Traditional LNG offloading arms technologies have been used for LNG transfers in open waters, while hose-based technologies for both side-by-side transfers in calm seas and tandem transfers in rough sea conditions are nearing maturity. However, the offloading arms are not fully utilized for side-by-side arrangement since they have a limited allowable motion envelope and there are not adequate numbers of model test validations. Thus, the proper selection of offloading system and methodology will minimize the capital and operational cost involved.

The hydrodynamic interactions between FLNG and LNGC vessels for side-by-side arrangement are strongly coupled resulting in high relative motions. These motions play a significant role in evaluating the availability for a successful offloading operation for a given metocean criteria. Thus, offloading availability and reliability remain highly uncertain.

In addition, the viscous roll damping has an obvious effect on both the roll and sway motion responses for LNGC since they are coupled. The heave and roll motions of LNGC are influenced by the roll motion of FLNG which indicate the hydrodynamics coupling between the two vessels. Finally, the single amplitude roll motion for the LNGC controls the offloading availability more than the relative lateral motion and combined lateral and vertical motions between the vessels.

The main objective of this paper is to present a new approach for the evaluation of the offloading availability for a given metocean criteria based on the calculated relative motion responses for the side-by-side FLNG and LNGC. In addition, the effect of lateral distance between the vessels on both the hydrodynamic interactions and the offloading availability is further investigated.

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