potential shift changes the environment around the metal
and inhibits the corrosion reaction as the steel structure
is
polarised. With sufficient polarisation, the
corrosion rate
for the metallic pipeline is reduced to virtually zero and
the corrosion risk is eliminated.
Though the vast majority of pipelines are carbon
steel and the technology is applicable for other metallic
pipelines, for the purpose of this article, metallic and
carbon steel are used interchangeably. Since pipeline
operators utilise coatings and CP to prevent their carbon
steel pipeline from corrosion damage that can adversely
affect the integrity of their assets, they should be very
interested in assessing the condition of the coating and
the effectiveness of the CP system. One of the most
effective tools to assess the effectiveness of a pipeline’s
CP system and the condition of the pipeline’s coating
system is the CIPS survey.
What does a CIPS survey do?
The goal of any CIPS survey is to measure the electrical
potential of the steel pipeline along the length of the
pipeline area being surveyed. As noted earlier, the goal of
CP is to polarise those exposed areas along the pipeline
that are not fully isolated from the environment by the
coating system. If the coating system for a 10 km long,
12 in. dia. pipeline were 99% effective and only had 1% of
holidays, there would be almost 96 m
2
of exposed steel
along the length of pipeline. Each of these coating defects
or holidays along the pipeline are potential corrosion
sites unless the CP system is properly polarising the
pipeline along its entire length.
While there are a variety of accepted criteria for what
constitutes proper polarisation, the two most widely
accepted are those detailed in NACE specifications –
the criteria descriptions are simplified and apply to the
majority of buried and submerged pipelines in typical
ambient temperature environments, where the criteria may
need to be adjusted for elevated operating temperatures
and especially corrosive chemical environments.
The first is the -850 MV polarised potential criteria.
NACE deems any pipeline that has a negative polarised
potential of -850 MV to meet the criteria for effective
CP. The second is to demonstrate that there is at least a
-100 MV shift of the polarised structure from its native
or depolarised potential. The data collected in the
CIPS survey allows the pipeline operator to assess the
polarised potential of the pipeline to ensure that it meets
criteria and that the CP system is working effectively in
conjunction with the coating system to prevent corrosion.
The CIPS survey can be conducted on buried or
submerged pipelines. At regular intervals along the
pipeline – typically every 2.5 ft (76 cm) – measurements
are taken of the potential or voltage difference between
the pipeline, and a reference electrode in contact with
the soil or water. That is a technician with the two poles.
Every step along the ROW one of the poles makes contact
with the ground, and the technician pushes a button that
submits data collected by the data logger in a backpack.
The data collected during a close interval survey can be
used to establish a baseline for CP on a new pipeline,
or assess the performance and operation of existing
underground pipeline CP systems. CIPS can also identify
areas with insufficient CP and may detect large coating
defects or holidays, however small defects may not be
detected.
Keys to a successful CIPS survey
Selecting a qualified survey crew
Whether the CIPS survey is being performed by the
pipeline operator’s personnel or by a supplier that
specialises in these surveys, the qualifications and
experience of the survey crew are critical to collecting
accurate data. When contracting the CIPS survey work
key considerations should include: safety record, makeup
of crew, whether crew members are full time employees
or subcontractors, the type of equipment they have,
engineering and IT support for data analysis and reporting,
and turnaround time for reporting.
Many operators have started to specify that at
least the crew leader must have NACE certification,
that the crews must be full time employees and not
subcontractors, and that a NACE CP or corrosion
specialist must review and stamp the final report. One
additional note on contracting CIPS surveys – some
survey companies compensate the field crew based on
miles per day. This model has led to data being collected
quickly, but often not correctly as the crew rushes down
the ROW. Many operators prefer to pay a fixed crew day
rate to avoid the danger of pay-per-mile incentives.
Advanced planning
Before any survey, it is important to properly plan.
Typical information required in advance of starting a CIPS
survey includes historical survey data, rectifier output
information, maps and alignment sheets detailing the
pipeline, surrounding terrain and structures. Also helpful
is information about any remediation done in the area,
such as construction activity, additional pipelines or
new power lines. Noting any changes or upgrades to the
Figure 2.
CIPS crew gathers potential data along pipeline ROW.
34
World Pipelines
/
FEBRUARY 2016
