Hydrodynamic Analysis For The Side-by-side Offloading Operation Of FLNG And LNGC

The concept of FLNG was put forward in order to develop the rich natural gas in the vast ocean more cost-effectively. As a kind of new gas development technology, there are many key technologies, one of which is the offloading of LNG. Due to the ultra-low-temperature characteristic of LNG, side-by-side offloading is always adopted. How to consider the hydrodynamics between FLNG and LNGC during offloading operation seriously and seek effective measures to avoid collision accidents between the two floating bodies are key factors for offloading design. These problems are studied thoroughly in this paper.Firstly, frequency domain hydrodynamic analysis was performed on FLNG individually. The RAO results of this paper and the model test was compared to verify the accuracy of the hydrodynamic model so as to lay a good foundation for the FLNG-LNGC multi-body hydrodynamic analysis.Secondly, on the basis of hydrodynamic analysis on FLNG, the further analysis on FLNG-LNGC system in the frequency domain was performed. The resonance of the water between FLNG and LNGC was discussed and parameter sensitivity analysis was performed correspondingly. The results show that huge difference is existed in the effect on the six freedoms of FLNG and LNGC as the wave directions are different; Relatively smooth nonresonance and resonance of the water between the two floating bodies are included in the hydrodynamic interactions; The impact of resonance region is different along with the distances between the two floating bodies and the drafts of the two floating bodies; Further research shows that the damping lids impose a kind of obvious inhibiting effect on the movement of the water resonance and little impact on the smooth nonresonance.Thirdly, on the basis of the frequency domain analysis, the FLNG-LNGC multi-body coupling hydrodynamic analysis was performed by time domain analysis. After verifying the accuracy of the time domain model, LNG offloading operation was simulated by changing the loading states of FLNG and LNGC. The results show that the FLNG/LNGC/mooring system/connecting system is safe and effective enough under different loading conditions and sea conditions, which can be a reference for mooring system design of side-by-side operation.Lastly, considering the engineering practice during side-by-side offloading operation of FLNG and LNGC, relevant work has done to analyze the influence of connecting system on the security of the FLNG-LNGC system. The key considerations are the arrangements, the pre-tension and the stiffness of the hawsers. The results show that, compared to the lateral relative movement, longitudinal relative movement is more sensitive to the changes in the arrangements, pre-tension and stiffness, while vertical relative motion is nearly not influced by the changes of the connecting system; Although the increasement of pre-tension and stiffness can reduce the relative motion, too large values of the pre-tension and stiffness can not achieve very good results either; Offloading arms should be arranged at amidships as much as possible instead of being laid in the bow direction.In summary, a relatively complete and reliable numerical prediction method was formed through the analysis of the calculated results in this paper so as to know the regular pattern of motion for FLNG and LNGC during side-by-side offloading operation. The research results have some reference and guidance for carrying out the relevant projects to some degree.