ECDA (External Corrosion Direct Assessment)

The ECDA Methodology was developed as an alternative tool to be used by pipeline operators who were not able to use ILI (intelligent pigging) as an integrity tool on their pipelines. It is therefore a very useful tool to be used on water pipelines, which traditionally are not evaluated using ILI.

The ECDA Methodology is intended to assess and evaluate the threat from external corrosion on a pipeline.

The ECDA Methodology is an iterative process and should be conducted at various time intervals (which are determined in the Post Assessment Phase of a project). It should not be viewed as a once-off process that is not repeated again.

The ECDA process follows four steps as follows:

• Pre-Assessment
• Indirect Inspection
• Direct Examination
• Post Assessment

1. Pre-Assessment

During this stage, the feasibility of whether ECDA is suitable for the intended pipeline is evaluated. This is achieved by reviewing all the historical information that is available and also current information. Should the analysis reveal that ECDA is feasible, then the ECDA regions would be defined and the combination of survey techniques that would be used for each ECDA region will be chosen. A minimum of two indirect inspection tools are required for each ECDA region.

2. Indirect Inspection

During this phase, above-ground inspections are undertaken using a variety of overline techniques, such as DCVG, ACVG, CIS, Depth of cover, soil resistivity etc. The locations of defects and all pipeline components are captured using differential GPS, as well as chainage using a surveyors wheel.

The main purpose of this activity is to accurately align all the field data, identify coating faults and size them accurately. All the defects are then re-sorted using the defect sizing and severity values from each individual overline technique, but in combination, to arrive at an overall ranking for all defects.

3. Direct Examination

During this phase, the overline data is analysed and a selection of defects of varying sizes are selected for excavation and detailed examination. At each excavation site, the soil, coating and any metal loss are measured and analysed further.

Measurements such as pH and SRB tests can be taken in situ. Soil samples are taken at pipeline depth for further analysis in the laboratory. The pipe to soil potential is also taken at the excavation site.

Thereafter, the coating is examined to determine what caused the coating defect that was detected with the overline survey. Photographic evidence is taken at each stage of the examination. The pH of any solutions present between the coating and metal substrate, is taken.

The coating is carefully removed and the thickness measured. Thereafter, the metal surface is examined. Any metal loss is measured accurately using appropriate NDT methods.

All the information gathered above needs to be carefully recorded and made available for later analysis. The information gathered in this phase is combined with other historical data to assess the threat from external corrosion on the pipeline.

4. Post Assessment

During this phase, the data collected in steps 1-3 is evaluated. The main purpose is to assess whether the ECDA process was effective and whether sites of external corrosion were identified and located using this method.

The re-assessment intervals are then calculated using the metal loss that was discovered and measured as a guideline.

It is important to note that ECDA is an on-going process and needs to be undertaken at intervals on a pipeline. During the first, or baseline survey, it is often not possible to calculate accurate corrosion rates as there is only one set of data. Subsequent ECDA evaluations can help to determine corrosion rates as metal loss information from two time intervals should be available.