Materials Performance

MAY 2015

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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Page 39 of 100

37 NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS PERFORMANCE MAY 2015 Continued on page 38 By applying coatings on subsea struc- tures, the number of anodes installed for cathodic protection (CP) can be decreased. Typically, the number of anodes is deter- mined by the lifetime of the structure and the bare metal surface to be protected. Te coating is assumed to degrade with time, and an equation is used to estimate the breakdown of the coating as function of time. CP design rules include a coating breakdown factor that represents the area of the coated structure that needs CP. Tis equation was developed for projects with a typical lifetime of 20 to 25 years. A study was conducted to evaluate coat- ing degradation on subsea ofshore installa- tions after long-term exposure of coatings, and to suggest a revised coating breakdown equation for use in CP design. Four diferent installations, each with more than 20 years of exposure time, were investigated: two sub- sea templates and two jackets. One of the templates was taken ashore and investigated there. For the three other installations, the coating condition was evaluated based on subsea survey reports. On the investigated structures, the con- dition of the coating shows that the coating breakdown models used in CP design stan- dards are very conservative. Te actual coat- ing breakdown on one of the jackets was es- timated to be less than 5% after 25 years, while the design rules used predicted 22%, about 4 to 5 times more coating breakdown than was actually observed. At the same time, however, the anode consumption on the installations was higher than expected, considering the good performance of the coatings. Terefore, it appears that that the design rules for CP overestimate coating breakdown, but underestimate other factors such as current drain to wells, pilings, etc., as well as the anode auto-corrosion. For struc- tures with a long lifetime, a higher fraction of the anode will be lost due to auto-corrosion, which will decrease the anode capacity. Te study and results are discussed in CORROSION 2015 paper no. 5537, "Use of Coatings in Combination with Cathodic Pro- tection—Evaluation of Coating Degradation on Ofshore Installations after 20+ Years," by O.Ø. Knudsen and S. Olsen. Design rules for CP overestimate coating breakdown and underestimate other factors, such as current drain to wells, pilings, etc., as well as auto-corrosion of the an ode.

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