Page 248 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Risk modeling 111225
Distribution systems differ fundamentally from transmis- tribution systems has historically been more focused on
sion systems by having a much larger number of end-users or leakibreak history. Coupled with the inability to inspect many
consumers, requiring specific equipment to facilitate product portions of an urban distribution system, this makes data col-
delivery. This equipment includes branches, meters, pressure lection for leaks and breaks even more critical to those risk
reduction facilities, etc., along with associated piping, fittings, management programs.
and valves. Curb valves, curb cocks, or curb shutoffs are addi- Several sections of this chapter and pages 301-302 of
tional valves usually placed at the property line to shut off serv- Chapter 14 further discuss the application of leakibreak data to
ice to a building. A distribution, gas, or water main refers to a risk assessment and risk management. Table 14.13 shows an
piece of pipe that has branches, typically called service lines, example of predicting leakibreak probabilities based on
that deliver the product to the final end-user. A main, therefore, relative risk assessment results.
usually carries more product at higher pressure than a service
line. Where required a service regulator controls the pressure System losses
to the customer from the service line.
The operating environments of distribution systems are Unaccounted-for gas or systent losses are terms common to
often materially different from that of transmission systems. distribution operators. Normally expressed as a percentage of
Normally located in highly populated areas. distribution sys- total system throughput, the terms relate to imbalances in quan-
tems are generally operated at lower pressures. built from dif- tities ofproduct received into the system and quantities of prod-
ferent materials, installed under and among other infrastructure uct delivered out. In a modern, well-maintained system. system
components such as roadways, and transport less hazardous losses will be a small percentage of the total quantities moved
materials. (Although natural gas is a hazardous material due to and are often not real loss of product, rather, they are caused in
its flammability. distribution systems do not normally transport large part by the accuracy limitations of flow measurement
the high-pressure, more hazardous toxic and flammable materi- devices. This is not surprising when it is noted that the measure-
als that are often seen in transmission lines.) Many distribution ment accuracy is dependent on the accuracy of several instru-
systems are much older than transmission lines and, hence, ments, not just one. Gas flow measuring instruments include a
employ a myriad of design techniques and materials that were volumetric meter, a pressure meter, sometimes a Btu meter, and
popular during various time periods. They also generally possibly others.
require fewer pieces of large equipment such as compressors Other sources of unaccounted-for product include inten-
(although water distribution systems usually require some tional and unintentional releases. Intentional releases of
amount of pumping). Operationally, significant differences pipeline products are often necessary to perform a variety of
from transmission lines include monitoring (SCADA, leak maintenance and construction activities on a pipeline.
detection, etc.), right-of-way (ROW) control, and some aspects Unintentional releases is, of course, primarily due to pipeline
of corrosion control. leaks.
Because of the smaller pipe size and lower pressures, leak Although the system loss number depends on many factors
sizes are often not as big in distribution systems as they are in such as system age, complexity, and operation practices, it can
transmission systems; however. because of the environment provide the risk evaluator with a general sense of how “tight”
(e.g.. in towns, cities, etc.), the consequences of distribution the distribution system is. The operator’s understanding and use
pipe breaks can be quite severe. Also, the number of leaks seen of these numbers to reach improvement goals may also provide
in distribution systems is often higher. This higher frequency is insight into the company philosophy and professionalism and
due to a number of factors that will be discussed later in this in that respect. might be as important as the numbers them-
chapter. selves.
111. Risk modeling
II. System integrity
Risk management efforts
Pipeline system integrity is often defined differently for hydro-
carbon transmission versus distribution systems. In the former, As noted risk management approaches for distribution sys-
the system must not allow any leakage (beyond the micro- tems seem to have been focused on pipeline break forecast-
scopic, virtually undetectable amounts), so integrity normally ing. Emphases have been on support for “repair versus
means “leak free.” This intolerance of even the smallest leak is replace” decision making and on strategies and models that
due to potential consequences from leaks ofany size. Many dis- estimate budgetary requirements for system maintenance in
tribution systems, on the other hand, tolerate some amount of future periods. Some programs have been implemented as a
leakage-system integrity is considered compromised only result of dramatic increases in the number of breaks for a
when leakage becomes excessive. municipality.
The higher leak tolerance leads naturally to a greater inci- Studies are available that describe programs in many parts of
dence of leaks in a distribution system. These are often docu- the world including Australia. Canada. and Europe (Italy,
mented and monitored and placed on “to be repaired’ lists. France, Switzerland, United Kingdom) as well as many US.
Knowledge of leaks and breaks is often the main source of sys- cities. Consider these overall observations from a general
tem integrity knowledge. It, rather than inspection information, literature review:
is usually the first alert of issues of corrosion of steel, graphiti-
zation of cast iron, loss of joint integrity, and other signs of The pipe material cast iron features prominently in many
system deterioration. Consequently, risk modeling in urban dis- studies. both for reasons of its common use during certain