The experts at LUSAS Consultancy Services explain how base-isolation can protect the structure of LNG tanks in tectonically active regions
Panama and Central America are tectonically active regions, and LNG tanks represent critical structures with strict design requirements under accidental and earthquake conditions. In this case, base-isolation systems have become a cost-effective solution for increasing seismic demands in the design of large storage tanks.
LUSAS Consultancy Services has extensive experience in carrying out a wide range of analyses for large LNG storage tanks. Over the years, the team’s specialist engineers have assisted numerous companies, including Korea Gas Technology Corporation (KOGAS-Tech), on various projects and analyses. This includes the seismic isolation analysis of a 180,000 m³ full- containment tank for the Costa Norte LNG Terminal in Panama. Completed in 2018, this became the first LNG reception terminal in Central America.
This tank has a 32.7 m high, 9% nickel steel, inner tank of 89 m diameter, where the LNG is stored under normal operating conditions. This is insulated from a 91 m inside-diameter post-tensioned concrete wall, intended to contain any accidental spillage of the product. The base insulation sits on a concrete base slab supported by 400 pedestal-mounted isolators, which are cast on top of a reinforced concrete piled raft foundation. The overall tank height to the top of the roof dome is 53.4 m.
Analyses Undertaken
LUSAS Consultancy developed several FE (finite element) models to perform a detailed seismic assessment of the LNG tank under different earthquake and accidental conditions. These included the following:
• Time history seismic analysis using lumped-mass stick (beam) models
• Static and peak seismic loading application using shell models
• Coupled thermal-structural analysis for spillage/aftershock event using solid models
Isolation System
Triple friction pendulum bearing (TFPB) isolators were located under the tank bottom slab in order to decouple the tank from the earthquake ground motion and reduce the transmission of seismic energy to the tank components. In all, 400 pedestal-mounted TFPB isolators were used, with four sliding concave surfaces and three friction coefficients.
These were modelled in LUSAS’ software using specialised joint materials which included hysteretic damping and the variation of friction with sliding velocity and normal pressure. As friction properties are variable with time and also axial force, lower and upper bound properties were used for the empty and full tank cases. Under peak conditions, the base isolation was able to provide an overall effective damping of up to 33%, with peak displacements circa 200 mm, well within the 605 mm bearing capacity.
Soil-Structure Interaction
The tank foundation included a large number of closely spaced steel piles, fully embedded in rock. Piecewise linear joint materials of varying depth were used in LUSAS’ software to simulate
the nonlinear response of soil, including lateral bearing capacity, skin friction and end bearing resistance.
Time-History Seismic Analyses
Lumped mass modelling was used for fluid/structure interaction of the LNG tank and for soil/structure interaction of the pile arrangement. The nonlinear hysteretic behaviour of the isolation system required a detailed dynamic analysis. Using nonlinear transient dynamic analyses in LUSAS’ software, time-history responses were obtained under simultaneous horizontal and vertical ground motion.
Bedrock input motions of 0.33 g (Operational Basis Earthquake) and 0.54 g (Safe Shutdown Earthquake) from the seismic hazard analysis provided by KOGAS-Tech were used to develop multiple ground motion records to satisfy code requirements. The peak averaged results from the dynamic analyses were combined with normal operation static loading to perform a detailed stress analysis on a 3D shell model of the structure.
Spillage And Aftershock
A critical design condition of LNG tanks is the aftershock (SSEaft) event following an accidental spillage, which is assumed as a result of a prior SSE earthquake that has damaged the inner tank. The aftershock earthquake was estimated as 50% of the SSE, and the time-history analysis was repeated on the lumped mass model considering that in this case the LNG is in direct contact with the outer tank. A 3D solid model that included a state-of-the-art nonlinear concrete material was developed to carry out a semi coupled steady state thermal analysis to assess the effects
of the spillage on the tank wall.
Similarly, as for the shell model, peak hydrodynamic pressures were subsequently applied to the solid model and the liquid tightness and collapse prevention of the concrete tank were assessed.
Conclusion
Using the most advanced finite element analysis/modelling techniques for isolated LNG tanks, LUSAS Consultancy Services was able to provide KOGAS-Tech with a design basis for the tank checks under OBE, SSE and SSEaft conditions; including relevant results for foundation forces, isolator response, freeboard, concrete tank forces, liquid tightness, crack widths, and more.
After completion of the project, KOGAS- Tech’s civil & arch dept manager, Jung-Hoe Kim, comments: ‘LUSAS has provided us with a powerful design capability and advanced technical
support for over 20 years. On the Costa Norte LNG Terminal project, the use of Lusas successfully verified our tank design for a highly seismic region. With LUSAS, we can always ensure that
our LNG tank designs meet the strict requirements of our clients.’
