Isolation of subsea stainless and carbon steel pipe

[pipelinermike]

I have a situation where I need to electrically isolate a segment of stainless steel pipe (A312-TN304L) from a segment of low alloy carbon steel pipe (API 5L). Does anyone have any good experience using isolation joints subsea? If so, please reply with some manufacturers. The operating pressure is 2220 psig (ANSI 900 class), the connection is near shore so external pressure should not be an issue. Pipe size is 24" OD.

Thanks

 

[Metalguy]

Given the conductivity of sea water the OD of the pipe will have to be coated very well. You'll need insulating flange gaskets and bolt sleeves & washers also.

 

[jay165]

A few thoughts:

1. As mentioned by Metalguy, the only way to "isolate" the 2 pipe materials is to install a flange wih a flange insulating kit. My experience with such kits is that 50% of them are installed incorrectly (eg, a bolt sleeve is left out or gets pinched) which renders them useless and this is ON LAND. If possible I'd make your spec break above ground if you wnat to do this.

2. Assuming you can't move the spec break, I'd check to see if galvanic corrosion will really be a problem. The potential difference between your 2 materials may be low enough to where galvanic corrosion will not be significant. Welding the 2 lines together and then applying a sufficeint coating system like Belzona over the welded area (and several feet upstream and downstream) may do the trick.

3. If the electrical potential is a problem and you want a belt and suspenders solution, you could weld an anode (or anodes) across the 2 materials to prevent galvanic corrosion.

In my opinion anodes and/or coating are going to be more reliable as well as cheaper to construct than a flange with an insulating kit.

 

[AndrewLindsay]

Pipelinermike,
Why do you need to isolate the 304 from the carbon steel?
I would have CP applied to the 304 section as well as the carbon steel, so making them continuous is a very pragmatic solution. If you are concerned about internal corrosion, then I would have to ask what is going to be carried internally in the pipeline. Under anoxic conditions (no oxygen), the potential difference between 304 stainless steel (depassivated) is similar to carbon steel, therefore the rate of corrosion I would expect to be very low.
If you are still in need of assistance, then please reply with more details of your concerns.

Regards

Andrew