As offshore structures around the world are aging and in many cases reaching the end of their useful lives, operators are looking for ways to reduce costs on subsea maintenance without increasing the risk of failure. This paper presents three case histories of platform and pipeline retrofits where innovation in design and installation methods resulted in successful cathodic protection (CP) retrofits at significant cost reductions over conventional methods. Several new concepts are presented.
Many offshore structures, platforms and pipelines, will require a cathodic protection retrofit in the next several years. The required life extension will vary from one or two years through adding over 20 years to the original design life of the equipment. The historical approach to retrofit has been to replace anodes on a one for one basis, but this approach is very costly and unnecessarily dangerous. There is a tendency for the industry to misinterpret the reasons why cathodic protection systems for new structures are designed the way they are: they are invariably designed to satisfy installation requirements.
For example, a pipeline bracelet anode is designed to look the way it does to facilitate pre-installation of the cathodic protection system on the pipeline. The shape of the anode allows for the pipe to be easily laid with the anodes already in place. In truth, from the CP engineer's standpoint (trying to maximize efficient use of the sacrificial alloy), the design of the bracelet anode is possibly the worst that it could be. The resistance is high, the utilization factor is low, the manufacturing cost is high and the "throwing power" is poor. But bracelet anode shapes do not inhibit the installation procedure, and in that way, the bracelet is quite suited.
Another example is the conventional platform anode; they 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 that will be used. Is this the best way to install an anode on a large bare steel structure, to maximize the amount of cathodic protection delivered per anode? .. Of course not; utilization is reduced, the standoff distance is not optimized and the cost of all those welds is very significant. When one is charged with designing a retrofit cathodic protection system, as the structure is already in place most of these original constraints are not relevant. Thus, one should not be constrained in any way by the original design methodology when designing the retrofit.
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 for anode attachment 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 during the 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 during the design phase.
Just as cathodic protection designs for new construction are made to facilitate installation of the asset, the cathodic protection design criteria for retrofit projects are designed to essentially to polarize a structure from native state potential and to provide adequate redundancy in design to allow for some system damage during installation (or for unknown environmental affects). As installation cost of the CP system itself is negligible for new construction, there is little incentive to "over-optimize" the design, if it entails any added risk. However, 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 from the original CP system, 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 (i.e. not protected for long).
2. Asset life extension requirements may only be a few years, in which case it may not be necessary to optimize protective potential levels in the design.
3. We have the benefit of being able to perform a survey, to accurately define the condition of the cathodic protection system, and to measure the existing polarization characteristics (current density vs. potential).
4. We have the advantage of being able to monitor both anode and cathode response during the retrofit to verify design predictions.
As a result of these differences, it is rarely (if ever) required when designing a CP retrofit 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 is prudent, significant savings can still be realized .
The value of a well conceived and executed survey cannot be over estimated. This is true of both platforms and pipelines but particularly so with buried or partially buried pipelines. Concerning pipeline survey: high resolution type surveys  are highly recommended, while remote or semi-remote (trailing wire or tow fish) type surveys provide little or no useful information. The most important data obtained from a detailed pipeline survey, in order of value, are:
1. Line location - To efficiently install a retrofit cathodic protection system, having an accurate position on the pipeline is essential (though not by any means given), particularly if the line is buried. The hourly rate for the offshore equipment necessary to effect a pipeline retrofit makes it quite unacceptable to waste any time trying to locate the pipeline during the installation process itself.
2. Line depth of cover - Knowing where the pipeline is exposed or has only minimal cover will save significant amounts of time attaching the retrofit anodes to the pipe. If a retrofit site where the pipeline is buried 2 m deep is selected during the design, it could take divers many hours to excavate the pipeline before being able to attach the new anodes. Once excavated, they would be further hindered working in a deep hole, where visibility and maneuverability could be extremely limited.
3. Knowing cathodic protection system performance - By measuring the field gradients as well as potential, the resilience of the cathodic protection system can be estimated. CP designers can also take into account areas of significant coating damage. If an ROV can fly the line, there is always the chance of obtaining a visual inspection of one or more anodes. Combined with accurate potential data, actually seeing the state of the current anodes can provide invaluable information to the CP designer.
4. Verification of environmental conditions - The survey will give a good indication of seabed conditions, current velocities etc., as well as giving accurate sea-water and more importantly, mud resistivity information.
Armed with this survey information, the cathodic protection 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 modeling 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 a cathodic protection system's performance. Again, structure potential data alone do not tell the whole story. Estimating anode depletion percentage is another area where mistakes are often made. Figure 1 shows a dimension vs. volumetric relationship on a typical platform anode. As can be seen, the first 12 mm of cross sectional reduction equates to 12.6% loss of metal. Near the end of an anode's life the same reduction represents only 7.1% volumetric loss. Thus it is important to take accurate measurements on a few cleaned (water blasted) anodes to get an accurate status on remaining anode material. The benefits of an intelligent platform inspection are, in order of value:
1. Polarization data - Knowing the existing maintenance current density on a structure gives the CP designer a precise benchmark from which to work. This will always result in a lower (but still safe) current density target for the retrofit anode design. This saves time and money without adding risk.
2. Anode performance - Knowing the current output range of the existing anodes and their average degree of consumption will allow a more precise prediction of the remaining life of the system. With this information, an operator could responsibly defer a retrofit for one or more seasons, again with no risk.
3. General platform condition - A typical survey will also include an evaluation of the seabed conditions, as well as information about silt / scour and seabed debris. These are invaluable data if a seabed pod or sled approach is considered, or if access to mud-line framing is required. Extent and thickness of marine growth will affect structural attachments. Verification of the type and location of the original anodes may prove useful if they still output significantly enough to be used to support new retrofit anodes. Ascertaining existing corrosion damage is also incredibly important, as some heavily corroded structures may not be candidates for certain kinds of retrofit.
Figure 1. Anode Consumption vs. Cross Section Loss
To illustrate more specifically some of the points mentioned in the above paragraphs, three case histories will be discussed. These projects were completed in the Gulf of Mexico in 2000, and while not all of the procedures follow the ideals outlined, the application of the basic principles is apparent, as are the documented performance data and cost savings.
Case History No. 1 - 12" Oil Pipeline
The Asset - This pipeline was installed in 1973 and runs from an offshore platform in 76 m (250 feet) of water to an onshore terminal 90 km (56 miles) distant. From the platform to about kilometer 24 (mile 15) the line is bottom laid, although the location of the line near the Mississippi River delta has resulted in much of the pipeline silting over. The next 32 km (20 miles) of pipe was laid in water that was less than 125 m (200 feet) deep and was buried to a depth of 1.5 m (5 feet). The final 34 km (21 miles) are laid through wetland (swamp) in a pipe canal and buried a minimum of 2 m (6 feet). The offshore section (56 km / 35 miles) of the line was originally protected with zinc anode bracelets; the wetland section was originally protected with a combination of impressed current CP and zinc bracelets. Precise records of where the bracelets started were not available. The pipeline has a 25mm (1") thick concrete weight coat.
Survey - In 1997 the offshore section of the pipeline was surveyed using a three electrode system . The survey results showed that the pipeline was still at protected potentials but that the anode bracelets were heavily depleted. The survey also showed the depth of cover on the pipeline ranged from exposed to > 3 m (10 feet). An accurate as-built plot was generated, which showed the actual position of the pipeline to be as much as 30 m (100 feet) off from the original "as built" survey.
Anode retrofit - In 2000 the pipeline owner decided to retrofit cathodic protection for the offshore section of the pipeline (56 km / 35 miles). The desired life extension for the asset was 10 years. Review of the riser potential showed a very small decline in protected potential from the survey conducted 3 years before.
The retrofit design utilized pairs of sleds located either side of the pipeline as shown in Fig. 2. Attenuation models were used to set the maximum spacing between anode sled installations; this resulted in a value of 3650 m (12,000 feet). Points were selected where the depth of cover was minimal or the pipe was exposed, and a total of 28 sleds were proposed for 14 sites along the pipeline. It was apparent that the majority of time on bottom would be taken up exposing the line and removing concrete (to obtain sufficient electrical connection with the pipe). The asset owner decided to use a continuity clamp (RetroClamp) designed by Deepwater Corrosion, that could be installed without having to expose 360 degrees of the pipeline or having to remove a large area of concrete weight coating.
Figure 2. Typical Pipeline Anode Sled Arrangement
The clamp system used Figure 3. allowed for only a small circle of concrete (100 mm / 4 inches in diameter) to be removed on the top of the pipeline, and required only 180 degrees of the pipeline to be exposed. It was also necessary that the clamp would pull off the line if snagged and that continuity could be assured over a wide range of temperatures without inducing a stress raising point on the pipeline. The clamp shown is a constant tension device with a hollow ground soft contact-tip that will indeed separate from the pipe if snagged, without damaging the pipeline itself.
Figure 3. Constant Tension Retrofit Clamp for Weight Coated Pipelines (remodeled RetroClamp™ shown right)
We also wanted to be able to verify system performance so we included a current measuring facility on the sleds (as shown in Figure 4).
Figure 4. Pipeline Anode Sleds with Current Monitoring Stab Shown (Inset)
This simple device used the tieback drain cables as shunts and measured the IR drop in the leads. The "shunt" was read through a diver held CP probe with dual readouts (Fig. 5) stabbed into the stab rails, and current and potential are displayed simultaneously on the readout.
Figure 5. Dual Readout CP Probe used for Potential and Current Measurement (CP Gun™ remodeled in 2007 shown right)
The results obtained as the retrofit progressed are documented in Figure. 6 and the project cost is detailed in Figure. 7. Plans are now underway to address the swamp portion of the pipeline.
Figure 6. Results Obtained During Pipeline Retrofit. All Potentials (-) Volts vs. Ag/AgCl sw.
All Currents are Amperes Dual Readout CP Probe used for Potential and Current Measurement.
Clamp Potential Pre-
|Pipe Potential Post Anode Attachment
Figure 7. Cost of Pipeline Retrofit (US Dollars)
|Item Description||Quantity||Unit Cost||Total Cost|
|Materials ( 2 Anode Sleds, Clamp &
Cables) Per Site.
|Engineering Fee||Lump Sum||Lump Sum||$ 5,000.00|
|4-Point Vessel Spread with Saturation
Diving Crew (Includes Mob — Demob)
|Total Installed Cost||$299,700.00|
Case History No. 2 - 90 M (300 feet) 8-Pile Drilling Production Platform
The Asset - This platform was installed in the late 1960's and was originally fitted with a impressed current system. At some point in time it was retrofitted with clamp-on aluminum anode pairs, but not before having sustained serious corrosion damage. Many of the weld areas had perforations and the entire structure was heavily pitted. The sacrificial retrofit was depleted and the platform was depolarized into the (-)0.780 to (-)0.820 V vs. Ag/AgCl sw. range. Based on the structure arrangement (Figure 8) with a congested center conductor bay, the high retrofit current required (estimated to be > 1000 Amperes), and the badly deteriorated condition of the structure, it was decided to use a hybrid retrofit approach. The desired asset life extension was 15 years.
Figure 8. Platform for Case History No. 2.
A saturation diving spread was contracted, with a shallow air capability also to handle the galvanic anodes and I-Tube installations. This proved to be cost effective because the deep work required was minimal and only a few diver rotations were required.
Anode retrofit - The majority of the current was to be provided by four semi-remote seabed deployed buoyant anode sleds deployed off either side of the jacket at a distance of 15 m (50 feet) off the structure and the same distance from any of the pipelines associated with the structure. Each sled (Fig. 9) is rated at 250 Amperes. The remainder of the current is provided by 16 x 400Kg (900 lb.) dual suspended aluminum anode arrays deployed from the first two subsea elevations of the structure, immediately around the conductor bay area.
Figure 9. Buoyant Anode Sled During Deployment (Left) and Model prototype (Inset)
Figure 9a. RetroBuoy™ - ICCP anode sled redesigned 2006-7
The vertically hung anodes reduce effective resistance and speed installation by allowing the suspension clamp to be pre-installed then the anodes, which have "shepherds crook" style hooks to be easily engaged with simple single point rigging. Dual anodes hung from each clamp improve current distribution and optimize installation time. Flexible jumper cables ensure low resistance structural continuity. Typical anodes are shown Fig. 10.
Figure 10. Anodes showing Shepherds Crook and Eye Configuration
Benefits of buoyant sled anode arrays:
1. Buoyant anodes are constantly held in sea-water with no possibility of being silted. Silting can decreases anode performance significantly.
2. The individual buoys will recoil from impact from dropped objects, and every anode is individually cabled. In the unlikely event that one of the anodes is damaged, the other 3 can operate independently with enough current to protect the structure for the design life.
3. Dual feed cables (double steel wire armored) are used and are completely independent.
4. All cable connections are made in the central oil-filled, pressure-compensated junction box.
5. The system is easy to install and cost effective to build.
Other features of the impressed current system include:
1. 4 individual I-Tubes installed to protect the cable for the splash zone transition.
2. Individual rectifiers for each sled (250A @ 24V).
3. Wet weld quick fit cable supports routed up each leg.
4. 6 permanent reference electrodes installed to allow accurate commissioning of the system and to facilitate on-line monitoring.
The results of the retrofit were excellent, in fact the current required was less than anticipated and the sleds are now operating at a little over 65% of their rated capacity. Even though the anodes were not truly remote, the effects of the impressed current system could be measured all over the platform. The installed cost comparison versus a conventional dual clamp-on anode approach is presented in Table 11. This pre-supposes that the structure would support the weight of so many additional anodes clamped on to the members. In truth, the poor condition of the structure would have required the old anode weight to have been removed, before retrofitting were possible. The cost of such removal is not included in the comparison below.
Figure 11. Comparative Platform (Hybrid vs. Conventional) Retrofit Cost
|Item Description (RetroBuoys and hanging anodes)||Cost|
|ICCP System Cost (Four Anode Sleds, Rectifiers, I-Tubes, Cables,
Reference Electrodes and Cable Clamps)
|Galvanic Materials (16 Dual Anode Assemblies with Clamps)||$ 30,000.00|
|Installation Cost (4 Days Subsea) plus topside Installation||$180,000.00|
|Total Installed Cost||$360,000.00|
|Comparable dual clamp-on anode system||Cost|
|Materials (122 Dual Assemblies @ 1080lb. Each)||$210,000.00|
|Installation Cost (14 Days Subsea Required)||$540,000.00|
|Total Installed Cost||$750,000.00|
Case History No. 3 - 90M (300 feet) Drilling / Production Platform
The Asset - This structure was the same vintage (1960's) as the previous example and was installed in the same field. The major difference is that this structure had a depleted anode system but was still polarized to average levels of (-)0.920 V vs. Ag/AgCl sw. An economic study showed that a galvanic system based on seabed pod arrays and shallow suspended anodes would provide the most cost effective solution. Required asset life extension was 15 years.
Anode Retrofit - The system required 24 suspended anode arrays (similar design to the previous example), again from the first two subsea elevations to minimize deep bottom time. The arrays were also 400 kg (900 lb.). In addition, 18 seabed anode pods were deployed in six (6) groups of three (3) around the structure's periphery, approximately 3 m (10 feet) off the structure. The pod's shape was designed to minimize mutual anode interference, while maintaining a shape and weight that was easy to handle offshore. Electro-mechanical tie-back to the structure was made using modified versions of the standard RetroClamp pipeline clamp (Fig. 13). A typical anode pod is shown Fig. 12.
Figure 12. 2000 lb. Anode Pod Assembly (RetroPod™)
Figure 13. Dual Clamp Assemblies for Anode Pod Attachment.
Each modified RetroClamp was designed to accommodate three (3) pods (two negative connection wires per pod), and there was facility to measure the current contribution from each pod using the same principle as the pipeline sled current monitor. The only difference was a cathode-based test point, rather than anode-based. Grab rails were also provided to facilitate ROV monitoring on future subsea inspections. Fig 14 shows this current monitoring test point and the probe for interrogation.
Figure 14. Anode Pod Current Measurement Stabs with Modified CP Probe.
Again, this approach worked well. The comparative installation costs are shown Table 15. Deployment of the anode pods went very smoothly; largely because of the diving support vessel which had an extending boom crane on the back deck. Fig. 16.
Figure 15. Cost Comparison Anode Pod / Hanging Anodes vs. Conventional Dual Retrofit System.
|Item Description (RetroPod and suspended anode system)||Cost|
|Anode Pods (24) with Cables and Clamps||$100,000.00|
|Suspended (32 Dual Anode Assemblies with Clamps)||$ 64,000.00|
|Installation Cost (3 Days Subsea)||$120,000.00|
|Total Installed Cost||$284,000.00|
|Comparable dual clamp-on anode system||Cost|
|Materials (75 Dual Assemblies @ 1080lb. Each)||$130,000.00|
|Installation Cost (7 Days Subsea Required)||$280,000.00|
|Total Installed Cost||$410,000.00|
Figure 16. Dive Support Vessel Showing Extender Crane on Back Deck.
Although the descriptions are brief, these three jobs demonstrate that structure specific design, good survey data, and innovative application of basic technology focused on reducing installation costs can save considerable sums of money. Even on relatively small retrofit projects such as these.
 Mateer M W, NACE International CORROSION 91. Paper No. 233
 Britton J N, NACE International CORROSION 92, Paper No. 422.
 Britton J N, NACE International CORROSION 98, Paper No. 729.
 Mateer M W, Kennelley K W, NACE International CORROSION 93, Paper No. 526