MDMT of unknown CS type: PMI, Hardness, etc

[Hansac]

Hi there everyone.

I have a group of ageing assets (offshore platforms, mostly), which has scant documentations at best. Recently our organization embarked on a project to determine if the carbon steel portion of the pipings and pressure vessels are adequately protected against brittle fracture. In order to do this, we have to know the minimum design metal temperature (MDMT). With inadequate documentations, it is not possible to find from records the actual type of carbon steel used in the pipings or pressure vessels (A106 Gr. B? A333 Gr 6? SA 516 Gr 60N? SA 283 or 285?).

It was proposed that a boat/scoop sample is taken from these equipment (pipings/pressure vessels) and impact testing be carried out in the lab using these samples...

I am thinking of a different approach and would need opinions (dissenting or otherwise) from this esteemed forum.

Can I just do positive material identification (PMI) using equipment available from the market? Together with portable hardness tester?

With PMI and portable hardness tester, I can:
1. Find out all material composition of the carbon steel, including carbon content.
2. Find out the strength of the material (correlated to the hardness of the material).
3. From item 1, find out carbon equivalent.

I have to first qualify whatever I am saying here, that my knowledge in material engineering is limited (mechanical engineering by training with about 15 years of inspection engineering under my belt).

Having said that, I *think* there is a correlation worth exploring between material composition of carbon steel, knowing carbon content thereof, hardness of the material and therefore the strength, weldability, WITH minimum design metal temperature (MDMT) of the material.

I have reservations with doing boat/scoop sampling because this:
1. introduces a notch into my equipment
2. removes much needed thickness for pressure containment
3. removes insulation/coating if any
4. also have issue with the impact test using the boat samples since the boat size sample is too small and even with standard size impact test test sample, a quantity of three samples is required to arrive at the energy required for the impact testing.

Therefore I am thinking something along the line of non-destructive testing to approach this issue of finding MDMT of unknown material.

If my argument holds, my organization stands to save a *substantial* amount of money (to the tunes of hundreds of millions of dollars if you factor in not just the testing exercises itself, but also other factors like preparation and reinstatement).

Any reply/discussion is very much appreciated and I thank you in advance.

Regards,

Ir. Hanafi

 

[stanweld]

You will not be able to establish the minimum Charpy V-notch toughness at a specific temperature by hardness testing and PMI. For decades off shore platforms have been designed to resist brittle fracture at minimum service temperatures. If you wish to use the items at temperatures below which they were originally designed, I suggest that you remove sections of material for impact testing and weld in flush patches to replace the material removed.

 

[rneill]

Agree with Stanweld, other than impact testing, I'm not aware of any other test you might perform that would give you an indication of the MDMT. Certainly, PMI would not do anything for you in this case - as far as I know, it can't even differentiate the types of carbon steel.

However, flanges and fittings should be stamped with the grade of material (whereas pipe is stenciled) and it is quite possible that despite painting, these markings may still be visible. The first thing I would do is try to establish the grade of flanges and fittings and then I might assume that the pipe were a corresponding grade of pipe. If you get lucky and the materials are a low temperature material, you may be able to save yourself some work. On the other hand, if the markings are for regular temperature materials you will be back where you started.

If I were going to remove materials for testing, I'd want to think carefully about the best way to do this. I'm not very keen on patches which used to be common in the pipeline industry but I believe they are rare today. The current pipeline code in Canada (CSA Z662) prohibits them.

One point that should be made is that prior to the mid 90's, ASME B31.3 accepted A106-B pipe down to -29 C regardless of thickness but about that time it was changed to the current curve B (so only -29 C if less than 0.5" thick). You never mentioned what your MDMT actually is but they might have used A106B meeting code for the required MDMT at the time but perhaps not meeting code for the MDMT today based on the code changes. The piping would be grandfathered in this case but you may want to think about what you would want to do with such piping.

 

[Hansac]

Gentlemen.

We only want to establish the MDMT during design, and not re-rate any equipment.

The assets are all in the tropics, so the lowest temperature will only be seen during blowdown. Assets include oil (& gas) and gas (only) producing platforms.

Seriously, I do not see any compelling engineering reason why this has to be carried out (finding MDMT when the service is not cold service). Perhaps this forum, too, can enlighten me on this.

 

[SJones]

This is the old 'how is the MDMT set and who is responsible for its determination' chestnut. This is a fitness for service exercise and an API 579-1 approach is probably required where the concepts of CET and MAT, using a conservative choice for the unknown materials, may be more useful than trying to set an MDMT.

Steve Jones
Materials & Corrosion Engineer

http://www.oilandgaspeople.com/cv/11499664

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

 

[rneill]

I guess I would emphasize that B31.3 does have a variety of paragraphs that address the need for ensuring that materials have an appropriate MDMT for the temperature to which they may be exposed while under pressure. Consequently, even during transient conditions such as blowdown, code does require consideration of the metal wall temperature that will be experienced (in light of the coincident pressures and corresponding stresses).

Recent editions of the code have provided reductions to the normally permitted MDMT for coincident pressures that result in stress levels less than permitted for piping systems / components and there is a general blanket exception that may apply once the coincident stress drops below 30% of the normally allowed stress. However, there is nothing that would exempt a blowdown case from having to be fully evaluated / considered just because it is a blowdown case and not a normally operating "cold" case.

That said, given that these are existing operating assets and B31.3 is a new construction code, I would concur with SJones recommendations.

 

[Hansac]

Gentlemen.

The crux of the issue is the inability to know what exactly is the type of the carbon steel we have in our hand, and from there trying to determine the MDMT.

If we know it is A106 Gr. B or SA516 Gr 60N, then we can readily find the MDMT from respective curves in the respective Codes.

How do we find out the MDMT without doing impact test on an unknown material?

That's a challenge I would throw to our metallurgists here...

Is that a valid challenge?

 

[SJones]

No it isn't because it should not be the material that sets the MDMT; it should be the responsible (process) engineer. What you should be trying to do is assess the risk of brittle fracture for a given low temperature not trying to determine that given low temperature!

Steve Jones
Materials & Corrosion Engineer

http://www.oilandgaspeople.com/cv/11499664

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

 

[rneill]

In my mind the process is normally ...

Step 1: the responsible process engineer determines the possible coincident temperature / pressure combinations that may occur and need to be considered.

Step 2: the responsible mechanical engineer evaluates the information provided by the process engineer, and attempts to establish that the materials to be used are suitable (have an appropriate MDMT) for those conditions in accordance with the applicable design code.

Only once you have established the requirement can you start to assess whether the material is suitable for that requirement. If nothing else, you have to know what temperature to do impact tests at ...