A crucial offshore operation is the side-by-side offloading of LNG (Liquefied Natural Gas) from an FLNG (Floating Liquefied Natural Gas) platform to an LNG carrier. The wave-structure interaction between the gap of two vessels is complex and requires significant attention. This study investigates the gap resonance between two floating rectangular vessels (with sharp bilges) under regular waves in beam sea conditions. In the present study, two vessels of unequal draught are allowed to move in the heave and roll direction. Hence, two different configurations are considered here: one with a shallow draught vessel on the weather side (SW configuration) and the other with a deep draught vessel on the weather side (DW configuration). Firstly, the multiphase computational fluid dynamic (CFD) is implemented to set up a 2D numerical wave tank (NWT) with two vessels. Here, the Volume of Fluid (VOF) is used to capture the free-surface interface, and the overset meshing approach is employed to capture the dynamic mesh around the vessels. Secondly, the results, including gap resonance, motion response and forces on floating vessels, are validated from the experimental data, carried out at the Department of Ocean Engineering, IIT Madras. The numerical results showed a significant deviation (or more than 35%) from experimental results.
With the increasing rise of oil fields in deep or very deep sea, away from onshore processing facilities, ‘offloading’ of liquefied natural gas (LNG) is becoming a better economically viable alternative to bring LNG to onshore. The offloading of LNG from floating LNG vessels to LNG carriers is carried out side-by-side, which creates a relatively narrow gap between the two vessels. The free-surface elevation in a narrow gap resonates in the presence of an incoming wave of a certain frequency. It is called ‘gap resonance’. The side-by-side offloading operations are usually performed under mild or calm sea conditions. The gap resonance becomes more violent as the gap between two carriers gets narrower (Hong et al. 2005).