The External Corrosion Direct Assessment Methodology was developed to assist pipeline operators in assessing the integrity of pipelines that could not be assessed using In Line Inspection (ILI or sometimes referred to as Intelligent Pigging).

The External Corrosion Direct Assessment (ECDA) Methodology was developed as an alternative method to determine, and prove, pipeline integrity for pipelines that could not be inspected using intelligent pigs.

It integrates all available pipeline data and allows the condition of the pipeline to be determined and the risk of failure due to external corrosion to be determined and addressed. ECDA is not intended for internal corrosion, nor for pipeline sections that cannot be inspected using traditional overline techniques (such as DCVG, PCM, CIS etc).

The ECDA Methodology has been incorporated into a NACE Recommended Practice RP0502. It is an iterative process designed to identify areas where external corrosion may be expected. ECDA is undertaken using a four step process as follows:

STEP 1 – Pre-assessment
STEP 2 – Indirect Inspection
STEP 3 – Direct Examination
STEP 4 – Post assessment

During this phase, all the available pipeline information is evaluated and assessed. This includes information from original design, construction, maintenance and operations. All the inspections and surveys carried out on the pipeline previously are also reviewed, along with corrosion and leak history.

The purpose of this first step is to determine whether ECDA would be suitable as an inspection methodology for a particular pipeline. Should the ECDA Methodology be suitable, then the Pre-Assessment Phase also determines the various ECDA regions on the pipeline and assigns which inspection tools will be used for each ECDA Region.

During this phase, the actual overline surveys are carried out. These will include combinations of DCVG, CIS, PCM, C-Scan, soil resistivity surveys and GPS surveys. A minimum of two techniques need to be used in combination for each ECDA region, and the chosen techniques must match the potential threats to integrity that were identified on the pre-assessment step.

Once the surveys have been completed, the results are analysed and integrated and a number of excavations need to be carried out to check whether the process has been successful. During this phase, measurements of the soil pH, resistivity, pipeline potential, presence of SRB and depth of cover are recorded. The condition of the coating is also assessed to determine the cause for the coating defect. Any metal loss is recorded and NDT done, if necessary. A minimum of two exploratory excavations must be completed per ECDA region.

This final step evaluates whether the ECDA process was successful and determines the re-inspection interval for the next survey. This is accomplished by determining the corrosion rate, estimating the remaining life and re-inspecting in half the time. However, sound engineering needs to be used when ECDA is being undertaken for the first time, as accurate corrosion rates may not be available.

It must be stressed that ECDA is an on-going activity and each subsequent survey greatly enhances the confidence in the integrity of the pipeline. ECDA needs to be implemented as a “continuous improvement” process.

PPT has extensive experience in undertaking ECDA Projects. PPT staff have surveyed many thousands of kilometres using all the standard overline techniques such as DCVG, ACVG, CIS, Soil resistivity, Depth of cover and GPS surveys.

PPT has developed specific software that allows Planning, Executing, Analysing and Storing the entire ECDA process and data on one software package. The software is called PECIMS and more details are available at www.pecims.com