Cathodic Protection Design Considerations for Offshore Structures in the Arctic Environment

Abstract

Abstract The Arctic imposes unique challenges for design of cathodic protection (CP) systems for offshore structures. These include alternative structure designs, and environmental factors such as pack ice, high water resistivity, extreme tidal velocities and amplitudes, and severe weather. Traditional approaches to CP design for offshore structures are typically not suitable for service in Arctic conditions. Adaptations include almost exclusive use of impressed current or hybrid CP systems, versus traditional sacrificial anode designs, specialized anode deployment and cabling systems, and design of CP monitoring equipment suitable for Arctic conditions. The effect of Arctic conditions on CP current requirement, anode current output, anode material selection, anode configuration, cabling systems and power supplies is discussed. Case histories of specialized CP designs for offshore structures in the Russian Arctic and Cook Inlet are presented. These case histories include relevant CP design, construction, installation and operating details. Introduction Cathodic protection using distributed weld-on aluminum alloy anodes has become the offshore industry standard for corrosion control on immersed surfaces of offshore structures in most oil producing areas of the world. Aluminum anodes with a design life equal to the expected service life of the platform are welded to the structure in the fabrication yard. Use of coatings is typically limited to the splash zone areas and the inside of ballast tanks on floating structures. These stand-alone sacrificial anode systems are essentially maintenance free, except in the rare instances where anode retrofit is required to extend the platform service life. The use of impressed current cathodic protection (ICCP) systems is generally limited to floating vessels such as FPSOs or drill ships, and for anode retrofit. With respect to cathodic protection design, offshore structures located in Arctic/subArctic regions subject to the presence of sea ice, frigid temperatures and other extreme environmental conditions often require a different approach. The demands of the Arctic environment impact cathodic protection design in two general ways - electrochemically and mechanically. Electrochemical impacts include, but are not limited to, increased current requirement, higher seawater resistivity, reduced anode performance and higher rectifier voltage requirements. Mechanical affects are considered those that impact the design of cathodic protection hardware with respect to resistance to physical damage. These include the damage from ice impact and/or abrasion, freezing and wave/current forces. In some cases, mechanical and electrochemical effects are combined, as in the mechanical removal of calcareous deposits by ice or wave action, which increases current requirement.