils.
By prescribing pressure distributions, one noncavitating foil section and one supercavitating foil section are designed for a condition of r/R =0.7 position of a trans-cavitating propeller. Cavitation performance as well as noncavitating performance are examined by the noncavitating and cavitating foil theories, and by the cavitation tunnel tests such as hydrodynamic force measurement and cavitation observation. From these test results, the foil performance and design method are evaluated as well as the accuracy of the theoretical prediction are discussed. The following results are obtained.
(1) The noncavitating foil section did not cavitate at the design condition as expected. High lift/drag ratio of 35.3 was obtained. Wide cavitation bucket of about 2 deg. was also achieved. Theoretical prediction gave slightly higher lift coefficient by 0.25 deg. in the angle of attack and narrower cavitation bucket than the experiment. The theory should be further improved in these points.
(2) The supercavitating foil section generated supercavitation at the design point as expected. High lift/drag ratio of 33.1 was obtained. The predictions by a linear supercavitating foil theory showed fine agreement with the experimental results.

View full abstract

This paper describes the experimental studies of propeller blade force during contact between a nozzle propeller and an ice block. The problem of propeller-ice interaction is very important for ice going vessels.
At first, the scenario of propeller-ice interaction is suggested. Following this scenario, it is considered that the interaction load consists of four components. They are a force due to ice failure, a hydrodynamic force, a force due to momentum change of ice block mass and a force due to added mass. Four kinds of experiments in water and air were carried out, and the propeller-ice interaction load could be divided into these components.
In the second place, using two model propellers, the same experiments were conducted. Each propeller had same geometry and different diameter. The results of experiments with two propellers showed a relation between each components of torque and propeller diameter, so a scale effect of four components could be estimated. Finally, this scaling laws were tried to apply the full-scale propeller concerning propeller-ice interaction, and it showed that ice force on full-scale propeller can be discussed from model experiments.

View full abstract

A simple surface panel method has been successfully applied to steady or unsteady wing problems. This method uses source distributions (Hess and Smith type) on the wing surface and vortex distributions arranged on the camber surface according to Lan's quasi-continuous vortex lattice method (QCM). We named this method SQCM (Source and QCM).
In this paper, we apply SQCM to the unsteady propeller problem by combining the source panels and unsteady QCM which has been successfully developed. We consider the unsteady Kutta condition that the pressures must coincide with each other on the upper and lower surfaces at the trailing edge. In the steady problem, SQCM satisfies the Kutta condition by setting zero normal velocity at the trailing edge on the camber surface. But it is not usually equal to zero in the unsteady problem but a finite normal velocity exists. In the unsteady two or three-dimensional wing problem, we obtained this normal velocity by iteration for the equal pressure condition at the trailing edge.
We show numerical results for two kinds of full-scale propellers (Seiun-maru's conventional and highly skewed propellers) in non-uniform flow. Pressure distributions obtained by the present method are in good agreement with experimental data and thrust fluctuations of one blade and the propeller are plausible.

View full abstract

At present time the computational models based on solution of perturbation potential flow are widely used to solve the Neumann's problem on the surface of a lifting body. Employing surface paneling discretization of the potential according to the curvature of body and wake surfaces, Fredholm's second kind integral equation is solved numerically, regarding the dipole strength of potential.
In the present variational surface panel method a decomposition of the boundary surface is carried out for analytical integration on body and wake panels by polyhedral subpaneling. Using Taylor's series expansion of the dipole strength in the vicinity of each analysed point and employing least squares minimization of its derivatives, the relevant minimal perturbation velocity field on the body surface is obtained. By means of the corresponding streamlines an optimisation of the body boundary surface discretization is done through new asymmetrical streamlines adapted repaneling (SAR). Numerical calculations with wings and propellers are performed in order to prove the agreement of the estimated circulation and pressure distribution, as well as induced downstream flow characteristics with their experimental data.
Special attention is paid to obtain a reliable estimation of the tip flow characteristics of wings and propellers.

View full abstract

The so-called multilevel model, which has been successfully used to simulate tidal current of global scale, is applied to compute current flow around a large floating structure called Mega-Float model moored off Oppama in Tokyo Bay. In order to get detailed knowledge on current flow around MegaFloat model, the sea area adjacent to the model is latticed by very fine square-grid with one side of 20 m. Outside this area, however, less fine grids are introduced ; grid size is increased larger and larger as the distance between the latticed area and Mega-Float model increases. With increase of grid size, numerical values of horizontal eddy viscosity coefficient are increased according to Richardson's 4/3 power law. Qualitative characteristic features of computed tidal current corresponding to M₂ tide agrees well with features of current observed around Mega-Float model and in Tokyo Bay as well. As a result of some additional computations, it is concluded that not only adequate choice of numerical values of horizontal eddy viscosity coefficient but also more elaborate modeling of sea bottom topography are very important to improve further agreement between computed and observed tidal current in such a narrow sea area as the area adjacent to Mega-Float model.

View full abstract

A new technique for PPS (performance prediction simulation) to design sailing boats is developed. The performance of a sailing boat with 6 degrees of freedom is predicted. The boat sails in the simulation with automatic steering to get steady state and with automatic sail trimming to get better VMG (Velocity Made Good). With this technique the steady performance of sailing boats is predicted with sufficient accuracy on various conditions. This system is applied to the design procedure for a IACC (International America's Cup Class) sailing boat for which the steady sailing attitude plays an important role for the sailing performance.

View full abstract

An experimental study has been made on the friction reduction of a model ship for a bulk carrier by means of air lubrication. We designed two types of devices to keep the supplied air in the hollow of the flat bottom of the model. For the even trim condition the air sheet is very robust and the maximum friction reduction is about 30%. The air supply requires energy by 5.4% of that for propulsion while its decrease due to the air lubrication is 20.1%. Therefore, 14.7% of the propulsion energy is saved for the model ship. However, different patterns of air separation, which deteriorate the effect of air lubrication, were observed for the positive and negative trim conditions, respectively. It is considered that these patterns are attributed to the distribution of flow pressure on the bottom of the model.

View full abstract

This paper deals with experimental and analytical analyses of fish pectoral fin motion and performance tests of apparatus of pectoral fin motion from viewpoint of maneuverability of underwater vehicles.
Observation of pectoral fin motion of Black Bass revealed that the combination of feathering motion and lead-lag motion of a pair of pectoral fins generates the fish motion of advancing, backward swimming, hovering and turning.
The apparatus making the feathering motion and the lead-lag motion of the pectoral fin generates thrust force in a certain range of phase difference between both motions.
The unsteady vortex-lattice method including the effect of viscosity can express fairly well the unsteady forces acting on a pectoral fin model with feathering and lead-lag motions in a range of the phase difference between both motions where it exerts thrust force.
The fish model consisting of fish body model and a pair of the apparatus of pectoral fin motion can not only swim forward and backward and turn in the same position, but also swim in the transverse direction.

View full abstract

This paper deals with a mathematical model of the ship hydrodynamic forces in maneuvering motion at slow speed with large drift angle and tight turning. The model is structurally composed of several force elements and can be used to estimate the hydrodynamic forces on a ship in turning motion through the analysis of hydrodynamic forces in oblique motion without requiring turning experiments.
The mathematical model was deduced from a vortex system, composed of a simple bound vortex system in/on the ship and a simple shedding vortex system around of the ship, based on the idea of slender body theory and the flow model of wing theory.
The vortex wake around the ship in the former paper took into account the longitudinal flow, but in the present paper the 3 D cross-flow wake considers the lateral which is added to the former vortex wake.
The trailing vortex containing the 3 D cross-flow wake effects the induced drag strongly in the longitudinal componet of ship hydrodynamic forces and most of the hydrodynamic forces are described by the vortex system in this paper, except frictional resistance and lift force due to asymmetric cross-flow.

View full abstract

A series of captive model tests was carried out to unmanned untethered submersible AUV-HMl testbed, which has a body of a streamlined section, developed at National Taiwan University. Furthermore, for estimating its lateral stability derivatives the same method, which is based on the empirical formula proposed by Bottaccini and used in previous paper, is applied. As a result, both of the two sets of stability indices evaluated with estimated values of stability derivatives as well as those obtained by measurement, show that AUV-HMl testbed is unstable on lateral mode. This result agrees well with the tendency shown by the vehicle responses for step heading commands in the pool test performed with the AUV-HMl testbed.

View full abstract

The purpose of this paper is to investigate the insight of capsizing modes in details and the probability of capsize in a random sea by making use of a mathematical model for simulations. Firstly a prediction method to calculate critical wave height leading to capsize are proposed for three modes of parametric resonance, harmonic resonance and pure loss of stability. Secondly according to Belenky's^4'5) simplified method to compute the probability of capsizing, the probabilities of capsizing caused by parametric resonance and harmonic resonance are evaluated for a ship running with the specified combination of heading angle and ship speed in random waves. Finally these computed outcomes are discussed for the 15000 GT container carrier.

View full abstract

This paper describes dynamic effects of flooded water on nonlinear roll motion of a ship in regular beam waves. Experiments using a ferry model demonstrate nonlinear roll response of a flooded ship in waves. It is found that two kinds of roll motion with different amplitudes and periods coexist and that one of them is particularly irregular and complicated even in regular waves of relatively moderate amplitude. The reconstructed attractor of the roll motion in a delay-coordinate state space shows that it is not simple harmonic motion. In order to further study this complicated roll motion, we derived model equations which include both the nonlinear effects of the restoring moment and the coupling effects of roll and flooded water. The dynamic motion of flooded water produces the coupling effects which have been neglected in most previous works. Numerical solutions of the model equations show that this coupled system yields nonlinear phenomena similar to the experimental results. In addition, we also experimentally studied motion of a box-shaped model with flooded water in regular waves. The experimental results show that the roll response varies with changing the wave height in a complicated manner which also depends on the amount of flooded water, and that some of complicated roll motion have typical properties of low-dimensional deterministic chaos. These results suggest that the nonlinearly coupled dynamics of a ship and flooded water is a key to solve this problem.

View full abstract

In order to develop a practical prediction method for shipping water volume, load and pressure due to deck wetness, a model test was carried out for a domestic tanker in regular head seas. Shipping water height, load and pressure were measured at various ship speed and wave steepness. As for the occurrence of deck wetness, N. S. M. calculation of relative water height at bow gives a good estimation if the bow top height is reduced by considering the static swell up.
A practical prediction method for shipping water height, load and pressure was proposed. The inputs of this method are relative water height at bow, pitching angle and bow vertical velocity, those can be calculated by strip theory and so on. The theory of “flood waves” was applied for evaluating the shipping water height from the relative water height at the bow and so on. This theory gives a better estimation of the water height distribution on deck than the conventional model of “dam collapsing” because it can include the effect of ship forward speed. On the shipping load and pressure, not only the static component but also the time derivative of the momentum of the shipping water were considered. It was clarified that the momentum component is important to estimate the peak of shipping water load and pressure. Having compared with measured data, it was confirmed that the combination of these two methods is practical enough for predicting shipping water load and pressure.

View full abstract

The wave spectral analysis technique is applied to predict the limiting wave conditions for a deep sea drilling riser in the hang-off mode. In this mode, the riser is disconnected from the subsea BOP stack, hangs from the riser tensioner wires, and the top of the riser moves with the drilling vessel. The limiting wave height then, is the highest wave which does not cause compression in the top of the riser. Calculation results are presented showing the limiting wave height as a function of averaged wave period for a 2, 000 m long deep sea riser. This method is then verified through comparisons with results of calculations performed using the time domain method in response to a random sea. Additionally, the influences of internal fluid mass in the riser and the associated fluid friction on the limiting wave height are discussed. Finally, applications of the method to the analysis and design of deep sea drilling risers are presented.

View full abstract

This paper examines application of active control technique to underwater installation as a new approach instead of the present method using crane vessels. In this paper, two kinds of active control problems are examined. The one problem is how to control a flexible structure on tracking the given trajectory or precise positioning on the operation of installation, not stimulating its elastic responses. The other is how to control a rigid body structure on tracking the given trajectory or precise positioning under the existence of unknown current.
The structural flexibility imposes problems of suppressing elastic responses and securing stability of control system. In this paper, H^∞ controller combined with LAC/HAC feedback controller is designed for the case that the structural flexiblity exists.
When unknown current exists, the dynamical effect of the current must be compensated somehow. This paper presents learning trajectory tracking control when a structure is transported repeatedly along the objective trajectory.
In order to confirm the control algorithm and verify the possibility of active control installation method, basin tests are executed using a neutrally buoyant flexible model with ultrasound ranging system and thrusters.

View full abstract

This paper deals with configuration of autonomous underwater vehicle (AUV) for inspection of underwater cables taking low hydrodynamic drag, sensor alignment, collision avoidance maneuver, turning maneuver and rolling motion into account. AUV consists of fuselage of body of revolution with low hydrodynamic drag, fore and aft horizontal wings, upper and lower vertical tails and a pair of horizontal thrusters at both sides of aft horizontal wings. The design process for the horizontal wings and the vertical wings is discussed performing model experiments and numerical simulations.
The shape of fore horizontal wing and that of aft horizontal wing can be determined by use of a non-linear optimization method under the constraints of magnetic sensor alignment for cable tracking, dynamic stability in vertical plane and performance of collision avoidance maneuver.
The performance of turning maneuver and rolling motion are attributed to thrust force difference between a pair of thrusters at both sides of aft horizontal wing, because the shapes of upper and lower vertical tails can be designed from the viewpoint of the dynamic stability in horizontal plane.

View full abstract

We had already reported the developed results on the hybrid type on board measuring method of directional wave spectrum. This method utilize the measured ship motions including relative wave heights and also uses information by radar images on board. By this method, ocean wave characteristics can be observed automatically and quantitatively on board of a running ship. But more data is required for evaluating this method.
This time we report about the compared results with that of newly developed directional wave buoy, and also with the experimental results carried out in our towing tank for the improvement of the accuracy of our method.

View full abstract

Needless to say, the accumulation of a reliable database of ocean wave characteristics is very important for safe ship design and navigation. Usually, such information has been collected from reports based on shipboard visual observations. On the other hand, a ship can be regarded as a type of wave recorder and, under the assumption of linear superposition, wave spectra can thus be estimated from measured ship motions. Based on this idea, the evaluation of one-dimensional wave spectra, as well as directional wave spectra, has been extensively carried out in recent years. However, when a ship is traveling with an advance speed in following seas, there is a range of the encounter frequency ω_e of the ship for which ω_e cannot be related to the wave frequency by a single-value transformation. Practically, this difficulty has been addressed by using an approximate method where only one solution ω of the relationship is retained by partitioning the ω-axis. When one keeps the lowest-frequency ω solution, however, resulting spectra tend to be narrower and displaced towards low frequencies.
In this study, we propose to design a wave spectrum by using a nonlinear programming method, in which a standard wave spectrum, such as the ISSC, is selected, and a significant wave height and average wave period are used as design parameters. The capability of this approach has been confirmed by comparing its results, based on the full-scale measurements of the motions of a container ship, with those from the so-called SMB method.

View full abstract

A row of discrete vortices is superposed to an air-sea interface to modelize the generation of surface waves by wind. A numerical method is developed in order to take into account the nonlinear effects which occur in the evolution of surface waves with finite steepness. The shape of the excited surface waves range from quasi sinusoidal to asymmetrical profiles close to breaking. Bi-orthogonal decomposition shows that resonant excitation (amplification of wave amplitude) may occur both at the fundamental frequency and at its second harmonic. This deterministic physical model is compared with experiments conducted in an air-sea tunnel. A vortex street is artificially created in the air flow. Using Fourier analysis and Wavelet transform it is found, in agreement with the numerical formulation, that non-sinusoidal disturbances advected over the water surface can have harmonic components that interact resonantly to generate surface waves, even thought the basic (fundamental) scale is non-resonant.

View full abstract

This paper describes the experimental results backscattering from wave surfaces by using microwave scatterometer. Active microwave remote sensing is considered to be very useful to observe the vast ocean at one time, while the ocean phenomena change quickly. The characterization of the microwave backscattering from wave surfaces is investigated in a wind wave tunnel and a square basin by using a C-band (5.2 GHz) microwave scatterometer. This paper clears the relationships between microwave backscattered power and wind driven waves. The significant wave height of wind wave and the average period, the wave direction contribute to Backscattering. The HH and VH-polarization are effective for monitoring the wind wave than the VV and HV-polarization.

View full abstract

Hydrodynamic forces including a wave drift damping for a slow drift motion of a four-column platform are investigated, both experimentally and theoretically.
It is well known that the 2nd order hydrodynamic forces acting on the marine structures composed of multiple cylinders in both current and waves are significantly influenced by the interaction between cylinders, and extremely vary with the wave number ka.
We measured the 2nd order wave forces and moment acting on a four-column platform model, in both slow current and regular wave trains of various incident angles. The experimental results were compared with the results of semi-analytical solutions based on the potential flow theory. In this solution, the interactions between cylinders are represented by additional waves emitting form adjacent cylinders towards the cylinders under consideration, and the drift forces as well as the yaw moment are evaluated by the far field method.
It was confirmed that the yaw moment is significantly induced by oblique waves and current, and that those phenomena are well explained by the theory.

View full abstract

This paper describes the real-time algorithm of ocean waves generated by the snake type wave maker in a test basin. The developed real-time algorithm forms a unified computation process against various kinds of waves. Moreover, a lattice filter like structure of this algorithm is shown.
The algorithm takes an iterative formulation on its calculation process. An iterative calculation generally has a progressive error when the number of iteration becomes large. However, the algorithm causes only a little error after the one million times iteration. Further, the spectrum profiles generated by this method and the conventional one are discussed.

View full abstract

A very large floating structure is now being considered as a possible candidate of an offshore airport. The structure is expected to be so large that the analysis of the hydrodynamic forces that will act on the structure by a conventional numerical technique is practically impossible because of the required huge computational burden. In order to overcome this difficulty, an approximate but quite accurate numerical method for the analysis of a diffraction problem was presented in the 1st report. The method exploits the fact that the structure is very large and assume a certain uniformity in the flow field around the structure except at the vicinity of the perimeters of the structure. The present paper extends the method to the analysis of a radiation problem (including that due to elastic motions) and shows that a similar approximate technique can be used in the analysis. In addition, as examples of the application of the presented method to some practical problems, an analysis of elastic responses of a very large floating structure in waves and an analysis of wave forces on a floating airport composed of two runway structures are carried out.

View full abstract

An analytical method to calculate motions of an elastic floating mat-like structure in wave is developed, using the method of velocity potential continuation to analyze the fluid region and thin plate theory. The deflections of the plate is approximated by an expansion in terms of the natural mode functions for free vibration of a free-free beam. Calculation method for steady wave drifting forces are also shown by using Maruo's far-field theory.
To verify the accuracy of presend method, calculated results for a rectangular plate are compared with the experimental results. The Calculated and measured amplitudes of the deflections and bending moments of the plate are in good agreement.

View full abstract

The decay characteristics of waves when they are incident to an array of a large number of cylindrical legs are investigated both experimentally and numerically. The number of legs supporting a very large floating structure which may be used for such purposes as an airport is expected to be so large (around 2000030000) that a direct application of conventional numerical techniques for the estimation of hydrodynamic forces that will act on the structure is practically impossible. However, in some previous works, it has been shown that the computational burden can be reduced drastically if a certain uniformity around a structure is exploited. To be more specific, the flow field around a very large floating structure may be assumed that it is the same as that around a floating structure which extends infinitely in horizontal direction. If a similar assumption can be made in the analysis of hydrodynamic forces on an array composed of a huge number of legs, the analysis may be carried out with a reasonable computational effort. In order to examine this possibility, extensive experiments are conducted. Through the examination of these results and also through the comparisons with theoretical computational results, the characteristics of the wave decay are clarified.

View full abstract

Very large floating structures (VLFS) are being considered for various applications such as floating airports, offshore cities and so on. A VLFS (Mega Float I) with length of 5 km, breadth of 1 km, and draft of 1 m was proposed by The Technological Research Association of Mega-Float in 1996. The elastic deformation of these structures in waves is important, because vertical dimension is small compared with horizontal. There are some numerical methods proposed to estimate its elastic behaviours and it is needed to validate these methods comparing with model test. Therefore, the authors carried out the tank test focusing on elastic response in wave using 1/100 scale model (VL 50) with 50 m length. The model was designed on the basis of the law of similitude and has elastically similar bending rigidity and weight with the full scale structure. The vertical displacement distributions are measured at 126 points along the center line by potentiometers, and the strains at 10 points on the upper surface at weather side and lee side respectively.
Some results of vertical displacement showed clearly long periodic deformations with the several node superposed on short periodic deformations that can be considered the cause of interaction between the model and the side walls of tank.
The elastic behaviors of VL 50 in wave are computed by the analysis method based on FEM and the pressure distribution method using the zero-draft Green function. The effect of the side walls are dealt as symmetric refraction.
The calculated results agree qualitatively with the experimental results. The side wall effect can be explained well by the present method.

View full abstract

We have developed the estimation method of the first-order hydroelastic responses of a very large floating structure. The deformation of the structure is small in the frequency ranges of the wind wave, because the wavelengths are very short comparing with the length of the structure. However, the lengths of the second-order long period waves that are the difference-frequency component of the second-order wave are very long. Generally, effects of that wave are very important for the design of the mooring system. But, the hydroelastic responses may be occurred by the second-order long period vertical wave loads.
In this paper, we examined the behavior of the hydroelastic responses of the very large floating structure due to the second-order wave forces. Then, the analysis method of the second-order wave loads on elastic modes is developed. The method is based on the pressure distribution method.

View full abstract

Very Large Floating Structures (VLFS) of several thousand meters long are being considered for various applications such as floating airports, offshore cities and so on. The VLFS recently designed have a thin mat-like configuration and very large horizontal size. A typical design, for example, is 5 Km long, 1 Km wide and 5 m high. Therefore this type of structure will be very flexible and the elastic deformation due to wave action will be more crucial than the rigid body motions.
For the analysis of the hydroelastic behaviors of VLFS in waves, the conventional numerical techniques can be applied in principle. However, the direct application of such techniques is practically impossible, because the wave length is very small relative to the horizontal size of the structure and it requires enormous computational burden. There are many calculation methods proposed to overcome this difficulty. However, it is still very difficult for the case of the length of structure is more than one hundred times of incident wave length.
Then the author propose a new numerical calculation method based on the 3-dimensional eigenfunction expansion method for the diffraction and radiation problems and the modal expansion method for the elastic response representation.
This paper describes, at first, a new calculation scheme proposed then the numerical results compared with the model experiments carried out at various institutions which indicate good agreement each others. Finally, based on the calculations, it shows the various features of hydroelastic behaviors of typical VLFS including the effects of water depth, elastic stiffness and so on.

View full abstract

The Mighty Whale, floating wave energy converter, has been studied and developed for the purpose of using coastal areas effectively by JAMSTEC (Japan Marine Science and Technology Center). The Mighty Whale will be installed at 1.5 km point from the coast of the Gokasyo-bay in Mie prefecture. In other words, the Mighty Whale will have to be moored in shallow water under severe environmental conditions by catenary mooring with chain.
Generally, in case of catenary mooring, slowly varying oscillations occur and mooring tensions become very large. Furthermore, if floating structures are set in shallow water, mooring tensions are raised by their vertical motions. Therefore, there have been few similar mooring cases such as the Mighty Whale.
In this paper, design methods of mooring system by approximate analysis, simulation analysis and model experiment are indicated, and concluded points through the design of the Mighty Whale are mentioned as follows ;
(1) It is feasible to moor large floating structures in shallow water under severe environmental conditions by catenary mooring.
(2) Intermediate weight on mooring line is effective in controlling displacement and mooring tensions.
(3) Approximate analyses for considering slowly varying oscillations and influences of vertical motions are available to design of real structures' mooring systems.
(4) Using these approximate analyses in initial design is simple and efficient, and useful for reducing time of design.

View full abstract

In order to meet with the requirement of the limitation of attitude fluctuation of semi submersible type floating airport, enhancement of bending rigidity will be a practical answer. But as another way, we studied on the possibility of attitude control by active control. Here, we considered the introduction of pneumatic actuator and carried out some numerical simulations for simplified elastic model as the first report.

View full abstract

The objective of paper is to develop a stochastic method to predict the slamming load acting on a very large pontoon-type structure with shallow draft in irregular waves. Threshold values of relative wave height and its velocity are important parameters to predict the slamming load acting on ships in irregular waves. In the case of the very large pontoon-type structure, however, it was difficult to determine the threshold values because the values were highly influenced by the rigidity of structure. This problem were related to development of a new stochastic method for prediction of slamming load without threshold values. Using the new stochastic method, the probability of occurrence of the slamming pressure acting on the very large pontoon-type structure were predicted.

View full abstract

The objectives of present paper are to derive the approximate solution for comparatively easily computing the sea shock forces on developing numerical simulation method of sea shock force and to clear the effect of sea shock forces on very large floating offshore structures by the present approximation. Therefore the approximation solution and the boundary integral equation method on frequency domain are derived to understand fundamental characteristics of sea shock force. Further the accuracy and the usefulness of the present approximation is confirmed by comparing the results by the present approximation with the numerical results by the boundary integral equation method.
Sea shock forces on very large floating structure are concretely understood by transforming numerical results in frequency domain into numerical results in time domain and basic data of earthquakeresistant design which is very important for implementation design is obtained.

View full abstract

Seaquake is caused by propagation of seismic motion of sea-bed through seawater. Although aseismatic design has been providing applicable marine structures for earthquake zone, the effects of seaquake on a floating structure may be one of an item to be taken into further consideration from a view point of the structure safety. Although the effects of vertical motion of sea-bed due to earthquake may not be small and seaquake forces on a floating structure should not be ignored, the study on effects of seaquake has not been performed much.
This paper is presenting a method of numerical calculation of seismic wave propagation in seawater. The hydrodynamic pressure caused by seaquake is analyzed numerically and its dynamic effects on the three-dimensional floating structure is investigated.
First of all, we are introducing Green function which is a solution of differential equation governing the seismic wave propagation and fulfills free surface condition and the sea bottom surface condition. Secondary, we are introducing the boundary integral equation by making use of Green function and Green's theorem. Finally, we are obtaining the pressure and the seaquake forces on a three-dimensional floating structure by solving the integral equation. And then we'll discuss the effects of seaquake on the floating structure.

View full abstract

This paper deals with the dynamic response of Very Large Floating Structures (VLFS) in regular waves considering the deflection wave propagation (VLFS-wave) using a simple beam modeling. Firstly, the dispersion relation of VLFS-wave is derived based on the analogy of linear water wave theory, and the relation between the incident wave length and VLFS-wave length is derived. The effects of water depth and rigidity of VLFS on wave length (or dispersion relation) of VLFS-wave are investigated. Next, the reflection wave due to the difference of dispersion relation between the incident wave and VLFS-wave is discussed under some particular assumptions. Finally, a simple method for the response analysis of VLFS is proposed. Using this method, the dynamic response analysis is carried out numerically, and its validity is shown as compared with the results of some other exact analyses. From these results, we can identify the following properties of VLFS : (1) VLFS-wave propagates faster than the incident wave. (2) VLFS-wave length is longer than the incident wave length. (3) When the water depth or the rigidity of VLFS increases, VLFS-wave length also increases. (4) When the incident wave number is sufficiently smaller than the characteristic wave number of VLFS, the wave length or the wave height of VLFS-wave becomes equal to those of the incident wave.

View full abstract

This paper deals with a progressive collapse simulation of multiple mooring dolphin system in a huge box shaped, shallow drafted floating structure which is now being proposed as one of the candidates of an offshore airport in Japan. The progressive collapse simulation is recently being well-used to check the robustness of the mooring system after an initial failure of one mooring dolphin. This recognizes the possibility of failure even though a design satisfies the requirements of the ultimate limit state.
Model experiments and time domain simulations have been carried out in order to investigate how the collapse of multiple mooring dolphins progresses after a first passage failure of one mooring dolphin and how the dynamic model of collapse process can be represented. The probability distribution of mooring system collapse time after the initial failure is studied.

View full abstract

For the structural analysis of a Very Large Floating Structure (VLFS) of several thousand meters long, a hierarchical system of structural analysis must be established, in which a macroscopic structural idealization is performed first to make an overall structural analysis practicable and then the local structural analysis is performed using a zooming technique. A rational structural modeling for overall analyses is most important in such an analysis system, because it governs the accuracy of subsequent structural analyses for local structures.
In this study, a two-dimensional grillage model is considered as one of the possible modeling for overall structural analysis of a VLFS, and a rational formulation of a torsional stiffness of grillage beam elements considering shear deformation of deck and bottom plating is discussed. Considering the characteristics of torsional deformation of plate-like structures and based on the concept of an equivalent strain energy, a new formulation of the equivalent torsional stiffness for beam elements is presented. Several static analyses of pontoon-type and semi-submersible type VLFSs are performed, and a validity of the proposed formulation is demonstrated.

View full abstract

For the people interested in the behaviors of a floating body in waves, exact nonlinear calculations of a 3-D body of arbitrary geometry in arbitrary waves must be the final goal. In order to achieve the purpose, we, in this paper, limit our attention to a 2-D body problem in arbitrary (non-breaking) waves in an inviscid and irrotational fluid. In an inviscid and irrotational fluid, it is well known that a velocity potential exists. Using the velocity potential, the equations to be solved are reduced to a single Laplace equation plus 6 equations of motions of a body and thus our computational burden is greatly reduced. However, it is hard to extend a potential theory to a body-motion in a viscous fluid, which is our final goal. Therefore we dare to avoid the use of a velocity potential and solve the continuity equation written in terms of the velocity and the Euler's equation of motions of water particles, so that the future extension to a viscous fluid problem can be carried out without total reformulation of a numerical scheme.
In this paper, we first compare a numerically produced regular wave train of small amplitude with analytical solutions. Secondly, we compare the response amplitude operator of a body in small regular waves calculated by the present method with those measured in experiments as well as with the linear potential theory. Next we compare the time history of the motion of a body in transient waves of large height with that measured in experiments. Lastly, we present results of bi-harmonic motions caused by a nonlinear restoring moment.

View full abstract

To realizing such a very large floating structure as an artificial island, for example, a floating airport, it is indispensable to examine comprehensively various expected environmental impacts on the sea area surrounding the floating structure. As a national research project in Japan, a large floating structure called Mega-Float model has been completed and moored off Oppama in Tokyo Bay in July 1996. For the purpose described in the beginning, measurement of temperature and salinity of the sea water in the vicinity of Mega-Float model was started in August 1996, and is still now under way, as of July 1997, in cooperation between a group of university researchers and Technological Research Association of Mega-Float. Based on the results of almost 11 month field measurement, difference in the vertical profile of water temperature at the center and at the northwest corner of the structure is discussed in order to manifest the primary effect of the presence of a very large floating structure on oceanophysical environment around the structure.

View full abstract

This paper deals with a new statistical prediction method for the propeller racing of ships sailing in rough seas. The propeller racing is one of the most important sea keeping quality in relation to the safety of main engine and shafting system. The trend of the racing has been investigated mainly in order to estimate allowable maximum propeller diameter, operability of ocean-going ships etc.. In those studies, the propeller racing generally and mainly means the situation (propeller exposed) in which the relative motion amplitude between ship hull and wave surface would exceed a depth of point in rotary disk propeller. Therefore, it seems that the magnitude of the amplitude and its exceeding frequency have been examined as a principal subject of study as usual. However, the time during which the amplitude exceeds a depth of point, that is, the propeller exposes in the air over sea surface, must be also one of most important factor affecting the trend of propeller racing. Then, this paper proposes a new practical method for estimating the time lasting of exposed propeller related to propeller racing in rough confused seas on the basis of the statistics.

View full abstract

Reliability of total ship handling system including human must be improved as by compensating the part where human make mistakes. Although compensation for hardware such as double hull can be effective for accidents, more reasonable and effective compensational way is to develop ship handling system that prevents accident. In other word, it would be reasonable to develop software concerning handling. In this paper, the support system is developed for berthing maneuver which is one of the most difficult maneuver. The present berthing support system can only display the items of current condition and forecast to the very limited extent of future. A rule based support in the information processing has not been realized, let alone a knowledge based support. The proposed berthing support system has realized leading support function and evaluation function in order to satisfy knowledge based support. The effectiveness of the proposed support system was confirmed through the experimental berthing maneuver utilizing ship handling simulator.

View full abstract

A study on psychological aspects is made focusing on two different senses which fall upon a captain in navigation. One sense is called menace which originates in encumbrance for the freedom of the seas with not objects such as ships or islands but the mirror images of the captain's own ship in the boundaries formed by the objects. An analytical hypothesis is set forth to express the menace field by applying the image principle in hydrodynamics for an incompressible fluid. Some important examples of the menace fields are shown to assure the proposed hypothesis. Navigation surroundings are assessed in view of maneuverability of ships. Another sense is called danger how menace is coming or going away. The sense of danger is represented by the time differentiation of the menace field. It is discussed that the sense of danger relates strongly with fears for ship collision.

View full abstract

In the previous papers, we proposed a structure of feed forward neural network and its learning process in order to simulate the dynamic behavior of underwater robots. The proposed network can estimate the time series of state variables when the initial variables and a series of manipulate signals are given. By introducing the proposed network into a neural network based control system proposed by Fujii et al., a quick on-line controller adaptation method can be realized. The method is examined through tank test and shows good performance. And an on-line adaptable controller system can be realized by the network with simulating capability and the on-line adaptation method. In this paper, adaptability of the controller system is investigated by heading keeping and path following experiments when the unknown disturbances are given to the robot.

View full abstract

For a close survey mission to a wreck such as the Russian tanker Nakhodka, the ROV is a powerful tool and provides us vivid images of the wreck. The ROV is controlled by human operators on the support vessel based on the visual information from the vehicle, knowledge of the ship, and operational know-how. Since the width of the visual field is limited, the operators may misunderstand the position of the vehicle. Moreover, the operation near the wreck is a significantly dangerous task and often causes the loss of the vehicle.
This paper proposes a world modeling system to display whale's-eye view of the target area in real time to help the ROV operators to understand the surrounding environment of the vehicle. The ranging SONAR data are accumulated and processed to update the world model while the vehicle is operated along a path which is determined based on the model.
Results of experiments in a testing pool and computer simulations represent that the proposed system can draw elevation maps and 3 D occupancy grid maps of the environment with complicated shapes of obstacles.

View full abstract

Based on the basic principles of genetic algorithm and movement features of Autonomous Underwater Vehicles (AUVs), this paper proposes a system that optimizes the path to a target point and velocity along it. Multi-objective optimization by the evaluation functions of location and speed precisions at the stopping point, and consumed energy is conducted using ε-limitation and two-time selection of pareto-optimal. The proposed system can include collision avoidance performance. The efficiency of the system is demonstrated in the simulation based on the AUV 'Twin-Burger'.

View full abstract

The AUV (Autonomous Underwater Vehicle) is not restricted by the umbilical cable and able to swim freely. Complex control is, however, significantly difficult, because of low rate and long delay in acoustic link between the operators and the vehicle. Since there are many complicated missions for underwater vehicle which need powerful operators control ability, it is desirable that unmanned untethered underwater robots in deep water could be remotely controlled by the operators on surface.
In this paper, the teleoperation system for unmanned untethered underwater robots, which cope with long time delay of acoustic communication, is proposed. A virtual robot which should immediately represents the result of remote control is simulated and shown to the operators on the graphical workstation as prediction computer graphics. Both the virtual robot and the real robot have the same “World Model” which has been constructed based on predetermined configuration of the seabed and obstacles. These robots generate their path and move themselves by receiving intermittently operators' commands. The real robot status and the information on encountering with unknown obstacles, which has not been written in the World Model, are sent to the workstation. The operators can decide how to operate the vehicle in support with three-dimensional computer graphics (3 DCG) on the workstation, which compares the motion of the virtual robot and that of the real robot in the World Model. The workstation provides the whale view and the virtual diver view to back up the operation. The proposed system is evaluated by the simulation experiments, and the results show that long delay in acoustic communication can be overcome and that remotely operation can be realized by the proposed system.

View full abstract