Civil Engineering Reference
In-Depth Information
s relationship to the
seabed, lowest astronomical tide level (LAT), mean water level, and
maximum operational conditions
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Extent of use of, and application of, protective coating
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Availability of electrical power
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Proposed construction schedule
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Structure fabrication methods and fabrication on site
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Any weight limitations/constraints on the installed CP system
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Safety requirements
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Constraints and limitations on the installation and in-service maintenance
and monitoring of the CP system
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High-strength steels or other metals used in the structure that may be
subject to a reduction in mechanical properties when under cathodic
protection
II. Offshore site location data
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Water depth, oxygen content, velocity, turbulence, temperature,
resistivity, tidal effect and suspended soil
General arrangement drawings showing the structure
'
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Chemical composition of water
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Pollutants, depolarizing bacteria or marine borers present in the water or
seabed
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Geological nature of the seabed and the probability that scour will occur
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Adjacent facilities, including pipelines, and details of their CP system
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Susceptibility to stratification of the water and the resultant effect on
resistivity, temperature and oxygen content
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Performance history of previous or existing CP systems in the same
environment
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Protective current density requirements to achieve the applicable protection
criteria, obtained from site surveys or reliable documentary sources
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Susceptibility to adherent marine fouling, including type, rate of growth
and variation with water depth
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6.1.3 Choice of System Type
There are three types of cathodic protection system, each of which, when cor-
rectly designed, installed and operated, can effectively protect a fixed offshore
steel structure for its design life. They are:
Sacrificial: anodes cast from reactive metals, usually zinc or aluminum
alloys, as they are more electronegative than the structures requiring protec-
tion and require no external source of power
Impressed current: anodes manufactured from materials that are essentially
inert and powered by an external source of direct current
Hybrid: a mixture of sacrificial anodes and externally powered impressed
current anodes.
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