Pipeline Cathodic Protection

[BRIS]

I have to review a specification for large diameter steel pipeline carrying raw water. The pipeline is through a limestone area with pH greater than 8 the soils are non-aggressive. I have no other soils data. The pipeline is specified to have either a 3 coat polyethylene coating to DIN 30 670 (Fusion bonded epoxy, adhesive copolyner layer and polyethylene) or a single high build polyurethane coating at the choice of the contractor.

The specification calls for the contractor to undertake field surveys and provide cathodic protection using sacrificial anodes.

The requirement is for a 50 year design life.

My questions are;
1) Is cathodic protection essential
2) Why should the specification favour sacrificial anodes rather than impressed current protection.

I would appreciate any comments or guidance - this is clearly not my field

Brian

 

[Metalguy]

The CP current requirement, and therefore the sac. anode consumption, is greatly affected by the cathodic surface area-the less area the better. Your coating will remove most, possibly 99+ % of the area, so sac. anodes should last a long time. An impressed current system is much more likely to fail because of rectifier/power supply problems, and if someone sets it too high it can disbond some of the coating.

*IF* you had a high current demand the imp. curr. system would probably be a better choice, but not for a very low current such as yours.

 

[pablo02]

I concur with MEtalguy -- another reason to use sacrificial anodes is location of the pipe -- many times the pipeline is located through an area where the expense of running a power source for the impressed current system is not economically feasible (and I am pleading ignorant here on any potential photo-voltaic sources)

we have used both types -- the sacrificial ones have generally been "set it and forget it" while we've had to monitor the impressed current ...

 

[BRIS]

Thanks for your response - I have now checked the AWWA manual M11 for steel pipes and that also favours sacrificial anodes.

My next question is how many anodes do we need to provide - order of - are they installed one per pipe length or at much larger centres.

Thanks Brian

 

[Sargardani]

Bris

The number of sacrificail anodes depends on the current requirement. For pipeline CP applications sacrificial (galvanic) anodes are generally used in cases where less than 1 A current is required and areas where soil resistivity is low < 10,000 ohm-cm. If the soil resistivity is high then the potential difference between the pipe and sacrificial anodes is not sufficient to force the CP current through the soil for corrosion prevention. Under these circumstances an impressed current system would be required.
Also its normal practice to favour impressed current susyems for long pipelines because it covers large distances. Sacrificial anodes are on the other hand used for short pipe spans, or where a leak has been repaired etc.

Hope it helps!

 

[saldanha]

An other factor which can motivate the use of impressed current cp is the existence of DC stray current and the chnage of pipe potential due earth magnetic field activity.
unfortunatly stray current can only be measured to determine its existence. stray current can originat from any impressed current cp or any other DC current source in the area. that you need to find out because it can cause serious damages.

 

[cmurdoch]

It is a common misnomer to assume that applying a good quality coating will meet all your corrosion requirements. A coating system must be supplemented with a suitable CP system. By applying appropriate standards from NACE, EN, etc it will be possible to design a CP system that takes into account the good quality of the coating.

Coating systems only remain in good condition for limited periods. 3-Layer PE coatings have only been around for 20 years and therefore we have no idea what their 50 year perfomance is. The most vulnerable location will be the field joints, where MIC and other severe corrosion mechanisms have been found. There has been experience in disbonding due to poor application of the FBE layer which has been applied at too low a temperature (Will not gel and wet the surface adequately). Third party damage is common and high temperature creep can also occur on these coatings.

DNV have just re written their offshore CP specs to allow for the better quality coatings being applied offshore, enabling much lower masses of anode and lower current densities. This has not been done onshore, due to the well established use of FBE coatings.

I would suggest you design an impressed current CP system, allow for regular monitoring and also ensure you cater for intelligent pigging at intervals to ensure that there are no further corrosion problems.

 

[march1971]

Think these contributions are enough for you as mine will be a repetition of the things already said. So combine these fragments of Knowledge and it will make the good volume of knowledge you require. If you are not satisfied write back so that I can give you more infomation.

 

[BRIS]

Thanks for the responses - I have stuck with the specification imposed on me - although I am not happy with it - my view is that some resistivity survey should be made pre tender to give the bidders some idea of what they are pricing for but I am told there is no money in the budget.

Brian

 

[PVRV]

BRIS,

1.0 What is the soil resistivity?

2.0 What is the pipeline diameter and lenght?

3.0 Any problems with the power source?
________________________________________________

The following items are premature without full details
however; IMO

a) I doubt if the coverage would extend beyond a pipeline
lenght. Wonder how long it would take during
construction not to mension the number of test posts
that are needed; else it may be the intension to lay
the pipeline and forget about it (perhaps maintenance
personnel / corrosion technicians would be inclined
towards such a notion)

b) Wonder how many tons of sacrificial anodes would be
required for 50 Years design life and how un-optimized
such an arrangement would be.

c) At any rate refer to BS 7361 Part 1.

d) ILI pigging was mentioned. It is a headache to utilize
an intelligent tool in water.

e) Are there any future plans to have other
pipelines in that area.

f) I would only consider sac. anodes offshore, short
lenghts else what I dont have a choice (power source)


Cheers

 

[quadswift]

I thought the sequence was:
- product and soil survey data
- required design life
- leading to material selection
- and determination of coating type and thickness / required CP system selection
Regards,
quadswift

 

[BRIS]

The pipeline in fact is a main water transmission pipeline in south Lebanon - it will supply irrigation water to the previously occuppied lands. There will be no maintenace.

Diameters range from 800 mm to 1700 mm. The majority of the pipeline is GRP but steel is used where pressures exceed 16 Bar - maximum pressure is 40 bar.

Brian

 

[PVRV]

Good Man,

1) Now that makes more sense.

2) BRIS I would NOT use 3-Layer HDPE or HDPP for large
diameter pipes; application would not be practicle.

3) Installing two GRP pipes may be a better idea; no CP
requirement, welding consumables, welding operator,
fuel, generators, welding machines, coating & wrapping.

4) Consider the flow rate requirement.


Regards,

 

[BRIS]

We have twin GRP pipes already - reservation widths prevent us going to 3 smaller pipes. The terrain is mountainous and the route torturous - welded steel is the right material for the difficult sections.

Single layer polyurethane coating appears to be a good alternative to 3-layer - any comment?

 

[PVRV]

1. Some GRP pipes have steel strip laminate to sustain high
pressures.

http://www.angelfire.com/biz/ameronagent/ssl.html

Alternatively, sac. anodes for such small sections
would be plauseable. An impressed current system with
the majority of the pipe being GRE is hehe
Refer to BS 7361 Part 1 for onshore sac. anodes
installation

2. Small diameter pipes HDPP/HDPE external coating is
excellent. Larger diameter pipes, heat distribution
during application diffucult to control; the coating
disbonds. Let me know if would like to see pictures of
a 48&quot; CS pipe with HDPE coating failure during
construction

Cheers,