Field Operations

External Corrosion Direct Assessment: When the Pipe Is Buried and the Data Is Indirect

Direct assessment is a method for buried pipelines when you cannot inspect the steel directly. It relies on voltage readings, coating condition and excavation data to estimate whether corrosion is happening. It is widely used on older lines where coating records are incomplete and in-line inspection tools cannot be run.

The four-step process

External corrosion direct assessment starts with pre-assessment. The operator collects coating, soil and history data for the segment. The second step is indirect inspection with close interval surveys, direct voltage gradient and alternating current voltage gradient. These surveys map coating holidays and current flow. The third step is excavation and measurement. The operator digs at selected sites, measures wall thickness and records pitting patterns. The fourth step is post-assessment, where the operator decides whether the segment needs another assessment interval or additional mitigation.

When direct assessment is required

Integrity management plans often include direct assessment for segments where in-line inspection cannot be run. A small diameter or a sharp bend can block the tool. When a pipeline was built before modern coating standards, direct assessment becomes the practical choice. Operators have to justify why the method is appropriate for the threat, show that the surveys were placed correctly and prove the anomalies are addressed. The justification is written, reviewed and kept in the integrity file. If a regulator asks why direct assessment was chosen instead of smart pigging, the answer is already documented.

Documentation and next steps

Direct assessment reports include survey maps, excavation photographs, metal loss measurements and root cause conclusions. The operator uses these findings to recalculate the next assessment date or add mitigation. If anomalies exceed the remaining strength threshold, the file needs a repair plan, excavation verification and a reassessment schedule. The report stays current as long as the segment is in service. Inspection logs, repair orders and follow-up tickets are exactly the kind of field data tracked in OpsFlo, ticket, dispatch, timesheet, approvals and document software built for field crews.

Sources and further reading

• eCFR 192.461: External corrosion control
• eCFR 192.631: Corrosion control
• eCFR 192.639: Integrity management
• API RP 571: Damage mechanisms