One of the key challenges associated with an FLNG facility is to optimise its offloading operability. LNG offloading operations typically use a side-by-side (SBS) configuration, where a carrier is moored parallel to the FLNG, leaving only a relatively small gap between the two vessels. In this paper, a numerical model for predicting the relative motion between an FLNG vessel and LNG carrier during offloading is developed using potential flow theory, and compared to wave basin experiments on simplified box geometries.

In particular, this study addresses the prediction of roll motion, which is critical to SBS operation but difficult to model within the potential theory framework due to its dependence on viscous damping. The Ikeda method, which is an industry standard method for calculating ship roll damping, is applied in the model. However, since the Ikeda method was developed for a single hull, a modification is proposed to deliver improved performance in the context of the SBS offloading operation. More specifically, the small clearance between the vessels will influence the flow field and subsequently modify the fluid velocity around the bilge. This study suggests that the relative velocity between the fluid and hull roll motion is a better input for the original Ikeda method.

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