ABSTRACT:

In this paper, the earthquake records observed at an underground crude oil storage facility in the rock ground are presented along with their spectral characteristics. Using the observed records, the dynamic response analysis is carried out for the rock ground model including the oil.tank with the aid of the two dimensional finite element method. As the result of analyses, it is found that the existence of the underground crude oil storage tank does not have much influence on the earthquake motion in the rock ground. Furthermore, it is concluded that the deformation of the service tunnel during earthquakes can be decomposed into two dominant modes: the uniform expansion and contraction in the vertical direction and the shearing type mode in the horizontal direction.

INTRODUCTION

The underground petroleum storage facility has a history of several decades in Europe and North America. In order to investigate its applicability in JAPAN where the geological structure is rather complex, a demonstration plant was constructed in the northwest of Shikoku Island in 1981 by Japan National Oil Corporation (Kato 1984). The plant has a horizontal-water-curtain-type underground crude oil storage tank of 25,000 kl capacity.

The earthquake observations have been carried out in the rock ground surrounding the underground storage cavern as well as the investigations on geomechanics and hydraulics (Miyashita et al. 1983). The dynamic characteristics of the rock ground surrounding the storage cavern, the deformation of the service tunnel during earthquakes and the safety of the underground storage cavern against earthquakes have been studied Using the observed earthquake records.

In this paper, we will present the outline of the earthquake observations in this demonstration plant and the dynamic characteristics of the storage cavern and its surrounding rock ground evaluated from the observed earthquake records. Also, the deformation of the service tunnel during earthquakes are presented and the dynamic response analysis is carried out for the rock ground model including the underground storage cavern with the aid of two-dimensional finite element method using the observed earthquake records.

OUTLINE OF THE EARTHQUAKE OBSERVATIONS

The demonstration plant is located in Kikuma, Ehime Prefecture in Japan as shown in Figure 1. The natural features of the area, including the site, exhibit a gentle slope and most of this area consists of granitic rocks. There are no major faults and fracture zones in this Figure 1. Location of the demonstration plant. area. The results of the elastic wave exploration at the bore hole in the site are shown in Table 1. The rock from the surface to about EL -5 m is weathered and the deeper rock is fresh. The primary wave velocity, the secondary wave velocity and Poisson's ratio of the fresh rock and the weathered stratum from EL ±O m to -4.98 m were obtained, but those of the weathered stratum from the surface to EL ±O m were not obtained. Table 1. Results of the elastic wave exploration. Elevation Vp*l Vs*2 Poisson's (m) (lan/s) (lan/s)Ratio *1 Primary wave velocity *2 Secondary wave velocity.

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