1、Item No. 24165NACE International Publication 7L192 (2009 Edition) This Technical Committee Report has been prepared by NACE International Task Group 269,* “Cathodic Protection Design Considerations for Deep Water Projects” Cathodic Protection Design Considerations for Deep Water Projects January 200
2、9, NACE International This NACE International (NACE) technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone from manufacturing, marketing, purchasing, or using pr
3、oducts, processes, or procedures not included in this report. Nothing contained in this NACE report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying o
4、r protecting anyone against liability for infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictable circu
5、mstances may negate the usefulness of this report in specific instances. NACE assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE report are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and for d
6、etermining their applicability in relation to this report prior to its use. This NACE report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this report. Us
7、ers of this NACE report are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of this
8、report. CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACE reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 years old. Purchasers of NACE reports
9、may receive current information on all NACE publications by contacting the NACE FirstService Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1 281-228-6200). Foreword This report presents an extensive review of literature that addresses the application of cathodic protectio
10、n (CP) in the oceans at depths greater than 300 m (1,000 ft) and insight from NACE members actively involved in offshore deep water activities. In some instances, review of literature addressing the application of CP in somewhat shallower water is presented as it affords a basis for comparison with
11、deeper water applications. Environmental factors encountered in deep water and their effects on cathodic polarization behavior of steel and calcareous deposit formation are discussed. Deep water field test results and operating experiences are presented. CP design considerations for deep water that
12、include initial and maintenance current densities, anode performance, and alternative approaches for corrosion protection in deep water are also discussed. Future data needs are identified. _ * Chair William H. Thomason, ConocoPhillips (retired), Ponca City, OK. NACE International 2 The anticipated
13、audience for this report is corrosion and project engineers responsible for the design, maintenance, and operation of external corrosion protection systems for submerged areas of deep water projects. The goal is to provide technical and practical information to assist these engineers. Another antici
14、pated audience is researchers involved with deep water for which the information, references, and data sources provided will be useful. This report was originally developed by NACE Task Group T-7L-8, “Corrosion in Deep Water,” a component of Unit Committee T-7L, “Cathodic Protection,” and was publis
15、hed by NACE in 1992 under the auspices of Group Committee T-7, “Corrosion by Waters.” NACE Task Group (TG) 269, “Cathodic Protection Design for Deep Water,” was formed to update the 1992 report and in doing this, the committee included new industry experience and research results. Both Task Group T-
16、7L-8 and TG 269 were composed of representatives from the offshore oil/gas production and corrosion control industries. This report supplements NACE SP0176, “Corrosion Control of Submerged Areas of Permanently Installed Steel Offshore Structures Associated with Petroleum Production.” This technical
17、committee report is issued under the auspices of STG 30, “Oil and Gas ProductionCathodic Protection.” NACE technical committee reports are intended to convey technical information or state-of-the-art knowledge regarding corrosion. In many cases, they discuss specific applications of corrosion mitiga
18、tion technology, whether considered successful or not. Statements used to convey this information are factual and are provided to the reader as input and guidance for consideration when applying this technology in the future. However, these statements are not intended to be requirements or recommend
19、ations for general application of this technology, and must not be construed as such. Introduction 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 explora
20、tion, 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 lo
21、ng-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 environm
22、ent 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 productio
23、n 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. Consequent
24、ly, 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
25、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 S
26、ea) on unpolarized steel immediately upon immersion in order to promote rapid cathodic polarization and formation of high-quality calcareous deposits.1,2,3,4Well-formed calcareous deposits reduce the rate of diffusion of dissolved oxygen in the seawater to steel surfaces and thus greatly reduce the
27、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 no
28、t as readily formed in deep water as in more shallow waters, this design approach would be altered. NACE International 3 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.5,6Although there is signif
29、icant 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 i
30、nfluence 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
31、 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-of-the-art technology in deep water CP design. Extensive references and a list of acronyms (Appe
32、ndix A) are included to aid the user. Historic Petroleum Exploration and Production(Data from Offshore Magazine, May 2008)05001,0001,5002,0002,5003,0003,5001960 1970 1980 1990 2000 2010 2020YearWater Depth (m)Platforms one is water depth. However, many other factors vary with depth, and their indivi
33、dual 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 interrelat
34、ed, 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 NACE International 4 analyzing the effects of
35、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 Culbersons8work is quite comprehensive for seawater chemistry variations with depth in the oceans and is espe
36、cially useful for this review. The sections below discuss the environmental factors and identify available literature for the reader to review. Sources of Oceanographic Data The National Oceanographic Data Center(1)provides oceanographic data collected at numerous sites and depths around the world.9
37、Other data sources are the Ocean Drilling Program (ODP)(2)and Texas A and calcite is normally modified by incorporation of 6 to 10 percent magnesium carbonate MgCO3).543. In addition to decreased temperature at depth, increased pressure also contributes to enhanced CaCO3solubility as a consequence o
38、f, first, lower pH at depth because of the effect of pressure on the equilibrium of the reactions in Equations (3), (4), and (5)55 (decomposition of sinking dead organisms also contributes to this pH decrease)8and, second, the effect of pressure on the apparent solubility product for the reaction in
39、 Equation (13).564. Based on analysis of CP data from offshore structures and ocean-deployed and laboratory specimens, it has been shown that current density decreases with time according to a power law expression because of, at least in part, the progressive formation of calcareous deposits, even a
40、fter 20-plus years.57Data scatter at any given time exceeded an order of magnitude with results for cold/deep water exposures occupying the upper bound and warm water ones the lower. Because of the temperature and pressure effects (see also items 2 and 3 above), greater current density is generally
41、used in order to provide a given level of cathodic polarization in deep ocean applications than in warm shallow ones. 5. Increased water movement tends to limit calcareous deposit formation. Both (1) a possible reduction in the interface pH to below 9.5 as a consequence of a reduced diffusional boun
42、dary layer thickness and resultant more rapid transport of hydroxide (OH-) from the interface, and (2) reduced boundary thickness, per se, irrespective of interface pH, are probably responsible. Depolarization of cathodically protected steel specimens at a 915 m (3,000 ft) Gulf of Mexico water depth
43、 has been reported in conjunction with onset of a loop current.50 Loop currents are discussed further in Loop Currents. 6. The slow precipitation kinetics of CaCO3in seawater (see above) are because of, in part, blockage of surface growth sites by adsorbed magnesium (Mg+2).58,59However, because the
44、ratio of major ions in seawater tends to be constant world-wide, this effect is of little practical significance in either near-surface or deep ocean, beyond the fact that it occurs. 7. Macrofouling does not occur in the deep ocean, but microfouling (bacteria) is expected. Presence of bacteria has b
45、een reported to favor formation of brucite, which is normally less protective than CaCO3. Biofilms have been reported to act as either a polarizer or depolarizer, depending on current density.60,61,62Any biological species dependence of calcareous deposit composition, structure, and properties is un
46、determined, however. The effect of dissolved organics63,64and phosphates65,66has an inhibiting effect on CaCO3precipitation, even in relatively small concentrations. The effect of such species is thought to be responsible for the lack of correlation between long-term laboratory tests in natural seaw
47、ater and instrumented deep water Gulf of Mexico exposures, in which temperature, pressure, dissolved oxygen concentration, and pH for the former were controlled to match those of the latter site.50,67Dissolved organics have also been projected to be responsible for the spatial variability in the lys
48、ocline (depth at which CaCO3dissolution starts to accelerate) and compensation depth (depth at which all CaCO3dissolves).49 8. Steel composition and surface roughness, to the extent that these have been investigated, have revealed little or no effect on calcareous deposit formation.68Lower current d
49、ensity demand has, however, been reported for rusted compared to clean steel surfaces, suggesting that the combination of corrosion products and calcareous deposits is NACE International 14 beneficial. The depolarization of steel in 915 m (3,002 ft) Gulf of Mexico waters in response to a loop current (see item 5 above) was less significant for atmospherically prerusted than for clean specimens.50Thus, while a qualitative description of calcareous deposit formation and how the process depends on some specific, individual factors has been established, the
