Nace CP 1, Course Manual

Page: 339 / 535

RP0176-2003
NACE International
17
made. The complete connection should be coated
to prevent atmospheric corrosion.
6.6.1.3 Conductive connections to other pipelines
or across insulating joints should be installed in ac-
cordance with Paragraph 6.6.1.1. All bond con-
nections should be readily accessible for testing.
Cables should have welded connections. Care
should be taken to ensure that all parts of the
structure are in proper electrical contact with one
another or are equipped with separate anodes.
6.7 Other Considerations
6.7.1 Insulating devices
6.7.1.1 Use of insulating flanges in pipelines as-
sociated with structures is covered in Paragraph
5.10.1. If insulating flanges above water are inac-
cessible, extension test leads should be installed
in accordance with Paragraph 6.6.1.1 for test or
bonding purposes.
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Section 7: Control of Interference Currents
7.1 Introduction
7.1.1 The purpose of this section is to describe the
possible detrimental effects of interference currents,
sometimes called stray currents, and to recommend
practices for their control.
7.1.2 Except during certain types of welding opera-
tions, the possibility of interference current corrosion on
offshore structures is minimal.
7.2 Effects
7.2.1 Corrosion occurs in areas at which the interfer-
ence current leaves the affected structure and enters
the electrolyte as it returns to its source.
7.2.2 The degree of damage is directly related to:
(a) the quantity of interference current and the current
density,
(b) the area of the affected structure where the current
leaves to enter the electrolyte, and
(c) the period of time when this effect occurs.
7.3 Short-Term Interference
7.3.1 The most common serious cause of short-term
interference to a structure is welding operations when
the welding machine is mounted on a barge or vessel
with the machine’s negative terminal grounded to the
vessel and with the electrodes positive. Some oper-
ators have reported serious damage to structures with
complete penetration of the submerged member as a
result of welding current interference. The following
measures should be implemented to prevent interfer-
ence current during welding operations from a barge
moored alongside during construction: (a) All welding
machines should be electrically isolated from the
barge, and (b) the grounding cable from each machine
should be connected to the structure. Alternatively and
preferably from a corrosion standpoint, the barge oper-
ator should place the welding machines on the struc-
ture.
7.3.2 When negative welding leads are bonded to the
structure, current return is ostensibly through negative
leads or cables connected to the structure, but is also
to some degree through the parallel water path from
structure to barge hull if the precautions of Paragraph
7.3.1 are not followed. Current return through the
water can be reduced by decreasing the resistance of
the negative cables, by increasing their number and/or
diameter, and by decreasing their length. Negative
cables should be connected to the structure at a point
as close to the barge as possible.
7.4 Long-Term Interference
7.4.1 There are normally no long-term interference
sources that would cause significant corrosion on a
structure, if all components of the structure are properly
bonded.
7.4.1.1 The submerged, bare structure area is
usually very large with respect to exposed pipeline
areas; consequently, pipeline protection currents
have a negligible interference effect on the struc-
ture.
7.4.1.2 The location of interference current sour-
ces and the physical arrangement of a protected
structure are usually such that there is no ten-
dency for significant interference to exist.
7.4.2 In certain cases, interference may be produced
on a foreign pipeline when it is electrically isolated from
a protected structure, because of the structure’s CP
system. Such interference can readily be controlled by
the use of a current drainage bond from the pipeline to
the structure, by installing CP on the isolated pipeline,
or by upgrading the existing CP system on the pipeline.
These mitigation procedures should be performed in
cooperation with other companies or operators, when
applicable.