DNV-RP-B401: Cathodic Protection Design

1 RECOMMENDED PRACTICEDET NORSKE VERITASDNV-RP-B401 Cathodic Protection DESIGNOCTOBER 2010 This document has been amended since the main revision (October 2010), most recently in April 2011. See Changes on page 3. The electronic pdf version of this document found through is the officially binding version Det Norske VeritasAny comments may be sent by e-mail to subscription orders or information about subscription terms, please use Typesetting (Adobe Frame Maker) by Det Norske VeritasThis service document has been prepared based on available knowledge, technology and/or information at the time of issuance of this document, and is believed to reflect the best ofcontemporary technology. The use of this document by others than DNV is at the user's sole risk.

2 DNV does not accept any liability or responsibility for loss or damages resulting fromany use of this NORSKE VERITAS (DNV) is an autonomous and independent foundation with the objectives of safeguarding life,property and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification andconsultancy services relating to quality of ships, offshore units and installations, and onshore industries worldwide, andcarries out research in relation to these service documents consist of amongst other the following types of documents: Service Specifications. Procedual requirements. Standards. Technical requirements. Recommended Practices. Standards and Recommended Practices are offered within the following areas.

3 A) Qualification, Quality and Safety MethodologyB) Materials TechnologyC) StructuresD) SystemsE) Special FacilitiesF) Pipelines and RisersG) Asset OperationH) Marine OperationsJ) Cleaner EnergyO) Subsea SystemsDET NORSKE VERITASA mended April 2011 Recommended Practice DNV-RP-B401, October 2010see note on front coverChanges Page 3 CHANGES GeneralAs of October 2010 all DNV service documents are primarily published order to ensure a practical transition from the print scheme to the electronic scheme, all documentshaving incorporated amendments and corrections more recent than the date of the latest printed issue, have beengiven the date October overview of DNV service documents, their update status and historical amendments and corrections maybe found through Main changes October 2010 Since the previous edition (January 2005)

4 , this document has been amended, most recently in April 2008. Allchanges have been incorporated and a new date (October 2010) has been given as explained under General . Amendments April 2011 Item has been amended and clarified concerning galvanic anode performance requirements. A new Guidance note has been added to item (Annex C). The layout has been changed to one column in order to improve electronic NORSKE VERITASR ecommended Practice DNV-RP-B401, October 2010 Amended April 2011 Page 4 Contentssee note on front .. and Structure .. to Other DNV .. (American Society for Testing and Materials) .. (Det Norske Veritas) .. (European Standards).. (International Organization for Standardisation).

5 International .. AND .. AND SYMBOLS .. CP Design CONSIDERATIONS (INFORMATIVE) .. of CP .. Parameters Affecting CP .. Potentials .. effects of CP .. Anode Geometry and Fastening of Coatings in Combination with CP .. Continuity and Current Drain .. Design Life .. Current Breakdown Factors for CP Design .. Anode Material Design Resistance Formulas .. and Sediment Resistivity .. Utilization Factor .. Drain Design Parameters .. CALCULATION AND Design PROCEDURES .. of CP Object .. Area NORSKE VERITASA mended April 2011 Recommended Practice DNV-RP-B401, October 2010see note on front cover Contents Page Demand Drain Calculations .. of Anode Type .. Mass Calculations .. of Number of Anodes.

6 Of Anode Resistance .. Design .. of for Electrical .. MANUFACTURE .. Procedure Qualification Testing .. Control of Production .. , Fabrication of Anode Inserts and Casting of Anodes .. and Testing of and , Storage and Shipping of OF ANODES .. Procedure Specification .. of and Handling of Anodes .. Installation and Provisions for Electrical of Anode .. A TABLES AND FIGURES .. and B LABORATORY TESTING OF GALVANIC ANODE MATERIALS FOR QUALITY CONTROL .. and Preparation of Test Specimens.. and Experimental Criteria and Re-Testing .. C LABORATORY TESTING OF GALVANIC ANODE MATERIALS FOR QUALIFICATION OF ELECTROCHEMICAL and Preparation of Test Specimens.. and Experimental.

7 37 DET NORSKE VERITASR ecommended Practice DNV-RP-B401, October 2010 Amended April 2011 Page 6 1. Generalsee note on front cover1. Cathodic Protection (CP) can be defined as electrochemical Protection by decreasing thecorrosion potential to a level at which the corrosion rate of the metal is significantly reduced (ISO 8044) or atechnique to reduce corrosion of a metal surface by making that surface the cathode of an electrochemical cell (NACE RP0176). The process of suppressing the corrosion potential to a more negative potential is referred toas Cathodic polarization . For galvanic anode CP systems, the anode of the electrochemical cell is a casting of an electrochemicallyactive alloy (normally aluminium, zinc or magnesium based).

8 This anode is also the current source for the CPsystem and will be consumed. Accordingly, it is often referred to as a sacrificial anode , as alternative to theterm galvanic anode consistently used in this Recommended Practice (RP). For impressed current CP, aninert (non-consuming) anode is used and the current is supplied by a rectifier. In this RP, the cathode of theelectrochemical cell ( the structure, sub-system or component to receive CP) is referred to as the protectionobject . For permanently installed offshore structures, galvanic anodes are usually preferred. The Design issimple, the system is mechanically robust and no external current source is needed. In addition, inspection andmaintenance during operation can largely be limited to periodic visual inspection of anode consumption andabsence of visual corrosive degradation.

9 However, due to weight and drag forces caused by galvanic anodes,impressed current CP systems are sometimes chosen for permanently installed floating structures. CP is applicable for all types of metals and alloys commonly used for subsea applications. It preventslocalised forms of corrosion as well as uniform corrosion attack, and eliminates the possibility for galvaniccorrosion when metallic materials with different electrochemical characteristics are combined. However, CPmay have certain detrimental effects, for example hydrogen related cracking of certain high-strength alloys andcoating disbondment as described in CP is primarily intended for metal surfaces permanently exposed to seawater or marine sediments. Still,CP is often fully effective in preventing any severe corrosion in a tidal zone and has a corrosion reducing effecton surfaces intermittently wetted by seawater.

10 This RP has been prepared to facilitate the execution of conceptual and detailed CP Design usingaluminium or zinc based galvanic anodes, and specification of manufacture and installation of such the requirements and recommendations are general, this document contains advice on how amendmentscan be made to include project specific requirements. The RP can also easily be amended to includerequirements or guidelines by a regulating authority, or to reflect Owner s general philosophy on corrosioncontrol by Some of the Design recommendations and methods in Sections 5, 6 and 7 are also valid for CP systemsusing other current sources such as magnesium anodes and rectifiers ( impressed current).