QUANTITATIVE RISK ASSESSMENT OF LNG ABOVEGROUNDTANKS BASED ON PAST OPERATING RECORDS OF LNG

REGASIFICATION TERMINALS AND LIFE CYCLE ASSESSMENT

MIYAZAKI Shinichi, Production Department, Tokyo Gas Co., Ltd.YAMADA Yoshihisa, LNG Engineering Department, Tokyo Gas Engineering Co., Ltd.

1. PrefaceIn 2001 Japan imported 54.1 million tons of LNG, which was slightly over half of the world’s

total LNG production of 100 million tons per year. About 60% of Japan’s LNG is stored inaboveground tanks and the rest is stored in-ground tanks at LNG regasification terminals in Japan.

In-ground tanks have excellent merits; high safety, effective land utilization and not disruptingscenery. On the other hand, in 1993, pre-stressed concrete (PC) was used for the first time in Japanto construct a new aboveground LNG storage tank. Currently almost all aboveground tanks in Japanare single containment, with structurally independent dikes. At present five PC tanks, with this newdesign, are in operation in Japan, and some are under construction or in the planning stage. InEurope and the United States aboveground PC tanks have been constructed since 1960s and was thecommon tank design of the 1980s.

Presented here is a Quantitative Risk Assessment (QRA) of LNG aboveground tank designsbased on the past operating records of LNG regasification terminals, along with a conventional singlecontainment tank design. The assessment was conducted after choosing two aboveground tankdesigns, among several, to study. One is the PC membrane tank without a structurally independentdike. It has a high, calculated safety probability. This design, however, has not been constructedanywhere in the world. The other is a Steel/PC double shell tank without a structurally independentdike. This design has been constructed only in Japan. A Life Cycle Assessment (LCA) was alsoconducted to compare both PC tank designs from the viewpoint of their global environmental impact.

2. Structure and Function of LNG Aboveground PC Tanks AssessedA summary of the structure and the function of both the PC membrane tank and the Steel/PC

double shell tank (both without a structurally independent dike) is given below. Also a conventionalsingle containment tank is described. In addition, in-ground tank construction is also referenced,though it is not assessed in this paper.

2.1. PC Membrane Tank without a Structurally Independent DikeA 2mm-thick stainless steel membrane serves as the gas and liquid-tight inner container. The

PC outer wall serves both as the structural member supporting the membrane and as a dike duringunexpected loss of LNG containment. Polyurethane foam (PUF) insulation installed between the PCouter wall and SS membrane transfers the LNG’s liquid pressure to the PC outer wall. The PC outerwall also supports the gas-tight, dome-shaped steel roof.

This tank design has been standardized by European Code, EN1473, “Installation andEquipment for Liquefied Natural Gas - Design of Onshore Installations”. However, every tank built sofar, however, has been constructed with the dike. There have been two 120,000 m3 tanks constructedin France and ten 100,000 m3 tanks, in South Korea. All of them are now in operation.

2.2. Steel/PC Double Shell Tank without a Structurally Independent DikeThis tank design consists of PC outer wall, perlite insulation and a structurally self-supported

steel inner container. For low-temperature service, this steel contains 9% nickel. The PC outer wallserves as a dike during unexpected loss of LNG containment. A liner generally made of metalinstalled inside the PC outer wall operates both as a moisture barrier and as a gas barrier to contain thenitrogen held in the inter-barrier space. Only by the primary container (not by the PC outer wall)supports the gas-tight, dome-shaped roof, made of low-temperature-service steels.

At present 5 tanks are in operation in Japan. The full containment tanks, similar to theSteel/PC double shell tank and without a structurally independent dike have been constructed in

Portugal, South Korea, India and other countries. There is a difference in the vapor control methodbetween a full containment tank and a Steel/PC double shell tank. The roof is supported by the PCouter wall in a full containment tank (as standardized by EN 1473) and this makes it possible to controlthe vapor in the case of loss of LNG containment.

In Japan, “The Preliminary Draft of Recommended Practice for PC LNG Aboveground Tank”was issued as the first technical standard for Steel/PC aboveground tanks by the Center of Promotionof Natural Gas in 1990. It was developed as the “Recommended Practice for LNG AbovegroundStorage” in August 2002.

GL

SS Membrane

RC Tank Body

2.4. In-ground Tank

GL

CS Outer shell

Inner Shell(9% Ni Steel or Al Alloy)

Dike

2.3. Single Containment Tank

GL

PC Outer Wall

SS Membrane

2.1. PC Membrane Tank 2.2. Steel/PC Double Shell Tank

GL

PC Outer Wall

Inner Shell(9% Ni Steel)

Figure 1 Comparison between PC Membrane Tank and Conventional LNG Tanks

2.3. Single Containment Tank with a Structurally Independent DikeThis tank design consists of an inner container made of low-temperature service steels (such

as steel containing 9% nickel or an aluminum-based alloy), perlite insulation and a carbon steel outerwall, which is surrounded by a structurally independent dikes. In case of an unexpected LNG spill, thesteel outer wall can not confine the leaked LNG; however, the surrounding dikes can prevent the LNGfrom spreading outside the immediate area.

This tank design was developed for LPG storage during the 1950s and is the most commondesign in the world. In Japan this design was first constructed in 1969 and since then about 100 tankshave been constructed, making up approximately 60 percent of those in service in Japan.

2.4. In-ground TankThis tank design has a 2-mm thick membrane as its primary container. The tank body, which

is installed below grade, is made of reinforced concrete (RC) because large earth and water pressureforces constantly act on side wall and bottom.

In-ground tanks have following excellent merits:1. The possibility of spilling large quantities of LNG onto the ground is almost ruled out since

the liquefied gas is stored underground. Auxiliary equipment and sometimes piping areabove ground but these contain limited quantities of LNG relative to the storage tanksthemselves.

2. Land is effectively utilized since storage is below ground level and no protective dikes areneeded around the tanks.

3. The dome-shaped roof does not disrupt the scenery of surrounding area, giving the

onlookers a sense of safety. For the advanced-class of in-ground tank that is constructedcompletely underground at Ohgishima terminal in Yokohama, Japan, the dome-shapedroof is covered with at least one meter of soil and so it is entirely hidden from view.

Approximately 70 tanks have been constructed in Japan since the first tank was constructed in1970. The in-ground tanks account for approximately 40% of LNG storage in Japan. They continueto win approval from local authorities for their high safety level/record. 14 units have been constructedin South Korea and in other countries.

2.5. Demanded Function for LNG Storage TanksThe functions demanded of a LNG storage tank are roughly classified into 3 categories:� the containment of LNG and boil-off gas,� withstanding the inner gas and liquid pressures, and� the prevention of liquid spread in case of an emergency LNG spill.Table 1 shows function of each component of PC membrane tank, Steel/PC double shell tank

single containment tank and in-ground tank.� In a PC membrane tank, the inner shell membrane operates as the primary container

only. The PC outer wall withstands inner gas and liquid pressure and prevents liquidspread in an emergency, acting as the secondary container.

� In the Steel/PC double shell tank, the inner shell, made of 9% Nickel Steel, functions notonly as primary container but also it must withstand the inner gas and liquid pressure.The PC outer wall operates as a secondary container, preventing liquid spread in anemergency.

� In a single containment tank, the inner shell operates in the same fashion as that in theSteel/PC double shell tank. The dike functions as a secondary container.

� For an in-ground tank, the inner shell membrane operates as primary container. Theouter PC wall together with earth pressure withstands the inner gas and liquid pressuresand intrinsically prevents liquid spread in an emergency by also acting as a secondarycontainer.

LNG Containment WithstandingLNG pressure Prevention of Liquid Spread

PC Membrane tank Primary Container Secondary ContainerSteel/PC Double Shell Tank Primary Container Secondary ContainerSingle Containment Tank Primary Container Dike (Secondary Container)In-ground Tank Primary Container Secondary Container and Earth

Table 1 Classification of the Functional Components of LNG tanks

3. Features of PC Membrane TankPC membrane tanks derive the following features from their structural characteristics listed

above.

3.1. High Economical Efficiency and Short Construction TimeSince its membrane is supported (through the PUF) by the PC wall behind it, a PC membrane

tank’s capacity is not limited by the strength of its thin-film membrane used as the inner shell.However, the capacity of a Steel/PC double shell tank is restricted by the strength of the maximumthickness of its 9% Nickel steel sheet. The larger capacity tank design enables the construction costper unit capacity to be cut. Because LNG storage facilities generally makes up approximately onethird of a LNG terminal’s cost, being able to cut the cost of the storage tanks is very important. Also,as capacity is increased, construction times of a PC membrane tank become much shorter comparedwith those of Steel/PC double shell tanks. For example in case of 140,000m3 tanks, a PC membranetank’s construction period is 25 months while Steel/PC double shell tank takes 34 months.

Figure 2 Comparison of Construction Period of LNG Tanks

3.2. High SafetyThe membrane is not a pressure part but a part of the liquid and gas tightness system.

Defects/flaws in the membrane can not grow rapidly because they instantly release the secondarystress that predominates on the membrane. Therefore, there is low probability of a LNG spill from amembrane tank. Even if it were to occur, the LNG would not spread out rapidly. Moreover it wouldbe easy to detect the gas’s escape, which has good gas permeability, because the tank’s PUFinsulation already fills the inter-barrier space (between membrane and PC outer wall) and thereforeinhibits the LNG from entering. The PC outer wall, operating as secondary container, could containany LNG spill within the tank structure. In addition, the inner shell, outer wall, foundation and roof canbe designed and constructed using proven existing technical standards and technology. The ability toconduct a hydrostatic test enables engineers to enhance the reliability of PC outer wall.

4. Quantitative Risk AssessmentA quantitative risk assessment was carried out in order to evaluate the safety of these two tank

designs. Additionally, the single containment tank design was also assessed for comparison.

4.1. Assessment ContentWhen doing a QRA, risk is synthetically evaluated from the viewpoints of probability of its

occurrence and the impact of the hazard. Then, in general, the following risk assessments areexecuted by QRA for the tank designs.

(a) Impact of Vapor LeakageEvaluation of the impacts and probabilities of damage to the roof, blow-off from one of thesafety valves and damage to the boil-off-gas (BOG) line (both from gas diffusion andradiant heat when ignited) are conducted.

(b) Impact of Liquid LeakageEvaluation of the impacts and probabilities from damage to the LNG receiving and feedinglines and from overfilling of the LNG tank itself (both from gas diffusion and radiant heatwhen ignited) are conducted.

(c) Impact of Tank RuptureEvaluation of the impact and probability of tank rupture are conducted.

In this report, the object is to evaluate the safety level of the PC membrane tank designcompared to that of the Steel/PC double shell and single containment tank designs. The above twocases, (a) and (b), do not exert a different result among these tank designs because assumed damageis irrelevant to tank designs. On the other hand, case (c) is dependent on tank designs and so is thefocus of this paper.

1214

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1412

34

28

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25

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35

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0 20,000 40,000 60,000 80,000 100,000 120,000 140,000 160,000Capacity （m3）

Con

stru

ctio

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riod

(Mon

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PC Membrane TankSteel/PC Double Shell Tank

4.2. Assessment MethodAlthough there are several other methods such as Event Tree Analysis (ETA) and Failure

Mode Effect Analysis (FMEA) that are also used in the field of QRA, FTA is employed in this report.FTA consists of both qualitative and quantitative analysis. Causes of the hazardous event, namely theloss of LNG containment, are deductively inferred in the qualitative analysis. The probabilities ofthese hazardous events based on the deduced qualitative analysis are then computed with failure ratedata, such as the probabilities of basic causes. Finally, a QRA of a tank system is completed bothwith the calculated probability of incidence and with an analysis of the hazardous consequences.However it is noted again that the analysis of the hazardous consequences is not addressed in thispaper adjusting the focus of the above object described in 4.1.

4.3. Cause for Loss of LNG ContainmentThe following four top events of possible causes for loss of LNG containment were determined

from qualitative analysis. Figure 3 shows Fault Tree of the four top events that could result in leakagefrom the tank rupture.

(1) Shortcomings of design and/or inspection (frost heave, welding failure, brittle failure,excessive vibrations, settling of insulation, wrong material(s) used)

(2) Natural disasters (earthquakes, near earth object (NEO) impact, lighting, excessive snowload, flooding)

(3) Failure in plant operations (under-pressure in the LNG tank, excessive pressure in the tank,procedure failures during a plant maintenance or in-tank pump drop, overfilling)

(4) Extra-ordinary events (Jet fire at the site, excessive blast load, missiles)The probabilities for above each four events were determined by references, or past operating

records of the terminal of Tokyo Gas, depending on the circumstances.

Shortcomings of design and/or inspection

P1

Natural disasters

P2

Failure in plant operation

P3

Extra-ordinary events

P4

Leakage from the tank

OR

Figure 3 Fault Tree of the Four Top Events

4.4. Investigation of Past operating Records of LNG Regasification TerminalsPart P3 of Figure 3 denotes the Fault Tree (FT) generated by evaluation of the loss of LNG

containment caused by failure in plant operations. Where possible, among the above four top events,the past operating records of Tokyo Gas were used to evaluate the events which led to LNG leaks dueto failure in plant operations.

That is to say, the operational records from actual plant are reflected in the following twofactors: rollover and excessive pressure in the tank during an unloading operation, which both cancause excessive pressure. Details are described below:

4.4.1. RolloverRollover occurs when four events occur simultaneously:� liquids with different densities are loaded into a tank,� unstable stratification occurs,� failure of detection (temperature measurement) and intervention, and� failure of pressure safety valves (PSVs).Then, the past operating records are reflected in the probability of failure of PSVs.

4.4.2. Excessive Pressure in the Tank during an Unloading OperationExcessive pressure occurs under conditions of:� overpressure protection failure, and� unloading operation ongoing.Overpressure protection failure occurs under simultaneous conditions of:� failure of PSVs� failure to remove boil-off-gas, and� failure of pressure control valve (PCV) to flare.Failure to remove boil-off-gas occurs under condition either of:� failure of first compressor and failure of take-over, or� common cause failure of compressors.Then the three probabilities: failure of PCV, failure of first compressor and failure of take-over,

and common cause failure of compressors are deduced from past operational records of the TokyoGas LNG regasification terminals.

Failure in plant operation

UnderpressureExcessivepressure

During anmaintenanceintervention

Overfilling

P3

OR

ORP28 P37 P38

Rollover

AND

During anunloadingoperation

Liquids withdifferentdensities

Occurrence ofunstable

stratification

Failure ofdetection

Failure of PSVs

P29 P30 P31 P32

Excessivepressure

protection fails

Failure ofoperation

AND

AND P33

Failure to removeboil-off- gas

Failure of PSVsFailure of PCV

to flare

Failure of firstcompressor and

take-over

Common cause of compressors

ORP32 P34

P35 P36

Figure 4 FT of the Loss of LNG Containment by Shortcomings in Plant Operations

4.5. ResultsTable 2 shows the probability of LNG leakage from each tank designs. QRA by FTA finds that

a PC membrane tank design (1X10-7 times/year) has the same safety level as the Steel/PC double shelltank design (1X10-7 times /year) and that it has higher safety level than single containment tank without

a dike (3X10-5 times /year).Among the top four events, extra-ordinary events and natural disasters exerted such large

influences (produced large leaks) that primary and secondary container would be simultaneouslydamaged. Then little probability distinction is made between two PC tank designs. On the otherhand, shortcomings of design and/or inspection and failure in plant operations gave such smallerimpact (small leaks) that only one of two containers is damaged. Then LNG leaks must beaccompanied by outer wall damage besides inner shell harm. In the PC membrane tank design, theprobability of shortcoming of design and/or inspection is higher than that of Steel/PC double shell tankdesign. The difference derives from the fact that the inner shell membrane is structurally incorporatedinto the outer wall of a PC membrane tank, while it is structurally independent in the Steel/PC doubleshell tank design. The probabilities difference, however, can be neglected because they are very low.Therefore, it is considered that the safety level of PC membrane tank is equivalent to that of a Steel/PCdouble shell tank.

For both the PC membrane and the Steel/PC double shell, natural disasters, dominated byfloods, are the critical events, while for a single containment tank, shortcomings of design and/orinspection, dominated by welding failure, is critical. This difference is derived from tank structurefeatures. In the case of leakage from inner shell, LNG can be confined by PC outer wall for both thePC membrane tank and for the Steel/PC double shell tank, while it would be spread out around acompromised single containment tank without a dike.

It should be noted, however, for a single containment tank design with a structurallyindependent dike around the tank that LNG spilled, due to loss of primary container is held within a dike.Then the probability of LNG spill due to shortcomings of design and/or inspection dominated by weldingfailure (3X10-5 times /year) is greatly reduced (3X10-9 times /year) because LNG leakage is not causedwithout simultaneous damage of a dike. Thus, the existence of a dike makes the critical events ofLNG spill over a dike (secondary container) replace shortcomings of design and/or inspection withnatural disasters dominated by floods. Furthermore the final probability of a LNG spill is largelydecreased (5X10-7 times /year). Therefore the tank design has the nearly same safety level as theother two tank designs, which shows us the significance of a dike for the single containment tankdesign.

The minimum allowable safety level is set as 1X10-6 times /year by reference to nuclear powerplant in the U.S. and land-use planning in the vicinity of major industrial hazard in U.K. Then, in orderto compare with the above allowable level, risks obtained for a member of public originally have to bestudied with considering hazardous impacts after LNG spill. However, hazardous impacts are notdescribed because the QRA is developed in this paper to clarify the relative safety differences amongseveral tank designs. In the strict sense probability of risks to an individual is obviously smaller thanthe analyzed probabilities, since LNG spill does not necessarily cause an actual hazard to public.However it is an adequate approach to assume that probability of risks to an individual is equivalent toLNG spill probability because it is quite a large hazard. A PC membrane tank and the Steel/PC doubleshell tank, even without structurally independent dike, can satisfy the required level (as can a singlecontainment tank with dike). The PC membrane tank design has the same safety level as theSteel/PC double shell, and has a little higher level than a single containment tank with a dike. Theseresults show that the structural dependence of the inner shell membrane on secondary container (PCouter wall) is little significant from the viewpoints of safety considerations.

EventTank Design

Shortcomings of Designand/or Inspection Natural Disasters Failure in Plant

OperationExtra-ordinaryEvents Total

PC Membrane 1X10-8 3X10-8 3X10-10 7X10-8 1X10-7

Steel/PC Double Shell 7X10-10 6X10-8 3X10-10 7X10-8 1X10-7

Single Containment *1(without dike) 3X10-5 4X10-7 3X10-10 7X10-8 3X10-5

Single Containment(with dike) 3X10-9 *2 4X10-7 3X10-14 *2 7X10-8 5X10-7

*1: LNG leak from a primary container (unit: times/year)*2: multiply as probability of unexpected loss of containment of dike

Table 2 Probability of LNG leakage from Secondary Container

5. Life Cycle AssessmentIn Life Cycle Assessment for an industrial product, the impact on the environment and natural

resources will quantitatively be assessed by studying all inputs, such as the required resources andenergy, and all output possibly impinging on environment during all stages of use: mining/extraction,manufacturing, operating and disposal. Here in this study, priority is given to comparison ofenvironmental impact between a PC membrane tank and the Steel/PC double shell tank.

LCA falls into two categories, the summation method and the inter-industry-relations method.The former needs detailed process event analysis because it quantitatively integrates material andenergy use as inputs, and environmental impact as its output, for each sub-process of production.The latter is called the Simple Appraisal Method because it estimates environmental impact based oninter-industry-relations table. A collection of statistic data concerning all industry in Japan, ispublished every five years by the Ministry of Public Management, Home Affairs, Posts andTelecommunications.

The LCA analytical tool as the summation method, JEMAI-LCA, was employed in this paperbecause insulation is such a special material that it was not listed on the inter-industry-relations table.The tool has been developed by JEMAI, the Japan Environmental Management Association. All typesof greenhouse gas emissions were converted to a carbon dioxide equivalent in order to access itsenvironmental impact from the viewpoints of global warming in this assessment.

Moreover, an additional study was executed in the use of state-of-the-art PUF foamed bycarbon dioxide as an alternative to using HCFC-141b, a hydrochlorofluorocarbon (HCFC). Thisenvironmentally friendly PUF was developed by Tokyo Gas four years ago and was installed in 2002during the construction of the 200,000m3 LNG underground storage tank at the Ohgishimaregasification terminal in Yokohama City.5.1. Assumed Tank Capacity for this Assessment

While an in-ground tanks with a maximum capacity of 200,000 m3 have been constructed, sofar, the 140,000 m3 tank size has been selected for LNG regasification terminals recently planned inJapan, since they can handle approximately 3X106 tons of LNG per year. Therefore, the 140,000m3

tank is assumed for this assessment.5.2. Limitation of the Components for this Assessment

In general, a LCA demands estimation of environmental impact for all components of LNG tank,such as primary container, secondary container, roof and foundation. This assessment, however, wasexecuted for the purpose of comparing the relative environmental impact among several tank designs.This enabled us to limit the LCA to those components (insulation, primary and secondary container)that make biggest differences in environmental impact. The foundation and roof can be neglectedbecause there is no differences in these components, between a PC membrane tank and a Steel/PCdouble shell tank.

5.2.1. Primary ContainerThe primary container of PC membrane tank is a 2-mm thick stainless-steel membrane, while

the Steel/PC double shell uses 9% Nickel-steel sheet that is about 30-mm thick.

5.2.2. Secondary ContainerSecondary containers of both tanks are made of prestressed concrete. In a PC membrane

tank, liquid pressure and larger thermal load constantly act on the secondary container, while they doso only in case of an emergency (LNG leak) in the Steel/PC double shell tank design. Rebar andprestressing tendons somewhat more extensively used in the PC membrane tank.

PC Membrane Tank Steel/PC Double Shell Tank

Primary Container Stainless-steel membrane2mm thick

9% Nickel Steel Sheetapproximately 30mm thick

Secondary ContainerPrestressed concreteConstant liquid pressure and thermalload

Prestressed concreteLiquid pressure and thermal loadonly in case of a LNG leak

Insulation 20cm thick PUF board 1m thick Perlite

Table 3 Materials Used During Construction

5.2.3. InsulationInsulation for a PC membrane tank is made of (approximately) 20-cm thick PUF board for both

the side and bottom, while the Steel/PC double shell tank uses 1-m thick perlite for the side and PUFfor the bottom.

5.3. Stage for AssessmentIn LCA, it is normal to estimate the environmental impact for all stages of a component’s life:

from mining/extracting the material out of the earth through to disposal of the manufacturing waste andthe material itself (at the end of its useful life) because Life Cycle stands for cradle-to-grave services.It is, however, enough to consider the stages from mining/extracting up to the end of operation,because a LNG storage tank has a long design life, 50 years. Moreover it is an appropriateapproximation to neglect the construction and operation stages because this assessment is executedfor the purpose of comparing the relative environmental impact between two tank designs and becauselarge environmental impacts are not made in these two stages.

It, however, should be noted that, no matter how effectively the conventionally foamed PUFwould be recovered and destructed by countermeasures (such as combustion in a rotary kiln), theHCFC-141b used to foam the conventional PUF, will be released during tank deconstruction and wouldadvance global warming at that time. The large impact due to the energy consumed by the kiln cannot be neglected either. The impact of HCFC-141b will be addressed in this paper.

5.3.1. Extracting the Material out of the EarthExtracting the raw materials out of the earth (e.g., iron-ore mining and oil drilling) were

considered.

5.3.2. ManufacturingEmission of greenhouse gas is computed by an inventory analysis based on an analysis of the

input-output specification, namely inventory, describing input material, energy, product and wasteduring the manufacturing process.

5.3.3. Transporting the MaterialsDuring this stage emissions were calculated based on transportation of the final products a

standard 500 km from their manufacturing plants to the construction site. In addition, ten times thegreenhouse gas emission per carrier is considered for insulation, both perlite and PUF, because it istoo light to take up the maximum authorized payload for the truck.

5.3.4. ConstructionEmission of greenhouse gas is neglected in this stage since there should be smaller energy

consumed by the construction machinery used to construct the two designs. Since time required forthe construction is less for a PC membrane tank, in a more complete analysis, this would show up also.

5.3.5. OperationNo relative distinction is made in this stage because insulation thickness is designed so that the

tanks have the same boil-off gas (BOG) rate. Therefore, emission of greenhouse gas is alsoneglected in this stage.

5.4. Estimation of Environmental ImpactGlobal warming was the focus of this study, although various other environmental impacts,

such as energy consumption, ozone depletion, atmosphere pollution, water contamination, and solidwaste generation could have been studied. Emission of greenhouse gas was computed concerningthe previously mentioned components during stages 5.3.1. through 5.3.3. listed in the previous sectionof this paper.

The quantity of each material used is described in Table 4.

Steel/PC Double Shell Tank PC Membrane Tank9% Nickel Steel Sheet 2,476.90 t 0.00 tStainless Steel 0.00 t 266.77 tConcrete 13,880.00 m3 14,160.00 m3

Rebar 3,090.00 t 3,210.00 tPrestressing tendons 530.00 t 520.00 tPerlite 1,046.29 t 0.00 tPUF 36.00 t 428.16 t

Table 4 Material Use for a 140,000 m3 LNG Tank

5.4.1. Per Unit Emission of Greenhouse Gas

(1) 9% Nickel Steel SheetComputing done by JEMAI-LCA

(2) Stainless SteelComputing done by JEMAI-LCA

(3) ConcreteRecommended value by LCA subcommittee of JSCE, Japan Society of Civil Engineering used.

(4) RebarRecommended value for electric furnace rebar by LCA subcommittee of JSCE used.

(5) Prestressing TendonsRecommended value for blast furnace hot-rolled steel by LCA subcommittee of JSCE used.

(6) PerliteComputing by JEMAI-LCA employing the value of limestone instead of perlite for exploiting and

import. The computed value is smaller because electricity for the grinder mill in plant was neglected.While coal oil, for on-site combustion, is considered, electricity (for other purposes) is neglected in thecomputing process.(7) Standard PUF

Unit emission is computed for conventional PUF foamed with HCFC-141b. It is calculated byJEMAI-LCA such that all HCFC-141b consumed during manufacturing process is contained andstabilized in the PUF.(8) PUF Foamed by Carbon Dioxide

The replacement of conventionally foamed PUF to carbon dioxide foamed PUF is acountermeasure to reduce the tank’s environmental impact. It was developed by Tokyo Gas and hasactually been installed in the 200,000m3 LNG underground storage tank at the Ohgishima regasificationterminal in Yokohama, Japan.

Unit Emission Unit9% Nickel Steel 2.34 t-CO2/tStainless Steel 4.24 t-CO2/tConcrete 0.3113 t-CO2/m3

Rebar 0.4693 t-CO2/tPrestressing Tendon 1.507 t-CO2/tPerlite 9.97 t-CO2/tStandard PUF 2.95 t-CO2/tCO2 foamed PUF 2.95 t-CO2/tHCFC-141b in Standard PUF 90.00 t-CO2/t

Table 5 Per unit Greenhouse Gas Contribution of each Component

5.5. Bottom-lineTable 6 shows the calculation results;

Extraction/miningthrough Manufacturing Unit Steel/PC Double

Shell Tank PC Membrane Tank PC Membrane Tankwith CO2 foamed PUF

9% Nickel Steel t-CO2 5,795.95 - -Stainless Steel t-CO2 - 1,131.10 1,131.10Concrete t-CO2 4,320.84 4,408.01 4,408.01Rebar t-CO2 1,450.24 1,506.56 1,506.56Prestressing tendon t-CO2 798.71 783.64 783.64Perlite t-CO2 10,431.51 - -Conventional PUF t-CO2 106.20 1,263.07 -CO2 foamed PUF t-CO2 - - 1,263.07Summation [1] t-CO2 22,903.45 9,092.38 9,092.38

Transporting t-CO2 450.71 319.12 319.12Summation [2] t-CO2 23,354.16 9,411.50 9,411.50

Considering effectof HCFC-141b t-CO2 - 38,534.40 -

Summation [3] t-CO2 23,354.16 47,945.90 9,411.50

Table 6 Global Warming Impact of the Designs

Summation [1] shows greenhouse gas emission from extraction/mining stage through themanufacturing stage. This result does not include the environmental impact such as disposal of theHCFC-141b, contained in the conventional PUF. The numbers for the conventional PUF and CO2foamed PUF are the same because the same amount of CO2-equivalent is embodied in the unfoamedcomponents for the PUF.

Next summation [2] shows greenhouse gas emission from the beginning up through thetransporting stage. Here we also see that the construction of a Steel/PC double shell tank would emitmore greenhouse gas than the construction of a PC membrane tank. It is quantitatively proved thatemissions of PC membrane tank are smaller. There was no difference at all between PC membranetank with conventional PUF and with state-of-the-art PUF because the HCFC-141b, is still stabilized inthe PUF.

Only the emission of HCFC-141b, in its disposal stage, is considered in summation [3]. Theenvironmental impact of PC membrane tank (with conventional PUF) is now larger than that ofSteel/PC double shell tank. It is possible, however, to minimize greenhouse gas emission by adoptingthe CO2 foamed PUF. Thus the environmental impact of a PC membrane tank is now smaller again.When CO2 foamed PUF is used, the environmental impact is only 48.7% of the Steel/PC tank and19.6% of a PC membrane tank made with conventional PUF.

The above LCA calculation shows that when CO2 foamed PUF is used, greenhouse gasemission of PC membrane tank (total embodied CO2 and CO2 equivalent for the component materials)is smaller than the other designs. Moreover the use of CO2 foamed PUF has enabled Tokyo Gas toeliminate the potential risk of greatly increased greenhouse gas emissions during the disposal stage.PC membrane tank with CO2 foamed PUF is ranked as an environmental-friendly LNG facility.

6. ConclusionThe high safety level of the PC membrane tank design and Steel/PC double shell tank design

have been proven (even without a structurally independent dike) by a Quantitative Risk Assessmentusing FTA, based on the past operating records of LNG regasification terminals. Also the LCAclarifies the inherently low environmental impact of a PC membrane tank with the CO2 foamed PUFdeveloped by Tokyo Gas.

From now on, in the century of the environment, PC membrane tank will join the mainstream of

LNG aboveground storage tank design, both in Japan and abroad, from the viewpoints of higher safety,higher reliability, lower cost, and their shorter construction times.

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