Cathodic Protection Design Considerations for Deep Water Projects
|Publication Date:||1 January 2009|
The development of offshore energy resources is rapidly moving into water depths of 3,000 m (10,000 ft) and greater. This trend to deep water energy resources is a direct response to successful exploration, as shown in Figure 1. Because steel is usually the major construction material for these facilities, effective corrosion control is utilized for the safe operation of long-term production facilities that are typically required to develop these resources. CP is probably used in some form for long-term corrosion protection of subsea steel components. Thousands of CP applications during the last 50 years have enabled the offshore industry to develop reliable and efficient CP systems for water depths less than 300 m (1,000 ft). However, changes in the characteristics of the seawater environment at greater depths raises the concern that CP design for shallow waters is not always adequate for deeper waters.
Oil and gas production in deeper waters commonly results in greater cost for the project and also increases the cost for any failures or remedial actions. Because deep water production facilities are large, expensive, and usually weight-sensitive, overdesign of the CP system to handle uncertainties is often very costly. Because of the great water depths involved, subsea repair or replacement of components is neither technically nor economically feasible in many cases. Consequently, the designer is forced to develop corrosion protection designs with a greater reliability than normally used for shallow depths. Increasing the weight of the CP system to achieve this reliability frequently has a dramatic effect in increasing structural requirements which, consequently, greatly multiplies the cost of additional CP. Therefore, for deep water structures, the optimal CP system is often designed in order to minimize weight and maximize reliability.
Presently, the optimum CP design is achieved by inducing a high current density (e.g., 320 mA/m2 [30 mA/ft2] in the southern North Sea) on unpolarized steel immediately upon immersion in order to promote rapid cathodic polarization and formation of high-quality calcareous deposits.1,2,3,4 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 that is normally used to maintain cathodic polarization. This method, as opposed to sacrificial anode systems not designed to produce high initial current densities, provides optimum corrosion protection and both weight and cost savings. If the high-quality calcareous deposits are not as readily formed in deep water as in more shallow waters, this design approach would be altered.
A thorough understanding of seawater chemistry as a function of water depth and its effect on long-term CP aids in optimizing CP designs for deep water.5,6 Although there is significant variability with seasons and in different oceans, at increased depth, calcium carbonate solubility is greater; fouling is less; velocity of sea currents is less; and dissolved oxygen concentration, temperature, salinity, and pH are also different than in shallow water. All of these variables influence CP, but their long-term cumulative effect is difficult to quantify, making it difficult for the CP designer to develop the optimal CP system.
This state-of-the-art report provides a summary of the information currently available in the literature regarding CP in deep water and the variables in seawater chemistry that affect CP and that are considered in a CP design. This report addresses environmental effects on CP, results of tests and field experience in deep water, and the present state-ofthe- art technology in deep water CP design. Extensive references and a list of acronyms (Appendix A) are included to aid the user.
In a marine CP system, the electrochemical behavior of the anode and cathode and the quality and solubility of the generated calcareous deposit are influenced by a range of factors; one is water depth. However, many other factors vary with depth, and their individual effects on the CP system are normally considered in order to determine a total effect. The environmental factors that affect the CP system and typically vary with depth include dissolved oxygen, temperature, salinity, pH, sea currents, pressure, and fouling. Many of these factors are interrelated, and nearly all of them influence calcareous deposit generation and quality, which is a feature of an optimized CP design. Variations in these factors produce a vertical galvanic gradient (potential variation with water depth). A valuable resource in analyzing the effects of these variables is the large body of information available on oceanography, seawater chemistry, and geochemical aspects of the natural formation of calcareous deposits in the sea. Dexter and Culberson's8 work is quite comprehensive for seawater chemistry variations with depth in the oceans and is especially useful for this review. The sections below discuss the environmental factors and identify available literature for the reader to review.
(1) National Oceanographic Data Center, 1825 Connecticut Avenue NW, Washington, DC 20235.
(2) Ocean Drilling Program (ODP), 1000 Discovery Dr., College Station, TX 77845-9547.
(3) National Science Foundation, 4201 Wilson Blvd., Arlington, VA 22230, and the Joint Oceanographic Institutions, Inc., 1201 New York Avenue, NW, Suite 400, Washington, DC 20005.
(4) American National Standards Institute (ANSI), 25 West 43rd St., 4th Floor, New York, NY 10036.
(5) International Organization for Standardization (ISO), 1 ch. de la Voie-Creuse, Case postale 56, CH-1211 Geneva, Switzerland.
(6) Det Norske Veritas (DNV), 1322 Veritasveien 1, Høvik, Oslo, Norway.
(8) ASTM International (ASTM), 100 Barr Harbor Dr., PO Box C700, West Conshohocken, PA 19428-2959.