Pipelines are subjected to both residual and applied tensile stresses, and can form near-neutral pH SCC (transgranular stress corrosion cracking) if the pipeline is exposed to a conducive environment and is made from a material that is susceptible to SCC. This transgranular SCC is an ongoing integrity concern for pipeline operators. As part of an SCC Integrity Management Program (IMP), it is necessary to perform integrity assessments and prioritize segments of the pipelines to manage the SCC threat. Ultrasonic crack detection in-line inspection tools have proven capable of locating SCC, but reliability of these tools is not absolute and the reduced probability of detection of subcritical flaws limits options for proactive management. Hydrostatic retesting is a very effective program for removing near-critical axial defects, such as SCC, but does not provide useful information as to the location of SCC along the pipeline. NACE Standard RP0204-2004 (SCC Direct Assessment Methodology or SCCDA) outlines factors to consider and methodologies to employ to predict where the SCC is likely to occur, but the standard acknowledges that there are no well-established methods for predicting the presence of SCC with a high degree of certainty. The trend in probabilistic modelling has been to focus on establishing deterministic relationships between environmental factors, tensile stress and SCC formation, and growth; these models have achieved varying degrees of success. The Statistical Predictive Model (SPM) was previously developed to predict the likelihood of occurrence of near-neutral pH Stress Corrosion Cracking (SCC) for the NPS 10 Alberta Products Pipeline (APPL). SPM Phase 5 uses selected predictor variables representing tensile stress, environmental, pipe-related, corrosion control and operational relevant factors to determine the Probability of Occurrence of SCC. Regression techniques were used to create multi-variable logistic regression models. The results for each model are checked at locations where SCC is known to be present or absent to assess predictive accuracy, then used to prioritize susceptible segments for field excavation. The relative strength of individual predictor variables provides insight into the mechanism of near-neutral pH SCC crack initiation.
Regulations for pipeline operators within the Oil and Gas Pipeline Industry are becoming increasingly rigorous especially in the fields of pipeline integrity and emergency response and as such the need for equally rigorous approaches to analyzing, understanding, managing, and reporting of the effects of a pipeline release has increased. Trans-Northern Pipelines (TNPI) undertook a detailed fate and transport modeling project for its Ontario and Quebec based pipeline systems. These pipeline systems are relatively complex products delivery systems that handle multiple fluids and, for some pipeline segments, allow for flow in either direction. The goals of the project included understanding and improving risk management of the systems through consequence reduction and to identify and quantify potential impact to surrounding areas and risk receptors within the vicinity of the pipeline systems. The receptors are the focal points that receive the negative impact if there is a leak (i.e. pin-hole, small and large scenarios) or rupture of the pipeline. These can include health/safety, environmental, property damage, reputation and public disruption, and financial impacts. The outcomes from the project included a continuum of potential release volumes along the pipeline systems for each product and operational scenario from which a spatial representation of the potential impact areas (due to overland flow) along the pipeline system were derived and locations were identified where potential release volumes (i.e. initial and drain down volumes) could affect streams and could possibly be transported along the stream. Potential impact areas and representations of the stream transport were used to identify possible risk receptors. A unique aspect to this modeling project was that it was undertaken utilizing a four dimensional (4D) model thus allowing for the results to be visualized within a GIS and as animations; both of which facilitated visualization of potential release impact over time (e.g. a 48 hour period). Utilizing the time domain provided unique insights that were used to augment TNPI’s emergency response plan. Based on a review of the preliminary results Trans-Northern undertook additional effort to investigate what-if scenarios for valve placement. The purpose of these what-if scenarios was to quantify the improvement of additional valves on lowering the overall potential impact and thus allowing for a quantitative basis for valve placement.
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