IMPRESSED CURRENT GROUND BEDS

Main keywords for this article are Impressed Current Ground Beds, Anode Cable Installation, Surface groundbeds, Deepwell groundbeds, Underwater anodes.

Impressed Current Ground Beds, Anode Cable Installation

IMPRESSED CURRENT GROUND BEDS

The location and the soil resistivity of the groundbed site should be verified to confirm the design parameters. During construction, test results may require modifications in the design, e.g. anode depth, location etc. These activities should be carried out by or in close co-operation with the design contractor of the groundbed.

Prior to installation, impressed current anodes shall be inspected for defects, conformance with the anode specification, and size and length of anode cable tails, and to ensure that anode caps, if used, are secure. Care must be exercised to avoid damaging anodes during handling and installation. Cable tails should be visually inspected for defects in insulation. Care must be taken to avoid damage to the cables or cable insulation. Each anode shall be tagged with a unique number or code.

Anodes should never be handled, lifted or lowered using the anode cable, unless specifically allowed by the Manufacturer in the installation instructions (e.g. stringed anodes).

Impressed current anodes are usually installed in carbonaceous backfill. If the backfill material is supplied loose, it should be installed properly, so that there are no voids around the anodes. Anodes may also be delivered pre-packaged with compacted backfill in metal canisters.

Surface groundbeds

Surface anode ground beds shall be constructed as directed by the design document.

The installation checklist for surface groundbeds (CATHDIC PROTECTION SYSTEM COMMISSIONING) should be used.

 Vertical groundbeds

For vertical anode installation, holes shall be excavated in appropriate diameters and depth. Power auger equipment may be used where available and where local conditions permit.

Pre-packaged anodes shall be lowered into the holes and backfilled with the original soil. The soil around the anodes shall be compacted by fluidizing the soil with water.

Non pre-packaged anodes shall be positioned in the centre of the holes on a layer of at least 10 cm of tamped backfill. The holes shall be filled with backfill, which shall be tamped in thin layers to approximately 10 cm above the top of the anode. Water may be used to fluidize the backfill. When multiple anodes are installed in one hole, the cable tails of the lower anodes shall be protected from damage during installation of the upper anode(s).

The holes shall be finished with the original soil or, if additional venting is required, with fine gravel. If required by the design, separate headworks shall be erected over each anode hole. Otherwise the original ground profile shall be restored.

Depending on the type of soil, different techniques may be used to install vertical anodes, such as by using water jets to drive the (pre-packaged) anodes into the soil. In such cases, the installation Contractor shall provide a description of the procedures to be followed.

Horizontal groundbeds

For horizontal anode installation a trench shall be excavated to the appropriate depth and length. At the bottom of the trench a slot of about 30 cm by 30 cm shall be excavated to accommodate the anodes and backfill.

The anodes shall be laid in the trench on a layer of tamped backfill material and shall be covered with a layer of backfill material, so that the anodes are positioned in the centre of the backfill.

Anode Cable Installation

Upon installation of the anodes, the cables shall be laid in the trench, not directly above the anodes, in a layer of soft sand. For underground cable installation see section. If a ring main feeder system is used, the anode cable tails shall be spliced into the ring cable using C-crimps or line taps without cutting the ring cable strands. The ring cable shall be connected to the feeder cable. If a central distribution box is used, the cable tails shall be laid in the trench and shall be terminated in the distribution box.

The trench shall be finished with original soil. On completion of the groundbed the resistivity of the groundbed and if possible the individual anodes shall be measured to remote earth using a low resistance earth Megger. If the groundbed resistivity is outside usable limits for operation within the design parameters, the Principal and Contractor shall investigate the cause and take remedial actions.

The location of the groundbed shall be indicated e.g. by the installation of groundbed marker posts at both extremities of the groundbed.

{Main keywords for this article are Impressed Current Ground Beds, Anode Cable Installation, Surface groundbeds, Deepwell groundbeds, Underwater anodes. }

Deepwell groundbeds

Groundbed design options

Various types of deep groundbeds are possible and any specific type is chosen to suit the local conditions and/or output requirements.

Options for the bore hole include:

  1. Various diameters, to facilitate the installation of large anode systems and depending on the available drilling equipment.
  2. Holes cased or uncased down their entire length or part of it, depending on open hole or closed hole option and sub-soil stability. Casings, if specified, can be made of steel, (perforated) plastic or a combination of these.

Options for the anode system include:

  1. Conventional anodes with individual cable tails (silicon iron, magnetite).
  2. String type anodes having various active sections with one feeder cable (linear distributed anodes).

Options for the total groundbed include:

  1. Open hole. Open holes can be used when anodes can be suspended in groundwater at depth.
  2. Closed hole. Closed holes are required when no groundwater is present. Closed holes require low resistivity (coke) backfill and venting facilities. Coke backfill can be supplied loose or pre-packaged with the anodes in metal canisters.

The selected design options shall be detailed in the design drawings.

Deepwell groundbeds Construction

  • A deepwell groundbed shall be constructed in accordance with the design document and drawings.
  • The installation checklist for deepwell groundbeds (Appendix 3) should be used.
  • A surface casing larger than the nominal diameter of the bore hole shall be set to a depth of approximately 5 m.
  • A hole of the specified diameter shall be drilled to the required depth.
  • All cuttings, mud and other foreign matter shall be removed from the bore hole.
  • A record shall be kept of the drilling. The various layers, total depth and water level shall be recorded. A bore hole resistivity profile should be made during drilling to confirm the boundaries of the low resistivity layers.
  • The form for recording the drilling of deep well ground beds (Appendix 4) should be used.
  • If specified, a casing shall be set in the bore hole. Normally a non conductive (plastic) casing is used for the inactive length of the groundbed.
  • A bore hole headwork shall be constructed in accordance with the design drawings.
  • Anodes shall be lowered on suspension ropes in the bore hole to the specified depth. Anodes shall never be handled using the cable tails. Anodes shall be provided with centralisers to centre them and to prevent cable damage of lower anodes. The anode lead wires shall be permanently identified before installation.
  • If specified, a venting pipe perforated over the active length shall be installed along with the lower anode.
  • If specified, a slurry of carbonaceous backfill shall be gradually poured or pumped into the bore hole to the required depth, so that there are no voids around the anodes. This may be done using a large diameter hose which is lowered down to the bottom of the hole and is retracted as the level of coke rises. The quantity of the backfill material shall be recorded. The top of bore hole shall be backfilled with graded soil or gravel to prevent cable isolation damage.
  • The anode cable tails shall be terminated in the anode connection box and the headwork cover set in place. Care shall be taken that anode cables have sufficient slack to cope with any backfill settlement.
  • On completion of installation, the resistance of each anode to remote earth shall be measured and recorded using a low resistance earth Megger. These anode details shall be recorded (see CATHDIC PROTECTION SYSTEM COMMISSIONING). After completion of the groundbed the total groundbed resistance shall be measured and evaluated against the design. Any major deviation from the design values shall be reported to the Principal.

Main keywords for this article are Impressed Current Ground Beds, Anode Cable Installation, Surface groundbeds, Deepwell groundbeds, Underwater anodes

Underwater anodes

Impressed current anodes may be used for immersed structures such as jetties, sheet piles, platforms, sub-sea (loading) pipelines etc. The required anode systems may be of different designs depending on local situations. The following guidelines should be followed to prevent premature failure of the anodes. It is emphasised that the integrity of impressed current anode cables and cable connections is vital for prolonged service. Failure of cables and cable connections is the most important cause of failure of offshore anodes and anode beds.

Underwater anodes may be installed fixed to the structure, suspended from the structure deck or roof or positioned on the bottom of the sea or other water. The location of the anodes shall be such that they are immersed at all times.

Anodes that are fixed to the structure shall be designed for that method of installation. The distance between the anode and the structure shall be as large as possible. The structure surface close to the anode shall be provided with a di-electric shield or/and be coated using a heavy duty isolating paint to avoid local overprotection and excessive current loss. Anode cables shall be connected to the underwater anode and sealed using pressure glands or by permanent moulding with epoxy resin.

Suspended anodes shall be suspended from the roof/deck hangers using special insulated wires or ropes. The electrical cables shall not be used for suspension unless the system is has been designed for that purpose. The anodes shall be located so that a maximum distance from all structure members is obtained and no interference with (shipping) operations occurs.

Anodes that are to be laid on the sea bed shall be installed on a suitable support raft to avoid movements due to current and wave action. The raft shall be positioned remote from the structure and outside shipping and fishing areas. Electrical connections shall be made using waterproof cable splice kits.

Main keywords for this article are Impressed Current Ground Beds, Anode Cable Installation, Surface groundbeds, Deepwell groundbeds, Underwater anodes.

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