B. Leak Volume

B. Leak Volume

For purposes here, the terms leak, spill, and release are used interchangeably and can apply to unintentional episodes of product escaping from a pipeline system, whether that product is in the form of liquid, gas, or a combination. The total spill quantity is the sum of leak volumes prior to system isolation (includes detection and reaction times), the leak volume after facility isolation (drain and/or depressure time), and mitigated leak volume (secondary containment). The following paragraphs discuss pipeline spills and suggest ways to model spill size for a relative risk assessment.

Leaked volume or spill size is a function of leak rate, reaction time, and facility capacities. It is a critical determinant of damage to receptors under the assumption that hazard zone size is proportional to spill size. This assumption is a modeling convenience and will not hold precisely true for all scenarios. Some leaks have a negative impact that far exceeds the impacts predicted by a simple proportion to leak rate. For example, in a contamination scenario, a 1 gal/day leak rate corrected after 100 days is often far worse than a 100 gal/day leak rate corrected in 1 day, even though the same amount of product is spilled in either case. Unknown and complex interactions between small spills, subsurface transport, and ground-water contamination, as well as the increased ground transport opportunity, account for the increased chronic hazard. On the other hand, from an acute hazard perspective, such as thermal radiation, the slower leak...