montbIanc
Civil/Environmental
- Mar 11, 2023
- 21
G’day.
I’m trying to learn the basics of sacrificial anode CP design for wharves, mostly piled structures and sheet pile walls. The situation is we’re a small structural firm that does some marine work. We subcontract out detailed design of CP but often need to do a comparison of options at concept design. The options are things like CP, paint, Denso wrap and encasement. Setting up a subcontract for just the concept stage is a pain (and usually makes us late) so I want to be able to do it in-house without being too far from the final answer when we sub it out if CP is selected.
We’re usually required to design to the Australian Standard AS 2832.3. Whilst looking into it, I’ve also come across the DNV standard DNV-RP-B401 which seems compatible/similar and provides more guidance on the values of parameters to use. Since DNV is more international I’ll refer to it hoping more people can help with my questions.
Basically, it looks to me like the concept design process has three criteria:
1) Initial current output
2) Final current output
3) Anode mass based on average current output
I’ve found two design example on the internet but they’re not very good in my opinion, with numbers coming out of nowhere with no explanation and some mathematical errors that make it hard to fill in the blanks. These are:
a) US Naval Academy:
The main issue I have is that neither of these documents checks all three criteria, but all three are checked between the two of them. So my basic question is whether I’m right in saying those are the three criteria.
The USNA examples (see section 8.1 in the document) use Criteria 1 & 3 from my list. The final current output isn’t checked. It looks to me that final current won’t need to be checked for bare steel because the initial current demand from new anodes will govern over final current from almost-depleted anodes. Is that correct?
The USNA then does an example for the same structure except it is painted. There is no text explanation of the process (just a spreadsheet printout), but they’ve used the mean current density so I think they’ve just checked total anode mass (criterion 3) without looking into criteria 1 or 2. I can understand omitting criterion 1 (initial current demand) because it will be small for painted structures, but for paint system 1 the coating is completely ineffective at end of life so shouldn’t the final current demand be checked?
There’s an error in the USNA painted example in that the total mass of anodes is not the product of individual anode mass and number of anodes. They’ve called that the cost which I believe is an error. It makes it hard to understand the example without any description of the process, because maybe they would have adopted a much larger anode mass if the calculation wasn’t mis-labelled.
The WBDG examples (see sections 3.3 and 3.4) calculate the initial current output from criterion 1 but don’t use it for anything. There’s no check of whether it’s adequate or not for the demand, whereas this is the critical factor in one of the USNA examples. Criterion 2 (final current output) is checked and criterion 3 appears to be checked twice using different methods (for example, the calculation above the yellow box on page 32, and step 5 on page 35. Unfortunately, the section number 3.3.2.3 is repeated twice, both above and below the yellow box – did I mention these examples aren’t great?)
Thanks for any clarification you can provide.
I’m trying to learn the basics of sacrificial anode CP design for wharves, mostly piled structures and sheet pile walls. The situation is we’re a small structural firm that does some marine work. We subcontract out detailed design of CP but often need to do a comparison of options at concept design. The options are things like CP, paint, Denso wrap and encasement. Setting up a subcontract for just the concept stage is a pain (and usually makes us late) so I want to be able to do it in-house without being too far from the final answer when we sub it out if CP is selected.
We’re usually required to design to the Australian Standard AS 2832.3. Whilst looking into it, I’ve also come across the DNV standard DNV-RP-B401 which seems compatible/similar and provides more guidance on the values of parameters to use. Since DNV is more international I’ll refer to it hoping more people can help with my questions.
Basically, it looks to me like the concept design process has three criteria:
1) Initial current output
2) Final current output
3) Anode mass based on average current output
I’ve found two design example on the internet but they’re not very good in my opinion, with numbers coming out of nowhere with no explanation and some mathematical errors that make it hard to fill in the blanks. These are:
a) US Naval Academy:
https://www.usna.edu/NAOE/_files/documents/Courses/EN380/Course_Notes/Cathodic Protection Design.pdf
b) Whole Building Design Guide: The main issue I have is that neither of these documents checks all three criteria, but all three are checked between the two of them. So my basic question is whether I’m right in saying those are the three criteria.
The USNA examples (see section 8.1 in the document) use Criteria 1 & 3 from my list. The final current output isn’t checked. It looks to me that final current won’t need to be checked for bare steel because the initial current demand from new anodes will govern over final current from almost-depleted anodes. Is that correct?
The USNA then does an example for the same structure except it is painted. There is no text explanation of the process (just a spreadsheet printout), but they’ve used the mean current density so I think they’ve just checked total anode mass (criterion 3) without looking into criteria 1 or 2. I can understand omitting criterion 1 (initial current demand) because it will be small for painted structures, but for paint system 1 the coating is completely ineffective at end of life so shouldn’t the final current demand be checked?
There’s an error in the USNA painted example in that the total mass of anodes is not the product of individual anode mass and number of anodes. They’ve called that the cost which I believe is an error. It makes it hard to understand the example without any description of the process, because maybe they would have adopted a much larger anode mass if the calculation wasn’t mis-labelled.
The WBDG examples (see sections 3.3 and 3.4) calculate the initial current output from criterion 1 but don’t use it for anything. There’s no check of whether it’s adequate or not for the demand, whereas this is the critical factor in one of the USNA examples. Criterion 2 (final current output) is checked and criterion 3 appears to be checked twice using different methods (for example, the calculation above the yellow box on page 32, and step 5 on page 35. Unfortunately, the section number 3.3.2.3 is repeated twice, both above and below the yellow box – did I mention these examples aren’t great?)
Thanks for any clarification you can provide.
