The integrity validation of small diameter, thin wall pipelines can be challenging. In-line inspection tool limitations for diameters below approximately 4.5-inch contribute significantly to this challenge. This paper will describe experiences related to the successful integrity validation of a 3-inch vintage high vapour pressure (propane) pipeline which was susceptible to external cracking as well as crack-like indications at the seam weld (e.g. hook cracks and lack of fusion). This line pipe was manufactured in 1967 with a low frequency electric resistance welding (ERW) process. The major threat was identified to be external corrosion since external metal loss up to 70% of the wall thickness was reported by in-line inspection (ILI). The cut-out samples obtained from integrity digs following the ILI were examined by magnetic particle inspection methods. Some external features on these cut-out samples were reported as cracks. The pipeline licensee involved Skystone to complete an engineering assessment and provide a short-term and long-term integrity plan to validate the integrity of the pipeline for continued service.
The cut-out samples were re-examined. This examination included cross-sectional metallography of the area suspected to contain crack-like indications. No external cracking was identified; however, crack-like indications such as lack of fusion were found on the ERW seam.
A pressure-spike test was designed as an integrity validation technique and completed successfully. This paper reviews the difference between the pressure-spike test and conventional pressure tests, the technical basis for the pressure-spike test, its limitations, and practical considerations that need to be considered. The regulatory recognition of spike testing as well as available recommended practices that refer to this approach will be discussed.
A long-term integrity plan was developed by establishing an interval for the next pressure-spike test using fatigue life estimation.