This Method Statement describes the specific safe working methods which will be used to carry out the construction of new storage tank and related facilities.
Document covers the scope of work required for the execution of the tank construction works.
Below is sequence of activities or steps that shall be followed for the steel storage tank installation and fabrication activities.
- Tank specific design and drawings
- Construction of new tank foundation
- Plate, structural steel and tank component procurement
- Floor, shell and roof plate fabrication
- Installation of new floor plate and double deck roof
- Fire protection system
- Instrumentation and electrical works
- Demolition of tank foundation, floor and roof
- Tank decommissioning
- Tank isolation
- Tank cleaning
- Tank conditional inspection survey
- Blasting and painting works
- Hydro-testing
- Calibration
- Hook up to existing process line, commissioning and demobilization
Codes, References & Recommended Practices
API 650: Welded steel storage tanks for Oil Storage
API MPMS 3.1B: Manual of Petroleum Measurement Standards Chapter 3 – Tank Gauging (Section 1B: Standard Practice for Level measurement of Liquid Hydrocarbons in Stationery Tanks by Automatic Tank Gauging)
14FZ: Recommended practice for Design and Installation of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities for Unclassified and Class1, Zone 0, Zone 1 and Zone 2 Locations
NFPA 30: Flammable and Combustible Liquid Code-2000 Edition
BS 8110: Structural use of Concrete in Building
Necessary Machinery and Equipment
30 ton mobile crane
Flat deck/low bed trailer
Semi automatic welding machine
DC welding machines (400A)
Air compressor
Sand blasting set
Spray painting set
Wheel loader
Tipper truck
Mini dumper truck
Fuel tanker
Power generating set (30kVA, 50kVA, 60kVA, 100kVA)
NDT Equipment
Man Lifting Equipment (25m Boom height)
Dewatering pumps
Pneumatic power winch
Pneumatic air winch
List of Relevant Human Resources: Numbers depend upon the site work
- Construction Manager
- Site Engineer
- HSE Officer
- QA/QC Officer / QA Engineer / QC Inspector
- Site Supervisor
- Pipe/Plate Welders
- Pipe/Plate Fitter
- Structural Welders
- Riggers
- Grit Blasters
- Operators
Work Methodology and Work Sequence for Tank Construction
Tank Specific Design and Drawings
The following elements design shall be considered for adequacy in accordance with the relevant codes and standards as may be applicable and any findings communicated to the client for review and approval before changes will be effected.
Tank foundation shall be verified in accordance with appendix B of API 650:2005 and BS 8110.
The floor and annular plates shall be verified in accordance with the requirements of clauses 4.4 and 4.5 of API 650:2005.
Shell plates shall be verified in accordance with the requirements of clauses 4.6 of API 650:2005.
Roof and other roof accessories shall be verified to meet the requirement of Clause 4.10 of API 650:2005.
Nozzles shall be verified to meet the requirement of clauses 4.7.6 for shell and 4.8.5 for roof of API 650:2005.
The piping system shall be verified to meet the requirement of ASME B31.3 6.2.
Plates, Structural Steel and Diesel Storage Tank Components Procurement
Steel plate and sections procured shall be accompanied with their mill certificates.
Samples shall be taken from the procured materials to confirm that the chemical and mechanical materials conform with the requirement of clause 4 of API 650: 2005 6.3.
Cutting, Forming and Fabrication Plate material shall be straight and/or flattened by pressing or rolling audtties shall be done before the material is processed in any way.
Marking
To enable the various fabricated components to be assembled together correctly on site, each part shall be marked with unique numbering system as shown on the “Approved for Construction” drawings.
The erection marks shall be clearly written in paints and shall be at least 50mm high.
Plate Edge Preparation
The edge of the tank plates shall be sheared, machine cut or beveled with a machine operated gas-cutting torch.
Shearing shall be limited to plates less than or equal to 10mm thick used for butt-welded joints and to plates less than 16mm thick used for lap-welded joints.
All shell plates shall be profiled to tolerance of ±2mm in length and ±1mm in width.
In addition, to ensure that plates are truly rectangular, the diagonals, measured across the rectangle formed by scrubbing lines 50mm from each edge, shall not differ by more than 3mm for lapped welded roof and bottom plates, normally mill tolerances are acceptable.
Curved edges of roof, bottom sketch plates and annular plates shall be cut to shape by the machine operated gas torch.
Various radius templates will be used for marking and cutting.
The resulting surface shall be uniform, smooth and free from scale or slag accumulations.
Beveled edges shall be ground to remove all the above accumulations.
Underside of tank bottom plates shall be blasted and coated as per procedure on blasting and painting.
Rolling of Shell Plate
Shell plates shall be shaped to the correct curvature of the tank.
Rolling will be developed in place, in a fabrication shop specifically designed and built for this purpose.
Plate bending machine having rolling capacity 16mm thick and 2500mm wide size shall be used.
Pre-bend 200mm of both ends, of shell plate edges to the required radius.
Plate shall be rolled within permissible localized variations as specified in ASTM A6 & A20 and shall be free from buckling, dents, excessive drawing, distortion or obvious imperfections.
Templates shall be used to measure the rolled plate’s curvature.
Rolling of shell plate giving required load stage by stage on rolling direction.
Check radius using the above template stage by stage until the required radius is obtained.
Rolled shell plates shall be stacked on radius saddles, which is same to that of the rolled plates for further process.
Plates dimension shall be verified prior to the start of edge preparation or rolling operations.
Clean-out Door
Clean out door plate cutting and marking shall be as per “Approved for Construction” drawings.
Machining of clean out door flange and cover shall be as per drawings.
Stress relief shall be carried out according to API 650 and Approved post weld heat treatment (PWHT).
Nozzles, Manhole and other Appurtenances
All nozzles, manholes and other appurtenances shall be fabricated & PWHT in the shop, in accordance with the approved construction drawings.
A protective shelter shall be used.
Pipe shall be pre-heated around its circumference and maintained on 12” sections on either side of the weld.
Each weld shall be started (root pass) and finished while the preheat temperature is maintained without interruption.
Pipe shall be covered immediately after welding and be permitted to cool at uniform rate.
Steel Storage Tank Erection Method
Before installation of the bottom plate, the tank foundation shall be inspected visually for evidence of cracks, damage etc.
A check of the dimension, level, reference point and foundation profile accuracy shall be carried out prior to commencement of tank bottom plate installation.
Spot level of the foundations shall be verified and tolerances shall be as per the limits specified in clause 4.3.1 and Appendix B API 650: 2005.
Exact centre point and orientation (0˚, 90˚, 180˚, 270˚) shall be marked.
Laying of Annular plates
The underside of the plates shall be painted with black bituminous paint before laying.
A backing plate shall be fixed to each annular plate.
Each annular plate shall be placed in accordance with the “Approved for Construction” drawings in correct orientation.
The radius of each annular plate outer edge shall be measured to ensure that the outer edges are the correct distance from the centre of foundation.
Fit-up between annular plates shall be adjusted as applicable or required.
Upon completion of installation and prior to welding of annular plates, a final check of the joint fit-up welding gaps and radius shall be made.
The weld area upon which the shell plates are to be erected shall be ground flush.
All welds shall be inspected by vacuum box technique and penetration method.
Laying of Bottom Plates
The bottom plates, starting from the centre plate shall be laid progressively towards the annular plates in accordance with the project drawings.
Water draw-off sump shall be installed and welded according to “Approved for Construction” drawings.
Floor welding shall start from the centre of floor with the short transverse lap joints working outwards each side of the centre to the periphery of the floor. This sequence shall be repeated for the strakes of plate each side of the centre strake.
This shall be repeated for the strake adjacent to the last welded until the transverse weld are completed.
The longitudinal joints shall now be welded starting at the centre of the floor and working outwards to the periphery from each side of the floor centre line which is transverse to the setting out line.
Where three plate thicknesses occur in the floor lap joints, the upper plate shall be joggled, or cut and joggled.
Tank Shell Assembling
Preparation before Assembling
The shell plates shall be placed around the tank foundation.
All required fixtures shall be installed on the inside of the shell plates.
All required fixtures shall be installed on the inside and outside of the annular plates to ensure that the shell plates are aligned with the scribed line.
Bracket hooks shall be fitted inside and outside each shell plate before it is lifted into position.
Shell Assembling Steps
Each shell plate shall be secured on the annular plate and aligned by using guides and wedges.
The first shell plate with its edge on the start mark shall be erected upright using a 100 mm channel support.
Strong back plates shall be employed to secure adjacent shell plates. An alignment and width check of joint gaps shall be carried out before tack welding the vertical seams.
The alignment fixtures shall be removed, all tacks cleaned and all vertical joints of the first shell course welded.
The shell course dimensions shall be checked for roundness at the bottom and top levels, vertically at a minimum of two points and for level at the shell plate top-level line.
When checking is completed the second course of plates shall be placed into position together with the first shell course.
For the erection of the second and subsequent courses, scaffold brackets and plates shall be attached to the shell plates and secured with rope.
The shell plates shall then be lifted into position.
On completion of assembling the shell plates handrail shall be installed on the inside of the scaffold
Wall ladders shall be attached on the inside wall of the tank.
The second course vertical joints shall be aligned and welded.
Aligning horizontal seam between 1st & 2nd courses, and fit up check shall be carried out after tack welding.
The alignment fixtures shall be removed, all tacks cleaned and horizontal joints shall be welded.
After completing welding of horizontal joint, 3rd shell course shall be erected.
The same sequence shall be followed to complete the assembly of the remaining shell courses.
Vertical seams shall be located, as to clear all shell appurtenances and their reinforcing pads.
Guying system shall be used during shell courses erection from 4th. Course and shall be moved up progressively.
Tank Shell Plumbness
To inspect the roundness of the first shell course, the distance from the auxiliary line scribed on the tank bottom inside the tank shell, to each bottom shell plate, shall be measured.
If the distance is not within tolerance it shall be corrected by means of the wedge guides and wedges.
The verticality of the first shell course shall be inspected by using an internal plumb line suspended from the course top and the distance between the plumb line and the inside of the tank shell at the bottom, measured at a minimum of two positions for each plate.
If the tank shell is found to be titled in excess of the tolerance limit, the temporary 100mm channel supports shall be removed from the first shell course and the tank shell verticality corrected by means of a hand lever block.
To inspect the level of the first shell course top line, a graduated level shall be installed inside the tank at the centre of the tank bottom, with its top aligned with the top of the shell course. The graduations shall be read at two or more positions per shell plate.
The level shall be corrected by driving wedges between the foundation and the tank bottom where the tank shell is low.
Space between the lifted tank bottom and the foundation shall be filled with non-shrink grout.
Shell Manholes
The manhole contour shall be marked on the shell plate in accordance with the “Approved for construction drawing”.
Client/consultant inspector shall inspect the scribed manhole lines before the manhole is cut out.
Tank Floating Roof
A grid formation of vertically adjustable scaffolds support (Acrows) shall be set to suit the final level of the underside of the roof pontoons and deck.
The supports shall be held securely in place with scaffold poles and clips.
Roof shall be completely erected and welded on these supports and all the roof support legs, nozzles, manholes and all other roof appurtenances shall be fitted to the roof.
Once the legs are in place and pinned in position, the supports and scaffolding shall be removed from the tank through the shell manholes.
The outer rim of the pontoons shall be supported off temporary brackets welded to the tank shell.
Staircase
Staircase shall be prefabricated, erected and welded according to “approved for construction drawing”
Top Hand-railing & Platform
Top hand-railing & platform shall be prefabricated, erected and welded according to approved drawing
Welding Method for Steel Tank Fabrication
All welding of tank plates, steel framing, structural attachments and mountings in both the fabrication shop and in the field shall conform to welding procedures and qualifications per section IX of ASME BPVC.
Sequence of Welding
The sequence employed for tack welding and final welding of the bottom, shell and roof plates shall be arranged to minimize distortion due to weld shrinkage.
Annular plate
All joints shall fit-up except one and welding shall start either odd or even joints first.
After that, welding shall start from any one joint to both directions i.e. clockwise and anti-clockwise.
Before welding of the final joint make sure the annular plate outer diameter as required.
Particular attention shall be given to the welding of butt-welded seams for the annular plates where these pass under the shell plates, to ensure that full penetration is obtained.
Necessary NDT shall be applied according to approved contract drawing & API 650.
Bottom plate
Short seam joints fit-up and welding will start first from bottom centre to side.
After welding of short seam joints the long seam fit-up and welding will starts from centre towards side.
Complete the welding of both long joints adjacent to the centre point, after that alternate joint.
Finally complete all remaining long seam joints in the same manner.
Keep open at least 8” from open ends of long seam with annular plates.
Annular plate to bottom sketch plates fit-up and welding shall start after completion of shell to annular welding.
Three lap joints shall be rounded and bent using horse shoe and wedge with hammering, direct hammering not allowed, and these joints shall be tested.
Shell plates Vertical joints
Fit-up and welding shall start at alternative joints. After that, the remaining joints fit-up and welding shall start from any one joint in both directions i.e. clockwise and anti-clockwise.
Before welding of the final joint the shell inner diameter shall be verified.
All other shell courses shall be completed as above.
Shell Plates Horizontal joints
After completion of first and second shell course vertical joints, horizontal joint fit-up and welding shall start from any point in both clockwise and anti-clockwise directions.
Welders shall be deployed at approximately equal distance, and welding progression shall be one direction.
All other horizontal joints will be completed by the same welding method.
Roof Plate
Short seam lap joints of each roof plate segments shall be completed first.
After erection of roof plates the fit-up and welding of alternate long seam lap joints will complete.
The remaining long seam welding will be completed in the same manner.
Three lap joints shall be rounded and bent using horse shoe and wedge with hammering, direct hammering not allowed, and these joints shall be tested.
Weather Conditions
Welding shall not be carried out during rain, sand storms or high winds, unless the welder and the work are properly shielded.
To avoid delayed cracking the base metal temperature should be kept in between zero F to 32o F, the base metal joint shall be preheated to temperature warm to hand before starting the welding within 3 “ of welded area.
During inclement weather following shall be considered;
- A protective shelter shall be used.
- Pipe shall be pre-heated around its circumference and maintained on 12” sections on either side of the weld.
- Each weld shall be started (root pass) and finished while the preheat temperature is maintained without interruption.
- Pipe shall be covered immediately after welding and be permitted to cool at uniform rate.
Welding Procedure
The welding procedure shall be as follows:
In multilayer welding, each layer of weld metal shall be thoroughly cleaned by removing all slag and other deposits before the next layer is applied.
All completed welds shall be free from slag, brushed and thoroughly cleaned before final inspection.
The reverse side of double welded vertical and horizontal butt joints shall be back-chipped, ground after the application of the first run of weld, in a manner which will leave the exposed surface accessible and satisfactory for the fusion of the weld metal to be added.
Back gouging or grinding to bright metal is required.
Undercutting and other weld defect shall be restricted to code permissible.
The weld metal on both sides of all butt joints, except offset faces of horizontal joints of unequal plate thickness, shall be built up in the form of an overlay so that the finished face in the area of fusion extends above the surface of the adjoining plates to a height of not more than 1.5 mm.
The edges of all welds shall merge with the surface of the adjoining plates without a sharp angle.
Automatic welding shall be used, subject to written approval from the client.
When automatic welding is used, details of plate edge preparation shall be specified.
Procedure qualification for shell- to-bottom joints shall be performed on the actual joint configuration, material and thickness to be used.
The hardness of the heat affected zone shall not exceed 280Hv.
Vertical joints in bottom shell courses shall be at least 300mm clear of any annular plate joint.
Vertical joints in adjacent shell courses shall be offset by at least one third of the plate length but with a minimum of 500mm where make-up plates are used.
Butt welds joining compression rings top angle or wind girder sections shall be at least 150mm away from any shell vertical seam.
All shell vertical joints shall be welded by manual Arc Welding.
Progressive RT & other NDT required shall be carried out as per contract & code requirements.
Bottom Plate Joints
All lap joints on bottom plates shall be full fillet welded on the topside only.
The annular plate’s radial seams shall be butt-welded and shall have complete penetration for the full thickness of the annular plate, with a backing strip of 6 mm thickness.
Special attention shall be given to the required gap between the plates to ensure full penetration.
Bottom to Shell Plate Joints
The shell plates shall be continuously welded on both sides to the annular plates with the leg length of both fillet welds equal to the thickness of the annular plate.
Shell Butt Joints
Shell butt joints shall be made as follows and as per API 650 latest available edition as applicable:
- All seams shall be butt welded from both sides of the plate.
- Vertical butt joints, whether square, single “V “or double “V’, shall have complete weld penetration through the full thickness of the parent plate.
- All horizontal square and single bevel butt joints shall have complete penetration through the full thickness of the parent plate.
- Horizontal double bevel butt joints shall have complete penetration through the full thickness of the parent plate.
Roof Plate Joints
All Roof plate joints shall be full fillet welded on the welded side.
All roof lap joints and nozzles penetration shall be seam welded and weld spatter shall be removed to give smooth internal surface.
Tank Fire Protection System
The tank fire protection system shall be installed in accordance with the “approved for construction drawings”.
Hydro Testing
Upon completion of mechanical works, each tank shall be prepared for hydro pressure test.
All the hydro test materials such as blind flanges, pumps, recorders, break tanks, temporary line filters, and screen and installed in position ready for hydrostatic test.
The appropriate filter size shall be installed for the hydro test.
Upon completion of a successful hydro test, the tank shall be drained, all blinds and/or spades shall be removed and all valves re-fit.
The manhole shall be opened and the tank internal areas made clean and dry in readiness for calibration.
Should a hydro test fail for whatever reason, necessary repairs shall be carried out, affected area and further hydro test shall be perform.
This remedial work shall be carried out at no cost to the client.
Blasting and Painting Works
Blasting and painting shall be carried out in accordance with the approved procedure for blasting and painting.
Calibration
Upon successful hydro test, the client shall be informed of readiness to calibrate the tank and make sure the presence of client representative during the calibration and measurements.
All calibration procedures together with any proposed sub-contractor shall be approved by the consultant and shall be in accordance with all regulatory requirements referenced in the contract.
Calibration shall be carried out using both optical reference and strapping methods.
Critical zone determination shall be determined by liquid metering.
Upon completion of the calibration work (field data measurements and table compilation), six copies of the calibration tables in US barrel units and one copy each in kiloliters with electronic copy each shall be submitted to the client.
Client / consultant shall review and approve the tables; the approved table shall be submitted to the local regulatory authorities together with an application for approval.
On approval of the calibration tables, 8 laminated sets and an electronic version of the approved table including laminated copies of approval letter shall be provided to the client/consultant.
Health and Safety Requirements
It is the intention of this document that all work methods and procedures are carried out in a safe manner in accordance with the Health and Safety At Work Act and any other relevant legislation.
All site employees will attend any Site Safety induction course that may be required for the specific activity the personnel will be involved in on site before commencement of such activity.
There can be assorted hazards associated with working on any site and site regulations as laid down by the Facility Operator should be observed.
Equipment and associated items may be heavy and care should be taken when handling manually.
It is generally preferable to reduce any risk by avoiding manual handling altogether.
When unavoidable, correct lifting procedure should be used.
Heavy items will include, but not limited to, roof and floor plates, stilling wells and rolling ladder.
Arrangements should be made for items such as these to be delivered as close as possible to the intended point of installation and be moved subsequently as much as possible by machine.
Mandatory Personnel Protective Equipment PPE
The minimum personal protective equipment is as follows:
- Approved safety helmet
- Approved earplugs
- Safety footwear
- Eye protection
- Gloves
- Overalls
- Respirators
And any other equipment deemed necessary by the specific task involved.
If any installation or activity needs to be done at a height above 2m, care should be exercised to prevent personnel falling from height. Man lifting equipment should be used for such activity/installation.
Specific hazards applicable to tank Rehabilitation and Modification
Crane and hydraulic Jacking of tanks
All cranes and lifting/jacking equipment shall undergo relevant test and thorough examination and certified okay. They shall be operated with the use of manufacturer’s handbook.
Only certified operators and trained riggers authorised by the contractor shall be allowed to operate cranes/lifting equipment.
Crane/lifting equipment operators and/or safety personnel shall carry out daily inspections and this shall be entered into the crane/equipment register.
Cutting and Welding Hazards and Precautions
All the manholes shall be opened to allow for adequate ventilation.
Fumes extractor shall be installed as applicable or as required.
Personnel involved with this operation shall use respirator.
Welding shield shall be worn.
Working in confined Spaces (such as Welding, Tank Cleaning and blasting and painting)
Personnel should not enter confined spaces without the authority and direct supervision of qualified personnel.
Site owners/operators should have their own procedures for entry into confined spaces and these should always be followed.
Always ensure the correct permit has been issued and that all correct permits has been issued and that all correct procedures have been carried out before continuing.
All manhole should be opened.
Under no circumstances should a person working alone enter a confined space.
There must always be someone outside the confined space to raise alarm in case of emergency.
Working on Mains & High Voltages
Care should be exercised when working with mains voltages (240V or 380V AC).
All appliances, equipment, and materials used for temporary electrical installations shall be constructed, installed, protected, worked and maintained, so as prevent danger of electrocution etc.
The work on electrical equipment shall be carried out by, or under the immediate supervision of, a qualified electrical craftsman.
Power in excess of 240V shall be transmitted by armoured cable, which shall either be buried or strung overhead.
Foundation Demolition
Before commencing any demolition and excavation, the area should be examined using a suitable pipe/cable locator to ensure that any buried services in the vicinity of the excavation are located and marked before the excavation begins.
Unless absolutely necessary, personnel should not enter the excavation.
Any excavation, regardless of depth can be dangerous and there is no “safe” working depth.
If the work cannot be completed without entering the excavation then suitable safe methods of exit and egress must be provided.
Any risk should be assessed and steps taken to ensure the safety of the excavation.
Under no circumstances should a person working alone enter an excavation.
There must always be someone outside the excavation to raise the alarm in case of emergency.
Control of Substances Hazardous to Health COSHH
Where applicable, substances hazardous to health shall be listed and itemized in the form of a register.
Health and Safety Data Sheets or MSDS for all hazardous substances shall be kept in a file for reference.
Site personnel shall be issued with copies of Health and Safety Data Sheets relevant to their work activities.
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