Cost Saving Offshore Cathodic Protection Retrofit Methods
by Jim Britton (2002)
Many offshore structures (largely platforms and pipelines) will require a cathodic protection retrofit at some point during their lifetime. This is due to the age limitations of the original cathodic protection anodes and operators' new desire to extend the life of existing offshore infrastructure. This paper explores the conventional methods of anode replacement and then compares them to the newer methods developed by Deepwater Corrosion Services and others. These new proprietary products and methodologies are designed to cut installation time, while still providing an excellent life extension for the asset of up to 25 years.
Many offshore structures and pipelines are reaching the end of the designed life of their original cathodic protection anode systems. The requirements of each life extension project will vary from adding one or two years through to adding over 20 years to the original design life of the equipment. The historical approach to cathodic protection retrofit has been to replace anodes on a one-for-one basis, using divers or ROV's to assist a crane in lowering clamp-on aluminum anodes down to the members. This approach is extremeely expensive and completely unnecessary. There is a tendency in the industry for asset owners to misinterpret the reasons why cathodic protection systems are designed differently for a retrofit than they are for new structures. While the designs can look very different, the driving force for both designs is the same: to satisfy installation requirements.
For example, a pipeline bracelet anode is designed to look the way it does to facilitate pre-installation (before pipe lay) on the pipeline; the shape of the anode is designed so that the pipe can be easily laid with the anodes in place. In truth, the bracelet anode is possibly the worst configuration that an anode could be (from the cathodic protection engineer's standpoint). The resistance is high, the utilization factor is low, the manufacturing cost is high and the "throwing power" is poor.
Another example is the conventional platform anode. Anodes are attached by welding extremely stout pipe cores to the structure. Why? To withstand launch forces and/or pile driving during installation. Again the cathodic protection design is predicated on the installation method. Is this the best way to install an anode on a large bare steel structure? Of course not; efficiency is reduced, the standoff distance is not optimized, and the cost of all those welds is significant.
When we are charged with designing an anode retrofit system, most of these constraints disappear, because the structure is already in place. As a result, when plannign a CP retrofit, a cathodic protection designer should not be limited in any way by the original design methodology.
The Cost of Retrofitting Anodes
When analyzing the cost of a retrofit project, the driver is always the same. Cost of installation always drives the project budget. Therefore, the design should focus on reduction of installation cost without sacrificing mechanical reliability. Some of the obvious ways in which this may be accomplished are:
1. Minimize the number of locations on the pipeline that have to be visited
2. Select areas where the depth of cover is minimal or the pipeline is exposed on the seabed
3. Minimize bottom time requirement at each location
4. On deeper projects, use ROV’s rather than saturation divers
5. Carefully evaluate and compare costs of 4-point moored systems vs. dynamically positioned equipment
6. Evaluate Impressed Current, Sacrificial Anode and Hybrid solutions at design phase
7. Have the flexibility to adjust the retrofit plan offshore based on survey results obtained as the installation progresses
1. Minimize the number of anode installations.
2. Minimize the amount of marine growth removal.
3. On deeper projects, use ROV’s rather than saturation divers. Use shallow diving support to accomplish high dexterity tasks such as marine growth removal, installation of splash-zone pull tubes etc.
4. Have the flexibility to adjust the retrofit plan offshore based on survey results obtained as the installation progresses.
5. Carefully plan topside rigging and set equipment prior to mobilization of the subsea installation spread.
6. Evaluate impressed current, sacrificial anode and hybrid solutions at design phase.
Designing Cathodic Protection Retrofits
Just as new construction cathodic protection designs are made to facilitate installation of the offshore platform or pipeline, the cathodic protection design criteria are designed to polarize a structure from native state potential, provide adequate redundancy in design to allow for some system damage during installation or for unknown environmental affects. In new construction there is little incentive to “over-optimize” if it entails any added risk. When considering a retrofit there are a number of major differences that should be reflected in the design criteria selection:
1. In all cases there will be some degree of polarization remaining, even if the structure has fallen below "protective potential criteria". In many cases the structure will still be adequately protected but will have heavily depleted anodes.
2. Design life requirement may be for only a few years, in which case it may not be necessary to optimize protective potential levels.
3. We have the benefit of being able to perform a survey to accurately define the condition, and to measure the existing polarization characteristics (current density vs. potential).
4. We have the advantage of being able to monitor both anode cathode response during the retrofit to verify design predictions.
So when designing a retrofit it is rarely if ever required to provide the same current density as one would for a new structure, and if existing maintenance current density can be demonstrated to be much lower than conventional wisdom would dictate, significant savings can be realized .
The Importance of Survey
The value of a well specified survey cannot be over estimated, this is true of both platforms and pipelines but particularly so with buried or partially buried pipelines. This is only true of a high resolution type survey , remote or semi-remote (trailing wire) type surveys provide little or no useful information. The most important information obtained from a detailed pipeline survey, in order of value, is:
Line location - Having an accurate position on the pipeline is essential, particularly if the line is buried. The hourly rate on the offshore equipment necessary to effect a pipeline retrofit is such that it is unacceptable to waste any time trying to locate the pipeline.
Line Depth of Cover - Knowing where the pipeline is exposed or has only minimal cover will save significant time and money. If a retrofit site is inadvertently selected where the pipeline is buried 2 meters deep, it could take divers many hours to excavate the pipeline, and then they would be forced to work in a deep hole where visibility would be essentially zero.
Knowing CP System Performance - By measuring the field gradients as well as potential, the resilience of the CP system can be estimated, as well as any areas of significant coating damage. Having an ROV fly the line there is always the chance of obtaining a visual inspection opportunity on one or more anodes, this can provide invaluable information to the cathodic protection designer.
Verification of Environmental Conditions - The survey will give a good indication of seabed conditions, current velocities etc., as well as giving accurate seawater and more importantly mud resistivity information.
Armed with this survey information, the designer can first select ideal sites for retrofit anode locations based on the depth of cover survey. Knowing the current density requirement and general coating condition facilitates accurate application of attenuation models to optimize spacing between retrofit sites. Knowledge of the mud resistivity allows accurate calculation of current outputs from various anode arrays.
On platforms it is the same story, using an intelligent survey approach , , will yield valuable information on cathodic protection system performance. Again, structure potential data alone do not tell the whole story. Estimation of anode depletion percentage is another area where mistakes are often made. Table 1. shows a dimension vs. volumetric relationship on a typical platform anode.