Cathodic Protection of Steel Structures in Deep Water: A Review

Abstract

ABSTRACT An extensive review of literature that addresses the application of cathodic protection (CP) in the oceans at depths greater than 300 meters is presented. The review discusses environmental factors encountered in deep water and their effect on cathodic polarization behavior of steel and calcareous deposit formation. Deep water field test results and operating experiences are also presented. An evaluation of available data and an identification of data needed for optimum cathodic protection design for deep water are included. Current CP design approaches and deep water CP studies are also discussed. INTRODUCTION The development of offshore energy resources is rapidly moving into water depths greater than 300 meters. This trend to deep water developments is a direct response to the successful exploration in deep water as shown in Figure 1. Since steel is usually the major construction material for these facilities, good corrosion control is essential for the safe operation of long term production facilities that will be required to develop these resources. Cathodic protection (CP) in some form will be required for long term corrosion protection of the subsea steel components. Over 40 years of successful applications of CP have enabled the offshore industry to develop reliable and optimized CP systems for water depths less than 300 meters. However, changes in the seawater environment at greater depths have raised the concern that the CP design approaches for shallow water may not be adequate for deeper waters. Since deep water production facilities are large, expensive, and usually weight sensitive, over design of the CP system to handle uncertainties can be very costly. Because of the great water depths involved, opportunities for subsea repair or replacement of components is neither technically or economically feasible in most cases. Consequently, the designer is forced to develop corrosion protection designs with a greater reliability than would be required for water depths that allow subsea success. Increasing the weight of the CP system to achieve this reliability frequently has a dramatic effect in increasing structural requirements which greatly multiplies the cost of the additional CP redundancy. Therefore, it is especially important for deep water structures that the designer optimize the CP system to minimize weight and maximize reliability. Presently, an optimum CP design using sacrificial anodes is achieved by inducing a high current density (e.g., 320 mA per square meter in the southern North Sea) on steel immediately upon immersion to promote rapid cathodic polarization and formation of high quality calcareous deposits (B78, A13, B68, A48). The well formed calcareous deposits reduce the rate of diffusion of dissolved oxygen in the seawater to steel surfaces and thus greatly reduce the current density necessary to maintain long term cathodic polarization. This approach provides optimum corrosion protection and provides both weight and cost savings over sacrificial anode systems that cannot produce the high initial current densities. If the high quality calcareous deposits can not be as readily formed in deep water as in more shallow waters, this design approach would have to be altered.