(570m) Dynamic Analysis of Offshore Natural Gas Conditioning: TEG Dehydration, Joule-Thomson Expansion and Membrane Separation

The natural gas that comes through the production risers may exhibit contaminants that can damage pipes and equipment. Therefore, the gas composition must be adjusted in order to guarantee a normal and secure flow. Depending on the contaminants levels and the gas exploration site, different processes should be used in primary conditioning on offshore platforms to meet the regulated requirements of transport and / or marketing of gas.

The offshore operation of gas treatment processes is naturally related to dynamic problems that may compromise the production and process safety. Such transient instabilities are due to high-frequency oscillations in the load flow, such as slug flow, besides other disturbances (non-scheduled stops and plant restarting, for instance). Therefore, process control of a natural gas conditioning plant must be sufficiently robust to ensure operational stability of the process.

The discoveries of new oil and natural gas reserves in the Brazilian pre-salt layer represents a new horizon for the national industry. The country is taken to a higher level in the international oil and gas market, creating expectations of doubling the production capacity by 2020. It also brings new operational challenges such as the high amount of CO₂ found in the preliminary analysis. Membrane separation technology is considered the most recommended process to treat gas with such composition in offshore platforms.  

This work initially proposes a natural gas conditioning process flowsheet, building a base case in steady state for a gas with composition similar to the pre-salt reserves. After the analysis of the optimum operating parameters to achieve the gas specifications, the process is transferred to the dynamic state, where the control loops are proposed and tuned. In addition, a membrane unit operation developed by H2CIN/EQ-UFRJ for use in Aspen HYSYS simulation software is adapted to a dynamic extension using VB programming language, so that it can be used both in steady and transient state simulations. The dynamic behavior of the membrane extension is considered close to the process reality.

Three different dynamic scenarios of feed flow oscillations - pulse, sine wave and slug flow - are analyzed for the proposed flowsheet and the overall robustness of the process is tested. The results point to good controllability of production and specifications of the natural gas.