External corrosion direct assessment (ECDA) has also been found to be a valuable supplement to other assessment methodologies since it also assesses the performance of the corrosion control systems that are being utilised to protect the pipelines and can identify areas in that system that are not completely functional.
Mears Integrity Solutions has specialised in the external corrosion direct assessment ECDA methodology since it was developed by NACE International in 2004 in response to the American natural gas industry’s need to assess pipelines that were difficult to assess with in-line-inspection or pressure testing. NACE Standard Practice 0502 specifies the requirements of conducting ECDA project and more recently, the revised Australian Standard AS 2885.3-2012 adopted the ECDA process in Section 9 – Anomaly Assessment and Defect Repair.
ECDA is a structured process consisting of four steps in the assessment of a pipeline segment. These steps are:
Step 1: Pre-assessment
This step gathers existing data regarding the pipeline. The data is from five categories of information: pipe, construction, soil and environment, corrosion and control, and operational. This data is integrated and analysed to determine if ECDA is feasible, which indirect inspection tools are appropriate for the assessment and identification of regions along the pipeline segment that have similar corrosion history and corrosion control characteristics. These are referred to as ECDA regions.
Step 2: Indirect inspection
The objective of this step is to identify locations on the pipeline that are most likely to have external corrosion. This is accomplished by performing different electrical measurements above the pipeline to identify areas of coating failure and levels of pipe polarisation (pipe-to-soil voltages).
Typical tools that are used in this step are interrupted close interval potential surveys, direct current voltage gradients (DCVG) and pipeline current mapping. This data is integrated and analysed to identify areas that may have external corrosion.
Step 3: Direct examination
Direct examination (DE) refers to exposing approximately 5-10 m of pipe to examine it for external corrosion. The objective of this step is to calibrate and validate the indications of corrosion identified in Step 2. During the DE, numerous tests are performed to determine coating condition, the level of cathodic protection and environmental factors that may influence the corrosion rate of the pipeline.
Step 4: Post-assessment
In Step 4, all of the data that is collected in the previous three steps is integrated and evaluated to determine: the root cause of corrosion that was found, estimate the remaining life of the area with corrosion, determine the reassessment interval for the pipe, and the overall effectiveness of the ECDA project.
It is expected that the ECDA process will continue to be a valuable assessment tool for pipeline operators around the world.
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