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1、Item No. 24196 NACE International Publication 7L198 This Technical Committee Report has been prepared by NACE International Task Group T-7L-16* on Offshore CP Design Design of Galvanic Anode Cathodic Protection Systems for Offshore Structures July 1998, NACE International This NACE International tec
2、hnical 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 products, processes, or procedures not included in this re
3、port. Nothing contained in this NACE International 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 or protecting anyone against liability for
4、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 circumstances may negate the usefulness of this
5、 report in specific instances. NACE International assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE International report are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and for determining the
6、ir applicability in relation to this report prior to its use. This NACE International 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 International 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 th
8、e use of this report. CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACE International reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 years old.
9、 Purchasers of NACE International reports may receive current information on all NACE International publications by contacting the NACE International Membership Services Department, P.O. Box 218340, Houston, Texas 77218-8340 (telephone +1 281228-6200). FOREWORD This NACE technical committee report s
10、ummarizes the approaches and experience of Task Group T-7L-16 on the design of galvanic anode cathodic protection systems for offshore structures. Cathodic protection system designers can use the report as a guide for recently published data and theoretical developments. Although the concepts discus
11、sed here were developed for galvanic anode cathodic 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 repor
12、t summarizes laboratory and field experimental data 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 proced
13、ures. The new design approach allows more precise design of cathodic protection systems, particularly in areas, such as deep water or new geographic areas, where extensive experience is not available. This report was prepared by Task Group T-7L-16, a component of Unit Committee T-7L on Cathodic Prot
14、ection, and is issued by NACE International under the auspices of Group Committee T-7 on Corrosion by Waters. _ *Chairman Dan Townley, Chevron Petroleum Technology, La Habra, California. Copyright NACE International Provided by IHS under license with NACELicensee=IHS Employees/1111111001, User=Wing,
15、 Bernie Not for Resale, 05/16/2007 01:04:05 MDTNo reproduction or networking permitted without license from IHS -,-,- NACE International 2 INTRODUCTION Historically, cathodic protection system design for offshore structures using galvanic anodes was based on a single nominal maintenance current dens
16、ity intended to protect a structure over the system design life, after polarizing it to a protected potential within several months. This maintenance current density was identified from service experience and was used simply to determine the amount of anode material to be used. Today, typical design
17、 practices incorporate three design current densities: initial, maintenance, and final. The reason behind using three design current densities compared with the earlier single maintenance current density approach lies in the technical and economic benefits derived from the rapid polarization resulti
18、ng 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 results in three different answers for the number of anodes required, indicating a fundamental inconsistency among the typical values. If the
19、three criteria are viewed as minimum criteria, the design for bare structures is normally driven by the initial current density criterion. For coated structures, the design is normally driven by the final or maintenance current density criteria. In this case, the actual amount of anode material calc
20、ulated 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 data and example designs in this report are appropriate for uncoated structures. Although the applicable standards1,2 have only recently be
21、en revised, published experimental data3,4,5 and theoretical developments provide a simpler and more universal empirical description of the polarization process of cathodically protected steel. This description leads to a simplified design procedure that incorporates both the rapid polarization and
22、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. In addition, these concepts are often applied to analysis of in-service cathodic protection survey data. Although polarization is a critica
23、l factor in the new offshore cathodic protection system design method, the details of the polarization process and deposition of calcareous films are beyond the scope of this report. SLOPE PARAMETER CONCEPT Theoretical Derivation of the Slope Parameter Fischer, et. al.6 (1988) applied Ohms Law to th
24、e galvanic couple to describe its electrical behavior in an equation similar to Equation (1): IaIcicA ca RxRcRa = + (1) Ia=total current provided by the anode, A Ic=total current provided to the cathode, A ic=current density on the cathode, A/m2 a=polarized anode potential, V c=polarized cathode pot
25、ential, V Ra=resistance from the anode to remote seawater, Rc=resistance from the cathode to remote seawater, Rx=external resistance (metallic, shunts, etc.), A=cathode area, m2 This equation is sometimes rewritten as shown in Equation (2): () c RxRcRa Aic a =+(2) Because galvanic anode materials ar
26、e chosen to be relatively nonpolarizable, the anode potential is assumed to be approximately constant during the polarization process 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 pr
27、edicted. The slope of the line is equal to the product of the circuit resistance and the cathode area, and is here called the 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. c Sic a =+(3) Optimizing the De
28、sign Gartland, et. al.7 have shown that the inter- dependence of current density and potential conforms at least in some cases to a sigmoidal trend, as illustrated by Figure 2. Figure 3 reproduces the sigmoidal behavior and also shows four superimposed design slope value alternatives. A design accor
29、ding to slope S1 results in an unprotected structure, because polarization does not achieve the minimum protection potential. A design according to slope S2 provides protection considered to be marginal, at a potential for which current density is relatively high. Slopes in the range S3 to S4 result
30、 in polarization to the potential range at which current density is minimum. Slopes less than S4 sometimes lead to overprotection, with increased current demand. Copyright NACE International Provided by IHS under license with NACELicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 0
31、5/16/2007 01:04:05 MDTNo reproduction or networking permitted without license from IHS -,-,- NACE International 3 Current Density (A/m2) Cathode Potential (V vs Ag/AgCl) a Initial Conditions Final Conditions Time Figure 1 Linear Polarization Trend Current Density (A/m2) Final Cathode Potential (V vs
32、 Ag/AgCl) Figure 2 Sigmoidal Polarization Behavior (after Gartland, et. al. 7) Copyright NACE International Provided by IHS under license with NACELicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 05/16/2007 01:04:05 MDTNo reproduction or networking permitted without license from
33、IHS -,-,- NACE International 4 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 potential more negative than prot, the protection potential. Thus protection near prot can occur as a quasi
34、-instability, because a small change in environmental conditions can alter either Ra or a or both, shifting the polarization process to a new line and resulting in either underprotection or additional rapid cathodic polarization. This polarization abruptness may not occur in situations that do not e
35、xhibit a pronounced sigmoidal curve, such as is likely with high water velocity or low water temperature. Typically, the definition of an optimal slope parameter for a particular design requires information regarding the shape of the long-term -i curve, particularly between prot and a, because only
36、on this basis is the minimum maintenance current density generally achieved. Figure 3 Sigmoidal Polarization 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
37、current recommended practices, as illustrated in Equation (4): NwimTAk=(4) N=number of anodes w=net mass of an individual anode, kg im=maintenance current density, A/m2 T=design life, y A=cathode area protected, m2 k=anode consumption rate, kg/A-y For a coated structure, the cathode area protected i
38、s often taken as the actual area times the percent coating breakdown, or percent bare. For 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. In the total circuit resistance for an offshore cathodic protec
39、tion system, the anode resistance dominates. The anodes are usually considered parallel 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
40、be protected, as shown in Equation (5): Current Density (A/m2) Cathode Potential (V vs Ag/AgCl) corr prot a S1S2S3 S4 i0,1i0,2i0,3i0,4 Steady State Curve range of slopes to attain minimum current density Copyright NACE International Provided by IHS under license with NACELicensee=IHS Employees/11111
41、11001, User=Wing, Bernie Not for Resale, 05/16/2007 01:04:05 MDTNo reproduction or networking permitted without license from IHS -,-,- NACE International 5 S RaA N =(5) S=design slope parameter, -m2 Ra=resistance of a single anode to remote seawater, A=cathode area protected, m2 N=number of anodes C
42、ombining Equations (4) and (5) to eliminate N yields the unified design equation in Equation (6): RawimTkS=(6) Everything on the right-hand side of Equation is a design choice. The left-hand side describes the specific anode chosen and allows the designer to determine whether the combination of mass
43、 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 Ra can be estimated using one of a number of anode resistance equations, or from a numerical model such as the boundary element method
44、. 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 CONCEPT Laboratory Data Wang, Hartt, and Chen3,4,5 employed the once- through seawater test cell illustrated in Figure 4 to inve
45、stigate 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 in.2 4,000 mm2) was connected through an external current-control resistor to an aluminum ring anode. Potential and current
46、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 graph shows potential as a function of time and the middle graph shows current density as a function of time. These plots show a
47、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 of the formation of the calcareous deposits and are beyond the scope of this report. The time factor is removed from considerati
48、on when the potential is plotted against current density, as illustrated in the lowest plot. Copyright NACE International Provided by IHS under license with NACELicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 05/16/2007 01:04:05 MDTNo reproduction or networking permitted without license from IHS -,-,- NACE International 6 Figure 4 Laboratory Test Cell (Hartt and Chen4) Copyright NACE International Provided by IHS under license with NACELicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 05/16/2007 01:04:05 MDTNo reproduction or networking permitted witho