There are complex motion responses for the two floating bodies with the small gap in side-by-side offloading operation. In this paper, a numerical analysis is carried out by potential flow theory with the damping lid method.
The Damping Lid is added to the free surface for suppressing non-realistic high wave elevation between two floating barges in close proximity. The Damping Lid coefficient is determined through comparing the wave elevation between CFD and BEM results. It is found that the Damping Lid coefficient is sensitive to the wave height and directions. Furthermore, when the characteristic distance of the gap exceeds a certain value, the effect of the Damping Lid will become weak. The hydrodynamic coefficients and motion present a more significant varies near the first-order resonance frequency compared with single barge results.
The side-by-side offloading is one type of configurations when the FLNG offloads the liquefied natural gas to the LNGC nearby, and the floating bodies is usually only 4-10 meters apart. It is a challenging problem because the complex motion responses for the two floating bodies due to the narrow gap, especially when the side-by-side system is under oblique or beam waves, the relative roll between the two bodies is intensified and the probability of collision is increased. Therefore, it is of great importance to predict the relative motion accurately in order to ensure the safety of engineering operation.
In the theoretical research and experimental study, it is observed that the resonance will happen inside the gap at certain incident wave frequencies. Model test and numerical simulation are two main methods for the gap resonance. In the early time, experimental studies have been crucial in the above work but mainly focused on 2D geometries. Pauw et al. (2007) and Molin et al. (2009) conducted the 3D model test about the side-by-side system, it is found that the hydrodynamic coefficients and wave exciting forces vary greatly near the resonance frequency of the gap, and some numerical method is needed to suppress the resonance. Recently Zhao et al. (2017,2018) conducted 3D experiments in transient wave groups and white noise wave group respectively for vessels with round and square bilges, and the higher order resonance response of the free surface was analyzed, it was suggested that the damping was almost entirely due to laminar boundary layers.