🎓 Lesson 13
D5
Cathodic Protection Design Calculations for Multi-Material Mooring Systems
Cathodic protection is a method that uses electricity to stop rust from forming on underwater metal parts like mooring chains and anchors.
🎯 Learning Objectives
- ✓ Calculate total current demand for a multi-material mooring system using surface area, current density, and material-specific polarization requirements
- ✓ Design a sacrificial anode system (mass, number, placement) that satisfies 25-year design life per ISO 15257 while accounting for galvanic coupling effects
- ✓ Analyze galvanic compatibility between dissimilar metals in seawater using the galvanic series and predict preferential corrosion pathways
- ✓ Apply correction factors for anode utilization, resistivity, and coating breakdown to refine CP system sizing
- ✓ Explain how stray current interference and sediment coverage impact CP performance in marine renewable energy (MRE) mooring environments
📖 Why This Matters
Offshore wind, wave, and tidal energy devices rely on mooring systems submerged for decades—yet corrosion failure of chains, connectors, or anchors can cause catastrophic loss of station-keeping, environmental damage, and millions in remediation costs. In multi-material systems (e.g., Grade 400 steel chain + 316 stainless shackle + aluminum fairlead), unintended galvanic couples accelerate corrosion unless cathodic protection is precisely engineered—not just installed. This lesson equips you to design CP that works *across materials*, not just for one component.
📘 Core Principles
Corrosion in seawater is an electrochemical process driven by potential differences between metals and their local environment. Cathodic protection forces the protected structure to become a cathode by supplying electrons—either from sacrificial anodes (Zn, Al, Mg alloys) or impressed current systems (ICCP). In multi-material systems, three critical phenomena dominate: (1) Galvanic coupling—when dissimilar metals contact seawater, the least noble (most anodic) corrodes preferentially; (2) Current demand heterogeneity—different materials require different current densities (e.g., coated steel: 0.01–0.03 mA/m²; bare steel: 110–150 mA/m²; aluminum: 15–25 mA/m²); and (3) Shielding and shadowing—geometry, sediment burial, and biofouling reduce effective current delivery. Design must therefore integrate electrochemical kinetics, Ohmic resistance, and long-term degradation modeling.
📐 Total Anode Mass Calculation
The minimum mass of sacrificial anode required is determined by balancing total current demand over design life, accounting for anode efficiency, utilization factor, and electrochemical capacity. This formula ensures compliance with ISO 15257:2017 for offshore CP system lifetime validation.
Anode Mass Requirement
M = (I_total × t × 8760) / (C × η × U)Determines minimum sacrificial anode mass needed to deliver required current over design life.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| M | Anode mass | kg | Total mass of sacrificial anode material required |
| I_total | Total current demand | A | From total current demand calculation |
| t | Design life | years | Service life target (typically 25 years for MRE) |
| C | Electrochemical capacity | A·h/kg | Charge delivered per unit mass (e.g., Al-Zn-In = 2700 A·h/kg) |
| η | Anode efficiency | dimensionless | Fraction of theoretical capacity realized (0.75–0.90) |
| U | Utilization factor | dimensionless | Fraction of anode consumed before failure (0.75–0.85 per ISO 15257) |
Typical Ranges:
Al-Zn-In anodes in open seawater: 2600 – 2800 A·h/kg
💡 Worked Example
Problem: A mooring system consists of 300 m of bare carbon steel chain (diameter = 80 mm), two 316 stainless steel shackles (total SA = 0.85 m²), and one aluminum fairlead (SA = 0.42 m²). Design life = 25 years. Seawater resistivity = 0.2 Ω·m. Use Al-Zn-In anodes (capacity = 2700 A·h/kg, efficiency = 0.85, utilization = 0.80). Current densities: bare steel = 120 mA/m²; stainless = 5 mA/m² (for crevice-prone zones); aluminum = 20 mA/m².
1.
Step 1: Calculate surface areas — Chain SA = π × 0.08 m × 300 m = 75.4 m²; Shackles = 0.85 m²; Fairlead = 0.42 m²
2.
Step 2: Compute current demand — Steel: 75.4 × 0.120 = 9.05 A; Stainless: 0.85 × 0.005 = 0.00425 A; Aluminum: 0.42 × 0.020 = 0.0084 A → Total I = 9.063 A
3.
Step 3: Apply design life & capacity — Required ampere-hours = 9.063 A × 25 yr × 365 d/yr × 24 h/d = 1,996,000 A·h
4.
Step 4: Solve for mass — M = (1,996,000 A·h) / (2700 A·h/kg × 0.85 × 0.80) = 1,085 kg
Answer:
The minimum anode mass required is 1,085 kg, distributed across ≥12 anodes (per DNV-RP-B-401 spacing guidelines) to ensure uniform current distribution and redundancy.
🏗️ Real-World Application
The 2021 Hywind Tampen project (Norway) deployed 11 floating wind turbines moored with 3×3-chain catenary systems. Each mooring included Grade R4 steel chain, duplex stainless shackles, and aluminum padeyes. DNV-certified CP design used segmented Al-Zn-In anodes welded directly to chain links every 25 m, with additional discrete anodes at shackle interfaces. Post-installation potential surveys confirmed −0.85 V vs. Ag/AgCl reference across all materials—validating coupled-potential control—and no galvanic attack was observed on stainless or aluminum after 3 years of monitoring.
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