Effects of sour, wet gas corrosion in Carbon Steel Pipelines 3

[ekannan]

Dear all,

I have a problem with one pipeline located in China. We have pigged the lines carrying Sour, Wet gas product. The corrosion anticipated were large internal pitting features. True enough, there are large internal pitting features in this pipeline but according to our data, these features are external instead of internal! How can this be? Could it be the soil acidity etc instead of the sour, wet gas? Any further details on this problem would be a great help. Thanking you all in advance.
Keep up the good work.

 

[PVRV]

1. All pigging data requires verfication. (sample digs,
excavations.)

2. Pipeline corrosion can be assessed with internal
monitoring (probes/coupons) Else externally with
potential readings else DCVG.

3. It is unlikely in my prespective that intelligent
pigging would highlight hydrogen damage.
(HIC/SOHIC/SSC/etc) unless ultrasonic tool was used
in place of MFL and it is very diffucult to provide
the cleaning requirement for UT pigging.

Cheers

 

[ekannan]

MFL Type pigging was used to detect the abovesaid pitting features. As the pipeline contains H2S, deep internal pitting features were anticipated. After MFL pigging was done and and evaluation of the data was completed, most large pitting features were external instead of internal to the pipeline. No dig-ups have been done as of yet. Any comments on why these features are external instead of internal would be greatly appreciated. Thank you in advance.
Kannan

 

[PVRV]

1. In wet H2S service PITTING corrosion would NOT be my first
concern.

2. Prior to assessing why corrosion presence is evident externally
in place of internally verifying the data is prudent.

3. Assuming, deep pitting has been verified externally; that
points towards the effectivness of the external coating system
employed to combat marshy area or soil with high salt content.

4. Please consider, that the method you employ to survey the
pipeline will only demonstrate indications that the method of
testing is able to present. So there may be much more serious
indications or anomolies that MFL is unable to detect.

 

[flames]

I agree with PVRV that you need to verify the indications with digs. The company you are working for should have guidelines for determining when an indication is a potential candidate to verify.

If you are suspecting external than the procedure of excavation should include methods of collecting groundwater adjacent to the pipe and sampling of the environment beneath any corrosion barrier. Your description of the damage can indicate either high chlorides or MIC, but this is all just speculation.

If you are dealing with wet H2S sour gas you can in fact get deep isolated pitting (looks like someone took a drill and started putting deep holes in the wall with a 1/4" or so drill bit).

 

[arbo]

probably not used the appropriate corrosion inhibitor
try to contact

http://wwww.arbochim.be

thks

 

[PVRV]

flames,

How does H2S influence deep pittings ?

 

[flames]

You will get a bunch of different answers to this questions as the mechanisms are not clear. One point I want to make is that although isolated pitting does occur, having an area of corrosion also occurs.

Typcially a wet H2S system is not overly corrosive due to the formation of the various protective iron sulphide films like mackinawite, pyrite and pyrohotite. If you can increase your H2S partial partial to achieve these last two your corrosion rate is very low (look at NACE 2002 paper by Smith for a good discussion on the thermodynamics of these sulphides).

Unfortunately its the contaminates in the system that forces a localized breakdown of the film that leads to pitting. A major one is chlorides which could be present in the formation water and another is sulphur. In a sour gas well the gas will contain hydrogen polysulphide and as the pressure in the system drops, it breaks down into H2S and sulphur. There are other factors, but this is just a start of a long topic.

If you look at a NACE Corrosion 96 and a paper by Bich and Goerz gives an example with pictures of a failure of a sour gas line due to these large isolated pits.

 

[PVRV]

Now that is commendable. thank you