Page 263 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

1 V240 Distribution Systems
other pressure control devices is critical. The evaluator Product hazard
should seek evidence that regulator activity is monitored and
periodic overhauls are conducted to ensure proper perform- Note that a chronic component of a product hazard is often
ance. Other pressure control devices should similarly be closely enhanced where a leaking product can accumulate in buildings,
maintained. The care of an odorization system in a gas distribu- beneath pavement, etc. This is generally considered when
tion system should also be included with maintenance proce- assigning RQ points to substances such as methane. The evalu-
dures. ator is encouraged to review pages 138-142 to ensure that the
Score the maintenance practices as described in Chapter 6. reasoning behind the RQ assignments are appropriate for the
evaluation.
In the case of water systems, the main product hazard will be
X. Sabotage related to the more mechanical effects of escaping water. This
includes flood, erosion, undermining of structures, and so on.
The risk of sabotage is difficult to fully assess because such The potential for people to drown as a result of escaping water
risks are so situation specific and subject to rapid change over is another consideration.
time, The assessment would be subject to a great deal ofuncer- The product hazard variable can be assessed as described on
tainty, and recommendations may therefore be problematic. pages 136142.
Note, however, that many current risk variables and possible
risk reduction measures overlap the variables and measures that Spill size and dispersion
are normally examined in dealing with sabotage threats. These
include security measures, accessibility issues, training, safety One of the chief concerns of gas distribution systems operators
systems, and patrol. is the potential for a hazardous material to enter a building
The likelihood ofapipeline system becoming a target of sab- intended for human occupancy. In a city environment, the
otage is a function of many variables, including the relationship potential is enhanced because gas can migrate for long dis-
of the pipeline owner with the community and with its tances under pavement, route through adjacent conduits (sewer,
own employees or former employees. Vulnerability to attack is water lines, etc.), permeable soils, or find other pathways to
another aspect. In general, the pipeline system is not thought to enter buildings.
be more vulnerable than other municipal systems. The motiva- For more catastrophic pipe break scenarios, and as a model-
tion behind a potential sabotage episode would, to a great ing simplification, spill size can be modeled as a function of
extent, determine whether or not this pipeline is targeted. only pipe diameter and pressure as discussed on pages 142-143.
Reaction to a specific threat would therefore be very situation The underlying assumption in most consequence assess-
specific. ments is that higher spill quantities result in higher potential
Guidance documents concerning vulnerability assessments damages. The drain volume and flow stoppage time (reaction
for municipal water systems are available and provide some time) are determining factors for total volume released on
potential input to the current risk model. An effort could be water systems. In simplist terms, low spots on large-diameter,
undertaken to gather this information and incorporate sabo- high-flow-rate pipelines can be the sites of largest potential
tage and terrorism threats into the assessment, should that be spills and larger diameter, higher pressure gas pipeline mains
desirable. can generally cause greater releases.
See Chapter 9 for more discussion on sabotage issues and As discussed in Chapter 7, leak size is also function ofthe fail-
ideas for risk assessments. ure mechanism and the material characteristics. Smaller leak
rates tend to occur at corrosion (pinholes) or some design failure
modes (mechanical connections). The most costly small leaks
XI. Leak impact factor occur below detection levels for long periods of time. Larger leak
rates tend to occur under catastrophic failures such as external
In general, the leak impact factor (LIF) for a distribution sys- force (equipment impact, earthquake, etc.), avalanche crack fail-
tem can be scored in a manner similar to that described in ures, and with system shocks to graphitized cast iron pipes.
Chapter 7. Some key points of consequence assessment are In assessing potential hole sizes, the failure mechanism and
reiterated and some considerations specific to distribution sys- pipe material properties would ideally be considered. As noted,
tems are discussed below. a failure mechanism such as corrosion is characterized by a
As in the transmission model, both multiplication and addi- slow removal of metal and, hence, is generally prone to produc-
tion operations can be used to better represent real-world rela- ing pinhole-type leaks rather than large openings. Outside
tionships. For example, a top-level equation, forces, especially when cracking is precipitated, can cause
much larger openings. The final size of the opening is a func-
LIF = @roduct hazard) x (spill) x (dispersion) x (receptors + outage), tion of many factors including stress levels and material proper-
ties such as toughness. Because so many permutations of
captures the idea that the overall consequences are propor- factors are possible, hole sizes can be highly variable.
tional to the spill size and product hazard. If either variable is The risk reduction benefits of a leak detection and response
zer+no spill or no product hazard-then there are no conse- system can be captured in the spillscore. The ability to reliably
quences. It also shows that locations where both receptor dam- minimize the exposure time or area of exposure needs to be
age and losses due to service interruption (outage) are high are measured, at least in some general way, in order to score
the most consequential. As either or both of these are reduced, these aspects. The leak detectiodreaction capabilities can be
so too is overall consequence. assessed at all points along the pipeline and are a function of