Nace CP 1, Course Manual

Page: 332 / 535

RP0176-2003
10
NACE International
5.2.4 To provide adequate allowance for anticipated
changes in structure current requirements and anode
current delivery capability from the standpoints of ag-
ing, additions to the structure, and possible changes in
the environment.
5.2.5 To place all CP system components such that
the possibility of disturbance or damage is minimal.
5.3 Information Necessary or Useful for Offshore Structure
CP System Design
5.3.1 Structure construction specifications and prac-
tices:
(a) Construction drawings,
(b) Onshore construction site, offshore construction
practices, and probable construction dates,
(c) Number, size, and locations of riser pipes,
including electrical cables,
(d) Coatings,
(e) Availability of electrical power,
(f) Corrosion control measurement points (test
stations, electrical bond locations, etc.),
(g) Safety
requirements,
(h) Installation accessibility and availability of structure
equipment for possible use in system installation
and/or replacement,
(i) Anticipated life of structure (design life),
(j) Water
level,
(k) Pile guides removed after piling is completed or
included in the CP design, and
(l) Location of isolation flanges or standoffs.
5.3.2 Offshore site conditions:
(a) Water depth, oxygen content, velocity, turbulence,
temperature, water resistivity, tidal effects, silt, sand,
and suspended solids (see Appendix A),
(b) Dependability of future monitoring and main-
tenance, including cost factors,
(c) Adjacent facilities, including pipelines,
(d) Existing and proposed CP systems, and
(e) Electrical isolation from foreign pipelines or
structures.
5.3.3 Field survey data, corrosion test data, and oper-
ating experience:
(a) Protective current requirements to meet appli-
cable criterion or criteria (see Paragraph 5.6),
(b) Water resistivity (considering possible strati-
fication of water properties),
(c) Changes in construction specifications,
(d) Electrical
isolation and/or continuity,
(e) Interference currents (see Section 7),
(f) Performance history of previous or existing CP
systems in the same general environment,
(g) Maximum operating temperatures of pipeline
risers, and
(h) Other maintenance and operating data.
5.3.4 Complete field survey work prior to actual appli-
cation of CP is not always required if previous experi-
ence and test data are available for estimating current
requirements and system performance.
5.4 Types of CP Systems
5.4.1 Galvanic anode systems
5.4.1.1 Galvanic anodes may be alloys of such
active metals as magnesium, zinc, or aluminum.
These alloys can be obtained in a variety of
shapes and sizes to deliver protective current to a
specific offshore structure with optimum current
distribution. The method used to attach the an-
odes to the structure should be governed by their
type and application, but low-resistance electrical
contact must be maintained throughout the oper-
ating life of the anodes.
5.4.1.2 The performance of galvanic anodes in
seawater depends critically on the composition of
the alloy, particularly in the cases of zinc and alu-
minum (see Appendix B).
5.4.1.3 Magnesium anodes are very active and
furnish high current outputs. For this reason, they
can provide CP with a minimum number of an-
odes. Because of their properties, they usually are
not used for long-life designs.
5.4.1.4 For coated structures, the galvanic anode
material should be capable of supplying current
even after many years of very low anodic current
density.
5.4.2 Impressed current anode systems