Abstract
Cathodic protection (CP) of submarine pipelines has generally been based on a conservative approach, as consequences of failure, to a great extent, are higher than the cost of installations. All design parameters and procedures are based on robust selections. Recent failures on pipelines with supermartensitic stainless steel (SMSS) materials have challenged this approach and laid the way for new design aspects with more realistic use of parameters. A new design concept has been introduced for the cathodic protection of offshore pipelines made from high strength stainless steel materials such as supermartensitic stainless steels (SMSS) with enhanced coating systems such as multilayer polypropylene coatings for combined corrosion protection and thermal insulation. The objectives are to reduce the susceptibility to hydrogen induced stress cracking (HISC) and coating damage. Conditions for the pipeline material, which can cause HISC have been studied and this has motivated the use of the new design concept. The study has been concerned with materials testing, CP-design parameters, CP installation and pipeline attachments, pipeline design, coating quality and direct electrical heating systems for hydrate control. The study has shown that the most critical factors for HISC are pipeline conditions with high stress and strain, especially stress concentrations, potential levels more negative than -0.80 V versus Ag/AgCl, low operating temperatures, coating failures and deep waters (high hydrostatic pressures). The paper gives a detailed overview of the new concept with regard to protection potentials for different pipeline materials; selection of design current density requirements; use of anodes with reduced driving potentials; use of subsea isolating joints; CP design principles; installation methods; compatibility with the direct electrical heating systems for hydrate control. The basis for the design has been the new ISO Draft International Standard 15889-2 on cathodic protection for offshore pipelines. Several of the new design parameters have been challenged and the new concept gives a significantly increased distance between anodes.