Two marine steel structures which are both largely uncoated except for the splash zone and
both fully submerged are described. Both are cathodically protected with Al/Zn/In sacrificial
anodes,. They are a Japanese structure installed in 1973 and based on the DNV
recommendations and a Chinese structure in 1996 according to the NACE recommendations.
The cathodic protection systems of these structures have been extensively monitored, anode
and structure potentials versus time, depth, anode output versus time, and these values are
compared with the values from the codes of practice to see how the expected and real
behaviour compare and contrast. It may be seen that the design criteria originally employed
provided insufficient initial current density according to the standards but that the steady state
design current density was very generous leading to a substantial over design of the
structures. The cathodic protection system for the older structure is shown to be at its end of
life in 2006 with the anodes almost fully consumed. Even so, the structure potential is still
within the recommended protection potential criterion of -800mV Ag/AgCl/sea water.
Key words: Steel, Offshore, Marine, fully submerged, aluminium sacrificial anodes, cathodic
protection, operating potentials, anode output, design life