Utilização de simulação dinâmica na avaliação de processos de blowdown em FPSOs dedicados a testes de longa duração: um estudo para o pré-sal brasileiro

Abstract

Oil processing facilities present serious risks concerning fire, explosions, and rupture of vessels. In offshore operations, rapid depressurization/blowdown is necessary when emergencies arise in which there is a fire or explosive gas escapes. High CO2 content blowdown scenarios can bring many problems, such as solid formation, pipeline and equipment fractures, clouds of dispersed hydrocarbon vapor, increase in flare radiation, among other consequences explored in the literature. The present work has the main objective of performing an analysis of the CO2 composition variation influence in depressurization/blowdown processes in FPSOs dedicated to Long Term Tests in Pre-Salt reservoirs, which can have up to 70% CO2 content. Five topside systems were chosen to be analyzed with real data from a FPSO dedicated to LTTs in a Pre-Salt reservoir. Four scenarios of different CO2 composition were selected to perform the study (0%, 10%, 30%, and 70%). Simulations were carried out to analyze the effect of the CO2 content in the depressurization time, in the maximum gas rate to flow through the restriction orifices (ROs), and in the minimum fluid and wall temperatures that the system material needs to support. With the results obtained, it was observed that, in general, the higher the CO2 content, the longer the depressurization will take, the higher the maximum flow rate in the ROs, and the lower the minimum fluid and wall temperatures. It was also possible to perceive the influence of the Joule-Thomson Effect in some depressurization scenarios. It was concluded that the higher the CO2 content, the larger the restriction orifice diameter that governs the system must be to guarantee the process's safety. Therefore, for FPSOs dedicated to LTTs in the Pre-Salt, a restriction orifice design that supports more limited ranges of CO2 composition is recommended (as the ones delimited in this work, from 0-10%, 10-30%, 30-70%). This design philosophy is essential not to exceed design boundary conditions in emergency depressurization systems, such as flare overload, compliance with API 521, and possible equipment and piping ruptures.