Carbon steel is the main construction material in HYDROFLUORIC ACID (HF) alkylation units. Carbon Steel has good corrosion resistance to anhydrous HF (AHF) below 160 degrees fahrenheit (71 C). The corrosion resistance is due to the formation of an inorganic iron fluoride scale on the carbon steel surface that protects the steel from futher corrosion. The presence of an adherent and continuous scale is essential in keeping the corrosion rate at a minimum.
Residual elements (RE) in carbon steel, not specifically included in the specified steel, appear to influence the corrosion rate under certain conditions, especially in services involving hydrofluoric acid (HF). The relative proportions of RE, specifically %C, %Ni, %Cu, and %Cr in carbon steel base and weld metals used in refineries, especially in alkylation processes with HF as the catalyst, significantly impact corrosion behavior. Studies described in the literature show corrosion damage with high RE (Cu + Ni + Cr >0.20) components as compared to low RE (Cu + Ni + Cr <0.20) components.
In this study, electrochemical corrosion testing was performed on a 3-inch pipe elbow section with high REs that had developed a through-wall leak in service. Test results were compared to those obtained on a similar pipe elbow section with lower REs. The samples were exposed to 50% HF at room temperature and at 65°C. Linear polarization resistance (LPR) corrosion rates were measured at both temperatures. Potentiodynamic (PD) polarization scans were performed on samples of low and high RE steel exposed to 50% HF at room temperature.
Test results indicated that LPR corrosion rates were higher for the high RE carbon steel samples than for low RE carbon steel samples at both temperatures. PD scans showed that the critical current densities were higher for high RE steel than for low RE steel.
Biocides are used to control problematic microorganisms in the oil and gas industry. High doses of biocides cause environmental and operational problems. Therefore, using biocide enhancers to make biocides more effective is highly desirable. 2,2-dibromo-3-nitrilopropionamide (DBNPA) is a popular biocide because it is broad-spectrum, effective, kills microorganisms immediately upon addition, and it degrades rapidly. D-amino acids are natural chemicals that have been used in lab tests to enhance biocides to treat biofilms. In this work, D-tyrosine was used to enhance DBNPA against Desulfovibrio vulgaris biofilm on C1018 carbon steel. After 7 days of incubation, the weight loss of coupons without treatment chemicals in culture medium was found to be 3.1 ± 0.1 mg/cm2. With a treatment of 150 ppm (w/w) DBNPA, the weight loss was reduced to 1.9 ± 0.1 mg/cm2 accompanied by a 1-log reduction in the sessile cell count. The combination of 150 ppm DBNPA + 1 ppm D-tyrosine achieved an extra 3-log reduction in sessile cell counts and an additional 30% reduction in weight loss compared with 150 ppm treatment of only DBNPA. The combination also led to a smaller maximum pit depth. Linear polarization resistance (LPR), potentiodynamic polarization and electrochemical impedance spectrometry (EIS) tests corroborated the enhancement effects.
Oil & Gas flowlines are paramount for safe and reliable production of hydrocarbons, ensuring their integrity is one of the key tasks for all operators. Petroleum Development of Oman (PDO) manages around 11,000 Km of flowlines and connects to production stations more than 400 wells across more than 50 fields every year. Applications for these flowlines vary from the traditional Oil & Gas transmission to other applications such as water injection, polymer flood and steam injection. All these diverse applications involve multiple operation environments ranging from benign fields with low CO2 < 0.5 mole% and minimum H2S to very aggressive environments with CO2 and H2S concentrations well above 10 mole%, combined with a wide range of salinities and water cuts, with chlorides concentrations ranging from 5,000 to 200,000 ppm and water cuts from almost nil to more than 95%. The challenges encountered in safe and economic development of these assets across such a wide range of conditions are numerous; therefore, the material selection and designed methods to manage flowlines integrity are complex with no single solution approach to address the myriad of different conditions. This paper present two cases of metallic and non-metallic materials installed in PDO flowlines showing their historical field performance and discusses the strategy adopted for the material selection of flowlines.