Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

The hydrodynamic analysis of side-by-side offloading including gap resonance
Tianwen Chen, Ju Fan
School of Naval Architecture, Ocean & Civil Engineering, Shanghai Jiao Tong University
Shanghai, China

exciting forces vary greatly near the resonance frequency of the gap, andsome numerical method is needed to suppress the resonance. RecentlyZhao et al. (2017,2018) conducted 3D experiments in transient wave
groups and white noise wave group respectively for vessels with roundand square bilges, and the higher order resonance response of the freesurface was analyzed, it was suggested that the damping was almost
entirely due to laminar boundary layers.
Many numerical results (Faltinsen (2007), Lu (2011)) show that theamplitude of the resonance frequency response will be overestimated by
the linear potential flow theory. However, the simplified 2-d model canignore the longitudinal resonant standing wave and cannot fully reflect
the real gap resonance. It is generally believed that the difference
between the theoretical and experimental results of linear potential flow
is due to the energy dissipation caused by viscosity. Wang et al (2019)conducted the numerical tank based on OpenFOAM to simulate two
identical side-by-side barges, and the energy variation of the wallboundary layer in the gap was investigated, it was found that thefrictional resistance in the boundary layer was the main cause of energy
dissipation.
Molin et al. (2002) believed that waves of certain frequencies can reflect
and form standing waves in the gap between two bodies, the gap issimilar to the moon pool and has similar resonance characteristics. Theresonance phenomenon includes three modes: Piston mode, longitudinalsloshing mode and transverse sloshing mode. The influence of viscosity
on the gap resonance calculation can be well considered by model test
and CFD method. But the preparation time of model test is long, the costis high, and the CFD method has a large amount of calculation andlimited efficiency, which usually need millions of grids for simulation,for instance, there are over 40 million cells and the simulation requires
approximately 1 million CPU hours in Wang et al (2019). For the sakeof efficiency and accuracy, the potential flow method is still a common
evaluation method in engineering field.
Due to the lack of viscosity, the amplitude of free surface elevation andhydrodynamic coefficients in the gap are often overestimated by the
traditional frequency domain potential flow method. In order to solve
this problem, many potential flow correction methods have beenproposed. The Damping Lid Method proposed by Newmann et al. (2001),Chen et al. (2005), is an effective way by adding artificial damping term
to the free surface boundary condition in the gap to restrain the abnormal

ABSTRACT
There are complex motion responses for the two floating bodies with thesmall gap in side-by-side offloading operation. In this paper, a numericalanalysis is carried out by potential flow theory with the damping lid
method.
The Damping Lid is added to the free surface for suppressing nonrealistic high wave elevation between two floating barges in closeproximity. The Damping Lid coefficient is determined throughcomparing the wave elevation between CFD and BEM results. It is foundthat the Damping Lid coefficient is sensitive to the wave height anddirections. 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.

KEY WORDS: Side-by-Side Offloading; CFD; Damping Lid; Gap
Resonance;

INTRODUCTION
The side-by-side offloading is one type of configurations when theFLNG offloads the liquefied natural gas to the LNGC nearby, and thefloating 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 isunder oblique or beam waves, the relative roll between the two bodies isintensified and the probability of collision is increased. Therefore, it isof great importance to predict the relative motion accurately in order toensure the safety of engineering operation.
In the theoretical research and experimental study, it is observed that theresonance 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-byside system, it is found that the hydrodynamic coefficients and wave

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