INTRODUCTION

In response to increasing demand for city gas in the metropolitan area, Tokyo Gas Co, Ltd. is now constructing a new LNG Terminal in Ohgishima, Tsurumi-Ward, Yokohama(Fig. 1). Construction of this terminal is now underway and includes many interesting features: the world's first embedment type underground LNG tanks, a sea berth type jetty, and a shield tunnel structure pipeway. The civil engineering work of these facilities is outlined below.

WHAT IS LNG?

LNG (Liquefied Natural Gas) is natural gas liquefied at -162? for ease of transportation and storage. Globally distributed natural gas is liquefied in producing countries, and transported to Japan using LNG tankers. At the terminal, LNG is unloaded, stored in LNG tanks, and vaporized on a supply and demand basis.

WORLD'S FIRST

EMBEDMENT TYPE

UNDERGROUND LNG TANKS

To keep harmony with the environment as well as preserve scenery, this plant will adopt the world's first embedment type underground LNG tank. As shown in Fig. 2, extensive greenery instead of the tank will be seen at its completion.

The tank, with an inner circumference of 72 m, liquid level of 49.2 m, and the world's largest capacity of 200,000 kl, is a giant cylindrical underground structure without partition, as shown in Figs. 3 and 4. The bottom is a 9.8 m thick super large reinforced concrete slab designed to withstand a liquid pressure of 0.6 MPa. The reinforced concrete domed roof is 1 m thick at the center and 2.5 m thick at the edges, to support the cover soil load of 40,000 tons and a dead weight of 15,000 tons. Generally, an embedment type underground tank costs less, the flatter its domed roof becomes, due to the reduction in excavation volume. Therefore, after the safety of the dome was verified through a buckling test and non-linear analysis, the dome was designed with a rise to span ratio of 1/10, as compared with the conventional ratio of 1/6.

Fig. 2 Conceptual drawing of Ohgishima Plant at its completion

The inner surfaces of the tank are covered with insulation and stainless steel membranes, like a giant thermos bottle. Hence, a high degree of accuracy becomes necessary for the inner surfaces of the 421-faced polyhedron RC domed roof: misalignment and level difference must be less than 6 mm. The RC domed roof is erected with trussed steel structural frames(Photo 1). Since it is difficult to assure accuracy when concrete is cast in several operations due to the difference in level caused by deformed trusses, 5,430 m³ of concrete was cast over the entire roof in one 28 hour operation. At adjacent construction yards, a side wall is being constructed for another 60,000 kl LNG tank, and excavation is being conducted for another 200,000 kl LNG tank (Photo 2).

PIPEWAY OF DEEP SHIELD TUNNEL STRUCTURE AND JETTY OF SEA BERTH TYPE

The unloading jetty, located 500 m off Ohgishima, is linked to shore by a shield tunnel. There is no pipeway bridge in order to avoid obstacles for vessel traffic (Photo 3). This type of jetty has been adopted for the first time in Japan. As the plant is located inland at Ohgishima, the tunnel stretches as much as 2 km (Fig. 1).

To avoid the settlement of the tunnel in an alluvial layer and keep it stable in earthquakes, the tunnel is designed to run through a diluvial soil layer, with a soil coverage of about 60 m at the deepest point subject to maximum soil and water pressure. (Fig. 5) The tunnel has an outer circumference of 8.9 m and an inner circumference of 7.2 m. High strength concrete with a design strength of 59 N/mm² is used for 550 mm thick RC tunnel segments. (Photo 4) Since the tunnel is used as an LNG pipeway, a high degree of water tightness and accuracy are required. Accordingly, assembly accuracy and deformation in earthquakes are taken into account in the design of seals between segments to keep the tunnel watertight. Furthermore, waterproof sheets are provided between segments and secondary lining elements. The area where pipes run is sealed with nitrogen gas for safety reasons(Fig. 6). After undersea excavation is completed a vertical shaft, consisting of an 18 m-diameter, 65 m deep steel shell caisson, is sunk into the seabed at the sea berth using earth anchors .

Fig. 4 Structure of underground LNG tank Photo 3 Sea berth type jetty

Photo 4 Shield tunnel under construction Fig. 6 Conceptual drawing of pipeway at its completion (inside diameter: 7.2 m)

POSTSCRIPT

Ohgishima Plant, the most advanced plant in terms of the underground tanks and other facilities, is scheduled to be completed in autumn, 1998. Civil engineering technology developed and adopted for this project is expected to be applied in other fields.