A numerical and experimental investigation is performed with respect to the seakeeping behavior of a prototype, twin-hull, LNG-FPSO and its hydromechanic interactions with the offtake (LNG) shuttle carrier during Side-by-Side offloading. The numerical model describing the SbS mooring system as well as the assumptions and calibration steps made towards the development of a robust numerical realization are presented herewith. Following calibration, the numerical model was tested against dedicated seakeeping experiments in order to assess the effectiveness of the proposed approach as well as the hydrodynamic performance of the overall offloading system. For a more realistic offloading-scenario case, three-component, i.e., wave, wind and current, offloading environments were used for validation purposes. For the time domain simulation, as far as the hydrodynamic (diffraction) database is concerned, the multibody response amplitude operators are implemented directly, instead of employing retardation functions, in order to observe whether such an approach can still yield robust results for such complicated hydromechanic system. Statistics of the relative motions between the LNG-FPSO and LNGC manifolds, fender loads and lines' tensions are obtained and presented, illustrating the good agreement between numerical and experimental results. Lessons learned and further recommendations are subsequently summarized and stated.
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