Alliance Pipeline (Alliance), an integrated Canadian and U.S. high-pressure rich natural gas transmission system administers a Geohazard Management Program (GMP) which identifies, investigates, monitors and assesses sites subject to risks from geohazards. Within the pipeline industry, recent flood events have shown that pipelines with a seemingly adequate depth of cover can become exposed and fail in a single flood event. As such, it is important to understand which water crossings could result in a pipeline failure if the pipeline were to become exposed in a flood (termed vulnerability).
Two complimentary methods were developed for evaluating the vulnerability of pipelines at water crossings. The first method is a mechanistic approach that compares the maximum allowable free span length (MAFSL) of an exposed pipeline to simple geomorphic properties of the water crossing. The MAFSL was determined by calculating the strain and fatigue limits of the pipeline from hydrodynamic loading and vortex shedding. The second approach is based on a statistical regression of historical pipeline performance, hydrotechnical inspection records and actual exposure rates to calculate a probabilistic estimate of pipeline vulnerability.
Utilizing the developed approaches, the vulnerabilities were combined with probability of exposure values to provide an improved risk estimation of the water crossings. Further analysis shows the calculated likelihood of failure at the water crossings has no correlation to the depth of cover (DOC). This suggests that the designation of an arbitrary DOC requirement at water crossings is incongruous with risk management principles. Instead, the DOC at water crossings should be maintained at a safe level based on the specific hydraulic and geomorphic characteristics of the site.