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
59 NACE INTERNATIONAL: VOL. 56, NO. 4 MATERIALS PERFORMANCE APRIL 2017 TABLE 1. SUMMARY OF SCHEMES Scheme Maintenance of Anodes Number of Total Anodes Number of New Installed Anodes Average Anode Life (y) Minimum Potential (mV) Maximum Potential (mV) Total Anode Current (A) 1 Actual state 16 0 3.86 –892 –735 82.3 2 Add five anodes in the places with 100% anode consumption 21 5 13.56 –947 –806 103 3 Replace the four anodes with 80% consumption 16 4 13.54 –944 –763 94.3 4 Combine Schemes 2 and 3 21 9 20.13 –967 –814 106 term curve. The unique polarization curve of bare steel was used for different arrange- ments and seawater depths. In comparison with the primitive de- sign using empirical CD demand, the retro- fitting scheme uses the same number of anodes; however, only nine of the anodes are new (added or replaced), and the other 12 existing anodes have consumption up to 70%. The anode resistance increases as the anode radius decreases, resulting in re- duced current output. The BEM-elected design requires less anode current and is, therefore, more cost effective. Additionally, while the total number of anodes is more dependent on the average life of the an- odes, the local protection potential is more sensitive to the distribution of anodes. Therefore, the BEM enables the designer to optimize both quantity and distribution of anodes, while anode distribution is usually designed in terms of the designer's experi- ence in industrial standards such as NACE SP0176-2007. 2 The potential distribution is greatly de- pendent on the polarization curve, which varies with calcareous deposits, season, and seawater depth. Therefore, it is diffi- cult to predict the CP state over a long time period. It is recommended to update the survey data and periodically estimate the actual state, and then a new plan of mainte- n a n c e c a n b e d e v e l o p e d w i t h h i g h reliability. Summary A series of simulations was performed using the BEM for optimizing the distribu- tion and quantity of anodes on an offshore platform. As an important boundary condi- tion, the polarization curve of the platform represented bare structural steel in a sea- water sample taken from the actual plat- form location, as first measured in the labo- rator y. The calcareous deposits on the of fshore str ucture were th en included using Hartt's sigmoidal curve. It was dem- on strat ed that th e b ound ar y el em ent method is computationally efficient for predicting the performance of SACP sys- tems. The number and distribution of an- odes can be optimized with cost- effective results. Acknowledgment The authors are grateful to IMP for the financial support through Project D.61014. References 1 W.H. Hartt, "2012 Frank Newman Speller Award: Cathodic Protection of O f fshore Structures—Histor y and Current Status," Corrosion 68 (2012): pp. 1,063-1,075. 2 NACE SP0176-2007, "Corrosion Control of Steel Fixed Offshore Platforms Associated with Petroleum Production" (Houston, TX: NACE International, 2007). 3 K . J. Ke n n e l l e y, M . W. Ma t e e r, C O R R O - SION/93, paper no. 523 (Houston, TX: NACE, 1993). 4 W.H. Hartt, S. Chen, D.W. Townley, "Sacrifi- cial Anode Cathodic Polarization of Steel in Seawater : Part 2—Design and Data Analysis," Corrosion 54 (1998): pp. 317-322. 5 D.W. Townley, "Unified Design Equation for O f fshore Cathodic Protection ," C O RRO- SION/97, paper no. 473 (Houston, TX: NACE, 1997). 6 NACE Publication 7L198, "Design of Galvanic Anode Cathodic Protection Systems for Off- shore Str uctures" (Houston , TX: NAC E, 1998). 7 R.A. Adey, S.M. Niku, "Computer Modeling of Corrosion Using th e B oundar y Elem ent Method," R.S. Munn, ed., Computer Modeling in Corrosion, STP 1154 (West Conshohocken, PA: ASTM, 1992): pp. 248-264. 8 C.A. Brebbia, "Topics in Boundary Element Research," Vol. 3, Electrical Engineering Ap- plications (Berlin, Germany : Springer-Verlag, 1990): pp. 34-64. 9 Z. Lan, et al., "Simulation of Sacrificial Anode Protection for Steel Platform Using Bound- ar y Element Method ," Eng. Anal . Bound . Elem. 36, 5 (2012): pp. 903-906. 10 W.J. Santos, J.A.F. Santiago, J.C.F. Telles, "Optimal Positioning of Anodes and Virtual Sources in the Design of Cathodic Protection Systems Using the Method of Fundamental Solutions," Eng. Anal. Bound. Elem. 46 (2014): pp. 67-74. 11 T. Yonezawa, V. Ashworth, R .P.M. Procter, "Pore Solution Composition and Chloride Effects on the Corrosion of Steel in Con- crete," Corrosion 44, 7 (1998): pp. 489-499. 12 W.H. Hartt, C.H. Culberson , S.W. Smith , "Calcareous Deposits on Metal Surfaces in Seawater—A Critical Review," Corrosion 40 (1984): pp. 609-618. HONGBO LIU is a researcher at the Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas Norte 152, San Bartolo Atepehuacan, G.A. Madero 07730, Mexico, email: hbliu@imp.mx. He has more than 15 years of experience in corrosion and CP. His research interests include the structure and properties of metallic materials such as liquid metal, crystalline Continued on page 60 Optimizing an Offshore Platform Cathodic Protection System Retrofit