Offshore Cathodic Protection System Management:
A 21st Century Approach
Jim Britton |
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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 risk of failure. This paper presents three case
histories of platform and pipeline retrofits where innovation
in design and installation methods have resulted in successful
CP retrofits at significant cost reductions over conventional
methods. Several new concepts of inspection, design, and
installation hardware are presented.
Keywords: Cathodic Protection, Retrofits, Offshore Platform,
Offshore Pipeline, Impressed Current Offshore, Buoyant Anodes |
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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. Historical approach to retrofit has been to
replace anodes on a one for one basis, 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.
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
that an anode could be (from the cathodic protection engineers
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, why .. to withstand launch forces and/or pile driving
during installation. Again the CP design is predicated on the
installation method. Is this the best way to install an anode
on a large bare steel structure .. well of course not,
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. |
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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:
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On Pipelines
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.
On Platforms
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. |
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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: |
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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. |
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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 [1]. |
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The value of a well conceived 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 [2], 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: |
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1. 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.
2. 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.
3. 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 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 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.
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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 do
not tell the whole story. Estimation of anode depletion
percentage is another area where mistakes are often made.
Fig. 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 life the same reduction represents only 7.1%
volumetric loss. 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: |
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1. 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.
2. 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.
3. 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. |
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Figure 1. Anode Consumption vs. Cross Section Loss
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Bearing the aforementioned paragraphs in mind, three case
histories of projects completed in the Gulf of Mexico in 2000
will be described, and while not all of the procedures follow
the ideals outlined, the application of the basic principles
is apparent, as are the documented performance and cost
savings. |
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Case History No. 1 - 12" Oil Pipeline |
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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
silting over of much of the pipeline. The next 32 Km. (20
miles) or so were laid in water that was less than 125 M (200
feet) deep and was buried to a depth of 1.5 meters (5 feet).
The final 34 Km. (21 miles) are laid through wetland (swamp)
in pipe canal and buried a minimum of 2 meters (6 feet). The
offshore section (56 Km. 35 miles) of the line was protected
with zinc anode bracelets, 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 25mm (1") thick concrete
weight coat.
In 1997, the offshore section of the pipeline was surveyed
using a three electrode system [2]. 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 meters (10 feet). An accurate as built plot was
generated which showed the actual position of the pipeline to
be as much as 30 meters (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 (56 Km. 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 3 years previous.
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 meters (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, that could
be installed without the need to either expose 360 degrees of
the pipeline or having to remove a large area of concrete
weight coating. |
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Figure 2. Typical Pipeline Anode Sled Arrangement |
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The clamp system used Figure 3. allowed only
a small circle 100 mm (4") diameter 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. |
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Figure 3. Constant Tension
Retrofit Clamp for
Weight Coated Pipelines |
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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). |
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Figure 4. Pipeline Anode Sleds with
Current Monitoring Stab Shown (Inset) |
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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. |
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Figure 5. Dual Readout CP Probe used for Potential and Current Measurement
(CP Gun remodeled in 2007, click here for details) |
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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. |
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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 |
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Figure 7. Cost of Pipeline Retrofit (US Dollars). |
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Case History No. 2 - 90 M (300 feet) 8-Pile Drilling-
Production Platform |
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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. System design life was 15 years. |
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Figure 8. Platform for Case History No. 2. |
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A saturation diving spread was contracted, with a shallow air
capability also 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
seabed deployed buoyant sleds deployed off either side
of the jacket at a distance of 15 meters (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. |
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Figure 9. Buoyant Anode Sled During
Deployment (Left) and Model prototype
(Inset)
(RetroBuoy Jr. Remodelled in 2005, click here for info) |
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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. |
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Figure 10. Anodes showing Shepherds
Crook and Eye Configuration
(vertically suspended anode technology has been retired as of 2005, click here for details) |
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Benefits of Buoyant Sled Anode Arrays:
1. Buoyant anodes are held in sea-water 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. 4 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. 6 permanent reference electrodes installed to allow
accurate commissioning of the system and to facilitate
on-line monitoring. |
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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
supposes that 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. |
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Figure 11. Comparative Platform (Hybrid vs. Conventional) Retrofit Cost |
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Case History No. 3 - 90M (300 feet) Drilling / Production
Platform |
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This structure 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 sw.
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. The arrays were also 400 Kg (900 lb.). In
addition, 18 seabed pods were deployed in 6 groups of three
around the structure periphery approximately 3 meters (10
feet) off the structure. A typical anode pod is shown Fig. 12. |
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Figure 12. 2000 lb. Anode Pod
Assembly |
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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 (Figure 13). |
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Figure 13. Dual Clamp Assemblies
for Anode Pod Attachment. |
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Each clamp accommodated 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, only this time it was cathode
based. Grab rails were also provided to facilitate ROV
monitoring on future subsea inspections. Figure 14. shows this
current monitoring arrangement and the probe used to
interrogate them. |
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Figure 14. Anode Pod Current
Measurement Stabs with Modified
CP Probe. |
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Again, this approach worked well and 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. |
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Figure 15. Cost Comparison Anode Pod / Hanging Anodes vs. Conventional
Dual Retrofit System. |
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Figure 16. Dive Support Vessel
Showing Extender Crane on
Back Deck. |
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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. |
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[1] Mateer M W, NACE International CORROSION 91. Paper No.
233
[2] Britton J N, NACE International CORROSION 92, Paper No.
422.
[3] Britton J N, NACE International CORROSION 98, Paper No.
729.
[4] Mateer M W, Kennelley K W, NACE International CORROSION
93, Paper No. 526 |