Medium-Scale Experiments on Stabilizing Riser-Slug Flow

Summary This is the second of two papers describing control experiments on a medium-scale slug rig. The first paper (Sivertsen et al. 2009) describes experiments performed on a small-scale laboratory rig built at the Norwegian University of Science and Technology (NTNU) Department of Chemical Engineering. These experiments showed that, despite noisy measurements, it is possible, with feedback control, to "stabilize the flow" (i.e., to achieve reasonably smooth flow in the normally riser-induced severe slug-flow region) using only topside measurements. The question to be answered is whether these results also apply for larger riser systems. In the present paper, we look at some results obtained from a 10-m-high, 3-in.-diameter medium-scale test rig located at the Statoil Research Centre in Porsgrunn, Norway. Several cascade control structures are tested and compared, both with each other and with the results obtained from the small-scale NTNU loop. The rig was also modeled and analyzed using a simple three-state dynamic model. The new experiments were successful and confirm the results of Sivertsen et al. (2009) from the small-scale rig. The valve opening with nonslug flow operation could be increased from approximately 12% with no control to almost 24% with control using topside measurements only. This makes it possible to produce with a larger production rate and increase the total recovery from the producing oil field. The valve opening with control could be further increased to approximately 28% using measurements from the bottom of the riser, but such measurements may not be available in many cases.