Short-Term Production Optimization in an Offshore Oil and Gas Processing Plant Using Nonlinear Model Predictive Control
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The main objective in this thesis is to study how nonlinear model predictive control (NMPC) can be used directly for short-term production optimization. Here the objective to maximize the production is directly included in the MPC optimization problem, as opposed to a two-layer RTO approach where a steady-state model is used to find optimal setpoints for the MPC. These setpoints define the maximal production, but this method needs two sets of models and is sub-optimal if a disturbance occurs. The study is based on a nonlinear model of an offshore oil and gas platform modeled in Dymola and Modelica by Cybernetica AS, and the objective is to increase total oil production on a daily basis by controlling well and pipeline chokes. The model has a set of wells ordered in clusters, and includes models for a three-phase flow of gas, oil and water. This makes the model stiff and complex with a large number of equations and states. Modifications on the model and methods for model reduction are studied in order to decrease simulation times, making it possible to run the NMPC application at least in real-time with a sampling time of one minute. The NMPC application used is CENIT, also developed by Cybernetica AS. The sequential optimization algorithm used in CENIT is studied and compared to simultaneous and multiple shooting methods. Two methods for production optimization are used. The first method studied is the unreachable setpoints method where an unreachable setpoint for oil production is used in order to maximize oil production. This method gives a quadratic cost, and the ideas from this method are combined with the exact penalty function for soft constraints, in order to get a linear cost in the objective function. These methods are applied in a series of case studies, where also traditional pressure control using NMPC is studied. The last case study shows that a maximal oil production under gas coning is achieved when the wells are producing with the same marginal gas/oil ratio.