RIASSUNTO
ABSTRACT
DNVGL-RP-B401 “Cathodic Protection Design” is widely used as a design code for cathodic protection (CP) of permanently installed offshore structures associated with production of oil and gas.1 CP design parameters in RP B401 are consistently selected using a conservative approach. Adherence to these values is therefore likely to provide a service life that exceeds the design life of the CP system. When selecting design parameters, asset owner should take into account additional investment cost for an overcapacity which may not be utilized. This extra investment cost should be compared with the potential additional costs associated with increased monitoring and maintenance during operation, including subsea retrofitting of anodes in case the design capacity initially installed proves to be insufficient.
The objective of this paper is to describe the basis for the design parameters in RP-B401 and discuss the present values in relation to new field experience and published laboratory test data. The paper discusses how the new knowledge may be employed to reduce the intrinsic conservatism of certain design parameters and still maintain an intentional overcapacity to ensure that the CP system will achieve its design life.
INTRODUCTION
Cathodic protection by sacrificial anodes continues to be used worldwide as a safe and reliable method to prevent corrosion on metallic components and structures. Cathodic protection system design according to DNVGL-RP-B401 has been field proven, showing that such CP systems are able to provide reliable protection for the specified design life. RP-B401 has gone through major revisions since it was first published in 1986 as described by Sydberger et al.2
EVOLUTION OF DNVGL-RP-B401
First version - 1986
DNV RP B401 “Cathodic Protection Design” was first issued in 1986 with the objective of providing general guidelines for the design, fabrication, installation and monitoring of cathodic protection systems for offshore pipelines and structures. 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 1976.3 As in the NACE(1) 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 anodes were defined as typical ranges and design current densities determining the CP current demand of bare steel surface 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(1) document, recommendations were also given regarding the effect of coatings on CP design referring to thin film coatings (<1 mm total coating thickness), thick film coatings (>1 mm total coating thickness) and coated pipeline systems. Hence, coating breakdown criteria (in %) were recommended for thin film coatings. Moreover, requirements were defined for the quality control of galvanic anode manufacture, including a procedure for verification of the electrochemical performance of anode material during production.