Page 261 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

11/238 Distribution Systems
Design Locating processes-finding and marking buried utilities
prior to excavation activities-are important for any subsurface
In general, the potential for human error in the design phase can system, but perhaps especially so for distribution systems that
be evaluated as described in Chapter 6 with some additional often coexist with many other subsurface structures. These pro-
considerations as discussed below. cedures may warrant additional attention in this evaluation.
In addition to the previously noted definitions of failure, With the high activity level commonly seen around urban
other failure modes-such as overpressure of the customer’s distribution systems, the operating company usually devotes a
facilities, infiltration of contaminants, service interruption, and significant amount of resources to receiving notifications of
the failure of a gas odorization system--can be especially digging activities and then marking owned facilities and
important in a distribution system risk assessment. communicating with the notifying party. Whereas the same
Because facilities designed to operate at various pressures evaluation technique used in transmission lines can be used for
are interconnected in most distribution systems, special atten- distribution lines, the evaluator of a distribution system should
tion should be paid to prevention of overpressure. This may be alert to a heavy reliance on drawings and records to locate
include overpressure protection for systems downstream of lines, and the discipline of the line locating program in general.
the distribution pipes, if the evaluation considers such risks. Any history of line strikes (lines being struck by excavating
A common design practice in distribution systems is the instal- equipment) after locating was done should be investigated.
lation of redundant pressure control to protect downstream
components from overpressure. This is accomplished via an SCADA/comrnunications
internal fail-safe feature in one regulator or through the use of
two regulators (or both). Installed in series, the second regula- As a means of early problem detection and human error
tor is designed to control pressure should the first regulator fail. reduction, the effectiveness of a SCADA system or control
Detection of a failed primary pressure control should be part of center or communications protocols can be scored as shown
a regular maintenance program. It is often (but not always) the in Chapter 6, with additional considerations as discussed
responsibility of the distribution system to protect the customer below.
from overpressure. When this is the case, the evaluator should As a means of reducing human errors in transmission
examine the system capabilities and safety systems designed to pipelines, the use of SCADA systems and/or other systems of
prevent overpressure of downstream equipment. regular communications between field operations and a central
The practice ofodorization ofgas in distribution systems is a control is a suggested intervention point for human error reduc-
leak detection provision used to reduce the impact of a pipeline tion. The nature of distribution systems, however, does not nor-
failure or to alert individuals of faulty or leaking equipment. mally benefit to the same degree from this error avoidance
As such, it is covered mostly in the leakimpactfactor section of measure. By their design, distribution systems operate at lower
the risk model (see Chapter 7). pressures and are intended to respond constantly to changing
conditions as customers increase and decrease their product
Construction use. Point values for this variable should reflect the somewhat
reduced role of SCADA and communications as a risk reducer
Complete construction records ofthe distribution facilities are in distribution systems.
often unavailable due to the age of many systems, the con-
struction philosophies of the past, and record-keeping prac- Drug testing
tices. Evidence to score construction-related items might have
to he accumulated from information such as 1eaWfailure histo- Score this item as described in Chapter 6.
ries, visual inspections of the systems, and comparisons with
similar systems in other areas. As previously discussed, pro- Safety programs
tection of the pipeline from third-party damage is critical in
most distribution systems. When part of the damage preven- Score this item as detailed in Chapter 6.
tion program relies on accurate drawings and records, the eval-
uator should examine the error potential of the documentation Surveys/maps/records
program. This includes as-built construction documentation in
particular. The role of surveys, maps, and records as potential error reduc-
The potential for human error during the construction phase ers is discussed in Chapter 6. The evaluation suggested there
can be generally evaluated as detailed in Chapter 6. applies for distribution systems as well. As a special type of sur-
vey in gas distribution systems, leak surveys are usually a nor-
Operations mal undertaking and may warrant special attention in the
evaluation. as discussed next.
The evaluation of operations-phase human error warrants some
discussion specifically for distribution systems. This variable is Leaksurveys The first determination for the risk role of leak-
best examined in several parts as are described below. age surveys is whether they play a role mostly in terms of fail-
ure avoidance or consequence minimization. It can be argued
Procedures that a leak detection survey should be scored in the leak impact
factor because such a survey acts as a consequence-limiting
Score as described in Chapter 6, with additional considerations activity-the leak has already occurred and, under special cir-
as discussed below. cumstances, early detection would reduce the potential conse-