7.4 Coating Repair, Holiday Inspection & Testing

Coating Repair, Holiday Inspection & Testing

Quick Answer: Holiday detection (jeeping) runs a high-voltage detector over the coated pipe; the test voltage is set from the coating thickness using a standard table such as NACE SP0188. Every pinhole or holiday is marked, repaired per the coating procedure (clean, prime, patch, re-jeep), and the pipe is not lowered into the trench until the inspector confirms all holidays are cleared and documented.

A holiday is any discontinuity in the coating — a pinhole, a thin spot, a mechanical damage point, or a disbonded area — that exposes or nearly exposes bare steel. Because the CP system is sized to protect a small number of holidays, not a bare pipe, the coating is jeeped before lowering-in so that defects are found and repaired while the pipe is still accessible on the surface.

High-Voltage Holiday Detection (Jeeping)

High-voltage holiday detection (often called jeeping from the trade name of the detector) works by applying a voltage between a sweeping electrode (a ring or spring that slides over the coated pipe) and the pipe steel. When the electrode crosses a pinhole or thin spot, the voltage arcs through the coating to the steel and the detector signals a holiday.

Why Voltage Depends on Coating Thickness

The test voltage must be high enough to break down the coating film at a defect but low enough not to puncture sound coating. The breakdown voltage of a dielectric film scales roughly with thickness, so thicker coatings require higher test voltages. The industry reference is NACE SP0188 (or the equivalent in the project specification), which tabulates test voltage against dry-film thickness.

The voltages above are representative of the NACE SP0188 / SP0490 approach. The inspector uses the voltage from the project specification or the coating manufacturer's data sheet, not a memorized number — the table concept is what the exam tests.

Wrong-Voltage Consequences

• Voltage too low — real pinholes are missed; the coating is falsely accepted and corrosion starts in service.
• Voltage too high — sound coating is punctured; good coating is turned into a holiday that then has to be repaired, wasting time and creating a repair that may be weaker than the original.

Marking & Repairing Holidays

When the detector signals a holiday, the location is marked with a weather-resistant marker or piece of tape. The inspector (or the coating contractor's QC) walks the pipe after jeeping and records the number, location, and size of holidays per joint.

The repair sequence is specified by the coating procedure and is typically:

1. Clean the holiday area — remove dirt, moisture, and loose coating
2. Prime — apply the specified primer if required by the repair procedure
3. Patch — apply the repair material (liquid epoxy, hot-melt stick, shrink patch, tape) per the manufacturer's instructions and the qualified repair procedure
4. Cure — allow the patch to cure per the procedure before re-testing
5. Re-jeep — re-run the holiday detector over the repaired area to confirm the holiday is closed

Repairs follow the same qualification rules as new coating: a qualified repair procedure, qualified personnel, surface preparation of the repair area (often a solvent wipe and light abrade, sometimes a spot blast), and cure verification. A repair made with an off-specification material or an unqualified applicator is a nonconformance.

Coating Damage Repair

Beyond pinholes, the coating can be mechanically damaged during handling, lowering-in, or backfill. Damage types and their repair path include:

• Bearing marks from belts or chains — clean, abrade, patch, re-jeep
• Impact damage (rock strike) — remove loose coating, check for steel indentation, patch or recoat the area
• Disbonded coating — cut back to sound coating, blast the exposed steel, recoat per procedure
• Tape wrinkles or lifting — remove the affected tape section, re-wrap with overlap per the tape procedure

A damage area large enough to require recoating of more than a small patch may require the joint to be re-coated over its full length, or, on plant-applied coatings, the pipe may be rejected and replaced.

Before Lowering-In

Holiday testing and repairs are completed before the pipe is lowered into the trench. Once the pipe is in the ditch and backfilled, the coating cannot be jeeped (a buried pipe cannot be reached by an electrode) and any remaining holiday becomes a long-term CP demand and a corrosion risk. The API 1169 inspector verifies:

• The holiday detector is calibrated and the test voltage matches the coating thickness and specification
• Every joint has been jeepeed over its full coated surface (including the field joint)
• All holidays are marked, repaired per procedure, and re-jeeped
• A holiday-test report is on file for each joint, signed by the testing technician
• The pipe is released for lowering-in only after the report is accepted

Documentation

The coating inspection record for each joint typically contains:

• Joint number and heat number
• Coating system and thickness (measured dry-film thickness)
• Jeep test voltage used
• Number and location of holidays found
• Repair material and procedure reference for each repair
• Re-jeep result
• Final acceptance signature

This record is part of the construction package that supports the CP survey and the as-built documentation. A missing or incomplete holiday-test record is a hold point — lowering-in does not proceed until the record is complete.