係留船の長周期動揺に関する数値シミュレーション

Abstract

Slow drift ship motions in irregular waves can be an important and practical problem. The large horizontal oscillations cause large forces in mooring lines, which may break in extreme cases, and difficulties in cargo handling even in less extreme sea state. This paper deals with a mathematical model of first- and second-order ship motions in irregular waves. Firstly, the first-order problem is calculated by the boundary integral equation method using Green's function. First-order quantities calculated there are then used as input to the second-order model based on the perturbation method. Both models derived from potential theory, are fully three-dimensional, and include the effect of a flat sea bed and arbitrary shape of the floating body. Some details about the model are given in the first section. There is also an examination of the facet sizes required for accurate estimation of second-order forces. The section after presents numerical results of second-order wave forces compared with a physical model of a barge. In the final section, the verification of the model is continued by comparisons between ship motions, including long period oscillations, predicted using the mathematical model and movements measured in physical model tests.