Deep Water: Considerations of the Cathodic Protection Design Basis
Authors: Fischer K.P. 
Published: October 2008

Presently the design guidelines for cathodic protection do not consider the effects of deep water. How variations in the seawater properties will effect the CP current demand can today only be established accurately by field-testing or through experience. The paper presents data from field testing on the Norwegian Continental Shelf at depths down to 1300 m. An approach to how the environmental data can be utilised to evaluate the CP current demand in deep water is also discussed.

Introduction

Today the approach to CP design for offshore structures in general is either to apply a high initial current density in order to get maximum benefits from the calcareous deposits or to combine CP with an organic coating. In relation to the CP in deep water there are uncertainties related to how the design basis needs to be changed compared to present guidelines and recommendations. The present paper addresses the CP current demand for deepwater.

The typical CP current density values for locations, which constitute deep-sea areas, are given in the guidelines from NACE and DNV1,2. At present, none of these guidelines specify any special requirements for structures in deep waters as compared to more shallow waters. The definition of term deep water is constantly changing. In the present paper the term deep water will imply a depth from 600 m and deeper. As the physiochemical properties of the sea can depend on the depth the corrosion engineer needs to evaluate oceanographic data for the prospective area prior to specifying the CP design basis. However there is no established procedure for how the physiochemical data (oceanographic data) can be utilised to optimise the CP current density requirements. Today the CP current demand can only be established by field-testing or from experience.

The approach to an efficient CP system for the protection of bare steel is based on achieving a polarisation as shown in Figure 1. Here by selecting a high initial design current density a rapid polarisation to potentials in the range -900 mV to -1000 mV Ag/AgCl will be achieved. A rapid polarisation will lead to maintenance current density of less than 0.10 A/m2. This is due to the build-up of a protective calcareous deposit. As given in Figure 1 the development of the calcareous deposit is a necessity for achieving a cost effective CP system.

For deep water there are two important questions concerning the CP current demand:Will there be an increase in the initial current densities for deep water?Will the final as well as the maintenance current density in deep waters be high due to a less protective calcareous deposit?

As given in Figure 1B a high initial current density is required to produce a low final current density. A "steady state" S-curve as given in Figure 1B indicates the development of a protective calcareous deposit for the polarisation behaviour of C-steel. A polarisation behaviour indicating the formation of protective calcareous deposit will occur when typically the maintenance current density is at 0.1 mA/m2 or lower for potentials in the range -900 to -1000 mV Ag/AgCl.

The calcareous deposits formed during CP in seawater are mainly CaCO3 either as calcite or aragonite. Brucite, Mg(OH)2, may be a constituent of calcareous deposits. The fact that the solubility of CaCO3 increases with decreasing temperature is the main reason why there is uncertainty on the protective properties of deep-water calcareous deposits. At the present time there is limited data on both composition and effectiveness of calcareous deposits in deep water (&gt;1000m)6.