Two-dimensional gap resonance between oppositely leaning floating twin barges in proximity subjected to normal incident waves is investigated by both model test and numerical simulation. The experimental data and numerical results of the gap resonance between twin oppositely leaning barges are in good agreement generally, and both of which show the resonant frequency of the fluid oscillation in the narrow gap decreases with the increase of the leaning angle relative to the vertical and the resonant wave height in the narrow gap decreases with the increase of the leaning angle.
Side-by-side arrangement of floating structures in proximity is often applied in marine field operations, for example, the offloading from an FPSO (Floating Production Storage and Offloading) to an oil tank or LNG (Liquid Natural Gas) ship. As the incident wave frequency is close to the natural frequency of the confined fluid bulk between floating structures, large amplitudes of fluid oscillation in the narrow gap can be observed. With a two-dimension assumption, the wave-induced gap resonance between fixed floating barges has been extensively studied by theoretical analysis, laboratory test and numerical simulation. Miao et al. (2000 and 2001) studied the wave interaction with two floating caissons with a small gap between them and examined the resonant phenomenon of the oscillation inside the gap theoretically. Saitoh et al. (2006) and Iwata et al. (2007) conducted two-dimensional experiment to investigate the gap resonance between two and three fixed boxes, respectively. Their experimental results indicated that the maximal amplitude of resonant wave motion excited in the narrow gap can approach up to about five times of the incident wave amplitude. As for the numerical simulation employed for the gap resonance, there are two main numerical models. One is the potential flow model and the other is the viscous flow model. Based on the potential flow theory, various numerical codes (Zhu et al., 2005; Li et al., 2005; Teng et al., 2006; He et al., 2006; Zhu et al., 2008; Sun et al., 2010) have been developed. The potential models can predict the resonance frequency at the gap between boxes accurately, and run very fast. It is widely used to find the resonant frequencies at the gaps of various floating bodies in practices. However, for predicting the wave amplitude in the narrow gap accurately, a viscous model must be used, as the physical energy dissipation due to fluid viscosity, vortex shedding and even turbulence plays a dominate role for the amplitude of resonant oscillation. Employing the viscous flow theory, Computational Fluid Dynamics (CFD) methods have also been utilized to investigate the fluid resonance in the narrow gaps between fixed bodies due to incident waves (Lu et al., 2008; Lu et al., 2010a; Lu et al, 2010b; Lu et al., 2011). The CFD models have a good performance in predicting both resonant frequency and wave height in the narrow gap. Though the fluid viscosity and vortex shedding are considered in those models, the application for the interaction between the incident waves and the moving floating barges are rarely seen due to the complex in meshing for moving and inclined bodies.
