Civil and Structural engineering

Case Study

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Seismic isolation analysis of an LNG storage tank

  • 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 plus aftershock event using solid models

Costa Norte LNG Terminal, Panama.

LUSAS Consultancy Services has extensive experience in carrying out a wide range of analyses for large storage LNG tanks. Over the years its specialist engineers have assisted Korea Gas Technology Corporation (KOGAS-Tech) on various projects and analyses including, more recently, the seismic isolation analysis of a 180,000m3 full-containment tank for the Costa Norte LNG Terminal project in Panama.  On completion in 2018, it became the first LNG reception terminal in Central America.

Overview

Panama and Central America are seismically active regions and LNG tanks represent critical structures with stringent design requirements under accidental and earthquake conditions. In this context, base-isolation systems have become a cost-effective solution to cater for increasing seismic demands in the design of large storage tanks.

This above-ground full containment tank comprises a 32.7m high, 9% nickel steel inner tank of 89m diameter - where the LNG is stored under normal operating conditions - insulated from a 91m inside diameter post-tensioned concrete outer tank intended to contain any accidental spillage of LNG 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 53.4m.

Analyses undertaken

LUSAS Consultancy developed several finite element models to perform a detailed seismic assessment of the LNG tank under different earthquake and accidental conditions, including:

  • 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 plus 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 were used, with 4 sliding concave surfaces and 3 friction coefficients. These were modelled in LUSAS 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 of around 200mm, well within the 605mm bearing capacity.

Installation of base isolation system 
(Image: AES Colon)

Isolator force-displacement curve Acceleration-time reponse in horizontal direction

Soil-structure interaction

The tank foundation included a large number of closely spaced steel piles, fully embedded in rock. 

Piecewise linear joints materials varying with depth were used in LUSAS to simulate the nonlinear response of soil, including lateral bearing capacity, skin friction and end bearing resistance.

Liquefaction potential, cyclic loading, group effects and soil damping were considered in the computation of the axial and lateral stiffness of the soil support.

Pile modelling

Time-history seismic analyses

Lumped mass modelling was used for fluid/structure interaction of the LNG 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, time-history responses were obtained under simultaneous horizontal and vertical ground motion.

Bedrock input motions of 0.33g (Operational Basis Earthquake) and 0.54g (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.

Lumped mass model deformation Shell model deformation

Hoop axial forces in shell model

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.

Solid model deformation | Cracking / crushing | Compression zone

Spillage temperature results and tank material modelling

In summary

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.

 

"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 design requirements of our clients."

Jung-Hoe Kim, Civil & Arch Dept Manager, KOGAS-Tech.


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