5.1–1

TECHNOLOGICAL CHALLENGES FOR THECONSTRUCTION OF THE OHGISHIMA LNG TERMINAL

DEFIS TECHNOLOGIQUES RELEVES LORS DE LACONSTRUCTION DU TERMINAL GNL D’OHGISHIMA

Junji UmemuraGeneral Manager

Sadao GotoGeneral Manager, Ohgishima Project

Keiichi SakataSenior Manager, Ohgishima Project

Masafumi NakanoSenior Manager, Ohgishima Project

Tokyo Gas Co., Ltd.Production Engineering Department

1-5-20 Kaigan, Minato-ku, Tokyo Japan

ABSTRACT

Tokyo Gas’ third LNG receiving terminal, the Ohgishima terminal, goes intooperation beginning October 1998 in the city of Yokohama. It is located adjacent to areaswhere city gas is in great demand. In the planning, designing, and construction of theLNG terminal, state-of-the-art technology has been fully incorporated and specialattention has been paid to improved safety and protection of the environment, includingthe appearance of the surrounding landscape. In particular, the LNG Receiving Facilitiesand Storage Facilities were designed utilizing a brand new type of system. Featuresinclude an offshore berth for LNG tankers without bridges for pipelines between theberth and shore, a 2-km shield tunnel connecting the sea berth and the terminal, pipelinesincluding 40B large-diameter LNG receiving piping accommodated in the tunnel, and a200,000-kl large-capacity LNG underground storage tank constructed fully below groundlevel, including the concrete domed roof.

The engineering concept realized in the Ohgishima LNG Terminal based on the mostadvanced technology will have great influence on LNG terminal projects in the 21stcentury and will provide valuable information for their development.

5.1–2

RESUME

Le 3ème terminal de réception de gaz naturel liquéfié (GNL) opéré par la sociétéTokyo Gas, le terminal d’Ohgishima, entrera en opération en octobre 1998 à Yokohama.Il est situé à proximité de zones où le gaz de ville fait l’objet d’une demande soutenue.Lors de la planification, de la conception et de la construction du terminal GNL, nousavons amplement utilisé des techniques de pointe et attaché une attention particulière àl’amélioration des conditions de sécurité et à la protection de l’environnement, y comprisl’aspect du paysage environnant. En particulier, les installations de réception et destockage du GNL ont été conçues au moyen d'un type de système complètementnouveau. Les principales caractéristiques comprennent un poste d'amarrage en mer pourles méthaniers sans pont pour les pipelines entre le poste d'amarrage et la terre, un tunnelblindé d'une longueur de 2 km reliant le poste d'amarrage en mer et le terminal deréception, des pipelines d'un diamètre de 40B pour la réception du GNL à l'intérieur dutunnel et un réservoir de stockage souterrain de grande capacité pouvant contenir 200.000kl de GNL totalement sous le niveau du sol, y compris le toit en coupole en béton.

Le concept d’ingénierie réalisé lors de la construction du Terminal GNL d’Ohgishimaet basé sur les techniques de pointe les plus avancées aura une grande influence sur lesautres projets de construction de terminaux GNL au XXIe siècle et fournira de précieusesinformations pour leur développement.

5.1–3

TECHNOLOGICAL CHALLENGES FOR THECONSTRUCTION OF THE OHGISHIMA LNG TERMINAL

1. INTRODUCTION

Tokyo Gas’ third LNG terminal, the Ohgishima Terminal, is located between theexisting Negishi Terminal and the Sodegaura Terminal, which face Tokyo Bay, as shownin Figure 1. In other words, the Ohgishima Terminal is located adjacent to a high-demandarea for city gas. This great advantage means that there is no need to newly construct agas distribution trunk line. The facility is ideally located in a position so that the existinggas pipeline can be utilized most effectively.

Figure 2 illustrates the environment of the Ohgishima Terminal which is located onOhgishima island. The rectangular premises of the terminal extends about 1,000 metersfrom east to west and about 300 meters from north to south. It faces lines of oil tanks onthe north and the Bay-Circulating expressway on the south. It is in Yokohama City whichis known as a port city. The landscape in the bay area has been changed for enhancedappearance in accordance with the city's long-term planning. In addition, the waters infront of the terminal serve several channels for marine traffic and many small-sizedvessels can be seen coming and going in the channels.

Because of these geographical conditions and the environment of the OhgishimaTerminal, planning for the terminal has demanded not only sufficient consideration interms of safety, appearance, and environmental protection but also the most effective useof the valuable urban space. As a result, the terminal employs new types of LNGfacilities making the most of the state-of-the-art technology as described below.

Figure 1. LNG Terminals of Tokyo Gas Figure 2. Environment of the Ohgishima

Ohgishima

Negishi

Sodegaura

TokyoBay

• î“ ‡• H• ê

Gas Trank Line

Highway

Shield Tunnel

Kawasaki CityYokohama City

LNG Berth

Ohgishima Terminal

Oil Terminal

TsurumiChannel

5.1–4

• The terminal’s berth takes the form of a sea berth, which is located 500 m off theshore of Ohgishima island. The berth and the LNG terminal are connected by a 2-kmshield tunnel which has a diameter of 7.2 m and maximum depth of 68 m. The designeliminates the need for a piping bridge or any other structure in or above the sea waterbetween the berth and the shore.

• Low temperature service pipelines, such as LNG receiving pipeline are installedinside the shield tunnel.

• The LNG underground storage tanks have large capacities of 200,000 kl and areinstalled fully below ground level including the roofs.

This paper deals mainly with the technological aspects of the above new systemsemployed in LNG facilities in the planning, designing, and construction phases.

2. GENERAL OVERVIEW OF THE OHGISHIMA LNG TERMINAL

The Ohgishima LNG Terminal is scheduled to receive its first LNG cargo in July1998 and send out city gas in October that year. Table 1 shows the planned terminalfacilities, while Figure 3 illustrates the construction schedule up to the beginning ofterminal operation. The Ohgishima Terminal under construction in October 1997 isshown in Figure 4. For other information about the terminal, refer to References on thelast page.

Table 1. Capacities of the Ohgishima LNG Terminal

Year 1998 2010 Remarks

LNG 0.5 3 mil. t/year

Gas Volume 700 4,000 mil. m3/year

LNG Tank 200,000 kl x 1 200,000 kl x 6 Space for 10 tanks

LPG Tank 60,000 kl x 1 60,000 kl x 1 Space for 2 tanks

Vaporizer 150 t/h2 150 t/h6 Space for 14 units

Year 93 94 95 96 97 98

Commissioning 5 9

LNG Tank 2 4

LPG Tank 10 4

Tunnel & Pipeline 2 5

Berth 5 5

Vaporizer 7 5

Figure 3. Construction Schedule of the Ohgishima Terminal

5.1–5

Figure 4. The Ohgishima Terminal Under Construction, Oct. 1997

3. LNG BERTH AND SHIELD TUNNEL

3.1 Outline

Due to conditions in the sea waters surrounding the LNG terminal, LNG tankerscannot be brought alongside the bank of the LNG terminal. The berth is constructedabout 2 km away from the terminal, and they are linked by means of a tunnel whichhouses necessary piping so that passage of small-sized vessels near the island will not beaffected. As the Tsurumi Sea Channel carries a large amount of small-sized vessel traffic,the berth should be placed at a distance from the channel. Accordingly, the tunnel isangled by 28 degrees at the island bank.

Normal means for access to the berth for operation is by shuttle boat. However,should the weather be rough, passage provided via the inside of the shield tunnel isavailable to reach the berth. Figure 5 shows the positions of the berth and the tunnel,together with a longitudinal sectional view of the tunnel.

3.2 LNG Tanker Berth

(1) Applicable types of vessels and unloading arms. LNG tankers and LPG tankerscan dock at the pier of the berth. All types of LNG tankers, up to 135,000 m3, can beaccommodated by the berth, and the berth is equipped with four 16B unloading arms forLNG liquid and one 16B for return gas. There are a 12B liquid arm and a 12B return gasarm for LPG.

5.1–6

Figure 5. Layout & Section of Offshore Berth, Tunnel and Shafts

(2) Structure of berth. The berth is composed of reinforced concrete structuressupported by steel pipe piles. Most of the piles are battered as called for by theearthquake-resistant design. The longest pile measures 90 m. Piles are monolithic typeand are driven by a large piling barge without applying on-site welding. On the platform,which is 50 m × 50 m wide, are installed an unloading arm rack, piping rack, and berthcontrol room building. The racks are made of reinforced concrete which has theadvantages of corrosion resistance to splashes of sea water. Also, it has the advantages oflow-temperature resistance to possible LNG spill from the arms, and cost performance incomparison with a steel structure.

(3) Facilities to aid tanker berthing and other facilities. To aid tanker berthing, theberth is equipped with a wave height meter, tidal current meter, anemometer, shipapproaching velocity meter, and tension meter for the ropes for mooring vessels. Sincethe berth is distanced from the LNG terminal, it is equipped with security equipment suchas gas detector, fire alarm, industrial TV monitor system, and intruder surveillance andalarm system with infrared ray and infrared ray cameras. Information obtained by thesedevices can be monitored and viewed in the berth control room and the central controlroom in the terminal.

Figure 6. shows the LNG offshore berth under construction.

5.1–7

Figure 6. LNG Offshore Berth Under Construction(No Pipeline Bridge over the Sea)

3.3 Shield Tunnel and Vertical Shaft

(1) Outline. The shield tunnel has an inner diameter of 7.2 m and a total length ofabout 2,000 m. The tunnel runs about 500 m under the sea and 1,500 m under OhgishimaIsland. At the berth and the terminal, vertical shafts for piping are provided. Forconstruction work, an intermediate shaft was prepared at the bank between the offshoreshaft and terminal shaft. The positions of these shafts are illustrated in Figure 5.

Two shield tunnelling machines with outer diameters of 9 m were used to dig thetunnel. Excavation for a length of 1,500 m under the island was performed from theterminal shaft and 500 m under the sea was made from the intermediate shaft. Theultimate accuracy in the escavation of the tunnel was 10 to 20 mm.

(2) Ground conditions. The shield tunnel is deepest at the intermediate shaft, wherethe thickness of earth covering over it measures 59 m. This depth has never beenachieved for a shield tunnel constructed in permeable ground. The ground at theintermediate shaft consists of thick deposits of soft soil, where settlement due toconsolidation is anticipated to be several tens of centimeters. Consequently, the tunnel isinstalled in the settlement free stable layer to avoid harmful influence by settlement onthe structure.

(3) Vertical shafts. The excavation for the intermediate shaft is 74 m deep. For thedeep excavation, a super deep slurry wall was employed as a cut-off and retaining wall.This is also used for construction of LNG underground tanks. The shield machines leftand reached the intermediate shaft under the unprecedentedly high underground waterpressure of around 0.6 MPa.

5.1–8

The offshore shaft was constructed by open caisson method. A concrete caisson linedwith steel plate was used for the bottom part of about 40 m which was prefabricated in afactory and then transported to the site by a carrier boat. It was installed in position by theuse of a floating crane. After installation, concrete was poured onto the side wall of thecaisson and underwater excavation inside the caisson was carried out. Twelve earthanchors each with a 250-ton jack were used for the sinking of the caisson. Anchors werefixed in the 100 m deep bed rock.

(4) Shield tunnel. This tunnel features a large cross section at a great depth. The outerdiameter and the inner diameter measure 9 m and 7.2 m each and the depth measures 59m at the deepest point. Figure 7 shows a cross section of the tunnel. The primary liningconsists of concrete segments with a thickness of 50 to 60 cm. The segments wereprefabricated at factories and assembled by means of bolts on the site. The segments bearearth pressure and ground water pressure. Non-woven geotextile and high quality water-proof sheet were installed inside the segments and then a secondary concrete lining with athickness of 20 to 40 cm was applied.

No water seepage into tunnels for LNG pipelines is acceptable, different from tunnelsfor roads or sewage. Detailed investigations were made on the complete water-proofdesign, structure and execution of work. High-strength concrete was used to reduce thethickness and the forms for concreting were made with high accuracy. Special attentionwas paid to sufficient curing of concrete. High quality water-expansive water-proofrubber was applied between the segments as sealing material. In addition, any seepagethrough segment joints would be collected via the geotextile into the water distributionpipe installed at the bottom of the tunnel. This would run into the bottom of theintermediate shaft and be finally pumped up. Water-proof sheets with a thickness of 2mm were used inside the geotextile. The sheets were jointed by doublethermocompression bonding method of which quality is confirmed by leakage tests. Thefixing jigs for the sheet never penetrate the sheet.

These measures resulted in complete water-proofing secured in the tunnel over thetotal length of 2 km.

Figure 7. Pipeline Tunnel with Complete Water Tightness

5.1–9

4. LONG LNG PIPELINES WITHIN TUNNEL

4.1 Outline of the Pipelines

Figure 8 shows the outline of the pipelines within the tunnel. Around 4/5 of the crosssection of the tunnel is occupied by the pipelines and the remaining 1/5 is used for theelectric and instrument cables and manway for inspection and passage. Eleven pipelinesare installed in the space including a 40B LNG receiving pipeline. An electric heater isinstalled at the lowest level in the pipe space to control the temperature inside.

4.2 Special Points Considered in the Planning and Design

For LNG liquid pipelines installed in the confined space of a tunnel, not onlysufficient performance of the pipelines, but also safety, maintenance, monitoring systems,and easy construction work should be guaranteed as follows.

• High level of safety : LNG liquid pipelines in a confined space or a tunnel must bestructured to be leakage free. Even when assuming the minimum amount of leakage,the safety of the facility must be secured.

• Easy and certain operation and maintenance of the facility : Since the pipelines insidethe tunnel are to be utilized for a long period of time after start-up, operation andmaintenance should be as easy and certain as possible.

• Monitoring inside the tunnel : The tunnel must be fully monitored in entirety for thepurpose of maintaining the safety and continuing good performance and maintenanceof the facility.

• Easy construction work : Because there are difficulties in installing many pipelines ina limited space, construction method should be as efficient, economical and time-saving as possible.

These points were taken into intensive consideration in the planning, designing, andconstruction of the pipelines.

Figure 8. Pipelines in the Tunnel

Piping Dia. (in.)Upper LPG Return-gas

LPG ReceivingLPG Circulation

1020 6

Middle LNG Receiving 40Lower LNG Return-gas

LNG Circulation20 8

Lowest LNG ventLPG ventInstrumentation airNitrogenWater

4 4 3 3 3

5.1–10

4.3 Design and Structural Considerations for Safety

(1) Design and structural considerations. The joints of the pipes installed in the tunnelwere welded instead of using flanges and no valves were provided. A stainless steel pipewith a length of 2 km contracts by 5.8 m when it is cooled from room temperature downto -162°C. The contraction is absorbed by self-flexible loops provided at intervals ofaround 50 m. No shrink absorbing device such as bellows or the like is installed. Thesemeasures lead to no chance of LNG leakage in the tunnel.

(2) N2 gas filling into the pipeline space. After the construction of pipelines isfinished, the pipeline space is filled with dry nitrogen gas. This process is employed onthe basis of the idea that, even assuming the smallest amount of leakage, LNG gas shouldnot be mixed with oxygen, which might produce a combustible situation inside.

4.4 Maintenance and Monitoring of the Pipelines

(1) Maintenance of the pipeline. Since the pipes and pipe racks are made of stainlesssteel and the pipeline space is filled with nitrogen gas, maintenance work like corrosionprevention treatment is basically unnecessary. A ventilation facility is provided to replacethe nitrogen gas with air in the case that inspection or any other work is needed.

(2) Monitoring facilities and manway. Thermometers, gas detectors, oxygendensitometers, pressure gauges, and other types of monitoring instruments are installed inboth the pipeline space and the manway. The information from the instrumentation is sentto the central control room in the terminal and monitored around the clock. When anydevice detects an abnormal condition, the display panels in the central control roomautomatically indicate the location and the conditions of the trouble.

The manway provided in the tunnel is mainly used for the calibration and inspectionof the said monitoring devices. However, it also serves as an access to the berth when theweather is too rough for the shuttle boat, or as an evacuation route in case of emergencysuch as fire on the berth. The manway is supplied with sufficient air by blowers. Thepressure of the air in the manway is raised a little higher than the pipeline space so that noentry of nitrogen gas into the manway is possible.

4.5 Plan for Piping and Construction Work

(1) Pipe layout and installation. In order to reduce construction costs, the size of thetunnel should be minimized. For this purpose, the number of pipes should be reduced andthe most efficient method for construction should be employed.

Concerning LNG receiving pipe, 40” × 1 was installed in the tunnel, while 32” × 2were provided on the berth and terminal premises. Piping for industrial water supply waseliminated by using drinkable water from a water tank installed on the berth, because ofits limited usage. Sewage water produced by workers is processed by a high-performancewater treatment system on the berth, which eliminated the piping for this purpose. Thenumber of the pipes in the tunnel was 11 in a final plan and they were installed on fourdifferent levels.

5.1–11

There are no other examples or experiences with such pipe installation in a limitedspace. A mock-up test for pipe installation was conducted as shown in Figure 9. Themodel of the tunnel was made of steel and was 7.2 m in diameter and 30 m in length.Pipes were actually assembled and welding, inspection and insulation work were carriedout in the model. Based on the results obtained from the test, pipe-layout, pipeinstallation procedure, pipe installing devices and construction schedule were reviewed.

Figure 9. Mock-up Test of Piping in the Tunnel

Figure 10. New Insulation System for Pipeline in the Tunnel

(2) New insulation system for piping. Working space is limited in the pipe space andconstruction materials can be delivered only through the shafts. The temperature in thepipe space is maintained at about 10°C and there is no influence caused by ultravioletrays as is the case with facilities exposed to open air. Considering these conditions, a newinsulation system for the pipeline was developed as shown in Figure 10. In conventionalsystems, an exterior barrier is attached over the insulation. However, in the new system,exterior barrier is integrated with the insulation which eliminates the chance of generatingdew condensation after the cooling down of the air between the exterior barrier and theinsulation. In addition, it eliminates the work for mounting the exterior barrier on the site.Black rubber was used for exterior barrier instead of the conventional steel plate. It actsas a surface reinforcing material during foaming, and a vapor barrier for the insulation.Conventional insulation made of rigid polyurethane foam (PUF) is prefabricated in

5.1–12

lengths of 1 m in a factory and joint foaming is executed on site. However, in the newsystem, the length was extended to 2 m for pipes of 20” or smaller to improve workingefficiency on the site. In addition, high density PUF supports were integrated with theinsulation to improve workability, quality, and cost performance.

(3) Module construction system for pipes and frames in the shafts. The terminal shafthas an inner diameter of 16 m and a depth of about 36 m, while the offshore shaft hasmeasurements of 13 m and about 56 m respectively. In the shafts, stair cases, elevators,and pipes are installed. It is very difficult to carry out frame assembly, piping, andinsulation work in these deep and narrow spaces. For making the work more efficient,more economical and safer, and to obtain higher quality, a module construction methodwas employed. Framing, piping and insulation work were conducted outside the shafts ina horizontal position and were assembled into a module unit. The module was installedinto the shaft with a crane. Figure 11 shows the pipeline module for the offshore shaft aslifted by a 4,000-ton large floating crane. It is 55 m high and weighs 180 tons. Themodule was assembled at a factory, and then transported to the site. Figure 11 also showsthe pipeline module being installed into the terminal shaft.

Figure 11. Pipeline Module for Shaft (left; Offshore 55 m H, right; Terminal 35 m H)

(4) New automatic welding systems for large-diameter stainless steel pipes. The LNGreceiving pipe installed in the tunnel is a large-diameter thin stainless steel pipe with adiameter of 1,000 mm and a thickness of 9 mm. The length of piping materials is limitedto 12 m as they are delivered through shafts. To ensure efficient welding with highlyreliable quality in a limited space, two types of automatic welding system weredeveloped. One is a high-performance TIG welding system, which was used for thelength of 1,500 m from the terminal shaft to the intermediate shaft. The other is a plasmawelding system, which was used for the length of 500 m from the intermediate shaft tothe offshore shaft.

High-performance TIG automatic welding system. This system was developed byimproving conventional TIG automatic welding system for more stable quality andhigher reliability. The features of this system are as follows.

5.1–13

• Welding conditions are managed with a memory card in which the data are input off-line. Stable and high-quality welding is achieved independent from the skill level ofindividual welding operator.

• Work efficiency is improved since the system can continue welding from the firstlayer to the final layer.

• Automatic welding operation is simplified with tracking system and CCD camera.• Maintenance efficiency is improved by employing a welding head module.

In addition, this system employs a pneumatic internal clamp which has great holdingpower and has a prevention mechanism from oxidation on penetration bead, and agrooving machine which produces highly accurate groove shapes. Figure 12 shows thehigh-performance TIG automatic welding system.

Plasma automatic welding system. Plasma welding method enables deep fusion sothat higher efficiency and higher speed in welding are expected. However, because of thedifficulty in the control of the fusion metal in vertical upward welding, it was not used forall position welding on large-diameter pipes in the past. Many tests and studies wereconducted on the welding current, welding speed, and composition and quantity ofplasma gas to produce appropriate penetration shapes. Suitable angle of torch and gapbetween torch and base material were also examined closely. Based on the valuesattained from the studies, a new system of plasma automatic welding was developed.This system ensured higher welding speed and wider allowable offset between thewelded pipes owing to V-shaped as opposed to U-shaped grooves. Consequently, theefficiency and workability of welding have been improved.

In the actual welding, welding current and the quantity of plasma were controlled onthe time axis by computer. The conditions at startup and end of welding were alsocontrolled which led to sufficient quality. Furthermore, the system is equipped with aconstant arc length holding device and high-performance welding line tracking system.The welding was carried out on both sides, TIG from the outside and plasma from theinside. Figure 12 also shows a plasma automatic welding machine.

Figure 12. Automatic Welding Machine (left; TIG, right; Plasma)

5.1–14

5. 200,000-kl LNG UNDERGROUND TANKS

5.1 Outline

Since the construction of the first 10,000-kl LNG inground storage tank in 1970,sixty(60) LNG inground tanks, with a total capacity of 4,800,000 kl have beenconstructed in Japan. LNG inground storage tanks provide extreme safety in storing largevolumes of LNG. They facilitate a highly effective utilization of land area, and have theadvantage of being in harmony with the surrounding environment. These merits haveresulted in the recent construction of increasing numbers of LNG inground storage tanksin Japan. Figure 13 shows total capacities of LNG tanks in Japan. Figure 14 shows theexpanding trend for LNG inground storage tank capacity in Japan.

0

2

4

6

8

10

12

14

69 72 74 76 78 80 82 84 86 88 90 92 94 96 98 0 2Year

Cap

acity

(m

il. k

l)

I ngr ound; 60t anks , 4. 8 mi l . kl . ' 98. 5

Above Gr ound; 95t anks , 7. 0 mi l . kl . ' 98. 5

Tot a l

Figure 13. Total Capacities of LNG Tanks in Japan

Figure 14. Increasing Capacity of LNG Inground Tanks

5.1–15

A new LNG tank with a capacity of 200,000 kl will go into operation this July andanother is scheduled for the year 2000 at the Ohgishima Terminal. These tanks are buriedcompletely underground, for the first time, including their concrete roofs. This newconcept has been adopted as it enhances safety and the surrounding scenery. The tanksdemonstrate the specific features of inground storage tanks above mentioned and alsohave additional economical advantages. In addition, due to reduction in the number oftank facilities, simple and easy operation and maintenance are possible.

5.2 Ground Condition, Tank Configuration and Functional Components

The ground mainly consists of sandy permeable soil with an impermeable silty softrock layer more than 60 m below the surface. Deep excavation for tanks is carried out byinstallation of high strength super deep slurry walls into the soft rock as retaining and cut-off walls.

Figure 15 shows the structural configuration and functional components of the200,000 kl LNG underground storage tank. The side wall is made of reinforced concreteand features a cylindrical configuration, a structural advantage for inground structureswhich retains pressure from earth and ground water. The bottom of the tank is made of athick reinforced concrete slab to withstand high ground water up-lift. The domed roof isalso made of reinforced concrete which supports the weight of soil covering it. On theinner side of the concrete roof, membrane and insulation are installed. This type of roofhas been adopted for the first time for this LNG storage tank. Installed inside areinsulation, which maintains the cryogenic conditions of the tank interior, and a stainlesssteel membrane with a thickness of 2 mm for liquid/gas tightness. Surrounding the sideand bottom, a heating system is provided to control ground freezing caused by theLNG’s cryogenic temperature.

Figure 15. Structural Configuration of 200,000 kl LNG Underground Tank

5.1–16

Major technological points for underground tank construction are focused on theconstruction method of a large underground structure, its strength and water tightness towithstand high pressure from earth and ground water. Moreover, precise execution ofwork required as a civil-mechanical complex structure, liquid/gas tightness, thermaldesign, seismic design and so on must be secured. In order to realize large undergroundstorage tanks, a number of innovative technologies have been developed and applied.

5.3 Civil Engineering Technology

With regard to civil engineering, the following technologies have been developed andapplied. They include technologies for high-strength high-quality slurry wall, high-strength concrete side wall, earth-covered low-rise reinforced concrete domed roof, thicklarge scale reinforced concrete bottom slab and so on.

(1) High Strength Super Deep Slurry Wall. At the Sodegaura LNG terminal, TokyoGas developed an advanced super deep slurry wall for which tolerance in digging of 100m depth is 5 cm for 130140,000 kl LNG inground storage tanks. For the construction ofthe 200,000 kl underground tanks at the Ohgishima Terminal, high strength super deepslurry walls were used as retaining and cut-off walls during construction. They are 70 min depth, 1.5 m in thickness, 76.4 m in inner diameter and 50 Mpa in design strength.Excavation was executed from the ground surface to the bottom all at once and thebottom slab and side wall were built from bottom to top as in normal concrete lining.Figure 16 shows the high strength slurry wall, which retains external high pressure duringthe construction of the bottom slab and the side wall. At the bottom part, the slurry wallwithstands a very high ground water pressure of 0.6 MPa acting on its outer side.

Figure 16. Underground Space Retained by High-strengthSlurry Wall (D 80 mH 60 m)

5.1–17

(2) Application of High Strength Concrete to Side Wall. High strength concrete wasapplied to the side wall to ensure water tightness and reduction of construction cost. Thedesign strength of concrete was 59 Mpa and was twice the normal concrete strength. Thethickness of the side wall is reduced to 2.2 m. Many tests were performed concerningconcrete mix proportion, structural member strength, workability of concrete and so on.The concrete was fluidified with an admixture and exceptional water tightness wassecured.

(3) Reinforced Concrete Bottom Slab. To resist ground water pressure, the bottomslab is made of concrete with a thickness of 9.8 m and diameter of 72 m. The shearstrength of reinforced concrete is known to decrease relatively as thickness increases.Tokyo Gas conducted many studies and tests and verified the scale effect of the concreteshear strength. Loading tests were also conducted, applying water pressure below actualtank slabs. The resulting test data which are shown in Figure 17 have been highlyevaluated by researchers and engineers all over the world. The bottom slab was designedbased on the study results. Figure 18 shows the arrangement of reinforcement bars, with adiameter of 51 mm, under construction for the bottom slab. Concrete pouring for the slabwas made in two lots to reach the thickness of 9.8m.

Figure 17. Scale Effect of SearStrength of Concrete

Figure 18. 51 mm Diameter Re-barArrangement for Bottom Slab

(4) Reinforced Concrete Domed Roof Covered with Soil. The roof is made ofreinforced concrete with insulation and membrane on its inner side and is covered withearth. The thickness of the domed roof is 1 to 2.5 m from the center to the circumference.It supports 15,000 tons of dead weight and 40,000 tons of soil weight.

The rise ratio of the doomed roof (height/diameter) is 1/10 and is very smallcompared to 1/6 for conventional steel roofs. Tests of a 1/20 scale model which is shownin Figure 19, non-linear analysis, and design study were carried out for the purpose ofconfirming the safety against buckling of low-rise domes. Figure 20 shows the concretedomed roof before being covered with earth.

5.1–18

Figure 19. Model Test on Dome

Figure 20. Concrete Domed Roof

5.4 Mechanical Engineering Technology

Concerning mechanical engineering, the following technologies have been developedand applied. These include an advanced automatic welding system, insulation andmembrane system for concrete domed roof, high-strength insulation using alternate Freonfor the deep liquid level, a super long piping system for LNG pumps and so on.

(1) Advanced Automatic Welding System using CCD Camera. For the large sizeLNG underground tank at Ohgishima, a highly advanced automatic pulse welding systemfor membrane was developed and utilized. This improves weld seam tracking and settingof weld starting point. A vision sensor system with CCD camera was developed as shownin Figure 21. A CCD camera is attached, in an inclined position, to the welding torch tomonitor the shape of arc light image on the membrane. Because of the level differencebetween the two membrane plates, the arc image shows a particular edge shape. TheCCD camera processes the image and detects an accurate welding line which leads to theaccurate welding position. For setting of welding starting point, a potential-meter was

5.1–19

adopted. Around 95% of the total length of 36,000 m of the welding line wasautomatically welded and the quality and reliability of the membrane system wereconfirmed.

(2) Membrane for Concrete Domed Roof. Membrane is provided beneath the concretedomed roof to maintain gas/liquid tightness. This was adopted for the first time for anLNG tank. Special equipment was developed to put insulation and membrane on to theunder side of the roof and the work was successfully completed. Figure 22 showsautomatic welding for the roof membrane. Figure 23 shows the inner side of theunderground tank lined with membrane including the domed roof.

Figure 21.Automatic WeldingSystem for Membrane

Figure 22. Automatic Weldingof Roof Membrane

Figure 23. Inside of LNG UndergroundTank

5.1–20

(3) Insulation for Deeper Liquid Level. An insulation system of rigid polyurethanefoam (PUF) was adopted to accommodate greater liquid pressure. Studies were carriedout concerning the strength and creep characteristics, function at low temperature, anchorsystem to the concrete and so on. The foaming of the PUF was made by alternate Freon(HCFC141b).

(4) Super Long Pipes for LNG Pumps. LNG pumps are installed near the tank bottomwith the length of the piping for pumps being over 60m. Pipes shrink when cooled downand suffer from severe vibrations during earthquakes. Studies were required regarding thesupporting structure of pipes allowing the shrinkage caused by the temperature changealong with dynamic analysis for seismic design.

5.5 Consideration on the Technology of LNG Underground Tank

Construction of 200,000 kl LNG underground storage tanks has been successfullyproceeding at the Ohgishima LNG terminal. After the completion of the tanks, they weretotally embedded in the ground and cannot be seen. It appears that they were easily andsimply constructed. However, this is not true. The construction required a high level oftechnique and ceaseless endeavor. Indeed, the required technology is composed of anaccumulation of various technologies. Both civil and mechanical parts play veryimportant roles in supporting gigantic earth and ground water pressure, keeping watertightness, bearing high liquid pressure, being liquid/gas tight and being precise andaccurate as a civil-mechanical complex structure. Moreover, the LNG inground storagetank is an excellent system for storing large amounts of LNG, because of its intrinsicsafety, effective land utilization, simple operation and maintenance, and harmony withthe environment.

6. CONCLUSION

Tokyo Gas has made a continual effort in the development, planning, design,construction, operation and maintenance of LNG facilities, including LNG ingroundstorage tanks, over the past 30 years. The Ohgishima LNG receiving terminal is the resultof long term efforts and accumulated experience. In the construction of the terminal, thenewest and most advanced technologies were applied and new types of LNG receivingand storing facilities were realized. Consequently, all the facilities in Ohgishima showhigh levels of safety, harmony with the environment, very high effectiveness in landutilization and with operational personnel, and economy.

The Ohgishima LNG Terminal starts operation in October this year, moving rapidlyinto the 21st century.

REFERENCE CITED

Sakata K., Aizawa I., Komatsubara T., Genma T., “Construction of a New LNG Terminal– Ohgishima Project,” LNG 11, 1995