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58 APRIL 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 4 where anode consumption was 80%. Figure 2(d) shows that the entire structure is cath- odically protected with Scheme 4, with a minimum potential of –967 mV and a maxi- mum potential of –814 mV. Besides the distribution of structure- seawater potentials, the CDs were calcu- lated for both the structure and the an- odes. Based on the CD and surface area of the anodes, the anode current was deter- mined for each anode. Additionally, the service life of each anode was estimated using Equation (1): = ⋅ ⋅ T m C A i (1) where T is the ser vice life of the anode (years), m is the mass of the anode (kg), C is the capacity of the anode (kg/y/mA), A is the area of the anode, and i is the CD for the anode (mA/m 2 ). Both the utilization factor and the efficiency of the anode were taken as unity in this study. Th e ser v i c e li fe of th e an o d e s for Schemes 1 through 4 are calculated and summarized in Table 1. For Scheme 1, the actual state, not all of the structure is cathodically protected and the average remaining life of the existing anodes is ~3.86 years. For Scheme 2, the complete structure is cathodically pro- tected and the total current provided by the anodes is 102.938 A. The average life of the anodes is ~13.56 years. For Scheme 3, not all of the structure is cathodically pro- tected. The total current provided by the anodes is 94.291 A and the average life of the anodes is ~13.54 years. For Scheme 4, the complete structure is cathodically pro- tected, the total current provided by the anodes is 105.889 A, and the average life of the anodes is ~20.13 years. In summar y, Schemes 2 and 4 provided adequate CP for the entire structure; however, in consider- ation of the retrofitting requirement for the life of the platform, only Scheme 4 is ex- pected to provide the 20-year extension of service life for the platform. The optimum SACP design was based on the polarization curve of the bare plat- form structural steel and the seawater sam- ple taken in the platform location. The ef- fect of calcareous deposits was considered using the dynamic polarization and long- FIGURE 2 Distribution of structure-seawater potentials for (a) Scheme 1: actual state; (b) Scheme 2: the addition of anodes in five places with 100% anode consumption; (c) Scheme 3: the replace- ment of four anodes with 80% consumption; (d) Scheme 4: the replacement of four anodes with 80% consumption as in Scheme 3, as well as the addition of five anodes as in Scheme 2. projected protection potential was found to be in reasonable agreement with the sur- vey data, which demonstrates the effective- ness of the method. For Scheme 2, five an- odes were added in places where anode consumption was 100%. As a result, all sur- faces of the structure were cathodically protected, with a minimum potential of –947 mV and a maximum potential of –806 mV. For Scheme 3, only the four anodes with 80% consumption were replaced. Con- sequently, not all portions of the structure were cathodically protected: the minimum potential was –944 mV, and the maximum potential was –763 mV. Scheme 4 was the combination of Schemes 2 and 3, with five new anodes added where anode consump- tion was 100% and the four anodes replaced MATERIALS SELECTION & DESIGN