Motion of a barge beside a fixed box: Improved relative velocity roll damping estimates, and experiments

Side-by-side offloading is currently the most common operation by which a Floating LNG (FLNG) facility transfers liquified cargo onto a carrier. Operability is limited by a range of criteria, of which the relative motion between the two vessels is one of the most critical. As the FLNG is relatively larger and does not move as much in response to waves, the prediction of carrier motion is thus of great importance in operability prediction. Roll motion is strongly influenced by viscous damping and is therefore typically difficult to predict within a potential flow framework. This study presents a comparison of a numerical model of vessel motions, based on linear potential flow theory, and experiments conducted in a deep water wave basin, where both vessels are simplified rectangular boxes. The roll damping formulation used to add viscous damping in the time domain is a particular focus of the numerical modelling. It is observed that using roll methods developed for isolated vessels may under-estimate roll damping for the scenario where two vessels are closely positioned, as the second hull in proximity significantly alters the surrounding flow field. This paper therefore proposes a modification to a conventional roll damping method, known as the Ikeda method, by introducing a relative velocity formulation that takes into account the flow field around the hull, which includes any possible effects from gap resonance. This modified method is shown to generate improved roll damping predictions compared to standard practice. Overall, the proposed method remains within the linear potential flow framework using standard coefficients, meaning that it can be readily integrated into engineering practice.