Aft Anode Sled Failure at a Floating Production Unit Hull
W Jamrok, R Franco, G Germer,
J Britton, A Luna
Fig 1 - View of ZP (foreground) and other operations
In 2002, the future on-location life of the ZP was assessed to be an additional 15 years. A survey completed in February 2002 indicated that the hull was no longer protected and replacement of the CP system was required. A study carried out identified that the optimum CP replacement system for the ZP was an impressed current system with anode arrays located on the seabed. The submerged portion of the ZP hull was protected against corrosion by a sacrificial anode system that was designed for a five-year life. Since the vessel had already been on station for approximately 5 years, and was expected to remain on station for an additional 15+ years, it became necessary to replace the current sacrificial anode cathodic protection system.
Fig 2 - Layout of ICCP system onboard the ZP
The impressed current system for the ZP is comprised of the following major components:
1. Two (2) - 400 amp skid mounted transformer rectifiers (T/R) located on the deck of the ZP
2. Two (2) - 400 amp anode sled arrays (Fig 3) designed to be placed on the seabed and that permit the anodes to be suspended above the seabed so as to maximize current output and prevent high anode consumption by seabed burial with time
3. Anode cabling suspended between the anode arrays and transformer rectifiers that is tolerant of the movement of the ZP on its moorings (Fig 4).
4. Two (2) permanent fixed reference electrodes
Fig 3 - One of two anode sled arrays prior to deployment
Fig 4 - Lazy S - Catenary cable configurations
There are some technically challenging problems to overcome when designing an ICCP system. The obvious challenge is that the floating production facility is constantly moving in reference to the remote anode array on the seabed. Cable fatigue and cable entanglement are two of the more obvious problems to overcome. The proposed system was based around a buoyant anode sled system as a modular impressed current system especially designed for platform and offshore facility CP retrofits. The ICCP system was designed to accommodate the installation of additional T/R and subsea anode sled assemblies if required later in the life to maintain full levels of cathodic protection.
Design Parameters
Although it was estimated that 1600 amps of current would be required at the end of 15 years, the system was designed so that each anode sled and rectifier provides a continuous DC current of 400 amps at a maximum operating voltage of 36 volts for a period of 15 years, which was the remaining life of the Zafiro Producer. This capacity is based on the calculated current requirement at the end of a 10-year period, which assumes that hull coating breakdown occurs progressively over the period. The actual current requirement at the end of life depends not only upon the amount of coating breakdown but also upon the extent of marine growth and calcareous deposits. Since the current demand of the vessel was small at the time of ICCP installation, only one-half of the calculated end of life equipment capacity was procured and installed. That is to say that, initially, anodes capable of providing only 800 amps were installed.
System Components
1. Transformer Rectifiers (T/R) - The transformer rectifiers are three-phase 460-volt AC input and are rated at a DC output of 400 amps at a maximum voltage of 36 volts. They are capable of operating continuously at their full rated output without overheating at the ambient temperatures that prevail offshore in Equatorial Guinea, and are designed and constructed for outdoor exposure in a marine environment. The outputs of the units are capable of manual adjustment continuously throughout the voltage range at stepped intervals of no greater than 35 amps. The primary AC input is protected with circuit breakers and lightning arrestors and the secondary AC is protected with fuses and surge suppressors. The DC outputs are protected with surge suppressors and lightning arrestors. One (1) T/R is located at the bow of the ZP and one (1) T/R at the aft of the ZP. These locations were based upon the consideration of hazardous area classification, available space, minimization of AC and DC cable run costs, and proximity to the location where the anode cables come onboard.
2. Buoyant Anode Arrays - The anodes are mixed metal oxide type or and are sized to provide the full rated current for a 15-year period. Individual anode cables are connected to the seven (7)-conductor cable in an underwater, pressure compensated junction box (Fig 5), which is protected from sea-water ingress. The anodes are mounted on two frames that protect and support the anodes during deployment. The frames anchor the cables when in position, and function as mud mats to prevent the anodes from being covered by seabed mud that could affect their current output.
Fig 5 - Internal view of anode junction box
One sled was located to the aft of the ZP, and the other towards the bow. Actual seabed locations were selected so that a minimum of 75-ft separation from anchor chains, existing and proposed flow lines, and umbilical equipment is achieved to avoid adverse interaction. Each anode array was situated 180 meters (600 feet) below the hull and approximately 300 meters (1000 feet) away.
3. Anode Feed Cables - The seven (7) conductor cables (Fig 6) connecting the anodes on the seabed to the transformer-rectifiers on the ZP are supported on the ZP by an arrangement that minimized on-site installation time. They were installed in a lazy S curve or catenary configuration that is tolerant of the movement of the ZP on its moorings. The layout was designed to avoid stress and or fatigue failure during the life of the cable, and was achieved by the fitting of flotation units to the cable.
Fig 6 - Seven (7) conductor anode cable
The cables were sized such that the rated current can be produced at the maximum voltage output of the transformer rectifiers. They are constructed to provide protection where required, such as through the splash zone and at points of high movement, such as the touchdown point on the seabed.
4. Reference Electrodes - Permanent reference electrodes are cabled back to the transformer rectifiers and are hung off the anode cables to permit easy measurement of the hull potential. The reference electrode assemblies are comprised of a zinc element electrode. Provision for reading the reference electrode assembly is incorporated within the T/R.
Fig 7 - Anode sleds on work-boat prior to installation
Fig 8 - ROV view of anode sled on seabed with one of four buoyant anode deployed
Fig 9 - Anode feed cable hang off assembly on the ZP
1. Visually inspect the rectifiers for damage
2. Inspect the AC Lightning arrestors
3. Inspect fuses
4. Verify that the internal DC negative (-) case grounding jumpers had been removed
5. Inspect the cable hang off for any visible mechanical damage
6. Perform a drop cell survey for the hull of the Zafiro Producer to verify continued cathodic protection of the entire hull.
Fig 10 - Abrupt decrease of current output from aft T/R
After the initial inspection of the rectifiers, it was determined that, the DC lightning arrestor on the aft unit was significantly damaged, while the forward rectifier had only minor damage to the DC arrestor. The DC arrestor on the aft rectifier was removed. Other than this damage, both rectifiers were operational. It was decided to turn off the forward rectifier until further testing could confirm that there was not any additional damage.
Maintenance personnel previously had removed the factory installed AC lightning arrestor on the forward rectifier. Inspection of this arrestor confirmed that it was damaged. The rectifier manufacture had indicated that the supplied AC arrestors were incompatible with the transformer oil. Replacement AC arrestors were supplied by the manufacturer. The new AC arrestor had been installed in the forward rectifier; however, the aft rectifier still had the original AC arrestor. This arrestor was disconnected but not removed from the rectifier and a new AC arrestor was installed.
The AC fuses in both units were inspected and found to be operational. The rectifiers were not installed with DC fuses, so new DC fuses were installed in both rectifiers. This fuse was installed in the positive (+) DC output leg of each rectifier. Initially, it was suggested that this fuse be installed in the negative (-) DC leg of the rectifier. The location of this fuse would still allow for a lightning strike to enter the rectifier and get shunted to the sub sea buoyant anode, thus still allowing for the possibility of its damage. The fuse was installed by first removing the 7 positive (+) DC conductors from the rectifier output studs. A buss bar was then fabricated and installed over these studs. A 500-amp fuse was then attached to this buss bar. Another buss bar was then attached to the other end of the 500-amp fuse and the 7 positive (+) DC conductors were then re-attached to this buss bar (Fig 11).
Fig 11 - Fabricated buss bars and fuse
The rectifiers were visually inspected for any other damage and were determined not to have sustained additional damage other than what was described above. ZP personnel also verified at this time that the internal DC negative (-) case grounding jumpers were removed.
The rectifiers were then energized to determine if they were operational. When each rectifier was energized, the DC voltage readings of each rectifier were the same as when the rectifiers were energized in August 2004.
The DC current output of the forward rectifier was measured to be 218.4 amps. Although this is significantly higher then the readings when the rectifier was energized in August 2004, this was the proper reading. The previous readings were skewed due to the internal DC negative (-) case grounding jumpers. These jumpers allowed for a parallel electrical path that bypassed the amperage meter, thus indicating lower amperage reading than the rectifier was actually providing. The rectifier was left operating at a tap setting of C-2, F-1 at 16.14 VDC and 218.4 ADC
The DC current output of the aft rectifier was measured through a temporary variable load resister. By changing the resistance of the load, the rectifier's DC current output changed accordingly to Ohm's law. This determined that the rectifier was once again operational.
The cable hang offs for each rectifier were visually inspected, and found to be satisfactory. The top portions of the hang offs have been filled with silicone sealant, as recommended, to prevent water from pooling on top of the hang off assemblies.
Prior to final energization of the forward rectifier, a drop cell survey of the hull was performed on June 2, 2005 to record the base line potentials of the hull. Although the rectifiers had been out of operation for some time, the potentials of the hull were still more negative than the base line potentials recorded in August 2004. This indicated that the hull was still showing evidence of polarization from the operation of the ICCP systems and remaining sacrificial anodes. The forward rectifier was then energized and allowed to polarize for approximately 3 hours before the final potential readings of the hull were recorded. These readings are tabulated in Table I. All readings, unless otherwise indicated, are referenced to an Ag/AgCl reference electrode:
It was not until a ROV inspection of the anode arrays was performed in May 2005 that the failure of the aft anode array was determined (Fig 12 - 14).
Fig 12 - Damaged anode sled
Fig 13 - Damaged junction box
Fig 14 - Damaged anode float assembly
In June 2005, a ROV retrieved several components of the failed aft anode sled;
1. Copper buss bar - The retrieved buss bar was less than 50% of its original mass and exhibited severe metal loss over its entire surface (Fig 15)
2. Steel junction box base plate - The retrieved junction box base plate was almost entirely consumed Fig (16). The power feed cables to the junction box were severed and exhibited burnt insulation (Fig 17).
3. PVC protective shroud - This shroud prevents the buss bar form contacting the inside of the steel junction box. It showed distortion and high temperature scorching (Fig 18).
Fig 15 - Damaged copper buss bar
Fig 16 - Remains of a 19" (482 mm) diameter blind flange
Fig 17 - Damaged power feed cable
Fig 18 - Damaged PVC shroud
It is highly unlikely that one of the anode float assemblies failed. It this was to have happened, the anode element would have to have been in contact with the frame of the anode sled. The steel at this point of contact would start acting as an anode by discharging current, thus causing metal loss of the steel framing. The other remaining anodes would also continue to discharge the majority of the current. The only anode that was found to be lying on top of the frame was the one by where the main cable entered the anode sled (Fig 12). One would expect to have the most corrosion and metal consumption near the anode contact to the frame. However, this was not the case. The majority of the remaining steel frame was at this location, hence these observations do not support float failure as the cause.
Shorted junction box
From the ROV survey, the protective PVC shroud was still intact around the buss bar contained inside of the junction box (Fig 13). If there were a direct contact from the buss bar to the inside of the junction box, the flange bolts would be the first objects to corrode away. Once these bolts were consumed, the metallic electrical path to the frame would be eliminated and all further corrosion of the frame would cease. With the bolts being the first contact from the painted junction box dome to the frame, the IR drop would be greater across this connection, and the bolts would corrode away preferentially. The flange bolts were all missing. Without these bolts serving as part of the electrical connection from the buss bar to the frame, the extent of corrosion to the steel frame does not support the shorted junction box theory.
Flooded junction box
If the junction box were flooded with sea-water, only part of the current would discharge off the buss bar into the sea-water inside of the junction box. The remaining current would still be discharging through the anodes. The current discharging off the buss bar would pass through the sea-water to the steel surfaces inside the junction box. This current would then pass through the steel framing and discharge off the exterior surfaces of the frame. Whenever electrical current discharges off from a metallic object, metal is consumed at the point of discharge (exactly how an anode operates). The amount of metal consumed is a function on the magnitude of the current and the consumption rate of the metal. The copper buss bar has a consumption rate almost twice that of steel anode sled framework. The only electrical path for this current is what is discharged off the buss bar. Therefore, the equivalent mass of copper required to account for the metal loss of the anode sled would be about twice that of the steel frame that was corroded away. This is a physical impossibility.
Shorted out anode cable not due to lightning
If one of the anode cables were shorted to the frame, this would have to occur at a pinch point in order to maintain electrical contact to the frame long enough for the mass of steel frame to corrode away. These cables are free hanging and are not under tension that would promote or sustain this type of contact to the frame. In addition, this type of contact would be an intermittent contact at best; however, the contact of the electrical cable to the sea-water would be constant and the exposed electrical cable would corrode away and eliminate the electrical connection long before the steel frame would have corroded away.
Direct connection of cable to anode sled frame
If a cable were directly and securely connected to the frame through some manufacturing error, which is the type of connection that would be necessary to sustain the current flow to the frame, the amount of steel that corroded away would have occurred in 2 months and not 7 months. The surface area of the anodes (3 ft2) (0.28 m2) compared to the surface area of the frame (200 ft2) (18.5 m2) is only 1.5%. Thus, the frame would discharge 98.5% of the current. Because the frame did not corrode uniformly, this does not support the presence of a single electrical contact to the frame.
Lightning strike
This is the most likely scenario. A lightning strike destroyed the DC lightning arrestor in the aft rectifier, the PVC insulating shroud inside of the junction box was burnt and deformed, and all the recovered cables going from the rectifier to the anode sled were burnt in two, approximately 12 inches (300 mm) below the junction box. The other cables were most likely also damaged in a similar fashion. The energy lightning has, or how it interacts discharging off a steel surface in very low resistivity sea-water is unknown. The anode sled was operating prior to March 9, 2005. The burnt cables effectively cut the anode sled out of the circuit. The small current discharge after that date is most likely due to current leaving the exposed ends of the copper cables and consuming the copper cables up into the insulation until the circuit resistance increased enough and the driving voltage was no longer sufficient to discharge any more current.
The rectifier does not have the energy or capability on its own to destroy the cables as we have observed (Fig 17).
Parallel paths for a lightning strike to ground include the ZP hull through the sea-water, the mooring chains from the ZP to the seabed, and the anode sled / power feed cable assembly.
1. The hull of the ZP is coated and does not present a low resistance path to ground.
2. The mooring chains consist of numerous high electrical resistances at the point-to-point mechanical contacts of the links. Experience has demonstrated that conductivity of long lengths of mooring chain is low.
3. The anode sled and cable assembly is designed to be low resistance, and it is situated on the seabed. This is the preferential path to ground for lightning.
We may never know what really happened underwater. Eyewitnesses confirmed lightning strikes to the ZP occurred during a severe storm the day before the drop in T/R current was discovered. Inspection of the subsea components supports the lightning strike theory. The investigation team concluded that a lightning strike is the only plausible mode of failure.
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