Fully 35% of the world's offshore infrastructure has now been in service for 20 years or more, which in many cases, exceeds the design lifetime of the original cathodic protection (CP) systems installed on jackets and pipelines. This rapidly aging infrastructure has necessitated CP technology innovations that reduce the cost of installation without sacrificing performance or reliability
Abstract
On platforms, conventional CP retrofits methods, generally using clamp-on anodes, are tedious, expensive and potentially dangerous for divers. First introduced in the U.S, Gulf of Mexico, remote or semi-remote sacrificial anode arrays or buoyed impressed current CP (JCCP) systems, set on bottom, are now widely accepted as the most effective and economical means of retrofitting platform CP systems.
Offshore pipelines are at the highest risk of catastrophic external corrosion failure if their CP systems are allowed to fall into disrepair. However, until the recent introduction of DNV-RP-103, the offshore industry had not even addressed the issue of CP retrofits on subsea pipelines. Such now provides for current attenuation modeling, and the use of remote anode sleds for retrofits.
By way of selected case histories, this paper will highlight the design and installation methodologies inherent in new generation CP retrofit practices; showing how improved performance, reliability and cost savings can go hand in hand.
Introduction
The historical approach to CP retrofits has been to effectively replace anodes on a one-for-one basis. That is, wherever an anode was installed during new construction, so too will another anode or anodes be installed 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. They are invariably designed to satisfy installation requirements, as opposed to CP considerations.
For example, a pipeline bracelet anode configuration is designed to facilitate pre-installation of the cathodic protection system on the pipeline, as well as accommodate pipe-laying operations. In truth, the bracelet anode is possibly the worst anode shape and placement that an anode could have fro m a CP 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. They are attached by welding extremely stout pipe cores to the structure in order to withstand launch forces and /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 additional anodes are required to offset these constraints. When we are charged with designing a retrofit system, most of these installation considerations 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.
CP retrofit strategies
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 performance or reliability. Some of the obvious ways in which this may be accomplished are:
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Pipelines
- Minimize the number of locations on the pipeline that have to be visited
- Select areas where the dept h of cover is either 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.
Platforms
- Minimize the number of anode installations.
- Minimize the amount of marine growth removal.
- On deeper projects, use ROV's rather than saturation divers.
- Have the flexibility to adjust the retrofit plan offshore based on survey result s obtained as the installation progresses.
- Carefully plan top side rigging and set equipment prior to mobilization of the subsea installation spread.
- Evaluate Impressed Current. Sacrificial Anode and Hybrid solutions at design phase.
CP retrofit system selection
As with any CP system, three basic options are available: impressed current CP (lCCP), sacrificial nodes, or hybrid systems, which employ an optimized combination of ICCP and sacrificial anodes. Although sacrificial anodes are by far the most common CP system used on offshore infrastructure, the suitability of ICCP merits consideration, and will be determined largely based on the following broad criteria:
Pipelines
Isolation status of the pipeline - if the pipeline is electrically isolated at one or both ends, the application of ICCP may be practical.
Practicality of applying ICCP - there are several sub-criteria that will determine the viability of using ICCP:
- Availability of suitable AC power supply
- Cost of access and maintenance
- Potential for stray current interference.
The length of the pipeline - ICCP can only be deployed at the ends of an offshore pipeline if it starts or terminates at a platform or on dry land .The distance that can be protected from an end current source will be limited by the linear resistance of the pipeline and the current required to protect it (coating condition). Generally speaking, ICCP offers no real advantage over sacrificial anodes when considering attenuation. For short lines, gene rally less than 8 km in length that run between two platforms, or otherwise provide access to a CP source at either end, the answer may be simply to electrically connect (short) the pipeline to those CP sources at either end (platforms). This is an ideal retrofit scenario when both platforms have been retrofitted with adequate current to accommodate short lines connected between them. Attenuation models, verified with a midpoint contact potential measurement will usually be sufficient to provide adequate retrofit capacity; at a very attractive price.
Platforms
As a rule of thumb, ICCP becomes the most cost effective CP retrofit option at water depths greater than 60 msw. As with pipelines, the main sub-criteria in determining the viability of ICCP is the availability of suitable AC power supply, the cost of access and maintenance, and the potential for stray current interference .
CP retrofit systems
Dual Clamp-On Anode System
The dual clamp-on anode system is a tried and tested, albeit inefficient method of deploying retrofitted sacrificial anodes to offshore plat forms. An optimized design, whereby mutual interference between the anodes is minimized, can provide as much 8.7 DC amperes of CP current for a cathode with a potential of (-) 0.800V vs. Ag/AgCl (sw) in 22 ohm-cm sea water for a design life of 10 - 20 years.
Anode Pod
The anode pod is a bottom set steel frame consisting of at least four 148 kg plat form-type anodes; capable of delivering at least 18.75 DC amperes of CP current for a cathode with a potential of (-) 0.800V vs. Ag /Cl (sw) in 22 ohm-cm seawater for a design life of up to 20 years. Ballast is provided by an articulated concrete mattress, mounted in the base frame.
Figure 1 Anode Pod on Bottom with Clamp Attached to Vertical Diagonal on Jacket
Pods are installed just inside or outside the base of the jacket. This semi-remote location greatly improves current distribution, and the installation is accomplished with divers or ROV, using an electromechanical clamping system.
Anode Sled
Anode sleds are designed for pipelines where anode burial below natural sea bed is required or anticipated. A sled consists of four 129 kg platform-type anodes on a steel structural frame, and is capable of delivering at
least 4 DC amperes of CP current for a cathode with a potential of (-) 0.800V vs. Ag /CP (sw) over the entirety of the CP design life, which is typically 15 to 20 years.
Sleds are installed at least 5 meters from the pipeline, with two electro-mechanical clamps for system redundancy.
Clamping System
The clamp and tie-back cables (2tie-back cables per clamp for system redundancy) are designed to maintain both mechanical integrity and electrical continuity between the galvanic anode array and the structure being protected for the life of the CP system, while accommodating varying pipe or structural member diameters due to thermal cycling and coating creep. The clamp is also designed with a calculated failure mechanism to allow it to pull off the structure or pipeline with a force that will not cause coating damage in the event of a snag or mechanical interference (e.g., trawling nets, anchors, etc.).
Figure 2 Anode Pods Protecting Subsea Production Equipment
Impressed Current Remote Sled
The ICCP sled is a modular, remote, bottom set system that can be customized to provide current capacities from 250 - 500 amperes for a 15 - 25 year CP design life. Anode elements are mixed metal oxide; buoyancy components are syntactic foam, depth rated to 305 msw; main feed cables are 120 mm2 HMWPE insulated and bedded, with a contra-helical double galvanized steel wire armor package; cable connections are made inside an oil-filled, pressure compensated junction box, with each anode element on a separate parallel circuit.
Figure 3 Anode Sled installed by ROV
Figure 4 Remote ICCP Shield
Figure 5 Clamping System on Pipeline with CP Monitoring Instruments Attached
Anode Mat
The anode mat consists of an articulated grid of interconnected concrete blocks. Depending on current required and design life; some to all of these blocks contain embedded 8 kg cylindrical anodes (electrically continuous).
Mats are capable of delivering at least 4 DC amperes of CP current for a cathode with a potential of (-) 0.800V vs. Ag/ Cl (sw) over the entirety of the CP design life.
Link Anode System
The link anode system is comprised of a series of cylindrical anodes cast onto a 'wire rope core, and is designed for retrofitting minimal, shallow water structures and caissons, or subsea pipelines. Strings are supplied in three standard lengths: 10m (5 anodes), 20m (10 anodes), and 30m (15 anodes).
Figure 6 Anode Mat Installed on Pipeline using Clamping System
Links are deployed in three basic ways:
- Surface suspended with trailing mud anodes. Used for 3 - 5 year CP of offshore structures in 30 msw or less. Used as riser or infield pipeline CP system with an insulated hang-off option.
- Subsea suspended with trailing mud anodes.
- Mud deployed on ultra shallow structures (e.g., wind turbine caissons) or subsea pipelines.
CP 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 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:
- In most 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.
- Design life requirement may be for only a few years, in which case it may not be necessary to optimise 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 have the advantage of being able to monitor both anode and cathode responses during the retrofit to verify design predictions.
So when designing a retrofit, it is rarely, if ever necessary, to provide the same current density as one would for a new structure, and if existing maintenance current densities can be demonstrated to be much lower than conventional wisdom would dictate, significant savings can be realized [1].
Importance of survey
The value of a well-specified survey cannot be overestimated. This is true of both platforms and pipelines, but particularly so with buried, or partially buried pipelines. This is only true, however, of a high resolution type survey [2]. Remote or semi-remote (trailing wire) type surveys provide little or no useful information.
Figure 7 Link Anode on a Single Well Caisson
Pipelines
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 retro fit is such that it is unacceptable to waste any time trying to locate the pipeline.
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 pipe line is buried 2 meter s 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.
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. In having an ROV fly the line there is always the chance of obtaining a visual inspection opportunity on one or more anodes. Such can provide invaluable information to the CP designer.
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.
Platforms
On platforms it is the same story; using an intelligent survey approach [3], [4]; will yield valuable information on CP system performance. Again, structure potential data alone does not tell the whole story. Estimation of anode depletion percentage is another area where mistakes are often made. Thus it is important to take accurate measurements on a few water blast-cleaned anodes to get a realistic status on remaining anode material. The benefits of an intelligent platform inspection are, in order of value:
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.
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.
A typical survey will also include evaluation of the sea bed condition, silt and scour and seabed debris. This is invaluable data if a sea bed 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 is also important. A heavily corroded structure may not be a candidate for certain kinds of retrofit.
Case histories
Cost and installation time assumptions are as follows:
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Description
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Cost Per Day (US$)
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Surface DSV (Diving <60m)
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87,000
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Saturation DSV (Diving >60m)
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110,000
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Topside Installation Crew (5 persons)
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7,500
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Description
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Unit Cost (US$)
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Installation Time (hours)
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Dual Clamp-On (20 yr)
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5,600
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1.00
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Anode Pod
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14,700
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0.75
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Retro Buoy
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145,000
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12.00
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Anode Sled
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12,500
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6.00
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Case history - ICCP (EI 296-B)
The EI 296-B complex is comprised of two structures with a connecting bridge. The structure sits in 66meters of water in the U.S .Gulf of Mexico. Both are standard 8-pile jackets; the drilling platform has 24 well conductors.
The cathodic protection systems on both jackets had depleted to the point where the potential readings were below the minimum required levels. The potentials on the structures ranged from (-) 0.680 to (-) 0.733 V vs. Ag / AgCl (sw) reference.
At these potentials the retrofit current was computed to be 1184Amperes. The customer wanted an additional 20-year life extension on the CP systems.
Based on an economic analysis, it was decided to use an impressed current retrofit solution as described below. There was also some concern about the additional weight that a retrofitted sacrificial anode system would add to the structures (approximately 176 MT) spread between 191 clamp-on anode arrays weighing almost 1 MT each.
It was decided to deploy 3x400Ampere RetroBuoy sled systems to protect both structures. Total installation time was extracted from the dive logs and is presented below.
Time Line from Dive Logs:
Sled 1
21Aug03 1005 Hrs Arrived EI-296
22Aug03 0536 Hrs Prod. Platform Complete (1 Sled, Clamps & I-Tube)
Elapsed Time 20 Hrs 21 Min
Sled 2
22Aug03 0806 Hrs Moor to E1296-B. Drill. 2141 Hrs 1 Sled, Clamps & I-Tube Installed
Elapsed Time 13 Hrs 35 Min
Sled 3
22Aug03 2210 Hrs Move to Second Sled Location
23Aug03 0636 Hrs Sled Installation Complete. Dive Crew Moved to Inspection Tasks
Elapsed Time 8 Hrs 26 Min
Total Subsea Installation Time = 42 Hrs 22 Min
The selected ICCP solution was 32% of the cost of a conventional clamp-on anode retrofit; a savings of US$1.8 million.
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System Type
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# Req’d.
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Material Cost (US$)
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Subsea Install Time(hr)
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Topside Install Time (hr)
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Total Install Cost (US$)
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Total Project Cost (US$)
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Dual Clamp-on
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191
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1,069,600
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191
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N/A
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1,604,400
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2,674,000
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RetroPod/Clamp on
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65/46
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955,500
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95
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N/A
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780,375
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1,735,875
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RetroBuoy (ICCP)
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3
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435,000
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42
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36
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408,000
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843,000
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Case history – Anode Pods (SS 216)
SS 216 is a 10-pile jacket in 34 meters of water in the U.S, Gulf of Mexico. The platform has 21 conductors in 24 slots. The client wanted a 10-year life extension of the CP system; the retrofit current was computed to be 305 amps.
The retrofit was originally planned for 2003, using standard anode pods, but was delayed a year. During which time, the low potential had fallen an additional 50 mV from (-) 0.730 V vs. Ag /Cl (sw) to a native state potential of (-) 0.680. This would normally have required a "booster" pod design; however the in-built redundancy in the system design assured full protection of the platform.
Time Line from Dive Logs:
27Aug04 1900 Hrs Arrived SS 216
2103 Hrs First Pod Launched
28Aug04 0926 Hrs Last CP Check Made
1010 Hrs Depart SS 216
Total Time at Platform: 16 Hours 10 Minutes
Average Pod Installation Time: 54 Minutes
The selected anode pod solution was 70% of the costs of a conventional clamp-on anode retrofit savings of US$179,000.
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System Type
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# Req’d.
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Material Cost (US$)
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Subsea Install Time(hr)
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Topside Install Time (hr)
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Total Install Cost (US$)
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Total Project Cost (US$)
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|
Dual Clamp-on
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40
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224,000
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40
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N/A
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367,000
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591,000
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RetroPod/Clamp on
|
18
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265,000
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16
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N/A
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147,000
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412,000
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RetroBuoy (ICCP)
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1
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145,000
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12
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12
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118,000
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263,000
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Case history – pipelines
Unfortunately, pipeline CP retrofits have been fairly limited to d ate. When undertaken, the methodologies deployed have been a re al mixed bag; some of which have worked; some not. As there is no conventional, industry standard retrofit methodology against which to compare our systems, such as clamp-on anodes for jackets, the following is a costing based on using anode sleds and the clamping system detailed in an earlier section. Such is based on an average spacing of 3,000m between sleds .The reader can compare these numbers against their own experiences:
> 60 m Water Depth US$22,500 / km
< 60 m Water Depth US$18,300 / km
Summary and conclusions
Within the next ten years, almost two thirds of the existing global offshore facilities will be 20 years and older. With rising oil prices driving deepwater developments, the need to extend the service life of this aging infrastructure well beyond the original CP design life may necessitate CP retrofits on a potentially grand scale.
Offshore CP designers cannot be constrained by the limits placed on us in new construction designs. The operators will demand that we optimize to reduce costs, yet will similarly insist that we not compromise performance and reliability. The necessary technology exists today. It is proven.
References
- M.W. Mateer, NACE International CORROSION 91, Paper No. 233
- J. N. Britton, NACE International CORROSION 92, Paper No. 422
- J. N. Britton, NACE International CORROSION 98, Paper No. 729
- K. W. Kennelly, M.W. Mateer, NACE International CORROSION 93, Paper No. 526
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 Petromin thanks M. Edgar Lewis of Wasco Energy, Kuala Lumpur, Malaysia for this paper, which was presented at the NACE East Asian & Pacific Regional Conference & Exposition 2008, Kuala Lumpur, Malaysia 12 - 14 May 2008.
M. Edgar Lewis is the Business Development Director of Wasco Energy Ltd. He has been involved in the offshore corrosion control industry for the past twenty-five years. He is an active member of NACE, SSPC and the ACA, and is a NACE Certified Coating Inspector. Mr. Lewis is a graduate of the University of Southern California.
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