RP0176-2003
NACE International 11
5.4.2.1 Impressed current anode materials may
include lead-silver alloy, platinum over various
substrates, mixed metal oxides, lead-platinum,
graphite, or silicon-iron. These anodes should be
connected with an electrically insulated conductor,
either singly or in groups, to the positive terminal of
a direct current source such as a rectifier or gener-
ator. The structure to be protected should be con-
nected to the negative terminal of the direct cur-
rent source.
5.4.3 Combination of galvanic anode and impressed
current systems
5.4.3.1 A combination CP system provides pro-
tection during structure construction and/or during
times when the impressed current system is inop-
erative.
5.4.3.2 If the galvanic portion of the system is
smaller than a conventional long-term system, it
requires careful design to ensure adequate
amount and distribution of current.
5.5 Considerations That Influence Selection of the Type of
CP System
5.5.1 Availability of electrical power
5.5.2 Dependability of the overall system
5.5.2.1 Galvanic anode systems have shown
dependability for long-term protection.
5.5.2.2 Impressed current systems are capable of
providing long-term protection but are less tolerant
of design, installation, and maintenance short-
comings than galvanic anode (sacrificial) systems.
Good service can be expected if proper attention
is paid to mechanical strength, connections, cable
protection (particularly in the wave or splash zone),
choice of anode type, and integrity of power
source. Adequate system monitoring should be
provided.
5.5.2.3 Built-in redundancy, the use of appropriate
design margins, and provisions at the design
stage for thorough maintenance can enhance the
dependability of an impressed current system.
5.5.3 Total protective current required
5.5.4 Given the current required, the feasibility of pro-
tection with galvanic anodes and selection of anode
material can be established by consideration of the fol-
lowing (see Appendixes):
(a) Resistivity of the electrolyte,
(b) Anode-to-structure potential (when structure is at
protected level),
(c) Freedom from mechanical damage in installation,
launching, and operation,
(d) Useful life expectancy of anode material, and
(e) Structure weight and structural limitations.
5.5.5 The physical space available for impressed cur-
rent anodes placed on the ocean floor should be deter-
mined by the proximity of other structures and pipelines
and by future construction and maintenance consider-
ations.
5.5.6 The economics of capital investment, mainten-
ance, inspection, depreciation, taxes, and corrosion
losses should be considered over the entire expected
life of a CP system to assess the impact of the various
alternatives properly.
5.5.6.1 A galvanic anode system requires a rela-
tively large initial capital investment but little main-
tenance expense.
5.5.6.2 An impressed current anode system typic-
ally requires a lower initial capital investment with
power and higher maintenance expenses added
over the life of the system.
5.6 Factors Determining Total Current Requirements for
CP
5.6.1 Total submerged areas:
5.6.1.1 Water-submerged area.
5.6.1.2 Area below mudline.
5.6.1.3 On structures that may contain wells, the
actual anticipated number of conductors to be
installed.
5.6.1.4 Uninsulated and unprotected foreign
structures.
5.6.2 Anticipated minimum protective design (or main-
tenance) current density requirements of submerged
structure area.
5.6.2.1 Typical long-term average water zone cur-
rent densities for CP of open-ocean structures that
have been employed historically range from 55 to
430 mA/m
2
(5 to 40 mA/ft
2
). For values typical of
11 offshore production areas, see Appendix A.
Alternatively, based on service data from offshore
structures and laboratory results from test speci-
mens, Hartt and Lemieux
10
proposed that design
mean current density, i
m
, conform to the expres-
sion used in Equation (1):
1b
T10
i
d
)ca(
m
+
×
=
σ+
(1)
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