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Cathodic Protection for Deepwater Pipelines
Introduction |
Background Design Information |
Design Decisions | Design
Basis | Cathodic Protection Design
Introduction Initial consideration was given to the use of an impressed current
system to protect the bundle assemblies because of the relatively short
line lengths (the longest 5.8 miles) and the inherent flexibility of an
impressed current system. If impressed current was used, one system could
be designed and installed at the shallow water facility to provide
protection to the SW jacket, the Bowline bundles, and the templates. The
impressed current design was not chosen, however, because the bundle was
to be towed a distance of approximately 500 miles, and some form of
cathodic protection would be required to protect the bundle during the
tow. Based on experience gained from previous tows, zinc anodes had
proven satisfactory for this purpose, and would be used for cathodic
protection during the tow. Because some form of protection was required
during tow operations, a sacrificial zinc anode system was selected to
protect the pipeline during its design life. Zinc anodes were decided
upon primarily because of successful use on previous bottom tows, the low
initial cost for materials and the relative ease with which zinc can be
cast directly onto casing pipe sections. Once the anode material was
chosen, calculations to determine the weight, size, and spacing required
to provide corrosion protection to the bundles (end effects were not
directly considered). These calculations were performed by the MANOD
computer program. Check calculations were made with the PIPE
program3, presented in paper 333, a CORROSION 84 paper by
Jerry Cochran and Fred Mayes. Using these programs, anode weights and
sizes were chosen for spacing in multiples of 40 feet, (a nominal joint
length) between 120 and 320 feet. To ensure high quality castings, anodes
were cast in foundry-like conditions near New Orleans and not at the
launch site (the Matagorda Peninsula), as was done on an earlier project.
Also, to reduce handling and the number of pipe joints to be shipped to
the casting location (and subsequently anode joints to the launch site),
more than one anode was cast per pipe joint. Calculations indicated an
anode length of about 12 inches would be suitable. Assuming a
center-to-center distance of eight feet between anodes, five anodes could
be cast per 40 foot joint of casing pipe. This multiple casting per joint
had a significant effect on the total cost of the anodes. To allow for
end effects (more surface area to be protected), one extra anode was
placed immediately behind the sleds and another placed at half the design
spacing from the end anode. In addition, to compensate for the
possibility that the bundle casing pipe temperature at the template end
of the bundle would not drop to ambient water temperature immediately,
extra anodes were installed. A double anode joint was installed near the
template sled, as the second casing joint from the sled. Two additional
double anode joints were placed uniformly at the design spacing. In
addition, four single anodes were placed at half the design anode
spacing. Single joint anodes were then placed at design spacing. Small
bolt-on anodes were used to protect each sled. These anodes would be
accessible by ROV for inspection and possible repair/replacement.
(back to top) After review of all design possibilities, the following decisions were made:
The following parameters, generally in accordance with NACE guidelines and Gulf of Mexico practice, are used in the CP system design. Bundle Parameters
Coating Breakdown (Per NACE Guidelines)
Environmental Parameters
Protection Requirements
Potential with Respect to Ag/AgCl Electrode
Anode Material Properties
Notes on Design Basis. The expected pipeline operating temperature
profiles for the casing pipe can generally be expected to be at or very
near the ambient seawater temperatures, which are approximately 44°F
at the templates and 44-70°F at the SWF, depending largely on the
location of the thermo cline. The first several hundred feet of casing
near the template may be somewhat warmer than the ambient water
temperature. Therefore, because zinc loses efficiency as the pipe
temperature increases above 80°F, anode spacing should be reduced
near the template end of the bundle casing. In general, seabed soils
range from silts at the SWF to highly saturated silts at the templates.
Some sinkage may occur in deeper water. This should not be significant,
considering the low submerged weight of the bundle in operation and the
fact that zinc is equally effective as a sacrificial anode when buried or
unburied. (back to
top) The CP system for the bundle assemblies and associated risers is
designed as a "stand alone" system and is not intended to provide
protection for any other structure. The design parameters used are
industry standards for the Gulf of Mexico. Because of the water depths in
which the bundles are to be installed and operated, a conservative design
was chosen. Anode dimensions will be based on 0% burial, a conservative
choice since at least some percentage of the line will be covered. A
minimum pipe-to-seawater potential of -800 mV to Ag/AgCl, the recommended
value given in NACE RP-01-76-83, is used. Fusion bonded epoxy (FBE) is a
proven pipeline coating with inherently low holiday potential and has a
long projected life. A design estimate of 5 % holidays in the coating was
chosen, but upon inspection after application of FBE, the line showed
less than 1% holidays. In fact, the open circuit voltage of the line when
tested in seawater before launch was -lOSOmV, the open circuit voltage
for Zn vs. Ag/AgC1 in seawater.
See Figure 2. This indicates a
defect-free coating. The line was also coated along the bottom half with
an abrasion resistant epoxy for protection against dragging along the sea
bottom. Usage of sacrificial anodes ensures complete protection during
tow to site. As a final safeguard, the cathodic protection system was
checked prior to towing to site. The DC power supply was not used to
initially polarize the system during nitrogen filling, because of the
extremely low incidence of holidays in the FBE coating. The approach used
in the CP design ensures that the following requirements are achieved: 1.
Sufficient weight of anode material is available for cathodic protection
of the pipeline throughout the design life. 2. Sufficient current is
available for protection of the pipeline at the end of the operational
period when the anodes are partially consumed. The cathodic protection
system is designed for compatibility by ensuring sufficient sacrificial
anode material is available throughout design life and sufficient output
current is developed by each anode for the end of life period. Burial in
the form of sinkage will be minimal because of the low pipeline submerged
weight. Near the SWF, the pipeline will be placed in a shallow plowed
trench approximately 18 inches deep for stability. As discussed
previously, temperature aerating is not required, although the
possibility of a higher than expected pipe temperature near the wellhead
is covered by addition of extra anodes. (back to top) Much of the CP design analysis was performed with the MANOD computer program. The program required as input the following design criteria:
MANOD calculates the size of anodes required at the specified spacing
based on both the mass of the anode required and the final current demand
criteria. If the pipeline temperature is expected to be above 85°F,
the temperature compensation option should be utilized. As the casing
wall temperature may become equal to the ambient water temperature only
after several hundred feet of the wellhead template, additional current
may be needed in this area. Doubling the anode weight requirement at the
spacing used in the calculation or halving the spacing required between
anodes will compensate for the additional anode weight requirement to
satisfy the extra current demand. (Check calculations assuming a casing
surface temperature of 100°F and a water temperature of 45°F to
verify that doubling the number of anodes near the wellhead template
would adequately compensate for possible anode degradation were made and
indicated that this approach was satisfactory.) The program checks
against maximum anode length available and then determines whether two
anodes should be cast to satisfy casting length restriction, which had
been set at 30 inches for a two inch thick, 22 inch inside diameter
anode. (back to top) American Corrosion Services, Inc., Belle Chasse, Louisiana, was awarded the work of casting the anodes. The final anode design called for a cast on zinc anode with a minimum zinc weight of 350 pounds. Overall anode length, as cast, was 8 inches. Each end of the anode was cast with a 2 inch tapered section, 0.25 inches at the outside end of the taper, and 1.5 inches thick at the center section. The casting molds were fabricated from heavy steel plate and were fitted with vents at the top, away from the pouring spout, to insure complete filling. Each pipe joint was shot blasted to remove the FBE coating and prepare the steel to near white condition for flame spray application of zinc metallizing wire prior to weighing. The following procedure was established to set standard anode weights:
This procedure established the capacity of each anode mold within the accuracy of the scales (which had been previously calibrated). The weight variation between anodes was less than the specified 5 percent tolerance. To prevent oxidation of the zinc metallized pipe prior to casting, the following procedure was adopted.
The following testing procedure was used.
After transport to the pipe assembly site on Matagorda Peninsula, the
anode joints were cut between anodes as required for inclusion into the
welding line. The 8 foot lengths with one anode located in the middle of
the joint provided ample separation from the weld to prevent
interference. Previous experience showed that an 18 inch or less
separation caused occasional welding problems.
(back to top) An ROV survey of the cathodic protection system is planned following
final connection of the flow line 9. bundles at the templates and at the
shallow water facilities platform. Data obtained from this survey will be
compared to design values and theoretical. (back to top)
TABLE 1 - Summarized output from MANOD
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