Predictions of hydrodynamics of a conceptual FLNG-LNG offloading system

The offloading operation between a floating liquefied natural gas (FLNG) facility and an LNG carrier are often of limited duration depending on the sea environment. In extreme seas or even in moderate sea states, strong hydrodynamic interactions between the FLNG and LNG may occur with resonant motions of the fluid in the gap between the two ships, leading to excessive ship motions and limiting the operability of on-board facilities. Taking an alternative approach of a conventional potential flow (PF) method, this study focuses on the application of solving viscous Reynolds-Averaged Navier-Stokes (RANS) equations for investigating the hydrodynamic interactions of a conceptual side-by-side FLNG-LNG offloading system. To tackle this complex engineering problem, the research has been built up systematically. Initially, predictions of the interaction forces and moments in steady current are carried out wit h a quasistatic approach. The feasibility of RANS computation is demonstrated through validations against existing benchmark experimental results. The effects of varying longitudinal and lateral offsets on the hydrodynamic interactions are analysed. When comparing model and full scale computations, scale effects are evident in the surge force but found to be less influential in the predictions of sway force, roll moment and yaw moment for the cases tested. For analysing the hydrodynamic behaviour of the FLNG-LNG system in waves, a two-phase volume of fluid (VOF) method is adopted together with the fifth-order Stokes wave theory in the unsteady RANS (URANS) computation. This investigation is firstly performed for predicting wave induced loads and motions on single FLNG and single LNG in regular waves for assessing the credibility of the numerical approach. The computed wave loads correlated well with experimental measurements performed at the AMC model test basin. Applying an analogous approach, URANS computations of FLNG-LNG interactions are carried out for different wave frequencies and lateral separations with the vessels constrained in 6 degrees of freedom (DOF) being fixed. Physical model tests on the FLNG-LNG interactions in regular waves are performed for validation. URANS computations show better accuracy over the PF calculations, especially at relatively high wave frequency conditions where the gap wave resonance occurs. It is seen that the gap wave resonance appears when the incident wave frequency approaches the natural frequency of the gap fluid, resulting in significant variation of wave loads in the directions of sway, heave, pitch and yaw. Meanwhile, the lateral separation is found to have an inverse relationship with the natural frequency of the gap fluid. Reduction in the lateral separation shifts the occurrence of gap wave resonance to a higher wave frequency and brings more significant exaggerations on the gap waves and wave loads. When comparing model and full scale wave loads and gap wave responses, the two series of data correlate well implying insignificant influence of scale effects. To investigate the global performance of the side-by-side FLNG-LNG system in a real world scenario, a case study based on time domain analysis is carried out when the system is coupled with mooring lines, fenders and hawsers in an irregular sea environment. The system is moored by an inner turret mooring system allowing weathervaning under external disturbances. A thorough overview of the relative motions between the two ships is presented as well as the mooring line and fender loads. The effects of varying hawser pretension and stiffness on the hydrodynamic performance of the system and the loads of the connection system are presented. As above, the presented work provides insights into the hydrodynamics of the FLNG-LNG interactions in steady current and regular waves, especially for quantifying the hydrodynamic loads and gap wave responses in head sea conditions. The results will assist safe manoeuvring and mooring of the LNG alongside the FLNG in respect of achieving minimal hydrodynamic loads and relative motions between the vessels. The information gathered can also be incorporated into the mathematical model of ship-handling simulators for crew training purposes in the near future.