Seismic analysis of a LNG tank with GST technology including SSI
Introduction: LNG tanks are very specific structures with high loads and specific requirements to be satisfied when subjected to seismic actions. When harsh seismic conditions and poor soil foundation properties are found, the seismic design becomes even more challenging from both geotechnical and structural points of view. Two different kinds of above ground LNG tanks are compared in this paper: full containment tanks and membrane tanks with GST® technology. In the full containment tank there is an outer reinforced concrete (r.c.) container and an inner 9%Ni steel container and the seismic actions due to the inertia of the liquid are carried by the steel layer (flexible tank). On the other hand, in the GST® membrane technology the seismic actions are received directly by the r.c. tank (rigid tank).
Case study and modeling technique: Inclusion of soil structure interaction in the seismic analysis of large LNG tanks in both technologies can lead to a reduction of the seismic forces when compared to the simple case of fixed base approach often employed in seismic design. A comparative study has been performed considering the two tank technologies resting on the same site prone to high seismic action, poor soil conditions and same liquid capacity of about 175.000 m3. Because of the effectiveness of the membrane technology, the gross volume of the tank with the GST® technology is smaller than that of the full containment: this leads to a lower centroid of the horizontal seismic actions with consequential benefit in the actions on the foundations. The soil conditions of the case study are very poor so stone columns and vertical drains are required to increase the capacity and reduce the settlements. The soil structure interaction and the fluid-structure interaction are considered in the finite element models and Response Spectrum Analysis is developed. The added mass approach is followed in order to include both the convective and the impulsive part of the fluid mass.
Conclusions: The analyses show that membrane tank is feasible without isolators and the foundation is verified after ground improvement. The amount of reinforcement in the concrete elements is within technical feasibility. On the contrary, full containment tank shows uplift during the seismic event and verification of walls buckling during uplift would require walls and base plate thicker than usual. The uplifting may be avoided by anchoring of the inner tank wall into the bottom slab but this implies penetration through the bottom insulation, the secondary tank bottom and the vapor barrier. Furthermore the welding for fixing the anchor straps on the inner tank shell causes additional stresses in a heavily loaded area.
In conclusion, analyses conducted demonstrate the feasibility of a GST® membrane tank of large capacity in poor soil conditions and severe seismic environment without seismic isolators. Differently from full membrane tank, with GST® technology there is no need for any specific disposition to cope with these harsh conditions for the specific case study.
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