CSA CAT II pipeline material question 4

[vesselguy]

There was a good thread going on in this forum regarding material substitution and someone said this,

"With respect to the sour service issue in Alberta, there seems to be a strong preference among our Alberta clients to default to Category II whenever a service is "sour", if only to get the slightly tighter chemistry and lower CE. "

I have a question regarding this point. Please don't get me wrong, I am not picking out someone and attacking. I just like some help to understand.

Can someone tell me why CAT II material is "strongly prefered" for sour service? I too hear that but I have not received any technically acceptable answers. CAT II is a notch toughness classification in CSA Z662/Z245.1. It has nothing to do with hardness. It also has no relationship to "tighter chemistry and CE". Chemistry is purely govern by table 5 in Z245.1 and it applies to all grade of piping, without reference to notch toughness Category.

Thanks for reading.

 

[SNORGY]

It was me who originally said that, although it was within the context of flanges rather than line pipe. Irrespective, as you point out, the comment as stated was in error. My apologies. It has been my personal observation, however, that as H2S concentration goes up, engineering and client specifications tend to start asking for lower CE and tighter restrictions on sulphur and phosphorus, thinking that such materials will likely be more resistant to SSCC, HIC and SOHIC. Further, since the issue of hardness restrictions comes up regularly in NACE MR0175 (ISO 15156) and CSA Z662 Clause 16 (and invoked Clauses in Z245.(xx)), the feeling in industry appears to be that hardness control is one of several measures used to attempt to gain greater comfort about resistance to these cracking mechanisms in a sour environment. The feeling appears to be that higher hardness loosely correlates with lower ductility and, for welds, higher residual stresses, which in turn are thought to correlate to a higher susceptibility to sour service cracking mechanisms.

Confusion further exists when those people than want to obtain exemptions from PWHT provided that hardness control to some magically soft number is ensured. While seldom an issue in pipeline line pipe, it does create confusion in the case of risers for which a material substitution has been made in accordance with Tables 5.2 and / or 5.3 in CSA Z662. From this philosophy is spawned the perception that low hardness equates to a resistance to cracking that is comparable to that afforded by applying PWHT. So, the chain of events in thinking appears to be that higher resistance to cracking is afforded by any or all of the above measures in combination, and from there the extrapolation is often made that (to paraphrase) "...if you meet Category II you are better off in sour service than if you just meet Category III or Category I...".

As you have correctly pointed out, notch toughness categorization does not have anything directly to do with CE or chemistry. In my earlier post, I incorrectly phrased a case of "perception is reality". Again, my apologies.

Regards,

SNORGY.

 

[vesselguy]

Hi Snorgy,

Yes, that's right it was your comment. And as I stated, I'm sorry to have picked your comment to use as example to help me get answer.

Yes, I completely agree with your first paragraph. Those are the remedies to limit HIC, SSCC/SOHIC. But the last sentence in your second paragraph (and I know it is not coming from you and that you are just paraphrasing), totally does not make technical sense.

I am reviewing a company's piping standard right now and I am questioning why they used CAT II pipe in sour service eventhough the MDT is only -29 and service is LVP. I get comments like "oh yeah, CAT II is for sour serivce piping..." I cannot find any link in the CSA standard that says sour service requires notch toughness nor chemistry restriction is linked to the notch toughness classification. I know enough metallurgy to be just dangerous, and I know the requirements for sour service treatment. But notch toughness is not one of them. Did I missed something??? I think that someone decided to use CAT II pipe on a project long ago and then people copied that project's spec without question, and then it became a myth that you should use CAT II pipe for sour service.

 

[SJones]

Is it a perception that the steel production processes and chemical compositions actually adopted in practice to meet Cat II requirements of energy and fracture appearance will automatically be beneficial for H2S resistance without having to run a HIC test verification exercise?

Steve Jones
Materials & Corrosion Engineer

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

 

[SNORGY]

Steve,

From my perspective, the perception among us "generalists" is almost precisely as you stated, although it is more the sulphide stress corrosion cracking that is the perceived concern than it is the HIC or SOHIC. I think we generalists are guilty of coming to broad conclusions about what is probably more accurately explained to us by more knowledgeable metallurgists, and the correct details and reasons why and how things fit together get lost or distorted in the translation.

But in general, for vessel steel or line skelp product, the message that most people hear, believe or propagate is that when you have steel made to fine grain practice, semi or fully killed, vacuum degassed, inclusion shape control, controlled sulphur and phosphorus, and lower CE, you probably have steel that would pass a HIC test (at least to Prudential's PS10 level of acceptance) if it was subjected to one. Then, when you read the NACE MR0175-2003 (or earlier) curves, people get the impression that, the more H2S they have (i.e., the more sour they are) the more they need "better" steel. However, NACE MR0175 up to 2003 was principally concerned with SSCC. HIC and SOHIC are different cracking mechanisms. Hence the different names *sulphide* stress corrosion cracking versus *hydrogen induced* cracking or *stress-oriented hydrogen induced* cracking.

So, in short, the impetus in opinion tends towards something along the lines of "...the better the steel, the more resistant it will tend to be in a sour service environment where hydrogen and sulphur are liberated and can promote cracking...". There might be truth in the correlation, but the reasons and their interrelationships are probably not completely understood the further away one is from being an expert.

About 4-5 years ago, I was asked to write a piping specification for pipe systems designed and built in accordance with CSA Z662 (as opposed to B31.3) for upstream oil and gas facilities - classic "single well tie-in" stuff. The more I studied the Code(s) and available pipe materials, the more permutations and variables crept into the effort. Long story short, not knowing whether to standardize on Grade 241, 290, 359, 386, not knowing what Class Location to standardize on, not knowing what notch toughness requirement to standardize on, not knowing whether to standardize on B36.10 (typically seamless) dimensions or Z245.1 (typically ERW) dimensions, and knowing that the wall thickness would be different depending on whatever combination of assumptions were made, I found myself faced, at the end of the day, with 72 Line Classes and some 200,000 wall thickness calculations. I ended up developing a spreadsheet that did all that stuff for me based on 30 data inputs, drew the material selection P&ID onto 11 x 17 paper, and then I created a subsequent Master Specification builder that could re-do it all from scratch and print a new 195 page specification in fifteen minutes. (I timed it.) I had to develop the tool to write the spec; but it turns out that this tool became more valuable than the spec anyway. The biggest points of confusion that it clarified (or attempted to) was how to make sense of Tables 5.1, 5.2 and 5.3 and pick the right Notch Toughness Category and, much more important, where to draw the spec break between B31.3 and Z662. Almost nobody knew; almost everyone was in the habit of substituting A-333-6 for CSA Category II above grade in line sizes 10" NPS and smaller because "it worked" but they didn't understand why. So much confusion existed, in fact, that in June 2006, the EUB, ABSA and Shell jointly authored the "Reference Tool For Interpreting Jurisdictional Relationships For Pipeline, Pressure Equipment and Pressure Piping". This publication was prodded largely due to my pestering those folks for clarifications and the growing awareness of the frustration and confusion in the industry. This development work occurred simultaneously with my suggesting - as internal guidelines - modified NACE Curves to define Service Severity Regimes, which then could provide recommendations for things like when to ask for controlled chemistry, when to substitute hardness testing for PWHT, and when to ask for HIC testing. This was accomplished prior to the rewrite of ISO 15156 which now explicitly provides definitions of sour service severity; that rewrite was spawned from a NACE paper. Interestingly, the modified NACE curves that I came up with were spawned on one correlation between the log of pH versus the log of H2S partial pressure, and when you cross-referenced the begin of test and end of test solution pH for a typical Solution A in NACE TM0284, it suggested that the 70 kPa partial pressure line appeared to be where the acceptance threshold would probably exist for steel to a Prudential PS10 HIC specification.

But...all I wanted to do was write a spec so people would stop bugging me...

Anyway, maybe I've learned my lesson: next time, I'll try not to let anyone know what I am trying to do.

Not that I am in any way bitter...

Regards,

SNORGY.

 

[SNORGY]

vesselguy,

I don't think you missed anything. As I understand it, the relationship between sour service and CSA Z662 Notch Toughness becomes clear when you reconcile all of the data and footnotes found in Table 4.2, Table 5.1, Table 5.2 and Table 5.3 in CSA Z662. As I explained briefly (and probably not clearly) in my post to SJones, I did go through all that. What I found is, simply, for a given MOP, then the lesser is your wall thickness, the higher is the required notch toughness because the higher is the corresponding DOS (Design Operating Stress). Therefore, the thinnest wall acceptable corresponds to the most stringent notch toughness. What falls out of it is that since the sour services have the lowest values for Location Factor L in Table 4.2, then for a given MOP (therefore DOS) you either need thicker wall or higher notch toughness. In other words, you can engineer your way into less stringent notch toughness qualification by increasing wall thickness, or you can engineer your way into thinner wall pipe through more stringent notch toughness qualification. While it is the ratio between DOS and PTSV that defines the notch toughness categorization, the interpretation - the wrong one, at that - is that "...Category II is better for sour service...", and people believe this almost as if they think that "Category II" is a pipe material property instead of what it really is, namely, a categorization of material qualification. Since nobody ever clarified the confusion effectively (although when you read it enough times you can see that the teaching was there but the students weren't listening), the myths propagated.

Kudos (and star) to you for trying to understand something by asking for a better answer than "...it's what someone else who seems to be smart enough did..." and just copying it yet again. Maybe if more engineers tried to actually understand something, rather than just copy it, we could regain a lot of the respect that we have lost and, if we were wrong, at least we were still trying to think.

Regards,

SNORGY.

 

[vesselguy]

Snorgy,

Gee, very good explanation!!!

You talk like two metallurgy guys I know, Roy and Mike in Calgary.

Yeah, I know what you mean when you are talking about degradation of engineering overall. Engineer skills are under valued and we are governed by high school grads (procurement and document control). Worst yet, you got young new grad engineers and they are put into project engineering position to lead the project. Some days I just have to close my door and bang my head on my table. I have 8 more yrs to early retirement and in some way, I can't wait till I don't do this anymore. In many ways I will miss the fun technical stuff.

Thank you for responding.

 

[SNORGY]

If by "Roy" you mean Roy Baguley in Calgary (I'll name drop because he is an author in the public domain who wrote a companion book to the B31.3 Code, etc.), then yes, I know him and I have a great deal of respect to him. Very, very smart and nice guy who sets an example for the rest of us "wannabes" like me.

Regards,

SNORGY.

 

[vesselguy]

Yes. Totally agree. I learned from him on how to work out the reason for selecting a metal or NDE. It is because of his example, that's why I could not accept the choice of CAT II material for sour service without a sound technical explaination.

 

[vesselguy]

Sorry, I hit the wrong button before I finished my writing.

There is nothing wrong to use a higher quality material but for the purpose of developing a company standard (in this case a pipe spec), it is wrong to blindly copy another project's commercial decision (or other project decision) into a "base" document which will end up being used on other corporate projects. A corporate spec must contain only the basic, technically correct and minimum requirements, and any project changes due to commercial issues such as availability/prices should then be noted in a project decision document/technical deviation for proper record keeping instead of just burying it into the piping specification. Of course, procuremnt people will have something to say about this.

 

[eadwine]

Check with the NACE and ASTM folks on their recommendations for "Sour Service" and then adjust your material spec. accordingly.

"Blindly following the Blind" can get you into trouble more often than not.

 

[vesselguy]

Thank you for your input eadwine. I have worked with NACE standard before. This thread topic is not a question on NACE but rather with CSAZ662.

 

[SNORGY]

Yes. Sorry about that...I dragged all of the NACE side-issues into it to try to explain (or hypothesize) why, in my experience, lots of people appear to be getting confused by the Z662 Code vis a vis Notch Toughness and its perceived link to sour service. Some of that NACE-related discussion may have been superfluous.

Regards,

SNORGY.

 

[eadwine]

If you don't like the U.S. versions, then use the Canadian Equivalents, the outcomes will be basically the same, even the notch toughness will be taken into account in their recommendations.

 

[SNORGY]

I think vesselguy and I are both Canadian, so the thread is centred on the Canadian Code.

Which we probably blatantly copied from the American Codes, so we sure hope you guys were right, or we might be "hooped" and end up becoming "stressed"...so to speak.

Regards,

SNORGY.

 

[eadwine]

Snorgy, It works, which is the bottom line. The big push is on for the use of "alloy clad steels" or "plastic/glass lined steel" these days. This can reduce costs by up to 70-80 percent in most applications. Not too mention, significant reductions in Inspection/Maintenance costs to check corrosion allowances and to effect repairs over the life of the Piping/Vessel systems.

Remember, "cost cutting is the soul of profitability" these days. It's all about the race to the bottom. Hopefully, we won't go too far ...

 

[rneill]

Question, is it possible that part of the desire to specify Cat II materials in sour service falls out of the NACE/ISO requirement that carbon steel materials be in a heat treated condition, something that is not necessarily certain with Cat I materials ?

I don't know but it may be that people are trying to simplify the process of ensuring compliance with that aspect of NACE/ISO ?

 

[SNORGY]

rneill,

That's certainly possible, but based on the scores of folks that I have talked to, that would be giving us way too much credit for the level of knowledge and thought that are usually behind the decision.

I'm blunt this morning...need coffee...

Regards,

SNORGY.