Corrosion

Assessing Internal Cathodic Protection for Seawater Collection Pipelines at Oil Platforms
WALMAR BAPTISTA, PETROBRAS/CENPES SIMONE L.D.C. BRASIL and JOSÉ CLAUDIO F. TELLES,
Federal University of Rio de Janeiro

This article discusses computer simulation and experimental test results of an impressed current internal cathodic protection system for seawater pipelines. A single anode located onthe axis of the pipeline was studied, and the current required to keep the minimum potential ~ –950 mV vs silver/silver chloride (Ag/AgCl) electrode was determined for 8-, 16-, and 24-in. (20-, 41-, and 61-cm)-diameter pipelines. Coating efficiencies of 0 and 80% and two stirring velocities were considered. Use of evenly spaced longitudinal anodes was analyzed to find the maximum allowable spacing.The correlation between computer simulation and experimental test results was good.

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MATERIALS PERFORMANCE April 2004

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arge amounts of seawater are collected on offshore oil platforms for two primary reasons: to cool the equipment used for initial oil processing and to inject seawater into the well to

counter the loss of productivity near the end of the service life of an oilfield (secondary recovery). During seawater collection, hypochlorite (ClO3–) is injected to control marine fouling and microbiological corrosion. Because of its chemical composition, significant velocity, and the injected hypochlorite, the seawater becomes highly corrosive. Seawater collection pipelines at oil platforms usually are made of Cu-Ni or fiberglass. Some platforms, however, use lines thatare carbon steel (CS) with an organic coating. Not surprisingly, such lines have experienced significant corrosion requiring frequent repairs. Because interrupting the water intake leads to a production shutdown, corrosion failures cause significant losses. Internal cathodic protection (CP) in these lines could solve corrosion problems without interrupting seawater collection. The followingissues need to be considered: • Survey of parameters to develop the basic CP design (type of anode, distance between anodes, etc.) • Development and design of a model of the anode and reference electrode configuration enabling installation with lines in operation and lasting throughout the CP system service life. In the design of a CP seawater system, a –0.800 V vs silver/silver chloride (Ag/AgCl)electrode (SSE) potential has been accepted as a CS protection criterion. The density of the cathodic current required to achieve this potential depends on environmental conditions, steel structure geometry, and surface conditions.1-2 When designing a CP system for the internal surface of seawater collection pipelines, the following need to be taken into account: seawater velocity inside the pipeline(2,200 m3/h flow), pipe diameter, coating efficiency, and free chloride concentration. Several references in the literature emphasize that the distance protected by each anode increases as the diam-

TABLE 1 eter of the pipeline to be protected increases. Nevertheless, the literature neither thoroughly presents nor clearly discusses the influence of seawater velocity, coating efficiency, thepresence of hypochlorite, and the formation of a calcareous layer. Therefore, the authors considered these parameters when carrying out their computer simulations and experimental tests. The results of these simulations and tests are presented here.

SINGLE ANODE—PIPELINE PROTECTED LENGTH (–0.80 TO –0.95 V VS SSE)
Conditions 80% efficiency With stirring Pipe Diameter (in.) 8 16 24 8 16 24 8 16 248 Pipeline Protected Length (m) 0.50 0.70 0.80 1.20 1.80 2.00 0.30 0.40 0.60 0.60 Applied Current (mA) 140 410 760 49 140 265 338 1,050 2,020 120

No stirring

No coating

With stirring

Computer Simulation

No stirring

16 1.00 350 CP system performance may be as24 1.40 800 3 sessed through computer simulations. Some advantages of using computer techniques are: FIGURE 1 • Precise...