ABSTRACT
The interpretation of indirect inspection data and selection of direct examination locations are crucial steps in conducting a successful Stress Corrosion Cracking Direct Assessment (SCCDA) process. This paper presents advances in the interpretation of indirect inspection data and selection of SCCDA direct examination locations and how the accurate selection of locations most susceptible to external corrosion would improve pipeline integrity. Comprehensive case studies from SCCDA digs, practical experience and lessons learnt from these case studies are fully presented.
Stress Corrosion Cracking Direct Assessment (SCCDA), specified in NACE SP0204-2008, was developed to improve pipeline safety and reduce the threat of stress corrosion cracking (SCC) on onshore petroleum pipeline systems. The SCCDA program can be utilized as a primary technique for difficult to pig pipelines and as a supplement technique for inline inspection (ILI) for piggable pipelines. SCCDA is a continuous improvement process that identifies and addresses locations where SCC has occurred, is occurring, or might occur.
The direct examination step of SCCDA involves prioritizing the findings of indirect inspections and excavation (for underground pipelines) of locations where SCC are most likely, the measurement of cracks, evaluation of crack severity, and root cause analysis.
The precise selection of direct examination locations would ensure that pipeline operators can reduce unnecessary and costly excavations while making sure locations most susceptible to stress corrosion cracking are comprehensively addressed to prevent future failure from SCC.
INTRODUCTION
SCCDA is a four-step process that includes pre-assessment, indirect inspection, direct examination, and post-assessment. Pre-assessment utilizes and requires a sufficient amount of data collection, integration and analysis of historic and current pipeline data to prioritize potentially susceptible segments of pipelines and ultimately help select specific sites for excavation. The indirect inspection step utilizes aboveground survey techniques to assist in the identification of locations most likely to have potential for SCC. The type of survey and the need to conduct indirect inspections depends on the nature and extent of the data collected in the pre-assessment step. CP CIPS, DCVG, ACVG, terrain conditions, soil resistivity surveys, environmental point surveys, and historical ILI records are used in conjunction with pre-assessment records to pinpoint susceptible SCC areas. The indirect inspection of SCCDA does not, however, identify cracks (unless the pipe is exposed), but rather the propensity for cracks to occur, and therefore may be considered as a proactive, or preventive risk assessment measure to ensure the future integrity of a pipeline. The next step, direct examination, is performed at likely locations susceptible to SCC. Direct examination prioritizes the findings of indirect inspections and involves bellhole excavation (in underground pipelines) of locations where SCC are most likely. The SCCDA process is concluded with a post-assessment that evaluates the effectiveness of the SCCDA technologies and defines and/or determines reassessment intervals.
