The system was carefully adjusted to avoid the overprotection concern during early stages; hence the total current output was restricted, yet the potentials at the lower elevations slowly polarised. Within a short period of energisation, protection levels were achieved on almost all, if not, the entire structure. It is deemed that over time as the potentials continue to improve, the current demand will be less and the output can be reduced.
If the surface area is taken as 80,000 m2 taking the 4200 A afforded to the structure, the current density value to achieve this is 52.5 mA/ m2. However, a couple of points to consider is that;
1. There are still some galvanic anodes that were operating prior to energisation and maybe providing some levels of current output, the total current required to maintain the structure could be higher.
2. The levels last measured showed that polarisation is ongoing and it is expected that the ICCP system current output will be reduced.
The galvanic CP system on a large offshore fixed structure in the North Sea lasted 30 years. However, once the CP system started degrading the speed at which the CP failed, escalated in a short period of time.
The most cost-effective method of extending the CP life of a structure of this magnitude is with a remote ICCP system.
The size of the ICCP system was optimised using data previously published for the North Sea.
The total current output of the ICCP system was limited due to the risk of overprotection. If the current output had been increased, the polarization levels may have been achieved much earlier and the final maintenance output could have been achieved, notwithstanding the inherent risks associated with that.
Polarisation is ongoing and could take another year, however a CP design based on a current density of 50 mA/m2 provides sufficient protection, half as given in the standards, and it is expected improvements will be made such that the final maintenance current may actually be reduced further.
In publishing this data, it is hoped operators can make better judgments as to whether to extend the life of their assets based on optimising a retrofit CP system.
The authors would like to point out, the data should be used for CP retrofit and not applied for CP designs of new structures, where higher initial currents in many cases will be required.
The authors of this paper would like to thank TAQA, whose UK business was incorporated in 2006 and is a wholly owned subsidiary of Abu Dhabi National Energy Company, (a diversified international energy group head-quartered in Abu Dhabi), for their faith and support throughout the entire process and for the permission to publish this paper.
Thanks are also due to Ronnie Toal, Graham Woodland, Andrew Wilson, David Milne and the entire project and construction team for their support on implementing the project.
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2. DNV RP B401 2010: “Cathodic Protection Design”
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