Severe Slugging Mitigation in an S-Shape Pipeline-Riser System With a Venturi for Increased Production and Recovery

Severe slugging is a cyclic flow regime which causes intermittent delivery of oil and gas which usually leads to flow separator flooding, production reduction, platform trips and plant shutdown. This paper presents a novel method for severe slugging mitigation. It describes the use of a Venturi for the improvement of system stability, increase in production and recovery. A Venturi is coupled to the pipeline-riser system upstream of the choke valve before the topside test separator for severe slug mitigation. Experiments were carried out in a 2″ pipeline-riser system which comprises of a 40 m long horizontal pipe connected to a 10.23 m high S-shape riser followed by a 5.2 m horizontal topside section. The effects of Venturi on severe slugging were investigated, gas perturbation method was used to analyse the effects of Venturi on the stability of the system, and the traditional choking technique and Hopf bifurcation technique were combined and used to investigate the stability and production increase performance of the pipeline-riser with Venturi applied. Experimental results show that with the Venturi applied, the system achieves stability quicker than without the Venturi in the pipeline-riser and reduced the pressure fluctuation range by 57 %. In addition, combining the Venturi with the choke valve to choke the pipeline-riser (bifurcation study) stabilised the system at higher valve opening and lower pressure compared to choking the pipeline-riser with the choke valve only. For the case studied (Vsl = 0.25 m/s and Vsg = 0.37 m/s) bifurcation (critical valve opening) occurred at 18 % valve opening and average riser base pressure value of 2.8 barg for the plain riser. However, with Venturi applied bifurcation occurred at a larger valve opening of 21% and a lower average riser base pressure value of 2.5 barg. The low loss of energy due to the gradual change in geometry of the Venturi may account for its ability to achieve stability at a lower riser base pressure. Thus, Venturi increased the valve opening by 17% and reduced the riser base pressure by 11%. These in practice translate to an increase in oil and gas production. This is a cost-effective severe slug mitigation method, its deployment at the topside is an additional advantage when compared with other methods that require subsea deployment. The increase in brown fields due to diminishing reserves of oil from reservoirs have made oil recovery very vital. This technique will help to extend the operational life of a reservoir, thus enhancing oil recovery and flow assurance.