Page 271 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION OF UNDERGROUND GAS AND LIQUID TRANSMISSION PIPELINES 249

Impressed-current CP systems are extensively used in transmission pipelines.
Impressed-current systems can be readily adjusted to compensate for changes in the
amount of current required to adequately protect the structures; however, they may
also contribute to the interference of other structures in the vicinity.
Depending on soil type pipe coating properties and pipe size, impressed-current
CP systems can be used to protect long pipelines. However, impressed-current CP
systems require more expensive installation and equipment, increased monthly
monitoring, and greater power consumption charges than that of sacrificial anode
systems. It is estimated that there are between 48,000 and 97,000 CP rectifiers
in operation at present:, the corresponding pipe length of 778,900 km (484,000
miles) with rectifiers every 8–16 km (5–10 miles) with an average installation cost
of $12,000 per rectifier/ground bed. The total investment in impressed-current CP
system is between $0.6 billion and $1.2 billion. It is estimated that the annual
investment by pipeline companies in impressed-current CP systems consisting of
new installations and replacement of existing system is $40 million.
Sacrificial anode systems are widely used to protect gas distribution pipelines
but are applied as a remedial measure for problem areas on transmission pipelines.
Sacrificial anodes are relatively inexpensive, do not require external power supply
and require no regular monitoring of the anodes because of their low driving volt-
ages, sacrificial anodes are not applicable in all environments and do not have the
power to protect long pipelines. Sacrificial anodes are often used to complement
impressed-current CP systems by providing protection to local areas where addi-
tional protection is required because of inadequate coating quality. It is estimated
that $30 million of sacrificial anode material (zinc and magnesium) are purchased
by the pipeline industry each year. Assuming 30% of the sales is for transmission
pipelines, the annual cost is $9 million. The major portion of $21 million may be
assigned to distribution pipelines.
Internal pipeline corrosion can be mitigated through various measures such as
dewatering, inhibition, cleaning (pigging), and internal pipeline coatings. Dewatering
consists of removal of corrosive fluids prior to their entry into the pipeline. Dewater-
ing components are generally located at pipeline compressor and pump stations. In
other cases, specific low points are selected along the pipeline right-of-way for the
installation of “drips” that allow the collection of corrosive fluids periodically and
removed from the line to prevent corrosion of the pipe downstream from the site.
Another method of mitigating internal corrosion of pipelines is by periodic clean-
ing of the line. This is known as “pigging” and consists of cleaning and scraping
pigs. Pigging involves insertion of different “pigs” into the line and propelling it
through the line with gas or another product. As the pig passes through the line, it
pushes and/or scrapes fluids, waxes, and debris from the line. These cleaning oper-
ations can also make use of various cleaning media, including solvents, biocides,
acids, and detergents to aid in cleaning effectiveness. Corrosion is reduced by the
elimination of the corrosive environment from the line. Costs of pigging process
include the cost of preparing the line for pigging (installation of pig launchers and