Method Statement - baixardoc

METHOD STATEMENT

1. INTRODUCTION

This Method Statement describes the specific safe working methods which will be used

by ------------------ to carry out the construction new storage facilities.

2. SCOPE

This document covers the scope of work required for the execution of the tank

construction. The method statement is in the following sequence as the work will be

carried out.

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 demobilisation

3. REFERENCES

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)

Method Statement 1 of 17

Recommended Practice 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

4. LIST OF 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

5. SITE PERSONNEL

Construction Manager 1

Site Engineer 2

HSE Officer 1

QA/QC Officer 1

QA Engineer 2

QC Inspector 2

Site Supervisor 1

Pipe/Plate Welders 6


Pipe/Plate Fitter 6

Structural Welders 4

Rigger 6

Grit Blasters 4

Operators 8

6. WORK SEQUENCE AND METHOD

6.1. 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

COMPANY for review and approval before changes will be effected.

The 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

The 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

showed 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 bevelled 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. The bevelled 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 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.

6.4. Tank Erection

Foundation tolerances

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 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” drawing

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.

Assembling

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. The 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”. The COMPANY’s authorized 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.

The 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

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 starts first from bottom centre to side. After

welding of short seam joints the long seam fit-up and welding will starts from centre

to 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 by DPI.


Method Statement - baixardoc

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