The paper gives a brief general requirement for cathodic protection of steel structures in sea water. The main part of the paper deals with specific designs of cathodic protection installations on drilling rigs at present under construction for use in the North Sea and also for drilling rigs at present in use in the North Sea.
Sacrificial anodes cast from aluminium, zinc or magnesium are more active in the electro-motive series than steel. The immersed anodes when connected to the steel structure produce a protecting current that suppresses the natural corrosion currents which occur. This forces all the naturally occurring anodic areas to be cathodic. In doing so the anode is consumed and sacrifices itself.
Impressed current systems require an external D.C. electrical source. The structure is connected to the negative D.C. source terminal whilst the positive terminal is connected to the immersed anode which discharges the protecting current into the water electrolyte.
The general requirement of anodes is that they must polarise the steel to a point where the steel will not corrode (or corrode only at an acceptable rate) for an acceptable period of time at an acceptable cost. While so doing the cathodic protection system must not interfere with or adversely affect the structure or other facility. The installation must of course satisfy statutory regulations and codes.
Bare or coated steel structures in sea water may be protected with sacrificial aluminium or zinc anodes or impressed current anodes. Sub-sea pipelines which rest in the sea bed should be protected with impressed current systems and/or zinc anodes. The use of magnesium is restructed to high resistivity conditions such as fresh water where the high driving voltage of magnesium is required.
It must be noted that the composition of all cathodic protection anodes is significant in determining their performance. Because of the critical nature of the alloying elements and their dispersion within the material, rigid quality control is required during manufacture. In order to meet anode specifications high grade primary metal must be used.
Anode variables which affect degree of cathodic protection and influence design are:-
Material composition
Manufacturing method
Physical shape, especially length and related output.
Electrical contact via the anode insert.
Anode output
Anode capacity
Anode potential
External variables which affect degree of cathodic protection and influence design are:-
Area of steel requiring protection
Quality of any coating and its anticipated breakdown rate.
System life or the period of time for which protection is required.
Chemical composition of electrolyte
Temperature of electrolyte
Flow rate of electrolyte
Aeration of electrolyte
Location or situation of the anode relative to the steelwork.