Deep water and closed seas are characterised by anoxic conditions with the proliferation of anaerobic bacteria, i.e. Sulphate Reducing Bacteria (SRB), which generate a significant amounts of H2S in the sediments. Oil and gas pipeline are normally made in carbon steel (CS) that may be susceptible to stress corrosion cracking (SCC) when exposed to sour environment. In this case the presence of H2S on the seabed environment may cause the diffusion of hydrogen into the pipeline steel. The susceptibility to SSC generated by the external environment is related of the following conditions: The probability to have a 'sour environment' (pH value and H2S concentration); The probability that a material can be exposed to a sour environment coupled with the coating breakdown; The steel material susceptibility to this corrosion phenomena. Normally the application of external anticorrosion coating provides the main protection for the steel. Nevertheless steel exposure may occur in those locations where the coating has been damaged by natural degradation or by external interferences. A methodology is proposed to quantify the risk of external SSC during engineering phase. A case study exemplification is presented using a typical water and sediments environmental data of deep water and closed seas (i.e. Back Sea, Baltic Sea) recovered by bibliographic sources. In order the assess the most reliable design value of H2S concentration and pH for pipeline design, representative distribution functions for both pH and H2S concentration in seabed mud are found, using Montecarlo statistical method to characterize the environmental conditions. On this basis, the external environment seen by an hypothetical pipeline as been classified following the NACE/ISO15156 approach as 'susceptible to sulphide stress cracking'. The application of the proposed methodology to this case data leads to 'a ready to use' considerations for a pipeline project.
Il rischio di tensocorrosione da H2S all'esterno di condotte sottomarine: una metodologia di valutazione quantitativa
Umweltbedingtes Risiko der sulfidischen Spannungsrissbildung für Off-Shore-Rohre: eine quantitative Bewertungsmethode
Sulphide stress cracking risk for offshore pipelines coming from external environment: a quantitative assessment methodology
Metallurgia Italiana ; 102 , 3 ; 13-21
2010
9 Seiten, 8 Bilder, 6 Tabellen, 23 Quellen
Aufsatz (Zeitschrift)
Italienisch
Valutazione del rischio e revisione dello strumento militare
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