Leaving The Girth Weld Uncoated in Sour Gas Pipeline

[Hajiwaji]

Dears

The pipeline will have sour gas with H2S and CO2 with little to no water phase. The predicted corrosion rate is small and can be controlled with corrosion inhibitor and scraping with bare CS. Curranty, internally FBE coated pipes are readily available. Is it acceptable not to fix coating of the girth weld of the spools? Will this create large cathode to anode surface area and induce high corrosion rate?

Thank you

 

[LittleInch]

Interesting - Not sure I've ever seen that before. I theory if you add CI and pig regularly to presumably remove any standing water? then it should be ok, with reduced corrosion due to the coating.

There is no anode cathode issue here...

Far far better to try and eliminate any free water....

Also with coated pipes, your choice of pig or "scraper" is more limited otherwise all the internal coating starts to flake off.

If you internally coat the welds then people will be under pressure not to inject expensive CI, but I've never liked relying on an internal coating for internal corrosion protection with CO2 and H2S around.

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

 

[EdStainless]

Or the weld metal may be the anode and adjacent base metal the cathode.
If this a dry application (no detectible water) then you could likely get away with it.
If there is any water at all it will collect at the weld ridges, and you could have some serious issues.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Hajiwaji]

Thank you LittleInch and EdStainless,

What I meant by large surface area of cathode to anode is that the system will be out of mass transport control. large amount of acid gas and small CS exposed to it. The gas is single phase with dew point control, so there will be little to no condensate.

 

[SJones]

If it is believed that an internal coating is required for corrosion control, then why leave areas of the pipeline open to the corrosive environment? If the environment is not corrosive, why coat at all?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[0707]

(Litle water) works as electrolit, so on girth welds botom, not coated the probability to have locally galvanic corrosion can´t be negligible.

regards

 

[Hajiwaji]

Hi Dr. Jones,

The internally FBE coated pipes are available in client's storge, and they want to use them. The service is corrosive as acid gas is present and partial pressure of CO2 is above 30 psig. very little to no water will be present during operation of the pipeline. The concern is that having girth weld uncoated will drive the system to the activation control state, and as a result the exposed area will experience high corrosion rate.

I hope you can correct my understanding of "mass transport" and "activation" controls concepts.

 

[EdStainless]

I wouldn't risk it.
If they want to rely on inhibitor, then are they going to be using ORP (or similar) sensors to measure the actual corrosion potentials in the pipeline?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[SJones]

It may help the discussion to break the projections down a little more:

1. The water will be condensed water with low conductivity
2. The electrochemical range of influence of the low conductivity water will be short
3. Enhanced corrosion will require the bare area to become a net anode
4. Item 3 will require the coating to be permeable, and the steel underneath to become a net cathode

I think that there are papers that suggest 4 is possible and can happen with FBE. I’m on the hunt for them when I have time.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[LittleInch]

Does the effectiveness of the CI depend on water being present?

So no water, the CI is not effective?

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

 

[Hajiwaji]

Thank you Dr. Jones,

The water if it ever condense, will be conductive since acid gas will dissolve in it. ECE pH calculation is about 4.

 

[rscosta1976]

I don't believe the suggestion of having uncoated girth welds is acceptable for the conditions you are describing. A CO2 partial pressure above 30 PSIG is usually considered quite corrosive, especially in your case (with a pH of 4).

As stated above by several professionals, this is a risky approach. See below some points for your consideration:

1) How exactly can you ensure no water will be present, neither produced or condensed? Please remember that any water in the system can be highly corrosive for such a small anodic areas (girth welds), especially if there is no pH buffering (in case of condensed water).

2) Even if "little water" is present, considerable amounts may still accumulate at low points between scrapping intervals, where corrosion inhibition efficiency is low.

The corrosion management strategy should mandate girth weld coating (which is possible for pipelines above 6") and have continuous Corrosion Inhibitor injection (to provide protection against some localized coating defects). Scrapping frequency should be adjusted accordingly.

 

[SJones]

Perhaps, further examination is required. The OP mentions “spools” which implies short sections of the pipeline rather than the complete pipeline. They also mention inhibition but the management of the chemical treatment process is not indicated. If it is a case of the coated pipe serving an opportunity purchase, and there is confidence in a high quality inhibition operation including pigging (scraping for those in certain parts of the world), then why not go ahead?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[rscosta1976]

I agree with the need to make a detailed assessment.

I see no issues in using FBE coating for the pipeline inner surface (coupled with a well designed and operated chemical injection system). But, in my view, the girth welds should be coated as well, especially for pipelines with several kilometers, where even small amounts of water carried with the main stream may accumulate at lower points. Coating the girth welds should not represent a big investment in the overall cost of the project, so why take the risk?
Also, some wells may see an increase in water cut in the future.

It all comes down to the quantity of water present and its source (condensed vs produced) and respective level of contaminants.

 

[brimmer]

With proper inhibition I don't see this as a huge issue, but you will get some corrosion at the girth welds eventually. The large cathode to anode scenario isn't really valid (at least until the coating starts to break down). There is going to be some edge affect where the coating stops, being that the area is lower and rougher than the surrounding coating, some water is going to accumulate there. if properly inhibited and it is only CO2 corrosion (i.e. there isn't a bunch of solids and set up of under deposit corrosion) it should be okay. Your operating history of other similar lines should give you an indication of what's happening (assuming you might have some bare steel in the system)