Numerical analysis of hydrodynamic response and operability of large moored LNG ships in swell waves

LNG ships are highly susceptible to longer periods, which intensify their motion, reduce (un)loading efficiency and pose safety risks. This study validated the developed numerical model through comparisons with analytical solutions and experimental results. It then employed time-frequency domain analysis to investigate relationships among motions, mooring line forces, and operability of large LNG ships under varying swell periods. Results indicated that motions were more pronounced under beam waves than head waves. Specifically, the roll resonance period of large LNG ships typically falls within the swell periods, which easily causes roll oscillations and roll-sway coupling effects. This coupling, observed in wavelet power spectrum, causing uneven distribution of mooring line forces. Integrating static and dynamic analyses of operability reveals that the first hour of (un)loading operations has the longest downtime and the most frequent stoppages. Moreover, the duration of each stoppage of large LNG ships under beam waves increases with increasing wave periods. This effect was particularly pronounced for swell periods at or above the roll resonance period. Frequent and prolonged exceedance of motion limits increases the risk of loading arm disconnection. Therefore, terminal designs must consider whether local wave periods approach or exceed the roll resonance period of the designed ship.