Cathodic Protection Retrofit Options Comparison of Current Distribution 1.0 Introduction There are three basic methods available for the deployment of anode systems when considering an offshore cathodic protection life extension. These are: Close Fitted Anode Systems Semi-Remote Anode Systems Remote Anode Systems The selection between these three options will normally be determined by the size of the cathodic protection system and the met-ocean / geo-technical conditions at the platform location. 2,0 Methods 2.1 Close Fitted Systems This type of system is usually fitted as the original CP method, the reason for this being the ability to pre-install the system prior to installation of the platform. The anodes are welded directly to the structure on heavy pipe cores, this is to allow the anode system to survive the launch and subsequent piling forces associated with the installation. Early practitioners naturally considered anode replacement that would mirror this initial concept, thus the clamp on anode systems were designed. In an attempt to reduce the number of subsea attachments, anodes were fabricated into dual clamp on arrays. Typical Dual Clamp-On Anode System Installed by Diver Deepwater Corrosion Services Inc. 10851 Train Court, Houston, TX. 77041 U.S.A.
The close fitted anodes do indeed provide a reliable solution, although current distribution is not optimized. Each anode array is only capable of delivering a small amount of current and thus the number of installations required is still quite high. This translates to a more costly installation with a number of potential safety concerns. The system has to be installed by divers, and this can present a problem as depths increase. Current distribution produces an uneven potential profile with areas in the close vicinity of the anodes being the most negative. Potential Distribution Gradients Close Fitted Anodes
2.2 Semi-Remote Systems It was recognized in the mid 90 s that a retrofitted anode system did not need to be close fitted to the structure, the launch and piling forces were no longer in play. It was further apparent that the current distribution could be improved if the anodes were situated at a distance of 2-3 meters away from the platform. This meant that anodes could be located on the seabed in larger arrays, and connected back to the platform with heavy connection cables and clamps. Thus several advantages were realized: More current could be made available from each array Less connections were required to the structure Installation was much quicker and safer ROV or Diver could accomplish the task Anode Pods Offshore Onboard Installation Vessel The location of the anodes on the seabed requires that the bottom stability be verified for local met-ocean and seabed soil conditions. From the current distribution perspective the bottom location in the higher resistivity colder water sets the anodes on the right side of the resistivity gradient, thus it is quite easy for the system to send current upwards to protect the shallower areas of the platform as shown:
Comparison of Offshore CP Retrofit Methods Potential Distribution from Bottom Installed Anode Pods The semi-remote location of the pods produces a slight potential gradient up the structure, however the locally high levels of the close fitted system are avoided.
2..3 Remote Systems Remote anode systems will usually be of the impressed current type. Sacrificial anode systems do not lend themselves to this type of deployment based on the excessive voltage losses that would be seen in the tieback cable systems. The ability to locate an anode system at a remote location allows for very large amounts of CP to be delivered from a single installation, impressed current systems can take full advantage of this. The benefits are: Reduced number of subsea installations Diver or ROV installation Lower cost 500A Remote Buoyant Anode Sled on Seabed The remote anode location allows current to flow evenly to all areas of the platform. This type of installation is limited to structures with available topside power.
Remote Anode Produces Even Current Distribution 3.0 Summary Cathodic protection retrofit systems are more efficient if installed remotely or semiremotely. If power is available on the structure a remote impressed current system is usually the most cost effective alternative.