On the Hydrodynamic Forces Acting on Box Type Vessels and Their Motions : (3rd Report) On practical Estimating Method

Abstract

In regard to a box type floating vessel as one of the fundamental configurations of a floating offshore structure, this study has been proceeded with to establish a practical estimating method of its motion in waves of high accuracy. The authors have carried out forced oscillation tests, wave exciting force tests, and motion tests in beam, head and oblique seas by use of box type vessels of low length-to-beam ratio (L/B) which is supposed to have three-dimensional effect, and made two-dimensional and three-dimensional theoretical calculations. The three-dimensional calculation gives accurate values of almost all hydrodynamic coefficients and wave exciting forces excluding damping term of rolling, etc., but it has a disadvantage that CPU time is long. Therefore, as a practical estimating method of the motion, the authors have developed a Modified Strip Method (M.S.M) on the basis of the strip method of short CPU time, taking account of experimental results and three-dimensional calculation results. The M.S.M. is as follows: 1) Longitudinal strip method is adopted as well as ordinary transverse strip method. 2) Strict Strip Method (or S.T.F.) is introduced for estimating wave exciting force. 3) Correction of three-dimensional effect is made, based on experimental results and three-dimensional calculation results. 4) Estimating method of eddy damping for roll is modified, taking three-dimensional effect and shallow water effect into consideration.