The reliable operation of a pipeline is dependent on maintaining the structural integrity of the system. Defects, which could compromise structural integrity, can be introduced into the pipeline at any point throughout its life cycle, that is, during pipe manufacture, construction, and operation. It is vital therefore that the significance of these defects is assessed, and this process forms part of the regulatory framework for conducting engineering assessments in many jurisdictions. The word defect is used in a general sense here; however it has a specific meaning to those involved in pipeline integrity, so before progressing further, it will be worthwhile examining the definitions provided in API 1163 (2005).

Indication: A signal from an ILI system. An indication may be further classified, or characterized, as an anomaly or imperfection

Anomaly: An unexamined deviation from the norm in pipe material, coatings, or welds

Feature: Any physical object detected by an ILI system (i.e., anomalies, components, nearly metallic objects, etc.)

Imperfection: An anomaly with dimensions and characteristics that do not exceed acceptable limits

Defect: A physically examined anomaly with dimensions, or characteristics, that exceed acceptable limits

This chapter describes a number of quantitative methodologies that are available to the pipeline integrity specialist to evaluate the severity of a defect or damage to the pipeline. These methodologies can be broadly categorized as “fitness for service,” or “fitness for purpose” assessments and their output is information, which is then used to decide whether to continue to operate the system, as is, in an altered state, or to repair, or replace the damaged pipe.