Microorganisms, including those that influence corrosion, are present almost everywhere in soil, freshwater, well-water, and seawater. The mere detection of microrganisms in an environment does not necessarily indicates a corrosion problem. What is important however, is the unique combination of microorganisms present, along with favourable conditions enhancing their growth in the sessile state, since microbiologically influenced corrosion (MIC) occurs directly on metal surfaces. This paper focuses on microbial growth requirements, and provides guidelines for evaluating the potential occurrence of MIC, based on operating pipeline parameters, and water chemistry influencing microbial growth.
Transmission pipelines play an extremely important role throughout the world as a means of transporting hydrocarbon products from their production sources to the ultimate consumers. However, like all other engineered structures, pipelines are also susceptible to failure due to various reasons. When a new pipeline is planned to be installed, its integrity is assured by providing sound and reliable engineering design, and by adopting a suitable corrosion protection system. Pipelines are susceptible to both internal as well as external corrosion, and engineering designs must take these threats into account. Internal corrosion in pipelines is influenced mainly by temperature, pH, carbon dioxide (CO2) and hydrogen sulfide (H2S) content, water chemistry, flow velocity, microbial contamination, oil or water wetting, and composition and surface conditions of the metal. Corrosion in liquid petroleum transmission lines as well as in the bottom of gas pipelines where liquid accumulates1, is often attributed to MIC factors2.
Microbiologically influenced corrosion occurs when a unique combination of biological factors is present simultaneously with other conditions, such as specific regimes of water chemistry, temperature, flow velocity, metallurgy, and organic and inorganic fouling materials. The biological factors involve the growth of microorganisms that may induce or initiate the corrosion mechanism. The involved microorganisms may be aerobic, heterotrophic bacteria; facultative anaerobic bacteria; sulfate-reducing bacteria (SRB), acid and exopolymer-producing bacteria.
Effective prevention and control of MIC involves not only an underlying knowledge of the MIC-related microorganisms but also, an understanding of the phenomena occurring over a broad range of length and time scales. This poses a tremendous challenge to both scientists and engineers dealing with this issue. Models that can describe or predict the threat or potential for MIC, thus present an attractive prospective.
The aim of this paper is to present an approach to predict threats of MIC inside oil and gas pipelines based on the operating conditions that have a major impact on microbial growth. It briefly describes the types of microorganisms, and reviews the existing models for predicting MIC.
Free-floating bacteria are commonly referred to as "planktonic" microorganisms, and depending on the type of industrial system, they may also include unattached algae, diatoms, fungi and other microorganisms that may be present in bulk fluids. In most cases, planktonic bacteria are the centre of monitoring for MIC since in general, sampling of system fluids is easier than sampling metallic surfaces.