Almost 35 percent of the global offshore infrastructure has exceeded the design lifetime of the original cathodic protection (CP) systems installed on jackets and pipelines. As much of this infrastructure is anticipated to remain in service for the foreseeable future, a flurry of activity is expected over the next several years in the retrofitting of these CP systems.

The opportunities for the diving industry are two-fold: providing CP retrofit installations that are not only safer than conventional retrofit methods, but also add greater value for clients, many of whom are faced with regulatory pressure to retrofit more facilities than they can realistically fund in any financial year. This is an especially acute problem for the many independents taking over aging facilities on the continental shelf.

The retrofit methods illustrated here typically cost 50 to 70 percent of what conventional "clamp-on" CP anode retrofits run. But why would a diving contractor be interested in installing CP retrofit systems that significantly reduce diving costs? The answer is their customers demand it, and usually turn right around and use those savings to buy even more retrofits. The demand is, and will continue to be, that great, particularly as greater emphasis will soon be placed on subsea pipelines.

A Little History
The historical approach to retrofitting has been to replace anodes on a one-for-one basis. That is, wherever an anode was installed during new construction, so too will it be during a retrofit.

This approach is very costly and completely unnecessary. There is a tendency for the industry to misinterpret the reasons why CP systems for new structures are designed the way they are, which is to satisfy installation requirements as opposed to CP considerations.

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 is designed so that the pipe can be easily laid with the anodes in place.

In truth, the bracelet anode is possibly the worst anode configuration and placement that an anode could have from a 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.

Conventional platform anodes are another example. They are attached by welding extremely stout pipe cores to the structure. Why? To withstand launch forces or pile driving during installation.

Again, the CP design is predicated on a less-than-desirable installation method. Anode utilization is reduced, the standoff distance is not optimized, and the cost of all those welds is very significant.

When we are charged with designing a retrofit system, most of these constraints disappear because the structure is already in place, so we should not be constrained in any way by the original design methodology when designing the retrofit.

The Cost of Retrofitting
When analyzing the cost of a retrofit project, the story is always the same: Cost of installation always drives the project budget. Therefore, the design should focus on reduction of installation cost without sacrificing performance or reliability. Some of the obvious ways in which this may be accomplished are as follows:

On Pipelines

• Minimize the number of locations on the pipeline that have to be visited.
• Select areas where the depth of cover is minimal, or the pipeline is exposed on the seabed.
• Minimize bottom time requirements at each location.
• On deeper projects, use ROV's rather than saturation divers.
• Carefully evaluate and compare costs of 4-point moored systems vs. dynamically positioned equipment.
• Evaluate Impressed Current, Sacrificial Anode and Hybrid solutions at design phase.
• Have the flexibility to adjust the retrofit plan offshore based on survey results obtained as the installation progresses.

• Minimize the number of anode installations, as well as the amount of marine growth removal.
• On deeper projects, use ROVs 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.
• Have the flexibility to adjust the retrofit plan as the installation progresses.
• Carefully plan topside rigging and set equipment prior to mobilization of the subsea installation spread.
• Evaluate impressed current, sacrificial anode and hybrid solutions during the project's design phase.

Retrofit Design
Just as new construction cathodic protection designs are made to facilitate installation of the offshore platform or pipeline, the cathodic protection design criteria are meant to polarize a structure from native state potential, as well as to provide adequate redundancy 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.

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.

The design life requirement may be for only a few years, and it may not be necessary to optimize protective potential levels.

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). We also have the advantage of being able to monitor both anode and 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 for both platforms and pipelines, particularly with buried or partially buried pipelines. This is only true of a high-resolution survey. Remote or semi-remote (trailing wire) 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 six feet (2m) deep, it could take divers many hours to excavate the pipeline. They would then 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, along with any areas of significant coating damage. Having an ROV fly the line offers the chance of obtaining a visual inspection opportunity on one or more anodes, providing valuable information to the CP designer.

Verifying Environmental Conditions: The survey will give a good indication of seabed conditions, current velocities, etc., as well as providing 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 CP 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.

The benefits of an intelligent platform inspection are, in order of value:

Polarization Data: Knowing the existing maintenance current density on a structure gives the designer a precise benchmark from which to work. This will always result in a lower (but still safe) current density value being applied for the retrofit. This saves time and money without adding risk.

Anode Performance: Knowing the current output range of the existing anodes and their average degree of consumption allows more precise prediction of remaining life. This could result in deferring a retrofit for one or more seasons; again with no risk.

General Platform Condition: A typical survey will also include evaluation of the seabed condition, silt and 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 original anodes may prove useful if original anodes are used to support retrofit anodes. Condition of the structure regarding existing corrosion damage. A heavily corroded structure may not be a candidate for certain kinds of retrofit.

In the following case histories, three Gulf of Mexico projects are outlined. While not all of the procedures follow the ideals presented in this article, the application of the basic principles is apparent, as are the documented performance and cost savings.

Case History No.1
This 12-inch oil pipeline was installed in 1973 and runs from an offshore platform in 250 feet (76m) of water to an onshore terminal 56 miles distant.

From the platform to about mile 15 the line is bottom laid, although the location of the line near the Mississippi river delta has resulted in silting over of much of the pipeline. The next 20 miles or so were laid in water that was less than 200 feet (61m) deep and was buried to a depth of five feet (1.5m). The final 21 miles are laid through wetland (swamp) in pipe canal and buried a minimum of six feet (2m).

The offshore section (35 miles) of the line was protected with zinc anode bracelets, while the wetland section was originally protected with a combination of impressed current and zinc bracelets. Precise records of where the bracelets started were not available. The pipeline has a one-inch-thick concrete weight coat.

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 greater than 10 feet. An accurate as-built plot was generated showing the actual position of the pipeline to be as much as 100 feet off from the original "as-built" survey.

In 2000, the pipeline owner decided to undertake a retrofit of the offshore section of the pipeline (35 miles). The extended design life was 10 years. Review of the riser potential showed a very small decline in protected potential from the survey three years previous.

The retrofit design utilized pairs of sleds located either side of the pipeline, using a 10- to 12-foot sled to pipeline spacing. Attenuation models were used to set the maximum spacing between anode sled installations, resulting in a value of 12,000 feet. Points were selected where the depth of cover was minimal or the pipe was exposed, and a total of 28 sleds at 14 sites were proposed.

It was apparent that the majority of time on bottom would be taken up exposing the line and removing concrete. It was therefore decided to use a continuity clamp, designed by Deepwater Corrosion Services, that could be installed without exposing 360 degrees of the pipeline or having to remove a large area of concrete weight coating. The clamp system allowed only a small four-inch-square of concrete 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 tip contact.

We also wanted to be able to verify system performance so we included a current measuring facility on the sleds. 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 stabbed into the stab rails, and current and potential are displayed simultaneously on the readout.

Case History No.2
This eight-pile dilling/production platform was installed in the late 1960s and was originally fitted with a impressed current system. At some point 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 range.

Based on the structure arrangement with a congested center conductor bay, the high retrofit current required (estimated to be greater than 1,000 Amperes), and the badly deteriorated condition of the structure, it was decided to use a hybrid retrofit approach. The system design life was 15 years.

A saturation diving spread was contracted, with a shallow air capability to handle the galvanic anode 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.

The majority of the current was to be provided by four semi-remote buoyant sleds deployed off either side of the jacket at a distance of 50 feet off the structure and the same distance from any of the pipelines associated with the structure. Each of the sleds was rated at 250 Amperes.

The remainder of the current was provided by 16 900-pound dual-suspended aluminum anode arrays deployed from the first two subsea elevations of the structure, immediately around the conductor bay area. The vertically hung anodes reduced effective resistance and sped installation by allowing the suspension clamp to be pre-installed. The anodes, which had "shepherds crook" style hooks, could be easily engaged with simple single point rigging. Dual anodes hung from each clamp improved current distribution and optimized installation time. Flexible jumper cables ensured low resistance structural continuity.

Benefits of buoyant sled anode arrays include:

1. Buoyant anodes are held in seawater with no possibility to silt (which greatly reduces anode performance).
2. The individual buoys will recoil from impact from dropped objects, and every anode is individually cabled so that loss of the entire system is a low risk.
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. Four individual I-tubes installed 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. Six permanent reference electrodes installed to allow accurate commissioning of the system and to facilitate on-line monitoring.

The results of this retrofit project were excellent. In fact, the current required was less than anticipated and the sleds are now operating at a little over 65 percent 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. This supposes the structure would support the weight. In truth, the cost would have been much higher because anodes would have to have been removed before the new ones could be installed.

Case History No.3
This 300-foot drilling and production platform was the same vintage 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. Required design life was 15 years.

An economic study showed that a galvanic system based on seabed pod arrays and shallow suspended anodes would provide the most cost-effective solution. The shallow suspended arrays were the same as used on the previous example, except that 24 arrays were deployed, again from the first two subsea elevations to minimize deep bottom time. In addition, 18 seabed pods were deployed in six groups of three around the structure periphery approximately 10 feet off the structure.

The pod geometry was designed to minimize mutual anode interference while maintaining a geometry and weight that was easy to handle offshore. Tie back to the structure was made through modified versions of the pipeline clamp. Each clamp accommodated three 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, only this time it was cathode based. Grab rails were also provided to facilitate ROV monitoring on future subsea inspections. Like the pipeline sleds, current output was monitored from each pod to verify design calculations.

Deployment of the anode pods went smoothly largely because of the diving support vessel, which had an extending boom crane on the back deck.

Significant Savings
Although these project descriptions are brief, these three jobs demonstrate that structure specific design, good survey data, and the 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. UW

M. Edgar Lewis is the Executive Vice President of Deepwater Corrosion Services in Houston, Texas (www.stoprust.com). A graduate of the University of Southern California, Edgar started in the industry as a diver in Indonesia's Java Sea, and has almost 20 years of domestic and international experience providing offshore corrosion control.