previous three steps to assess the effectiveness of the DA
process and determine the necessary reassessment intervals.
There are six DA standards developed by National
Association of Corrosion Engineers (NACE) and they include:
)
)
2002 - NACE SP0502-2010 ECDA.
)
)
2004 - NACE SP0204-2008 SCC-DA (stress corrosion
cracking DA).
)
)
2006 - NACE SP0206-2006 DG-ICDA (dry gas internal
corrosion DA).
)
)
2008 - NACE SP0208-2008 LP-ICDA (liquid petroleum
internal corrosion DA).
)
)
2010 - NACE SP0110-2010 WG-ICDA (wet gas internal
corrosion DA).
)
)
2010 - NACE SP0210-2010 ECCDA (external corrosion
confirmatory DA).
DA is also covered in ASME B31.8S (Section 6.4). In the
US, DA is covered in US Code of Federal Regulation CFR 49
Part 192.923 (for natural gas pipelines) and 195.888 (for liquid
hazardous pipelines). It is now one of the three accepted
inspections (ILI and hydrotesting being the other two) allowed
for oil and gas pipelines.
Identifying pipeline anomalies using DA
When completing a DA inspection, there are three types of
anomalies that owners are aiming to identify:
)
)
EDCA and ECCDA.
)
)
ICDA.
)
)
SCCDA.
Due to the serious consequences of corrosion and leaks
in underground pipelines, ECDA, and ECCDA were developed
in an attempt to proactively prevent external corrosion and
ensure the integrity of oil and gas pipelines that are difficult
to pig.
ECDA is a continuous improvement process intended to
identify and address locations at which corrosion activity has
occurred, is occurring, or might occur. For instance, ECDA
identifies areas where coating defects have already formed,
and can ascertain where CP is insufficient and corrosion is
possible, before major repairs are required.
The success of any ECDA requires strong knowledge of the
soil/environment, pipeline material, coating, CP, and foreign/
interference current on the pipeline. Also, the accurate
selection of susceptible areas for external corrosion relies
on using at least two complementary advanced aboveground
inspection techniques. These aboveground indirect inspection
techniques may include: direct current voltage gradient
(DCVG) or alternating current voltage gradient (ACVG) surveys,
a CP close interval potential survey (CP CIPS), alternating
current-current attenuation (ACCA) and side drain (for bare
or ineffectively coated pipelines) surveys. Normally these
aboveground inspections are used in conjunction with pipe
locating.
The development of internal corrosion in pipelines is
partly because of its complex nature and interaction between
constituents that are found in transported gas and liquid products
(e.g. oxygen, carbon dioxide, hydrogen sulfide, chloride, bacteria,
etc.). When in the presence of water, these contaminants can lead
to conditions conducive to the occurrence of internal corrosion.
The susceptible locations for internal corrosion are usually
where liquids, solids and gas accumulate. In order to ensure that
susceptible locations along the pipeline are prevented from
internal corrosion, ICDA methodology is implemented.
ICDA methodology has been developed to verify pipeline
integrity, especially for pipelines that are not able to accept
ILI tools. ICDA includes WG-ICDA, DG-ICDA and LP-ICDA.
WG-ICDA is used in pipelines that assumes that water, or a
combination of water and hydrocarbons, can be present in
the pipeline. It is intended for onshore and offshore systems
where the liquid to gas ratio is small. It tends to identify
locations in the pipeline where corrosion is expected to be
severe. DG-ICDA is applicable to pipelines that transport gas
that is normally dry but may suffer infrequent upsets, which
may introduce water to the pipeline. LP-ICDA is employed to
assess the susceptibility of internal corrosion on pipelines that
transport incompressible liquid hydrocarbons that normally
contain less than 5% base sediment and water. The success of
any ICDA process is dependent on using an accurate corrosion
model to predict a precise elevation profile in order to
determine susceptible locations for internal corrosion.
DA technology has also proven successful in SCCDA,
offering pipeline operators a comprehensive pipeline integrity
management portfolio. SCCDA is a proactive structured
process that seeks to improve pipeline safety by assessing
and reducing the impact of stress corrosion cracking. It can
occur at neutral or high pH when susceptible pipeline material
is exposed to stress, specific susceptible temperature and a
corrosive environment.
The benefits of DA
DA is non-intrusive and inspections can be completed during
normal operation of the pipeline. DA is also a proactive
integrity management tool that can find anomalies before they
become critical defects, while traditional ILI tools are reactive
in that they identify existing pipeline damage.
While hydrotesting and ILI tools are an important part
of integrity management, the development of DA provides
pipeline owners with another solution to identify at-risk areas
of pipe before they become a major problem. A combined
integrity approach that employs DA can help pipeline owners
ensure containment and prevent costly, reputation-harming
pipe failures.
References
1.
Pipeline Research Committee, American Gas Association, NG-18 Report No. 111
(3 November 1980).
2.
NACE 35100, In-Line Inspection of Pipelines, NACE International, May 2012.
3.
NACE Training Course, Direct Assessment, NACE International, November 2012.
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World Pipelines
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MARCH 2016
