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Over the past 10 years inspections (site visits, boat based surveys or diver surveys) have been completed at nearly 20,000 pipeline watercourse crossings for 20 different pipeline owners. During the last 10 years there have been 721 unique locations where an exposed pipeline was found and at 213 of these locations a freespan was encountered. Only one of the freespans resulted in the failure (loss of product) of the pipeline. This record illustrates what is now become widely accepted, that pipeline exposure does not necessarily lead to pipeline failure. The record adds to this, pipeline freespan does not necessarily lead to failure. This highlights that the relevant question for “water loading caused pipeline failure” is: Does the combination of freespan length and water velocity exceed a combination that would lead to vortex induced vibration or the exceedance of the static load limit of the pipe?

Oil and gas pipelines are subjected to multiple types of geohazards which cause pipeline failures (loss of containment); two of the most common types occur at watercourse crossings and at landslides. At watercourse crossings, the most common geohazard which causes pipeline failures is flooding during which excessive scour may result in the exposure of the buried pipeline and if the exposure results in a free spanning pipeline, then this may fail due to fatigue caused by cyclic loading from vortex-induced vibration. Fortunately the free span length and water velocity combinations that lead to failure can be defined and can be used to identify the flood discharge that should be monitored for in order to trigger actions to manage the hazard and avoid failure. Most watercourse crossings in a pipeline network are on ungauged watercourses and necessitate the use of a proxy gauged watercourse. The “proxy” gauged watercourse is used to infer whether flooding is occurring on the ungauged crossing, and the owner can take appropriate actions. Often the proxy gauged watercourse is too far away or the watercourse may not be representative of the crossing of concern (e.g. large difference in the drainage areas). Real-time rainfall data can be used in conjunction with streamflow monitoring to determine when extreme precipitation has occurred within the ungauged watercourses catchment which may result in flooding. Where pipelines cross landslide prone areas, large scale movements can be initiated, or slow on-going movement rates increased when extreme rainfall occurs. The definition of the extreme rainfall event for slope sites is the key component of providing a suitable warning of potentially dangerous conditions; shallow slides can be caused by short term events from sub-hourly to 3 day duration precipitation events whereas large deep seated (creeping) landslides can be driven by annual and intra-annual rainfall amounts. Monitoring of real time rainfall can be used to determine when extreme rainfall occurs at a landslide site. The density of in-situ weather stations collecting real-time rainfall data prevents the application along remote sections of pipeline routes and within large sections of Canada. Gridded real time rainfall from quantitative precipitation estimations which integrate a multiple data sources including in-situ, numerical weather prediction, satellite and weather radar, can be used to overcome this problem and provide warnings when pre-determined rainfall thresholds are exceeded on a site-specific basis.

Free span assessment in watercourse crossings for the on-shore pipeline industry has become a more and more important part of pipeline integrity practice. One reason is that it has become increasingly well known that the dominant cause of pipeline failures in watercourse crossings is fatigue failure due to vortex induced vibrations at pipeline free spans. Recognition of this is now being identified in industry recommended practices and owners are incorporating this type of assessment into their pipeline integrity management practice. On shore pipelines are not designed with an allowable free span as is the practice with off-shore pipelines, but are buried. Design codes specify minimum depths of cover when constructed and indicate that pipelines should be maintained so that no excessive loads occur. In the past the no excessive loads requirement has been interpreted that the pipeline must remained buried. As experience from the off-shore environment and increasingly from experience on-shore has shown that most exposed and/or free spans do not fail. Due to various river erosion mechanisms; scour, bank erosion or avulsion, previously buried pipelines do develop free spans. Some of the free spans fail and release products directly into the watercourse. Failures, particularly for liquid products, are very expensive due to the economic loss, repair costs and environment clean-up of the watercourse and its banks. Similarly, costs associated with pipeline replacement for free spanning pipelines or repair of pipelines that might develop free spans are relatively high. It is important to develop an understanding of the probability of the pipeline failing due to a free span, or put another way, determine which free span is a threat to integrity. This paper discusses some of the challenges with assessing free spans in watercourse crossings as part of integrity programs and highlights experiences in assessing this threat to integrity. The objective of this paper is to discuss some of the key uncertainties related to the management of the threat due to free spans. These uncertainties are due to the reliability of information about the free span, water velocity and condition of the pipelines.

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.

Essentially all areas of Canada are affected by significant seasonal frost penetration. The effect of the frost penetration on roads and highways, especially the amount of total frost heave and the effect of the subsequent thaw, varies with soil type. Road design and construction must consider the effects of frost heave and subsequent spring thaw. The location and amounts of differential frost heave are especially important to pavement performance. Frost heave behavior can be manifest at pipeline crossings as “reverse frost heave”, where the ground above the pipeline does not heave as much as the adjacent ground. If the amount of differential heave within pipeline crossing is large, it can lead to rough, and potentially unsafe, driving conditions. In the case of pipeline crossings, if rough road surface conditions occur in the right of way, highway agencies typically request compensation for additional maintenance costs from the pipeline company. A case study that shows the effects of differential frost heave at a pipeline road crossing is presented.

The Transcanada Pipelines Ltd. (TCPL)mainline system carries natural gas from the Alberta/Saskatchewan border to delivery points near the Quebec/Vermont border. The system contains pipelines that traverse approximately 4,000 kilometres (2,500 miles) of terrain and delivers about 2,900 Bcf of gas per year. As part of the integrity program this system has been subjected to a geotechnical hazard identification and assessment program over the last several years. The geotechnical hazard assessment process has identified hazardous sites and specific studies at these sites have been undertaken. TCPL has recently begun a transition from performing Hazard Management to an overall Risk Management approach for pipeline integrity. As part of this process TCPL required that a geotechnical risk methodology be developed to analyze the risk posed to the pipeline due to landslides. Therefore a new landslide failure probability estimation process was developed to complete this program.