Resonant fluid motion between an FLNG vessel and LNG carrier may play a critical role in side-by-side offloading operations in open seas. There have been many studies investigating aspects of gap resonance, a number of which have been focused on wider gaps (e.g. 12 m, 30 m) which may be less relevant to side-by-side offloading operations. In this study, we investigate resonant fluid motion in a narrow gap (i.e. 4 m) which is relevant to a typical side-by-side offloading configuration. Both numerical and wave basin tests are reported, and we consider round and square bilge cases. In the experiments, transient wave groups are used which avoids reflections from wave basin walls. Numerical models based on potential flow theory are developed using linear frequency domain, linear time domain and nonlinear time domain approaches. Unlike the majority of existing studies which have focused on reproduction of response amplitude operators, we attempt to reproduce the experimental time histories. This requires accuracy in simulating both amplitude and phase for each wave component, arguably providing a better guide as to whether the relevant physics has been captured in the models. In this study, vessels with identical shapes except for round and square bilges are investigated, and we find that different levels of damping beyond the radiation damping component are necessary to obtain good fits. The comparison between the numerical simulations and the experimental data reveals the difficulty in re-producing the time history of a resonant response with a combination of different resonant modes.

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