Abstract
Considering the mooring systems, second-order wave forces and potential nonlinear gap resonance, the intricate relative motions between Floating Liquefied Natural Gas (FLNG) and Liquified Natural Gas Carrier (LNGC) during the offloading operation of Liquified Natural Gas (LNG) significantly increase the risk of collision. The present research mainly focuses on the numerical analysis of the forces of the side-by-side offloading system. Based on the three-dimensional potential theory modified by the Damping Lid Method, hydrodynamic coefficients such as the added mass, the quadratic transfer function (QTF) of drift forces and so on in the frequency domain are calculated. The Damping Lid is added to the free surface to suppress the overestimated wave elevation between two floating bodies in the close proximity. Using the Frequency to Time-Domain Transformation Method, the time-domain simulation can then be performed, through which the forces and the relative motions are subsequently obtained. The numerical calculation results indicate that if the incident wave frequency is close to the resonant frequency of the gap, a violent resonance phenomenon will occur in the liquid surface motion. Similarly, when the frequency of the low-frequency drift force is close to the natural frequency of the system’s horizontal movement, it can also cause resonance. The hydrodynamic coefficients and the motions of the side-by-side system, as well as the tensions of the mooring system, exhibit significant variations near both types of resonance frequencies.
