Design of sacrificial anode cathodic protection (SACP) systems sometimes involves use of arrays of anodes mounted on one or more sleds with the sled connected to the structure by cable to provide a return path. Such a solution is frequently adopted for retrofit life-extension programmes of marine structures.
This type of design may allow for a lower installed system cost or may be desirable for other reasons. However two factors can limit the effectiveness of sled-mounted anode arrays. Firstly the IR drop along the return path cable can significantly reduce the effective driving potential. Secondly close proximity of anodes can reduce the output of the array to well below what would be expected if the anodes were well separated.
The effect of cable resistance can easily be estimated using known cable resistance and anticipated current. However the available analytical techniques which take into account effects of interference between multiple sacrificial anodes for calculation of the maximum theoretical output are limited to fairly simple groupings of anodes.
Since a typical anode sled design may include two or more layers of anodes with possibly several anodes (which typically tend to be long and thin) in each layer establishing the theoretical maximum output of such a sled is only practically possible through the use of mathematical modelling performed using numerical techniques. A range of numerical methodologies can be applied to simulation of galvanic effects and cathodic protection and of these it is the boundary element method that is applied in this work. The boundary element simulation requires an accurate model of the anode array which should be positioned appropriately relative to the seabed so that influence of different seawater/seabed resistivity can be taken into account. Although not normally necessary (since its effect is minor) a model of the structure of the sled itself may be created. The simulation takes into account the effects of return path cable resistance.
The main objective of this paper is to quantify interference effects between sled-mounted sacrificial anodes and thereby identify the overall output of a sled. Firstly however the paper takes results obtained from the analytical techniques most generally used within the industry (as applied to configurations for which those techniques have been corroborated) and compares these with results from modelling.
Simulation is then applied to typical anode sled layouts and results presented showing total sled output as well as output of individual anodes on the sled and the nature of the electric field around the sled. The way in which the electric field influences anode and hence sled output is discussed. A parameter study is used to investigate effects on sled output of design variables such as spacing between layers of anodes.