RIASSUNTO
Development of oil and gas facilities in subsea Black Sea presents unique corrosion and materials challenges due to the unusual environmental conditions of deep Black Sea waters. At depths starting around 200 m, the Black Sea is anoxic, slightly sour (containing 10-15 ppm dissolved H2S), and corrosive due to the presence of anaerobic microorganisms. While the industry has experience in handling sour fluids containing corrosive bacteria on the inside of structures such as pipelines, there is very limited experience on how such fluids influence external components (e.g. wellhead and christmas trees, coatings, sacrificial anodes), which are not typically exposed to these conditions. In order to facilitate safe and reliable development in the Black Sea, a comprehensive laboratory and field study was conducted to establish proper corrosion control and materials selection strategies for externally exposed subsea structures and equipment.
This paper will discuss the approaches used and the performance data of the different materials evaluated under Black Sea conditions. First, a field sampling campaign was conducted to accurately characterize water chemistries and microbial populations at various locations and depths in the Black Sea. Next, new laboratory tools and capabilities were developed to study the effects of anoxic, sour conditions and anaerobic corrosive microorganisms on cathodic protection requirements, hydrogen embrittlement of different metals, and coatings integrity. Finally, field testing of the different materials was carried out using a uniquely designed, modular test skid deployed into the Black Sea over a six- month period at a water depth of 1000 m.
Key findings resulting from this work include:
Field campaign showed that deep Black Sea waters contained up to 8ppm dissolved H2S as expected, but the sediments at the same location were significantly more sulfidic, containing up to three times higher H2S.
The study showed that the Black Sea waters and sediments contained relatively small fractions of corrosive sulfate reducing bacteria (SRB), but the biofilms which formed on steel samples were completely dominated by members of SRB.
Cathodic protection using sacrificial anodes was found to be a viable strategy for controlling sour and microbial corrosion, but the anode capacity was reduced. The degree of reduction varied based on anode type.
Some high strength low alloy steels (e.g. SAE 4340 martensitic steel) and corrosion resistant alloys (e.g. S17400 precipitation hardened martensitic stainless steel) were found to be susceptible to hydrogen embrittlement
Data from field testing confirmed laboratory results, illustrating the success of the novel experimental techniques utilized in this study.