204 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION
and effort on the insertion, removal, and collection of
fi eld monitoring data and not enough time analyzing the
meaning of the data collected. Widely publicized oilfi eld
failures have been associated with systems where hun-
dreds of thousands of coupons had been collected, yet
unexpected leaks on major equipment still occurred.
The unfortunate consequences of reduced production
rates are that oil fi elds become more corrosive at the
same time that aging equipment and increased corrosiv-
ity require more corrosion control and monitoring.
Monitoring cannot replace inspection. The two pro-
cedures are complementary. Appropriate corrosion
monitoring can reduce the need for inspections and
indicate potential locations where problems are occur-
ring and additional inspections are warranted.
TESTING
Testing is used in two senses in oilfi eld applications.
Hydrostatic testing is commonly used to “ proof ” newly
constructed or altered equipment to insure that the
equipment will be safe to operate under the intended
temperature and pressure conditions. The other use of
the term is for relatively short - term laboratory or fi eld
trials to determine materials compatibility, the effec-
tiveness or corrosion inhibitors, and so on.
Hydrostatic Testing
Hydrostatic testing is required on pipelines and storage
tanks after construction and major repairs. Incomplete
removal of the hydrotest water after pipeline testing can
result in major corrosion problems. While it would be
desirable to use clean water for these tests, as a practical
matter, river water or seawater is often used. If this
water is not removed and the pipeline dried, then micro-
bial colonies can form and corrosion can be noted in a
matter of days. Treatment of the test water with bio-
cides can minimize this problem, but disposal of the
biocide - containing test water can become a problem.
Industry standards on the treatment and disposal of
hydrotest water are available.
41
Laboratory and Field Trial Testing
Laboratory and fi eld trial testing procedures have been
developed to provide short - term evaluation methods
for new or replacement materials or chemical treat-
ments prior to their adoption for fi eld use. The tests
tend to concentrate on potential weaknesses or vulner-
abilities of the materials being tested, for example, H
2
S
compatibility for metals and decompression resistance
of polymeric liners.
of biocide treatments.
40
The standard provides guidance
on effective sampling and culture procedures for both
planktonic, freely fl oating, and sessile, surface - attached,
bacteria. A number of commercially available fi eld
testing technologies are available to determine bacterial
populations and activity.
Planktonic bacteria, bacteria freely fl oating in the
liquid, are collected using sampling devices similar to
those shown in Figures 7.28 and 7.29 . It is important that
any liquid samples be contained in clean glass or plastic
containers. Samples should be analyzed as soon as pos-
sible and, if delays of more than 1 h are unavoidable,
the samples should be kept in air - tight glass containers.
Refrigeration of samples kept for more than 4 h is also
recommended.
Sessile bacteria grow in biofi lms on metal surfaces.
The standard discusses coupons for collecting these bio-
fi lms, often at the 6 o ’ clock position in oil and gas piping.
Sampling is recommended just prior to and after
biocide treatments. Bacteria can then be cultured and
assessed for responses to biocides.
Additional Comments on Monitoring
Additional monitoring methods such as the fi eld signa-
ture method for monitoring corrosion rates on subsea
pipelines and topside piping are available and discussed
in NACE and other publications.
40
The purpose of monitoring is to identify when
changes in corrosion rates are occurring and to correlate
these changes with corrosion control procedures.
4
No
monitoring method can identify actual corrosion rates.
Oilfi eld systems are too complicated, the sizes of samples
used to monitor corrosion are much smaller than the
surface areas of exposed equipment and piping, and
the metallurgical conditions of probe materials are dif-
ferent from the conditions on complicated structures
which have welds, stresses, and other complications not
replicated in monitoring samples. Once changes in cor-
rosion rates are noted, causes and corrective action
can be identifi ed, for example, the need for additional
corrosion inhibitors or changes in the corrosivity of
produced fl uids or injection water. Corrective actions
can then be taken if necessary.
Many organizations suffer from too much monitor-
ing. It is common to have chemical and inhibitor sup-
pliers responsible for the application of the appropriate
chemicals and also for the application and analysis of
monitoring coupons or probes used to determine the
effectiveness of these treatments. Aside from the
obvious confl icts of having suppliers monitor their
effectiveness, it is often the case that too many coupons
or other sources of data are collected. This has been
known to lead to organizations spending too much time
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