Page 247 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

11/224 Distribution Systems
the other grids of streets, sewers, electricity lines, phone lines, pipeline operations of all types. However, municipal distribu-
and other utilities. tion systems, both water and gas, usually have much more leak
This chapter offers ideas and guidance for risk assessments data available than hydrocarbon transmission systems. There
primarily for natural gas distribution systems and water trans- appears to be a readily identifiable reason for this difference, as
mission and distribution systems. Historically, operators of nat- discussed later in the pipeline integrity section ofthis chapter.
ural gas distribution systems have been more aggressive in A common complaint among most distribution system oper-
applying risk management practices, specifically addressing ators is the incompleteness of general system data relating to
repair-and-replace strategies for their more problematic com- material types, installation conditions, and general perfom-
ponents. These strategies incorporate many risk assessment ance history. This situation seems to be changing among all
and risk management issues, including the use of scoring mod- operators, most likely driven by the increased availability and
els for screening and risk assessments. Many of these concepts utility of computer systems to capture and maintain records as
will also generally apply to wastewater systems and any other well as the growing recognition of the value of such records.
pipeline operations in predominantly urban environments. Despite companies' increased data availability, it is difficult
to make meaningful correlations among all of the factors
Comparisons believed to play a significant role in accident frequency and
consequence. These factors, however, can be identified and
All pipeline systems share similar risk profiles. All are vulnera- considered in a somewhat qualitative sense, pending the acqui-
ble to varying degrees from external loadings including third- sition of more comprehensive data. For these reasons, and for
party damage, corrosion, fatigue, overstressing (often due to the benefits of consistency, an indexing approach for distribu-
high internal pressures), and human error. When the pipelines tion lines that parallels the basic pipeline risk analysis (trans-
are in similar environments (buried versus aboveground, urban mission pipelines) is recommended.
versus rural, etc.) and have common materials (steel, polyethyl- The primary differences, from a risk perspective, among
ene, etc.), the similarities become even more pronounced. pipeline systems include:
Similar risk mitigations techniques are commonly chosen to
address similar risks. Materials and components
There are often only a few areas of the risk assessment tech- Pressure/stress levels
nique that must be modified for either a distribution or trans- Pipe installation techniques
mission system. Unfortunately, safety data are limited for Leak tolerance.

n

Meter and Large industriai
regulator customer

District District
regulator regulator

; Low pressure
' 0.5 psig

I----- r----
*
___- __-__.
- 4
+-4 - - - - _j_ - - - -.
I Medium pressure
I 0.5-25 psig
Figure 11.1 Typical gas distribution system.