Souring and Microbial-influenced Corrosion in Produced Water Re-injection Systems
detected in biofilms of Tests 2 and 5, while polysulphides have not been determined in this study.
The corrosion products were composed of iron sulphides and iron oxides. Greigite (Fe3S4) was found in biofilms from Tests 4, 5 and 6, while pyrite (FeS2) was present in biofilms from Tests 4 and 5. Indications of the presence of elemental sulphur (S8) were found in Tests 2 and 5. The iron oxides goethite (FeOOH) and magnetite (Fe3O4) were found in biofilms from Tests 1, 4 and 5.
The strongest attack predominantly occurred across the long edges of the metal coupons where they touched the coupon holders during the experiment. These areas experienced reduced nutrient availability and reduced shear forces due to the low flow. This protected biofilm from detaching and could have provided niches in which corrosive bacteria persisted and proliferated. Deep pits were also present on the edges of two neighbouring coupons, probably as a result of galvanic corrosion between the metal coupons and the accumulating corrosion products present. Results obtained here can be extrapolated to the field where weak points in pipelines occur, such as welding joints and connections, as well as failures and impurities in the steel matrix (e.g. manganese sulphide inclusions).5
Such areas are most prone to biofouling and corrosion and particularly MIC.
Control of Microbial Activity Souring Control
The continuous addition of nitrate in Tests 1–4 kept the sulphide level in the bulk water low, and sulphide was removed from the water within three weeks after nitrate addition in Tests 5 and 6. Consistent with the
Black arrows indicate the direction of the surface scan. Scale bar = 100 µm.
Figure 5: Biofilms and Coupons before (Top) and after (Middle) Combined Addition of Corrosion Inhibitor and Nitrate (Test 5) and 3D Profilometry Results at the End of the Experiment (Bottom)
Before
After
End
We have never experienced so few computer problems during the fi rst year of operation. This is important since most of our vessels are operated far from base. The advantage was that both the DP and PMS were tested thoroughly early in the building process, which has not been possible before.
Jon Helge Ulstein Superintendent, Bourbon Offshore
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