Aseawater injection (SWI) system

Transcription

1 Corrosion 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 management (CM) components. Accordingly, any shortcomings or failures associated with such an IMS could be either CEbased 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 distinguishing them from the CE-based ones. Finally, the article presents a list of CMrelated 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 corrosion (MIC) is believed to be the most insidious threat to SWIs because of these field observations: Measured localized corrosion rates caused by MIC are ~6.0 mm/y. 68 MATERIALS PERFORMANCE August 2009 August 2009 MP.indd 68

2 table 1 The main integrity threats, areas affected by them, and the common corrosion engineering mitigation measures for a typical SWI system Integrity 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 Chlorination upstream of deaerator (DA) towers Biocide injection downstream of DA towers 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 H 2 S levels in the reservoir 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 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 Use of internally clad CS injection flow lines 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 Once bacterial control has been lost, it can take years to reinstate. The required chlorination, biocide treatment, and bacterial enumeration 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 (seawater is used for diesel displacement), produced water and produced oil (seawater is used for vessel sandwashing), cooling water, and fire water. These points demonstrate the criticality of SWI and the significance of maintaining an efficient seawater treatment process. While occasional dissolved oxygen 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 systems 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 (H 2 S) within the reservoir, gradually souring the hydrocarbon content. Production reduction: the bacterial growth could block pores within the reservoir, leading to formation plugging. Such plugging can eventually decrease production rates. Dissolved Oxygen and Erosion-Corrosion Corrosion from dissolved oxygen and erosion-corrosion are considered the next worst integrity threats (after MIC), respectively. 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 injected into the reservoir. This threat is not covered in this article, however, as it does not apply to all SWI systems. Observed Corrosion Shortcomings A proper SWI integrity management system comprises both CE and CM components. 1 Thus, any observed shortcoming associated with this IMS could be either CE-based or CM-related. The August 2009 MATERIALS PERFORMANCE 69 August 2009 MP.indd 69

3 Corrosion for Seawater Injection Systems 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 Failure to correctly enumerate the bacterial population (both planktonic and sessile types) Failure to create and associate a corrosion KPI system with the water treatment process to monitor seawater treatment performance and effectiveness Regular monitoring of their performance Assessment of their effectiveness Inability to produce a fully risk-based inspection scope for the SWI system 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 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 CE-based shortcomings are more of a physical activity in nature and closely associated with the following four seawater 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 distinguish whether an integrity management shortcoming is CE-based or CMrelated, the responsible corrosion engineer should use the CM concept definition. According to this definition, CM for any 70 MATERIALS PERFORMANCE August 2009 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 considerations or principles 2) Regular monitoring of their performance 3) Assessment of their effectiveness Applying the above to any SWI integrity management system would enable the corrosion engineer to identify the existing shortcomings from a CM standpoint. North Sea experience has illustrated 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 categories with their main consequences. Table 2 also provides the link between each category and the element of CM associated with it. Proposed Corrosion Improvements Table 3 lists all the proposed CMrelated activities deemed to be required August 2009 MP.indd 70

4 table 3 Proposed CM-related activities associated with each basic CM element and the pertinent integrity threat Integrity Threat MIC Oxygen Reviewing the Applied CE Considerations 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. 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. Regular Monitoring of their Performance Use liquid sampling to enumerate planktonic bacteria populations regularly. Use a sidestream to enumerate sessile bacteria populations regularly. 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. 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. Assessing their Efficiency Based on the measured bacterial populations, determine whether the existing chlorination and biociding processes are functioning efficiently and in an acceptable manner. 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. 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. Use UT wall thickness inspection to measure wall loss and determine corrosion rate. 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. Erosion-corrosion etc. Review the flow velocity for coppernickel alloy sections. During corrosion coupon retrievals, check for grooving or erosion signs on the retrieved coupons and then determine if the identified erosion rate is acceptable. Ensure that filters are in place to remove any suspended solids and macro fouling. 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 Review An integrity review enables the corrosion engineer to determine the inspection, mitigation, and monitoring requirements based on the design, operational, and integrity parameters. The outcome of the integrity review process will help to determine existing gaps in the inspection, mitigation, and monitoring strategies. Shortcomings can then be addressed and corrected. Furthermore, the review will provide one with a comprehensive list of activities that have to be carried out (mostly on a regular basis) to maintain and improve integrity managements for the SWI system concerned. This activity list is often referred to as the Corrosion Control Matrices document, which provides the backbone for any future key performance indicator August 2009 MATERIALS PERFORMANCE 71 August 2009 MP.indd 71

5 C H E M I C A L T R E AT M E N T Corrosion for Seawater Injection Systems (KPI) system. For more information on whether the incumbent biocide treatment integrity review process and its products, has been effective or whether it requires please refer elsewhere.1-2 improvement in the form of changing the chemical, injection frequency, concentracorrosion Key Performance tion, etc. Furthermore, it is strongly recindicator System ommended to carry out an independent A corrosion KPI system should be an annual bacterial survey by a specialist indispensable tool in the regular perfor- third party. This will provide the chemist mance monitoring and the effectiveness or the corrosion engineer with a second assessment of any seawater treatment set of data that has been produced (allegprocess.1-3 A KPI system enables the cor- edly) by a more professional body in the rosion engineer to instantly identify the field of MIC and bacterial enumeration. extent and magnitude of any potential shortcomings in the seawater treatment Corrosion Coupons process. The system also improves the Field experience has demonstrated corrosion engineer s supervision over that corrosion rate (or online) probes recritical activities and identifies the indi- quire regular servicing and cleaning, viduals responsible for these activities. which makes them an expensive choice. Any incompliance(s) identified through Fouling and short circuiting of online the corrosion KPI system can further corrosion probes is another issue that streamline the relevant inspection and makes them less desirable. On the conmonitoring activities/strategies. For more trary, corrosion coupons are easier to detailed information on how to deter- maintain and don t need any regular mine, calculate, and report corrosion servicing and cleaning, only retrieval KPIs, please refer elsewhere.3 when corrosion rate information is required. Corrosion coupons could also Enumeration of provide information on both general and Bacterial Populations localized corrosion rates, organic or bacbacterial enumeration on some assets terial presence, and erosion. Furthercould often be erratic or incorrect. Fur- more, taking bacterial samples from them thermore, the enumeration process fails could help to determine bacterial populato assess the impact and effectiveness of tions per unit surface area. Such improvethe biociding activity on the bacterial ments in the quality and quantity of activity and population on a long-term bacterial information could in turn help basis. This could be achieved by having to further optimize both the chlorination in place an improved procedure for re- and biociding activities on a regular basis. moving the sidestream studs on a regular basis for sessile sampling processes. Such Ultrasonic Testing Wall a procedure should specify that some Thickness Inspections studs have to be removed less frequently While corrosion coupons could pro(than the normal ones that are removed, vide accurate general and localized cornormally on a weekly basis) and prefer- rosion rates, it is also strongly recomably on a monthly, three-month, and mended to carry out UT wall thickness six-month basis. This method of stud inspection on different areas of the SWI removal will enable the corrosion engi- system based on the available risk-based neer to identify or determine the long- inspection scope. The generated corroterm effects (or the effectiveness) of regu- sion rate (and remaining life) information lar biociding (e.g., on a weekly basis) on would supplement those gathered from the bacterial growth and population. the corrosion coupons and act as a second Thereafter, the responsible chemist or the source of useful integrity data. Such UT corrosion engineer can determine inspections could also be highly useful in measuring corrosion rates within the existing deadleg areas in the SWI system where the risk of failure (due to a synergy between MIC and under deposit corrosion mechanisms) 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 divided into the following three categories: Failure to perform an integrity review and to determine the associated corrosion control matrix Failure to enumerate the bacterial population correctly Failure to use a corrosion KPI to monitor and assess the seawater treatment performance and effectiveness on a regular basis Accordingly, various CM-related activities 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. References 1 A. Morshed, Offshore Assets: From Corrosion Engineering to Corrosion, MP 46, 10 (2007): p A. Morshed, Corrosion for Oil and Gas Assets, MP 47, 8 (2008): p A. Morshed, Improving Asset Corrosion 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., ali.morshed@ psnworld.com. He has years of experience protecting oil and gas assets, specializing in producing assetspecific CM systems. He received a Ph.D. grant from BP to conduct research on corrosion of carbon steel sweet oil transfer pipelines ( ), has an M.S. degree in corrosion engineering materials from Imperial College (London, 1997), and has authored several publications. 72 MATERIALS PERFORMANCE August 2009 August 2009 MP.indd 72