Pipeline operators often deal with the large numbers of dents that require further consideration, and are required to prioritize these dents for further investigation and repair. Code guidance is clear on the relative severity of dents based on a depth or associated with welds, corrosion, gouging or cracking. It allows the pipeline operators to prioritize their dig lists and limit the number of “investigative digs” by omitting plain and shallow dents. However, the dent depth criterion has limitations, experiences learned in the past have shown that a dent prioritization based on depth alone, may still leave a significant number of dents in the pipeline which may pose a threat, particularly from local static strain and fatigue. In recent years, strain and fatigue analyses have been included in the assessment of severity of dents in order to better prioritize and effectively repair dents which represent a threat to the structural integrity of pipelines. However, strain analyses require tedious calculation of dent curvature at each data point from ILI reported dent profile. When a large numbers of dents require to be prioritized by strain severity, using this detail calculation is impractical. Therefore, a screening methodology is required to reliably identify candidate dents that require detail strain assessment. In this paper, an aspect-ratio based screening methodology is developed and applied to over 7,000 dents reported by an In-Line Inspection (ILI) caliper tool for a 265 miles long pipeline section. A total of 263 shallow dents which could have injurious strain level were identified and ranked for detail strain and fatigue calculations. In-ditch investigation of 20 dents with LaserScan profiling showed that the developed screening methodology provides an effective tool to capture all the dents with strain equal and 6.5% at 95% confidence level.
Recent failures in seam weld pipe have raised concerns within the pipeline industry over the integrity of such welded pipe. Low-Frequency (LF) Electric Resistance Welded (ERW) pipe manufactured prior to 1970, in particular, can be susceptible to failures caused by hook cracks, lack of fusion and other planar defects should the weld area exhibit low toughness. Integrity management regulations and Pipeline operators are evaluating potential methodologies to address and mitigate the LF-ERW seam weld threat. A program has been initiated at Williams Northwest Pipeline GP (NWPGP) to address the integrity management of its pre-70s ERW pipelines. In this case study, as part of an overall integrity management program, a hydrostatic test and fatigue analysis based methodology for addressing the LF-ERW seam weld threat is presented. The methodology was applied to 15 pre-1970’s natural gas pipelines. The results and findings are summarized in terms of the integrity threat mitigation and maintenance strategies.
A 22-inch onshore natural gas transmission pipeline experienced an in-service rupture in January of 2018. The rupture was caused by an axial crack that occurred 150 Km (93 miles) downstream of the closest compression station. While the root cause analysis of the rupture was in progress, excavations of indications reported by a Hard Spot In-Line Inspection (ILI) were undertaken starting approximately 42 km (26 miles) downstream of the rupture. These excavations discovered predominantly circumferential crack colonies, with a number of through-wall cracks being leak. The root cause analysis concluded that both the axial and circumferential external surface cracks are consistent with hydrogen environment assisted cracking generated by the impressed-current cathodic protection system, operated near the pipe-to-soil recommended potential limit of −1200 mV as measured with respect to a saturated copper/copper sulfate reference electrode (CSE). Transportadora de Gas del Norte (TGN) implemented a series of preventive and corrective measures that included pressure testing, cathodic protection improvement, coating rehabilitation programs and In-Line Inspection programs among others. This paper presents the background and approach used in the root cause analysis, summarizes the methodology, results and findings, and discusses their implications in the management of the Pipeline Integrity of the pipeline system operated by TGN.
The Cactus-San Fernando gas pipeline system was constructed between 1977–79 in 42 and 48-inch diameter pipe. The pipeline extends for a total length of 650 km in 10 piggable sections. This paper details the work conducted to allow the pipeline to be re-validated to its original design pressure. The overall aim of the project was to demonstrate the future integrity of the pipeline system. The scope of the study was extensive and required innovative project management techniques. It encompassed caliper inspections and high-resolution magnetic inspection, the provision of global positioning surveys (GPS), Pipeline Integrity Assessment including repair schedules, future corrosion prevention and reinspection strategy and pipeline rehabilitation including non-destructive examination (NDE), material testing and repairs. The whole process was reviewed and ratified by an approved Certifying Company who issued a Certificate of Mechanical Integrity, valid for a fixed period of 2 1/2 years.
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