Seiji Takezawa

A comparison of methods for calculating the motion of a semi-submersible

Abstract The 17th ITTC Ocean Engineering Committee undertook the comparison of methods for calculating semi-submersible wave motion, and 34 programs from 28 different organizations participated in the project. The summary of the results are reported in the Technical Report of the 17th ITTC (ITTC, 1984). In this paper, the details of the project are described. Namely, almost all the calculation results of the 34 programs are shown and examined from the viewpoints of the validity of the program itself and of the correlation between the differences in the various calculation methods and the differences in the calculation results. The calculation results are also compared with the experiments, the details of which are also illustrated.

Free Running Model Experiment on Speed Loss in Directional Spectrum Waves

For evaluating speed loss, self-propulsion test and free running test are carried out in directional spectrum waves with a full ship model in the towing tank of Yokohama National University. Such experiments will be the first time in the world. In the free running test, the model has six degrees of freedom as in real condition, and it is controlled to run straightly by using rudder controlling system, therefore the speed loss can be derived directly by considering the relative speed against carriage by using optical position sensor. It is found that the speed loss in short-crested waves is bigger than the speed loss in long-crested waves in short wave length. The experimental results of speed loss in directional spectrum waves are compared with the results of estimation methods which are originated from the added resistance obtained by experiment and theoretical calculation.

Full Scale Measurements on the Seakeeping Performance of a Plant Barge

A 220m long and 45m wide square Plant Barge was towed for about three months from Japan to Brazil covering the distance of 13, 000 sea miles.During this towing, some of the items concering motions and strength of the abrge were measured for 40 minutes per day. Observations of environmental conditions were made simultaneously, and, in some cases, wave heigths were measured using throw-in type wave meters.The measured results were analyzed by means of statistical analysis and spectral analysis.The feature of short term responses was investigated, and the spectral form thus obtained was compared with that of Pierson-Moskowiz type wave spectrum.As for the frequency response function of motions, the calculated values obtained by using the measured power spectra of wave heigth were compared with the measured ones and experimental results in model test basin, and a good agreement was found between the measured values and experimental results, and also the calculated values indicated to agree fairly well with the measured ones.