68 MATERIALS PERFORMANCE August 2009

Corrosion Management for Seawater

Injection Systems

Ali Morshed, Production Services Network (PSN), Aberdeen, U.K.

Seawater injection (SWI) is important to maintain

oilfield production and to provide various systems

with treated seawater. Any shortcomings in seawater

treatment will have serious effects on production and

asset integrity. This article examines SWI systems

integrity management from a corrosion management

point of view. It reviews major shortcomings and

makes recommendations to rectify them and improve

overall integrity management of a typical SWI system

from a corrosion management standpoint.

Aseawater injection (SWI) system plays an important role for its asset and associated reservoir(s). It maintains reservoir pressure

for hydrocarbon production and feeds other systems as well. Any shortcomings in the seawater treatment process can adversely affect hydrocarbon production, the integrity of the SWI system itself, and those systems that receive seawater from it. A proper integrity management system (IMS) is therefore indispensable for any SWI system.

The IMS should include corrosion engineering (CE) and corrosion manage-ment (CM) components. Accordingly, any shortcomings or failures associated with such an IMS could be either CE-based or CM-related. This article focuses on the latter and explains how using the CM concept definition could help the corrosion engineer in determining such CM-related shortcomings and distin-guishing them from the CE-based ones. Finally, the article presents a list of CM-related proposals that are intended to improve the overall SWI system integrity management, mainly based on the U.K.’s North Sea experience.

Common Integrity Threats and their Mitigation

There are three main integrity threats often associated with SWI systems. Table 1 lists these threats and some of the more common mitigation methods that are used to rectify them.

Microbiologically Influenced Corrosion

Microbiologically influenced corro-sion (MIC) is believed to be the most in-sidious threat to SWIs because of these field observations:

• Measured localized corrosion rates caused by MIC are ~6.0 mm/y.

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• Once bacterial control has been lost, it can take years to reinstate.

• The required chlorination, biocide treatment, and bacterial enumera-tion are often inadequate, erratic, or incorrect.

• The process and utility systems fed by SWI are often contaminated and experience MIC to various degrees. Such systems include diesel (sea-water is used for diesel displace-ment), produced water and pro-duced oil (seawater is used for vessel sandwashing), cooling water, and fire water.

These points demonstrate the critical-ity of SWI and the significance of main-taining an efficient seawater treatment process. While occasional dissolved oxy-gen excursions would cause transient high localized corrosion mainly in the SWI system itself, inefficient or inadequate

bacterial control would often lead to prolonged and acute corrosion issues (and sustained high localized corrosion rates)within many of the aforementioned sys-tems that are fed by the SWI system. Once such MIC issues appear, they are often too complicated and too expensive to rectify.

Poor bacterial control can also lead to bacteria being injected into the reservoir with these adverse effects:

• Reservoir souring: this occurs when bacteria produce increasing levels of hydrogen sulfide (H2S) within the reservoir, gradually souring the hydrocarbon content.

• Production reduction: the bacte-rial growth could block pores within the reservoir, leading to formation plugging. Such plugging can eventually decrease produc-tion rates.

Dissolved Oxygen and Erosion-Corrosion

Corrosion from dissolved oxygen and erosion-corrosion are considered the next worst integrity threats (after MIC), re-spectively.

There is sometimes a fourth threat category, which is applicable only to those facilities where the seawater and produced water phases are mixed before being in-jected into the reservoir. This threat is not covered in this article, however, as it does not apply to all SWI systems.

Observed Corrosion Management Shortcomings

A proper SWI integrity management system comprises both CE and CM com-ponents.1 Thus, any observed shortcom-ing associated with this IMS could be either CE-based or CM-related. The

TAbLE 1

The main integrity threats, areas affected by them, and the common corrosion engineering mitigation measures for a typical SWI systemIntegrity Threat Systems or Areas Affected Common CE Measures

MIC The SWI system itself plus the following systems that may be fed by the SWI system: diesel, produced water, produced oil, cooling water, and fire water. The drain system may not receive water from the SWI system directly, but it will do so eventually and become acutely contaminated through some of the above systems. The reservoir could also be contaminated by the bacteria through water injection. This can lead to formation plugging and increased H2S levels in the reservoir

Chlorination upstream of deaerator (DA) towers Biocide injection downstream of DA towers

Oxygen Any exposed carbon steel (CS) surfaces, mainly within the SWI system itself, but also any other system that receives water from it

Use of copper nickel alloys upstream of the deaerator towers Deaeration via DA towers Injection of oxygen scavenger Use of internally coated CS piping Use of internally clad CS injection flow lines

Erosion-corrosion Copper nickel pipework (if the flow velocity is higher than their maximum velocity threshold) Upstream and downstream of the seawater booster and main injection pumps; in particular around bends, reducers, and where pipework geometry creates higher turbulence Erosion due to sand or suspended solids; in particular at areas where flow changes direction, such as bends

Flow velocity control for copper nickel pipework upstream of DA towers Flow control upstream and downstream of SWI pumps to minimize liquid erosion in bends, reducers, etc. Use of filters to remove suspended solids and biological macrofouling

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C H E M I C A L T R E A T M E N T Corrosion Management for Seawater Injection Systems

CE-based shortcomings are more of a physical activity in nature and closely associated with the following four seawa-ter treatment activities:

• Filtration• Chlorination• Deaeration• Biociding However, CE-based shortcomings are

not the focus of this article; hence they are not discussed hereinafter. To better dis-tinguish whether an integrity manage-ment shortcoming is CE-based or CM-related, the responsible corrosion engineer should use the CM concept definition. According to this definition, CM for any

asset (or any system within that asset) is “the process of reviewing the applied CE considerations, the regular monitoring of their performance, and the assessment of their effectiveness post-commissioning.” Based on this definition, the three main elements of any corrosion management system should sequentially be:

1) Reviewing the applied CE consid-erations or principles

2) Regular monitoring of their perfor-mance

3) Assessment of their effectivenessApplying the above to any SWI integ-

rity management system would enable the corrosion engineer to identify the

existing shortcomings from a CM stand-point. North Sea experience has illus-trated that such CM shortcomings for a typical SWI system—while so numerous and diverse—could be divided into three major categories. Table 2 lists these cat-egories with their main consequences. Table 2 also provides the link between each category and the element of CM associated with it.

Proposed Corrosion Management Improvements

Table 3 lists all the proposed CM- related activities deemed to be required

TAbLE 2

The main three categories of observed CM-related shortcomings, their association with the pertinent CM element, and their main consequences within a SWI system CM Shortcoming Category The Associated CM Element Consequences

Failure to perform an integrity review and produce a corrosion control matrix for the SWI system

Reviewing the initial CE considerations Inability to recognize or determine all those activities indispensable to maintaining the SWI system integrity Inability to select the appropriate individual corrosion KPI activities due to the lack of a SWI-based corrosion control matrix Inability to determine or identify all the existing CE-based and CM-related shortcomings within the SWI system Inability to produce a fully risk-based inspection scope for the SWI system

Failure to correctly enumerate the bacterial population (both planktonic and sessile types)

Regular monitoring of their performance Inability to determine the effectiveness of chlorination and biociding treatments Inability to determine when biocide shock dosing is required Inability to determine the extent of both planktonic and sessile contaminations within the SWI system

Failure to create and associate a corrosion KPI system with the water treatment process to monitor seawater treatment performance and effectiveness

Assessment of their effectiveness Inability to determine and monitor the effectiveness of seawater treatment in regard to CE-based activities of chlorination, deaeration, and biociding Inability and difficulty in communicating (or reporting) the effectiveness of the water treatment process within the SWI system Inability to highlight and quantify the existing CE-based issues to others; in particular to the senior management

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TAbLE 3

Proposed CM-related activities associated with each basic CM element and the pertinent integrity threat

Integrity Threat Reviewing the Applied CE Considerations

Regular Monitoring of their Performance

Assessing their Efficiency

MIC Review design parameters—geometry, material selection, corrosion allowance, internal coating type, internal cladding type, etc.

Use liquid sampling to enumerate planktonic bacteria populations regularly. Use a sidestream to enumerate sessile bacteria populations regularly.

Based on the measured bacterial populations, determine whether the existing chlorination and biociding processes are functioning efficiently and in an acceptable manner.

Review chemical treatment parameters—chemical type, treatment type (i.e., batch or continuous), frequency, concentration, injection location, etc.

Measure residual chlorine levels. Use corrosion coupons to determine localized and general corrosion rates and the presence of organic colonies. Use UT wall thickness inspection to measure wall loss and determine corrosion rates.

Based on the observed corrosion rates, determine whether the existing chlorination and biociding processes are functining efficiently. Select residual chlorine level, bacterial population (either planktonic or sessile or both), and biociding as corrosion KPIs.

Oxygen Review design parameters—geometry, material selection, corrosion allowance, internal coating type, internal cladding type, etc. Review chemical treatment parameters—chemical type, treatment type (i.e., batch or continuous), frequency, concentration, injection location, etc.

Use oxygen probes/monitors to measure the dissolved oxygen concentration downstream of DA towers and oxygen scavenger injection point. Use corrosion coupons to determine localized and general corrosion rates. Use UT wall thickness inspection to measure wall loss and determine corrosion rate.

Based on the observed corrosion rates, determine whether the deaeration process is functioning efficiently. Select oxygen concentration in the deaerated seawater phase as a corrosion KPI.

Erosion-corrosion Review design parameters—geometry, material selection, corrosion allowance, etc. Review the flow velocity for copper- nickel alloy sections. Ensure that filters are in place to remove any suspended solids and macro fouling.

In case of frequent failures within the copper-nickel alloy sections, determine if the flow velocity is higher than the maximum threshold velocity for such alloys.

During corrosion coupon retrievals, check for grooving or erosion signs on the retrieved coupons and then determine if the identified erosion rate is acceptable.

to improve a SWI system’s integrity from a corrosion management standpoint. This table also illustrates the link between any of the aforementioned three integrity threats, the three CM elements, and the proposed CM-related activities. The recommendations can be grouped into five main categories:

Integrity ReviewAn integrity review enables the corro-

sion engineer to determine the inspec-tion, mitigation, and monitoring require-ments based on the design, operational, and integrity parameters. The outcome of the integrity review process will help to determine existing gaps in the inspec-tion, mitigation, and monitoring strate-gies. Shortcomings can then be ad-

dressed and corrected. Furthermore, the review will provide one with a compre-hensive list of activities that have to be carried out (mostly on a regular basis) to maintain and improve integrity manage-ments for the SWI system concerned. This activity list is often referred to as the “Corrosion Control Matrices” docu-ment, which provides the backbone for any future key performance indicator

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C H E M I C A L T R E A T M E N T Corrosion Management for Seawater Injection Systems

(KPI) system. For more information on integrity review process and its products, please refer elsewhere.1-2

Corrosion Key Performance Indicator System

A corrosion KPI system should be an indispensable tool in the regular perfor-mance monitoring and the effectiveness assessment of any seawater treatment process.1-3 A KPI system enables the cor-rosion engineer to instantly identify the extent and magnitude of any potential shortcomings in the seawater treatment process. The system also improves the corrosion engineer’s supervision over critical activities and identifies the indi-viduals responsible for these activities. Any incompliance(s) identified through the corrosion KPI system can further streamline the relevant inspection and monitoring activities/strategies. For more detailed information on how to deter-mine, calculate, and report corrosion KPIs, please refer elsewhere.3

Enumeration of Bacterial Populations

Bacterial enumeration on some assets could often be erratic or incorrect. Fur-thermore, the enumeration process fails to assess the impact and effectiveness of the biociding activity on the bacterial activity and population on a long-term basis. This could be achieved by having in place an improved procedure for re-moving the sidestream studs on a regular basis for sessile sampling processes. Such a procedure should specify that some studs have to be removed less frequently (than the normal ones that are removed, normally on a weekly basis) and prefer-ably on a monthly, three-month, and six-month basis. This method of stud removal will enable the corrosion engi-neer to identify or determine the long-term effects (or the effectiveness) of regu-lar biociding (e.g., on a weekly basis) on the bacterial growth and population. Thereafter, the responsible chemist or the corrosion engineer can determine

whether the incumbent biocide treatment has been effective or whether it requires improvement in the form of changing the chemical, injection frequency, concentra-tion, etc. Furthermore, it is strongly rec-ommended to carry out an independent annual bacterial survey by a specialist third party. This will provide the chemist or the corrosion engineer with a second set of data that has been produced (alleg-edly) by a more professional body in the field of MIC and bacterial enumeration.

Corrosion CouponsField experience has demonstrated

that corrosion rate (or online) probes re-quire regular servicing and cleaning, which makes them an expensive choice. Fouling and short circuiting of online corrosion probes is another issue that makes them less desirable. On the con-trary, corrosion coupons are easier to maintain and don’t need any regular servicing and cleaning, only retrieval when corrosion rate information is re-quired. Corrosion coupons could also provide information on both general and localized corrosion rates, organic or bac-terial presence, and erosion. Further-more, taking bacterial samples from them could help to determine bacterial popula-tions per unit surface area. Such improve-ments in the quality and quantity of bacterial information could in turn help to further optimize both the chlorination and biociding activities on a regular basis.

Ultrasonic Testing Wall Thickness Inspections

While corrosion coupons could pro-vide accurate general and localized cor-rosion rates, it is also strongly recom-mended to carry out UT wall thickness inspection on different areas of the SWI system based on the available risk-based inspection scope. The generated corro-sion rate (and remaining life) information would supplement those gathered from the corrosion coupons and act as a second source of useful integrity data. Such UT inspections could also be highly useful in

measuring corrosion rates within the exist-ing deadleg areas in the SWI system where the risk of failure (due to a synergy between MIC and under deposit corrosion mecha-nisms) is higher.

Conclusions and Recommendations

MIC, dissolved oxygen, and erosion are the three main integrity threats to SWI systems. The majority of failures are either CE-based or CM-related. The latter is di-vided into the following three categories:

• Failure to perform an integrity review and to determine the associated cor-rosion control matrix

• Failure to enumerate the bacterial population correctly

• Failure to use a corrosion KPI to monitor and assess the seawater treat-ment performance and effectiveness on a regular basis

Accordingly, various CM-related ac-tivities have been proposed to improve SWI integrity. Such activities are divided into five main categories:

• Perform integrity reviews. • Create and use a corrosion KPI

system.• Enumerate the bacterial population.• Use corrosion coupons.• Perform UT wall thickness in spections.

References1 A. Morshed, “Offshore Assets: From

Corrosion Engineering to Corrosion Management,” MP 46, 10 (2007): p. 34.

2 A. Morshed, “Corrosion Management for Oil and Gas Assets,” MP 47, 8 (2008): p. 54.

3 A. Morshed, “Improving Asset Corrosion Management Using KPIs,” MP 47, 5 (2008): p. 50.

ALI MORSHED is the principal corrosion engineer at Production Services Network (PSN), Wellheads Place, Dyce, Aberdeen, AB21 7GB, U.K., e-mail: ali.morshed@psnworld.com. He has years of experience protecting oil and gas assets, specializing in producing asset- specific CM systems. He received a Ph.D. grant from BP to conduct research on corrosion of carbon steel sweet oil transfer pipelines (1997-2001), has an M.S. degree in corrosion engineering materials from Imperial College (London, 1997), and has authored several publications.

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