Recent Advances in Offshore Cathodic Protection Monitoring
by Jim Britton, John P. LaFontaine and Grant T. Gibson (2000)
Offshore production facilities and pipelines are being installed in new, more hostile environments. Advances in Cathodic Protection Monitoring for new structure types in deepwater as well as for high temperatures are discussed. New portable ROV instrumentation as well as permanent monitoring of parameters affecting cathodic protection system performance are reviewed.
The factors driving the use and design of cathodic protection (CP) monitoring equipment have not changed since first use, however our understanding of the process of cathodic protection has. And so has information technology. Recently proposed new design approaches (1), have increased the importance of area-specific steady-state polarization data. More complex monitoring systems are being deployed to generate these data. Improvements in remote data transmission have revolutionized collection of and access to the information, thus making it more available to the right people. Today, structures in deep water are engineered to the highest standards, leaving little room for error in the corrosion protection systems. Corrosion resistant and high-strength materials used are often susceptible to damage if exposed to cathodic protection, magnifying the need for accurate, real time monitoring. Ongoing improvements in remotely operated vehicle (ROV) technology affects the way cathodic protection systems are being monitored and maintained, and how information is gathered and transmitted. Finally the need to understand how temperature and water velocity affect cathodic protection design is challenging the imagination of monitoring system designers.
Application of New Design Methodology
The slope parameter cathodic protection design method will no doubt replace or at least enhance the design of many sea-water immersed cathodic protection systems in the future. The method requires an understanding of the polarization curve of the construction material in a particular oceanographic environment. Variation of the curve shape with current density, sea-water flow rate, temperature and resistivity are not accurately known for all values measurable in sea-water. The best way to obtain these curves is to monitor actual installations and develop real polarization curves for a particular area. Thus the use of shunted current density sensors (Figure 1) and anode current monitors (Figure 2) is growing in popularity. The parameters measured from these instruments offer obvious benefits. First, the operator is provided with critical information on the monitored structure. Second, the system will yield information that will almost certainly allow a safe reduction on the size and cost of the next cathodic protection systems deployed in that particular area. Monitors can interface directly with using new cable-less diver and ROV tools. New cable-less designs reduce the cost of monitoring by 50%. Because continuous monitoring is not necessary to develop the polarization data needed, the sensors can be interrogated during already scheduled regulatory subsea inspections. Since their introduction in 1998, four systems have been installed offshore.