The focus is on an LNG carrier moored alongside a bottom mounted offshore terminal, and exposed to incoming waves. When designing mooring and fendering systems, the coupled motions of the ship and of the entrapped fluid column between the ship and the terminal should be analyzed. Linear potential theory, suitable for the study such large volume structures, may largely over-estimate ship and fluid responses near resonant conditions. Flow separation at the ship bilges is the cause to the discrepancy between model tests and linear potential flow theory near resonance. In the present work we investigate the gap-resonance problem in a two-dimensional setting by a Navier-Stokes solver (CFD). We study two different problems, one with free decay of the fluid column, and one with forced heave of the ship. The CFD code is validated against existing experiments and an existing Boundary Element Method with vortex tracking (BEM+VT). The comparison involves the accuracy of the solution, robustness, stability and computational cost. The CFD-based method is found relevant for calibrating dissipation models in potential flow solvers aimed at solving gap problems.
Bottom mounted LNG offshore terminals are usually located some kilometers away from land. The ensure offloading of LNG carriers (see Figure 1), and storage of their cargo. The LNG is then regasified at the terminal, and transported via pipelines to land, where it is injected into the network. The fact that these terminals are located offshore reduces port congestion, and facilitates the approach and the accommodation of steadily larger LNG carriers. Large wave-induced motions of the ship may affect the availability and profitability of the terminal. A careful analysis has therefore to be carried out when designing the loading arm, the mooring and fendering systems, as well as when defining operational limits for the offloading operation.
