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SUBSEA FLANGED JOINT FOR HDPE PIPES

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chrysa1975

chrysa1975

Materials
Nov 14, 2007
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For subsea flanged connections of HDPE sea water intake chlorinated pipes we need to specify the material for the flanges/bolts/nuts with or without cathodic protection. System will be covered in its full length with backfill sand. Proposed materials are HDG with cathodic protection, 316L with cathodic protection, duplex and super duplex with or without cathodic protection. CP, if any, will be through sacrificial anodes. Design life is 50 years.
My queries are:
1.For the material selection do we have to consider impact resistance, bacteria growth, hydrogen embrittlement except from resistance to pitting corrosion?
2. Can you help me with the evaluation for the proposed materials?
3. 316L can receive CP through sacrificial anodes?
4. What is the appropriate material selection for 50 years design life? Thank you all very much in advance.
 
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1. Each material has different failure mechanisms. For SS and Ni alloys pitting and crevice corrosion are important, especially with biofouling. A steel would have HE and impact concerns.
2.Has anyone presented support data for a 50 year design? Sacrificial anodes are not lasting 50 years.
3. Yes you can CP SS alloys.Current draw is lower since the corrosion rates are lower, but in crevices you can still get attack.
4. I would suggest looking to older literature.
The only alloys that I would look are Ni based, either Monel 400 or C276.
The problem with bolts is that you don't really have a corrosion allowance. Once you get any crevice corrosion the flanges loose clamping. So it isn't like a bulk stucture.

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P.E. Metallurgy
 
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Just to go outside of the box, have you considered silicon bronze? It's much less expensive than any of the materials listed, resists corrosion in sea water and is not subject to crevice corrosion. High strength shouldn't be a requirement if you're using plastic piping. Silicon bronze fasteners are an off-the-shelf option as well.
 
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Start with reviewing EEMUA 194 for guidance on materials and corrosion control. Then work through the degradation mechanisms listed in API RP 571, eliminating those deemed as not credible for the various materials options.

Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.
 
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Thank you all very much! Some queries on your replies:
1. Sand backfill will enhance biofouling?
2. HDG will face HE due to the CP? Is it a viable option? It is already installed in a nearby contruction with huge Aluminium alloy sacrificial anodes...
3. Is there a possibility CP sacrificial anodes to revert and sacrify the cathode (flanged connection) under conditions described, i.e. backfill & subsea? Especially for the 316LSS. What will be the material of the anodes? Aluminium?
4. HDPE bidders have quoted for HDG with CP or 316LSS with CP or 1.4410. CP will be maintained to last 50 years. But I need the opinion of a specialist in metals, not in plastics!
5. Service will be chlorinated seawater, should it be considered for the material selection?
6. What are the grades for NiAl bronze flange/bolt/nut? There shall be all the same grade?
A search has shown no pitting/crevice/galling/biofouling. Maybe this will be my solution since no other metals will be near for galvanic corrosion issues.
I will also review EEMUA and API when I archive them, super references, thanks!
 
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Read, and digest, the documents that you will be "archiving" (hopefully not filching illicit copies), and also check DNVGL-RP-B401. Incidentally, it's the steel substrate, not the hot dip galvanising, that could have a susceptiblity to hydrogen embrittlement.



Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.
 
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The NAB is alloy 630 (C63000) and some derivatives.
The three digit numbers are usually the old CDA (Copper Development Association) designations. You can go to the CDA web site and look up a lot of technical info.

By being buried you assure an anaerobic environment, so the bugs will be different. And alloys like SS which require oxygen for passive film development will have more problems.
I cannot picture anodes or HDG holding up for 50 years, 20 yes.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy
 
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The inner service will be chlorinated sea waste but that's irrelevant. Your flange is buried subsea.

Really not sure how you plan to add CP to a flange??

Forget CP for a 50 yr life. Replacing it is a nightmare.

Just use Super Duplex or Aluminium Bronze.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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LittleInch, thank you very much for your reply! I know nothing about cathodic protection and how you can add sacrificial anodes on a flange. The HDPE bidders have quoted so!, this is why I need an expert approach corrosion point of view. Mr. EdStainless said above that crevice corrosion should be considered for super duplex. If I go with Nickel Alu Bronze, it will be affected from the fact that it will be backfilled with sand?
 
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Ed will know a lot more than me so maybe, but I think you need to look at the service and determine the stresses and what happens if the bolt became loose or even "failed". For a seawater intake line this will be lower pressure than the sea to make it flow, unless you have a subsea pump??

~Or is there a way to get rid of the flange completely? PE fuses really well and unless this flange is being connected subsea?? then there are alternatives,

But use or find a good metalurgist. 50 years in seawater is a loooong time for any material, even something as realtively bullet proof as Super Duplex.

The only thing you find intact from wrecks are the bronze fittings....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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Dear all, just to inform you that I have contacted copper.org for the suitability of copper alloys and I will share any feedback. Furthermore I have just finish reading Corrosion Performance of Metals for the Marine Environment EFC 63: A Basic Guide, where a lot of queries found a reply.
 
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