The coupon is a retrievable piece of metal and the probe is an electronic instrument.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

Don't put either one in a piggable line.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

I'd go even farther and say that both were developed for lines that are liquid full, and you shouldn't put either one in a primarily gas line, because they are never where the corrosion happens.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

Even in liquid lines, corrosion seldomly happens right down the center of the pipeline

If it ain't broke, don't fix it. If it's not safe ... make it that way.

Yeah, they go from not-good in liquid lines to horrible in gas lines. I have never gotten useful information from either probes or coupons. I guess people run them to make them feel like they are doing SOMETHING about corrosion.

Same with corrosion chemicals in gas lines--they accumulate at the first sag because the gas can't move them farther and the chemicals never get to where the corrosion cells are, but we keep pumping that crap into gas lines to the tune of a few billion dollars/year. The attitude that "doing something probably ineffective is better than doing nothing" is pretty pervasive. I've cut open lines at sags that were pure (toxic or poisonous) chemicals and I have to disagree with the idea that injecting chemicals in gas lines is better than doing nothing. It is actually far worse than worthless.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

Gents, Gents - spoken like true pipeline engineers. There are many variants of the two corrosion monitoring types, including flush mounted with the pipe wall to both 'get where the corrosion action is' and keep out of the way of pigs. Selection of specific types is based on what you are trying to monitor and what you are trying to achieve with the results. Gas line inhibition should also be reliant on maintenance pigging as part of the corrosion control strategy - that would shift the stuff down the line. My money is on your experience being based on pipelines where the instrument engineer or the mechanical engineer has been tasked with corrosion monitoring system design and execution: generally a sure bet for failure!

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

The first class I had on corrosion was 1971. I've had several more since then (been a NACE member for 20 years). As a consultant I see a lot more issues where the corrosion "mitigation" was controlled by the Integrity Management department than when it is in the ham-handed control of pipeline engineers.

I've kept track of the corrosion failures I've been asked to investigate over the years and over 80% of the failures have been on "protected" lines (some combination of cathodic and/or chemical injection) that were not pigged. "Protected" lines make up about 30% of the lines I see in my practice, but they make up the lion's share of the failures. I have never seen a single failure on a line that was pigged regularly (and adding corrosion chemicals to the pig run just costs money without providing any real benefit since pigged lines don't rot).

As an aside; at a conference a decade ago I had occasion to ask the Chief Metallurgist of a major Oil & Gas chemical supplier to join me for a drink. After far too many drinks I asked him what was the transport mechanism for biocides in a low pressure gas line. He was just drunk enough to say "quarterly profit and loss statements". In vino veritas.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

Yes as you allude, the biggest problem is keeping up the gas flow to get good inhibitor transport in lower pressure lines out at the edge of the field where the water starts creeping in.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

In few cross country water pipelines (not piggable), we've had "pipe monitoring arrangements". A short section parallel to the main pipe like a by pass at where you want to have your assessment. The main pipe can be inspected when needed (perhaps once every few years) to assess the corrosion and the lining of the pipeline by bypassing the flow through the by pass arrangement. I think at least you're inspecting the "real" pipe.

I have resorted to culverts with UT monitoring on some of the main transmission lines I manage, an expensive way to get corrosion rates, but much better than coupons or probes.

I don't know how the question became a discussion on chemical inhibition, but David I have certainly seen my share of corrosion failures on lines that were pigged regularly (twice per month) with no chemical inhibition. I can also say if I stopped applying chemical to most of the sour multiphase pipelines I manage I would see a spike in the number of failures, as this was the case before applying the inhibitor with everything else equal. Like anything else though, you have to know the details of what you are doing rather than just put chemicals in the line and hope for the best.

Bimmer,
I would love to know the details on that. I've consulted on some pretty sour wet gas lines that were regularly pigged and when we've cut them open for tie-ins, the pipe looked fine every time. Some of the non-piggable laterals were a very different story, but in the absence of standing water there just isn't any place for the gas to facilitate the creation of a corrosive cell (either acid attack or MIC). It seems like twice monthly pigging would be adequate to disturb the liquid pools before they become a problem (I often have to pig more often than that for pressure-drop reasons, but not for corrosion).

I'm not saying you haven't seen what you've seen, it is just so counter to my experience that I would really like to know more.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

There's a number of factors involved in H2S corrosion. Some are lucky and get a good protective sulphide film; other's aren't so lucky, particularly with high chloride water, and get corrosion failures.

The Canadians have the best experience with sour gas corrosion control

http://www.capp.ca/getdoc.aspx?DocId=155644&DT...

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04