Vidar Alstad

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This paper deals with the optimal selection of linear measurement combinations as controlled variables, c 1⁄4 Hy. The objective is to achieve ‘‘self-optimizing control”, which is when fixing the controlled variables c indirectly gives near-optimal steady-state operation with a small loss. The nullspace method of Alstad and Skogestad [V. Alstad, S.(More)
This paper considers the selection of controlled variables for the unconstrained degrees of freedom, such that near-optimal operation is achieved with constant setpoints (“self-optimizing control”). From a second-order Taylor approximation around the optimal point, we derive an exact local method. This may be used to find the optimal linear combination of(More)
In this paper, we compare two solutions for suppressing slug flow in pipes conveying a diphasic fluid by active feedback control of the outlet (production) choke. Conventionnaly, a non-collocated PI controller is used to stabilize the measured inlet pressure. Unfortunately, this method lacks robustness to changes in operating conditions and reveals(More)
This paper addresses the problem of reproducing oscillations generated by the well-known slugging phenomenon in multiphase flow. Reported investigations show how to determine the parameters of a recently proposed ordinary differential equations system, so that it captures the characteristics of actually observed slugging oscillations. A tuning procedure(More)
This paper studies the multiphasis slugging flow phenomenon occurring in oil wells and flow lines. The main contribution is a low-dimensional distributed parameters model, comprising the gas mass fraction, the pressure, and gas velocity as states. Along with appropriate boundary conditions, on the one-dimensional space domain, it constitutes a well-posed(More)
Indirect control is commonly used in industrial applications where the primary controlled variable is not measured. This paper considers the case of “perfect indirect control” where one attempts to control a combination of the available measurements such that there is no effect of disturbances on the primary outputs at steady-state. This is always possible(More)
This paper illustrates the potential of nonlinear model-based control applied for stabilization of unstable flow in oil wells. A simple empirical model is developed that describes the qualitative behavior of the downhole pressure during severe riser slugging. A nonlinear controller is designed by an integrator backstepping approach, and stabilization for(More)