method - it is kind of the
sheet by sheet erection method. First must be done erection and welding
bottom and the first (lower) course. The shell must be collected course
course on the insider part of the first course and only the vertical
between sheets must be made. After the welding of the last course, the
must be mounted and welded to the last one. The access for coming in
must be assured through one or two part of the roof cover plates by
use of erecting stairs inside and outside.
The lifting began from the last course together
with the roof. The jacking system type “Laterna” (10 – 15 t) must be
hydraulic jacking system evenly positioned on the perimeter of the
insider course must be lift up to the level in which the horizontal lap
between upper course and the next one to be 60 mm of the entire
necessary to be made double lap joint with minimum 2 layers per weld.
weld must be made outside in bottom position and then inside in ceiling
position. The whole shell of the tank is executed as consecutive
welding of the erecting horizontal joints. The correct lifting of the
must be controlled (the different courses must have the same axis) and
bending of the shell as a whole must be avoided.
This type of lifting method, with lap welds on horizontal joints, is
not in application now. According all standards around the world every
one shell joint have to be made as but, with full penetration and
Increasing of capacity of the tank with lifting method and butt welds is often used in Bulgaria. The diameter D
of the tank is unchanged but the highness of shell H
Technical sequence is as follow:
- lifting installation is fixed on project position;
- fillet welds between shell and bottom are removed;
- the shell and roof are jacked up more than new shell course;
- sheets in new shell course are mounted and fixed on project position under the lifted tank;
- vertical joints between steel sheets are performed;
- the tank lowers and horizontal butt joints between existing shell and new course is performed;
- if it is necessary operations could be repeated - jacking up,
mounting of new course, welding of vertical joints, welding of
- welding of fillet welds between shell and bottom.
Increasing of capacity of AST by lifting method
The technology of the Swedish company BYGGING-UDDEMANN AB
must be mentioned as one of the kinds of the lifting method. The
succession of the operations is the following:
- upon the erected bottom must be constructed
the upper shell course of the tank. The roof must be mounted and then
to the shell. The top angle must be made. The course and the connected
roof must be lifted up to the level which allows to put the course
lifted one through hydraulic jacking system. First the vertical joints
executed and then horizontal joint between the two shell. All
be executed as butt weld with full penetration and fusion. The operation must be
till the completion of the construction of the whole shell. The last
is between shell and bottom.
The advantages of the method are as follow:
- the welding operations are executed on the
ground, which increases their quality and assures the security for the
- easier access and easier control to the welded
- scaffolding is not needed;
- when the roof of the tank is erected and the
upper supporting ring is made, the constructors do not need any
stabilizing of the shell against the loss of stability in the radial
caused by wind pressure during the erection work;
parts of the welding joints are executed under the erected roof.
method. This is a method which is
combination from sheet by sheet method and the rolls method, where the
made by rolled sheets, the shell is made by sheet by sheet method and
the roof is made
shields. This method combines the advantages of the rolls, sheet by
This technology allows the acceleration of the erection and the use of
relatively light mechanization.
e) positive tendencies and solutions connected with erection of AST
mounting of tank roofs as one part
The roofs will be mounted in accordance with accepted constructive
solution – as shields or as separate radial and circular girders
covered with plates in cone or dome roofs. Mounting of the roof element by element on the site when is relevant to the roofs where cover plates are not connected to supporting structures.
Roofs mounted shield by shield on the site is used in roll method of
erection. In the same time when the rolls of shell are being
unfolded the mounting of the first, middle and closing
roof shield is
of the two so called classical methods for roof erection obviously
require a lot of time and a lot of people are engaged. Many of
mounting works must be done at big height.
In order to make mounting
faster, to increase the quality of the joints and in order to make
easier control there is a trend roofs to be mounted in
their designed shape on the earth and after that to be put as whole element
above the shell.
Mounting of dome roof on the shell as one assembly
mounting of AST with floating of self-supporting roof
This method can be applied in tanks with floating
roofs and in tanks with self supporting cone or dome roofs.When the bottom and several courses in the shell are
mounted the roof mounts inside the tank. When the roof is finished it must be
stabilized to not turn or overturn.The water is poured gradually and
the roof bigins to float. The level of the water must remain such that
allows the confortable mounting operations and in same time to assure
stability of the shell.
In tanks with fixed roofs and the shell is
already mounted the top angle have to be mounted and then the
roof must be welded to it.
The advantages of this method are as follow:
- the roof is used as floating work platform on wich the workers move. So the scaffolding is not needed;
- the static water load makes the soil tighter. Depending on
geological conditions the process of soil setlement can become faster or can
be completely finished till the moment of tank connection with
outsider infrastructure of pipes. The risk of big deformation and
turning in the nozzle zone is diminished;
- the time needed for hydrotest of the tank can be minimized because the time needed for filling AST also is diminished.
The following disadvantages can be noted:
- the water must be poured gradually inside the tank during the whole
process of erection, not at once, which can cause a problem when the
tank has big capacity;
- the lack of necessary quality of fresh water is possible and it makes
this kind of consolidation impossible. Sea water accelerates metal
corrosion and it is not advisable a contact between the steel and
sea water for a long time;
- the shell of the tank which is not stabilized with stiffening rings behaves inappropriately during the earthquake when is full with water .
Erection of AST with floating of dome roof