Reliability-Based Evaluation of Corrosion Assessment Safety Factors for Pipelines Designed to Risk-Based Safety Class System

Abstract A risk-based pressure design approach for designing onshore transmission pipelines has been developed and incorporated in the Annex C of the 2023 edition of Canadian Standard Association (CSA) Z662 standard. This approach uses a set of hoop stress factors (termed class factors) calibrated for six consequence-based safety class system to design the required minimum wall thickness of the pipeline to limit the pipeline risk at a consistent acceptable level. This non-mandatory approach provides an alternative to the more established experience-based hoop stress factors (product of design and location factors) described in Clause 4.3.5 of the standard. As written in the CSA standard, Clause 4.3.5 hoop stress factors are also used as safety factors to assess corrosion features as prescribed in Clause 10 of the standard. Following the same principle, Annex C class factors described in Clause C.5 are also prescribed to be used for the Clause 10 corrosion assessment. This paper evaluates the reliability implications of applying the class factors for corrosion assessment of pipelines designed to the Annex C safety class approach. Three gas pipeline cases were considered in this evaluation, one in undeveloped area and the two others in developed areas. All pipelines were considered in two scenarios: 1) designed using the Clause 4.3.5 approach, and 2) designed using the Clause C.5 approach. Similar corrosion populations representative of decades-long operation were simulated for each scenario using information in public domain literature, and corrosion assessments to determine repairs were carried out using the approach described in Clause 10.10.2.5 of the CSA standard. In this assessment, each scenario utilized the corresponding hoop stress factors for the pipeline design as the corrosion safety factors (i.e. design and location factors for scenario 1, class factors for scenario 2). The implication of using these safety factors was evaluated by assessing the reliability of the pipelines with the remaining corrosion features that passed the Clause 10.10.2.5 assessments and subsequently their overall safety. Comparison was made between the Clause 4.3.5 and Clause C.5 scenarios to demonstrate the implied safety with both approaches.