Severe slugging and unstable flows in an S-shaped riser
Severe slugging and unstable flow in an S-Shaped riser has been investigated in three research themes - experiments, criterion modelling and transient code modelling. A series of experiments were carried out on the Cranfield University Three Phase Facility and Riser Rig using a 10 m high riser over the pressure range 2,4 and 7 bar(a). The collected information was used to characterise the unstable flows in terms of pressure cycling, riser liquid inventory and fluid production characteristics. From analysis of the flow behaviour, it was found that in terms of slug characteristics, transitional severe slugging and oscillation flows are as potentially problematic as 'classical' severe slugging. This is due to the magnitude of peak flow in excess of the average fluid throughput in the riser and the size of the liquid slugs generated. A criterion for the occurrence of unstable flows in an S-Shaped riser was developed based upon considerations of bubble penetration at the riser base. This proved to be successful at predicting the experimental results from this work and an independent source. Comparisons were made between the experimental results and a transient code. The code could predict the occurrence of 'classical' severe slugging however the detailed characteristics of the experiments were not predicted by the code. The results for transitional severe slugging and oscillation flow showed further differences between the code prediction and the experiments. Drawing all simulation results together it was concluded that the prediction of pipeline behaviour and the propagation of flow regimes local to the riser base and in the curved riser pipe were significant sources of error in the simulation.