Materials Performance

OCT 2016

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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24 OCTOBER 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 10 level of current for maintenance once the str ucture was prop erly polari zed . Th e actual time to fully polarize the structure would depend on several site variables, but was expected to begin in as little as one week. A f t e r c o m p l e t i n g t h e c o n c e p t u a l design for both the galvanic anode and impressed current options, a rough budget estimate was developed based on both systems being designed for a 30-year oper- ating life (Table 2), and presented to the owner. Based on the significantly shorter installation time and the overall cost sav- ings associated with the impressed cur- rent option, the owner chose to proceed with the MMO anode system. Several addi- tional factors, however, impacted the cost evaluation and the decision-making pro- cess. These included: a) Quantity of anodes—with the gal- vanic CP system, over 95,000 kg of aluminum anodes would have been required. The material costs and the freight and handling efforts involved for this quantity would be significant. Conversely, the impressed current anode system only required six anode sleds—each weighing ~1,300 kg—and three rectifiers weighing 300 kg each. Total system weight of the impressed current alternative was <10% of the w e i g h t of th e re q u i re d ga lv a n i c anodes. b) Installation time—for the galvanic CP system, it would require a diver to physically secure each massive anode to one of the jetty sheet piles. Assum- ing tw o anode installation crew s operating in parallel with a produc- tion rate of four anodes/crew-day, the installation would take ~30 work days to install 247 anodes weighing 400 kg each. With the MMO sleds, the entire installation could be com- pleted in less than one week using a single crew with a crane, and a single diver to help the sled settle properly on the sea f loor and then route the anode cables along the sea floor and up the side of the jetty piles to the rectifier. c) S afety—w ith th e greatly reduced quantity of anodes required, along with the simplicity of the sled instal- TABLE 2. GALVANIC AND IMPRESSED CURRENT COST COMPARISON Galvanic System Impressed Current System Materials $365,000 $270,000 Installation $120,000 $30,000 Total cost $485,000 $300,000 FIGURE 2 Anode sled installation. FIGURE 3 CP system rectifiers. would provide more than sufficient anode life, even when briefly operating at 100% of their rated capacity during the initial sys- tem polarization. For the impressed current design, the rectifier capacity would be sized to meet the initial polarization output require- ments and then be operated at a reduced CATHODIC PROTECTION

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