Abstract
To achieve an optimal cathodic protection (CP) design for a bare steel structure, a high initial current density is required to give a rapid polarisation and a subsequent formation of a calcareous deposit. The formation of the calcareous deposit will lead to low maintenance and final current density demand. Today, field experience or field testing at the given location and depth are the only way the CP design current densities can be accurately established. Obviously, for the deepwater installations inadequate or over conservative design can lead to serious economic ramifications. It is therefore important to establish the actual requirements for design of CP systems for deep water well in advance of a field development. The present paper presents the results of testing in the Barents Sea area at depths down to about 500 m. The results show that very high initial current densities are required to give polarisation to a potential more negative than -0.80 V (Ag/AgCl). In these areas adequate CP can be achieved only if high initial current densities are applied . The current demand is highest close to the surface and will decrease with depth.
The formation of the calcareous deposit on the steel reduces the CP current demand. To establish if such deposits are stable in the deep water areas a review of the characteristics of the calcareous deposits in the deep oceans is presented. The dominant calcareous deposit in cold waters is calcite, CaCO3. It is the kinetics which control the precipitation and dissolution of CaCO3 in the sea. Calcite which is the dominant CaCO3 mineral formed in deep cold water will not dissolve before the degree of saturation is less than 0.6. For most ocean areas the CaCO3 saturation (Ω) will be higher than 0.6. However, for some ocean areas (e.g. off West Africa) the saturation will be less than 0.6. In such areas the design of the CP system will require special considerations.
