🎓 Lesson 9
D5
Cathodic Protection Design Using Ohm’s Law for Marine Structures
Cathodic protection is like giving a metal structure a 'sacrificial shield'—a more reactive metal that corrodes instead of the structure, keeping it safe underwater.
🎯 Learning Objectives
- ✓ Calculate required anode mass and circuit resistance using Ohm’s Law and Faraday’s law for submerged steel structures
- ✓ Design a sacrificial anode CP system for an offshore wind substation jacket foundation, selecting appropriate alloy type, number, and placement based on design life and environmental resistivity
- ✓ Analyze polarization potential measurements to verify CP adequacy against NACE SP0169 criteria
- ✓ Explain the relationship between anode driving voltage, circuit resistance, and protective current density in seawater environments
- ✓ Apply marine resistivity data and coating breakdown factors to adjust current demand estimates
📖 Why This Matters
Offshore wind substations operate in aggressive marine environments for 25–30 years—yet unplanned corrosion failures can cause catastrophic structural degradation, costly unplanned maintenance, and even grid instability. In 2022, a North Sea substation experienced premature pitting corrosion on its jacket leg welds due to undersized anode arrays and unverified current distribution—resulting in €4.2M in remediation. Cathodic protection isn’t optional; it’s the primary corrosion defense for submerged carbon steel components—and Ohm’s Law is the indispensable tool that bridges theory to reliable, code-compliant design.
📘 Core Principles
Corrosion in seawater occurs via electrochemical cells where anodic sites (e.g., scratches in coating) dissolve metal while cathodic sites consume oxygen. CP forces the entire protected surface to become cathodic by supplying electrons from a sacrificial anode—whose higher electrochemical activity (more negative corrosion potential) drives protective current flow. Ohm’s Law (V = I × R) governs the current delivery: the anode’s driving voltage (ΔE) must overcome the total circuit resistance (R_total = R_anode + R_electrolyte + R_cable + R_structure) to deliver the minimum current density (typically 0.01–0.03 A/m² for coated steel in seawater) required for polarization. Environmental variables—seawater resistivity (15–30 Ω·cm), temperature, salinity, and coating quality (holiday rate)—directly impact R_total and thus anode sizing and spacing.
📐 Ohm’s Law-Based Current Demand & Anode Sizing
The fundamental CP design equation combines Ohm’s Law with Faraday’s law to determine anode mass and quantity. First, calculate required protective current (I_req); then use Ohm’s Law to verify if the anode’s driving voltage can drive that current through the circuit resistance; finally, size the anode mass using capacity and consumption rate.
💡 Worked Example
Problem: Design CP for a 3-leg jacket substation foundation (submerged surface area = 850 m²). Steel is coated (99% intact, holiday rate = 1%). Seawater resistivity = 22 Ω·cm. Use Al-Zn-In anodes (driving voltage vs. Ag/AgCl = 0.25 V; capacity = 2,700 Ah/kg; consumption rate = 3.8 kg/A·yr). Design life = 25 years. Target current density on bare areas = 0.15 A/m².
1.
Step 1: Calculate bare surface area: 1% of 850 m² = 8.5 m².
2.
Step 2: Compute required current: I_req = 0.15 A/m² × 8.5 m² = 1.275 A.
3.
Step 3: Estimate circuit resistance: R_total ≈ R_electrolyte = ρ / (2πd) where d = effective anode-to-structure distance (~2 m assumed) → R ≈ 22 / (2π×200) ≈ 0.0175 Ω (using cm units consistently).
4.
Step 4: Verify driving voltage sufficiency: V_drive = I_req × R_total = 1.275 A × 0.0175 Ω ≈ 0.022 V — well below 0.25 V → feasible.
5.
Step 5: Size anode mass: Total charge needed = I_req × t × 3600 s/hr = 1.275 A × 25 yr × 8760 h/yr = 279,000 Ah. Mass = 279,000 Ah / 2,700 Ah/kg = 103.3 kg. With 14 kg anodes (typical), require ⌈103.3 / 14⌉ = 8 anodes.
Answer:
The result is 8 Al-Zn-In anodes (14 kg each), which delivers sufficient current with margin. The calculated circuit resistance (0.0175 Ω) confirms the 0.25 V driving voltage is more than adequate—validating passive CP feasibility without impressed current.
🏗️ Real-World Application
The Hornsea Project Three Offshore Wind Substation (UK, commissioned 2024) uses a 3-leg jacket foundation with 12 x 14 kg aluminum-zinc-indium sacrificial anodes welded to legs and braces. Design was validated using DNV-RP-B401 guidelines and verified via potential mapping: -1.05 V vs. Ag/AgCl (seawater) at all critical weld zones after 6 months immersion—exceeding NACE SP0169’s -1.00 V criterion. Post-installation IR-drop corrected measurements confirmed uniform current distribution, avoiding localized under-protection near mudline where resistivity increases.
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📋 Borssele III & IV Substation Jacket Corrosion Remediation
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