During field inspection of Christmas (X-mas) trees in a giant oil offshore field in Abu Dhabi, a small subset of wells were reported to be severely corroded on X-mas tree studded outlets imposing high HSE risks due to possible loss of containment. A holistic analysis was conducted to identify corroded X-mas trees, establish the root-cause of corrosion and recommend a remedial action plan to control future corrosion damage and reduce HSE risk exposure.
Advanced modeling tools and lab tests were used to analyze the flow behavior and field samples, respectively. Advanced modeling was performed to analyze inorganic scale potential, identify flow regimes and calculate corrosion rates in the X-mas trees to correlate with operating conditions. Solid samples from X-mas trees were analyzed using advanced microscopy techniques to identify the elemental composition and phases. Water samples were also analyzed to check bacteria content. Available data on historical operating conditions, modeling and lab analysis results were segregated into in-favor and against factors for each of the mechanisms to identify the potential root-cause of corrosion. Modeling results were used in conjunction with actual field data such as corrosion feature morphology, historical operating conditions, etc. to evaluate corrosion damage. Based on corrosion feature morphology, wells were categorized into different groups to compare the flow behavior and operating conditions with observed corrosion patterns.
A thorough analysis of corrosion feature morphology and operating conditions identified flow-induced localized corrosion (FILC) as the root-cause of corrosion in severely corroded X-mas trees. X-mas tree design, fluid properties and operating parameters such as well head pressure (WHP), wellhead temperature (WHT) and flow rate were found to be key contributing factors of accelerated corrosion. Results of computational fluid dynamics (CFD) modeling showed that the horizontal section of X-mas tree is exposed to higher turbulence, water wetting and transient gas bubble formation/collapse phenomena than the vertical section due to changes in flow direction and gravity effects. Several mitigation strategies were implemented to control corrosion in the X-mas tree flange area, and reduce likelihood of leakage through the X-mas tree flange. Findings from this work led to development of an Excel based tool which can be used to assess and predict the corrosion risks to X-tree based on operating conditions.