Cathodic Protection Design

[rinshonsan]

Working on CP design using DNV RP B401, I am noticing that the anode mass requirements when opting for a large flush mounted anode (258 lb) are higher compared to a small flush mounted anode (12 lb). In other words with 258 lb anode the total anode mass required is 700 kg however with 12 lbm anodes its nearly half of that ~300kg for the same cathode surface area. I am trying to understand why this is the case. I know with smaller anodes one would need larger quantity but don't understand why total mass is less. My calculations show that the 258lbm anode has low final current output and so to meet the final current demand and to meet the criteria in 7.8.4, the numbers adjust to a high total mass. Also, for both anodes, as per the spec, I am assuming the final area to be the same as the initial area of the anode touching the structure. Has any one experienced this before and explain why? Also is it better to have fewer large mass anodes than having a higher number of small mass anodes?

 

[BigInch]

Don't remember a whole lot about this, however a higher number of smaller anodes, shorter distances apart along a pipeline results in more efficient coverage and less overall total anode weight.