Engineering Critical Assessment (ECA) of Onshore Natural Gas Pipelines Using Partial Safety Factors (PSFs)

Abstract

Pipeline design codes recognize the potential risks posed by gas pipelines by relating the factors which affect the probability of failure to consequences in particular locations. To maintain more or less uniform risk level, ASME B31.8 Code adopted risk based concepts indirectly through location classifications by specifying different design factors (DFs). Engineering Critical Assessment (ECA) of onshore natural gas pipelines in accordance with pipeline-specific methods such as API 1104 allow much deeper defects in higher class pipe, which is in contrary to the basic design concept. This is due to the lack of consideration for the higher consequences in the higher classes even though they were considered at the design stage. To ensure the failure probability within a target value, generic fitness for service standards such as API 579-1/ASME FFS-1 and BS 7910 recommend partial safety factors (PSFs) to key variables. However, there is no correlation between the design factors used during the pipeline design stage and the PSFs used for the ECA. To achieve the basic intent of design code i.e., to maintain risk level as per location classifications, this paper proposes to use PSFs based on class location. Few case studies are presented to demonstrate the proposed methodology.