Microbes live everywhere including the oceans. Sulfate-reducing bacteria (SRB) are considered microbes. SRB is one of many common bacteria species that live throughout our ecosystem. Seawater is a primary source of SRB where a single gallon of seawater has roughly a few billion different kinds of bacteria. While most bacteria are aerobic and require oxygen to survive, SRB is among a group of species that are anaerobic and thrive only in the absence of oxygen. While SRB exist in the wild and does not present a danger to humans or animals, its effects can cause damage to metals and concrete. Offshore structures either resting on the seabed or penetrating through, whether made of concrete or steel are exposed to those attacks by SRB. Offshore structures, such as oil and gas platforms, monopiles foundations and concrete gravity base structures for wind turbines, subsea pipelines, and subsea manifolds are potential targets for those bacteria attacks.
This attack can be described as a phenomenon called Microbiologically Induced (or Influenced) Corrosion (MIC). MIC is the degradation of structures because of the activity of various microorganisms. So, the attack comes in a form of corrosion and can be called, microbial corrosion, bacterial corrosion, biological corrosion, or concrete/metal eating bacteria.
SRB perform anaerobic respiration utilizing sulfate (SO42) as a terminal electron acceptor, reducing it to hydrogen sulfide (H2S), which is a highly corrosive compound to concrete, steel, and other elements such as copper. Another set of microbes turns hydrogen sulfide to sulfuric acid, which can cause pitting of steel or concrete and hence reducing the service life of the structures. In other words, these sulfidogenic microorganisms “breathe” sulfate rather than oxygen and the results are highly corrosive substance.
Cathodic protection (CP) is one of the corrosion control methods mostly used with satisfactory results; however, where the phenomenon is microbiologically influenced corrosion (MIC) related, the efficiency of such control methods decreases significantly, due to several factors, (e.g. MIC may increase the kinetics of the corrosion reactions, which in turn increases the CP current necessary to achieve a given level of polarization.) The microorganism can attack pipeline coating for example, increasing exposed metal surface area and further increasing the CP current required to achieve the desired polarization. Hence, the use of coatings with high resistance in the presence of bacteria is a must.
Corrosion protection of offshore structures according to several organizations such as DNV (a European based advisor for the maritime industry) must include a holistic approach including corrosion allowance, cathodic protection, corrosion protective coatings and use of corrosion resistant materials. Measurement of corrosion rate with techniques such as linear polarization resistance (LPR) may also be very useful especially both for new and old structures.
Although SRB is a very tiny organism we cannot see, it can lead to potentially catastrophic consequences and costly operational shutdowns and should be taken seriously in the design of any offshore structures.
Assem Elsayed is the Vice President and Practice Area Leader of GeoStructural Engineering at Geocomp. Assem has extensive experience with waterfront and marine structures, design of monopiles for wind farms, and support of deep excavation.