Finding Coating defects on burried pipes with local cathodic protection

[DBreyer]

Hi all,

I'm not sure if this is the best Forum for the question, so please tell me if you know of a better one.

We have a burried pipe in a tank farm, that is protected with local cathodic protection. As such there are no Isolation Flanges to the attached storage tanks.

We want to assess the coating integrity of the pipe. In a typical CP-protected pipe we would use CIPS to do so. But since our pipe is not electrically insulated from the neighbouring structures we were advised that CIPS won't work. We want to avoid shutting down the line, and opening all the flanges. The pipe is not piggable, so that is also not an option.

Do you know/have experience with other techniques for assessing the coating integrity untere these conditions?


I initially posted the question in the QC, Inspection & Testing forum, but removed it form there, since it might fit better here.

 

[LittleInch]

OK, I'll repeat for others that I don't think you will be able to do any inspection based on CP type methods, CIPS or DVCG etc without isolating the pipe electrically.

Also at the moment you won't have a "local cathodic protection" system as the tanks will simply ground the pipeline.

Has anyone actually checked the CP system / voltages? If so what are they / type of CP?

You might be able to dig a series of holes and use long range UT to check for pipe corrosion, but I think you just need to bite the bullet and install isolation flanges. Then you can use CIPS / DCVG if you can isolate the incoming voltage from a power supply or anodes and also your CP system will actually work.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MJCronin]

You state:... "The pipe is not piggable, so that is also not an option."

Why is it not piggable ? ... too expensive ?... too long ... too small ?

So what is the diameter, wall thickness and length ? ... More information equals better answers !!!

Have you considered some of the piping inspection robots out there ?..... They can evaluate pipe wall thickness dow to about a 4"NPS diameter

https://nexxis.com/custom-solutions/

MJCronin
Sr. Process Engineer

 

[DBreyer]

I'll try to answer your questions:

@littleinch: local cathodic protection is a bit different from classical CP. A good explanation is found here

https://www.sciencedirect.com/science/article/pii/B9780884150565500198

So while it is true that a huge portion of the current will go to other structures, the pipe will still have the correct voltage.
This is also tested at least once a year.

@MJCronin: The pipe is not piggable as there are a lot of Tees and changes in diameter. We supply some crude tanks from this line.
Main pipe size is 36", I think around 2km in length total.

Has someone expirience with this technique?

https://empit.com/en/

It seems to even work on burried pipes without active CP, so might be an option.

 

[LittleInch]

Well it seems very odd. How is it "tested"?

I had a look at that site and can't find much about it. All looks a bit too good to be true to me and no real explanation of how it works is always troubling.

But see how much it is and give it a go?

Biggest issue isn't finding some defects but missing others. So scan say 50m then dig the whole lot up to see??

Is it the same as this?

https://www.eng-tips.com/viewthread.cfm?qid=497468

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Dhurjati Sen]

How long is the pipe?

Dig manually at few places to expose the pipe.

Check the condition of the coating visually. Carry out holiday testing also.

This would help you decide future course of action.

DHURJATI SEN
Kolkata, India

 

[GarryB1957]

As a CP Engineer I would say it may be possible to carry out non-invasive surveys to establish the coating integrity. However, much information would be needed to make an informed decision on the best way forward. I am assuming the CP is an Impressed Current system and also protects the underside of the tanks.

Although CIPS appears to be the go to survey for non CP engineers when coating defect surveys are being considered. This is not actually the case, as a CIPS survey only establishes the level of CP protection being applied to the structure. Although it may indicate the possiblity of a coating defect, this indication may also be caused by many other reasons.

The main coating defect surveys would be DCVG, Pearson, Audible Pearson and AC current attenuation surveys. I would suggest the Pearson, Audible Pearson and AC curent attenuation would not produce the best or even any meaningful data, for a variety of reasons.

The DCVG survey would produce meaningful data over the majority of the pipe to be inspected. There are however several caviates to this statement. To many for a short post.

My advice is have a competant CP engineer carryout an isnpection on site to establish the best way forward. The cost of this is significantly less than digging 1 inspection hole to examine the coating on a small section of pipe, when there could be a major coating defect 2m away.

As a footnote with regards to CIPS and anomalies in structure potentials, CIPS is remarkably accurate in detecting variations in structure potentials. However these are only variation in the potential, with reason not necessarily due to coating defect.

 

[brimmer]

Use DCVG and ACVG, it will detect holiday locations in the coating, this is assuming enough current is actually reaching the pipe you can reasonably detect. What are CP levels on this line currently? CIS won't necesarily tell you much as if there is no CP system for this line, you are just getting some current from tank ends and the entire line may be low, or very high in current pick up areas, you will probably need to get a basic understanding of where current is coming from and levels to be able to answer your question, then re-ask the question.