As per Indian regulations (OISD-STD-141, OISD-2014-SOP, PNGRB T4S, PNGRB IMS etc.), the mandatory requirement for the operators is to perform Cathodic Protection (CP) and/or Coating Integrity above ground survey every five (5) years.
The individual Indirect Inspection (IDi) technique or techniques used for such surveys are Closed Interval Survey (CIP) On/ Off, Direct Current Voltage Gradient (DCVG), Current Attenuation Testing (CAT) and Alternating Current Voltage Gradient (ACVG). These techniques primarily assist in evaluating the CP performance, coating condition of the pipeline and locations of probable DC/ AC interferences. Usually these surveys are performed separately and integrated by their respective GPS coordinates to get a common chainage.
As per prevailing practice in India, typically the pipeline operators perform a Closed Interval Survey (CIP) On/ Off survey and after reviewing the reports subsequently plan for conducting DCVG, CAT and/or ACVG for certain stretches only, where CIP indications are found. By the time the team is re-mobilized for these surveys the pipeline or environment conditions may be totally different. This may be due to season variability, accuracy of GPS (mapping, if at all conducted during the coating integrity survey), climatic conditions, access to right of way (RoW) due to cultivation / farming cycles, water table variance and eventually fluctuations in the insitu soil resistivity. In addition, interference from other CP sources in the RoW, which may have occurred during interim. All of this can lead to misalignment or incomplete analysis of the integrated consolidated survey data.
In addition, these “indirect inspection” surveys are majorly dependent on the experience and training of the surveyor, resulting in extensive subjectivity on the survey results with very limited traceability of collected data. This is unlike the other integrity tools for assessing integrity of a pipeline, such as In-line Inspection (ILI) as well as hydrostatic testing, wherein it is compulsory to provide the pipeline owner recorded footprint of the raw data collected for authenticity.
For the CP and coating integrity surveys, if these can be performed simultaneously along with workable recorded raw logs for each survey with recorded GPS position of the surveyor, for further analysis, this does lead to eradicating the subjectivity from the IDi surveyor and providing “true” authentic repeatable results. This paper provides case studies wherein results of legacy IDi surveys are compared to the results of performing all surveys together along with recorded raw logs.