Corrosion Engineering, Subsea Pipelines & Offshore Structures
Sacrificial-Anode Cathodic Protection Mass & Count Formula
Calculate DNV-RP-B401 Galvanic Cathodic Protection Mean Current Demand (A), Required Net Anode Mass (kg), Minimum 150 kg Al-Zn-In Anode Count, and Gross Installed Anode Mass (kg).
Never omit the DNV anode utilization factor u = 0.85—the last 15% stub cannot protect steel!
Under DNV-RP-B401 offshore cathodic protection design, galvanic Al-Zn-In or zinc sacrificial anodes corrode preferentially to polarize submerged carbon steel below -0.80 V vs Ag/AgCl. Mean Current Demand is I_mean = Area_m2 * i_bare * f_coating, and Required Net Anode Alloy Mass over Design Life t_yrs is M_net = (I_mean * t_yrs * 8760) / (Capacity_Ah_kg * u)! Forgetting the Utilization Factor u (0.85 for slender stand-off anodes, 0.75–0.80 for flush-mounted bracelets) assumes 100% of the alloy dissolves smoothly—when in reality the anode detaches from its welded steel insert stub once 85% consumed, leaving subsea jackets unprotected for the final ~4 years of a 25-year field life!
Verify on a 25-Year Subsea Jacket Structure—Submerged_Steel_Area = 4,000 m² (B2), Seawater_Bare_Current_Density = 0.10 A/m² (100 mA/m², B3), Mean_Coating_Breakdown_Factor f_c = 0.05 (5.0% bare equivalent, B4 -> Mean Current I_mean = 20.00 A in E2), Design_Life = 25 years (B5), Al_Zn_In_Capacity = 2,000 A·hr/kg (B6), and Stand-Off Utilization_Factor u = 0.85 using 150 kg individual anodes (B7): Required Net Alloy Mass equals (20 * 25 * 8,760) / (2,000 * 0.85) = 2,576.47 kg (E3), rounding UP via ROUNDUP(2,576.47 / 150, 0) to 18 Anodes (E4, 2,700.00 kg gross installed alloy E5)!
| DNV-RP-B401 Cathodic Protection KPI (4,000 m², 0.10 A/m², 5% Breakdown, 25 Yrs) | Formula | Verified Output |
|---|---|---|
| Mean Protection Current I_mean (A) | Steel_Area_m2 * Bare_A_m2 * Coating_Breakdown | 20.00 A (40.00 A @ 10% Breakdown) |
| Required Net Anode Mass (kg) | (I_mean * Years * 8760) / (Ah_per_kg * u) | 2,576.47 kg (5,152.94 kg @ 10%) |
| Minimum 150 kg Stand-Off Anodes | ROUNDUP(Required_Net_Mass_kg / 150, 0) | 18 Anodes (35 Anodes @ 10%) |
| Gross Installed Alloy Mass (kg) | Min_Anode_Count * 150 | 2,700.00 kg (5,250.00 kg @ 10%) |
DNV Galvanic Anode Alloy Mass & ROUNDUP Count Formula
Validate Steel_Area > 0, Bare_Current > 0, Coating_Breakdown in (0, 1], Design_Years > 0, Capacity_Ah_kg > 0, and Utilization_u in (0.50, 0.95]!
=LET(ar_s, B2, i_br, B3, fc_m, B4, yr_d, B5, cp_a, B6, ut_f, B7, IF(OR(ar_s <= 0, i_br <= 0, fc_m <= 0, fc_m > 1, yr_d <= 0, cp_a <= 0, ut_f <= 0.5, ut_f > 0.95), "Invalid Cathodic Protection Input", LET(i_mn, ar_s * i_br * fc_m, ROUND((i_mn * yr_d * 8760) / (cp_a * ut_f), 2))))Degraded Coating 10% Breakdown Sensitivity Check (Set Coating Breakdown = 0.10)
Double Mean_Coating_Breakdown B4 from 0.05 to 0.10 (harsher splash/riser coating wear): Mean Current E2 doubles to 40.00 A, Net Anode Mass E3 rises to 5,152.94 kg, and Minimum 150 kg Anode Count E4 jumps to ROUNDUP(34.35, 0) = 35 anodes!
=IF(ISTEXT(E3), E3, ROUNDUP(((B2 * B3 * B4 * B5 * 8760) / (B6 * B7)) / 150, 0))Try the example
Use the original values or change them to test your understanding.