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M AT E R I A L S S E L E C T I O N & D E S I G N TAbLE 1 Major element comparison between typical natural seawater and untreated GoM seawater Ca2+ (ppm) Na+ (ppm) Cl– (ppm) F– (ppm) SO42– (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 Major element comparison between typical natural seawater and Qurrayah seawater(A) Na+ (ppm) SO42– (ppm) TOC (ppm) Typical natural seawater 10,500 to 10,770 2,655 to 2,715 <1 to 2 Qurrayah seawater 16,580 4,330 498 (A) + 2– 4 The assay for Na and SO in Qurrayah seawater was done by ENC Labs (Albuquerque, NM), and TOC was assayed by San Antonio Testing Laboratory, Inc. (San Antonio, Texas). croorganisms. Natural seawater contains viruses, prokaryotes, protists (mainly fagellates), and algae.5 Water used in hydrotesting is sometimes treated with biocides. Even treated water can be a source of sulfate-reducing bacteria (SRB) inoculum.6 Two other methods to treat the hydrotest water are adjusting pH and using water sources without sulfate.7 The pH adjustment (within a basic range), however, 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 diffcult, if possible at all. Sulfate-Reducing Bacteria Metabolism It has been known that some SRB are able to utilize hydrocarbons or even live on carbon dioxide-hydrogen (CO2-H2) autotrophically,8 which means they can live without organic carbon intakes. Rossmoore9 found that a variety of bacteria have the capability to reduce in size, decreasing energy consumption during starvation and residing in smaller pores. These bacteria can then wait to thrive when the appropriate environmental NACE International, Vol. 52, No. 5 conditions are met. This unique feature of bacteria makes predicting and preventing the MIC in hydrotesting diffcult. Steel corrosion in seawater sometimes has been misdiagnosed as attack induced only by conventional chloride corrosion. Borenstein 1 found that microorganisms contained in a stagnant chloride-bearing medium can cause steel failure much faster than in conventional chloride crevice corrosion alone. This increased corrosion rate may come from sulfate and other nutrients in the seawater, which cause souring and pipeline corrosion due to SRB activities. Use of Oxygen Scavengers In the feld, oxygen scavengers are usually added to the hydrotesting water to prevent oxygen-caused 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, nutrients (organic and inorganic), microorganisms, and pressure. The majority of SRB can thrive at pH ranges from 5 to 9, and except for thermophiles, are unable to thrive at temperatures >45 °C. Availability of a carbon source is usually considered to be the most important factor for SRB growth; SRB growth will be se- verely restricted if utilizable carbon in organic nutrients in the form of volatile fatty acids such as formate, acetate, and propionate, is <20 ppm.10 Pots, et al.10 also indicated that SRB growth would be the most prominent if the ratio of carbon to utilizable nitrogen was 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. Fermentative acid-producing bacteria (APB) should be considered in MIC forensics, especially in zero-sulfate and low-sulfate environments. Laboratory Testing Performing MIC tests in a laboratory setting for hydrotest has always been a challenge. Pipeline fluids (especially those in subsea pipelines) can be at very high local pressures. Barophilic SRB are adapted to this kind of pressure.8 In a laboratory, however, it is diffcult and cost prohibitive to perform many tests in high-pressure reservoir simulators. It is possible that laboratory tests at one atmosphere may be able to simulate SRB growth at a high pressure because it has been reported that barophilic SRB isolated from a high-pressure oil reservoir May 2013 MATERIALS PERFORMANCE 65