Katsuji Tanizawa

A Three-Dimensional Numerical Analysis Code for Shipping Water on Deck Using a Particle Method

A three-dimensional numerical analysis code for shipping water on deck was developed using MPS (Moving Particle Semi-implicit) method [1–2]. MPS is a particle method for incompressible flows. A ship motion model was developed using the particle method. Shipping water was analyzed using this code. The fluid behavior, the waveform of pressure, and the time integration of pressure on an imitated deck were in agreement with experiment. A visualization system for calculation result was also developed.Copyright

Development of a Weather Adaptive Navigation System Considering Ship Performance in Actual Seas

Aiming to reduce fuel consumption and CO2 emission a new navigation system called WAN has been developed. The system provides optimum route and engine revolution with constraints of schedule and seakeeping criteria. In this paper, simulations of a container liner on transpacific route are demonstrated using WAN. Weather data used here are composed of 8 items; significant height, period and peak direction of wind wave, significant height, period and peak direction of swell, and mean speed and direction of wind. Ship responses; i.e. ship speed, fuel consumption and vertical acceleration at F.P., are calculated based upon the enhanced unified theory. To optimize route and engine revolution the augmented Lagrange multiplier method is applied. The objective function of minimization is fuel consumption with constraints of the schedule and the service limit. From the simulations, the effectiveness of WAN resulted very high and it is shown that the reduction of the fuel becomes 26.1% on average. Concerning the schedule keeping, accuracy of weather forecast must be examined. The influence on the system is evaluated using two kinds of weather data; one is a forecast received at departure and the other is the dataset extracted from the sequential forecasts of every 24 hours. From the simulations it is found that the fluctuation of fuel consumption due to updating the weather forecast is much smaller than the reduction of the fuel by WAN. However, from the viewpoint of ship safety, it is necessary to execute the system again whenever the weather forecast updated.Copyright

On the Generation of Spatially Periodic Breather in a Wave Tank

Spatially periodic modulational wave train was generated at the National Maritime Research Institute, Actual Sea Model Basin. The 76 m by 36 m basin of depth 4.5 m is surrounded by 382 paddle type wave generators with wave absorbing capacity. Taking advantage of this unique wave basin, we have compared two generation methods, first controlling the wave makers at both upwind and downwind sides and second controlling just the upwind side. The wave generator signal was computed a-priori by High-Order Spectral Method (HOSM hereafter). The HOSM provided temporally non-periodic time series of the unstable wave train as the wave-maker control signal. A number of wave wires located in the tank were used to compare the wave forms between two different generation methods. Our test result indicates that it is not necessary to provide signal at both ends, because the spatial evolution of the wave train remains periodic in space if the wave maker signal is appropriately controlled. The physical experiment and HOSM simulation seem to agree better with high order of nonlinearity.Copyright

Freak Wave Generation in a Wave Basin With HOSM-WG Method

A method to produce freak waves with arbitrary spectrum in a fully directional wave basin is presented here. This is an extension of Waseda, Houtani and Tanizawa at OMAE 2013[1], which used “HOSM-WG” based on the higher-order spectral method (HOSM). We used the following three methods to improve the HOSM-WG in [1]: “separation of free waves from bound waves,” “using Biesel’s transfer function in wavenumber space” and “using Schaffer’s 2nd-order wave maker control method.” Modulational wave trains, freak waves in unidirectional irregular waves and freak waves in short-crested irregular waves were generated in a wave basin. The experimental results using the improved HOSM-WG were compared to the HOSM simulation, and good agreements were found. The effectiveness of the improved HOSM-WG was ascertained. We showed that the difference between HOSM-WG and HOSM simulations became larger as wave steepness, frequency bandwidth of the spectrum or directional spreading became larger.Copyright

Water Surface Impact Load Acts on Bulbous Bow of Ships

As a part of the project study concerning on ship structural design for oil spill prevention by collision, optimal design of flexible bulbous bow structure is studied to absorb kinetic energy of the colliding ship. Such a flexible bulbous bow is named as buffer bow. To formulate design criteria of the buffer bow, the hydrodynamic impact forces acts on the bulbous bow was studied theoretically and experimentally. For theoretical study, shape of the bulbous bow was approximated by an ellipsoid and von Karman’s momentum theory was applied to estimate the slamming impact loads with given impact velocity of the emerged bow to free surface. Wagner’s impact theory was also applied to study the effect of free surface swell up. Based on the momentum theory, an estimation method of the impact loads was proposed. To validate the estimation method, a experiment was conducted at the 80m square tank of NMRI. A self-propelling container ship model was used for the experiment. The vertical and lateral shearing forces and bending moments at the root of the bulbous bow were measured. In this paper, the results of the study are presented.Copyright

Experimental and numerical investigations of temporally and spatially periodic modulated wave trains

A number of studies on steep nonlinear waves were conducted experimentally with the temporally periodic and spatially evolving (TPSE) wave trains and numerically with the spatially periodic and temporally evolving (SPTE) ones. The present study revealed that, in the vicinity of their maximum crest height, the wave profiles of TPSE and SPTE modulated wave trains resemble each other. From the investigation of the Akhmediev-breather solution of the nonlinear Schrodinger equation (NLSE), it is revealed that the dispersion relation deviated from the quadratic dependence of frequency on wavenumber and became linearly dependent instead. Accordingly, the wave profiles of TPSE and SPTE breathers agree. The range of this agreement is within the order of one wave group of the maximum crest height and persists during the long-term evolution. The findings extend well beyond the NLSE regime and can be applied to modulated wave trains that are highly nonlinear and broad-banded. This was demonstrated from the numerical w...

Study of Ship Responses and Wave Loads in the Freak Wave

In this study, we conducted tank experiment to measure ship responses to the Freak wave, Quasi Freak waves were generated utilizing focusing wave packet superposed on a regular wave train. Motion of the wave maker was determined in reference to wave simulations by our fully nonlinear numerical wave tank, NWT2D. An elastic model of a container ship was used to measure ship motions and whipping response. A time domain nonlinear strip method, SRSLAM, was used to simulate ship responses in the same incident waves. In this paper, the experimental and numerical results are presented and ship responses to the Freak wave are discussed.

Green sea loads on hatch covers of bulk carrier

A series of model tests in waves were conducted to measure the green sea loads that act on deck and hatch covers due to shipping water. A model of bulk carrier was used. The tests were carried out in irregular waves of which significant height is 10.6 meters and peak period is 14 seconds. In order to discuss the effects of wave heading and ship forward speed on the green sea loads, the model tests were made in several combinations of wave heading and ship speed conditions. It was confirmed that the deck wetness and green sea loads will be reduced considerably if the wave heading is altered to the quarter or beam seas or the ship speed is reduced.In order to assess the experimental results quantitatively, green sea loads were estimated by the practical estimation methods that were developed by the one of authors. Having compared with measured results, it was found that the estimated results were in good agreement with measured ones. It is concluded that the experimental results were rational.Comparison of green sea loads between measured results and present rules, ICLL66 and IACS UR-S21, was made. Although it is difficult to directly correlate measured values with the rules, mean values tend to be larger than the ones of ICLL66. It is also found that the green sea loads defined in IACS UR-S21 is ranked to somewhere between 1/10 and 1/3 significant values in relation to the measured results.