Dry dock final

T

he dry dock of the Madero Maritime Terminal is the largest dry dock in Latin America, ca­ pable of handling ships up to 250 m. It is a strategic installation for the operation of the maritime terminal and for the distribution department of PEMEX Refinación. The dry dock is located on the Panuco River in Tamau­ lipas State under environmental condi­ tions that favor the corrosion of steel structures and the reinforcing steel of concrete structures. The dry dock con­ sists mainly of sheet piling and a steel lock gate.

There are four main metallic struc­ tures subjected to corrosion: 1) the sheet piling, which supports the dry dock, bay dock, and ship repair dock; 2) the lock gate, which is considered a floating ship; 3) the dolphin supporting piles; and 4) the dry dock gate columns, which are made of reinforced concrete.

Different inspections were conducted to obtain an integral corrosion evaluation of the steel structures. Conditions found on each structure demanded complex solutions to improve the actual corrosion control systems. For the atmospherically exposed areas, the solution was based on applying special coatings to irregular surfaces with many cavities, which are a challenge to cover with regular paints.

For the submerged and buried struc­ tures, the best solution was the applica­ tion of cathodic protection (CP), but the geometry made it difficult to predict the current behavior and the magnitude of the nonvisible metallic structures ne­ cessitated field tests and numerical simu­ lation to estimate the total current requirements.

Inspections and Results The Lock Gate The lock gate is a steel structure di­ vided internally into five chambers of different capacities. The chambers are NACE International, Vol. 51, No. 3

Corrosion Assessment and CP Design for a Large Dry Dock in the Gulf of Mexico Hernan Rivera and Arturo Godoy, Centro de Investigación en Ingeniería y Ciencias Aplicadas, UAEM, Toluca, Mexico Lorenzo M. Martinez-dela-Escalera, Jorge Canto, Lorenzo Martínez, and José A. Padilla, Corrosión y Protección Ingeniería S.C., Morelos, Mexico Carlos G. Lopez and Cecil H. Knight, PEMEX Refinación, Huasteca, Mexico Leonardo De Silva-Muñoz, Instituto de Investigaciones Eléctricas, Morelos, Mexico Jorge A. Ascencio, Instituto de Ciencias Físicas, Universidad Nacional, Autonoma de Mexico, Mexico City, Mexico

This article reports the engineering work and numerical modeling for the complete diagnosis and refurbishing of the cathodic protection system for the largest dry dock in Mexico.

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Corrosion Assessment and CP Design for a Large Dry Dock in the Gulf of Mexico

figure 1

Electrochemical potential distribution according to the three-dimensional model of the lock gate.

figure 2

A satellite image of the ship repair dock, the dry dock, and the bay dock. The yellow line shows the location of the sheet piling.

filled and emptied with water according to need during the operation of the gate, but when the gate is not operating, the chambers are always filled with water.

The lock gate is in constant contact with water from the Panuco River except for the wall that faces the dry dock, which is also exposed to the atmosphere when the dock is emptied. The lock gate is pro­ tected from corrosion by anticorrosion

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coatings and by sacrificial anodes for both the external and internal walls of the structure.

The lock gate is an extremely impor­ tant element of the dry dock. When the gate is closed, it must keep the dock dry by resisting the high pressure generated by the river water. A lock gate failure could cause uncontrollable flooding of the dry dock, as occurred in Dubai in 2002.1

When the lock gate of that dry dock failed, the incoming water took the lives of more than 20 people performing main­ tenance work on five docked ships and caused vast material damages.

The principal risks that threaten the lock gate integrity are ship collisions with the gate and structural failures that can be caused by the deterioration of key structural components from corrosion.

Inspection of the internal and external walls of the lock gate showed that the coating was in good condition except for some areas with coating deterioration and subsequent localized corrosion.

Corrosion was observed along sharp edges, such as manhole borders and square beam edges. Deterioration of the coating due to maintenance work was also observed.

Three-dimensional modeling of elec­ trochemical potential distribution of the lock gate was performed. The model considered the physical dimensions of the gate, the local water properties, the elec­ trochemical properties of the zinc sacrifi­ cial anode, and spatial distribution. Re­ sults shown in Figure 1 indicate that there is a sharp difference between the poten­ tials near the zinc anodes (blue zones) and the potentials on the rest of the structure.

Experimental validation of the model was performed by measuring the potential of the structure at various points.

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figure 3

Results of the three-dimensional modeling of the (a) ship repair dock and (b) bay dock.

figure 4

Results of the three-dimensional modeling of a section of the two parallel sheet piling sections in the cofferdam area.

The Sheet Piling

m2) of the sheet piling surface is protected

by CP systems. Figure 2 shows the three areas where the sheet piling is located.

Physicochemical analysis of the water showed a high concentration of chlorine (16,000 ppm). This has caused a general metal thickness loss of >25% due to corro­ sion on the unprotected face of the sheet piling, which is in contact with soil. In ad­ dition, 300 corrosion pits were discovered, some of which have penetrated through the steel sheets. Crevice corrosion was also present in almost every joint between the steel sheets forming the sheet piling.

Dock sheet pilings are usually exposed to three different media: 1) the bottom of the sheet piling is immersed in the under­ water soil, 2) the middle part is exposed to water on one side and soil on the other side, and 3) the top part is exposed to the air. This exposure to different media can cause potential gradients that can pro­ mote corrosion processes on the structure.

The Madero Maritime Terminal has a steel sheet piling divided into three zones: 1) the bay dock, 2) the dry dock, and 3) the ship repair dock. Altogether, the piling comprises an area of 30,085 m2, which means that 60,170 m2 of metallic Sheet Piling Protection To stop the generalized corrosion of structure have to be protected against corrosion. Nevertheless, only 8% (4,606 the sheet piling, an impressed current CP NACE International, Vol. 51, No. 3

(ICCP) system using distributed semideep anodes was proposed. Current de­ mand measurements were performed showing that ~0.48 A/linear meter of sheet piling was needed, meaning a total of 614 A. In addition, a three dimensional model of the sheet piling and its sur­ rounding media was constructed using the boundary elements method. The optimum distribution of the anodes was determined using the simulations. Figure 3 shows, in a color scale, the potential distribution calculated with the numerical modeling.

The sheet piling is supported with anchors and steel beams buried behind it. The area known as the cofferdam is a corridor with two parallel sheet piling sections attached to each other by steel beams (Figure 4). By modeling this struc­ ture, it was possible to propose the instal­ lation of anodes between the sheet piling sections to protect both structures and the anchor beams.

The Dolphin The dolphin is a steel-reinforced con­ crete structure supported by 50 steel piles.

The piles are 18-m long, with 8 m im­ mersed in water and the remainder bur­ ied under the river base soil. The two different views in Figure 5 show the dis­ tribution of the dolphin’s piles.

The steel piles are exposed to corro­ sion risks. The total metallic surface of the piles is 1,866 m2; 829 m2 are exposed to March 2012 MATERIALS PERFORMANCE 3

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Corrosion Assessment and CP Design for a Large Dry Dock in the Gulf of Mexico

figure 5

A three-dimensional model of (a) the dolphin and (b) potential distribution according to a three-dimensional simulation of the system.

figure 6

(a) A damaged anode and (b) the articulated design of the anode support.

the water and 1,036 m2 are buried. The corrosion of the piles is controlled by a 12 V/80 A ICCP system. Four platinized titanium anodes, 1.5-m long, are sus­ pended under the dolphin. The anodes are evenly distributed among the piles.

Computer simulations of the system showed that the anode distribution was appropriate. The principal problem with the system was the anode integrity. Float­ ing debris periodically damaged one or more anodes. This has caused the system to be out of order for long time periods.

The proposed solution was to replace the fiberglass anode supports with articu­ lated reinforced nylon bars that can bend when floating debris is passing by. The weight of the anode support allows recov­ ering the vertical position (Figure 6).

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Conclusions Highly complicated structures such as a dry dock require the use of advanced numerical modeling to design adequate CP. Traditional design leaves vital struc­ tures without protection, such as structures lying beneath the ocean floor and internal faces of the sheet piling. Accidents in the past have shown the importance of critical elements such as a watertight lock gate. Its protection has required the input of cor­ rosion specialists from several disciplines.

Anode support, monitoring, and connec­ tions require special designs that fulfill the unique technology requirements of corro­ sion control systems in a dry dock.

For the buried and submerged steel surfaces, CP is the best solution to mitigate corrosion deterioration. Most of the steel

structures are not accessible for repainting or replacement. The combination of dis­ tributed anodes in submerged surfaces and remote anodes for buried structures are intended to provide adequate protection.

Many calculations and field tests will be necessary to avoid possible interference with other structures such as pipelines be­ cause of the increased amount of current proposed in the CP design.

The coating system proposed for the internal walls of the dry dock is capable of filling the gaps between each steel sheet, and has very good mechanical resistance to the sandblasting used during ship mainte­ nance. The coating application will be done after pitting repairs, which includes welding steel sheets in areas where low metal thick­ ness was detected.

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Acknowledgments This work was made possible by the technical support of Jaime Cervantes, Juan Mario Bautista, Francisco Javier Pérez, Edgar Maya, Maritza Paola Lo­ pez, Jose Roberto Ramirez, and Jorge Alberto García.

References 1 M. Leduc, “Sudden Drydock Flooding, a Tragic Event in Dubai, March 2002,” Martin’s Marine Engineering Page, http://www.dieselduck.ca/images/ drydock/index.htm (Jan. 23, 2012).

2 L.M. Martinez de la Escalera, J. Canto, A. Rios, H. Carrillo Calvet, H.C. Albaya, J.A. Ascencio, L. MartinezGomez, “Hybrid CP System for an Airport Jet Fuel Pipeline,” MP 48, 8 (2009): pp. 40-45.

Hernán Rivera is a project manager at Corrosion y Proteccion Ingenieria, Rio Nazas 6, Cuernavaca, Morelos, CP 62290, Mexico, e-mail: hrivera@ corrosionyproteccion.com. He is a mechanical engineer with a Ph.D. in materials science and a NACE International-certified Cathodic Protection Specialist.

A member of NACE, he has six years of experience in corrosion control and integrity management projects in South America and the United States.

Arturo Godoy Simon is a project manager at Corrosion y Proteccion Ingenieria, e-mail: arturogodoy@corrosionyproteccion.com. His experience involves CP system troubleshooting, geographic information system (GIS) analysis, risk analysis using GIS tools, development of simulation methodology, and geo-databases. He has a Ph.D. in materials technology, is a GIS specialist, and is a NACE-certified Cathodic Protection Technologist and Coating Inspector Level 2. He is a member of NACE. Lorenzo M. Martínez de la Escalera is the executive director of Corrosion y Proteccion S.C., Rio Nazas 6 Vista Hermosa, Cuernavaca, Morelos, 62290, Mexico, e-mail: lmm@corrosionyproteccion. com. His engineering experience includes designing, installing, and surveying CP systems for 25 Mexican airports and 3,000 km of PEMEX right-of-way. He has a B.S. degree in finance and an M.S. degree and Ph.D. in materials science. A member of NACE, he is a NACE-certified Cathodic Protection Specialist and Coating Inspector Level 2.

Jorge J. Canto is the engineering director at Corrosion y Proteccion S.C., e-mail: canto@ corrosionyproteccion.com. For the last six years, he has been involved in a nationwide survey of corrosion control installations in Mexico. He has a B.S. degree in mechanical engineering and a Ph.D. in materials science. A NACE member and chair of the NACE International, Vol. 51, No. 3

NACE Central Mexico Section, he is a NACE-certified Cathodic Protection Specialist and Coating Inspector Level 2.

Lorenzo Martinez is director of the Corrosion y Proteccion Ingenieria, e-mail: lmg@ corrosionyproteccion.com. He has 30 years of experience in engineering projects involving corrosion control and integrity management. He served on the NACE Board of Directors as director of the Latin America Area. A 20-year member of NACE, he received the Mexican National Prize of Science and Technology, the Latin American Science and Technology Award of the Organization of American States, and the J.S. Guggenheim Fellowship Award.

He is a member of the National Council of Science and Technology of the President of Mexico.

Jose A. Padilla is a project manager at Corrosion y Proteccion Ingenieria, e-mail: padilla@ corrosionyproteccion.com. He is in charge of promoting the coating consulting services of the company. He has more than 24 years of experience in the coatings industry, is a NACE-certified Coating Inspector Level 3, and has been a member of NACE since 1989.

Carlos G. Lopez Andrade is a project director at PEMEX, Marina Nacional 329, Huasteca, DF 11311, Mexico. A pipeline maintenance specialist, he has participated in multiple program implementations at PEMEX. His experience includes methodologies for evaluating maintenance and corrosion control systems for pipelines, including CP, pipeline integrity, and risk assessment.

Cecil H. Knight is a project manager at PEMEX. He has several years of experience with the company in corrosion control project management, including pipelines, storage facilities, and maritime terminals in Mexico.

Leonardo De Silva Munoz is a researcher at the Instituto de Investigaciones Eléctricas, Reforma 113, col. Palmira, Cuernavaca, Morelots, 62490, Mexico, e-mail: leonardodesilva@yahoo.com.mx. He has four years of experience in the study of electrochemical systems such as electrolysers, fuel cells, and CP systems. He has a Ph.D. in process engineering from INPT in Toulouse, France.

Jorge A. Ascencio is a researcher at the Instituto de Ciencias Fisicas, Universidad Nacional Autónoma de Mexico, Avenida Universidad s/n, Col. Chamilpa, Cuernavaca, Morelos, 62210, Mexico, e-mail: ascencio@fis.unam.mx. He specializes in materials science and solutions development, with experience in metals, characterization methods, new materials design, and the application of nanotechnology in multiple fields. He is a member of NACE, Materials Research Society, Mexican Academy of Sciences, and Mexican Academy of Materials Science and Technology. He received the Mexico State Award in 2005.

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