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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.