Galvanic Anodes for an Offshore Jacket in Warm Seawater
A four-leg steel jacket in warm seawater (about 24 C, resistivity 20 ohm-cm) supports a platform. The submerged steel area is 3,200 m2, of which 2,900 m2 is bare and 300 m2 is coated. Design current density for bare steel in warm seawater is 1.3 mA/ft2; coated area is credited at 10% of bare. The owner proposes aluminum-indium galvanic anodes. A previous jacket in similar water used 250 kg anodes and reached criterion, but that jacket had 20% less submerged area. Target is -0.90 V Ag/AgCl.
1. Estimate the anode mass required and explain why the previous jacket's anode count cannot simply be scaled by area.
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Effective bare-equivalent area = 2,900 m2 + 0.10 * 300 m2 = 2,930 m2 = 31,540 ft2. Current demand = 31,540 * 1.3 mA/ft2 = about 41 A.
Anode mass follows from current demand, design life, and anode consumption rate. Aluminum-indium in seawater has a consumption rate of roughly 3.5-4 kg/A-yr (verify against the anode supplier's certified data). For a 25-year design life: 41 A * 3.7 kg/A-yr * 25 yr = about 3,800 kg of anode mass, before a utilization factor (typically 0.8-0.9 for standoff anodes). That is the mass basis.
Why not scale the old jacket? The old jacket's anode count reflected its own area, its own design life, and its own coating condition. Area scaling ignores design life, current density assumptions, anode utilization, and the number of anodes needed to distribute current over the jacket geometry. Anode count is set by both total mass and current distribution (each anode has a maximum practical output in 20 ohm-cm water), so the two jackets are not proportional.
2. How does the 20 ohm-cm warm seawater affect anode output and anode placement?
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Low resistivity means each anode can deliver high current, so the current capacity is not the constraint; the constraint is distributing current evenly over the jacket. In 20 ohm-cm water, a single anode's output can be several amps, so a small number of large anodes may overprotect nearby steel while leaving remote members under-protected.
Placement should follow the geometry: anodes near the mudline, at mid-depth, and near the surface, distributed around all four legs and the bracing, so each member is within a reasonable current-throw distance. Check the potential on remote members (e.g., inner faces of legs, deep bracing) with a temporary reference electrode survey after installation. Also confirm the anode alloy is suitable for warm seawater; some aluminum alloys lose performance or passivate at elevated temperature, so verify the supplier's temperature rating.
3. What monitoring would you specify to confirm the design?
Read the worked answer
Install permanent reference electrodes on representative members (near anodes and at remote points), and plan a diver or ROV survey at commissioning and periodically. Measure potentials at multiple depths and around all four legs, not just one. Confirm criterion with a 100 mV decay test on a representative member. Track anode consumption over time by visual inspection of a sample anode to validate the design life assumption.