Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 434

From a practical point of view, the anodes used to provide cathodic protec-
tion of surfaces buried in sediments are located freely exposed to the sea.
To avoid interaction effects that reduce the current output, there should be
enough spacing between the anodes. As far as practical, anode locations should
be selected to cover the surface area of the steel structure but to be away from
structural members that may reduce the current output. Note that the closest
anode is problematic, especially in case of welding joints.
Shielding and interference effects become insignificant at a distance of
about 0.5 m or more, except for very large anodes. Our design tends to be con-
servative, so that we can consider two adjacent anodes as one long anode, or as
one wide anode, depending on their location in relation to each other, if it is
expected the anodes will interfere with each other.
It is important to avoid locating anodes near welding or in areas with high
fatigue loads. In addition, the location of anodes should take into account the
restrictions from fabrication, installation and operation.
Sequential priority for anode placement is based upon the following
considerations.
1. Anode distribution should normally begin with placement upon larger mem-
bers, such as legs near nodes, and continue to minor members.
2. Placement should consider future added components, which are usually new
conductors and risers as operations require.
3. Locations where attenuation effects will reduce good current distribution,
such as well conductors and complex pile guides, will require a higher
anode concentration.
4. Anodes are placed within a minimum distance of 150 mm of adjacent struc-
tural welds.
5. Anodes are placed within 100 mm of surfaces to be protected.
6. No anodes are to be located closer than 600 mm to tubular joints.
Proper anode placement should be performed as an iterative process, in
which initial design drafts are adjusted to an optimum configuration, using
the above guidelines. The placement process may result in conservative adjust-
ments to anode dimensions in addition to the calculated total number to ensure
adequate current distribution.
Anodes designed to protect the buried surface area are all installed on the
bottom elevation of the structure. Figures 6.7 and 6.8 present examples of the
distribution of anodes.
6.5 DESIGN EXAMPLE
Table 6.16 sets forth the steps of CP design for an offshore structure platform.
The table can be useful in a spreadsheet for the design of a CP system.