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CHAPTER 4 - Tanks and reservoires 169
LIGHTWEIGHT STRUCTURES IN CIVIL ENGINEERINGPROCEEDINGS OF THE INTERNATIONAL SYMPOSIUM
Warsaw, Poland, 24-28 June , 2002
PART I Structures 170
CHAPTER 4 - Tanks and reservoires 171
PART I Structures 172
CHAPTER 4 - Tanks and reservoires 173
LIGHTWEIGHT STRUCTURES IN CIVIL ENGINEERINGPROCEEDINGS OF THE INTERNATIONAL SYMPOSIUM
Warsaw, Poland, 24-28 June , 2002
B.F. Belyaev, Head of Department
Yu.L. Bormot, Chief Specialist
G.P. Kandakov, Deputy Director
(Melnikov Central Research and Design Institute of Structures, Russia)
Abstract: This paper presents a new structural concept of a single-deck floating roof for a vertical cylindrical tank. The new
design of the floating roof secures the desired form with a slope towards the center and guaranteed removal of atmospheric
precipitates.
The single-deck floating roof for a vertical cylindrical tank
consists of a pontoon ring located around the periphery, radial
beams, a central ring, a deck, supports and cantledges.
The pontoon ring comprises sealed factory-made boxes
securing the roof’s buoyancy in case of damage (seal failure)
of two neighboring boxes and of the central part of the roof.
The central part of the roof is formed by radial beams, the
central ring and the deck. The ends of the radial beams are
made fast to the pontoon ring and the central one. The deck
consisting of separate sectors is manufactured at a factory as a
whole panel and supplied in roll form. The assembly
components (sectors) of the deck are marked and cut out of the
panel at the construction site. The deck sectors are welded to
the radial beams, the pontoon ring and the central support ring.
The central ring is also covered by a cone-shaped deck and
may be supplied from the factory as a finished product or
assembled at the erection site.
When the roof is below (at the bottom), it rests on the supports
made fast to the radial beams, the pontoon boxes and the
support ring. In the said position constant and temporary roof
loads are transmitted through the beams and the supports to the
bottom of the tank. Beams and supports of appropriate sections
are selected depending on the load values. For removal of
atmospheric precipitates from the roof there is a rain
catchment device in the center thereof and atmospheric
precipitates are removed beyond the tank through a floodgate
system.
When the roof is afloat, the surface of the roof with a slope
towards the center is formed by the cantledges made fast to the
supports or the radial beams. The total mass of the cantledges
is determined by means of special calculations depending on
the diameter of the tank, the slope gradient and the product
density. Distribution of the cantledges along the beams and on
the central ring is also determined by means of calculations.
The surface of the deck (membrane) within a sector is formed
by the buoyancy force of the product caused by submergence
of the central part of the roof. Therefore, the membrane is
deflected upwards and thus secures the flow of precipitates
towards the radial beams and the center of the roof. The
central ring is covered by a deck in the form of a rigid cone-
shaped cover with the rain catchment device in the center
thereof.
All the components of the floating roof are supplied from the
factory of origin by the following dispatch assembly units:
- sealed boxes, tested at the factory, for assembly of the
pontoon ring;
- radial beams and supports;
- rolled deck panel;
- central ring including the cone-shaped membrane; and
- cantledges.
Assembly of the roof frame sectors begins with assembly of a
part of the pontoon ring simultaneously with the radial beams
and the central ring resting on the supports. Towards the end
of assembly a deck sector is cut out of the rolled panel, placed
on the radial beams and welded thereto along the edges. When
doing that its necessary to exclude the possibility of beam
turning away from the plane because of the emerging torque.
The design envisages the use of removable cantledges
weighing about 60 kg each which makes it possible to install
them, for instance, on the supports upon completion of
assembly of the whole roof as well as remove them when
performing repair work inside the tank.
Thus the presented design of a single-deck floating roof of the
frame and membrane type has advantages compared to the
traditional concept in that it guarantees removal of
atmospheric precipitates due to its form.
Compared to a double-deck floating roof, the suggested design
implies substantial reduction in metal consumption.
_________________________________
B.F. Belyaev, Yu.L. Bormot, G.P. Kandakov
Melnikov Central Research and Design Institute of Structures
Str. Arh. Vlasov, 49, Moscow, Russia.
PART I Structures 174
LIGHTWEIGHT STRUCTURES IN CIVIL ENGINEERINGPROCEEDINGS OF THE INTERNATIONAL SYMPOSIUM
Warsaw, Poland, 24-28 June , 2002
NEW PROJECT FOR CONSTRUCTION OF LNG
STORAGE TANKS, 120,000 3
By B.V. POPOVSKY 1, V.A. NADEIN
2, V.I. GUREVICH
3.
1Professor, VNIImontazhspetsstroy, Moscow, Russia. 2Vice President, LLC SGS-Energodiagnostika, Moscow, Russia.
3Candidat Sc. Tech., LLC SGS-Energodiagnostika, Moscow, Russia.
ABSTRACT: The report sets out the main provisions of the Project Specific Technical Standard (PSTS) for the design and
construction of liquefied natural gas (LNG) full containment closed storage tanks 120 000 m3.
1. PRIME PROVISIONS.
The confirmed global gas reserves comprise 120,000 milliard m3.
The annual global gas production amounts to 1,900 milliard m3. In
case of uninterrupted natural gas production its explored reserves
will last for 60 years. In Russia the annual natural gas production is
approximately 500 milliard m3, which is 25% of the global
production. Presently one of the new directions in natural gas
production is the development of gas fields in the continental shelf of
the Sakhalin Island.
For the export of natural gas to the world market it is presupposed to
build a LNG plant in the Sakhalin Island.
According to the project for the construction of the LNG plant at the
southern part of the Sakhalin Island, two isothermic tanks 120,000
m3 will be built for the storage of LNG under -1650 C.
The PSTS was developed in order to determine the main
specifications for the design and construction of isothermic tanks,
which have no analog in Russia. The British standard BS 7777 for
the design and construction of low-temperature tanks was the basis of
PSTS development. Besides, in the course of PSTS development a
wide use was made of the specifications by Shell Global Solutions
(the Netherlands) which acquired a vast experience in the
construction of isothermic tanks in various countries.
Shell Global Solutions representatives provided valuable assistance
to the authors in the course of PSTS development.
On the basis of the developed and approved by the Russian Federal
bodies PSTS Shell Global Solutions is presently carrying out the
detail designing of the LNG storage tanks.
The Sakhalin Island is characterized by the following climatic
conditions:
• Minimal temperature -330
• Maximal temperature +340
• Relative humidity 78.9-87.4%
• Icing 11.0 mm
• Snow coating 162 mm
• Maximal wind velocity 29.2 m/sec
• Soil frost zone 1.96 m
• Allowable value of earthquake 8 (Eurocode) (0.18 g).
The main structural components of the tank are given in Fig. 1.
Judging by Fig. 1 the tank is made as a closed full containment
cylinder.
The internal shell is made of special cold-resistant 9% nickel alloy
steel. The bottom of the internal shell is made of the same material.
The suspended ceiling made of 9% nickel steel is located in the upper
part of the internal shell and hosts the thermal insulation.
The space under the suspended ceiling is open for the vapor over the
surface of LNG that fills the internal metallic shell.
Fig. 1.
The external shell is absolutely hermetic and is made of
ferroconcrete. The availability of fully hermetic concrete shell allows
refusing the earlier used bordering the isothermic tanks with the aim
to restrict the LNG spill area during emergency rupture of the tank.
The bordering did not excluded the opportunity of LNG vapor cloud
formation, which is hazardous for the environment.
The space between the tank internal and the external shells is filled
with thermal insulation.
Tank diameter – 70 m.
Tank height – 34 m.
2. TANK STRENGTH ANALYSIS
The strength analysis of the internal metallic shell of the tank under
operational loads is made by the standard methods on the basis of
selecting the allowable stresses according to BS 7777 standard.
Proceeding from the specifications for constructing such isothermic
tanks, during the strength analysis of the internal metallic shell along
with normal operational loads the following additional loads are
taken into account:
1. Loads resulting from uneven settlement of the foundation in the
radial and tangential directions;
CHAPTER 4 - Tanks and reservoires 175
2. Loads on the internal metallic lining of the ceiling resulting
from the weight of ferroconcrete during the construction of
ferroconcrete roof when the metallic lining is used as a formwork for
the ferroconcrete.
3. Loads on the internal metallic shell including the base of
foundation resulting from the estimated earthquake.
In the course of designing the isothermic tanks of big volumes the
main danger for the environment is the evaporation of large amounts
of LNG resulting from the external shell rupture and the methane
vapors discharge into atmosphere with their possible inflammation.
In this connection during designing a full containment tank a special
attention should be paid to the external ferroconcrete shell strength
analysis.
The external shell includes the ferroconcrete foundation, the
prestressed ferroconcrete wall and the ferroconcrete roof. The wall
has a monolithic concrete connection with the foundation.
The strength calculation of the prestressed ferroconcrete wall is
performed by the finite-element method with due account of two
marginal states:
• Absolute marginal state;
• Operational marginal state.
The analysis of the prestressed ferroconcrete external wall in fire and
during compression loads on the concrete shell due to reinforcement
tension is calculated according to absolute marginal state. The
tension in prestressed steel reinforcement during fire is calculated
with due account of the variations in the yield point of the steel at
wall temperature rise.
During calculations according to the absolute marginal state, the
general marginal elasto-plastic deformation of the prestressed steel
reinforcement is accepted as 1%.
The PSTS specifies the accepted safety factors according to loads
and according to materials under operational and additional loads in
an absolute marginal state. The safety factors depend on favorable or
unfavorable combination of operational and additional loads.
In the course of calculations according to the operational marginal
state, it is necessary to calculate the concrete cracking and the
concrete shell flexure. It is necessary to consider the planned
operational loads with due account of the following additional loads
under the planned emergencies:
• Loads on prestressed ferroconcrete shell under the possible
LNG leakage through the internal shell.
• Loads on prestressed ferroconcrete shell under planned
emergency earthquake.
To ensure the durability of the ferroconcrete shell, the minimal
allowable width of a crack in the concrete is accepted as 0.2 mm. In
case of product leakage, it is necessary to consider the temperature
stresses in the concrete shell as a result of local cooling in the
leakage zone up to -1650 C.
The PSTS specifies the safety factors of the materials under
operational marginal state of the ferroconcrete shell. In this case the
safety factor of loads is equal to 1.
In the course of calculating the ferroconcrete and metallic shells for
dynamic impact of emergency earthquakes, the design seismic load
was accepted as 0.4 g, i.e. earthquake measured 9.0 on the Richter
scale.
The forecast seismic load during the normal operational mode of the
tank is accepted as 0.18 g according to the actual analysis of the
earthquake intensity in the construction region.
In the course of calculation due account was given to dynamic loads
on the tank concrete shell as a result of impact of a metallic object
(valve) weighing 50 kg and flying at a speed of 45 m/sec.
3. TANK DESIGN
Taking into account the impact of low temperature (-1650 C) on the
internal and external shells of the tank, the high cold resisting
property requirements are applied to the materials used in
construction.
The internal shell metal constructions are made of special cold-
resistant ferritic steel with 9% nickel content according to British
standard BS 1501.
It should be noted that the impact strength of the steel under - 1960 C
is 100 J. For comparison, steel grade A-553 type 1 according to
ASTM of the similar chemical composition has the impact strength
under the same temperature equal to 34 J.
The impact strength test is carried out on the samples cut out from
each sheet.
The sheet products are delivered after double temper quenching. The
sulfur cut in the steel does not exceed 0.003%, which is an order
lower than in steel A-553 type 1.
Each sheet over 12 mm thickness is subjected to 100% ultrasonic
testing for foliation.
In order to prevent magnetic field impact on the welding arc, after
rolling the steel is subjected to degaussing, while the residual
magnetism in steel should not exceed 50 gauss.
In the course of the tank internal shell welding it is necessary to
apply the welding materials of nickel alloys, Inconel and Hastelloy,
which composition is 80% nickel. These nickel alloys have the same
linear expansion factor as 9% nickel steel, which fact allows
reducing the internal temperature stresses between the weld and near
weld zone under loading LNG into the tank.
The PSTS envisages application of a special method of ultrasonic test
of welds with high content of nickel by using the ultrasonic
compression wave of reduced frequency.
The main design difference of this tank is construction of the closed
external ferroconcrete shell, which should protect the environment
from liquid product spillage.
For safety reasons, in the construction of the ferroconcrete shell the
use is made of a special cold-resistant steel reinforcement capable of
working under -1700 C in case of the product leakage from the tank
internal shell.
The temperature and the pressure of vapors over the LNG surface
should remain constant. This is ensured by the uninterrupted
measuring the temperature and pressure in the tank and by
uninterrupted automatic control of these parameters with the help of
the dirigible outlet electric safety valves or by supply of additional
cooled gas in the tank under pressure drop within the tank.
Along with outlet valves the tank roof hosts vacuum safety valves
that supply air to the upper part of the tank in case of inadmissible
vacuum under quick product level lowering as a result of quick
emptying of the tank and inadequate speed of gas supply under the
tank roof.
It is necessary to constantly measure the density and temperature
distribution at different levels of LNG in order to monitor the hints of
stratification.
Under LNG stratification the product mixing system is switched on.
The temperature measurement in the wall of the internal shell is
effected by 6 thermocouples vertically located on the shell wall and
by 12 thermocouples located on the diagonals of the tank bottom.
In order to detect the possible LNG leakage, the inter-shell annulue is
equipped with two independent systems for detecting the product
leakage with the help of the leak sensors.
In case of LNG leakage from the internal metallic shell, an alarm
signal appears on the control panel. This signal also appears in case
of failure in leakage control circuits.
In order to prevent frost penetration and heaving in the soil under the
tank, there is a system of the electric heating of the foundation with
uninterrupted monitoring of heating temperature.
PART I Structures 176
The tank location area is equipped with 2 sensors that detect the
possible gas leakage through the tank roof.
All mortise-and-tenon joints of the tank nozzles are located at the
tank roof to prevent weakening of the most loaded lower belt of the
tank internal metallic shell.
The tank roof in the valves location zone is equipped with
camcorders, which ensure valves operation monitoring from the
control panel.
In case of a fire at the tank roof as a result of discharge and ignition
of gas coming from the safety valves, the valves location zone is
equipped with roof automatic water reflux system, which is started
on by the ultraviolet and thermal sensors actuation. The water reflux
of the concrete roof increases its resistance in case of fire.
The strength analysis carried out by Shell Global Solutions shows
that in case of fire inside the tank the ferroconcrete wall may
preserve its operational integrity for 72 hours since the start of the
fire.
4. PRODUCT LEAKAGE HAZARD STATISTICAL
APPRAISAL.
The probabilistic assessment of LNG leak danger through the side
wall of the external shell and gas vapors leakage through the
ferroconcrete roof of full containment tanks was carried out for
various emergencies. The registered natural gas vapors leaks through
the external shell as a result of the roof or process pipelines damage
were characterized by the following reasons:
1. Foundation plate heating system failure 4.6x10-5
occurrence/year;
2. Unrevealed defect in the external shell 1.5x10-5 ccurrence/year;
3. Vacuum inside the tank 5.6x10-8 occurrence/year;
4. Aircraft crash 4.7x10-8 occurrence/year.
The registered LNG leaks through the external shell were
characterized by the following reasons:
1. Emergencies related to the dip of the submersible gas pumping
pump inside of the tank 1.3x10-7 occurrence/year;
2. Foundation plate heating system failure 4.6x10-7
occurrence/year;
3. Vacuum inside the tank 8.0x10-10 occurrence/year;
4. Aircraft crash 9.4x10-9 occurrence/year.
The cases of less than 10-6 occurrence/year are not considered in the
further analysis.
The statistical appraisal of emergencies showed that the most
probable emergencies were related to the possible LNG vapors leaks
through the roof of the external shell.
The possibility of liquid product leak through the sidewall of the
ferroconcrete external shell of the tank is extremely unlikely and is
not considered in further analysis.
The further analysis related to the tank’s strength under emergencies
attention was paid only to the emergencies connected to possible
impact on isothermic tank of a fire or explosion at the adjacent
facilities close to the tank. In such cases most attention was paid to
determining safe distances from the isothermic tank to other highly
explosive and fire risk industrial facilities.
Summary: The full containment tank 120,000 m3 with the
ferroconcrete external shell is more reliable and safe in operation in
comparison to the one-shell and two-shell metallic isothermic tanks.
B.V. Popovsky, VNIImontazhspetsstroy, 9 Perovo Pole 2-nd Lane,
111141 Moscow, Russia.
V.A. Nadein, LLC SGS-Energodiagnostika, 11 Khavskaya str.,
113162 Moscow, Russia.
V.I. Gurevich, LLC SGS-Energodiagnostika, 11 Khavskaya str.,
113162 Moscow, Russia.
CHAPTER 4 - Tanks and reservoires 177
LIGHTWEIGHT STRUCTURES IN CIVIL ENGINEERINGPROCEEDINGS OF THE INTERNATIONAL SYMPOSIUM
Warsaw, Poland, 24-28 June , 2002
1. INTRODUCTION
The present exploring project of load-carrying and frame covering
structures of the aluminium dome of the tank with the capacity of
20 000 m3 with a pontoon for oil storage in Nizhny Novgorod was
worked out in the year 2000. The tank has a stationary roof and a
pontoon. The diameter is 40,34 m, the wall height – 18.0 m.
Specific technological requirements to the dome:
ventilation – naturally ventilated (joint density is not required);
gas impermeability – is not required;
water impermeability – is required;
drainage system – is not provided.
Requirements to the dome material:
Home produced aluminium alloys meeting the requirements to
weldability, durability, heat stability and corrosion-resistance.
2. THE TECHNICAL CHARACTERISTICS OF THE LOAD-
CARRYING FRAMEWORK AND COVERAGE
The aluminium dome 40 340 mm in diameter has a spherical shape
along the support axles with the radius of 29 000 mm and a lift
arrow of 8250 mm. The covering construction is made from
aluminium sheets which are 1,5 mm. The load-carrying framework
is designed in the grid structure. (fig. 1)
Fig. 1
Unlike the system “ULTRAFLOT” the core grid of the load-
carrying framework has been designed in the star-shaped system
and has 64 supports which are jointed to the steel tank supporting
stiffening ring and prevent
displacements of the dome supports in relation to the tank wall. All
the elements of the dome framework without exception are made
in the shape of rectilinear cores from extruded H-beams 240 mm in
height. Here two types of section are used: one type of section for
all the elements of the dome, and the second, much more powerful
section for the support cores of the dome.
All the elements of the dome framework are bolt jointed. The joint
of the dome elements was developed on high-strength bolts M16
(which is a replacement proof unit – fig.2), which requires less
accuracy of manufacturing than a unit on shear bolts of
“ULTRAFLOT” system. The general stability of the dome is
secured by the structure of the framework in combination with the
structure of the unit joints.
The fastening of the assembly elements of the covering to the
framework elements is done in the overlapped way on aluminium
bolts. In case the width of the sheet metal of the covering is
smaller than the external dimension of the covering element, the
sheets welded at the manufacturing plant.
Prefabrication of the framework elements and unit elements
(power cover plates) should be made taking into account the
following
Fig. 2
requirements to the accuracy of manufacturing. The length of the
framework elements – with due accuracy (concerning
manufacture).
For joint elements and framework elements when joined with
high-strength bolts M16 in openings 18 mm:
- distance between groups of openings with accuracy of 0
+0.5 mm
- distance between openings in a group with accuracy of 0 ±
0.2 mm
- opening of 18 mm in diameter – with accuracy of 0 ± 0.2
mm.
Marking and drilling of the hole must be done on the conductor.
The unit plates are cone-shaped with the angle of backing-off
having a design value that makes it possible standard type of the
plate for the whole of the dome. The given joint has been patented.
Priority from 01. 2001 (fig 3).
The assembly of the dome structure is recommended to be done in
the following order:
1. The exterior ring 38 990 mm in diameter is installed on the
assembly lining on the tank bottom.
2. Inside the assembled dome’s exterior ring on the tank bottom
are installed inventory
telescopic supports for the grid structure joints and they are moved
forward to the required height level. After that the lower joint
plates are installed on the supports and the elements of the grid
Dr. I.L. Roujanski
Chief of the Department Melnikov Central Research and Design Institute of Steel Structures, Moscow, Russia
ABSTRACT
The paper considers the structural conception of the 40-m diameter aluminium dome for a 20 000 m3 capacity steel tanks developed by the Melniko
Institute in the year 2000. The dome is spherical having the radius 29 m and the rise of about 8 m. The load-carrying framework of the dome is designe
in the grid structure and is unlike the “ULTRAFLOT” structure both in structure of the grid itself and in the structure of the dome joints and th
supporting joints on the tank wall.
Key words: aluminum done, tank, high-strength bolt, constructions, erection, calculation
PART I Structures 178
structure (from 5 to 8 elements per joint) are mounted on the lower
joint plates.
3. The upper plate is installed after the assembly of all the
elements in the given joint.
4. The locking order of the joint may be either ring or sector.
Bolt tightening in every joint must be done only after
assembling of all the adjoining units. Replacement proof
joint with high-strength bolts must have their contact surface
elements treated with a sand-stream. The friction coefficient
between the joined elements is µ = 0.45
5. After locking the grid structure and forming all the joints the
covering sheets are mounted on the dome framework.
6. The dome is lifted and installed on the supporting elements
in its designed position, after that the dome units are joined
to the supporting elements.
7. The edging covering elements are installed.
3. THE DOME CALCULATIONS
The calculation of the dome was made in accordance with the
following loads and their combinations: the weight of the
construction itself; snow;
Fig. 3
temperature overfall 800 C. As the supporting joints cannot be
displaced in respect to the tank wall, combined work of the dome
and the tank wall were taken into account in calculations. Critical
combinations include the following loading combinations: the
weight itself + symmetrical snow and the weight itself +
asymmetrical snow. To design this construction the variability of
the annual reports for the snow cover weight was thoroughly
analyzed. Four local weather stations contributed to this research
providing data. The approximation of this variability was
calculated by means of Gumbel’s limiting distribution. The
received data suggested that that the figure of 280 kgs/m2 should
be taken as the designed meaning of the snow loads. (It is the
average data provided by four weather stations occurring once in
25 years ).
Calculation diagram of snow loading
Symmetrical snow loading of the tank coating was established
according to the figure 4.
µ1= cos 1.6
– coating inclinations in degrees for all -angles
Asymmetrical loading. Calculating the one-side coating the zero
loading was established, while the other side is determined by the
following formula:
µ2 = ( 2 /700 + sin 48 ) * sin
– coating inclinations in degrees of the dome;
– angle counted from the stationary radius of the coating as to
the radius passing through the projection of the considered points
of the coating. For example, in section II –II coefficient µ2 = 0. In
section I-I on the diameter µ2 = 2 (fig 4).
Calculations were carried out by means of the finite-element
method of the KATRAN program developed in the Moscow
Institution of the Transport Engineering under the guidance of the
Doctor of Sciences Professor Shaposhnikov N.N. . The “dome-
tank construction” is a space system consisting of plate and core.
The position of each joint has six levels of flexibility. Two types
of finite elements were used for the design model assembling:
space core and rectangular plate.
Numerical experiments showed that below the third belt of the
tank (approximately 6 m of height) disturbances from the tense
and
deformed condition of the dome-and-wall work disappear.
Fig. 4
As a result of the calculations two upper wall belts 8 mm thick,
total height about 4 m, were strengthened by the stiffening ribs.
The construction fully meets domestic requirements of the
deformability, durability and production and system assembling
conditions.
At present the construction is at the stage of prefabrication.
I.L.Ruganski, Melnikov Central Research and Design Institute of
Steel Structures, Architect Vlasov str. 49 117393 Moscow, Russia