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A CFD Analysis of Decommissioned Oil Platforms Jackets on the Brazilian Coast

Every facility reaches the last phase in its life cycle, which is decommissioning. Since the last decade, this subject has been gaining importance in Brazil’s offshore oil and gas companies. For jacket type rigs, one of the methods widely applied after idling the production is the conversion of these structures into artificial reefs (ARs). There are several critical aspects for choosing the best strategy for cutting and sinking a platform jacket, ensuring the success of an AR from a biological point of view. One of them is the influence of marine currents and their fluid-structure interaction which, by maximizing local upwelling and back vortex effects, favours the growth of aggregated flora and fauna. This study consists in the application of computational fluid dynamics (CFD) techniques for studying the marine flow around a disassembled and sunk jacket in the seabed for the purpose of converting it into an artificial reef. An FVM (Finite Volume Method) from a commercial software (most recent version of ANSYS FLUENT®) is applied with the upwind scheme. A k-ε turbulence model on steady state is chosen. Field data about Brazilian coastal currents are collected and analysed from the amount of information available on a Brazilian Navy's meteoceanographic program. Next, different combinations of cutting and sinking a jacket are studied, always keeping a minimum 55m free water column. The objective is to verify where the formation of local upwelling regions - that is, where the vertical velocity component reaches values equal or greater than 10% of the magnitude of the free flow velocity - is more significant, without decreasing back eddy formation. It is observed that the dismemberment of the jacket with the positioning of its parts in an increasing height sequence in the direction of the prevailing current is favourable to generate local upwelling while tipping the structure at 90° to the prevailing current results in the most voluminous back eddy region.