As the first element of the overall PoF measurement, the estimate of ‘exposure’ (or ‘attack’) is anything that can damage a very weak pipeline component. This includes all loadings that can impact the stress-carrying capacity of a pipeline component. Separately, the role of mitigation–systems or actions that protect against exposures–is handled separately.
Use this page as a jumping off point to either find specific aspects of interest or to find deeper overall discussions.
- Sample list of threats/exposures/failure mechanisms
- Measurements vs Estimates
- The “Tin Can” Analogy
- Test of Time Evidence
- Assessing Corrosion Potential
- AC Induced Corrosion
- Threat Interactions
- Threat Dismissal
- Facilities
- Statistics
- Geohazards
- Impacts
- Case Studies
- Published exposure rates
- Loads vs stresses vs resistance
Threat Dismissal
Threat dismissal is discussed here under Exposure for a very important reason. If aspects of mitigation or resistance must be included in an analyses to exclude a threat, then that threat, almost by definition, cannot be dismissed. Only failure mechanisms with virtually no plausible exposure scenario can be completely dismissed.
The question: “what constitutes ‘insignificant’?” must also be addressed.
Statistics
Historical event frequencies can be important determinants of exposure estimates. They will usually, however, require a separation of mitigation and resistance effects from the overall event rate.
Test of Time Evidence
In the absence of more compelling evidence, an appropriate starting point for the exposure estimation may be the fact that a component or collection of components has not failed after x years in service. This involves the notion of having ‘withstood the test of time’. A component having survived a threat, especially for many years, is evidence of the exposure level. This is best illustrated by example. If 10 miles of pipe, across an area with landslide potential, has been in place for 30 years without experiencing any landslide effects, then a failure tomorrow perhaps suggests an event rate of 1/(10 miles x 30 years) = 1/300 mile years.
This simple estimate will not address the conservatism level. The estimator will still need to determine if this value represents more of a P50 estimate or perhaps a more conservative P90+ value. Conservatism would dictate that the 1/300 per mile-year rate is more of a P50 and that, for a normal P90+ risk assessment, the rate should be increased, perhaps considerably.
Part of this consideration in using the ‘test of time’ evidence would logically be the extent to which conditions have remained the same over the time period used. In the above example, possible recent changes in rainfall, drainage patterns, soil conditions, etc, should be considered before the 30 year experience is used at face value. Even without recent changes, the 30 year experience likely does not include 100 year rainfall events, for example.
In some cases, the evidence is actually of the mitigated exposure level. That is, the component has survived the threat, but perhaps at least partially due to the presence of effective mitigation. This makes the separation of exposure more challenging.
Despite the lack of complete clarity, this ‘test of time’ rationale can be a legitimate part of an exposure estimate. Afterall, there is still ‘evidence’ in the fact that damages have not occurred in the past under similar conditions.
This is akin to, but should not be confused with, an assertion that “its never happened before” can be taken as a conclusion that it cannot happen in the future. Very rare events are being modeled in many cases, so the weight of any evidence is proportional to the mile-years of history that is available. In the previous example, if the 10 mile pipeline had only been in service for 2 years, then the evidence (1/20 per year) is much weaker.
Again, this is recommended only in the absence of better data–perhaps to set approximate and temporary risk levels pending acquisition of better data.
Published Exposure Rates
| excavation frequencies | Mechanical Damage |
| specs/manuf/construction | Resistance |
| Geohazs | Geohazs Rates |
| Weather events | Geohazs |
| Human Error | Incorrect operations |