Development and Implementation of the Control System for an Autonomous Choke Valve for Downhole Flow and Pressure Control
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Drilling operations in the oil and gas industry calls for tight control of well pressure. A drilling fluid is used to control the pressure in the well, remove cuttings, and lubricate and cool the drill bit. If the well pressure becomes either too low or too high, it can cause cave ins or fracturing of the well. When drilling operations are executed from a floating drilling rig, the drill string may move in and out of the well with the ocean waves. This will cause pressure fluctuations in the well. As of today, the downhole pressure is controlled from topside equipment, but due to the length of the drill string, significant delay makes pressure control difficult. A proposed solution to remedy this problem is to install a choke valve downhole, right above the drill bit. This choke valve and its control system has been named HeaveLock. The aim of this thesis is to make the HeaveLock autonomous, and capable of controlling the downhole pressure in a lab at NTNU. The hardware and software that makes up the HeaveLock has been carefully selected and developed with autonomous operations in mind. The software that controls the HeaveLock is designed as a concurrent real-time program. It estimates the velocity of the HeaveLock based on acceleration, and uses the estimated velocity to control flow through the choke valve. By controlling the flow, pressure attenuation can be obtained. Both the velocity estimation and flow control algorithms are based on previous work, and has been discretized and adjusted to fit the purpose of the HeaveLock. A graphical user interface for a computer has been developed to enable effortless configuration and live data acquisition. Every aspect of the HeaveLock has been tested both individually and collectively. The HeaveLock estimates velocity with good precision, and operates as intended. Unfortunately, due to the lab being occupied with other experiments, it was not possible to perform the final test in the lab within the deadline of this thesis. However, a suggestion for how such a test should be performed has been attached to this thesis. With the exception of the pressure attenuation test of the HeaveLock in the lab, all objectives of this thesis have been met. The system is ready for the final test before being used in experiments.