Inspection of offshore pipelines is essential to provide vital information required for integrity assessment, to allow data driven engineering decisions to be made and prevent costly failures. To identify and quantify anomalous features and defects including internal corrosion damage, it is common to use inline inspection (ILI) pigs.
Conventional ILI requires deployment of a tool which has a combination of characteristics that allow the passage of a free-swimming assembly through the pipeline without disruption to normal operation. However, some pipelines deviate from conventionally "piggable" pipelines, so inspection for internal corrosion could be complex and/or unfeasible. To establish the integrity of these difficult to pig pipelines, Internal Corrosion Direct Assessments have been performed to ascertain the condition of these pipelines with considerable success.
This paper presents experience from applying Internal Corrosion Direct Assessment (ICDA) for offshore pipelines in the North Sea. The paper focuses on how these assessments have been performed, challenges experienced, results and lessons learnt.
Most operators have a pipeline integrity management system (PIMS) which consists of a set of instructions implemented to manage the safe and reliable operation of pipelines. For offshore pipelines a variety of non-destructive testing (NDT) techniques are typically used to measure how internal corrosion has progressed and will normally provide the following information:
1. Measured wall thickness to allow comparison to predicted corrosion rates.
2. Confirm that process parameters are within expected operational envelope.
3. Determine level of compliance with corrosion control performance targets.
4. Establish integrity assurance and identify datum points against which corrosion damage can be quantified.
Inline inspection (ILI) is a popular method used for inspecting offshore pipelines and involves the inspection of a pipeline from its interior by the deploying a device capable of detecting and sizing metal loss defects by one or more NDT techniques. Some pipelines are designed with pig launch and receipt facilities that allow the ILI tool to be deployed during normal operations. However, some pipelines fall into the “difficult to pig” category and any pigging operation may pose considerable operational risks and cost. For these pipelines, the NACE Liquid Petroleum Internal Corrosion Direct Assessment (LP-ICDA) methodology1 has been used as an alternative to identify sections exposed to high internal corrosion risks with considerable success although in some cases there were unexpected results.