Cathodic Protection for Subsea Structure Piling 1

[SethParisGrace]

Hello everyone,

My question is this:

For a subsea open end pile, is there a recommended limit to the pile depth which contributes to the structure's buried surface area (and therefore its current drain)?

The DNV-RP-B401 addresses pile contribution to the structure's submerged surface area in the following:

"For open pile ends, the top internal surface shall be included for a distance of 5 times the diameter and shall be regarded as seawater exposed."

There is no similar prescription for evaluating the surface of the buried end up to a specific depth.

Surely, the buried pile surface area is not evaluated across its entire depth, correct? I would assume that after a depth of at most 20 or 30m, the corrosive effects of salt water would be inhibited.

Thank you.

 

[SJones]

Read the first sentence of 6.9.2. It does not make the same assumption that you are making. Naturally, you are still free to make that assumption in your design. It is not a question of whether there is corrosion, it is a question of whether the design can accommodate the current that will be drawn by the metallic structure. You might find your anodes wearing out a little more quickly than you planned even if you aren't expecting any corrosion, and then what will you do for the rest of the structure?

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[SethParisGrace]

Thanks Steve,

That makes sense. In order to prevent any corrosion on the structure, the driving potential between the pile and sediment would have to be negated regardless of whether iron is actually oxidizing at the buried end of the pile. This is my understanding of your response at least.

 

[strider6]

you're correct, below a certain depth there will be no oxygen and the corrosion rate will be negligible. i'm not aware of any standard that set a limit but i think that 20-30 mt is a good assumption.

S

Corrosion & Rust Prevention Control

 

[SJones]

Not quite. The DNV document is simply, possibly arbitrarily, accounting for the current that could be drawn by the pile surface in sizing the sacrificial anodes or the impresssed current system. It is not advocating that this current is required for corrosion protection.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[48as52]

here is no similar prescription for evaluating the surface of the buried end up to a specific depth.

Eng Tips Rocks

thanks,
kk

http://cubuilt.com