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 58 of 84

56 APRIL 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 4 MATERIALS SELECTION & DESIGN M The boundary element method, com- bined with the determination method for calcareous formation, was em- ployed to optimize the cathodic pro- tection (CP) system for the retrofit of an ASTM A36 (UNS K02600) carbon steel offshore platform. Based on the platform structure and anode distri- bution and consumption, a series of retrofitting schemes was evaluated to maximize CP for the entire structure. The optimization process provided an insight on how the distribution of an- odes affects the structure-seawater potential and the life of the anodes. Many cathodic protection (CP) designs are based upon trial and error or empirical methods. These include the original single and subsequent triplicate current density (CD) design methods. 1-2 Recently, the slope- parameter method , 3 based on the first- principle and unif ied design equation concepts, 4-6 was attempted. More precise CP design , eith er sacrificial anode CP (SACP) or impressed current CP (ICCP), involves the solution of Laplace's equation with the relevant boundary conditions to provide the current and potential distribu- tions in the solution domain . Even for rel atively simpl e ge om etr y, a sp e ci f i c analytical solution usually is not possible. Therefore, various numerical approaches have been developed and the boundar y element method (BEM) has been found to be the most effective for modeling marine structures. 7-9 In this study, the system modeling tech- nique was used to optimize the distribution of anodes to provide service for 20 years for an offshore platform. The environmental conditions and formation of calcareous de- posits were expected to vitally impact CP of the offshore platform. 10-12 Methodology BEM software (BEASY Corrosion Mod- eling † ) was used to assess and retrofit the offshore platform, made from ASTM A36 (UNS K02600) carbon steel (CS). In sea- water, the calcareous deposits on cathodic surfaces result in a sigmoidal shape of the long-term potential-CD curve. 1 This curve shows the relationship between CD and po- tential on wetted steel surfaces on which calcareous deposits could form. It is appli- cable to low and mild CS in the tempera- ture range of 23 to 25 °C. Therefore, the sig- moidal curve was used for the platform located in the warm zone of the Gulf of Mexico. Each point on the curve shows the CD, which will eventually be reached if a steel sample is held at that particular po- tential for a long period of time to allow the formation of calcareous deposits. To simu- late the calcareous deposit effect, three curves were prepared as shown in Figure 1: the sigmoidal curve (long-term curve), the "no calcareous deposit" dynamic polariza- tion curve, and the "good calcareous de- Optimizing an Offshore Platform Cathodic Protection System Retrofit H.B. Liu, J. A LA mi LLA , A. Contrer A s, A nd e . s os A , Instituto Mexicano del Petróleo, Mexico City, Mexico J. m . m AL o, Instituto Nacional de Electricidad y Energías Limpias, Cuernavaca, Morelos, Mexico † Trade name.

Articles in this issue

Archives of this issue

view archives of Materials Performance - APR 2017