Numerical Study of Wave-Induced Responses of a Moored LNG Carrier

Large LNG carriers are sensitive to wave-induced motions at berth, where excessive motions and mooring-line loads compromise operations and safety. This study investigates the wave-induced responses of a fully loaded 177,000 m3 LNG carrier using the time-domain program SHIP-MOORINGS, examining the effects of significant wave height, peak period, wave direction, spectral peak enhancement factor and directional spreading on the six-degree-of-freedom motions and mooring-line forces. The model is validated against two physical model tests (172,000 m3 carrier in ballast, beam waves, Tp = 8.4 s), with maximum-motion differences within 0.05 m (0.05°), line-force differences below 10% and the same spectral content; results beyond this range, obtained from a single irregular-wave realization, indicate trends. Significant wave height scales the responses in approximate proportion without altering their pattern, whereas peak period and wave direction govern both their magnitude and the dominant motion patterns and mooring-load distribution. Under long-period waves, the dominant frequencies of heave, roll and pitch approach their natural frequencies, while surge is dominated by low-frequency motion. The peak enhancement factor changes the maximum motions by no more than 5%, whereas directional spreading produces larger, component-dependent variations. The critical wave direction for the mooring system is the oblique heading of 75°, at which Line 4 is the critical line; sway governs the loss of motion-based operability.