What Is an Underwater Cathodic Protection Survey?
An underwater cathodic protection (CP) survey measures the electrochemical potential of submerged steel to confirm that its corrosion-protection system is actually working. Every steel structure sitting in seawater — a ship or FPSO hull, an offshore jacket, jetty and berth piles, a sheet-pile quay wall, or a subsea pipeline — is fitted with a CP system precisely because bare steel corrodes in the marine environment. The survey is the direct, in-water evidence that the system is holding the steel at a protective potential and that the anodes are doing their job.
Cathodic protection works by shifting the steel to a more negative (cathodic) potential, at which the electrochemical reactions that drive corrosion are suppressed. That shift is produced either by sacrificial anodes — blocks of a less-noble alloy such as aluminium or zinc that corrode preferentially and feed protective current to the steel — or by an impressed-current cathodic protection (ICCP) system that drives the same current from a power supply through inert anodes. Either way, protection is invisible from the surface. The only way to know it is adequate is to go underwater and measure it.
That is what a CP survey delivers. Using a reference electrode read across the steel-seawater interface, we quantify the actual potential at points along the structure and compare it against the protection criteria set out in the governing standards. Where the potentials fall short, the survey pinpoints the under-protected zones, the depleted anodes and the coating breakdown responsible — turning an invisible electrochemical condition into a mapped, actionable dataset for the asset's integrity programme.
Why Cathodic Protection Surveys Matter
Submerged steel does not stop corroding; cathodic protection simply outcompetes the corrosion reaction by keeping the metal below a threshold potential. The moment protection falls away, corrosion resumes — and because it happens underwater, it does so entirely unseen. A CP survey exists to catch that drift before it turns into thickness loss, deep pitting, or a breach.
Anodes are consumable by design. A sacrificial anode is meant to waste away over the asset's service life, and an ICCP system depends on power, cabling and anodes that all degrade. If anodes deplete faster than expected, if an ICCP rectifier faults, or if coating breakdown increases the bare-steel area demanding current, the system can quietly slip below its protection criterion. Nothing is visible topside until damage is already advanced. Regular potential measurement is the early-warning signal that lets operators intervene — replacing anodes or adjusting ICCP output — while it is still routine maintenance rather than emergency repair.
The stakes are highest on containment and load-bearing steel. Under-protection on a subsea pipeline can progress to a wall-thickness loss and, ultimately, a leak; on a jetty pile or offshore jacket it erodes the structural margin the asset was designed to carry. A documented CP survey is also part of the evidence base that classification societies and asset owners expect for continued in-service certification, which is why it sits within UWILD / in-water survey scopes and formal integrity-management programmes.
The Two Types of CP Survey
There are two established ways to measure cathodic protection underwater. The right choice depends on the structure, the level of detail required, and whether the priority is spot verification or continuous coverage.
Contact (stab) potential measurement. A reference electrode — most commonly silver/silver-chloride (Ag/AgCl) in seawater — is placed in direct contact with the steel at selected points, and the structure-to-electrolyte potential is read in millivolts. Each reading is a discrete, high-confidence measurement of the potential at that location. Stab readings are ideal for verifying representative points on a hull, at anode positions, at node connections on a jacket, and wherever a defensible single-point value is needed against the protection criterion. On a hull inspection, a grid of stab readings characterises protection across the whole wetted surface.
Proximity (field-gradient) survey. Here the reference electrode is moved continuously through the water a fixed distance off the structure, logging potential and the electric-field gradient as it travels. Because the current flowing to and from anodes and coating defects sets up measurable gradients in the surrounding seawater, a proximity survey does more than confirm potential — it locates individual anode outputs, finds coating holidays and defects, and reveals under-protected stretches without touching every point. It is typically ROV-mounted for efficient continuous coverage of pipelines and large structures, making it the standard method for long subsea runs where a stab-only approach would be impractical.
In practice the two are complementary: a proximity survey maps the whole structure and flags anomalies, and targeted stab readings confirm the absolute potential at the points that matter most.
What the Survey Reveals
A CP survey is not simply a pass/fail reading. Properly interpreted, it produces a corrosion-protection picture of the whole structure, each finding located and quantified for the integrity engineer.
| What we assess | How it is measured | Why it matters |
|---|---|---|
| Protection potential | Reference-electrode reading (mV vs Ag/AgCl) at stab points and along proximity runs | Confirms whether the steel meets the protection criterion or is under-protected. |
| Anode consumption & remaining life | Field-gradient current output plus visual/dimensional wastage checks | Shows which anodes are depleting and supports anode-replacement planning. |
| Coating breakdown / holidays | Field-gradient anomalies and close visual survey | Increased bare steel raises current demand and accelerates local corrosion. |
| Under-protected zones | Mapped potentials more positive than the criterion | Identifies areas needing anode addition or ICCP adjustment before damage occurs. |
| ICCP system performance | Potential distribution around impressed-current anodes and reference cells | Verifies the rectifier and anodes are delivering an even, adequate protective field. |
| Over-protection | Excessively negative potentials, especially near ICCP anodes | Can drive coating disbondment and hydrogen effects on high-strength steel. |
How We Carry Out the Survey
Every CP survey is planned around the structure's own protection design so the measurements can be judged against the right criteria. We work from the anode layout, coating breakdown assumptions and design potentials, then set out the stab points and proximity run lines that will characterise the system.
Measurement is delivered by ROV-mounted or diver-held reference electrodes, chosen to suit the asset and the water. An ROV with a calibrated Ag/AgCl electrode and a contact (stab) probe efficiently covers hulls, pipelines and large structures, logging position with every reading; where close manipulation is needed, a diver-held probe places the electrode precisely at anodes, connections and defect sites. Readings are geo- or event-referenced so that each potential can be located back to the structure. The results are then mapped along the hull, pile line or pipeline route, so the potential profile is read as a continuous picture rather than a scatter of isolated numbers.
The deliverable is a CP dataset that feeds directly into integrity management: potential plots against the protection criterion, anode-condition findings, coating-defect locations and clear identification of any under-protected zones. That is exactly the evidence an operator needs to schedule anode replacement, tune an ICCP system, or confirm that no action is required this cycle.
Where a CP Survey Fits
Cathodic protection surveying is rarely a standalone job — it is one measurement within a broader in-water inspection or integrity programme, and it dovetails with the visual, thickness and defect work carried out on the same dive or ROV mobilisation.
Ships & UWILD/IWS
On vessels, CP readings are a standard element of the in-water survey (UWILD) alongside hull, propeller and rudder inspection, evidencing anode and hull-protection condition for class.
Jetty & Port Piles
Berth and jetty piles rely on anodes or ICCP to survive the tidal and immersion zones. CP surveying is central to port infrastructure integrity programmes.
Offshore Structures
Jackets, risers and mooring systems carry designed anode arrays. Periodic potential and anode-wastage surveys confirm the protection margin is intact across the structure's life.
Subsea Pipelines
Continuous field-gradient surveys along subsea pipelines confirm protection, locate coating defects and detect anode depletion over long routes.
Standards We Work To
CP survey results only mean something when they are judged against recognised criteria. We plan and interpret surveys against the international standards that govern marine cathodic protection, so the data is defensible for class, operators and their integrity engineers.
- ISO 13174 — cathodic protection of harbour and port installations, the reference for jetty, berth and quay-wall CP.
- ISO 15589 — cathodic protection of pipeline systems, covering subsea pipeline CP criteria and survey practice.
- DNV-RP-B401 — the recommended practice for sacrificial anode design, against which anode output and consumption are assessed.
- NACE/AMPP SP0176 & SP0169 — corrosion control of offshore steel structures and of buried/submerged pipelines, including protection potential criteria.
- IMCA guidance — for the safe and consistent conduct of diver and ROV survey operations underpinning the measurements.
We cite these standards as the yardstick for interpretation; the formal assessment and any continued-fitness determination rest with the operator's competent integrity engineer or classification surveyor, to whom we deliver complete, criterion-referenced CP data.
What a CP Survey Covers
A complete cathodic protection survey brings four distinct measurements together into one corrosion-protection assessment of the structure.
Potential Measurement
Structure-to-electrolyte potential read in millivolts against a calibrated Ag/AgCl reference electrode, at stab points and along proximity runs, checked against the protection criterion.
Anode Condition & Wastage
Assessment of sacrificial anode consumption, output and remaining life — the data that drives anode-replacement scheduling before protection lapses.
Coating-Defect Detection
Field-gradient anomalies and close visual survey locate coating holidays and breakdown, where bare steel raises current demand and localises corrosion.
ICCP Verification
Mapping of the potential field around impressed-current anodes to confirm even, adequate protection and to flag both under- and over-protection.
Why Choose SG Drone Inspections
We are a specialist marine and underwater inspection provider in Singapore, delivering ROV and diver survey capability across the full range of submerged-steel assets.
Full ROV & Diver Capability
We deploy ROV-mounted electrodes for efficient continuous coverage and diver-held probes for precise stab readings at anodes, connections and defects — matched to the asset and the water.
Standards-Referenced Interpretation
Surveys are planned and read against ISO 13174, ISO 15589, DNV-RP-B401 and AMPP SP0176/SP0169, so the results are meaningful to class and to your integrity engineer.
Mapped, Integrity-Ready Data
Potentials plotted along the structure, anode findings and coating-defect locations — delivered as a dataset that feeds straight into anode-replacement and integrity planning.
One Mobilisation, Full Scope
CP surveying combines cleanly with visual, thickness and defect inspection on the same dive or ROV spread, from UWILD to pipeline and port-infrastructure integrity work.
Frequently Asked Questions
Get a Cathodic Protection Survey Quote
Send us your structure type, location and CP system details — sacrificial anode or ICCP — and we will reply with a survey scope and quote. ROV or diver, hull, piles or pipeline.