Abstract

Transmission pipeline operators regularly inspect their assets using in-line inspection (ILI) tools to monitor for potential internal and external threats to the system. When these tools identify features that meet excavation criteria, the operators will complete mitigation activities to reduce or remove the threat. Typically, these mitigation activities include excavation of the pipeline, removal of the coating, and non-destructive examination at the targeted feature. Upon completion of the maintenance activities, the pipeline is then re-coated and the backfill restored.

During the maintenance work, the pipeline's coating at the ends of the excavation is exposed to atmospheric conditions (e.g., sun light, humidity, etc.). Moreover, the pipeline is then exposed to disturbed soil with varying moisture content after being backfilled. Depending on the coating type, these conditions may increase the corrosivity of the localized environment at the existing coating (that was left as-is) and at the pipe ends.

Approximately 16,000 digs in Canada and the United States of America were analyzed to determine the impact of maintenance activities on the future integrity of transmission pipelines. A re-visit rate at previously excavated locations of less than 1% was observed based on this analysis. Typically, the re-visit occurs 4 years after the initial visit. As expected, most of the revisits were associated with pipelines that were originally coated with polyethylene tape. Within this paper, strategies to reduce the revisit rate will also be discussed.

Introduction

Transmission pipeline companies utilize successive ILI tool runs to identify, size, and determine the corrosion growth rate (CGR) of pipeline features that may be detrimental to the operation of the pipeline [1, 2]. The importance of back-to-back ILI tool runs in monitoring and maintaining the safe operation of a pipeline is further supported by the methodologies' incorporation by US regulators for high consequence areas. The calculated CGR from the ILI tool runs is used to determine maintenance digs required to mitigate the identified features prior to the next scheduled re-inspection interval.

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