ABSTRACT

Standards and other guidance documents covering design and installation of offshore cathodic protection (CP) systems have been available for over two decades. However, during this period, increased service experience has lead to a refinement in the requirements specified in the relevant codes and standards. New coating systems, some designed for substrate temperatures of 120°C, and new operating environments (e.g. deepwater) have also acted as input to the evolution of these documents, to ensure that guidance remains relevant to modern-day subsea developments, pipelines and structures. For subsea pipelines in particular, this has reduced the conservatism inherent in CP design which affords two main advantages. Firstly, the cost of the CP system may be significantly reduced when fewer anodes have to be installed. Secondly, for certain linepipe materials the risk of cracking failure modes associated with hydrogen absorption induced by cathodic protection can be reduced, especially if CP can be provided from anodes located on adjacent structures. The development of DNV recommended practices for cathodic protection is described in detail, with reference to significant changes made for certain design parameters and the reasons behind such amendments. Comparison is also made with other CP codes, both in terms of the general methodology and by direct numerical comparison, using some illustrative examples.

INTRODUCTION

Cathodic protection (CP) by the use of sacrificial galvanic anodes continues to be used worldwide, as a reliable and cost-effective method of protecting subsea metallic components from the deleterious effects of corrosion. Some consideration is required in specifying the quantity, design and materials of construction of the galvanic anodes, in order to ensure the ?protection object? receives adequate CP current throughout its design life. This has driven the development of a number of standards and recommended practices, giving both general guidance and specific methodologies for design of marine CP systems and for the manufacturing and installation of galvanic anodes. This paper charts the development of a selection of these documents and uses illustrative examples to compare the anode requirement using some different CP design codes, including DNV RP-F103 and ISO 15589-2 for subsea pipelines and DNV RP-B401 for other marine structures.

EVOLUTION OF DNV RP-B401

DNV RP-B401 Cathodic Protection Design (first version - 1986)

DNV RP-B401 Cathodic Protection Design was first issued by Det Norske Veritas (DNV) in 1986 with the objective of providing general guidelines for the design, fabrication, installation and monitoring of cathodic protection systems for offshore structures and pipelines. Prior to this, the only offshore design code which addressed these objectives was the NACE RP-01-76 Control of Corrosion on Steel, Fixed Offshore Platforms Associated with Petroleum Production, first issued in 1976, in which galvanic anode cathodic protection was treated rather superficially. As in the NACE code, the first issue of RP-B401 did not describe the CP calculation procedures in any detail; CP design parameters related to the performance of galvanic anode parameters were defined as typical ranges, and ?design current densities? determining the CP current demand of bare steel surfaces were only provided for a limited number of geographical regions. For the North Sea, however, the new RP provided specific design current densities for areas south of 57°N and for 57°N ? 62°N, respectively. Contrary to the NACE document, recommendations were also given regarding the effect of coatings on CP design, referring to thin film coatings (=1 mm total thickness), thick film coati

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