flexibilities of lng storage in lined rock cavern (lrc) with high operating pressure of lng... ·...

AIChE Spring Meeting: Flexibilities of LNG Storage in LRC with High Operating Pressure

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Flexibilities of LNG Storage in Lined Rock Cavern (LRC) with High Operating Pressure

Do-Youn Kim, Ph. D.

Process Engineer

Joseph H. Cho, Ph. D., P. E.

Director of Gas Tech. Center

Sang-Woo Woo

General Manager, Civil Engineering

Dae-Hyuk Lee, Ph.D.

Team leader of GSUC

SK Engineering & Construction Co., Ltd.

2010 AIChE Spring Meeting

10th Topical Conference on Gas Utilization

San Antonio, TX, March 21-25, 2010

ABSTRACT

Natural gas consumption is expected to grow significantly in the next

decades. The need for building LNG import terminals with significant storage

capacity is quite often a critical aspect due to restriction of land in the area of

interest and environmental constraints. A new concept for the storage of LNG in

underground mined rock caverns has been developed as very efficient in terms of

land occupation, environmental and visual impact at ground surface, safety and cost.

The concept consists of the combination of two well-proven technologies: the

storage of gas and liquid hydrocarbons in underground mined cavern and the

membrane containment system used for conventional LNG tanks and ocean

carriers. The advantages of underground LNG storage in rock caverns are the

following: Safety – storage less vulnerable to external hazards, Security – high

protection against terrorism, Footprint – very limited surface impact, Environment –

no visual impact, and Size – virtually no limitation of size.


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This paper describes the concept of storage system and the features of the

process. The location flexibility and various unconventional LNG transferring

methods to be allied to the Lined Rock Cavern (LRC) System have also been

discussed. The unique features of LNG terminal process have been presented with

reference to a 450,000 m3 storage and a 750 t/h of send-out capacity.

INTRODUCTION

General trend of the public is to enjoy using energy as they need, but not

allowing building of energy facilities. It is typically expressed as “Not in My Back

Yard (NIMBY)”. Recently, this trend has evolved into a more serious social

opposition to any development for example, when project developers advocate

infrastructure such as new roads, energy facility, power plants, etc. Currently, local

emotional opposition against project development is well presented by the term

“BANANA”, an acronym for “Build Absolutely Nothing Anywhere Near Anything (or

anyone)”. This term is often used to criticize the ongoing opposition of certain

interest groups to land development.

Every body needs energy everyday. However, if a development project of

energy infrastructure is shut down by local opposition, the next question will then be

“where to build energy infra”. Many LNG import terminals have not materialized due

to strong opposition from local people taken up to Capitol Hill.

If energy facility is far from the massive energy demanding area, energy

transport costs are going to be significantly high regardless of energy forms, be it

electricity, liquid or natural gas. Meeting the demand of public energy with low costs

and ensuring the safety of the energy facility shall be a primary goal of those

working for the energy sector.

In order to provide energy to the public safely, economically, and at the

same time mitigate the public emotional reaction to oppose the project, such as

NIMBY or BANANA, underground LRC LNG storage system has been developed.

This storage technology adopts two well proven technologies: Underground mined

rock cavern and membrane LNG storage.

Underground mined rock technology has been widely used for strategic

energy storage: crude oil, gasoline, LPG, etc. The membrane technology has been

used for LNG storage and LNG carriers. This new underground storage system can


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mitigate the public safety concerns which oppose the project development.

This paper discusses the concept of the lined rock carven system. The

possible location of rock carven system is directly governed by the available rock

mass and sometimes it might be far from the shore line. The longer unloading line

and its associated operation issues will be also discussed. This paper also presents

advantages of the flexible operating pressure of the storage system, which will be a

higher operating pressure than that of the conventional storage system.

DESCRIPTION OF LINED ROCK CAVERN (LRC) LNG STORAGE SYSTEM

Underground mined rock caverns are commonly used and safely operated

since many decades to store petroleum products like crude oil, propane and butane

either compressed or refrigerated. The attempts to store LNG in underground rock

caverns with a similar approach have not been deemed satisfactory due to large

boil-off rate and the low LNG temperature acting on rock wall being liable to

generate cracks in the rock mass. On the other hand, the storage of LNG using

aboveground tanks and in a limited extent using in ground tanks is now a well

proven technology.

The concept developed by SK E&C, Géostock, and Saipem-sa is a simple

combination of both the underground mined rock cavern and the aboveground tank

technologies. The underground storage is of particular interest towards reducing the

land occupation, enhancing safety and security aspects. This is also economically

attractive.

The concept consists of protecting the host rock against the extreme low

temperature and providing a liquid and gas tight liner (see Figure 1) using insulating

panels fixed on a concrete lining and a corrugated stainless steel membrane.

Similar containment systems are used in LNG carriers since 30 years without any

troubles. The thermal characteristics and thickness of the insulation is designed in

such a way to achieve allowable minimum temperature in the rock mass for the

design life of the storage. A boil-off rate around 0.05 to 0.1% per day is expected [1].

A dedicated water drainage system made of boreholes drilled from the surface

and/or dedicated drainage galleries installed around the cavern allows controlling

the hydrostatic pressure and the ice formation in the rock mass during the cooling

down process (see Figure 2). Process and equipment to operate the storage are


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similar to aboveground or in-ground tanks.

Fig. 1- Sectional View of LRC Containment System

Fig. 2 - Main Components of the LRC Storage System

CONCRETELINING

INSULATINGPANELS

SHAFT

CONCRETEPLUG

PIPING TOWER

STAINLESS STEELMEMBRANE

ROCKMASS

SHAFT

DRAINAGEGALLERIES

ACCESS GALLERIES(FOR CONSTRUCTION)

STORAGEUNIT

DRAINAGEHOLES


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ADVANTAGES AND ECONOMICS OF UNDERGROUND STORAGE

There are many advantages of underground storage in terms of safety,

security and environmental acceptability compared with aboveground tanks and in-

ground tanks. Underground storage is much safer in consideration of fire on plant or

decreased potential damages in case of industrial accident nearby. This is due to its

multi-component barrier with liner and ice ring. It is less vulnerable to earthquake

and typhoon. Regarding security aspects, underground storage can easily survive

acts of sabotage or terrorism.

Because there is no need of large reclaimed areas and less earthworks at

ground level, underground storage is environmentally friendlier, and eventually it will

become a better acceptable proposition to people located nearby. The other major

advantage is the minimum plot space requirement for an LNG terminal due to the

fact that the LNG storage is about 50 m underground. This represents a huge cost

saving especially in seashore areas where industries are already developed and the

limited available real estate is very expensive. It is also the case in areas whose

topography needs expensive reclaimed land.

Small galleries should be avoided wherever possible due to their poorer

capacity/area ratio. Geometrical studies show that underground storage in the form

of a gallery of around 20 m width by 30 m height cross section is the most favorable

in terms of cost versus rock behavior [2]. Moreover, mining technologies and

membrane containment system have such flexibility that unit storage capacity has

no limits. As Crude oil caverns are up to 4,500,000 m3 which are operating in Korea,

it is possible that volume of LNG lined caverns can also be designed to such

capacities. The comparative cost estimate between aboveground and cavern

storage is only the storage itself and its equipment. It does not take into account the

substantial cost saving which could be made, in the case of the cavern storage, for

the safety equipment (impounding basin, peripheral retention wall, fire fighting

systems, etc.) and possibly for the reduction in piping length and terminal plot area.

Moreover, reduction in operation costs, including maintenance cost is also would be

attractive towards cavern storage.

In 2008, a national forum for cost comparison among conventional

aboveground and in-ground LNG tank, and underground storage was held in Korea

with the participation of Ministry of Knowledge and Economics (MKE), Korea Gas

Corp. (KOGAS), Korea National Oil Corp. (KNOC), Korea Institute of Geosciences


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and Mineral Resources (KIGAM), experts of engineering consultants and reputed

academicians steered by Congress committee. Costs of varying stored volume from

200,000 m3 to 1,000,000 m3 with increment of 200,000 m3 were evaluated and

compared with reference price as of March 2006 in Korea. In all cases,

underground storage is the most economic over the stored volume of 300,000 m3,

and at 400,000 m3, cost for underground storage can be economical by 8%

compared to aboveground tank [3]. The relative relationship among storage types

are illustrated in Figure 3.

Intrinsically, the underground storage is cheaper than in-ground storage tank.

Moreover, operation cost for underground storage units are highly competitive as

compared to aboveground and in-ground tanks as systems like slab heating or fire

water are not necessary or can be tremendously reduced. Based on Korean

reference which has been implemented on crude oil storage by Korea National Oil

Company, operation cost of underground storage is 63% less than that of

aboveground one [4].

Fig. 3 - Comparison of Construction Cost


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TERMINAL PROCESS AND MAIN FACILITIES

The process philosophy and the main equipment needed to operate the

storage gallery and above ground facilities of the receiving terminal are discussed

below. The cryogenic caverns and the above ground process shall fulfill the

following basic function:

1) Cryogenic cavern

A. Store the required Liquefied Natural Gas (LNG) volume in a safe and

economical manner utilizing a dedicated containment system, designed

to limit heat ingress into the storage

B. Resist the loads generated by LNG, soil and seismic effects

C. Control the conditions of different spaces by use of dedicated

instruments

2) Above ground process

A. Unload the LNG from carriers berthed at the jetty to fill the storage

caverns

B. Pump the LNG from the cavern to the above ground re-gasification

process

C. Vaporize the LNG and take it to the required send-out temperature

D. Gather the Boil-off Gas (BOG) from the storage caverns and route it to

the ship or recycle it into the process

E. Provide the utilities required for the site operation

Terminal General Overview

LNG is transferred from the LNG carrier to the cryogenic caverns via

unloading system with the use of the LNG carrier pumps. Unloaded LNG is stored in

lined rock cryogenic caverns for an extended period of time. Each storage cavern is

equipped with removable submerged LP pumps, which deliver LNG to vaporization

system at the required send-out rate.

LNG from the storage caverns is routed to a recondenser vessel. It is then

routed to high pressure (HP) send-out pumps that increase the LNG pressure up to

the grid pressure. High pressure LNG is routed to vaporizers where LNG is heated


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and vaporized. Two types of vaporizers are installed in site: Fuel gas fired

Submerged Combustion Vaporizer (SCV) and Open Rack Vaporizer (ORV), which

derive the energy required for LNG vaporization from seawater.

BOG is naturally generated in the caverns due to heat ingress from

environment and gas displacement during filling. The BOG is compressed and re-

condensed into the LNG send-out stream in a recondenser vessel.

A flare or vent stack is provided to safely dispose of any emergency

hydrocarbon release.

Fig. 4 - Process Flow Diagram (PFD)


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HIGH OPERATING PRESSURE ALLOWS SITE FELXIBILITY

One of challenging issues on LNG storage is boil-off gas (BOG) from stored

liquid. This is due to heat transfer into the storage tank from surroundings. Most of

LNG tanks have a boil off rate in the range of 0.05 to 0.1%/day. The BOG rate of the

LRC can also be achieved by a proper thickness of insulation panels.

However, operation of the storage system should consider not only normal

BOG rate, but also the BOG generated during unloading operation. LNG will be

considered saturation point when the LNG carrier is arriving at the port of the import

terminal, LNG is pumped by the cargo pumps from the carrier to the onshore LNG

tanks through the unloading lines.

Enthalpy of the transferring LNG increases because of heat gain from

pipelines (mainly unloading line and other cargo and tankage area piping) and

unloading arms. Pressurized LNG by the cargo pumps also increases its enthalpy.

The increased enthalpy causes “Flash vapor” when LNG enters into the storage

tanks.

Length of unloading line(s) is quite site specific. If the tide difference is

considerable, the required length is long. In some cases port condition allow short

jetty and trestle line, resulting in a short unloading line.

The possible location of the LRC may be near the shore or far which is

depending on the available rock mass. It should be noted that with lengthy

unloading lines there will be a significant amount of the heat leak into the flowing

LNG.

During Unloading operation, the amount of BOG is governed by the following

factors:

Liquid displacement of unloaded LNG

Flash Vapor

General BOG because of heat leak through the tank roof and wall (bottom as

well)

Liquid pumping (negatively acting on the BOG rate)

A sudden change of barometric pressure


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If the flash vapor is well controlled, the amount of vapor generated during unloading

operation can be significantly reduced. This can be the answer to why High

operating pressure of the LRC can contribute in reducing the BOG handling system.

Maximum operating pressure and design pressure of LNG storage tanks is

confined by tank type associated with its design. However, LRC can increase its

design pressure up to 500 mbarg, which is the max design pressure specified in EN

14620 [5]. Table 1 summarizes the LNG tank operating pressure and design

pressure.

Table 1 – LNG Tank Operating and Design Pressure

Tank Type Operating Press Design Pressure

Single Containment 50-75 mbarg 120-150 mbarg

Double Containment 50-75 mbarg 120-150 mbarg

Full Containment 250 mbarg 290 mbarg

Membrane Tank 170 - 250 mbarg 190 - 290 mbarg

Lined Rock Cavern 400 mbarg 500 mbarg

One of the advantages of the full containment system, which has a higher operating

pressure than any other tank type (except the LRC) is to reduce BOG rate during

unloading operation. As a result, the size of the BOG handling system, such as

BOG compressors and the recondenser) can be significantly reduced.

As shown in Table 1, the LRC’s high operating pressure can provide enough

suppression pressure of flash vapor when LNG is entering into the tank. This benefit

allows flexibility of the LRC site location. Our study reveals that about 10 – 12 km of

LNG transfer lines does not impact BOG generation during unloading operation.

The benefit can also reduce its construction cost because storage site flexibility can

facilitate less expensive construction options and reduce construction infrastructure.

UNCONVENTIONAL UNLOADING LINES

When the possible LRC location is far from the shore line, the required

distance of the unloading line may be considerable. Then the cost for construction


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of the unloading lines will be significantly high assuming that the unloading lines are

built with the conventional methods: jetty and trestle, concrete support structure,

steel structure lined with fire proof concrete, etc. The design of conventional

unloading lines also requires consideration of LNG leaks along the lines. According

to the international codes and standards generally applied to the import terminal

facility design, impounding basin and associated leaked LNG path to this

impounding basin are mandatory.

In order to provide high flexibility in site location of the LRC, unconventional

unloading lines have been investigated. These include Vacuum Insulated Pipe (VIP)

and Pipe-in-Pipe system (PIP)

Design Configuration of VIP

The configuration that is shown in Figure 5 depicts the simplest VIP configuration.

The 16” LNG process pipe is jacketed with a 28” x 0.625” thick wall pipe that serves

as both carrier pipe and vacuum insulation enclosure. There are two (2) major

benefits in utilizing VIPTM over more traditional pipeline installations.

The vacuum jacket / carrier pipe is combined with the process pipe. The

factory assembled, insulated, and tested sections are sent to the field in 24.4

meter lengths ready for installation.

The fact that the vacuum annulus of the 24.4 meter sections are isolated

from each other gives this assembly a unique compartmentalization feature

compared to previously designed subsea piping systems.

This feature, a fundamental part of the VIP™ design, shares the principle of

compartmentalization which is common to marine vessels, confining potential

damage to only a small section.


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Fig. 5 - VIP™ with Combined Carrier and Vacuum Jacket

Design Configuration of PIP

The key technical features of cryogenic pipe-in-pipe systems which have

been identified as being fundamental and governing the successful implementation

of the concept are summarized below:

The levels of thermal insulation required for the longer unloading lines are

higher than those of the shorter above water conventional designs to limit

undesirable flashing, or boil-off, to the same low levels. Heat transfer issues

are therefore of fundamental importance to the design of cryogenic pipeline

systems.

The requirement for re-circulating the LNG during waiting periods between

ship visits indicates provision of at least one other cryogenic line.

All insulation materials and concepts available for cryogenic service require

dry operating conditions. This dictates adoption of pipe-in-pipe configuration

concepts with the provision of a steel outer pipe to provide a sealed annular

environment by excluding seawater from contacting the annular insulation.

All pipe-in-pipe designs require mechanically ‘locking’ or connecting the inner

and outer pipes together with stiff bulkheads at least at the two ends of the

pipelines, and also, sometimes at additional intermediate locations as well.

Figure 6 illustrates the simplest VIP configuration.


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Fig. 6 – Simplest Configuration of PIP

Advantages of VIP and PIP

The advantages of long subsea cryogenic pipeline systems over that of

conventional above water trestle based lines of the same length are:

Logistics and viability

Cost effectiveness

Environmentally more friendly

The major advantages of using PIP or VIP to replace conventional stainless steel

piping and insulation systems are:

Reduced schedule for installing the fully insulated lines at site. Installing

conventional polyurethane insulation systems require several steps (weld

pipes, install special pipe supports, pressure test, paint, install insulation

segments, install vapor barrier, install cladding, seal joints, etc.) which are

labor intensive and involve several crafts. PIP and VIP eliminate many of

these steps.

Elimination of high density PUF pipe supports and other special supports

required for conventionally insulated systems. These supports are often


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difficult to install and prone to failure.

Reduced cost due to the reduction in field labor.

Lower heat gain and lower thermal mass compared to conventional

insulation systems. This results in lower boil off which is particularly

important during initial cool down of the facilities and in between ship

loadings.

PIP and VIP can be buried, and can provided an extra level of containment in

sensitive areas such as road crossings, tunnels, culverts, or other areas

where the public may come near.

A much longer lifetime is expected for VIP and PIP, whereas conventional

insulation systems degrade over time due to water ingress and other aging

effects.

Disadvantages of VIP and PIP

The main disadvantages of replacing conventional stainless steel piping and

insulation systems are:

Limited experience in LNG applications for VIP (ALNG: 4” recirculation line,

Darwin LNG: 30” and 24” for LNG and BOG line (7000-ft), Egypt LNG: 30”

and 24” for LNG and BOG line, tank riser pipes and some of LNG run down

pipes.

Lack of similar design competition for either product. Both are essentially

single source, although it may be possible to develop alternates without

violating patents.

CONCLUSION

As compared with the conventional aboveground and in-ground storage

tanks, the use of LRC LNG storage system at the LNG terminals can be more

economical in terms of CAPEX and OPEX. In addition, it has also the advantage of

safety, security and environmental acceptability, compared to the conventional tanks.

Lined Rock Cavern LNG storage system can be realized in due course at

some countries which have suffered from the shortage of storage capacity of LNG

and seasonal extreme variation of domestic demand, and where industries are


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already developed and remaining vacant areas are small and expensive.

High operating pressure of the LRC LNG storage system provides benefits

to allow long unloading lines, which will be flexible in selecting the LRC project site.

High operating pressure benefits to reduce BOG rate during unloading operation,

and thus reduces Capital expenditure and Operating Cost by reducing BOG

handling facility.

Lined Rock Cavern LNG storage system could be a great candidate where

security of the energy facility is the national top priority because its storage system

is totally located in safe underground below 30 – 50 m from the ground.

Since the LRC is an intrinsically safe LNG storage system, it can mitigate

the public’s concerns on facility’s safety and have less local opposition to the project

development.

REFERENCE

1. SKEC, Geostock, and Saipem: “Taean LNG Receiving Terminal Project Pre-

Feasibility Study Report”, 2007.

2. H.Y. Kim, S.W. Woo, D.H. Lee, J. Cho, “Economical and Technical Challenges in

Lined Rock Cavern LNG Storage System”, AIChE Spring Meeting, Tempa, USA,

2009.

3. SKEC, Geostock, and Saipem: “Proceedings of International Symposium on

LNG Storage in Line Rock Caverns”, Seoul, Korea, 2004.

4. S.K. Chung, E.S. Park, K.C. Han, “Feasibility study of underground LNG storage

system in rock cavern”, presented at the11th ACUUS Conference, Athens,

Greece, 2007.

5. European Norm 14620, “Design and manufacture of site built, vertical,

cylindrical, flat-bottomed steel tanks for the storage of refrigerated, liquefied

gases with operating temperatures between 0 °C and -165 °C”, 2006.

flexibilities of lng storage in lined rock cavern (lrc) with high operating pressure of lng... ·...

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