Resonance behavior and collision could possibly occur in the process of material replenishment and/or rescue procedures between multiple vessels in ocean environment. In this paper, a numerical model consisting of a free-floating barge and a cargo ship in parallel is built up to study the collisions between them in waves. Hydrodynamic interactions and wave-exciting forces on the vessels are calculated based on 3-D linear potential theory, and near field approach is adopted to deal with the second-order drift forces, which are found to be critically distance-dependent and wave direction-dependent. By synthesizing both the first- and second-order response, the relative motion trajectory of two vessels is obtained, which is applied to calculate the collision time in regular waves with different spacing distances and wave angles. Second-order drift forces are found dominant with respect to collision time. Results indicate distance between vessels and wave direction play the sensitive roles in prediction of collision time while the influence of various collision points can be ignored.

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