First step in optimizing LNG storages for offshore terminals

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The development of new LNG offshore facilities addresses new technical challenges that need to be deeply investigated to overcome the new constraints raised from the offshore operability. One of the key issues concerns the management of LNG storages. As future offshore LNG projects could be implemented in areas with harsh environmental conditions, the motions of the floating structure combined to the design and operating parameters should widely affect the behavior of the LNG inside partially filled tanks. Therefore, the knowledge available from onshore LNG storages has to be completed taking into account the offshore environment. The first concern deals with the production of Boil-Off Gas (BOG) during LNG’s unloading and storage operations under offshore conditions, where specific offshore parameters would contribute in increasing the production of BOG. For that purpose, a new optimized LNG storage process based on an increased operating pressure ( P = 500 mbarg) in the TRITON FSRU’s tanks has been investigated during LNG’s unloading and storage operations with LNG emission. It mainly showed that by operating at 500 mbarg, the production of BOG is nearly 4 times reduced during unloading operations compared to LNG carrier standard operating pressure (100 mbarg) and nearly suppressed during the storage phase. The second concern deals with managing the rollover risk under offshore conditions. In this context, GDF SUEZ performed a pioneer qualitative assessment to highlight the impact of the liquid motion on the rollover mechanisms in the TRITON FSRU’s tanks. The liquid motion completely modifies the mixing mechanisms in LNG stratified layers. The mixing is no longer governed by the molecular diffusion induced by the thermal effects but by the dynamic flow effects induced by the fluid wave movement into the layers. Moreover, this assessment showed that the time needed for a stratification to evolve up to a complete mix is severely reduced compared to the onshore LNG storage while the assessment of the induced BOG peak depends as usual on the energy stored in the stratification when being mixed and could be of the same order of magnitude as for onshore rollover.