Storage Tank Construction

Welding and assembly works are performed based on the Work Execution Plan for the oil storage tank construction. It is the basic technological document, worked out by a specialized engineering design organization on the basis of metal frameworks project.

Work Execution Plan should include and provide:

• Master plan of the construction site stipulating the materials-handling equipment and its positioning;
• Description of procedures, aimed at providing the necessary assembling accuracy and the tank elements’ space stability in the course of top assembly and installing to the project position;
• Measures, providing the load-bearing capacity of the construction elements in the course of storage tank erection;
• Requirements for the quality of welding and assembly works for each procedure of oil storage tank construction;
• Types and the scope of control measures for the storage tank erection;
• The order of tank testing procedures;
• Safety regulations and labor protection norms;
• Environmental protection requirements.

Work Execution Plan sets the order of oil tank elements’ installation, including the use of special facilities and equipment. The project also stipulates measures, aimed at providing the needed geometrical accuracy of the tank frameworks and reducing deformation process resulting from the shrinkage loss of the welding seams.

Technological standards for welding, included in the Work Execution Plan, should stipulate:

• Requirements for preparation of edges for welding;
• Requirements for joints’ assembly for welding;
• Welding methods and regimes;
• Welding materials;
• The order of procedures;
• The order of welding passes and welding of joints;
• Requirements for heating the junctions depending on the air temperature and the tempo of their cooling;
• The need for protection cover in the welding zone;
• The necessity of after-welding heat treatment of the junction;
• Necessary equipment and technological facilities for the oil storage tank construction;
• Methods and the scope of control of the welding seams.

The Register of operational control is to be seen as the essential part of Work Execution Plan, as it sets the requirements for quality control of the welding and assembly procedures.

3.1. Plate-by-plate method of bottom installation

If the bottom is supplied by the producer in plates, it is assembled in the following way:

There are 1 meter-long logs with rectangular or semi-circular cross-section, arranged in squares on the installed and accepted foundation (picture 14), the cross-section area is 0.1×0.1 m. The top row of the squares is better to make of 1.2-1.3 meters’ logs. The height of the squares is equal to 0.8 m, to ensure the possibility to weld the flow-joints and tar the bottom. The centerlines of the squares should be located at not more than 3 meters’ distance from each other, while the distance between the centerlines of the squares’ rows should be twice as big as the width of the sheets minus two widths of the bottom seams’ shaping. Boards (planks) are put along the squares, the bottom is installed on them.

The two elements of the bottom – segmental ring with the welded first ring of the shell and the central part are assembled and welded independently. The welding joint, connecting them together, — the so-called “expansion” seam – is welded only after the complete installation of each separate element.

Assembly of the central part of the bottom starts with the line (zone) leading through the basement center of the tank. Then all the lower sheets of the bottom are assembled in the direction from the center to the edges. Butt-welds of the sheets are tacked in 6-7 positions, the end tacks are located at 50 mm distance from the edges and are made flush. The butt joints are welded after assembling the whole band. The ends of the joints of 50 mm are welded flush so as to ensure the tight contact between the upper and lower bands. After the lower bands are welded, the upper bands are assembled and welded in the same way. The overlap between the bands should not be less than 50 mm.

Assembly of the central part of the bottom is started from the central bands. They are overlapped and assembled on tack-welds, located at the same time at the top and at the bottom on both sides of the workpiece every 250-300 mm in the direction from the center to the edges of the bands. To ensure the adjustment of the bands of the central part of the bottom, the edges of the end-sheets are left without tacks at the length of 750-800 mm in the course of joining the bottom with the segmental ring.

Welding of the bands is done with the overlap seam from the middle of the band to its edges, using the backstep procedure with the step length of 200-250 mm. At first all top overlap seams are welded, then the lower, overhead joints are welded afterwards. After this the butt-joints of the bands are backwelded by overhead weld.

Segmental sheets of the edges are assembled on 10-12 supporting stands, placed along the perimeter of the basement. Segmental ring is assembled in a way to put the two butt-joints on the centerline of the central band and to ensure that the gap clearance between the elements of the ring will not exceed 3-4 mm. After thorough check of the horizontal parameters of the segmental ring the butt-welds are tacked at the ends of the joints; the inside is left without tacks so as to have the possibility to level the segmental ring in the strictly horizontal position in case of distortion in the course of welding procedures.

Prior to assembly of the lower angle joint it is important to weld the parts of the butt-joints of the segments, to which the angle joint is placed. The welding is done in two layers, scrapping the slag and backwelding the overhead seams. The beads are cut down with chipping chisel flush with the surface of the sheets of the segmental ring.

After two circular guidelines are drawn on the segmental ring, which correspond to the inside and outside diameters of the circumference angle, the first angle section is installed and tack-welded. Tacking is done along the outside circle perimeter from the angle center to the ends every 500-600 mm in areas 30-40 mm long. The ends of the angle section are left without tacks at the length of 600-700 mm to ensure the possibility of easy adjustment of the other parts. The other angle sections are assembled on both sides of the first one. They are installed with the overlap of 3 mm, after which they are butt-welded. Then the attached sections are adjusted by the guidelines with tacks to the segmental ring from the butt-joints to the free ends. The final section, which is not less than 1 m long, is adjusted and cut on-site. The vertical panel of the angle should be strictly perpendicular to the segmental ring. The first sheet of the first ring is installed on segmental ring in strictly vertical position after chipping the edges in the lower corners at the height of the corner block and at 1 mm depth to let the butt-joint be afterwards welded to the vertical panel of the corner block. The first sheet is tack-welded at the same time both to the segmental ring and to the angle in checkerboard order from the middle of the sheet to the edges every 400-600 mm in areas 40-50 mm long. The ends of the first sheet are left without tacks at the length of 600-700 mm to ensure the possibility of easy adjustment of the other sheets. The other sheets of the first ring are installed on both sides of the first sheet with a gap clearance of 2-3 mm between the sheets and connection of the edges. The tack-welding is started from the butt-joint with the first sheet, the tacks are made in 4-6 positions each is 60-75 mm long. Then the tack welding is done along the lower edge of the sheets from the tack-welded butt-joints to the free ends. The final sheet of the first ring is adjusted and cut on-site.

The welding of the bottom, assembled in this way, and the first tank ring is done in the following order:

1. All joints of the first ring are welded at the height of 200-300 mm from the segmental ring and at 50 mm from the edge in the upper part, flush with the surface of the sheets to ensure the tight contact of the second ring’s sheets in the course of the successive installation.
2. All circular seams are welded: the first ring is welded to the segmental ring by double seam, after that the circumference angle is welded by single seam: first to the segmental ring, then to the first ring of the tank.
3. The butt-joints of the segmental ring elements are checked and undercut (if necessary) to eliminate waviness and to install 3-4 mm gaps, after which the butt-joints are welded with overhead back-welding and reinforcing with steel straps of sheet steel 8-10 mm thick. At the same time the butt-joints of the circumference angle are reinforced by welding pin of corner steel.

Prior to welding the central part of the bottom with the segmental edge, the joints’ edges of the lower bands are planned, cut with a 2-3 mm gap, tacked and then welded with overhead back-welding. Then the ends of the upper bands are planned, cut with an overlap not less than 30 mm, tack-welded at the long parallel edges at first and then to the segmental ring. The welding is done in the same order as tacking. The welding operations at the points of seams’ crossing should be done only by highly-professional welders.

3.2. Assembling coiled bottoms

Bottoms of tanks of loading capacity up to 2000 m³ and up to 12 m in diameter, as a rule, are completely welded at production site and coiled. The coil is then rolled to the basement in a way to ensure that the middle of the coil is positioned along the centerline of the basement. The bottoms of larger tanks, having more than 12 m in diameter, cannot be fully loaded to the open wagon, which is 13.66 m long. They are made of several parts, placed on each other while coiling.

The coil with the bottom, consisting of two parts, is placed on the basement in a way to provide that the first half of the bottom, which is the outside cover of the coil, will be installed in the planned position after uncoiling. In this case the second half of the bottom will be put on the first one.

Panels, fixing the coil, are cut with oxygen, after which the coil is unfolded when the loop of the cable rope is eased off. In case the coil failed to unfold spontaneously (under the influence of elastic forces) the further uncoiling is done with the help of truck tractor or hauling winch. When the coil is fully unrolled the center of the circular edge of the top half-bottom has a brace welded on it, meant for fixing the end of the cable rope for moving the second part of the bottom to its project position with truck tractor or hauling winch. Then the butt-joint of the two halves of the bottom is assembled for welding. It is always done with an overlap. It is fixed with tack welds from the center of the bottom to its edges, tightly pressing both panels to each other.

If the bottom is assembled from three strip panels successively coiled, the first coil is unfolded to its project position, then it is loaded to the sledge together with the other two and they are moved by truck tractor in a way, enabling to uncoil the second strip panel to the project position. After this the last coil is again loaded to the sledge and transported to the other side of basement to unroll the third one.

Pictures 7-13 show the order of assembly operations for coiled bottom of a tank of 400 m³ loading capacity.

3.3. Extreme deviations of dimensions and shapes of the installed bottom

Regardless the method of the bottom installation, the deviations of its size and shape should not exceed the following rates:

• Maximum allowable height of local bulges and buckles in the central part of the bottom is determined by the formula: f ≤ 0,1R ≤ 80 mm, where f stands for maximum pointer – сarrow dents of a bulk or a bulge on the bottom, mm; R stands for the radius of inscribed circle at any area of a bulk or a bulge, mm. Cutting (sharp) bends and wrinkles are not allowed.
• Local deviations from the planned shape in the zones of radial assembly welds of the edges’ circle (angularity):±3 mm (the measurements are done with the pattern with 200 mm base).
• The rise of edges in the connection zone with the central part of the bottom is determined by the formula:

• fa ≤0,03L for bottom’s diameter equal to 12-25 m;
• fa ≤0,04L for bottom’s diameter more than 25 m, where fa – the height of the edge’s rise, mm, L — the width of the edge, mm.

• The point of the outer profile of the bottom:

Empty tank:	Tank diameter
	up to 12 m	12-25 m	25-40 m	40 m
The difference in the marks of neighboring points at a distance of 6 m around the perimeter	10 mm	15 mm	15 mm	20 mm
Difference of marks of any other points	20 mm	25 mm	30 mm	40 mm

 

Water-filled tank:	Tank diameter
	up to 12 m	12-25 m	25-40 m	40 m
The difference in the marks of neighboring points at a distance of 6 m around the perimeter	10 mm	15 mm	15 mm	20 mm
Difference of marks of any other points	20 mm	25 mm	30 mm	40 mm

3.4. Corrosion preventive treatment of the bottom:

After thorough cleaning of the lower surface of the bottom with wire brushes a cool prime coat is applied – a thin layer of primer (a solution of stearine pitch in benzol or bitumen in petrol).

After the primer becomes dry the bottom is covered with two layers of hot bitumen added with filling material, as it is done in the course of pipelines’ insulation.

To ensure applying the coat to the whole bottom’s surface the squares are replaced from one position to another.

5.1. Installation of fixed roofs.

Depending on the construction scheme of a fixed roof either of the following procedures is performed:

• Installation of framed conical and spheric roofs – with the central supporting stand;
• Installation from the top, without central supporting stand: this is applied for frameless conical and spheric roofs and for framed conical and spheric roofs with separated elements of framework and decking;
• Installation from inside the tank, without the central supporting stand; this is applied for roofs with separated elements of framework and decking;
• Installation of framed spheric roofs inside the tank with their subsequent lifting to the planned position.

In the course of working out the technology for installation of roofs it is essential to consider the assembly load on the roof as a whole and on its construction elements. It may be needed to install temporary bracing beams, ties and other devices, meant to prevent deformation.

The height marks of the central board and the assembly stand on tanks with framed spheric roof should be determined considering the planned height and camber, stipulated by the design project.

Extreme deviations of the installed roof’s size and shape should not exceed the following rates, stipulated in the table:

Name of the parameter and notes	Limit deviation, mm, if the diameter of the tank
	12 m	12-25 m	25-40 m	40 m
The mark of the top of conical and spherical roofs (measurements are carried out through the central branch pipe)	±30		±50
The difference between the marks adjacent nodes top of radial and girders: — in the interface zone with the shell: — in the interface zone with the central shield: — in the area of joining of radial beams of spherical roofs:	20 10 15
Deviation from the design radius of spherical roofs. Clearance between the template and the curved surface (measurements are carried out on each radial beam and truss).	5,0

 5.2. Installation of floating roofs and pontoons

A pontoon or a floating roof is installed at the bottom of the tank after it is assembled and controlled to be leak proof.

Extreme deviations of the installed roof’s size and shape should not exceed the following rates, stipulated in the table:

Name of the parameter and notes	Limit deviation, mm, if the diameter of the tank
	12 m	12-25 m	25-40 m	40 m
Mark the upper edge of the outer ring sheet (Board): — the difference between the marks of the neighboring pixels at a distance of 6m on the perimeter: — the difference between the marks of the other points:	30 40
Deviation of the outer ring sheet from the vertical height of the sheet (the measurements are carried out at least every 6 m along the entire perimeter)	±10
Deviation guides from the vertical to the entire height of the guides N, mm, in the radial and tangential directions	1/1000 Н
The gap between the top edge of the outer ring sheet and the vessel wall (dimension spend every 6m on the perimeter. Position — pontoon at the bottom)	10
The gap between the sending and pipe in a pontoon or a box floating roof (position — pontoon at the bottom)	15
Deviation of supports from the vertical when resting on them pontoon or a floating roof	30

7.1. Methods of quality control of the welding joints in the course of assembly works

Quality control of the welding joints in the course of tank construction should imply:

• The use of welding methods, ways and scope of control of the welding joints, applicable to the tank’s importance level;
• The use of effective technological welding procedures and materials according to the stipulations of metal frameworks plan and Work Execution Plan;
• Carrying out technical and construction design supervision.

The table contains information on methods of quality control of the welding joints, applicable depending on the controlled element of the tank:

The scope of physical control of welding joints (in percentage to the joint’s length) of the tank shell, depending on the tank’s class of hazard, should correspond to the requirements of the table: 

VOLUMES of the PHYSICAL CONTROL of  WELDED JOINTS of  TANK SHELL, as % of the length of the seam
Control zone	hazard class
	IV	III		II	I
		1 000 – 10000m³	10000 –20000m³
vertical welded connections:
in zones 1-2	20%	25%	50%	100%	100%
in zones 3-4	5%	10%	25%	50%	100%
in zones 5-6	2%	5%	10%	25%	50%
the zones above the 6-th	-	-	5%	10%	25%
horizontal welds:
in zones 1-2	3%	5%	10%	15%	20%
in zones 3-4	1%	2%	5%	5%	10%
in zones 5-6	-	-	2%	2%	5%
in other zones	-	-	-	2%	2%

While choosing the areas of control the prevailing attention should be paid to the areas of joints’ crossing.

Assembly butt-joints of the tanks installed with coiled method with the loading capacity from 1000 m³ and more must be controlled on the full length of the joints.

The results of tests and quality control of the welding joints are fixed in certificates and acts and represent the essential supplements to the documents for the tank.

7.2. Final tests after the oil storage tank construction

The final stage of the tank testing is the hydraulic pressure test, meant to check the tightness of connections and the durability of construction on the whole.

Tanks with fixed roof without a pontoon are additionally subject to extra pressure and relative under pressure from the inside.

The table below stipulates the tests that need to be done for tanks of different types (1 - tanks with fixed roof without a pontoon, 2 - tanks with fixed roof and a pontoon, 3 - tanks with floating roofs).

№	type of test	1	2	3
1	Watertight test	+	+	+
2	Strength test of tank body under hydrostatic load	+	+	+
3	Air pressure tightness test of a fixed roof	+	-	-
4	Test the stability of the tank body by creating a relatively rarefaction inside the tank	+	-	-
5	Buoyancy test of floating roof	-	+	+
6	Rolling ladder performance test	-	-	+
7	Testing the stability of the base of the tank with the determination of absolute and uneven draft along the contour of the bottom, roll of the tank profile of the central part of the bottom	+	+	+

Hydraulic pressure test of the tanks with floating roofs or a pontoon are done before installing the rim seals.

The tests of a tank of any type are carried out based on the testing program, included in the metal framework design project and Work Execution Plan.

The testing program should include:

• Stages of testing, stipulating the level of loading/discharge of water and curing time;
• Rates of excessive pressure and relative under pressure, test curing time;
• Scheme of visual inspection;
• Demands for measuring the necessary geometrical parameters of the construction elements of the tank and its foundation;
• Testing results’ processing, checking calculations (if necessary), final report on usability and operating regime of the tank.

7.2.1. Hydraulic pressure tests of a tank

In the course of hydraulic pressure test the tank is gradually filled with water to the planned level. The loading is performed in stages with certain time gaps, meant for supervising its settlement and the state of welding joints, and for carrying out other measuring and inspection, stipulated by the testing program.

If leaking from under the edge of the bottom or in the first ring of the shell is revealed in the course of testing, the water is fully discharged (removed). If cracks are revealed in the shell seams the water is discharged to the level lower than the discovered defect. If the defect appears in the 2^nd-6^th ring, the water is removed to the level one ring lower the defect. If the defect appears in the 7^th ring and higher – the water is removed to the 5^th ring. After the defects are eliminated the testing goes on.

Tanks for storing liquid materials with the mass density, exceeding that of water, as well as the tanks, installed at sites without water availability, can be tested with the product itself. Prior to such tests all welding joints of the shell, bottom, roof and manway plates/fitting pipes, and also connections of shell with the roof and the bottom should be tested for leak tightness.

The tank filled to the planned level must be kept under load within the following period:

• Vertical steel tank V≤10000 m³ — 24 hours;
• Vertical steel tank V=10000-20000 m³ — 48 hours;
• Vertical steel tank V≥20000 m³ — 72 hours.

The tank is considered to pass the test if no leaking appears on its surface of at the edges of the bottom within the test period, if the level of test product does not decrease, if the settlement (sinking) of the foundation and basement becomes stable.

The hydraulic pressure test is recommended to be carried out at the temperature level not less than +5 °С. If the test is done in winter period the water should be heated, or its permanent circulation should be provided so as to prevent its freezing in the pipes and gate valves, and to prevent frosting of the tank shell.

7.2.2. Pressure/under pressure testing for the tank’s body and roof:

Fixed roof of a tank without a pontoon is tested for excessive pressure with the tank filled to the level, which is 10 % lower the planned level, with keeping the load for 30-minutes. The pressure is provided by the flow of water with all roof manway plates tightly closed. In the course of testing the full visual inspection of welding joints of the fixed roof is done.

The steadiness of the tank body is checked by relative under pressure inside the tank, filled with water to the level of 1.5 m, keeping the tank under the load within 30 min. The relative under pressure is provided by the discharge of the liquid product with all manway plates tightly closed. If no signs of unsteadiness are revealed (no canning, no buckles), the shells and roofs are considered to pass the test for relative under pressure.

The excessive pressure is applied at the rate of 25 % higher the planned level, the relative under pressure – 50 % higher the planned level, if the design documents do not contain other demands.

After the acceptance test is done, it is not allowed to weld any construction elements to the tank. It is possible to perform corrosion preventive procedures, heat insulation and installation of equipment, stipulated in the design documents.

After the hydraulic pressure testing is done, the actual technical condition of the tank’s metal framework, basement and foundation should be evaluated.

7.2.3. The basic demands for the arranging and performing the tests:

The final testing of a tank for durability, steadiness and leak tightness is done after all welding and assembly procedures are over, the quality of all construction elements, including welding seams, is checked and accepted by the technical supervision organization.

All tests are done according to the technological chart, included in Work Execution Plan. Technological chart should include:

• The order and regime of the hydraulic pressure test;
• The order and regime of testing for excessive pressure and vacuum;
• Arrangement of temporary pipelines for water loading and discharging with safety and shutoff valves’ installation;
• Control panel;
• Labor safety requirements in the course of testing.

Temporary pipeline for water loading and discharging should be arranged outside the area of embanking. The scheme of water discharge is worked out for each particular case. In the course of testing the water is often pumped from one tank to another, and from the last one to the firefighting tank of temporary water body.

Diameter of the pipeline for loading and discharging water should correspond to the estimated level of loading-discharge operations’ efficiency.

Apart from the operation scheme of flow and discharge of water, an emergency unloading scheme should be foreseen in case cracks appear on the tank’s body. For emergency purposes it is recommended to use one of the intake-dispensing fitting pipelines and technological pipeline with a valve outside the area of embanking.

It is essential to set and specially mark with preventing and safety signs the borders of danger zone for the testing period. If an embanking or a protective shell is arranged round the tank, they are considered to be the border of the danger zone. In case the tank is installed without embanking, the limits of the danger zone are determined by the radius, drawn from the center of the tank for the distance equal to two diameters of the tank.

The testing is done by the assembly organization accompanied by representatives of technical supervision service and construction design supervisor. After the tests are finished, the installation specialists and the ordering customer sign a certificate (an act), stating the finish of the metal frameworks’ installation and acceptance of the tank for further corrosion preventive procedures, installing equipment and other works.