Nace CP 1, Руководство по эксплуатации

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RP0176-2003
NACE International
5
3.4.1.8 Well-casing conductors must be electric-
ally connected to the structure when the well-cas-
ing conductors are considered necessary to the
integrity of the well casings.
3.4.1.9 All steel to be protected must have elec-
trical continuity with the CP system (preferably by
welded contact). This electrical continuity should
be capable of lasting for the lifetime of the struc-
ture.
3.5 Submerged Zone—Internal Areas
3.5.1 Corrosion normally is negligible on the internal
surfaces of structural members or in compartments that
are sealed and have no contact with either the atmos-
phere or the seawater. Whenever possible, the design
should provide for sealed compartments.
3.5.2 During offshore structure launching and tilt-up
operations, some structural members are flooded and
remain flooded for the life of the structure. To prevent
internal corrosion, the flooding valves should be closed
after flooding, to isolate the flooded chambers from
contact with atmospheric oxygen. In compartments in
which circulation of seawater is possible, provisions
should be made to curtail internal corrosion. CP or a
combination of CP and coatings should be used.
Tanks and compartments with a gas phase should be
coated internally where condensation may occur and
CP is not effective. The use of impressed current sys-
tems in these compartments should be avoided due to
development of toxic and corrosive chlorine gas by the
anode.
3.5.2.1 Piping, valves, submerged pumps, and
other special equipment must also be protected;
particularly when different metallic materials are
used there is high risk of galvanic corrosion. This
is important, for example, when corrosion-resistant
alloy piping is routed through structural steel bulk-
heads.
3.5.3 In flooded compartments with a source of org-
anic nutrients, bacterial growth may generate organic
acids, carbon dioxide, and/or hydrogen sulfide (H
2
S)
that can cause corrosion. Bacteria-related corrosion
can be controlled by the use of internal CP, coatings,
and/or microbiocides. Caution: H
2
S is toxic, even at
low concentrations.
3.5.4 Pipelines are sometimes installed in pull tubes,
or “J” tubes. To restrict contact with seawater and at-
mospheric oxygen, the pull-tube annulus should be
sealed at the above-water end with a suitable nonwick-
ing packing after the lines have been pulled into posi-
tion.
3.5.5 Floating production units have mooring systems
intended for long-term service. Special considerations
are required for the corrosion protection of these moor-
ing lines.
3.5.5.1 Chains: Chains are protected by corro-
sion allowance. Corrosion allowance is added on
the diameter of the chain (bar), such that twice the
corrosion rate at the location must be used when
calculating the additional material. In the splash
zone a higher corrosion allowance is required.
Depending on chain handling, the wear could be
substantial. Many permanent floating production
units require minimal handling of the mooring sys-
tems. Instead of full corrosion allowance in the
splash zone, the splash zone chain segment can
be “turned” once during the lifetime; thus the sub-
merged part becomes the splash zone part and
vice versa after half the intended design life.
Experience has shown that CP on the structure is
effective approximately 30 to 60 m (100 to 200 ft)
out from the structure. The distance depends on
the connection of the chain to the structure, the
size of the chain, and the line tension. This CP
should not be included in the mooring line design,
but additional CP must be applied to prevent
underprotection of the steel fairleads and steel
structure.
3.5.5.2 Wire ropes: Wire ropes are normally
made from galvanized wires and have an outer ex-
truded corrosion protection jacket from polyure-
thane, polyethylene, or a similar material. In addi-
tion, grease is injected to prevent water from pene-
trating into the rope and ease movement between
the individual wires. Sacrificial zinc wires may be
included. Because steel wires have very high
strength, hydrogen effects should be evaluated as
part of any CP of the ropes.
It is important that the external jacket completely
cover the connection of the wire rope to the socket
end and prevent water from penetrating into the
wire rope.
3.5.5.3 For mooring lines and anchors, bacterial
corrosion in the parts exposed to bottom sedi-
ments must be evaluated and, if necessary, pro-
tection measures taken in the form of CP, coating,
or corrosion allowance.