Study on a Combined Isolation Method for Large LNG Storage Tanks in Soft Site

The fundamental natural frequency that can determine the seismic response of large LNG storage tank is usually within 2Hz∼10Hz, it is in the range of predominant frequency of strong ground motion. It has been proved that the isolation control technique can be adopted to mitigate the seismic action on the liquid storage tank in the hard site, but in the soft site, if the conventional isolation design methods are still be used for seismic mitigation of the large LNG storage tank, the problems that the isolation period is too long, and the horizontal displacement of the isolation layer is too large under strong earthquake will be produced. For these problems, a combined isolation design method that is suitable for soft site is proposed in this paper. Based on the Malhotra’s simplified elastic-wall model of liquid storage tank, the simplified multi-mass mechanical model of combined isolated large LNG storage tank is established, and the seismic response of the LNG storage tank under the case of SSE (safety shutdown earthquake) is analyzed. The results show that the seismic-reduced effectiveness of the combined isolated LNG storage tank is obvious, and the horizontal displacement of isolation layer can be controlled in an acceptable range.