Due to the serious consequences of corrosion and leaks in underground pipelines, external corrosion direct assessment (ECDA), as described in ANSI(1)/NACE(2) SP0502 was developed in an attempt to proactively prevent external corrosion and ensure the integrity of oil and gas pipelines that are difficult to pig. This standard requires a minimum of two indirect inspections to confirm the most susceptible locations on a pipeline for external corrosion to occur.

Cathodic protection close interval survey (CP CIPS) and direct current voltage gradient (DCVG) are usually complementary indirect inspection techniques used for assessing the effectiveness of the cathodic protection and coating conditions. DCVG detects coating anomalies due to voltage gradient created as a result of direct current flow to coating anomaly. Coating severity index or DCVG % IR is used to prioritize coating anomalies for repair with the intent of improving the cathodic protection performance.

Coating anomalies with large DCVG %IR are thought to have greater risk for external corrosion during ECDA; possibly due to large exposed metal to corrosive environment. However, considering that a large coating anomaly would consume more CP current, the tendency for corrosion is expected to be minimal.

This paper will show improvements in interpretation of DCVG % IR and how accurate selection of pipeline coating rehabilitation location based on this concept would lead to improvements in cathodic protection performance. Case studies from previous ECDA digs are presented.

Subject
Electric current, Indirect inspection, External corrosion direct assessment, Coatings, Piping, Voltage, Cathodic protection, Pipeline integrity management, Pipelines, Anomalies, External corrosion, Soil, Remote earth
Keywords:
External Corrosion Direct Assessment, DC voltage gradient, close interval survey, cathodic protection, coating anomaly, data interpretation, severity classification
© 2016 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
2016
Association for Materials Protection and Performance (AMPP)
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