Civil Engineering Reference
In-Depth Information
The objective of the design calculations is to compute by iteration the num-
ber of anodes and anode dimensions to verify the following:
the initial current, final current, current capacity and current distribution
requirements of the structure are fulfilled
●
the lowest installed cost for the CP system, including cost of anode materi-
als, molds, installation and coating systems (if used), is achieved.
●
To ensure adequate and efficient use of the anode material, the submerged
structure should be divided into zones to be protected. Three zones are normally
classified:
Zone 1: This zone starts from mean sea level to just above the first horizon-
tal member below it. The depth of this zone is either the trough depth of a
100-year extreme wave or 20 m, whichever is larger. The calcareous coat-
ings of this zone are subjected to damage by storm waves.
●
Zone 2: The zone below zone 1 and above sea bottom.
●
Zone 3: The zone below the sea bottom mud line.
●
In zone 3, instead of calculating the surface area for well-casing strings
extending beyond the end of the conductor, an allowance of 2.5
3amperes
per well should be provided. This allowance is provided in addition to the sur-
face area contributions from other components, such as conductors and piles.
The external surface area of the buried part of piles and conductors is
included in the calculations only up to a depth of 30 m into the seabed.
The current drain to open skirt piles, sumps and caissons should cover an
internal surface area equivalent to 10 times
-
π
times their respective diameter.
The internal surfaces of all flooded structural members with unsealed flooding
holes are included in the surface area calculations.
6.7 ANODE MANUFACTURE
Anodes are manufactured in accordance with the required specification and the
supplier usually submits detailed anode and steelwork manufacturing drawings,
complete with tolerances, as well as manufacturing and testing procedures for
the client
s approval.
The stand-off supports are formed from single lengths of carbon steel seam-
less tube or bar, bent using purpose-built bending equipment of proven perfor-
mance to provide the minimum clearance specified between the centerline of
the anode and the nearest jacket member. The tubular supports have their
ends square to the anode axis and to each other within a tolerance of
'
2mm.
The bending method to be used for tubular inserts is proven to the purcha-
±
ser
s satisfaction to achieve consistent tolerances, with a maximum flattering
(ovality) of 10% of the tube outer diameter. All production bends are checked
for general dimensional tolerances and a minimum of 20% of bends are checked
for flattering tolerance.
'
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