Project Detail

Threats to the integrity of underground oil and gas pipelines are broadly grouped into those that are time-related and those that are not. Threats that are not time-related include third party damage, original manufacturing or construction threats, and other singular events over which the pipeline operator has limited control through preventative measures. Time-related threats can be evaluated and mitigated with considerable success. These include the series of corrosion threats: external corrosion, internal corrosion, stress corrosion cracking, and AC corrosion: a threat to pipeline integrity that crosses over both time-dependent and singular event categories. AC Corrosion is related to influences from high voltage AC power lines that often parallel pipeline rights-of-way and will be discussed separately.

Several of these time-dependent corrosion-related threats to pipeline integrity can be evaluated using a relatively new combination of technologies known as Direct Assessment. Standard Practices developed by NACE International began with RP0502 (now SP0502), “Pipeline External Corrosion Direct Assessment Methodology”. This ECDA standard served as the framework for the subsequent development of the following Standard Practices:

  •  SP0204 – Stress Corrosion Cracking Direct Assessment Methodology
  •  SP0206 – Internal Corrosion Direct Assessment for Pipelines Carrying Normally Dry Gas
  •  SP0208 – Internal Corrosion Direct Assessment for Liquid Petroleum Pipelines

Each of these Direct Assessment methodologies is patterned around a structured four-step process, utilizing multiple inspection tools, and is intended for use by pipeline operators in establishing integrity and managing deterioration of pipelines resulting from corrosion. The process is designed to reduce existing and future risks by locating and assessing areas that may contain corrosion or be likely sites for future corrosion. These techniques are especially important as alternatives for assessing the integrity of gas pipelines because in-line inspection is often not an available option due to changes in pipe diameters and sharp-angle bends. In contrast, liquids pipelines are typically designed for use of scouring pigs that are similar in configuration to in-line inspection tools and use similar launching and retrieval facilities.

The Four Step Process

Each of these Direct Assessment methodologies is patterned around a structured four-step process, utilizing multiple inspection tools, and is intended for use by pipeline operators in establishing integrity and managing deterioration of pipelines resulting from corrosion.



Pre-Assessment utilizes operational and historical records to determine corrosion-based threats to pipeline integrity. The primary objectives of Pre-Assessment are:

1. Evaluate the available information to determine whether it is sufficient and feasible to proceed with the assessment process or can be adequately supplemented by field investigations.

2. Identify piping with similar characteristics and threats for grouping into “Regions” that can be reasonably evaluated by application of the same Indirect Inspection Tools (IITs).

3. Select complimentary IITs that are suitable for the assessment of each Region. In general, IITs are well-established aboveground survey techniques that have been shown to provide reliable information about the condition of buried piping. Complimentary IITs provide different types of information or may provide similar information based on different principles of tool operation. For example, in an External Corrosion Direct Assessment, IITs might be selected from a list that includes:

  • Interrupted Close Interval Potential Survey (CIS) to provide information about the effectiveness of cathodic protection (CP) and general trends in coating condition.
  • Direct Current Voltage Gradient (DCVG to provide information about specific locations of exposed pipe metal.
  • AC Current Attenuation (PCM) to provide information regarding the general pattern of protective current distribution and identify contacts to foreign structures.
  • Soil Resistivity Surveys to provide general information regarding the aggressiveness of the soil environment.
  • Depth Surveys to provide information regarding vulnerability to third party damage.
  • Other Surveys to provide specific information regarding additional threats that are suggested by the Pre-Assessment data. The combination of IITs selected for a particular Region would depend upon the threats and operating history that are identified during the collection of Pre-Assessment information.


Indirect Inspection consists of conducting the IIT surveys, using the combination of tools selected during Pre-Assessment, to characterize the severity of threats to pipeline integrity at specific locations. Analysis of this data considers the combination of indications from all the tools that are used, but the results from a single tool may dominate particular sites. The analysis may be influenced by information collected during the Pre-Assessment, such as historical failures or operational irregularities such as known sources of oil or gas that include corrosive components that could accelerate internal corrosion or interruptions in cathodic protection operation that could affect external corrosion.

The ultimate objective of Indirect Inspection is to select representative sites for Direct Examination in order to validate the field data that has been collected. Validation provides hands-on information about the pipe and coating condition to see whether it matches the predictions of the Indirect Inspections. Pipeline sites are typically prioritized from greatest anticipated risk to least anticipated risk as Immediate, Scheduled, or Monitor. In general, all high risk (Immediate) locations are excavated along with a sampling of lesser-risk sites to validate the range of predicted conditions.


Direct Examination of the selected sites is intended to examine the actual pipe surface, internal or external, for the existence, extent, and severity of corrosion. This information is compared with the conclusions drawn from analysis of the Indirect Inspection data. If this correlation is good, the original Direct Examination plan will suffice; however, if the results of Direct Examination do not agree with the Indirect Inspection predictions, additional testing or excavations may be justified.

Data collected during Direct Examination includes physical measurements of metal loss or evidence of cracking in addition to specific information about the corrosivity of the environment and other factors that may have contributed to the pipe condition.


Post-Assessment involves the analysis of all project data to evaluate the effectiveness of the overall process. This includes consideration of the completeness of Pre-Assessment information, the suitability and accuracy of the selected Indirect Inspection Tools, the categorization of IIT indication severity, prioritization of sites for Direct Examination, and the correlation between IIT predictions and the actual condition of the excavated piping. Post-Assessment also includes Root Cause Analysis of any metal losses that were examined, reviews the actions that were taken to minimize these corrosion mechanisms in the future, and determines whether the original grouping of pipeline sections into Regions accurately reflects the common corrosion influences throughout that piping.

Finally, evaluation of the overall threat to pipeline integrity is used to establish the schedule for future re-assessments, and lessons learned may be applied to the planned Direct Assessments for other similar piping.