SUBSEA FLANGED JOINT FOR HDPE PIPES

[chrysa1975]

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.

 

[chrysa1975]

We may go with 1.4410 to avoid cathodic protection. If anyone has any kind objection, please do so. Thank you all for your help!

 

[EdStainless]

The reason that we mentioned Cu based alloys is that they have a long term track record in seawater. The main reason is that the Cu in them slowly dissolves and is toxic to marine life, this serves to prevent/minimize fouling.
I don't know that 2507 would not work, it certainly should, but I don't know of any long term sub-sea data.

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P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

The biggest concern with copper alloys in salt water is sulfide corrosion. If the back fill has a high concentration of microorganisms then this could be an issues. Otherwise, copper alloys are very reliable.

 

[chrysa1975]

The concerns about Copper alloys are:
1. if they can be fabricated in DN2300 required for the flange backing ring.
2. What will be the material grade for the three mating items, flange, bolts, nuts. Can we have NAB flange with silicon bronze bolts and what shall be the material for nuts. Will any galvanic corrosion present?
3. if they will face high corrosion rate being immersed and buried under sand backfill.
On the other hand, same applies for the Super duplex as well! I am really in trouble...

 

[chrysa1975]

TugboatEng, I saw your reply after my submittal. Thank you both indeed, so helpful posts! Can I request for the backfill to be clean? Or the bacteria will be self born?

 

[LittleInch]

Well you never said it 2.3m diameter pipe!!

I think you might be over thinking this a little bit. Super Duplex in this sort of service is about as bullet proof as you're going to get and doesn't need CP.

What is this for - A power plant water channel or something else?

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

 

[chrysa1975]

You are correct, I have not mentioned the NPS. It is an HDPE Seawater Intake pipeline.

 

[EdStainless]

Well at least you grasp the challenge.
Location is a big factor here. If this is deep and away from populated areas you have a very good chance.
If it is shallow and near lots of people (more pollution) then the risk is higher.
Have you looked at non-metallic options? Could you weld the tube or use FRP flanges?

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

 

[chrysa1975]

Locations will be 2 km from coast and 10 meters depth. So I guess there will be a pollution.
Welding the long length HDPE pipes is an option that I have suggested.
Non metallic backing rings according to ISO 9624 have been suggested as well, but we have a concern regarding ease of installation at site, bolt torquing requirements, gaskets, bolts materials etc.