NACE -51300-10404-SG

2010

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PROTECTION AGAINST MICROBIOLOGICALLY INFLUENCED CORROSION BY EFFECTIVE TREATMENT AND MONITORING

DURING HYDROTEST SHUT-IN

Joseph E. Penkala, Jennifer Fichter, and Sunder Ramachandran

Baker Petrolite Corporation 12645 West Airport Blvd. Sugar Land, TX 77478

ABSTRACT During hydrotesting, a pipeline may be shut-in for a significant period of time. If unprotected, the pipeline becomes susceptible to corrosion due to bacteria, oxygen, and saline conditions of the hydrotest water, typically obtained from surface waters and seawater. Most notably, indigenous bacterial populations in source water used to fill the pipeline can proliferate in the stagnant shut-in condition and attach to the pipe wall forming biofilms. These sessile biofilms may contain sulfate reducing bacteria (SRB) and/or acid-producing bacteria (APB) which can contribute to microbiologically influenced corrosion (MIC). In addition, SRB generate hydrogen sulfide (H2S) which is hazardous, corrosive, and can form iron sulfide solids. To protect against these adverse effects, three types of chemicals are generally recommended to be added to the hydrotest water during the pipeline fill: an oxygen scavenger, a biocide, and a corrosion inhibitor. Chemical treatment to prevent corrosion invariably deals with balancing risk, damage to the environment, and cost of the program. The method of applying chemical is important to ensure protection for extended shut-in times. Monitoring is important to ensure that protection is being provided during shut-in and that risk is minimized after the pipeline is brought into operation. The paper will discuss different methods for discharging the water into the environment as well as procedures to ensure corrosion protection for shut-in times that exceed the original target discharge date.

©2010 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACEInternational, Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper aresolely those of the author(s) and are not necessarily endorsed by the Association.

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Paper No.

10404

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In support of this discussion, field data is presented from a hydrotest application that was monitored for bacterial growth and biocide residual during shut-in. This data highlights the importance of protecting the pipeline during shut-in and emphasizes that a quality monitoring program is the key in determining if a chemical program is providing control over the targeted parameters. A discussion of best practices for MIC-protection during hydrotest is provided. Keywords: hydrotest, microbiologically influenced corrosion (MIC), biocide, sulfate-reducing bacteria (SRB), acid-producing bacteria (SRB), monitoring

INTRODUCTION

Before a new or rehabilitated pipeline is placed into service it must be tested for integrity at a pressure above its designed working pressure. This testing is typically done with water (hydrostatic testing), which may remain in the system for an extended shut-in period. This period may be longer than initially targeted due to unforeseen complications in commissioning. The water used for hydrostatic testing in offshore installations usually comes from filtered seawater; whereas, onshore facilities obtain water from surface waters (rivers, lakes, and ponds), aquifers, or potable water. Water from any of these sources can cause corrosion due to the introduction of bacteria, dissolved oxygen, and or chlorides into the pipeline.1-3 The severity of the problem is dependent upon the type and quality of water that is used, the length of time the water remains in the line, and the ambient temperature of the system. The build-up of corrosion and bacterial by-products on the pipe walls during shut-in can cause problems at later stages of a pipeline’s operational life. Additionally, once a bacterial biofilm is formed, if it is not properly removed before starting operation, it may remain active and cause damage to the pipeline, which may not be discovered until years later. Other potential forms of damage are under-deposit corrosion and contamination of fluids passing through the pipe. In order to protect against these adverse effects, three types of chemicals may be added to the hydrotest water during the pipeline fill: an oxygen scavenger, a biocide, and a corrosion inhibitor. It is advisable to treat the hydrotest water, at a minimum, with biocide and oxygen scavenger. It is important to choose products that are environmentally friendly and/or easily neutralized should the line integrity become compromised during the shut-in period, and to allow for discharge of the fluids at the end of the hydrotest. A quality monitoring program is the key to determining if a chemical program is providing control over the targeted parameters. Due to the nature of hydrotest applications (i.e. offshore pipelines, limited or non-existent sampling points, etc.), a monitoring program is often overlooked. Monitoring becomes critical especially when water remains in the pipeline longer than intended. This paper discusses factors than should be considered in designing water treatment programs during hydrotesting. As an illustration, a field study is described that shows how monitoring data was used to estimate the length of time a pipeline system that was filled with seawater was protected from bacterial incursion by the chemical treatment. The monitoring data were also used to predict when it would be necessary to re-treat the pipeline system after the water shut-in period exceeded its target discharge date. Finally, a discussion of important factors to consider for MIC-protection during hydrotest shut-in is given.

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FACTORS CONTRIBUTING TO CORROSION DURING SHUT-IN Bacteria Of the three key factors that can initiate pipeline corrosion during shut-in, bacteria are the most formidable. All MIC problems stem from establishment of biofilm on the pipe wall consisting of a multispecies consortium of microorganisms and associated biopolymer matrix and accumulated solids. In addition to MIC, microbiologically induced souring (MIS) can also occur. Two of the more common groups of bacteria associated with MIC are sulfate-reducing bacteria (SRB) and acid producing bacteria (APB). Uncontrolled growth of SRB during the stagnant shut-in period can create severe operational, environmental, and safety problems, resulting from corrosion, solids production, and hydrogen sulfide (H2S) generation. The APB can degrade dissolved organic material in the water, producing volatile fatty acids (VFAs) which can cause under-deposit, acid-induced corrosion pitting. Moreover, VFAs can also provide nutrients for SRB, thereby creating a synergism between these two classes of bacteria. Use of seawater and high total dissolved solids (TDS) brines can increase the potential for corrosion relative to fresh water due to the higher conductivity and increased sulfate concentration (SRB respiration nutrient). Although the industry emphasizes the role of SRB and APB in MIC because of identified corrosion mechanisms and relative ease of study and monitoring, several other types of bacteria have been identified that either contribute to or are associated with MIC: slime-forming bacteria, iron-oxidizing bacteria, iron-reducing bacteria, methanogenic bacteria, and sulfur-oxidizing bacteria. These organisms and their roles in MIC have been documented in the literature and discussed elsewhere.1,4-6 However, the current consensus is that MIC is not limited to these organisms and in fact a number of other species may be intimately involved. Several known and even unknown species are being implicated in MIC, e.g., members of the Domain Achaea. Identifying these organisms and elucidating their roles in MIC remains the task of future investigations as new approaches emerge in the fields of molecular microbiology and biocorrosion. During shut-in with untreated water, it is expected that the biofilm will be established in the absence of preventative measures. This will ultimately give rise to initiation of MIC processes, as the surface of the pipe wall is colonized and exposed to the plethora of metabolic processes taking place in the biofilm. Furthermore, once established, it is likely that this biofilm will persist both during and after shut-in, thereby not only incurring risks to pipeline integrity during shut-in, but also during the operational life of the pipeline. Biofilms are the most difficult corrodants to remove once they have formed, and because bacterial incursion can never be completely eradicated, only controlled, the rest of the paper will be focused on managing this corrosion threat during shut-in. It should be remembered that no pipeline system can be sterilized and the continued threat of re-establishment of bacteria within a pipeline is real and ongoing. Therefore, protection of pipeline integrity against MIC must be comprehensive for the entire life of the hydrotest period.


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Oxygen Oxygen is a corrodant present in significant concentrations in all waters utilized for hydrostatic testing, and therefore is an immediate corrosion risk. However, if the oxygen is removed at the start of hydrotest, the risk is eliminated and there is minimal chance that this risk will reoccur during shut-in. Since oxygen reacts with iron solubilized from the pipe wall to form iron oxide and hydroxides, then in a closed system the oxygen concentration is reduced. The reduction in dissolved oxygen levels in various hydrotest waters in different diameter pipelines during the initial hours of shut-in is shown in Table I. It can be seen that oxygen levels achieve concentrations of 18 to 48 ppb within 18 to 48 hours in this study.

Table I INITIAL OXYGEN REDUCTION DURING HYDROTEST 7

Faced with additional chemical cost and compatibility issues between oxygen scavengers and biocides, many operators choose to follow this logic and bypass the use of oxygen scavenger for hydrotesting. If the operator chooses to use a scavenger, the application of the scavenger and biocide must be carefully engineered. Both glutaraldehyde and THPS react with bisulfite which would negate the oxygen scavenger and significantly reduce the initial biocide concentration. If the scavenger is applied sufficiently upstream of the biocide injection point to allow reaction with the oxygen during filling, then it minimizes the amount reaction that might occur between these two chemicals.

TREATMENT OPTIONS FOR HYDROTEST FLUIDS

Cost vs Risk To treat or not to treat is not a trivial decision facing the operator. Large volumes of water are utilized in hydrotesting, resulting in a high up front cost. An example of the program chemical volumes is given in Table II for a 50 mile pipe segment with 30-in diameter treated with 225 ppm of biocide. This table does not include volumes of corrosion inhibitor, oxygen scavenger

207.0fresh2

484.5seawater10

304.0seawater4

184.5seawater2

266.8fresh4487.2fresh10

Time (Hrs): O2Reduced to ppb

Levels

Initial O2Concentration

(ppm)

Water Salinity

Line Size (in)

207.0fresh2

484.5seawater10

304.0seawater4

184.5seawater2

266.8fresh4487.2fresh10

Time (Hrs): O2Reduced to ppb

Levels

Initial O2Concentration

(ppm)

Water Salinity

Line Size (in)


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or chemical required for biocide neutralization at discharge. However, this cost must be weighed against the risk of not treating. If biofilm develops during the shut-in period, which is highly likely, and pitting corrosion is initiated, then an irreversible process has begun and the pipeline integrity has already become compromised. Of course, the severity of the damage depends on the duration of the shut-in. If a short-term (< 1 week) hydrotest is performed using potable water with line-drying after discharge, the requirement for treatment is not generally warranted. However, when environmental water (seawater, fresh surface waters or produced waters) is used for filling and the shut-in period exceeds 1 month, it becomes advisable to treat. In many instances, the predicted shut-in times are under-estimated due to altered commissioning schedules or unforeseen delays, resulting in under-treatment due to degradation of the biocide over time. As will be seen in the case history discussed in the last section of this paper, microbiological activity in a shut-in pipeline is a real risk with real repercussions.

Table II

EXAMPLE OF BIOCIDE VOLUMES REQUIRED DURING HYDROTEST

Pipe Length (mi) 50 Pipe Length (ft) 264,000 Pipe Diameter (in) 30 Pipe Radius (ft) 1.25 Pipe Volume (cu ft) 1,295,906 Pipe Volume (gal) 9,694,049 Biocide Conc. (ppm) 225 Biocide Volume (gal) 2181 Biocide Volume (drums) 40

Biocide Treatment If the shut-in time exceeds one month, biocide treatment is strongly recommended to ensure integrity of the pipeline. In general hydrotest biocides must meet the following criteria: • be effective against a broad spectrum of bacteria. • have good persistence, i.e. long half-life • be cost effective with respect to cost and concentration. • be relatively safe to handle and provide minimal environmental impact (low ecotoxicity

profile). • be compatible with system fluids and with other treatment chemicals. • satisfy handling requirements such as corrosivity and stability. • be capable of being readily and rapidly neutralized. Based on these criteria, two commonly used biocide chemistries for hydrotest applications are • Tetrakishydroxymethyl Phosphonium Sulfate (THPS) • Glutaraldehyde


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The initial concentration of biocide required for treating a hydrotest fluid is typically calculated using the following parameters: 1) the predicted duration of the hydrotest; 2) the half-life of the biocide at a given system pH and temperature; and 3) a targeted end biocide residual, typically 20 to 50 ppm active biocide. This concentration is also subject to demands of the system and issues arising over discharge. For instance, kill studies may indicate that it is prudent for a relatively short shut-in time to increase treating rate to at least 100 ppm in order obtain significant initial kill of the microorganisms in the water. THPS and glutaraldehyde have significantly different half-lives in water; therefore their persistence during shut-in will vary accordingly. Table III shows the varying half-lives of glutaraldehyde based on pH and temperature. Figure 1 shows a degradation of THPS in seawater based laboratory testing. Based on the different half-lives seen in Table III, it is apparent that glutaraldehyde would have more utility in longer term hydrotest shut-ins since it has 6-fold greater stability than THPS. Conversely, THPS is better suited for short-term applications because the terminal residual will be much lower at the end of an application. There is always a balance of maintaining a sufficiently high initial concentration for effective bacterial kill and insurance of an endpoint residual vs. trying to minimize the residual biocide for discharge.

Table III HALF-LIFE OF GLUTARALDEHYDE AT DIFFERENT PHS FOR DIFFERENT

TEMPERATURES 8

Temperature 10˚C Starting Concentration (ppm)

500 1000 pH Half-Life (Days) 6.0 29,155 29,005 6.5 25,841 25,474 7.0 19,010 18,660 7.5 10,504 10,073 8.0 4,305 4,063 8.5 1502 1427 9.0 491 465 9.5 157 149

10.0 50 47 11.0 5 4.7

Temperature 15˚C Starting Concentration (ppm) 250 500

pH Half-Life (Days) 6.0 16,512 16,467 6.5 14,091 14,299 7.0 9,841 9,687 7.5 4,916 4,796 8.0 1,904 1,847 8.5 648 627


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9.0 210 203

Temperature 25˚C Starting Concentration (ppm) 250 500

pH Half-Life (Days) 6.0 5,648 5,632 6.5 4,288 4,353 7.0 2,488 2,457 7.5 1,051 1,034 8.0 372 365 8.5 122 120 9.0 39 38 9.5 12.4 12.2

10.0 3.9 3.9 10.5 1.2 1.2 11.0 0.4 0.4

Ultimately, the key to biocide treatment is to successfully control microbiologically influenced corrosion. Concurrently, there must be consideration for environmental impact both during shut-in (contingency for leak or failure) and during discharge (at the end of the test). Treating after Hydrotest One treatment philosophy is to suspend use of chemicals in the hydrotest water and treat the pipeline after the hydrotest period is complete. It should be cautioned that as a stand alone treatment this procedure is only effective again MIC if the hydrotest period is only a few weeks up to one month. Beyond that timeframe, the pipeline can become significantly compromised in the absence of biocide treatment. Occasionally the operators will combine the two treating philosophies and treat the hydrotest fluid in addition to doing post-hydrotest treatment. The post hydrotest treatment consists of draining the hydrotest fluid from the line, followed by a pigging cleaning program. It is advisable to include a biocide/corrosion inhibitor slug during the pigging operation. The pigging is aimed at: 1) removing water from low spots and sags, 2) removing built-up solids, and 3) disturbing and/or removing biofilm. Following the pigging operation, the pipeline is generally dried using nitrogen or air before being placed into operation. Discharge of Hydrotest Water and Neutralization of Biocide As stated, biocide requirements during hydrotest are fairly rigorous, aside from efficacy, economics and relative low ecotoxicity, the biocide must be persistent during treatment but safe for discharge into the environment at the end of hydrotest. The latter requirement presents a conflict. To ensure a biocide residual and adequate treatment until the end of the test period dictates that the discharge water will contain residual biocide. Therefore, a neutralization scheme must be implemented. This requires measurement of the concentration of biocide residual remaining in the hydrotest water using a field biocide residual kit,


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application of a neutralization agent, and often, since neutralization is not instantaneous, requires the collection of the discharge water in a tank to allow sufficient reaction time of the neutralizing agent with the biocide. The biocide residual must be reduced to within the Environmental Protection Agency’s National Pollutant Discharge Elimination System (NPDES) specifications for the target environmental system. Field biocide residual kits can again be utilized after the neutralization process to verify the biocide residual has been reduced sufficiently. Table IV demonstrates for the amount of hydrogen peroxide required to neutralize 50 ppm active THPS at the end of the hydrotest using the 50 mile pipeline segment from Table I.

Table IV Chemical requirement for neutralization during hydrotest

Pipe Length (mi) 50 Hydrotest Water Volume (gal) 9,694,049 Endpoint THPS (ppm active) 50

Endpoint THPS (gal) 485 Peroxide Required (ppm active) (1) 8.5 Peroxide Required (gal 3% product) 2750 Peroxide Required (drums 3% product) 50

Monitoring during Shut-In Essential to any bacterial mitigation program is a monitoring program to evaluate the effectiveness of the treatment. Monitoring is the best method to assess the level of bacterial control during the hydrotest and to determine whether the biocide residual is still present at effective concentrations, especially if the shut-in period is extended for longer than planned. This aspect of hydrotest applications is virtually ignored in the industry because of failure to anticipate such requirement in the design and construction of the pipeline system. Typically, a shut-in pipeline during hydrotest has no access port for sampling. Offshore systems are particularly susceptible to this issue. The only solution to provide access is to install a sampling port during construction of the line. Typically, such a design feature is not considered because 1) the need for treatment and monitoring is not understood, 2) it is not anticipated that the line may be shut-in beyond its target commission date, 3) it is not anticipated that the line may require additional hydrotesting after commissioning, or 4) it is difficult to identify where to install suitable sample points due to location of the line (e.g., subsea), 5) may be a weak point and a potential leak path. In the next section, a case history is presented highlighting the value of a monitoring program. The monitoring program provided key information that was used to make decisions not only about the subject pipeline but also for a parallel pipeline system without a monitoring system. (1) This value is based on a ratio of 17 ppm hydrogen peroxide: 100 ppm THPS (active) according to technical information from Rhodia. 9 Demand for amount of peroxide required can vary according to composition of water being treated.


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CASE HISTORY: MONITORING DURING HYDROTEST ENABLES EFFECTIVE BACTERIAL TREATMENT

This section describes a hydrotest program for a portion of a subsea pipeline system in an offshore setting. This pipeline system consisted of several segments, two of which will be focused on in this study. The first pipeline held approximately 237,000 bbls of fluid (“Pipeline A”). The second pipeline held approximately 260,000 bbls of fluid at capacity (“Pipeline B”). Two different scenarios are presented in this study. Newly laid Pipeline A was filled with filtered seawater treated with biocide, oxygen scavenger, and corrosion inhibitor targeted for a 16 week shut-in period. Thereafter, it became apparent that there would be an extended shut-in period for the pipeline system. Due to the extended shut-in period, it was anticipated that the biocide residuals might have dropped to levels that would no longer be lethal to bacteria in the pipeline. A plan was implemented for sampling the pipeline to determine the status of the hydrotest fluid and actions were taken based on the results of the sampling. Pipeline B was flooded with untreated seawater during installation. Based on the information gained from monitoring Pipeline A, it became apparent that Pipeline B was at risk. After six months shut-in with the untreated water, Pipeline B was drained, pigged, and refilled with chemically treated water (biocide, oxygen scavenger, and corrosion inhibitor) to provide adequate protection for the remainder of the shut-in period. Treatment and Sampling Overview The timeline, presented below and summarized in Table V, begins with month 0, initiation of the study and logs each event for Pipeline A or B with respect to this starting point. Month 0: Initiation of treatment in Pipeline A. The Pipeline A segment was filled with filtered seawater that had been treated with 267 ppm of a tetrakishydroxymethyl phosphonium sulfate (THPS)-based biocide, 100 ppm catalyzed oxygen scavenger, and 100 ppm corrosion inhibitor. The THPS concentration was calculated using a half-life estimate of 55 days for a targeted shut-in period of 16 weeks. The treatment targeted a minimum of 50 ppm active biocide remaining at the end of the hydrotest period to ensure control over any surviving bacteria in the hydrotest waters. THPS was chosen as the preferred biocide for this application for the following reasons: 1) it can be rapidly neutralized by the addition of hydrogen peroxide or oxygenation; 2) it has a moderately long half-life of 55 to 65 days under anaerobic, neutral pH conditions; and 3) its environmental toxicity profile is slightly better than other potential biocides. Since THPS is chemically incompatible with the oxygen scavenger, the oxygen scavenger was added upstream of the biocide to allow time for the scavenger to react with the dissolved oxygen prior to adding the biocide. Month 8: Pipeline A initial sample. Due to an extended shut-in period past the initially targeted 16 week hydrotest, a plan was created to design a sampling regime for the pipeline and determine a plan of action if the biocide residuals had dropped to levels that would no


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longer be able to control bacteria growth in the pipeline. Because pipeline A had no existing sampling points, plans were made to section the pipeline at the onshore facility. Four sets of samples were collected from this location at month 8: 1) water flowing from the cut in of the surface section of the pipe, 2) water pulled from the subsurface section of the pipeline using PVC tubing and suction by a pump after flushing for 10 minutes, 3) a second sample from the subsurface section of the pipeline after flushing the PVC tubing for an additional 10 minutes, and 4) a solids sample scraped from the interior of the cut section of pipe (Figure 2a-c). Enumeration of SRB and APB by serial dilution into bacteria culture media, dissolved H2S, and THPS biocide residual analyses were conducted on site. Additional samples were collected to perform water analyses, corrosion inhibitor residuals, and total bacterial counts by microscopy. During the repair of the pipeline cut, a Latrolet® (2) sampling valve was installed in the 30” Pipeline A for future sampling (Figure 2d). Month 9: Pipeline B installation. Pipeline B installed and received untreated seawater prior to shut-in. Month 11, 13 and 14: Pipeline A sampling. During the next sampling periods, month 11 and month 13, water samples were collected through the Latrolet® valve (Figure 2d). These samples showed increasing bacteria concentrations due to the low biocide residual and increasing ambient temperatures. A sample of the bay water used to fill Pipeline A was collected to determine the background bacteria concentrations in this water prior to chemical treatment. Month 15: Pipeline B discharge, sampling, refill and treat. Based on the 13-month monitoring results for Pipeline A, it became apparent that the untreated pipeline B was at risk. After approximately six months of shut-in (month 15 of study), Pipeline B was drained, pigged and refilled with chemically treated, filtered seawater (biocide, oxygen scavenger, and corrosion inhibitor). During the draining/refilling operation, samples of the displaced water were collected from the beginning, middle, and end of the operation and analyzed for the level of bacterial contamination. In addition, a suspended solids sample was collected from the envelope of fluid just before the first pig in the pigging train. Field reports indicated that the pipeline water had a rotten egg odor indicative of hydrogen sulfide. Month 15: Pipeline A decision to discharge, refill and retreat. Based on the increasing bacteria concentrations observed from months 8-14 for Pipeline A, as well as the high bacteria concentrations and presence of hydrogen sulfide in the Pipeline B discharge water, it was decided to drain and refill the pipeline with treated water. In addition, it was decided that a detailed monitoring program would be implemented if the second time period extended longer than six months. Therefore, based on a 55-day THPS half-life and a six month shut-in period, targeting 50 ppm biocide remaining at the end of the hydrotest period, it was calculated that the pipeline should be treated with 405 ppm of THPS-based product. In addition, 50 ppm corrosion inhibitor and 100 ppm oxygen scavenger were utilized for the pipeline refill. Month 15: Pipeline A discharge and refill. Three sets of samples were collected during the hydrotest water discharge: 1) 3 samples early in the discharge, 2) 3 samples in the middle

(2) Latrolet is a trademark of Cedar Investments, Inc.


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of the discharge, and 3) 2 samples at the end of the discharge period. During the discharge process, a caliper pig was utilized to push the contaminated water out of the line and to remove any biofilm and other solids that were attached to the pipeline during the extended hydrotest period. The residual THPS was neutralized with hydrogen peroxide just prior to discharge. Pipeline A was refilled with filtered seawater that had been treated with the biocide, corrosion inhibitor, and oxygen scavenger. Month 20: Pipeline A sampling. Approximately five months after the refill, samples were collected from the onshore sample point to determine the effect of the biocide treatment on the bacterial concentration in the seawater.

Table V Schedule for Pipeline System Hydrotests

Month Pipeline Segment Action Taken

0 (Initial) A Installed and Filled with Treated Seawater

8 A First Monitoring

11 A Second Monitoring

13 A Third Monitoring

14 A Fourth Monitoring

15 A Drain and Refill with Treated Seawater Water

20 A Monitoring after Refill

Month Pipeline Segment Action Taken

9 B Installed and Filled with Untreated Seawater 15 B Drain and Refill with Treated Seawater

Pipeline Monitoring Results

Pipeline A. As expected based on half-life calculations and the extended shut-in period, the THPS biocide residuals were already low at months 8 and 11, averaging 15 to 17 ppm (Figure 3). Despite the low biocide residuals, there continued to be very low levels of APB (10-100 APB/mL) and undetectable levels of SRB, indicating that the small amounts of biocide and the cooler winter temperatures had limited the growth of any surviving bacterial populations (Figure 3). Analysis of the samples showed that the seawater samples contained 95 to 101 ppm of total organic carbon (food source for the acid-producing bacteria); however, the sample contained very low concentrations of VFAs (<5 ppm), verifying a lack of bacterial activity. Dissolved H2S was not detected in any of the samples. The 13-month sample showed a significant increase in the APB concentration, increasing from 100 APB/mL at month 11 to 10,000 APB/mL at month 13 (Figure 3). This bacterial growth


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corresponded to the low residual of THPS (14 ppm) and the increasing ambient temperature as the season proceeded from winter into spring. The APB activity resulted in a significant increase in concentration of acetic, formic, and propionic acid (a total of 160 ppm VFA) during the five-month time period from month 8 to month 13 (Figure 3). VFAs are the preferred carbon source for SRB and can alter the pH of poorly buffered waters such as seawater and fresh water. The presence of these VFAs decreased the pH of the hydrotest water by one pH unit from 6.7 to 5.4. The presence of the VFAs also stimulated the growth of SRB with 10 SRB/mL detected at 13 months. Because the APB concentrations were rapidly increasing and producing VFAs and viable SRB were detected, the team made a decision that it was time to discharge the water in the pipeline and refill the pipeline with newly treated water. A seawater sample collected during month 13 sampling to give a representation of the bacterial concentration in the hydrotest water before chemical treatment showed very high concentrations of APB (1,000,000 APB/mL) and 100 SRB/mL and had a pH of 6.96. During discharge, three sets of samples (early, middle, and late in the discharge period) all showed further growth of the APB and SRB during the period from month 13 to month 15. The samples contained between 100,000 to 1,000,000 APB/mL and 100 to 1,000 SRB/mL. The VFA concentration dropped significantly presumably due to consumption by SRB. Hydrogen sulfide was not detected, indicating that any sulfide produced by the SRB had reacted with dissolved iron to precipitate as iron sulfide in the pipeline. These results verify the importance of maintaining a significant biocide residual in the seawater throughout the entire hydrotest period. A sample set was collected from Pipeline A five months after the month-15 refill with treated fluid (405 ppm active THPS, oxygen scavenger, and corrosion inhibitor). The samples showed 114 to 122 ppm active THPS corresponding to a 64-day half-life (Figure 1). The samples contained low concentrations of APB (5 APB/mL) and undetectable levels of SRB. The VFA concentrations were very low with only a small amount of acetic and formic acid detected. Using the calculated 64-day half-life of THPS in Pipeline A and the results from the first hydrotest treatment cycle, it was projected that the biocide should maintain control over the bacteria for an additional seven months (Figure 1).

Pipeline B. During the discharge of the Pipeline B water at month 15, each sample contained high levels of APB (100,000 to ≥ 1,000,000 APB/mL) and SRB (10,000 to 100,000 SRB/mL) (Figure 4). Suspended solids collected from the pigging fluids showed very high concentrations of SRB and APB (106 to 108 bacteria/mL) indicating that sessile bacterial biofilm had become established in the pipeline during the extended shut-in period with untreated fluid. These results verify the importance of a quality biocide program in maintaining control over the bacterial contamination present in seawater. Findings Revealed by Hydrotest Case History The monitoring program for Pipeline A was the key to the successful implementation of the hydrotest chemical program. When Pipeline A was required to be shut-in for at least 12 months longer than anticipated, the monitoring program clearly indicated when it was time to drain the pipeline’s hydrotest fluid, pig the pipeline to remove any established biofilm and other deposition, and refill the pipeline with freshly treated seawater. The results obtained from the


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Pipeline A monitoring program also provided the pipeline team critical information required to decide to drain and refill the untreated Pipeline B. The main limitation to establishing a quality monitoring program in hydrotest situations is the inability to access sampling points in the pipelines. In offshore scenarios, it is critical to install sampling points in each pipeline segment to ensure that information can be gained on each segment of a pipeline system. In the subject pipeline system, Pipeline A comes onshore; thus it could be used to gain access to the fluids for the first sampling period and, thereafter, a sampling point to could installed for monitoring the pipeline fluids for the remainder of the hydrotest period. In the first phase of the Pipeline A hydrotest, the acid-producing bacteria were the first bacterial group to respond to the decrease in biocide residuals. This occurred because the seawater contained significant concentrations of total organic carbon (food source for the APB), but very low concentrations of VFAs (preferred food source for the SRB). Therefore, as the ambient temperature increased, the APBs began to grow utilizing the dissolved organic carbon from the seawater as a food source and degrading it to VFAs, which the SRB could then consume. The VFAs stimulated SRB growth and lowered the pH of the poorly buffered seawater. In Pipeline A, SRB activity was not detected until the APB produced the SRB’s preferred food source, VFAs. No hydrogen sulfide was detected at any point in the Pipeline A hydrotest periods because the water was discharged before the SRB numbers reached concentrations sufficient to produce significant quantities of H2S. Conversely, in the untreated Pipeline B hydrotest fluids, high concentrations of SRB were present (>10,000 SRB/mL) and a rotten egg odor indicative of hydrogen sulfide was detected by operators during the discharge of the untreated Pipeline B hydrotest fluids. This study clearly demonstrates the importance of having a quality chemical and bacterial monitoring program during hydrotest applications. Because of the high concentrations of bacteria present in most of the water sources utilized for hydrotest applications, the stagnant nature of hydrotest applications, and the length of time over which many of the hydrotests occur, it is imperative to maintain adequate biocide residuals to ensure that the pipeline integrity is maintained and to avoid contamination of the produced fluids once the pipeline is brought into service.

CONCLUSIONS

Proper biocide treatment of the seawater used for hydrotesting is critical to prevent the growth of bacteria and the formation of biofilm that may seriously jeopardize the integrity of the pipeline.

When pipelines are left filled with seawater for a long time period, even when treated

with biocide, the conditions inside the pipeline need to be monitored to be able to determine if and when it is necessary to clean and re-treat the pipeline.

The availability of access points for sampling is critical and should be considered when

designing and laying the pipeline.


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During a monitoring program, it is important to remember that the concentration of sessile bacteria (the most damaging group of bacteria) may be orders of magnitude higher than the number of planktonic bacteria found in water samples.

One integral parameter to be considered when deciding the concentration of biocide to

be used is its degradation with time. It is a good practice to design the biocide treatment using half life data to provide for an active biocide residual (20 to 50 ppm) at the end of a hydrotest period to prevent re-growth of bacteria in a system. If a biocide residual is maintained, it is also important to plan for biocide neutralization during discharge if the concentration exceeds the NDPES permit specification.

Zero detection in the early portion of the treatment program does not imply a complete

kill, and surviving bacterial cells that may escape detection by monitoring can proliferate as the biocide concentration decreases over time.

REFERENCES

1. Little, B.J. and Lee, J.S. 2007. Microbiologically Influenced Corrosion. Wiley and Sons, New York, 279 pp.

2. Stein, A.A. 1993. MIC in the power industry. In: Kobrin, G. (ed) A Practical Manual on

Microbiologically Influenced Corrosion. Houston, TX: NACE International, pp. 9.1-9.30.

3. Stoecker, J.G. 1981. Penetration of stainless steel following hydrostatic test. Materials

Performance 20(8): 43-44.

4. Borenstein, S.W. 1994. Microbiologically Influenced Corrosion Handbook. Industrial Press, Inc., New York, 288 pp.

5. Videla, H.A. 1996. Manual of Biocorrosion. CRC Press, Inc., New York, 273 pp.

6. Ollivier, B. and Magot, M. 2005. Petroleum Microbiology. ASM Press, Washington,

D.C., 378 pp.

7. Baker Hughes. 2009. Technical data.

8. Dow Chemical. 2009. Technical data.

9. Rhodia. 2009. Technical data.


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FIGURES

FIGURE 1 - THPS Degradation (ppm) over Time (months) in Seawater

Months

0 5 10 15 20

ppm

0

50

100

150

200

5000

10000

15000

20000

THPS 8 month dosageTHPS 11 month dosage

Caculations were done for pH 7, 25° C


16

FIGURE 2. Pipeline A Sampling point at Onshore Facility a) Cutting and Sampling 30” Pipeline(3)after 8-month Shut-in b) Moving Sectioned Pipe to Insert PVC Tubing c) Pump Connected to PVC for Upstream Sampling Inside Pipe d) Latrolet® Fitting Inserted for Future Sampling during Shut-in

(3) One in = 2.54 cm


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FIGURE 3 - Treated Pipeline A: Bacteria and Biocide Residual Monitoring

100

1000

10000

100000

1000000

10000000

100000000

No

Bac

teria

per

mL

0

160

21.51

10

100

1000

10000

100000

1000000

Initi

al

8 m

onth

s

11 m

onth

s

13 m

onth

s

14 m

onth

s

15 m

onth

s

20 m

onth

s

Log

No.

Bac

teria

per

mL

0

50

100

150

200

250

300

350

400

450

THPS

Res

idua

l (pp

m)

APBSRBTHPSVFAs

InitialTreatment

Refill and Retreat

ProjectedEnd ofShut-in

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FIGURE 4 - Untreated Pipeline B: Bacterial Monitoring during Discharge following 6-month Shut-in


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