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Investigation of
research needs
regarding the storage
of hydrogen gas in
lined rock caverns
Prestudy for Work Package 2.3 in
HYBRIT Research Program 1
FREDRIKJOHANSSON
JOHANSPROSS
DAVIDAMASCENO
JANJOHANSSON
HÅKANSTILLE
Technical report, 2018
KTH Royal Institute of Technology
School of Architecture and the Built Environment
Department of Civil and Architectural Engineering
Division of Soil and Rock Mechanics
SE-100 44, Stockholm, Sweden
Report number
TRITA-ABE-RPT-182
© Authors, 2018
Summary
The objective of HYBRIT RP1 is to explore and assess pathways to fossil-
free energy-mining-iron-steel value chains and thereby provide a basis for
industrial development activities and the necessary future transformative
change in this field. A large-scale storage capacity for hydrogen gas is an
important component of the proposed HYBRIT concept. Underground
storage in lined rock caverns provides a reasonable option: a large-scale
demonstration plant for storage of natural gas was constructed in Sweden
in 2002 and has operated safely since then. Considering that this lined
rock cavern facility was constructed for natural gas, the present report
investigates the current research needs to allow for underground storage
of hydrogen gas in such a facility. This will serve as a basis for the
research in Work Package 2.3 of HYBRIT RP1.
Studying the experiences from decades of Swedish and international
research and practice on the construction of underground gas storage
facilities, the conclusion is that the lined rock cavern concept seems a
reasonable way forward. In terms of rock engineering research, there are
currently no critical research issues; however, a development of a
previously proposed risk-based design framework for lined rock caverns
may further strengthen the ability to manage risks related to
underground gas storage facilities. The report identifies several potential
research questions on this topic to be further studied: development of a
risk-based design approach using subset simulation, the optimization
potential of the concrete thickness in the lining, and the effect of spatial
variation of rock mass properties on a location’s suitability for the storage
facility.
Additionally, the report identifies the potential effect of hydrogen
embrittlement on the steel lining as a critical research issue to ensure safe
storage of hydrogen gas in lined rock caverns. However, as this issue is
not related to rock engineering, but a material issue, it will not be covered
further in Work Package 2.3.
Keywords: LRC, lined rock caverns, hydrogen gas, gas storage
Sammanfattning
Syftet med HYBRIT RP1 är att undersöka och utvärdera möjliga vägar till
att göra värdekedjorna för energi-gruva-järn-stål fossilfria och därigenom
ge en grund för industriella utvecklingsarbeten och den framtida
omställningen. En viktig del i HYBRIT-konceptet utgörs av behovet av
lagring av stora volymer vätgas. Lagring i inklädda bergrum är ett möjligt
alternativ: en storskalig demonstrationsanläggning för lagring av
naturgas byggdes 2002 i södra Sverige och har använts sedan dess.
Eftersom denna anläggning konstruerades för naturgas, är syftet med
denna rapport att undersöka det nuvarande forskningsbehovet för att
kunna lagra vätgas i en sådan typ av anläggning. Detta kommer att utgöra
basen för det fortsatta arbetet inom delprojekt 2.3 i HYBRIT RP1.
Efter att ha studerat resultaten från svensk och internationell
forskning, samt erfarenheterna från byggnation av inklädda bergrum för
gaslagring, är slutsatsen att inklädda bergrum utgör ett rimligt alternativ
för lagring av vätgas. Avseende bergmekanik finns det för närvarande
inga kritiska frågeställningar. Däremot finns möjlighet att vidareutveckla
riskbaserade dimensioneringsmetoder för inklädda bergrum, vilket kan
stärka förmågan till god riskhantering vid byggnation av sådana
anläggningar. Rapporten identifierar flera forskningsuppslag inom detta
område att arbeta med inom delprojekt 2.3: utveckling av en riskbaserad
dimensioneringsmetod med hjälp av subset-simulering, studie av
optimeringspotentialen för betongliningens tjocklek, samt hur
bergmassans rumsliga variation påverkar en plats lämplighet för
anläggandet av ett inklätt bergrum.
Avseende materialfrågor finns dock en kritisk frågeställning för
underjordisk vätgaslagring: vätgasförsprödning av stålliningen ses som
ett möjligt problem och bör studeras vidare. Men eftersom detta inte är
relaterat till bergmekanik kommer det inte att studeras vidare inom
delprojekt 2.3.
Nyckelord: LRC, inklädda bergrum, vätgas, gaslagring
Preface
This prestudy was carried out at the Division of Soil and Rock Mechanics
at KTH Royal Institute of Technology during October 2017 to February
2018. The study was initiated as a part of the Work Package 2.3 of the
HYBRIT RP1 (Hydrogen Breakthrough Ironmaking Technology:
Research Program 1). HYBRIT is a joint initiative of the three companies
SSAB, LKAB, and Vattenfall with the aim of developing the world's first
fossil-free ore-based steelmaking route. HYBRIT RP1 was initiated in July
2017. A key issue is the possibility to store large quantities of hydrogen
gas to ensure continuous steel production when there is little production
in renewable energy. Lined rock caverns have been put forward as the
main alternative for storage of hydrogen gas. This is the main focus of
Work Package 2.3.
The lined rock cavern concept has been successfully demonstrated for
natural gas in Sweden through the construction of the Skallen
demonstration plant in Halland. Recognising the success of the Skallen
plant and the preceding decades of research, a project team and a
reference group with significant previous experience from work with
lined rock caverns were formed for the prestudy. The objective was to
identify the current research needs to be addressed to allow for
underground storage of hydrogen gas.
The project team consisted of the report authors, while the reference
group consisted of Robert Sturk, Skanska; Per Tengborg, Rock
Engineering Research Foundation (BeFo); Nicklas Simonsson, Vattenfall;
and Bojan Stojanovic, Vattenfall. Their contributions to the project work
are gratefully acknowledged.
We gratefully acknowledge financial support from the Swedish Energy
Agency.
Stockholm, March 2018
Fredrik Johansson, Johan Spross, Davi Damasceno, Jan Johansson, and
Håkan Stille
Contents
1. Introduction ......................................................................................... 1
1.1. Background ............................................................................................ 1
1.2. Objective of the prestudy ...................................................................... 3
1.3. Outline of report ..................................................................................... 3
2. General principles for underground storage of hydrogen gas ...... 5
2.1. External requirements on the facility ................................................... 5
2.2. Development of the LRC concept ........................................................ 6
2.2.1. Early development....................................................................................... 6
2.2.2. The Grängesberg pilot tests ........................................................................ 8
2.2.3. The demonstration plant at Skallen ........................................................... 10
2.3. General description of the LRC concept ........................................... 10
2.3.1. Structural components .............................................................................. 10
2.3.2. System for detection and collection of potential gas leaks ........................ 12
2.3.3. Design limitations related to temperature effects ...................................... 12
2.4. Rock mass – properties and behaviour ............................................. 13
2.5. Hydrogen gas – properties and behavior .......................................... 14
2.5.1. Hydrogen gas compared to other gases ................................................... 14
2.5.2. Technical aspects to consider for the storage system ............................... 17
2.6. Legal issues ......................................................................................... 18
2.7. State-of-the-art in underground storage of gas ................................ 20
2.7.1. Overview ................................................................................................... 20
2.7.2. Development in structural analysis methodology for gas storage ............. 21
2.7.3. Other storage concepts for hydrogen gas ................................................. 23
3. Design principles for pressurised caverns and gas-tight linings 27
3.1. Chapter overview ................................................................................. 27
3.2. Failure modes of the storage facility ................................................. 27
3.3. Large-scale failure of the facility caused by uplift ........................... 28
3.4. Failure modes in the rock during excavation ................................... 29
3.4.1. General considerations ............................................................................. 29
3.4.2. Experiences from the Skallen storage facility ............................................ 31
3.5. Failure modes of the cavern wall lining ............................................. 34
3.5.1. Low cycle fatigue of the steel lining ........................................................... 34
3.5.2. Local failure of the steel lining due to structural weakness ........................ 40
3.6. Groundwater management and gas leakage detection ................... 40
3.6.1. The drainage system ................................................................................. 40
3.6.2. Pre-grouting of the rock mass ................................................................... 41
3.7. Advantages of using a risk-based design approach ........................ 41
3.8. Risk management of design, construction and operation ............... 43
4. Discussion .......................................................................................... 45
4.1. General .................................................................................................. 45
4.2. Potential research questions for WP 2.3 ........................................... 46
4.2.1. Application of subset simulation for risk-based design .............................. 46
4.2.2. Optimization of concrete thickness in the lining ......................................... 47
4.2.3. Effect of spatial variation of rock mass properties on location suitability ... 48
4.3. Additional important research questions and design issues .......... 48
4.3.1. Hydrogen embrittlement of the steel lining ................................................ 48
4.3.2. Long-term behavior of the sliding layer ...................................................... 49
4.3.3. Case study of the application of risk management frameworks ................. 49
4.3.4. Control of temperature variation in the cavern ........................................... 50
4.3.5. Gas behavior in the rock mass and design of gas detection system ......... 50
5. Conclusions ....................................................................................... 53
References ............................................................................................. 54
INTRODUCTION | 1
1. Introduction
1.1. Background
HYBRIT (Hydrogen Breakthrough Ironmaking Technology) is a joint
initiative of the three companies SSAB, LKAB and Vattenfall with the aim
of developing the world's first fossil-free ore-based steelmaking route. In
traditional steelmaking from iron ore, carbon is a decisive component
required for the chemical reduction of the (oxidic) ore in the production
of (metallic) iron. Hydrogen replacing carbon as reductant is the most
sustainable and technically promising option for the iron and steel
industry.
The objective of HYBRIT Research Program 1 (RP1) is to explore and
assess pathways to fossil-free energy-mining-iron-steel value chains and
thereby provide a basis for industrial development activities and future
transformative change. The six work packages within the project address
the central research issues associated with a transition to hydrogen and
the resulting fundamental changes in energy systems, iron ore and steel-
making process technologies, as well as markets and policies (Figure 1).
One of the challenges when replacing coal with hydrogen in iron and
steel production is the envisaged need of a hydrogen storage facility. The
storage acts as an accumulator and buffer. This enables the iron and steel
production to have access to hydrogen at demand without requiring a
hydrogen production equivalent to the peak demand. The other
advantage is the ability to procure and store hydrogen when process
demand is low and electrical production is high. In turn, it is envisaged
that such facilities can enable the national electrical system to
additionally dampen future fluctuations, due to a presumed increase in
intermittent electrical production, e.g. wind power.
2 | INTRODUCTION
Figure 1. Schematic layout of the HYBRIT project, showing industrial process flows, work
packages and domains. As can be seen, a vital step from electrical energy to iron
& steel is the hydrogen production, distribution and storage. Storage is included in
WP2. (© Hybrit AB, with permission)
For Swedish conditions, previous research and current practice show
that underground storage in lined rock caverns (known as the LRC
concept) likely is a viable option for such an energy system. A lined rock
cavern currently stores natural gas at Skallen on the Swedish West coast.
The development of a storage concept for hydrogen gas therefore takes its
basis in the technology that was developed for the facility at Skallen;
though, other storage options are investigated in another work package of
the HYBRIT project.
The basic principles of the LRC concept are outlined in Figure 2. The
respective components are covered in the main chapters of this report.
INTRODUCTION | 3
Figure 2. The components of the lined rock cavern concept for storage of gas. (Figure by
authors).
1.2. Objective of the prestudy
The purpose of this prestudy is to investigate the current research needs
regarding storage of hydrogen gas in lined rock caverns. The objective is
to establish the most pressing rock engineering research issues that are to
be addressed in the execution of WP 2.3 in HYBRIT RP1.
1.3. Outline of report
The report is outlined as follows. Chapter 2 describes the general
principles of the LRC concept in light of both the external conditions that
the planned use of the hydrogen storage facility implies and the current
Swedish legislation that is applicable. An overview of the international
state-of-the-art in underground gas storage is also presented. Chapter 3
discusses the design principles that are available for large, lined
4 | INTRODUCTION
underground rock caverns, with focus on structural safety and risk
management concepts. Chapter 4 identifies and discusses the current
research need. Chapter 5 summarizes the main conclusions of the report.
GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS | 5
2. General principles for underground storage of
hydrogen gas
2.1. External requirements on the facility
Using the Skallen facility as a baseline, the present scope of the expected
external requirements on the LRC design is that the facility may need to
accommodate a geometric volume in the approximate range of 50 000 –
150 000 m3 and withstand an internal maximum pressure of at least
200 bar (20 MPa). The gas cycling frequency is expected to be less than 1
per week, if feasible, corresponding to approximately 1500–2000 storage
cycles for a life-span of 30–35 years. According to tentative assessments,
a design equivalent to the existing Skallen facility would approximately
suffice to supply a full-scale HYBRIT steelmaking facility with hydrogen
for about 50 hours. (For reference, the Skallen facility was designed for 1
cycle per month over 30 years, which adds up to close to 400 cycles in
total.)
The Skallen facility is 51 m high and 35 m in diameter, giving a
geometrical volume of 40,000 m3. The rock cover is 115 m. The geometric
shape is shown in Figure 3. The pressure range is 20–200 bar, giving the
facility a total gas volume of 10 MNm3. The working gas capacity is
approximately 90% and the cushion gas 10%. It takes 20 days to fill the
facility with gas and 10 days to withdraw (Mansson & Marion 2003).
Considering the wide target values regarding the external
requirements at this point, the requirements should be interpreted as a
starting point for the research project and not as technical limitations.
6 | GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS
Figure 3. Vertical section and basic data of the demo plant at Skallen in south-west of
Sweden. (© Johansson, 2003, with permission).
2.2. Development of the LRC concept
2.2.1. Early development
Worldwide, there are several concepts for storage of gas. The alternatives
include storage in existing geological formations, such as depleted oil and
gas fields; storage of liquefied gas in insulated tanks (natural gas can be
stored at –163°C); and storage in lined or unlined excavated caverns in
hard rock. The gas tightness of such caverns can, in principle, be ensured
by several different methods, as presented in Figure 4. The methods
categorized as being based on permeability control ensure gas tightness
by providing sufficiently low permeability around the storage, where one
option is the LRC concept. The methods categorized as being based on
control by hydrodynamic containment use water pressure to prevent gas
migration through the rock mass (Kjørholt 1991).
GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS | 7
With respect to Swedish conditions, there are no suitable existing
geological formations and insulated tanks are generally expensive to run,
which leaves the lined alternative or any of the unlined alternatives. Out
of these, the lined rock cavern (LRC) concept has been found more
suitable than the unlined alternatives, as the LRC concept is more cost-
effective for shallow depths. Additionally, it allows a rather large variety
in the geological conditions, although the weight of the rock mass must
prevent overburden uplifting (see section 3.3). The shallow location
implies, however, that the tightness of the storage must be kept at all
times, as the internal pressure is higher than the pressure of the
formation, i.e. the rock and the groundwater are not capable of
preventing the gas from escaping in case of a leak (Kovári 1993,
Johansson 2003).
Control by hydrodynamic containment needs a large groundwater
pressure to balance the pressurized gas; for a storage pressure of 15 MPa,
the equivalent depth would be 1500 m if natural groundwater pressure is
used. As a consequence, the storage volume needs to be very large, for the
facility to be economical. However, underground gas storages with
artificially pressurized water curtains have been successfully used in
Norway as air cushion surge chambers in headrace pressure tunnels at
hydropower plants. The Norwegian experience shows that a properly
Figure 4. Methods for ensuring gas tightness in underground storage facilities. (After
Kjørholt, 1991).
8 | GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS
designed water curtain eliminated all leakage for air pressures in the
tested range of 4–8 MPa (Kjørholt & Broch 1992). However, for
substantially larger pressures, e.g. in the same range as the pressure in
the Skallen facility, the required water pressure in the curtain would
cause hydraulic fracturing because of the relatively low in-situ stress in
the rock mass.
Note also that storing gas in rock caverns that previously were used for
storing oil is generally not possible, because such caverns have an
unfavorable shape with too large horizontal cross-sectional area. The
larger the horizontal cross-sectional area, the deeper the storage facility
has to be placed, considering the risk for uplift failure.
The LRC concept has to a large extent been developed in Sweden since
the mid-1980s, as an effect of the introduction of natural gas in the
Swedish energy system. Though, even before that, there was plenty
experience in constructing underground storage facilities for oil; see e.g.
Calminder & Hahn (1982). Main stakeholders in the research on natural
gas storage were major Nordic energy companies and contractors. Pilot
tests were conducted in Grängesberg to investigate the effectiveness of
different lining concepts, evaluate the effect of different pressures and
temperatures, as well as study the failure mechanism in the surrounding
rock and the consequences of leakage through the lining. The results of
the pilot tests were published in an extensive report by Johansson et al.
(1995), as well as in several conference papers, e.g. Stille et al. (1994). A
summary of the findings is presented in the following. A timeline for the
development of the lined rock cavern concept is presented in Figure 5.
2.2.2. The Grängesberg pilot tests
The Grängesberg pilot tests involved three different steel–concrete lined
caverns with 9 m high and 4.4 m diameter at 50 m depth in a fractured
granite formation. The first cavern (Room 1) was designed with a 0.4 mm
thick lining of austenitic stainless steel, but problems with the quality of
the lining welding method were observed, limiting the evaluation of this
cave because of leakage problems.
GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS | 9
Figure 5. Timeline for the development of the lined rock cavern concept in Sweden.
The second cavern (Room 2) had a 6 mm carbon steel lining with a
0.6 m unreinforced concrete lining and an asphalt sliding interface
between them. Tests in this cavern at pressures far above the in situ
conditions (52 MPa) and close to the compressive strength of the concrete
resulted in a maximum radial displacement of 5.65 mm in the rock mass,
and fracturing of the concrete; however, the cavern functionality was not
disturbed by this fracturing, because the concrete was designed mainly to
transfer the compressive load to the surrounding rock.
The last cavern (Room 3) was first equipped with a lining of 10 mm
thick fusion welded thermoplastic sheets installed on 0.3 m reinforced
concrete (without a sliding layer). Unfortunately, the lining failed during
the first water filling due to low ductility in the welds and brittle failure
behavior. The thermoplastic sheets were therefore substituted by 0.5 mm
stainless steel on the reinforced concrete. The maximum tested pressure
in this cave was 28 MPa with 3.2 mm maximum deformation and the
concrete lining stayed in very good conditions with only thin cracks being
observed.
The number of cyclic loads was over 200 for Room 2 and 91 for
Room 3 with a plastic strain-hardening deformation pattern; that is, the
deformability of the rock mass decreased as it deformed irreversibly.
10 | GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS
During these cyclic tests, opening and closing of rock joints were
experienced.
2.2.3. The demonstration plant at Skallen
After further studies of the technical details of constructing a lined rock
cavern for underground storage of natural gas, as well as the economic
aspects of the project, a demonstration plant was commissioned at
Skallen close to Halmstad in south-west of Sweden. The operation started
in 2002 (Glamheden & Curtis 2006, Mansson et al. 2006).
2.3. General description of the LRC concept
2.3.1. Structural components
The LRC storage system consists of two parts: the surface-bound facility
and the underground facility. The surface-bound facility consists of a
compressor station, heating and cooling equipment, piping, valves,
metering, and a control system, similarly to that of other underground
gas storages. The underground facility consists of one or several storage
caverns and a system of tunnels connecting the caverns with the ground
surface. The caverns are excavated as vertical cylinders with rounded top
and bottom (Figure 3). Typical dimensions are 35–40 m in diameter and
a height of 60–100 m. Typical gas pressures ranges between 15–30 MPa
(Johansson 2003).
The key element in a rock cavern for gas storage is the lining, which
consists of three structural components with different purpose: a sealing
layer to contain the gas in the facility, a pressure-distributing part to
transfer the load from the gas to the rock mass, and, of course, the rock
mass, which carries the load from the gas pressure. These structural
components work together in a complex interaction as the cavern is
pressurized (Johansson et al. 1995).
Having studied the behaviour of different lining concepts in the
Grängesberg experiments, a working concept was developed with the
following components (Figure 2), listed from the inside and outwards
(Johansson et al. 1995, Johansson 2003):
GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS | 11
An inner steel lining, which ensures gas tightness and bridges
minor cracks in the underlying concrete.
A sliding layer in between the steel lining and the concrete,
reducing the friction between these components. This layer
also provides some corrosion protection to the steel and
assists in sealing off the concrete surface in case of gas leaks in
the lining.
A concrete lining, which transmits the gas pressure to the rock
mass, distributing the deformations in a uniform way. It is
also needed to provide a smooth base for the steel lining. The
concrete lining is reinforced with a welded mesh to distribute
the tangential strain into many small concrete cracks.
A layer of low strength permeable shotcrete, which protects
the drainage system, allows water to seep between the rock
and the casted concrete to the drains, and reduces the
interlocking between the concrete lining and the rock surface.
A drainage system, which consists of perforated drainpipes
around the cavern. The purpose of the drainage system is to
reduce the water pressure against the steel lining when the gas
pressure in the facility is low. It is also part of the monitoring
system for gas leaks. Referring to confidential project reports,
Johansson (2003) states that the drainage system should not
circulate water under normal conditions, but be activated if a
gas leak has been indicated. This reduces the risk for chemical
and biological clogging. It is also favorable from corrosion
point of view to maintain stagnant conditions and avoid
introducing oxygen near the steel lining.
Surrounding rock, which carries the load from the gas
pressure. The occurring deformations in the lining will mainly
depend on the gas pressure, the deformation modulus of the
rock mass, the rock mass strength, and the size and shape of
the cavern. Additionally, the deformations may also be
affected by the presence of joints and weakness zones in the
rock mass and its in-situ stresses.
12 | GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS
The structural considerations of these components are further
discussed in chapter 3. Note that a US patent covers the design principle
of the lining; see Johansson (2002).
2.3.2. System for detection and collection of potential gas leaks
The facility is designed to be completely gas tight during its lifetime. This
is primarily ensured by a high level of quality control of the welds during
construction, directed equally on testing weld strength and weld
tightness. Additionally, the mechanical integrity of the lining is tested
with a water pressure test before the cavern is put into operation. Thus,
any leakage during operation of the facility would likely be caused by
corrosion of the steel lining or fatigue, which for the storage of natural gas
was deemed as possible but unlikely (see further discussions on the effect
of hydrogen embrittlement on steel in section 2.5.2).
A leakage detection system is integrated into the drainage pipes, which
also are used for groundwater, so should any gas leakage occur, it will be
detected by this system. Any leakage will increase the pressure in the
drainage system, which provides a warning signal. The leakage detection
concept was tested in the pilot plant in Grängesberg and the result
showed that leakage was clearly detectable (Johansson & Lindblom 1995).
The leakage detection system (including several independent leak
detection methods) was also successfully tested at Skallen. The design of
the drainage pipe system is further discussed in section 3.6.1.
2.3.3. Design limitations related to temperature effects
Reducing the pressure of a gas leads to temperature reduction of the gas;
however, the cavern walls must not be exposed to temperatures below
0 °C, because freezing may damage the concrete lining, if it is saturated
by water. For the Skallen plant, full ground water saturation of the
concrete lining was expected within 5–10 years of operation. This means
that restrictions must be put on the gas withdrawal rate to avoid
temperatures below zero, to avoid frost heave and internal frost damage
in the concrete.
GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS | 13
The LRC concept developed in Sweden also includes a circulation
heating and cooling system that can control the temperature in the
cavern. The main purpose is to increase the effective working gas volume;
see further details on this heating system in the US patent by Hall (2002)
(for which GDF Suez and E.ON Sverige AB currently are assignees).
However, the temperature can, if needed, be controlled through the
operation procedure only.
2.4. Rock mass – properties and behaviour
In this report, we assume that the rock mass is a jointed hard crystalline
rock mass. This type of rock can be exemplified with the Scandinavian
granite or gneiss of the very old Baltic Shield, which covers most parts of
Sweden; the main exceptions are the sedimentary rocks of Skåne, Öland,
Gotland, and the Scandinavian mountain range, which generally are
softer and weaker. The jointed hard crystalline rock of the Baltic Shield is
more favourable to the LRC concept than sedimentary rock types. In fact,
locations with such hard crystalline rock are generally feasible for an LRC
facility; the better rock mass quality, the higher gas pressure can be used.
Though, detailed analyses with respect to local heterogeneities in the rock
mass and anisotropic in situ stresses are always required before the
feasibility of a location can be evaluated.
The excavation of a large rock cavern will cause deformation to the
rock mass because of the surrounding gravitational (vertical) and tectonic
(horizontal) stresses. On a large scale, the deformations both during
excavation and operation are expected to behave analogously to the rock
mechanical concept known as the convergence–confinement method (the
ground reaction curve), which describes how the rock mass deformation
around the opening depends on the counter-pressure from, for example,
support measures. The analysis of such structural interaction is well
described in many textbooks; see e.g. Palmström & Stille (2010).
Affecting parameters on the deformation pattern of a highly pressurized
cavern are the rock mass deformation modulus, the rock mass strength,
14 | GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS
the in situ stress situation, the cavern shape, and the pressure of the
stored gas (Johansson 2003).
In addition to the large-scale analysis, the effect of local
heterogeneities in the rock mass (e.g. rock joints) needs to be considered
in the design of the storage facility. The reason is that such local effects
may strain the lining in addition to the large-scale deformations. Thus,
the design of the facility needs to consider both the general deformation
pattern of the rock mass and the effect of local heterogeneities in the rock
mass on the behaviour of the lining. In particular, largely anisotropic
horizontal stress conditions are unfavourable for the LRC concept, as
isotropic conditions provide the facility with a uniform prestressing of the
rock wall that is favourable in the pressurising of the facility. In addition,
largely anisotropic horizontal stresses may also make excavation more
difficult. The effect of anisotropy on the LRC concept is discussed in detail
in Johansson (2003).
2.5. Hydrogen gas – properties and behavior
2.5.1. Hydrogen gas compared to other gases
While hydrogen gas is a promising renewable and clean energy, it
requires specific safety considerations for its storage. Hydrogen is a gas in
ambient conditions and it is the lightest known molecule in the universe;
therefore, the tightness of the storage needs to be ensured. It is
undetectable by human senses (colourless, odourless and tasteless) and
sulphur or another odorant compound cannot be added, as in safety
demands for natural gas, given the drastic difference in the densities of
the different gases. On the other hand, hydrogen rises at almost 20 m/s
(in fact, it is 14 times lighter than air and 8 times lighter than natural
gas); this buoyancy effect is a safety advantage for rapid dispersion in an
open environment. The accumulation of hydrogen in a closed
environment may cause asphyxiation, even though it is not poisonous.
The flammable concentration of hydrogen varies within a wide range,
from 4 to 75%, against 5 to 15% of the natural gas; thus, the operational
area should be free of heat flames and sparks. Hydrogen has an optimum
GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS | 15
combustion at 29% concentration and its minimum ignition energy is
very low, 0.02 mJ, compared to natural gas, 0.29 mJ. On the other hand,
hydrogen carries less energy per volume than methane, achieveing 2.5 vs.
8.0 GJ/m3 at 20 MPa and 10.5 vs. 32.0 GJ/m3 at 80 MPa (Makridis
2016).
Makridis (2016) summarizes how hydrogen behaves differently than
most other gases and that classical gas theory may not be applicable. One
example can be observed during the decompression of hydrogen gas to
atmospheric pressure. For most other gases, with the exception of helium
and neon, the gas cools down as a result of expansion; however, hydrogen
gas heats up. This behaviour is named the Joule-Thomson effect and is
quantified by a top limit inversion temperature, for which gases change
from cooling down to heating up at expansion. For instance, at
atmospheric pressure, the hydrogen inversion temperature is about
202 K (–71 °C) while for the air it is above 700 K (427 °C); that is,
hydrogen gas needs to be cooled down below –71 °C to behave like most
other gases. This effect has been studied by several authors; e.g. McCarty
et al. (1981), Maytal & Shavit (1997), Woolley et al. (1948), and Johnston
et al. (1946).
The well-known ideal gas assumption is often used for simplicity in
thermodynamic analyses of gases: , where P is the gas pressure,
V is the gas volume, n is the amount of substance (in mole), R is the
universal gas constant, T is the gas temperature. However, for a real gas,
the compressibility factor, Z, should also be taken into account, such that
. Thus, Z is given by the ratio
, (1)
where is the density of the gas, and is the molecular weight of the
gas. (Consequently, Z = 1 for the ideal gas assumption.)
Maslan & Littman (1953) present compressibility factor charts for
hydrogen gas. The closer the gas is to phase change, by changes in
16 | GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS
Figure 6. Change in compressibility factor (Z) with pressure (P) for ideal gas, natural gas
(CH
4
) and hydrogen gas (H
2
) in different isotherms.
pressure and temperature, the more Z deviates from the ideal gas
behaviour. Figure 6 shows the change in compressibility factor with
respect to pressure for the ideal gas, natural gas, and hydrogen gas at
different isotherms. It can be observed that below 35 MPa and at
atmospheric temperatures (20–27°C), the Z of hydrogen is always greater
than 1, while methane has a Z less than 1. Therefore, as the
thermodynamic relationship of real gases shows that the density of the
gas is inversely proportional to Z, it can be expected that the density of
hydrogen gas will be less than the density of the natural gas for the same
temperature and pressure. The effect on the storage capacity of a rock
cavern is that the stored amount of hydrogen gas would be less than the
stored amount of natural gas in terms of Nm
3
, given that temperature and
pressure is the same in the storage facility.
Figure 7 shows a comparison of the density of hydrogen gas and
methane for different pressures and isotherms. Clearly, stored hydrogen
gas will have a much smaller density than methane for the same pressure
and temperature.
GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS | 17
Figure 7. Density variation of methane and hydrogen gas for varying pressure and
isotherms.
2.5.2. Technical aspects to consider for the storage system
The behavior of the steel lining is a crucial aspect of the facility, as it
provides the barrier against gas leakage. For natural gas, a relatively
thick, welded steel lining made of low-alloyed ductile carbon steel with
moderate yield strength was considered the most favorable option, based
on the Grängesberg pilot tests (see section 2.2.2). The substantial
thickness also allowed the steel lining to act as a formwork for the casting
of the concrete between the steel and the rock.
Hydrogen, however, differs from natural gas in the ability to
chemically react with steel, as hydrogen under some circumstances may
cause the degradation phenomenon known as hydrogen embrittlement.
Hydrogen atoms may promote localized plastic processes and enhance
crack propagation in the steel (Zheng et al. 2012). It is therefore critical to
assess the potential effect of any chemical reaction between the hydrogen
18 | GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS
and the steel lining, as degradation caused by hydrogen embrittlement
may reduce the life length of the lining. According to Kanezaki et al.
(2008), hydrogen embrittlement is expected to affect for example tensile
strength, ductility, and crack growth under static loading. Generally, it is
believed that austenitic stainless steel is less susceptible to hydrogen
embrittlement; therefore, steel with the austenitic crystalline structure is
believed to be a better candidate for hydrogen fuel cell applications.
However, hydrogen embrittlement may occur also for austenitic steel.
The performed research shows that hydrogen makes the fracture process
more complex; in fact, Zheng et al. (2012) regard hydrogen embrittlement
to be “one of the most controversial issues of all fracture related
phenomena up to now”.
Mechanical tests have proven crucial to be able to select construction
materials that have good compatibility with hydrogen. Therefore, to
support the needs of the hydrogen community, the Sandia National
laboratories have issued a report (San Marchi & Somerday 2012)
containing an extensive technical reference of hydrogen compatibility
with various materials. Additional references on materials testing are also
available in Zheng et al. (2012).
2.6. Legal issues
This section does not intend to provide an exhaustive cover of the
applicable Swedish laws, rules, and regulations for underground storage
of gas, but an overview of issues that may be relevant for further
investigation.
From a legal point of view, underground storage of hydrogen gas in
lined rock caverns is in Sweden governed by several laws. They can be
divided into two categories: the construction of the underground cavern
and the handling of the explosive gas.
Regarding the construction, the underground facility must be planned,
design, and constructed in accordance with the laws that govern any type
of civil engineering structure. This includes a ruling [miljödom] from the
Environmental court regarding any potential effects on the environment,
GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS | 19
a building permit [bygglov], and a design that satisfies the society’s
structural safety requirements in the Ordinance for planning and building
[Plan- och byggförordningen]. A concession in accordance with the Act
on Some Pipelines (1978:160) [Lag om vissa rörledningar] may also be
required.
Regarding the handling of explosive gas, the relevant laws are the
Flammables and Explosives Act (2010:1011) and the Flammables and
Explosives Ordinance (2010:1075), which deal with the import and
handling of flammable and explosive goods. The aim of the legislations is
to prevent and limit loss of life, injury, and damage to environment and
property as a result of fire or explosion. The Flammables and Explosives
Act also permits the Swedish Civil Contingencies Agency (MSB) to issue
detailed rules on this matter. The Swedish Work Environment Authority’s
Rules for pressure-carrying devices (AFS 2016:1) [Arbetsmiljöverkets
föreskrifter om tryckbärande anordningar] does not cover “devices for
underground storage that are intended to contain and/or control the gas
pressure”, according to its paragraph 2§h.
Currently, the MSB rule SÄIFS 2000:4 – Rules and general advice
regarding tanks, domes, rock caverns, and pipelines for flammable gas
[Föreskrifter och allmänna råd om cisterner, gasklockor, bergrum och
rörledningar för brandfarlig gas] is applicable to storage of hydrogen
gas in lined rock caverns. SÄIFS 2000:4 does not, however, provide
detailed rules; rather, it states requirements such as “Rock caverns that
contain air shall be put into operation in a safe manner” and “A
contingency action plan for emergency situations during filling shall be
developed”. Additionally, MSB’s rules on explosion hazardous
environment in management of flammable gases and liquids [Statens
räddningsverks föreskrifter om explosionsfarlig miljö vid hantering av
brandfarliga gaser och vätskor, SRVFS 2004:7] may also be applicable.
A separate MSB handbook is available for these rules [MSB Handbok om
tillstånd till hantering av brandfarliga gaser och vätskor].
Note that new rules currently are under development; though, the new
rules will not explicitly cover rock caverns, because such facilities were
not expected to be commonly built in the foreseeable future. Though, in
20 | GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS
the case of application for a permit for such a facility, the old rules (SÄIFS
2000:4) will likely be used for the rock cavern in this special case,
according to Lars Synnerholm at MSB (2017, personal communication).
Any external gas-related parts of the energy system (e.g. pipelines, etc.)
will however be governed by the applicable parts of the new rules, once
they have been issued.
Permitting public authority for handling of flammable gas is the board
that the local municipal authority (kommun) has appointed for this
purpose. In many municipalities, this is the local Emergency Services
(Räddningstjänsten). Because of the large scale of the project, the
permitting authority may seek consultation from MSB or other public
authorities in reviewing the application (Lars Synnerholm, 2017, personal
communication).
Additionally, Sweden has implemented the Seveso II directive
(96/82/EG) as the Act on preventive and limiting measures in case of
serious chemical accidents (1999:381) [Lagen om åtgärder för att
förebygga och begränsa följderna av allvarliga kemikaliolyckor]. This
implies that the facility requires a comprehensive safety report including
a plan for contingency actions and an emergency preparedness plan to be
submitted to the County board (Länsstyrelsen).
Regarding the operation of the facility, the Swedish Work
Environment Authority’s new Rules on use and control of pressurised
devices (AFS 2017:3) [Arbetsmiljöverkets föreskrifter om användning
och kontroll av trycksatta anordningar] may be applicable, as
underground storage facilities are not explicitly exempted.
2.7. State-of-the-art in underground storage of gas
2.7.1. Overview
As discussed in section 2.2, a concept for storing natural gas in lined rock
caverns has been developed in Sweden. However, to the authors’
knowledge, this concept has not previously been studied or tested for
storage of hydrogen gas. This literature review therefore covers studies
GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS | 21
both on the use of lined rock caverns for other gases and on experiences
from alternative storage methods for hydrogen gas.
Rutqvist et al. (2012) discuss a Japanese experiment for rock caverns
lined with concrete and synthetic rubber seal that showed acceptable
compressed air leakage for operational pressures of 4–8 MPa (cf. the
Skallen facility with pressure up to 20 MPa). Also, other LRC experiments
have been conducted in Korea as stated by Tunsakul et al. (2017);
however, these reports are not available in English. Regarding
Compressed Air Energy Storage (CAES), only two projects are under
operation today and they are related to unlined salt rock formations. One
of these storage facilities was built in 1978 in Huntorf, Germany, with
storage volume of 310000 m3 (Crotogino et al. 2001, Raju & Kumar
Khaitan 2012) and the other facility is in operation in McIntosh, USA,
since 1991 with storage of 500000 m3 (de Biasi 2009).
2.7.2. Development in structural analysis methodology for gas
storage
In terms of methods for structural analysis of underground storage of
pressurized gases, there has been some development during the last
decades, despite high investment costs and extensive laboratory
experiments. Additionally, numerical simulation tools are often used
along with field observations in order to overcome practical barriers.
Numerical design tools for general rock mechanic applications have
been well defined by Jing & Hudson (2002) and Jing (2003); though
since then, there has been a significant development of numerical
methods such as the Finite Element Analysis and the Discrete Element
Analysis toward efficient handling of complex three-dimensional thermo-
hydro-mechanical (THM) simulations. For modelling of underground gas
storage facilities, researchers have used different methods and they are
briefly covered in the following.
Xia et al. (2015) derived a simplified analytical solution for
temperature and pressure variations for thermodynamic analysis of CAES
systems. Another solution for CAES coupled THM analysis of jointed
hard rock was derived by Zhuang et al. (2014). Probabilistic method
22 | GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS
combined with numerical approximation was used by Park et al. (2013) to
design an LRC in South Korea to resist high internal gas pressure.
Park et al. (2012) proposed a two-dimensional numerical simulations
including coupled thermodynamic, multiphase fluid flow and heat
transport analysis that have investigated the LRC concept for CAES with
respect to gas leakage and energy-balance disregarding geomechanical
changes. Their results showed that a cavern at 100 meters depth and
operational pressure varying from 5 to 8 MPa achieves acceptable air
leakage for concrete lining with permeability 10-18 m2. The energy-balance
study showed that the energy loss in a daily pressure cycle depends on the
air pressure and heat loss to the surrounding media.
Rutqvist et al. (2012) used a thermomechanical two-dimensional
CAES model to compare an LRC that has a low permeability concrete
lining to an LRC with an impermeable thin lining. Fracture patterns were
estimated from results of strain and permeability; however, the model
was rather simplified disregarding mechanical interaction with and
within the lining, rock fractures and drainage zone, as well as the use of a
coarse numerical mesh. They concluded that the mass loss from using a
low permeability concrete is acceptable for CAES and a tight lining may
be more important to contain explosive fuels. In another study performed
by the same group (Kim et al. 2013), they used geophysical surveying to
characterize the excavation damaged zone (EDZ) and numerical
thermodynamic and geomechanical modelling to evaluate the EDZ
influence on the storage geomechanical stability.
Glamheden & Curtis (2006) report on how they used the FLAC code
for the numerical simulation of the Skallen facility. They aimed to analyse
the rock mass response to the excavation and pressurization of the cavern
in two different geometries: one horizontal profile and another vertical
profile of the storage. However, the conclusion was that the two-
dimensional assumption was unrealistic for the cylindrical storage and
that three-dimensional models are necessary.
Other two-dimensional numerical analyses were conducted by Kim et
al. (2012, 2013) and Rutqvist et al. (2012) with the linking of two
established codes (TOUGH-FLAC) applied to CAES in a LRC. Zhuang et
GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS | 23
al. (2014) presented numerical models with a coupled THM analysis for
unlined caverns that could represent better the physical processes
involved in the LRC concept; however, several simplifications and
assumptions were necessary due to the computational complexity of the
problem.
2.7.3. Other storage concepts for hydrogen gas
Züttel (2003) presented a review on hydrogen storage methods. Among
all hydrogen storage concepts, the most commonly used is the
compressed gas cylinder at surface level. The cylinder must stand
pressures in the order of 20 MPa; therefore, its material composition
should have very high tensile strength, low density and be inert to
hydrogen.
Storing compressed gas in cylinders involves severe safety concerns
due to the highly flammable characteristic of the hydrogen gas and high
operational pressures. The gas is compressed using piston-type
compressors and the work required for that is high due to the
temperature change. The method is well established, but high gas
pressures are generally required, considering the relatively low hydrogen
gas density.
Higher volumetric densities of 70.8 kg/m3 are achieved for liquid
hydrogen at atmospheric pressures (compared to <40 kg/m3 for the
compressed gas at 80 MPa of pressure); however, the hydrogen
temperature must then be decreased and maintained below its critical
temperature of –252 °C. Constant heat leakage from the cryogenic tank
narrows down the usage of this method to cases in which the hydrogen is
consumed after a low storage period, such as for aerospace applications.
Other methods are the storage of hydrogen by surface interaction
within carbon nanotubes (which is known as physisorption or physical
adsorption) and within the molecular structure of metal and complex
hydrides. In addition, hydrogen can be stored in the molecule of water
(H2O) and released by chemical reaction with sodium (Na); however,
although these are established technologies, they still require
development for industrial application (Züttel 2003). Other concepts are
24 | GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS
currently under development for the hydrogen storage on the molecular
level (Rosi et al. 2003, Schmitt et al. 2006, Duriska et al. 2009).
Although compressed gas tanks have been the most popular method of
storing hydrogen, the cost for large-scale storage would be substantial.
Geologic storage of hydrogen gas in salt caverns has been proposed as a
cheaper and sufficiently safe solution. The impermeable characteristic of
salt caverns makes them excellent for storing gas; however, these
formations are not available in most geographical locations.
Currently, two salt caverns are being used for this purpose in Texas,
USA, (580000 m3; Leighty, 2008) and three small ones in Teeside, UK
(150000 m3 each; Crotogino and Huebner, 2008; Panfilov et al., 2006).
Hydrogen gas can also be stored in other types of geological storages
rather than salt caverns, e.g. as 50-60% H
2 town gas (Fasanino &
Molinard 1989, Panfilov et al. 2006) and helium have been stored
successfully in aquifers (Tade 1967). Large-scale salt caverns related to
gas fields and aquifers have been used as hydrogen storage in Turkey
(Ozarslan 2012).
According to Lord et al. (2011), the popularity of LRC is expected to
increase with increasing demand for gas storage worldwide and due to its
versatility in implementation. Lord (2009) and Lord et al. (2010, 2011,
2014) conducted robust economic and deliverability studies for large-
scale storage of hydrogen comparing salt caverns, depleted hydrocarbon
reservoirs, aquifers and hard rock caverns. They developed the Hydrogen
Geological Storage Model (H2GSM) simulator, which is used to analyse
the cost per kg of hydrogen stored for the geological storages in USA. The
results in Lord et al. (2014) showed that depleted hydrocarbon reservoirs
and aquifers are more attractive economically (US$1.23–1.29 /kg H2). It
was observed that the location of a given city with respect to geological
storage formations had significant influence on the estimated hydrogen
cost because of storage volume and transportation logistics. The current
analysis did not account for the possibility of increasing the number of
cavern cycling per year and that would make the salt cavern the most
economic option (currently estimated as US$1.61/kgH2). Lined hard rock
caverns, on the other hand, are generally able to be cycled multiple times
GENERAL PRINCIPLES FOR UNDERGROUND STORAGE OF HYDROGEN GAS | 25
per year and that would decrease the calculated levelized cost of
US$2.77/kgH2 for the storage; however, this decrease was not estimated.
Lord et al. (2014) suggested that an update of this simulation could
include additional parameters such as dehydration units and steel liner
costs. Furthermore, it was proposed to consider lined caverns in soft rock
for future studies, as they would be easier and cheaper to excavate, while
they could be operated in a similar fashion to salt caverns.
DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS | 27
3. Design principles for pressurised caverns and
gas-tight linings
3.1. Chapter overview
With the structural components as described in section 2.3.1, a lined rock
cavern for gas storage has three main aspects that need to be considered
in the design work. These main aspects are discussed in this chapter:
establishment of sufficient depth below the ground surface to avoid large-
scale uplift failure (section 3.3); design of temporary support to be
installed during the excavation of the cavern (section 3.4); and design of
the gas-tight and pressure-distributing lining (section 3.5).
Additional key aspects in the design work concern management of
groundwater and gas leakage (section 3.6), the use of a risk-based design
approach (section 3.7), and applied risk management procedures (section
3.8).
3.2. Failure modes of the storage facility
In principle, the depth below ground surface of the cavern may affect the
structural integrity of the storage facility in three ways, according to
Johansson (2003):
1. Global failure of the rock mass, caused by too large uplift
pressure from the stored gas, so that it exceeds the vertical
resisting forces of the overburden.
2. Rupture of the steel lining caused by too large, one-time
deformation of the surrounding rock mass, as it is loaded by
the gas pressure.
28 | DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS
3. Low-cycle fatigue of the steel lining for caverns located at such
depth where the influence of the free ground surface will
enlarge the strain range in the steel lining.
As discussed by Johansson (2003), these failure modes could be
considered to be different stages of the same phenomenon. The global
failure of the rock mass (failure mode 1) must therefore have been
preceded by a rapid increase of the lining strain (failure mode 2 and 3).
How to select a sufficient depth with respect to uplift is further discussed
in section 3.3.
In addition to the first three failure modes, Johansson (2003) also
states that even at sufficient depth, structural failure may also occur
under the following two circumstances.
4. Low cycle fatigue of the steel lining for caverns located deep
enough to avoid the influence of the free ground surface.
5. Local failure of the steel lining because of some structural
weakness. Causes can be a local weakness in the rock mass,
the concrete, the welds, or possibly pipe lead-throughs.
Corrosion may also be a causative factor.
It should be noted that structural failure here is defined as something
that causes a measurable gas leak such that the facility is no longer
sufficiently safe or economically justifiable to operate. The reason is that
the LRC concept is very robust, so that failure in one structural
component not necessarily implies failure of the gas tight lining and
subsequent gas leakage.
3.3. Large-scale failure of the facility caused by uplift
Referring to the global failure of the rock mass, caused by too large uplift
pressure from the stored gas that exceeds the vertical resisting forces of
the overburden, some simplified analytical methods are available to
assess the resisting forces (Johansson et al. 1995). One of them considers
the resistance as the weight of a rigid upside-down rock mass cone, where
the cone tip has been cut off where the cross-sectional diameter equals
that of the storage facility. An overview of some simplified models for
DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS | 29
ground uplift is presented in Figure 8: the log-spiral model, which is
based on pullout tests of soil anchors; the rigid cone model; and the
straight failure-plane-geometry model, as compiled in Tunsakul et al.
(2017).
In addition to the analytical methods discussed above, it is also
possible to perform numerical analyses to assess the resistance against
uplift. In these analyses the pressure in the rock cavern can be increased
until a complete plastic state of the rock mass above the cavern is
reached. At this stage, the deformations rapidly increase for small
increases of pressure in the cavern.
Johansson (2003) reports that a general methodology to assess the
safety against uplift was developed for the demo plant in Skallen and
presented in a confidential project report. The general recommendation
was that the cavern should be placed deep enough to make the influence
from the ground surface on the structural behaviour negligible.
3.4. Failure modes in the rock during excavation
3.4.1. General considerations
In addition to the failure modes of the facility, the general failure modes
of rock masses are relevant to analyse in the design of the facility
Figure 8. Different models for ground uplift evaluation above the silo-shaped pressurized
storage in the past. (© Tunsakul et al., 2017, with permission from Springer).
30 | DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS
to enable a stable cavern before the lining is installed. Such failure modes
are described in most basic textbooks on rock mechanics, to which we
refer for further details; see e.g. Palmström & Stille (2010).
In jointed hard crystalline rock, the most common failure mode is
block instability. The strike and dip of the joints of the rock mass create
blocks of different shape and volume. Depending on the stress state
surrounding the cavern and the characteristics of the rock joints in terms
of frictional resistance, different degree of support is required. The
support usually consists of rock bolts combined with shotcrete, where
rock bolts are used to secure larger block volumes while the shotcrete
stabilizes potential loose block between the rock bolts.
In Sweden, the initial stresses in the rock mass are rather high,
especially the horizontal ones. For example, at a depth of 100 m,
horizontal stresses in the order of 10–15 MPa are common, while the
vertical stresses are roughly around 3 MPa. Due to the shape of the
cavern, its excavation results in stress concentrations near its boundary.
Thus, it is necessary to analyse the expected stresses and strains in the
rock mass around the cavern and compare them against the strength of
the rock mass. If local overstressing of the rock mass occurs, it can result
in need for additional support. In rock mass of very good quality the
strength could be sufficient, but other types of failure modes related to
the strength of the intact rock, such as spalling, might be necessary to
consider. Spalling implies that thin fragments of rock with time detach
from the boundary of the cavern. Other potential instability problems
related to high stresses in competent rock include rock burst, where rock
blocks with high velocity are pushed out of the cavern wall due to high
stress conditions in the rock mass. If conditions for spalling or rock bursts
exist, this has to be accounted for in the design of the rock support.
The relation between horizontal and vertical initial stresses together
with the diameter of the cavern also governs the height of the
compressive arch that keeps the roof of the cavern stable. In the case of a
low horizontal/vertical initial stress ratio, the height of the compressive
arch might exceed the arch of the cavern roof, resulting in unstable
volumes of rock mass in need of support. Another potential failure mode
DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS | 31
concerning the arching stability of the cavern roof is sliding along rock
fractures, which occurs when the direction of the compressive arch in the
rock mass is directed at an angle near parallel to the rock fracture.
Analyses are therefore needed to verify that the arching stability of the
roof is sufficient; if not, adequate support is needed.
The aforementioned failure modes imply that information is needed
on the initial stress state in the rock mass, the cavern shape, the
orientation of the rock fractures and their frictional resistance, together
with the rock mechanical parameters describing the strength and
deformation characteristics of the rock mass, in order to perform a design
of the necessary rock support needed to stabilize the cavern. However, the
rock support only needs to be designed for the time period until the lining
is installed; thus, there are no long-term requirements.
3.4.2. Experiences from the Skallen storage facility
Glamheden & Curtis (2006) report that the rock support at the Skallen
facility was designed with the Q-system (Barton et al. 1974), although the
design was verified with numerical modelling. The cupola was supported
by fibre-reinforced shotcrete and systematic bolting. The cavern walls
were first covered in unreinforced shotcrete, to which fibre-reinforced
shotcrete and systematic bolting were added. The bolts aimed to prevent
large slab failures caused by the dominant north–south vertical joint set.
Glamheden & Curtis (2006) state that the amount of support was
considered conservative with respect to the present rock mass quality;
however, considering the potential impact of large-scale failures and the
considerable wall height, which limited the access to the upper walls, the
support effort was deemed reasonable.
A possible construction sequence of a lined rock cavern for gas storage
in a hard crystalline Scandinavian rock mass is also described in
Glamheden & Curtis (2006). An informative video clip is also available on
Youtube (Johansson 2012). We only provide a very brief outline of the
procedure in the following.
All excavation is made with conventional drilling and blasting. The
excavation sequence is presented in Figure 9.
32 | DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS
Figure 9: Vertical section of the cavern showing the steps and levels in the excavation
sequence at Skallen. Ground surface on level +135 m. (Reprinted from
Glamheden and Curtis, 2006, with permission from Elsevier).
First, access tunnels were excavated to three levels in the facility.
Starting at the top (I), a pilot hole for the vertical shaft to the ground
surface was bored. Then, the cavern was excavated, advancing in two
spirals along the edges from level +8 m up to the top of the cupola (II and
Figure 10), then excavating from the upper level to the lower level, using a
central shaft (VI) for loading and hauling out the rock (Figure 11). Lastly,
the bottom was excavated (X).
At Skallen, the lining was constructed by using the steel layer as a
formwork for the casting of the concrete. This implies that the steel lining
has to be thick enough to carry its own weight before the concrete lining
has been casted.
The experience from the construction of the demonstration plant
Skallen also shows that the constructability needs to be carefully assessed,
as the steel welds need to be inspected from both sides to ensure gas
tightness and the concrete reinforcement must be possible to put in place
between the rock wall and the steel plates in an efficient manner. Using
DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS | 33
Figure 10. Excavation of the cupola at Skallen, equivalent to step II in Figure 9. Photo:
Storengy (ENGIE) and E.ON, with permission.
Figure 11. Downward excavation from the upper level at Skallen, equivalent to step VII and
VIII in Figure 9. The rock was hauled out through a central shaft. The drainage
system is clearly visible on the naked rock walls, before it was covered with
shotcrete (section 3.6.1). Photo: Storengy (ENGIE) and E.ON, with permission.
34 | DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS
additional space provided by work niches in the rock wall at one level and
then hoisting the structure upward step-by-step allowed efficient
assembly without having space for workers all around the cavern walls
(Figure 12).
3.5. Failure modes of the cavern wall lining
3.5.1. Low cycle fatigue of the steel lining
The internal gas pressure in the storage causes an expansion of the cavern
due to deformations in the rock mass. The principle behaviour is
illustrated in Figure 13.
Depending on the internal pressure, the stress situation in the
surrounding rock mass close to the cavern boundary can be divided into:
an elastic state with compressive stresses in the rock mass (up to an
Figure 12. Work niche to allow for assembly of the steel plates and concrete reinforcement
at Skallen. Photo: Storengy (ENGIE) and E.ON, with permission.
DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS | 35
Figure 13. Illustration of the principle behavior due to a pressurization of the cavern: as the
pressure increases, rock joints start to open up (© Johansson, 2003, with
permission).
internal pressure equal to two times the initial stresses in the rock mass),
an elastic state with tangential tensile stresses in the rock mass, a plastic
state with radial compressive stresses equal to the uniaxial compressive
strength of the rock mass. The different stages are illustrated in Figure 14.
As previously described in chapter 2.4, the deformations and strains in
the rock mass are analyzed according to the principles of the ground
reaction curve concept and is mainly a function of the rock mass
Figure 14. Principle of elasto-plastic stress states of the rock mass (© Johansson, 2003,
with permission).
36 | DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS
modulus, rock mass strength, initial stresses in the rock mass, the cavern
shape and the internal pressure in the storage.
The tangential deformation in the rock mass will mostly be due to
opening and shearing of existing joints in the rock mass. These local
strains will generate cracks in the concrete lining concentrated to the
opening of the rock joints. If the concrete lining is unreinforced, there will
be few but large cracks, which influence the strain in the steel lining
negatively. In order to make the crack pattern more evenly distributed
reinforcement should preferable be installed in the concrete lining,
located in the outer parts of it (towards the steel lining).
The cracking of the concrete will cause a tangential strain in the steel
lining. How this strain is distributed depends to a large extent on the
friction coefficient of the sliding layer. If there no friction at all between
the steel and the concrete, the stress in the steel lining will, in principle,
be evenly distributed and proportional to the tangential strain. Under
such conditions the stress in the steel lining will be governed by global the
factors mentioned above (i.e. rock mass modulus etc.). However, if the
friction is high, concentrations of high stresses will occur at the locations
of the cracks while they will be as lowest in the middle between two
cracks. The principle is illustrated in Figure 15.
Based on chosen reinforcement, concrete quality and thickness of the
concrete lining, together with the expected opening of the rock joints and
their spacing, the average concrete crack spacing can be calculated. From
this, the average crack width can be determined.
Once the average crack width is known together with the friction of the
sliding layer, limit equilibrium in the horizontal direction derived from a
piece of the steel-lining makes it possible to calculate the maximum strain
over the crack; see Johansson (2003) for an account of the principles.
In order to analyse the effect of the sliding layer, its frictional
resistance has to be known. It is also necessary that it could function
properly under the varying temperatures and pressures that will exist in
the cavern during the operation of the facility. Another important design
aspect is also to ensure long-term durability and that it can handle the
cyclic loading it will be exposed to.
DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS | 37
Figure 15. Illustration of the influence from friction between the concrete- and steel lining
(© Johansson, 2003, with permission).
Since the storage will be filled and emptied a large number of times
during its operational lifetime, the maximum strain in the lining has to be
controlled with respect to fatigue. According to Johansson (2003), the
material for the steel lining should be a high quality grade, fully killed
carbon steel with high ductility, excellent weldability and impact
properties. The yield strength should be in the range of 300 to 400 MPa.
With respect to fatigue, two different types of behavior can be
distinguished depending on the strain range: (1) ϵy < strain range < 2ϵy
and (2) strain range >2ϵy (where ϵy is the yield strain).
If the strain range is of the first type, it yields in tension during the
first loading cycle. During unloading, the tension is relaxed and at some
point changes to compression. However, the whole unloading is elastic
and the yield limit is not reached in the compressional stage. During the
next load cycles, the steel will work elasticly in the whole strain range.
The principle is illustrated in Figure 16a. This means that it will be an
38 | DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS
elastic fatigue situation with a large number of cycles (high-cycle fatigue)
until failure occurs.
If the strain range is larger than two times the yield limit, the steel will
yield in both tension and compression in the first cycle. In the following
load cycles, yielding will also take place in each of the cycles. The
principle is illustrated in Figure 16b. In this type of situation, the fatigue
limit is significantly smaller (low-cycle fatigue).
As an example, the fatigue limit was investigated within the Skallen
storage project based on tensile testing on a steel (S355J2G3) with a
thickness of 12 mm. Based on this data, an ϵ-N diagram was constructed
based on Manson’s universal slopes equation (Jergéus 1982, Dieter 1988,
Rask & Sunnersjö 1992), see Figure 17.
Figure 16. Illustration of steel behavior for two different strain ranges for steel with yield limit
355 MPa with a) Strain range 3‰ and b) Strain range 5‰(© Johansson, 2003,
after Rask and Sunnersjö, 1992, with permission).
DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS | 39
In the curve, it can be observed that the fatigue limit is approximately
10 000 cycles at a strain of 5‰ and 100 000 cycles at a strain of 3‰. In
addition, when the fatigue limit is analysed, it has to be considered that
the LRC cavern has a biaxial stress state, meaning that the strain in both
directions along the steel has to be accounted for in the design. It should
be noted that the curve in Figure 17 does not include the effects from
welds. If the effects from welds are included, the curve will be different
(Johansson 2003).
In the calculations performed on the Skallen facility by Fredriksson &
Persson (2000), as reported in Johansson (2003), it was found that for a
cavern pressure of 20 MPa and initial stresses in the rock mass equal to 5
MPa, the maximum strain was approximately 1.25‰. This strain
corresponds to a fatigue limit larger than 1 000 000 cycles. However, as
previously discussed, the strain in the steel lining depends on several
factors and is a design question that has to be taken seriously and
analyzed for each specific case.
Figure 17. Fatigue diagram based on Manson’s universal slopes equation and data from a
tensile test on steel S355J2G3 (© Johansson, 2003, with permission).
40 | DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS
3.5.2. Local failure of the steel lining due to structural weakness
In the methodology described in the previous subchapter, the general
assumptions are that the rock mass is a homogeneous isotropic material;
however, spatial variations of the properties or the influence from large
weakness zones can imply that local parts of the cavern experience larger
deformations than other parts. As a consequence, it is important to have
knowledge not only on the average properties but also on the variation of
the mechanical properties. This is particularly important if the scale of
fluctuation of the properties is close to the size of the cavern, since this
can imply local additional straining of the steel lining.
Other possible causes for failure due to structural weakness could be if
the concrete is of an uneven quality, resulting in spatially varying
mechanical properties. This has to be controlled through comprehensive
quality assurance programs at site, making sure that the concrete is of
sufficient quality during the casting process.
Another important aspect, which could cause failure, is if the welds are
not properly performed. Therefore, all welds should be thoroughly
checked as a part of the quality assurance program.
Any pipe lead-throughs into the cavern or access roads into the
chamber are also potential points of weaknesses and has to be designed
accordingly.
3.6. Groundwater management and gas leakage
detection
3.6.1. The drainage system
The purpose of the drainage pipes is multiple. The drainage system is
used to drain groundwater during construction and to avoid the
groundwater pressure from damaging the lining when the gas pressure in
the cavern is low. The system is also used as a safety system with the
ability to detect, collect, and evacuate leaking gas. Should any leakage of
gas occur from the cavern, it will be collected in the grid of drainage pipes
DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS | 41
and safely vented at the ground surface. The drainage system is described
in detail in Johansson & Lindblom (1995). The main components are:
a rhombic pattern grid of sub-vertical perforated drainage pipes
covering the complete cavern wall with 1–2 m spacing,
two ring-shaped horizontal gas collector pipes (one at the top and
one at the bottom of the wall), which collect the gas from the
drainage pipes, and
several large gas evacuation pipes, leading any leaking gas toward
the ground surface from the horizontal collector pipes.
Under normal conditions, the drainage system will be filled with water
to prevent chemically or biologically induced clogging and to prevent
oxygen for corrosion of steel lining. The drainage system needs to be
protected during the constructions phase, e.g. during the pouring of the
concrete. The technical solution for the Skallen plant was to cover the
drainage system in heavy geotextile and spray a special low-strength and
permeable shotcrete on top (Figure 18). This design concept is part of a
US patent: see Johansson (2002) for further details.
3.6.2. Pre-grouting of the rock mass
Although the LRC concept implies an impermeable steel lining (which not
only prevents gas from escaping from the cavern, but also prevents
groundwater from entering), pre-grouting of the rock mass may need to
be considered if the inflow to the excavation is expected to be substantial.
Flowing or dripping water during concrete casting and welding of the
steel lining will affect the construction process and quality negatively.
Additionally, the acceptable groundwater inflow to the excavation from
an environmental point of view will be governed by the environmental
permit issued by the Environmental Court.
3.7. Advantages of using a risk-based design approach
For the construction of the Skallen demo plant, considerable effort was
spent on assessing the involved risks in the project. The risks associated
42 | DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS
Figure 18. Installation of the drainage system at Skallen. Photo: Storengy (ENGIE) and
E.ON, with permission.
with the construction and operation of an underground storage for gas
concern both the overall structural safety of the facility and the effects of
gas leakage. As detailed in section 2.6, a number of laws and other
regulations ensure that the safety of the facility is satisfactory. Notably, as
a general rule, these laws and regulations aim to manage the involved
risk, which can be seen as the combination of likelihood and
consequences; the larger the consequences of an event are, the lower the
acceptable probability of the event is. This principle is valid both for most
modern structural design codes and for the regulations on management
of chemicals in the Seveso II directive. For example, the Eurocodes for
design of structures suggest acceptable failure probabilities in the range
of 10-5–10-7, depending on the magnitude of the consequence of failure;
though, the respective nation may prescribe other levels (CEN 2002).
Following this principle – to correspond the structural safety level
toward a specified risk level – implies that it will be favorable to adopt a
DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS | 43
risk-based design approach, which directly addresses the risk associated
with structural failure and gas leak. Using a risk-based design approach
also circumvents the issue that no specific guidelines on the structural
safety of underground gas storage facilities are available currently.
Additionally, the great natural variability of rock mass properties gives
risk-based design approaches an advantage over traditional deterministic
design approaches, as highlighted in Johansson et al. (2016). Using a
risk-based design approach to satisfy the structural safety requirements
makes it also easier to link the structural safety aspects to all other safety-
related issues that need to be considered in the overall safety assessment,
which is part of satisfying the Seveso II directive. Thereby, a more
stringent risk management can be achieved for the construction and
operation of the facility.
3.8. Risk management of design, construction and
operation
As discussed in the previous section 3.7, the construction and operation
of the underground gas storage facility will carry risks that need to be
managed. For this context, risk management is an established term in
ISO 31000 (ISO 2009) for the coordinated activities that direct and
control an organization with regard to the associated risks.
From a general perspective, the management of risks in a project can
be seen as a part of the overall project management. In the context of
underground gas storage facilities, the relevant risks may concern for
example the structural safety, workers’ safety during construction,
environmental impact of the construction, economic aspects of the
project, the handling of the hydrogen gas, the possibility of gas leakage,
etc. Clearly, the scope is huge; therefore, a structured approach to manage
all relevant risks is necessary to ensure sufficient quality in the project.
The aforementioned ISO standard 31000 provides a general framework
for structured risk management. Its applicability to the geotechnical
aspects of civil engineering works has recently been studied (Spross et al.
44 | DESIGN PRINCIPLES FOR PRESSURISED CAVERNS AND GAS-TIGHT LININGS
2017, Spross et al. 2015) and the Swedish Geotechnical Society has
adopted a general methodology for this purpose (SGF 2017).
Given the complexity of the underground gas storage facility, a
structured and carefully executed risk management is essential to the
project. Application of the risk-based design approach, which is discussed
in section 3.7, may contribute to facilitate high quality in the management
of both the design-related risks. However, the scope of the risk
management is clearly much larger and substantial effort must
consequently be put into this work. For example, an essential part of the
construction of an LRC facility is extensive and thorough quality control
of all structural components and their interaction both with the rock mass
and with each other. As discussed by Stille (2017), rock engineering
projects require a dual approach to quality assurance: in addition to
doing things right, which is the focus of traditional quality systems for
manufacturing processes, the rock engineer must also ensure that the
right things are done (with respect to the geological conditions at the
site). For a project such as a lined rock cavern for gas storage, quality
assurance during construction becomes extremely important, because of
the extreme difficulties in correcting errors after completion.
DISCUSSION | 45
4. Discussion
4.1. General
Due to the geological conditions in the Scandinavian bedrock, the most
favorable solution for storing large quantities of hydrogen gas (50 000–
100 000 m3) is through the LRC concept. Today, only one such type of
facility exists in the world: the Skallen facility in the south west of
Sweden, which stores natural gas. The successful construction and
operation of the plant show that this type of storage is possible to
construct. However, the main problem in storing hydrogen gas in a steel-
lined rock cavern subjected to high pressure compared to natural gas is
the possible embrittlement of the steel caused by the hydrogen gas.
According to the Swedish steel manufacturer Sandvik, this problem is
most likely possible to solve, although it requires careful testing of both
the steel and the welds of the steel before a proper steel type can be
chosen. However, the issue of choosing a proper steel type for an LRC
facility for hydrogen is not within the scope of WP 2.3, but will be a key
question to solve before a storage facility can be constructed within the
HYBRIT project. Additionally, the degradation process of the sliding layer
between the steel and concrete in the lining may require further studies,
in particular for longer life lengths and higher gas pressures than applied
in the design of the Skallen plant.
The fact that the LRC concept has been developed and proven
successful means that it today does not exist any critical research
question that needs to be solved before a storage facility can be built,
except for the hydrogen embrittlement and degradation of the sliding
layer. However, the design and construction of an LRC facility in the rock
mass is a complex engineering task, which requires a good understanding
46 | DISCUSSION
on how the different components interact and with what uncertainties
they are associated.
In order to fulfill the legal requirements in the design, such as the
Seveso directive and the Eurocodes, design with risk-based methods is
preferable. Even though a risk-based design methodology was used in the
design of the Skallen facility, the experience and knowledge of using risk-
based methods in design is still limited. This is generally the case for rock
engineering problems and in particular for LRC-storages. Further
development of these methods would therefore be beneficial for the LRC
concept. The use of such methods would also enable an optimization of
the design with respect to the present uncertainties. A logical way forward
within WP 2.3 would therefore be to further develop the use of risk-based
methods in the design and construction of an LRC facility for hydrogen in
order to achieve a safe, economic, and environmentally sustainable
storage.
4.2. Potential research questions for WP 2.3
4.2.1. Application of subset simulation for risk-based design
The LRC concept consists of several components where the rock mass
acts as load-carrying medium. The deformations caused by the gas
pressure will follow a behavior similar to the behavior when excavating a
circular tunnel in an isotropic rock mass, according to the principles of
the ground reaction curve (Hoek & Brown 1980). However, due to the
cavern geometry, the analytical solutions for the two-dimensional
problem of an infinite tunnel is not suitable to use for the analysis of
stresses and strains in the rock mass and the lining. Instead, three-
dimensional numerical analyses are needed in the assessment of global
stresses and strains. When risk-based methods are used in the design
together with numerical calculations, the assessment of the probability of
failure requires a large number of computations, unless specific search
algorithms are used for more efficient sampling around the design point.
Subset simulation, which is based on a Markov-chain Monte Carlo
algorithm, is one example; see e.g. Au & Wang (2014). With this method,
DISCUSSION | 47
the location of the design point is located with an iterative search
algorithm and the sampling of the parameters around this point is
weighted with respect to their probability of occurrence, leading to a more
efficient Monte Carlo simulation.
Approximate techniques can also be used, such as the point estimate
method or the modified point estimate method (Langford & Diederichs
2013); however, the accuracy of this technique is questionable, especially
for complex limit states with low probabilities of failure.
A potential research question is therefore the implementation of
subset simulation together with numerical analysis, in order to reduce the
necessary calculation time and improve the accuracy of the numerical
analyses used to assess the structural safety.
4.2.2. Optimization of concrete thickness in the lining
When the cavern is pressurized, tensile stresses tangential to the cavern
will develop at a certain internal pressure. At which pressure this occur
depends on the prevailing tangential compressive stresses around the
cavern due to the initial stresses in the rock mass. These tensile stresses
open up existing fractures in the rock mass. Such fractures constitute
potential areas with high local strains in the impermeable lining. To
reduce these local strains, a reinforced concrete lining together with a
sliding layer between steel and concrete is used. In the Skallen facility, the
thickness of the concrete layer was not optimized from a structural
perspective, but mainly chosen with respect to practical reasons. The
space between the rock mass and the steel liner should allow for welding
and non–destructive testing of the steel, to pour the concrete, and to
install reinforcement mesh, etc. (Johansson 2003).
Detailed analysis of the minimum required thickness would therefore
constitute a potential research question within WP 2.3, with respect to
the ability of the lining to reduce local strain effects caused by the opening
of rock fractures. This optimization of the concrete thickness should
preferably be performed using a risk-based approach combined with
numerical methods.
48 | DISCUSSION
4.2.3. Effect of spatial variation of rock mass properties on location
suitability
The spatial variation of the rock mass properties could significantly
influence the occurring strain in the rock close to the wall and in the
lining. Depending on allowable strain limits in the steel, the shape of the
cavern, and the in-situ stress conditions, the spatial variation of the
surrounding rock mass properties may influence the design. Today,
analysis of stresses and strains in the rock mass are usually made
assuming a homogeneous and isotropic rock mass.
How a spatial variation due to different scale of fluctuations of the
rock mass properties influences the cavern behavior is of interest,
because it is not only the average properties that govern the cavern
behavior, but also their spatial distribution, especially if the scale of
fluctuation is relatively large compared to the cavern size. Another
important aspect is also the presence of weakness zones, which are
relatively frequently occurring in the rock mass. The effect of the spatial
variation in the rock mass on the cavern wall may affect how close the
cavern can be located to different types of identified weakness zones.
These topics are worth further research, since they constitute important
input in the selection of suitable cavern locations.
4.3. Additional important research questions and design
issues
In the course of the work with this report, a number of additional
research questions related to underground storage of hydrogen gas have
been recognized. Although they may be worth further investigation, or
even be critical to the design, we have not deemed it reasonable to cover
them in WP 2.3 for various reasons, which are discussed in each
subsection. They are therefore listed here separately.
4.3.1. Hydrogen embrittlement of the steel lining
As discussed in section 2.4 and 4.1, the effect of hydrogen gas on steel
properties, which is known as hydrogen embrittlement, is a critical issue
DISCUSSION | 49
to study and solve for the design of an LRC facility for hydrogen gas. The
selection of a suitable steel type is, however, a metallurgic issue rather
than a rock engineering issue. We suggest that the potential effect of
hydrogen embrittlement is studied separately.
4.3.2. Long-term behavior of the sliding layer
A specially developed sliding layer was used in the Skallen facility to even
out shear stresses in the interface between steel and concrete. The layer
was 6 mm thick and consisted of polymer-modified bitumen with textile
reinforcement in between. The surface of the layer, which was applied
against the concrete, was covered in sand and the layer was applied on
epoxy-primed steel. Several shear tests were performed on the layer
under different temperatures and the normal stress ranged up to 25 MPa.
The shear stress was found to be dependent on both normal pressure and
shear resistance. It may be possible that cyclic loading with time
gradually decrease the ability of the layer to reduce shear stresses.
However, no test was performed on the long-term behavior of the sliding
layer with respect to cyclic loading at high pressure. Additional testing
under cyclic loading at different temperatures would therefore be of
interest to further increase the understanding of the characteristics of the
sliding layer over time. However, similarly to hydrogen embrittlement,
this is also an issue related to selection of manufactured construction
materials and should therefore be studied separately.
4.3.3. Case study of the application of risk management frameworks
Although high-quality and substantial risk management will be a
necessity in the design and construction of the storage facility, we see no
critical need for further development of the available risk management
methods. Well-established risk management procedures are already
available through the ISO 31000 standard and application guidelines for
geotechnical aspects of construction work have been published. However,
there are few publicly available high-quality case studies on the practical
application of such risk management frameworks; in fact, for rock
engineering structures, there is none, to our knowledge. Therefore, we
50 | DISCUSSION
believe that a case study of the design and execution of the planned
underground gas storage facility from a risk management perspective
would be of high value. However, as this is not a theoretical study to
enable the construction of the facility, but a report of the experience from
the actual work, we believe that such a study is out of the scope of WP 2.3.
4.3.4. Control of temperature variation in the cavern
Due to the cyclic filling and emptying of the storage during commercial
operation, the temperature in the cavern will vary. Very high
temperatures may be harmful for the sliding layer. Temperatures below
0 °C could freeze the groundwater causing damage to the concrete lining.
Restriction of the operation during gas injection and withdrawal is
necessary to avoid too high or too low temperatures. How to control the
temperature in the cavern is an important question; see also the US
patent by Hall (2002).
4.3.5. Gas behavior in the rock mass and design of gas detection
system
In principle, there are two options to manage a possible gas leakage in the
design of the cavern: design the drainage and gas detection system with
respect to a “worst case” leakage scenario or only design the drainage
system with respect to the water leakage during the construction phase of
the cavern.
In the design for a “worst case” gas leakage scenario, it is necessary
that the leakage drainage and detection system is properly designed in
order to be able to collect and transfer the gas to specific locations on the
surface. The dimensions and spacing of this system is mainly dependent
on gas pressure and the expected size of the crack in the steel lining.
If the system has insufficient dimensions, which for example is the
case if the leakage system only has been designed for water leakage
during the construction phase, hydrogen gas may leak into the rock mass.
If the cavern is situated relatively shallowly, the distribution of gas in the
rock fractures could result in jacking (uplift of the rock mass due to
pressurized rock fractures) and uncontrolled leakage to the surface. How
DISCUSSION | 51
gas is transmitted in a rock mass has been studied by a number of
researchers, see e.g. Kjørholt (1991), but further research on this topic
with respect to hydrogen gas and the LRC concept may be of interest;
however, such a study would need to start off on a rather basic research
level and the expected outcome over 4 years is not believed to contribute
as much to the advances of the HYBRIT project, as the studies listed in
section 4.2.
CONCLUSIONS | 53
5. Conclusions
The most suitable way of storing large quantities of hydrogen gas in
Sweden is concluded to be the use of lined rock caverns (LRC). The LRC
concept has been proven successful through the construction and
operation of the Skallen demonstration plant. No critical research
questions exist today for the construction of such facility with the
exception of the issue concerning hydrogen embrittlement of the steel
lining. Even though this question will not be covered within WP 2.3, it is a
key question that needs to be solved before the storage can be designed
and constructed.
In the design of an LRC facility, the use of risk-based methods is
preferable. However, the experience of using such methods in rock
engineering in general – and for lined rock caverns in particular – is
limited. Future research questions within WP 2.3 are therefore suggested
to further develop the use of risk-based methods in the design and
construction of the LRC-concept. Within this context, several potential
research questions has been identified, which mainly focus on the use of
numerical methods in combination with risk-based design methods in
rock engineering:
Implementation of subset simulation together with numerical
analysis to reduce the necessary calculation time and improve the
accuracy of the numerical analyses with respect to reliability.
Optimization of the concrete lining thickness with respect to the
ability of the lining to reduce local strain effects caused by the
opening of rock fractures.
Influence from spatial variation due to different scale of fluctuations
of the rock mass properties and from possible weakness zones in the
rock mass on the cavern behavior.
54 | REFERENCES
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Figure credits
Figure 1. Reprinted with permission from Hybrit AB.
Figure 9. Reprinted with permission from High pressure storage of gas in
lined rock caverns – cavern wall design principles. Licentiate
thesis by Johansson, J. 2003. KTH Royal Institute of Technology.
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Figure 8 Reprinted with permission from Springer: Acta Geotechnica,
”Evaluation of rock fracture patterns based on the element-free
Galerkin method for stability assessment of a highly pressurized
gas storage cavern”, Tunsakul et al. 2017.
Figure 9. Reprinted from Tunnelling and Underground Space
Technology, 21, Glamheden & Curtis, Excavation of a cavern for
high-pressure storage of natural gas, pp. 56-67, Copyright
(2006), with permission from Elsevier.
Figure 10. Reprinted with permission from Storengy (ENGIE) and E.ON.
Figure 11. Reprinted with permission from Storengy (ENGIE) and E.ON.
Figure 12. Reprinted with permission from Storengy (ENGIE) and E.ON.
Figure 13. Reprinted with permission from High pressure storage of gas
in lined rock caverns – cavern wall design principles. Licentiate
thesis by Johansson, J. 2003. KTH Royal Institute of Technology.
Figure 14. Reprinted with permission from High pressure storage of gas
in lined rock caverns – cavern wall design principles. Licentiate
thesis by Johansson, J. 2003. KTH Royal Institute of Technology.
Figure 15. Reprinted with permission from High pressure storage of gas
in lined rock caverns – cavern wall design principles. Licentiate
thesis by Johansson, J. 2003. KTH Royal Institute of Technology.
Figure 16. Reprinted with permission from High pressure storage of gas
in lined rock caverns – cavern wall design principles. Licentiate
thesis by Johansson, J. 2003. KTH Royal Institute of Technology.
Figure 17. Reprinted with permission from High pressure storage of gas
in lined rock caverns – cavern wall design principles. Licentiate
thesis by Johansson, J. 2003. KTH Royal Institute of Technology.
Figure 18. Reprinted with permission from Storengy (ENGIE) and E.ON.
