The fluid response at resonance in a narrow gap between two identical fixed barges is investigated for three typical wave headings including: beam sea, quartering sea and head sea conditions. A potential-flow model with energy dissipation effects is developed based on the boundary element method. The dissipation surface is devised at the bottom opening as well as two end openings of the gap, and both linear and quadratic damping terms are accounted for. Satisfactory agreement with experiments demonstrates that the response in the gap at resonance exhibits nonlinear correlation with the wave amplitude indicating the importance of the quadratic damping.
Liquefied Natural Gas (LNG) is an attractive source of clean energy, and it features easy transportation and relatively low carbon dioxide emission. The offloading of LNG from a floating LNG (FLNG) facility to a LNG carrier in a side-by-side configuration in the open sea is widely applied in offshore industry (Zhao et al., 2018a). Due to the presence of a narrow gap between the FLNG and LNG carrier, resonance of the partially-entrapped water column may occur under certain wave frequencies leading to large fluid motions in the gap. The consequence of large fluid motion may influence the relative motions of vessels, and induce large drift forces which in turn may pose various hazards that affect the cargo transfer operations (Kristiansen, 2009). The gap response is of particular interest because of strong resonant phenomenon where viscous dissipation effects may exert a significant influence.
The study of natural frequencies of standing wave patterns in the gap spurs the interests of researchers. Due to the narrow nature of the gap in between two vessels, the energy is trapped resulting in large response at resonance. Compared to the width of gap, the beam of the LNG carrier and FLNG can be assumed to be infinite. Under this assumption, Molin (2001) analytically studied resonant frequencies and natural modes for a side-by-side configuration in two dimensions and a rectangular moonpool in three dimensions, and explicit expressions were presented. To incorporate the end effect of two vessels, Newman and Sclavounos (1988) suggested that homogeneous Dirichlet conditions can be simply imposed at the ends (velocity potential equals to zero). Based on such assumption, Molin et al. (2002) derived an analytical formula to estimate the natural frequencies of gap resonance. Sun et al. (2010) verified Molin's formula numerically, and reported the sensitive hydrodynamic effects due to gap resonance. Zhao et al. (2017) experimentally studied the resonant fluid response in the gap driven by a transient focussed wave group, and observed that the duration of the liquid motion in the gap is much longer indicating the energy is partially trapped in the gap.