Anode Depletion on a Jacket Leg Below the Mudline
A four-leg jacket installed in 1987 in 110 m of water uses aluminum-indium sacrificial anodes. A 2023 CP survey reports structure-to-electrolyte potentials of -0.78 V versus Ag/AgCl/seawater at the -20 m elevation and -0.82 V at -60 m, both more positive than the -0.90 V criterion. The survey also notes that several anodes on the north face show visible consumption approaching the core, and that the mudline-to-buried-depth potentials are -0.72 V. The operator has budget for one intervention this year and is debating between (a) retrofitting a remote impressed-current system with a surface sled, (b) clamping new sacrificial anodes by diver, or (c) doing nothing because the readings are only slightly off criterion and the structure has 36 years of service with no reported wall loss.
1. Assess whether the potential readings justify intervention, and explain the significance of the mudline readings.
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Yes, intervention is justified. The -0.90 V Ag/AgCl/seawater criterion is the standard protective threshold for steel in seawater; readings of -0.78 V and -0.82 V are 80–120 mV positive of criterion, which is a meaningful underprotection, not measurement noise. The trend matters more than a single survey: with anodes visibly consumed to the core on the north face, the system is at end of life and potentials will continue to drift positive.
The mudline readings of -0.72 V are the most concerning result. Buried and driven steel in mud is typically the hardest zone to protect because of the higher resistivity and the geometry; a potential that positive suggests the buried steel is effectively unprotected. That zone is also where external corrosion is hardest to inspect, so the absence of reported wall loss is weak evidence - it likely reflects limited inspection coverage rather than absence of corrosion.
2. Compare the two intervention options (a) and (b) for this structure, including practical constraints.
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Option (b), diver-clamped sacrificial anodes, is the conventional and lower-risk choice for a jacket at this water depth. It directly restores current to the legs and braces, is well understood by offshore crews, and does not require a power feed or control system. Its weakness is that it does not reach the buried steel below the mudline, and it requires diver time in 110 m water, which is at the edge of - and generally beyond - routine air-diving limits, so saturation or ROV-assisted work would be needed. That raises cost and schedule risk.
Option (a), a remote impressed-current system with a surface sled, can deliver current to the whole structure including the lower and buried zones, and avoids saturation diving. But it introduces a powered system with rectifiers, cables, and control electronics that must survive the marine environment, and the current distribution to a complex jacket is harder to verify. It also has a longer design and commissioning lead time, which conflicts with a single-year budget window.
Given the mudline deficiency, a defensible plan is a hybrid: clamp sacrificial anodes to the accessible upper structure now, and design the ICCP retrofit for the buried and deep zones as a follow-on. If only one action is possible this year, the ICCP retrofit better addresses the worst deficiency but carries more execution risk.
3. The operator argues for option (c), doing nothing. What is the strongest counterargument, and what monitoring would make a deferral defensible?
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The strongest counterargument is that 'no reported wall loss' is not the same as 'no wall loss.' The structure is 36 years old with anodes at end of life, and the zones most at risk - the mudline and buried steel - are precisely the zones with the least inspection coverage. Underprotection there can proceed for years before it is detected, and by the time wall loss is found the repair is a structural intervention rather than a CP retrofit.
A deferral could be defensible only with: (1) a close-interval potential survey confirming the readings are stable rather than drifting, (2) targeted wall-thickness measurements at the mudline and at the most anodic locations, (3) an anode consumption assessment confirming remaining life, and (4) a defined re-survey interval of no more than 12 months. If any of those cannot be met, deferral is not defensible.