Materials Performance

SEP 2014

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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CATHODIC PROTECTION S This article presents design consider- ations and some of the challenges a corrosion engineer encounters while designing a cathodic protection (CP) system for an offshore wind turbine foundation. This article is based on the authors' experience in designing CP systems for offshore wind farms in Europe. Several European countries (Denmark, Germany, The Netherlands, Sweden, United Kingdom, Belgium, Ireland, and Finland) and two Asian countries (China and Japan) have offshore wind farms (OWFs). 1-2 As of 2013, there were no OWFs in the United States, but projects are under development in wind-rich areas of the East Coast, the Great Lakes, and the Pacific coast. 3-4 Figure 1 provides an overview of types of offshore wind turbine foundations. The most common foundations in Europe are the monopile, gravity-based, and jacket- type foundations. The durability of gravity-based founda- tions is essentially covered by concrete durability measures. For the monopile and jacket-type foundations that compri se steel, however, the offshore environment p o s e s a h i g h r i sk of c o r ro si o n . C o n - sequently, a well-designed corrosion pro- tection system must be applied to these types of foundations. OWF foundations in this article will refer to monopile and jacket-type foundations comprising steel. The corrosion protection system design must ensure practicality in installation and operation as well as meet the service life requirements (usually 20 to 30 years). For corrosion protection, OWF operators in Europe mostly specify DNV-OS-J101, 5 or in some cases GL codes (such as GL-IV-7-2 6 ), and in oth er cases th eir own sp ecif ic requirements. The corrosion protection system for an OWF foundation typically consists of either corrosion allowance, a coating system , cathodic protection (CP), or a combination of the three. Table 1 shows corrosion pro- tection system requirements according to DNV-OS-J101. Cathodic Protection Design and Considerations Choice of System We have found that operators prefer a galvanic anode CP (GACP) system over an impressed current CP (ICCP) system for OWFs. This tendency for OWFs is attrib- uted to the fact that a GACP system is more established in the offshore industry than an ICCP system. The advantages seen with a GACP sys- tem are minimal maintenance, protection from day one, and no risk of hydrogen- induced cracking (HIC) embrittlement due to overpolarization . The disadvantages Cathodic Protection Design for Offshore Wind Turbine Foundations Sameer ayyar, Jacob JanSSon, and ruth SørenSen, COWI A/S, Copenhagen, Denmark 26 SEPTEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 9

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