Materials Performance

APR 2017

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.

Issue link: http://mp.epubxp.com/i/804522

Contents of this Issue

Navigation

Page 39 of 84

37 NACE INTERNATIONAL: VOL. 56, NO. 4 MATERIALS PERFORMANCE APRIL 2017 and the area adjacent to that anode is less than the R e between the anode and other elements of the structure. Considering Equation (9), the protec- tive current provided to the area adjacent to an anode is greater than the current pro- vided to other areas. According to the rele- vant literature, strong evidence from off- shore potential measurements indicates that the R c value may be locally high at complex nodes and locally low in areas adjacent to an anode. By Ohm's law, the protective CD is maximized in the areas adjacent to anodes, while it is minimized in the areas adjacent to complex nodes. Pro- tection potentials shift to more negative values with increases in the CD received. With the uniform distribution of CD illustrated in Figure 2, the highest and low- est mean current outputs of the anodes in the optimum design are 113 and 93 mA, respectively. Since the current output of a sacrificial anode is directly related to its mass loss rate, 14 and the ratio value of the highest to lowest anode mass loss rates for this design is 1.22, all anode consumption is relatively uniform during the design life. In the optimum design , the sacrificial anode system produced the maximum CD output for the anodes. Using CD outputs, the total current output from the sacrificial anode system can be calculated . 14 The mean current output of the anodes is 1 A. Since a greater spread of potentials pro- duces increased ri sk, th e vari ation of assum ed simulation conditions w ould cause corrosion. In addition to this consid- eration, the uniformity of the anodes' mass loss rates was assessed, since the highest efficiency of the sacrificial anode system design indicat es that al l anodes have reached the utilization factor simultane- ously. This helps to avoid early installation of costly retrofit systems, which may be needed if the consumption rate of some anodes is faster than others. 3,14 Since the potential distribution on the jacket and the mass loss rates of all anodes were uniform, it was concluded that this design provides the optimum arrangement of anodes on the jacket. Sacrificial Anode System Modeling in the Caspian Sea The contour plots for the optimum cal- culated potential and protective CD distri- FIGURE 4 CD output of anodes contour plot (A·m –2 ) and protective CD contour plots (A·m –2 ) for the optimum design in the Caspian Sea. FIGURE 5 Comparison of the calculations and field-measured potentials at selected sites in the Caspian Sea. bution on the jacket model in the Caspian Sea are illustrated in Figures 3 and 4, respec- tively. The protective CDs on the jacket here are lower than those for the Persian Gulf. The protection potentials shift in the elec- tropositive direction compared to those in the Persian Gulf and the potential over the jacket could be below a critical value. The mean current output of the anodes is 0.52 A. Although thi s represents a significant reduction compared to the results for the Persian Gulf model, the anode mass loss rates in the Caspian Sea are uniform and appropriate. This difference in current out- put is probably related to the greater con- ductivity of the Persian Gulf water. Comparison of Field Measurements and Finite Element Method Calculation Results Th e f i el d vali d ation m easurem ents were carried out to justify the applicability of FEM calculations to simulate the sacri- ficial anode system. The potential values of selected sites obtained from the FEM model and the field measurements for the platform jacket in the Caspian Sea are shown in Figure 5. The potential of the structure gradually reaches a constant value, corresponding to steady state. The sam e trend obtain ed by calcul ation i s found in situ for each part of the jacket. In c ontra st to th e m o d eli n g re sult s , th e Modeling the Cathodic Protection System for a Marine Platform Jacket

Articles in this issue

Archives of this issue

view archives of Materials Performance - APR 2017