Page 276 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

254 CORROSION CONTROL AND PREVENTION

4.12 GAS DISTRIBUTION

The gas distribution pipeline sector is part of the oil and gas industry. The different
components of natural gas production, transmission, storage, and distribution sys-
tems are production wells, gathering lines within the production fields, processing
plants, transmission pipelines, compressor stations (periodically along the transmis-
sion pipelines), storage wells and associated gathering pipelines, metering stations
and city gate at distribution centers, distribution piping, and meters at residential or
industrial sites. A schematic representation of natural gas production transmission
and distribution is as follows:

Natural Gathering Distribution
gas → pipelines → Transmission → Processing → pipelines
wells pipelines plant

Several different materials have been used for main and service distribution piping.
Summary of miles of gas distribution main and number of services by material are
noted in Table 4.24.
A large percentage (57%) of mains and services (46%) is metal (steel, cast iron
or copper), and corrosion is a major issue. For distribution pipe, external corrosion
is of primary importance, although internal corrosion has been noted in some cases.
The methods of monitoring corrosion on cathodically protected pipe are similar to
those in the transmission pipeline sector, including pipe-to-soil potential and coating
surveys. One difference is that in distribution systems, leak detection is an acceptable
method of monitoring for these pipelines without CP (nearly 15% of the steel mains).
For gas distribution piping, corrosion mitigation is primarily sacrificial CP. Tech-
niques such as in-line inspection are typically not an option for the relatively complex
network of distribution mains and services, which makes integrity assessment of the
piping difficult if not impossible.
There are two different costs, namely, funding for corrosion control in maintaining
the existing piping system and the cost for replacing the infrastructure. The average
cost of main replacement (1993 dollars) ranged from $328/m in urban areas to $482/m
in developed areas. The average cost of a service replacement was $950 per service.

TABLE 4.24 Summary of Miles of Gas Distribution Main and Number of Services by
Material
Material Total Miles of Main by Material Total Number of Services

Steel 583,711 23,814,222
Cast iron 46,023 –
Copper – 1,497,638
Plastic PVC 21,526 1,198,017
Plastic polyethylene 439,907 27,308,110
Other 8,041 1,151,019
Total by size 1,099,208