Page 175 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

71152 Leak Impact Factor
To fully analyze a liquid spill scenario, a host of variables Adding leak detection and emergency response considera-
must be assessed: tions impacts the volumes released and adds a level ofresolu-
tion to any of the above analyses. It is especially important to
Product characteristics
consider leak detection capabilities for scenarios involving
Product viscosity toxic or environmentally persistent products. In those cases, a
Product vapor pressure full line rupture might not be the worst case scenario. Slow
Product flow rate leaks gone undetected for long periods can be more damaging
Product pressure than massive leaks that are quickly detected and addressed. A
Product solubility leak detection capability curve (see Figure 7.7) can be used to
Product miscibility establish the largest potential volume release.
Evapotranspiration rate. The more complex analyses are becoming more common-
Pipeline Characteristics place given the increased availability of powerful computing
environments and topographical information in electronic
Root cause of failure databases. An important benefit of the more complex analysis
Hole dimensions approaches is the ability to better characterize the receptors that
Proximity to isolation valves are potentially exposed to a spill-those that are actually “in
Time to recognize event harm’s way.” In many cases, receptors may be relatively close
Time to confirm release to, but upslope of, the pipeline and hence at much less risk.
Time to close block valves Focusing on the locations that are more at risk is obviously an
Initial release volume advantage in risk management.
Stabilization release volume. Spills in soil or water are the most common pipeline environ-
Environment Characteristics mental concern. Such spills also carry the potential for ground-
water contamination. Product movement through the soil
Soil infiltration rate depends on such soil factors as adsorption, percolation, mois-
Drainage pathways ture content, and bacterial content. Soil characteristics can be
Weather patterns best assessed by using one of the common soil classification
Proximity to ignition sources systems, such as the USDA soil classification system, which
Vegetative cover effects incorporates physical, chemical, and biological properties of
Slope effects the soil. For simplicity, only one soil characteristic-perme-
Groundwater flow patterns ability-is considered in some risk evaluations. This is also the
Proximity to surface waters. soil characteristic that is used in the EPA hazard ranking system
(HRStpermeability of geologic materials [ 141.
In identifying all possible liquid leak impact ranges, it Releases into surface waters are the second potential type of
may not be necessary to fully evaluate all of the potential environmental insult and pathway to population receptors. The
interplays among each of these variables. The added complexi- size of the body of water and its uses determine the severity of
ties and modeling costs often outweigh the benefits of such the hazard. Ifthe water is used for swimming, fishing, livestock
detailed calculations. A range of leak analysis options is avail- watering, irrigation, or drinking water, pollution concentra-
able, each of which might be appropriate for a certain type of tions must be kept quite low. Spills into water should take into
evaluation. account the miscibility of the substance with water and the
Topography aspects will be a critical determinant in most liq- water movement. A spill of immiscible material into stagnant
uid spill scenarios. It is difficult to generate a universally appli- water would be the equivalent of a relatively impermeable soil.
cable scoring table for topography. Which is preferable-rapid, A highly miscible material spilled into a flowing stream is the
wide surface dispersion or limited surface transport but more equivalent of a highly permeable soil. (See later section dealing
rapid ground penetration? The unfortunate (from a modeling with spills into waterways.)
perspective) answer is that “it depends.”
In some cases, a concentrated spill (limited dispersion) poses Thermal effects
less risk, while in other cases, even at the same location, the
opposite is true. A rapid and wide dispersion might reduce igni- Addmg to the physical extent of the spilled product are the poten-
tion probability and burn time, should ignition occur. In other tial thermal effect distances arising from pools of ignited and
cases, ignition might be preferable, thereby eliminating con- burning product. These are more fully discussed in Chapter 14
tamination potential or preventing migration of the spill to under the calculation of hazard zones. Potential thermal effects
other receptors. The possible receptor interactions are critical are largely dependent on the size of the pool created from the
elements of topographical considerations. This includes recep- spilled product. Pool growth can be simulated using a calculation
tors of ground and surface water, in addition to other environ- method specified by the EPA, the Federal Emergency Manage-
mental receptors and population density and property. mentAgency, or the US. Department oflransportation (DOT) [5,
It is difficult to find simplifying assumptions to use in rank- 861. This correlation relates the release size to the pool area:
ing potential liquid spill scenarios, given the widely varying
threats accompanying the many differences in terrain and Log (A) = 0.492 log (M) + 1.617
topography and product characteristics. A range of analysis
options is available, of which several methods are listed inTable where M represents the total liquid mass spilled in pounds
7.6, from the simpler to the more complex: and A is the pool area in square feet.