Stabilization of two-phase flow in risers from reservoirs - Robust anti-slug control strategies

This thesis is an experimental study of robust ant-slug control strategies for riser systems. The main objective of this thesis was to develop a robust anti- slug control scheme to increase the stability of riser systems. Similar research has been done previously, but is repeated in this thesis using a new closed loop tuning method by Skogestad et al.[24]. Using this systematic approach ensures that the results from the different experiments are comparable. A number of experiments have been carried out using a small scale two phase flow riser loop. The robustness of the different control schemes was compared by slowly increasing the choke valve opening (setpoint) of the closed loop system until instability was reached. Since the inflow is depended on the system pressure, the control scheme with highest robustness was the one which was stable at the largest choke valve opening. Mainly, single loop control schemes were tested successfully. The general trend was that the best controllable variables, with respect to robustness, are measurements upstream the riser low point. The best control scheme was able to stabilize the riser system until a choke valve opening of 27 %. Top side measurements were in general found to be difficult to use in anti-slug control schemes. Density measurements using an optical slug sensor was able to stabilize the riser system, but no tuning method could be implemented. Measuring the topside flow rate using a venture tube and a DP-cell installation was tested with no success. The equipment used in the experiments was oversized and resulted in no signal output. Since no systematic tuning method could be implemented to the topside control schemes, no cascade anti-slug schemes were tested. Inspired by the density valve action, a square wave signal was used as an input to the choke valve. The experiments proved that the riser system stability region was extended when using a square wave as input