LIGHTWEIGHT STRUCTURES IN CIVIL...

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






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.




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;


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.


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.,


V.I. Gurevich, LLC SGS-Energodiagnostika, 11 Khavskaya str.,





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


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

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