This paper covers various methods for dealing with sources of errors affecting the accuracy of the Direct Current Voltage Gradient (DCVG) coating defect identification and sizing tool. Topics reviewed in detail include: eliminating the influence of local soil resistivity and varying pipe depth on the calculation of the %IR severity ranking index and using the DCVG in conjunction with an AC Current Attenuation (ACCA) survey.
The accuracy of the indirect inspection data and the quality of the data interpretation are critical factors in conducting a successful ECDA program. Knowing the limitations of each indirect inspection tool, identifying the sources of error and finally adapting the data processing and interpretation to deal with these sources of errors are essential aspects of the continuous improvement of the ECDA process.
The DCVG survey is now considered the only tool which can accurately evaluate the size of a holiday, using the percentage IR (%IR). The %IR is calculated as a ratio between gradient and pipe-to-soil potentials, both measured with respect to remote earth, as described in NACE Standard TM0109-2009.1
However, differences between the soil resistivity at the test station where the pipe-to-soil potential was measured with respect to remote earth and the local soil where the DCVG detected the holiday can result in significant errors. Furthermore, the calculated percentage IR does not account for the pipeline depth.
An additional challenge is how to assess the coating quality and the protection level at inaccessible areas, such as highways, rivers, etc.
This paper covers lessons learned when dealing with these challenges.
Each challenge lesson will be presented based on the original survey case, complete with recommendations for avoiding possible errors.
ACCOUNTING FOR PIPELINE DEPTH IN %IR CALCULATIONS
An ECDA process was conducted on a NPS16 gas pipeline in Canada in 2005.2 At the time, a proprietary formula was developed to calculate the %IR using the lateral gradient measured at 3 m and the pipe-to- soil potential measured on top of the pipe. However, the formula used a ”standard” cover of 1.2 m. A moderate DCVG indication (i.e., 55.2%IR) was identified at a ditch crossing, with the pipeline fully protected along its entire length (i.e., no CIPS indications). The high %IR was attributed in part to the gradient distortion produced by the reduced soil cover (i.e., 0.5 m). Considering the difficulties associated with excavating the pipe it was imperative to re-assess the severity of the coating damage under reduced cover. Subsequently, a different technical approach was developed for this re-assessment.