Proceedings of the Institution of

Civil Engineers
Energy 160
May 2007 Issue EN2
Pages 79–86
doi: 10.1680/ener.2007.160.2.79
Paper 600001
Received 03/11/2006
Accepted 29/08/2007
Keywords: Narasimhan Raghavan Siva R. K. V. Kosuri Kankipati V. Bhaskar Patrick de Laguerie Pierre Roux Philippe Vaskou André Saint
drilling & drillholes/excavation/rock Vice President and Head, Regional Project Project Manager, Project Manager, Site Underground Senior Geologist, Site Underground
mechanics Hydel & Nuclear Sector, Manager, L&T-ECC, L&T-ECC, India Géostock, Manager, Géostock, Géostock, Manager, Géostock,
Larsen & Toubro Limited, India France France France France
ECC Division, India

Visakhapatnam underground LPG storage cavern, India

N. Raghavan, S. R. K. V. Kosuri, K. V. Bhaskar, P. de Laguerie, P. Roux, P. Vaskou and A. Saint

One of the world’s deepest underground liquefied (a) safety from natural calamities and various forms of sabotage
petroleum gas (LPG) storage cavern projects has been including bombings
built near the city of Visakhapatnam in India. Storage is (b) safety from the hazards of leakage of the inflammable gas
based on the hydraulic containment principle using water (c) spatial economy (economy in usage of surface land area),
pressure. This paper discusses the design aspects and lower capital cost (also owing to savings in piping,
various phases involved in construction. The general instrumentation, etc.) and lower operating costs compared
outline, significance of the facility and basic principles of with conventional pressurised spheres and refrigerated tanks
storage are described, including the initial exploration used for surface storage
works, complementary site investigations, basic
engineering design and the design changes made during This mined cavern will be one of the world’s deepest LPG caverns.1
construction. Health, safety and environment as well as Mined caverns for LPG storage are usually designed and operated
quality aspects are considered. The sequence and methods either for butane gas or for propane gas, for which the depth
adopted to construct diaphragm walls for the foreshafts, needed is 100 m to 160 m. This cavern would store either butane
sinking for the rock shafts, installation of the water curtain or/and propane blended as LPG. The depth required is over 162 m
tunnel and excavation for the main storage cavern are below mean sea level in order to meet the pressure requirement of
described. The scope and construction methodologies are storing pure propane and to take account of the high geothermal
explained and the challenges and complexities of the temperature prevailing at the site (around 308C) since the pressure
underground project, a first for south Asia, are of the contained fluid would increase with higher temperatures.
considered. Achievements and areas where there is scope The facility is also provided with a water curtain system that
for improvements are discussed. Finally, the advantages of provides saturated conditions around the cavern. While for
such projects, critical success factors and special issues caverns at shallower depths an inclined approach through tunnel/
needing attention are presented. ramps would be preferred for economic reasons (given adequate
surface geology and land space), for deeper caverns a vertical
1. INTRODUCTION approach through vertical shafts only would be more economical.
Construction of one of the world’s deepest underground storage The cavern is connected to the surface through a 6.5 m dia. access
caverns for storage of liquefied petroleum gas (LPG) is underway shaft and a 4 m dia. operation shaft. A typical process flow
in the city of Visakhapatnam (‘Vizag’ for short) in India. This diagram of the facility is shown in Fig. 1.
paper discusses the various aspects involved in the project. The
cavern, with a capacity of 125 000 m3 , is located 186 m below The project is located in a 13 acre site at the foot of a famous
ground level. It is unlined and the petroleum product is retained by hillock, the Dolphin’s Nose, which juts out into the Bay of Bengal
the water containment principle, that is, the rock mass at Visakhapatnam port, in the state of Andhra Pradesh on the east
surrounding the cavern is saturated by water percolating from a coast of India. The owner is South Asia LPG Company Private
number of horizontal holes drilled from a water curtain gallery Limited (SALPG), a 50 : 50 joint venture between Hindustan
located above the cavern. The cavern is accessed by two vertical Petroleum Corporation Ltd (HPCL) and Total Gas and Power India
shafts of 4 m and 6.5 m dia. The entire rock excavation is carried (TGPI), a subsidiary of the French oil major, Total. Géostock of
out by drilling and blasting. The project is unique for the speed at France, well experienced in underground storage cavern design, is
which such a large cavern was excavated through only two small providing consultancy to the owner including geological,
diameter shafts and for the stringent safety measures employed. hydrogeological, geomechanical and process designs. Larsen &
Construction started in November 2003 is expected to complete by Toubro Limited (L&T) of India is the contractor for both the
mid 2007. underground and above-ground works.

2. THE FACILITY AND ITS SIGNIFICANCE Visakhapatnam was chosen as the location for the project because
The project envisages the underground storage of LPG, the first of suitable ground conditions for cavern construction covering
project of its kind in south Asia. Bulk storage of LPG in rock- rock quality, lesser incidence of cracks and fissures and water
mined caverns, in preference to surface tankage installations, table position. The selected site offers the advantages of having an
offers many advantages such as existing LPG-handling infrastructure, high demand for LPG in the

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 Ocean tankers Dispatch through
LPG pipelines/
loading arms

LPG unloading
arms
Metering unit

LPG boosting
pump station
LPG boosting
pump station

Blending unit
Odorant
injection system
Seawater
heating system
Ground level Fig. 2. Aerial view of site of the Visakhapatnam LPG storage
cavern project
Access Operation
shaft shaft

measurements. The pipes are installed in a vertical shaft or in

Cavern storage specific boreholes linking the cavern to the surface and are also
equipped with special downhole safety devices. The steel casings
Fig. 1. Process flow diagram of Visakhapatnam LPG storage cavern are vertically supported on concrete sealing plugs located in the
project shafts just above the cavern and laterally restrained through a
number of steel brackets provided in the shafts. Steel tubes used
for transporting LPG or water, or for housing instrumentation and
region along the east coast of India and potential for re-exporting so on are located inside these casings to permit future raising and
LPG to south-east Asia. An aerial view of the project is shown in lowering.
Fig. 2.
The surface installations include product injection and withdrawal
3. BASIC PRINCIPLES OF LPG STORAGE IN MINED systems and the specific units needed for product metering and
ROCK CAVERNS treatment (dehydration). Seepage water pumped out of the cavern
Mined rock caverns are purpose-built underground spaces is also treated before disposal. Throughout the operation, the
excavated in rock with access from vertical shafts or inclined stability of the structure is checked by continuous surveillance
drifts. Although conventional civil engineering techniques are using geophones (based on acoustic emissions) and the LPG-
used, efficient operations are required to ensure speedy tightness is checked by a hydro-geological monitoring system
construction and overall economy. Sites are selected essentially through observation wells fitted with piezometers. Temperatures
on the basis of suitability of local geological conditions. For this inside the cavern would be monitored by resistance temperature
purpose the rock must be strong enough for the cavern to be detectors.
stable. Stability can also be improved by installing appropriate
strengthening measures, such as rock bolts, shotcreting with 4. DESCRIPTION OF THE FACILITY
wire mesh or with integral steel fibres and structural steel ribs. The construction of the facility was contracted in two
A wide range of different rock types are suitable, such as packages: underground and above ground. The above-ground
igneous (granite, diorite, etc.), metamorphic (gneiss, schists, package comprises the electromechanical installations required
hornfels, etc.) and even sedimentary rocks (sandstone, for injection and withdrawal of the product and for the
limestone, chalk, shale, etc.). The geometry and arrangement of monitoring systems. The storage facility will be linked to the
the caverns are selected on the basis of the geotechnical nearby Visakhapatnam Port Trust (VPT) jetty where large
properties of the rock. Cross-sectional areas vary from a few refrigerated shipment parcels will deliver the LPG to the
tens to several hundreds of square metres. cavern. LPG will be sent from the cavern to the nearby HPCL
refinery through an existing pipeline or to smaller pressurised
The cavern wall is not lined with concrete and the rock is left vessels at the VPT jetty for re-exporting. The project involves
exposed except for shotcreting for strengthening the exposed rock the construction of two loading arms at the existing VPT jetty,
faces. The LPG is prevented from escaping using the principle of chilling and booster pumps, a seawater heat exchanger to heat
hydraulic containment, whereby the cavern is located at such a the refrigerated LPG prior to entry into the cavern and allied
depth that the water naturally present in the surrounding rock ancillary systems. The planned storage volume is 125 000 m3 .
flows all around and towards the caverns, preventing the LPG Design pressure is 14 bars (pure propane) and the operating
from escaping. The depth of the facility is governed by the vapour pressure for the butapro mixture is around 6 bars. Design
pressure of the LPG: it typically varies from 70 to 200 m. The water temperature range is 78C to 338C. Operation of the cavern will
pressure in the rock is generally enhanced artificially by special be achieved through casings and tubings installed in the
water supply systems (water curtains). operating shaft and access shaft, submerged pumps for LPG
and for seepage water, injection line, instrumentation casings
The storage and retrieval facilities are operated using a system of (level, pressure and temperature) and vent lines. Once all the
tubes within outer casings installed in the operation shaft to inject equipment is installed, the two shafts are sealed with concrete
and withdraw the LPG, and to carry out necessary monitoring and plugs just above the cavern.

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 including the hydrogeological
and geomechanical parameters
of the rock formation was built
up. From this model a
Water curtain gallery Access shaft Operation shaft preliminary layout and cavern
4 × 4 m D-shaped 6·5 m dia. 4·0 m dia. design for the project were
Water curtain bore holes proposed.

5.2. Complementary site

investigations and basic
engineering design
When SALPG decided to
launch the project in 2000,
prior to the execution of
the basic engineering design a
complementary site
investigation programme was
Storage cavern 20 × 19 m
carried out. This programme
160 m consisted of four additional
cored boreholes to investigate
the area of the two shafts and
to further narrow down the
location of the cavern in order
Fig. 3. Isometric view of the cavern to detect any major faults or
jointed zones. Fortunately no
The underground contract package comprises the construction of major rock problems were encountered in the site.
the cavern, the water curtain gallery and the connecting shafts.
The layout and general arrangement for the underground cavern The design of the underground works focuses on the stability
are shown in an isometric view in Fig. 3. This package consists of of the rock and the hydrogeological aspects and involves the
civil works for 125 000 m3 of hard rock excavation, 34 000 m2 of setting-up of numerical models for modelling of the cavern
shotcreting, 115 t of rock bolting and 175 t of grouting. The water both during the excavation phase and during its operating life
curtain gallery is a 4 m wide invert D-shaped tunnel located 15 m when it is exposed to LPG. Once the depth and the cross-
above the main cavern. From this tunnel horizontal boreholes are sectional shape of the storage cavern have been selected, the
drilled laterally on both sides, with a maximum length of about layout has to take into account the geological constraints (rock
82.5 m. These holes are charged with bacteria-free water from the strength, jointing, risk of fault zones, orientation of main in
surface and the water seeping down from these boreholes ensures situ stresses), the constructional aspects (sequence of
that the cavern has a curtain of water that saturates the excavation, maximum slope of internal access ramp, heights of
surrounding rock, preventing the escape of the contained LPG. heading and benches for excavation, etc.) and above-ground
This hydrostatic containment is the basic principle behind aspects (the facility to remain within the limit of the plot, the
underground LPG storage. The water curtain gallery or tunnel are shafts to be kept at minimum safety spacing during operation
accessed from both the shafts. The cavern has two limbs, each as the exit line is through access shaft and inlet is through
19 m high, 20 m wide, 160 m long and 64 m apart from each other. operation shaft, etc.).

The cavern is located in hard garnet silliminite gneiss bedrock, The underground part of the design covered aspects such as
with the invert at a depth of 181 m below mean sea level. At their
deepest point the shafts reach down to 200 m below the collar (a) layout and cross-sections for the main cavern and water
elevation. A central inclined ramp runs between the two cavern curtain
limbs leading to the access shaft and with three connecting (b) rock support systems for various types of rock mass
galleries between the two limbs at three different levels. The plan classification
and typical geometrical details of the cavern are shown in Fig. 4. (c) rock instrumentation to check the stability and in situ stress
measurements
5. ENGINEERING SYSTEMS (d) hydrogeological monitoring networks
5.1. Initial exploration works (e) a set of specifications for all the site works: surveys,
After a pre-feasibility study which concluded that the geology at instrumentation, drilling and blasting techniques, installation
this location would be favourable, an exploration programme was of rock bolts and shotcrete, grouting, cavern tests, etc. As per
launched in 1998 covering four cored boreholes (two vertical and the design, detailed engineering was developed for rock
two inclined) to a depth of 200 m. Hydrogeological tests (water supports, monitoring piezometer status, appropriate
loss, injection/relaxation and interference tests) and wireline construction methods and so on.
logging were performed on the four boreholes. In addition,
comprehensive laboratory tests were conducted on the samples 5.3. Design change during construction
selected from the cores. Typical data from these tests are shown in In order to validate the initial designs it was planned to measure
Table 1. Based on these results, a geological model of the site the in situ stresses more precisely during construction in the

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 162 m

20 m
CMG (west)

8 m UC IC LC

AR

64 m
8m
AS
(6·5 m dia.) WCG

OS (4·0 m dia.)

CMG (east)

214 m

(a)

Legend
Heading AS Acess shaft
OS Operation shaft
CMG Cavern main gallery
19 m

Benching-1 WCG Water curtain gallery

UC Upper connection
6m

IC Intermediate connection
4m

LC Lower connection
Benching-2 AR Access ramp

20 m 8m 4m

Detail of CMG Detail of AR Detail of WCG

(Area = 340 m2) (Area = 41 m2) (Area = 14 m2)
(b)

Fig. 4. Typical geometrical details of the cavern: (a) plan view; (b) geometrical details of cavern

water curtain gallery using over-coring methodology. If, ratio of horizontal-to-vertical stresses reaching a factor of 3 to
however, the results were made available only after the water 4. Taking this into account and the existence of sub-horizontal
curtain gallery was excavated, this would have had a serious joints, it was decided to modify the initial design, especially the
impact on the construction programme or sequence, considering shape of the main storage cavern (which was reduced in height
the potential impact on the design of the main storage cavern at and enlarged in width, in order to have rounded walls and to
that stage. It was therefore decided to make the measurements limit the tension on sub-horizontal joints as much as possible).
at an earlier stage by a special methodology: a borehole The plan of the cavern was changed from a single continuous
previously dedicated to hydrogeological monitoring was horse-shoe shape (Fig. 5) to two individual cavern limbs with
deepened and stress measurements were carried out by rectilinear layout, as already shown in Fig. 3.
hydraulic fracturing. This was done while the two shafts were
still being excavated. The results of hydrofrac stress
measurements done through an inclined borehole showed a

Class Class 1
Quality Massive
24 m

Dry density 2853 kg/m3

19 m

Uni-axial compressive strength 106–156 MPa

P-wave velocity 5.195–5.931 km/s
Young’s modulus 35.03–44.34 GPa 15 m
Poisson’s ratio 0.24–0.35
Cohesion 18.86 MPa
Internal angle of friction 51.348
20 m
Porosity 1.433% 13·3 m
Permeability 10ÿ1 md (a) (b)

Table 1. Typical rock test data Fig. 5. Shape of cavern (a) before and (b) after change in design

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 6. HEALTH, SAFETY AND ENVIRONMENT AND
QUALITY ASPECTS
6.1. Health, safety and environment aspects
Health, safety and environment aspects are critical during
execution of such projects in view of the limitations of access and
the working environment. There are also statutory health, safety
and environment aspects to be implemented. Such aspects covered
were workmen management, hazard analysis, incidence analysis,
emergency response, medical services, mechanical integrity
checks, motor vehicle safety, industrial hygiene, personnel
training, personnel protection equipment, management of
chemicals, air, water and wastes and so on. Job safety analysis and
job hazard analysis studies were developed along with the
required mitigation plans for each sequence of activities and these
Fig. 7. Typical blast pattern for top heading of cavern
were then implemented at site. A work permit procedure was put
in place to ensure the safety of ongoing works during the usage of
the existing adjacent process facility. As a result of stringent
safety monitoring there were no accidents during the execution of parameters for rock bolting are strength and elongation. Trial
the underground works. tests have been conducted for initial and routine pull out tests
(specified strength of 15 t) for different lengths of rock bolts. For
6.2. Quality aspects the drilling of the water curtain boreholes, deviation limits were
The main items of work for the underground construction were specified to ensure that they do not approach the storage
drilling and blasting for rock excavation, shotcreting, rock locations. Quality assurance plans and inspection test plans
bolting, concrete paving of the floor and borehole drilling. The have been developed for the above parameters and implemented
National Institute of Rock Mechanics (NIRM), a leading public at site.
sector research institute in India, was involved in the blast design
for various sections. The quality parameters governing the drilling 7. METHODOLOGIES ADOPTED FOR
and blasting works for rock excavation are smooth blasting, UNDERGROUND WORKS
ensuring the construction tolerances for the profile, vibration 7.1. Fore shaft: diaphragm wall and excavation
limits as per national standards for each existing structure and Initially a polygonal diaphragm wall enclosure was constructed
rock fragmentation. Blast design and mapping were carried out by through the overburden above the rock level for the foreshaft as
expert blasting engineers and geologists at the site, who have been the first stage for each of the two shafts. The reinforced concrete
performing the geological mapping to classify the rock diaphragm wall panels extended into weathered rock strata for
encountered and suggesting the rock support systems to be adequate keying in. The diaphragm wall was constructed with
implemented. Typical blast hole patterns for shaft and caverns are eight panels for the operation shaft and ten panels for the access
shown in Figs 6 and 7. It was possible to satisfy all the specified shaft. Trenching of each panel in the overburden soil and within
criteria successfully. the depth of anchoring into the rock below was carried out by the
reverse mud-circulation method. In the trenches stop-ends were
The parameters for shotcreting are strength, sprayability and inserted at both ends of the panel before lowering the
thickness of application. Accordingly the mix design has been reinforcement cage. The panels were concreted using concrete
developed for the shotcrete (M35 grade 35 MPa compressive from a nearby L&T ready mix concrete plant.
strength at 21 days) and established with trial tests for
implementing at site. The applied thickness varied from 50 mm The depths of these foreshafts were 20 m at the operation shaft
to 100 mm based on the rock mass classification. The and 16 m at the access shaft respectively. After completing all
the panels of the diaphragm wall a capping beam was cast at
the top to integrate all the panels. Then a grout curtain was
formed all around below the diaphragm wall (in hard rock) in
order to reduce the seepage while excavating the foreshaft.
After installing the capping beam, a 75 t crane with a clamshell
was used to remove the overburden soil. A mini excavator and
a rock breaker were deployed for the excavation. While
excavating the foreshaft, intermediate reinforced cement
concrete (RCC) ring beams and a bottom ring beam have been
cast to stabilise the diaphragm wall panels. Typical diaphragm
wall details are given in Fig. 8.

7.2. Shaft sinking

Below the foreshaft, the rock shaft was excavated first in
weathered rock and then in hard rock. Drilling, charging and
controlled blasting were carried out in the rock shaft up to about
Fig. 6. Typical blast pattern for shaft 20 m depth below the ring beam. The muck generated was
manually loaded into a bucket and the bucket lifted up using a 75 t

Energy 160 Issue EN2 Visakhapatnam underground LPG storage cavern, India Raghavan et al. 83

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 EL +5·00 m
Ground level

Sea sand with pebbles and boulders

Diaphragm wall
Weathered rock

Ring beam
EL –12·70 m

Rock bolts

EL –18·00 m
Rock shaft

Hard rock

Shaft

Fig. 8. Diaphragm wall of shaft

crane with a clamshell arrangement for muck removal. After drills and charging with cartridge explosives. A skid steer loader
reaching about 20 m depth below the ring beam, a moving (Bobcat) and an Ajax Fiori dumper were used for muck
scaffold operated by two 10 t winches and a winder transportation from the face. The rock bolting and shotcreting
arrangement for bucket hoisting were established. A suitable methodologies were the same as that for shaft sinking. A total
head frame was provided to support the sheave pulleys that of 43 horizontal bore holes were drilled on either side of the
carry the ropes of the winder and the winches. Drilling was main gallery at 1.5 m above the gallery base (Fig. 10) and
carried out with hand-held rock drills and charging with another nine holes were drilled further to improve the water
cartridge explosives. Muck loading into the bucket was done curtain efficiency.
manually in the smaller operation shaft, and by using a small
excavator in the larger access shaft. The loaded bucket was 7.4. Storage cavern
hoisted up by using the winder bucket and tipped into an The initial part of the storage cavern was excavated by manual
inclined chute available on the headframe (Fig. 9), which drilling to create enough space for parking, manoeuvring and
discharges into a tipper truck for removal. Rock bolting was protecting the equipment. Then the drilling, mucking and
carried out by manual drilling, inserting cement capsules and shotcrete equipment were lowered and a muck-hoisting system
pushing the rockbolts in by using a hand-held machine. was established. For the cavern excavation, drilling was carried
out with drilling jumbos. Charging was done by power bulk, a bulk
7.3. Water curtain gallery explosive (combination of base emulsion—a mixture of
The water curtain gallery was 3.5 m high and 4 m wide. Since ammonium nitrate and fuel oils with other chemicals and
the size was small, drilling was carried out with hand-held rock emulsifiers that impart strength, pumpability and stability, being a

84 Energy 160 Issue EN2 Visakhapatnam underground LPG storage cavern, India Raghavan et al.

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 Road
Barricade

Diaphragm wall
Platform rope

Ring beam

Cross-section

Fig. 9. Headframe, winch and working platform

non-explosive material—and a sensitising solution of a mixture of

nitrite and dilute acid, which altogether form an explosive
material) by pumping explosives using a pumping unit into the
drilled holes, after loading the priming cartridge. Shotcreting was
carried out using a Robojet machine. Rock bolt drilling was done
using the jumbos. The finished cavern is shown in Fig. 11. The
cavern excavation alone was completed in about 17 months after
the shafts were completed.

8. SIGNIFICANT ACHIEVEMENTS
The shaft-sinking progress increased progressively over the
period, and a national record of 33 m per month was established.
Cavern excavation also progressed well and over 16 500 m3 of
muck was removed in one month. The liquid explosive method
Fig. 10. View of water curtain gallery was used for the first time in the country for underground
construction. This resulted in reduction in cycle times and
increased safety. Health, safety and environment practices in
many areas have been audited by many parties including Total of
France, HPCL, India and L&T and have been found to be
satisfactory. Independent third-party checks for quality were also
conducted by the NIRM, SGS India Private Limited and Lloyds
Register Asia.

9. CONCLUDING REMARKS
Underground cavern construction is an elegant solution—
technically feasible and commercially viable—for storage of large
volumes of LPG or crude oil. At present many such caverns are
being planned for the strategic storage of crude oil. The
construction of the underground facility is a technically
challenging job. The successful completion of such a project
Fig. 11. View of finished cavern depends on the deployment of the right kind of equipment,
preparedness to face different eventualities, and great teamwork.

Energy 160 Issue EN2 Visakhapatnam underground LPG storage cavern, India Raghavan et al. 85

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 While a project with access from the shaft alone involves many savings in time is generally justified, even with additional costs
complexities, some of the problems of construction can be involved, for reducing the time period. Projects of this kind
mitigated by providing an inclined access provided the cavern is provide good solutions in the face of rising demand for open land
located at higher levels below the ground. Providing an inclined and for safe storage of hydrocarbons in proximity to urbanised
access depends, however, on the presence of good rock conditions areas.
in the subsurface region and on the availability of required space
as large horizontal distances have to be travelled with an easy REFERENCE
slope for the ramp. The fixed establishment cost for such deep 1. Newspaper report. LPG cavern trial run at Visakhapatnam by
underground projects is high during the construction compared next year-end. The Hindu Business Line (Financial Daily),
with variable costs and provision of additional resources to ensure 23 November 2005.

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