2010

LABORATORY INVESTIGATION OF MIC IN HYDROTESTING USING SEAWATER

Kaili Zhao and Tingyue Gu (Speaker) Department of Chemical and Biomolecular Engineering

Ohio University Athens, Ohio 45701

Phone: 740-593-1499; Fax: 740-593-0873 gu@ohio.edu

Ivan Cruz

Saudi Aramco PO Box 6891, Dhahran, 31311

Saudi Arabia

Ardjan Kopliku BP America Inc., EPT - Mechanical Engineering Team

501 Westlake Park Boulevard, Westlake 1, Room 18130 Houston, Texas 77079

ABSTRACT

Microbiologically Induced Corrosion (MIC) is a potential threat associated with hydrotesting. It has been established that Sulfate Reducing Bacteria (SRB) can utilize hydrocarbons or even live on CO2 – H2 autotrophically. Pitting due to MIC during hydrotesting itself may not be a serious problem, because its duration is limited to several days or months. The biofilms left behind after the hydrotest may present a serious threat once the pipelines are commissioned and used for many years, because pipeline fluids may contain a sufficient amount of nutrients for biofilms to flourish. This laboratory investigation was conducted to study the MIC threat in hydrotests using seawater. Arabian and Gulf of Mexico (GoM) seawater samples were collected from offshore locations. Quantitative PCR (Polymerase Chain Reaction) analysis was used to detect SRB in seawater samples. It was found that offshore GoM “clean seawater” did not contain a sufficient amount of organic carbons to support the rapid growth of biofilms. Enriched seawater spiked with SRB was used to speed up biofilm growth. An MIC prediction software program based on the mechanistic Biocatalytic Cathodic Sulfate Reduction (BCSR) theory was able to predict longer term SRB pitting using short-term pitting data in laboratory experiments for MIC in hydrotest. Keywords: hydrotest, SRB, MIC, seawater, THPS, model

©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.

1

Paper No.

10406

INTRODUCTION Hydrotesting is a common practice to assess pipeline integrity before service. It is different from pneumatic testing, which is used only for leak testing, whereas hydrotesting is applied to test for both leaks and strength. During hydrotesting, a pipeline is filled with a liquid and pressurized to a pressure greater than the anticipated future operating pressure. In general, hydrotesting itself lasts only 8 to 10 hours. In the oil and gas industry, it is often the case that water is left in the system afterwards for many months before the system is actually commissioned. During this holding time, or when the pipeline is first exposed to an aqueous environment, e.g., wet lay-up, corrosion due to MIC can commence1. When the system makes contact with the ground2 or is even exposed to air3, there are further possibilities for microbial contamination. Reuse of water also increases chances for MIC. Improper hydrotesting practices can result in MIC causing pitting attack and also the so-called “black powder” problem4. MIC pitting during the hydrotest itself may not be a serious problem due to the limited hydrotest time frame. The biofilms left behind during the hydrotest may present a serious threat once the pipelines become operational, because fluids transported in pipelines may contain sufficient nutrients for bacteria to flourish and during the decades that a pipeline is expected to be operational.

Seawater is routinely used in the hydrotesting of sub-sea pipelines. Occasionally other water sources may be used and they mainly come from aquifer water and/or produced water. Any water source for hydrotesting can contain microorganisms. Natural seawater contains viruses, prokaryotes, protists (mainly flagellates) and algae5. Water used in hydrotesting is usually treated with biocides. Even treated water can be a source of SRB inoculum6,7. Two other methods to treat the hydrotest water are adjusting pH and using water sources without sulfate8. The pH adjustment (within a basic range) could increase the possibility of mineral scale formation on the pipe surface, and using a large amount of water without sulfate is usually costly and inconvenient when hydrotesting takes place offshore. Furthermore, the method of pipeline laying or water filling makes water treatment very difficult if at all possible. It has been known for some time that SRB are able to utilize hydrocarbons or even live on CO2 – H2 autotrophically7, i.e., they can live without an organic carbon source. Herbert9 reported that a variety of bacteria have the capability to reduce in size; decreasing energy consumption during starvation and reside in smaller pores. Some SRB can even form spores that are highly heat resistant7. These bacteria can then wait to thrive when the appropriate environmental conditions are met. This makes predicting and preventing the MIC during and after hydrotesting difficult. Steel corrosion in seawater sometimes has been misdiagnosed as attack exacerbated only by conventional crevice corrosion. Borenstein1 found that microorganisms contained in stagnant chloride bearing-media can result in steel failure much faster than in conventional chloride crevice corrosion alone. This increased corrosion rate may result from sulfate and other nutrients in the seawater, which cause souring and pipeline corrosion due to SRB activity. In the field, oxygen scavengers are sometimes added to the hydrotest water to prevent oxygen corrosion. This provides an anaerobic environment for anaerobic bacteria such as SRB. MIC occurs when several favorable factors are present simultaneously, such as suitable water chemistry, temperature, availability of nutrients (organic and inorganic), presence of microorganisms, and pressure. The majority of SRB can thrive at pH ranges from 5-9 and are unable to grow well at temperatures above 45oC. Thermophilic SRB prefers higher

2

temperatures7. Availability of a carbon source is usually considered to be the most important factor for SRB growth, and SRB growth will be severely restricted if utilizable carbon in organic nutrients such as formate, acetate and propionate, is below 20 ppm10. Pots et al.10 indicated that SRB growth would be the most prominent if the ratio of carbon to utilizable nitrogen is 10:1. Synergistic microorganisms can enrich the nutrients (such as organic carbons) in the local environment and thus promote SRB growth and accelerate the MIC process even though the initial environmental conditions are not suitable for SRB growth. Performing MIC tests in a laboratory setting simulating hydrotesting has always been a challenge. Pipeline fluids (especially those in subsea pipelines) can be at very high local pressures. In a laboratory, it is difficult and cost prohibitive to perform many tests in high-pressure reservoir simulators. It has been reported that barophilic SRB isolated from a high-pressure oil reservoir grew well at 1 atm and also at 300 bar in a laboratory7. Therefore, it may be acceptable that laboratory tests at 1 atm may be able to simulate SRB growth at much higher pressures.

EXPERIMENTAL METHODS 100 ml anaerobic vials (Figure 1) were used for the experiments. A glove box deoxygenated with N2 gas provided an anaerobic environment. X65 carbon steel coupons were used. These coupons had typical dimensions of 4.76×1.09×0.16cm (Figure 1). Prior to use, the coupon surfaces were polished successively with 200 and 400 grit SiC abrasive papers, rinsed with alcohol, and then sonicated in a beaker filled with alcohol. The ratio of coupon surface to liquid volume is close to that in 0.30m (12″) ID pipes. All liquids in the tests were deoxygenated using N2 sparging for at least 30 min before use, to reflect oxygen scavenger use in the field. Planktonic SRB bacterial counts were determined by manual counting under an optical microscope at 400X using a hemacytometer. Only motile SRB were counted. If needed, a Rodine HCl solution was applied to remove any films on the coupon surfaces. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometry (EDS) were employed to perform surface analyses. An oxygen test kit (www.chemetrics.com, product code: K-7540) was used to test the oxygen concentration in the experimental vials. Desulfovibrio desulfuricans subsp. aestuarii, ATCC (American Type Culture Collection) 14563 was used in this work as a laboratory strain of SRB. Some experimental results as indicated were obtained by enriching artificial seawater and natural seawater samples with 1g/L yeast extract, 3.5g/L sodium lactate and 200ppm Fe2+. For biofilm observations under the SEM, unless mentioned specifically, coupons were pretreated according to the following procedure: coupons were removed from vials and were immediately treated with 4% w/w glutaraldehyde for around 1 hour (to kill and immobilize the biofilm), and then were dehydrated with 30% (v/v), 50%, 75% and 100% alcohol in sequence. Before observing the biofilm, the coupons were first treated using a critical point dryer (BAL-TEC, CPD 030) and then coated with a gold film.

RESULTS AND DISCUSSION Table 1 shows that the Gulf of Mexico (GoM) seawater has a similar chemical composition to that of typical natural seawater. The total organic carbon (TOC) in the first GoM sample was less than 1 ppm compared to a TOC of 1 to 2 ppm for typical seawater, while the TOC of a

3

second GoM sample was 4.6 ppm. The GoM seawater samples were actually very clean in terms of total bacteria concentrations, and the SRB cell count was below the detection limit of 1 to 3 SRB cells per liter using Polymerase Chain Reaction (PCR) (Table 2). It was believed to be untreated and was taken from an offshore platform. When Hardy and Hamilton11 measured seven samples from two similar locations of the North Sea, he obtained SRB numbers from 0 to 90 cells/ml, the average was 22 SRB/ml. Lee et al.12, using the MPN method, detected around 101 SRB/ml and 102 SRB/ml in Arabian Gulf and Florida Key West seawaters, respectively. These two water samples came from 1.2 to 1.5 meters deep and near-shore (within 100 meters) locations that could be contaminated by wastes. Table 3 shows Na+, SO4

2- and total organic carbon (TOC) in a comparison between typical natural seawater and Qurayyah seawater in Saudi Arabia (SA). It is clear that Na+ and SO4

2- concentrations in Qurayyah seawater are almost 1.6 times higher than in typical seawater, and TOC concentration, which is very important for microbial growth, can be 500 times higher. Figure 2 shows how temperature affected planktonic SRB growth, where 37ºC is the optimum growth temperature for the laboratory strain SRB. Compared to the full nutrient medium (ATCC 1250 modified Baar’s medium), the enriched artificial seawater with limited nutrients is an acceptable environment for SRB growth, especially at 37ºC, and those added chemicals provided adequate nutrients for SRB growth. In general mesophilic SRB grow well at 37oC. Thermophilic SRB prefer an even higher temperature, but 37oC is likely sufficiently high for shallow seabed in a hot climate. This means increased SRB growth with increasing temperature is generally expected in practical situations. It should be pointed out that planktonic cell counts may be used to help indicate the likelihood of sessile cell health in laboratory tests, but the cell counts should not be used to correlate with sessile cell counts.

No microbial growth was detected after one month and six months in vials containing untreated GoM seawater. Figure 3 shows SEM images of a coupon surface with one-month exposure in a vial at 37oC. The entire coupon surface became rough after the coupon was exposed to the untreated GoM seawater after six months at 25oC (Figure 4). Due to lack of microbial activity and H2S smell at the end of the test, the roughness was likely caused by leaked oxygen and perhaps some other factor(s). Similar roughness was also observed in tests using heat sterilized GoM seawater. The Qurayyah seawater from the Arabian Gulf is much saltier than the GoM seawater as seen in Table 3. In-house quantitative PCR testing showed no detectable SRB count. Figure 5 shows that a mineral layer covered the coupon surface after a three-month exposure at 37oC. The EDS analysis of the surface indicates the absence of elemental sulfur, which means that SRB activity was likely absent. Figure 6 shows scattered pits after the coupon surface were cleaned. They were likely due to oxygen and chloride attack, because no microbial activity was detected. Due to the lack of native viable microbes and the lack of nutrients, no MIC pitting was observed in untreated seawater samples in the time frame for lab testing. To simulate a contaminated hydrotest fluid and to speed up laboratory testing, worst-case scenario tests were carried out by enriching seawater samples and spiking them with the laboratory SRB strain. Figure 7 shows the SEM image of the biofilm on a 1-week old coupon. Numerous kidney bean-shaped SRB cells are clearly visible. An EDS analysis indicates the presence of iron sulfide. Pits characteristic of MIC attack were revealed after acid cleaning of the coupon surface as seen in Figure 8.

4

Until recently, there has been a lack of mathematical mechanistic models for MIC pitting prediction due to the complicated and controversial MIC mechanisms. Gu et al.13 introduced an electrochemical kinetics-based mechanistic model using a new biocatalytic cathodic sulfate reduction (BCSR) theory. It assumes that a corrosive SRB biofilm is present on an iron surface causing the following reactions to go forward due to biofilm biocatalysis by the sessile SRB cells that are on or very close to the iron surface. Anodic: 4Fe 4Fe2+ + 8e- (Iron dissolution) (1) Cathodic: SO42- + 8H+ + 8e- HS- + OH- +3H2O (BCSR) (2) By using charge transfer and mass transfer theories and electrochemical kinetics, a mechanistic model has been established and solved numerically13. MICORP MIC prediction software has been created. It incorporates BCSR, proton reduction and organic acid reduction to account for the low pH in the bottom of a pit due to the presence of organic acids. Figure 9 shows the model prediction and experimental data. The model used calibration of biofilm aggressiveness from a single pit depth data to predict longer term pitting. Longer term testing will be required to validate this prediction. Tetrakis Hydroxymethyl Phosphonium Sulfate (THPS) is a biodegradable biocide that is most often proposed for hydrotest fluid treatment. A minimum dosage is needed to prevent biofilm establishment. Tests were carried out in anaerobic vials to evaluate the THPS degradation profiles in artificial seawater, GoM seawater and Qurayyah seawater. A mechanistic model of THPS degradation under alkaline condition was obtained as follows14,

( ) t)pH75.1exp(15.273T

10161exp101.3tpH,TkCCln 7

0

ו×⎥⎦⎤

⎢⎣⎡

+−

××−=•−=⎟⎟⎠

⎞⎜⎜⎝

⎛ (3)

In Eq. (3), pH stands for the stabilized pH of the seawater after THPS introduction and C is the THPS concentration in ppm. T is the temperature in oC and t is the time in days.

CONCLUSIONS This work provided a framework for laboratory testing of MIC in hydrotest conditions. Arguments were made for laboratory testing at atmospheric instead of the high pressure expected in a subsea pipeline during hydrotesting. Offshore seawater samples from the Gulf of Mexico and the Arabian Gulf were found to lack native viable microbial activities. The seawater samples were enriched and spiked with a laboratory strain of SRB to simulate contaminated seawater used in hydrotesting. Biofilms and MIC pits were observed in the accelerated tests. A software package based on the BCSR theory was able to predict the pit growth.

ACKNOWLEDGEMENTS

The authors would like to thank Saudi Aramco and BP America for their financial support of this research and for their permission to present the results.

5

TABLE 1 MAJOR ELEMENT COMPARISON BETWEEN TYPICAL NATURAL SEAWATER

AND UNTREATED GOM SEAWATER

Ca2+

(ppm) Na+

(ppm) Cl- (ppm)

F-

(ppm) SO4

2-

(ppm) K+

(ppm) TOC (ppm)

Typical natural seawater

400 to 412

10,500 to 10,770

18,800 to 19,300

1.2 to 1.3

2,655 to 2,715

380 to 390

<1 to 2

GoM seawater

421

10,800

19,700

1.41

2,655

398

Not detected <1

TABLE 2 QUANTITATIVE PCR ANALYSIS OF GOM SEAWATER

GoM seawater Total bacteria concentration 13.3 cells/ml

SRB None detected

TABLE 3 MAJOR ELEMENT COMPARISON BETWEEN TYPICAL NATURAL SEAWATER

AND QURAYYAH SEAWATER

Na+ (ppm) SO42- (ppm) TOC (ppm)

Typical natural seawater 10,500~10,770 2,655~2,715 <1 to 2 Qurayyah seawater

16,580

4,330

498

*The Na+ and SO4

2- assays of Qurayyah seawater were done by ENC Labs (Albuquerque, NM), and TOC was assayed by San Antonio Testing Laboratory, Inc. (San Antonio, TX)

6

FIGURE 1 - A 100ml Anaerobic Vial With A Chewing Gum-Shaped Carbon Steel Coupon FIGURE 2 - Planktonic SRB Growth In Enriched Artificial Seawater And Full Nutrient Medium At Different Temperatures

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

0 3 6 9 12 15 18Time/days

Cel

l con

cent

ratio

n/(c

ells

/ml)

In enriched artificial seawater at 10°C In enriched artificial seawater at 25°CIn enriched artificial seawater at 37°CIn full nutrient medium at 37°C

7

(a) SEM at 28X (b) SEM at 348X FIGURE 3 - One-Month Exposure To The Untreated Second Shipment Of GoM Seawater At 37ºC (SEM Analysis Of Coupon Surface After Acid Cleaning)

(a) SEM at 29X (b) SEM at 353X FIGURE 4 - Six-Month Exposure To The Untreated GoM Seawater At 25ºC (SEM Analysis Of Coupon Surface After Acid Cleaning)

8

(a) SEM at 40X (b) SEM at 2000X (c) Composition on the coupon surface analyzed by EDS FIGURE 5 - Three-Month Exposure To The Untreated Qurayyah Seawater At 37 ºC (SEM And EDS Analyses Of Coupon Surface Before Acid Cleaning)

500 μm 10 μm

9

(a) SEM at 40X (b) SEM at 750X FIGURE 6 - Three-Month Exposure To The Untreated Qurayyah Seawater At 37 ºC (SEM Analysis Of Coupon Surface After Acid Cleaning)

500 μm 20 μm

10

(b) Composition on the coupon surface analyzed by EDS FIGURE 7 - One-Week Exposure To The Enriched Qurayyah Seawater Spiked With SRB At 37ºC (SEM And EDS Analyses Of Coupon Surface Before Acid Cleaning)

5 μm

SRB

11

(a) SEM at 40X (b) SEM at 750X FIGURE 8 - One-Week Exposure To The Enriched Qurayyah Seawater Spiked With SRB At 37 ºC (SEM Analysis Of Coupon Surface After Acid Cleaning)

FIGURE 9 - BCSR Model Prediction Of 2-Week And 1-Month Pit Depths Using Biofilm Aggressiveness Calibrated From The 1-Week Pit Depth (18μm)

500 μm 20 μm

12

REFERENCES

1. W.S. Borenstein, P.B. Lindsay, “MIC failure of 304L stainless steel piping left stagnant after hydrotesting,” Materials Performance, 41, 70-73, 2002.

2. H.A. Videla, Manual of Biocorrosion, Lewis Publishers, Boca Raton, Florida, 1996.

3. A., Parra, J.J. Capio, I. Martinez, “Microbial corrosion of metals exposed to air in tropical

marine environment,” Materials Performance, 35 (10), 1996.

4. R. M. Baldwin, “Black powder in the gas industry – sources, characteristics and treatment,” Mechanical and Fluids Engineering Division, Southwest Research Institute, May 1998.

5. M. G. Weinbauer, D.F. Wenderoth, “Microbial Diversity and Ecosystem Functions – the

Unmined Riches,” Electronic Journal of Biotechnology, April 15, 2002.

6. P.F. Sanders, “Overview of souring, corrosion and plugging due to reservoir organisms,” UK Corrosion 98, Paper No. 15, Sheffield, UK, Oct. 20-21, 1998.

7. J. T. Rosnes, T. Torsvik, T. Lien, “Spore-Forming Thermophilic Sulfate-Reducing

Bacteria Isolated from North Sea Oil Field Waters,” Applied and Environmental Microbiology, 57, 2302-2307, 1991.

8. R. Prasad, “Chemical treatment for hydrostatic test,” US Patent No. 6815208 B2, 2004.

9. B.N. Herbert, “Biocide in oil field operations,” in Handbook of Biocide and Preservative

Use, H.W. Rossmoore (editor), p. 198, Blackie Academic & Professional, Chapman & Hall, UK, 1995.

10. B.F.M. Pots, R.C. John, J.J. Rippon, M.J.J. Simon, S.D.K. Kapusta, M.M. Girgis, T.

Whitham, “Improvements on DeWaard-Milliams corrosion prediction and applications to corrosion management,” CORROSION/2002, Paper No. 02235. (Houston, TX: NACE, 2002).

11. J.A. Hardy, W.A. Hamilton, “The oxygen tolerance of sulfate-reducing bacteria isolated

from North Sea waters,” Current Microbiology, 6, 259-262, 1981.

12. J.S. Lee, R.I. Ray, “Comparison of Key West and Persian Gulf seawaters,” CORROSION/2007, Paper No. 518. (Houston, TX: NACE, 2007).

13. T. Gu, K. Zhao, S. Nesic, “A Practical Mechanistic Model for MIC Based on a

Biocatalytic Cathodic Sulfate Reduction Theory,” CORROSION/2009, Paper No. 09390. (Houston, TX: NACE, 2009).

14. K. Zhao, J. Wen, T. Gu, A. Kopliku, I. Cruz, "Mechanistic Modeling of Anaerobic THPS

Degradation Under Alkaline Condition in the Presence of Mild Steel," Materials Performances, 62-66, August 2009.

13