Cathodic Protection(DNV vs NACE)

[victor007]

Hi, Has somebody done the CP calcs per both DNV-RP-B401 and NACE-SP0607?
I am wondering why the results are soooo different per these codes. For the same surface area DNV comes up with waaaay higher number for anodes comparing to NACE. (i.e. per DNV B401 you need 13 of 150lb anodes but per NACE you just need 2).
I digged more into the detail, and figured out this wide gap btwn the results is comming from big differences between these codes on the coating breakdown factors. NACE0607 Coating breakdown factors are way lower than DNV, and the final result is so sensetive to those factors.

Can somebody shed more light on this issue, and help me understand why these two codes are so different (conceptually)? I can't understand the concept behind "coating breakdown factors", and the concept behind this big gap.

I appreciate the time and energy you gonna spend to educate me.
Victor

 

[BigInch]

Perhaps the NACE applied coating inspection and testing requirements are more demanding.

Independent events are seldomly independent.

 

[LittleInch]

I know this might sound condescending, but make sure you're using the latest DNV code. It changed relatively recently and resulted in a significant decrease of anodes. These are free to download from the dnv website.

Coating breakdown is usually a historical based figure and maybe dnv is just using either data from the North Sea where conditions are a bit rougher or just a bit out of date.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SJones]

Firstly, try comparing pipeline CP standards instead of structures with pipelines. There is one factor straightaway that will explain the difference: coatings on pipelines and structures are vastly different. Try rerunning the calculation with DNV-RP-F103 instead and see what you get. The coating breakdown factors in DNV-RP-F103 are derived from DNV's belief in the reliability of coatings complying with their other standards RP-F102 and RP-F106 whereas the NACE (ISO 15589-2) standard doesn't quite have the same belief. Maybe the revised version will come closer to DNV if they account for perceived coating reliability instituted by the ISO 21809 suite of standards?

http://www.ptil.no/getfile.php/Tilsyn på nettet/wi_oppsummeringsrapp.pdf

Steve Jones
Corrosion Management Consultant

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

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

 

[victor007]

@BigInch, make sense, but I do not know. I wish you could tell me. Thanks for your response.

 

[victor007]

@LittleInch, My DNV calcs was based on DNV-RP-B401 OCTOBER 2010, Modified on April 2011. I looked at DNV website and it was the latest one. Thanks for your response.

 

[victor007]

Steve, Thanks a lot for sharing the knowledge. The report was great too. I appreciate your time.
Victor