Assessing Hit Rate Reduction Benefits of Mechanical Damage Prevention for Application in Risk-Based Pressure Design

Abstract A risk-based pressure design approach has been developed as an alternative to the class location approach currently used in the Canadian Standards Association’s (CSA’s) Standard Z662. This new approach was adopted in Annex C of the 2023 edition of CSA Z662 as an optional alternative to the pressure design approach in the main body of the standard. The risk-based design approach uses a set of hoop stress factors to calculate the minimum wall thickness from the pipe’s pressure, diameter, and specified minimum yield strength. The hoop stress factors are calibrated to meet reliability targets based on undamaged pipe burst and third-party mechanical damage failure limit states. These reliability targets are defined for a set of consequence-based safety classes to achieve a consistent, broadly acceptable, level of risk for all pipelines. Since expected failure rates due to third-party mechanical damage were directly used in developing this approach, the benefits of enhanced mechanical damage prevention measures, such as deeper cover depth, additional signage, and installation of mechanical protection, can be reflected in the pressure design. The benefits of enhanced mechanical damage prevention are accounted for via a “hit rate factor,” which can increase the total hoop stress factor, allowing for the use of smaller wall thicknesses. The value of the hit rate factor is dependent on the “hit rate reduction factor,” which is a ratio quantifying the reduction in expected hit rate from third-party activities on the pipeline alignment given the design’s mechanical damage prevention measures, as compared to the minimum requirements. In the 2023 edition, Annex C of CSA Z662 provides little prescriptive guidance on how to estimate the hit rate reduction factor. However, the hit rate fault tree approach is identified as an appropriate option. This paper describes estimates of the hit rate reduction benefits of various mechanical damage prevention measures in terms of a hit rate reduction factor using a fault tree model. The predictions of multiple industry-leading fault tree models are compared, and the pressure design implications are quantified for representative scenarios in terms of the resulting risk-based hoop stress factors.