ABSTRACT Experiments were performed where steel specimens were cathodically polarized in natural sea water by galvanic coupling through an external resistor to an aluminum anode. Temperature was either ambient or 5°C; and pressure was atmospheric or 8.96 + 0.14 MPa (1,300 + 20 psi), which is equivalent to a water depth of 899 m (2,950 feet). For some experiments, dissolved oxygen concentration was controlled at 5.5 + 0.2 mg/1 and pH was 7.8. These corresponded to values that were measured at the above depth for a specific Gulf of Mexico site. The apparent steady-state potential (¢c) and current density (ic) for the different experiments were compared with previously reported ambient temperature and pressure data. Calcareous deposits that formed on specimens from each of the test categories were viewed and analyzed. The long-term Oc-i~ trend for the different tests was the same at the two pressures and for 5.5 compared to 9 mg/l 02. Also, i~ was independent of 0~ over the potential range investigated (approximately --0.80 to -1.10 VSCE) despite differences in the calcareous deposit structure and composition. The results are discussed in terms of, first, design criteria for deep water cathodic protection and, second, experimental testing to develop such criteria.
INTRODUCTION Increasingly, the offshore petroleum production industry has focused upon deep water exploration and production, particularly in the Gulf of Mexico. However, while recommended practices regarding cathodic protection (cp) and criteria pertaining thereto have been developed for near-surface ocean waters (1,2)*, comparable information is not available for deep water. This statement must be qualified in view of the recent advent of the slope parameter approach to cp design (3-6) and the unified design equation that was developed there from. Also, appropriateness of existing design current densities has recently been brought into question (7).
While the -0.80 VAg/AgCI potential criterion for corrosion protection is considered to apply equally to both deep and shallow water, the current density to achieve this potential and, in particular, to establish the most effective and efficient level of cathodic protection can at the present time only be confidently determined by long-term, in-situ deep water experiments involving prototype specimens, an approach that is both costly and time consuming. This inability on the part of technologists to define the design current density for a particular deep water site results from uncertainties regarding protectiveness of any calcareous deposits that are likely to form (assuming that deposits do form) and, there from, to project oxygen availability and the rate of oxygen reduction. Thus, while considerable past research activity has addressed the precipitation kinetics of calcareous deposits and how these are affected by influential variables (8-14), still the present state-of-knowledge falls short of permitting design current density prediction for frontier deep ocean sites. Reasons for this include the fact that 1) past research programs and experiments were mostly either potentiostatic or galvanostatic, whereas galvanic anode cp systems are of a mixed mode or free running type, 2) addressment of how cp system design is affected by environmental variables relevant to deep water has not been extensive, and 3) uncertainties exist regarding the interactive influence of variables. Based upon current understanding, calcareous deposit formation is most strongly affected by temperature, pH, water composition, and flow; and ocean depth is important primarily as it influences these factors (9). Values for the first three (temperature, pH, and water composition) determine the extent to whi