Towards hydrodynamic modelling of ship-to-ship LNG bunkering in waves with focus on gap resonance

Ship-to-ship bunkering of liquid fuel, e.g., LNG, outside the port can be a flexible and cost-efficient solution without having to upgrade the infrastructures at the port. The occurrence of fluid resonance in the narrow gap between side-by-side receiving vessel and bunkering vessel under wave actions may greatly influence the dynamic responses of the vessels through hydrodynamic interaction. The proximity of marine structures can generate drastic wave elevation in the narrow gap, which may reduce the operational time window of the bunkering and even risk the safety of the crew. In this paper, the fluid resonance inside the gap between two non-identical ship cross-sections in side-by-side configuration is studied under various wave conditions. The bunkering scenario that a larger cross-section on the upstream is fixed while a small cross-section free to heave in the downstream is considered as more realistic. A numerical wave tank is set up based on the commercial CFD package STAR-CCM+. The unsteady Reynolds averaged Navier-Stokes turbulence model is required to consider viscous dissipation. The volume of fluid method is applied to capture the position of the free surface, and regular waves are generated and absorbed using forcing zones at upstream and downstream boundaries, respectively. The overset meshing technique is employed near the downstream section to enable the heave motion. The heave response of the downstream section and its effect on the linear and nonlinear harmonics of wave elevation and resonant frequency inside the gap are investigated and compared with the fixed-sections scenario. It can be concluded that the heave motion reduces the gap resonance to a large extent by modulating the frequency of wave elevation inside the gap, thus avoiding the occurrence of second-order excitation.