2010 diagnosis numeral modeling and development of the cathodic protection refurbishung of the metal

Paper No.

10393

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· · ··~ NACE CORROSION 2010 I N

T ER N

A T ION A L

CONFERENCE & EXPO

DIAGNOSIS, NUMERICAL MODELlNG ANO DEVELOPMENT OF THE CATHODIC

PROTECTION REFURBISHING OF THE METAL SHEET PILlNG OF THE DOCKS OF

QUETZAL MARINE TERMINAL IN GUATEMALA

Lorenzo M. Martinez_dela_Esealera , Osear Alvarez , Jorge Canto , Hernan Rivera , Arturo Godoy, Hugo E. Ventura

Corrosion y Proteccion Ingeneria, S.C. Rio Nazas 6, Cuernavaca, Morelos. Mexico. 62290.

Jorge A. Aseeneio, and Lorenzo Martínez

Instituto de Ciencias Físicas, Universidad Nacional Autonoma de Mexico, Universidad 1001 ,

Col. Chamilpa , Cuernavaca , Morelos. 62210. Also at Corrosion y Proteccion Ingenieria SC

ABSTRACT

This paper reports the refurbishing of the 1200 A cathodic protection (CP) system on a 2-mile

(3200 m) long , 100-foot (30.48m) deep steel sheet piling wall after 30 years service. Included

are the diagnosis of the sheet piling by underwater ultrasonic measurements; the analysis of

the performance of the mixed metal oxides (MMO) anode array along the docks; and the

effects of cable defects that exposed the copper sheathing to aggressive environments on the

anodic side of the circuit and rapid deterioration. The paper also reports the performance of the

high current rectifiers ; the replacement of the MMO anodes; the redesign of the anode

mountings; and , the wiring details of the anode connector cables and rectifier modifications.

The paper includes results of the tests of the CP current distribution in the wiring as well as the

return current from the protected structure to the rectifier. The status of the CP system

performance after the refurbishing is presented. The work included developing a numerical

model of the CP system as designed. The limitations of the original design are discussed in

view of all the metall ic structures involved, other than the steel sheet pilings, serving in

environments other than the main dock sea front.

KEY WORDS Cathodic Protection system, Steel Sheet Piling Wall.

©2010 by NA CE Intern ational. Requests for pe nni ssi on to pu blis h th is ma nu scri pt in any for m , in part or in whole. mU Sl be in writing to N ACE ln tern ational, Pu blica tions D iv is ion , 1440 So uth Creek Orive, Ho uston, Texas 77084. The mate n al present ed and th e vie ws expressed in th is paper are solely th ose aY lhe au th or(s) an d are no t necessaril y en dorsed by the Associatio n.

INTRODUCTION

Quetzal Marine Terminal was built after a requirement of the Guatemala government in 1979 year, inaugurating the Quetzal Port during the year 1983. Its location is latitude 13째 55" NI longitude 90째 47". Quetzal port is considered one of the main ports in the Republic of Guatemala, because the amount of imported/exported products handled through it. It is administrated by the "Empresa Portuaria Quetzal". The port complex involves 835 .15 hectares, divided into 10 zones, which are used with commercial goals that are at the disposal of the interested entities in developing projects related to international trade . Its strategic geographic location provides service mainly to the Pacific Basin and the West Coast of the American Continent. But, beca use of its proximity to the Panama channel, it can be accessed from any place around the world (see figure 1a and 1b). The port has modern infrastructure, machinery, equipment and specialized installations, which provide complete port services, at competitive costs, large area for commercial and industrial development and access to tourist attractions in the country. [1]

Figure 1. Generalities of Quetzal port. a) Satellite image to identify the loeation of the port (marked with the arrow), b) aerial view of the Quetzal port, e) physieal inspeetion of the dock and d) identifieation of the site with metal strueture to be proteeted.

In the search of solutions to preserve the optimal conditions of the docks, the "Empresa Portuaria Quetzal" , contracted "Corrosion y Protection Ingenieria" to develop a plan to install a new cathodic protection I?ystem for docks of Quetzal Port. This project involved diagnostic, identification of particular corros ion problems and the implementation of solutions to enhance the cathodic protection system.

The authors were contracted to implement a project

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concerning the renovation of the cathodic protection for docks of the Port Quetzal. This became a particularly interesting goal beca use of the variety of metallíc structures íncluded in the main dock, serving a wide range of ship types: solid bulk carriers, container ships, tankers, transporters, and specialists in refrigeration, general chargers, roll on - roll off, cruisers, among others.

APPROACH: TASKSANDMETHODOLOGY In order to develop the tasks related to the renovation of the cathodic protection system, several tasks were identified to form a complete methodology. The included: • General evaluation of the components of the cathodic protection : rectifiers, impressed current anodes, connections network, anode connectors' network, test stations, electrolyte-structure potential performance, cathodic protectíon current distribution and evaluation of structure isolation.

• Physical dimensional survey of the metal/lc structure of docks in Quetzal port that are protected by the cathodic protection system.

• Physical trace and diagnostic of continuity in the electric network of anode and cathode connections with the help of a global positioning system that included differential precísion by satellite.

• Evaluation of physical conditions and conductance of the electrolyte­ structure interface.

• Physical and chemical properties measurements of soil and marine media, that are relevant to the design of the CP system; and evaluation of corrosion damage in vertical metallic construction elements of the dock.

In addition to the above tasks for enhancing the system:

work included:

• Cathodic protection engineering to verify the design function and the adequacy of the current distribution for corrosion control over the metallíc surfaces of docks in the Quetzal port under the actual physical conditions.

• Repositioning of anode and cathode wire networks of the CP system, in order to improve energy conservation.

• Reposition of the 100% mixed metal oxide (MMO) anodes for the impressed current network.

• Repair and activation of the automatic control systems for the rectifíers.

• Rehabilítation and maintenance of rectifiers including the reserve rectifier.

• tests and commissioning of cathodic protection system rehabílitation.

• Elaboration of plans (as-built) of cathodic protection system with the rehabilitation related installations.

• Delívery of the installation and trainíng on CP system operatíon to technicians of Quetzal port.

SITE ANO PREVIOUS CONDITIONS Dock diagnostic

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The dock of the Quetzal port uses a stake-board of approximately 1,773 meters as the mechanical element for ship mooring. It is divided into four sections; the Roll On-Off dock, the East dock, the South dock and the Fishing dock. Additionally, on the seaward side, the stake足 board has a face in contact with the soil below the docks, which is also joined to another stake足 board (Iocated approximately 30 meters away) by using steel tieso So, assuming cathodic protection for the whole structure electrically linked to the stake-board, the four individual sections that represent a total of 87,090 square meters must be considered as is illustrated in table 1.

TABLE 1.

AREA OF THE ST AKE-BOARD FOR THE DIFFERENT SECTIONS.

Length [m)

Area [m2)

External face of external plate

1773

26595

External face of internal plate

1773

External face of internal plate

1130

26595 16950

External face of external plate

1130

16950

Total area

87090

Using a GPS differential correction system, the dimensions of the installation and metal structures were verified . The measurement of the dimensions of the distribution and placement of the anodes and the water level (against the metal structure) provided a clear picture of the area of the stake-board that must be protected. This sizing was accomplished by the work of specialized diving and cabinet calculations (see figure 1c). The total metal surface that had to 2 be considered for the design of the cathodic protection systems was established as 26,595 m of uncoated stake-board along the dock as it is illustrated in figure 1d. The structure dimensional data were reproduced with a geographic information system (GIS) for conveyance to modeling programs. Recorded findings also included cathodic protection relevant parameters.

Cathodic protection evaluation Originally (before this project), the face of the external plate was the only one with a cathodic protection system. Its design was based on a system of impressed current controlled by 8 rectifiers with a capacity of delivering 300 amperes of direct current, and with 50 Volts (up to 15 kw) of tension each. The rectifiers were Universal equipments model OSOI with 3-phase power of 230/450 Volts at 60 Hz AC the current input ranges from 18.8 and 8.9 amps. Teams are immersed in 110 gallons of dielectric oil which comply with the regulations ANSI7 ASTM 0足 3487 and NEMA TR - PB - 1975.

The 8 installed rectifiers are of single output. The main anode cables conducts of re-routing to cash anode, where the connection to secondary cables is located . The cathode cables are connected to the masses (physical masses on the stake-board sUrface, which are used as an easy monitoring connection point) independent to close circuit with the stake-board as shown in figure 2a. It is worth mentioning that all the inspected connections to masses were considerably sulphated; so the circuit resistance was significantly increased before CP system renovation . The 8 rectifiers can also be operated manually, with the capability of measuring the structure/ soil potential on a different screen . At the time of the inspection, however, these were found to working manually, because the automatic control cards were damaged. It was

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discovered that the original instrumentation for the automatic control used a fixed silver-silver chloride reference cell that had become encrusted by marine growth, shown in figure 2b, greatly reducing its monitoring capacity.

Figure 2. Identified problems in the dock eathodie proteetion system. a) Insta/lation of the eleetrieal, sea life around elements and e) damage on the wires and metal/ie strueture exposed to the sea water.

Electrolyte evaluation To achieve the correct redesign of the CP system it was necessary to measure the resistivity of the water directly in contact with the stake-board . In the performed tests , the average resistivity was 25 ohm-cm. This corresponds to a highly corrosive electrolyte with high electrical conductivity. The underwater inspections determined that a considerable number of anodes were damaged in their mechanical integrity, and that some had ceased to operate due to consumption of the connecting copper wire. Also, it was determined that the joints between anodes and conductor wires were accompl ished in the field with no formal procedure. This should have been required because the complexity of the process. The observed damage in some of the anodes corresponds to impacts by ships , resulting in the loss of mixed oxide coating integrity, consequently reducing the useful life of the anodes . Examples of the observed damage to the anodes are shown in figure 2c.

Besides the previously mentioned problems , some cables that were over the structure surface from the rectifier to the connection registries with the anodes showed isolation failures . This resulted in insufficientcurrent flow of the associated anodes . Using the current mapping systems with omni-directiQnal antennas , it was possible to locate precisely these failure points of mechanical coating, even when they corresponded to small pores of just a few millimeters.

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Electrical network tracing and study The physical tracing of electrical networks by GPS technology was focused on locating cables, their verification and their continuities, as well as their depth. Geo-positioning with differential correction capability was also used to generate an advanced diagnostic of the electrical networks of anode and cathode connections. The location of the network was carried out along whole dock and on the original drawings the circuit of anodic and cathodic networks. Efforts were made to apply the information on path of networks to generate a complete map including the circuits that were found with electrical when discontinuities caused by the damage to the wires and bad circuit conditions. was valid information about the location of the cable conductors inside the circuitry for the Quetzal port dock, it was important to have a record of the geo-referenced position of component of structure, including their depth and relative position inside the dock. trace study and locatíon of the electrical circuitry of the CP system for Quetzal Port confirmed a total of 8 rectifiers, 209 anodes, 36 connection points at the stake­ board and distributed along the dock as it is shown in figure 1.

Metal structure potential evaluation The control of the impressed current was achieved with a portable direct current source with a current interrupter. This setup allowed the connectíon of a temporary anode bed to assess degree of current impression The rectifiers were to control the curren!, using the iron extensions, with continuity, as monitoring stations that are commonly called cathodic masses. For the evaluation of this activity, it was necessary to use devices for synchronization vía satellite that allowed measuring the ON-OFF potentials at points with the Ag/AgCI cell as . ."'~,-..."'....

TABLE 2.

STRUCTURE POTENTIAL ORIGINAL VALUES FOR DIFFERENT POINTS IN REFERENCE TO AN Ag/AgCI

CELL.

behavior was slighÜy different for each rectifier. The original structure potential in reference to the Ag/AgCI cell for each rectifier and mass is shown in table The most

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significant variation of data was recorded for rectifier 7. However, it is clear that there is gradual reduction of potentials at several of the evaluated sites.

Corros ion damage evaluation

In addition to the main objectives of evaluating the corrosion damage in the vertical metallic structural elements of the Quetzal dock were the verification and the thickness measurements for the stake-board . This was accomplished by means of ultrasonic inspection to determine the thickness profile. In order to achieve this, visual inspection of the stake-board was conducted by a CP systems certified engineer. In addition to visual inspection , photographic and submarine videos, and ultrasonic measurements of the walls were performed since dock walls had been subject to corrosion . i. e. the loss of material that results in decreasing wall thickness to the point of reaching structurally critical conditions.

On the basis of the dock depth , a total of 52 points were selected, covering 1.9 kilometers and distributed approximately every 30 meters. For each point of sampling , three measurements were obtained, resulting in a total of 156 collected records. Table 3 shows the average thicknesses at four depths (O, 4 , 8 and 12 meters), as well as an overall average thickness determined for the stake-board .

TABLE 3.

AVERAGE THICKNESS MEASUREMENTS.

ID

A

O

B

4 8

C

O

AVERAGE VALUE OF STAKE足 BOARD THICKNESS (in) 0.788 0.785 0.887 0.961

OEPTH (m)

12 GENERAL AVERAGE

0.825

Based on these results, the average thickness at their respective depths , as well as a general average thickness found on the stake-board . Note that at shallow depth, there was a wall thickness less than 0.788" Near the surfrace there is greater exposure of the stake-board to factors such as a high concentration of dissolved oxygen , resulting in depolarization at the cathode defects, related to ncreased movement of water in contact with the protected structure , as well as increased aeration . Other factors may include various substances that could be in the seawater near the surface that are likely to accelerate the corros ion processes. Also it must be considered that the area with the greatest structural integrity is found around 12 meters depth with 0.961" thickness, 18% more than at O meters. The aboye results produce an overall average of 0.825 " that is acceptable as wall thickness for the stake-board .

Cathodic protection system modeling

An analysis was implemented looking for a better understanding of the fundamental parameters that affect the operation of the cathodic protection system, and to identify the way to apply enough electrical current for a sufficiently electronegative polarization of the metallic structure. To achieve protection the criteria is -900 mV instant off against a Cu/CuS reference cel!. Modeling mechanisrr;s are really useful ; particularly the application of approaches where the different elements can be considered by using the boundary e/ement method and the rinde

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element method based on numerical analysis and empirical data. This methodology allows adaptation of a particular calculation to real data and to use the computer as a virtual lab. The utility of this system is a graphical illustration of the potential distribution over the metallic structure as a function of the that play a role such as rectifier and anode configuration, soil and the conditions at the metal I eletrolyte interface.

generated of the original system From the anode data a model can the optimal performance and to evaluate the function of the proposed conditions for the operation of refurbished network.

Old system model

From the anode behavior data, and the previously obtained values for geometry, structure dimensions (confirmed by the submetric geo-positioning), number and characteristics of anodes, impressed current by the anodes, polarization potentiai criteria, media resistivity for lts seawater and soils, and electric isolation, the system is reproduced as a model performance before rehabilitation. The model produces calculated profiles for the two sections of the stake-board in the seawater and the corresponding burled reglons. It is that both show a common behavior with similar for the calculation in seawater and burled conditions. potential values, however, are very dependent on position along the metal structure. The lower potential values occur where the electrolyte is earth (Figure 3).

From profiles it is evident that multiple with insufficíent polarization to protect the dock are more in than in seawater locations. For first section the high risk sites appear in number and are also to the dimensions of the segment.

Potencial oH ac:tual

Pot.ndal off actual

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Figure 3. CaJculated

off profiles for the stake-board. First section in seawater and b) buried; additionally, the second section e) in seawater and d) buríed.

8

Model for the improved conditions Fol/owing a similar procedure for the case of an analysis for candidates improve the operation the CP system, are shown of comparable profiles against the conditions of stake-board. Figure 5 shows the calculated profiles where the implementation are of solutions allowed control on the polarization potentials and the protected increased to fulfill -850 mV . (This value is for the CU/CUS04 cel!. corresponds to silver-silver chloride (Ag-AgCI) reference value of -900mV). This calculation anodes, and related instrumentatíon, for new was based on the substitution of the cathodic protection system but with anode locations, wíth the goal of determining the required current to the whole stake-board.

Potencial off después de rest1Il1l1lción

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4. Calculated potentíal off profiles for the stake·board. First section in seawater and b) additionally, a second section c) in seawater and d) buried.

real impact on the cathodic protection operation can be identified with the help of the plots in figure where the calculated potential distribution around the stake-board structure in the sol! is shown for case, the original and modified conditions. plots compare the potential distribution behavior. The post renovation, enhanced system performances shows considerable improvement in the uniformity of the potential distribution.

Through the simulation of the cathodic protection system installed in the dock of Quetzal port, it was found that the design is suitable. However, its requirements for maintenance resulted in deficiencies in the system. The reasons were attributable to problems with part of the cabling system as well as damaged anodes, which had by various oftime, impacting negatively on performance. The above could be factors over a determined form original condition model, which showed that the amount of current

9

provided by anodes varies significantly along the stake-board; this means that while some were delivering slightly more than 10 amps, there were several others that were just delivering hundreds of milliamps and others further outside acceptable operation. These results show that the variation in the amount of current delivered by the anodes caused different areas of the stake-board to be inadequately protected as the recommended potentials were not reached.

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Figure 5. Gradient plot of potential distribution for the stake-board a) before and b) after restoration.

Thus, to achieve the proper performance of the CP system, as is seen in the results for the modined model, it was recommended that damaged connections and anodes be restored or replaced. The need is current deliver capacity of 6.5 amps per anode, given the same location as in the original anode distribution. This would achieve uniform dispersion and therefore adequate potentials to comply with cathodic protection international regulations al! along the stake-board, optimal in both, the submerged and the subsea buried areas.

CONCLUSION: IMPROVEMENT OF CATHODIC PROTECTlON SYSTEM Corrective actions

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From the previously mentioned results concerning diagnostic and modeling efforts, the replacement of anode and cathode cable networks, the need for rectifier maintenance was determined and implemented. The anode and cathode network replenishment required a detailed methodology for cable replacement, circuit repair and rectifier enhancement. Figure 6a shows this work in progress. The installation work was undertaken by engineers certified by NACE International.

As part of the activities in the anode cleaning and replacement, the existing joint between the MMO anode and the box of anodes (type A:n connection) was disconnected in order to allow the submerged dive can pull the guide and remove the cable. Later the cables for the new anodes were fixed to the guides to allow divers pulling and arranging the cables of the new anodes (figure 6b). The new anodes were placed into PVC holder mounts, which have a nominal capacity of 25 amps as it is shown in figure 6c. As part of the manipulation procedure, prisoner screws were taken off by dives to subsequently remove the hydraulic PVC Stoppers as in figure 6d.

Figure 6. Images of the cQfrective actions of the cathodic protection; a) cables substitution, b) besides the submerged cables manipulation to allow the anode substitution, c) the new mixed oxide anode into PVC holders, extraction of the prisoner screw to subsequently remove the hydraulic PVC Stoppers, and e) maintenance and rehabilitation of rectifiers.

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rehabilitation of the cathodic protection rectifiers was performed by changing the dielectric oil, the extraction of total used oíl and the replacement with Univolt N61 oíl through pumping. Rectifiers were also cleaned, rehabilitated multiple components were physical replacement (electrical terminals, components connections, of fixation and hardware in general) as it is illustrated in figure 6e. The appropriate cleaning and replenishment of anode and cathode cables of DC power was done.

Polarization potentials values

After the design of the cathodic protection system and the different corrective actions, the on­ site evaluation of the polarization potentials the values shown in table 4. beneficial impact of the rehabilitation work is evident. The polarizatíon potential values fulfill the -850mV criterion in all and there is an evident tendency of more negative values in the expected because the of the seawater superficial evaluated sites, which can resistivity.

From the two main goals (diagnostic and rehabilitation), we can conclude that the impact to the dock protection with the new cathodic protection system is positive and the real capability to reduce the corrosion processes over the stake-board in the two sections and the two conditions (seawater and buried) are based on the fulfilling of the -850mV criterion.

TABLE 4.

STRUCTURE POTENTIAL ENHANCED VALUES FOR DIFFERENT POINTS AGAINST AN Ag/AgCI CELL

The contribution of the different techniques of characterization, together the computer modeling of the original and the proposed conditions, and the solutions implementation (focused to the cables, an.odes and cathodes network, and existing rectifiers) are showed in this work. This set of tasks as a complete process allowed identifying the contribution of one for the understanding and well sustained solutions suggestion.

j

2

rehabilitation of the Quetzal port will allow the operation of this maritime terminal for a longer time and with the even under the different conditions that affect the metallic structure of the dock. This analysis allowed studying the effect of the water conductivity and the configuration of cathodic protection system.

REFERENCES [1] http://www.puerto-quetzal.com/php/ L. M., J., A., Camilo Calve!, H., Albaya, H. System for an Airport Fuel Pipeline.

W. A. Cathodic Protection For Steel Bulkheads Under Relieving Platforms. Materials 30 (11), 20-23,1991.

rrrl<lnl"a

, Albaya, H.

L. M., Rivera, H., Godoy, A., Betancourt, Norberto,Ascencio, J. A., Martinez-Gomez, L. Pipeline Survey in 100-mV Polarization CP Criterion.

48, (4), 32-36,

Ordaz, JM Corrosion Performance of Concrete Columns After Localized Repairs 11 2009.

Environment.

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