Abstract
The offshore de-oiling process is a vital part of current oil recovery, as it separates the profitable oil from water and ensures that the discharged water contains as little of the polluting oil as possible. With the passage of time, there is an increase in the water fraction in reservoirs that adds to the strain put on these facilities, and thus larger quantities of oil are being discharged into the oceans, which has in many studies been linked to negative effects on marine life. In many cases, such installations are controlled using non-cooperative single objective controllers which are inefficient in handling fluctuating inflows or complicated operating conditions. This work introduces a model-based robust H ∞ control solution that handles the entire de-oiling system and improves the system’s robustness towards fluctuating flow thereby improving the oil recovery and reducing the environmental impacts of the discharge. The robust H ∞ control solution was compared to a benchmark Proportional-Integral-Derivative (PID) control solution and evaluated through simulation and experiments performed on a pilot plant. This study found that the robust H ∞ control solution greatly improved the performance of the de-oiling process.
Highlights
Since the late 1960s, the North Sea has been home to a booming Oil and Gas industry and has provided substantial quantities of petroleum products [1]
During scenario 7 (Figure 9), the PID control solution performs well with respect to tracking the l reference with a maximum steady state error of approximately 5.6 mm, which is translated in an aggressive actuation of Vu, between 0% and 60%. This has a direct impact on Vo and the pressure drop ratio (PDR), where Vo experiences severe saturation in the fully open position, which accounts for 45% of the experiments duration with the longest period being 174 s
The reference tracking of l often results in saturation of Vu which is of great consequence for the PDR, as the Vo is less dominant and PID control solution cannot compensate for this effect due to the saturated Vo
Summary
Since the late 1960s, the North Sea has been home to a booming Oil and Gas industry and has provided substantial quantities of petroleum products [1]. The limit is exceeded occasionally, for example, in 2008, there were 25 instances where the monthly average limit was exceeded in the Danish sector of the North Sea, the number of such instances has fallen drastically and the limit was exceeded just once in 2012 [8]. It is, vital to keep in mind that the OiW in the discharge is measured offline using a Gas Chromatography with Flame-Ionization Detection (GC-FID) instrument, where at least two samples per day in accordance with the OSPAR Convention’s OSPAR reference method
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