ABSTRACT

Gap amplification effects between two floating bodies in close proximity under beam sea condition for regular wave is simulated using Coupled FNPT-DualSPHysics model. Numerical and experimental studies have been compared for two different configurations SW (shallow water) and DW (deep water) of floating bodies. The numerical simulations are performed for two-dimensional case while the experimental set-up is forced to mimic the two-dimensional case by fixing the length of floating bodies similar to the width of flume. The floating bodies have been allowed only to heave and pitch by restricting other degrees of freedom. The wave elevations at different locations including the gap between floating vessels have been compared with experimental results and have shown a good agreement with the same. The floating body responses along with horizontal and vertical forces on each body for both configurations have been compared with experimental results. Salient conclusions have been drawn for horizontal and vertical forces on each body for different configurations. This work is a contribution to the benchmark "Comparative Study on Wave Interaction with Twin Floating Vessels in Beam Sea" set up for the ISOPE-PACOMS’2024 conference.

INTRODUCTION

Over the past few years, the increased demands of Liquefied natural gas (LNG) as the cleanest form of energy have spurred increased transportation through maritime routes. The LNG transportation from extraction site to onshore poses few challenges in its initial phase as the offloading of LNG from FLNG (Floating Liquefied Natural Gas) (Zhao et al. 2018), a floating platform to extract LNG above natural gas field, to the LNGC (Liquefied Natural Gas Carrier), a carrier ship to transport LNG from FLNG to the onshore and further to industries. A minimum gap is maintained between both FLNG and LNGC in order to avoid any collision between them to avoid drifting. The large relative motions between two structures during offloading causes large wave forces on both vessels (Li and Teng, 2021) causing the damage to mooring lines and loading arms. Molin (2001) gave the first mathematical expression to model gap resonant frequencies using potential flow method which over-predicted the gap resonant responses. Experiments were also performed, Peric and Swan (2015), to investigate the gap resonances between a fixed GBS (Gravity Based Structure) and between side-by-side floating barges respectively. Although some studies suggests that the calibration for artificial damping is not required, Clauss et al. (2013) concluded through models tests at full scale that no artificial damping was required for gap fluid responses. Molin et al. (2002) reported the effect of draft over the wavelength through experiments in wave flume for a floating body in front of vertical wall in a channel. Lewandowski (2008) predicted the gap resonant frequency for two vessels in close proximity and confirmed the drastic changes in hydrodynamic forces and responses of vessels at critical frequencies using BEM. Faltinsen (2007) performed the theoretical and experimental studies in wave flume for twin rectangular hulls in calm water and reported the influence of vortex shedding, used local discrete-vortex method, at the corners of vessels under piston mode of excitation. Gao et al. (2020) investigated the effect of bathymetry slope on maximum horizontal force, maximum vertical force and moment on the body in front of a vertical wall and found the inverse relationship between the resonant frequency and slope of bathymetry. Saitoh et al. (2006) investigated the two-dimensional gap resonance for two floating structures based on potential flow theory and reported the effect of varying the gap width on resonant peak. Recently Zou et al. (2024) studied the effect of varying the gap width over barge breadth using AQWA for twin floating vessels under beam sea conditions and calibrated a damping coefficient for potential flow solver. Vineesh and Sriram (2023) performed the numerical investigation of gap resonance between two floating bodies for different configurations.

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