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Underground Fuel Tank Back Filling and Cathodic Protection Installation Procedure

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Below is a brief method statement for Fuel Oil System, that covers the backfilling and cathodic protection of underground pressure vessel / fuel storage tank.

Overall project manager is responsible for the implementation of project quality and safety requirements and achieving the best quality workmanship for the scope of work.

All project site, QA/QC and HSE engineers, foremen, chargehand and workers shall be responsible to work as per project documents and their job descriptions on the project site.

All required Equipment’s, Tools and tackles shall be deployed to meet the work Schedule and satisfactory cathodic protection installation.

Necessary measurement tools shall be made available to enable correct installation and inspection of structures.

All lifting tools and Tackles shall be tested and certified by independent testing authority, and records shall be maintained.

A Pre-construction conference will take place to brief all operatives on this method statement.

All operatives will attend the Project Safety & quality induction prior to commencing work on site.

The minimum Personal Protective Equipment (PPE) on site is:

References and standards

Handling & Storage of Materials

Adequate protection shall be provided to prevent mechanical damage and atmospheric corrosion at job site.

CP materials and components shall be completely free of water, dry and shall be secured to prevent damage during lifting or unloading .

Cathodic protection materials and components shall be protected to safeguard against all adverse environments, such as humidity, moisture, rain, dust, dirt, sand mud, salt, air and seawater.

All CP materials shall be packed and protected to withstand extended period of storage at the job site.

HSE Requirements

During installation of the CP equipment, health and safety practices in accordance with the client and contractor safety rules, codes and regulations shall be applicable.

Safety is the responsibility of everyone and therefore shall be adhered to at all times.

Sequence of Work for Fuel Tank Placement & Backfilling

Sand Bedding , Back Filling & Cathodic Protection installation details for the Tanks – 1st Stage

Geo textile material to be placed on the concrete pad floor.

A Sand bedding of minimum 6 inches thickness is to be placed on the Geo textile material.

The tank shall be placed on the sand bedding.

During tank placement and subsequent backfilling operations care shall be taken to ensure that no foreign materials enter.

After placing and levelling of the tank, sand shall be placed around the side of the tank at minimum 6 inches thickness.

Geo textile material to be used to maintain a minimum distance of at least 6 inches around the tank circumference. This gap to be filled with sand.

The remaining backfill material, which is further than 6 inches away from the tank, size 20mm of crushed stone can be used.

Dead man weight is used, place for anchoring, Align the tanks with anchors for proper placement of anchor straps, tanks should not be sloped.

Install anchor straps on locations marked on the tank ribs by the arrowhead symbol.

Make sure that during anchoring there is no deflection to the tank, straps must be uniformly tight.

Back filling at both sides can be done step by step up to 400mm from the tank bottom.

The first layer of anode shall be placed 400mm from tank bottom and maximum distance  of 500 mm away from tank surface, shall be placed around the tank, sand shall be placed around the anode minimum 6 inches thickness.

Back filling at both sides can be completed step by step, these steps are repeated up to below 50% of tank diameter are covered.

The remaining area should be sufficient and safe for piping fabrication, testing and inspection activities.

Tanks should be protected properly using scaffolding materials during top slab concreting.

During Form work and concreting operations care shall be taken to ensure that there is no damage on the tank, no fall of material on the tank & no load transfer to the fuel tank.

All Fiberglass Sumps and Manways should be installed in proper places.

Install all pipes and pipe supports, all connection to tank shall be as properly sealed.

Pipe connected to the tank opening through sump, pipe shall be tested.

Mechanical, Electrical and I&C works should be completed and inspected prior to 2nd stage back filling.

2nd Stage Backfilling

Back filling at both sides can be done step by step up to 1500 mm from the first layer of anode.

The second layer of anode shall be 1500mm up from 1st layer and maximum distance of 500 mm away from tank surface, shall be placed around the tank, sand shall be placed around the anode minimum 6 inches thickness.

Back filling at both sides can be done step by step up to 650 mm from the second layer of anode.

A reference electrode for testing purpose (size approx. 300 m long and 50 mm dia rod) shall be placed and maximum distance of 500 mm away from tank surface, sand shall be placed around the anode minimum 6 inches thickness.

Back filling at both sides can be done step by step up to tank top neck.

Complete the backfilling as per the approved drawing.

General Instructions

Do not use rammer type compactors over the top of the tank.

Use a non metal tamping rod long enough to reach beneath the tank to push material under the tank body and dome until solid resistance is felt, all voids are filled and the  tank is fully supported.

An object like long wooden shovel handle is a practical choice as a tamping rod.

All Fiberglass sumps and manways should be installed in proper places.

Installation of Cathodic Protection System for Storage Tanks

The cathodic protection system for the underground tank external surface of the new tank is based on an impressed current system using mixed metal oxide (MMO) coated titanium wire anode installed in an array beneath & surrounding the tank base.

Cathodic protection system is designed to protect the external base plates of the underground tank.

Objective of this document is to describe the procedures and inspection methods to be followed for the installation of a permanent cathodic protection system.

The design shall be in accordance with the following established codes and engineering practices:

NACE SP 0285: External Corrosion Control of Underground Storage Tank Systems by Cathodic Protection

Brief Description of Cathodic Protection System

The Permanent Cathodic protection system is based on an impressed current system designed for 30 years.

In order to overcome possible interference effects from the CP system on the plant earthing system or other underground metallic objects, the permanent CP system for the underground tanks shall be based on a close anode system.

With a close anode system the CP anodes are located in close proximity (approx. 0.5m) to the structure to be protected and distributed along its surface.

Current emitted from the anodes preferentially impresses onto the nearest structure i.e. the tanks to be protected, without significant current losses to other buried structures thus significantly reducing possible interference effects.

The general layout of the CP system shall comprise an oil cooled transformer providing current to the anodes through positive distribution boxes (PDB). The anodes shall be based on various lengths, 1.5mm dia mixed metal oxide wire anode connected at nominally 5.0m intervals to a 16 mm2 KYNAR/HMWPE header cable all of which is encased in a fabric sock filled with petroleum coke backfill.

The circuits within the PDB shall include shunts to allow monitoring of each individual anode current output.

Header cables spliced to large diameter feeder cables at each end, which shall then be routed to and terminated within the PDBs.

The CP system designed for the buried tanks is based on the assumption that all buried tanks shall be electrically isolated from station earthing, any above ground piping connections and concrete reinforcing steel.

Monitoring of the effectiveness of the CP system shall be undertaken by use of above ground test facilities installed at discrete locations around the facility. Each test facility shall comprise a terminal box containing 2 No. test cables connected to the tank, 1 No. cable from a permanent reference electrode (PRE).

The use of permanent reference electrodes enables more accurate monitoring of potentials on the tanks and ensures continued monitoring capabilities in areas where surface paving may have been laid as part of the plant construction layout.

Cathodic Protection Installation Method

The installation of Permanent CP materials and equipment shall include the following activities:

Transformer Rectifier Installation

The direct current (DC) power supply for the impressed current cathodic protection systems (ICCP) shall be provided by transformer rectifiers (T/R) powered from AC supplies.

Proposed new transformer rectifier units shall be installed at locations shown in the layout drawings.

A suitable sized concrete plinth (Supplied by civil contractor) shall be constructed for the installation of each T/R.

The concrete plinth location and orientation to the plant shall have an easy access for monitoring and adjustment.

Plinth shall be cast in concrete and provided with fixing studs on the top surface to suit the fixing holes of the steel channel on the underside of the transformer rectifier.

Alternatively, the plinth shall be marked out and provided with “rawlbolt” fixings.

Cable ducts or appropriate PVC conduit (Supplied by contractor) shall be cast into the plinth to route the DC (Positive and negative cables) and AC power supply cables.

When the concrete plinth is cured, the transformer rectifier shall be lifted in the upright position into location on the plinth using the lifting lugs provided on the unit, with the sunshade removed.

The unit shall be bolted down level to the plinth using the fixing studs in the plinth.

With the isolator/residual device (AC & DC) of the transformer rectifier in the ‘OFF’ position, install positive and negative DC cables through the appropriate cable duct or conduit in the plinth and the cable glands supplied with the unit.

Suitable cable markers shall be fitted to the DC cables within the transformer rectifier. AC mains cable shall be installed and terminated similarly.

The main positive header cable shall be routed from the positive connection point in the transformer rectifier to the relevant positive distribution box (PDB) connection point.

Main negative header cable shall be routed from negative connection point in the transformer to the relevant) connection point at tanks as shown in installation CP layout drawing.

Header cables shall be connected to the appropriate terminals, with the cable lugs and nuts, locknuts and washers supplied with the transformer rectifier.

Cable lugs shall be mechanically connected to the cable conductor using a crimping tool. The cables shall be terminated with sufficient slack.

On completion of the cable connections, all glands shall be tightened, with sufficient slack in the cables to permit cables to be removed from connection terminals.

The opening where the cables exit the duct for buried cables shall be sealed properly.

An earth lead shall be connected to the stud of the transformer rectifier earthing terminal using the cable lug, nut, locknut and washers supplied with the unit. (Earthing shall be provided by the subcontractor).

The positive DC cable shall be removed from the positive terminal and left DISCONNECTED until the transformer rectifier is commissioned.

MMO Piggy Back Anode Installation Procedure

It is critical that the linear anode be electrically isolated from all underground metallic structures.

The anode shall not be installed closer than 500 mm for any metallic structures including; the cathodically protected tank, grounding / earthing facilities, electrical conduits, instrumentation lines, reinforcing steel, hydrant risers etc.

Electrical continuity tests shall be carried after each Section of MMO linear anode has been installed and before the lead cables are terminated in the junction box.

The trenches for the MMO Linear anode shall be free from sharp stones or other materials, which might injure the jacket of the MMO piggy back anode.

Anodes shall be unreeled from a portable support rack and laid into the bottom of the trench. This unreeling shall be done carefully so as not to damage the jacket of the anode. Pulling the cable from stationary reel or dragging the cable into the trench is not permitted.

The anode shall be surrounded by clean native soil, free from large rocks and sharp objects.

Anode feeder cables shall be spliced to anode header cable using splice kit & encapsulating material. The anode lead cables shall be terminated in the junction boxes as shown on the project drawings. The end of each cable shall be permanently labelled with the ID numbers.

MMO Linear anode shall be inspected in accordance with approved checklists/forms.

Junction Box Installation

Positive Distribution boxes shall be installed in areas as shown in the GP system – layout drawings.

The positive distribution box including the galvanized mounting frame shall be cast into a concrete plinth complete with cable duct.

Anode cable tails and the main positive cable from the transformer rectifier shall be terminated at the appropriate terminals in the positive distribution box.

The cable entry shall be through the conduit hub installed through the bottom gland plate of the junction box.

Cables shall be provided with permanent identification tag at the designated terminals.

Details of the positive distribution box are shown in the the detailed drawing.

This drawing is to be read in conjunction with the CP system – Layout drawings which gives the locations of the PDB’s in relation to the other CP equipment:

The positive distribution box shall be inspected in accordance with approved checklist/ form.

Installation of Test Stations

Test stations are used for monitoring of the CP system provided to the buried tanks. STP’s are used for monitoring of the CP potentials. Details of the STP and its installation are given in the detailed drawings.

The following cables shall be terminated in the test station:

Test stations shall be inspected in accordance with approved checklists/forms.

Permanent Reference Electrode Installation Method

Permanent Cu/CuSO4 reference electrodes are used for monitoring tank-to-soil potential measurements at STP’s. The details of the reference electrode are given in the approved drawing.

The location of the permanent reference electrodes shall be marked out at site and it shall be confirmed that the area is free of any underground metallic services, cables etc.

Permanent reference electrode shall be installed at the mid-point level of the buried tank and with a perpendicular distance of 100mm – 300mm from the tank.

The permanent reference electrode shall be backfilled with soil and wetted with potable water to provide better soil contact and compaction.

After backfilling, the permanent reference electrode shall be tested to confirm correct functioning by measuring its potential with a portable reference electrode.

The permanent reference electrode cable tail and monitoring cable shall be routed and terminated inside the respective test station with appropriate cable tags.

Reference electrodes shall be inspected and results recorded on approved checklists/forms.

Cable to Tank Connection

Cable to tank connection shall be carried out using Cadweld & welded pad connections.

Field Test and Inspection Methods

Field testing and inspection shall be carried out to ensure proper installation of the CP equipment and facilities.

The following equipment/instruments shall be available on site during installation of the cathodic protection system as a minimum:

All testing and inspection shall be performed by qualified CP Engineers to ensure high quality system performance.

These inspections shall be carried out at various stages during the course of CP installation.

The following inspections shall be performed after each stage of installation works:

A visual inspection of all CP materials and equipment shall be performed to ensure correct and proper installation in accordance with the project specification, design document and drawings.

Inspections shall be performed to ensure that each CP component, materials, equipment and facilities have not been damaged during or after each  installation stage.

Any discrepancies shall be noted and necessary corrective actions shall be taken.

All CP cables are to be checked for proper identification, terminal connections and routing.

Potential Measurements

Potentials shall be measured with a portable Cu/CuS04 reference electrode placed in the ground and connected to the “common” terminal of the multimeter.

The test lead connection shall be connected between the positive terminal of the multimeter and the tank being tested.

Multimeter shall be set to read DC volts. In this setup, potential values displayed will be negative (-ve).

For the permanent Cu/CuS04 reference electrode installed at the STP’s, potential measurements shall be undertaken as for the portable Cu/CuS04 reference electrode.

Checklists & Forms

Transformer Rectifier

MMO Piggy Back Anode (MMO Linear anode)

Junction Box

Test Station

Permanent Reference Electrode

 

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