Nace CP 1, Course Manual

Page: 462 / 535

RP0200-2000
NACE International 5
5.4 Clearing of Shorted Casings: The point of metallic
contact between the carrier pipe and the casing (such as
contact with the metallic portions of end seals, isolating
spacers, bond wires or straps, test leads, debris, or the
casing itself) can often be removed. Established
construction techniques shall be used to realign the pipe
or casing to eliminate pipe metallic contact. Equipment
used in this situation includes hydraulic jacks, tripods, air
bags, side-boom slings, and belts. The pipe and casing
shall be maintained in this realigned position by the use
of supports such as compacted earth, sandbags, or
concrete piers placed under the pipe. In certain
situations, the elimination of a metallic contact may be
accomplished by removing a portion of the casing. Once metallic contact is eliminated, spacing materials,
end seals, vents, and test leads shall be reinstalled.
CAUTION: Engineering, metallurgical, and operational
concerns and regulatory requirements shall be
considered before moving the carrier pipe.
5.5 Filling of Casings: In certain situations, casings may
be filled with dielectric material or corrosion inhibitors in
an attempt to eliminate a corrosive environment. Casing-
filling procedures are provided in Appendix A.
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Section 6: Monitoring
6.1 If a casing is suspected of compromising the integrity
of the pipeline, the following methods may be used to
determine whether the casing is shorted to the carrier
pipe:
6.1.1 A recommended initial test for a shorted
casing is to compare the casing-to-electrolyte
potential with the pipe-to-electrolyte potential. A
shorted casing may exist if there is a small
differential or if there is no differential between the
pipe-to-electrolyte potential and casing-to-electrolyte
potential. If a test suggests that the casing may be
shorted, subsequent testing should be performed.
NOTE: If cathodic protection is applied to the
casing, the cathodic protection system normally must
be disconnected from the casing and allowed to
depolarize before any tests are conducted. The
presence of direct-connected galvanic anodes on the
casing during the test may negate the test results.
6.2 Monitoring: One or more of the following basic
electrical test methods should be used to monitor
pipelines and casings on an annual basis. Procedures
are provided in Appendix B.
6.2.1 Potential Survey: This method is the initial test
conducted to identify a shorted casing. A voltmeter
and a reference electrode are used to conduct the
test.
6.2.2 Internal Resistance: This method indicates
whether direct metal-to-metal contact exists between
a carrier pipe and the casing pipe by measuring
electrical resistance. The value is obtained using
standard corrosion field instrumentation. A battery
and a combination volt-ammeter or four-pin soil
resistance meter along with four test leads (two on
casing, two on carrier pipe) are used to conduct the
test.
This is an especially useful test immediately after
construction and before the carrier pipe is tied in. If
this test is performed after the carrier pipe is tied in,
the resistance of the parallel path through the soil
must be considered. A low resistance value
indicates a short, and that the pipe should be
removed or the short cleared.
6.2.3 Four-Wire IR Drop: This method may indicate
the existence and location of a short. A battery,
suitable test leads or probe bars, and a combination
volt-ammeter are used to conduct the test. A four-
pin resistance meter or megger to determine the as-
found resistance between the carrier and casing may
also be used as part of this test.
6.2.4 Cycling Rectifier: Cycling the cathodic
protection rectifier is a method used to evaluate the
electrical isolation between pipe and casing. For this
test, the nearest influencing rectifier(s) is interrupted.
An interrupter, voltmeter, and reference electrode are
used to conduct the test. If the pipe-to-electrolyte
potentials taken on the pipe and the casing are
identical during both the rectifier “on” and “off” cycles
(with the reference electrode at the same position), a
shorted casing is indicated.
6.2.5 Casing Depolarization: This technique verifies
isolation status by discharging a direct current (DC)
from the casing. A significant potential difference
occurs between the casing and carrier pipe if the two
structures are not in metallic contact. A temporary
cathode or ground electrode, variable DC power
source, and combination voltammeter are used to
conduct the test.
6.2.6 Direct Resistance Measurement: This
technique uses a four-pin resistance meter or
megger to determine the as-found resistance
between the carrier and casing.