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aper No.

559 CORROSIONOL

The NACE International Annual Conference and Exposition

INTERNAL CATHODIC PROTECTION OF SEAWATER PIPING SYSTEM BY THE

USE OF THE RCP METHOD

by

Roy Johnsen, Per O. Gartland, Stein Valen

CorrOcean as, Teglgaarden, N-7005 Trondheim, Norway

and

John M. Drugli

SINTEF Corrosion Centre, N-7005 Trondheim, Norway

ABSTRACT

During the last ten years high alloyed stainless steels with 6% molybdenum (6Mo) or 25%Cr (super

duplex) have been the most popular materials for seawater systems on offshore installations in the

North Sea. The basis for this material selection was to obtain rnaintcnancc free systems with long

lifetime. However, practical experience has shown that corrosion failures can occour. This paper

presents a simple and economical method to avoid corrosion problcrns internally in piping systems

transporting chlorinated seawater. The method is called RCP – Resistor controlled Cathodic

Protection. Principles of the method including protection potential, current density rcquircmcnts and

anode design in addition to different practical applications arc dcscribcd.

INTRODUCTION

Since early eighties high alloyed stainless steels like austcnitic steels with about 6% molybdenum

(called 6Mo-steel) and duplex stainless steels with 25% Cr (called super duplex) have been widely

used in seawater systems in connection with oil– and gas production. Based on laboratory tests

these highly alloyed stainless steel qualities were found to bc corrosion resistant in chlorinated

seawater at temperatures up to about 3@C. In practice, however, the use of these alloys has not

been entirely without problcmsl-2. On a small percentage of the thousands of flanges installed

crevice corrosion attacks have been observed. Corrosion attacks have also been observed in

threaded connections and in systems with tcmpcraturcs cxcccding 30°C permanently or for a short

period.

Copyright

@Mgg6yNACE International. Requests for permission to publish this manuscript in any form, in part or In whole must be made in writing to NACE

International, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in this

paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

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As a result of the corrosion problems, the Norwegian oil companies have rcduccd the safe upper

limit for 6M0– and 25% Cr steel to 15°C in regions with crcviccs.

Normally parts of the seawater piping systems operate at tcmpcraturcs above 15°C and actions have

to be taken to avoid corrosion problems when high alloyed stainless steels arc used. One possibility

is to use alternative materials e.g. titanium for critical components, and weld overlay with highly

alloyed Ni–alloys on existing flanges. Zkese solutions can be rather costly.

During the last few years, the trend has been to optimize lifetime cost of equipment by using more

standardized and cheaper material. By using stainless steel Iikc AISI 316 combined with sacrificial

anodes the total costs can be significantly rcduccd. However, conventional cathodic protection

design results in unacceptable anode consumption.

In this paper, an alternative way of solving the problems is prcscntcd. It is based on cathodic

protection, but with a new design principle. 7he method is called RCP - Resistor controlled

Cathodic Protection.

The RCP–method is based on a patent claim 3 “Method and arrangement to hinder local corrosion

and galvanic corrosion in connection with stainless steels (SS) and other passive materials.” The

method is now being commercialized by CorrOccan in cooperation with SINTEF.

THE RCP PRINCIPLE

The RCP is a means to prevent local corrosion of stainless steels or other passive alloys in piping

systems with various types of saline waters, in which a critical combination of the potential and the

temperature which cause corrosion may bc cxcccdcd.

The basic principle of the method is to apply cathodic protection to a stainless steel pipe system

using a resistor in series with the anode to control both the potential on the stainless steel and the

anode output. The principle is shown schematically in fig. 1.

The method is further based on the observation that the protection potential for prevention of

localized corrosion on stainless steel is much more positive than the typical potentials of sacrificial

anodes. The voltage drop over the resistor is thcrcfor designed to obtain sufficiently but not

excessively negative polarization of the stainless steel.

The resistor control thus keeps the stainless steel in a protective potential range, where the current

requirements are very small in many saline environments, e.g. in chlorinated seawater, in producedwater from the oil and gas production and in natural seawater above 3@C.

Due to the very low current requirements in the relevant potential range, a single anode can protect

large lengths of a pipe systcm at a very low anode consumption rate.

This paper describes the RCP method used in chlorinated seawater. The method can, however, also

been used in produced water or water injcetion systems with low oxygen content.

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DESIGN PAMMETERS

Protection potentials

Stainless steels exposed to natural or chlorinated seawater may suffer from either pitting or crevice

corrosion when critical values of temperature and/or residual chlorine arc excccdcd.. In chlorinated

seawater the potential on a passive stainless steel rises to above 600 mV SCE within a few hours.

Exposure experiments have given the relation between critical potentials and temperatures leading

to initiation of localized corrosion of different stainless steels. Fig. 2 contains curves for

6Mo/25%Cr and AISI 316. As can be seen from the figure, 6Mo/25%Cr normally operate up to

30°C without suffering corrosion in chlorinated seawater, while AISI 316 even at temperatures down

to O°C suffers from corrosion.

A more important aspect of fig. 2 is that both 6Mo/25%Cr and AISI 316 can withstand a higher

temperature before they start to corrode when the potential is lowered. At 80°C the initiation

potential will be about 100 mV SCE for 6Mo/25%Cr and -100 mV SCE for AISI 316.

Current density requirements

A key parameter in CP design is the current density requirements as a function of potential and

environmental parameters like velocity and residual chlorine lCVCI. This typc of information is not

reported in the literature and standards like e.g. DnV RPB 401 does not contain such information.

From R&D work performed for a number of oil companies and steel suppliers, a data base

containing this actual type of information has been built up. Data shown in fig. 3 is an example

of data that has been measured on 6M0 stainless steel during short term tests. Our data base

contains all necessary information for a set of different parameters both from laboratory tests and

systems in operation.

A very important aspect of the data in fig. 3 is the relatively low current requirements at high

potentials. Since we already concluded that a sufficient protection potential is -100 mV SCE, the

current load on a sacrificial anode can be low at this potential compared to what sacrificial anodes

normally experience under protection of carbon steel.

DESIGN OF A SYSTEM

Resistor control of the pipe potential and the anode current output

A sacrificial anode of zinc or an aluminum alloy directly coupled to the pipe system will polarize

the nearby stainless steel to much more negative potentials than required. As a result the current

requirements will be increased considerably and lead to a very rapid consumption of the anode. Zhe

solution with the Resistor controlled Cathodic Protection – RCP - is there for to control the

potential by use of a resistor as indicated schematically in fig. 1. With R as the total resistance in

series and I the current output from the anode, the potential, E,, on the pipe near the anodes

becomes:

(1)

where E, is the anode potential.

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For the more distant parts of the pipe the potential, E~,on the pipe bccomcs:

(2)

where E,~,Wis the potential drop between the near and far position caused by the current transport

in the seawater.

By knowing the current densi~ requirements together with a suitable choice of the resistor, the CP

system can be designed such that both E. and E~are slightly negative to the required protection

potential where the current load is small.

Potential variation along a pipe

Due to the low current densities in the potential range of interest for protection, the potential drop

in the seawater along the pipe will be relatively small compared to traditional design. This makes

it possible to protect relatively long pipe sections with a small number of anodes. The maximum

spacing between the anodes will be determined from a calculation of the potential profile along the

pipe utilizing realistic boundary conditions. The rcquircmcnt is that the most positive potential, E[,

shall be lower than the relevant protection potential.

A computer program called GALVKORR was originally dcvclopcd to perform calculations for a

simple piping system with actual boundary conditions and pipe design as input parameters. With

a recently developed program called RCPSim, it is possible to perform calculations for a rather

complex piping system including pumps, heat cxchangcrs, filters etc. Both programs arc based on

the Finite Difference Mcthod(FDM).

Anode consumption rates

By using these computer programs it is possible to optimize the anode spacing and select a suitable

value for the resistor that results in a minimum ancxie consumption rate. Table 1 contains typical

lengths to be protected from one anode as a function of pipe dimension. In addition anode

consumption rates are given.

In practice, the anodes may be put on a systcm at shorter distances than the maximum calculated.

This is because the seawater systems typically consist of a number of pipe sections that run in

parallel, branching sections and sections separated by valves which may bc in a closed position

some of the time. Current drain to nearby heat exchangers duc to galvanic couplings will also have

an influence on the anode positioning. Due to this it is important to prepare a proper design by

skilled personnel before a system is installed.

EXAMPLES OF APPLICATION OF TIIE RCP PRINCIPLE

Seawater cooling system with max. temperature 60°

The seawater systcm on the Draugcn platform, operated by A/S Norskc Shell on the Norwegian

continental shelf, is made from 6M0 stainless steel. The max. design tcmpcraturc was 30°C. After

start up temperatures up to 60°C were measured in heat exchangers bctwccn in the cooling lines.The high temperature resulted in localized weld– and crcvicc corrosion.

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A technical and economical evaluation between the two alternatives:

a) rcplaccmcnt of the piping with titanium

b) repair of the corroded areas and installation of RCP anodes to prevent future attacks

resulted in the selection of alt. b).

Detail design work carried out by our RCP experts showed that tcn anodes were required to protect

the actual part of the seawater cooling system.

In this case the Draugen Operator dccidcd to install the anodes in standard access fittings similar

to those used to install traditional corrosion monitoring equipment on the platform. In this way the

anodes can be replaced during normal operation of the cooling systcm using a hydraulic retrieval

tool.

The standard 2“ fitting available today limits the maximum diameter of the anode to about 25 mm.

By increasing the anode length such that the anode enters the main pipe, the lifetime of the anode

can be up five years depending on the diameter of the pipe and the anode spacing. The anode has

a steel core which gives the anode the required strength to withstand the mechanical forces from

the fluid flow in the pipe and an insulating material with drilled holes covering the external surface.

This prohibits fragments of the anode from mixing with the main stream. Figure 4 shows a

schematic view of the access fitting/anode assembly.

The RCP anodes were installed in the piping systcm in Fcbr. –95. Some of the anodes are

continuously monitored by measuring the current delivery, while others arc manually measured once

a month. Fig. 5 shows the current from the anodes after 70 days in operation together with the

calculated values from design. Based on these values the systcm works in accordance with the

design.

Fig. 6 shows an alternative anode assembly solution. In this solution the anode is placed in a

container welded or flanged to the pipe wall. This design offers the usc of anodes with much larger

volume than with standard 2“ access fittings. The rcsu!t is incrcascd rcplaccmcnt interval for the

anodes and up to thirty years lifetime can bc obtained. Onc drawback with this solution is that the

anode replacement has to be done under shutdown of the systcm.

Work on developing new anode designs is now in progress. In onc project that is currently running,

the goal is to design a RCP systcm for an existing platform where localized corrosion on 6M0stainless steel has been observed. For this installation hot work is not allowed and special anode

assemblies have to bc designed.

Local protection of pumps and valves

Due to the corrosion problems that have been observed with the usc of 6M0– and 25%Cr steels

especially in regions with high tcmpcraturc, there has been an incrcascd interest in the use of

titanium alloys, especially Grade 2, as rcplaccrncnt material. For piping and fittings the price

difference between the two alternatives is in the range of 20-30%. For valves and pumps with

dimensions above 3“, however, the cost for titanium components is much higher duc to the limitednumber of suppliers and special requirements for the casting process.

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An interesting alternative is to use pumps and valves made from stainless steels that may suffer

from corrosion in the environment and locally protect the equipment with RCP anodes. One

operator in the Norwegian sector of the North Sea has decided to install pumps made from 25%Cr

stainless steel in a seawater system with temperatures up to 45°C. Each pump is individually

equipped with one RCP anode. The anode will catholically protect the internal parts of the pump.

In addition the titanium piping connected to the pump will drain current. Since the anode is an

integrated part of the pump, the anode lifetime is four years which is in accordance with the

maintenance interval for the pump.

The main advantage of installing RCP anodes to locally protect stainless steel (or other materials)

is the potential cost saving; onc example has been given above. However, if RCP anodes are

installed even cheaper alloys like AISI 316 can be used. We have started discussions with different

oil companies in Norway to replace valves made from titanium, 6M0–, 25%Cr or 22%Cr with AISI

316L combined with RCP anodes. Fig. 7 shows a 10” valve with an anode installed near the inlet

of the valve. The lifetime of the anode is ten years.

Local protection of galvanic corrosion

On several occasions we have been faced with problems related to galvanic couplings between

stainless steel and CuNi alloys in seawater systems, e.g. couplings between seawater piping in 6M0

and deluge skids made from CuNi. In this connection stainless steel will act as a cathode, while

CuNi will be the anode normally suffering from localized corrosion in this environment. To avoid

or reduce the galvanic corrosion problem, an “insulation pipe” with ().5 m length has often been

used. However, since CuNi suffers from localized corrosion, the problem cannot be prcvcntcd by

installing an insulation pipe of that length, as demonstrated and elaborated in a forthcoming

papcrb.

By installing RCP anodes close to the connection bctwccn the two materials in the galvanic

coupling, corrosion attacks can be prcvcntcd on the less noble alloy. The key point is to keep the

potential in a region where corrosion is avoided on the CuNi alloy,

CONCLUSIONS

Recent field experience has shown that high alloyed stainless steels suffer from Iocalizcd corrosion

in seawater when the temperature and/or chlorination lCVC1Cxcccds Certain limits. A new rncthod

of cathodic protection has been developed to eliminate such corrosion problems. The method is

based on resistor control of the anode current output. The resistor control allows cathodicpolarization of the stainless steel to below a defined protection potential for the material, but at the

same time the resistor control prohibits cxccssivc cathodic polarization and too rapid consumption

of the anode. The method is called RCP – Resistor controlled Cathodic Protection.

This method can be used to:

* expand the operating parameter window (temperature, chlorine lCVC1)for 6Mo/25%Cr

stainless steels in piping system transporting chlorinated seawater

*

replace special equipment like pumps and valves made in titanium for temperaturesexcccding 30°C, with pumps and valves made from stainless steel (6M0, 25%Cr, 22%Cr,

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AISI 316) protected by RCP anodes

* locally prevent galvanic corrosion in couplings bctwccn different alloys

* usc AISI 316 in complct piping systems

By performing a proper design the anode lifetime can be as long as thirty years and the lifetime

cost for a system can be dramatically reduced compared to a traditional design.

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REFERENCES

R.Johnsen: “Corrosion Failures in Seawater Piping Systems Offshore”. Paper on the Int.

Symposium on Marine and Microbial Corrosion, EFC Event no. 184, Stockholm, scpt. -91.

R.Mollan: “Snorre Project - Materials Engineering Expcricncc”, Kursdagcne, NTH

(Norway), Jan. -93.

Patent claim no. 91.0093 from SINTEF “Method and arrangement to hinder local corrosion

and galvanic corrosion in connection with stainless steels (SS) and other passive materials”.

P.O.Gartland and J.M. Drugli; “Crcvicc corrosion of High Alloyed Stainless Steels in

Chlorinated Seawater”, Part I - Practiclc aspects, Paper no. 510 at NACE CORROSION’91,

Cincinnati, Ohio, 1991.

P.O.Gartland and J.M.Drugli; “Methods for evaluating and prevention of local and galvanic

corrosion in chlorinated seawater pipelines”. Paper no. 408 at NACE CORROSION’92,

Nashville, 1992.

R.Johnsen, P.O.Gartland, J.M.Drugli and T.Rognc; “How to Prevent Galvanic Corrosion in

Seawater Piping Systems”. Paper no. 496 at NACE CORROSION’96, Denver Colorado,

1996.

TABLE 1

TYPICAL ANODE DISTANCE AND CONSUMPTION FOR CATHODIC PROTECTION OF

STAINLESS STEEL PIPING IN CHLORINATED SEAWATER AT 40”C.

DIAMETER DISTANCE CONSUMPTION LIFETIME

(mm) (m) (g/yr) 1 dm3 Zn anode

(y)

50 16 22 319

80 21 56 125

150 29 143 49

200 34 440 16

250 38 516 11

350 44 1018 7

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R 10 R Ia

Ef

A)

(

FIGURE 1 – schematics of the RCP method applied to a piping system.

1200

1000- No ‘6 Mo/25Cr

acorrosion

800-

% Po[cntial MI 6Mn/25Cr in

>600-

~ chlorirm[crl Scawnlcr400- ,

z q

.-E 200- ‘-. .al?5

\—o-

for 6Mu/25Cr at 80 “c

n.I fur 316 at 80 ‘c

xL—L_—l—o 20 40 60 80 100

Temperature(C)

FIGURE 2- Critical combinations of tcmpcraturcs and potentials leading to initiation of localized

corrosion on 6Mo/25%Cr stainless steel and AISI 316.

800

600

400

g

VI200

~o

z.--200

;a~ .400

–600

–800

0.01 0.1 1 10 100 1000

Current density (mA/m2)

FIGURE 3 – Typical current density requirements for 6M0 stainless steel in chlorinated seawater

dependent on the potential. The scattcrband covers typical variations in the chlorination level andflow velocity.

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PROTECTIVE

COVER

HYDRAULIC

ACCESS FITTING

PIPE

SACRIFICIALANODE

FIGURE 4- Schematic drawing of an RCP

fitting.

Draugen

anode installed in a 2“ hydraulic flareweld access

platform

50

45

5

0

2 3 4 5 7 8 9 10

Anode number

FIGURE 5- Measured current from each anodes after 70 days in operation on the Draugen Field.

Measurements compared to design values.

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.—. —.

---t""--"-------"-"--------------------"-`----------

ANODE

1 I 11 I1 I !1 1

T ~(

.-

—

—

FIG”URE 6 – Pipe spool with external “container” to install RCP anode into.

7 – Valve with a RCP anode

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