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.
Submerged and uncoated steel marine structures regularly employ sacrificial anode cathodic protection for their corrosion control. To assist the cathodic protection designer in the task of determining what anodes to use, what size they should be and where they might be located. Having installed the specified number of anodes, the structure is put into service and typically an annual inspection will be carried out to assess the efficacy of the CP system. Occasionally a body of annual inspection data is gathered together in a single publication; the Marine Technology Directorate, MTD in the UK; (now sadly defunct), produced such a document [1]. The appendices of this document contains quantities of inspection data gathered in 1988 of the performance of some 58 platforms and offshore structures in the North Sea some after 20 years of operation, most between 15 and 10 years. To our knowledge this exercise has not been repeated. Great interest is currently being shown towards the end of the design life of the structure especially if the structure owner wishes to extend the useful life of the structure beyond the design life of the CP system. Questions arise such as “How much more life do I have in these anodes?” and “what do I need to do to get a further ten years from my structure?” Very occasionally, structures are installed with a carefully designed monitoring system and much more information can be obtained. In particular the real behaviour of the cp system can be compared with the anticipated behaviour and the relevant codes of practice can be assessed and if necessary updated. In this paper, two such structures are described, one is a berthing terminal in the north of Japan and installed in 1973 with a design life of 10y. The 16 year behaviour of this structure was published in 1989 in a conference proceeding in Kobe, Japan[3].
