1、 Item No. 24196 NACE International Publication 7L198 (2009 Edition) This Technical Committee Report has been prepared by NACE International Specific Technology Group 30,*“Oil and Gas ProductionCathodic Protection.” Design of Galvanic Anode Cathodic Protection Systems for Offshore Structures February
2、 2009, 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 usin
3、g products, 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 indemnifyi
4、ng or 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 c
5、ircumstances 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 f
6、or determining 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
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8、his 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 NACE reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 years old.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 repo
9、rts may receive current information on all NACE International publications by contacting the NACE FirstService Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1 281-228-6200). Foreword This NACE technical committee report summarizes the approaches and experience of Task Gro
10、up T-7L-16 on the design of galvanic anode cathodic protection systems for offshore structures. Cathodic protection system designers can use this report as a guide for recently published data and theoretical developments. Although the concepts discussed here were developed for galvanic anode cathodi
11、c protection systems for offshore structures, some of the concepts may be applicable to other cathodic protection systems. The first part of this report describes a new design method based on first principles derivations. The second part of the report summarizes laboratory and field experimental dat
12、a related to the new design approach. The third part gives examples of how existing design criteria are incorporated into the new design equation and presents two example designs using the new equation. Appendix A presents an example of design procedures. The new design approach allows more precise
13、design of cathodic protection systems, particularly in areas such as deep water or new geographic areas where extensive experience is not available. _ * Chair Ian Rippon, Shell Global Solutions UK Ltd., Aberdeen, UK. NACE International 2 This report was originally prepared in 1998 by Task Group T-7L
14、-16, a component of Unit Committee T-7L, “Cathodic Protection,” under the auspices of Group Committee T-7, “Corrosion by Waters.” The report was reviewed and reaffirmed in 2009 by Specific Technology Group (STG) 30, “Oil and Gas ProductionCathodic Protection.” It is published under the auspices of S
15、TG 30. 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 mitigation technology, whether considered successful or not. Statements used to convey this informati
16、on 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 recommendations for general application of this technology, and must not be construed as such. Introduction Historically
17、, cathodic protection system design for offshore structures using galvanic anodes was based on a single nominal maintenance current density intended to protect a structure over the system design life, after polarizing it to a protected potential within several months. This maintenance current densit
18、y was identified from service experience and was used simply to determine the amount of anode material to be used. Today, typical design practices incorporate three design current densities: initial, maintenance, and final. The reason behind using three design current densities compared to the earli
19、er single maintenance current density approach is because of the technical and economic benefits derived from the rapid polarization resulting from application of an initially high current density. Unless an effort is made to optimize anode size and shape, the use of three design criteria usually re
20、sults in three different answers for the number of anodes required, indicating a fundamental inconsistency among the typical values. If the three criteria are viewed as minimum criteria, the design for bare structures is normally driven by the initial current density criterion. For coated structures
21、, the design is normally driven by the final or maintenance current density criteria. In this case, the actual amount of anode material calculated is greater than has been shown in the past to provide the needed design life. Therefore, the result is increased life, not reduced cost. The experimental
22、 data and example designs in this report are appropriate for uncoated structures. Although the applicable standards1,2have been revised, published experimental data3,4,5and theoretical developments provide a simpler and more universal empirical option to describe the polarization process to cathodic
23、ally protected steel in seawater. This description leads to a simplified design procedure that incorporates both the rapid polarization and long-term maintenance current concepts into a single equation. The final current density concept is also sometimes included in the framework of this new method.
24、 In addition, these concepts are often applied to analysis of in-service cathodic protection survey data. Although polarization is a critical factor in the new offshore cathodic protection system design method, the details of the polarization process and deposition of calcareous films are beyond the
25、 scope of this report. NACE International 3 Slope Parameter Concept Theoretical Derivation of the Slope Parameter Fischer, et. al6(1988) applied Ohms Law to the galvanic couple to describe its electrical behavior in an equation similar to Equation (1): aRcRxRa cAcicIaI+= (1) Where: Ia = total curren
26、t provided by the anode, A Ic= total current provided to the cathode, A ic= current density on the cathode, A/m2a= polarized anode potential, V c= polarized cathode potential, V Ra= resistance from the anode to remote seawater, c= resistance from the cathode to remote seawater, Rx= external resistan
27、ce (metallic, shunts, etc.), A = cathode area, m2This equation is sometimes rewritten as shown in Equation (2): ()a cAiaRcRxRc+= (2) Because galvanic anode materials are chosen to be relatively nonpolarizable, the anode potential is assumed to be approximately constant during the polarization proces
28、s over the range of current densities encountered, as long as the anode does not passivate. Thus, a linear dependence between cathode potential and current density is predicted. The slope of the line is equal to the product of the total circuit resistance and the cathode area, and is here called the
29、 slope parameter, S. The intercept on the potential axis is equal to the anode potential, as shown in Equation (3) and illustrated in Figure 1. a cSic+= (3) Optimizing the Design Gartland, et. al7have shown that the interdependence of current density and potential conforms at least in some cases to
30、a sigmoidal trend, as illustrated by Figure 2. Figure 3 reproduces the sigmoidal behavior and also shows four superimposed design slope value alternatives. The initial points on these design lines link the cathode free corrosion potential, corrto the selected initial current density, i0.A design acc
31、ording to slope S1results in an unprotected structure, because polarization does not achieve the minimum protection potential. A design according to slope S2provides protection considered to be marginal, at a potential for which current density is relatively high. Slopes in the range S3to S4result i
32、n polarization to the potential range at which current density is minimum. Slopes less than S4sometimes lead to overprotection, with increased current demand. NACE International 4 Current Density (A/m2)CathodePotential(V vsAg/AgCl)aInitial ConditionsFinal ConditionsTimeFigure 1: Linear polarization
33、trend. Current Density (A/m2)Final Cathode Potential(VvsAg/AgCl)Figure 2: Sigmoidal polarization behavior (after Gartland, et. al7). Figure 3 suggests that it can be unusual to protect an offshore structure cathodically without achieving at least some degree of rapid polarization, with a final poten
34、tial more negative than prot, the protection potential. Thus protection near protcan occur as a quasi-instability, because a small change in environmental conditions can alter either Raor aor both, shifting the polarization process to a new line and resulting in either underprotection or additional
35、rapid Cathode Potential. Current Density Current Density FinalCathode PotentialNACE International 5 cathodic polarization. This polarization abruptness may not occur in situations that do not exhibit a pronounced sigmoidal curve, such as is likely with high water velocity or low water temperature. T
36、ypically, the definition of an optimal slope parameter for a particular design requires information regarding the shape of the long-term -i curve, particularly between protand a, because only on this basis is the minimum maintenance current density generally achieved. Figure 3: Sigmoidal polarizatio
37、n behavior (Hartt and Chen4). Unified Design Equation The unified design equation is usually derived by first considering the total amount of anode material needed, in the same manner as in the current recommended practices, as illustrated in Equation (4): TAkmiNw = (4) Where: N = number of anodes w
38、 = net mass of an individual anode, kg im= maintenance current density, A/m2T = design life, y A = cathode area protected, m2k = anode consumption rate, kg/A-y For a coated structure, the cathode area protected is often taken as the actual area times the percent coating breakdown or percent bare. Fo
39、r these systems, the end-of-life conditions may be critical to the design, as the coating breakdown is normally taken to be greatest at that time. Current Density (A/m2)CathodePotential(VvsAg/AgCl) corr prot aS1 S2 S3S4i0,1 i0,2 i0,3 i0,4Steady State Curverange of slopes to attainminimum current den
40、sityCathode Potential corrprotaSteady State Curve Range of slopes to attain minimum current density 0,1 0,2 i0,3 i0,4 Current Density NACE International 6 In the total circuit resistance for an offshore cathodic protection system, the anode resistance dominates. The anodes are usually considered par
41、allel resistors, so that the total resistance of the anode ensemble to seawater is equal to the resistance of a single anode divided by the number of anodes. The design slope is then this value times the total area to be protected, as shown in Equation (5): NAaRS = (5) Where: S = design slope parame
42、ter, -m2Ra = resistance of a single anode to remote seawater, A = cathode area protected, m2N = number of anodes Combining Equations (4) and (5) to eliminate N (and A) yields the unified design equation in Equation (6): TkSmiwaR = (6) Everything on the right-hand side of Equation (6) is a design cho
43、ice. The left-hand side describes the specific anode chosen and allows the designer to determine whether the combination of mass and shape for that anode are appropriate for the design. Alternatively, for a given anode, the equation describes the performance that can be expected. The value of Racan
44、be estimated using one of a number of anode resistance equations, or from a numerical model such as the boundary element method. The value of k is often a function of the exposure environment (including temperature) and of the current density on the anode itself. Experimental Basis of Design Slope C
45、oncept Laboratory Data Wang, Hartt, and Chen3,4,5employed the once-through seawater test cell illustrated in Figure 4 to investigate the polarization behavior of cathodically protected steel in seawater. A 1.0 in (25 mm) diameter by 2.0 in (51 mm) high carbon steel cathode specimen (surface area 6 i
46、n24,000 mm2) was connected through an external current-control resistor to an aluminum alloy ring anode. Potential and current were recorded as a function of time. Experiments have been run with the resistor ranging from 75 to 5,750 . A typical experiment is illustrated in Figure 5, where the top gr
47、aph shows potential as a function of time and the middle graph shows current density as a function of time. These plots show a continual decrease in the current density on the cathode accompanied by a continual shift to more protected potential. The details of the plots are related to the kinetics o
48、f the formation of the calcareous deposits and are beyond the scope of this report. The time factor is removed from consideration when the potential is plotted against current density, as illustrated in the lowest plot. NACE International 7 Figure 4: Laboratory test cell (Hartt and Chen4). seawater
49、in reference electrode seawater out to computer data acquisition specimen anode ring plastic frame coated steel bar electrical wire NACE International 8 Figure 5: Typical laboratory polarization data, Al-Zn-Hg anode, ambient temperature (Hartt and Chen4). POTENTIAL V, SATURATED CALOMEL ELECTRODE (SCE)NACE International 9 Figure 5 illustrates the approximately linear relationship between current density and potential, with each experiment beginning with high current density and potential at the upper right and mo
